WO2023090310A1

WO2023090310A1 - Carbon fiber bundle and production method therefor

Info

Publication number: WO2023090310A1
Application number: PCT/JP2022/042352
Authority: WO
Inventors: 石川透; 沖嶋勇紀; 末永和真
Original assignee: 東レ株式会社
Priority date: 2021-11-19
Filing date: 2022-11-15
Publication date: 2023-05-25
Also published as: CN117999385A

Abstract

Provided are: a carbon fiber bundle that has a high total fineness and excellent strength, elastic modulus, and handleability during high-level processing; and a method for producing the carbon fiber bundle. The carbon fiber bundle is substantially not twisted, includes filaments in a quantity of 24,000-72,000, has an initial elastic modulus of 240-279 GPa, and satisfies formula (2) regarding a relationship between a coefficient A determined from approximation formula (1) that is nonlinear in a range in which stress is 0-3 GPa in a stress σ-strain ε curve in a resin-impregnated strand tensile test, and the degree of crystal alignment Π (%) in wide-angle X-ray diffraction measurement. (1): ε = Aσ2 + Bσ + C (2): -410 ≤ (0.0000832Π2 - 0.0184Π + 1.00)/A ≤ -310, where A, B, and C each represent a coefficient of a second-order function of the stress σ and the strain ε, and Π represents the degree of crystal alignment.

Description

Carbon fiber bundle and its manufacturing method

The present invention relates to a carbon fiber bundle that has a high total fineness, yet has excellent strength, elastic modulus, and workability when subjected to high-order processing, and a method for producing the same.

Because carbon fiber bundles have high specific strength and specific modulus, they are used in a wide range of applications, including aerospace applications, as reinforcing fibers for composite materials. Recently, it is also being used in industrial applications such as automobile parts and wind power generation. Especially in wind power generation, light weight and rigidity are required, so carbon fiber bundles with excellent specific elastic modulus are often used, and the demand for carbon fiber bundles for wind power generation is increasing in recent years.

In industrial applications, there is a strong demand for cost reduction, and carbon fiber bundles with 24,000 or more filaments are often used due to their excellent productivity. Further, when carbon fiber bundles are used to produce prepregs, towpregs, intermediate base materials such as woven fabrics and sheet molding compounds (SMC), and carbon fiber composite materials such as pultruded materials, high-order workability is considered important. In order to improve high-order processability, the carbon fiber bundle should have less fluff and excellent opening properties, and the entire carbon fiber bundle or single carbon fiber when unwound from the bobbin and run through the manufacturing process. No breakage and good runnability are particularly important.

In general, carbon fiber bundles are produced by oxidizing polyacrylonitrile-based precursor fibers obtained by fiberizing polyacrylonitrile-based copolymers in the air at 200-300°C. It is produced through a preliminary carbonization step of heating in an atmosphere and a carbonization step of heating in an inert atmosphere at a maximum temperature of 1,200 to 3,000°C.

Until now, techniques have been proposed for producing carbon fibers with high strength, high modulus, and excellent high-order workability for industrial use (Patent Documents 1 to 4). In Patent Document 1, when a polyacrylonitrile-based precursor fiber bundle having a total fineness of 40,000 dtex or more is flameproofed, the precursor fiber bundle runs in a flameproofing furnace by defining the shape and arrangement of folding rolls. A technology that suppresses the twisting of the fiber bundle when doing so, stably maintains the shape of the fiber bundle, suppresses yarn breakage and fluffing during the flameproofing process, and enables the stable production of high-quality carbon fiber bundles. is disclosed. Patent Literature 2 discloses a technique for improving resin impregnability and spreadability during composite material molding by controlling the diameter and surface state of carbon fibers within a specific range. Patent Document 3 discloses a carbon fiber bundle having a semi-permanent twist and an elastic modulus of 200 GPa or more, which is excellent in handleability and high-order workability as a fiber bundle, and has a high reinforcing effect for fiber-reinforced composite materials. A carbon fiber bundle is disclosed. In Patent Document 4, a carbon fiber bundle capable of obtaining a high-performance carbon fiber reinforced composite material having excellent tensile strength by controlling the nonlinearity of the stress σ-strain ε curve in a resin-impregnated strand tensile test to a specific range. is disclosed.

JP 2014-214386 A JP-A-2002-69754 JP 2019-151956 A WO2016/068034

However, the background technology has the following issues.

In Patent Document 1, it is shown that by setting the yarn density in the flameproofing process to a specific range, the effect of suppressing the occurrence of twisting and "groove skipping" (dropping of the yarn from the roller) in the flameproofing process. However, the effect of improving the quality of the resulting carbon fiber bundle has not been demonstrated, and the operability cannot be improved when subjected to the process of higher processing.

In Patent Document 2, although the strength expression rate of the obtained molding material is improved by improving the resin impregnating property when molding the pressure vessel, the operability when the obtained carbon fiber bundle is subjected to the process of high-order processing did not improve

In Patent Document 3, although the handleability can be improved by leaving a semi-permanent twist in the carbon fiber bundle, there is no specific effect on the workability when the obtained carbon fiber bundle is subjected to a process of high-order processing. There is no disclosure or suggestion, but there is a problem that the presence of twist disturbs the orientation of the fibers in the resulting carbon fiber reinforced composite material, making it difficult to develop mechanical properties.

In Patent Document 4, by controlling the heat treatment method in the flameproofing process, the nonlinearity of the stress σ-strain ε curve in the resin-impregnated strand tensile test is controlled to a specific range, thereby improving the fracture toughness value, which is effective for improving strength. , There is no suggestion about the workability when subjected to a high-order processing process of carbon fiber bundles with a high total fineness, and the initial elastic modulus in the resin-impregnated strand tensile test is as high as 315 GPa, and when subjected to a high-order processing process However, it was not possible to expect an improvement in the operability of the plant. Furthermore, in order to obtain a carbon fiber bundle with excellent productivity, it is effective to increase the total fineness of the polyacrylonitrile-based precursor fiber bundle. There are limitations, and the method described in the patent document has the problem that it is difficult to stably control the nonlinearity of the stress σ-strain ε curve.

As described above, in the prior art, techniques for improving the mechanical properties of carbon fiber bundles and techniques for improving operability during the production of carbon fiber bundles have been proposed. There is no disclosure of a technology capable of suppressing troubles such as fuzz caused by rubbing against rollers or guides and breakage occurring in part or all of the carbon fiber bundle. In the present invention, although it has a high total fineness, it is excellent in strength, elastic modulus, and workability when subjected to high-order processing, and when it is made into a carbon fiber reinforced composite material in a substantially untwisted manner, mechanical properties are likely to appear. An object of the present invention is to provide a carbon fiber bundle and a method for manufacturing the same.

In order to achieve this object of the present invention, the present invention mainly has the following configuration.

That is, the present invention provides the coefficient A obtained from the nonlinear approximation formula (1) in the stress σ-strain ε curve in the resin-impregnated strand tensile test in the range of 0 to 3 GPa, and the degree of crystal orientation in wide-angle X-ray diffraction measurement. The relationship between Π (%) satisfies the formula (2), the initial elastic modulus is 240 to 279 GPa, the number of filaments is 24,000 to 72,000, and the substantially untwisted carbon fiber bundle is.
ε=Aσ ² +Bσ+C (1)
−410≦(0.0000832Π ² −0.0184Π+1.00)/A≦−310 (2)
Here, A, B, and C are the coefficients of the quadratic function of stress σ and strain ε, and Π is the degree of crystal orientation.

The present invention also provides a method for producing the above carbon fiber bundle,
A flameproofing step of heat-treating a substantially untwisted polyacrylonitrile-based precursor fiber bundle having a filament number of 24,000 to 72,000 in an oxidizing atmosphere at a temperature of 220 to 280°C, and a flameproofing step obtained by the flameproofing step. a preliminary carbonization step of heat-treating the flameproofed fiber bundle in an inert atmosphere at a maximum temperature of 300 to 1,000 ° C.; A carbonization step of heat treatment at a maximum temperature of 1,000 to 1,600 ° C.,
The draw ratio in the preliminary carbonization step is 1.05 to 1.20, the draw ratio in the carbonization step is 0.960 to 0.990, and stretching in the preliminary carbonization step and the carbonization step The product of magnification is 1.020 to 1.180,
In the flameproofing step, the polyacrylonitrile-based precursor fiber bundles are stepwise processed in a plurality of heat treatment furnaces set to mutually different temperatures, or in a plurality of heat treatment sections provided in the heat treatment furnace and set to mutually different temperatures. and the temperature of the heat treatment furnace or heat treatment section with the lowest temperature in the flameproofing step is set to less than 230 ° C., and the temperature of the heat treatment furnace or heat treatment section with the highest temperature is set to 280 ° C. or less.
A method for producing a carbon fiber bundle.

According to the present invention, a carbon fiber bundle that is excellent in strength, elastic modulus, and workability when subjected to high-order processing while having a high total fineness, and that easily exhibits mechanical properties when made into a carbon fiber reinforced composite material. can get.

In order to achieve this purpose, the present invention has the following configuration.

The carbon fiber bundle of the present invention is a coefficient obtained by introducing the stress σ-strain ε curve obtained by measuring the carbon fiber bundle by a resin-impregnated strand tensile test into the following nonlinear approximation formula (1): The value of A satisfies the following equation (2).
ε=Aσ ² +Bσ+C (1)
−410≦(0.0000832Π ² −0.0184Π+1.00)/A≦−310 (2)
Here, Π indicates the degree of crystal orientation (%) obtained by measuring the carbon fiber bundle by wide-angle X-ray diffraction measurement. The degree of crystal orientation is obtained by a method for measuring the degree of crystal orientation Π of carbon fibers, which will be described later.

The value of the middle term in the formula (2) is -410 to -310, preferably -406 to -343, more preferably -386 to -352.

In formula (1), the coefficient A indicates the nonlinearity of the stress σ-strain ε curve. The coefficient A is obtained by fitting a stress σ-strain ε curve obtained by measuring a carbon fiber bundle by a resin-impregnated strand tensile test to the approximate expression (1) within a stress range of 0 to 3 GPa. As described above, the stress σ-strain ε curve of a carbon fiber bundle generally shows a downwardly convex curve when the stress σ (GPa) is on the vertical axis and the strain ε (−) is on the horizontal axis. The coefficient A obtained from the approximate expression (1) takes a negative value. That is, the closer the coefficient A is to 0, the smaller the nonlinearity.

Further, the present inventors have found that the correlation with the shear modulus of carbon fibers is not necessarily sufficient only with the nonlinearity of the stress σ-strain ε curve. Theories related to stress and deformation in carbon fibers are described, for example, in "Carbon" (Netherlands), Elsevier, 1991, Vol. 29, No. 8, pp. 1267-1279. ing. However, this is an academic study, and was difficult to use for practical studies to control the shear modulus of carbon fibers. As a result of repeated studies based on these theories, the present inventors have found that from a practical point of view, the degree of crystal orientation Π, which is relatively easy to measure, and the above formula derived from the coefficient A of the approximate formula (1) It was found that the value (0.0000832 Π ² -0.0184 Π + 1.00)/A in the middle term of (2) has an extremely high correlation with the shear modulus of carbon fiber. More specifically, the shear modulus decreases as the value of the middle term in formula (2) increases, and the shear modulus increases as the value in the middle term of formula (2) decreases.

The shear modulus is an index of how easily a single fiber deforms when stress is applied in the bending or compressive directions, and is important for improving workability in advanced processing processes. When the value of the middle term in the above formula (2) is -410 to -310, the fiber is moderately deformed when subjected to bending or compressive stress in the high-order processing step, resulting in single fiber breakage and Wrapping around subsequent rollers and guides can be suppressed. The coefficient A in the formula (1) can be controlled by the draw ratio in the flameproofing step, the draw ratio in the preliminary carbonization step, and the draw ratio in the carbonization step. Further, the degree of crystal orientation Π can be controlled by the draw ratio in the preliminary carbonization step, the draw ratio in the carbonization step, and the temperature in the carbonization step.

In addition, the carbon fiber bundle of the present invention has an initial elastic modulus of 240 to 279 GPa, preferably 245 to 269 GPa, more preferably 245 to 260 GPa. The initial modulus of elasticity is an index of the ease of initial deformation when stress is applied to a single fiber in the tensile direction, and is important for improving workability in advanced processing steps. If the initial elastic modulus is 240 to 279 GPa, the fibers will deform appropriately when subjected to stress in the tensile direction in the advanced processing step, and breakage of single fibers and subsequent winding around rollers and guides can be suppressed. The initial elastic modulus is calculated as the reciprocal 1/B of the coefficient B obtained by fitting the stress σ-strain ε curve measured by the resin-impregnated strand tensile test described later with the approximation formula (1). Such an initial elastic modulus can be controlled by the draw ratio in the flameproofing step, the draw ratio in the preliminary carbonization step, the draw ratio in the carbonization step, and the temperature in the carbonization step.

The carbon fiber bundle of the present invention has a filament number of 24,000 to 72,000, preferably 36,000 to 60,000, more preferably 48,000 to 50,000. The number of filaments is the number of single fibers that make up the carbon fiber bundle, and the more the number, the better the productivity of the carbon fiber reinforced composite material. The mechanical properties of carbon fiber reinforced composite materials may deteriorate. If the number of filaments is 24,000 to 72,000, the productivity in molding the composite material is excellent, and it can be suitably used for industrial applications. The number of filaments can be controlled by adjusting the number of holes in the spinneret, the splitting of the yarn, and the doubling of the yarn in the spinning process of the polyacrylonitrile-based precursor fiber bundle.

The carbon fiber bundle of the present invention is substantially untwisted. The term "substantially untwisted" as used herein means that the twist of the carbon fiber bundle is 0.5 turns or less per meter. If the carbon fiber bundle is substantially untwisted, it is possible to suppress disordered orientation of the fibers in the carbon fiber reinforced composite material, thereby improving the reinforcing effect of the carbon fiber reinforced composite material.

The carbon fiber bundle of the present invention preferably has a crystallite size Lc of 1.80 to 2.20 nm. The crystallite size Lc is the size of graphite crystals in the carbon fiber in the [002] direction. If the crystallite size Lc is from 1.80 to 2.20 nm, carbon fibers with better balance between strength and elastic modulus can be obtained. Such a crystallite size Lc can be evaluated by a method for measuring the crystallite size Lc, which will be described later, by wide-angle X-ray diffraction measurement. Such crystallite size Lc can be controlled by the temperature of the carbonization step.

The carbon fiber bundle of the present invention preferably has a single fiber fineness of 0.63 to 1.35 dtex, more preferably 0.67 to 1.35 dtex, still more preferably 0.74 to 1.20 dtex. Single fiber fineness is the mass per unit length of single fiber. If the single fiber fineness is 0.63 to 1.35 dtex, both productivity and mechanical properties can be achieved. The single fiber fineness can be evaluated by measuring the mass per unit length by the method described later. Such single fiber fineness can be controlled by the discharge amount and draw ratio of the polyacrylonitrile-based polymer in the spinning process of the polyacrylonitrile-based precursor fiber bundle.

The carbon fiber bundle of the present invention preferably has a single fiber cross-sectional circularity of 0.86 to 0.98, more preferably 0.87 to 0.96, still more preferably 0.87 to 0.96. 93. The circularity of the single fiber cross section is defined as follows from the perimeter L and the area _Acs of the single fiber cross section.
(Circularity)=4πA _cs /L ² .

If the roundness of the single fiber cross section is 0.86 to 0.98, both bundling property and abrasion resistance during high-order processing can be achieved more reliably, and workability during high-order processing is superior. The roundness of the cross section of the single fiber can be evaluated from the image of the cross section obtained by cutting the single fiber vertically by the method described later. The roundness of the single fiber cross section can be controlled by the shape of the ejection hole of the spinneret in the spinning process and the conditions of the coagulation process.

Next, a method for producing a carbon fiber bundle that is preferable for obtaining the carbon fiber bundle of the present invention will be described.

When manufacturing the carbon fiber bundle, the polyacrylonitrile-based precursor fiber bundle is spun. A polyacrylonitrile-based polymer is preferably used as the raw material for the production of the polyacrylonitrile-based precursor fiber bundle. In the present invention, the polyacrylonitrile-based polymer refers to a polymer in which at least acrylonitrile is the main component of the polymer skeleton, and the main component usually accounts for 90 to 100% by mass of the polymer skeleton. Refers to constituents. The polyacrylonitrile-based polymer preferably contains a copolymer component such as itaconic acid, acrylamide, and methacrylic acid from the viewpoint of improving the spinning property and efficiently performing the flame-resistant treatment. A method for producing a polyacrylonitrile-based polymer can be selected from known polymerization methods. In the production of the polyacrylonitrile-based precursor fiber bundle, the spinning dope is prepared by adding the above-mentioned polyacrylonitrile-based polymer to a solvent in which polyacrylonitrile is soluble, such as dimethylsulfoxide, dimethylformamide, dimethylacetamide, or an aqueous solution of nitric acid, zinc chloride, and rhodan soda. It is dissolved.

The method for producing the polyacrylonitrile-based precursor fiber bundle used in the present invention is not particularly limited, but wet spinning is preferably used, followed by drawing, washing with water, application of oil, drying and densification, and post-drawing if necessary. It can be obtained through processes such as The number of holes in the spinneret in the production process of the polyacrylonitrile-based precursor fiber bundle is preferably 3,000 to 200,000 holes in order to achieve the above-mentioned number of filaments in the carbon fiber bundle. A polyacrylonitrile-based precursor fiber bundle having a predetermined number of filaments can be obtained.

In the production of polyacrylonitrile-based precursor fiber bundles, the coagulation bath preferably contains a solvent such as dimethylsulfoxide, dimethylformamide, and dimethylacetamide used as the solvent for the spinning dope, and a so-called coagulation accelerating component. As the coagulation accelerating component, a component that does not dissolve the polyacrylonitrile-based polymer and is compatible with the solvent used for the spinning dope can be used. Water is preferably used as the clot-promoting ingredient.

In the production of the polyacrylonitrile-based precursor fiber bundle, it is preferable that the water washing step uses a water washing bath consisting of multiple stages at a temperature of 30 to 98°C. Further, in the water washing step, it is also preferable to set the draw ratio to 2 to 6 times.

After the water-washing process, an oil such as silicone is preferably applied to the threads in order to prevent the single fibers from sticking to each other. Such a silicone oil agent is preferably a modified silicone, and preferably contains an amino-modified silicone having high heat resistance.

A known method can be used for the dry heat treatment step (the above-described dry densification step). For example, the drying temperature is 100-200°C.

The single fiber fineness of the polyacrylonitrile-based precursor fiber bundle in the carbon fiber bundle production method of the present invention is preferably 1.20 to 2.40 dtex, more preferably 1.20 to 2.20 dtex, and even more preferably 1.40 to 1.80 dtex. Single fiber fineness is the mass per unit length of single fiber. If the single fiber fineness is 1.20 dtex or more, a carbon fiber bundle can be obtained with sufficiently high productivity. A carbon fiber bundle with high mechanical properties is obtained. Such single fiber fineness can be controlled by the discharge amount and draw ratio in the spinning process.

The polyacrylonitrile-based precursor fiber bundle in the carbon fiber bundle production method of the present invention preferably has a single fiber cross-sectional circularity of 0.86 to 0.98, more preferably 0.87 to 0.96. and more preferably 0.87 to 0.93. The circularity of the single fiber cross section is defined as follows from the perimeter L and the area _Acs of the single fiber cross section.
(Circularity)=4πA _cs /L ² .

If the roundness of the single fiber cross section is 0.86 to 0.98, the bundling property and abrasion resistance of the obtained carbon fiber can be more reliably achieved, and the obtained carbon fiber bundle can be processed in a higher order. better than The roundness of the single fiber cross section of such a polyacrylonitrile-based precursor fiber bundle can be evaluated from the image of the cut surface obtained by vertically cutting the single fiber by the method described later. The roundness of the single fiber cross section of such a polyacrylonitrile-based precursor fiber bundle can be controlled by the shape of the ejection hole of the spinneret in the spinning process and the conditions of the coagulation process.

The number of filaments of the polyacrylonitrile-based precursor fiber bundle in the carbon fiber bundle production method of the present invention is preferably 24,000 to 72,000, more preferably 36,000 to 60,000, and still more preferably 48,000 to 50,000. The number of filaments is the number of single fibers that make up the polyacrylonitrile-based precursor fiber bundle. , If it is too large, the unevenness of the treatment in the flameproofing process, the preliminary carbonization process, and the carbonization process will increase, and the dynamics of the carbon fiber reinforced composite material obtained from the viewpoint of the spreadability of the obtained carbon fiber bundle and the resin impregnation property. characteristics may deteriorate. If the number of filaments in the polyacrylonitrile-based precursor fiber bundle is 24,000 to 72,000, the productivity of the carbon fiber bundle and the carbon fiber reinforced composite material is excellent, and the carbon fiber bundle can be suitably used for industrial applications. can get. The number of filaments in such a polyacrylonitrile-based precursor fiber bundle can be evaluated by counting the number of single fibers constituting the polyacrylonitrile-based precursor fiber bundle. The number of filaments can be controlled by the number of holes in the spinneret in the spinning process, the number of divisions of the fiber bundle extruded from the spinneret, and the number of doubling of the fiber bundle.

In the method for producing a carbon fiber bundle of the present invention, the substantially untwisted polyacrylonitrile-based precursor fiber bundle as described above is heat-treated at a temperature of 220 to 280°C in an oxidizing atmosphere (flameproofing step). The temperature in the flameproofing step is preferably 220-280°C. If the temperature of the flameproofing treatment is 220° C. or higher, a flameproofed fiber bundle having sufficient flame resistance can be produced, so the occurrence of fluff due to lack of flame resistance can be suppressed, and the resulting carbon fiber bundle can be improved. Excellent workability during advanced processing. When the flameproofing temperature is 280° C. or lower, the heat generation rate does not become excessively high, so that the temperature unevenness in the flameproofed fiber bundle can be reduced, and a carbon fiber bundle having excellent mechanical properties can be obtained. The temperature of such flameproofing treatment can be determined by inserting a thermometer such as a thermocouple into the flameproofing furnace and measuring the furnace temperature. calculates the simple average temperature. The temperature of such flameproofing treatment can be controlled by heating output in a heating method used in a known flameproofing furnace. For example, in the case of a hot air circulation type flameproof furnace, the output of the heater used for heating the oxidizing atmosphere may be changed.

In the flameproofing step, a plurality of heat treatment furnaces set to different temperatures from each other, or a plurality of heat treatment sections provided in the heat treatment furnace and set to different temperatures, are used to stepwise polyacrylonitrile-based precursor fibers. Subjecting the bundle to a heat treatment (hereinafter such heat treatment furnaces and heat treatment sections may be referred to as "heat treatment furnaces/heat treatment sections"). In the present invention, the temperature may be different between at least two heat treatment furnaces/heat treatment sections among the plurality of heat treatment furnaces/heat treatment sections, for example, two heat treatment furnaces/ The heat treatment sections may be at the same temperature. In the present invention, the temperature of the heat treatment furnace or heat treatment section, which is the lowest in the flameproofing process, is less than 230°C, preferably 225°C or less, more preferably 223°C or less. By setting the temperature of the lowest heat treatment furnace or heat treatment section to less than 230 ° C., heat treatment unevenness that tends to occur in polyacrylonitrile-based precursor fiber bundles with a high total fineness can be reduced, and stretching in the preliminary carbonization step and carbonization step described later. High quality can be maintained in If the temperature of the lowest heat treatment furnace or heat treatment section is 230° C. or higher, heat treatment unevenness increases in the flameproofing step, and the grade deteriorates due to stretching in the preliminary carbonization step and the carbonization step.

In addition, in the present invention, the temperature of the heat treatment furnace or heat treatment section, which is the highest in the flameproofing process, is 280°C or lower, preferably 275°C or lower, and more preferably 270°C or lower. By setting the temperature of the highest heat treatment furnace or heat treatment section to 280 ° C. or less, it is possible to reduce the heat treatment unevenness that tends to occur in the polyacrylonitrile-based precursor fiber bundle with a high total fineness, and the stretching of the preliminary carbonization step and the carbonization step described later. High quality can be maintained in If the temperature of the heat treatment furnace or heat treatment section exceeds 280° C., heat treatment unevenness increases in the flameproofing step, and the quality is lowered by stretching in the preliminary carbonization step and the carbonization step.

Following the polyacrylonitrile-based precursor fiber bundle production step and the flameproofing step, pre-carbonization is performed. In the preliminary carbonization step, the flameproof fiber bundle obtained as described above is heated in an inert atmosphere at a maximum temperature of 300 to 1,000° C., preferably to a density of 1.5 to 1.8 g/cm ³ . heat-treated until

Carbonization is performed following the preliminary carbonization. In the carbonization step, the pre-carbonized fiber bundle is heat-treated at a maximum temperature of 1,000 to 1,600° C. in an inert atmosphere.

In addition, in the present invention, a plurality of heat treatment furnaces or heat treatment sections may be used in the preliminary carbonization step and the carbonization step and set to temperatures different from each other. Therefore, the temperature of the heat treatment furnace or heat treatment section with the highest temperature in each step is referred to as the "maximum temperature".

In the method for producing a carbon fiber bundle of the present invention, the draw ratio in the preliminary carbonization step is 1.05 to 1.20, the draw ratio in the carbonization step is 0.960 to 0.990, and the preliminary The product of the carbonization step and the draw ratio in the carbonization step is 1.020 to 1.180.

The draw ratio in the preliminary carbonization step is preferably 1.10 to 1.20, more preferably 1.10 to 1.15.

The draw ratio in the carbonization step is preferably 0.975-0.990, more preferably 0.975-0.985.

The product of the draw ratio in the preliminary carbonization step and the draw ratio in the carbonization step is preferably 1.040 to 1.130, more preferably 1.070 to 1.130.

The draw ratio in the preliminary carbonization step is 1.05 or more, the draw ratio in the carbonization step is 0.960 or more, and the product of the draw ratio in the preliminary carbonization step and the draw ratio in the carbonization step is 1.020 or more. By controlling, the value of the middle term of the above formula (2) and the initial elastic modulus of the obtained carbon fiber bundle can be controlled within an appropriate range. On the other hand, the draw ratio in the preliminary carbonization step is 1.20 or less, the draw ratio in the carbonization step is 0.990 or less, and the product of the draw ratio in the preliminary carbonization step and the draw ratio in the carbonization step is 1.180 or less. By controlling in such a manner, it is possible to suppress yarn breakage due to drawing, thereby suppressing a decrease in workability during carbon fiber production and an increase in the number of fluffs in the obtained carbon fiber bundle.

The carbon fiber bundles obtained as described above are preferably subjected to oxidation treatment to introduce oxygen-containing functional groups in order to improve adhesion with the matrix resin. Gas-phase oxidation, liquid-phase oxidation, and liquid-phase electrolytic oxidation are used as the oxidation treatment method. Liquid-phase electrolytic oxidation is preferably used from the viewpoint of high productivity and uniform treatment. The liquid-phase electrolytic oxidation method is not particularly specified, and a known method may be used.

After such electrolytic treatment, a sizing treatment can be applied to impart bundling properties to the obtained carbon fiber bundles. As the sizing agent, a sizing agent having good compatibility with the matrix resin used in the composite material can be appropriately selected according to the type of the matrix resin.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to these.

<Resin Impregnated Strand Tensile Test of Carbon Fiber Bundle>
The tensile elastic modulus of the resin-impregnated strand of the carbon fiber bundle (strand elastic modulus E (GPa)), the tensile strength of the resin-impregnated strand (strand strength (GPa)), and the stress σ-strain ε curve are calculated according to JISR7608 (2008) "Resin impregnation Strand test method”. The strand elastic modulus E is measured over a strain range of 0.1-0.6%. A test piece is prepared by impregnating a carbon fiber bundle with the following resin composition and subjecting it to curing conditions of heat treatment at a temperature of 130° C. for 35 minutes.

[Resin composition]
· 3,4-epoxycyclohexylmethyl-3,4-epoxy-cyclohexane-carboxylate (100 parts by mass)
・ Boron trifluoride monoethylamine (3 parts by mass)
・ Acetone (4 parts by mass)
The number of strands to be measured is 6, and the arithmetic mean value of each measurement result is taken as the strand elastic modulus and strand strength of the carbon fiber bundle.

<Analysis of stress σ-strain ε curve>
Analysis of the stress σ-strain ε curve obtained by the resin-impregnated strand tensile test is performed by plotting the strain ε (-) on the vertical axis and the stress σ (GPa) on the horizontal axis, and fitting according to the following equation (1). The coefficients A, B, and C are calculated by The fitting is performed for a region where the stress σ is 0 to 3 GPa in the stress σ-strain ε curve obtained by the measurement. Fitting is performed using Microsoft's "Excel" using a quadratic function.
ε=Aσ ² +Bσ+C (1).

<Initial elastic modulus (GPa)>
The initial elastic modulus of the carbon fiber bundle is calculated as follows using the coefficient B obtained by fitting according to the equation (1) by analyzing the stress σ-strain ε curve described above.
Initial modulus (GPa) = 1/B.

<Degree of crystal orientation Π (%) of carbon fiber bundle>
Carbon fiber bundles to be measured are aligned and solidified using a collodion/alcohol solution to prepare a square pole measuring sample having a length of 4 cm and a side length of 1 mm. A prepared measurement sample is measured using a wide-angle X-ray diffraction device under the following conditions.
・X-ray source: CuKα ray (tube voltage 40 kV, tube current 30 mA)
・ Detector: goniometer + monochromator + scintillation counter 2θ = Obtained using the following formula from the half width H (°) of the diffraction intensity distribution obtained by scanning the peak appearing in the vicinity of 25 to 26 ° in the circumferential direction .
Crystal orientation degree Π (%) = [(180-H) / 180] × 100
In the examples, XRD-6100 manufactured by Shimadzu Corporation was used as the wide-angle X-ray diffraction device.

<Crystallite size Lc (nm)>
・X-ray source: CuKα ray (tube voltage 40 kV, tube current 30 mA)
・Detector: goniometer + monochromator + scintillation counter ・Scanning range: 2θ = 10 to 40°
• Scanning mode: step scan, step unit 0.02°, counting time 2 seconds.

In the obtained diffraction pattern, the half-value width is determined for the peak appearing near 2θ=25 to 26°, and from this value, the crystallite size is calculated by the following Scherrer's formula.

Crystallite size (nm) = Kλ/β ₀ cos θ _B
however,
K: 1.0, λ: 0.15418 nm (X-ray wavelength)
β ₀ : (β _E ² - β ₁ ² ) ^1/2
β _E : Apparent half width (measured value) rad
β ₁ : 1.046×10 ⁻² rad
θ _B : Bragg's diffraction angle.

<Roundness measurement (-)>
The polyacrylonitrile-based precursor fiber bundle or carbon fiber bundle is cut perpendicular to the fiber axis direction with a single-edged razor, and the obtained cross section is scanned with a scanning electron microscope (SEM) "S-4800" manufactured by Hitachi High Technologies. , observed from the direction perpendicular to the fiber cross-section. The acquired image is analyzed using the image analysis software "ImageJ", and the roundness of the single fiber included in the fiber cross section is calculated from the perimeter and area of the cross section of the single fiber according to the following definition. This measurement is randomly repeated for 25 single fibers in one cross section, and the average roundness is taken as the roundness of the cross section of the single fiber.
The circularity of the single fiber cross section is defined as follows from the perimeter L and the area _Acs of the single fiber cross section.
(Circularity)=4πA _cs /L ² .

<Evaluation of high-order workability>
A carbon fiber bundle bobbin is placed on a creel and pulled out with a tension of 1.6 mN / dtex. take up. At this time, the generated fluff is counted for 10 minutes just before the drive roller and evaluated according to the following indices.
A: less than 10/m
B: 10 or more and less than 50/m
C: 50 or more/m.

(Examples 1 to 4)
A polyacrylonitrile-based copolymer composed of acrylonitrile and itaconic acid was polymerized by a solution polymerization method using dimethylsulfoxide as a solvent to produce a polyacrylonitrile-based copolymer to obtain a spinning dope. The obtained spinning dope was coagulated by a wet spinning method in which it was introduced into a coagulation bath comprising an aqueous solution of dimethyl sulfoxide through a spinneret with 50,000 holes to form a fiber bundle. This fiber bundle was washed with water at 30 to 98° C. in a conventional manner, and drawn at that time. Subsequently, an amino-modified silicone oil agent was applied to the fiber bundle after washing and stretching, and a drying and densification treatment was performed using a heating roller at 130° C., resulting in a single fiber count of 50,000 and a single fiber fineness of 1.5. A 50 dtex polyacrylonitrile precursor fiber bundle was obtained. The polyacrylonitrile-based precursor fiber bundle was not twisted.

The obtained polyacrylonitrile-based precursor fiber bundle was treated with a flameproofing step, a preliminary carbonization step, and a carbonization step under the conditions shown in Table 1 to obtain a carbon fiber bundle. In each of the flameproofing process, the preliminary carbonization process, and the carbonization process, heat treatment was performed by increasing the temperature stepwise using a plurality of heat treatment furnaces having different temperatures. No twisting treatment was performed in the flameproofing process, the preliminary carbonization process, or the carbonization process. Table 2 shows the properties of the obtained carbon fiber bundles.

(Example 5)
A polyacrylonitrile-based precursor fiber bundle having a single fiber fineness of 1.65 dtex was obtained by changing the discharge amount of the spinning stock solution, and the conditions of the subsequent preliminary carbonization step and carbonization step were changed as shown in Table 1. It was carried out analogously to Example 1.

(Example 6)
A polyacrylonitrile-based precursor fiber bundle having a single fiber fineness of 2.40 dtex was obtained by changing the discharge amount of the spinning stock solution, and the conditions of the subsequent preliminary carbonization step and carbonization step were changed as shown in Table 1. It was carried out analogously to Example 1.

(Example 7)
The procedure was carried out in the same manner as in Example 1, except that the conditions shown in Table 1 were changed for the flameproofing temperature, the stretching ratio in the preliminary carbonization step, and the stretching ratio in the carbonization step.

(Comparative example 1)
A carbon fiber bundle was produced in the same manner as in Example 1 except that the draw ratio in the preliminary carbonization step was changed to 1.00, the draw ratio in the carbonization step was changed to 0.960, and the product of the draw ratios was changed to 0.960. Obtained. The obtained carbon fiber bundle had a value of -307 in the middle term of the formula (2) and an initial elastic modulus of 213 GPa, indicating poor workability during high-order processing.

(Comparative example 2)
A carbon fiber bundle was produced in the same manner as in Example 1, except that the draw ratio in the preliminary carbonization step was changed to 1.01, the draw ratio in the carbonization step was changed to 0.955, and the product of the draw ratios was changed to 0.965. Obtained. The obtained carbon fiber bundle had a value of -286 in the middle term of the formula (2) and an initial elastic modulus of 215 GPa, indicating poor workability during high-order processing.

(Comparative Example 3)
A carbon fiber bundle was produced in the same manner as in Example 1, except that the draw ratio in the preliminary carbonization step was changed to 1.02, the draw ratio in the carbonization step was changed to 0.950, and the product of the draw ratios was changed to 0.969. Obtained. The obtained carbon fiber bundle had a value of −287 in the middle term of the formula (2) and an initial elastic modulus of 220 GPa, indicating poor workability during high-order processing.

(Comparative Example 4)
A polyacrylonitrile-based precursor fiber bundle having a single fiber fineness of 0.80 dtex was obtained by changing the discharge amount of the spinning stock solution, and the draw ratio in the preliminary carbonization step was 1.05, and the draw ratio in the carbonization step was 0.950. , a carbon fiber bundle was obtained in the same manner as in Example 1, except that the product of the draw ratios was changed to 0.998. The obtained carbon fiber bundle had a value of -290 in the middle term of the formula (2) and an initial elastic modulus of 218 GPa, indicating poor workability during high-order processing.

(Comparative Example 5)
A polyacrylonitrile-based precursor fiber bundle having a single fiber fineness of 3.00 dtex was obtained by changing the discharge amount of the spinning stock solution, and the draw ratio in the preliminary carbonization step was 1.00, and the draw ratio in the carbonization step was 0.955. , a carbon fiber bundle was obtained in the same manner as in Example 1, except that the product of the draw ratios was changed to 0.955. The obtained carbon fiber bundle had a value of -277 in the middle term of the formula (2), an initial elastic modulus of 225 GPa, and was inferior in workability during high-order processing.

(Comparative Example 6)
The polyacrylonitrile-based precursor fiber bundle was carried out in the same manner as in Example 1, except that the stock solution for spinning was once expelled into the air from a spinneret and then introduced into a coagulation bath consisting of an aqueous solution of dimethyl sulfoxide for coagulation by a dry-wet spinning method. Carbon was obtained in the same manner as in Example 1 except that the draw ratio in the preliminary carbonization step was changed to 1.01, the draw ratio in the carbonization step was changed to 0.965, and the product of the draw ratios was changed to 0.975. A fiber bundle was obtained. The obtained carbon fiber bundle had a value of -290 in the middle term of the formula (2) and an initial elastic modulus of 223 GPa, indicating poor workability during high-order processing.

(Comparative Example 7)
Example 1 was repeated except that the draw ratio in the preliminary carbonization step was changed to 1.23.

(Comparative Example 8)
In the same manner as in Example 1, except that the draw ratio in the preliminary monocarbonization step was controlled to 1.05, the draw ratio in the carbonization step to 1.000, and the product of the draw ratios to 1.050, carbon The fiber bundle was broken in the curing step, and no carbon fiber bundle was obtained.

(Comparative Example 9)
The temperature in the flameproofing step was changed to the conditions shown in Table 1, and the draw ratio in the pre-carbonizing step was changed to 1.05. , the quality deteriorated significantly, and the subsequent steps could not be operated, and no carbon fiber bundle was obtained.

(Comparative Example 10)
The temperature in the flameproofing step was changed to the conditions shown in Table 1, and the draw ratio in the pre-carbonizing step was changed to 1.05. , the quality deteriorated significantly, and the subsequent steps could not be operated, and no carbon fiber bundle was obtained.

Claims

In the stress σ-strain ε curve in the resin-impregnated strand tensile test, the stress is in the range of 0 to 3 GPa. satisfies formula (2), the initial elastic modulus is 240 to 279 GPa, the number of filaments is 24,000 to 72,000, and the carbon fiber bundle is substantially untwisted.
ε=Aσ 2 +Bσ+C (1)
−410≦(0.0000832Π 2 −0.0184Π+1.00)/A≦−310 (2)
Here, A, B, and C are the coefficients of the quadratic function of stress σ and strain ε, and Π is the degree of crystal orientation.
The carbon fiber bundle according to claim 1, having a single fiber fineness of 0.63 to 1.35 dtex.
3. The carbon fiber bundle according to claim 1 or 2, wherein the single fiber cross section has a roundness of 0.86 to 0.98.
A method for producing the carbon fiber bundle according to any one of claims 1 to 3,
A flameproofing step of heat-treating a substantially untwisted polyacrylonitrile-based precursor fiber bundle having a filament number of 24,000 to 72,000 in an oxidizing atmosphere at a temperature of 220 to 280°C, and a flameproofing step obtained by the flameproofing step. a preliminary carbonization step of heat-treating the flameproofed fiber bundle in an inert atmosphere at a maximum temperature of 300 to 1,000 ° C.; A carbonization step of heat treatment at a maximum temperature of 1,000 to 1,600 ° C.,
The draw ratio in the preliminary carbonization step is 1.05 to 1.20, the draw ratio in the carbonization step is 0.960 to 0.990, and stretching in the preliminary carbonization step and the carbonization step The product of magnification is 1.020 to 1.180,
In the flameproofing step, the polyacrylonitrile-based precursor fiber bundles are stepwise processed in a plurality of heat treatment furnaces set to mutually different temperatures, or in a plurality of heat treatment sections provided in the heat treatment furnace and set to mutually different temperatures. and the temperature of the heat treatment furnace or heat treatment section with the lowest temperature in the flameproofing step is set to less than 230 ° C., and the temperature of the heat treatment furnace or heat treatment section with the highest temperature is set to 280 ° C. or less.
A method for producing a carbon fiber bundle.
5. The method for producing a carbon fiber bundle according to claim 4, wherein the polyacrylonitrile-based precursor fiber bundle has a single fiber fineness of 1.20 to 2.40 dtex.
6. The method for producing a carbon fiber bundle according to claim 4 or 5, wherein the polyacrylonitrile-based precursor fiber bundle has a single fiber cross-sectional circularity of 0.86 to 0.98.