KR20240097812A - Carbon fiber bundle and its manufacturing method - Google Patents

Abstract

In order to provide a carbon fiber bundle with high total fineness and excellent strength, elastic modulus, and operability when subjected to high-level processing, and a manufacturing method thereof, the stress in the stress σ-strain ε curve in the tensile test of the resin-impregnated strand is In the range of 0 to 3 GPa, the relationship between the coefficient A obtained from the approximation equation (1) for nonlinearity and the crystal orientation degree Π (%) in wide-angle X-ray diffraction measurement satisfies equation (2), and the initial elastic modulus is 240 It is ~279GPa, has a filament count of 24,000~72,000, and is made of a substantially lead-free carbon fiber bundle.
ε=Aσ ² +Bσ+C … (One)
-410≤(0.0000832Π ² -0.0184Π+1.00)/A≤-310 … (2)
Here, A, B, C are the coefficients of the quadratic function of stress σ and strain ε, and Π is the crystal orientation.

Description

Carbon fiber bundle and its manufacturing method

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

Since carbon fiber bundles have high specific strength and specific elastic modulus, they are being used as reinforcing fibers for composite materials in a wide range of applications, including aviation and space applications. Recently, it has also been deployed for industrial purposes such as automotive components and wind power generation. In particular, since lightness and rigidity are required in wind power generation, carbon fiber bundles with excellent specific elastic modulus are often used, and in recent years, the demand for carbon fiber bundles for wind power generation has expanded.

In industrial applications, there is a strong demand for cost reduction, and carbon fiber bundles with an excellent productivity of 24,000 or more filaments are often used. In addition, high-order processability is considered important when producing carbon fiber composite materials such as prepreg, towpreg, intermediate substrates such as fabric and sheet molding compound (SMC), and extractants from carbon fiber bundles. In order to improve high-order processability, the carbon fiber bundle should have less fluff and excellent openability, as well as not break the entire carbon fiber bundle or carbon fiber single fiber when unwound from the bobbin and run during the manufacturing process, and have good operability. becomes particularly important.

In general, carbon fiber bundles are produced through a flameproofing process in which polyacrylonitrile-based precursor fibers obtained by fiberizing polyacrylonitrile-based copolymers are oxidized in air at 200 to 300°C and heated in an inert atmosphere with a maximum temperature of 500 to 1,200°C. It is manufactured by going through a preliminary carbonization process and a carbonization process of heating in an inert atmosphere at a maximum temperature of 1,200 to 3,000°C.

Until now, technologies for producing carbon fiber with high strength, high modulus of elasticity, and excellent high-order processability for industrial use have been proposed (Patent Documents 1 to 4). In Patent Document 1, in the flameproofing treatment of a polyacrylonitrile-based precursor fiber bundle with a total fineness of 40,000 dtex or more, the shape and arrangement of the folding roll are defined so that when the precursor fiber bundle runs inside the flameproofing furnace. A technology is disclosed that suppresses twisting of the fiber bundle, stably maintains the shape of the fiber bundle, suppresses thread breakage and fluff generation during the flameproofing process, and enables stable production of high-quality carbon fiber bundles. Patent Document 2 discloses a technology for improving resin impregnation and expandability during composite material molding by controlling the diameter and surface state of carbon fibers to a specific range. Patent Document 3 discloses a carbon fiber bundle with a semi-permanent twist and an elastic modulus of 200 GPa or more, which is excellent in handling and high-order processability as a fiber bundle, and has a high reinforcing effect of a fiber-reinforced composite material. It is done. Patent Document 4 discloses a carbon fiber bundle that can obtain a high-performance carbon fiber reinforced composite material with 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. .

Japanese Patent Publication No. 2014-214386 Japanese Patent Publication No. 2002-69754 Japanese Patent Publication No. 2019-151956 International Publication No. 2016/068034

However, the background technology has the following problems.

Patent Document 1 shows the effect of suppressing the occurrence of twisting and “home jump” (thread falling off from the roller) in the flame resistance process by setting the yarn density in the flame resistance process to a specific range, but the obtained The effect of improving the quality of the carbon fiber bundle was not shown, and it was not possible to improve the operability when used in a high-level processing process.

In Patent Document 2, the resin impregnation property was improved when molding a pressure vessel, and the strength development rate of the obtained molding material was improved, but the operability was not improved when the obtained carbon fiber bundle was subjected to a high-level processing process.

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

In Patent Document 4, the fracture toughness value effective for strength improvement is improved by controlling the nonlinearity of the stress σ-strain ε curve in the resin-impregnated strand tensile test to a specific range by controlling the heat treatment method in the flame resistance process. , there is no suggestion about the operability when a high total fineness carbon fiber bundle is used in a high-order processing process, and the initial elastic modulus in the tensile test of a resin-impregnated strand is also high at 315 GPa, and when used in a high-order processing process, It was not possible to expect any improvement in operability. In addition, 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, but there are restrictions on the heat treatment method in the flameproofing process due to reasons such as thermal runaway, and the above patent There was a problem that it was difficult to stably control the nonlinearity of the stress σ-strain ε curve with the method described in the literature.

As described above, in the prior art, technologies for improving the mechanical properties of carbon fiber bundles and technologies for improving operability during the production of carbon fiber bundles have been proposed, but rollers and guides during high-order processing for carbon fiber bundles with a large total fineness are proposed. There is no disclosed technology that can suppress troubles such as fluff caused by abrasion or fracture occurring in part or all of the carbon fiber bundle. In the present invention, a carbon fiber bundle that has a high total fineness but is excellent in strength, elastic modulus, and operability when subjected to high-order processing, is substantially lead-free, and is prone to exhibit mechanical properties when used as a carbon fiber reinforced composite material, and the same. The purpose is to provide a manufacturing method.

In order to achieve this purpose of the present invention, the present invention mainly has the following structure.

That is, the present invention relates to the coefficient A obtained from the approximate equation (1) of nonlinearity in the stress range of 0 to 3 GPa in the stress σ-strain ε curve in the resin-impregnated strand tensile test, and the wide-angle X-ray diffraction measurement The relationship between crystal orientation and Π (%) satisfies Equation (2), the initial elastic modulus is 240 to 279 GPa, the number of filaments is 24,000 to 72,000, and it is a substantially lead-free carbon fiber bundle.

ε=Aσ ² +Bσ+C … (One)

-410≤(0.0000832Π ² -0.0184Π+1.00)/A≤-310 … (2)

Here, A, B, C are the coefficients of the quadratic function of stress σ and strain ε, and Π is the crystal orientation.

In addition, the present invention is a method for producing the carbon fiber bundle,

A flameproofing process of heat-treating a substantially lead-free polyacrylonitrile-based precursor fiber bundle with a filament count of 24,000 to 72,000 at a temperature of 220 to 280°C in an oxidizing atmosphere, and heat-treating the flameproofing fiber bundle obtained in the flameproofing process in an inert atmosphere. , a preliminary carbonization process of heat treatment at a maximum temperature of 300 to 1,000°C, and a carbonization process of heat treatment of the preliminary carbonized fiber bundle obtained from the preliminary carbonized fiber bundle in an inert atmosphere at a maximum temperature of 1,000 to 1,600°C. do,

The draw ratio in the preliminary carbonization process is 1.05 to 1.20, the draw ratio in the carbonization process is 0.960 to 0.990, and the product of the draw ratios of the preliminary carbonization process and the carbonization process is 1.020 to 1.180. and

In the flameproofing process, the polyacrylonitrile-based precursor fiber bundle is subjected to heat treatment step by step in a plurality of heat treatment furnaces set to mutually different temperatures, or in a plurality of heat treatment sections formed within the heat treatment furnace and set to mutually different temperatures, In the flameproofing process, the temperature of the heat treatment furnace or heat treatment section with the lowest temperature 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.

According to the present invention, a carbon fiber bundle is obtained that has a high total fineness and is excellent in strength, elastic modulus, and operability when subjected to high-level processing, and that exhibits mechanical properties that are easy to develop when used as a carbon fiber reinforced composite material.

To achieve this purpose, the present invention has the following structure.

In the carbon fiber bundle of the present invention, the value of the coefficient A 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 nonlinearity approximation equation (1) is as follows: Equation (2) is satisfied.

ε=Aσ ² +Bσ+C … (One)

-410≤(0.0000832Π ² -0.0184Π+1.00)/A≤-310 … (2)

Here, Π represents the crystal orientation (%) obtained by measuring the carbon fiber bundle by wide-angle X-ray diffraction measurement. The degree of crystal orientation is obtained by the method of measuring the degree of crystal orientation Π of carbon fiber described later.

The value of the central term in the above equation (2) is -410 to -310, preferably -406 to -343, and more preferably -386 to -352.

In equation (1), coefficient A represents 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 equation (1) in the stress range of 0 to 3 GPa. As described above, the stress σ-strain ε curve of a carbon fiber bundle generally shows a downward convex curve with stress σ (GPa) as the vertical axis and strain ε (-) as the abscissa, so the above approximate equation (1 ) The coefficient A obtained from ) takes a negative value. In other words, the closer the coefficient A is to 0, the smaller the nonlinearity is.

In addition, the present inventors discovered that simply the nonlinearity of the stress σ-strain ε curve is not necessarily sufficient for correlation with the shear modulus of carbon fiber. Regarding the theory related to stress and strain in carbon fiber, for example, "Carbon" (Netherlands), Elsevier, 1991, Volume 29, No. 8, p.1267- It is explained in 1279, etc. However, this is an academic review, and it is difficult to use in practical studies to control the shear modulus of carbon fiber. As a result of repeated studies based on these theories, the present inventors have found that the crystal orientation degree Π, which is relatively easy to measure from a practical point of view, and the value of the central term of the above equation (2) derived from the coefficient A of the above approximate equation (1) It was found that (0.0000832Π ² -0.0184Π+1.00)/A has a very high correlation with the shear modulus of carbon fiber. More specifically, as the value of the central term in equation (2) increases, the shear modulus decreases, and as the value of the central term in equation (2) decreases, the shear modulus increases.

Shear modulus is an indicator of the easiness of deformation when stress in the bending or compression direction is applied to single fibers, and is important for improving operability in high-order processing processes. If the value of the central term in the above equation (2) is -410 to -310, the fiber is moderately deformed when subjected to bending or compression stress in the high-order processing process, resulting in fracture of the single fiber or winding on the roller or guide following it. can be suppressed. The coefficient A in the above equation (1) can be controlled by the draw ratio of the flameproofing process, the draw ratio of the preliminary carbonization process, and the draw ratio of the carbonization process. In addition, the crystal orientation degree Π can be controlled by the stretching ratio of the preliminary carbonization process, the stretching ratio of the carbonization process, and the temperature of the carbonization process.

Additionally, the carbon fiber bundle of the present invention has an initial elastic modulus of 240 to 279 GPa, preferably 245 to 269 GPa, and more preferably 245 to 260 GPa. The initial elastic modulus is an indicator of the initial easiness of deformation when stress in the tensile direction is applied to single fibers, and is important for improving operability in high-order processing processes. If the initial elastic modulus is 240 to 279 GPa, the fiber is moderately deformed when subjected to stress in the tensile direction in a high-order processing process, and fracture of single fibers and subsequent wrapping around rollers or guides can be suppressed. This initial elastic modulus is calculated as 1/B, the reciprocal of the coefficient B when fitting the stress σ-strain ε curve measured by the resin-impregnated strand tensile test described later with the approximate equation (1). This initial elastic modulus can be controlled by the draw ratio of the flameproofing process, the draw ratio of the preliminary carbonization process, the draw ratio of the carbonization process, and the temperature of the carbonization process.

The carbon fiber bundle of the present invention has a filament count of 24,000 to 72,000, preferably 36,000 to 60,000, and more preferably 48,000 to 50,000. The number of filaments is the number of single fibers that make up a carbon fiber bundle. The larger the number, the better the productivity of the carbon fiber reinforced composite material. However, if it is too large, the dynamics of the carbon fiber reinforced composite material obtained from the viewpoint of the expandability of the carbon fiber bundle and the resin impregnation ability will be affected. There are cases where the characteristics deteriorate. If the number of filaments is 24,000 to 72,000, productivity when molding composite materials is excellent and can be used suitably for industrial purposes. This number of filaments can be controlled by the number of holes in the spinneret, division of yarn, and plying in the spinning process of the polyacrylonitrile-based precursor fiber bundle.

The carbon fiber bundles of the present invention are substantially lead-free. Substantially lead-free as used herein means that the twist of the carbon fiber bundle is less than 0.5 turns per meter. If the carbon fiber bundle is substantially lead-free, the orientation disorder of the fibers in the carbon fiber reinforced composite material can be suppressed, and the reinforcing effect of the carbon fiber reinforced composite material becomes good.

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 the graphite crystals in the carbon fiber in the [002] direction. If the crystallite size Lc is 1.80 to 2.20 nm, carbon fiber with a better balance of strength and elastic modulus can be obtained. This crystallite size LC can be evaluated by a method of measuring the crystallite size Lc described later using wide-angle X-ray diffraction measurement. This crystallite size LC can be controlled by the temperature of the carbonization process.

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

In the carbon fiber bundle of the present invention, the roundness of the single fiber cross section is preferably 0.86 to 0.98, more preferably 0.87 to 0.96, and still more preferably 0.87 to 0.93. The roundness of the single fiber cross section is defined as follows from the circumferential length L and the area A _cs of the single fiber cross section.

(Roundness)=4πA _cs /L ² .

If the roundness of the single fiber cross section is 0.86 to 0.98, the convergence and abrasion resistance during high-order processing can be achieved more reliably, and the operability during high-order processing is more excellent. The roundness of the cross section of such a single fiber can be evaluated from an image of a cross section obtained by cutting the single fiber vertically by a method described later. The roundness of the cross section of these single fibers can be controlled by the shape of the discharge hole of the spinneret in the spinning process or the conditions of the coagulation process.

Next, a preferred carbon fiber bundle manufacturing method for obtaining the carbon fiber bundle of the present invention will be described.

In the production of carbon fiber bundles, polyacrylonitrile-based precursor fiber bundles are spun. As a raw material used in the production of the polyacrylonitrile-based precursor fiber bundle, a polyacrylonitrile-based polymer is preferably used. In addition, in the present invention, a polyacrylonitrile-based polymer refers to one in which at least acrylonitrile is the main component of the polymer skeleton, and the main component is a component that usually accounts for 90 to 100% by mass of the polymer skeleton. says that The polyacrylonitrile-based polymer preferably contains a copolymerization component such as itaconic acid, acrylamide, or methacrylic acid from the viewpoint of improving spinning properties and efficiently performing anti-chlorination treatment. As a method for producing the polyacrylonitrile-based polymer, it can be selected from among known polymerization methods. In the production of polyacrylonitrile-based precursor fiber bundles, the spinning solution is prepared by mixing the above-described polyacrylonitrile-based polymer with polyacrylates such as dimethyl sulfoxide, dimethylformamide, dimethylacetamide, or aqueous solutions of nitric acid, zinc chloride, and rhodanate soda. Ronitrile is dissolved in an available solvent.

There is no particular limitation on the manufacturing method of the polyacrylonitrile-based precursor fiber bundle used in the present invention, but wet spinning is preferably used, followed by processes such as stretching, water washing, emulsion application, dry densification, and, if necessary, post-stretching. It can be obtained through The number of holes in the spinning spindle in the manufacturing 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, and is predetermined by division or plying. A bundle of polyacrylonitrile-based precursor fibers can be obtained.

In the production of a polyacrylonitrile-based precursor fiber bundle, the coagulation bath preferably contains a solvent such as dimethyl sulfoxide, dimethylformamide, and dimethylacetamide used as a solvent for the spinning stock solution, and a so-called coagulation promoting component. As a coagulation promotion component, one that does not dissolve the polyacrylonitrile-based polymer and is compatible with the solvent used in the spinning solution can be used. Preferably, water is used as a coagulation promoting component.

In the production of polyacrylonitrile-based precursor fiber bundles, the water washing process preferably uses a water washing bath consisting of multiple stages at a temperature of 30 to 98°C. Additionally, in the water washing process, it is also preferable to set the stretching ratio to 2 to 6 times.

After the water washing process, for the purpose of preventing adhesion between single fibers, an emulsion preferably made of silicon or the like is applied to the yarn. This silicone emulsion is preferably a modified silicone, and preferably contains an amino-modified silicone with high heat resistance.

The dry heat treatment process (the dry densification process described above) can use a known method. For example, the drying temperature is 100 to 200°C.

The single fiber fineness of the polyacrylonitrile-based precursor fiber bundle in the method for producing a carbon fiber bundle of the present invention is preferably 1.20 to 2.40 dtex, more preferably 1.20 to 2.20 dtex, and still more preferably 1.40. It is ~1.80dtex. Single fiber fineness is the mass per unit length of a single fiber. If the single fiber fineness is 1.20 dtex or more, a carbon fiber bundle can be obtained with sufficiently high productivity, and if the single fiber fineness is 2.40 dtex or less, processing unevenness in heat treatment after the flameproofing process is reduced, and carbon fibers have high mechanical properties. A bunch is obtained. This single fiber fineness can be controlled by the discharge amount or draw ratio in the spinning process.

The polyacrylonitrile-based precursor fiber bundle in the method for producing a carbon fiber bundle of the present invention preferably has a roundness of the single fiber cross section of 0.86 to 0.98, more preferably 0.87 to 0.96, and still more preferably 0.87. It is ~0.93. The roundness of the single fiber cross section is defined as follows from the circumferential length L and the area A _cs of the single fiber cross section.

(Roundness)=4πA _cs /L ² .

If the roundness of the single fiber cross section is 0.86 to 0.98, the coherence and abrasion resistance of the obtained carbon fiber can be more reliably achieved, and the operability during high-level processing of the obtained carbon fiber bundle is more excellent. The roundness of the cross section of the single fibers of such a polyacrylonitrile-based precursor fiber bundle can be evaluated from an image of a cross section of the single fibers cut vertically by a 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 discharge hole of the spinneret in the spinning process or the conditions of the coagulation process.

The number of filaments of the polyacrylonitrile-based precursor fiber bundle in the method for producing a carbon fiber bundle 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 constituting the polyacrylonitrile-based precursor fiber bundle. The larger the number, the better the productivity of carbon fiber bundle production and the productivity of the resulting carbon fiber reinforced composite material using the carbon fiber bundle. However, if it is too large, it will become flame resistant. Processing unevenness in the process, preliminary carbonization process, or carbonization process may increase, or the mechanical properties of the obtained carbon fiber reinforced composite material may deteriorate from the viewpoint of the expandability of the resulting carbon fiber bundle or resin impregnation. If the number of filaments of 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 a carbon fiber bundle that can be suitably used for industrial purposes is obtained. The number of filaments of such a polyacrylonitrile-based precursor fiber bundle can be evaluated by counting the number of single fibers constituting the polyacrylonitrile-based precursor fiber bundle. This 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 discharged from the spinneret, and the number of plying yarns of the fiber bundle.

In the method for producing a carbon fiber bundle of the present invention, the substantially lead-free polyacrylonitrile-based precursor fiber bundle as described above is heat-treated at a temperature of 220 to 280 ° C. in an oxidizing atmosphere (salting process). The temperature in the flameproofing process is preferably 220 to 280°C. If the temperature of the flame resistance treatment is 220°C or higher, a flame-resistant fiber bundle with sufficient flame resistance can be produced, so the generation of fluff due to lack of flame resistance can be suppressed, and the operation during high-level processing of the resulting carbon fiber bundle can be improved. The castle is excellent. If the temperature for flameproofing treatment is 280°C or lower, the heat generation rate does not become excessively high, so temperature unevenness within the flameproofing fiber bundle can be reduced, and a carbon fiber bundle with excellent mechanical properties can be obtained. The temperature of this flameproofing treatment can be determined by inserting a thermometer such as a thermocouple into the flameproofing furnace to measure the temperature inside the furnace. If there is temperature unevenness or temperature distribution when measuring the temperature inside the furnace at several points, a simple average temperature is calculated. The temperature of this flameproofing treatment can be controlled by the heating output in a heating method used in a known flameproofing furnace. For example, in a hot air circulation type flame retardant furnace, the output of the heater used for heating the oxidizing atmosphere may be changed.

In the flameproofing process, a plurality of heat treatment furnaces set to mutually different temperatures, or a plurality of heat treatment sections formed within the heat treatment furnace and set to mutually different temperatures are used, and heat treatment is performed on the polyacrylonitrile-based precursor fiber bundle in stages. (Hereinafter, such a heat treatment furnace and heat treatment section may be referred to as “heat treatment furnace/heat treatment section”). In addition, in the present invention, the temperature may be different between at least two heat treatment furnaces/heat treatment sections among a plurality of heat treatment furnaces/heat treatment sections, for example, two heat treatment furnaces/heat treatment sections among three heat treatment furnaces/heat treatment sections. This may be the same temperature. In the present invention, the temperature of the lowest heat treatment furnace or heat treatment section in the flameproofing process is set to less than 230°C, preferably less than 225°C, and more preferably less than 223°C. 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 high total fineness can be reduced, and the preliminary carbonization process and carbonization described later The quality can be maintained at a high level throughout the process. Additionally, if the temperature of the lowest heat treatment furnace or heat treatment section is 230°C or higher, heat treatment unevenness in the flameproofing process increases, and the quality deteriorates due to stretching in the preliminary carbonization process and carbonization process.

Additionally, in the present invention, the temperature of the highest heat treatment furnace or heat treatment section in the flameproofing process is set to 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 lower, heat treatment unevenness that tends to occur in polyacrylonitrile-based precursor fiber bundles with high total fineness can be reduced, and the preliminary carbonization process and carbonization described later The quality can be maintained at a high level throughout the process. Additionally, if the temperature of this heat treatment furnace or heat treatment section exceeds 280°C, heat treatment unevenness in the flameproofing process increases, and the quality deteriorates due to stretching in the preliminary carbonization process and carbonization process.

Continuing from the polyacrylonitrile-based precursor fiber bundle manufacturing process and flameproofing process, preliminary carbonization is performed. In the preliminary carbonization process, the flame-resistant fiber bundle obtained as described above is heat-treated in an inert atmosphere at a maximum temperature of 300 to 1,000°C, preferably until the density reaches 1.5 to 1.8 g/cm3.

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

In addition, in the present invention, in the preliminary carbonization process and the carbonization process, a plurality of heat treatment furnaces or heat treatment sections may be used, and they may be set to mutually different temperatures. Therefore, the temperature of the heat treatment furnace or heat treatment section with the highest temperature in each process is called the “highest 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 carbonization step and the carbon The product of the stretching ratio of the chemical process is 1.020 to 1.180.

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

The draw ratio in the carbonization process is preferably 0.975 to 0.990, more preferably 0.975 to 0.985.

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

Carbon obtained by controlling the draw ratio in the preliminary carbonization process to be 1.05 or more, the draw ratio in the carbonization process to be 0.960 or more, and the product of the draw ratio in the preliminary carbonization process and the draw ratio in the carbonization process to be 1.020 or more. The value of the central term of the above equation (2) and the initial elastic modulus of the fiber bundle can be controlled to an appropriate range. Meanwhile, the draw ratio in the preliminary carbonization process is controlled to be 1.20 or less, the draw ratio in the carbonization process is 0.990 or less, and the product of the draw ratio in the preliminary carbonization process and the draw ratio in the carbonization process is 1.180 or less. By suppressing thread breakage due to stretching, a decrease in operability during carbon fiber production and an increase in the number of fluffs in the obtained carbon fiber bundle can be suppressed.

The carbon fiber bundle obtained as described above is preferably subjected to oxidation treatment to improve adhesion to the matrix resin and introduces an oxygen-containing functional group. As oxidation treatment methods, gas phase oxidation, liquid phase oxidation, and liquid phase electrolytic oxidation are used, but liquid phase electrolytic oxidation is preferably used from the viewpoint of high productivity and uniform treatment. There is no particular designation regarding the method of liquid electrolytic oxidation, and any known method may be used.

After this electrolytic treatment, sizing treatment may be performed in order to provide bundling properties to the obtained carbon fiber bundle. As a sizing agent, a sizing agent that has good compatibility with the matrix resin can be appropriately selected depending on the type of matrix resin used in the composite material.

(Example)

Hereinafter, the present invention will be described in more detail through examples. However, the present invention is not limited to these.

<Tensile test of resin-impregnated strands of carbon fiber bundles>

The tensile elastic modulus (strand elastic modulus E (GPa)) of the resin-impregnated strand of the carbon fiber bundle, the tensile strength (strand strength (GPa)) and the stress σ-strain ε curve of the resin-impregnated strand are according to JIS R7608 (2008) “Resin-impregnated strand test” Obtained in accordance with the law. Strand elastic modulus E is measured in a strain range of 0.1 to 0.6%. Additionally, the test piece is produced by impregnating the carbon fiber bundle with the following resin composition and curing it by 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)

Additionally, the number of measured strands is set to 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 strain ε(-) on the vertical axis and stress σ(GPa) on the horizontal axis, and fitting the coefficients A and B using the following equation (1). , yields C. Fitting is performed for the region where stress σ is 0 to 3 GPa among the stress σ-strain ε curve obtained from the measurement. Fitting is performed using a quadratic function using “Excel” manufactured by Microsoft.

ε=Aσ ² +Bσ+C … (One).

<Initial elastic modulus (㎬)>

The initial elastic modulus of the carbon fiber bundle is calculated as follows by analyzing the stress σ-strain ε curve described above and using the coefficient B obtained by fitting Equation (1).

Initial elastic modulus (㎬) = 1/B.

<Crystal orientation of carbon fiber bundle Π (%)>

The carbon fiber bundles to be used for measurement are neatly collected and hardened using a collodion/alcohol solution to prepare a measurement sample of a square pillar with a length of 4 cm and a side length of 1 mm. Measurement is performed on the prepared measurement sample using a wide-angle X-ray diffractometer under the following conditions.

·X-ray source: CuKα ray (tube voltage 40 kV, tube current 30 mA)

·Detector: goniometer + monochromator + scintillation counter

The half width H (°) of the diffraction intensity distribution obtained by scanning the peak that appears around 2θ = 25 to 26° in the circumferential direction is obtained using the following equation.

Crystal orientation Π(%)=[(180-H)/180]×100

In addition, in the examples, as the wide-angle X-ray diffraction device, XRD-6100 manufactured by SHIMADZU CORPORATION was used.

<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=10~40°

·Scanning mode: step scan, step unit 0.02°, counting time 2 seconds.

In the obtained diffraction pattern, the half width is determined for the peak that appears around 2θ = 25 to 26°, and the crystallite size is calculated from this value using the following Scherrer equation.

Crystallite size (㎚)=Kλ/β ₀ cosθ _B

step,

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.

<Measurement of roundness (-)>

A polyacrylonitrile-based precursor fiber bundle or a carbon fiber bundle was cut perpendicularly to the fiber axis direction with a single-edged razor, and the obtained cross section was examined using a scanning electron microscope (SEM) "S-4800" manufactured by Hitachi High-Tech Corporation. , observed from the vertical direction of the fiber cross section. The acquired image is analyzed using image analysis software "ImageJ", and for the single fiber included in the fiber cross section, the roundness is calculated from the circumferential length and area of the cross section of the single fiber according to the following definition. This measurement is repeated for 25 single fibers at random in one cross section, and the average roundness is taken as the roundness of the single fiber cross section.

The roundness of the single fiber cross section is defined as follows from the circumferential length L and the area A _cs of the single fiber cross section.

(Roundness)=4πA _cs /L ² .

<Evaluation of high-order machinability>

The bobbin of the carbon fiber bundle is installed on the creel, pulled out with a tension of 1.6mN/dtex, interposed with 10 free rollers, then rubbed against 5 fixed guides, taken up by a driving roller at a speed of 10m/min, and then wound by a winder. wind it up At this time, the fluff generated is counted for 10 minutes immediately before the drive roller, and evaluated using the following indicators.

A: Less than 10 pieces/m

B: 10 or more but less than 50/m

C: More than 50 pieces/m.

(Examples 1 to 4)

A polyacrylonitrile-based copolymer composed of acrylonitrile and itaconic acid was polymerized by solution polymerization using dimethyl sulfoxide as a solvent to prepare a polyacrylonitrile-based copolymer, thereby obtaining a spinning stock solution. The obtained spinning stock solution was coagulated by a wet spinning method in which it was introduced into a coagulation bath made of an aqueous solution of dimethyl sulfoxide from a spinneret with 50,000 holes, thereby forming a fiber bundle. This fiber bundle was washed with water at 30 to 98°C according to a conventional method, and then stretched. Subsequently, an amino-modified silicone emulsion is applied to the fiber bundle after washing and stretching, and a dry densification treatment is performed using a heating roller at 130° C., and polyacrylonitrile with a single fiber count of 50,000 and a single fiber fineness of 1.50 dtex is obtained. A system precursor fiber bundle was obtained. In addition, false twisting treatment was not performed on the polyacrylonitrile-based precursor fiber bundle.

The obtained polyacrylonitrile-based precursor fiber bundle was subjected to a flameproofing process, preliminary carbonization process, and carbonization process under the conditions shown in Table 1 to obtain a carbon fiber bundle. In addition, in each of the flameproofing process, preliminary carbonization process, and carbonization process, heat treatment was performed by gradually increasing the temperature using a plurality of heat treatment furnaces having different temperatures. In addition, no false combustion treatment was performed in the flameproofing process, preliminary carbonization process, and carbonization process. The properties of the obtained carbon fiber bundle are shown in Table 2.

(Example 5)

Example 1, except that the discharge amount of the spinning solution was changed to obtain a polyacrylonitrile-based precursor fiber bundle with a single fiber fineness of 1.65 dtex, and the conditions of the subsequent preliminary carbonization process and carbonization process were changed as shown in Table 1. It was carried out similarly.

(Example 6)

Example 1, except that the discharge amount of the spinning solution was changed to obtain a polyacrylonitrile-based precursor fiber bundle with a single fiber fineness of 2.40 dtex, and the conditions of the subsequent preliminary carbonization process and carbonization process were changed as shown in Table 1. It was carried out similarly.

(Example 7)

The same procedure as in Example 1 was carried out except that the flame resistance temperature, the draw ratio of the preliminary carbonization process, and the draw ratio of the carbonization process were changed to the conditions shown in Table 1.

(Comparative Example 1)

A carbon fiber bundle was obtained in the same manner as in Example 1, except that the draw ratio of the preliminary carbonization process was changed to 1.00, the draw ratio of the carbonization process was changed to 0.960, and the product of the draw ratio was changed to 0.960. The value of the central term in equation (2) of the obtained carbon fiber bundle was -307, the initial elastic modulus was 213 GPa, and the operability during high-order processing was poor.

(Comparative Example 2)

A carbon fiber bundle was obtained in the same manner as in Example 1, except that the draw ratio of the preliminary carbonization process was changed to 1.01, the draw ratio of the carbonization process was changed to 0.955, and the product of the draw ratio was changed to 0.965. The value of the center term in equation (2) of the obtained carbon fiber bundle was -286, the initial elastic modulus was 215 GPa, and the operability during high-order processing was poor.

(Comparative Example 3)

A carbon fiber bundle was obtained in the same manner as in Example 1, except that the draw ratio of the preliminary carbonization process was changed to 1.02, the draw ratio of the carbonization process was changed to 0.950, and the product of the draw ratio was changed to 0.969. The value of the center term in equation (2) of the obtained carbon fiber bundle was -287, the initial elastic modulus was 220 GPa, and the operability during high-order processing was poor.

(Comparative Example 4)

By changing the discharge amount of the spinning solution, a polyacrylonitrile-based precursor fiber bundle with a single fiber fineness of 0.80 dtex was obtained, and the draw ratio of the preliminary carbonization process was 1.05, the draw ratio of the carbonization process was 0.950, and the product of the draw ratio was 0.998. A carbon fiber bundle was obtained in the same manner as in Example 1 except that this was changed. The value of the central term in equation (2) of the obtained carbon fiber bundle was -290, the initial elastic modulus was 218 GPa, and the operability during high-order processing was poor.

(Comparative Example 5)

By changing the discharge amount of the spinning solution, a polyacrylonitrile-based precursor fiber bundle with a single fiber fineness of 3.00 dtex was obtained, and the draw ratio of the preliminary carbonization process was set to 1.00, the draw ratio of the carbonization process was set to 0.955, and the product of the draw ratio was set to 0.955. A carbon fiber bundle was obtained in the same manner as in Example 1 except that it was changed to . The value of the center term in equation (2) of the obtained carbon fiber bundle was -277, the initial elastic modulus was 225 GPa, and the operability during high-order processing was poor.

(Comparative Example 6)

A bundle of polyacrylonitrile-based precursor fibers was obtained in the same manner as in Example 1, except that the spinning stock solution was once discharged into the air from the spinning nozzle and then coagulated by a dry and wet spinning method in which it was introduced into a coagulation bath made of an aqueous solution of dimethyl sulfoxide. A carbon fiber bundle was obtained in the same manner as in Example 1, except that the draw ratio of the carbonization process was changed to 1.01, the draw ratio of the carbonization process was changed to 0.965, and the product of the draw ratio was 0.975. The value of the central term in equation (2) of the obtained carbon fiber bundle was -290, the initial elastic modulus was 223 GPa, and the operability during high-order processing was poor.

(Comparative Example 7)

As a result of carrying out the same procedure as in Example 1 except that the draw ratio of the preliminary carbonization process was changed to 1.23, the fiber bundle broke in the preliminary carbonization process and no carbon fiber bundle was obtained.

(Comparative Example 8)

As a result of the same procedure as in Example 1, except that the draw ratio of the preliminary single carbonization process was controlled to be 1.05, the draw ratio of the carbonization process was controlled to be 1.000, and the product of the draw ratio was controlled to be 1.050, the fiber bundle was broken in the carbonization process, resulting in a carbon fiber bundle. This was not obtained.

(Comparative Example 9)

As a result of performing the same procedure as in Example 1 except that the temperature of the flameproofing process was changed to the conditions shown in Table 1 and the draw ratio of the preliminary carbonization process was set to 1.05, the fluff of the preliminary carbonized fiber bundle increased and the quality deteriorated significantly. As a result, subsequent processes could not be performed and no carbon fiber bundles were obtained.

(Comparative Example 10)

As a result of performing the same procedure as in Example 1 except that the temperature of the flameproofing process was changed to the conditions shown in Table 1 and the draw ratio of the preliminary carbonization process was set to 1.05, the fluff of the preliminary carbonized fiber bundle increased and the quality deteriorated significantly. As a result, subsequent processes could not be performed and no carbon fiber bundles were obtained.

Claims (6)

In the stress σ-strain ε curve in the resin-impregnated strand tensile test, the coefficient A obtained from the approximate equation (1) for nonlinearity in the stress range of 0 to 3 GPa, and the crystal orientation Π in the wide-angle X-ray diffraction measurement A carbon fiber bundle whose (%) relationship satisfies Equation (2), has an initial modulus of elasticity of 240 to 279 GPa, a filament count of 24,000 to 72,000, and is substantially lead-free.
ε=Aσ ² +Bσ+C … (One)
-410≤(0.0000832Π ² -0.0184Π+1.00)/A≤-310 … (2)
Here, A, B, C are the coefficients of the quadratic function of stress σ and strain ε, and Π is the crystal orientation. According to claim 1,
A bundle of carbon fibers with a single fiber fineness of 0.63 to 1.35 dtex. The method of claim 1 or 2,
A bundle of carbon fibers with a circularity of the single fiber cross section of 0.86 to 0.98. A method for producing the carbon fiber bundle according to any one of claims 1 to 3, comprising:
A flameproofing process of heat-treating a substantially lead-free polyacrylonitrile-based precursor fiber bundle with a filament count of 24,000 to 72,000 at a temperature of 220 to 280°C in an oxidizing atmosphere, and heat-treating the flameproofing fiber bundle obtained in the flameproofing process in an inert atmosphere. , a preliminary carbonization process of heat treatment at a maximum temperature of 300 to 1,000°C, and a carbonization process of heat treatment of the preliminary carbonized fiber bundle obtained from the preliminary carbonized fiber bundle in an inert atmosphere at a maximum temperature of 1,000 to 1,600°C. do,
The draw ratio in the preliminary carbonization process is 1.05 to 1.20, the draw ratio in the carbonization process is 0.960 to 0.990, and the product of the draw ratios of the preliminary carbonization process and the carbonization process is 1.020 to 1.180. and
In the flameproofing process, the polyacrylonitrile-based precursor fiber bundle is subjected to heat treatment step by step in a plurality of heat treatment furnaces set to mutually different temperatures, or in a plurality of heat treatment sections formed within the heat treatment furnace and set to mutually different temperatures, In the flameproofing process, the temperature of the heat treatment furnace or heat treatment section with the lowest temperature 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. According to claim 4,
A method for producing a carbon fiber bundle in which the single fiber fineness of the polyacrylonitrile-based precursor fiber bundle is 1.20 to 2.40 dtex. The method of claim 4 or 5,
A method of producing a carbon fiber bundle wherein the roundness of the cross-section of the single fiber of the polyacrylonitrile-based precursor fiber bundle is 0.86 to 0.98.