Carbon fiber bundle production method and smc production method

Abstract

To provide a carbon fiber bundle that can be easily divided into carbon fiber bundles with 10,000 or fewer filaments when cut short.SOLUTION: A production method for carbon fiber bundles, wherein carbon fiber bundles are produced by supplying acrylic fiber bundles with a hook drop value of 15 cm or more when measured in an untwisted state to a calcination process, comprises adding a twist of 1 turn / m or more and 10 turns / m or less to the acrylic fiber bundles before supplying them to the calcination process. The method of adding twist to the acrylic fiber bundles is preferably performed by rotating a package or bobbin winding of untwisted acrylic fiber bundles.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing a carbon fiber bundle. [Background technology]

[0002] Conventionally, a method for producing carbon fiber bundles has been known in which a carbon fiber precursor acrylic fiber bundle (hereinafter also referred to as "precursor fiber bundle") made of acrylic fiber or the like is converted into a flame-resistant fiber bundle by heat treating the bundle in an oxidizing atmosphere of 200°C or higher and 400°C or lower (hereinafter also referred to as "flame-resistant process"), followed by a carbonization process in an inert atmosphere of 1000°C or higher (hereinafter also referred to as "carbonization process". The "flame-resistant process" and the "carbonization process" are collectively referred to as a baking process), to obtain a carbon fiber bundle.

[0003] When producing a carbon fiber bundle from a precursor fiber bundle, in the flame-proofing process, a large number of fiber bundles arranged in a sheet shape are folded back through a roll and passed through a flame-proofing furnace in multiple stages while changing the running direction by 180 degrees. When the running direction is changed, the filaments may wrap around the roll or adjacent fiber bundles may interfere with each other, which may cause a deterioration in the mechanical properties and quality of the final carbon fiber bundle. In order to prevent such filaments from wrapping around the roll or adjacent fiber bundles from interfering with each other, it is necessary for a single fiber bundle composed of a large number of filaments to be in a bundled state without falling apart, and various studies have been conducted so far to maintain this bundled state.

[0004] As a means for forming a precursor fiber bundle into a bundled state, there have been proposed a means for performing an entanglement process (a process for entangling filaments constituting the precursor fiber bundle within the fiber bundle) and maintaining the bundled state of the fiber bundle. For example, Patent Document 1 proposes a means for performing an entanglement process on the fiber bundle by continuously running the precursor fiber bundle through an entangler having a flat rectangular cross section and arranged in one stage or multiple stages that ejects air, thereby maintaining the fiber bundle into a bundled state.

[0005] Carbon fiber bundles obtained by subjecting the precursor fiber bundles obtained by the above-mentioned method to a sintering process have excellent mechanical properties and are therefore widely used industrially as reinforcement fibers for composite materials, such as sheet molding compounds (hereinafter referred to as SMC). When heated and pressurized in a mold, SMC is an intermediate material that is advantageous for obtaining molded products of various shapes, such as those with partially different thicknesses and those with ribs and bosses, because the reinforcing fibers and resin composition flow together to fill the cavity. On the other hand, the carbon fiber bundles in SMC have a short fiber length and a random orientation, which makes it difficult to increase the mechanical strength. Furthermore, there is a drawback in that the mechanical strength varies greatly depending on the location. In addition, Non-Patent Document 1 shows that the strength and elastic modulus of SMC molded products decrease as the number of filaments in the carbon fiber bundle increases.

[0006] Therefore, the use of finely divided thick fiber bundles has been considered. Patent Document 2 discloses an apparatus for dividing a carbon fiber bundle having 48,000 (48K) filaments into thin carbon fiber bundles (split tows) having several thousand filaments by passing the bundle through a spreading bar with a crown and a splitting bar with grooves.

[0007] Patent Document 3 discloses a method for dividing a carbon fiber bundle, in which a continuous carbon fiber bundle is continuously run while being subjected to tension, and the carbon fiber bundle is widened with a widening tool disposed midway through the run, and simultaneously, or after the widening, the carbon fiber bundle in the widened state is divided while cutting some of the carbon fiber filaments with a cutting blade rotating parallel to the running direction of the fiber bundle.

[0008] However, in the carbon fiber bundle obtained by carrying out the entanglement treatment as in Patent Document 1, the filaments constituting the fiber bundle are strongly entangled with each other, and it is not possible to divide the thick fiber bundle into small pieces as described in Patent Document 2, nor to widen the fiber bundle as described in Patent Document 3, and as a result, it is difficult to obtain a high-performance SMC molded product. Furthermore, when an attempt is made to divide or widen the fiber bundle by applying a large external force, the entangled filaments are cut and become fluff, degrading the quality of the SMC molded product. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] JP 2002-294517 A [Patent Document 2] U.S. Patent No. 6,385,828 [Patent Document 3] JP 2006-219780 A [Non-patent literature]

[0010] [Non-Patent Document 1] N. Tsuchiyama, “The Mechanical Properties of Carbon Fiber SMC”, Proceedings of the Fourth International Conference on Composite Materials (ICCM-IV), 1982, p.497-503 Summary of the Invention [Problem to be solved by the invention]

[0011] In view of these problems with the conventional technology, the present invention aims to provide a method for producing a carbon fiber bundle that is suitable for obtaining a high-performance SMC, i.e., a method for splitting the carbon fiber bundle into fine carbon fiber bundles (split fiber bundles) having 10,000 or less filaments by cutting the carbon fiber bundle into lengths of 20 to 50 mm. [Means for solving the problem]

[0012] In order to achieve the above object, the present invention has the following features. [1] A method for producing a carbon fiber bundle by supplying an acrylic fiber bundle having a hook drop value of 15 cm or more measured in an untwisted state to a baking process to produce a carbon fiber bundle, in which a twist of 1 turn / m or more and 10 turns / m or less is added to the acrylic fiber bundle before supplying it to the baking process. [2] The method for producing a carbon fiber bundle according to [1], wherein a method for adding a twist of 1 turn / m or more and 10 turns / m or less to the acrylic fiber bundle comprises storing the carbon fiber precursor acrylic fiber bundle in an untwisted state in a packaging package, and rotating the packaging package while pulling the bundle out of the packaging package to add a twist of 1 turn / m or more and 10 turns / m or less to the carbon fiber precursor acrylic fiber bundle. [3] The method for producing carbon fibers according to [1], wherein the packaging package of the acrylic fiber bundle is a bobbin winding, and the acrylic fiber bundle, which is wound around a bobbin in an untwisted state, is unwound from the bobbin winding while the bobbin winding is rotated to give the acrylic fiber bundle a twist of 1 turn / m or more and 10 turns / m or less. Effect of the Invention

[0013] According to the present invention, by adding twist to the acrylic fiber bundle, the fiber bundle can be stably passed through the sintering process in a bundled state even if the filaments are less entangled. In addition, the carbon fiber bundle obtained by the manufacturing method of the present invention is extremely easy to widen and divide in the SMC manufacturing process, and high-performance SMC molded products can be stably manufactured. [Brief description of the drawings]

[0014] [Figure 1] 1 is an example of an apparatus for processing carbon fiber bundles produced according to the present invention into SMC. [Diagram 2]1 is an example of a rope pin cutter used for cutting carbon fiber bundles short when the carbon fiber bundles produced by the present invention are processed into SMC. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Preferred embodiments of the present invention will now be described. (Precursor fiber bundle) The precursor fiber bundle used in one embodiment of the present invention may be an acrylic fiber bundle spun by a known technique, specifically, an acrylic fiber bundle obtained by spinning an acrylonitrile-based polymer.

[0016] An acrylonitrile-based polymer is a polymer obtained by polymerizing acrylonitrile as a main monomer. The acrylonitrile-based polymer may be a homopolymer obtained only from acrylonitrile, or an acrylonitrile-based copolymer obtained by using other monomers in addition to acrylonitrile as a main component.

[0017] The content of acrylonitrile units in the acrylonitrile copolymer is preferably 96.0% by mass or more and 98.5% by mass or less in terms of preventing thermal fusion of fibers in the baking process, heat resistance of the copolymer, stability of the spinning dope, and quality when made into carbon fiber. When the acrylonitrile units are 96.0% by mass or more, the excellent quality and performance of the carbon fiber can be maintained without inviting thermal fusion of the fibers in the baking process when converted into carbon fiber, which is preferable. In addition, the heat resistance of the copolymer itself is not reduced, and adhesion between single fibers can be avoided in the process of drying the fibers or stretching with a heated roller or pressurized steam when spinning the precursor fiber. On the other hand, when the acrylonitrile units are 98.5% by mass or less, the solubility in the solvent is not reduced, the stability of the spinning dope can be maintained, and the precipitation and coagulation of the copolymer is not increased, which is preferable because it enables stable production of the precursor fiber.

[0018] In the case of using a copolymer, the monomer other than acrylonitrile can be appropriately selected from vinyl monomers copolymerizable with acrylonitrile. It is preferable to select from monomers such as acrylic acid, methacrylic acid, itaconic acid, or alkali metal salts or ammonium salts thereof, acrylamide, etc., which have the effect of promoting the flame retardancy reaction, since the flame retardancy can be promoted.

[0019] As the vinyl monomer copolymerizable with acrylonitrile, a carboxyl group-containing vinyl monomer such as acrylic acid, methacrylic acid, itaconic acid, etc. is more preferable. The content of the carboxyl group-containing vinyl monomer unit in the acrylonitrile copolymer is preferably 0.5% by mass or more and 2.0% by mass or less. These vinyl monomers may be used alone or in combination of two or more kinds.

[0020] In spinning, the acrylonitrile polymer is dissolved in a solvent to prepare a spinning solution. The solvent can be appropriately selected from known solvents such as organic solvents such as dimethylacetamide, dimethylsulfoxide, and dimethylformamide, or aqueous solutions of inorganic compounds such as zinc chloride and sodium thiocyanate. Among these, dimethylacetamide, dimethylsulfoxide, and dimethylformamide, which have a fast coagulation rate, are preferred from the viewpoint of improving productivity, and dimethylacetamide is more preferred.

[0021] In order to obtain a dense coagulated yarn, it is preferable to prepare the spinning dope so that the polymer concentration in the spinning dope is at a certain level or higher. % by mass or more, and more preferably 19% by mass or more. Since the spinning dope needs to have appropriate viscosity and fluidity, the polymer concentration is preferably in the range not exceeding 25% by mass.

[0022] As the spinning method, a known spinning method can be appropriately adopted, such as a wet spinning method in which the above-mentioned spinning dope is directly spun into a coagulation bath, a dry spinning method in which coagulation is performed in air, and a dry-wet spinning method in which the dope is spun once into air and then coagulated in a bath. In order to obtain a carbon fiber bundle having higher performance, the wet spinning method or the dry-wet spinning method is preferred.

[0023] The spinning shaping by the wet spinning method or the dry-wet spinning method can be carried out by spinning the spinning dope into a coagulation bath from a nozzle having a circular cross-sectional hole. As the coagulation bath, it is preferable to use an aqueous solution containing the solvent used in the spinning dope from the viewpoint of ease of solvent recovery.

[0024] As the spinning method, a known spinning method can be appropriately adopted, such as a wet spinning method in which the above-mentioned spinning dope is directly spun into a coagulation bath, a dry spinning method in which coagulation is performed in air, and a dry-wet spinning method in which the dope is spun once into air and then coagulated in a bath. In order to obtain a carbon fiber bundle having higher performance, the wet spinning method or the dry-wet spinning method is preferred.

[0025] The spinning shaping by the wet spinning method or the dry-wet spinning method can be carried out by spinning the spinning dope into a coagulation bath from a nozzle having a circular cross-sectional hole. As the coagulation bath, it is preferable to use an aqueous solution containing the solvent used in the spinning dope from the viewpoint of ease of solvent recovery.

[0026] When an aqueous solution containing a solvent is used as the coagulation bath, the solvent concentration in the aqueous solution is preferably 50% by mass or more and 85% by mass or less, and the temperature of the coagulation bath is preferably 10° C. or more and 60° C. or less, for reasons such as that a dense structure without voids can be formed to obtain a high-performance carbon fiber bundle, and that stretchability can be ensured, resulting in excellent productivity.

[0027] The polymer or copolymer is dissolved in a solvent, and the resulting spinning solution is discharged into a coagulation bath to form a coagulation fiber. The resulting coagulated fiber can then be stretched in a coagulation bath or a stretching bath. Alternatively, the fiber may be partially stretched in the air and then stretched in a bath. The fiber may be washed with water before, after or simultaneously with the stretching to obtain a precursor fiber bundle in a water-swollen state.

[0028] The bath drawing is usually carried out in a water bath at 50° C. or higher and 98° C. or lower in one or more divided stages, and the coagulated yarn is drawn so that the total drawing ratio of the air drawing and the bath drawing is 2 times or higher and 10 times or lower, which is preferable from the viewpoint of the properties of the resulting carbon fiber bundle.

[0029] To apply an oil to the coagulated yarn (water-swollen precursor fiber bundle) stretched in a bath, it is preferable to use an oil treatment liquid for carbon fiber precursor acrylic fibers (hereinafter, also simply referred to as "oil treatment liquid") in which an oil composition containing an oil is dispersed in water. The average particle size of the dispersed oil composition is preferably 0.01 μm or more and 0.3 μm or less. When the average particle size of the dispersed oil composition is within the above range, the oil can be applied more uniformly to the surface of the precursor fiber bundle.

[0030] The precursor fiber bundle in a water-swollen state to which the oil agent has been applied is dried and densified in the subsequent drying step. The drying and densifying process must be carried out at a temperature higher than the glass transition temperature of the fiber bundle to be dried and densified, but the glass transition temperature differs between when the fiber bundle to be dried and densified is in a wet state and when it is in a dry state. For example, the densifying and drying process is preferably carried out by a method using a heating roller at a temperature of 100° C. to 200° C. In this case, the number of heating rollers may be one or more.

[0031] The densely dried fiber bundle is preferably subjected to a pressurized steam stretching treatment. The stretching treatment can further increase the density and the degree of orientation of the resulting precursor fiber bundle. The pressurized steam drawing treatment is a method for drawing a fiber bundle in a pressurized steam atmosphere. The pressurized steam drawing treatment enables drawing at a high ratio, which allows for faster and more stable spinning and also contributes to improving the denseness and orientation of the resulting fibers.

[0032] In the pressurized steam stretching treatment, it is preferable to control the temperature of the heating roller immediately before the pressurized steam stretching device to 120°C or higher and 190°C or lower, and the fluctuation rate of the steam pressure in the pressurized steam stretching to 0.5% or lower. By controlling the temperature of the heating roller and the fluctuation rate of the steam pressure in this way, it is possible to suppress fluctuations in the draw ratio in a small section and the resulting fluctuations in the total fiber bundle fineness in a small section. If the temperature of the heating roller is less than 120°C, the temperature of the fiber bundle to be subjected to the pressurized steam stretching treatment does not rise sufficiently, and the stretchability is likely to decrease.

[0033] The precursor fiber bundle obtained by drying and densifying, or the precursor fiber bundle obtained by drying and densifying and then subjecting it to pressurized steam drawing treatment, is preferably subjected to an entanglement treatment, in which the precursor fiber bundle is guided to a grooved guide or a grooved roll, the width of which is made constant, and passed through an entanglement device having a flat rectangular cross section that blows air in an untwisted state, thereby providing entanglement.

[0034] The materials for the groove guide, grooved roll, and intertwining device are not particularly limited, but in consideration of durability and cost, stainless steel, titanium, ceramic, etc. are preferable. Furthermore, in order to reduce the performance degradation caused by the friction between the grooved roll and the precursor fiber bundle, those that have been plated are preferable.

[0035] The entangled precursor fiber bundle is wound onto a bobbin by a winder, or fed into a can and stored.

[0036] The hook drop value of the acrylic fiber bundle used in the present invention measured in an untwisted state is preferably 15 cm or more. If the hook drop value is 15 cm or more, sufficient mechanical properties can be obtained in the final SMC molded product, and it is more preferable that the hook drop value is 20 cm or more, and further preferably 25 cm or more.

[0037] The acrylic fiber bundle used in the present invention preferably has a total fineness of 10,000 dtex or more and 100,000 dtex or less from the viewpoint of productivity when converted into a carbon fiber bundle, and more preferably has a total fineness of 30,000 dtex or more and 100,000 dtex or less, and further preferably has a total fineness of 50,000 dtex or more and 100,000 dtex or less, from the viewpoint of productivity in the SMC production process.

[0038] The packaging of the acrylic fiber bundle is not particularly limited, but from the viewpoint of productivity, it is preferable that the acrylic fiber bundle is wound around a bobbin in an untwisted state, or that the acrylic fiber bundle is placed in a container having an opening. The number of acrylic fiber bundles packed in one packaging package may be one bundle, or two or more bundles.

[0039] In one embodiment of the present invention, the acrylic fiber bundle is pulled out from the packaging and transferred to a baking process, where it is subjected to flame retardant treatment, carbonization treatment, and if necessary, graphitization treatment, surface treatment, and sizing treatment to become a carbon fiber bundle.

[0040] When the untwisted acrylic fiber bundle is pulled out from the packaging and fed to the baking process, it is preferable to add a twist of 1 turn / m to 10 turns / m, more preferably 2 turns / m to 5 turns / m. If there is a twist of 10 turns / m or less, the fiber bundle can pass through the sintering process in a coherent state without falling apart, and a high-quality carbon fiber bundle can be obtained. Also, if the twist is 10 turns / m or less, the oxidation treatment in the flame-resistant process can be performed evenly, and the mechanical properties of the obtained carbon fiber bundle will be good.

[0041] The packaging of the acrylic fiber bundle used in the present invention is not particularly limited, but from the viewpoint of production, it is preferable that the acrylic fiber bundle is housed in a can or wound around a bobbin. In the present invention, a method for twisting the acrylic fiber bundle is preferably carried out by pulling out the acrylic fiber bundle from the package while rotating the package, from the viewpoint of production. In addition, when the package is wound on a bobbin, it is preferable to pull out the precursor fiber bundle by taking it longitudinally from the bobbin.

[0042] In the method for producing carbon fibers of the present invention, the flame-retarding step is a step of converting the precursor fiber bundle into a flame-retardant fiber bundle by heat treating the precursor fiber bundle in an oxidizing atmosphere. The conditions for the flameproofing step are that the density is preferably 1.28 g / cm under tension in an oxidizing atmosphere. 3 More than 1.42g / cm 3 Less than or equal to 1.29 g / cm 3 More than 1.40g / cm 3 It is recommended to heat the material to between 200°C and 300°C until the density is below 1.28g / cm 3 If the density is 1.42 g / cm or more, adhesion between the single fibers can be prevented during the next carbonization process, and the carbonization process can be produced without any trouble. 3 When the temperature is equal to or less than this, the flame-proofing step does not become too long, which is economical. As the oxidizing atmosphere, a known oxidizing atmosphere such as air, oxygen, nitrogen dioxide, etc. can be used, but air is preferable from the viewpoint of economy.

[0043] The flame-retardant fiber bundle is continuously introduced to the carbonization process. In the carbonization process, the flame-retardant fiber bundle is carbonized in an inert atmosphere to obtain a carbon fiber bundle. The carbonization is carried out in an inert atmosphere with a maximum temperature of 1000°C or higher. The inert atmosphere may be any of nitrogen, argon, helium, etc., but it is preferable to use nitrogen from an economical point of view.

[0044] In the early stage of the carbonization process, i.e., at a processing temperature of 300°C to 400°C, scission and crosslinking reactions occur in the flame-retardant polyacrylonitrile copolymer, which is a component of the fiber. In this temperature range, it is preferable to slowly increase the temperature of the fiber at a heating rate of 300°C / min or less in order to improve the mechanical properties of the carbon fiber bundle finally obtained. In addition, at a processing temperature of 400°C to 900°C, a graphite structure is gradually constructed. In the stage of constructing this carbon structure, it is preferable to treat it while applying stretching under tension in order to promote the regular orientation of the carbon structure. In order to control the temperature gradient and stretching (tension) at 900°C or less, it is more preferable to install a pre-process (pre-carbonization process) separate from the final carbonization process.

[0045] At a treatment temperature of 900° C. or higher, the remaining nitrogen atoms are released and the graphite structure develops, causing the fiber to shrink as a whole. Even in heat treatment at such high temperatures, it is preferable to treat the fiber under tension in order to achieve good mechanical properties in the final carbon fiber.

[0046] The carbon fiber bundle thus obtained may be subjected to a graphitization treatment, if necessary, which increases the elastic modulus of the carbon fiber bundle.

[0047] As for the conditions of graphitization, it is preferable to carry out the process in an inert atmosphere with a maximum temperature of 2000°C or higher, while elongating the bundle at an elongation rate in the range of 3% to 15%. When the elongation rate is 3% or higher, a highly elastic carbon fiber bundle (graphitized fiber bundle) having sufficient mechanical properties can be obtained. This is because, This is because, when attempting to obtain a carbon fiber bundle with a certain elastic modulus, the lower the elongation rate, the higher the treatment temperature required. On the other hand, when the elongation rate is 15% or less, the difference in the effect of promoting the growth of the carbon structure due to elongation is small between the surface layer and the interior, forming a uniform carbon fiber bundle and obtaining high-quality carbon fiber. After the above-mentioned calcination step, the carbon fiber bundle is preferably subjected to a surface treatment suitable for the final use.

[0048] Although there is no limitation on the method of surface treatment, a method of electrolytic oxidation in an electrolyte solution is preferable. Electrolytic oxidation is a method of modifying the surface by generating oxygen on the surface of the carbon fiber bundle to introduce oxygen-containing functional groups onto the surface.

[0049] As the electrolyte, acids such as sulfuric acid, hydrochloric acid, and nitric acid, and salts thereof can be used. As conditions for the electrolytic oxidation, the temperature of the electrolytic solution is preferably not more than room temperature, the electrolyte concentration is from 1% by mass to 15% by mass, and the quantity of electricity is preferably not more than 100 coulombs / g. The carbon fiber bundle thus obtained may be appropriately provided with a sizing agent for carbon fiber. The amount of the sizing agent attached to the carbon fiber bundle is preferably 0.6% by mass or more and 2% by mass or less, more preferably 1% by mass or more and 1.6% by mass or less, based on the total mass of the carbon fiber bundle and the sizing agent. If the amount of the sizing agent attached is 0.6% by mass or more, when the SMC is molded into a carbon fiber reinforced composite material, the functionality such as toughness can be fully exhibited. On the other hand, if the amount of the sizing agent attached is 2% by mass or less, the carbon fiber bundle can be prevented from becoming hard. In addition, if the amount of the sizing agent applied is within the above range, the carbon fiber bundles will have excellent bundling properties and abrasion resistance.

[0050] The carbon fiber bundle obtained by the present invention has excellent mechanical properties and is extremely easy to widen and split, making it particularly suitable for producing high-performance SMC molded products. An example of an SMC manufacturing apparatus suitable for use with the carbon fiber bundle obtained by the present invention will be described with reference to Fig. 1. The SMC manufacturing apparatus comprises a creel (not shown) that rotatably holds a plurality of carbon fiber bundles wound on bobbins, a roving cutter 31 that cuts each of the drawn carbon fiber bundles 1 to a desired fiber length, an unwinder 21 that unwinds the upper and lower films, a daughter blade 12 that applies the resin composition 11 to the film in a desired thickness, a group of rollers 24 that apply pressure to impregnate the carbon fiber bundles with the resin composition, a mesh belt 22 that transports the SMC during impregnation, and a winder 25 that winds up the SMC after impregnation has been completed.

[0051] 1 and 2, the roving cutter 31 is composed of a rubber roller 32 and a cutter roller 33. Furthermore, the cutter roller is provided with blades 34 spaced apart from each other by a desired cutting length on its circumferential surface. EXAMPLES

[0052] The present invention will be described in more detail below with reference to examples. However, the present invention is not limited to these examples. Various measurement methods and evaluation methods used in the examples are as follows.

[0053] [Example 1] <Acrylic fiber bundle hook drop evaluation> A 50g weighted hook is hooked onto a carbon fiber precursor acrylic fiber bundle that is hanging vertically with a 1kg weight attached, and the bundle is then released and allowed to fall. The length from where the hook is hooked to where the hook stops (descent distance) is measured. This operation is repeated 100 times for every 1m in the length direction of the acrylic fiber bundle, and the average descent distance is calculated and used as the hook drop value (HD value) of the acrylic fiber bundle. The hook drop value is measured to confirm the level of entanglement of the precursor fiber bundle. This indicates that the higher the value, i.e., the longer the distance that the hook hooked on the acrylic fiber bundle falls, the less entangled the acrylic fiber bundle is.

[0054] <Production of Carbon Fiber Precursor Acrylic Fiber Bundle> The acrylic fiber bundle used is prepared by the following method: An acrylonitrile copolymer (composition ratio: acrylonitrile / acrylamide / methacrylic acid = 96.5 / 2.7 / 0.8 (mass ratio)) is dispersed in dimethylacetamide at a ratio of 21 mass%, heated and dissolved to prepare a spinning dope, and discharged from a spinning nozzle with a hole size (diameter) of 45 μm and 24,000 holes into a coagulation bath at 38°C filled with a dimethylacetamide aqueous solution with a concentration of 67 mass% to form a coagulated yarn. The coagulated yarn is desolvated in a water washing tank and stretched to 3 times its size to form a precursor fiber bundle in a water-swollen state.

[0055] The precursor fiber bundle in a water-swollen state is introduced into an oil treatment tank filled with an oil treatment liquid, and an oil is applied thereto. The precursor fiber bundle with the oil applied thereto is then dried and densified with a roller having a surface temperature of 150°C, and then stretched 5 times in water vapor at a pressure of 0.3 MPa to obtain a carbon fiber precursor acrylic fiber bundle. The filament count of the obtained acrylic fiber bundle is 24,000, and the single fiber fineness is 1.0 dTex. Thereafter, the acrylic fiber bundle is taken out in a form wound around a bobbin in an untwisted state without being subjected to an entanglement treatment.

[0056] <Manufacturing of carbon fiber bundles> The acrylic fiber bundle is rotated while still wound on a bobbin so that it is twisted at 1 turn / m, and is supplied to a baking process. The twisted acrylic fiber bundle is made flame-resistant by passing it through a flame-resistant furnace having a temperature gradient in the range of 220°C to 260°C for 70 minutes to produce a flame-resistant fiber bundle. The flame-resistant fiber bundle is then made flame-resistant by passing it through a carbonization furnace having a temperature gradient in the range of 400°C to 1400°C in a nitrogen atmosphere for 3 minutes to produce a carbon fiber bundle.

[0057] <Determination of passability of firing process> If the acrylic fiber bundle is supplied to the baking process and a carbon fiber bundle is obtained, it is judged as "OK", and if no carbon fiber is obtained, it is judged as "NO".

[0058] <Determination of splittability of carbon fiber bundle> The carbon fiber bundle obtained as described above is supplied to the SMC manufacturing process shown in FIG. 1, cut using a roving cutter shown in FIG. 2, and then divided into fine fiber bundles consisting of 10,000 filaments or less. If the bundle is not divided, the result is judged as "OK". If the bundle is not divided, the result is judged as "NO". The total fineness, entanglement treatment, HD value, packaging, twisting method, number of twists, passability through the baking process, and splittability of the carbon fibers are summarized in Table 1.

[0059] [Examples 2 to 20] Acrylic fiber bundles and carbon fiber bundles were obtained in the same manner as in Example 1, except that the total fineness, entanglement treatment, packaging package, twisting method, and twist number of the acrylic fiber bundles were changed as shown in Table 1, and further, each measurement and evaluation were performed after passing through the SMC production process. The total fineness, entanglement treatment, HD value, packaging, twisting method, number of twists, passability through the baking process, and splittability of the carbon fibers are summarized in Table 1.

[0060] [Table 1]

[0061] As is clear from each example, the acrylic fiber bundles to which appropriate twisting has been applied can be used to obtain carbon fibers without causing any trouble in the sintering process even if the filaments are weakly entangled. Furthermore, when the carbon fiber bundles are cut in the SMC manufacturing process, they are divided into extremely fine fiber bundles.

[0062] [Comparative Examples 1 to 20] Acrylic fiber bundles and carbon fiber bundles were obtained in the same manner as in Example 1, except that the total fineness, entanglement treatment, packaging package, twisting method, and twist number of the acrylic fiber bundles were changed as shown in Table 1, and further, each measurement and evaluation were performed after passing through the SMC production process. The total fineness, entanglement treatment, HD value, packaging, twisting method, number of twists, passability through the baking process, and splittability of the carbon fibers are summarized in Table 1.

[0063] As is clear from Comparative Examples 1, 2, 4, 5, 7, 8, 10, and 11, when the filaments in a carbon fiber precursor acrylic fiber bundle having no twist or a small number of twists are weakly entangled with each other, the filaments are taken up by the roll during the baking process, and ultimately the fiber bundle is wound around the roll, causing a problem that no carbon fiber is obtained.

[0064] Moreover, as is clear from Comparative Examples 3, 6, 9, 12, 16, and 20, if the twist is too great, the carbon fiber precursor acrylic fiber bundle is supplied to the baking step in an extremely dense state, causing problems such as heat accumulation and breakage of the carbon fiber precursor acrylic fiber bundle in the flame-proofing step, and no carbon fibers are obtained.

[0065] In addition, as is clear from Comparative Examples 13, 14, 15, 17, 18, and 19, when the filaments are tightly entangled in the carbon fiber precursor acrylic fiber bundle to which an appropriate twist has been imparted, carbon fibers can be obtained without causing any trouble in the baking process. However, when the carbon fiber bundle is cut in the SMC manufacturing process, it cannot be divided into fine filaments. [Industrial Applicability]

[0066] The carbon fiber manufacturing method of the present invention can produce carbon fibers suitable for obtaining high-performance SMC, which could not be achieved by conventional techniques, that is, carbon fibers that can be divided into fine filaments of several thousand pieces when the carbon fiber bundles are cut. [Explanation of symbols]

[0067] 1 Carbon fiber bundle 4 Short cut carbon fiber bundles 11 Resin composition 12 Doctor Blade 21 Unwinding machine 22 Mesh Belt 24 Roller Group 25 Winding machine 31 Roving cutter 32 Rubber Roller 33 Cutter roller 34 Blades

Claims

1. A method for producing a carbon fiber bundle by supplying an acrylic fiber bundle to a baking process to produce a carbon fiber bundle, in which an acrylic fiber bundle that has been subjected to an entanglement treatment is twisted at 1 turn / m or more and 10 turns / m or less, and then the carbon fiber bundle is supplied to the baking process.

2. 2. The method for producing a carbon fiber bundle according to claim 1, wherein the acrylic fiber bundle has a total fineness of 10,000 dtex or more and 100,000 dtex or less.

3. 3. The method for producing a carbon fiber bundle according to claim 1 or 2, wherein the method for twisting the acrylic fiber bundle with 1 turn / m or more and 10 turns / m or less comprises storing the acrylic fiber bundle in an untwisted state in a packaging package, and rotating the packaging package while pulling the acrylic fiber bundle out of the packaging package to twist the acrylic fiber bundle with 1 turn / m or more and 10 turns / m or less.

4. The method for producing carbon fibers according to any one of claims 1 to 3, wherein the acrylic fiber bundle subjected to the entanglement treatment has a hook drop value of 15 cm or more.

5. The method for producing carbon fibers according to any one of claims 1 to 4, further comprising subjecting the precursor fiber bundle obtained by the pressurized steam drawing treatment to an entanglement treatment.

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