JP2015048561A - Electrically conductive ultrathin carbon fiber excellent in homogeneity and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pitch-based carbon fiber that has a uniform fiber diameter and has excellent in dispersion in an aqueous solvent and an aqueous polymer.SOLUTION: The pitch-based carbon fiber has an average fiber diameter of 0.01-5 μm, a CV value of 0-10% which shows the variation in fiber diameter and a surface oxygen concentration ratio (O/C) of 0.02 or more which is defined by an abundance ratio of oxygen atom to carbon atom on the surface of the carbon fiber as measured by an X-ray photoelectron spectroscopy apparatus.

Description

  The present invention relates to a pitch-based ultrafine carbon fiber having excellent conductivity and excellent uniformity.

  Carbon fiber has excellent properties such as high mechanical properties, chemical resistance, high conductivity, and high thermal conductivity, and is used in various fields such as aircraft, automobiles, leisure goods, and general industries for the purpose of improving mechanical strength. Widely used. Furthermore, in recent years, attempts have been made to develop carbon fibers having an ultrafine fiber diameter, and an excellent performance effect with a large specific surface area can be expected, and the ultrafine carbon fibers having excellent quality are not only used as reinforcing fillers. Applications in many fields such as a material for imparting conductivity to a resin, an electromagnetic shielding material, and a conductive aid for electrodes are expected.

  For example, as a method for providing ultrafine carbon fibers, a gas phase method (Patent Literature 1, Patent Literature 2), a melt blow method (Patent Literature 3, Patent Literature 4), an electrospinning method (Patent Literature 5, Patent Literature 6), In addition, a mixed melt spinning method (Patent Document 7, Patent Document 8, and Patent Document 9) in which a thermoplastic resin and a carbon source resin are mixed has been reported.

  The ultra-fine carbon fiber obtained by the vapor phase method has a high cohesiveness due to its very small fiber diameter, and it is difficult to uniformly disperse due to the high hydrophobicity of the fiber surface. was there.

  In the melt blow method, the electrospinning method, and the mixed melt spinning method by mixing a thermoplastic resin and a carbon source resin, there is a problem that variations in fiber diameter are large and a product having excellent uniformity cannot be obtained.

  In order to exhibit uniform performance when used as a filler and an auxiliary agent, the fiber diameter and physical properties of carbon fibers must be high and they must be uniformly dispersed.

  If the fiber diameter is varied, unevenness occurs in the oxidation reaction at the time of infusibilization, so that a homogeneous carbon fiber cannot be obtained. Moreover, since uneven density after dispersion occurs, local bias may occur in characteristics such as conductive performance.

  As a method for obtaining ultrafine fibers having a uniform fiber diameter, a composite spinning method using a sea-island type composite nozzle has been conventionally used. After forming the sea-island type composite fiber, the sea component is removed to obtain an ultrafine fiber composed of the island component. Patent Document 10 discloses an ultrafine carbon fiber having a uniform fiber diameter. However, since this method uses a cellulosic raw material as a carbon fiber source, sufficient electrical conductivity and mechanical properties cannot be obtained.

  On the other hand, in order to impart performance with good homogeneity, in addition to the uniformity of the fiber diameter, it is required that they are uniformly dispersed. In particular, the use of an aqueous solvent is desired while the reduction of environmental load in recent years has been sought, and it is very useful to be able to disperse uniformly in an aqueous solvent and an aqueous polymer solution. However, since carbon fibers are generally highly hydrophobic, it is difficult to disperse them in an aqueous solvent and an aqueous polymer solution as they are.

  As a method for satisfactorily dispersing ultrafine carbon fibers in an aqueous solvent, a method of chemically modifying the carbon fiber surface (Patent Document 11) and a method of coating a polymer or the like on the surface (Patent Document 12) are disclosed. This method has a problem that when the surface is modified, the conductivity of the carbon fiber is partially impaired, or a complicated process is required, so that the productivity is not excellent. Moreover, although the method using a dispersing agent such as a surfactant (Patent Document 13, Patent Document 14, Patent Document 15) is disclosed, a dispersion with a sufficient concentration cannot be obtained, or as an electronic member such as an electrode. When used, there is a problem that the remaining dispersant may adversely affect the performance and the application is limited.

JP-A-60-54998 Japanese Patent No. 2778434 Japanese Patent No. 2680183 JP 2000-8227 A JP 2009-203565 A Japanese Patent No. 4697901 JP 2008-169511 A JP-A-2005-060882 JP 2005-248371 A JP 2010-1000096 A JP 2006-265151 A JP-T-2004-506530 JP 2001-048511 A JP 2003-238126 A Japanese Patent Laying-Open No. 2005-263608

  An object of the present invention is to provide a pitch-based ultrafine carbon fiber having excellent conductivity and excellent homogeneity.

  As a result of intensive studies to solve the above problems, the present inventors have made a mixture of pitch and a meltable polyvinyl alcohol polymer with an island component, and another meltable polymer with a sea component, After obtaining a sea-island type composite fiber by using the sea-island type composite fiber, it was found that an ultrafine carbon fiber excellent in homogeneity can be obtained by removing a sea component from the sea-island type composite fiber and subjecting it to a carbonization treatment. .

  That is, the first embodiment of the present invention is a pitch-based carbon fiber having an average fiber diameter of 0.01 to 5 μm, a CV value indicating variation in fiber diameter of 0 to 10%, and X-ray photoelectron spectroscopy. This is a pitch-based carbon fiber having a surface oxygen concentration ratio (O / C), which means an abundance ratio of oxygen atoms to carbon atoms on the surface of the carbon fiber, measured by an apparatus, is 0.02 or more.

  The pitch-based carbon fiber has an intensity ratio Id / Ig between the D band and the G band in a Raman spectrum of 0.1 or more and less than 1.0, and a crystal lattice plane spacing in an X-ray diffraction method is 0.37 nm or less. Preferably there is.

Further, the pitch-based carbon fiber preferably has a volume specific resistance value of 5 × 10 ⁻² Ω · cm or less.

The second embodiment of the present invention includes a method for producing pitch-based carbon fibers.
The manufacturing method uses a mixture of a polyvinyl alcohol-based polymer that can be melted as an island component and pitch, uses another molten polymer as a sea component, and separates the island component polymer and the sea component polymer separately. A process of melting and forming each melt flow, and then discharging from a sea-island type composite nozzle to obtain a sea-island type composite fiber, and a step of removing sea components from the sea-island type composite fiber to obtain ultrafine fibers made of island components And an infusibilization step for subjecting the sea-island type composite fiber to an infusibilization treatment, and a step of carbonizing and graphitizing the sea-island type composite fiber to obtain a pitch-based carbon fiber.

  In the production method, the meltable polyvinyl alcohol polymer is preferably an ethylene-vinyl alcohol copolymer.

  In the manufacturing method, the pitch is preferably a mesophase pitch.

  Further, the production method preferably includes a drawing step of further drawing the spinning yarn obtained in the spinning step, and the drawn yarn is subjected to sea component removal treatment and infusibilization treatment.

  Or in the said manufacturing method, it is preferable that a graphitization process is performed at 1200 to 3000 degreeC.

  A third embodiment of the present invention includes a conductive fabric composed of the pitch-based carbon fiber.

  The fourth embodiment of the present invention includes a conductive material composed of the pitch-based carbon fiber.

  According to the present invention, it is possible to provide a pitch-based ultrafine carbon fiber having not only excellent conductivity but also a uniform fiber diameter and good dispersibility in an aqueous solvent and an aqueous polymer solution. Such a carbon fiber can be dispersed in an aqueous solvent and an aqueous polymer solution alone without going through a chemical modification step or a step using a dispersant, etc., and has excellent productivity and handling properties. Various physical properties such as uniform mechanical properties, thermal properties, and electrical properties can be provided in applications. And the conductive material excellent in homogeneity is obtained by using the carbon fiber of this invention which has a uniform fiber diameter and favorable dispersibility.

  Hereinafter, the present invention will be specifically described.

The present invention is a pitch-based carbon fiber having an average fiber diameter of 0.01 to 5 μm, a CV value indicating variation of the fiber diameter of 0 to 10%, derived from oxygen atoms present on the surface, The surface oxygen concentration ratio (O / C), which means the abundance ratio of oxygen atoms to carbon atoms on the carbon fiber surface, measured by an X-ray photoelectron spectrometer is 0.02 or more.
This pitch-based carbon fiber can be produced by the following method.

(Kneading process)
The manufacturing method of the fiber of this invention is demonstrated. First, a polyvinyl alcohol polymer that can be melted in advance (hereinafter sometimes abbreviated as PVA polymer) and pitch are kneaded to prepare a mixed resin. The pitch may be kneaded in a molten state or mixed in a dispersed state.

  First, a PVA polymer that can be melted will be described. The PVA polymer has a vinyl alcohol unit as a main component, and the vinyl alcohol unit can be obtained, for example, by saponifying a polyvinyl ester obtained by polymerizing a vinyl ester. Examples of vinyl esters include vinyl acetate, vinyl formate, vinyl propionate, vinyl butyrate, vinyl pivalate, vinyl versatate, vinyl laurate, vinyl stearate, vinyl benzoate and the like. Of these, vinyl acetate is preferred from the viewpoint of productivity.

  The degree of polymerization of the PVA polymer is not particularly limited, but considering the process passability and the mechanical properties and dimensional stability of the obtained fiber, the average degree of polymerization determined from the viscosity of the 30 ° C. aqueous solution is 300. Those of ˜20,000 are desirable. From the viewpoints of polymer production cost, fiberization cost, etc., the average degree of polymerization is more preferably 500 to 3000.

  Although the saponification degree of the PVA polymer is not particularly limited, it is 88 mol% or more, preferably 95 mol% or more, more preferably 98 mol or more, from the viewpoint of mechanical properties of the obtained fiber. preferable. When a meltable PVA polymer having a too low saponification degree is used, it is not preferable in terms of mechanical properties, process passability, production cost, and the like of the obtained fiber.

  In the PVA polymer, examples of the structural unit that can melt the PVA polymer include olefins such as ethylene, propylene, and butylene (preferably α-olefins having 2 to 20 carbon atoms). These structural units may be used alone or in combination of two or more. Preferred are α-olefins having 2 to 5 carbon atoms, with ethylene being particularly preferred. In particular, as the PVA polymer of the present invention, an ethylene-vinyl alcohol copolymer is preferable from the viewpoint of kneading with a pitch and spinnability.

  The content of olefins in the PVA-based polymer can be selected from a wide range of 2 to 90 mol% with respect to the structural units of the entire PVA-based polymer, depending on the type and required properties. Preferably it may be 10-80 mol%, more preferably 20-70 mol%.

  Furthermore, the PVA polymer may have other structural units as desired as long as the effects of the present invention are not impaired. Examples of such structural units include acrylic acid and salts thereof and acrylic esters such as methyl acrylate, methacrylic acid and salts thereof, methacrylic esters such as methyl methacrylate, acrylamides such as acrylamide and N-methylacrylamide, and the like. Derivatives, methacrylamide derivatives such as methacrylamide, N-methylol methacrylamide, N-vinyl amides such as N-vinyl pyrrolidone, N-vinyl formamide, N-vinyl acetamide, allyl ethers having polyalkylene oxide in the side chain, methyl Examples include vinyl ethers such as vinyl ether, nitriles such as acrylonitrile, vinyl halides such as vinyl chloride, unsaturated dicarboxylic acids such as maleic acid and salts thereof, anhydrides and esters thereof, and the like. These structural units may be used alone or in combination of two or more. Such a modified unit may be introduced by copolymerization or post-reaction. In addition, an additive such as an antioxidant, an antifreezing agent, a pH adjusting agent, a concealing agent, a coloring agent, an oil agent, and a special functional agent is included in the polymer as long as the effect of the present invention is not impaired. It may be.

  The melting point of the PVA polymer is not particularly limited as long as it can be kneaded with pitch, and may be, for example, about 110 ° C. to 250 ° C. in order to ensure stability in the kneading and spinning process with the pitch. May be 120-230 ° C, more preferably 130-210 ° C.

  Next, the pitch will be described. Pitch is obtained from distillation residue of coal or petroleum, polycyclic aromatic hydrocarbons such as naphthalene and phenanthrene, and isotropic pitch that shows optical isotropy in the molten state, and molten state There is a mesophase pitch that can form an optically anisotropic phase (liquid crystal phase). From the viewpoint of high crystallinity and improvement in conductivity, it is preferable to use mesophase pitch in the present invention. The mesophase rate of the mesophase pitch to be used is not particularly limited, but may be, for example, 70 to 100%, preferably 90 to 100%, from the viewpoint of obtaining a carbon fiber having a high crystallinity and a high carbonization rate. The mesophase rate of the mesophase pitch can be determined by observing the pitch in the molten state with a polarizing microscope.

  The softening point of the pitch to be used is not particularly limited, and can be selected from a wide range of 110 ° C. to 400 ° C., for example, but may be 160 to 350 ° C. from the viewpoint of productivity.

  A known method can be used for kneading the PVA polymer and pitch, and examples thereof include a uniaxial melt kneading extruder, a biaxial melt kneading extruder, a mixing roll, and a Banbury mixer. Among these, a biaxial melt extruder is preferably used for the purpose of favorably microdispersing the PVA polymer and pitch. The kneading temperature can be appropriately selected according to the type of PVA polymer and pitch to be used, and is not particularly limited, but from the viewpoint of suppressing the thermal decomposition of the PVA polymer and promoting the micro-dispersion of the PVA polymer and pitch, For example, 150 degreeC-350 degreeC may be sufficient, Preferably 180-300 degreeC may be sufficient. The kneading time is appropriately selected depending on the mixing ratio of the PVA polymer and pitch and the target fiber diameter, and is not particularly limited.

  The mixing ratio of the PVA polymer and pitch is not particularly limited, but the mixing ratio (weight ratio) of the PVA polymer and pitch may be, for example, PVA polymer / pitch = 98/2 to 40/60. Preferably, PVA polymer / pitch = 90 / 10-50 / 50 may be used. When the weight ratio of pitch is less than 2, the yield after firing is very low, the productivity is remarkably poor, and it becomes difficult to obtain highly crystalline carbon fibers. When the weight ratio of pitch exceeds 60, spinning is performed. In this case, the spinnability and handling properties may be impaired.

(Spinning process)
Next, this mixed resin and other meltable polymer are extruded in a molten state from a sea-island composite nozzle, and a sea-island composite fiber is melted with the island component as a mixture of PVA polymer and pitch and the sea component as another melt polymer. Spin. Here, when melt spinning, it is important to perform composite spinning using a sea-island composite nozzle, and ultrafine carbon fibers having a uniform fiber diameter can be obtained from the sea-island composite fiber thus obtained. The average fiber diameter of the sea-island type composite fiber can be controlled to a desired shape by the hole diameter of the spinning nozzle. The temperature at the time of melt spinning is not particularly limited, but is preferably in the range of 150 ° C to 350 ° C, and more preferably in the range of 180 ° C to 330 ° C.

  Further, the molten polymer used for the sea component of the sea-island composite fiber of the present invention is not particularly limited as long as it is a polymer that can be melted. For example, a polymer that can be removed by water and a solvent or a polymer that can be decomposed and removed by heat is used. Can be used. Examples of the polymer that can be removed with water and a solvent include PVA-based polymers, polyethylene glycol and / or modified polyester containing a sulfonic acid alkali metal salt as a copolymerization component, and polyethylene oxide. In view of this, a PVA polymer is preferably used. As a polymer that can be removed by thermal decomposition, for example, a polyolefin-based polymer such as polyethylene and polypropylene can be used.

(Stretching process)
Further, depending on the purpose, the diameter of the island component can be reduced by stretching the fiber obtained from the spinning nozzle in a molten state or a softened state. The draw ratio can be selected from a wide range of 1.1 to 30 times, for example, and preferably 3 to 20 times, more preferably 5 to 15 times. By appropriately selecting the draw ratio, it is possible to control the diameter of the island component.

(Sea component removal process)
By removing the sea component polymer from the sea-island composite fiber thus obtained, ultrafine fibers made of a mixture of PVA polymer and pitch can be obtained. Although the removal method of a sea component is not specifically limited, The method of elution removal using the solvent which can elute a sea component, the method of removing by thermal decomposition, etc. can be used. The removal of the sea component may be performed before the infusibilization process described later, may be performed simultaneously with the infusibilization process, or may be performed after the infusibilization process.

(Infusibilization process)
The island component consisting of a mixture of a PVA polymer and pitch is imparted with heat stability by applying an infusible treatment, and a carbon fiber precursor is obtained. The infusibilization treatment is performed by heat treatment under a gas stream of air, oxygen, ozone, nitrogen dioxide, halogen, etc., but it is preferably air or oxygen alone or a mixed gas containing these for ease of handling. . As a specific method of infusibilization under a gas stream, heat treatment is preferably performed at a temperature of 100 to 350 ° C., more preferably 130 to 300 ° C. for about 0.5 to 24 hours.

(Carbonization and graphitization process)
The pitch-based carbon fiber in the present invention is obtained by further carbonizing the pitch-based carbon fiber precursor obtained by removing and infusibilizing the sea component as described above in an inert gas atmosphere. The carbonization is preferably performed at a maximum temperature of 500 ° C to 1500 ° C, and more preferably 600 ° C to 1000 ° C. The carbonized fiber can be converted into a graphitized fiber by graphitizing at a temperature of preferably 1200 ° C. to 3000 ° C., more preferably 1500 ° C. to 2500 ° C. At this time, if the treatment temperature is less than 1200 ° C., highly crystalline carbon fibers having high electrical conductivity cannot be obtained, and if it exceeds 3000 ° C., the residual amount of surface functional groups is extremely small, and it is difficult to obtain good dispersibility. Become.

  Note that the pitch-based carbon fiber of the present invention may be obtained by continuously infusibilizing, removing sea components, and carbonizing / graphitizing under tension using a sea-island composite fiber. Alternatively, the pitch-based carbon of the present invention can be obtained by once producing a woven or non-woven fabric with sea-island type composite fibers and using the woven or non-woven fabric for infusibilization, removal of sea components, carbonization / graphitization. Fibers may be obtained.

(Pitch carbon fiber)
The pitch-based carbon fiber of the present invention is important in that the average fiber diameter is 0.01 to 5 μm from the viewpoint that high performance can be exhibited without hindering handling properties and quality such as processability and workability, By appropriately adjusting the hole diameter and the draw ratio of the spinning nozzle, it is possible to appropriately adjust the average fiber diameter of the carbon fibers after the graphitization treatment. Preferably it is 0.05-3 micrometers, More preferably, it is 0.1-2 micrometers.

  Further, in the present invention, by using the sea-island type composite nozzle, it is possible to obtain ultrafine carbon fibers with little variation in fiber diameter. In the pitch-based carbon fiber of the present invention, it is important that the CV value indicating the variation in fiber diameter is 0 to 10%. The average fiber diameter in such a range and the ultrafine carbon fiber having the variation are electrically conductive. In addition, there is little variation in performance such as mechanical characteristics, and it is possible to provide homogeneous performance to the conductive material. Preferably it is 0 to 9%, More preferably, it is 0 to 8%.

  Furthermore, the pitch-based carbon fiber of the present invention has a surface oxygen concentration ratio (O / C) of 0.02 or more, which means an abundance ratio of oxygen atoms to carbon atoms on the carbon fiber surface, as measured by an X-ray photoelectron spectrometer. It is important that As described above, since the polyvinyl alcohol-based polymer is contained in the island component of the sea-island type composite fiber in the present invention, some polar functional groups remain, so the aqueous solvent and the aqueous polymer solution are uniform. It is possible to disperse the conductive material in a uniform manner and to impart a homogeneous performance to the conductive material. Preferably it is 0.025 or more, More preferably, it is 0.03 or more.

  Here, the average fiber diameter, CV value, and surface oxygen concentration ratio (O / C) of the pitch-based carbon fiber of the present invention described above are values measured by the method described in Examples described later.

  The pitch-based carbon fiber of the present invention may be a long fiber or a short fiber (chopped fiber, milled fiber, etc.), and the fiber length can be appropriately selected according to the purpose. In the case of short fibers, the fiber length can be adjusted by cutting, pulverizing, etc. the fibers. For example, from the viewpoint of enhancing dispersibility, the carbon fiber may have an average fiber length of 10 μm or less, preferably 7 μm or less, more preferably 5 μm or less.

On the other hand, in the Raman spectrum of the carbon material, the D band derived from the defective portion of the amorphous carbon and the like in the vicinity of 1350 cm ^-1, and the G band derived from the crystalline graphite near 1590 cm ^-1 it is observed. The intensity ratio of these band peaks, Id / Ig, is used as an index of crystallinity. In particular, in applications such as a material for imparting conductivity to a resin and a conductive additive, high crystallinity is required to exhibit high conductivity. Therefore, the pitch-based carbon fiber of the present invention preferably has a specific crystal structure, and the intensity ratio Id / Ig between the D band and the G band in the Raman spectrum is 0.1 or more and less than 1.0. Is more preferable, and more preferably 0.1 or more and less than 0.5. Here, the intensity ratio Id / Ig between the D band and the G band indicates a value measured by a method described in Examples described later.

  Further, the crystal lattice spacing d (002) by X-ray diffraction is also used as an index of the crystallinity of graphite. The pitch-based carbon fiber of the present invention preferably has a crystal lattice spacing of 0.37 nm or less in the X-ray diffraction method, more preferably 0.35 or less. Such a highly crystalline carbon fiber is a material having excellent conductivity. Here, the spacing between the crystal lattice planes indicates a value measured by a method described in Examples described later.

In addition, the volume specific resistance value of the pitch-based carbon fiber in the present invention is preferably 5 × 10 ⁻² Ω · cm or less, and more preferably 4 × 10 ⁻² Ω · cm or less. As described above, when the pitch is contained in the island component of the sea-island type composite fiber in the present invention, the obtained ultrafine carbon fiber is excellent in conductivity. The lower limit value of the volume resistivity value is not particularly limited, but may be about 1 × 10 ⁻⁵ . Here, the volume resistivity value indicates a value measured by a method described in Examples described later.

(Conductive fabric)
In addition, the pitch-based carbon fiber of the present invention obtained in this way is suitably used for producing a conductive fabric. For example, the conductive fabric may be (i) a conductive fabric obtained by forming a fabric in the state of a sea-island type composite fiber, and making it infusible, carbonized, or graphitized, and (ii) long fibers The conductive fabric obtained by using a part or all of the carbon fiber made of the above may be used, and (iii) the conductive fabric obtained by mixing the short fiber made of the pitch-based carbon fiber with the fabric base material. It may be a fabric. In the case of (iii), the mixing ratio is preferably 0.1 to 50% with respect to the entire fabric from the viewpoint of the effect of improving the conductivity.

  Examples of the conductive fabric include woven fabric, knitted fabric, woven and knitted fabric, and non-woven fabric, but are not necessarily limited thereto.

(Conductive material)
The pitch-based carbon fiber of the present invention has high conductivity and good dispersibility in an aqueous solvent and an aqueous polymer solution. Therefore, it can be suitably used as a conductive material in various fields where application of carbon fiber has been difficult in the past. For example, materials such as a conductivity imparting agent to a resin, an electromagnetic wave shielding material, a conductivity imparting material for an electrode, etc. Can be mentioned.

  As such a conductive material, a material having high homogeneity is suitably used, and it is desirable that the CV value of the volume specific resistance value indicating variation in the conductivity is 10% or less. The pitch-based carbon fiber of the present invention is preferably used because it has a uniform fiber diameter and good dispersibility in an aqueous solvent and an aqueous polymer solution in addition to high electrical conductivity. Material is obtained.

  EXAMPLES The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the examples.

[Average fiber diameter (μm) and CV value (%)]
The graphitized pitch-based carbon fibers were observed and photographed using a scanning electron microscope, and an average value, standard deviation value, and CV value of fiber diameters at 50 randomly selected locations were calculated. The CV value is a value obtained by dividing the standard deviation value by the average value and expressed as a percentage (%), thereby evaluating the uniformity of the fiber diameter.

[Surface oxygen concentration ratio (O / C)]
The surface oxygen concentration ratio (O / C) can be determined by an X-ray photoelectron spectrometer (XPS) according to the following procedure. For the measurement, Quantera SXM manufactured by ULVAC-PHI Co., Ltd. was used. The measurement was carried out with a monochromatic AlKα ray (25 W 15 kV) and a photoelectron extraction angle of 45 degrees. The binding energy was corrected by adjusting the peak top position of the signal attributed to C-C in the C1s spectrum to 284.8 eV. The O _1s peak area was determined by drawing a baseline according to the Shirley method in the range of 528 to 540 eV, and the C _1s peak area was determined by drawing the baseline according to the Shirley method in the range of 282 to 292 eV. The surface oxygen concentration ratio O / C on the fiber surface was calculated from the intensity ratio obtained by correcting the O _1s peak area and the C _1s peak area with a sensitivity coefficient specific to each device.

[D band and G band intensity ratio Id / Ig]
The intensity ratio Id / Ig between the D band and the G band can be obtained by a laser Raman apparatus according to the following procedure. For the measurement, LabRAM ARAMIS (laser wavelength: 532 nm, irradiation time: 10 seconds, integration number: 5 times) manufactured by Horiba, Ltd. was used. Id / Ig, after peak separation the resulting Raman spectrum was calculated as the ratio of D band intensity of 1360 cm ^-1 relative to G band intensity of 1580 cm ^-1.

[Crystal lattice spacing d (002) (nm)]
The crystal lattice plane distance d (002) can be obtained by a wide angle X-ray diffractometer. For measurement, Rigaku Co., Ltd. Smartlab was used, and CuKα rays were measured as an X-ray source. The enclosure tube voltage was 45 kV and the current was 200 mA. From the diffraction profile obtained by the measurement, it was calculated from the diffraction peak corresponding to the carbon 002 plane appearing in the vicinity of the diffraction angle 2θ = 24 to 26 degrees using the Bragg equation.

[Volume resistivity of carbon fiber (Ω · cm)]
The volume specific resistance value of the pitch-based carbon fiber is calculated by the following formula from the resistance value when the sample is consolidated to 0.8 g / cm ³ in a cylinder having a diameter of 0.5 cm and a voltage of 10 V is applied to both ends. did.
Volume resistivity (Ω · cm) = resistance value (Ω) × sample cross-sectional area (cm ² ) / length between samples (cm)
For the measurement of the resistance value, a digital multimeter PM3 manufactured by Sanwa Denki Keiki Co., Ltd. was used, and an average value of 10 volume specific resistance values selected at random was calculated to evaluate the conductivity of the carbon fiber. did.

[Dispersibility of carbon fiber]
10 parts of a water-soluble PVA polymer (PVA-217 manufactured by Kuraray Co., Ltd.) was dissolved in 90 parts of water to prepare a 10% by weight PVA aqueous solution. To 100 parts of the prepared PVA aqueous solution, 1 part of the obtained carbon fiber was added to prepare a dispersion, which was allowed to stand after stirring for 10 minutes, and the dispersion state of the fibers after 30 minutes was visually determined. The state in which the fibers were uniformly dispersed throughout was expressed as “◯”, and the state in which aggregates were observed and not uniformly dispersed was expressed as “x”.

[Volume Specific Resistance Value (Ω · cm) and CV Value (%) of Conductive Film]
The dispersion was cast on a polyethylene terephthalate plate and naturally dried, and then dried at 50 ° C. under vacuum for 8 hours to produce a conductive film having a thickness of 0.4 mm. The volume specific resistance value of the conductive film was determined by applying the silver paste to both ends of a sample cut to 5 mm width × 50 mm length and applying a voltage of 10 V to both ends. It calculated and calculated | required by the method similar to a specific resistance value. Regarding the volume resistivity value of the conductive film, an average value, standard deviation value, and CV value of 10-point measurement were calculated. The CV value is a value obtained by dividing the standard deviation value by the average value and expressed as a percentage (%). This evaluated the electroconductivity of the electroconductive film, and its homogeneity.

[Example 1]
An ethylene-vinyl alcohol polymer (melting point: 161 ° C.) having an ethylene modification amount of 44 mol% and a saponification degree of 99.5 mol% and a mesophase pitch (MCP-165 manufactured by JFE Chemical Co., Ltd.) having a softening point of 170 ° C. The mixture was melt-kneaded at 0 ° C. to obtain a mixture of an ethylene-vinyl alcohol polymer and mesophase pitch. The mixing ratio of the ethylene-vinyl alcohol polymer and mesophase pitch was 85/15 by weight.

  A mixture of the ethylene-vinyl alcohol polymer and mesophase pitch is used as an island component, a water-soluble polyvinyl alcohol polymer having a viscosity average polymerization degree of 200, an ethylene modification amount of 8.5 mol%, and a saponification degree of 98.5 mol% as a sea component. Using a sea-island type composite nozzle having 100 islands, melt composite spinning was performed at 240 ° C. By treating this in hot water at 80 ° C., sea components were removed, and ultrafine fibers having an average fiber diameter of 3.0 μm were obtained. This was treated with sulfuric acid to improve heat resistance, and then kept in air at 300 ° C. for 4 hours to obtain an infusible fiber.

  Next, this fiber is heated to 800 ° C. at a temperature rising rate of 2 ° C./min in a nitrogen atmosphere, carbonized, and further fired to 2500 ° C. in an argon gas atmosphere, thereby obtaining an ultrafine carbon fiber. Got. The obtained ultrafine carbon fiber had an average fiber diameter of 2.2 μm, a fiber diameter CV value of 6.8%, and a surface oxygen concentration ratio O / C of 0.036. The intensity ratio Id / Ig between the D band and the G band was 0.248, and the crystal lattice spacing was 0.344 nm.

The average value of the volume resistivity value of the obtained fiber was 3.2 × 10 ⁻² Ω · cm. Moreover, when the dispersion state of the fiber in PVA aqueous solution was evaluated visually, it was uniformly disperse | distributed to the whole and was a favorable dispersion state.

  Furthermore, with respect to the conductive film obtained from this dispersion, 10 points were sampled and the volume resistivity value was measured. The average value, standard deviation value and CV value are shown in Table 1. From these measurement results, it can be said that a conductive film having a small variation in conductivity and excellent in homogeneity was obtained.

[Example 2]
An ethylene-vinyl alcohol polymer (melting point: 161 ° C.) having an ethylene modification amount of 44 mol% and a saponification degree of 99.5 mol%, and a mesophase pitch having a softening point of 350 ° C. (manufactured by JFE Chemical Co., Ltd.) MCP-350) was kneaded at 250 ° C. to obtain a mixture of an ethylene-vinyl alcohol polymer and mesophase pitch. The mixing ratio of the ethylene-vinyl alcohol polymer and mesophase pitch was 85/15 by weight.

  A mixture of the ethylene-vinyl alcohol polymer and mesophase pitch is used as an island component, a water-soluble polyvinyl alcohol polymer having a viscosity average polymerization degree of 200, an ethylene modification amount of 8.5 mol%, and a saponification degree of 98.5 mol% as a sea component. Using a sea-island type composite nozzle having 100 islands, melt composite spinning was performed at 240 ° C. The obtained fiber was further stretched 10 times at 140 ° C. to obtain a sea-island type composite fiber. By treating this in hot water at 80 ° C., sea components were removed, and ultrafine fibers having an average fiber diameter of 2.2 μm were obtained. This was treated with sulfuric acid to improve heat resistance, and then kept in air at 300 ° C. for 4 hours to obtain an infusible fiber.

  Carbonization and graphitization were performed in the same manner as in Example 1, the average fiber diameter was 1.8 μm, the CV value of the fiber diameter was 7.2%, and the surface oxygen concentration ratio O / C was 0.036. An ultrafine carbon fiber was obtained. The resulting fiber had a D band to G band intensity ratio Id / Ig of 0.232 and a crystal lattice spacing of 0.342 nm.

The average volume resistivity value of the obtained fibers was 2.9 × 10 ⁻² Ω · cm. Moreover, when the dispersion state of the fiber in PVA aqueous solution was evaluated visually, it was uniformly disperse | distributed to the whole and was a favorable dispersion state.

  Furthermore, a conductive film was prepared from the dispersion, and the volume resistivity value was measured. The average value, standard deviation value, and CV value are shown in Table 1. From these measurement results, it can be said that a conductive film having a small variation in conductivity and excellent in homogeneity was obtained.

[Example 3]
A polyvinyl alcohol polymer having a viscosity average polymerization degree of 200, an ethylene modification amount of 8.5 mol% and a saponification degree of 98.5 mol%, and a mesophase pitch having a softening point of 350 ° C. (MCP manufactured by JFE Chemical Co., Ltd.) 350) at 250 ° C. to obtain a mixture of a polyvinyl alcohol polymer and mesophase pitch. The mixing ratio of the polyvinyl alcohol polymer to the mesophase pitch was 85/15 by weight.

  A mixture of the polyvinyl alcohol polymer and mesophase pitch was used as an island component and polypropylene as a sea component, and melt compound spinning was performed at 240 ° C. using a sea-island type composite nozzle having 100 islands. The obtained fiber was further stretched 10 times at 140 ° C. to obtain a sea-island type composite fiber having an average fiber diameter of 2.6 μm. This is heated in air from 160 ° C to 200 ° C over 24 hours, treated at 200 ° C for 7 hours, and further treated at 400 ° C for 5 hours in an inert gas atmosphere to thermally remove the sea components. At the same time, the island components were infusibilized.

  The obtained infusible fiber was carbonized and graphitized in the same manner as in Example 1, the average fiber diameter was 2.0 μm, the fiber diameter CV value was 6.9%, and the surface oxygen concentration ratio O An ultrafine carbon fiber having a / C of 0.034 was obtained. The resulting fiber had a D band to G band intensity ratio Id / Ig of 0.234 and a crystal lattice spacing of 0.342 nm.

The average value of the volume resistivity of the obtained fiber was 3.0 × 10 ⁻² Ω · cm. Moreover, when the dispersion state of the fiber was visually evaluated in the PVA aqueous solution, it was uniformly dispersed throughout and was in a good dispersion state.

  Furthermore, a conductive film was prepared from the dispersion, and the volume resistivity value was measured. The average value, standard deviation value, and CV value are shown in Table 1. From these measurement results, it can be said that a conductive film having a small variation in conductive performance and excellent in homogeneity was obtained.

[Comparative Example 1]
A polyvinyl alcohol polymer having a viscosity average polymerization degree of 200, an ethylene modification amount of 8.5 mol%, a saponification degree of 98.5 mol%, and a mesophase pitch (MCP-165 manufactured by JFE Chemical Co., Ltd.) having a softening point of 170 ° C. The mixture was melt-kneaded at 250 ° C. to obtain a mixture of a polyvinyl alcohol polymer and mesophase pitch. The mixing ratio of the polyvinyl alcohol polymer to the mesophase pitch was 95/5 by weight. This was melt-mixed and spun at 240 ° C. with a twin-screw extruder, and further stretched 10 times at 140 ° C. to obtain a sea-island composite fiber in which the sea component was a polyvinyl alcohol polymer and the island component was a mesophase pitch. By treating this in hot water at 80 ° C., sea components were removed, and ultrafine fibers having an average fiber diameter of 0.9 μm were obtained. This was treated with sulfuric acid to improve heat resistance, and then kept in air at 300 ° C. for 4 hours to obtain an infusible fiber.

  When carbonization and graphitization were performed in the same manner as in Example 1, the average fiber diameter was 0.7 μm, the CV value of the fiber diameter was 48.2%, and the surface oxygen concentration ratio O / C was 0.00. An ultrafine carbon fiber of 036 was obtained. The intensity ratio Id / Ig between the D band and the G band was 0.246, and the crystal lattice spacing was 0.344 nm.

The average value of the volume resistivity value of the obtained fiber was 3.2 × 10 ⁻² Ω · cm. Further, when the dispersion state of the fibers was visually evaluated in the PVA aqueous solution, the fibers were uniformly dispersed throughout, and the dispersibility was as good as that of the example.

  Furthermore, a conductive film was prepared from the dispersion, and the volume resistivity value was measured. The average value, standard deviation value, and CV value are shown in Table 1. The conductive film had a large variation in conductivity and resulted in poor homogeneity.

[Comparative Example 2]
A water-soluble polyvinyl alcohol system having a pitch of 290 ° C. (FP-280 manufactured by JFE Chemical Co., Ltd.) with an island component, a viscosity average polymerization degree of 200, an ethylene modification amount of 8.5 mol%, and a saponification degree of 98.5 mol%. The polymer was used as a sea component, and melt compound spinning was performed at 240 ° C. using a sea-island type composite nozzle having 100 islands. By treating this in hot water at 80 ° C., sea components were removed, and ultrafine fibers having an average fiber diameter of 3.0 μm were obtained. This was treated with sulfuric acid to improve heat resistance, and then kept in air at 300 ° C. for 4 hours to obtain an infusible fiber.

  Carbonization and graphitization were performed in the same manner as in Example 1, the average fiber diameter was 1.9 μm, the fiber diameter CV value was 7.8%, and the surface oxygen concentration ratio O / C was 0.008. An ultrafine carbon fiber was obtained. The resulting fiber had a D band to G band intensity ratio Id / Ig of 0.332 and a crystal lattice spacing of 0.351 nm.

The average value of the volume resistivity value of the obtained fiber was 3.9 × 10 ⁻² Ω · cm. In addition, when the dispersion state of the fibers was visually evaluated in the PVA aqueous solution, a large number of aggregates were observed, and they were not uniformly dispersed throughout. This is because the surface oxygen concentration ratio O / C is low.

  Furthermore, a conductive film was prepared from the dispersion, and the volume resistivity value was measured. The average value, standard deviation value, and CV value are shown in Table 1. The conductive film had a large variation in conductivity and resulted in poor homogeneity.

[Comparative Example 3]
Average fiber diameter is 0.18 μm, fiber diameter CV value is 29.5%, surface oxygen concentration ratio O / C is 0.009, D band to G band intensity ratio Id / Ig is 0.101, crystal When the volume resistivity value of vapor grown ultrafine carbon fiber (VGCF-H manufactured by Showa Denko KK) having a lattice spacing of 0.339 nm was measured at 10 points, the average value was 8.0 × 10 ⁻³ Ω · cm. In addition, when the dispersion state of the fibers was visually evaluated in the PVA aqueous solution, a large number of aggregates were observed, and they were not uniformly dispersed throughout. This is due to the low surface oxygen concentration ratio O / C, as in Comparative Example 2.

  Furthermore, a conductive film was prepared from the dispersion, and the volume resistivity value was measured. The average value, standard deviation value, and CV value are shown in Table 1. The conductive film had a large variation in conductivity, resulting in poor conductivity homogeneity.

  According to the present invention, a method for producing an ultrafine carbon fiber having excellent conductivity, uniform fiber diameter, excellent dispersibility in an aqueous solvent and an aqueous polymer solution, and less variation in performance. Therefore, it is suitably used for applications such as a conductivity-imparting agent for resin, an electromagnetic shielding material, and a conductive aid for electrodes.

Claims (10)

  Pitch-based carbon fiber having an average fiber diameter of 0.01 to 5 μm, a CV value indicating variation in fiber diameter of 0 to 10%, and carbon atoms on the surface of the carbon fiber measured by an X-ray photoelectron spectrometer A pitch-based carbon fiber having a surface oxygen concentration ratio (O / C) which means an abundance ratio of oxygen atoms with respect to 0.02 or more.   The intensity ratio Id / Ig between the D band and the G band in the Raman spectrum is 0.1 or more and less than 1.0, and the crystal lattice spacing in the X-ray diffraction method is 0.37 nm or less. Pitch-based carbon fiber. The pitch-based carbon fiber according to claim 1 or 2, wherein the volume resistivity value is 5 × 10 ^-2 Ω · cm or less. Using a mixture of a polyvinyl alcohol polymer that can be melted as an island component and pitch, and using another molten polymer as a sea component, the polymer that becomes the island component and the polymer that becomes the sea component are melted separately, After forming the melt flow, spinning from the sea-island composite nozzle to obtain a sea-island composite fiber; and
Removing sea components from the sea-island type composite fiber to obtain ultrafine fibers composed of island components;
An infusibilization step for infusibilizing the sea-island type composite fiber;
A step of carbonizing and graphitizing the carbon fiber precursor to obtain pitch-based carbon fibers;
A method for producing pitch-based carbon fiber comprising:   The production method according to claim 4, wherein the meltable polyvinyl alcohol polymer is an ethylene-vinyl alcohol copolymer.   The manufacturing method of Claim 4 or 5 whose pitch is a mesophase pitch.   The spinning process according to claim 4, further comprising a drawing step for drawing the spinning yarn obtained in the spinning step, wherein the drawn yarn is subjected to a treatment for removing sea components and an infusibilization treatment. Manufacturing method.   The manufacturing method as described in any one of Claims 4-7 with which a graphitization process is performed at 1200 to 3000 degreeC.   The electroconductive cloth comprised with the pitch-type carbon fiber as described in any one of Claims 1-3.   The electroconductive material comprised with the pitch-type carbon fiber as described in any one of Claims 1-3.