CARBON FIBER PRECURSOR, METHOD OF PRODUCING CARBON FIBER PRECURSOR, STABILIZED FIBER, METHOD OF PRODUCING STABILIZED FIBER, AND METHOD OF PRODUCING CARBON FIBER

Abstract

A means for resolution is a carbon fiber precursor containing a crosslinked diene-based polymer and having a gel fraction of 40% or more, and its application.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2023-017205 filed on Feb. 7, 2023 and Japanese Patent Application No. 2023-179852 filed on Oct. 18, 2023. The above application is hereby expressly incorporated by reference, in its entirety, into the present application.

BACKGROUND

Technical Field

The present disclosure relates to a carbon fiber precursor, a method of producing a carbon fiber precursor, a stabilized fiber, a method of producing a stabilized fiber, and a method of producing a carbon fiber.

Related Art

Carbon fibers are lightweight and have excellent mechanical properties, and therefore have been developed into various applications such as aerospace applications, automobile applications, and building material applications.

For example, Japanese Patent Application Laid-Open (JP-A) No. S48-82199, Japanese Patent Application Laid-Open (JP-A) No. S48-92699, and Japanese Patent Application Laid-Open (JP-A) No. S49-106490 disclose a method of producing a carbon fiber using a 1,2-polybutadiene-based fiber.

JP-A No. S48-82199 discloses a method of obtaining a stabilized fiber by irradiating a 1,2-polybutadiene-based fiber with ultraviolet light for 2 hours or more, followed by a stabilization treatment at 200° ° C. for 8 hours or more. JP-A No. S48-92699 discloses a method of obtaining a stabilized fiber by subjecting a 1,2-polybutadiene-based fiber to an acid treatment to make the 1,2-polybutadiene-based fiber insoluble and infusible, followed by a stabilization treatment. JP-A No. S49-106490 discloses a method of obtaining a stabilized fiber by immersing a 1,2-polybutadiene-based fiber in an organic solvent in which a Lewis acid is dissolved or suspended to make the 1,2-polybutadiene-based fiber infusible, followed by a stabilization treatment.

Japanese Patent Application Laid-Open (JP-A) No. 2020-59619 discloses a method of producing a carbon material in which a carbon material precursor including a ring structure-containing polymer containing at least one of specific structural units is subjected to a stabilization treatment at a temperature of from 320 to 450° C. in an oxidizing atmosphere, followed by a carbonization treatment.

SUMMARY OF THE INVENTION

In a long-term stabilization treatment as described in JP-A No. S48-82199, energy required for production increases and productivity decreases, leading to an increase in production cost. In the irradiation with ultraviolet light, only the surface of the fiber and the portion irradiated with ultraviolet light are cured, and the central portion of the fiber and the portion not irradiated with ultraviolet light cannot be sufficiently cured. This tendency is remarkable when the fiber diameter is large, and furthermore, in the case of a fiber bundle including many fibers, it is difficult to sufficiently cure the fibers present inside. As a result, fusion between fibers occurs or thread breakage due to melting occurs during the stabilization treatment, and the obtained stabilized fiber tends to be brittle. When a carbon fiber is produced using such a stabilized fiber, thread breakage (rupture) due to a tension applied during carbonization, thread breakage due to thermal decomposition at a high temperature, or the like occurs, and carbonization stability tends to be low.

An acid treatment as described in JP-A No. S48-92699 and immersion in a liquid containing a Lewis acid as described in JP-A No. S49-106490 require a large amount of washing solvent in the subsequent washing treatment, leading to an increase in production cost. In addition, in the acid treatment and the immersion in a liquid containing a Lewis acid, only the surface of the fiber is made infusible, and the central portion of the fiber cannot be sufficiently made infusible. As a result, fusion between fibers occurs or thread breakage due to melting occurs during the stabilization treatment, and the obtained stabilized fiber tends to be brittle.

In JP-A No. 2020-59619, an attempt has been made to improve a carbonization yield, but it is sometimes required to further prevent fusion between fibers in the stabilization treatment.

An object of an embodiment of the disclosure is to provide a carbon fiber precursor capable of preventing fusion between fibers in a stabilization treatment and having excellent carbonization stability, and a method of producing the carbon fiber precursor.

An object of another embodiment of the disclosure is to provide a stabilized fiber in which fusion between fibers is prevented and which has excellent carbonization stability, and a method of producing the stabilized fiber.

An object of another embodiment of the disclosure is to provide a method of producing a carbon fiber using a stabilized fiber in which fusion between fibers is prevented and which has excellent carbonization stability.

Specific means for achieving the objects are as follows.

<1>

A carbon fiber precursor containing a crosslinked diene-based polymer and having a gel fraction of 40% or more.

<2>

The carbon fiber precursor according to <1>, wherein a swelling magnification after immersion in toluene at 60° C. for 2 hours is 9.5 times or less.

<3>

The carbon fiber precursor according to <1> or <2>, wherein the crosslinked diene-based polymer is a crosslinked product of a diene-based polymer containing a structural unit represented by Formula (1) below:

• • wherein R is a hydrogen atom or an organic group having from 1 to 20 carbon atoms.

<4>

The carbon fiber precursor according to <3>, wherein R in Formula (1) is a hydrogen atom or a methyl group.

<5>

A method of producing a carbon fiber precursor including irradiating a polymer fiber including a diene-based polymer containing a structural unit represented by Formula (1) below with a radiation having a dose of 20 kGy or more:

• • wherein R is a hydrogen atom or an organic group having from 1 to 20 carbon atoms.

<6>

The method of producing a carbon fiber precursor according to <5>, wherein R in Formula (1) is a hydrogen atom or a methyl group.

<7>

A method of producing a stabilized fiber including subjecting a fiber of the carbon fiber precursor according to any one of <1> to <4> to a heat treatment in an oxidizing atmosphere.

<8>

A method of producing a stabilized fiber including:

• • producing a carbon fiber precursor by the method of producing a carbon fiber precursor according to <5> or <6>; and
  • subjecting a fiber of the carbon fiber precursor to a heat treatment in an oxidizing atmosphere.

<9>

A method of producing a carbon fiber including:

• • producing a stabilized fiber by the method of producing a stabilized fiber according to <7>; and
  • subjecting the stabilized fiber to a carbonization treatment.

<10>

A method of producing a carbon fiber including:

• • producing a carbon fiber precursor by the method of producing a carbon fiber precursor according to <5> or <6>;
  • producing a stabilized fiber by subjecting a fiber of the carbon fiber precursor to a heat treatment in an oxidizing atmosphere; and
  • subjecting the stabilized fiber to a carbonization treatment.

<11>

A stabilized fiber containing a structure derived from a crosslinked diene-based polymer and having a fusion rate of 30% or less.

<12>

A method of producing a carbon fiber including subjecting the stabilized fiber according to <11> to a carbonization treatment.

According to an embodiment of the disclosure, a carbon fiber precursor capable of preventing fusion between fibers in a stabilization treatment and having excellent carbonization stability, and a method of producing the carbon fiber precursor are provided.

According to another embodiment of the disclosure, a stabilized fiber in which fusion between fibers is prevented and which has excellent carbonization stability, and a method of producing the stabilized fiber are provided.

According to another embodiment of the disclosure, a method of producing a carbon fiber using a stabilized fiber in which fusion between fibers is prevented and which has excellent carbonization stability is provided.

DETAILED DESCRIPTION OF THE INVENTION

In the disclosure, a numerical range indicated using “to” includes numerical values described before and after “to” as a minimum value and a maximum value, respectively.

In the numerical ranges described in stages in the disclosure, the upper limit or the lower limit described in one numerical range may be replaced with the upper limit or the lower limit of a numerical range described in another stage. In addition, in a numerical range described in the disclosure, the upper limit or the lower limit of the numerical range may be replaced with a value shown in a synthesis example.

In the disclosure, each component may contain a plurality of types of corresponding substances. When a plurality of types of substances corresponding to each component are present in the carbon fiber precursor, the content percentage or content amount of each component means the total content percentage or content amount of the plurality of types of substances present in the carbon fiber precursor unless otherwise specified.

In the disclosure, the “carbon fiber precursor” means a fiber from which a carbon fiber can be obtained by a carbonization treatment, or a stabilizaion treatment and a carbonization treatment.

<Carbon Fiber Precursor>

The carbon fiber precursor of the disclosure contains a crosslinked diene-based polymer and has a gel fraction of 40% or more.

The carbon fiber precursor of the disclosure can prevent fusion between fibers in the stabilization treatment.

The reason why the above-mentioned effect is exhibited is presumed as follows, but is not limited thereto.

When the carbon fiber precursor of the disclosure contains a crosslinked diene-based polymer and has a gel fraction of 40% or more, the crosslinking density is high, and therefore it is presumed that fusion between fibers in the stabilization treatment is prevented.

The carbon fiber precursor may be a single fiber or a fiber bundle, but from the viewpoint that a carbon fiber used for structural members such as aerospace applications, automobile applications, and building material applications exhibits high mechanical properties, the carbon fiber precursor is preferably a fiber bundle including a plurality of single fibers. As for the number of fibers of the carbon fiber precursor of the disclosure, when the carbon fiber precursor is a fiber bundle, the number of filaments per bundle is not particularly limited, but is preferably from 10 to 360,000, more preferably from 20 to 180,000, still more preferably from 30 to 72,000, and particularly preferably from 50 to 36,000 from the viewpoint of the productivity and the mechanical properties of the stabilized fiber and the carbon fiber. In addition, it is possible to prevent the occurrence of firing unevenness during the stabilization treatment or the carbonization treatment by setting the number of filaments per bundle to 360,000 or less.

From the viewpoint of further preventing fusion between fibers in the stabilization treatment, the gel fraction of the carbon fiber precursor is preferably 50% or more, more preferably 60% or more, still more preferably 80% or more, yet still more preferably 85% or more, particularly preferably 90% or more, and most preferably 95% or more.

The upper limit of the gel fraction of the carbon fiber precursor is not particularly limited. The gel fraction of the carbon fiber precursor may be 100%. The gel fraction of the carbon fiber precursor is preferably 99.9% or less from the viewpoint of reducing the production cost by reducing energy required for production in order to increase the gel fraction.

In the disclosure, the gel fraction is measured by the following method.

First, 0.2 g of a sample is cut out from the carbon fiber precursor. After the sample is dried at 80° C. for 4 hours, the mass is precisely weighed using a precision electronic balance and taken as the initial mass (g).

Subsequently, the sample is immersed in 30 ml of toluene and left to stand in a hot air circulating oven at 60° ° C. for 8 hours.

The sample after being left to stand is subjected to suction filtration using a membrane filter having a pore size of 1.0 μm to separate a gel component. The residue remaining on the membrane filter without being dissolved in toluene corresponds to a gel component. As the membrane filter, for example, an Omnipore TM membrane filter JAWP04700 manufactured by Merck KGaA can be used.

The separated gel component is air-dried together with the membrane filter in the air in a draft for 12 hours or more, and further left to stand in a hot air circulating oven at 90° ° C. for 12 hours to remove toluene.

The mass of the gel component and the membrane filter after being left to stand are precisely weighed using a precision electronic balance, and the gel fraction is determined from the following formula.

$\mathrm{Gel}\mathrm{fraction}\left(\%\right)=\left\{\left(\mathrm{mass}\mathrm{of}\mathrm{gel}\mathrm{component}\mathrm{and}\mathrm{membrane}\mathrm{filter}\left(g\right)-\mathrm{mass}\mathrm{of}\mathrm{membrane}\mathrm{filter}\left(g\right)\right)/\mathrm{initial}\mathrm{mass}\mathrm{of}\mathrm{sample}\left(g\right)\right\}\times 100$

The swelling magnification of the carbon fiber precursor of the disclosure after immersion in toluene at 60° C. for 8 hours is preferably 9.5 times or less, more preferably 8 times or less, still more preferably 7 times or less, particularly preferably 4 times or less, and most preferably 3 times or less from the viewpoint of further preventing fusion between fibers in the stabilization treatment. The lower limit of the swelling magnification is not particularly limited, but is preferably 1.05 times or more, and more preferably 1.1 times or more from the viewpoint of the stretchability of the carbon fiber precursor.

In the disclosure, the swelling magnification is measured by the following method.

First, 0.2 g of a sample is cut out from the carbon fiber precursor. After the sample is dried at 80° ° C. for 4 hours, the mass is precisely weighed using a precision electronic balance and taken as the initial mass (g).

Subsequently, the sample is immersed in 30 ml of toluene and left to stand in a hot air circulating oven at 60° C. for 8 hours.

The sample after being left to stand is subjected to suction filtration using a membrane filter having a pore size of 1.0 μm to separate a gel component, and the mass of the recovered gel component (swollen gel) is precisely weighed using a precision electronic balance. Subsequently the swollen gel is air-dried in the air in a draft for 12 hours or more, and further dried at 90° C. for 12 hours, and then, the mass of the gel after drying (dried gel) is weighed, and the swelling magnification is determined from the following formula.

Swelling magnification (times)=(mass of swollen gel [g])/(mass of dried gel [g])

(Crosslinked Diene-Based Polymer)

The carbon fiber precursor of the disclosure contains a crosslinked diene-based polymer. The carbon fiber precursor of the disclosure may contain two or more crosslinked diene-based polymers.

The crosslinked diene-based polymer is a crosslinked product of a diene-based polymer, and is preferably a crosslinked product of a diene-based polymer containing a structural unit represented by Formula (1) below. The crosslinked diene-based polymer is obtained, for example, by irradiating a diene-based polymer with a radiation to cause intramolecular or intermolecular crosslinking.

The diene-based polymer may contain only one structural unit represented by Formula (1) or may contain two or more structural units represented by Formula (1).

In Formula (1), R is a hydrogen atom or an organic group having from 1 to 20 carbon atoms.

The number of carbon atoms in the organic group represented by R is preferably from 1 to 10, more preferably from 1 to 6, and still more preferably from 1 to 3 from the viewpoint of improving the yield of the carbon fiber to be obtained.

Examples of the organic group represented by R include a hydrocarbon group and a group in which at least some of the atoms included in a hydrocarbon group are substituted with a halogen atom (for example, a chlorine atom, a bromine atom, or a fluorine atom), an oxygen atom, a nitrogen atom, or a sulfur atom.

The hydrocarbon group may be linear, branched, or may contain a ring structure.

The hydrocarbon group may be an aliphatic hydrocarbon group or an aromatic hydrocarbon group.

Among them, the hydrocarbon group is preferably an aliphatic hydrocarbon group, more preferably an alkyl group, and still more preferably a linear alkyl group. Specifically, the hydrocarbon group is preferably a hydrocarbon group having from 1 to 10 (preferably from 1 to 6) carbon atoms, and examples thereof include a methyl group, an ethyl group, a n-propyl group, an isopropyl group, a n-butyl group, an isobutyl group, a tert-butyl group, a sec-butyl group, a pentyl group, an isopentyl group, a neopentyl group, a hexyl group, a 2-methylpentyl group, a 3-methylpentyl group, a 2,2-dimethylbutyl group, a 2,3-dimethylbutyl group, a heptyl group, a 2-methylhexyl group, a 3-methylhexyl group, a 2,2-dimethylpentyl group, a 2,3-dimethylpentyl group, a 2,4-dimethylpentyl group, a 3-ethylpentyl group, a 2,2,3-trimethylbutyl group, an octyl group, a methylheptyl group, a dimethylhexyl group, a 2-ethylhexyl group, a 3-ethylhexyl group, a trimethylpentyl group, a 3-ethyl-2-methylpentyl group, a 2-ethyl-3-methylpentyl group, a 2,2,3,3-tetramethylbutyl group, a nonyl group, a methyloctyl group, a 3,7-dimethyloctyl group, a dimethylheptyl group, a 3-ethylheptyl group, a 4-ethylheptyl group, a trimethylhexyl group, a 3,3-diethylpentyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, and an eicosyl group.

From the viewpoint of further preventing fusion between fibers in the stabilization treatment, R is preferably a hydrogen atom or a methyl group.

Examples of a raw material for producing a diene-based polymer containing a structural unit represented by Formula (1) include 1,3-butadiene, isoprene, 2-ethyl-1,3-butadiene, 2-propyl-1,3-butadiene, 2-butyl-1,3-butadiene, 2-pentyl-1,3-butadiene, 2-hexyl-1,3-butadiene, 2-phenyl-1,3-butadiene, 2-methoxy-1,3-butadiene, and myrcene.

Examples of the raw material for producing a diene-based polymer containing a structural unit in which R in Formula (1) is a hydrogen atom include 1,2-butadiene.

Examples of the raw material for producing a diene-based polymer containing a structural unit in which R in Formula (1) is a methyl group include isoprene.

The raw material for producing a diene-based polymer containing a structural unit represented by Formula (1) is preferably at least one compound selected from the group consisting of 1,2-butadiene and isoprene.

In the diene-based polymer, the content amount of the structural unit represented by Formula (1) is preferably 10 mol % or more, more preferably 30 mol % or more, still more preferably 40 mol % or more, particularly preferably 50 mol % or more, more particularly preferably 60 mol % or more, and most preferably 70 mol % or more. The upper limit of the content amount of the structural unit represented by Formula (1) is not particularly limited. The content amount of the structural unit represented by Formula (1) may be 100 mol %.

The diene-based polymer preferably contains a structural unit represented by Formula (1), but may contain a structural unit derived from another conjugated diene-based monomer other than the structural unit represented by Formula (1).

Examples of another conjugated diene-based monomer include 1-pentyl-1,3-butadiene, 1-hexyl-1,3-butadiene, 1-heptyl-1,3-butadiene, 1-octyl-1,3-butadiene, 2,3-dimethyl-1,3-butadiene, 1-hexyloxy-1,3-butadiene, 1,3-pentadiene, 2-methyl-1,3-pentadiene, 1,3-hexadiene, 4,5-diethyl-1,3-octadiene, and 3-butyl-1,3-octadiene.

The diene-based polymer containing a structural unit represented by Formula (1) may contain a structural unit derived from another polymerizable monomer in addition to the structural unit represented by Formula (1).

Examples of Another Polymerizable Monomer Include

• • aromatic vinyl-based monomer such as styrene, α-methylstyrene, α-methyl-p-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, 2,4-dimethylstyrene, ethylstyrene, p-tert-butylstyrene, o-chlorostyrene, m-chlorostyrene, p-chlorostyrene, p-bromostyrene, 2-methyl-1,4-dichlorostyrene, 2,4-dibromostyrene, and vinylnaphthalene;
  • linear olefinic monomers such as ethylene, propylene, and 1-butene;
  • cyclic olefinic monomers such as cyclopentene and 2-norbornene;
  • non-conjugated diene-based monomers such as 1,5-hexadiene, 1,6-heptadiene, 1,7-octadiene, cyclopentadiene, dicyclopentadiene, and 5-ethylidene-2 norbornene;
  • α,β-unsaturated carboxylate esters such as methyl (meth)acrylate, ethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, monomethyl maleate, monomethyl itaconate, dimethyl itaconate, ethyl itaconate, and diethyl itaconate;
  • vinyl cyanide monomers such as acrylonitrile, methacrylonitrile, and ethacrylonitrile;
  • acrylamide-based monomers such as (meth)acrylamide and dimethylaminoethyl (meth)acrylamide;
  • α,β-unsaturated carboxylic acids such as (meth)acrylic acid, maleic acid, fumaric acid, and itaconic acid;
  • α,β-unsaturated carboxylic acid anhydrides such as maleic anhydride and itaconic anhydride;
  • sulfo group-containing vinyl-based monomers such as vinylsulfonic acid;
  • vinyl halide-based monomers such as vinyl chloride;
  • vinyl carboxylates such as vinyl acetate, vinyl butyrate, and vinyl pivalate;
  • and vinyl alcohol. As another polymerizable monomer, only one kind may be used or two or more kinds may be used in combination. In the disclosure, the “(meth)acryl” refers to methacryl or acryl.

In the diene-based polymer, when “R” in Formula (1) is a hydrogen atom, the 1,2-bond content proportion is not particularly limited, and a cis-1,4-bond or a trans-1,4-bond may be contained. In the diene-based polymer, the content amount of 1,2-structural unit (1,2-bond content proportion) when “R” in Formula (1) is a hydrogen atom is preferably 1 mol % or more, more preferably 5 mol % or more, still more preferably 10 mol % or more, yet still more preferably 30 mol % or more, particularly preferably 50 mol % or more, more particularly preferably 80 mol % or more, and most preferably 90 mol % or more from the viewpoint of reducing the fusion rate and improving the carbonization yield during the stabilization treatment by the progress of an intramolecular cyclization reaction and an intermolecular crosslinking reaction through radiation irradiation. The 1,2-bond content proportion in the diene-based polymer may be 100 mol %, but is preferably 99.5 mol % or less, and more preferably 99 mol % or less from the viewpoint of reducing the production (polymerization) cost for increasing the 1,2-bond content proportion in the diene-based polymer. The “1,2-bond content proportion” refers to the proportion of 1,2-structural unit (1,2-bond) when the total content proportion of cis-1,4-structural unit (cis-1,4-bond), trans-1,4-structural unit (trans-1,4-bond), and 1,2-structural unit (1,2-bond) included in the diene-based polymer is taken as 100 mol %. The content proportion of 1,2-bond can be checked by ¹H-nuclear magnetic resonance (NMR) or 13C-NMR.

In the diene-based polymer, when “R” in Formula (1) is a methyl group, the content proportion of 3,4-bond corresponding to the diene-based monomer unit (1) is not particularly limited, and a cis-1,4-bond, a trans-1,4-bond, or a 1,2-bond may be contained. In the diene-based polymer, the content amount of 3,4-structural unit (3,4-bond content proportion) when “R” in Formula (1) is a methyl group is preferably 1 mol % or more, more preferably 5 mol % or more, still more preferably 10 mol % or more, yet still more preferably 30 mol % or more, particularly preferably 50 mol % or more, more particularly preferably 80 mol % or more, and most preferably 90 mol % or more from the viewpoint of reducing the fusion rate and improving the carbonization yield during the stabilization treatment by the progress of an intramolecular cyclization reaction through radiation irradiation. The 3,4-bond content proportion in the diene-based polymer when “R” in Formula (1) is a methyl group may be 100 mol %, but is preferably 99.5 mol % or less, and more preferably 99 mol % or less from the viewpoint of reducing the production (polymerization) cost for increasing the 3,4-bond content proportion in the diene-based polymer. The “3,4-bond content proportion” refers to the proportion of 3,4-bond when the total content proportion of 3,4-bond, cis-1,4-bond, trans-1,4-bond, and 1,2-bond included in the diene-based polymer when “R” in Formula (1) is a methyl group is taken as 100 mol %. The content proportion of 3,4-bond can be checked by ¹H-NMR or ¹³C-NMR.

In the disclosure, the stereoregularity of the diene-based polymer is not particularly limited, and the diene-based polymer may be isotactic, syndiotactic, or atactic. The proportions thereof are not particularly limited.

The weight average molecular weight of the diene-based polymer is not particularly limited, and is usually 10 million or less, but is preferably 5 million or less, more preferably 4 million or less, still more preferably 3 million or less, particularly preferably 2 million or less, more particularly preferably 1 million or less, and most preferably 500,000 or less from the viewpoint of the molding processability of the carbon fiber precursor.

The weight average molecular weight of the diene-based polymer is usually 10,000 or more, but is preferably 20,000 or more, more preferably 30,000 or more, and particularly preferably 40,000 or more from the viewpoint of the strength of the carbon fiber precursor and the carbon fiber.

In the disclosure, the weight average molecular weight is measured by gel permeation chromatography under the following conditions. As the measuring device, HLC-8220 GPC manufactured by Tosoh Corporation or a device similar thereto can be used.

(Measurement Conditions)
Column: TSKgel SuperHM-H × 2 columns, SuperH2500 × 1 column
Eluent: chloroform
Eluent flow rate: 0.6 ml/min
Column temperature: 40° C.
Molecular weight standard: standard polystyrene
Detector: differential refractive index detector

From the viewpoint of further preventing fusion between fibers in the stabilization treatment, the content amount of the crosslinked diene-based polymer is preferably 30 mass % or more, more preferably 40 mass % or more, still more preferably 50 mass % or more, yet still more preferably 60 mass % or more, particularly preferably 70 mass % or more, more particularly preferably 80 mass % or more, and most preferably 90 mass % or more with respect to the total mass of the carbon fiber precursor. The upper limit of the content amount of the crosslinked diene-based polymer is not particularly limited. The content amount of the crosslinked diene-based polymer may be 100 mass %.

The presence of the crosslinked diene-based polymer in the carbon fiber precursor can be checked by infrared spectroscopy, solid NMR, ¹H-NMR and 13C-NMR analysis of a dissolved portion, or the like.

(Another Polymer)

The carbon fiber precursor of the disclosure may contain another polymer other than the crosslinked diene-based polymer. Another polymer may be mixed with the diene-based polymer before crosslinking by irradiation with a radiation (diene-based polymer before crosslinking). Use of a mixture containing the diene-based polymer and another polymer can improve the spinnability, and the fiber diameter can be reduced while thread breakage is prevented.

Another polymer is not particularly limited, and examples thereof include olefinic polymers, petroleum resins, aromatic vinyl-based polymers, acrylic polymers (such as poly(meth)acrylate esters (such as polymethyl acrylate and polymethyl methacrylate), poly(meth)acrylic acid, and (meth)acrylate ester/(meth)acrylic acid copolymers), polyesters (such as polyethylene terephthalate, polybutylene terephthalate, and polylactic acid), polyamides, polyvinylidene chloride, polyphenylene sulfide, polyimides, polycarbonates, polyacrylonitrile-based polymers containing a vinyl cyanide monomer unit such as acrylonitrile as a main component (such as polyacrylonitrile, acrylonitrile/itaconic acid copolymers, and acrylonitrile/methyl acrylate copolymers), acrylamide-based polymers containing an acrylamide-based monomer unit such as acrylamide as a main component (such as polyacrylamide and acrylamide/acrylonitrile copolymers), vinyl alcohol-based polymers containing a vinyl alcohol-based monomer as a main component (such as polyvinyl alcohol and vinyl alcohol/vinyl acetate copolymers), and phenolic polymers (such as novolac-type phenolic resins and lignin). From the viewpoint of improving the spinnability and stretchability of the diene-based polymer and the thread breakage prevention property of the crosslinked diene-based polymer during stabilization, at least one polymer selected from the group consisting of an olefinic polymer, a petroleum resin, and an aromatic vinyl-based polymer is preferable. When a mixture of the diene-based polymer and another polymer is used and irradiated with a radiation, not only crosslinking between molecules of the diene-based polymer but also crosslinking between the diene-based polymer and another polymer and crosslinking between molecules of another polymer proceed. In the disclosure, when the gel fraction is calculated, a residue remaining on a membrane filter without being dissolved in toluene is used as a gel component. Therefore, when the crosslinked product formed by crosslinking between the diene-based polymer and another polymer remains on a membrane filter without being dissolved in toluene, the crosslinked product is also included in the calculation of the gel fraction as a gel component. The olefinic polymer may be either linear or branched. The olefinic polymer is not particularly limited, and examples thereof include homopolymers and copolymers of an olefinic monomer. Examples of the olefinic monomer include ethylene, propylene, 1-butene, cis-2-butene, trans-2-butene, isobutene, 1-pentene, 2-pentene, 2-methyl-1-butene, 3-methyl-1-butene, 2,3-dimethyl-2-butene, 1-hexene, 1-octene, 1-nonene, 1-decene, cyclopentene, and 2-norbornene. The olefinic monomers may be used singly or in combination of two or more kinds thereof.

Examples of another polymerizable monomer other than the olefinic monomer include diene-based monomers such as 1,2-propanediene, methylarene, butadiene, isoprene, 2,3-dimethylbutadiene, 1,3-pentadiene, 1,4-pentadiene, cyclopentadiene, dicyclopentadiene, chloroprene, 1,5-hexadiene, 1,4-hexadiene, 1,4-cyclohexadiene, 1,6-heptadiene, 1,7-octadiene, and 5-ethylidene-2 norbornene;

• • aromatic vinyl-based monomers such as styrene, α-methylstyrene, α-methyl-p-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, 2,4-dimethylstyrene, ethylstyrene, p-tert-butylstyrene, o-chlorostyrene, m-chlorostyrene, p-chlorostyrene, p-bromostyrene, 2-methyl-1,4-dichlorostyrene, 2,4-dibromostyrene, and vinylnaphthalene;
  • α,β-unsaturated carboxylate esters such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, monomethyl itaconate, dimethyl itaconate, ethyl itaconate, and diethyl itaconate;
  • vinyl cyanide-based monomers such as acrylonitrile, methacrylonitrile, and ethacrylonitrile;
  • acrylamide-based monomers such as (meth)acrylamide and dimethylaminoethyl (meth)acrylamide;
  • α,β-unsaturated carboxylic acids such as (meth)acrylic acid, maleic acid, fumaric acid, and itaconic acid;
  • α,β-unsaturated carboxylic acid anhydrides such as maleic anhydride and itaconic anhydride;
  • sulfo group-containing vinyl-based monomers such as vinylsulfonic acid;
  • vinyl halide-based monomers such as vinyl chloride;
  • vinyl carboxylates such as vinyl acetate, vinyl butyrate, and vinyl pivalate;
  • and vinyl alcohol.

As another polymerizable monomer, only one kind may be used or two or more kinds may be used in combination.

Examples of the copolymer having a structural unit derived from an olefinic monomer include ethylenic copolymers such as an ethylene-propylene copolymer, an ethylene-1-butene copolymer, an ethylene-4-methyl-1-pentene copolymer, an ethylene-1-hexene copolymer, an ethylene-propylene-dicyclopentadiene copolymer, an ethylene-propylene-5-vinyl-2-norbornene copolymer, an ethylene-propylene-1,4-hexadiene copolymer, an ethylene-propylene-1,4-cyclohexadiene copolymer, a polystyrene-poly(ethylene/propylene) block copolymer (SEP), a polystyrene-poly(ethylene/propylene)-polystyrene block copolymer (SEPS), a polystyrene-poly(ethylene/butylene)polystyrene block copolymer (SEBS), a polystyrene-poly(ethylene-ethylene/propylene)-polystyrene block copolymer (SEEPS), and an ethylene-1-octene copolymer;

• • propylene-based copolymers such as a propylene-1-butene-4-methyl-1-pentene copolymer and a propylene-1-butene copolymer; and
  • a 1-hexene-4-methyl-1-pentene copolymer and a 4-methyl-1-pentene-1-octene copolymer.

Preferable examples of the petroleum resin include a C5-based petroleum resin, a C9-based petroleum resin, copolymerized petroleum resins of respective fractions such as a C5/C9-based petroleum resin, alicyclic petroleum resins containing a cyclopentadiene-based compound as a main raw material (such as a dicyclopentadiene-based petroleum resin), and hydrogenated petroleum resins obtained by hydrogenating these petroleum resins (such as a partially hydrogenated petroleum resin and a completely hydrogenated petroleum resin). Here, the C5-based petroleum resin is a petroleum resin (mainly an aliphatic petroleum resin) obtained using a C5 fraction of petroleum as a raw material, the C9-based petroleum resin is a petroleum resin (mainly an aromatic petroleum resin) obtained using a C9 fraction of petroleum as a raw material, and the C5/C9-based petroleum resin is a petroleum resin (a copolymerized petroleum resin) obtained using a C5 fraction and a C9 fraction of petroleum as raw materials. Here, as the C5 fraction and the C9 fraction, also analogs thereof are included. Examples of the C5 fraction include 1,3-pentadiene (piperylene), 2-methyl-2 butene, cyclopentadiene, dicyclopentadiene, methylcyclopentadiene, dimethylcyclopentadiene, isoprene, and 2-butyne. Examples of the C9 fraction include styrene, methylstyrene, vinyltoluene, ethylstyrene, dimethylstyrene, indene, and methylindene. As the C5-based petroleum resin and the C5/C9-based petroleum resin, those containing dicyclopentadiene derived from cyclopentadiene, which is a kind of C5 fraction, in the structure are preferable.

The aromatic vinyl-based polymer is not particularly limited, and examples thereof include homopolymers and copolymers of an aromatic vinyl-based monomer. Examples of the aromatic vinyl-based monomer include styrene, α-methylstyrene, α-methyl-p-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, 2,4-dimethylstyrene, ethylstyrene, p-tert-butylstyrene, o-chlorostyrene, m-chlorostyrene, p-chlorostyrene, p-bromostyrene, 2-methyl-1,4-dichlorostyrene, 2,4-dibromostyrene, vinylnaphthalene, and indene. The aromatic vinyl-based monomers may be used singly or in combination of two or more kinds thereof. Preferable examples of the aromatic vinyl-based polymer include a styrene-butadiene-styrene block copolymer (SBS), a styrene-isoprene-styrene block copolymer (SIS), an acrylonitrile-butadiene-styrene (ABS) resin, a methyl (meth)acrylate-acrylonitrile-butadiene-styrene (MABS), and a methyl (meth)acrylate-butadiene-styrene (MBS) resin.

The content amount of another polymer is not particularly limited, but is preferably 70 mass % or less, more preferably 60 mass % or less, still more preferably 50 mass % or less, particularly preferably 40 mass % or less, more particularly preferably 30 mass % or less, and most preferably 20 mass % or less with respect to the total mass of the carbon fiber precursor from the viewpoint of preventing fusion during the stabilization treatment of the carbon fiber precursor.

The carbon fiber precursor of the disclosure need not contain another polymer other than the crosslinked diene-based polymer. The carbon fiber precursor of the disclosure preferably contains another polymer from the viewpoint of improving the thread breakage prevention property of the crosslinked diene-based polymer during stabilization and the strength of the carbon fiber. The content amount of another polymer is preferably 0.1 mass % or more, more preferably 0.5 mass % or more, still more preferably 1 mass % or more, particularly preferably 2 mass % or more, and most preferably 3 mass % or more with respect to the total mass of the carbon fiber precursor.

(Other Additives)

In addition to the crosslinked diene-based polymer and another polymer, the carbon fiber precursor of the disclosure may contain an additive such as an antioxidant, an oxidizing agent, a release agent, a lubricant, a plasticizer, a colorant, a crosslinking agent, a crosslinking aid, a crosslinking accelerator, a crosslinking retarder, a reinforcing material (a filler such as a carbon nanotube, graphene, a cellulose nanofiber, carbon black, a glass fiber, or a metal fiber), an antiaging agent, a light stabilizer (such as an ultraviolet absorber or an ultraviolet scattering agent), a light shielding agent, a softening agent, an antistatic agent, or a compatibilizer as long as the effect of the disclosure is not inhibited.

<Method of Producing Carbon Fiber Precursor>

The method of producing a carbon fiber precursor of the disclosure includes irradiating a polymer fiber including a diene-based polymer containing a structural unit represented by Formula (1) above with a radiation having a dose of 20 kGy or more: Details of the diene-based polymer containing a structural unit represented by Formula (1) are as described above. The polymer fiber including a diene-based polymer containing a structural unit represented by Formula (1) preferably contains the above-mentioned another polymer before irradiation with a radiation from the viewpoint of reducing the fiber diameter while thread breakage is prevented when the diene-based polymer is spun, and from the viewpoint of improving the stretchability, the thread breakage prevention property of the crosslinked diene-based polymer during stabilization and the strength of the carbon fiber. Another polymer is preferably at least one polymer selected from the group consisting of the olefinic polymer, the petroleum resin, and the aromatic vinyl-based polymer described above. The polymer fiber including a diene-based polymer containing a structural unit represented by Formula (1) may contain another polymer described above after irradiation with a radiation, but preferably contains another polymer before irradiation with a radiation from the viewpoint that crosslinking between the diene-based polymer and at least part of another polymer also proceeds, and the thread breakage prevention property of the crosslinked diene-based polymer during stabilization is improved. When the above-mentioned another polymer is contained before irradiation with a radiation, the upper limit of the content amount of another polymer is preferably 70 mass % or less, more preferably 60 mass % or less, still more preferably 50 mass % or less, particularly preferably 40 mass % or less, more particularly preferably 30 mass % or less, and most preferably 20 mass % or less with respect to the total mass of the diene-based polymer from the viewpoint of increasing the gel fraction and preventing fusion during the stabilization treatment of the carbon fiber precursor. The lower limit of the content amount of another polymer is preferably 0.1 mass % or more, more preferably 0.5 mass % or more, still more preferably 1 mass % or more, particularly preferably 2 mass % or more, and most preferably 3 mass % or more with respect to the total mass of the diene-based polymer from the viewpoint of improving the spinnability and stretchability of the diene-based polymer and the thread breakage prevention property during stabilization.

Examples of the radiation include an X-ray, a γ-ray, an α-ray, a β-ray, an electron beam, a neutron beam, a proton beam, and a heavy particle beam. Among them, the radiation is preferably an electron beam from the viewpoint of preventing fusion between fibers in the stabilization treatment and preventing rupture of the fiber in the carbonization treatment.

The dose of the radiation is preferably 30 kGy or more, more preferably 60 kGy or more, still more preferably 80 kGy or more, particularly preferably 150 kGy or more, and most preferably 400 kGy or more from the viewpoint of increasing the gel fraction of the crosslinked diene-based polymer obtained by crosslinking of the diene-based polymer. The upper limit of the dose of the radiation is not particularly limited. The dose of the radiation is preferably 50 MGy or less, more preferably 10 MGy or less, still more preferably 5 MGy or less, particularly preferably 2,000 kGy or less, and most preferably 1,000 kGy or less from the viewpoint of reducing energy cost and reducing damage to the polymer fiber.

By irradiating the polymer fiber with a radiation having a dose of 20 kGy or more, the entire fiber from the surface to the center can be crosslinked. As a result, a carbon fiber precursor containing a crosslinked diene-based polymer having a high gel fraction can be obtained, and fusion between fibers in the stabilization treatment can be prevented. In addition, the stabilized fiber obtained by the stabilization treatment is less likely to be broken even when being subjected to the carbonization treatment.

When an electron beam is used as the radiation, the dose is measured using a film dosimeter. As the film dosimeter, FWT-60 manufactured by Toyo Medic Co., Ltd. or a device similar thereto can be used.

The acceleration voltage of the radiation applied to the polymer fiber is preferably adjusted to an acceleration voltage at which preferably 20% or more, more preferably 50% or more, and still more preferably 80% or more of the radiation applied passes through the polymer fiber from the viewpoint of preventing fusion between fibers in the stabilization treatment and improving the tensile strength, and from the viewpoint of shape stability.

When an electron beam is used as the radiation, the transmittance of the electron beam is calculated by measuring the dose before and after transmission. The transmittance may be calculated using a relationship diagram between the transmission depth generally disclosed and the relative dose.

Specifically, the acceleration voltage is preferably from 50 kV to 10 MV, more preferably from 100 kV to 3 MV, and still more preferably from 150 kV to 1 MV.

The irradiation with a radiation may be performed batchwise or continuously.

The apparatus used for the irradiation with a radiation is not particularly limited, and in a case in which the irradiation with a radiation is performed batchwise, CB250/30/20 mA manufactured by Iwasaki Electric Co., Ltd. or an apparatus similar thereto can be used. In a case in which the irradiation with a radiation is performed continuously, an electron beam irradiation apparatus EBC800-35 manufactured by NHV Corporation or an apparatus similar thereto can be used.

The irradiation with a radiation may be performed in a nitrogen atmosphere or in an air atmosphere.

When a radiation is applied, it is preferable to apply a tension of 0.03 mN/dtex or more to the polymer fiber. It is presumed that when a tension of 0.03 mN/dtex or more is applied, the molecules are easily oriented in the fiber axis direction, and when the molecules are oriented, the cyclization reaction in the oriented molecules and the crosslinking between molecules highly proceed. As a result, a carbon fiber precursor containing a crosslinked diene-based polymer having a higher gel fraction can be obtained, and fusion between fibers in the stabilization treatment can be prevented. In addition, the stabilized fiber obtained by the stabilization treatment is less likely to be broken even when being subjected to the carbonization treatment.

The tension applied to the polymer fiber is more preferably 0.05 mN/dtex or more, still more preferably 0.1 mN/dtex or more, particularly preferably 0.3 mN/dtex or more, and most preferably 0.5 mN/dtex or more from the viewpoint of preventing fusion between fibers in the stabilization treatment and improving carbonization stability.

The polymer fiber may be a commercially available product or may be produced by a conventionally known method.

The polymer fiber can be produced by spinning a diene-based polymer containing a structural unit represented by Formula (1) or a polymer composition including a diene-based polymer containing a structural unit represented by Formula (1) and another component described above.

The spinning method is not particularly limited, but melt spinning, spunbonding, melt blowing, or centrifugal spinning is preferable from the viewpoint of reducing the production cost and reducing the load on the environment. The spinning method may be dry spinning, wet spinning, dry-wet spinning, gel spinning, flash spinning, or electrospinning.

The polymer fiber may be a single fiber or a fiber bundle.

When the polymer fiber is a fiber bundle, the number of filaments per bundle is not particularly limited, but is preferably from 10 to 360,000, more preferably from 20 to 180,000, still more preferably from 30 to 72,000, and particularly preferably from 50 to 36,000 from the viewpoint of the productivity and the mechanical properties of the stabilized fiber and the carbon fiber.

In addition, it is possible to prevent the occurrence of firing unevenness during the stabilization treatment by setting the number of filaments per bundle to 360,000 or less.

The fineness of the polymer fiber is not particularly limited, but the fineness per single fiber of the polymer fiber is preferably from 1×10⁻⁷ dtex/fiber to 1,200 dtex/fiber, more preferably from 1×10⁻⁵ dtex/fiber to 700 dtex/fiber, further preferably from 1×10⁻³ dtex/fiber to 300 dtex/fiber, still further preferably from 1×10⁻² dtex/fiber to 100 dtex/fiber, particularly preferably from 4×10⁻² dtex/fiber to 25 dtex/fiber, and most preferably from 1×10⁻¹ dtex/fiber to 10 dtex/fiber.

By setting the fineness of the polymer fiber to 1×10⁻⁷ dtex/fiber or more, the occurrence of thread breakage can be prevented, whereby the case of winding the polymer fiber and the stability in the stabilization treatment can be improved.

By setting the fineness of the polymer fiber to 1,200 dtex/fiber or less, the difference between the structure near the surface layer and the structure near the center of the stabilized fiber obtained by the stabilization treatment can be reduced, and the tensile strength and the tensile modulus of the carbon fiber can be improved.

In the disclosure, as for the fineness per single fiber (dtex/fiber) of the polymer fiber, when the polymer fiber is a single fiber, the mass of the polymer fiber is measured, and the mass per 10,000 m is calculated as the fineness of the fiber. When the polymer fiber is a fiber bundle including a plurality of single fibers (hereinafter sometimes referred to as a polymer fiber bundle), the mass of the polymer fiber bundle is measured, the mass per 10,000 m is calculated as the fineness (dtex) of the fiber bundle, and the fineness per single fiber (dtex/fiber) is calculated by dividing the fineness of the fiber bundle by the number of single fibers included in the fiber bundle.

The average fiber diameter of one fiber (single fiber) of the polymer fiber (hereinafter also simply referred to as “the average fiber diameter of the polymer fiber”) is not particularly limited, but is preferably from 3 nm to 400 μm, more preferably from 30 nm to 300 μm, still more preferably from 300 nm to 200 μm, particularly preferably from 1 μm to 100 μm, more particularly preferably from 2 μm to 60 μm, and most preferably from 3 μm to 40 μm.

By setting the average fiber diameter of the polymer fiber to 3 nm or more, the stability in the stabilization treatment can be improved. By setting the average fiber diameter of the polymer fiber to 3 nm or more, the occurrence of thread breakage can be prevented, whereby the case of winding the polymer fiber and the stability in the stabilization treatment can be improved.

By setting the average fiber diameter of the polymer fiber to 400 μm or less, the difference between the structure near the surface layer and the structure near the center of the stabilized fiber obtained by the stabilization treatment can be reduced, and the tensile strength and the tensile modulus of the carbon fiber can be improved.

In the disclosure, the average fiber diameter of the polymer fiber can be determined by observing the side surface or the cross section of the fiber with a microscope, a scanning electron microscope, or the like, but when the polymer fiber is a fiber bundle, the average fiber diameter is calculated by the following method.

The polymer fiber bundle is vacuum-dried at 80° C. for 12 hours. Thereafter, the density of the polymer fiber bundle is measured using a dry automatic densitometer (“AccuPyc II 1340” manufactured by Micromeritics), and the average fiber diameter (μm) of single fibers included in the fiber bundle is determined according to the following formula.

$D={\left\{\left(\mathrm{Dt}\times 4\times 100\right)/\left(\rho \times \pi \times n\right)\right\}}^{1/2}$

In the formula,

• • D represents an average fiber diameter (μm) of single fibers included in the fiber bundle,
  • Dt represents the fineness (dtex) of the fiber bundle,
  • ρ represents the density (g/cm³) of the fiber bundle, and
  • n represents the number (fibers) of single fibers included in the fiber bundle.

Note that π is 3.14.

The polymer fiber may have a surface to which a conventionally known oil agent is applied.

By applying the oil agent to the surface of the polymer fiber, the bundling property and handling of the fiber can be improved, and fusion between fibers can be prevented.

In addition, by crosslinking the oil agent together with the diene-based polymer, fusion between fibers can be more effectively prevented.

The oil agent is not particularly limited, and for example, a conventionally known silicone-based oil agent can be used. The oil agent may be an oil agent having a functional group that is crosslinked by irradiation with a radiation. Examples of such an oil agent include a silicone-based oil agent having a functional group that is crosslinked by irradiation with a radiation. The oil agent may contain an additive such as an organic solvent, a surfactant, a crosslinking agent, a crosslinking accelerator, a smoothing agent, a moisture absorbent, a viscosity modifier, a plasticizer, a release agent, a spreader, an antioxidant, an antibacterial agent, a preservative, a rust inhibitor, or a pH adjusting agent.

<Method of Producing Stabilized Fiber>

The method of producing a stabilized fiber of the disclosure includes subjecting a fiber of the carbon fiber precursor to a heat treatment in an oxidizing atmosphere. Hereinafter, a treatment in which a heat treatment is performed in an oxidizing atmosphere is also referred to as “stabilization treatment”.

The temperature in the stabilization treatment is not particularly limited, but is preferably in a range of from 120° ° C. to 500° C., more preferably in a range of from 130° C. to 490° C., still more preferably in a range of from 140° ° C. to 480° C., yet still more preferably in a range of from 150° ° C. to 470° C., particularly preferably in a range of from 160° C. to 460° C., and most preferably in a range of from 170° C. to 450° C.

The temperature includes not only the highest temperature in the stabilization treatment described later but also temperatures in the temperature raising process up to the stabilization treatment temperature.

The highest temperature in the stabilization treatment is preferably 290° C. or higher, more preferably 300° C. or higher, still more preferably 310° C. or higher, yet still more preferably 320° ° C. or higher, particularly preferably 330° ° C. or higher, and most preferably 340° ° C. or higher from the viewpoint of improving the carbonization stability and reducing the production cost by reducing the time. The upper limit of the stabilization treatment temperature is not particularly limited, and the stabilization treatment temperature is, for example, preferably 500° C. or lower, and more preferably 490° C. or lower.

The heating time at the stabilization treatment temperature is not particularly limited, and may be 4 hours or more, but is preferably from 1 minute to 4 hours, more preferably from 2 minutes to 2 hours, still more preferably from 3 minutes to 110 minutes, particularly preferably from 4 minutes to 100 minutes, and most preferably from 4 minutes to 90 minutes. The carbonization yield is improved by setting the heating time at the stabilization treatment temperature to 1 minute or more. The production cost can be reduced by setting the heating time at the stabilization treatment temperature to 4 hours or less.

Examples of the oxidizing atmosphere include oxygen, ozone, air, a nitrogen oxide, a halogen, and sulfurous acid gas; a mixed gas thereof; and a mixed gas of oxygen, ozone, air, a nitrogen oxide, a halogen, or a sulfurous acid gas with an inert gas. Among them, the oxidizing atmosphere is preferably air, a mixed gas of oxygen with air, a mixed gas of oxygen with an inert gas, or a mixed gas of air with an inert gas, and particularly preferably air from the viewpoint of reducing the production cost.

In the temperature raising process up to the stabilization treatment temperature, a tension may or need not be applied to the carbon fiber precursor, but it is preferable to apply a tension from the viewpoint of improving the tensile strength of the stabilized fiber.

A tension may be applied from an initial stage of the temperature raising process, or may be applied from a stage in the middle of the temperature raising process. A tension of a magnitude different for each temperature may be applied.

The tension applied to the carbon fiber precursor is preferably from 0.005 mN/dtex to 200 mN/dtex, more preferably from 0.01 mN/dtex to 100 mN/dtex, and still more preferably from 0.02 mN/dtex to 50 mN/dtex.

<Stabilized Fiber>

The stabilized fiber of the disclosure contains a structure derived from a crosslinked diene-based polymer and has a fusion rate of 30% or less. The crosslinked diene-based polymer is preferably a crosslinked product of a diene-based polymer containing a structural unit represented by Formula (1) above. Details of the diene-based polymer containing a structural unit represented by Formula (1) are as described above.

The presence of a structure derived from the crosslinked diene-based polymer in the stabilized fiber can be checked by infrared spectroscopy, solid NMR measurement, elemental analysis, or the like. The structure derived from the crosslinked diene-based polymer is, for example, a polycyclic structure in which a plurality of rings are further condensed after the crosslinked diene-based polymer is cyclized in a molecule. The polycyclic structure preferably includes any one or more of a structure in which a substituent containing oxygen, such as a carbonyl group or a hydroxy group, is formed by crosslinking between molecules or oxidation during stabilization, and a conjugated structure in which a double bond of a carbon atom is formed.

Since the stabilized fiber of the disclosure has a fusion rate of 30% or less, rupture of the fiber in the carbonization treatment can be prevented.

The fusion rate is preferably 25% or less, more preferably 20% or less, still more preferably 15% or less, yet still more preferably 10% or less, particularly preferably 5% or less, and most preferably 0%.

In the disclosure, the fusion rate is calculated by the following method.

A fiber for evaluation having a length of 3 cm is cut out from the stabilized fiber, a cross section of the fiber for evaluation is observed using a microscope (“Digital microscope VHX-7000” manufactured by Keyence Corporation), and the number of fibers is counted. At this time, the number of fibers fused and the number of all fibers are counted. The number of fibers fused is determined to be, for example, two when two fibers are fused to each other. The number of all fibers is counted by separating the fused fibers into a state before fusion. The fusion rate is calculated based on the following formula.

$\mathrm{Fusion}\mathrm{rate}\left(\%\right)=\left(\mathrm{number}\mathrm{of}\mathrm{fibers}\mathrm{fused}/\mathrm{number}\mathrm{of}\mathrm{all}\mathrm{fibers}\right)\times 100$

<Method of Producing Carbon Fiber>

The method of producing a carbon fiber of the disclosure preferably includes producing a stabilized fiber by the method of producing a stabilized fiber of the disclosure, and subjecting the stabilized fiber to a carbonization treatment.

In addition, the method of producing a carbon fiber of the disclosure preferably includes subjecting the stabilized fiber of the disclosure to a carbonization treatment.

Examples of the method of subjecting the stabilized fiber to a carbonization treatment include a method of subjecting the stabilized fiber to a heat treatment in an inert gas atmosphere at a temperature equal to or higher than the temperature in the stabilization treatment. Examples of the inert gas include nitrogen, argon, and helium.

By subjecting the stabilized fiber to the carbonization treatment, the stabilized fiber is carbonized to obtain a carbon fiber.

The temperature in the carbonization treatment is preferably 500° C. or higher, more preferably 1,000° C. or higher, still more preferably 1,100° C. or higher, particularly preferably 1,200° ° C. or higher, and most preferably 1,300° C. or higher. The upper limit of the temperature in the carbonization treatment is not particularly limited. The temperature in the carbonization treatment is preferably 3,000° C. or lower, and more preferably 2,500° ° C. or lower from the viewpoint of reducing the production cost by reducing energy involved in the production.

In the disclosure, the “carbonization treatment” may include “graphitization” generally performed by heating at a temperature of from 2,000° C. to 3,000° C. in an inert gas atmosphere.

The carbonization treatment may include a plurality of times of heat treatment.

For example, it is possible to first perform a heat treatment (hereinafter also referred to as “pre-carbonization treatment”) at a temperature lower than 1,000° C., then perform a heat treatment (carbonization treatment) at a temperature of 1,000° C. or higher, and further perform a heat treatment (graphitization treatment) at a temperature of 2,000° C. or higher.

The carbonization treatment time is not particularly limited, but is preferably from 30 seconds to 120 minutes, more preferably from 30 seconds to 60 minutes, and still more preferably from 1 minute to 30 minutes. From the viewpoint of reducing the production cost, the carbonization treatment time is particularly preferably 20 minutes or less, and most preferably 10 minutes or less.

The average fiber diameter of the single fiber of the carbon fiber is not particularly limited, but is preferably from 3 nm to 300 μm, more preferably from 30 nm to 150 μm, still more preferably from 100 nm to 60 μm, yet still more preferably from 1 μm to 40 μm, particularly preferably from 2 μm to 30 μm, and most preferably from 2.5 μm to 25 μm.

When the average fiber diameter of the single fiber of the carbon fiber is 3 nm or more, in the case of preparing a composite material using a resin or the like as a matrix, the resin or the like is easily impregnated into the carbon fiber even if the viscosity of the matrix is high, and the tensile strength of the composite material is improved. When the average fiber diameter of the single fiber of the carbon fiber is 300 μm or less, the tensile strength of the carbon fiber is improved.

EXAMPLES

Hereinafter, the embodiments will be specifically described with reference to examples, but the embodiments are not limited to the examples.

<Diene-Based Polymer a1>

Syndiotactic 1,2-polybutadiene (product name “RB 840”, manufactured by ENEOS Materials Corporation, content amount of 1,2-structural unit (1,2-bond content proportion): 94 mol %, melting point: 126° C.) was used as a diene-based polymer a1.

<Diene-Based Polymer a2>

Synthesis of FeCl₃(TBTP)

As a polymerization catalyst that enables selective 3,4-addition polymerization of isoprene, FeCl₃(TBTP) in which tri-tert-butyl-terpyridine (TBTP) was coordinated to FeCl₃ was synthesized according to the following procedure.

In 50 mL of anhydrous tetrahydrofuran, 0.2 g of FeCl₃ was dispersed. To the dispersion, 0.5 g of TBTP was added, and the mixture was stirred at room temperature (25° C.) for 10 hours. The resulting solution was left to stand for 15 hours and a crude product was precipitated. Thereafter, the crude product was subjected to suction filtration with washing with tetrahydrofuran. The obtained powder was dried under vacuum at 30° C. for 3 days, thereby obtaining FeCl₃(TBTP) in a yield of 93%.

Synthesis of Diene-Based Polymer a2

To a flask containing 225.5 mg (0.4 mmol) of FeCl₃(TBTP), 2.75 mL of anhydrous toluene was added in a nitrogen atmosphere. Then, 17.25 mL of an anhydrous toluene solution containing 2.516 g (40 mmol) of modified methylaluminoxane (MMAO, [(CH₃)_0.95(C₈H₁₇)_0.05AlO]) (MMAO concentration: 16.3 mass %) was added dropwise thereto. Then, 8.0 mL (80 mmol) of isoprene was added dropwise thereto, and polymerization was allowed to proceed at 25° C. for 3 hours. The solution after polymerization was diluted by adding 20 mL of toluene. The solution was added dropwise to 500 mL of methanol containing 395.9 mg of 2,6-di-tert-butyl-p-cresol (BHT) to form a precipitate. The precipitate was washed with 500 mL of methanol containing 395.9 mg of BHT. The washing operation was performed three times, thereby obtaining a white solid. The obtained solid was dried under vacuum at 25° C. for 3 days, thereby obtaining an isoprene-based polymer (diene-based polymer a2) in a yield of 99%.

As a result of analyzing the composition of the obtained diene-based polymer a2 by 1H-NMR and 13C-NMR, the content amount of 3,4-structural unit (3,4-bond content proportion) was 74.4 mol %, the content amount of trans-1,4-structural unit was 5.6 mol %, the content amount of cis-1.4 structural unit was 20.0 mol %, and the content amount of 1,2-structural unit was 0.0 mol %. The 3,4-structural unit is a structural unit, which is represented by Formula (1), and in which R is a methyl group.

<Polymer Fiber Bundle (b1)>

Melt spinning (the number of holes of a nozzle of a melt spinning machine: 36) was performed at a temperature of 150° ° C. so that the fineness per fiber (single fiber) was 21 dtex using the diene-based polymer (a1), thereby obtaining a polymer fiber bundle of 36 fibers/bundle (fineness of single fiber: 21 dtex). Subsequently, the polymer fiber bundle was stretched by 3.5 times at 50° C., and then 6 bundles were aligned to form a fiber bundle of 216 fibers/bundle, thereby obtaining a polymer fiber bundle (b1) (average fiber diameter: 29 μm, fineness of single fiber: 6 dtex).

<Polymer Fiber Bundle (b2)>

Melt spinning (the number of holes of a nozzle of a melt spinning machine: 36) was performed at a temperature of 150° ° C. so that the fineness per fiber (single fiber) was 24.5 dtex using the diene-based polymer (a2), thereby obtaining a polymer fiber bundle of 36 fibers/bundle (fineness of single fiber: 24.5 dtex). Subsequently, the polymer fiber bundle was stretched by 3.5 times at 50° C., and then 6 bundles were aligned to form a fiber bundle of 216 fibers/bundle, thereby obtaining a polymer fiber bundle (b2) (average fiber diameter: 31 μm, fineness of single fiber: 7 dtex).

<Polymer Fiber Bundle (b3)>

Melt spinning (the number of holes of a nozzle of a melt spinning machine: 36) was performed at a temperature of 150° ° C. so that the fineness per fiber (single fiber) was 3 dtex using the diene-based polymer (a1), thereby obtaining a polymer fiber bundle of 36 fibers/bundle (fineness of single fiber: 3 dtex). Subsequently, 6 bundles were combined to form a fiber bundle of 216 fibers/bundle, and then the fiber bundle was stretched by 2 times at normal temperature, thereby obtaining a polymer fiber bundle (b3) (average fiber diameter: 14 μm, fineness of single fiber: 1.5 dtex).

<Polymer Fiber Bundle (b4)>

Melt spinning (the number of holes of a nozzle of a melt spinning machine: 36) was performed at a temperature of 150° ° C. so that the fineness per fiber was 2 dtex using a mixture containing 91 mass % of the diene-based polymer (a1) and 9 mass % of polyethylene (product name “Polyethylene Wax PE 520”, manufactured by Clariant AG, weight-average molecular weight: 5,500, viscosity at 140° C.: about 0.65 Pa·s, melting point: from 117 to 123° C., density: 0.93 g/cm³) as another polymer, thereby obtaining a polymer fiber bundle of 36 fibers/bundle (fineness of single fiber: 2 dtex). Subsequently, 6 bundles were combined to form a fiber bundle of 216 fibers/bundle, and then the fiber bundle was stretched by 2 times at normal temperature, thereby obtaining a polymer fiber bundle (b4) (average fiber diameter: 12 μm, fineness of single fiber: 1 dtex).

<Polymer Fiber Bundle (b5)>

To 100 parts by mass of the diene-based polymer (a1), 2 parts by mass of 4,4′-bis(dimethylamino)benzophenone as a photosensitizer was added, and melt spinning (the number of holes of a nozzle of a melt spinning machine: 36) was performed at a temperature of 150° C. so that the fineness per fiber (single fiber) was 21 dtex, thereby obtaining a polymer fiber bundle of 36 fibers/bundle (fineness of single fiber: 21 dtex). Subsequently, the polymer fiber bundle was stretched by 3.5 times at 50° C., and then 6 bundles were aligned to form a fiber bundle of 216 fibers/bundle, thereby obtaining a polymer fiber bundle (b5) (average fiber diameter: 29 μm, fineness of single fiber: 6 dtex).

With respect to the obtained polymer fiber bundles (b1) to (b5), the average fiber diameter and fineness were calculated by the following methods.

(Average Fiber Diameter of Polymer Fiber Bundle)

The obtained polymer fiber bundle was dried under vacuum at 60° C. for 12 hours. Thereafter, the density of the polymer fiber bundle was measured using a dry automatic densitometer (“AccuPyc II 1340” manufactured by Micromeritics), and the average fiber diameter (μm) of single fibers included in the fiber bundle was determined according to the following formula.

$D={\left\{\left(\mathrm{Dt}\times 4\times 100\right)/\left(\rho \times \pi \times n\right)\right\}}^{1/2}$

In the formula,

• • D represents an average fiber diameter (μm) of single fibers included in the fiber bundle,
  • Dt represents the fineness (dtex) of the fiber bundle,
  • ρ represents the density (g/cm³) of the fiber bundle, and
  • n represents the number (fibers) of single fibers included in the fiber bundle.

Note that π is 3.14.

(Fineness of Polymer Fiber Bundle)

The mass of the obtained polymer fiber bundle was measured, the mass per 10,000 m was calculated as the fineness of the fiber bundle, and the fineness per single fiber included in the fiber bundle was calculated.

<Production of Carbon Fiber Precursor>

In Examples 1 to 4 and 11 to 14, irradiation with an electron beam was performed batchwise.

In Examples 5 to 10, 15, and 16, irradiation with an electron beam was performed continuously.

Batch Method

A polymer fiber bundle shown in Table 1 was fixed onto a base paper while a tension shown in Table 1 was applied, and the polymer fiber bundle on the base paper was irradiated with an electron beam at a dose shown in Table 1 using an electron beam irradiation apparatus CB250/30/20 mA manufactured by Iwasaki Electric Co., Ltd. in an atmosphere shown in Table 1 at an acceleration voltage set to 250 kV, thereby obtaining a carbon fiber precursor containing a crosslinked diene-based polymer. In the electron beam treatment at a dose of 50 kGy, an acceleration voltage was 250 kV, a beam current was 3.2 mA, a conveyance speed was 5 m/min, and the treatment was performed once (3.6 seconds). In the electron beam treatment at a dose of 100 kGy, an acceleration voltage was 250 kV, a beam current was 6.3 mA, a conveyance speed was 5 m/min, and the treatment was performed once (3.6 seconds). In the electron beam treatment at a dose of 300 kGy, an acceleration voltage was 250 kV, a beam current was 6.3 mA, a conveyance speed was 5 m/min, and the treatment was performed 3 times (10.8 seconds in total of 3 times of treatment).

Continuous Method

A polymer fiber bundle shown in Table 1 was irradiated with an electron beam at a dose shown in Table 1 for 2 minutes using an electron beam irradiation apparatus EBC800-35 manufactured by NHV Corporation at a conveyance speed of 10 m/min in an atmosphere shown in Table 1 at an acceleration voltage set to 800 kV while a tension shown in Table 1 was applied, thereby obtaining a carbon fiber precursor containing a crosslinked diene-based polymer.

With respect to the obtained carbon fiber precursor, the gel fraction and swelling magnification were calculated by the following method. The calculation results are shown in Table 1.

(Gel Fraction)

From the carbon fiber precursor, 0.2 g of a sample was cut out. After the sample was dried at 80° ° C. for 4 hours, the mass was precisely weighed using a precision electronic balance and taken as the initial mass (g).

Subsequently, the sample was immersed in 30 ml of toluene and left to stand in a hot air circulating oven at 60° C. for 8 hours.

The sample after being left to stand was subjected to suction filtration using a membrane filter having a pore size of 1.0 μm (Omnipore TM membrane filter JAWP04700, manufactured by Merck KGaA) to separate a gel component.

The separated gel component was air-dried together with the membrane filter in the air in a draft for 12 hours or more, and further left to stand in a hot air circulating oven at 90° ° C. for 12 hours to remove toluene.

The mass of the gel component and the membrane filter after being left to stand were precisely weighed using a precision electronic balance, and the gel fraction was determined from the following formula.

$\mathrm{Gel}\mathrm{fraction}\left(\%\right)=\left\{\left(\mathrm{mass}\mathrm{of}\mathrm{gel}\mathrm{component}\mathrm{and}\mathrm{membrane}\mathrm{filter}\left(g\right)-\mathrm{mass}\mathrm{of}\mathrm{membrane}\mathrm{filter}\left(g\right)\right)/\mathrm{initial}\mathrm{mass}\mathrm{of}\mathrm{sample}\left(g\right)\right\}\times 100$

(Swelling Magnification)

From the carbon fiber precursor, 0.2 g of a sample was cut out. After the sample was dried at 80° C. for 4 hours, the sample was immersed in 30 ml of toluene and left to stand in a hot air circulating oven at 60° ° C. for 8 hours.

The sample after being left to stand was subjected to suction filtration using a membrane filter having a pore size of 1.0 μm to separate a gel component, and the mass of the recovered gel component (swollen gel) was precisely weighed using a precision electronic balance. Subsequently the swollen gel was air-dried in the air in a draft for 12 hours or more, and further dried at 90° C. for 12 hours, and then, the mass of the gel after drying (dried gel) was determined by weighing. The swelling magnification was determined from the following formula.

$\mathrm{Swelling}\mathrm{magnification}\left(\mathrm{times}\right)=\left(\mathrm{mass}\mathrm{of}\mathrm{swollen}\mathrm{gel}\left[g\right]\right)/\left(\mathrm{mass}\mathrm{of}\mathrm{dried}\mathrm{gel}\left[g\right]\right)$

<Production of Stabilized Fiber>

Examples 1 to 16

The obtained carbon fiber precursor was continuously conveyed into a heat treatment oven in an air stream while a tension of 60 cN (600 mN) was applied to the carbon fiber precursor. A heat treatment (stabilization treatment) was performed while the temperature was raised to from 200° ° C. to 350° C. over 60 minutes. Thereafter, the carbon fiber precursor was further subjected to a stabilization treatment at 350° C. for 30 minutes while the tension applied to the carbon fiber precursor was maintained. Stabilized fibers were continuously conveyed from the outlet of the heat treatment oven, thereby obtaining stabilized fibers.

Comparative Example 1

A stabilization treatment was performed under conditions similar to those in Examples 1 to 16 except that the polymer fiber bundle (b1) was used. However, when the polymer fiber bundle (b1) was conveyed into the heat treatment oven, the fibers were melted and broken

Comparative Example 2

Both ends of the polymer fiber bundle (b1) were fixed by applying a tension (0.02 mN/dtex) to such an extent that slack does not occur to the polymer fiber bundle (b1). In this state, the polymer fiber bundle (b1) was immersed in nitric acid (concentration: 61%) at 20° C. for 20 hours. Thereafter, the polymer fiber bundle (b1) was washed with water and dried. Subsequently, the obtained fiber bundle was subjected to a stabilization treatment in an air stream using a heat treatment oven under conditions similar to those in Examples 1 to 16, thereby obtaining stabilized fibers.

Comparative Example 3

The both ends of the polymer fiber bundle (b1) were fixed by applying a tension (0.02 mN/dtex) to such an extent that slack does not occur to the polymer fiber bundle (b1). In this state, the polymer fiber bundle (b1) was immersed in a solution in which 2 g of aluminum chloride was dissolved in 100 mL of p-xylene at 20° C. for 1 hour. Thereafter, the resulting material was washed with methanol containing benzene and a small amount of hydrochloric acid, and further washed with methanol, and then dried. Subsequently, the obtained fiber bundle was subjected to a stabilization treatment in an air stream using a heat treatment oven under conditions similar to those in Examples 1 to 16, thereby obtaining stabilized fibers.

Comparative Example 4

The polymer fiber bundle (b5) was fixed onto a base paper while a tension (0.02 mN/dtex) was applied thereto, and the polymer fiber bundle on the base paper was irradiated with ultraviolet light for 2 hours in an air atmosphere at an illuminance of 200 mW/cm² and a wavelength of 365 nm using a UV-LED irradiator (HLDL-350×270) manufactured by CCS, Inc., thereby obtaining a carbon fiber precursor containing a crosslinked diene-based polymer. Subsequently, the obtained fiber bundle was subjected to a stabilization treatment in an air stream using a heat treatment oven under conditions similar to those in Examples 1 to 16, thereby obtaining stabilized fibers.

With respect to the obtained stabilized fibers, the fusion rate was calculated by the following method, and the carbonization stability was evaluated by the following method. The calculation results and the evaluation results are shown in Table 1. In Comparative Example 1, stabilized fibers could not be obtained, and therefore “−” is shown in Table 1.

(Fusion Rate)

Fibers for evaluation having a length of 3 cm were cut out from the stabilized fibers, the cross sections of the fibers for evaluation were observed using a microscope (“Digital microscope VHX-7000” manufactured by Keyence Corporation), and the number of fibers was counted. At this time, the number of fibers fused and the number of all fibers were counted. The number of fibers fused was determined to be, for example, two when two fibers are fused to each other. The number of all fibers was counted by separating the fused fibers into a state before fusion. The fusion rate was calculated based on the following formula.

$\mathrm{Fusion}\mathrm{rate}\left(\%\right)=\left(\mathrm{number}\mathrm{of}\mathrm{fibers}\mathrm{fused}/\mathrm{number}\mathrm{of}\mathrm{all}\mathrm{fibers}\right)\times 100$

(Carbonization Stability)

While a tension of 20 cN was applied to each stabilized fiber (216 fibers), the fibers were carried into a heat treatment oven adjusted to 800° ° C. in a nitrogen stream, and subjected to a pre-carbonization treatment for 3 minutes. When the fiber bundle was completely broken during the pre-carbonization treatment, the evaluation result was determined to be E. When pre-carbonized fibers were obtained by the pre-carbonization treatment, fibers for evaluation having a length of 10 cm were cut out from the pre-carbonized fibers, and the presence or absence of fuzz caused by breakage of single fibers during the pre-carbonization treatment was observed visually and using a microscope (“SKM-S20B-PC” manufactured by Saito Optical Science Co., Ltd.). The evaluation criteria are as follows.

• • A: No fuzz due to breakage occurred.
  • B: Fuzz due to breakage of 1 to 3 single fibers occurred.
  • C: Fuzz due to breakage of 4 to 10 single fibers occurred.
  • D: Fuzz due to breakage of 11 or more single fibers occurred.
  • E: The fiber bundle was completely broken during the pre-carbonization treatment.

<Production of Carbon Fiber>

While a tension of 20 cN was applied to the stabilized fibers (216 fibers, average fiber diameter: 25 μm) of Example 9, the stabilized fibers were conveyed into a heat treatment furnace adjusted to 800° C. in a nitrogen stream, and subjected to a pre-carbonization treatment for 3 minutes, thereby obtaining pre-carbonized fibers (216 fibers, average fiber diameter: 22 μm). While a tension of 20 cN was applied to the pre-carbonized fibers, the pre-carbonized fibers were conveyed into a heat treatment furnace adjusted to 1,400° C. in a nitrogen stream, and subjected to a carbonization treatment for 3 minutes, thereby obtaining carbon fibers (c1) (average fiber diameter: 20 μm). In the carbon fibers (c1), fuzz due to breakage of single fibers did not occur, and fusion between fibers also did not occur.

While a tension of 20 cN was applied to the stabilized fibers (216 fibers, average fiber diameter: 26 μm) of Example 14, the stabilized fibers were conveyed into a heat treatment furnace adjusted to 800° ° C. in a nitrogen stream, and subjected to a pre-carbonization treatment for 3 minutes, thereby obtaining pre-carbonized fibers (213 fibers, average fiber diameter: 23 μm). While a tension of 20 cN was applied to the pre-carbonized fibers, the pre-carbonized fibers were conveyed into a heat treatment furnace adjusted to 1,400° C. in a nitrogen stream, and subjected to a carbonization treatment for 3 minutes, thereby obtaining carbon fibers (c2) (average fiber diameter: 21 μm). In the carbon fibers (c2), fuzz due to breakage of single fibers did not occur, and fusion between fibers also did not occur.

While a tension of 20 cN (200 mN) was applied to the stabilized fibers (216 fibers, average fiber diameter: 10 μm) of Example 15, the stabilized fibers were conveyed into a heat treatment furnace adjusted to 800° C. in a nitrogen stream, and subjected to a pre-carbonization treatment for 3 minutes, thereby obtaining pre-carbonized fibers (216 fibers, average fiber diameter: 8 μm). While a tension of 20 cN was applied to the pre-carbonized fibers, the pre-carbonized fibers were conveyed into a heat treatment furnace adjusted to 1,400° C. in a nitrogen stream, and subjected to a carbonization treatment for 3 minutes, thereby obtaining carbon fibers (c3) (average fiber diameter: 8 μm). In the carbon fibers (c3), fuzz due to breakage of fibers did not occur, and fusion between fibers also did not occur.

A single fiber was taken out from the carbon fibers (c3), and the single fiber was subjected to a tensile test (gauge length: 25 mm, tensile speed: 1 mm/min) in accordance with JIS R 7606 using a micro-strength evaluation tester (“Microautograph MST-I” manufactured by Shimadzu Corporation) to measure the tensile modulus and the tensile strength, and an average of 5 measurements was calculated. The tensile modulus was 71 GPa, and the tensile strength was 1.1 GPa.

While a tension of 20 cN (200 mN) was applied to the stabilized fibers (216 fibers, average fiber diameter: 9 μm) of Example 16, the stabilized fibers were conveyed into a heat treatment furnace adjusted to 800° ° C. in a nitrogen stream, and subjected to a pre-carbonization treatment for 3 minutes, thereby obtaining pre-carbonized fibers (216 fibers, average fiber diameter: 7 μm). While a tension of 20 cN was applied to the pre-carbonized fibers, the pre-carbonized fibers were conveyed into a heat treatment furnace adjusted to 1,400° C. in a nitrogen stream, and subjected to a carbonization treatment for 3 minutes, thereby obtaining carbon fibers (c4) (average fiber diameter: 7 μm). In the carbon fibers (c4), fuzz due to breakage of fibers did not occur, and fusion between fibers also did not occur.

A single fiber was taken out from the carbon fibers (c4), and the single fiber was subjected to a tensile test (gauge length: 25 mm, tensile speed: 1 mm/min) in accordance with JIS R 7606 using a micro-strength evaluation tester (“Microautograph MST-I” manufactured by Shimadzu Corporation) to measure the tensile modulus and the tensile strength, and an average of 5 measurements was calculated. The tensile modulus was 80 GPa, and the tensile strength was 1.3 GPa.

	TABLE 1
	Example	Example	Example	Example	Example	Example	Example
	1	2	3	4	5	6	7
Type of polymer fiber bundle	b1	b1	b1	b1	b1	b1	b1
Electron beam	Acceleration voltage (kV)	250	250	250	250	800	800	800
irradiation	Dose (kGy)	50	100	100	300	100	300	450
conditions	Atmosphere	Nitrogen	Nitrogen	Air	Nitrogen	Air	Air	Air
	Tension (mN/dtex)	0.07	0.07	0.07	0.07	3.0	3.0	3.0
Carbon fiber	Gel fraction (%)	73.0	87.0	79.0	98.5	92.5	98.6	98.8
precursor	Swelling magnification	9.2	7.3	8.8	4.9	5.0	2.7	2.1
	(times)
Evaluation	Fusion rate (%)	20	0	0	0	0	0	0
	Carbonization stability	C	B	C	B	B	A	A

	TABLE 2
	Exam-	Exam-	Exam-	Exam-	Exam-	Exam-	Exam-	Exam-	Exam-
	ple	ple	ple	ple	ple	ple	ple	ple	ple
	8	9	10	11	12	13	14	15	16
Type of polymer fiber bundle	b1	b1	b1	b2	b2	b2	b2	b3	b4
Electron beam	Acceleration voltage (kV)	800	800	800	250	250	250	250	800	800
irradiation	Dose (kGy)	600	600	800	50	100	100	300	550	550
conditions	Atmosphere	Air	Air	Air	Nitrogen	Nitrogen	Air	Nitrogen	Air	Air
	Tension (mN/dtex)	1.5	3.0	3.0	0.07	0.07	0.07	0.07	3.0	3.0
Carbon fiber	Gel fraction (%)	99.3	99.7	99.8	76.0	88.5	77.0	97.5	99.7	99.5
precursor	Swelling magnification	1.8	1.8	1.8	9.4	7.0	8.3	4.4	1.8	1.8
	(times)
Evaluation	Fusion rate (%)	0	0	0	20	0	10	0	0	0
	Carbonization stability	A	A	A	C	B	C	B	A	A

	TABLE 3
	Comparative	Comparative	Comparative	Comparative
	Example 1	Example 2	Example 3	Example 4
Type of polymer fiber bundle	b1	b1	b1	b5
Treatment method	Not irradiated	Treated with	Treated with	Irradiated with
		with electron	nitric acid	aluminum chloride	UV
		beam
Carbon fiber	Gel fraction (%)	0	25.0	30.0	33.3
precursor	Swelling magnification	—	>10.0	>10.0	>10.0
	(times)
Evaluation	Fusion rate (%)	—	50	40	35
	Carbonization stability	—	E	D	D

As shown in Tables 1 and 2, in Examples 1 to 16, since the carbon fiber precursor contains a crosslinked diene-based polymer and has a gel fraction of 40% or more, it was found that fusion between fibers in the stabilization treatment is prevented.

Meanwhile, in Comparative Example 1, the gel fraction was 0%, and the fiber was melted and broken, and thus stabilized fibers could not be obtained. In Comparative Examples 2 and 3, since the gel fraction was less than 40%, it was found that fusion between fibers is likely to occur in the stabilization treatment. It is presumed that when the polymer fiber was immersed in a solution containing nitric acid or aluminum chloride, the polymer fiber was not sufficiently cured up to the center of the fiber, and thus the gel fraction was low.

In Comparative Example 4, fusion between fibers in the stabilization treatment was not prevented even by irradiation with ultraviolet light for 2 hours, whereas in Examples 1 to 16, fusion between fibers in the stabilized treatment was prevented and the carbonization stability was also improved only by irradiation with an electron beam for a short time. From the results, it was found that the production method of the disclosure is excellent also from the viewpoint of production efficiency and energy saving.

Comparison of Examples 1, 2, and 4 and comparison of Examples 11, 12, and 14 showed that when the dose of the electron beam is increased, the gel fraction increases, the swelling magnification decreases, fusion between fibers in the stabilized treatment is prevented, and the carbonization stability is improved.

Comparison between Example 2 and Example 3 showed that in the case of irradiation with an electron beam in a nitrogen atmosphere, the gel fraction increases, the swelling magnification decreases, and the carbonization stability is improved as compared with the case of irradiation with an electron beam in an air atmosphere.

Comparison between Example 12 and Example 13 showed that in the case of irradiation with an electron beam in a nitrogen atmosphere, the gel fraction increases, the swelling magnification decreases, fusion between fibers in the stabilization treatment is prevented, and carbonization stability is improved as compared with the case of irradiation with an electron beam in an air atmosphere.

Comparison between Example 3 and Example 5 showed that when the acceleration voltage and the tension are increased, the gel fraction increases, the swelling magnification decreases, and the carbonization stability is improved.

Comparison between Example 8 and Example 9 showed that when the tension is increased, the gel fraction increases. It is presumed that by applying a specific tension in the fiber axis direction at the time of irradiation with an electron beam, cyclization and crosslinking proceed while molecules are oriented to increase the gel fraction.

Comparison between Example 15 and Example 16 showed that, in Example 16 in which the diene-based polymer contains an olefinic polymer as another polymer, the spinnability is improved and a carbon fiber precursor containing a single fiber having a smaller fiber diameter can be obtained, and the carbon fiber precursor has a high gel fraction even if another polymer is contained. Furthermore, the fiber diameter of the obtained carbon fiber also decreased, and the tensile strength and the tensile modulus increased.

Claims

1. A carbon fiber precursor, comprising a crosslinked diene-based polymer and having a gel fraction of 40% or more.

2. The carbon fiber precursor according to claim 1, wherein a swelling magnification after immersion in toluene at 60° C. for 2 hours is a factor of 9.5 or less.

3. The carbon fiber precursor according to claim 1, wherein the crosslinked diene-based polymer is a crosslinked product of a diene-based polymer containing a structural unit represented by the following Formula (1):

wherein R is a hydrogen atom or an organic group having from 1 to 20 carbon atoms.

4. The carbon fiber precursor according to claim 3, wherein R in Formula (1) is a hydrogen atom or a methyl group.

5. A method of producing a carbon fiber precursor, the method comprising irradiating a polymer fiber, including a diene-based polymer containing a structural unit represented by the following Formula (1), with radiation having a dose of 20 kGy or more:

wherein R is a hydrogen atom or an organic group having from 1 to 20 carbon atoms.

6. The method of producing a carbon fiber precursor according to claim 5, wherein R in Formula (1) is a hydrogen atom or a methyl group.

7. A method of producing a stabilized fiber, the method comprising subjecting a fiber bundle of the carbon fiber precursor according to claim 1 to a heat treatment in an oxidizing atmosphere.

8. A method of producing a stabilized fiber, the method comprising:

producing a carbon fiber precursor by the method of producing a carbon fiber precursor according to claim 5; and

subjecting a fiber of the carbon fiber precursor to a heat treatment in an oxidizing atmosphere.

9. A method of producing a carbon fiber, the method comprising:

producing a stabilized fiber by the method of producing a stabilized fiber according to claim 7; and

subjecting the stabilized fiber to a carbonization treatment.

10. A method of producing a carbon fiber, the method comprising:

producing a carbon fiber precursor by the method of producing a carbon fiber precursor according to claim 5;

producing a stabilized fiber by subjecting a fiber of the carbon fiber precursor to a heat treatment in an oxidizing atmosphere; and

subjecting the stabilized fiber to a carbonization treatment.

11. A stabilized fiber, comprising a structure derived from a crosslinked diene-based polymer and having a fusion rate of 30% or less.

12. A method of producing a carbon fiber, the method comprising subjecting the stabilized fiber according to claim 11 to a carbonization treatment.