JP2005273069A - Carbon fiber and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultra fine carbon fiber having improved electric conductivity, and to provide a method for producing the same. <P>SOLUTION: This carbon fiber having a fiber diameter of 0.001μm to 2μm and a B element content concentration of 0.5 to 100 ppm, and substantially having a branched structure. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to carbon fibers. More specifically, the present invention relates to an ultrafine carbon fiber produced from a mixture comprising a thermoplastic resin and a thermoplastic carbon precursor.

  Carbon fibers are used as fillers for high-performance composite materials because they have excellent properties such as high strength, high elastic modulus, high conductivity, and light weight. Its use is not limited to conventional reinforcing fillers intended to improve mechanical strength, but is expected to be used as a conductive resin filler for electromagnetic shielding materials and antistatic materials, taking advantage of the high conductivity of carbon materials. ing. In addition, it is expected to be used as a field electron emission material such as a flat display by utilizing the characteristics of chemical stability, thermal stability and fine structure as a carbon material.

  As such a carbon fiber for a high-performance composite material, a carbon fiber using a vapor phase method has been reported.

  As a production method using a gas phase method, for example, an organic compound such as benzene is used as a raw material, and an organic transition metal compound such as ferrocene is introduced into a high-temperature reactor as a catalyst together with a carrier gas, and is generated on a substrate (for example, , See Patent Document 1), a method of generating carbon fibers by a vapor phase method in a floating state (see, for example, Patent Document 2), or a method of growing on a reaction furnace wall (see, for example, Patent Document 3). ) And the like.

  However, in general, graphite produced by these methods does not contain an atom that can be an acceptor or a donor, so that the valence band and the conduction band slightly overlap and become a semimetal. For this reason, there existed a problem that electrical conductivity was low.

As a method for solving the above, Hishiyama et al. Embedded a graphite sheet produced by roll forming expanded graphite in a mixed powder of B ₄ C and 3000 ° C.-treated petroleum coke, and heat-treated at 2500 ° C. for 30 minutes in an argon stream. By doing so, graphite sheets having different boron solid solution amounts were produced, and it has been reported that the electrical resistivity decreases as the boron doping amount increases (see Non-Patent Document 1).

  However, there has been no report so far applied to a system of a carbon fiber for use in a high-performance composite material, particularly an ultrafine carbon fiber having no branched structure.

JP-A-60-27700 (page 2-3) JP 60-54998 A (page 1-2) Japanese Patent No. 2778434 (page 1-2) Koji Hishiyama, Carbon Alloy Research Results Report, (1998), P251

  An object of the present invention is to provide an ultrafine carbon fiber with improved electrical conductivity and a method for producing the same, which solves the problems of the prior art.

As a result of intensive studies in view of the above prior art, the present inventors have completed the present invention.
That is, the object of the present invention is to
This is achieved by carbon fibers having a fiber diameter (D) of 0.001 μm to 2 μm and a B element-containing concentration of 0.5 to 100 ppm and having substantially no branched structure.

Moreover, in this invention, B element content is 1.0-50 ppm, B element content is 2.0-10 ppm, between fiber length (L) and fiber diameter (D). It is included that the following relational expression (1) holds.
30 <L / D (1)

Another object of the present invention is to
(1) Precursor from a mixture comprising 1 to 150 parts by weight of a thermoplastic resin and 100 parts by weight of a thermoplastic resin and at least one thermoplastic carbon precursor selected from the group consisting of pitch, polyacrylonitrile, polycarbodiimide, polyimide, polybenzoazole and aramid A step of forming a body fiber, (2) a step of forming a stabilized precursor fiber by subjecting the precursor fiber to stabilization treatment in an oxygen or oxygen / iodine mixed gas atmosphere, and (3) a stabilized precursor fiber. This is achieved by a method for producing carbon fiber that includes a step of forming a fibrous carbon precursor by removing the thermoplastic resin from (4), and a step of carbonizing or graphitizing the fibrous carbon precursor.

熱可塑性樹脂がポリ−４−メチルペンテン−１またはその共重合体であること、熱可塑性樹脂がポリエチレンであること、熱可塑性炭素前駆体がメソフェーズピッチ、ポリアクリロニトリルからなる群より選ばれる少なくとも一種であること、メソフェーズピッチが多環芳香族化合物由来のものであること、多環芳香族化合物がナフタレンまたはナフタレン誘導体であること、メソフェーズピッチのＢ元素含有量が５ｐｐｍ以下であること、も包含される。 Furthermore, in the production method of the present invention, the thermoplastic resin is represented by the following formula (I):

The thermoplastic resin is poly-4-methylpentene-1 or a copolymer thereof, the thermoplastic resin is polyethylene, and the thermoplastic carbon precursor is at least one selected from the group consisting of mesophase pitch and polyacrylonitrile. It is also included that the mesophase pitch is derived from a polycyclic aromatic compound, that the polycyclic aromatic compound is naphthalene or a naphthalene derivative, and that the B element content of the mesophase pitch is 5 ppm or less. .

  Since the carbon fiber of the present invention has superior electrical conductivity as compared with the conventional carbon fiber made of graphite, it can be used as a field electron emission material such as a flat display and a filler for high performance composite materials for antistatic.

Hereinafter, the present invention will be described in detail.
The carbon fiber of the present invention is achieved by a carbon fiber having a fiber diameter (D) of 0.001 μm to 2 μm and a B element-containing concentration of 0.5 to 100 ppm and having substantially no branched structure.

  In general, graphite is a semimetal with a slight overlap between the valence band and the conduction band. If boron with one electron is substituted and dissolved in this graphite structure, it becomes a hole-type metal, and an improvement in electrical conductivity can be expected. It is known that boron substituted and dissolved actually becomes an acceptor and the hole concentration increases. The amount of boron that can be substituted and dissolved as a thermodynamic equilibrium is very low, but it is far larger than the number of graphite carriers, and it is known that the effect of slight boron-substituted solid solution on the physical properties is very large. In order to achieve the intended effect of the invention, the content must be 0.5 ppm or more. On the other hand, if the B element content exceeds 100 ppm, the high crystallinity of the ultrafine carbon fiber finally obtained is destroyed, resulting in a decrease in electrical conductivity, which is not preferable.

  In order to obtain excellent electric conduction characteristics, the B element content is preferably 1.0 to 50 ppm, more preferably 2.0 to 10 ppm. Moreover, the carbon fiber of the present invention has substantially no branched structure. The carbon fiber obtained by the vapor phase growth method has many branched structures, and because of the branching, the graphite structure is disturbed, that is, the grain structure is observed, and there is a problem that the electrical conductivity is lowered. However, in the carbon fiber of the present invention, no branched structure was observed, and it was found from the transmission electron microscope and electron diffraction that the grain structure observed in the carbon fiber obtained by the vapor phase growth method was very small. .

  The carbon fiber of the present invention needs to have a fiber diameter (D) in the range of 0.001 μm to 2 μm. When the fiber diameter of the carbon fiber is larger than 2 μm, the performance as a high-performance composite material filler is remarkably lowered, which is not preferable. On the other hand, if the fiber diameter is less than 0.001 μm, the bulk density becomes very large and handling becomes difficult, which is not preferable.

In the carbon fiber of the present invention, the following relational expression (1) is preferably established between the fiber length (L) and the fiber diameter (D).
30 <L / D (1)

  When L / D is 30 or less, when the carbon fiber is used as a reinforcing material for a negative electrode of a lithium battery, its performance is remarkably lowered, which is not preferable. More preferably, L / D is 50 or more.

  Carbon fibers satisfying the above conditions are produced from a mixture of a thermoplastic resin and a thermoplastic carbon precursor. The following describes (1) the thermoplastic resin and (2) the thermoplastic carbon precursor used in the present invention, and then (3) a method for producing a mixture from the thermoplastic resin and the thermoplastic carbon precursor, and then (4 ) The method for producing carbon fiber from the mixture will be described in detail in this order.

(1) Thermoplastic resin The thermoplastic resin used in the present invention needs to be easily removed after the production of the stabilized precursor fiber. For this reason, it is decomposed | disassembled to 15 wt% or less of the initial weight, more preferably 10 wt% or less, and further 5 wt% or less by hold | maintaining for 5 hours at the temperature of 350 degreeC or more and less than 600 degreeC in oxygen or an inert gas atmosphere. It is preferable to use a thermoplastic resin.

  As such a thermoplastic resin, a polyacrylate polymer such as polyolefin, polymethacrylate, or polymethyl methacrylate, polystyrene, polycarbonate, polyarylate, polyester carbonate, polysulfone, polyimide, polyetherimide, or the like is preferably used. Among these, as a thermoplastic resin that has high gas permeability and can be easily thermally decomposed, for example, a polyolefin-based thermoplastic resin represented by the following formula (I) or polyethylene is preferably used.

  Specific examples of the compound represented by the formula (I) include poly-4-methylpentene-1 and a copolymer of poly-4-methylpentene-1, such as poly-4-methylpentene-1. Examples of the polymer copolymerized with vinyl monomers and polyethylene include polyethylene homopolymers such as high-pressure low-density polyethylene, medium-density polyethylene, high-density polyethylene, and linear low-density polyethylene. Or a copolymer of ethylene and an α-olefin; a copolymer of ethylene and another vinyl monomer such as an ethylene / vinyl acetate copolymer;

  Examples of the α-olefin copolymerized with ethylene include propylene, 1-butene, 1-hexene, 1-octene and the like. Examples of other vinyl monomers include vinyl esters such as vinyl acetate; (meth) acrylic acid, methyl (meth) acrylate, ethyl (meth) acrylate, n-butyl (meth) acrylate ( And meth) acrylic acid and alkyl esters thereof.

  In addition, the thermoplastic resin of the present invention can be easily melt-kneaded with a thermoplastic carbon precursor, so that when amorphous, the glass transition temperature is 250 ° C. or lower, and when crystalline, the crystalline melting point is 300 ° C. or lower. Preferably there is.

(2) Thermoplastic carbon precursor The thermoplastic carbon precursor used in the present invention is maintained at 200 ° C. or higher and lower than 350 ° C. for 2 to 30 hours in an oxygen or oxygen / iodine mixed gas atmosphere, and then 350 ° C. or higher. It is preferable to use a thermoplastic carbon precursor in which 80 wt% or more of the initial weight remains by holding at a temperature of less than 500 ° C. for 5 hours. Under the above conditions, if the residual amount is less than 80% of the initial weight, carbon fibers cannot be obtained with a sufficient carbonization rate from the thermoplastic carbon precursor, which is not preferable.

  More preferably, 85% or more of the initial weight remains under the above conditions. Specific examples of the thermoplastic carbon precursor that satisfies the above conditions include rayon, pitch, polyacrylonitrile, poly α-chloroacrylonitrile, polycarbodiimide, polyimide, polyetherimide, polybenzoazole, and aramids. Among these, pitch, polyacrylonitrile, and polycarbodiimide are preferable, and pitch is more preferable.

  Of the pitches, mesophase pitches which are generally expected to have high strength and high elastic modulus are preferred. The mesophase pitch refers to a compound that can form an optically anisotropic phase (liquid crystal phase) in a molten state. As raw materials for mesophase pitch, distillation residue of coal or petroleum may be used, and organic compounds may be used, but fragrances such as naphthalene and naphthalene derivatives are used for ease of stabilization, carbonization or graphitization. It is preferable to use mesophase pitch made of a group hydrocarbon.

  Moreover, it is preferable that B element content of a thermoplastic carbon precursor is 5 ppm or less. If the B element content exceeds 5 ppm, the high crystallinity of the ultrafine carbon fiber finally obtained is destroyed, resulting in a decrease in electrical conductivity, which is not preferable.

  The thermoplastic carbon precursor may be used in an amount of 1 to 150 parts by weight, preferably 5 to 100 parts by weight, based on 100 parts by weight of the thermoplastic resin.

(3) Manufacture of the mixture which consists of a thermoplastic resin and a thermoplastic carbon precursor The mixture used by this invention is manufactured from a thermoplastic resin and a thermoplastic carbon precursor. In order to produce carbon fibers having a fiber diameter of less than 2 μm from the mixture used in the present invention, the dispersion diameter of the thermoplastic carbon precursor in the thermoplastic resin is preferably 0.01 to 50 μm. When the dispersion diameter of the thermoplastic carbon precursor in the thermoplastic resin (I) deviates from the range of 0.01 to 50 μm, it may be difficult to produce carbon fibers for high-performance composite materials. A more preferable range of the dispersion diameter of the thermoplastic carbon precursor is 0.01 to 30 μm. Moreover, it is preferable that the dispersion diameter in the thermoplastic resin of a thermoplastic carbon precursor is 0.01-50 micrometers after hold | maintaining the mixture which consists of a thermoplastic resin and a thermoplastic carbon precursor at 300 degreeC for 3 minute (s). .

  Generally, when a mixture obtained by melt-kneading a thermoplastic resin and a thermoplastic carbon precursor is kept in a molten state, the thermoplastic carbon precursor aggregates with time, but due to the aggregation of the thermoplastic carbon precursor, If the dispersion diameter exceeds 50 μm, it may be difficult to produce carbon fibers for high performance composite materials. The degree of the aggregation rate of the thermoplastic carbon precursor varies depending on the type of the thermoplastic resin and the thermoplastic carbon precursor used, but is more preferably 5 minutes or more at 300 ° C., more preferably 10 minutes or more at 300 ° C. It is preferable to maintain a dispersion diameter of 0.01 to 50 μm. The thermoplastic carbon precursor in the mixture forms an island phase and is spherical or elliptical. The dispersion diameter referred to in the present invention is the spherical diameter of the thermoplastic carbon precursor or the length of the ellipsoid in the mixture. It means the shaft diameter.

  The usage-amount of a thermoplastic carbon precursor is 1-150 weight part with respect to 100 weight part of thermoplastic resins, Preferably it is 5-100 weight part. If the amount of the thermoplastic carbon precursor used exceeds 150 parts by weight, a thermoplastic carbon precursor having a desired dispersion diameter cannot be obtained, and if it is less than 1 part by weight, the target carbon fiber can be produced at low cost. This is not preferable because problems such as inability to occur occur.

  As a method for producing a mixture from a thermoplastic resin and a thermoplastic carbon precursor, kneading in a molten state is preferable. Melting and kneading of the thermoplastic resin and the thermoplastic carbon precursor can be carried out by using a known method as required. It is done. Among these, a co-rotating twin-screw melt kneading extruder is preferably used for the purpose of satisfactorily microdispersing the thermoplastic carbon precursor in the thermoplastic resin.

  The melt kneading temperature is preferably 100 to 400 ° C. When the melt kneading temperature is less than 100 ° C., the thermoplastic carbon precursor is not in a molten state, and micro-dispersion with the thermoplastic resin is difficult, which is not preferable. On the other hand, when the temperature exceeds 400 ° C., the decomposition of the thermoplastic resin and the thermoplastic carbon precursor proceeds, which is not preferable. A more preferable range of the melt kneading temperature is 150 ° C to 350 ° C.

  The melt kneading time is 0.5 to 20 minutes, preferably 1 to 15 minutes. When the melt kneading time is less than 0.5 minutes, it is not preferable because micro dispersion of the thermoplastic carbon precursor is difficult. On the other hand, when it exceeds 20 minutes, the productivity of carbon fibers is remarkably lowered, which is not preferable.

  In the present invention, when a mixture is produced from a thermoplastic resin and a thermoplastic carbon precursor by melt-kneading, it is preferably melt-kneaded in a gas atmosphere having an oxygen gas content of less than 10%. The thermoplastic carbon precursor used in the present invention reacts with oxygen to be modified and infusible at the time of melt kneading, which may inhibit micro-dispersion in the thermoplastic resin. For this reason, it is preferable to perform melt kneading while circulating an inert gas to reduce the oxygen gas content as much as possible. The oxygen gas content during melt kneading is more preferably less than 5%, and further less than 1%. By implementing said method, the mixture of a thermoplastic resin and a thermoplastic carbon precursor for manufacturing the nonwoven fabric which consists of carbon fibers can be manufactured.

(4) Method for Producing Carbon Fiber The carbon fiber of the present invention can be produced from a mixture comprising the above-described thermoplastic resin and a thermoplastic carbon precursor. That is, the carbon fiber of the present invention comprises (4-1) a step of forming a precursor fiber from a mixture of 100 parts by weight of a thermoplastic resin and 1 to 150 parts by weight of a thermoplastic carbon precursor, (4-2) a precursor. A step of stabilizing the thermoplastic carbon precursor in the precursor fiber to form a stabilized precursor fiber by subjecting the fiber to a stabilization treatment; (4-3) removing the thermoplastic resin from the stabilized precursor fiber; And a step of forming a fibrous carbon precursor, and (4-4) a step of carbonizing or graphitizing the fibrous carbon precursor. Each step will be described in detail below.

(4-1) Step of forming precursor fibers from a mixture comprising a thermoplastic resin and a thermoplastic carbon precursor In the present invention, a mixture precursor fiber obtained by melt-kneading a thermoplastic resin and a thermoplastic carbon precursor is formed. To do. Examples of the method for producing the precursor fiber include a method obtained by melt spinning a mixture of a thermoplastic resin and a thermoplastic carbon precursor from a spinneret. The spinning temperature for melt spinning is 150 ° C to 400 ° C, preferably 180 ° C to 350 ° C. The spinning take-up speed is preferably 10 m / min to 2000 m / min.

  Moreover, the method of forming precursor fiber by the melt blow method from the mixture obtained by melt-kneading a thermoplastic resin and a thermoplastic carbon precursor as another method can also be illustrated. As the conditions for melt blowing, a range in which the discharge die temperature is 150 to 400 ° C. and the gas temperature is 150 to 400 ° C. is preferably used. Although the gas blowing speed of the melt blow affects the fiber diameter of the precursor fiber, the gas blowing speed is usually 2000 to 100 m / s, more preferably 1000 to 200 m / s.

  When a mixture of a thermoplastic resin and a thermoplastic carbon precursor is melt-kneaded and then discharged from the die, it is melt-kneaded and then fed in the molten state in a molten state and continuously sent to the discharge die. Preferably, the transfer time from melt-kneading to spinneret discharge is preferably within 10 minutes.

(4-2) A step of subjecting the precursor fiber to stabilization treatment to stabilize the thermoplastic carbon precursor in the precursor fiber to form a stabilized precursor fiber In the second step in the production method of the present invention, The precursor fiber prepared above is subjected to a stabilization treatment to stabilize the thermoplastic carbon precursor in the precursor fiber to form a stabilized precursor fiber. Stabilization of the thermoplastic carbon precursor is a necessary process for obtaining carbonized or graphitized carbon fibers. If the thermoplastic resin is removed as the next process without carrying out this process, the thermoplastic carbon is removed. Problems such as thermal decomposition and fusion of the precursor occur.

  The stabilization can be performed by a known method such as a gas flow treatment with oxygen or a solution treatment with an acidic aqueous solution, but infusibilization under a gas flow is preferable from the viewpoint of productivity. As the gas component to be used, oxygen and / or from the viewpoint of permeability to the thermoplastic resin and adsorption to the thermoplastic carbon precursor, and from the point that the thermoplastic carbon precursor can be quickly infusibilized at a low temperature. A mixed gas containing a halogen gas is preferred. Examples of the halogen gas include fluorine gas, chlorine gas, bromine gas, and iodine gas. Among these, bromine gas, iodine gas, and particularly iodine gas are preferable. As a specific method of infusibilization under a gas stream, it is preferable to perform the treatment in a desired gas atmosphere at a temperature of 50 to 350 ° C., preferably 80 to 300 ° C., for 5 hours or less, preferably 2 hours or less. .

  Further, although the softening point of the thermoplastic carbon precursor contained in the precursor fiber is remarkably increased by the infusibilization, the softening point is preferably 400 ° C. or higher for the purpose of obtaining a desired ultrafine carbon fiber. More preferably, the temperature is higher than or equal to ° C. By carrying out the above method, it is possible to stabilize the thermoplastic carbon precursor in the precursor fiber and obtain a stabilized precursor fiber.

(4-3) Step of forming a fibrous carbon precursor by removing the thermoplastic resin from the stabilized precursor fiber The third step in the production method of the present invention is the thermoplastic resin contained in the stabilized precursor fiber. It is removed by pyrolysis. Specifically, the thermoplastic resin contained in the stabilized precursor fiber is removed, and only the stabilized fibrous carbon precursor is separated to form the fibrous carbon precursor. To do. In this step, it is necessary to suppress the thermal decomposition of the fibrous carbon precursor as much as possible, to decompose and remove the thermoplastic resin, and to separate only the fibrous carbon precursor.

  The removal of the thermoplastic resin may be performed in either an oxygen-existing atmosphere or an inert gas atmosphere. When removing the thermoplastic resin in an oxygen-existing atmosphere, it is necessary to remove it at a temperature of 350 ° C. or higher and lower than 600 ° C. The term “in the presence of oxygen” as used herein refers to a gas atmosphere having an oxygen concentration of 1 to 100%. In addition to oxygen, inert gases such as carbon dioxide, nitrogen, and argon, and inert gases such as iodine and bromine are used. It may contain gas. Among these conditions, it is particularly preferable to use air particularly from the viewpoint of cost.

  When the temperature for removing the thermoplastic resin contained in the stabilized precursor fiber is less than 350 ° C., the thermal decomposition of the fibrous carbon precursor can be suppressed, but the thermoplastic resin cannot be sufficiently decomposed, which is not preferable. . If the temperature is 600 ° C. or higher, the thermoplastic resin can be sufficiently thermally decomposed, but the fibrous carbon precursor is also thermally decomposed, resulting in carbonization of the carbon fiber obtained from the thermoplastic carbon precursor. The yield is lowered, which is not preferable.

  The temperature at which the thermoplastic resin contained in the stabilized precursor fiber is decomposed is preferably 380 to 500 ° C. in an oxygen atmosphere, particularly in the temperature range of 400 to 450 ° C. for 0.5 to 10 hours. Is preferred. By performing the above treatment, the thermoplastic resin is decomposed to 15 wt% or less of the initial weight used. Moreover, 80 wt% or more of the initial weight used for the thermoplastic carbon precursor remains as a fibrous carbon precursor.

  Moreover, when removing a thermoplastic resin in inert gas atmosphere, it is necessary to remove at the temperature of 350 degreeC or more and less than 600 degreeC. The term “inert gas atmosphere” as used herein refers to a gas such as carbon dioxide, nitrogen, or argon having an oxygen concentration of 30 ppm or less, more preferably 20 ppm or less. Note that a halogen gas such as iodine or bromine may be contained.

  In addition, as an inert gas used at this process, a carbon dioxide and nitrogen can be used preferably from the relationship of cost, and nitrogen is especially preferable. When the temperature at which the thermoplastic resin contained in the nonwoven fabric composed of the stabilized precursor fibers is removed is less than 350 ° C., the thermal decomposition of the fibrous carbon precursor can be suppressed, but the thermoplastic resin can be sufficiently decomposed. Not preferable. Further, when the temperature is 600 ° C. or higher, the thermoplastic resin can be sufficiently thermally decomposed, but the fibrous carbon precursor is also thermally decomposed. As a result, the thermoplastic resin is made of carbon fibers obtained from the thermoplastic carbon precursor. Since the carbonization yield of a nonwoven fabric is reduced, it is not preferable.

  The temperature for decomposing the thermoplastic resin contained in the stabilized precursor fiber is preferably set to 380 to 550 ° C. in an inert gas atmosphere, particularly in the temperature range of 400 to 530 ° C., for 0.5 to 10 hours. It is preferable to do this. By performing the above-mentioned treatment, it is decomposed to 15 wt% or less of the initial weight of the used thermoplastic resin. Moreover, 80 wt% or more of the initial weight of the used thermoplastic carbon precursor remains as a fibrous carbon precursor.

  Further, as another method of removing the thermoplastic resin from the stabilized precursor fiber to form the fibrous carbon precursor, a method of removing the thermoplastic resin with a solvent may be adopted. In this method, it is necessary to suppress dissolution of the fibrous carbon precursor in the solvent as much as possible, decompose and remove the thermoplastic resin, and separate only the fibrous carbon precursor. In order to satisfy this condition, in the present invention, it is preferable to remove the thermoplastic resin contained in the fibrous carbon precursor with a solvent having a temperature of 30 to 300 ° C. When the temperature of the solvent is less than 30 ° C., it takes a lot of time to remove the thermoplastic resin contained in the precursor fiber, which is not preferable. On the other hand, if it is 300 ° C. or higher, it is possible to remove the thermoplastic resin in a short time, but not only dissolve the fibrous carbon precursor and destroy the fiber structure, but also the carbon fiber finally obtained This is not preferable because it reduces the carbonization yield of the raw material. The temperature at which the thermoplastic resin is removed from the stabilized precursor fiber with a solvent is particularly preferably 50 to 250 ° C, more preferably 80 to 200 ° C.

(4-4) Step of carbonizing or graphitizing the fibrous carbon precursor The fourth step in the production method of the present invention is a fibrous carbon precursor obtained by removing the thermoplastic resin to 15 wt% or less of the initial weight. Carbon fiber is produced by carbonization or graphitization in an inert gas atmosphere. In the present invention, the fibrous carbon precursor is carbonized or graphitized by high-temperature treatment in an inert gas atmosphere to form a desired carbon fiber. The fiber diameter of the obtained carbon fiber is preferably 0.001 μm to 2 μm.

  Carbonization or graphitization of the fibrous carbon precursor can be performed by a known method. Examples of the inert gas used include nitrogen and argon, and the temperature is 500 ° C to 3500 ° C, preferably 800 ° C to 3000 ° C. Note that the oxygen concentration during carbonization or graphitization is preferably 20 ppm or less, and more preferably 10 ppm or less. By carrying out the above method, carbon fibers can be produced.

EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention does not receive any limitation by this. In addition, each characteristic in a present Example was calculated | required by the method as described below.
Dispersion particle diameter of carbon precursor organic compound in thermoplastic resin and fiber diameter of fibrous molded body containing carbon fiber precursor:
Measurement was performed with a scanning electron microscope (“S-2400” manufactured by Hitachi, Ltd.).
Mesophase pitch and carbon fiber B element content:
1.0 g of a sample was weighed into a platinum crucible, 4 ml of 3% calcium hydroxide aqueous solution was added, mixed and wetted with the sample, and then incinerated at 880 ° C. (according to the method described in JIS R7223).
The ash was dissolved and diluted in dilute hydrochloric acid to obtain a measurement solution. About this solution, B element was quantified by ICP emission spectrometry ("ICPS-8000" manufactured by Shimadzu Corporation) to determine the content in the sample.

[Example 1]
100 parts by weight of poly-4-methylpentene-1 (TPX: grade RT-18 [manufactured by Mitsui Chemicals, Inc.) as a thermoplastic resin and mesophase pitch AR-HP (manufactured by Mitsubishi Gas Chemical Co., Ltd.) as a thermoplastic carbon precursor 11.1 parts were melt-kneaded with the same direction twin screw extruder (TEX-30, manufactured by Nippon Steel Works, barrel temperature 290 ° C., under nitrogen stream) to prepare a mixture. The dispersion diameter of the mixture obtained under these conditions into the thermoplastic resin of the thermoplastic carbon precursor was 0.05 to 2 μm. Further, this mixture was held at 300 ° C. for 10 minutes, but aggregation of the thermoplastic carbon precursor was not observed, and the dispersion diameter was 0.05 to 2 μm. The B content in the mesophase pitch AR-HP was 1.2 ppm.

  Next, the mixture was made into a non-woven fabric by the melt blow method, but in that case, it was discharged from the discharge hole at 330 ° C., and the air just below the discharge hole was blown at 350 ° C. and 500 m / min of air to the molten fiber, A nonwoven fabric composed of precursor fibers having a fiber diameter of 0.5 to 5 μm was prepared.

  Stabilize with air in a 1 liter pressure glass so that 0.5 parts by weight of iodine is contained per 10 parts by weight of the nonwoven fabric composed of this precursor fiber and hold at 180 ° C. for 20 hours. The nonwoven fabric which consists of a stabilization precursor fiber was created by processing.

  Next, the nonwoven fabric made of the fibrous precursor is made by removing the thermoplastic resin by heating the nonwoven fabric made of the stabilized precursor fiber to 550 ° C. at a temperature rising rate of 5 ° C./min in a nitrogen gas atmosphere. did. Carbon fibers were prepared by heating the nonwoven fabric composed of the fibrous carbon precursor from room temperature to 2800 ° C. in 3 hours in an argon gas atmosphere. The obtained carbon fiber diameter (D) is about 100 to 600 nm, the carbon fiber length (L) is 100 μm or more, L / D is larger than 30, and the fact that a branched structure is not substantially observed. This was confirmed by microscopic observation. The B element content in the ultrafine carbon fiber was 2.3 ppm.

Claims (12)

  Carbon fiber having a fiber diameter (D) of 0.001 μm to 2 μm and a B element-containing concentration of 0.5 to 100 ppm and having substantially no branched structure.   The carbon fiber according to claim 1, wherein the B element content is 1.0 to 50 ppm.   The carbon fiber according to claim 1, wherein the B element content is 2.0 to 10 ppm. The carbon fiber according to claim 1 or 2, wherein the following relational expression (1) is established between the fiber length (L) and the fiber diameter (D).
30 <L / D (1)   (1) Precursor from a mixture comprising 1 to 150 parts by weight of a thermoplastic resin and 100 parts by weight of a thermoplastic resin and at least one thermoplastic carbon precursor selected from the group consisting of pitch, polyacrylonitrile, polycarbodiimide, polyimide, polybenzoazole and aramid A step of forming a body fiber, (2) a step of forming a stabilized precursor fiber by subjecting the precursor fiber to stabilization treatment in an oxygen or oxygen / iodine mixed gas atmosphere, and (3) a stabilized precursor fiber. A process for producing a carbon fiber through a step of forming a fibrous carbon precursor by removing the thermoplastic resin from (4) carbonizing or graphitizing the fibrous carbon precursor. The method for producing carbon fiber according to claim 5, wherein the thermoplastic resin is represented by the following formula (I).

  The method for producing carbon fiber according to claim 6, wherein the thermoplastic resin is poly-4-methylpentene-1 or a copolymer thereof.   The method for producing carbon fiber according to claim 6, wherein the thermoplastic resin is polyethylene.   6. The method for producing carbon fiber according to claim 5, wherein the thermoplastic carbon precursor is at least one selected from the group consisting of mesophase pitch and polyacrylonitrile.   The method for producing a carbon fiber according to claim 9, wherein the mesophase pitch is derived from a polycyclic aromatic compound.   The method for producing carbon fiber according to claim 10, wherein the polycyclic aromatic compound is naphthalene or a naphthalene derivative.   The method for producing a carbon fiber according to claim 9, wherein the B element content of the thermoplastic carbon precursor is 5 ppm or less.