JP4429765B2 - Carbon fiber and method for producing the same - Google Patents

Description

  The present invention relates to carbon fibers, particularly ultrafine carbon fibers, and more particularly to carbon fibers produced from a mixture of 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 application is not limited to conventional fillers for the purpose of improving mechanical strength, but by taking advantage of the high conductivity of carbon materials, as an electromagnetic shielding material, a conductive resin filler for antistatic materials, or Use as a filler for electrostatic coatings on resins is expected. 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.

  Two methods for producing such carbon fibers for high performance composite materials have been reported: (1) carbon fiber production method using vapor phase method, and (2) method of producing resin composition from melt spinning. ing.

  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, , Refer to Patent Document 1), a method of generating VGCF in a floating state (for example, refer to Patent Document 2), a method of growing on a reactor wall (for example, refer to Patent Document 3), and the like. ing. However, although the carbon fiber obtained by these methods has high strength and high elastic modulus, there are many branches, and because of the branching, the disorder of the graphite structure, that is, the grain structure is observed, and the elastic modulus and strength are reduced. There is a problem, and there is a problem that the dispersibility of the blend into the resin is lowered due to the entanglement of the carbon fibers due to branching, and the performance as a reinforcing filler is very low. In addition, there is a problem of high costs.

  On the other hand, as a method for producing carbon fibers from melt spinning of a resin composition, a method for producing ultrafine carbon fibers from a composite fiber of a phenol resin and polyethylene (for example, see Patent Document 4) is disclosed. In the case of the method, a carbon fiber having a small branched structure can be obtained, but since the phenol resin is completely amorphous, it is difficult to form an orientation and is hardly graphitized, so that the strength and elastic modulus of the obtained ultrafine carbon fiber are low. There was a problem that expression was not expected.

JP-A-60-27700 (page 2-3) JP 60-54998 A (page 1-2) Japanese Patent No. 2778434 (page 1-2) JP 2001-73226 A (page 3-4)

An object of the present invention is to provide a high-strength and high-modulus carbon fiber having no branch structure from a thermoplastic carbon precursor, more specifically, an ultrafine carbon fiber having a fiber diameter of 0.001 μm to 2 μm. .
Furthermore, the other object of this invention is to provide the method of manufacturing the said ultrafine fiber.

As a result of intensive studies in view of the above-described prior art, the present inventors have completed the present invention. That is, the object of the present invention is to
A carbon fiber comprising a plurality of graphite layers converged and having a structure in which a graphite layer existing at an end of the carbon fiber is closed in a loop shape can be achieved by the carbon fiber. .

Further, the present invention has a structure in which a plurality of graphite layers are oriented substantially in the fiber axis direction, and the graphite layers on the surface other than the carbon fiber end portions are not closed in a loop shape, the fiber diameter of the carbon fibers ( It is also included that D) is 0.001 μm to 2 μm, and that the following relational expression (1) holds between the fiber length (L) and the fiber diameter (D) of the carbon fiber.
30 <L / D (1)

Another object of the present invention is to
(1) Precursor consisting of 100 parts by weight of thermoplastic resin and 1 to 150 parts by weight of 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. removing the thermoplastic resin to form a fibrous carbon precursor from step (4) achieved by the method for producing a carbon-containing fiber according to the the fibrous carbon precursor Ru through the step of carbonization or graphitization Is done.

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.

Since the carbon fiber of the present invention has less fiber branching structure than the carbon fiber obtained by a conventionally known vapor phase growth method, the graphite is less disturbed and has high strength and high elastic modulus.
Moreover, since it does not have a branched structure, there is almost no entanglement between the carbon fibers, and the blend dispersibility to the resin is very good. For this reason, it is very useful as a reinforcing filler.

Hereinafter, the present invention will be described in detail.
The carbon fiber of the present invention is a carbon fiber formed by converging a plurality of graphites, and has a structure in which the graphite present at the ends of the carbon fibers is closed in a loop layer shape.

  Here, the carbon fiber formed by converging a plurality of graphites means that the graphite layers are bundled together or bundled in a disorderly manner, thereby taking a fiber shape as a whole of the plurality of graphite layers, The structure in which the graphite present at the end of the carbon fiber is closed in a loop shape is that the end of the graphite layer itself is not exposed at the end of the carbon fiber, the graphite layer is curved in a substantially U shape, and the curved portion is carbon. It refers to the state exposed at the fiber end.

In the present invention, since the graphite layer has such a structure, the disturbance of the graphite layer of the entire carbon fiber is suppressed, and a carbon fiber having a high elastic modulus and high strength can be obtained.
In the carbon fiber of the present invention, it is preferable that a plurality of graphite layers are oriented substantially in the fiber axis direction, and the graphite layers on the surface other than the ends of the carbon fibers are not closed in a loop shape.

Here, the plurality of graphite layers are oriented substantially in the fiber axis direction, which means that the plurality of graphite layers are in a fiber shape as a whole in a state where the graphite layers are aligned and bundled. The fact that the graphite layer on the other surface is not closed in a loop shape means a state where the above-described substantially curved U-shaped portion is not exposed except at the end of the carbon fiber.
If it is such a structure, disorder of the graphite layer of the whole carbon fiber will be suppressed further, and a high elastic modulus and high intensity | strength carbon fiber can be obtained.

The carbon fiber of the present invention preferably has 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, it should be noted that when used as a filler for a high performance composite material, the performance is remarkably lowered.
On the other hand, if the fiber diameter is less than 0.001 μm, the bulk density becomes very small, and handling becomes difficult.

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)

  Here, when L / D is 30 or less, when using as a filler for high performance composite materials, it is necessary to note that the performance is remarkably deteriorated. More preferably, L / D is 50 or more. When it is within this range, a carbon fiber having further excellent mechanical properties as a high-performance composite filler can be obtained.

  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 inert gas atmosphere. It is preferable to use a thermoplastic resin. As such a thermoplastic resin, a polyacrylate polymer such as polyolefin, polymethacrylate, polymethylmethacrylate, polystyrene, polycarbonate, polyarylate, polyester carbonate, polysulfone, polyimide, polyetherimide and the like are 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. It is preferable to use a mesophase pitch using as a raw material. 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 1 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 of the thermoplastic carbon precursor in the thermoplastic resin is 0.01 to 50 μm after holding a mixture of the thermoplastic resin and the thermoplastic carbon precursor at 300 ° C. for 3 minutes. .

  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. The melt kneading of the thermoplastic resin and the thermoplastic carbon precursor can be carried out using a known method as required, such as a uniaxial melt kneading extruder, a biaxial melt kneading extruder, a mixing roll, a Banbury mixer, etc. 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 the thermoplastic resin and thermoplastic carbon precursor for manufacturing carbon fiber 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, precursor fibers are obtained from a mixture obtained by melt-kneading a thermoplastic resin and a thermoplastic carbon precursor. Form. 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 the melt blow, a discharge die temperature of 150 to 400 ° C. and a gas temperature of 150 to 400 ° C. are preferably used. The gas blowing speed of the melt blow affects the fiber diameter of the precursor fiber, but 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” here 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, when the temperature is 300 ° C. or higher, the thermoplastic resin can be removed in a short time, but the fibrous carbon precursor (I) is dissolved and the fiber structure is not only destroyed, but finally obtained. This is not preferable because the carbonization yield of the carbon fiber raw material is lowered. 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 a nonwoven fabric composed of a fibrous carbon precursor The fourth step in the production method of the present invention is fibrous carbon obtained by removing the thermoplastic resin to 15 wt% or less of the initial weight. The precursor is carbonized or graphitized in an inert gas atmosphere to produce a carbon fiber. 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 1 μm.

  Carbonization or graphitization of a nonwoven fabric made of a 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.

(5) Carbon fiber The carbon fiber of the present invention is a carbon fiber formed by converging a plurality of graphite layers, and has a structure in which the graphite layer present at the end of the carbon fiber is closed in a loop shape, A plurality of graphite layers are oriented substantially in the fiber axis direction, and the graphite layers on the surface other than the ends of the carbon fibers are carbon fibers having a structure that is not closed in a loop shape. The carbon fiber of the present invention has a carbon fiber fiber diameter (D) of 0.001 μm to 2 μm, and the following relational expression (1) between the fiber length (L) of the carbon fiber and the fiber diameter (D): Holds.
30 <L / D (1)

Examples of the present invention will be described below. In addition, this invention is not limited by the content described below.
The dispersion particle diameter and precursor fiber of the thermoplastic carbon precursor in the thermoplastic resin, and the fiber diameter of the carbon fiber were measured with a scanning electron microscope (“S-2400” manufactured by Hitachi, Ltd.). Further, the observation of the graphite layer on the surface other than the end portions of the carbon fibers was performed with a transmission electron microscope (“H-9000UHR” manufactured by Hitachi, Ltd.).

[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.

  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 photograph taken with the transmission electron microscope of the obtained carbon fiber edge part and the surface is published.

  As shown in FIG. 1, the graphite structure at the end of the carbon fiber has a closed loop structure. From FIG. 2, it was confirmed that the graphite layer was oriented substantially in the fiber axis direction, and the graphite on the carbon fiber surface was not closed in a loop shape.

  In addition, it was confirmed that the obtained carbon fiber diameter (D) was around 100 to 600 nm, the carbon fiber length (L) was 100 μm or more, and L / D was larger than 30.

It is the photograph (photographing magnification of 2.5 million times) image | photographed with the transmission electron microscope (Hitachi Ltd. "H-9000UHR") of the carbon fiber end obtained by operation of Example 1. FIG. It is the photograph (photographing magnification of 37.550,000 times) image | photographed with the transmission electron microscope (Hitachi Ltd. "H-9000UHR") near the carbon fiber surface obtained by operation of Example 1. FIG.

Claims (9)

  A carbon fiber comprising a plurality of graphite layers converged and having a structure in which a graphite layer existing at an end of the carbon fiber is closed in a loop shape.   2. The carbon fiber according to claim 1, wherein the plurality of graphite layers are oriented substantially in the fiber axis direction, and the graphite layers on the surface other than the ends of the carbon fibers have a structure not closed in a loop shape.   Carbon fiber of Claim 1 whose fiber diameter (D) of carbon fiber is 0.001 micrometer-2 micrometers. The carbon fiber according to claim 1, wherein the following relational expression (1) is established between the fiber length (L) and the fiber diameter (D) of the carbon fiber.
30 <L / D (1) (1) Precursor consisting of 100 parts by weight of thermoplastic resin and 1 to 150 parts by weight of 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. step of removing the thermoplastic resin to form a fibrous carbon precursor from (4) a step of carbonizing or graphitizing the fibrous carbon precursor, through the method for producing a carbon-containing fiber according to claim 1 . 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.   The method for producing a carbon fiber according to claim 1, wherein the thermoplastic carbon precursor is at least one selected from the group consisting of mesophase pitch and polyacrylonitrile.

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