Method for producing flame-resistant fiber, method for producing carbon fiber, and flame-resistant fiber

Abstract

The problem to be solved by the present invention is to provide a method for producing a flame-resistant fiber with which it is possible to produce a flame-resistant fiber which can be a carbon fiber having excellent tensile strength and in which fusion between fibers is suppressed.　The present invention is a method for producing a flame-resistant fiber, the method comprising subjecting a carbon fiber precursor fiber containing a diene-based polymer to a flameproofing treatment while subjecting the carbon fiber precursor fiber to a stretching treatment at a stretching ratio of 0.9-100 times.

Description

Method for producing flame-resistant fiber, method for producing carbon fiber, and flame-resistant fiber

[0001] The present disclosure relates to a method for producing a flame-resistant fiber, a method for producing a carbon fiber, and a flame-resistant fiber.

[0002] Carbon fiber is lightweight and has excellent mechanical properties, and therefore carbon fiber composite materials are being developed for a variety of applications, including aerospace, automotive, and building materials.

[0003] As a method for producing carbon fibers, a method is known in which a fiber bundle of carbon fiber precursor fibers obtained by spinning polyacrylonitrile or pitch is subjected to a flame retardant treatment and then subjected to a carbonization treatment (see Patent Documents 1 to 3).

[0004] Polyacrylonitrile-based carbon fibers, produced from fibers obtained by spinning polyacrylonitrile, are the most widely used due to their high mechanical properties. The production of polyacrylonitrile-based carbon fibers involves wet spinning or dry-wet spinning using organic solvents such as dimethyl sulfoxide or dimethylformamide. This poses a problem: energy is required to recycle the organic solvent, resulting in high production costs. Additionally, toxic gases such as hydrogen cyanide are generated during the flame-retardant and carbonization processes.

[0005] On the other hand, when pitch is used as a raw material, melt spinning can be performed without using an organic solvent, and hydrogen cyanide is not generated during the flame-proofing treatment and carbonization treatment. However, the melt spinning temperature of pitch-based fibers generally needs to be high (for example, 250°C or higher). In addition, the carbon fiber precursor fiber itself is relatively brittle, and during the flame-proofing treatment, fusion between the carbon fiber precursor fibers and breakage of some of the carbon fiber precursor fibers are likely to occur, making it difficult to handle (see Patent Document 3).

[0006] Therefore, 1,2-polybutadiene fibers have been disclosed as precursors that can be melt-spun without using organic solvents and that do not generate hydrogen cyanide (see Patent Documents 4 to 6).

[0007] Patent Document 1: JP 2006-183159 A Patent Document 2: JP 2008-202208 A Patent Document 3: JP 6-10215 A Patent Document 4: JP 48-82199 A Patent Document 5: JP 48-92699 A Patent Document 6: JP 49-106490 A

[0008] If 1,2-polybutadiene fibers are subjected to a heat treatment at a temperature equal to or higher than the melting point (for example, 150°C or higher), the 1,2-polybutadiene fibers will melt and break. Patent Document 4 discloses a method for obtaining flame-resistant fibers by irradiating 1,2-polybutadiene fibers with ultraviolet light for a long period of time (for example, 2 hours or more) to cure the fibers, and then performing a flame-resistant treatment at a relatively low temperature for a long period of time (for example, at 200°C for 8 hours or more).

[0009] A long flame-proofing treatment leads to an increase in production costs due to an increase in energy consumption and a decrease in productivity, so a short flame-proofing treatment (for example, 300°C or higher for 2 hours or less) is more preferable even if it is performed at a relatively high temperature.

[0010] However, ultraviolet light only cures the surface and the irradiated portion of the carbon fiber precursor fiber, while the central portion of the cross section perpendicular to the fiber axis direction of the carbon fiber precursor fiber (hereinafter simply referred to as the "central portion") and the portion opposite the irradiated portion are not sufficiently cured. Therefore, it has been difficult to increase the gel fraction by ultraviolet light irradiation. As a result, the carbon fiber precursor fiber is likely to soften during flame-resistant treatment at temperatures higher than 250°C (particularly temperatures of 300°C or higher). This may cause fusion between the carbon fiber precursor fibers or thread breakage due to melting, resulting in a brittle flame-resistant fiber. Furthermore, when carbon fibers are produced using this flame-resistant fiber, the central portion of the fused fiber formed by the fusion of two or more flame-resistant fibers (i.e., single fibers) is insufficiently flame-resistant. This reduces the strength and heat resistance of the flame-resistant fiber. As a result, thread breakage (i.e., fracture) due to the tension applied during the carbonization treatment and thread breakage due to thermal decomposition at high temperatures occur, resulting in a problem of low tensile strength of the carbon fiber.

[0011] Meanwhile, methods have been disclosed in which carbon fiber precursor fibers are immersed in an acid such as nitric acid, and in which carbon fiber precursor fibers are immersed for a long period of time in an organic solvent containing a Lewis acid to render them infusible (see Patent Documents 5 and 6). These methods use organic solvents containing highly toxic Lewis acids such as aluminum bromide or aluminum chloride. Furthermore, the carbon fiber precursor fibers must be immersed in these organic solvents for a long period of time, and a large amount of cleaning solvent must be used to remove these organic solvents from the carbon fiber precursor fibers. This results in high production costs and damage to the surface of the carbon fiber precursor fibers. Furthermore, while the surface of the carbon fiber precursor fibers can be hardened, as with ultraviolet irradiation, there is a risk that the center of the carbon fiber precursor fibers may not be sufficiently flame-resistant. Therefore, there are problems such as fusion between some of the carbon fiber precursor fibers during the flame-resistant treatment, fusion between some of the flame-resistant fibers during the carbonization treatment, and fiber breakage. Furthermore, the tensile strength of the carbon fibers is also insufficient.

[0012] The problem to be solved by one embodiment of the present disclosure is to provide a method for producing a flame-resistant fiber that can produce a carbon fiber having excellent tensile strength and in which fusion between fibers is suppressed.The problem to be solved by another embodiment of the present disclosure is to provide a method for producing a carbon fiber that can produce a carbon fiber having excellent tensile strength.The problem to be solved by another embodiment of the present disclosure is to provide a flame-resistant fiber that can produce a carbon fiber having excellent tensile strength and in which fusion between fibers is suppressed.

[0013] The present inventors have conducted extensive research to prevent fusion of flame-resistant fibers obtained by carbonizing flame-resistant fibers obtained from diene polymers and to improve the strength of the carbon fibers. As a result, it has been discovered that by performing a flame-resistant treatment while drawing a diene polymer fiber at a predetermined draw ratio, a flame-resistant fiber in which fusion between carbon fiber precursor fibers (i.e., single fibers) is suppressed can be obtained, and further, that such a flame-resistant fiber has a peak ratio in a specific range in the infrared absorption spectrum. The peak ratio is determined by the absorption peak (1715 cm) of the C=O stretching vibration of the carbonyl group generated by oxidation of the six-membered carbon ring structure. －１ ~1730cm －１ ) absorption peak of C=C stretching vibration of the carbon six-membered ring structure (1600 cm －１ ~1620cm －１ The present inventors have found that by subjecting such flame-resistant fiber to carbonization treatment, a carbon fiber having high carbonization treatment resistance and high tensile strength can be obtained.

[0014] Specific means for achieving the object are as follows. <1> A method for producing a flame-resistant fiber, comprising: drawing a carbon fiber precursor fiber containing a diene polymer at a draw ratio of 0.9 to 100; and performing a flame-resistant treatment while drawing the carbon fiber precursor fiber at a draw ratio of 0.9 to 100. <2> A method for producing a flame-resistant fiber according to <1>, wherein the drawing treatment is a treatment of drawing the carbon fiber precursor fiber so that the average fiber diameter of the flame-resistant fiber is 10 nm to 40 μm. <3> A method for producing a flame-resistant fiber according to <1> or <2>, wherein the flame-resistant treatment is a treatment of heating the carbon fiber precursor fiber at a temperature in a range of 120°C to 500°C in an oxidizing atmosphere. <4> A method for producing a flame-resistant fiber according to any one of <1> to <3>, wherein the carbon fiber precursor fiber contains a crosslinked diene polymer obtained by crosslinking the diene polymer. <5> A method for producing a carbon fiber, comprising: producing a flame-resistant fiber by the method for producing a flame-resistant fiber according to any one of <1> to <4>; and carbonizing the flame-resistant fiber. <6> A method for producing a flame-resistant fiber, comprising: drawing a carbon fiber precursor fiber containing a diene polymer while performing a flame-resistant treatment, and the infrared absorption spectrum of the flame-resistant fiber contains a peak at 1600 cm －１ ~1620cm －１ There is an absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure, which is confirmed by the following formula: Ａ / I Ｂ a treatment of heating the carbon fiber precursor fiber in an oxidizing atmosphere so that the ratio (I) falls within the range of 0.5 to 5.0; Ａ / I Ｂ ) is detected at 1715 cm in the infrared absorption spectrum of the flame-resistant fiber. －１ ~1730cm －１ The intensity of the absorption peak B of the C=O stretching vibration of the carbonyl group (I Ｂ ) to 1600 cm －１ ~1620cm －１ The intensity of the absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure (I Ａ <7> A method for producing a flame-resistant fiber, which has a ratio (I Ａ / IＢ ) is in the range of 0.5 to 5.0, and the ratio (I Ａ / I Ｂ ) is observed in the infrared absorption spectrum at 1715 cm －１ ~1730cm －１ The intensity of the absorption peak B of the C=O stretching vibration of the carbonyl group (I Ｂ ) to 1600 cm －１ ~1620cm －１ The intensity of the absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure (I Ａ ) ratio.

[0015] According to one embodiment of the present disclosure, there is provided a method for producing a flame-resistant fiber that can produce a carbon fiber having excellent tensile strength and in which fusion between fibers is suppressed. According to another embodiment of the present disclosure, there is provided a method for producing a carbon fiber that can produce a carbon fiber having excellent tensile strength. According to another embodiment of the present disclosure, there is provided a flame-resistant fiber that can produce a carbon fiber having excellent tensile strength and in which fusion between fibers is suppressed.

[0016] In the present disclosure, numerical ranges indicated using "to" include the numerical values ​​before and after "to" as the minimum and maximum values, respectively. In numerical ranges described in stages in the present disclosure, the upper or lower limit value described in one numerical range may be replaced with the upper or lower limit value of another numerical range described in stages. Furthermore, in numerical ranges described in the present disclosure, the upper or lower limit value of that numerical range may be replaced with the value shown in a synthesis example.

[0017] In the present disclosure, each component may contain multiple corresponding substances. When multiple substances corresponding to each component are present in a carbon fiber precursor fiber, the content or amount of each component means the total content or amount of the multiple substances present in the carbon fiber precursor fiber, unless otherwise specified.

[0018] (1) First Embodiment (1.1) Manufacturing Method of Flame-Resistant Fiber A manufacturing method of a flame-resistant fiber according to a first embodiment of the present disclosure includes performing a flame-resistant treatment on a carbon fiber precursor fiber (hereinafter also referred to as a "diene polymer fiber") containing a diene polymer (hereinafter also referred to as a "diene polymer (a)") while performing a drawing treatment at a draw ratio of 0.9 to 100 (hereinafter also referred to as a "flame-resistant step").

[0019] The term "carbon fiber precursor fiber" refers to a fiber from which carbon fiber can be obtained by carbonization, or flame-retardant treatment and carbonization. The term "flame-retardant fiber" refers to a fiber after the flame-retardant process but before the carbonization process. Details of the carbonization process will be described later. The term "diene polymer" refers to a polymer obtained by using a conjugated diene compound (e.g., 1,3-butadiene, isoprene, 2-ethyl-1,3-butadiene, etc.) as at least a part of the monomers. The term "drawing process" refers to a process in which tension or the like is applied to a carbon fiber precursor fiber to stretch the fiber in the fiber axis direction. When the flame-retardant process is performed continuously using a heat treatment device (i.e., when the carbon fiber precursor fiber is subjected to the flame-retardant process by transporting the carbon fiber precursor fiber through the heat treatment device with multiple rollers), the term "draw ratio" refers to the ratio (Lb / La) of the length (Lb) of the diene polymer fiber after the drawing process to the length (La) of the diene polymer fiber before the drawing process. Even when the drawing step is performed batchwise, the "draw ratio" refers to the ratio (Lb / La) of the length (Lb) of the diene polymer fiber after drawing to the length (La) of the diene polymer fiber before drawing. When the drawing step is performed in multiple steps (i.e., multiple stages), the "draw ratio" refers to the product of the draw ratios of each of the multiple steps. The "draw ratio" can be adjusted to a desired value by appropriately changing the ratio (Vb / Va) of the roller speed (Vb) (m / min) at the outlet of the conveying device to the roller speed (Va) (m / min) at the inlet of the conveying device, or by changing the applied tension. The roller is not particularly limited, and examples include a feed roller and a nip roller. When the flame-resistant step is performed batchwise using a heat treatment device, the "draw ratio" refers to the ratio (L2 / L1) of the length (L1) of the carbon fiber precursor fiber before drawing to the length (L2) of the flame-resistant fiber after drawing. The term "flameproofing treatment" refers to subjecting the carbon fiber precursor fiber to a heat treatment in an oxidizing atmosphere. The heating temperature in the flameproofing treatment is preferably within the range of 120°C to 500°C.

[0020] The method for producing a flame-resistant fiber according to the first embodiment has the above-described configuration, and therefore can produce a carbon fiber with excellent tensile strength and suppress fusion between fibers. The reasons for the above effects are presumed to be, but are not limited to, the following: When a carbon fiber precursor fiber containing a diene polymer is subjected to a flame-resistant treatment without being stretched, the resulting flame-resistant fiber shrinks. Specifically, the ratio of the length of the flame-resistant fiber to the length of the carbon fiber precursor fiber before the flame-resistant treatment is typically about 0.8. In the first embodiment, a carbon fiber precursor fiber containing a diene polymer is stretched at a stretch ratio of 0.9 to 100 times while undergoing the flame-resistant treatment. This facilitates the formation of continuous six-membered carbon ring structures within the molecular chain of the carbon fiber precursor fiber. The carbon fiber precursor fiber is oxidized, and hydroxyl groups and carbonyl groups (C═O) are formed in the chemical structure of the molecular chain constituting the carbon fiber precursor fiber. Dehydration reactions of these functional groups facilitate the formation of carbon-carbon double bonds (C═C). Therefore, in the obtained flame-resistant fiber, the ratio of the carbon double bonds (C═C) to the carbonyl groups (C═O) formed by oxidation falls within the range of the present disclosure. As a result, it is presumed that the method for producing the flame-resistant fiber according to the first embodiment can produce a carbon fiber having excellent tensile strength and a flame-resistant fiber in which fusion between fibers is suppressed.

[0021] (1.1.1) Flame-resistant treatment process The method for producing a flame-resistant fiber according to the first embodiment includes a flame-resistant treatment process. In the flame-resistant treatment process, a carbon fiber precursor fiber containing a diene polymer (a) is subjected to a flame-resistant treatment while being stretched at a draw ratio of 0.9 to 100 times. This process results in a flame-resistant fiber.

[0022] The flame-proofing step may be carried out using a known heat treatment device. The flame-proofing step may be carried out in a batch system or a continuous system.

[0023] (1.1.1.1) Flameproofing Treatment In the flameproofing treatment, the carbon fiber precursor fiber is heated in an oxidizing atmosphere.

[0024] Examples of the oxidizing atmosphere in the flame-proofing step include oxygen, ozone, air, nitrogen oxides, halogens, sulfur dioxide gas, mixed gases thereof, and mixed gases of these with an inert gas. Among these, air, mixed gases of oxygen and air, mixed gases of oxygen and an inert gas, and mixed gases of air and an inert gas are preferred, and air is particularly preferred from the viewpoint of cost reduction.

[0025] The treatment temperature for the flame retardant treatment is preferably 120° C. to 500° C., more preferably 150° C. to 490° C., even more preferably 170° C. to 480° C., particularly preferably 180° C. to 470° C., and most preferably 200° C. to 460° C. The maximum temperature for the flame retardant treatment is not particularly limited, but from the viewpoints of improving the tensile strength of the carbon fiber and reducing the production cost by shortening the treatment time, it is preferably 290° C. or higher, more preferably 300° C. or higher, and particularly preferably 330° C. or higher.

[0026] The flame-proofing time (i.e., the heating time at the maximum temperature) is not particularly limited and may be long (for example, more than 4 hours), preferably 1 minute to 4 hours, more preferably 2 minutes to 2 hours, even more preferably 3 minutes to 100 minutes, particularly preferably 4 minutes to 90 minutes, and most preferably 4 minutes to 60 minutes. By setting the heating time in the flame-proofing treatment to 1 minute or more, the carbonization yield can be improved. By setting the heating time in the flame-proofing treatment to 4 hours or less, the production cost can be reduced.

[0027] The flame-retardant treatment is preferably a treatment in which the carbon fiber precursor fiber is heated in an oxidizing atmosphere at a temperature in the range of 120°C to 500°C. A temperature of 120°C or higher can promote a dehydration reaction after at least one of a hydroxyl group and a carbonyl group (C=O) is formed in the chemical structure of the molecular chain that constitutes the carbon fiber precursor fiber, improving the heat resistance of the flame-retardant fiber and reducing the fusion rate. A temperature of 500°C or lower can suppress thermal decomposition in an oxidizing atmosphere.

[0028] The flame retardant treatment is carried out by adjusting the ratio of the flame retardant fiber (I Ａ / I ＢIt is preferable that the carbon fiber precursor fiber is heated in an oxidizing atmosphere so that the ratio (I Ａ / I Ｂ ) is found at 1715 cm in the infrared absorption spectrum of the flame-resistant fiber. －１ ~1730cm －１ The intensity of the absorption peak B of the C=O stretching vibration of the carbonyl group (I Ｂ ) to 1600 cm －１ ~1620cm －１ The intensity of the absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure (I Ａ This improves the carbonization resistance, carbonization yield, and tensile strength of the carbon fiber. Ａ / I Ｂ From the viewpoint of improving the carbonization resistance, the ratio (I) is more preferably 0.60 or more, even more preferably 0.65 or more, particularly preferably 0.70 or more, and most preferably 0.75 or more. Ａ / I Ｂ The ratio (I) may be 0.9 or greater, 1.0 or greater, 1.1 or greater, or greater than 1.1. Ａ / I Ｂ From the viewpoint of improving carbonization resistance and tensile strength, the ratio (I) is more preferably 4.8 or less, even more preferably 4.6 or less, particularly preferably 4.5 or less, and most preferably 4.0 or less. Ａ / I Ｂ The ratio (I) may be 2.0 or less, may be 1.6 or less, or may be 1.4 or less. Ａ / I Ｂ ) may be 1.2 to 1.6.

[0029] 1600 cm －１ ~1620cm －１ The C═C stretching vibration of the six-membered carbon ring structure confirmed in is the carbon double bond (C═C) of the diene polymer (infrared absorption frequency: 1640 cm －１The carbon-carbon double bond is not derived from a carbon-carbon double bond (near the carbon-carbon double bond) but from a carbon-carbon double bond in the structure of a flame-resistant fiber obtained by crosslinking or flame-resistant treatment of a diene polymer. Examples of the carbon-carbon double bond in a flame-resistant fiber include carbon-carbon double bonds formed by a dehydration reaction originating from a carbonyl group introduced by oxidation into a six-membered carbon ring generated by the flame-resistant treatment or a hydroxyl group bonded by oxidation to a six-membered carbon ring, and carbon-carbon double bonds formed by a dehydrogenation reaction of hydrogen in a six-membered carbon ring generated by the flame-resistant treatment.

[0030] (1.1.1.2) Drawing Treatment In the drawing treatment, tension is applied to the carbon fiber precursor fiber.

[0031] The stretching ratio during the flame-proofing treatment is 0.9 times or more, preferably 0.95 times or more, more preferably 1.0 times or more, even more preferably 1.05 times or more, particularly preferably 1.1 times or more, and most preferably 1.2 times or more. The stretching ratio during the flame-proofing treatment may be 1.3 times or more, may be 1.4 times or more, may be 1.5 times or more, may be 1.7 times or more, may be 1.8 times or more, may be 2.8 times or more, or may be 4.0 times or more. The stretching ratio during the flame-proofing treatment is 100 times or less, preferably 50 times or less, more preferably 30 times or less, even more preferably 20 times or less, particularly preferably 19.0 times or less, and most preferably 17.0 times or less. The stretching ratio during the flame-proofing treatment may be 15.0 times or less, may be 13.0 times or less, or may be 9.0 times or less. The draw ratio during the flame-retardant treatment may be 1.8 to 3.0 times, 4.0 to 16.0 times, 4.0 to 7.0 times, or 4.0 to 6.0 times. If the draw ratio is less than 0.9 times, the molecular chains constituting the flame-retardant fiber will not be sufficiently oriented in the fiber axis direction, which may increase the fusion rate of the flame-retardant fiber or cause breakage of some of the flame-retardant fibers among the plurality of flame-retardant fibers (i.e., reduce resistance to the flame-retardant treatment). If the draw ratio is more than 100 times, damage may be caused to the carbon fiber precursor fiber during the drawing treatment, which may cause breakage of some of the flame-retardant fibers among the plurality of flame-retardant fibers (i.e., reduce resistance to the flame-retardant treatment) or reduce resistance to the carbonization treatment.

[0032] The method of the drawing treatment is not particularly limited and is appropriately selected depending on the method of carrying out the flame-proofing treatment. When the method of carrying out the flame-proofing treatment is continuous, the drawing treatment may be carried out by adjusting the roller speed (Va) at the inlet of the heat treatment device and the roller speed (Vb) at the outlet of the heat treatment device. In addition, the carbon fiber precursor fiber may be drawn by applying an appropriate tension to the carbon fiber precursor fiber using a weight, a spring, an air cylinder, hydraulic pressure, or the like.

[0033] During the temperature rise process up to the maximum temperature in the flame retardant treatment, tension may or may not be applied to the carbon fiber precursor fiber. From the viewpoint of fully obtaining the effect of applying tension, tension is preferably applied also during the temperature rise process. Tension may be applied from the initial stage of the temperature rise process, or from an intermediate stage.

[0034] The drawing treatment is preferably a treatment of drawing the carbon fiber precursor fiber so that the average fiber diameter of the flame-resistant fiber becomes 10 nm to 40 μm, thereby reducing the fusion rate of the flame-resistant fiber and improving the carbonization resistance and strength of the carbon fiber.

[0035] (1.1.1.3) Carbon fiber precursor fiber Carbon fiber precursor fiber is a fiber produced by fiberizing a diene polymer composition. The diene polymer composition contains a diene polymer (a). The carbon fiber precursor fiber may be a monofilament. A plurality of carbon fiber precursor fibers may be used as a fiber bundle (hereinafter, a fiber bundle of carbon fiber precursor fibers may also be referred to as a "carbon fiber precursor fiber bundle"). The carbon fiber precursor fiber is preferably used for structural members in aerospace applications, automotive applications, building materials, and the like. From the viewpoint of exhibiting high mechanical properties, the carbon fiber is preferably a carbon fiber precursor fiber bundle.

[0036] In the carbon fiber precursor fiber bundle, the number of single fibers (i.e., carbon fiber precursor fibers) per bundle is not particularly limited, but from the viewpoints of the productivity and mechanical properties of the flame-resistant fiber and carbon fiber, 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. By setting the number of single fibers per bundle to 360,000 or less, it is possible to suppress the occurrence of uneven sintering during the flame-resistant treatment and carbonization treatment. In order to increase the number of single fibers per bundle, it is preferable to bundle multiple bundles made of multiple single fibers. From the viewpoint of reducing production costs, it is preferable to spin a large number of fibers at one time. In melt spinning, the number of holes in the nozzle may be 1 hole, preferably 10 holes or more, more preferably 20 holes or more, even more preferably 30 holes or more, particularly preferably 36 holes or more, and most preferably 50 holes or more.

[0037] The fineness of the carbon fiber precursor fiber is not particularly limited, but is preferably 1 × 10 －８ tex / thread to 100 tex / thread is preferable, and 1×10 －６ The fineness of the carbon fiber precursor fiber is more preferably 1×10 tex / fiber to 60 tex / fiber, even more preferably 0.001 tex / fiber to 40 tex / fiber, even more preferably 0.01 tex / fiber to 10 tex / fiber, particularly preferably 0.02 tex / fiber to 2 tex / fiber, and most preferably 0.03 tex / fiber to 0.4 tex / fiber. －８ When the fineness of the carbon fiber precursor fiber is 100 tex / fiber or more, thread breakage is unlikely to occur, and stable winding and flame-retardation can be easily performed. When the fineness of the carbon fiber precursor fiber is 100 tex / fiber or less, the difference between the chemical structure near the surface layer of the flame-retardant fiber and the chemical structure near the center of the cross section perpendicular to the fiber axis direction of the flame-retardant fiber (hereinafter simply referred to as the "center") does not become too large, and the tensile modulus of the obtained carbon fiber can be well maintained. The method for measuring the fineness of the carbon fiber precursor fiber is the same as the method described in the Examples.

[0038] The average fiber diameter of the carbon fiber precursor fiber is not particularly limited, but may be 3 nm to 300 μm, 30 nm to 250 μm, 500 nm to 200 μm, 1 μm to 100 μm, 2 μm to 60 μm, 3 μm to 40 μm, 3 μm to 40 μm, 4 μm to 15 μm, 4 μm to 8 μm, or 4 μm to 7 μm. When the average fiber diameter of the carbon fiber precursor fiber (i.e., single fiber) is 3 nm or more, thread breakage is less likely to occur, and stable winding and flame-resistant treatment are easily performed. When the average fiber diameter is 300 μm or less, the difference between the chemical structure near the surface layer of the flame-resistant fiber and the chemical structure near the center of the flame-resistant fiber is not too large, and the tensile modulus of the resulting carbon fiber can be maintained at a good level. The method for measuring the average fiber diameter of the carbon fiber precursor fiber is the same as that described in the Examples.

[0039] Carbon fiber precursor fibers can be obtained by spinning a diene polymer composition. Examples of spinning methods include melt spinning, dry spinning, wet spinning, dry-wet spinning, gel spinning, and electrospinning. From the viewpoint of producing carbon fiber precursor fibers at low cost and under environmentally friendly conditions, melt spinning is preferred.

[0040] (1.1.1.3.1) The diene polymer carbon fiber precursor fiber comprises a diene polymer (a).

[0041] The diene polymer (a) preferably contains a structural unit represented by the following formula (I) (hereinafter also referred to as "structural unit (I)"). By containing the structural unit (I), at least one of an intramolecular cyclization reaction and an intermolecular crosslinking reaction can be promoted by an external stimulus. This suppresses fusion between carbon fiber precursor fibers in the flame-resistant treatment step, and further improves the tensile strength of the resulting carbon fiber. Examples of external stimuli include radiation treatment, electron beam treatment, ultraviolet treatment, heat treatment, acid treatment, addition of a crosslinking agent, and addition of a radical initiator.

[0042] The diene polymer (a) may contain only one type of structural unit represented by formula (I), or may contain two or more types.

[0043]

[0044] In formula (I), R represents a hydrogen atom or an organic group having 1 to 20 carbon atoms, and n represents an integer of 1 or more.

[0045] 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, from the viewpoint of improving the yield of the obtained carbon fiber. Examples of the "organic group" represented by R include hydrocarbon groups. At least a portion of the carbon atoms constituting the hydrocarbon group may be substituted with oxygen atoms, nitrogen atoms, or sulfur atoms. Hydrogen atoms constituting the hydrocarbon group may be substituted with halogen atoms (for example, chlorine atoms, bromine atoms, fluorine atoms, etc.). The hydrocarbon group may be linear, branched, or may contain a ring structure.

[0046] The hydrocarbon group may be an aliphatic hydrocarbon group or an aromatic hydrocarbon group. Among these, 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), such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an 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 o-methyl ... octyl 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.

[0047] From the viewpoint of suppressing thread breakage and fusion of fibers during the flame-proofing step, R is preferably a hydrogen atom or a methyl group.

[0048] Examples of raw materials for producing a diene polymer (a) containing the structural unit (I) 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-heptyl-1,3-butadiene, 2-octyl-1,3-butadiene, 2-phenyl-1,3-butadiene, 2-methoxy-1,3-butadiene, 2-ethoxy-1,3-butadiene, 2-propoxy-1,3-butadiene, 2-isopropoxy-1,3-butadiene, 2-hexyloxy-1,3-butadiene, and myrcene. Only one type of raw material may be used for producing a diene polymer (a) containing the structural unit (I), or two or more types may be used in combination.

[0049] An example of a raw material for producing a diene polymer (a) containing a structural unit (I) in which "R" in formula (I) is a hydrogen atom is 1,3-butadiene. An example of a raw material for producing a diene polymer (a) containing a structural unit (I) in formula (I) in which "R" is a methyl group is isoprene. The raw material for producing a diene polymer (a) containing the structural unit (I) is preferably at least one selected from the group consisting of 1,3-butadiene and isoprene.

[0050] The content of the structural unit (I) is not particularly limited. The content of the structural unit (I) is preferably 1 mol% or more, more preferably 5 mol% or more, even more preferably 10 mol% or more, even more preferably 30 mol% or more, particularly preferably 40 mol% or more, even more particularly preferably 50 mol% or more, particularly preferably 60 mol% or more, and most preferably 70 mol% or more, relative to the total amount of the diene polymer (a). The upper limit of the content of the structural unit (I) is not particularly limited. The content of the structural unit (I) may be 100 mol%.

[0051] The diene polymer (a) may contain, in addition to the structural unit (I), a structural unit derived from another conjugated diene monomer. Examples of the other conjugated diene monomer include 1,2-butadiene, 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.

[0052] The diene polymer (a) may contain structural units derived from other polymerizable monomers, such as 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, and indene; linear olefin monomers such as ethylene, propylene, and 1-butene; cyclic olefin monomers such as cyclopentene and 2-norbornene; non-conjugated diene monomers such as 1,5-hexadiene, 1,6-heptadiene, 1,7-octadiene, dicyclopentadiene, and 5-ethylidene-2-norbornene; Examples of the polymerizable monomer include 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 (meth)acrylonitrile; nitrogen-containing vinyl 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 anhydrides such as maleic anhydride and itaconic anhydride; sulfo group-containing vinyl monomers such as vinyl sulfonic acid; halogenated vinyl monomers such as vinyl chloride; vinyl carboxylates such as vinyl acetate, vinyl butyrate, and vinyl pivalate; and vinyl alcohol. The other polymerizable monomers may be used alone or in combination of two or more.

[0053] In the diene polymer (a), when "R" in formula (I) is a hydrogen atom, the 1,2-bond is not particularly limited, and may contain at least one of a cis-1,4-bond and a trans-1,4-bond. In the diene polymer (a), when "R" in formula (I) is a hydrogen atom, the 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, 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 in the flame-proofing step in which an intramolecular cyclization reaction occurs due to an external stimulus. The 1,2-bond content of the diene polymer (a) may be 100 mol %, but from the viewpoint of reducing the production (polymerization) cost for increasing the 1,2-bond content of the diene polymer (a), it is preferably 99.5 mol % or less, and more preferably 99 mol % or less. The "1,2-bond content" refers to the proportion of 1,2-bonds when the total of the cis-1,4-bonds, trans-1,4-bonds, and 1,2-bonds constituting the diene polymer (a) is taken as 100 mol %. The 1,2-bond content is １ H-nuclear magnetic resonance (NMR) or １３ This can be confirmed by C-NMR.

[0054] In the diene polymer (a), when "R" in formula (I) is a methyl group, the 3,4-bond corresponding to the structural unit (I) is not particularly limited, and may contain at least one selected from the group consisting of a cis-1,4-bond, a trans-1,4-bond, and a 1,2-bond. In the diene polymer (a), when "R" in formula (I) is a methyl group, the 3,4-bond content is preferably 1 mol% or more, more preferably 5 mol% or more, even more preferably 10 mol% or more, even more preferably 30 mol% or more, particularly preferably 50 mol% or 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 in the flame-resistant step in which an intramolecular cyclization reaction occurs due to an external stimulus. When "R" in formula (I) is a methyl group, the 3,4-bond content of the diene polymer (a) may be 100 mol %, but from the viewpoint of reducing the production (polymerization) cost for increasing the 3,4-bond content of the diene polymer (a), it is preferably 99.5 mol % or less, and more preferably 99 mol % or less. The "3,4-bond content" refers to the proportion of 3,4-bonds when the total of the 3,4-bonds, cis-1,4-bonds, trans-1,4-bonds, and 1,2-bonds constituting the diene polymer (a) when "R" in formula (I) is a methyl group is taken as 100 mol %. The 3,4-bond content is １ H-nuclear magnetic resonance (NMR) or １３ This can be confirmed by C-NMR. In the present disclosure, the stereoregularity of the diene polymer (a) is not particularly limited, and may be any of isotactic, syndiotactic, and atactic. The ratio of these is not particularly limited.

[0055] The weight average molecular weight of the diene polymer (a) is usually 10,000 or more, and from the viewpoint of the strength of the carbon fiber precursor fiber and the carbon fiber, it is preferably 15,000 or more, more preferably 20,000 or more, and particularly preferably 25,000 or more.

[0056] (1.1.1.3.2) Polymer other than diene-based polymer The carbon fiber precursor fiber may further contain, in addition to the diene-based polymer (a), a polymer other than a diene-based polymer (hereinafter also referred to as "non-diene-based polymer (b)").

[0057] Examples of the non-diene polymer (b) include olefin polymers, petroleum resins, aromatic vinyl polymers, acrylic polymers (e.g., poly(meth)acrylic acid esters (e.g., polymethyl acrylate and polymethyl methacrylate), poly(meth)acrylic acid, and (meth)acrylic acid ester / (meth)acrylic acid copolymers), polyesters (e.g., polyethylene terephthalate, polybutylene terephthalate, and polylactic acid), polyamides, polyvinylidene chloride, polyphenylene sulfide, polyimides, polycarbonates, and polymers containing vinyl cyanide monomer (e.g., acrylonitrile) units as the main component. Examples of the polymer include polyacrylonitrile-based polymers (e.g., polyacrylonitrile, acrylonitrile / itaconic acid copolymer, and acrylonitrile / methyl acrylate copolymer) containing acrylamide-based monomer units (e.g., acrylamide) as the main component (e.g., polyacrylamide and acrylamide / acrylonitrile copolymer), vinyl alcohol-based polymers (e.g., polyvinyl alcohol and vinyl alcohol / vinyl acetate copolymer) containing vinyl alcohol-based monomers as the main component, and phenol-based polymers (e.g., novolac-type phenolic resin and lignin). One or more of these may be used.

[0058] From the viewpoints of improving the spinnability of the diene polymer composition, suppressing fiber breakage and fiber fusion during the flame-proofing process, and improving the tensile strength of the resulting carbon fiber, it is more preferable that the non-diene polymer (b) contains at least one selected from the group consisting of olefin polymers, petroleum resins, and aromatic vinyl polymers. When the diene polymer (a) is mixed with the non-diene polymer (b) containing at least one selected from the group consisting of olefin polymers, petroleum resins, and aromatic vinyl polymers, fiber breakage during melt spinning (particularly at high speed spinning) is likely to be suppressed, fiber breakage and fiber fusion during the flame-proofing process are likely to be suppressed, and the tensile strength of the resulting carbon fiber is likely to be improved.

[0059] When a carbon fiber precursor fiber containing a non-diene polymer (b) is irradiated with active energy rays (for example, radiation, electron beams, ultraviolet rays, etc.), crosslinking reactions may occur between diene polymers (a), between olefin polymers, between petroleum resins, between aromatic vinyl polymers, between diene polymers (a) and olefin polymers, between diene polymers (a) and petroleum resins, and between diene polymers (a) and aromatic vinyl polymers. These crosslinking reactions tend to suppress thread breakage and fiber fusion during the flame-resistant treatment step, and also tend to improve the tensile strength of the resulting carbon fiber.

[0060] The olefin-based polymer and the aromatic vinyl-based polymer may be either linear or branched. The olefin-based polymer may be a hydrogenated olefin-based polymer obtained by hydrogenation. The aromatic vinyl-based polymer may be a hydrogenated aromatic vinyl-based polymer obtained by hydrogenation.

[0061] (1.1.1.3.2.1) Olefin-Based Polymer The olefin-based polymer is not particularly limited, and examples thereof include homopolymers and copolymers of olefin-based monomers.

[0062] Examples of olefin-based monomers include ethylene, propylene, 1-butene, cis-2-butene, trans-2-butene, isobutene, 1-pentene, 2-pentene, 2-methyl-1-butene, 2-methyl-2-butene, 3-methyl-1-butene, 2,3-dimethyl-2-butene, 1-butene, 1-hexene, 1-octene, 1-nonene, 1-decene, cyclopentene, and 2-norbornene. One type of olefin-based monomer may be used alone, or two or more types may be used in combination.

[0063] The weight average molecular weight of the olefin polymer is not particularly limited, but from the viewpoint of improving spinnability, it is preferably 150,000 or less, more preferably 100,000 or less, even more preferably 20,000 or less, particularly preferably 10,000 or less, and most preferably 9,000 or less.

[0064] (1.1.1.3.2.2) Petroleum Resins Examples of petroleum resins include copolymerized petroleum resins of each fraction (e.g., C5 petroleum resins, C9 petroleum resins, and C5 / C9 petroleum resins), alicyclic (dicyclopentadiene) petroleum resins, and hydrogenated petroleum resins obtained by hydrogenating these petroleum resins (e.g., partially hydrogenated petroleum resins and fully hydrogenated petroleum resins).

[0065] "C5 petroleum resin" is a petroleum resin made from the C5 fraction of naphtha. "C9 petroleum resin" is a petroleum resin made from the C9 fraction of naphtha. "C5 / C9 petroleum resin" is a petroleum resin made from the C5 fraction and C9 fraction of naphtha.

[0066] The C5 fraction and the C9 fraction also include their analogs. Examples of the C5 fraction include 1,3-pentadiene, 2-methyl-2-butene, cyclopentadiene, methylcyclopentadiene, dimethylcyclopentadiene, isoprene, and pentane. Examples of the C9 fraction include styrene, methylstyrene, vinyltoluene, ethylstyrene, dimethylstyrene, indene, and methylindene. As the C5 petroleum resin and the C5 / C9 petroleum resin, those containing a dicyclopentadiene (DCPD) skeleton derived from cyclopentadiene, a type of C5 fraction, are preferred.

[0067] (1.1.1.3.2.3) Aromatic vinyl polymer The aromatic vinyl polymer is not particularly limited, and examples thereof include 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, and indene. Aromatic vinyl monomers may be used alone or in combination of two or more.

[0068] Examples of aromatic vinyl polymers include styrene-butadiene-styrene block copolymers (SBS), styrene-isoprene-styrene block copolymers (SIS), acrylonitrile-butadiene-styrene (ABS) resins, methyl (meth)acrylate-acrylonitrile-butadiene-styrene (MABS) resins, methyl (meth)acrylate-butadiene-styrene (MBS), acrylonitrile-ethylene propylene rubber-styrene (AES) resins, and acrylonitrile-acrylic rubber-styrene (AAS) resins.

[0069] (1.1.1.3.2.4) Blending ratio In the diene polymer composition, the blending ratio (mass ratio) of the diene polymer (a) to the non-diene polymer (b) (diene polymer (a):non-diene polymer (b)) may be 99.5:0.5 to 10:90, preferably 99.0:1.0 to 25.0:75.0, more preferably 98.0:2.0 to 50.0:50.0, even more preferably 97.0:3.0 to 50.0:50.0, particularly preferably 97.0:3.0 to 60.0:40.0, and most preferably 97.0:3.0 to 70.0:30.0. When the blending ratio (mass ratio) of the diene polymer (a) to the non-diene polymer (b) is 99.5:0.5 to 10.0:90.0, at least a portion of the non-diene polymer (b) is likely to form a dispersed phase, or the diene polymer (a) and the non-diene polymer (b) are likely to be compatible with each other. The proportion of the diene polymer (a) to the diene polymer (a) and the non-diene polymer (b) may be 89% by mass to 93% by mass, or may be 82% by mass to 89% by mass.

[0070] In the carbon fiber precursor fiber, the blending ratio (volume ratio) of the diene polymer (a) to the non-diene polymer (b) (diene polymer (a):non-diene polymer (b)) may be 99.5:0.5 to 10.0:90.0, preferably 99.0:1.0 to 25.0:75.0, more preferably 98.0:2.0 to 50.0:50.0, even more preferably 97.0:3.0 to 50.0:50.0, particularly preferably 97.0:3.0 to 60.0:40.0, and most preferably 97.0:3.0 to 70.0:30.0. When the blending ratio (volume ratio) of the diene polymer (a) to the non-diene polymer (b) is 99.5:0.5 to 10.0:90.0, at least a portion of the non-diene polymer (b) is likely to form a dispersed phase, or the diene polymer (a) and the non-diene polymer (b) are likely to be compatible with each other. The ratio of the diene polymer (a) to the diene polymer (a) and the non-diene polymer (b) may be 89% by volume to 93% by volume, or may be 82% by volume to 89% by volume.

[0071] The phase structure of the diene polymer composition and the carbon fiber precursor fiber may be a compatible structure or a sea-island structure. In the diene polymer composition and the carbon fiber precursor fiber, it is preferred that at least a portion of the non-diene polymer (b) forms a dispersed phase, or that the diene polymer (a) and the non-diene polymer (b) are compatible with each other. This makes it easier to draw the fiber bundle during the flame-retardant treatment. The temperature during the flame-retardant treatment while drawing is preferably 150°C to 500°C, more preferably 180°C to 490°C, even more preferably 200°C to 480°C, and particularly preferably 220°C to 450°C.

[0072] When at least a portion of the non-diene polymer (b) forms a dispersed phase, the shape of the dispersed phase is not particularly limited and may be a shape other than spherical (such as an oblate spheroid, a striated shape, or other irregular shapes). The average particle size of the dispersed phase is not particularly limited, but is preferably 1 μm or less, more preferably 900 nm or less, even more preferably 500 nm or less, particularly preferably 200 nm or less, and most preferably 100 nm or less. The average particle size of the dispersed phase is preferably 1 nm or more, more preferably 3 nm or more, even more preferably 5 nm or more, particularly preferably 7 nm or more, and most preferably 10 nm or more. The average particle size of the dispersed phase may be 75 μm to 88 μm, or may be 89 μm to 94 μm.

[0073] The average particle size of the dispersed phase may be measured as follows. An ultrathin section of a cross section perpendicular to the fiber axis direction of a carbon fiber precursor fiber made of a diene polymer composition is prepared, and the phase structure is observed using a transmission electron microscope (e.g., H-7650 manufactured by Hitachi High-Technologies Corporation). When a dispersed phase containing a non-diene polymer (b) is dispersed in a diene polymer (a), 20 dispersed phases are randomly selected, the primary particle sizes of the dispersed phases are measured, and the average value is taken as the average particle size of the dispersed phase.

[0074] (1.1.1.3.3) Photopolymerization initiator The carbon fiber precursor fiber may further contain a photopolymerization initiator (hereinafter also referred to as "photopolymerization initiator (c)") in addition to the diene polymer (a), or may not contain the photopolymerization initiator (c).

[0075] Examples of the photopolymerization initiator (c) include methyl 2-benzoylbenzoate, benzophenone, 4,4'-bis(dimethylamino)benzophenone, 4-benzoyl-4'-methyldiphenyl sulfide, acetophenone, diethoxyacetophenone, isopropylthioxanthone, diethylthioxanthone, ethyl-4-(diethylamino)benzoate, benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropyl ether, benzoin isobutyl ether, benzoin benzoic acid, benzoin dimethyl ketal, 2,4-diethylthioxanthone, 1-hydroxycyclohexyl phenyl ketone, 2- Examples of the photopolymerization initiator (c) include methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1,2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butanone-1,2-hydroxy-2-methyl-1-phenylpropan-1-one, 1-hydroxycyclohexyl phenyl ketone, alkylphenyl glyoxylate, benzyl diphenyl sulfide, tetramethylthiuram monosulfide, azobisisobutyronitrile, 2-chloroanthraquinone, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide, and bis(2,4,6-trimethylbenzoyl)-phenyl-phosphine oxide. Only one type of photopolymerization initiator (c) may be used, or two or more types may be used in combination.

[0076] When the carbon fiber precursor fiber contains a photopolymerization initiator (c), the content of the photopolymerization initiator (c) is not particularly limited, and is preferably 0.05 to 5 parts by mass, and more preferably 0.1 to 3 parts by mass, relative to 100 parts by mass of the total of the diene polymer (a) and the non-diene polymer (b). From the viewpoint of improving spinnability, the content of the photopolymerization initiator (c) is further preferably 0.3 parts by mass or more, and particularly preferably 0.5 parts by mass or more, relative to 100 parts by mass of the resin component of the carbon fiber precursor fiber.

[0077] (1.1.1.3.4) Additives The carbon fiber precursor fiber may contain additives other than the photopolymerization initiator (c) to the extent that the effects of the present disclosure are not impaired. Examples of such additives include antioxidants, release agents, lubricants, plasticizers, colorants, crosslinking aids (e.g., ultraviolet crosslinking aids), crosslinking catalysts (e.g., acid catalysts and base catalysts), crosslinking retarders, reinforcing materials (e.g., graphite, carbon nanotubes, graphene, cellulose, cellulose nanofibers, carbon black, boron nitride, boron nitride nanotubes, boron nitride nanosheets, glass fibers, and metal fibers), ultraviolet absorbers, light shielding agents, light stabilizers, antistatic agents, and compatibilizers.

[0078] When the carbon fiber precursor fiber contains a release agent, the release agent may be a known agent (for example, carnauba wax). The content of the release agent is not particularly limited, and is preferably 0.05 to 5 parts by mass, and more preferably 0.1 to 3 parts by mass, per 100 parts by mass of the diene polymer (a) and the non-diene polymer (b) combined. From the viewpoint of improving spinnability, the content of the release agent is more preferably 0.3 parts by mass or more, and particularly preferably 0.5 parts by mass or more, per 100 parts by mass of the diene polymer (a).

[0079] When the carbon fiber precursor fiber contains a plasticizer, the content of the plasticizer is not particularly limited, and is preferably 0.1 parts by mass to 100 parts by mass relative to 100 parts by mass of the diene polymer (a) and the non-diene polymer (b). From the viewpoint of improving the spinnability and drawability of the carbon fiber precursor fiber and suppressing fusion during flame retardation, the content of the plasticizer is preferably 0.5 parts by mass to 70 parts by mass, more preferably 1 part by mass to 60 parts by mass, even more preferably 2 parts by mass to 50 parts by mass, even more preferably 3 parts by mass to 40 parts by mass, particularly preferably 4 parts by mass to 30 parts by mass, and most preferably 4 parts by mass to 20 parts by mass relative to 100 parts by mass of the diene polymer (a) and the non-diene polymer (b).

[0080] (1.1.1.4) The crosslinked diene polymer precursor fiber for carbon fiber preferably contains a crosslinked diene polymer (hereinafter also referred to as "crosslinked diene polymer (a)") obtained by crosslinking a diene polymer (a). This suppresses yarn breakage and fiber fusion during the flame-proofing process. In addition, the tensile strength of the resulting carbon fiber is improved.

[0081] The "crosslinked diene polymer" has a structure formed by at least one of an intramolecular crosslinking reaction (including an intramolecular cyclization reaction between adjacent vinyl groups) and an intermolecular crosslinking reaction.

[0082] Hereinafter, the carbon fiber precursor fiber containing the crosslinked diene polymer (a) is also referred to as a "crosslinked diene polymer fiber."

[0083] The carbon fiber precursor fiber preferably contains a crosslinked diene polymer (a) and has a gel fraction of 30% or more, more preferably 50% or more, even more preferably 60% or more, particularly preferably 70% or more, and most preferably 80% or more. When the crosslinking treatment of active energy ray irradiation and heat treatment is carried out in air, oxygen radicals are introduced into the chemical structure of the crosslinked diene polymer (a) (such as the α-position of the vinyl group) to form carbonyl groups or hydroxyl groups. Furthermore, a cyclic structure may be formed by an intramolecular cyclization reaction between adjacent vinyl groups (a polymerization reaction of vinyl groups when the structural unit represented by the formula (I) is introduced consecutively (as a chain) as repeating units). In the infrared absorption spectrum of the flame-resistant fiber, －１ ~1620cm －１ The C=C stretching vibration of the six-membered carbon ring structure confirmed in is presumed to be derived from the carbon double bond obtained by flame retardation treatment of the structure formed by at least one of an intramolecular crosslinking reaction (including an intramolecular cyclization reaction between adjacent vinyl groups) and an intermolecular crosslinking reaction of the crosslinked diene polymer (a), and the carbon double bond is generated by the dehydration reaction of the carbonyl group and the hydroxyl group introduced into or bonded to the six-membered ring structure during the flame retardation treatment. －１ ~1620cm －１The C=C bond of the six-membered carbon ring structure confirmed in －１ ~1620cm －１ It is presumed that the peak of the C═C stretching vibration of the six-membered carbon ring structure confirmed in

[0084] The content of the crosslinked diene polymer (a) in the carbon fiber precursor fiber is not particularly limited, but is preferably 30% by mass or more, more preferably 40% by mass or more, even more preferably 50% by mass or more, even more preferably 60% by mass or more, particularly preferably 70% by mass or more, especially preferably 80% by mass or more, and most preferably 90% by mass or more, relative to the total mass of the carbon fiber precursor fiber. The content of the crosslinked diene polymer (a) is not particularly limited. The content of the crosslinked diene polymer (a) may be 100% by mass. As a method for confirming that the carbon fiber precursor fiber contains the crosslinked diene polymer (a), １ H-nuclear magnetic resonance spectroscopy (NMR) and １３ Examples of the methods include C-NMR analysis, infrared spectroscopic analysis, and measurement of gel fraction. －１ ~1645cm －１ At 1445 cm, there is a peak due to the C=C stretching vibration of the diene. －１ ~1460cm －１ Furthermore, when crosslinking is carried out in the presence of oxygen, it is preferable that oxygen radicals are added to the diene polymer or the crosslinked diene polymer. In this case, the crosslinked diene polymer (a) has a peak at 1715 cm in its infrared absorption spectrum. －１ ~1730cm －１ The C═O bond of the carbonyl group contributes to a dehydration reaction during flame retardation and pre-carbonization, leading to the formation of a C═C bond and the expansion of a cyclic structure.

[0085] From the viewpoint of suppressing thread breakage and fiber fusion in the flame-proofing step and from the viewpoint of suppressing fiber breakage in the carbonization step, the gel fraction is preferably 30% or more, more preferably 40% or more, even more preferably 50% or more, even more preferably 60% or more, particularly preferably 80% or more, and most preferably 90% or more. The gel fraction is not particularly limited, but is preferably 100% or less, and from the viewpoint of reducing the cost of the crosslinking process, it is more preferably 99.9% or less.

[0086] The crosslinked diene polymer (a) can be obtained by subjecting a carbon fiber precursor fiber made of the diene polymer (a) to a crosslinking treatment, which is at least one of irradiation with active energy rays and heat treatment.

[0087] Examples of active energy rays include X-rays, γ-rays, α-rays, β-rays, electron beams, neutron beams, proton beams, heavy particle beams, ultraviolet rays, infrared rays, and visible light. Among these, electron beams or ultraviolet rays are preferred from the viewpoints of easily adjusting the gel fraction within a specific range and suppressing thread breakage and fusion of fibers in the flame-stabilizing step, and electron beams are more preferred from the viewpoint of improving the tensile strength of the resulting carbon fiber.

[0088] In the case of irradiating with an electron beam, the dose of the electron beam to be irradiated is, from the viewpoint of increasing the gel fraction, preferably 20 kGy or more, more preferably 30 kGy or more, even 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. The dose is not particularly limited, but, from the viewpoint of reducing energy costs and reducing damage to the carbon fiber precursor fiber, it 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.

[0089] When irradiating with ultraviolet light, the illuminance of the ultraviolet light is preferably 5 mW / cm ２ ~1000mW / cm ２, more preferably 10 mW / cm ２ ~900mW / cm ２ , more preferably 50 mW / cm ２ ~800mW / cm ２ The ultraviolet irradiation time is not particularly limited, but from the viewpoint of reducing production energy, it is preferably 6 hours or less, more preferably 4 hours or less, even more preferably 2 hours or less, particularly preferably 1 hour or less, and most preferably 20 minutes or less. The ultraviolet irradiation time depends on the output of the device, but is preferably 1 second or more, more preferably 5 seconds or more, even more preferably 10 seconds or more, and particularly preferably 30 seconds or more. By including the non-diene polymer (b), the carbon fiber precursor fiber can preferably exhibit the effects of the present disclosure even with a short ultraviolet irradiation time.

[0090] When irradiating the carbon fiber precursor fiber with at least one of an electron beam and ultraviolet light, tension may be applied. The tension may be, for example, 0.03 mN / dtex or more, preferably 0.05 mN / dtex or more, 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. The tension may be 1.5 mN / dtex or less, or 1.0 mN / dtex or less. By setting the tension to 0.03 mN / dtex or more, at least one of an intramolecular crosslinking reaction and an intermolecular crosslinking reaction can proceed while the molecular chains of the carbon fiber precursor fiber are oriented in the fiber axis direction, and the degree of crosslinking (gel fraction) tends to increase. When irradiating the carbon fiber precursor fiber with at least one of an electron beam and ultraviolet light, the tension is not particularly limited, but from the viewpoint of suppressing fiber breakage, it is preferably 20 mN / dtex or less, more preferably 15 mN / dtex or less, and even more preferably 10 mN / dtex or less.

[0091] (1.1.1.5) Oil Agent The carbon fiber precursor fiber may be coated with an oil agent (for example, a silicone-based oil agent) from the viewpoint of improving fiber bundling and handling, and preventing adhesion of fibers to each other.

[0092] (1.1.2) Spinning step The method for producing an flame-resistant fiber according to the first embodiment may further include a spinning step. In the spinning step, the diene polymer composition described above is spun to produce a carbon fiber precursor fiber containing a diene polymer. The spinning step is performed before the flame-resistant step.

[0093] The spinning method may be a known method. Examples of spinning methods using a melt of a diene polymer composition include melt spinning, spunbond spinning, meltblown spinning, and centrifugal spinning. Examples of spinning methods using a solution obtained by dissolving a diene polymer composition in a solvent include dry spinning, wet spinning, dry-wet spinning, gel spinning, flash spinning, centrifugal spinning, and electrospinning. From the viewpoint of producing a carbon fiber precursor fiber at low cost and under environmentally friendly conditions, the spinning method is preferably melt spinning, spunbond spinning, meltblown spinning, or centrifugal spinning.

[0094] The average fiber diameter of the carbon fiber precursor fiber obtained in the spinning step is not particularly limited, and may be 3 nm to 300 μm, 500 nm to 200 μm, 1 μm to 100 μm, 3 μm to 40 μm, 10 μm to 20 μm, or 20 μm to 40 μm. The method for measuring the average fiber diameter of the carbon fiber precursor fiber obtained in the spinning step is the same as the method described in the Examples.

[0095] (1.1.3) Drawing Step The method for producing an oxidation-resistant fiber according to the first embodiment may further include a drawing step. In the drawing step, from the viewpoints of preventing the carbon fiber precursor fiber from melting and breaking during drawing and increasing the draw ratio, the drawing step is preferably performed at a temperature of -50°C to 200°C, more preferably at a temperature of 0°C to 150°C, and even more preferably at a temperature of 10°C to 120°C. The carbon fiber precursor fiber may also be subjected to a drawing treatment at room temperature (23°C). The draw ratio (hereinafter also referred to as the "draw ratio after spinning") is preferably 1.0 to 100, more preferably 1.1 to 50.0. The drawing step is performed before the oxidation step. When the method for producing an oxidation-resistant fiber according to the first embodiment includes a spinning step, the drawing step may be performed after the spinning step.

[0096] Since the method for producing an oxidation-resistant fiber according to the first embodiment includes a drawing step, the method for producing an oxidation-resistant fiber can produce an oxidation-resistant fiber that can be made into a carbon fiber having superior tensile strength.

[0097] The draw ratio after spinning is preferably 1.05 times or more, more preferably 1.1 times or more, from the viewpoint of improving the tensile strength of the carbon fiber. The draw ratio after spinning may be 1.2 times or more, or may be 1.5 times or more. From the viewpoint of preventing fiber breakage during the drawing treatment of the carbon fiber precursor fiber, the draw ratio after spinning is preferably 30.0 times or less, more preferably 20.0 times or less, and even more preferably 10.0 times or less. The draw ratio after spinning may be 8.0 times or less, 7.0 times or less, or 6.0 times or less. The draw ratio after spinning may be 1.4 times to 5.0 times, 2.0 times to 5.0 times, 3.0 times to 4.0 times, or 1.05 times to 4.0 times.

[0098] The stretching method in the stretching step may be the same as that exemplified as the stretching method in the flame-proofing step.

[0099] The average fiber diameter of the carbon fiber precursor fiber after the drawing treatment is not particularly limited, and may be 3 nm to 300 μm, 3 μm to 40 μm, 10 μm to 18 μm, or 18 μm to 35 μm. The method for measuring the average fiber diameter of the carbon fiber precursor fiber after the drawing treatment is the same as the method described in the Examples.

[0100] (1.1.4) Crosslinking step The method for producing an oxidized fiber according to the first embodiment may further include a crosslinking step. In the crosslinking step, a carbon fiber precursor fiber is subjected to a crosslinking treatment. This results in a carbon fiber precursor fiber containing the above-described crosslinked diene polymer (a). The crosslinking step is carried out before the oxidization step. The crosslinking step may be carried out after the drawing treatment. When the method for producing an oxidized fiber according to the first embodiment includes a spinning step and a drawing step, the crosslinking step may be carried out after the drawing treatment. When the method for producing an oxidized fiber according to the first embodiment includes a spinning step, the crosslinking step may be carried out after the spinning treatment.

[0101] Examples of the crosslinking method include the same as those exemplified as the crosslinking method in the flame-stabilizing step. In the crosslinking step, the crosslinking may be performed while the carbon fiber precursor fiber is being stretched, or the carbon fiber precursor fiber may be crosslinked without being stretched. Examples of the stretching method in the crosslinking step include the same as those exemplified as the stretching method in the flame-stabilizing step. Examples of the tension when the carbon fiber precursor fiber is stretched in the crosslinking step include the same as those exemplified as the tension in the flame-stabilizing step.

[0102] (1.2) Flame-resistant fiber In the first embodiment, the flame-resistant fiber is produced by the flame-resistant fiber production method of the first embodiment. The flame-resistant fiber includes a structure derived from a diene polymer (a). The diene polymer (a) is preferably a crosslinked diene polymer (a) including a structural unit represented by the above formula (I). Details of the diene polymer (a) including a structural unit represented by formula (I) are as described above.

[0103] The presence of a structure derived from the diene polymer (a) in the flame-resistant fiber can be confirmed by infrared spectroscopy, solid-state NMR, elemental analysis, or the like. The structure derived from the diene polymer (a) is, for example, a polycyclic structure formed by condensation of multiple rings after at least one of intramolecular cyclization, intermolecular crosslinking, and oxidation has progressed in the diene polymer (a). The polycyclic structure preferably includes at least one of a structure having an oxygen-containing substituent (e.g., a carbonyl group, a hydroxyl group, etc.) introduced or bonded by intermolecular crosslinking and oxidation during flame-resistant treatment after flame-resistant treatment, and a conjugated structure in which a double bond of carbon atoms is formed. The polycyclic structure more preferably includes a carbonyl group and a conjugated structure in which a double bond of carbon atoms is formed. In the flame-resistant fiber of the present disclosure, the stretching vibration of the carbonyl group in the infrared absorption spectrum is 1715 cm －１ ~1730cm －１ The stretching vibration of C═C in the conjugated structure (preferably a conjugated ring structure) of the carbon atom is 1600 cm －１ ~1620cm －１ Here, the stretching vibration of the double bond of the carbon atom of the diene of the diene polymer (a) is 1640 cm －１ Therefore, it is possible to distinguish between the double bond of the carbon atom in the six-membered carbon ring structure introduced after the flame retardant treatment and the double bond of the carbon atom of the diene in the diene polymer (a).

[0104] From the viewpoint of suppressing breakage of the fibers during carbonization, the fusion rate of the flame-resistant fiber is preferably 15% or less, more preferably 12% or less, even more preferably 10% or less, particularly preferably 6% or less, and most preferably 0%. The method for measuring the fusion rate is the same as that described in the examples.

[0105] The average fiber diameter of the flame-resistant fiber is not particularly limited, but is preferably 10 nm to 40 μm, more preferably 30 nm to 30 μm, even more preferably 100 nm to 20 μm, and particularly preferably 1 μm to 10 μm. When the average fiber diameter of the flame-resistant fiber is 10 nm or more, the carbonization treatment resistance is improved. When the average fiber diameter of the flame-resistant fiber is 40 μm or less, the carbonization treatment resistance and the tensile strength of the carbon fiber are improved. The method for measuring the average fiber diameter of the flame-resistant fiber is the same as the method described in the examples.

[0106] The flame-retardant fiber has a ratio (I Ａ / I Ｂ ) is preferably in the range of 0.5 to 5.0. Ａ / I Ｂ ) has a peak at 1715 cm in the infrared absorption spectrum. －１ ~1730cm －１ The intensity of the absorption peak B of the C=O stretching vibration of the carbonyl group (I Ｂ ) to 1600 cm －１ ~1620cm －１ The intensity of the absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure (I Ａ ) ratio. Ａ / I Ｂ When the ratio (I) is within the range of 0.5 to 5.0, the carbonization resistance and the tensile strength of the carbon fiber are improved. Ａ / I Ｂ From the viewpoint of improving the resistance to carbonization treatment, the ratio (I) is preferably 0.6 or more, more preferably 0.65 or more, particularly preferably 0.7 or more, and most preferably 0.75 or more. Ａ / I Ｂ From the viewpoint of improving carbonization resistance and tensile strength, the ratio (I) is preferably 4.8 or less, more preferably 4.6 or less, particularly preferably 4.5 or less, and most preferably 4.0 or less. Ａ / I Ｂ ) may be 1.2 to 1.6.

[0107] Ratio (I Ａ / I Ｂ) within the range of 0.5 to 5.0, for example, the stretch ratio during the flame retardant treatment may be set within the range of 0.9 to 100 times, and the treatment temperature during the flame retardant treatment may be set within the range of 130°C to 500°C.

[0108] (1.3) Manufacturing Method of Carbon Fiber The manufacturing method of carbon fiber includes manufacturing a flame-resistant fiber by the manufacturing method of a flame-resistant fiber (i.e., a flame-resistant step), and subjecting the flame-resistant fiber to a carbonization treatment (hereinafter also referred to as a "carbonization step").

[0109] The term "carbonization treatment" refers to a treatment in which a carbon precursor fiber or a flame-resistant fiber is heated in a low-oxygen atmosphere (preferably an oxygen-blocked environment) to carbonize the fiber.

[0110] (1.3.1) Carbonization step In the carbonization step, the flame-resistant fiber is preferably heated in an inert atmosphere (in an inert gas such as nitrogen, argon, or helium) at a temperature higher than that in the flame-resistant step. This carbonizes the flame-resistant fiber, resulting in the desired carbon fiber.

[0111] The heating temperature in the carbonization step 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. The heating temperature in the carbonization step is preferably 3000° C. or lower, more preferably 2500° C. or lower.

[0112] The heating time in the carbonization step is not particularly limited, but is preferably 30 seconds to 240 minutes, more preferably 30 seconds to 60 minutes, and even more preferably 1 minute to 30 minutes. From the viewpoint of reducing production costs, the heating time is further preferably 20 minutes or less, and particularly preferably 10 minutes or less.

[0113] The carbonization step may generally include a "graphitization step" performed by heating at 2000°C to 3000°C in an inert gas atmosphere. In the carbonization step, the flame-resistant fiber may first be heated at a temperature less than 1000°C (hereinafter also referred to as a "pre-carbonization step"), and then heated at a temperature of 1000°C or higher. In the carbonization step, the flame-resistant fiber may be heated at a temperature less than 1000°C, then heated at a temperature of 1000°C or higher, and then further heated at a temperature of 2000°C or higher. In the carbonization step of the present disclosure, heating may be performed multiple times.

[0114] (1.3.2) Carbon Fibers The average fiber diameter of carbon fibers (i.e., single fibers) 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, even more preferably 1 μm to 40 μm, particularly preferably 2 μm to 30 μm, and most preferably 2.5 μm to 25 μm. The average fiber diameter of carbon fibers may be 2.5 μm to 10 μm, 2.5 μm to 6 μm, or 2.5 μm to 5 μm. When the average fiber diameter of carbon fibers (i.e., single fibers) is 3 nm to 300 μm, when a composite material is produced using a resin or the like as a matrix, even if the viscosity of the matrix is ​​high, insufficient impregnation of the matrix into multiple carbon fibers is unlikely to occur, and the tensile strength of the composite material is unlikely to decrease. In addition, the tensile strength of carbon fibers (i.e., single fibers) tends to be less likely to decrease.

[0115] (2) Second embodiment (2.1) Method for producing flame-resistant fiber The method for producing a flame-resistant fiber according to a second embodiment of the present disclosure is a method for producing a flame-resistant fiber. The method includes subjecting a carbon fiber precursor fiber containing a diene polymer to a flame-resistant treatment while subjecting the fiber to a drawing treatment (hereinafter also referred to as a "flame-resistant step"). In the infrared absorption spectrum of the flame-resistant fiber, －１ ~1620cm －１ There is an absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure, which is confirmed by the above-mentioned flame-resistant treatment. Ａ / I ＢThe carbon fiber precursor fiber is heated in an oxidizing atmosphere so that the ratio (I) falls within the range of 0.5 to 5.0. Ａ / I Ｂ ) is found at 1715 cm in the infrared absorption spectrum of the flame-resistant fiber. －１ ~1730cm －１ The intensity of the absorption peak B of the C=O stretching vibration of the carbonyl group (I Ｂ ) to 1600 cm －１ ~1620cm －１ The intensity of the absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure (I Ａ ) ratio.

[0116] "In the infrared absorption spectrum of the flame-resistant fiber, －１ ~1620cm －１ The presence of an absorption peak A of the C=C stretching vibration of the six-membered carbon ring structure confirmed by the flame retardant treatment indicates that the flame retardant fiber obtained by carrying out the flame retardant treatment has, in its molecular chain, a carbon double bond formed by a dehydration reaction between a carbonyl group and a hydroxyl group that have been introduced or bonded by oxidation to the six-membered carbon ring produced by the flame retardant treatment, and a carbon double bond (C=C) formed by a dehydrogenation reaction of hydrogen introduced into the six-membered carbon ring produced by the flame retardant treatment. －１ ~1620cm －１ The C═C stretching vibration of the six-membered carbon ring structure confirmed in －１ It does not originate from the vicinity.

[0117] The phrase "absorption peak A is present" means that a distinguishable peak is present when the sample preparation conditions and measurement conditions are optimized so that the largest peak intensity in the infrared absorption spectrum is approximately 1 Abs in terms of absorbance (approximately 10% in terms of transmittance).

[0118] The method for producing an oxidation-resistant fiber according to the second embodiment has the above-described configuration, and therefore can produce a carbon fiber having excellent tensile strength and an oxidation-resistant fiber in which fusion between fibers is suppressed. The reason for the above-described effect is presumed to be as follows, but is not limited to this. By performing a drawing treatment during the oxidation-resistant treatment, the molecular chains constituting the carbon fiber precursor fiber are oriented in the fiber axis direction, which is the drawing direction, and the cyclization reaction easily proceeds, and the fiber diameter becomes thinner during the drawing treatment. Therefore, the oxidation reaction is promoted up to the center of the carbon fiber precursor fiber, and the cyclization reaction is suppressed at 1715 cm in the infrared absorption spectrum. －１ ~1730cm －１ A carbonyl group (C=O) is formed in the molecular chain. The dehydration reaction between this carbonyl group and a hydroxyl group occurs within the same molecular chain. In addition, a dehydration reaction between a carbonyl group and a hydroxyl group also occurs between different molecular chains oriented in the fiber axis direction. As a result, a carbon-carbon double bond (C=C) is formed, and a bond length of 1600 cm is obtained. －１ ~1620cm －１ The intensity of the absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure (I Ａ ) increases. As the number of carbon-carbon double bonds (C=C) increases, the molecular chain structure of the flame-resistant fiber becomes rigid, and the number of conjugated structures increases while the molecular chain remains oriented in the fiber axis direction, improving heat resistance during flame-resistant treatment and carbonization treatment and suppressing fusion. In addition, it is estimated that the number of graphite structures (ladder polymers) in the resulting carbon fiber increases, and the graphite crystal size increases, resulting in an increase in tensile strength.

[0119] The method for producing a flame-resistant fiber according to the second embodiment is the same as the method for producing a flame-resistant fiber according to the first embodiment, except for the following points (i) to (iii). Therefore, the method for producing a flame-resistant fiber according to the second embodiment will be explained by citing the explanation for the method for producing a flame-resistant fiber according to the first embodiment, and the explanation will be omitted. (i) The production method according to the second embodiment does not need to include a drawing treatment at a draw ratio of 0.9 to 100 times. (ii) In the infrared absorption spectrum of the flame-resistant fiber according to the second embodiment, －１ ~1620cm －１(iii) In the second embodiment, the flame-resistant treatment is carried out by increasing the ratio (I Ａ / I Ｂ ) is a treatment of heating the carbon fiber precursor fiber in an oxidizing atmosphere so that the value of

[0120] In the second embodiment, the ratio of the flame retardant treatment (I Ａ / I Ｂ From the viewpoint of improving the resistance to carbonization treatment, the ratio (I) is preferably 0.6 or more, more preferably 0.65 or more, even more preferably 0.7 or more, and most preferably 0.75 or more. Ａ / I Ｂ From the viewpoint of improving carbonization resistance and tensile strength, the ratio (I) is preferably 4.8 or less, more preferably 4.6 or less, particularly preferably 4.5 or less, and most preferably 4.0 or less. Ａ / I Ｂ ) may be 1.2 to 1.6.

[0121] In the second embodiment, the stretch ratio during the flame-proofing treatment is preferably 0.9 times or more, more preferably 0.95 times or more, even more preferably 1.0 times or more, particularly preferably 1.05 times or more, and even more preferably 1.1 times or more. The stretch ratio during the flame-proofing treatment may be 1.2 times or more, 1.3 times or more, 1.4 times or more, 1.7 times or more, 1.8 times or more, 2.8 times or more, or 5.8 times or more. The stretch ratio during the flame-proofing treatment is 100 times or less, 9.0 times or less, 5.8 times or less, 3.5 times or less, 3.0 times or less, 2.8 times or less, or 2.5 times or less. The stretch ratio during the flame-proofing treatment may be 1.8 times to 3.0 times, or 5.8 times to 10.0 times.

[0122] (2.2) Flame-resistant fiber In the second embodiment, the flame-resistant fiber is produced by the flame-resistant fiber production method of the second embodiment.

[0123] The flame-resistant fiber of the second embodiment is similar to the flame-resistant fiber of the first embodiment, and therefore the description of the flame-resistant fiber of the first embodiment will be used to describe the flame-resistant fiber of the second embodiment, and further description will be omitted.

[0124] The method for producing the flame-resistant fiber according to the second embodiment may further include at least one of a spinning step, a drawing step, and a crosslinking step. Examples of the spinning step, the drawing step, and the crosslinking step include the same steps as those exemplified in the method for producing the flame-resistant fiber according to the second embodiment.

[0125] (2.3) Method for producing carbon fiber The method for producing carbon fiber includes producing a flame-resistant fiber by the method for producing a flame-resistant fiber (i.e., a flame-resistant step), and subjecting the flame-resistant fiber to a carbonization treatment.

[0126] The method for producing a carbon fiber according to the second embodiment is similar to the method for producing a carbon fiber according to the first embodiment, and therefore the description of the method for producing a carbon fiber according to the first embodiment will be used to describe the method for producing a carbon fiber according to the second embodiment, and the description will be omitted.

[0127] (3) Third embodiment The flame-resistant fiber according to the third embodiment of the present disclosure has a ratio (I Ａ / I Ｂ ) is in the range of 0.5 to 5.0. Ａ / I Ｂ ) has a peak at 1715 cm in the infrared absorption spectrum. －１ ~1730cm －１ The intensity of the absorption peak B of the C=O stretching vibration of the carbonyl group (I Ｂ ) to 1600 cm －１ ~1620cm －１ The intensity of the absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure (I Ａ ) ratio.

[0128] The flame-resistant fiber according to the third embodiment has the above-described configuration, and thus can be a carbon fiber having excellent tensile strength and suppressing fusion between fibers. The reasons for the above effects are presumed to be as follows, but are not limited to this. By performing a drawing treatment during the flame-resistant treatment, the molecular chains constituting the carbon fiber precursor fiber are oriented in the fiber axis direction, which is the drawing direction, and the cyclization reaction easily proceeds, and the fiber diameter becomes thinner during the drawing treatment. Therefore, the oxidation reaction is promoted up to the center of the carbon fiber precursor fiber. A dehydration reaction between a hydroxyl group formed in this oxidation reaction and a carbonyl group (C=O) proceeds within the same molecular chain. In addition, a dehydration reaction between a carbonyl group and a hydroxyl group between different molecular chains oriented in the fiber axis direction also proceeds. As a result, a carbon-carbon double bond (C=C) is formed, and the 1600 cm －１ ~1620cm －１ The intensity of the absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure (I Ａ ) increases. As the number of carbon-carbon double bonds (C=C) increases, the molecular chain structure of the flame-resistant fiber becomes rigid, the number of conjugated structures increases while the molecular chain remains oriented in the fiber axis direction, and heat resistance during carbonization treatment improves. In addition, it is presumed that the number of graphite structures (ladder polymers) in the obtained carbon fiber increases, and the tensile strength increases due to the increase in graphite crystal size.

[0129] The flame-resistant fiber according to the third embodiment is the same as the flame-resistant fiber according to the first embodiment except for the following points (iv) to (v). Therefore, the flame-resistant fiber according to the third embodiment will be described by citing the description of the flame-resistant fiber according to the first embodiment, and the description will be omitted. (iv) The flame-resistant fiber according to the third embodiment does not have to be manufactured by the method for manufacturing the flame-resistant fiber according to the first embodiment. (v) In the third embodiment, the ratio (I Ａ / I Ｂ ) is within the range of 0.5 to 5.0

[0130] The method for producing the flame-resistant fiber according to the third embodiment is not particularly limited, and examples thereof include the method for producing the flame-resistant fiber according to the first embodiment or the method for producing the flame-resistant fiber according to the second embodiment.

[0131] The above embodiment will be specifically described below using examples, but the above embodiment is not limited to these examples.

[0132] [1] Raw Materials As raw materials for the flame-resistant fiber, the following diene polymer (a), non-diene polymer (b), and photopolymerization initiator (c) were prepared.

[0133] [1.1] Diene polymer (a) Diene polymer (a-1): Syndiotactic 1,2-polybutadiene (manufacturer: ENEOS Material Corporation, product number: RB840, 1,2-bond content: 94 mol%, melting point: 126 ° C, melt flow rate (MFR, ASTM D1238 compliant, temperature 150 ° C, load 21.2 N): 9 g / 10 min, density: 0.914 g / cm ３ )

[0134] [1.2] Non-diene polymer (b) Non-diene polymer (b-1): Polyethylene wax (manufacturer: Clariant, product number: Polyethylene wax PE520, weight average molecular weight: 5500, viscosity at 140 ° C: approximately 0.65 Pa s, melting point: 117 ° C to 123 ° C, density: 0.93 g / cm ３ Non-diene polymer (b-2): hydrogenated dicyclopentadiene / C9 type petroleum resin (manufacturer: ENEOS Material Corporation, product number: T-REZ HB103, aromatic content (measured in accordance with TSTM4030): 8% to 11%, weight average molecular weight: 720, softening point: 103°C, density: approximately 1.06 g / cm ３ )

[0135] [1.3] Photopolymerization initiator (c) Photopolymerization initiator (c-1): methyl 2-benzoylbenzoate (manufacturer: Fujifilm Wako Pure Chemical Industries, Ltd.)

[0136] [2] Examples and Comparative Examples [2.1] Examples 1 to 5 and Comparative Examples 1 and 2 A diene polymer (a-1) and a non-diene polymer (b-1) were charged into the chamber of a Labo Plastomill (manufacturer: Toyo Seiki Seisaku-sho, Ltd., product number: 10C100, chamber: R100H, blade shape: roller type) in the mixing ratios shown in Tables 1 and 2, and melt-kneaded at a screw speed of 100 rpm (revolutions per minute) at a temperature of 150°C for 5 minutes to obtain a diene polymer composition.

[0137] A melt spinning apparatus equipped with a single-screw extruder (screw diameter: 20 mm) and a nozzle (nozzle hole diameter: 200 μm, number of holes: 36 holes) was prepared. A diene-based polymer composition was charged into the hopper of the single-screw extruder and melt-spun at 150°C to obtain a first carbon fiber precursor fiber bundle (average fiber diameter: 15 μm, fineness: 1.6 dtex / fiber, number of single fibers per bundle: 36 / bundle). The first carbon fiber precursor fiber bundle consisted of a plurality of diene-based polymer fibers (i.e., carbon fiber precursor fibers).

[0138] The first carbon fiber precursor fiber bundle was subjected to a drawing treatment at room temperature to obtain a second carbon fiber precursor fiber bundle (average fiber diameter: 12 μm, fineness: 1.0 dtex / fiber, number of single fibers per bundle: 36 / bundle). The draw ratio after spinning was set to 1.7 times. Thereafter, 10 bundles of the second carbon fiber precursor fiber bundles were aligned to obtain a third carbon fiber precursor fiber bundle (number of single fibers per bundle: 360 / bundle).

[0139] Next, using an electron beam irradiation device (manufacturer: NHV Corporation, product number: EPS-800), the third carbon fiber precursor fiber bundle was conveyed at a speed of 20 m / min under an air atmosphere with a tension of 0.6 mN / dtex applied, and the third carbon fiber precursor fiber bundle was subjected to electron beam irradiation to obtain a fourth carbon fiber precursor fiber bundle (average fiber diameter: approximately 12 μm, number of single fibers per bundle: 360 / bundle). For electron beam irradiation, the acceleration voltage was set to 800 kV, and the electron beam dose was set to 550 kGy. The fourth carbon fiber precursor fiber bundle contained a plurality of crosslinked diene-based polymer fibers.

[0140] Two fourth carbon fiber precursor fiber bundles (number of single fibers per bundle: 360 / bundle) were aligned to obtain a fifth carbon fiber precursor fiber bundle (number of single fibers per bundle: 720 / bundle). The fifth carbon fiber precursor fiber bundle was fed into a heat treatment device having a temperature gradient of 200°C to 300°C under an air flow, and passed through the heat treatment device for 60 minutes, thereby subjecting the fifth carbon fiber precursor fiber bundle to a heat treatment (i.e., a flame-retardant treatment (first stage)). The draw ratio during the flame-retardant treatment (first stage) was set to a value obtained by dividing the value shown in "Draw ratio for flame-retardant treatment" in Tables 1 and 2 by 1.0. The fifth carbon fiber precursor fiber bundle was then subjected to an additional flame-retardant treatment (second stage) at 350°C for 30 minutes to obtain a flame-retardant fiber. The draw ratio during the flame-retardant treatment (second stage) was set to 1.0.

[0141] The total stretch ratio in the flame retardant treatment is expressed as the product of the stretch ratio in the first flame retardant treatment and the stretch ratio in the second flame retardant treatment. The total stretch ratio in the flame retardant treatment is shown in Tables 1 and 2.

[0142] The flame-retardant fiber was subjected to a tension of 30 cN and conveyed to a heat treatment device controlled at 800°C under a nitrogen stream, where the flame-retardant fiber was subjected to a preliminary carbonization treatment for 3 minutes to produce a pre-carbonized fiber. Next, the pre-carbonized fiber was subjected to a tension of 30 cN and conveyed to a heat treatment device controlled at 1400°C under a nitrogen stream, where the pre-carbonized fiber was subjected to a carbonization treatment for 3 minutes to obtain a carbon fiber.

[0143] [2.2] Examples 6 to 8 Diene polymer (a-1) and non-diene polymer (b-2) were mixed in the ratios shown in Table 1 and fed into a single-screw extruder (manufacturer: Toyo Seiki Seisakusho Co., Ltd., product number: D2020, screw shape: full-flight, screw length / screw diameter (L / D): 20) attached to a Laboplastomill (manufacturer: Toyo Seiki Seisakusho Co., Ltd., product number: 10C100), and melt-kneaded at a screw rotation speed of 90 rpm and a temperature of 145°C to obtain a diene polymer composition. The shear force of the melt-kneading in Examples 6 to 8 was higher than that of Examples 1 to 5.

[0144] A melt spinning apparatus equipped with a single-screw extruder (screw diameter: 20 mm) and a nozzle (nozzle hole diameter: 200 μm, number of holes: 36 holes) was prepared. A diene-based polymer composition was charged into the hopper of the single-screw extruder and melt-spun at 150°C to obtain a first carbon fiber precursor fiber bundle (average fiber diameter: 15 μm, fineness: 1.6 dtex / fiber, number of single fibers per bundle: 36 / bundle). The first carbon fiber precursor fiber bundle consisted of a plurality of diene-based polymer fibers (i.e., carbon fiber precursor fibers).

[0145] Next, the first carbon fiber precursor fiber bundle was subjected to a drawing treatment at room temperature to obtain a second carbon fiber precursor fiber bundle (average fiber diameter: 12 μm, fineness: 1.0 dtex / bundle, number of single fibers per bundle: 36 / bundle). The draw ratio after spinning was set to 1.7 times. Thereafter, 10 bundles of the second carbon fiber precursor fiber bundles were aligned to obtain a third carbon fiber precursor fiber bundle (average fiber diameter: 12 μm, number of single fibers per bundle: 360 / bundle).

[0146] Next, using an electron beam irradiation device (manufacturer: NHV Corporation, product number: EPS-800), the third carbon fiber precursor fiber bundle was conveyed at a speed of 20 m / min under an air atmosphere with a tension of 0.6 mN / dtex applied, and the third carbon fiber precursor fiber bundle was subjected to electron beam irradiation to obtain a fourth carbon fiber precursor fiber bundle (average fiber diameter: approximately 12 μm, number of single fibers per bundle: 360 / bundle). For electron beam irradiation, the acceleration voltage was set to 800 kV, and the electron beam dose was set to 550 kGy. The fourth carbon fiber precursor fiber bundle contained crosslinked diene-based polymer fibers.

[0147] Thereafter, the fourth carbon fiber precursor fiber bundle was subjected to the same treatment as in the manufacturing methods of Examples 1 to 5 to obtain flame-resistant fibers and carbon fibers.

[0148] [2.3] Example 9 and Comparative Example 3 A melt spinning apparatus equipped with a single-screw extruder (screw diameter: 20 mm) and a nozzle (nozzle hole diameter: 200 μm, number of holes: 36 holes) was prepared. A diene polymer (a-1) was charged into the hopper of the single-screw extruder and melt-spun at 150°C to obtain a first carbon fiber precursor fiber bundle (average fiber diameter: 15 μm, fineness: 1.6 dtex / fiber, number of single fibers per bundle: 360 / bundle). The first carbon fiber precursor fiber bundle was composed of a plurality of diene polymer fibers (i.e., carbon fiber precursor fibers).

[0149] The first carbon fiber precursor fiber bundle was subjected to a drawing treatment at room temperature to obtain a second carbon fiber precursor fiber bundle (average fiber diameter: 12 μm, fineness: 1.0 dtex / fiber, number of single fibers per bundle: 36 / bundle). The draw ratio after spinning was set to 1.7 times. Thereafter, 10 bundles of the second carbon fiber precursor were aligned to obtain a third carbon fiber precursor fiber bundle (number of single fibers per bundle: 360 / bundle).

[0150] Next, using an electron beam irradiation device (manufacturer: NHV Corporation, product number: EPS-800), the third carbon fiber precursor fiber bundle was conveyed at a speed of 20 m / min under an air atmosphere with a tension of 0.6 mN / dtex applied, and the third carbon fiber precursor fiber bundle was subjected to electron beam irradiation to obtain a fourth carbon fiber precursor fiber bundle (average fiber diameter: approximately 12 μm, number of single fibers per bundle: 360 / bundle). For electron beam irradiation, the acceleration voltage was set to 800 kV, and the electron beam dose was set to 550 kGy. The fourth carbon fiber precursor fiber bundle contained crosslinked diene-based polymer fibers.

[0151] Two fourth carbon fiber precursor fiber bundles (number of single fibers per bundle: 360 / bundle) were aligned to obtain a fifth carbon fiber precursor fiber bundle (number of single fibers per bundle: 720 / bundle). The fifth carbon fiber precursor fiber bundle was unwound under an air stream into a heat treatment device having a temperature gradient of 200°C to 300°C, and passed through the heat treatment device for 60 minutes, thereby subjecting the fifth carbon fiber precursor fiber bundle to a heat treatment (i.e., a flame-retardant treatment (first stage)). The draw ratio during the flame-retardant treatment (first stage) was set to a value obtained by dividing the value shown in "Draw ratio for flame-retardant treatment" in Table 2 by 1. Furthermore, a tension of 50 cN was applied, and an additional flame-retardant treatment (second stage) was performed on the fifth carbon fiber precursor fiber bundle at 350°C for 30 minutes to obtain a flame-retardant fiber. The draw ratio during the flame-retardant treatment (second stage) was set to 1.0.

[0152] The flame-retardant fiber was subjected to a tension of 30 cN and conveyed to a heat treatment device controlled at 800°C under a nitrogen stream, where the flame-retardant fiber was subjected to a preliminary carbonization treatment for 3 minutes to produce a pre-carbonized fiber. Next, the pre-carbonized fiber was subjected to a tension of 30 cN and conveyed to a heat treatment device controlled at 1400°C under a nitrogen stream, where the pre-carbonized fiber was subjected to a carbonization treatment for 3 minutes to obtain a carbon fiber.

[0153] [2.4] Example 10 A diene polymer (a-1) and a non-diene polymer (b-1) were charged into the chamber of a Labo Plastomill (manufacturer: Toyo Seiki Seisaku-sho, Ltd., product number: 10C100, chamber: R100H, blade shape: roller type) in the mixing ratio shown in Table 2, and melt-kneaded at a screw rotation speed of 100 rpm at a temperature of 150°C for 5 minutes to obtain a diene polymer composition.

[0154] A melt spinning apparatus equipped with a single-screw extruder (screw diameter: 20 mm) and a nozzle (nozzle hole diameter: 200 μm, number of holes: 36 holes) was prepared. A diene-based polymer composition was charged into the hopper of the single-screw extruder and melt-spun at 150°C to obtain a first carbon fiber precursor fiber bundle (average fiber diameter: 15 μm, fineness: 1.6 dtex / fiber, number of single fibers per bundle: 36 / bundle). The first carbon fiber precursor fiber bundle was composed of diene-based polymer fibers (i.e., carbon fiber precursor fibers).

[0155] Ten first carbon fiber precursor fiber bundles were aligned to obtain a second carbon fiber precursor fiber bundle (number of single fibers per bundle: 360 / bundle).

[0156] Next, using an electron beam irradiation device (manufacturer: NHV Corporation, product number: EPS-800), the second carbon fiber precursor fiber bundle was conveyed at a speed of 20 m / min under an air atmosphere with a tension of 0.6 mN / dtex applied, and the second carbon fiber precursor fiber bundle was subjected to electron beam irradiation to obtain a third carbon fiber precursor fiber bundle (average fiber diameter: approximately 15 μm, number of single fibers per bundle: 360 / bundle). For electron beam irradiation, the acceleration voltage was set to 800 kV, and the electron beam dose was set to 550 kGy. The third carbon fiber precursor fiber bundle contained crosslinked diene-based polymer fibers.

[0157] Two third carbon fiber precursor fiber bundles (number of single fibers per bundle: 360 / bundle) were aligned to obtain a fourth carbon fiber precursor fiber bundle (number of single fibers per bundle: 720 / bundle). The fourth carbon fiber precursor fiber bundle was unwound under an air stream into a heat treatment device with a temperature gradient of 200°C to 300°C, and passed through the heat treatment device for 60 minutes, thereby subjecting the fourth carbon fiber precursor fiber bundle to a flame-resistant treatment (first stage). The draw ratio during the flame-resistant treatment (first stage) was set to 3.0 times. Furthermore, the fourth carbon fiber precursor fiber bundle was unwound under an air stream into a heat treatment device set at 350°C, and the fourth carbon fiber precursor fiber bundle was subjected to an additional flame-resistant treatment (second stage) at 350°C for 30 minutes, thereby obtaining a flame-resistant fiber. The draw ratio during the flame-resistant treatment (second stage) was set to 1.2 times.

[0158] The total stretching ratio in the flame retardant treatment was 3.6 times (stretching ratio in the first flame retardant treatment: 3.0 times, stretching ratio in the second flame retardant treatment: 1.2 times).

[0159] The flame-retardant fiber was subjected to a tension of 30 cN and conveyed to a heat treatment device controlled at 800°C under a nitrogen stream, where the flame-retardant fiber was subjected to a preliminary carbonization treatment for 3 minutes to produce a pre-carbonized fiber. Next, the pre-carbonized fiber was subjected to a tension of 30 cN and conveyed to a heat treatment device controlled at 1400°C under a nitrogen stream, where the pre-carbonized fiber was subjected to a carbonization treatment for 3 minutes to obtain a carbon fiber.

[0160] [2.5] Example 11 Carbon fibers were obtained in the same manufacturing method as in Example 10, except that in the flame retardant treatment (second stage), the temperature was changed to 370°C and the treatment time was changed to 60 minutes.

[0161] [2.6] Example 12 A melt spinning apparatus equipped with a single-screw extruder (screw diameter: 20 mm) and a nozzle (nozzle hole diameter: 200 μm, number of holes: 36 holes) was prepared. A diene-based polymer composition having the same composition as the diene-based polymer composition produced in Example 10 was charged into the hopper of the single-screw extruder and melt-spun at 150°C to obtain a first carbon fiber precursor fiber bundle (average fiber diameter: 26 μm, fineness: 4.9 dtex / fiber, number of single fibers per bundle: 36 / bundle). The first carbon fiber precursor fiber bundle was composed of diene-based polymer fibers (i.e., carbon fiber precursor fibers).

[0162] The first carbon fiber precursor fiber bundle was subjected to a drawing treatment at room temperature to obtain a second carbon fiber precursor fiber bundle (average fiber diameter: 25 μm, fineness: 4.5 dtex / fiber, number of single fibers per bundle: 36 / bundle). The draw ratio after spinning was set to 1.1 times. Ten bundles of the second carbon fiber precursor fiber bundles were aligned to obtain a third carbon fiber precursor fiber bundle (number of single fibers per bundle: 360 / bundle).

[0163] Next, using an electron beam irradiation device (manufacturer: NHV Corporation, product number: EPS-800), the third carbon fiber precursor fiber bundle was conveyed at a speed of 20 m / min under an air atmosphere with a tension of 0.6 mN / dtex applied, and the third carbon fiber precursor fiber bundle was subjected to electron beam irradiation to obtain a fourth carbon fiber precursor fiber bundle (average fiber diameter: approximately 25 μm, number of single fibers per bundle: 360 / bundle). For electron beam irradiation, the acceleration voltage was set to 800 kV, and the electron beam dose was set to 550 kGy. The fourth carbon fiber precursor fiber bundle contained crosslinked diene-based polymer fibers.

[0164] Two fourth carbon fiber precursor fiber bundles (number of single fibers per bundle: 360 / bundle) were aligned to obtain a fifth carbon fiber precursor fiber bundle (number of single fibers per bundle: 720 / bundle). The fifth carbon fiber precursor fiber bundle was unwound in an air stream into a heat treatment device having a temperature gradient of 200°C to 280°C, and passed through the heat treatment device over 30 minutes, thereby subjecting the fifth carbon fiber precursor fiber bundle to a flame retardant treatment (first stage). The draw ratio during the flame retardant treatment (first stage) was set to 2.0 times. Furthermore, the fiber bundle after the flame retardant treatment (first stage) was unwound in an air stream into a heat treatment device set to 280°C to 300°C, and passed through the heat treatment device over 30 minutes, thereby subjecting the fifth carbon fiber precursor fiber bundle to a flame retardant treatment (second stage). The draw ratio during the flame retardant treatment (second stage) was set to 2.0 times. The fiber bundle after the second flame retardant treatment was then fed into a heat treatment device set at 300°C under an air flow, and subjected to a third flame retardant treatment at 300°C for 30 minutes to obtain a flame retardant fiber. The draw ratio during the third flame retardant treatment was set to 2.0 times.

[0165] The total stretch ratio in the flame retardant treatment was 8.0 times (stretch ratio in the first flame retardant treatment: 2.0 times, stretch ratio in the second flame retardant treatment: 2.0 times, stretch ratio in the third flame retardant treatment: 2.0 times).

[0166] The flame-retardant fiber was subjected to a tension of 30 cN and conveyed to a heat treatment device controlled at 800°C under a nitrogen stream, where the flame-retardant fiber was subjected to a preliminary carbonization treatment for 3 minutes to produce a pre-carbonized fiber. Next, the pre-carbonized fiber was subjected to a tension of 30 cN and conveyed to a heat treatment device controlled at 1400°C under a nitrogen stream, where the pre-carbonized fiber was subjected to a carbonization treatment for 3 minutes to obtain a carbon fiber.

[0167] [2.7] Examples 13 and 14 Diene polymer (a-1), non-diene polymer (b-1), and photopolymerization initiator (c-1) were charged into the chamber of a Labo Plastomill (manufacturer: Toyo Seiki Seisaku-sho, Ltd., product number: 10C100, chamber: R100H, blade shape: roller type) in the mixing ratios shown in Table 2, and melt-kneaded at a screw rotation speed of 100 rpm and a temperature of 150°C for 5 minutes to obtain diene polymer compositions.

[0168] A melt spinning apparatus equipped with a single-screw extruder (screw diameter: 20 mm) and a nozzle (nozzle hole diameter: 200 μm, number of holes: 36 holes) was prepared. The diene polymer composition was charged into the hopper of the single-screw extruder and melt-spun at 150°C to obtain a first carbon fiber precursor fiber bundle (average fiber diameter: 15 μm, fineness: 1.6 dtex / fiber, number of single fibers per bundle: 36 / bundle).

[0169] The first carbon fiber precursor fiber bundle was subjected to a drawing treatment at room temperature to obtain a second carbon fiber precursor fiber bundle (number of single fibers per bundle: 36 / bundle). The draw ratio after spinning was set to 1.7 times. Thereafter, 10 bundles of the second carbon fiber precursor fiber bundles were aligned to obtain a third carbon fiber precursor fiber bundle (average fiber diameter: 12 μm, fineness: 1.0 dtex / fiber, number of single fibers per bundle: 360 / bundle). The first carbon fiber precursor fiber bundle was composed of a plurality of diene-based polymer fibers (i.e., carbon fiber precursor fibers).

[0170] Next, using a UV-LED irradiator (manufacturer: CCS, product number: HLDL-350x270), the third carbon fiber precursor fiber bundle was irradiated with ultraviolet light for 60 minutes under an air atmosphere with a tension of 0.07 mN / dtex to obtain a fourth carbon fiber precursor fiber bundle (average fiber diameter: approximately 12 μm). The illuminance of the ultraviolet light irradiation was 200 mW / cm. ２ and the wavelength was set to 365 nm. The fourth carbon fiber precursor fiber bundle included crosslinked diene-based polymer fibers.

[0171] Next, the fourth carbon fiber precursor fiber bundle was subjected to the same treatment as that in the manufacturing method of Example 1, except that the draw ratio for the flame retardant treatment was the ratio shown in Table 2, to obtain flame retardant fibers (average fiber diameter: about 9 μm) and carbon fibers.

[0172] [2.8] Example 15 A melt spinning apparatus equipped with a single-screw extruder (screw diameter: 20 mm) and a nozzle (nozzle hole diameter: 200 μm, number of holes: 36 holes) was prepared. A diene-based polymer composition was charged into the hopper of the single-screw extruder and melt-spun at 150°C to obtain a first carbon fiber precursor fiber bundle (average fiber diameter: 26 μm, fineness: 4.9 dtex / fiber, number of single fibers per bundle: 36 / bundle). The first carbon fiber precursor fiber bundle consisted of a plurality of diene-based polymer fibers (i.e., carbon fiber precursor fibers).

[0173] The first carbon fiber precursor fiber bundle was subjected to a drawing treatment at room temperature to obtain a second carbon fiber precursor fiber bundle (average fiber diameter: 25 μm, fineness: 4.5 dtex / fiber, number of single fibers per bundle: 36 / bundle). The draw ratio after spinning was set to 1.1 times. Thereafter, 10 bundles of the second carbon fiber precursor fiber bundles were aligned to obtain a third carbon fiber precursor fiber bundle (number of single fibers per bundle: 360 / bundle).

[0174] Next, using an electron beam irradiation device (manufacturer: NHV Corporation, product number: EPS-800), the third carbon fiber precursor fiber bundle was conveyed at a speed of 20 m / min under an air atmosphere with a tension of 0.6 mN / dtex applied, and the third carbon fiber precursor fiber bundle was subjected to electron beam irradiation to obtain a fourth carbon fiber precursor fiber bundle (average fiber diameter: approximately 25 μm, number of single fibers per bundle: 360 / bundle). For electron beam irradiation, the acceleration voltage was set to 800 kV, and the electron beam dose was set to 550 kGy. The fourth carbon fiber precursor fiber bundle contained a plurality of crosslinked diene polymer fibers.

[0175] Two fourth carbon fiber precursor fiber bundles (number of single fibers per bundle: 360 / bundle) were aligned to obtain a fifth carbon fiber precursor fiber bundle (number of single fibers per bundle: 720 / bundle). The fifth carbon fiber precursor fiber bundle was unwound in an air stream into a heat treatment device having a temperature gradient of 200°C to 280°C, and passed through the heat treatment device for 30 minutes, thereby subjecting the fifth carbon fiber precursor fiber bundle to a flame-retardant treatment (first stage). The draw ratio during the flame-retardant treatment (first stage) was set to 2.5 times. Furthermore, the fiber bundle after the flame-retardant treatment (first stage) was unwound in an air stream into a heat treatment device set at 280°C to 300°C, and passed through the heat treatment device for 30 minutes, thereby subjecting the fifth carbon fiber precursor fiber bundle to a flame-retardant treatment (second stage), thereby obtaining a flame-retardant fiber. The draw ratio during the flame-retardant treatment (second stage) was set to 2.0 times.

[0176] The total stretching ratio in the flame retardant treatment was 5.0 times (stretching ratio in the first flame retardant treatment: 2.5 times, stretching ratio in the second flame retardant treatment: 2.0 times).

[0177] Next, the flame-resistant fiber was subjected to the same treatment as in the manufacturing method of Example 1 to obtain a carbon fiber.

[0178] [2.9] Example 16 A melt spinning apparatus equipped with a single-screw extruder (screw diameter: 20 mm) and a nozzle (nozzle hole diameter: 200 μm, number of holes: 36 holes) was prepared. A diene-based polymer composition was charged into the hopper of the single-screw extruder and melt-spun at 150°C to obtain a first carbon fiber precursor fiber bundle (average fiber diameter: 26 μm, fineness: 4.9 dtex / fiber, number of single fibers per bundle: 36 / bundle). The first carbon fiber precursor fiber bundle consisted of a plurality of diene-based polymer fibers (i.e., carbon fiber precursor fibers).

[0179] The first carbon fiber precursor fiber bundle was subjected to a drawing treatment at room temperature to obtain a second carbon fiber precursor fiber bundle (average fiber diameter: 14 μm, fineness: 1.4 dtex / fiber, number of single fibers per bundle: 36 / bundle). The draw ratio after spinning was set to 3.5 times. Thereafter, 10 bundles of the second carbon fiber precursor fiber bundles were aligned to obtain a third carbon fiber precursor fiber bundle (number of single fibers per bundle: 360 / bundle).

[0180] Next, using an electron beam irradiation device (manufacturer: NHV Corporation, product number: EPS-800), the third carbon fiber precursor fiber bundle was conveyed at a speed of 20 m / min under an air atmosphere with a tension of 0.6 mN / dtex applied, and the third carbon fiber precursor fiber bundle was irradiated with an electron beam to obtain a fourth carbon fiber precursor fiber bundle (average fiber diameter: approximately 14 μm, number of single fibers per bundle: 360 / bundle). For electron beam irradiation, the acceleration voltage was set to 800 kV, and the electron beam dose was set to 550 kGy. The fourth carbon fiber precursor fiber bundle contained a plurality of crosslinked diene polymer fibers.

[0181] Two fourth carbon fiber precursor fiber bundles (number of single fibers per bundle: 360 / bundle) were aligned to obtain a fifth carbon fiber precursor fiber bundle (number of single fibers per bundle: 720 / bundle). The fifth carbon fiber precursor fiber bundle was unwound in an air stream into a heat treatment device having a temperature gradient of 200°C to 280°C, and passed through the heat treatment device for 30 minutes, thereby subjecting the fifth carbon fiber precursor fiber bundle to a flame-retardant treatment (first stage). The draw ratio during the flame-retardant treatment (first stage) was set to 2.5 times. Furthermore, the fiber bundle after the flame-retardant treatment (first stage) was unwound in an air stream into a heat treatment device set at 280°C to 300°C, and passed through the heat treatment device for 30 minutes, thereby subjecting the fifth carbon fiber precursor fiber bundle to a flame-retardant treatment (second stage), thereby obtaining a flame-retardant fiber. The draw ratio during the flame-retardant treatment (second stage) was set to 2.0 times.

[0182] The total stretching ratio in the flame retardant treatment was 5.0 times (stretching ratio in the first flame retardant treatment: 2.5 times, stretching ratio in the second flame retardant treatment: 2.0 times).

[0183] Next, the flame-resistant fiber was subjected to the same treatment as in the manufacturing method of Example 1 to obtain a carbon fiber.

[0184] [2.10] Example 17 A fifth carbon fiber precursor fiber bundle (number of single fibers per bundle: 720 / bundle) was obtained by the same production method as in Example 16. The fifth carbon fiber precursor fiber bundle was unwound into a heat treatment device with a temperature gradient of 200°C to 280°C under an air stream, and the fiber bundle was passed through the heat treatment device over 30 minutes, thereby subjecting the fifth carbon fiber precursor fiber bundle to a flame-retardant treatment (first stage). The draw ratio during the flame-retardant treatment (first stage) was set to 3.0 times. Furthermore, the fiber bundle after the flame-retardant treatment (first stage) was unwound into a heat treatment device set at 280°C to 300°C under an air stream, and the fiber bundle was passed through the heat treatment device over 30 minutes, thereby subjecting the fifth carbon fiber precursor fiber bundle to a flame-retardant treatment (second stage). The draw ratio during the flame-retardant treatment (second stage) was set to 3.0 times. The fiber bundle after the second flame retardant treatment was then fed into a heat treatment device set at 300°C under an air flow, and subjected to a third flame retardant treatment at 300°C for 30 minutes to obtain a flame retardant fiber. The draw ratio during the third flame retardant treatment was set to 2.0 times.

[0185] The total stretch ratio in the flame retardant treatment was 18.0 times (stretch ratio in the first flame retardant treatment: 3.0 times, stretch ratio in the second flame retardant treatment: 3.0 times, stretch ratio in the third flame retardant treatment: 2.0 times).

[0186] Next, the flame-resistant fiber was subjected to the same treatment as in the manufacturing method of Example 1 to obtain a carbon fiber.

[0187]

[0188]

[0189] In Tables 1 and 2, "(a)" indicates diene polymer (a). "(b)" indicates non-diene polymer (b). "(c)" indicates photopolymerization initiator (c). In Examples 7 to 8, "with" in "high shear kneading" indicates that the shear force of melt kneading was higher than that of Examples 1 to 6 and Examples 9 to 14. "without" in "high shear kneading" indicates that the shear force of melt kneading was not higher than that of Examples 7 to 8. "Before crosslinking" indicates before the crosslinking treatment. "After crosslinking" indicates after the crosslinking treatment. "Average particle size of dispersed phase" indicates the average particle size of the dispersed phase of non-diene polymer (b). "Crosslinking" indicates the active energy ray used in the crosslinking treatment. "Stretching ratio" in the flame-resistant treatment indicates the total stretching ratio in the flame-resistant treatment.

[0190] [3] Measurement method [3.1] Average fiber diameter of each fiber The side of each fiber bundle of carbon fiber precursor fiber, flame-resistant fiber, and carbon fiber was observed using a microscope ("Digital Microscope VHX-7000" manufactured by Keyence Corporation). The diameters of 12 randomly selected single fibers in each fiber bundle were measured, and the average value was taken as the average fiber diameter.

[0191] [3.2] Fineness of carbon fiber precursor fiber The mass of the fiber bundle of the carbon fiber precursor fiber was measured, and the mass of the fiber bundle of the carbon fiber precursor fiber per 10,000 m was defined as the fineness [dtex] of the fiber bundle of the diene polymer fiber.

[0192] [3.3] Phase structure of carbon fiber precursor fiber and average particle size of dispersed phase Before and after crosslinking treatment, ultrathin sections of a cross section perpendicular to the fiber axis direction of each carbon fiber precursor fiber were prepared, and the phase structure of the ultrathin sections was observed using a transmission electron microscope (manufacturer: Hitachi High-Tech Corporation, product number: H-7650). When a dispersed phase containing a non-diene polymer (b) was dispersed in a diene polymer (a), 20 dispersed phases were randomly selected, the particle sizes of the dispersed phases were measured, and the average value was taken as the average particle size of the dispersed phase. In the particle size measurement of the dispersed phase, when the shape of the observed dispersed phase was other than circular (elliptical, streaky, or other irregular shape), the lengths of the long side (longest part) and short side (shortest part) of the dispersed phase were measured, and the value (average value) obtained by dividing the sum of the long side length and the short side length by 2 was taken as the average particle size.

[0193] [3.4] Degree of crosslinking (gel fraction) after crosslinking treatment A 0.2 g sample was cut out from the diene polymer fiber (crosslinked diene polymer fiber) after electron beam or ultraviolet irradiation. The sample was dried at 80°C for 4 hours, and then its mass was precisely measured using a precision electronic balance, which was defined as the initial mass (g) of the sample.

[0194] Next, the sample was immersed in 30 ml of toluene and allowed to stand in a hot air circulation 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 (manufacturer: Merck, product name: Omnipore™ membrane filter, product number: JAWP04700) to separate the gel fraction. The separated gel fraction together with the membrane filter was air-dried in the atmosphere in a draft chamber for 12 hours or more, and then allowed to stand in a hot air circulation oven at 90°C for 12 hours to remove the toluene. The masses of the gel fraction and membrane filter after standing were precisely weighed using a precision electronic balance, and the gel fraction was calculated using the following formula (A). Formula (A): Gel fraction (%) = {(mass of gel fraction and membrane filter (g) - mass of membrane filter (g)) / initial mass of sample (g)} x 100

[0195] Using the calculated gel fraction, the degree of crosslinking of the crosslinked carbon fiber precursor fiber was evaluated according to the following evaluation criteria. The results are shown in Tables 1 and 2.

[0196] A: The gel fraction was 90% or more. B: The gel fraction was 80% or more and less than 90%. C: The gel fraction was less than 80%.

[0197] [3.5] Flame-resistant treatment resistance (yarn breakage prevention) of carbon fiber precursor fiber To evaluate the flame-resistant treatment resistance when the flame-resistant treatment was performed, the presence or absence of breakage of the carbon fiber precursor fiber during the flame-resistant treatment was confirmed. Specifically, a fiber bundle for evaluation (length: 3 cm) was cut from a fiber bundle of the flame-resistant fiber obtained by the flame-resistant treatment. Using a microscope (manufacturer: Keyence Corporation, product number: Digital Microscope VHX-7000), the presence or absence of breakage of the evaluation fiber bundle was observed and evaluated according to the following criteria. The results are shown in Tables 1 and 2.

[0198] A: No breakage of the carbon fiber precursor fibers occurred. B: Breakage of 1 to 10 carbon fiber precursors (i.e., single fibers) was confirmed. C: Breakage of 11 or more carbon fiber precursors (i.e., single fibers) was confirmed.

[0199] [3.6] Fusion Rate of Fiber Bundle of Flame-Retardant Fiber A fiber bundle for evaluation, 3 cm in length, was cut out from a fiber bundle of flame-retardant fiber. The cross section of this fiber bundle for evaluation was observed using a microscope (manufacturer: Keyence Corporation, product number: Digital Microscope VHX-7000), and the number of fibers was counted. At this time, the number of fused fibers and the number of all fibers constituting the fiber bundle for evaluation were counted. The number of fused fibers was counted as two, for example, when two fibers were fused to each other. The number of all fibers was counted by separating the fused fibers into their pre-fusion state. The fusion rate was calculated based on the following formula (B). The results are shown in Tables 1 and 2. Formula (B): Fusion rate (%) = (number of fused fibers / total number of fibers) × 100

[0200] An acceptable fusion rate is 15% or less.

[0201] [3.7] Flame retardant fiber ratio (I Ａ / I ＢThe surface of the flame-resistant fiber was measured by the ATR (Attenuated Total Reflection) method of Fourier transform infrared spectroscopy (FT-IR). The measurement device used was an FTIR spectrophotometer (manufacturer: Thermo Fisher Scientific, product number: Nicolet iS50). The measurement was performed by the ATR method using a Ge prism. From the obtained infrared absorption spectrum, －１ ~1620cm －１ The intensity of the absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure (I Ａ ) and 1715 cm －１ ~1730cm －１ The intensity of the absorption peak B of the C=O stretching vibration of the carbonyl group (I Ｂ ) and the ratio (I Ａ / I Ｂ The results are shown in Tables 1 and 2. Formula (C): Ratio (I Ａ / I Ｂ ) = Intensity of absorption peak A (I Ａ ) / intensity of absorption peak B (I Ｂ )

[0202] [3.8] Carbonization yield of flame-retardant fiber A sample of approximately 2 mg was cut out from the flame-retardant fiber. Using a high-temperature differential thermobalance (manufacturer: Rigaku Corporation, product number: Thermo plus EV02 TG-DTA / H), the sample was heated from room temperature to 1000°C at a temperature increase rate of 10°C / min under a nitrogen gas flow of 1000 ml / min, and the carbonization yield was calculated using the following formula (D): Formula (D): Carbonization yield (%) = (mass of fiber (carbon fiber) after carbonization at 1000°C / mass of flame-retardant fiber at room temperature) x 100

[0203] The carbonization yield was evaluated according to the following criteria. The results are shown in Tables 1 and 2.

[0204] A: The carbonization yield (%) was 60% or more. B: The carbonization yield (%) was 50% or more and less than 60%. C: The carbonization yield (%) was less than 50%.

[0205] [3.9] Carbonization resistance of flame-retardant fiber The side of the carbon fiber obtained after the carbonization treatment was observed visually and using a microscope (manufacturer: Keyence Corporation, product number: Digital Microscope VHX-7000), and evaluated according to the following criteria. The results are shown in Tables 1 and 2.

[0206] A: No fluffing occurred. B: Fluffing occurred due to the breakage of 1 to 10 carbon fibers (i.e., single fibers). C: Fluffing occurred due to the breakage of 11 or more carbon fibers (i.e., single fibers).

[0207] [3.10] Tensile strength of carbon fiber Five carbon fibers (i.e., single fibers) were taken out from the obtained carbon fiber bundle, and a tensile test (gauge length: 25 mm, pulling speed: 1 mm / min) was performed on each carbon fiber (i.e., single fiber) at room temperature in accordance with JIS R7606 using a micro strength evaluation tester (manufacturer: Shimadzu Corporation, product number: Micro Autograph MST-I) to measure the tensile strength and calculate the average value. The results are shown in Tables 1 and 2.

[0208] The acceptable tensile strength of carbon fiber is 0.9 GPa or more.

[0209] [4] Results In Comparative Examples 1 to 3, the draw ratio in the flame-resistant process was not within the range of 0.9 to 100 times. In Comparative Examples 1 to 3, the flame-resistant treatment was carried out at a ratio of the flame-resistant fiber (I Ａ / I Ｂ In Comparative Examples 1 to 3, the ratio (I Ａ / I Ｂ) was not within the range of 0.5 to 5.0. The fusion rates of the flame-resistant fibers of Comparative Examples 1 to 3 were not 15% or less. The tensile strengths of the carbon fibers of Comparative Examples 1 to 3 were not 0.9 GPa or more. As a result, it was found that the manufacturing methods of the flame-resistant fibers of Comparative Examples 1 to 3 are not "manufacturing methods of flame-resistant fibers that can produce carbon fibers having excellent tensile strength and in which fusion between fibers is suppressed." It was found that the manufacturing methods of the carbon fibers of Comparative Examples 1 to 3 are not "manufacturing methods of carbon fibers that can produce carbon fibers having excellent tensile strength." It was found that the flame-resistant fibers of Comparative Examples 1 to 3 are not "manufacturing methods of carbon fibers that can produce carbon fibers having excellent tensile strength."

[0210] In Examples 1 to 17, the draw ratio in the flame-resistant treatment was in the range of 0.9 to 100 times. Ａ / I Ｂ In Examples 1 to 17, the ratio (I Ａ / I Ｂ ) was in the range of 0.5 to 5.0. The fusion ratios of the flame-resistant fibers of Examples 1 to 17 were 15% or less. The tensile strengths of the carbon fibers of Examples 1 to 17 were 0.9 GPa or more. These results demonstrate that the manufacturing method of the flame-resistant fibers of Examples 1 to 17 is "a manufacturing method of a flame-resistant fiber that can produce a carbon fiber having excellent tensile strength and in which fusion between fibers is suppressed." The manufacturing method of the carbon fibers of Examples 1 to 17 is "a manufacturing method of a carbon fiber that can produce a carbon fiber having excellent tensile strength." The flame-resistant fibers of Examples 1 to 17 are "a flame-resistant fiber that can produce a carbon fiber having excellent tensile strength and in which fusion between fibers is suppressed."

[0211] In Examples 13 and 14, ultraviolet irradiation was performed instead of electron beam irradiation as the crosslinking treatment. In Examples 13 and 14, it was found that by performing electron beam irradiation while applying a specific tension (0.07 mN / dtex or more), a carbon fiber with a crosslinking degree of "A" and excellent tensile strength could be obtained, and a flame-resistant fiber in which fusion between fibers was suppressed could be produced.

[0212] The phase structures of the carbon fiber precursors of Examples 7 and 8 exhibited a compatible structure and did not form a sea-island structure. Therefore, the fusion rates of Examples 7 and 8 were higher than that of Example 6. In other words, the flame-resistant treatment resistance of Examples 7 and 8 was lower than that of Example 6. This is presumably because, during the stretching treatment during the flame-resistant treatment, the effect of improving stretchability due to plasticization of the islands (diene-based polymer (a)) of the sea-island structure was exhibited in Example 6, while this effect of improving stretchability was not exhibited in Examples 7 and 8. The diene-based polymer compositions of Examples 5 and 15 to 17 had the same composition, but the draw ratios after spinning and the draw ratios during the flame-resistant treatment were different. The flame-resistant fibers of Examples 5, 15, and 16 had higher flame-resistant treatment resistance, lower fusion rates, and higher tensile strengths of the carbon fibers than the flame-resistant fiber of Example 17. This is presumably because the draw ratios during the flame-resistant treatment in Examples 5, 15, and 16 (1.3 times, 5.0 times, and 5.0 times, respectively) were able to orient the molecular chains more in the fiber axis direction while suppressing damage to the carbon fiber precursor fiber, compared to the draw ratio (18 times) during the flame-resistant treatment in Example 17. In particular, in Example 16, the drawing treatment was performed at a draw ratio after spinning of 3.5 times, and at the time of the flame-resistant treatment, the drawing treatment was performed at a draw ratio of 5.0 times. Therefore, the flame-resistant fiber of Example 16 had a ratio (I Ａ / I Ｂ ) was 1.3, and the fusion rate of the flame-resistant fiber was the lowest, and the carbonization yield and tensile strength of the carbon fiber were the highest.

[0213] The disclosure of Japanese Patent Application No. 2024-129213, filed on August 5, 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. A method for producing a flame-retardant fiber, comprising: subjecting a carbon fiber precursor fiber containing a diene polymer to a stretching treatment at a draw ratio of 0.9 to 100 times while subjecting the precursor fiber to a flame-retardant treatment.

2. The method for producing an oxidation-resistant fiber according to claim 1, wherein the drawing treatment is a treatment of drawing the carbon fiber precursor fiber so that the average fiber diameter of the oxidation-resistant fiber becomes 10 nm to 40 μm.

3. The method for producing an oxidation-resistant fiber according to claim 1, wherein the oxidation treatment is a treatment of heating the carbon fiber precursor fiber in an oxidizing atmosphere at a temperature in the range of 120°C to 500°C.

4. The method for producing an oxidation-resistant fiber according to claim 1, wherein the carbon fiber precursor fiber comprises a crosslinked diene polymer obtained by crosslinking the diene polymer.

5. A method for producing a carbon fiber, comprising: producing a flame-resistant fiber by the method for producing a flame-resistant fiber according to any one of claims 1 to 4; and subjecting the flame-resistant fiber to a carbonization treatment.

6. A method for producing a flame-resistant fiber, comprising: drawing a carbon fiber precursor fiber containing a diene polymer while performing a flame-resistant treatment; and in the infrared absorption spectrum of the flame-resistant fiber, －１ ~1620cm －１ There is an absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure, which is confirmed by the following formula: Ａ / I Ｂ a treatment of heating the carbon fiber precursor fiber in an oxidizing atmosphere so that the ratio (I) falls within the range of 0.5 to 5.0; Ａ / I Ｂ ) is detected at 1715 cm in the infrared absorption spectrum of the flame-resistant fiber. －１ ~1730cm －１ The intensity of the absorption peak B of the C=O stretching vibration of the carbonyl group (I Ｂ ) to 1600 cm －１ ~1620cm －１ The intensity of the absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure (I Ａ ) ratio.

7. Ratio (I Ａ / I Ｂ ) is in the range of 0.5 to 5.0, and the ratio (I Ａ / I Ｂ ) is observed in the infrared absorption spectrum at 1715 cm －１ ~1730cm －１ The intensity of the absorption peak B of the C=O stretching vibration of the carbonyl group (I Ｂ ) to 1600 cm －１ ~1620cm －１ The intensity of the absorption peak A of the C=C stretching vibration of the carbon six-membered ring structure (I Ａ ) ratio.

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