TWI396785B - Acrylonitrile swollen yarn for carbon fiber, precursor fiber bundle, flameproof fiber bundle, carbon fibre bundle and manufacturing method thereof - Google Patents

Abstract

Provided is a carbon fiber bundle for obtaining a fiber-reinforced plastic having high mechanical characteristics. An acrylonitrile swollen fiber for a carbon fiber having openings of 10 nm or more in width in the circumference direction of the swollen fiber at a ratio in the range of 0.3 openings/µm 2 or more and 2 openings/µm 2 or less on the surface of the swollen fiber, and the swollen fiber is not treated with a finishing oil agent. A precursor fiber obtained by treating the swollen fiber with a silicone-based finishing oil agent has a silicon content of 1700 ppm or more and 5000 ppm or less, and the silicon content is 50 ppm or more and 300 ppm or less after the finishing oil agent is washed away with methyl ethyl ketone by using a Soxhlet extraction apparatus for 8 hours. The fiber is preferably an acrylonitrile copolymer containing acrylonitrile in an amount of 96.0 mass % or more and 99.7 mass % or less and an unsaturated hydrocarbon having at least one carboxyl group or ester group in an amount of 0.3 mass % or more and 4.0 mass % or less.

Description

Acrylonitrile swelling yarn for carbon fiber, precursor fiber bundle, flame-proof fiber bundle, carbon fiber bundle, and manufacturing method thereof

The present invention relates to a carbon fiber bundle for obtaining a high-quality, high-performance fiber-reinforced resin having excellent mechanical properties, particularly for aircraft use, industrial use, and the like, and a swelled yarn and precursor used for producing the carbon fiber bundle. Fiber bundles and flame-resistant fiber bundles.

In order to improve the mechanical properties of the resin-based molded article, a method in which a fiber is used as a reinforcing material and a resin is usually used. In particular, since a molding material obtained by combining a carbon fiber having a specific strength and a specific modulus with a high-performance resin has excellent mechanical properties, the molding material is actively used as a structural material such as an airplane or a high-speed moving body. In addition, the material is required to have higher strength and higher rigidity, and also requires the material to have superior specific strength and specific rigidity. Therefore, the performance of the carbon fiber is required to achieve higher strength and high modulus of elasticity.

In order to produce such a high-performance carbon fiber, it is necessary to obtain an acrylonitrile precursor fiber bundle for carbon fibers excellent in strength and to calcine the precursor fiber bundle under the most suitable conditions. In particular, the following aspects are being studied: the structure densification of the precursor fiber bundle is performed; the starting point of the defect point formation is completely eliminated; and the calcination condition or the like which is not easily formed into a defect point is set. For example, Patent Document 1 proposes a method of improving the structure by extending a coagulated yarn in a solvent-containing state in a solvent-containing stretching bath when a precursor fiber bundle is obtained by a dry-wet spinning method. Uniformity of alignment. The method of extending the coagulated filament in a bath containing a solvent is a well-known method as a solvent extension technique, and is a method of achieving stable elongation treatment by solvent plasticization. Therefore, those skilled in the art consider this method to be a very excellent method for obtaining fibers having higher structure and alignment uniformity. However, by extending the fiber bundle in a swollen state containing the solvent, the solvent existing inside the filament is rapidly ejected from the inside of the monofilament as it extends, and thus the obtained monofilament easily forms a loose structure. It is impossible to obtain a fiber having a dense structure as a target. As a result, it is difficult to obtain a carbon fiber bundle having high strength.

In addition, Patent Document 2 proposes a technique of obtaining a precursor fiber excellent in strength discovery property by focusing on the pore distribution of the coagulated filament and drying and densifying the coagulated filament having a high densification structure. The pore distribution obtained by the mercury penetration method reflects the bulk property including the surface layer from the monofilament to the inside, and is an excellent method for evaluating the compactness of the overall structure of the fiber. A high-strength carbon fiber in which formation of a defect point is suppressed can be obtained by using a precursor fiber bundle whose overall density is at a certain level or higher. However, if the fracture state of the carbon fiber is observed, it can be found that a very high proportion of the fiber is in the vicinity of the surface layer as the starting point of the fracture. This phenomenon indicates that there is a defect point near the surface layer. That is, this technique is insufficient for producing a precursor fiber bundle excellent in denseness in the vicinity of the surface layer.

Patent Document 3 proposes a method for producing an acrylonitrile-based precursor fiber bundle having high density of the entire fiber and extremely high density of the surface layer portion. Further, in Patent Document 4, it is proposed that the oil agent penetrates into the surface layer portion of the fiber to inhibit densification, so that attention is paid to the microvoid in the surface layer portion to suppress the penetration of the oil agent. However, techniques for suppressing penetration of oil agents and techniques for suppressing formation of defective spots require very complicated steps, and thus it is difficult to obtain practical applications. Therefore, the current state of the art under study is that the effect of stably inhibiting the penetration of the oil agent into the surface layer of the fiber is not sufficient, and the effect of increasing the strength of the carbon fiber is not sufficient.

[Previous Technical Literature]

[Patent Literature]

Patent Document 1: Japanese Patent Laid-Open No. Hei 5-5224

Patent Document 2: Japanese Patent Laid-Open No. Hei 4-91230

Patent Document 3: Japanese Patent Special Fair No. 6-12522

Patent Document 4: Japanese Patent Laid-Open No. Hei 11-124744

An object of the present invention is to provide a carbon fiber bundle for obtaining a fiber-reinforced resin having high mechanical properties.

In order to solve the problem, the inventors of the present invention have developed an appropriate form and properties of acrylonitrile swellable filaments and precursor fiber bundles for carbon fibers, and adjusted the solidification conditions and elongation conditions of the spun fibers to appropriate. A swelled filament having a dense internal structure and inhibiting penetration of an oil agent in the vicinity of the surface layer.

The above problems are solved by the following group of the present invention.

According to a first aspect of the invention, there is provided an acrylonitrile expanded yarn for carbon fibers, which is not treated with an oil agent, and has an opening portion in a range of 0.3/μm ² or more and 2/μm ² or less on the surface of the single fiber, and the opening portion It has a width of 10 nm or more in the circumferential direction of the fiber.

A second invention is a method for producing a swollen silk comprising the following steps:

[1] Copolymerization of acrylonitrile 96.0 wt% or more, 99.7 wt% or less, and an unsaturated hydrocarbon having one or more carboxyl groups or ester groups of 0.3 wt% or more and 4.0 wt% or less as essential components The acrylonitrile-based copolymer is dissolved in an organic solvent in a concentration range of 20 wt% or more and 25 wt% or less to prepare a spinning dope having a temperature of 50 ° C or more and 70 ° C or less;

[2] After the spinning dope is temporarily ejected from the ejection hole into the air by the dry-wet spinning method, the inclusion temperature is -5 ° C or more and 20 ° C or less, and the organic solvent concentration is 78.0 wt% or more and 82.0 wt. The spinning dope is solidified in a coagulation bath of an aqueous solution of less than %, to obtain a coagulated tow containing the above organic solvent;

[3] The coagulated tow is stretched in the range of 1.0 times or more and 1.25 times or less in the air, and further extended in a warm aqueous solution containing an organic solvent, wherein the total stretching ratio of the two extensions is 2.6. Extending more than double and 4.0 times or less; and

[4] Next, the solvent is removed in warm water, and then extended to 0.98 times or more and 2.0 times or less in hot water.

The third invention is a precursor fiber bundle for carbon fibers, comprising 96.0 wt% or more and 99.7 wt% or less of acrylonitrile, 0.3 wt% or more, 4.0 wt% or less of an unsaturated hydrocarbon having one or more carboxyl groups or ester groups. The acrylonitrile copolymer obtained by copolymerization as an essential component is treated with an oil agent containing a polyoxyxasiloxane as a main component, and has a cerium content of 1700 ppm or more and 5000 ppm or less, and a Soxhlet extractor is used. After 8 hours of oil cleaning with methyl ethyl ketone, the cerium content is 50 ppm or more and 300 ppm or less.

According to a fourth aspect of the invention, there is provided a method for producing a precursor fiber bundle for a carbon fiber, comprising: 0.8 wt% or more and 1.6 wt% or less with respect to 100 wt% of the oil component of the swelled silk yarn; The polyxanthene compound is dried as an oil component as a main component, and then extended by a heat stretching method or a steam stretching method in a range of 1.8 times or more and 6.0 times or less.

According to a fifth aspect of the invention, in the method of producing a flame-resistant fiber bundle, the precursor fiber bundle is allowed to pass through a flame-retardant flame-retardant furnace at 220 ° C to 260 ° C for 30 minutes or longer and 100 minutes or shorter. The rate is set to 0% or more and 10% or less by heat treatment in an oxidizing atmosphere to produce a flame-resistant fiber bundle that satisfies the following four conditions: (1) a peak A in the equatorial direction obtained by fiber bundle wide-angle X-ray measurement (2θ) The intensity ratio (B/A) of the peak region B (2θ=17°) is 1.3 or more; (2) the alignment degree of the peak B is 80% or more; and (3) the alignment degree of the peak A is 79% or more. (4) The density is 1.335 g/cm ³ or more and 1.360 g/cm ³ or less.

The sixth invention is a carbon fiber bundle having a resin impregnated strand strength of 6000 MPa or more, and a strand elastic modulus of 250 GPa to 380 GPa as measured by the American Society for Testing and Materials (ASTM) method. The ratio of the major axis to the minor axis (long diameter/short diameter) of the cross section perpendicular to the fiber axis direction of the single fiber is 1.00 to 1.01, and the diameter of the single fiber is 4.0 μm to 6.0 μm in the direction of the single fiber and the fiber axis. In the vertical cross section, there are one or more and 100 or less voids having a diameter of 2 nm or more and 15 nm or less.

According to a seventh aspect of the invention, in the method of producing a carbon fiber bundle, the precursor fiber bundle is formed into a flame-resistant fiber bundle having a density of 1.335 g/cm ³ or more and 1.355 g/cm ³ or less by heat treatment in an oxidizing atmosphere. In an inert gas atmosphere, the first carbonization furnace having a temperature gradient of 300° C. or higher and 700° C. or lower is heated for 1 minute or more and 3.0 minutes or less while applying an elongation of 2% or more and 7% or less, and then in an inert atmosphere. In one or more carbonization furnaces having a temperature gradient from 1000 ° C to the calcination temperature, heat treatment is performed for 1.0 minute or more and 5.0 minutes or less while applying elongation of -6.0% or more and 2.0% or less.

[Effects of the Invention]

The swelling yarn of the present invention can inhibit the penetration of the silicone oil, which is a main component of the oil agent, into the surface layer portion of the precursor fiber. The carbon fiber bundle obtained by subjecting the precursor fiber bundle to flame-proofing and carbonization treatment is excellent in mechanical properties, and a fiber-reinforced resin having high mechanical properties can be obtained.

The above and other objects, features and advantages of the present invention will become more <RTIgt;

In the present invention, the coagulated wire refers to a step yarn which is extracted from the coagulating liquid and has not been supplied to the elongation treatment. The term "swelled silk" refers to a step yarn after performing the elongation treatment and the cleaning treatment on the coagulated yarn, and is a step yarn before the oil agent adhesion and drying treatment.

[swelling silk]

The carbon fiber acrylonitrile swelled yarn (hereinafter, referred to as "swelling yarn" as appropriate) of the present invention has a thickness of 0.3 / μm ² or more and 2 / μm ² or less on the surface of the single fiber in the state before the oil treatment. In the opening portion of the range, the opening portion has a width of 10 nm or more in the circumferential direction of the fiber. The swelled silk will adhere to an oil agent containing a Silicone compound and be dried, and then supplied to the step of stretching to form a precursor fiber bundle, which can greatly suppress the oil agent by having such a surface of the swelled silk. The ingredients penetrate into the surface layer of the swollen silk.

The polymer constituting the swelled silk is preferably an acrylonitrile unit of 96.0 wt% or more and 99.7 wt% or less, and an unsaturated hydrocarbon unit having one or more carboxyl groups or ester groups of 0.3 wt% or more and 4.0 wt% or less. An acrylic copolymer of the composition. By setting the content of the acrylonitrile to be 96.0 wt% or more and 99.7 wt% or less, the structural irregularity of the ladder polymer formed by the flameproofing reaction can be reduced, and the subsequent high temperature treatment can be suppressed. At the time of the decomposition reaction, a dense carbon fiber having less defects due to a decrease in strength can be obtained. Further, it is known that an unsaturated hydrocarbon component having a carboxyl group or an ester group is a starting point of a flame-retarding reaction in the flame-proofing step, and the content of the unsaturated hydrocarbon component having a carboxyl group or an ester group is 0.3 wt% or more and 4.0 wt. Below %, a flame resistant wire suitable for obtaining a carbon fiber containing a graphene laminate structure having irregular structure or less defects can be obtained in a high yield.

For the swelled silk, the swelled silk is subjected to a predetermined amount of adhesion treatment by using an oil agent containing a specific polyoxo compound, and after drying and densification, the oil is extracted and cleaned with methyl ethyl ketone for 8 hours. Then, the residual polyoxo compound was quantified to evaluate whether or not the swelling yarn had a surface portion capable of suppressing penetration of the oil component.

[Evaluation of oil permeability of expanded silk]

The oil permeability of the swelled silk can be evaluated by the following manner.

First, the following (1) amine-based modified polyoxyxane oil is mixed with (2) an emulsifier, and an aqueous dispersion (aqueous fiber oil agent) is prepared by a phase inversion emulsification method. The aqueous fiber oil agent is attached to the swelling yarn.

(1) Amine-based modified polyfluorene: KF-865 (manufactured by Shin-Etsu Chemical Co., Ltd., grade 1 side chain type, dynamic viscosity 110 cSt (25 ° C), amine equivalent weight 5,000 g/mol), 85 Wt%;

(2) Emulsifier: NIKKOL BL-9EX (manufactured by Nikko Chemicals Co., Ltd., polyoxyethylene (POE) (9) lauryl ether), 15 wt%.

Then, it was dried by a drying roll to completely evaporate the water, and then stretched twice between the heat rolls. In this manner, a fiber bundle having a cerium content of 1700 ppm or more and 5000 ppm or less obtained by a fluorescent X-ray device was obtained. Then, the ruthenium content of the fiber bundle after the 8-hour oil-extraction cleaning using methyl ketone using a Soxler extractor was measured by a fluorescent X-ray apparatus.

The swelling yarn of the present invention preferably has a cerium content (residual amount) after extraction and washing of the oil agent of 50 ppm or more and 300 ppm or less. The value is more preferably 50 ppm or more and 200 ppm or less.

The cerium content of the fiber bundle after the oil extracting and cleaning is more than 300 ppm, which means that the denseness of the surface layer portion which inhibits the penetration of the oil component into the surface layer portion is insufficient, and the carbon fiber obtained by the calcination step contains a large amount of voids in the surface layer portion. As a result, high-strength carbon fibers as targets are not obtained. On the other hand, the A value of less than 50 ppm means that the amount of oil permeation in the surface layer of the swollen silk is very small, and it is considered that the reason is that an extremely dense skin layer is formed in the surface layer portion of the fiber in the coagulation bath.

Moreover, it is more preferable that the swelling yarn of the present invention has a degree of swelling of 80 wt% or less as measured by [2. The method for measuring the degree of swelling of the swelling yarn] described later. The degree of swelling exceeds 80% by weight, indicating that the compactness of the inner layer structure of the swollen filament is slightly lowered. At this time, even in the surface layer portion, formation of defective spots can be suppressed, but the possibility of forming defective spots in the inner layer portion is improved, and as a result, carbon fibers having high mechanical properties cannot be obtained. A better degree of swelling is 75 wt% or less.

Further, the compactness of the swelled yarn can also be evaluated by measuring the pore distribution inside the fiber. The expanded pores of the present invention preferably have an average pore size of 55 nm or less and a total pore volume of preferably 0.55 ml/g or less. The average pore size is preferably 50 nm or less, more preferably 45 nm or less. Further, the total pore volume is more preferably 0.50 ml/g or less, still more preferably 0.45 ml/g or less. There is no large-sized void inside the fiber of such a swelled filament, and the proportion of the void is relatively small and dense. If a dense skin layer is formed on the surface of the fiber in the coagulation bath, there is a possibility that the pore size or the pore volume inside the fiber increases. In order to obtain the target high-strength carbon fiber, as described above, it is preferable to satisfy the following two aspects: densifying the surface portion of the swollen filament to suppress the penetration of the oil agent; and making the swollen filament have a dense structure in which the inner void of the fiber is less. In addition, the pore distribution of the swelled silk is measured by [4. Method for measuring pore size distribution of swelled silk] which will be described later.

[Manufacturing method of swelling yarn]

The swelling yarn of the present invention can be produced by wet spinning or dry-wet spinning a spinning dope comprising an acrylonitrile-based copolymer and an organic solvent.

The acrylonitrile-based copolymer is a copolymer obtained by copolymerizing acrylonitrile and an unsaturated hydrocarbon having one or more carboxyl groups or ester groups as essential components. Examples of the unsaturated hydrocarbon having one or more carboxyl groups or ester groups include acrylic acid, methacrylic acid, itaconic acid, methyl acrylate, methyl methacrylate, and ethyl acrylate. . It is preferred to use any one of the unsaturated hydrocarbons in an amount of 0.3% by weight or more and 4.0% by weight or less, or two or more kinds of the unsaturated hydrocarbons, and a total of 96.0% by weight or more and 99.7% by weight or less of acrylonitrile. The resulting acrylonitrile copolymer was polymerized. A more desirable acrylonitrile content is 98 wt% or more.

It is known that an unsaturated hydrocarbon component having a carboxyl group or an ester group is a starting point of a flame-proofing reaction in the flame-proofing step, and if the content of the unsaturated hydrocarbon component is too small, the flame-proofing reaction cannot be sufficiently performed, and the structure of the flame-proof fiber Formation creates obstacles. On the other hand, when the content of the unsaturated hydrocarbon component is too large, a rapid reaction occurs due to the presence of a plurality of reaction starting points, and as a result, a rough structural form is formed, and a carbon fiber having high performance cannot be obtained. When the content of the unsaturated hydrocarbon component is 0.3% by weight or more and 4.0% by weight or less, the balance between the flame-retarding reaction starting point and the reaction rate is good, and the structure is dense, and formation can be suppressed as a defect in the carbonization step. The structural irregularity of the point. Further, since it has moderate reactivity, the flame-proof reaction can be generated in a relatively low temperature region, and the flame-proof treatment can be performed in terms of economy and safety. Therefore, it is possible to obtain a flame resistant wire which is suitable for obtaining carbon fibers containing a graphene laminate structure having irregular structure or less defects in a high yield.

As the third component, an acrylamide derivative such as acrylamide, methacrylamide, N-methylol acrylamide or N,N-dimethyl acrylamide, vinyl acetate can be used. Vinyl acetate and the like. A suitable method for copolymerizing a mixture of monomers may be, for example, redox polymerization in an aqueous solution, suspension polymerization in a heterogeneous system, and emulsion polymerization using a dispersant, and any other polymerization method, the present invention It is not limited by the differences in the polymerization methods.

In the spinning step, the acrylonitrile-based copolymer is first dissolved in an organic solvent at a concentration of 20% by weight to 255% by weight to prepare a spinning dope having a temperature of 50 ° C to 70 ° C. The solid content concentration of the spinning dope is preferably 20% by weight or more, more preferably 21% by weight or more. By setting the solid content concentration to 20% or more, the amount of solvent which moves from the inside of the monofilament to the outside during solidification can be reduced, and a coagulated yarn having a desired denseness can be obtained. Further, by setting the solid content concentration to 25 wt% or less, an appropriate stock solution viscosity can be obtained, and the stock solution can be stably ejected from a nozzle, and the coagulated yarn can be easily produced. That is, by setting the solid content concentration to 20 wt% to 25 wt%, it is possible to stably produce a coagulated yarn having a dense and uniform structure.

Further, by setting the temperature of the spinning dope to 50 ° C or higher, an appropriate liquid viscosity can be obtained without lowering the solid content concentration, and the temperature of the spinning solution can be reduced by setting the temperature of the spinning dope to 70 ° C or lower. difference. In other words, by setting the temperature of the spinning dope to 50 to 70 ° C, the coagulated yarn having a high density and uniform structure can be stably produced.

The organic solvent is not particularly limited, and it is more preferred to use dimethylformamide, dimethyl acetamide or dimethyl sulfoxide. More preferably, it is dimethylformamide which is excellent in the dissolving ability of an acrylonitrile-type copolymer.

The spinning method can be any of wet spinning, dry wet spinning. More preferably dry and wet spinning. The reason is that it is easy to form a dense coagulated yarn by dry-wet spinning, and in particular, the denseness of the surface portion can be improved. In the dry-wet spinning, the prepared spinning dope is temporarily spun from a spinning nozzle provided with a plurality of nozzle holes into the air, and then ejected to a coagulating liquid filled with the mixed solution of the temperature-controlled organic solvent and water. Solidified in the middle, and the solidified silk is extracted. Among them, the coagulating liquid preferably has a temperature of -5 ° C to 20 ° C and an organic solvent concentration of 78 wt% to 82 wt %. The reason is that it is easy to form a dense coagulated yarn within this range, and in particular, the denseness of the surface layer portion can be improved. A more preferable temperature range is -5 ° C to 10 ° C, and a more preferable organic solvent concentration is in the range of 78.5 wt% or more and 81.0 wt% or less. By setting the organic solvent concentration of the coagulation liquid to 81.0 wt% or less, the denseness of the surface layer portion can be maintained, and the penetration of the oil agent into the surface layer portion of the fiber can be suppressed. Further, by setting the organic solvent concentration to 78.5 wt% or more, it is possible to suppress rapid solidification of the surface layer during solidification, suppress formation of the skin layer, and since the solidification rate is relatively slow, the internal compactness does not decrease. That is, by setting the organic solvent concentration of the coagulating liquid to 78.5 wt% to 81.0 wt%, a dense coagulated yarn can be obtained both on the surface and inside of the fiber.

The coagulation wire is stretched and cleaned. The order of extension and cleaning treatment is not particularly limited, and it can be cleaned after stretching, and can be extended and cleaned at the same time. Further, the cleaning method may be any method as long as it can be desolvated. A particularly advantageous extension and cleaning treatment for the coagulated filaments is to extend in a front extension trough having a lower solvent concentration and a higher temperature than the coagulating liquid prior to washing. Thereby, the coagulated filaments can be formed into a uniform fibril structure.

Previously, the extension of the coagulated filaments in a bath containing a solvent is a well-known method as a solvent extension technique, which achieves a stable elongation treatment by solvent plasticization, and as a result, a coagulated filament having a high uniformity in structure and orientation can be obtained. . However, since the fiber bundle containing the solvent is directly stretched in a swollen state, the alignment of the structure due to the formation and elongation of the fibril structure is insufficient, and since the oil agent is also rapidly extruded from the inside of the monofilament, Therefore, the obtained monofilament easily forms a loose structure, and a monofilament having a dense structure as a target cannot be obtained. In the present invention, by performing the most suitable setting on the temperature and concentration of the spinning dope and the coagulating liquid, and performing the solvent stretching treatment in combination with the conditions of the most suitable solvent extending groove and the stretching ratio, the dense forming can be formed. Fibrous structure.

First, the coagulated tow containing the organic solvent is extended in the air, and then the stretching treatment is carried out in an extension tank containing a warm aqueous solution containing an organic solvent. The temperature of the warm aqueous solution is preferably in the range of 40 ° C or more and 80 ° C or less. By setting the temperature to 40 ° C or higher, good elongation can be ensured, and a uniform fibril structure can be easily formed. Further, when the temperature is 80 ° C or lower, since excessive plasticization does not occur, the solvent removal on the surface of the yarn is moderately performed, and the elongation is relatively uniform, and the quality of the expanded yarn is good. More preferably, the temperature is 55 ° C or more and 75 ° C or less.

Further, the concentration of the organic solvent in the warm aqueous solution containing the organic solvent is preferably 30% by weight or more and 60% by weight or less. This concentration is a range that provides a stable extension treatment to form a dense and uniform fibril structure inside and on the surface of the coagulated filament. A more suitable concentration is 40 wt% or more and 50 wt% or less.

A preferred method of extending the coagulated yarn is to set the stretching in the air to 1.0 times or more and 1.25 times or less, and to set the total stretching ratio in the air to the warm aqueous solution to be 2.6 times or more and 4.0 times or less. The coagulated filament has a fibril structure which is swollen with a large amount of solvent. By setting the extension of the coagulated filament composed of such a structure in the air to 1.0 times or more and 1.25 times or less, formation of a loose fibril structure can be avoided. Further, by setting the extension in the above air to 1.0 times or more, uneven shrinkage can be suppressed.

Further, by setting the total stretching ratio in the air to the warm aqueous solution to 2.6 or more, sufficient stretching can be performed, and a fibril structure aligned in the desired fiber axis direction can be formed. In addition, by setting the total stretching ratio to 4.0 times or less, the fibril structure itself is not broken, and a precursor fiber bundle composed of a dense structural form can be obtained. That is, in a range of 2.6 times or more and 4.0 times or less, a dense fibril structure aligned in the axial direction of the fiber can be formed. The total extension ratio is preferably 2.7 times or more and 3.5 times or less.

Further, a more preferable extension method is to set the stretching ratio in the organic solvent warm aqueous solution to 2.5 times or more. The reason is that since the elongation in the organic solvent warm aqueous solution is extended at a relatively high temperature, by extending the above-described stretching ratio to 2.5 times or more, the stretching can be performed without causing structural damage. Therefore, the elongation distribution in the air and the organic solvent warm aqueous solution is preferably such that the distribution of the elongation in the organic solvent warm aqueous solution is set to be high. The extension in the air is more preferably 1.0 times or more and 1.15 times or less.

Thus, a dense swelled silk having a surface layer portion can be obtained, and in order to obtain a more dense swelled silk, a swelled yarn can be produced by the above-described stretching method by using a coagulated tow containing an organic solvent having a swelling degree of 160 wt% or less. The reason is that the inner structure of the coagulated filament is dense.

After the elongation treatment, the fiber bundle is washed with warm water of 50 ° C or more and 95 ° C or less to remove the organic solvent. Further, after the cleaning, the fiber bundle in a swollen state in which the solvent-free component is stretched can be further extended in hot water to further increase the alignment of the fibers, and a plurality of slacks can be applied to eliminate the strain of elongation. It is preferred to carry out the stretching of 0.98 times or more and 2.0 times or less in hot water having a temperature of 70 to 95 °C. The stretching ratio of the stretching ratio of 0.98 times or more to less than 1.0 times is a relaxation treatment. By extending the elongation strain of the fiber bundle supplied at a high stretching ratio in the previous stage, it is effective for obtaining a stable extension in the subsequent stretching step. When the stretching ratio is in the range of 1.0 times or more and 2.0 times or less, the degree of orientation of the fibril structure can be improved and the denseness of the surface layer can be improved. More preferably, it is extended by 0.99 times or more and 1.5 times or less.

The swelling yarn can be obtained by performing the stretching treatment and the cleaning treatment on the coagulated yarn in the above manner.

[dry heat extension]

The swelled silk is subjected to a predetermined amount of adhesion treatment of the oil agent, and is dried and densified. The drying densification can be dried and densified by a known drying method, and is not particularly limited. Preferably, a method of passing the swelled filament through a plurality of heating rolls is employed. The dried and densified fiber bundle is further extended in a pressurized steam of 130° C. to 200° C., a dry heat medium of 100° C. to 200° C., or a heating roll of 150° C. to 220° C. or a hot plate. After the alignment and densification, the precursor fiber bundle is obtained by winding up.

[precursor fiber bundle]

The precursor fiber bundle for carbon fibers (hereinafter referred to as "precursor fiber bundle" as appropriate) contains acrylonitrile, 0.3 wt% or more, and 4.0 wt% or less of 96.0 wt% or more and 99.7 wt% or less. The acrylonitrile copolymer obtained by copolymerization of the above-mentioned carboxyl group or ester group unsaturated hydrocarbon as an essential component is treated with an oil agent containing a polyfluorene-based compound as a main component, and the cerium content is 1700 ppm or more and 5000 ppm or less. After 8 hours of oil cleaning with methyl ethyl ketone using a Soxler extractor, the cerium content was 50 ppm or more and 300 ppm or less. The cerium content can be determined using a fluorescent X-ray device. In addition, the cerium content after the oil agent cleaning is a measured value from the evaluation of the adhesion from the oil agent to the oil agent in the above [Evaluation of oil permeability of the swelled silk].

If the cerium content of the precursor fiber bundle after the treatment with the oil agent is 1700 ppm or more and 5000 ppm or less, the fusion between the filaments does not occur in the flame-proofing step, and on the other hand, the polysiloxane compound due to the surface layer can be prevented. Excessively, oxygen diffuses into the interior of the monofilament, and a portion where the flame-proof reaction is insufficient is not generated, and the occurrence of breakage in the carbonization step of the higher-temperature treatment can be suppressed. As a result, stable step passability can be maintained.

After the precursor fiber bundle of the present invention is subjected to oil extraction and cleaning, the fiber bundle has a cerium content of 300 ppm or less. Among them, the cerium content exceeding 300 ppm means that the polyoxygenated compound oil penetrates into the surface layer portion and is present in a large amount. As a result, in the flame-proofing and pre-carbonization step (below 800 ° C) of the calcination step, the polysulfonated oil residue existing in the surface layer portion does not scatter, but scatters in the later carbonization step (over 800 ° C), thereby The final carbon fiber surface A large number of voids are formed in the portion. Therefore, the target high-strength carbon fiber cannot be obtained. On the other hand, the cerium content of the fiber bundle after the oil-extraction cleaning is 300 ppm or less, indicating that the polyfluorene oxide adhered to the precursor fiber penetrates into the surface layer portion and exists in the vicinity of the surface layer, but the extraction is difficult. The proportion is small, and polyoxynitride is present in the outermost layer. In such a state, in the flame-proofing step or the carbonization step of the calcination step, the polyfluorene-based compound can be scattered from the outermost layer portion without forming a defect. The cerium content after oil extraction and washing is more preferably 200 ppm by weight or less.

In the precursor fiber bundle, the fineness of the single fiber is preferably 0.5 dtex or more and 1.0 dtex or less, and the ratio of the major axis to the minor axis (long diameter/short diameter) of the fiber cross section of the single fiber is 1.00 or more and 1.01 or less. The surface uneven structure extending along the fiber axis direction of the single fiber, the height difference (Rp-v) between the highest part and the lowest part is 30 nm or more and 100 nm or less, and the center line average roughness (Ra) is 3 nm or more and 10 nm. the following. The present inventors believe that if the (Rp-v) value is 30 nm or more, or the (Ra) value is 3 nm or more, the smoothness of the surface of the precursor fiber monofilament is not excessive. Thereby, there is no possibility that the surface layer fibrils are slightly broken due to the formation of the skin layer in the solidification step, resulting in a low elongation in the spinning step, and formation of minute defects can be avoided. Further, it is also possible to prevent the fiber bundle which is an aggregate of the monofilaments from being excessively bundled, and to prevent the flame-proof treatment from being uneven due to the diffusion of oxygen into the inside of the monofilament in the flame-proofing step. On the other hand, by setting the (Rp-v) value to 100 nm or less or the (Ra) value to 10 nm or less, the compactness of the structure in the vicinity of the surface layer can be made sufficiently high. That is, when the surface has a (Rp-v) value of 30 nm or more and 100 nm or less and a (Ra) value of 3 nm or more and 10 nm or less, the denseness of the structure near the surface layer can be obtained sufficiently. The level, and also has a sufficiently extensible structure, in the spinning step to the calcining step, the possibility of forming defect points near the surface layer can be reduced. As a result, a high-strength carbon fiber bundle can be obtained.

Here, the surface uneven structure extending in the fiber axis direction means a wrinkle structure which is substantially parallel to the fiber axis direction and has a length of 0.6 μm or more. Generally, the acrylonitrile fiber bundle is subjected to volume shrinkage by solidification and subsequent elongation treatment, and a wrinkle structure extending in the fiber axis direction is formed on the surface. The formation of the wrinkle structure can be suppressed by suppressing the formation of a strong skin layer in the solidification step and achieving a slow volume shrinkage. Further, it is known that dry-wet spinning can suppress the formation of the wrinkle structure to a large extent. The precursor fiber bundle preferably does not have such a wrinkle structure having a length of 0.6 μm or more.

A fiber having a ratio of a long diameter to a short diameter (long diameter/short diameter) of a single fiber cross section of 1.00 to 1.01 is a single fiber having a cross section of a perfect circle or a nearly perfect circle, and structural uniformity near the fiber surface of the single fiber. Excellent. The ratio of the long diameter to the short diameter (long diameter/short diameter) is preferably from 1.00 to 1.005.

The fiber having a single fiber fineness ranging from 0.5 dtex to 1.0 dtex has a small fiber diameter, so that the structural unevenness in the cross-sectional direction generated in the calcination step can be reduced. The fineness of the single fiber is more preferably from 0.5 dtex to 0.8 dtex.

[Method of Manufacturing Precursor Fiber Bundle]

The above-mentioned predetermined amount of the cerium content precursor fiber bundle can be produced by attaching an oil agent containing a polyoxynitride as a main component to the swelled yarn of the present invention and drying it, by heat stretching or steam stretching. Implement extension processing.

The polyoxosiloxane as a main component of the oil agent is not particularly limited, and from the viewpoint of interaction with the acrylonitrile-based copolymer, it is preferred to use an amine-modified polydimethyl siloxane or an epoxy. Modified polydimethyl siloxane. In particular, since the surface layer portion of the swelled silk of the present invention has high compactness, it is preferably an amine-based modified polydimethyl oxime from the viewpoint of ease of coating of the surface layer and ease of detachment from the surface layer. alkyl.

Further, from the viewpoint of heat resistance, an amino group-modified polydimethyl siloxane having a part of a methyl group of a polydimethyl siloxane skeleton substituted with a phenyl group is excellent. The most suitable amine-modified polydimethyloxane is an amine-based modified polydimethyl group having a dynamic viscosity of 50 cst to 5,000 cst at 25 ° C and an amine equivalent weight of 1,700 g/mol to 15,000 g/mol. Oxane.

The type of the amino group modification is not particularly limited, and a first-order side chain type, a first-order side chain type, and a two-end modified type are suitable. In addition, a hybrid type of these may be used, or a plurality of types may be used in combination. If the dynamic viscosity at 25 ° C is 50 cst or more, it has a sufficient molecular weight which is not volatile, and the amine-modified polydimethyl methoxyalkane can be inhibited from being scattered from the fiber in the entire flame-proofing step to exert the original step oil. The function of the agent, so that the carbon fiber can be stably produced. Further, by setting the dynamic viscosity at 25 ° C to 5000 cst or less, in the flame-proofing step, a part of the oil agent is transferred from the fiber bundle to the roll or the like, and the viscosity is increased by receiving the heat treatment for a relatively long period of time. Adhesiveness results in frequent failure of a portion of the bundle of fibers attached to the roll. Further, by making the amine group equivalent of 1,700 g/mol or more, the thermal reactivity of the polyfluorene is suppressed, and a part of the oil agent can be prevented from being transferred from the fiber bundle to the roll or the like, and a part of the fiber bundle is wound onto the roll. The fault on the. By making the amine group equivalent of 15,000 g/mol or less, the precursor fiber has sufficient affinity with the polyfluorene oxide, and the polyfluorene oxide is prevented from scattering from the fiber throughout the flameproofing step. That is, as long as the dynamic viscosity at 25 ° C of the oil agent is from 50 cst to 5,000 cst and the amine equivalent is in the range of from 1,700 g/mol to 15,000 g/mol, no transfer of the oil agent to the roll or the like occurs. In the case where the fiber wrap is broken, and the oil agent is rushed and scattered during the flame-proofing step, the operation can be continuously and stably performed from the spinning to the flame-proof treatment for a long period of time.

The amine-modified polydimethyl siloxane of the first-stage side chain type may, for example, be KF-864, KF-865, KF-868 or KF-8003 (all manufactured by Shin-Etsu Chemical Co., Ltd.). Examples of the amine-based modified polydimethyl siloxane of the 1,2-stage side chain type include KF-859, KF-860, KF-869, and KF-8005 (all manufactured by Shin-Etsu Chemical Co., Ltd.). The two-terminal modified amino-modified polydimethyl siloxane can be exemplified by Silaplane FM-3311, Silaplane FM-3221, Silaplane FM-3325 (all manufactured by Chisso Co., Ltd.) or KF-8012 (Shin-Etsu Chemical) Manufactured by an industrial company).

The oil agent includes a surfactant for forming an oil agent as an aqueous emulsion, or a softening agent or a lubricating agent which imparts excellent step passability. As the surfactant, a nonionic surfactant is mainly used, and an ethylene oxide/propylene oxide (EO/PO) adduct of a Pluronic type or a higher alcohol can be used. Particularly suitable are Newpol PE-78, Newpol PE-108, Newpol PE-128 (all products of Sanyo Chemical Industry Co., Ltd.) as a polyoxyethylene/polyoxypropylene block polymer.

As the softener or lubricant, an ester compound or a urethane compound or the like is used. The content of the polyoxosiloxane in the oil agent is from 30 wt% to 90 wt%. When the content of the polyoxyxene compound is 30% by weight or more, the welding can be sufficiently suppressed in the flame-proofing step. Further, when the content of the polyoxyxene compound is 90% by weight or less, the stability of the emulsion of the oil agent can be easily achieved to a sufficient level, and the precursor fiber can be stably produced. That is, as long as the content of the polyfluorene-based compound in the oil agent is from 30% by weight to 90% by weight, even the surface-densified precursor fiber as in the present invention can sufficiently exert its effect in the flame-proofing step. The effect of welding can achieve stability in the oil adhering step and uniformity of the attached state, and thus the performance of the obtained carbon fiber can be found to be stable.

The amount of the oil agent having a polyoxyxene compound as a main component is from 0.8 wt% to 1.6 wt%. After the oil agent adhesion treatment, the fibers are supplied to dry densification. Drying and densification can be dried and densified by a known drying method, and is not particularly limited. A method of passing the fibers through a plurality of heating rolls is preferred. By setting the amount of the oil agent to be 0.8 wt% to 1.6 wt%, it is possible to reduce the irregularity of the flame-proof structure caused by the insufficient adhesion of the oil agent or the insufficient diffusion of oxygen due to excessive adhesion. Carbon fibers with high strength can be produced.

The fiber bundle after drying and densification is further extended by 1.8 to 6.0 times in pressurized steam or dry heat medium at 130 ° C to 200 ° C or between heating rolls or heating plates to further improve alignment and density. The precursor fiber bundle is obtained. A more preferable stretching ratio is 2.4 to 6.0 times, more preferably 2.6 to 6.0 times.

[Method of Manufacturing Flame-Resistant Fiber Bundles]

The precursor fiber bundle is subjected to heat treatment in an oxidizing atmosphere by setting the elongation to 0% or more and 10% or less in a hot air circulation type flameproof furnace of 220 ° C to 260 ° C for 30 minutes or more and 100 minutes or less. Thereby, a flame-resistant fiber bundle having a density of 1.335 g/cm ³ or more and 1.360 g/cm ³ or less can be obtained. In the flameproofing reaction, there is a cyclization reaction caused by heat and an oxidation reaction caused by oxygen, so that the balance of the two reactions is important. In order to balance the two reactions, the flameproofing treatment time is suitably 30 minutes or longer and 100 minutes or shorter. When the flameproofing treatment time is less than 30 minutes, a portion where the oxidation reaction does not sufficiently occur is present inside the single fiber, so that a large structural unevenness occurs in the cross-sectional direction of the single fiber. As a result, the carbon fiber obtained had a non-uniform structure, and high mechanical properties could not be found. When the flameproofing treatment time exceeds 100 minutes, more oxygen is present in the portion close to the surface of the single fiber, and the heat treatment at a high temperature thereafter causes a reaction in which excess oxygen disappears, forming a defect point, and thus cannot be obtained high. Strength of carbon fiber.

A more preferable flameproofing treatment time is 40 minutes or more and 80 minutes or less. When the flameproof filament density is less than 1.335 g/cm ³ , the flameproofing is insufficient, and the decomposition reaction occurs by the heat treatment at a high temperature thereafter to form a defect point, so that high-strength carbon fibers cannot be obtained. When the flameproof filament density exceeds 1.360 g/cm ³ , the oxygen content of the fiber increases. Therefore, by the heat treatment at a high temperature thereafter, excessive oxygen disappears and a defect is formed, so that high-strength carbon fibers cannot be obtained. A more preferable range of the flameproof filament density is 1.340 g/cm ³ or more and 1.350 g/cm ³ or less.

In order to maintain and improve the alignment of the fibril structure forming the fibers, it is necessary to carry out moderate elongation in the flameproof furnace. If the elongation is less than 0%, the alignment of the fibril structure cannot be maintained, and the alignment of the carbon fibers is insufficient in the formation of the fiber axis, and excellent mechanical properties cannot be found. On the other hand, when the elongation exceeds 10%, the fibril structure itself is broken, and the structure of the carbon fiber after the damage is formed, and the breaking point becomes a defect point, and high-strength carbon fibers cannot be obtained. A more preferable elongation is 3% or more and 8% or less.

In a preferred method for producing a flame-retardant fiber bundle, the flame-retardant fiber bundle obtained by heat-treating the precursor fiber bundle in the above-described oxidizing environment, that is, the equatorial direction obtained by the fiber bundle wide-angle X-ray measurement is obtained. The intensity ratio (B/A) of the peak A (2θ=25°) to the peak B (2θ=17°) is 1.3 or more, the alignment degree of the peak A is 79% or more, and the alignment degree of the peak B is 80% or more. It is 1.335 g/cm ³ or more and 1.360 g/cm ³ or less.

The crystal structure derived from the polyacrylonitrile (100) reflection of the peak B (2θ = 17°) is closely related to the structural formation of the carbon fiber. Further, once the crystal orientation or crystallinity is lowered in the production process of the carbon fiber, it is difficult to recover as it is, and it is possible to lower the performance of the carbon fiber. Here, the above (100) represents a crystal orientation. In particular, the flame-proofing step is a step of greatly changing the structure of the precursor fiber, and is also a step of forming a basis of the basic structure of carbon fiber, that is, graphite crystal. The crystal structure of the peak of the polyacrylonitrile (100) reflected by the peak B (2θ=17°) is particularly changed by the flame-proofing step, and the crystal structure is different depending on the conditions of the flame-proofing process. The degree of change is significantly different. In order to obtain a flame-retardant fiber having a high alignment, it is necessary to carry out an appropriate treatment, and the degree of orientation is closely related to the crystallinity, and as the degree of orientation is lowered, the crystallinity is remarkably lowered. On the other hand, if the high alignment can be maintained, a flame retardant fiber having high crystallinity can be obtained. For the above reasons, it is preferable to have a flame-retardant fiber bundle having a crystal structure having an intensity ratio (B/A) of 1.3 or more, an orientation of the peak A of 79% or more, and an orientation of the peak B of 80% or more.

By using the precursor fiber bundle of the present invention, the flame-proof fiber bundle as described above can be obtained relatively easily. Further, in the step of heat-treating the precursor fiber bundle in an oxidizing atmosphere, it is preferred to divide the elongation processing conditions into at least three blocks, and set the following flameproof conditions, that is, the fiber density is 1.200 g. In the range of /cm ³ or more and 1.260 g/cm ³ or less, elongation of 3.0% or more and 8.0% or less is performed, and then 0.0% or more is carried out in a range of density of 1.240 g/cm ³ or more and 1.310 g/cm ³ or less. The elongation is 3.0% or less, and then the elongation is -1.0% or more and 2.0% or less in the range of 1.300 g/cm ³ or more and 1.360 g/cm ³ or less.

[carbon fiber]

Then, in a first carbonization furnace having a temperature gradient of 300° C. or more and 800° C. or less in an inert atmosphere such as nitrogen gas, the flame-retardant fiber bundle is applied for an elongation of 2% or more and 7% or less, and is performed for 1.0 minute. Heat treatment for 3.0 minutes. Suitable treatment temperatures are from 300 ° C to 800 ° C and are processed under a linear gradient. If the temperature of the flameproofing of the previous step is considered, the initial temperature is preferably 300 ° C or higher. If the maximum temperature exceeds 800 ° C, the fibers become very brittle and it is difficult to proceed to the next step. A more suitable temperature range is from 300 ° C to 750 ° C. A more preferable temperature range is from 300 ° C to 700 ° C.

The temperature gradient is not particularly limited, and is preferably set to a linear gradient. If the elongation is less than 2%, the alignment of the fibril structure cannot be maintained, and the alignment of the carbon fibers is insufficient in the formation of the fiber axis, and excellent mechanical properties cannot be found. On the other hand, when the elongation exceeds 7%, the fibril structure itself is broken, and the structure of the carbon fiber after the damage is formed, and the breaking point becomes a defect point, and high-strength carbon fibers cannot be obtained. A more preferable elongation is 3% or more and 5% or less. A suitable treatment time is from 1.0 minutes to 3.0 minutes. If the treatment is less than 1.0 minute, a severe decomposition reaction occurs as the temperature rises rapidly, and high-strength carbon fibers cannot be obtained. When the treatment is carried out for more than 3.0 minutes, there is a tendency that the plasticization in the early stage of the step is affected, and the degree of alignment of the crystal tends to decrease, and as a result, the mechanical properties of the obtained carbon fiber are impaired. A more suitable heat treatment time is from 1.2 minutes to 2.5 minutes.

Next, in a second carbonization furnace which can set a temperature gradient in an inert environment such as nitrogen gas in a range of 1000 ° C to 1600 ° C, heat treatment is performed in a tension state to obtain carbon fibers. Further, if necessary, it is added to a third carbonization furnace having a desired temperature gradient in an inert atmosphere, and heat treatment is performed in a tensioned state. The setting of the temperature of the carbonization process depends on the modulus of elasticity required for the carbon fiber. In order to obtain carbon fibers having high mechanical properties, it is preferred that the maximum temperature of the carbonization treatment is low, and since the modulus of elasticity can be increased by making the treatment time longer, the maximum temperature can be lowered. Further, by making the processing time longer, the temperature gradient can be set to be gentler, and the formation of defective spots can be suppressed.

The temperature of the second carbonization furnace is also affected by the temperature setting of the first carbonization furnace, but it may be 1000 ° C or more. The temperature of the second carbonization furnace is preferably 1050 ° C or higher. The temperature gradient is not particularly limited, and is preferably set to a linear gradient. The treatment time is suitably from 1.0 minutes to 5.0 minutes. The treatment time is preferably from 1.5 minutes to 4.2 minutes. In this heat treatment, since the fiber bundle is accompanied by a large degree of shrinkage, it is important to perform heat treatment in a tension state. The elongation is suitably -6.0% to 2.0%. If the elongation is less than -6.0%, the orientation in the fiber axis direction of the crystal is poor, and sufficient performance cannot be obtained. On the other hand, when the elongation exceeds 2.0%, the structure formed so far is destroyed by itself, the defect point is remarkably formed, and the strength is largely lowered. A more suitable elongation is in the range of -5.0% to 0.5%.

The carbon fiber bundle thus obtained is supplied to a surface oxidation treatment. The surface treatment method may be a known method, that is, any one of methods may be employed by oxidation treatment such as electrolytic oxidation, chemical oxidation, and air oxidation. The industrially widely practiced electrolytic oxidation treatment achieves stable surface oxidation treatment and is the most suitable method from the viewpoint of controlling the surface treatment state by changing the amount of electricity. At this time, even under the same amount of electricity, depending on the electrolyte used and its concentration, the surface state will be greatly different. It is preferred to use carbon fiber as the anode and 10 Coul in the alkaline aqueous solution having a pH of more than 7. Coulomb) / g ~ 200 Coul / g of electricity for oxidation treatment. The electrolyte is suitably used: ammonium carbonate, ammonium bicarbonate, calcium hydroxide, sodium hydroxide, potassium hydroxide or the like.

Then, the carbon fiber bundle is supplied to the sizing treatment. The sizing agent is obtained by dissolving a solution obtained by dissolving in an organic solvent or an emulsion obtained by dispersing in water with an emulsifier or the like, and applying it to a carbon fiber bundle by a roll dipping method, a roll contact method, or the like, and drying the carbon fiber bundle. Sizing treatment. Further, the amount of the sizing agent adhered to the surface of the carbon fiber can be adjusted by adjusting the concentration of the sizing agent liquid or adjusting the amount of squeezing. Further, the drying can be carried out by using hot air, a hot plate, a heating roll, various infrared heaters or the like. Then, after the sizing agent is attached and dried, it is taken up on a bobbin to obtain a carbon fiber bundle.

By using the precursor fiber bundle or the flame-proof fiber bundle of the present invention and applying the above-described calcination method, a carbon fiber bundle excellent in mechanical properties can be obtained.

The carbon fiber bundle of the present invention has a resin impregnated strand strength of 6000 MPa or more, a strand elastic modulus measured by the ASTM method of 250 GPa to 380 GPa, and a long diameter and a short diameter of a section of the single fiber perpendicular to the fiber axis direction. The ratio (long diameter/short diameter) is 1.00 to 1.01, and the diameter of the single fiber is 4.0 μm to 6.0 μm. In the cross section perpendicular to the fiber axis direction of the single fiber, there are one or more and 100 or less diameters of 2 nm. Above, below 15 nm gap. Since the gap is less than 100, it can have a very high strand strength. In particular, a high elastic modulus carbon fiber bundle can also be found in high strand strength. More preferably, the above-mentioned voids are 50 or less carbon fiber bundles.

A more preferable carbon fiber bundle is such that the average diameter of the void having a diameter in the range of 2 nm to 15 nm observed in a cross section perpendicular to the fiber axis direction of the single fiber is 6 nm or less. The average diameter is 6 nm or less, which means that the oil agent is uniformly present on the precursor fiber bundle, and the oil-free agent partially penetrates more. By ensuring this 6 nm or less, stable carbon fiber strength discovery can be achieved.

In the carbon fiber bundle of the present invention, the total area A (nm ² ) of the voids existing in the cross section perpendicular to the fiber axis direction of the single fibers is preferably 2,000 nm ² or less. Further, it is preferable that a void corresponding to 95% or more of the total A (nm ² ) exists between the surface of the fiber and a position having a depth of 150 nm. The single fiber has such a structure that it is represented in the precursor fiber bundle, and the oil agent exists only in the surface layer portion near the surface layer.

In the present invention, it is preferred that the tensile breaking stress of the carbon fiber bundle of the nodule is divided by the sectional area of the fiber bundle (weight and density of the carbon fiber bundle per unit length) to have a knot strength of 900 N/mm ² or more. The knot strength is more preferably 1000 N/mm ² or more, and still more preferably 1100 N/mm ² or more. The knot strength can be used as an index reflecting the mechanical properties of the fiber bundle in a direction other than the fiber axis, and in particular, the performance in a direction perpendicular to the fiber axis can be easily seen. Composite materials often form materials by quasi-isotropic lamination to form complex stress fields. At this time, in addition to the tensile and compressive stress in the fiber axis direction, stress in a direction other than the fiber axis is generated. Further, in the case where a relatively high-speed strain is applied, such as an impact test, the stress state inside the material is very complicated, and the strength in the direction different from the fiber axis direction becomes important. Therefore, if the knot strength is less than 900 N/mm ² , the quasi-isotropic material cannot find sufficient mechanical properties.

[Example]

Hereinafter, the present invention will be specifically described by way of examples. Further, the measurement and evaluation of the properties of the various fibers in the present example were carried out by the following methods.

[1. Determination of swelling degree of coagulated silk]

The fiber bundle moved in the spinning step was collected and placed in a sealed polyethylene bag, and immediately stored in a refrigerator at 5 ° C or lower. The time from the start of storage to the end of the measurement of the degree of swelling was set to be within 8 hours.

After weighing the pre-dried weighing bottle with a direct reading balance, about 3 g of the sample was taken from the above fiber bundle and placed in a weighing bottle and weighed. The sample was placed in a dehydration cylinder of a table centrifuge and placed in a centrifuge. After centrifugation (crude dehydration) for 10 minutes at 3000 rpm, the dehydrated sample was transferred to a weighing bottle and weighed. This weight was taken as the wet weight A.

In the case where the sample after coarse dehydration still contains a solvent, it is sufficiently washed with water and then dehydrated. The crude dehydrated or washed and dehydrated sample was transferred to a weighing bottle, and dried in a dryer at 105 ° C for 3 hours while removing the lid. The dried sample was directly transferred to a desiccator in a state of being placed in a weighing bottle, and after slowly cooling for 20 minutes to 30 minutes, the weight of the symmetric measuring bottle was weighed. This weight was taken as the dry weight B.

The degree of swelling is calculated by the following formula.

Degree of swelling (%) = (A-B) / B × 100%

[2. Method for measuring the swelling degree of swollen silk]

The swelled silk collected in the spinning step was used as a sample. The measurement was carried out by the same method as the measurement of the degree of swelling of the coagulated filament.

[3. Observation of surface morphology of swollen silk]

The swelled silk collected in the spinning step was used as a sample. The solvent contained in the swollen silk is replaced with t-butanol, and after the swollen silk is rapidly frozen in liquid nitrogen, the temperature is maintained at -30 ° C to -25 ° C, and the pressure is reduced at about 3 Pa. The fiber sample was lyophilized for 24 hours. After the dried fiber sample was fixed on a scanning electron microscope (SEM) observation sample table with a carbon paste, the platinum was sputter-plated to a thickness of about 3 nm using a sputtering apparatus. The surface morphology was observed under the conditions of an acceleration voltage of 3 kV and an observation magnification of 50,000 times by a scanning electron microscope (Japan Electronics Co., Ltd., product name: JSM-7400F).

The width in the circumferential direction was measured for the voids which were opened at the surface of the fiber, and the number of voids having a width exceeding 10 nm was counted. The same measurement was performed on 50 or more swelling wires, and the total number of voids and the observation area were measured, and the average value (the average number of openings) per the area (1 μm ² ) of the voids was determined.

[4. Method for determining pore distribution of swollen silk]

The swelled silk collected from the spinning step was dried in the following manner. That is, the swelled silk is fixed to a fixed length so that the swelled silk does not undergo shrinkage deformation during the drying process, and the mixing ratio of the water/third butanol is 80/20, 50/50, 20/80, 0/100. The mixture was separately immersed for 30 minutes to replace the solvent contained in the swelled filament with the third butanol. Then, the sample of the expanded silk was placed in a flask, and after rapid freezing in liquid nitrogen, the sample was kept at a temperature of -30 ° C to -20 ° C, and freeze-dried for 24 hours under a reduced pressure of 100 Pa or less. ~72 hours.

The lyophilized swollen tow sample was cut into a length of about 10 mm with a blade and weighed about 0.15 g by a mercury porosimeter (Shimadzu Corporation, product name: Autopore IV) at atmospheric pressure~ The pore distribution was measured under conditions of a maximum pressure of 30,000 psia. The average pore size (nm) was determined as the volume average pore size obtained by weighting the pore volume to the pore volume. In addition, the mercury intrusion amount V1 (ml/g) at a pressure corresponding to a pore size of 500 nm and the mercury intrusion amount V2 (ml/g) at a pressure corresponding to a pore size of 10 nm are used in the following formula. The total pore volume V (ml/g) was determined.

V=V2-V1

[5. Determination of strontium content of precursor fiber bundles]

[Measurement device]

Fluorescent X-ray analyzer: manufactured by Rigaku Motor Industry Co., Ltd., product name: ZSX100e

Target: Rh (end window type) 4.0 kW

Spectroscopic crystallization: RX4

Detector: PC (proportional counter, proportional counter)

Slit: Std.

Optical aperture: 10 mmφ

2θ: 144.681 deg

Measuring radiation: Si-Kα

Excitation voltage: 50 kV

Excitation current: 70 mA.

[test methods]

On the acrylic resin plate having a length of 20 mm, a width of 40 mm, and a width of 5 mm, the precursor fiber bundle was uniformly wound without leaving a gap to prepare a measurement sample, which was placed in a measuring apparatus. The intensity of the fluorescent X-ray of erbium is measured by a usual fluorescent X-ray analysis method. From the obtained fluorescent X-ray intensity of the precursor fiber bundle, the enthalpy content of the fiber bundle was determined using a calibration curve. The number of measurements n=10, and the average value of the ten measurement samples was obtained as a measured value.

[6. Determination of surface uneven structure of precursor fiber]

Both ends of the single fiber of the precursor fiber bundle were fixed to a metal sample fixing plate attached to a scanning probe microscope apparatus with a carbon paste, and the measurement was performed under the following conditions using a scanning probe microscope. First, the shape image of a single fiber was measured by a scanning probe microscope. Image analysis was performed to measure the cross-sectional distribution of the measurement image in the direction perpendicular to the fiber axis by 10 points, and the height difference (Rp-v) and the center line average roughness Ra of the highest and lowest portions of the contour curve were obtained. Ten single fibers were measured and the average value was determined.

[Measurement conditions]

Device: SII NanoTechnology SPI4000 probe station, SPA400 (unit)

Scan mode: Dynamic Force Mode (DFM) (shape image measurement)

Probe: SI-DF-20 manufactured by SII NanoTechnology

Rotation: 90° (scanning in the vertical direction for the fiber axis direction)

Scanning speed: 1.0 Hz

Number of pixels: 512 × 512

Measurement environment: room temperature, atmosphere.

Under the above conditions, one image was obtained for one single fiber, and the image was analyzed using the image analysis software (SPIWin) under the following conditions.

[Image Analysis Conditions]

The obtained shape image is subjected to [flat processing], [median 8 processing], and [three-slope correction] to obtain an image in which the surface fitting is corrected to a plane. By analyzing the surface roughness of the plane-corrected image, the profile distribution in the direction perpendicular to the fiber axis is measured, and the height difference (Rp-v) and the center line average roughness of the highest and lowest portions of the contour curve are obtained. Degree Ra.

[flat processing]

The process is to remove strain, undulation in the Z-axis direction appearing in the image data by lifting, vibration, creep of the scanner, etc., and to remove the scanning probe microscope (scanning probe) Microscope, SPM) measures the data strain caused by equipment.

[median 8 processing]

This processing is performed by replacing the Z data of S with a 3×3 window (matrix) centering on the data point S to be processed, between S and D1 to D8. Get filtering effects such as smoothing or noise removal.

The median 8 processing is a process of calculating the median value of the Z data at 9 points of S and D1 to D8 to replace S.

[Three slope correction]

The slope correction is to correct the slope by obtaining a surface by a least square approximation method and fitting it according to the total data of the image to be processed. (1 time) (2 times) (3 times) indicates the number of times the surface is fitted, and 3 times means that the surface is fitted 3 times. By the three-slope correction process, the fiber curvature of the data can be eliminated to obtain a flat image.

[7. Determination of X-ray diffraction intensity and crystal orientation of flame-retardant fiber bundles]

The flame-retardant fiber bundle was cut into fiber lengths of 5 cm at any location, and 12 mg was collected to precisely align the sample fiber axes in a precise parallel manner. Further, the fiber bundles having a width of 2 mm in the direction perpendicular to the longitudinal direction of the fibers and having a uniform thickness in the direction perpendicular to the width direction and the longitudinal direction of the fibers are arranged. A vinyl acetate/methanol solution was impregnated at both ends of the fiber bundle and fixed so that the shape of the fiber bundle was not changed, and the obtained fiber bundle was used as a sample fiber bundle for measurement.

The sample fiber bundle for measurement is fixed on a wide-angle X-ray diffraction sample stage, and the diffraction intensity in the equatorial direction is measured by a transmission method to obtain a diffraction intensity distribution (vertical axis: diffraction intensity) , horizontal axis: 2θ (unit: °)). From the obtained distribution map, the value of 2θ=17° corresponding to the reflection of polyacrylonitrile (100) and the peak of the diffraction intensity near 2θ=25° of graphite (002) reflection were detected, and this value was taken as Peak intensity.

Further, the diffraction distribution in the azimuthal direction was measured at the peak position of each reflection, and the half value width W (unit: °) of the peak was obtained, and the crystal orientation was calculated by the following formula.

Crystalline alignment (%) = [(180-W) / 180] × 100

The crystal orientation was measured by collecting three sample fiber bundles in the longitudinal direction of the fiber bundle to be measured, and the crystal orientation was measured and distributed to determine the average value.

In addition, the X-ray diffraction measurement uses a CuKα ray (using a Ni-filter) X-ray generator (trade name: TTR-III, a target X-ray generator) manufactured by Rigaku Corporation as an X-ray source. The diffraction intensity distribution was detected by a scintillation counter manufactured by Rigaku Corporation. The power is set to 50 kV-300 mA.

[8. Evaluation of cross-sectional shape of precursor fiber and carbon fiber]

The ratio of the major axis to the minor axis (long diameter/short diameter) of the fiber cross section of the single fibers constituting the fiber bundle is determined as follows.

After the fiber bundle for measurement was passed through a tube made of a vinyl chloride resin having an inner diameter of 1 mm, the fiber bundle was cut into a disk by a knife to prepare a sample. Then, the sample was placed on the SEM sample stage with the fiber profile facing upward, and then gold (Au) was sputtered at a thickness of about 10 nm, and then an electron microscope (manufactured by Philips, product name: XL20 scanning type) was used. The fiber profile was observed under the conditions of an acceleration voltage of 7.00 kV and an actuation distance of 31 mm, and the long diameter and the short diameter of the fiber cross section of the single fiber were measured.

[9. Evaluation of strand physical properties of carbon fiber bundles]

A strand test body impregnated with a resin-impregnated carbon fiber bundle was prepared in accordance with JIS R7608 and the strength was measured. Among them, the elastic modulus is calculated using the strain range according to ASTM.

[10. Evaluation of voids in the cross section of carbon fiber bundles]

The single fiber is extracted from the carbon fiber bundle, and the platinum is sputtered to a thickness of 2 nm to 5 nm by a sputtering apparatus, and the carbon is plated to a thickness of 100 nm to 150 nm using a carbon plating apparatus. After that, using a focused ion beam processing apparatus (manufactured by Hitachi High-Technologies Co., Ltd., product name: FB-2000A), a tungsten protective film having a thickness of about 500 nm was deposited, and a focusing voltage of 30 kV was used. The ion beam is etched, thereby obtaining a cross section of the fiber (thickness of 100 nm to 150 nm).

The sheet was observed by a transmission electron microscope (manufactured by Hitachi High-Technologies Co., Ltd., product name: H-7600) at an acceleration voltage of 100 kV at a magnification of 150,000 to 200,000 times. Cross section of the fiber.

Then, using an image analysis software (manufactured by Nippon Roper Co., Ltd., product name: Image-Pro PLUS), a portion of the transmission electron microscope (TEM) image which is bright in the image is extracted. The entire section is counted for the number of voids N, and the area of each void is measured, and the projected area diameter d (nm) is calculated. Further, the total area A (nm ² ) of the voids and the average void diameter D (nm) were determined.

Further, the depth T (nm) of the void was obtained. T is a value obtained by sequentially accumulating the area from the void near the surface of the fiber, and the cumulative value reaches a distance of 95% of the area A from the surface of the fiber. In other words, the radius of the circle having the area of 0.95 A on the outer circumference side is set to r, and the radius of the single fiber is set to R, and the radius of the single fiber is set to R. T is obtained by the following formula.

T = R-r.

The above measurements were carried out on five fibers, and the average value was determined.

[11. Determination of nodule strength of carbon fiber bundles]

A grip of 25 mm in length was attached to both ends of a 150 mm long carbon fiber bundle as a test body. When the test piece was produced, a load of 0.1 × 10 ^-3 N/denier was applied to align the carbon fiber bundles. A knot was formed in a substantially central portion of the test piece, and the measurement was carried out under the condition that the crosshead speed at the time of stretching was 100 mm/min. The value obtained by dividing the tensile breaking stress by the sectional area of the fiber bundle (weight and density of the carbon fiber bundle per unit length) was taken as the knot strength. The number of tests was 12, and the minimum value and the maximum value were removed, and the measured values were represented by the average value of 10 pieces.

(Example 1 and Comparative Example 1 to Comparative Example 3)

[Preparation of Expanded Silk and Precursor Fiber]

Acrylonitrile and methacrylic acid were polymerized by aqueous suspension polymerization to obtain an acrylonitrile-based copolymer containing acrylonitrile unit/methacrylic acid unit = 98 wt% / 2 wt%. The obtained polymer was dissolved in dimethylformamide to prepare a spinning dope having a concentration of 23.5 wt%.

The spinning dope was temporarily spun into the air from a spinning nozzle equipped with a discharge hole having a diameter of 0.13 mm and a number of holes of 2000, and then passed through a space of about 4 mm, and then ejected to a temperature of 15 ° C. The coagulating solution containing an aqueous solution of 79.5 wt% of dimethylformamide was solidified, and the coagulated filament was taken. Then, after extending 1.1 to 1.3 times in the air, it was extended 1.1 to 2.9 times in an extension tank filled with an aqueous solution containing 30 wt% of dimethylformamide at a temperature of 60 °C. After the extension, the fiber bundle containing the solvent was washed with clean water, and then extended by 1.2 times to 2.2 times in hot water at 95 °C.

Next, an oil agent containing an amine-based modified polyfluorene as a main component was imparted to the fiber bundle in an amount of 1.1 wt%, and dried and densified. The dried densified fiber bundle was stretched 2.2 times to 3.0 times between the heating rolls at 180 ° C to further increase the alignment and densification, and then wound up to obtain a precursor fiber bundle. The precursor fiber has a fineness of 0.77 dtex. Further, the ratio of the major axis to the minor axis (long diameter/short diameter) of the fiber cross section of the single fiber was 1.005.

Among them, the following oil agents were used as the oil agent containing the amine-based modified polyfluorene as a main component.

‧ Amine-based modified polyoxo: KF-865 (manufactured by Shin-Etsu Chemical Co., Ltd., grade 1 side chain type, viscosity 110 cSt (25 ° C), amine equivalent weight 5,000 g / mol, 85 wt%;

‧ Emulsifier: NIKKOL BL-9EX (manufactured by Nikko Chemicals Co., Ltd., POE (9) lauryl ether), 15 wt%.

[Flameproof, carbonized]

Then, a plurality of precursor fiber bundles are introduced into the flameproof furnace in a state of being aligned in parallel, and air having a heat of 220 ° C to 280 ° C is applied to the precursor fiber bundle, thereby preventing flame retardation of the precursor fiber bundle. A flame resistant fiber bundle having a density of 1.342 g/cm ³ was obtained. Wherein, in the range of a density of 1.200 g/cm ³ to 1.250 g/cm ³ , an elongation of 5.0% is performed, and then an elongation of 1.5% is performed in a range of a density of 1.250 g/cm ³ to 1.300 g/cm ³ . Then, an elongation of -0.5% was carried out in a range of a density of 1.300 g/cm ³ to 1.340 g/cm ³ . The total elongation was set to 6%, and the flameproofing treatment time was set to 70 minutes.

Then, in a first carbonization furnace having a temperature gradient of 300 ° C to 700 ° C in nitrogen gas, a flame retardant fiber bundle was allowed to pass through the first carbonization furnace while applying an elongation of 4.5%. The temperature gradient is set to a linear gradient. The processing time is set to 1.9 minutes.

Then, a second carbonization furnace set to a temperature gradient of 1000 ° C to 1250 ° C was used in a nitrogen atmosphere, and heat treatment was performed at an elongation of -3.8%. Next, a third carbonization furnace set to a temperature gradient of 1,250 ° C to 1,500 ° C was used in a nitrogen atmosphere, and heat treatment was performed at an elongation of -0.1% to obtain a carbon fiber bundle. The total elongation of the second carbonization furnace and the third carbonization furnace was -3.9%, and the treatment time was 3.7 minutes.

[surface treatment of carbon fiber]

Then, the carbon fiber bundle was passed through a 10 wt% aqueous solution of ammonium hydrogencarbonate, and the carbon fiber bundle was used as an anode, and the carbon fiber bundle was energized between the carbon fiber bundle and the opposite electrode with respect to an electric quantity of 40 Coul per 1 g of the treated carbon fiber. It is washed with warm water of 90 ° C and dried. Then, 0.5 wt% of Hydran N320 (manufactured by DIC Corporation) was attached to the carbon fiber bundle, and wound up on a bobbin to obtain a carbon fiber bundle. In the example 1 and the comparative examples 1 to 3, the ratio of the major axis to the minor axis (long diameter/short diameter) of the fiber cross section of the single fiber of the carbon fiber was 1.005, and the diameter was 4.9 μm.

[Production of one-way prepreg]

The 156 carbon fiber bundles drawn from the bobbin are aligned on the release paper coated with the B-staged epoxy resin #410 (180 ° C curing type), and the carbon fiber bundle is impregnated by heating the pressure roller. Epoxy resin. A protective film is laminated thereon to prepare a unidirectional aligned prepreg having a resin content of about 33 wt%, a carbon fiber basis weight of 125 g/m ² and a width of 500 mm (hereinafter referred to as "UD prepreg" ).

[Forming and Mechanical Properties Evaluation of Laminates]

The laminate was formed using the above UD prepreg, and the 0° tensile strength of the laminate was measured by an evaluation method in accordance with ASTM D3039.

The elongation conditions in the spinning step are shown in Table 1.

[Evaluation of fiber]

The degree of swelling of the obtained coagulated silk and the swelled silk, the measurement of the surface opening width of the swelled silk, the wide-angle X-ray measurement of the precursor fiber bundle, the evaluation of the thermomechanical analysis (TMA), and the wide-angle X-ray of the flame-proof wire The carbon fiber strand strength, elastic modulus, cross-sectional void observation, and nodule strength were measured. The results are shown in Table 2. Carbon fiber confirmed to have high mechanical properties in Example 1.

(Example 2 to Example 16 and Comparative Example 4 to Comparative Example 9)

A part of the conditions of the spinning step was changed, and a swollen filament and a precursor fiber bundle were obtained in the same manner as in Example 1. The fineness of the precursor fiber was set to 0.77 dtex, and the ratio of the long diameter to the short diameter (long diameter/short diameter) of the fiber cross section of the single fiber was set to 1.005. Next, a carbon fiber bundle was produced under the same calcination conditions. The ratio of the major axis to the minor axis (long diameter/short diameter) of the fiber cross section of the carbon fiber single fiber was 1.005 and the diameter was 4.9 μm.

The conditions of the spinning step are summarized in Table 1, and the evaluation results of various fiber bundles are summarized in Table 2.

(Example 17 to Example 20)

Using the precursor fiber bundle obtained in Example 14, only the heat treatment conditions in the second and third carbonization furnaces were changed, and carbon fiber bundles were produced in the same manner as in Example 14 except for the other conditions. Table 3 shows the heat treatment conditions and the properties of the carbon fiber bundle.

(Example 21 to Example 25 and Reference Example 1 and Reference Example 2)

The precursor fiber bundle obtained by changing only the fineness of the single fiber under the same spinning conditions as in Example 14 except that only the heat treatment conditions in the second and third carbonization furnaces in the calcination conditions of Example 15 were changed, A carbon fiber bundle was produced under the same calcination conditions as in Example 15. The precursor fiber, the heat treatment conditions, and the properties of the carbon fiber bundle are shown in Table 4.

(Examples 26 to 28 and Reference Example 3 and Reference Example 4)

A precursor fiber bundle was produced under the same conditions as in Example 14 except that the type of the amine-based polyoxymethylene of the oil agent was changed, and then a carbon fiber bundle was produced.

The properties of the amine-based modified polyoxonium species, precursor fibers, and carbon fiber bundles used are shown in Table 5.

(Example 29 to Example 31)

A part of the conditions of the spinning step was changed, and a swollen filament and a precursor fiber bundle were obtained in the same manner as in Example 1. The fineness of the precursor fiber was 0.77 dtex, and the ratio of the long diameter to the short diameter (long diameter/short diameter) of the fiber cross section of the single fiber was 1.005. Next, a carbon fiber bundle was produced under the same calcination conditions. The ratio of the major axis to the minor axis (long diameter/short diameter) of the fiber cross section of the carbon fiber single fiber was 1.005 and the diameter was 4.9 μm.

Table 1 shows the conditions of the spinning step, and Table 2 shows the evaluation results of various fiber bundles.

While the present invention has been described in its preferred embodiments, the present invention is not intended to limit the invention, and the present invention may be modified and modified without departing from the spirit and scope of the invention. The scope of protection is subject to the definition of the scope of the patent application.

Claims (21)

An acrylonitrile expanded yarn for carbon fiber which is not treated with an oil agent and has an opening portion in a range of 0.3/μm ² or more and 2/μm ² or less on the surface of the single fiber, and the opening portion is on the circumference of the fiber It has a width of 10 nm or more in the direction. The acrylonitrile expanded yarn for carbon fiber according to claim 1, wherein the pore size obtained by the mercury infiltration method has an average pore size of 55 nm or less and a total pore volume of 0.55 ml/g. the following. The acrylonitrile swelled yarn for carbon fiber according to claim 1 or 2, wherein the polymer constituting the swelled silk is 96.0 wt% or more and 99.7 wt% or less of acrylonitrile unit, and 0.3 wt% or more. 4.0 wt% or less of an acrylonitrile-based copolymer having an unsaturated hydrocarbon unit having one or more carboxyl groups or ester groups as an essential component. A method for producing a swelled silk, comprising the steps of: [1] acrylonitrile having 96.0 wt% or more and 99.7 wt% or less, and 0.3 wt% or more and 4.0 wt% or less having one or more carboxyl groups or ester groups The acrylonitrile-based copolymer obtained by copolymerization of an unsaturated hydrocarbon as an essential component is dissolved in an organic solvent in a concentration range of 20 wt% or more and 25 wt% or less to prepare a spun temperature of 50 ° C or more and 70 ° C or less. [2] using the dry-wet spinning method, after the spinning dope is temporarily ejected from the ejection hole into the air, the temperature is -5 ° C or higher, 20 ° C or lower, and the organic solvent concentration is 78.0 wt % or more. 82.0 wt% or less of the aqueous solution is solidified in a coagulation bath to obtain a coagulated tow containing the above organic solvent; [3] the coagulated tow is 1.0 times or more and 1.25 times in air. After extending in the lower range, the stretching is further carried out in a warm aqueous solution containing an organic solvent, wherein the stretching is performed at a total stretching ratio of 2.6 times or more and 4.0 times or less; and [4] The solvent is removed from the water and then extended in the hot water by 0.98 times or more and 2.0 times or less. The method for producing a swelled silk according to the fourth aspect of the invention, wherein the organic solvent is any one of dimethylformamide or dimethylacetamide. The method for producing a swelled yarn according to the fourth aspect of the invention, wherein the stretching ratio in the warm aqueous solution is 2.5 times or more and 4.0 times or less. A precursor fiber bundle for carbon fibers, comprising: acrylonitrile of 96.0 wt% or more and 99.7 wt% or less, 0.3 wt% or more, 4.0 wt% or less of an unsaturated hydrocarbon having one or more carboxyl groups or ester groups as an essential component; The acrylonitrile copolymer obtained by copolymerization is subjected to an oil agent containing a polyxanium oxide as a main component, and the cerium content is 1700 ppm or more and 5000 ppm or less, and is subjected to methyl ethyl ketone for 8 hours using a Soxler extractor. After the oil is cleaned, the cerium content is 50 ppm or more and 300 ppm or less. The precursor fiber bundle for carbon fibers according to claim 7, wherein the single fiber has a fineness of 0.5 dtex or more and 1.0 dtex or less, and a ratio of a long diameter to a short diameter of the fiber cross section of the single fiber (long diameter/short diameter) ) is 1.00 or more and 1.01 or less, and has no surface uneven structure extending along the fiber axis direction of the single fiber, and the height difference (Rp-v) between the highest part and the lowest part is 30 nm or more and 100 nm or less, and the center line average roughness ( Ra) is above 3 nm, 10 nm the following. A method for producing a precursor fiber bundle for a carbon fiber, which is obtained by using a tow of a swelled yarn obtained by the method for producing a swelled yarn according to any one of claims 4 to 6 An oil agent containing a polyxanthene compound as a main component and having a 100 wt% oil component of 0.8% by weight or more and 1.6 wt% or less is dried and then 1.8 times or more by a heat stretching method or a steam stretching method. The extension is implemented in the range of 6.0 times or less. The method for producing a precursor fiber bundle for carbon fibers according to claim 9, wherein an amine-based modified polyoxo compound satisfying the following conditions (1) and (2) is used as the polyoxyxide compound: (1) The dynamic viscosity at 25 ° C is 50 cst or more and 5000 cst or less; and (2) the amine equivalent is 1,700 g / mol or more and 15,000 g / mol or less. A method for producing a precursor fiber bundle for a carbon fiber, which is an oil agent containing a polysiloxane as a main component, and is attached to an acrylonitrile for carbon fiber according to any one of claims 1 to 3. On the tow of the swelled silk. A method for producing a flame-retardant fiber bundle, which is a precursor fiber bundle obtained by a method for producing a precursor fiber bundle for carbon fibers according to claim 11, wherein the hot air circulation is performed at 220 ° C to 260 ° C In the flameproof furnace of the type, the flame resistance is set to 0% or more and 10% or less in an oxidizing atmosphere for a period of 30 minutes or more and 100 minutes or less, thereby producing a flame resistant fiber bundle that satisfies the following conditions: (1) The intensity ratio (B/A) of the peak A (2θ=25°) and the peak B (2θ=17°) in the equatorial direction obtained by the fiber bundle wide-angle X-ray measurement is 1.3 or more; (2) the alignment of the peak B The degree is 80% or more; (3) the peak A has an alignment degree of 79% or more; and (4) the density is 1.335 g/cm ³ or more and 1.360 g/cm ³ or less. A method for producing a flame-retardant fiber bundle, which is obtained by using a precursor fiber bundle for carbon fibers according to claim 7 or 8 in a hot air circulation type flameproof furnace of 220 ° C to 260 ° C After 30 minutes or more and 100 minutes or less, the elongation is set to 0% or more and 10% or less, and heat treatment is performed in an oxidizing atmosphere to produce a flame-resistant fiber bundle that satisfies the following conditions: (1) Wide-angle X-ray by fiber bundle The intensity ratio (B/A) of the peak A (2θ=25°) and the peak B (2θ=17°) in the equatorial direction measured is 1.3 or more; (2) The degree of alignment of the peak B is 80% or more; (3) The degree of alignment of the peak A is 79% or more; and (4) the density is 1.335 g/cm ³ or more and 1.360 g/cm ³ or less. The method for producing a flame-resistant fiber bundle according to claim 12, wherein the elongation treatment condition is divided into at least three sections, and the fiber density is 1.200 g/cm ³ or more and 1.260 g/cm. ^In the range of ³ or less, the elongation is 3.0% or more and 8.0% or less, and the fiber density is 1.240 g/cm ³ or more and 1.310 g/cm ³ or less, and the elongation is 0.0% or more and 3.0% or less. The elongation of -1.0% or more and 2.0% or less is carried out in the range of 1.300 g/cm ³ or more and 1.360 g/cm ³ or less. A carbon fiber bundle having a resin impregnated strand strength of 6000 MPa or more, a strand elastic modulus measured by the ASTM method of 250 GPa to 380 GPa, and a long diameter and a short diameter of a section of the single fiber perpendicular to the fiber axis direction The ratio (long diameter/short diameter) is 1.00 to 1.01, and the diameter of the single fiber is 4.0 μm to 6.0 μm. In the cross section perpendicular to the fiber axis direction of the single fiber, there are one or more and 100 or less diameters of 2 nm or more. , voids below 15 nm. The carbon fiber bundle according to claim 15, wherein the void has an average diameter of 6 nm or less. The carbon fiber bundle according to claim 15, wherein the total area A (nm ² ) of the voids is 2,000 nm ² or less. The application of the carbon fiber bundle patentable scope of clause 17, wherein the void is equivalent to the fiber cross section perpendicular to the axial direction of single fibers present in the sum of the areas A (nm ²⁾ 95% or more voids present in the fiber from Surface to a depth of 150 nm between the positions. The carbon fiber bundle according to any one of claims 15 to 18, which is a carbon fiber having a knot strength of 900 N/mm ² or more. A method for producing a carbon fiber bundle, which is formed by a heat treatment in an oxidizing atmosphere to form a precursor fiber bundle having a density of 1.335 g/cm ³ or more and 1.355 g/cm ³ or less as described in claim 8 of the patent application. After the flame fiber bundle, the first carbonization furnace having a temperature gradient of 300° C. or higher and 700° C. or lower in an inert atmosphere is heated for 1.0 minute or longer and 3.0 minutes or shorter while applying an elongation of 2% or more and 7% or less. In one or more carbonization furnaces having a temperature gradient from 1000 ° C to the calcination temperature in an inert atmosphere, heat treatment is performed for 1.0 minute or more and 5.0 minutes or less while applying elongation of -6.0% or more and 2.0% or less. A method for producing a carbon fiber bundle, which is formed by using a heat treatment in an oxidizing atmosphere to form a precursor fiber bundle obtained by the production method according to claim 9 or 10, to 1.335 g/cm ³ or more. After the flame-retardant fiber bundle of 1.355 g/cm ³ or less, the first carbonization furnace having a temperature gradient of 300 ° C or more and 700 ° C or less is heated in an inert atmosphere while applying an elongation of 2% or more and 7% or less. 1.0 or more and 3.0 minutes or less, and then, in one or more carbonization furnaces having a temperature gradient from 1000 ° C to the calcination temperature in an inert atmosphere, 1.0 is applied while being stretched by -6.0% or more and 2.0% or less. Heat treatment for more than one minute and less than 5.0 minutes.