Carbon fiber precursor, method for producing carbon fiber precursor, flame-resistant fiber, method for producing flame-resistant fiber, and method for producing carbon fiber

Abstract

To provide a carbon fiber precursor which can suppress fusion between fibers on flame-resistant treatment and has excellent carbonization resistance, and a manufacturing method of the carbon fiber precursor, etc.SOLUTION: A carbon fiber precursor including a crosslinked diene-based polymer and having a gel fraction of 40% or over and its application are disclosed.SELECTED DRAWING: None

Description

[Technical Field]

[0001] This disclosure relates to carbon fiber precursors, methods for producing carbon fiber precursors, flame-resistant fibers, methods for producing flame-resistant fibers, and methods for producing carbon fibers. [Background technology]

[0002] Because carbon fiber is lightweight and has excellent mechanical properties, it is being increasingly used in a wide range of applications, including aerospace, automotive, and building materials.

[0003] For example, Patent Documents 1 to 3 disclose a method for producing carbon fibers using 1,2-polybutadiene fibers.

[0004] Patent Document 1 discloses a method for obtaining flame-resistant fibers by irradiating 1,2-polybutadiene fibers with ultraviolet light for more than 2 hours, followed by flame-retardant treatment at 200°C for more than 8 hours. Patent Document 2 discloses a method for obtaining flame-resistant fibers by treating 1,2-polybutadiene fibers with acid to make them insoluble and infusible, followed by flame-retardant treatment. Patent Document 3 discloses a method for obtaining flame-resistant fibers by immersing 1,2-polybutadiene fibers in an organic solvent containing dissolved or suspended Lewis acid to make them infusible, followed by flame-retardant treatment.

[0005] Furthermore, Patent Document 4 discloses a method for producing a carbon material, characterized by subjecting a carbon material precursor, which consists of a ring structure-containing polymer containing at least one of specific structural units, to flame-retardant treatment at a temperature of 320 to 450°C under an oxidizing atmosphere, and then subjecting it to carbonization treatment. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Unexamined Patent Publication No. 48-82199 [Patent Document 2] Japanese Patent Application Laid-open No. 1983-92699 [Patent Document 3] Japanese Unexamined Patent Publication No. 49-106490 [Patent Document 4] Japanese Patent Publication No. 2020-59619 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The long-duration flame-retardant treatment described in Patent Document 1 increases the energy required for manufacturing and reduces productivity, leading to increased manufacturing costs. Furthermore, with ultraviolet irradiation, only the surface of the fiber and the parts irradiated with ultraviolet light harden, while the core of the fiber and parts not irradiated with ultraviolet light do not harden sufficiently. This tendency is particularly pronounced when the fiber diameter is large, and in the case of fiber bundles consisting of many fibers, it is difficult to sufficiently harden the fibers located inside. As a result, during the flame-retardant treatment, fusion between fibers occurs, or thread breakage occurs due to melting, and the resulting flame-retardant fibers tend to be brittle. Moreover, when attempting to manufacture carbon fibers using such flame-retardant fibers, thread breakage (fracture) due to tension applied during carbonization, thread breakage due to thermal decomposition at high temperatures, etc., tend to occur, resulting in low carbonization resistance.

[0008] Furthermore, acid treatment as described in Patent Document 2, and immersion in a Lewis acid-containing solution as described in Patent Document 3, require a large amount of cleaning solvent in the subsequent washing process, leading to increased manufacturing costs. In addition, with acid treatment and immersion in a Lewis acid-containing solution, only the surface of the fiber is made infusible, and the core of the fiber is not sufficiently made infusible. As a result, during flame-retardant treatment, fusion of fibers occurs, or the fibers break due to melting, and the resulting flame-retardant fibers tend to be brittle.

[0009] On the other hand, while Patent Document 4 attempts to improve the carbonization yield, there were cases where further suppression of fiber fusion during flame-retardant treatment was required.

[0010] The problem to be solved by one embodiment of the present disclosure is to provide a carbon fiber precursor capable of suppressing fusion between fibers in a flame-retardant treatment and having excellent carbonization resistance, and a method for producing the carbon fiber precursor. The problem to be solved by another embodiment of the present disclosure is to provide a flame-retardant fiber in which fusion between fibers is suppressed and having excellent carbonization resistance, and a method for producing the flame-retardant fiber. The problem to be solved by another embodiment of the present disclosure is to provide a method for producing carbon fiber using a flame-retardant fiber in which fusion between fibers is suppressed and having excellent carbonization resistance.

Means for Solving the Problem

[0011] The specific means for achieving the problem are as follows. <1> A carbon fiber precursor containing a crosslinked diene polymer and having a gel fraction of 40% or more. <2> The carbon fiber precursor according to <1>, having a swelling ratio of 9.5 times or less after being immersed in toluene at 60 °C for 2 hours. <3> The carbon fiber precursor according to <1> or <2>, wherein the crosslinked diene polymer is a crosslinked body of a diene polymer containing a structural unit represented by the following formula (1).

Chemical formula

Chemical formula

[0012] According to one embodiment of the present disclosure, a carbon fiber precursor that can suppress fusion between fibers in flame-retardant treatment and has excellent carbon resistance, and a method for producing the carbon fiber precursor are provided. Other embodiments of this disclosure provide flame-resistant fibers in which interfiber fusion is suppressed and which have excellent carbon resistance, as well as a method for producing flame-resistant fibers. According to other embodiments of the present disclosure, a method for producing carbon fibers using flame-resistant fibers that have suppressed fusion between fibers and excellent carbon resistance is provided. [Modes for carrying out the invention]

[0013] In this disclosure, the numerical range indicated using "~" includes the numbers before and after "~" as the minimum and maximum values, respectively. In numerical ranges described in stages within this disclosure, the upper or lower limit of one numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in numerical ranges described within this disclosure, the upper or lower limit of that range may be replaced with the values ​​shown in the composite examples.

[0014] In this disclosure, each component may contain multiple types of the corresponding substance. If multiple types of the substance corresponding to each component are present in the carbon fiber precursor, the content or amount of each component means the total content or amount of the multiple types of substances present in the carbon fiber precursor, unless otherwise specified.

[0015] In this disclosure, "carbon fiber precursor" means a fiber from which carbon fibers can be obtained by carbonization treatment, or by applying flame-retardant treatment and carbonization treatment.

[0016] <Carbon fiber precursor> The carbon fiber precursor of this disclosure comprises a crosslinked diene polymer and has a gel fraction of 40% or more.

[0017] The carbon fiber precursor of this disclosure can suppress the fusion between fibers during flame-retardant treatment.

[0018] The reasons for the above effects are presumed to be as follows, but are not limited to these. The carbon fiber precursor of this disclosure contains a crosslinked diene polymer and has a gel fraction of 40% or more, resulting in a high crosslink density, which is presumed to suppress fusion between fibers during flame-retardant treatment.

[0019] The carbon fiber precursor may be a single fiber or a fiber bundle, but for carbon fibers used in structural components such as aerospace, automotive, and building materials, it is preferable that the carbon fiber is a fiber bundle consisting of multiple single fibers from the viewpoint of exhibiting high mechanical properties. The number of fibers in the fiber bundle of the carbon fiber precursor in this disclosure is not particularly limited, but from the viewpoint of productivity of flame-resistant fibers and carbon fibers, as well as mechanical properties, it is preferably 10 to 360,000 fibers, more preferably 20 to 180,000 fibers, even more preferably 30 to 72,000 fibers, and particularly preferably 50 to 36,000 fibers. Furthermore, by limiting the number of filaments per bundle to 360,000 or less, it is possible to suppress the occurrence of uneven firing during flame-resistant treatment and carbonization treatment.

[0020] From the viewpoint of further suppressing fusion between fibers in flame-retardant treatment, the gel fraction of the carbon fiber precursor is preferably 50% or more, more preferably 60% or more, even more preferably 80% or more, still more preferably 85% or more, particularly preferably 90% or more, and most preferably 95% or more. There is no particular upper limit to the gel fraction of the carbon fiber precursor. The gel fraction of the carbon fiber precursor may be 100%. From the viewpoint of reducing manufacturing costs by decreasing the energy required for production to increase the gel fraction, it is preferable that the gel fraction of the carbon fiber precursor be 99.9% or less.

[0021] In this disclosure, the gel fraction is measured by the following method. First, a 0.2 g sample is cut from the carbon fiber precursor. After drying at 80°C for 4 hours, its mass is precisely measured using a precision electronic balance and recorded as the initial mass (g). Next, the sample is immersed in 30 ml of toluene and left to stand in a 60°C hot air circulating oven for 8 hours. After standing, the sample is subjected to suction filtration using a membrane filter with a pore size of 1.0 μm to separate the gel component. The residue remaining on the membrane filter without dissolving in toluene corresponds to the gel component. For example, the Omnipore™ membrane filter JAWP04700 manufactured by Merck can be used as the membrane filter. The separated gel is air-dried in a fume hood with a membrane filter for at least 12 hours, and then left to stand in a 90°C hot air circulating oven for 12 hours to remove toluene. The mass of the gel and membrane filter after standing is precisely weighed using a precision electronic balance, and the gel fraction is determined from the following formula. Gel fraction (%) = {(Mass of gel and membrane filter (g) - Mass of membrane filter (g)) / Initial mass of sample (g)} × 100

[0022] From the viewpoint of further suppressing interfiber fusion in flame-retardant treatment, the carbon fiber precursor of this disclosure preferably has a swelling ratio of 9.5 times or less, more preferably 8 times or less, even more preferably 7 times or less, particularly preferably 4 times or less, and most preferably 3 times or less, after immersion in toluene at 60°C for 8 hours. The lower limit of the above swelling ratio is not particularly limited, but from the viewpoint of the stretchability of the carbon fiber precursor, it is preferably 1.05 times or more, and more preferably 1.1 times or more.

[0023] In this disclosure, the swelling ratio is measured by the following method. First, a 0.2 g sample is cut from the carbon fiber precursor. After drying at 80°C for 4 hours, its mass is precisely measured using a precision electronic balance and recorded as the initial mass (g). Next, the sample is immersed in 30 ml of toluene and left to stand in a 60°C hot air circulating oven for 8 hours. After the sample has been allowed to stand, the gel component is separated by suction filtration using a membrane filter with a pore size of 1.0 μm, and the mass of the recovered gel component (swollen gel) is accurately weighed using a precision electronic balance. Next, the swollen gel is air-dried in the atmosphere in a fume hood for more than 12 hours, and then dried at 90°C for another 12 hours. The mass of the dried gel (dried gel) is weighed, and the swelling ratio is determined using the following formula. Swelling ratio (times) = (mass of swollen gel [g]) / (mass of dry gel [g])

[0024] (Cross-linked diene polymer) The carbon fiber precursor of this disclosure comprises a crosslinked diene polymer. Furthermore, the carbon fiber precursor of this disclosure may also comprise two or more crosslinked diene polymers.

[0025] The crosslinked diene polymer is a crosslinked diene polymer, and is preferably a crosslinked diene polymer containing a structural unit represented by the following formula (1). A crosslinked diene polymer can be obtained by, for example, irradiating the diene polymer with radiation, causing intramolecular or intermolecular crosslinking of the diene polymer.

[0026] The diene polymer may contain only one structural unit represented by formula (1), or it may contain two or more.

[0027] [ka]

[0028] In formula (1), R is a hydrogen atom or an organic group having 1 to 20 carbon atoms.

[0029] From the viewpoint of improving the yield of the resulting carbon fibers, the number of carbon atoms in the organic group represented by R is preferably 1 to 10, more preferably 1 to 6, and even more preferably 1 to 3.

[0030] Examples of organic groups represented by R include hydrocarbon groups and groups in which at least some of the atoms constituting the hydrocarbon group are substituted with halogen atoms (e.g., chlorine atoms, bromine atoms, and fluorine atoms), oxygen atoms, nitrogen atoms, or sulfur atoms.

[0031] The hydrocarbon group may be linear, branched, or contain a ring structure.

[0032] The hydrocarbon group may be an aliphatic hydrocarbon group or an aromatic hydrocarbon group.

[0033] In particular, the hydrocarbon group is preferably an aliphatic hydrocarbon group, more preferably an alkyl group, and even more preferably a linear alkyl group. Specifically, the hydrocarbon group is preferably a hydrocarbon group having 1 to 10 carbon atoms (preferably 1 to 6 carbon atoms), for example, methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, tert-butyl group, sec-butyl group, pentyl group, isopentyl group, neopentyl group, hexyl group, 2-methylpentyl group, 3-methylpentyl group, 2,2-dimethylbutyl group, 2,3-dimethylbutyl group, heptyl group, 2-methylhexyl group, 3-methylhexyl group, 2,2-dimethylpentyl group, 2,3-dimethylpentyl group, 2,4-dimethylpentyl group, 3-ethylpentyl group, 2,2,3-trimethylbutyl group, and Examples of groups include ctyl group, methylheptyl group, dimethylhexyl group, 2-ethylhexyl group, 3-ethylhexyl group, trimethylpentyl group, 3-ethyl-2-methylpentyl group, 2-ethyl-3-methylpentyl group, 2,2,3,3-tetramethylbutyl group, nonyl group, methyloctyl group, 3,7-dimethyloctyl group, dimethylheptyl group, 3-ethylheptyl group, 4-ethylheptyl group, trimethylhexyl group, 3,3-diethylpentyl group, decyl group, undecyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, octadecyl group, nonadecyl group, and eicosyl group.

[0034] From the viewpoint of further suppressing fusion between fibers during flame-retardant treatment, R is preferably a hydrogen atom or a methyl group.

[0035] Examples of raw materials for producing diene polymers containing the 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.

[0036] 1,2-butadiene is a raw material for producing diene polymers containing a structural unit in formula (1) where R is a hydrogen atom.

[0037] Isoprene is a raw material for producing diene polymers containing a structural unit in formula (1) where R is a methyl group.

[0038] The raw material for producing a diene polymer containing the structural unit represented by formula (1) is preferably at least one selected from the group consisting of 1,2-butadiene and isoprene.

[0039] In diene polymers, the content of the structural unit represented by formula (1) is preferably 10 mol% or more, more preferably 30 mol% or more, even more preferably 40 mol% or more, particularly preferably 50 mol% or more, also particularly preferably 60 mol% or more, and most preferably 70 mol% or more. There is no particular upper limit to the content of the structural unit represented by formula (1). The content of the structural unit represented by formula (1) may be 100 mol%.

[0040] Furthermore, while the diene polymer preferably contains the structural unit represented by formula (1), it may also contain structural units derived from other conjugated diene monomers other than the structural unit represented by formula (1).

[0041] Other conjugated diene monomers 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.

[0042] A diene polymer containing the structural unit represented by formula (1) may also contain structural units derived from other polymerizable monomers in addition to the structural unit represented by formula (1).

[0043] Other polymerizable monomers include, for example, Aromatic vinyl 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, vinylnaphthalene, etc. Chain-like olefin monomers such as ethylene, propylene, and 1-butene; Cyclopentene, 2-norbornene, and other cyclic olefin monomers; Non-conjugated diene monomers such as 1,5-hexadiene, 1,6-heptadiene, 1,7-octadiene, cyclopentadiene, dicyclopentadiene, and 5-ethylidene-2-norbornene; α,β-unsaturated carboxylic acid 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 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; Sulfogroup-containing vinyl monomers such as vinyl sulfonic acid; Vinyl halogenated monomers such as polyvinyl chloride; Vinyl carboxylates such as vinyl acetate, vinyl butyrate, and vinyl pivalate; Examples include vinyl alcohol. Only one polymerizable monomer may be used, or two or more may be used in combination. In this disclosure, (meth)acrylic refers to methacrylic or acrylic. In diene polymers, when "R" in formula (1) is a hydrogen atom, the 1,2-bond content is not particularly limited and may include cis-1,4-bonds and trans-1,4-bonds. In diene polymers, when "R" in formula (1) is a hydrogen atom, the content of 1,2-structural units (1,2-bond content) is preferably 1 mol% or more, more preferably 5 mol% or more, even more preferably 10 mol% or more, still more preferably 30 mol% or more, particularly preferably 50 mol% or more, especially preferably 80 mol% or more, and most preferably 90 mol% or more, from the viewpoint of reducing the fusion rate during flame-retardant treatment and improving the carbonization yield due to the progress of intramolecular cyclization and intermolecular crosslinking reactions by radiation. The 1,2-bond content of the diene polymer may be 100 mol%, but from the viewpoint of reducing the manufacturing (polymerization) cost to increase the 1,2-bond content of the diene polymer, it is preferable to have a content of 99.5 mol% or less, and more preferably 99 mol% or less. "1,2-bond content" refers to the proportion of 1,2-structural units (1,2-bonds) when the total of cis-1,4-structural units (cis-1,4-bonds), trans-1,4-structural units (trans-1,4-bonds), and 1,2-structural units (1,2-bonds) constituting the diene polymer is set to 100 mol%. The 1,2-bond content is:1 H nuclear magnetic resonance (NMR) and 13 This can be confirmed by 13C NMR. In diene polymers, when "R" in formula (1) is a methyl group, the content of 3,4-bonds corresponding to the diene monomer unit (1) is not particularly limited and may include cis-1,4-bonds, trans-1,4-bonds, and 1,2-bonds. In diene polymers, when "R" in formula (1) is a methyl group, the content of 3,4-structural units (3,4-bond content) is preferably 1 mol% or more, more preferably 5 mol% or more, even more preferably 10 mol% or more, still more preferably 30 mol% or more, particularly preferably 50 mol% or more, especially preferably 80 mol% or more, and most preferably 90 mol% or more, from the viewpoint of reducing the fusion rate during flame treatment due to the progress of intramolecular cyclization reactions by radiation and improving the carbonization yield. When "R" in formula (1) is a methyl group, the 3,4-bond content of the diene polymer may be 100 mol%, but from the viewpoint of reducing the manufacturing (polymerization) cost to increase the 3,4-bond content of the diene polymer, it is preferable to have 99.5 mol% or less, and more preferably 99 mol% or less. "3,4-bond content" refers to the proportion of 3,4-bonds when the total of 3,4-bonds, cis-1,4-bonds, trans-1,4-bonds, and 1,2-bonds constituting the diene polymer when "R" in formula (1) is a methyl group is set to 100 mol%. The 3,4-bond content is 1 H-NMR and 13 This can be confirmed by 13C-NMR. In this disclosure, the stereoregularity of the diene polymer is not particularly limited and may be isotactic, syndiotactic, or atactic. The ratio of these is not particularly limited.

[0044] The weight-average molecular weight of the diene polymer is not particularly limited and is usually 10 million or less, but from the viewpoint of the moldability of the carbon fiber precursor, it is preferably 5 million or less, more preferably 4 million or less, even more preferably 3 million or less, particularly preferably 2 million or less, also particularly preferably 1 million or less, and most preferably 500,000 or less. Furthermore, while the weight-average molecular weight of the diene polymer is usually 10,000 or more, it 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 carbon fiber. In this disclosure, the weight-average molecular weight is measured by gel permeation chromatography under the following conditions. A Tosoh HLC-8220GPC or a similar instrument can be used as the measuring device. (Measurement conditions) • Columns: TSKgel SuperHM-H x 2, SuperH2500 x 1 • Eluent: Chloroform ·Eluent flow rate: 0.6ml / min Column temperature: 40°C • Molecular weight standard material: Standard polystyrene • Detector: Differential refractive index detector

[0045] From the viewpoint of further suppressing fusion between fibers during flame-retardant treatment, the content of the cross-linked diene polymer is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, still more preferably 60% by mass or more, particularly preferably 70% by mass or more, particularly still preferably 80% by mass or more, and most preferably 90% by mass or more, based on the total mass of the carbon fiber precursor. There is no particular upper limit to the content of the cross-linked diene polymer. The content of the cross-linked diene polymer may be 100% by mass.

[0046] The presence of cross-linked diene polymers in the carbon fiber precursor was confirmed by infrared spectroscopy, solid-state NMR, and dissolution analysis.1 H-NMR and 13 This can be confirmed by analysis using 1C-NMR, etc.

[0047] (Other polymers) The carbon fiber precursor of this disclosure may contain polymers other than crosslinked diene polymers. The other polymers may be mixed with the diene polymer before crosslinking by radiation (pre-crosslinked diene polymer). By using a mixture consisting of a diene polymer and other polymers, spinnability is improved and the fiber diameter can be reduced while suppressing yarn breakage.

[0048] Other polymers are not particularly limited and include, for example, olefin polymers, petroleum resins, aromatic vinyl polymers, acrylic polymers (poly(meth)acrylic acid esters (polymethyl acrylate, polymethyl methacrylate, etc.), poly(meth)acrylic acid, (meth)acrylic acid ester / (meth)acrylic acid copolymers, etc.), polyesters (polyethylene terephthalate, polybutylene terephthalate, polylactic acid, etc.), polyamides, polyvinylidene chloride, polyphenylene sulfide, polyimide, polycarbonate, acrylonitrile, etc., mainly vinyl cyanide monomer units. Examples of components include polyacrylonitrile polymers (polyacrylonitrile, acrylonitrile / itaconic acid copolymer, acrylonitrile / methyl acrylate copolymer, etc.), acrylamide polymers mainly composed of acrylamide monomer units such as acrylamide (polyacrylamide, acrylamide / acrylonitrile copolymer, etc.), vinyl alcohol polymers mainly composed of vinyl alcohol monomers (polyvinyl alcohol, vinyl alcohol / vinyl acetate copolymer, etc.), and phenolic polymers (novolac-type phenolic resin, lignin, etc.). From the viewpoint of improving the spinnability and stretchability of diene polymers and the resistance to yarn breakage during flame treatment of crosslinked diene polymers, at least one selected from the group consisting of olefin polymers, petroleum resins, and aromatic vinyl polymers is preferred. By using a mixture of diene polymers and other polymers and irradiating it with radiation, crosslinking not only between diene polymers but also between diene polymers and other polymers and between other polymers proceeds. In this disclosure, when calculating the gel fraction, the residue remaining on the membrane filter without dissolving in toluene is used as the gel fraction. Therefore, if the crosslinked material formed by crosslinking between the diene polymer and other polymers remains on the membrane filter without dissolving in toluene, it is included in the calculation of the gel fraction as gel content. The olefin polymer may be linear or branched. Furthermore, the olefin polymer is not particularly limited, and examples include homopolymers and copolymers of olefin monomers.Examples of olefin monomers 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-butene, 1-hexene, 1-octene, 1-nonene, 1-decene, cyclopentene, and 2-norbornene. Only one olefin monomer may be used, or two or more may be used in combination. In addition, other polymerizable monomers besides olefin monomers include, for example, diene monomers such as 1,2-propanediene, methylalene, 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 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, vinylnaphthalene, etc. α,β-unsaturated carboxylic acid 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 monomers such as acrylonitrile, methacrylonitrile, and ethacrylonitrile; Acrylamide 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; Sulfogroup-containing vinyl monomers such as vinyl sulfonic acid; Vinyl halogenated monomers such as polyvinyl chloride; Vinyl carboxylates such as vinyl acetate, vinyl butyrate, and vinyl pivalate; Examples include vinyl alcohol, etc. Other polymerizable monomers may be used individually or in combination of two or more.

[0049] Examples of copolymers having structural units derived from olefin monomers include ethylene-propylene copolymers, ethylene-1-butene copolymers, ethylene-4-methyl-1-pentene copolymers, ethylene-1-hexene copolymers, ethylene-propylene-dicyclopentadiene copolymers, ethylene-propylene-5-vinyl-2-norbornene copolymers, ethylene-propylene-1,4-hexadiene copolymers, ethylene-propylene-1,4-cyclohexadiene copolymers, polystyrene-poly(ethylene / propylene) block copolymers (SEP), polystyrene-poly(ethylene / propylene)-polystyrene block copolymers (SEPS), polystyrene-poly(ethylene / butylene)polystyrene block copolymers (SEBS), polystyrene-poly(ethylene-ethylene / propylene)-polystyrene block copolymers (SEEPS), ethylene-1-octene copolymers, and other ethylene-based copolymers; Propylene-based copolymers such as propylene-1-butene-4-methyl-1-pentene copolymer and propylene-1-butene copolymer; Examples include 1-hexene-4-methyl-1-pentene copolymers and 4-methyl-1-pentene-1-octene copolymers.

[0050] Examples of petroleum resins that can be preferred include copolymerized petroleum resins of various fractions such as C5 petroleum resins, C9 petroleum resins, and C5 / C9 petroleum resins, alicyclic petroleum resins (such as dicyclopentadiene petroleum resins) that use cyclopentadiene compounds as the main raw material, and hydrogenated petroleum resins (partially hydrogenated petroleum resins, fully hydrogenated petroleum resins) obtained by hydrogenating these petroleum resins. Here, C5 petroleum resins are petroleum resins made from the C5 fraction of petroleum (mainly aliphatic petroleum resins), C9 petroleum resins are petroleum resins made from the C9 fraction of petroleum (mainly aromatic petroleum resins), and C5 / C9 petroleum resins are petroleum resins made from the C5 and C9 fractions of petroleum (copolymerized petroleum resins). Here, the C5 fraction and C9 fraction also include their analogues. 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. C5-based petroleum resins and C5 / C9-based petroleum resins are preferably those that contain dicyclopentadiene, derived from cyclopentadiene, a type of C5 fraction, in their skeleton.

[0051] Aromatic vinyl polymers are not particularly limited and include, for example, homopolymers and copolymers of aromatic vinyl monomers. Examples of aromatic vinyl monomers 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, indene, and the like. Aromatic vinyl monomers may be used individually or in combination of two or more. Examples of preferred aromatic vinyl polymers include styrene-butadiene-styrene block copolymer (SBS), styrene-isoprene-styrene block copolymer (SIS), acrylonitrile-butadiene-styrene (ABS) resin, methyl (meth)acrylate-acrylonitrile-butadiene-styrene (MABS), and methyl (meth)acrylate-butadiene-styrene (MBS) resin.

[0052] The content of other polymers is not particularly limited, but from the viewpoint of suppressing fusion during flame-retardant treatment of the carbon fiber precursor, it is preferably 70% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less, particularly preferably 40% by mass or less, especially preferably 30% by mass or less, and most preferably 20% by mass or less, based on the total mass of the carbon fiber precursor.

[0053] The carbon fiber precursor of this disclosure does not necessarily have to contain polymers other than the crosslinked diene polymer. However, from the viewpoint of preventing thread breakage during flame treatment of the crosslinked diene polymer and improving the strength of the carbon fiber, it is preferable that the carbon fiber precursor of this disclosure contains other polymers. The content of other polymers is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, particularly preferably 2% by mass or more, and most preferably 3% by mass or more, based on the total mass of the carbon fiber precursor.

[0054] (Other additives) Furthermore, in addition to the crosslinked diene polymer and other polymers, the carbon fiber precursor of this disclosure may also contain additives such as antioxidants, oxidizing agents, mold release agents, lubricants, plasticizers, colorants, crosslinking agents, crosslinking aids, crosslinking accelerators, crosslinking retarders, reinforcing materials (fillers such as carbon nanotubes, graphene, cellulose nanofibers, carbon black, glass fibers, and metal fibers), anti-aging agents, light stabilizers (UV absorbers, UV scatterers, etc.), light shielding agents, softeners, antistatic agents, compatibilizers, and solvability agents, to the extent that they do not impair the effects of this disclosure.

[0055] <Method for producing carbon fiber precursors> The method for producing a carbon fiber precursor according to this disclosure includes a step of irradiating a polymer fiber containing a diene polymer containing the structural unit represented by formula (1) with radiation at a dose of 20 kGy or more. Details of the diene polymer containing the structural unit represented by formula (1) are as described above. Furthermore, the polymer fiber containing the diene polymer containing the structural unit represented by formula (1) preferably contains the above-mentioned other polymers before the radiation irradiation step, from the viewpoint of reducing the fiber diameter while suppressing yarn breakage during spinning of the diene polymer, improving stretchability, preventing yarn breakage when the crosslinked diene polymer is flame-resistant, and improving the strength of the carbon fiber. The other polymer is preferably at least one selected from the group consisting of the above-mentioned olefin polymers, petroleum resins, and aromatic vinyl polymers. Polymer fibers containing a diene polymer containing the structural unit represented by formula (1) above may contain other polymers after the radiation irradiation step, but it is preferable to contain other polymers before the radiation irradiation step because crosslinking between the diene polymer and at least a portion of the other polymers also progresses, improving the resistance to thread breakage during flame treatment of the crosslinked diene polymer. When other polymers are contained before the radiation irradiation step, the upper limit of the content of other polymers is preferably 70% by mass or less, more preferably 60% by mass or less, even more preferably 50% by mass or less, particularly preferably 40% by mass or less, especially preferably 30% by mass or less, and most preferably 20% by mass or less, based on the total mass of the diene polymer, from the viewpoint of suppressing fusion during flame treatment of the carbon fiber precursor with a high gel fraction. The lower limit of the content of other polymers is preferably 0.1% by mass or more, more preferably 0.5% by mass or more, even more preferably 1% by mass or more, particularly preferably 2% by mass or more, and most preferably 3% by mass or more, based on the total mass of the diene polymer, from the viewpoint of improving the spinnability and stretchability of the diene polymer and the ability to prevent yarn breakage during flame treatment.

[0056] Examples of radiation include X-rays, gamma rays, alpha rays, beta rays, electron beams, neutron beams, proton beams, and heavy ion beams. Among these, electron beams are preferred from the viewpoint of suppressing fusion between fibers in flame-retardant treatment and suppressing fiber breakage in carbonization treatment.

[0057] From the viewpoint of increasing the gel fraction of the crosslinked diene polymer obtained by crosslinking the diene polymer, the radiation dose is preferably 30 kGy or more, more preferably 60 kGy or more, even more preferably 80 kGy or more, particularly preferably 150 kGy or more, and most preferably 400 kGy or more. There is no particular upper limit to the radiation dose. From the viewpoint of reducing energy costs and reducing damage to polymer fibers, the radiation dose is preferably 50 MGy or less, more preferably 10 MGy or less, even more preferably 5 MGy or less, particularly preferably 2000 kGy or less, and most preferably 1000 kGy or less.

[0058] By irradiating polymer fibers with radiation at a dose of 20 kGy or more, the entire fiber, from the surface to the core, can be crosslinked. This allows for the production of carbon fiber precursors containing crosslinked diene polymers with a high gel fraction, which suppresses fusion between fibers during flame-retardant treatment. Furthermore, flame-retardant fibers obtained through this treatment are less prone to breakage even after carbonization.

[0059] When an electron beam is used as the active ray, the dose is measured using a film dosimeter. A Toyo Medic FWT-60 or a similar device can be used as the film dosimeter.

[0060] From the viewpoint of suppressing fusion between fibers during flame-retardant treatment, improving tensile strength, and maintaining shape stability, it is preferable to adjust the acceleration voltage of the radiation irradiated onto the polymer fibers to such an acceleration voltage that preferably 20% or more, more preferably 50% or more, and even more preferably 80% or more of the irradiated radiation penetrates the polymer fibers.

[0061] When electron beams are used as radiation, the transmittance of the electron beam is calculated by measuring the dose before and after transmission. Alternatively, it may be calculated from a diagram showing the relationship between the transmission depth and the relative dose, which is generally disclosed. Specifically, the acceleration voltage is preferably 50kV to 10MV, more preferably 100kV to 3MV, and even more preferably 150kV to 1MV.

[0062] Radiation irradiation may be carried out in batches or in a continuous manner. The equipment used for radiation irradiation is not particularly limited, but for batch-type radiation irradiation, Iwasaki Electric's CB250 / 30 / 20mA or a similar device can be used. For continuous radiation irradiation, NHV Corporation's electron beam irradiation device EBC800-35 or a similar device can be used.

[0063] Radiation irradiation may be carried out under a nitrogen atmosphere or under an air atmosphere.

[0064] When irradiating with radiation, it is preferable to apply a tension of 0.03 mN / dtex or higher to the polymer fibers. By applying a tension of 0.03 mN / dtex or higher, the molecules are more likely to orient in the direction of the fiber axis, and it is presumed that this orientation leads to a high degree of intramolecular cyclization reaction and crosslinking between molecules. As a result, it is possible to obtain carbon fiber precursors containing crosslinked diene polymers with a higher gel fraction, and the fusion between fibers in flame-retardant treatment can be suppressed. Furthermore, flame-retardant fibers obtained by flame-retardant treatment are less prone to breakage even when subjected to carbonization treatment.

[0065] The tension applied to the polymer fibers is more preferably 0.05 mN / dtex or higher, even more preferably 0.1 mN / dtex or higher, particularly preferably 0.3 mN / dtex or higher, and most preferably 0.5 mN / dtex or higher, from the viewpoint of suppressing fusion between fibers and improving carbon resistance during flame-retardant treatment.

[0066] The polymer fibers may be commercially available products or may be manufactured by conventionally known methods.

[0067] Polymer fibers can be produced by spinning a diene polymer containing a structural unit represented by formula (1), or a polymer composition containing a diene polymer containing a structural unit represented by formula (1) and the other components mentioned above.

[0068] The spinning method is not particularly limited, but from the viewpoint of reducing manufacturing costs and the burden on the environment, melt spinning, spunbond, meltblown, or centrifugal spinning are preferred. The spinning method may also be dry spinning, wet spinning, wet-dry spinning, gel spinning, flash spinning, or electrospinning.

[0069] The polymer fibers may be single fibers or fiber bundles.

[0070] When polymer fibers are in the form of fiber bundles, the number of filaments per bundle is not particularly limited, but from the viewpoint of productivity and mechanical properties of flame-resistant fibers and carbon fibers, it is preferably 10 to 360,000, more preferably 20 to 180,000, even more preferably 30 to 72,000, and particularly preferably 50 to 36,000. Furthermore, by limiting the number of filaments per bundle to 360,000 or less, it is possible to suppress the occurrence of uneven firing during flame-retardant treatment.

[0071] The fineness of polymer fibers is not particularly limited, but the fineness of a single polymer fiber is 1 × 10⁻⁶. -7It is preferably 1200 dtex / strand, more preferably 1×10 -5 dtex / strand to 700 dtex / strand, even more preferably 1×10 -3 dtex / strand to 300 dtex / strand, still even more preferably 1×10 -2 dtex / strand to 100 tex / strand, yet still even more preferably 4×10 -2 dtex / strand to 25 dtex / strand, particularly preferably 1×10 -1 dtex / strand to 10 dtex / strand is most preferred.

[0072] By setting the fineness of the polymer fiber to 1×10 -7 dtex / strand or more, the occurrence of yarn breakage can be suppressed, thereby improving the ease of winding of the polymer fiber and the stability of the flame-retardant treatment. By setting the fineness of the polymer fiber to 1200 dtex / strand or less, the difference between the structure near the surface and the structure near the center of the flame-retardant fiber obtained by the flame-retardant treatment can be reduced, and the tensile strength and tensile modulus of the carbon fiber can be improved.

[0073] In the present disclosure, for the fineness (dtex / strand) of a single polymer fiber, when the polymer fiber is a single fiber, its mass is measured, and the mass per 10000 m is calculated as the fineness of the fiber. Further, when the polymer fiber is a fiber bundle composed of a plurality of single fibers (hereinafter sometimes referred to as a polymer fiber bundle), the mass of the polymer fiber bundle is measured, and the mass per 10000 m is calculated as the fineness (dtex) of the fiber bundle, and then divided by the number of single fibers constituting the fiber bundle to calculate the fineness (dtex / strand) per single fiber.

[0074] The average fiber diameter of a single polymer fiber (hereinafter also simply referred to as "average fiber diameter of polymer fibers") is not particularly limited, but is preferably 3 nm to 400 μm, more preferably 30 nm to 300 μm, even more preferably 300 nm to 200 μm, particularly preferably 1 μm to 100 μm, also particularly preferably 2 μm to 60 μm, and most preferably 3 μm to 40 μm.

[0075] By setting the average fiber diameter of the polymer fibers to 3 nm or more, the stability of the flame-retardant treatment can be improved. Furthermore, by setting the average fiber diameter of the polymer fibers to 3 nm or more, the occurrence of fiber breakage can be suppressed, thereby improving the ease of winding the polymer fibers and the stability of the flame-retardant treatment. By setting the average fiber diameter of the polymer fibers to 400 μm or less, the difference between the structure near the surface and the structure near the center of the flame-resistant fibers obtained by flame-retardant treatment can be reduced, thereby improving the tensile strength and tensile modulus of the carbon fibers.

[0076] In this disclosure, the average fiber diameter of polymer fibers can be determined by observing the side or cross-section of the fibers using a microscope or scanning electron microscope, but if the polymer fibers are in the form of a fiber bundle, it is calculated by the following method. The polymer fiber bundles are vacuum-dried at 80°C for 12 hours. Then, the density of the polymer fiber bundles is measured using a dry automatic densimeter (Micromeristics "AccuPic II 1340"), and the average fiber diameter (μm) of the individual fibers constituting the fiber bundles is determined using the following formula. D = {(Dt × 4 × 100) / (ρ × π × n)} 1 / 2 [During the ceremony, D represents the average fiber diameter (μm) of the individual fibers that make up the fiber bundle. Dt represents the fineness (dtex) of the fiber bundle. ρ is the density of the fiber bundle (g / cm³). 3 ) represents, n represents the number of individual fibers that make up the fiber bundle. Note that π is 3.14.

[0077] The polymer fibers may have a conventionally known oil coating applied to their surface. By applying an oil to the surface of the polymer fibers, the bundleability and handling of the fibers can be improved, and fusion between fibers can be suppressed. Furthermore, by crosslinking the oil agent together with a diene polymer, the fusion between fibers can be suppressed more effectively. There are no particular restrictions on the oil, but for example, conventionally known silicone-based oils can be used. The oil may also be an oil having a functional group that crosslinks upon irradiation with radiation. Examples of such oils include silicone-based oils having a functional group that crosslinks upon irradiation with radiation. The oil may also contain additives such as organic solvents, surfactants, crosslinking agents, crosslinking accelerators, smoothing agents, hygroscopic agents, viscosity modifiers, plasticizers, mold release agents, spreading agents, antioxidants, antibacterial agents, preservatives, rust inhibitors, and pH adjusters.

[0078] <Method for manufacturing flame-resistant fibers> The method for producing flame-resistant fibers according to this disclosure includes a step of heat-treating the carbon fiber precursor fibers described above under an oxidizing atmosphere. Hereinafter, the heat-treating process under an oxidizing atmosphere will also be referred to as the "flame-resistant treatment."

[0079] The temperature of the flame-retardant treatment is not particularly limited, but is preferably in the range of 120°C to 500°C, more preferably in the range of 130°C to 490°C, even more preferably in the range of 140°C to 480°C, still even more preferably in the range of 150°C to 470°C, particularly preferably in the range of 160°C to 460°C, and most preferably in the range of 170°C to 450°C. Furthermore, the above temperature includes not only the highest temperature in the flame-retardant treatment described later, but also the temperature during the heating process leading up to the flame-retardant treatment temperature.

[0080] The maximum temperature in the flame-retardant treatment is preferably 290°C or higher, more preferably 300°C or higher, even more preferably 310°C or higher, 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 carbonization resistance and reducing manufacturing costs by shortening the time. The upper limit of the flame-retardant treatment temperature is not particularly limited, and the flame-retardant treatment temperature is preferably, for example, 500°C or lower, and more preferably 490°C or lower.

[0081] The heating time at the flame-retardant treatment temperature is not particularly limited and may be 4 hours or more, but 1 minute to 4 hours is preferred, 2 minutes to 2 hours is more preferred, 3 minutes to 110 minutes is even more preferred, 4 minutes to 100 minutes is particularly preferred, and 4 minutes to 90 minutes is most preferred. Heating the time at the flame-retardant treatment temperature for 1 minute or more improves the carbonization yield. Heating the time at the flame-retardant treatment temperature for 4 hours or less can reduce manufacturing costs.

[0082] Examples of oxidizing atmospheres include oxygen, ozone, air, nitrogen oxides, halogens, sulfur dioxide; mixtures thereof; and mixtures of oxygen, ozone, air, nitrogen oxides, halogens, or sulfur dioxide with an inert gas. Among these, air, a mixture of oxygen and air, a mixture of oxygen and an inert gas, or a mixture of air and an inert gas are preferred as the oxidizing atmosphere, and air is particularly preferred from the viewpoint of reducing manufacturing costs.

[0083] During the heating process up to the flame-retardant treatment temperature, tension may or may not be applied to the carbon fiber precursor, but from the viewpoint of improving the tensile strength of the flame-retardant fiber, it is preferable to apply tension. Furthermore, tension may be applied from the initial stage of the heating process, or from an intermediate stage in the heating process. Also, different amounts of tension may be applied depending on the temperature.

[0084] The tension applied to the carbon fiber precursor is preferably 0.005 mN / dtex to 200 mN / dtex, more preferably 0.01 mN / dtex to 100 mN / dtex, and even more preferably 0.02 mN / dtex to 50 mN / dtex.

[0085] <Flame-resistant fiber> The flame-resistant fibers of this disclosure contain a structure derived from a crosslinked diene polymer and have a fusion rate of 30% or less. The crosslinked diene polymer is preferably a crosslinked diene polymer containing the structural unit represented by formula (1) above. Details of the diene polymer containing the structural unit represented by formula (1) are as described above.

[0086] The presence of structures derived from crosslinked diene polymers in flame-resistant fibers can be confirmed by infrared spectroscopy, solid-state NMR, elemental analysis, etc. Structures derived from crosslinked diene polymers are, for example, polycyclic structures formed when multiple rings are condensed after intramolecular cyclization of the crosslinked diene polymer. It is preferable that the polycyclic structure includes one or more of the following: structures formed by intermolecular crosslinking, structures in which oxygen-containing substituents such as carbonyl groups and hydroxyl groups are formed by oxidation during flame-resistant treatment, and conjugated structures in which double bonds are formed between carbon atoms.

[0087] Since the flame-resistant fibers of this disclosure have a fusion rate of 30% or less, fiber breakage during carbonization treatment can be suppressed.

[0088] The fusion rate is preferably 25% or less, more preferably 20% or less, even more preferably 15% or less, still more preferably 10% or less, particularly preferably 5% or less, and most preferably 0%.

[0089] In this disclosure, the fusion rate is calculated by the following method.

[0090] A 3cm length of evaluation fiber is cut from the flame-resistant fiber, and the cross-section of this evaluation fiber is observed using a microscope (Keyence Corporation, "Digital Microscope VHX-7000") to count the number of fibers. At this time, the number of fused fibers and the total number of fibers are counted. The number of fused fibers is counted as 2, for example, if two fibers are fused to each other. The total number of fibers is counted after separating the fused fibers into their pre-fusion state. The fusion rate is calculated based on the following formula. Fusion rate (%) = (Number of fused fibers / Total number of fibers) × 100

[0091] <Manufacturing method for carbon fiber> The carbon fiber manufacturing method of the present disclosure preferably includes the steps of manufacturing flame-resistant fibers by the flame-resistant fiber manufacturing method of the present disclosure and subjecting the flame-resistant fibers to a carbonization treatment. Furthermore, the method for producing the carbon fibers of this disclosure preferably includes a step of subjecting the flame-resistant fibers of this disclosure to a carbonization treatment.

[0092] One method for carbonizing flame-resistant fibers is to heat them in an inert gas atmosphere at a temperature higher than the temperature used for flame-retardant treatment. Examples of inert gases include nitrogen, argon, and helium.

[0093] By applying a carbonization treatment to flame-resistant fibers, the flame-resistant fibers are carbonized, and carbon fibers are obtained.

[0094] The carbonization temperature is preferably 500°C or higher, more preferably 1000°C or higher, even more preferably 1100°C or higher, particularly preferably 1200°C or higher, and most preferably 1300°C or higher. Furthermore, there is no particular upper limit to the carbonization temperature. From the viewpoint of reducing manufacturing costs by reducing the energy required for manufacturing, the carbonization temperature is preferably 3000°C or lower, and more preferably 2500°C or lower.

[0095] In this disclosure, "carbonization treatment" may generally include "graphitization," which is carried out by heating at a temperature of 2000°C to 3000°C under an inert gas atmosphere.

[0096] Furthermore, the carbonization process may include multiple heat treatments. For example, a heat treatment can be performed first at a temperature of less than 1000°C (hereinafter also referred to as "pre-carbonization treatment"), followed by a heat treatment at a temperature of 1000°C or higher (carbonization treatment), and then a heat treatment at a temperature of 2000°C or higher (graphitization treatment).

[0097] The carbonization treatment time is not particularly limited, but is preferably 30 seconds to 120 minutes, more preferably 30 seconds to 60 minutes, and even more preferably 1 minute to 30 minutes. From the viewpoint of reducing manufacturing costs, the carbonization treatment time is particularly preferably 20 minutes or less, and most preferably 10 minutes or less.

[0098] The average fiber diameter of a single carbon fiber is not particularly limited, but is preferably 3 nm to 300 μm, more preferably 30 nm to 150 μm, even more preferably 100 nm to 60 μm, still more preferably 1 μm to 40 μm, particularly preferably 2 μm to 30 μm, and most preferably 2.5 μm to 25 μm.

[0099] When the average fiber diameter of a single carbon fiber is 3 nm or greater, the resin or other material can easily impregnate the carbon fiber into the carbon fiber even if the viscosity of the matrix is ​​high, improving the tensile strength of the composite material. When the average fiber diameter of a single carbon fiber is 300 μm or less, the tensile strength of the carbon fiber improves. [Examples]

[0100] The above embodiments will be described in detail below with reference to examples, but the above embodiments are not limited to these examples.

[0101] <Diene polymer a1> As the diene polymer a1, syndiotactic 1,2-polybutadiene (product name "RB840", manufactured by ENEOS Material Co., Ltd., 1,2-structural unit content (1,2-bond content): 94 mol%, melting point 126°C) was used.

[0102] <Diene polymer a2> -Synthesis of FeCl3(TBTP)- As a polymerization catalyst that enables selective 3,4-addition polymerization of isoprene, FeCl3(TBTP), which is formed by coordinating tri-tert-butyl-terpyridine (TBTP) to FeCl3, was synthesized using the following procedure. 0.2 g of FeCl3 was dispersed in 50 mL of anhydrous tetrahydrofuran. 0.5 g of TBTP was added to this dispersion and the mixture was stirred at room temperature (25°C) for 10 hours. The resulting solution was allowed to stand for 15 hours to precipitate the crude product. The solution was then washed with tetrahydrofuran and filtered by suction. The resulting powder was vacuum-dried at 30°C for 3 days to obtain FeCl3(TBTP) in 93% yield.

[0103] -Synthesis of diene polymer a2- To a flask containing 225.5 mg (0.4 mmol) of FeCl3 (TBTP), 2.75 mL of anhydrous toluene was added under a nitrogen atmosphere. Then, 2.516 g (40 mmol) of modified methylaluminoxane (MMAO, [(CH3) 0.95 (C8H 17 ) 0.05 17.25 mL of anhydrous toluene solution (MMAO concentration: 16.3% by mass) containing AlO was added dropwise. Then, 8.0 mL (80 mmol) of isoprene was added dropwise, and polymerization was carried out at 25°C for 3 hours. The polymerized solution was diluted with 20 mL of toluene. This 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. This precipitate was washed with 500 mL of methanol containing 395.9 mg of BHT. This washing operation was repeated three times to obtain a white solid. The obtained solid was vacuum-dried at 25°C for 3 days to obtain an isoprene-based polymer (diene polymer a2) in 99% yield. The composition of the obtained diene polymer a2 is 1 H-NMR and 13 Analysis by 13C-NMR revealed that the content of 3,4-structural units (3,4-bond content) was 74.4 mol%, the content of trans-1,4-structural units was 5.6 mol%, the content of cis-1,4-structural units was 20.0 mol%, and the content of 1,2-structural units was 0.0 mol%. The 3,4-structural unit is represented by formula (1), where R is a methyl group.

[0104] <Polymer fiber bundle (b1)> Using a diene polymer (a1), melt spinning was performed at 150°C (with 36 holes in the melt spinning apparatus nozzle) to obtain polymer fiber bundles of 36 fibers / bundle (single fiber fineness 21 dtex) so that the fineness per fiber (single fiber) was 21 dtex. Next, after stretching 3.5 times at 50°C, six bundles were joined together to form a fiber bundle of 216 fibers / bundle, obtaining polymer fiber bundle (b1) (average fiber diameter 29 μm, single fiber fineness 6 dtex).

[0105] <Polymer fiber bundle (b2)> Using a diene polymer (a2), melt spinning was performed at 150°C (with 36 holes in the melt spinning apparatus nozzle) to obtain polymer fiber bundles of 36 fibers / bundle (single fiber fineness 24.5 dtex) with a fineness of 24.5 dtex per fiber. Next, after stretching 3.5 times at 50°C, six bundles were joined together to form a fiber bundle of 216 fibers / bundle, obtaining polymer fiber bundle (b2) (average fiber diameter 31 μm, single fiber fineness 7 dtex).

[0106] <Polymer fiber bundle (b3)> Using a diene polymer (a1), melt spinning was performed at a temperature of 150°C (36 holes in the nozzle of the melt spinning apparatus) to obtain polymer fiber bundles of 36 fibers / bundle (single fiber fineness 3dtex). Next, six bundles were combined to form a fiber bundle of 216 fibers / bundle, and then doubled at room temperature to obtain polymer fiber bundle (b3) (average fiber diameter 14 μm, single fiber fineness 1.5 dtex).

[0107] <Polymer fiber bundle (b4)> This product contains 91% by mass of a diene polymer (a1) and another polymer, polyethylene (product name "Polyethylene Wax PE520", manufactured by Clariant, weight-average molecular weight 5500, viscosity at 140°C: approximately 0.65 Pa·s, melting point 117-123°C, density 0.93 g / cm³). 3 Using a mixture consisting of 9% by mass of , melt spinning was performed at a temperature of 150°C (36 holes in the nozzle of the melt spinning apparatus) so that the fineness per fiber was 2 dtex, to obtain polymer fiber bundles of 36 fibers / bundle (fineness of single fiber 2 dtex). Next, 6 bundles were combined to form a fiber bundle of 216 fibers / bundle, and then doubled at room temperature to obtain polymer fiber bundle (b4) (average fiber diameter 12 μm, fineness of single fiber 1 dtex).

[0108] <Polymer fiber bundle (b5)> 100 parts by mass of diene polymer (a1) was mixed with 2 parts by mass of 4,4'-bis(dimethylamino)benzophenone as a photosensitizer. Melt spinning was performed at 150°C (using a melt spinning apparatus with 36 holes in the nozzle) to obtain polymer fiber bundles of 36 fibers / bundle (single fiber fineness 21 dtex) so that the fineness per fiber (single fiber) was 21 dtex. Next, the bundles were stretched 3.5 times at 50°C, and then 6 bundles were joined together to form a fiber bundle of 216 fibers / bundle, obtaining polymer fiber bundle (b5) (average fiber diameter 29 μm, single fiber fineness 6 dtex).

[0109] The average fiber diameter and fineness were calculated for the obtained polymer fiber bundles (b1) to (b5) using the following method.

[0110] (Average fiber diameter of polymer fiber bundles) The obtained polymer fiber bundles were dried in a vacuum dryer at 60°C for 12 hours. Then, the density of the polymer fiber bundles was measured using a dry automatic densimeter (Micromeristics "AccuPic II 1340"), and the average fiber diameter (μm) of the individual fibers constituting the bundles was determined using the following formula. D = {(Dt × 4 × 100) / (ρ × π × n)} 1 / 2 [During the ceremony, D represents the average fiber diameter (μm) of the individual fibers that make up the fiber bundle. Dt represents the fineness (dtex) of the fiber bundle. ρ is the density of the fiber bundle (g / cm³). 3 ) represents, n represents the number of individual fibers that make up the fiber bundle. Note that π is 3.14.

[0111] (Fineness of polymer fiber bundles) The mass of the obtained polymer fiber bundles was measured, and the fineness of the fiber bundle was calculated as the mass per 10,000 m, and the fineness of each individual fiber constituting the fiber bundle was calculated.

[0112] <Preparation of carbon fiber precursors> In Examples 1-4 and 11-14, electron beam irradiation was performed in a batch manner. In Examples 5-10, 15, and 16, electron beam irradiation was performed in a continuous manner.

[0113] -Batch type- Polymer fiber bundles shown in Table 1 were fixed onto a mounting board while maintaining the tension shown in Table 1. These polymer fiber bundles on the mounting board were irradiated with an electron beam at the dose shown in Table 1 using an electron beam irradiation device CB250 / 30 / 20mA manufactured by Iwasaki Electric Co., Ltd., under the atmosphere shown in Table 1, with the acceleration voltage set to 250kV, to obtain carbon fiber precursors made of cross-linked diene polymers. Here, when using an electron beam with an acceleration voltage of 50kGy, the beam current was 3.2mA, the transport speed was 5m / min, and one treatment (3.6 seconds) was performed. When using an electron beam with 100kGy, the acceleration voltage was 250kV, the beam current was 6.3mA, the transport speed was 5m / min, and one treatment (3.6 seconds) was performed. When using an electron beam with 300kGy, the acceleration voltage was 250kV, the beam current was 6.3mA, the transport speed was 5m / min, and three treatments (totaling 10.8 seconds) were performed.

[0114] -Continuous- Using an electron beam irradiation device EBC800-35 manufactured by NHV Corporation, the polymer fiber bundles shown in Table 1 were irradiated with an electron beam for 2 minutes at the dose shown in Table 1 while applying the tension shown in Table 1, at a transport speed of 10 m / min, under the atmosphere shown in Table 1, and with an acceleration voltage set to 800 kV, thereby obtaining carbon fiber precursors made of cross-linked diene polymers.

[0115] The gel fraction and swelling ratio of the obtained carbon fiber precursor were calculated using the following method. The calculation results are shown in Table 1.

[0116] (Gel fraction) A 0.2 g sample was cut from the carbon fiber precursor. After drying at 80°C for 4 hours, the mass was precisely measured using a precision electronic balance and recorded as the initial mass (g). Next, the sample was immersed in 30 ml of toluene and left to stand in a 60°C hot air circulating oven for 8 hours. After standing, the sample was subjected to suction filtration using a 1.0 μm pore size membrane filter (Omnipore™ membrane filter JAWP04700, Merck), and the gel component was separated. The separated gel was air-dried in a fume hood with a membrane filter for more than 12 hours, and then left to stand in a 90°C hot air circulating oven for 12 hours to remove toluene. The mass of the gel and membrane filter after standing was precisely weighed using a precision electronic balance, and the gel fraction was determined using the following formula. Gel fraction (%) = {(Mass of gel and membrane filter (g) - Mass of membrane filter (g)) / Initial mass of sample (g)} × 100

[0117] (Swelling ratio) A 0.2 g sample was cut from the carbon fiber precursor. After drying 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. After standing, the sample was subjected to suction filtration using a membrane filter with a pore size of 1.0 μm to separate the gel component, and the mass of the recovered gel component (swollen gel) was accurately weighed using a precision electronic balance. Next, the swollen gel was air-dried in a fume hood for more than 12 hours, and then dried at 90°C for another 12 hours. The mass of the dried gel (dried gel) was then weighed and determined. The swelling ratio was calculated using the following formula. Swelling ratio (times) = (mass of swollen gel [g]) / (mass of dry gel [g])

[0118] <Production of flame-resistant fibers> -Examples 1 to 16- The obtained carbon fiber precursor was continuously fed into a heat treatment apparatus under an airflow while applying a tension of 60 cN (600 mN) to the carbon fiber precursor. Heat treatment (flame retardation treatment) was performed by raising the temperature from 200°C to 350°C over 60 minutes. Subsequently, while maintaining the tension on the carbon fiber precursor, a further flame retardation treatment was performed at 350°C for 30 minutes. The flame retarded fibers were continuously discharged from the outlet of the heat treatment apparatus to obtain flame retarded fibers.

[0119] -Comparative Example 1- Flame-retardant treatment was performed under the same conditions as in Examples 1 to 16, except that polymer fiber bundles (b1) were used. However, when polymer fibers (b1) were placed in the heat treatment apparatus, the fibers melted and broke.

[0120] -Comparative Example 2- A tension of 0.02 mN / dtex was applied to the polymer fiber bundle (b1) to prevent sagging, and both ends of the polymer fiber bundle (b1) were fixed. In this state, the polymer fiber bundle (b1) was immersed in nitric acid (61% concentration) at 20°C for 20 hours. After that, it was washed with water and dried. Next, the obtained fiber bundle was subjected to flame-retardant treatment under an airflow using a heat treatment device under the same conditions as in Examples 1 to 16 to obtain flame-retardant fibers.

[0121] -Comparative Example 3- A tension of 0.02 mN / dtex was applied to the polymer fiber bundle (b1) to prevent sagging, and both ends of the polymer fiber bundle (b1) were fixed. In this state, the polymer fiber bundle (b1) was immersed in a solution of 2 g of aluminum chloride dissolved in 100 mL of p-xylene at 20°C for 1 hour. After that, it was washed with methanol containing benzene and a small amount of hydrochloric acid, washed again with methanol, and then dried. Next, the obtained fiber bundle was subjected to flame-retardant treatment under an airflow using a heat treatment apparatus under the same conditions as in Examples 1 to 16 to obtain flame-retardant fibers.

[0122] -Comparative Example 4- A polymer fiber bundle (b5) is fixed to a mounting board while tension (0.02 mN / dtex) is applied, and this polymer fiber bundle on the mounting board is irradiated with a UV-LED irradiator (HLDL-350×270) manufactured by CCS Corporation at an irradiation intensity of 200 mW / cm². 2 Next, ultraviolet light was irradiated at a wavelength of 365 nm for 2 hours in an air atmosphere to obtain a carbon fiber precursor made of a cross-linked diene polymer. Then, the obtained fiber bundles were subjected to flame-retardant treatment using a heat treatment device under an airflow, under the same conditions as in Examples 1 to 16, to obtain flame-retardant fibers.

[0123] The fusion rate of the obtained flame-resistant fibers was calculated using the following method, and the carbonization resistance was evaluated using the following method. The calculation and evaluation results are shown in Table 1. In Comparative Example 1, flame-resistant fibers could not be obtained, so "-" is indicated in Table 1.

[0124] (Fusion rate) A 3cm length of evaluation fiber was cut from the flame-resistant fiber, and the cross-section of this evaluation fiber was observed using a microscope (Keyence Corporation, "Digital Microscope VHX-7000") to count the number of fibers. At this time, the number of fused fibers and the total number of fibers were counted. The number of fused fibers was counted as 2, for example, if two fibers were fused to each other. The total number of fibers was counted after separating the fused fibers into their pre-fusion state. The fusion rate was calculated based on the following formula. Fusion rate (%) = (Number of fused fibers / Total number of fibers) × 100

[0125] (Carbonization resistance) Each flame-resistant fiber (216 strands) was subjected to a tension of 20 cN and then transported to a heat treatment device adjusted to 800°C under a nitrogen atmosphere for a 3-minute pre-carbonization treatment. If the fiber bundle completely broke during the pre-carbonization treatment, the evaluation result was set to E. If pre-carbonized fibers were obtained during the pre-carbonization treatment, a 10 cm evaluation fiber was cut from the pre-carbonized fibers, and the presence or absence of fuzz caused by the cutting of single fibers during the pre-carbonization treatment was observed visually and using a microscope (SKM-S20B-PC, manufactured by Saito Optical Co., Ltd.). The evaluation criteria are as follows. A: No fuzz was generated by cutting. B: Fuzzing occurred due to the cutting of 1 to 3 single fibers. C: Fuzzing occurred due to the cutting of 4 to 10 single fibers. D: Fuzzing occurred due to the cutting of 11 or more single fibers. E: The fiber bundle completely broke during the preliminary carbonization treatment.

[0126] <Fabrication of carbon fibers> The flame-resistant fibers (216 strands, average fiber diameter 25 μm) from Example 9 were subjected to a tension of 20 cN and transported to a heat treatment device adjusted to 800°C under a nitrogen stream for a 3-minute pre-carbonization treatment to obtain pre-carbonized fibers (216 strands, average fiber diameter 22 μm). The pre-carbonized fibers were then subjected to a tension of 20 cN and transported to a heat treatment device adjusted to 1400°C under a nitrogen stream for a 3-minute carbonization treatment to obtain carbon fibers (c1) (average fiber diameter: 20 μm). The carbon fibers (c1) showed no fuzzing due to the cutting of individual fibers, and there was no fusion between fibers.

[0127] The flame-resistant fibers (216 fibers, average fiber diameter 26 μm) from Example 14 were subjected to a tension of 20 cN and transported to a heat treatment device adjusted to 800°C under a nitrogen stream for a 3-minute pre-carbonization treatment to obtain pre-carbonized fibers (213 fibers, average fiber diameter 23 μm). The pre-carbonized fibers were then subjected to a tension of 20 cN and transported to a heat treatment device adjusted to 1400°C under a nitrogen stream for a 3-minute carbonization treatment to obtain carbon fibers (c2) (average fiber diameter: 21 μm). The carbon fibers (c2) showed no fuzzing due to the cutting of individual fibers, and there was no fusion between fibers.

[0128] The flame-resistant fibers (216 fibers, average fiber diameter 10 μm) from Example 15 were subjected to a tension of 20 cN (200 mN) and transported to a heat treatment device adjusted to 800°C under a nitrogen stream for a 3-minute pre-carbonization treatment to obtain pre-carbonized fibers (216 fibers, average fiber diameter 8 μm). The pre-carbonized fibers were then subjected to a tension of 20 cN and transported to a heat treatment device adjusted to 1400°C under a nitrogen stream for a 3-minute carbonization treatment to obtain carbon fibers (c3) (average fiber diameter: 8 μm). The carbon fibers (c3) showed no fuzzing due to fiber cutting and no fusion between fibers. Single fibers were extracted from carbon fiber (C3), and tensile tests were performed on the single fibers in accordance with JIS R 7606 using a micro-strength evaluation tester (Shimadzu Corporation's "Micro Autograph MST-I") (gauge between gauge marks: 25 mm, tensile speed: 1 mm / min). The tensile modulus and tensile strength were measured, and the average of five measured values ​​was calculated. The tensile modulus was 71 GPa, and the tensile strength was 1.1 GPa.

[0129] The flame-resistant fibers (216 fibers, average fiber diameter 9 μm) from Example 16 were subjected to a tension of 20 cN (200 mN) and transported to a heat treatment device adjusted to 800°C under a nitrogen stream for a 3-minute pre-carbonization treatment to obtain pre-carbonized fibers (216 fibers, average fiber diameter 7 μm). The pre-carbonized fibers were then subjected to a tension of 20 cN and transported to a heat treatment device adjusted to 1400°C under a nitrogen stream for a 3-minute carbonization treatment to obtain carbon fibers (c4) (average fiber diameter: 7 μm). The carbon fibers (c4) did not exhibit fuzzing due to fiber cutting, nor did any fusion between fibers. Single fibers were extracted from carbon fiber (C4), and tensile tests were performed on the single fibers in accordance with JIS R 7606 using a micro-strength evaluation tester (Shimadzu Corporation's "Micro Autograph MST-I") (gauge between gauge marks: 25 mm, tensile speed: 1 mm / min). The tensile modulus and tensile strength were measured, and the average of five measured values ​​was calculated. The tensile modulus was 80 GPa, and the tensile strength was 1.3 GPa.

[0130] [Table 1]

[0131] [Table 2]

[0132] [Table 3]

[0133] As shown in Tables 1 and 2, in Examples 1 to 16, it was found that because the carbon fiber precursor contained a cross-linked diene polymer and had a gel fraction of 40% or more, inter-fiber fusion during flame-retardant treatment was suppressed. On the other hand, in Comparative Example 1, the gel fraction was 0%, and the fibers melted and broke, making it impossible to obtain flame-resistant fibers. Furthermore, in Comparative Examples 2 and 3, the gel fraction was less than 40%, indicating that fusion between fibers was likely to occur during the flame-retardant treatment. It is presumed that when polymer fibers were immersed in a solution containing nitric acid or aluminum chloride, the gel fraction was low because the fibers were not sufficiently hardened to the core. Furthermore, in Comparative Example 4, fusion between fibers during flame-retardant treatment was not suppressed even after irradiation with ultraviolet light for two hours, whereas in Examples 1 to 16, fusion between fibers during flame-retardant treatment was suppressed and carbon resistance improved with only short-term irradiation with electron beams. From these results, it was found that the manufacturing method of the present disclosure is superior from the viewpoint of production efficiency and energy saving.

[0134] Furthermore, comparisons of Examples 1, 2, and 4, and Examples 11, 12, and 14, revealed that increasing the electron beam dose increased the gel fraction, decreased the swelling ratio, suppressed interfiber fusion during flame-retardant treatment, and improved carbon resistance.

[0135] A comparison of Example 2 and Example 3 revealed that when irradiated with an electron beam under a nitrogen atmosphere, the gel fraction increased, the swelling ratio decreased, and carbonization resistance improved compared to when irradiated with an electron beam under an air atmosphere.

[0136] A comparison of Example 12 and Example 13 revealed that when irradiated with an electron beam under a nitrogen atmosphere, the gel fraction increased, the swelling ratio decreased, interfiber fusion during flame-retardant treatment was suppressed, and carbon resistance improved compared to when irradiated with an electron beam under an air atmosphere.

[0137] A comparison of Example 3 and Example 5 revealed that increasing the acceleration voltage and tension increased the gel fraction, decreased the swelling ratio, and improved carbonization resistance.

[0138] A comparison of Example 8 and Example 9 revealed that increasing the tension increased the gel fraction. It is presumed that applying a specific tension in the fiber axis direction during electron beam irradiation caused cyclization and crosslinking to proceed while the molecules were oriented, resulting in an increase in the gel fraction.

[0139] A comparison of Example 15 and Example 16 revealed that in Example 16, where the diene polymer was replaced with an olefin polymer as another polymer, spinnability was improved, and a carbon fiber precursor with a smaller single fiber diameter was obtained. Even with the inclusion of other polymers, the carbon fiber precursor had a high gel fraction. Furthermore, the resulting carbon fibers had smaller fiber diameters, and their tensile strength and tensile modulus increased.

Claims

1. The process includes irradiating polymer fibers containing a diene polymer containing a structural unit represented by the following formula (1) with radiation at a dose of 20 kGy or more. A method for producing a carbon fiber precursor, wherein when irradiating the polymer fibers with the aforementioned radiation, a tension of 1.5 mN / dtex or more is applied to the polymer fibers. 【Chemistry 1】 In formula (1), R is a hydrogen atom or an organic group having 1 to 20 carbon atoms.

2. The method for producing a carbon fiber precursor according to claim 1, wherein R in formula (1) is a hydrogen atom or a methyl group.

3. A step of producing a carbon fiber precursor by the method for producing a carbon fiber precursor described in claim 1 or claim 2, A method for producing flame-resistant fibers, comprising the step of subjecting the carbon fiber precursor fibers to heat treatment under an oxidizing atmosphere.

4. A step of producing a carbon fiber precursor by the method for producing a carbon fiber precursor described in claim 1 or claim 2, A step of producing flame-resistant fibers by subjecting the carbon fiber precursor fibers to heat treatment in an oxidizing atmosphere, A step of applying a carbonization treatment to the flame-resistant fiber, A method for manufacturing carbon fibers, including

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