Carbon fiber bundle manufacturing method and SMC manufacturing 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. and SMC manufacturing method Regarding. [Background technology]

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

[0003] When producing carbon fiber bundles from precursor fiber bundles, in the flame-proofing process, a large number of fiber bundles arranged in a sheet form are folded back via a roll and passed through a flame-proofing furnace in multiple stages while changing their running direction by 180 degrees. This change in running direction can cause the filaments to wind around the roll or adjacent fiber bundles to interfere with each other, which can lead to a deterioration in the mechanical properties and quality of the final carbon fiber bundle. To prevent such filaments from winding 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 remain unfolded and remain together. Various studies have been conducted to date to determine how to maintain this togetherness.

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

[0005] Carbon fiber bundles obtained by subjecting precursor fiber bundles obtained by the above-described method to a sintering process have excellent mechanical properties and are widely used industrially, particularly as reinforcing fibers for composite materials, such as sheet molding compounds (hereinafter referred to as SMC). When heated and pressurized in a mold, SMC allows the reinforcing fibers and resin composition to flow together and fill the cavity. This makes SMC an advantageous intermediate material for producing molded products of various shapes, such as those with partially different thicknesses or ribs and bosses. However, the carbon fiber bundles in SMC have the drawback of being short and randomly oriented, limiting their mechanical strength. Furthermore, the mechanical strength also has significant spatial variations. Non-Patent Document 1 also 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, studies are being conducted on finely dividing thick fiber bundles for use. Patent Document 2 discloses an apparatus for dividing a carbon fiber bundle having 48,000 (48K) filaments into thin carbon fiber bundles (split tow) each having several thousand filaments by passing the bundle through a crowned spreading bar and a grooved splitting bar.

[0007] Patent Document 3 also discloses a method for dividing a carbon fiber bundle, in which a continuous carbon fiber bundle is continuously run while applying tension to the bundle, and the carbon fiber bundle is widened with a widening jig placed midway through the run, and simultaneously, or after the widening, the widened carbon fiber bundle is divided by cutting some of the carbon fiber filaments with a cutting blade that rotates parallel to the running direction of the fiber bundle.

[0008] However, in the carbon fiber bundle obtained by the entanglement treatment as in Patent Document 1, the filaments constituting the fiber bundle are strongly entangled with each other, making it impossible to divide the thick fiber bundle into small pieces as described in Patent Document 2, and it is also impossible to widen the fiber bundle as described in Patent Document 3. As a result, it has been 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, causing fluff, which reduces the quality of the SMC molded product. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-294517 [Patent Document 2] U.S. Patent No. 6,385,828 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-219780 [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 of the prior art, 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, wherein a twist of 1 turn / m or more and 10 turns / m or less is added to the acrylic fiber bundle before the carbon fiber bundle is supplied to the baking process. [2] The method for producing a carbon fiber bundle according to [1], wherein the 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 out the carbon fiber precursor acrylic fiber bundle from 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 wound package, and the acrylic fiber bundle, which is wound around a bobbin in an untwisted state, is unwound from the bobbin while the bobbin is rotated to twist the acrylic fiber bundle by 1 turn / m or more and 10 turns / m or less. [Effects 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 baking process in a cohesive 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 explanation 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. [Figure 2]1 is an example of a rope pinch cutter used to cut carbon fiber bundles short when processing the carbon fiber bundles produced by the present invention into SMC. DETAILED DESCRIPTION OF THE INVENTION

[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 polymer.

[0016] An acrylonitrile polymer is a polymer obtained by polymerizing acrylonitrile as a main monomer. The acrylonitrile polymer may be a homopolymer obtained only from acrylonitrile, or an acrylonitrile copolymer obtained by combining other monomers with acrylonitrile as the main component.

[0017] The content of acrylonitrile units in the acrylonitrile-based copolymer is preferably 96.0% by mass or more and 98.5% by mass or less from the viewpoints of preventing thermal fusion of the fibers during the calcination process, the heat resistance of the copolymer, the stability of the spinning dope, and the quality of the resulting carbon fiber. An acrylonitrile unit content of 96.0% by mass or more is preferred because it prevents thermal fusion of the fibers during the calcination process when converted into carbon fiber, thereby maintaining the excellent quality and performance of the carbon fiber. Furthermore, the heat resistance of the copolymer itself is not reduced, and adhesion between single fibers can be avoided during processes such as drying the fibers or stretching with a heated roller or pressurized steam when spinning the precursor fiber. On the other hand, an acrylonitrile unit content of 98.5% by mass or less is preferred because it prevents a decrease in solubility in solvents, maintains the stability of the spinning dope, and prevents the precipitation and coagulation of the copolymer from increasing, enabling stable production of precursor fiber.

[0018] In the case where a copolymer is used, the monomer other than acrylonitrile can be suitably selected from vinyl monomers copolymerizable with acrylonitrile, and 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 this can promote the flame retardancy.

[0019] As the vinyl monomer copolymerizable with acrylonitrile, a carboxyl group-containing vinyl monomer such as acrylic acid, methacrylic acid, or itaconic acid is more preferred. 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.

[0020] During 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, dimethyl sulfoxide, and dimethylformamide, or aqueous solutions of inorganic compounds such as zinc chloride and sodium thiocyanate. Among these, dimethylacetamide, dimethylsulfoxide, and dimethylformamide are preferred from the viewpoint of improving productivity, as they have a fast coagulation rate, and dimethylacetamide is more preferred.

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

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

[0023] Spinning and shaping by the wet spinning method or the dry-wet spinning method can be carried out by spinning the spinning dope through a nozzle having a circular cross-section into a coagulation bath. 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, any known spinning method can be appropriately adopted, such as a wet spinning method in which the spinning dope described above is directly spun into a coagulation bath, a dry spinning method in which coagulation is carried out in air, or a dry-wet spinning method in which the dope is spun once into air and then coagulated in a bath. However, in order to obtain a carbon fiber bundle with higher performance, the wet spinning method or the dry-wet spinning method is preferred.

[0025] Spinning and shaping by the wet spinning method or the dry-wet spinning method can be carried out by spinning the spinning dope through a nozzle having a circular cross-section into a coagulation bath. 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 forming a dense structure without voids to obtain a high-performance carbon fiber bundle, ensuring stretchability, and providing 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 resulting fiber can be washed with water before, after, or simultaneously with the stretching to obtain a precursor fiber bundle in a water-swollen state.

[0028] The in-bath drawing is usually carried out in a water bath at 50°C or higher and 98°C or lower, either once or in multiple stages of two or more, and the coagulated yarn is drawn so that the total drawing ratio of the in-air drawing and in-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) drawn in a bath, it is preferable to use an oil treatment solution for acrylic precursor fibers for carbon fibers (hereinafter simply referred to as "oil treatment solution") 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 above 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 using a heated roller at a temperature of 100°C to 200°C. In this case, the number of heated rollers may be one or more.

[0031] The densified and dried fiber bundle is preferably subjected to a pressurized steam drawing treatment. The drawing treatment can further increase the density and degree of orientation of the resulting precursor fiber bundle. Here, the pressurized steam drawing treatment is a drawing of a fiber bundle performed 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 density and orientation of the resulting fibers.

[0032] In the pressurized steam drawing treatment, it is preferable to control the temperature of the heating roller immediately before the pressurized steam drawing device to 120°C or higher and 190°C or lower, and the fluctuation rate of the steam pressure in the pressurized steam drawing 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 fineness of the fiber bundle in a small section. If the temperature of the heating roller is lower than 120°C, the temperature of the fiber bundle to be subjected to the pressurized steam drawing treatment does not rise sufficiently, and the drawability is likely to decrease.

[0033] The precursor fiber bundle obtained by drying and densifying, or the precursor fiber bundle obtained by drying and densifying followed by pressurized steam drawing, is preferably subjected to an entanglement treatment, in which the precursor fiber bundle is introduced into a grooved guide or grooved roll to keep its width constant, and is passed through an entanglement device with a flat rectangular cross section that blows air in an untwisted state to impart entanglement.

[0034] The materials for the groove guide, grooved roll, and intertwining device are not particularly limited, but considering durability and cost, stainless steel, titanium, ceramic, etc. are preferred. Furthermore, plated materials are preferred to reduce performance degradation due to friction between the grooved roll and the precursor fiber bundle.

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

[0036] The acrylic fiber bundle used in the present invention preferably has a hook drop value of 15 cm or more when measured in an untwisted state, since sufficient mechanical properties can be obtained in the final SMC molded product. A hook drop value of 15 cm or more is more preferably 20 cm or more, and even more 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 from the viewpoint of productivity in the SMC production process, more preferably 30,000 dtex or more and 100,000 dtex or less, and even more preferably 50,000 dtex or more and 100,000 dtex or less.

[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 in an untwisted state. 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 drawn out from the packaging and transferred to a baking step, where it is subjected to a flame retardant treatment, a carbonization treatment, and optionally a graphitization treatment, a surface treatment, and a 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 or more and 10 turns / m or less, more preferably 2 turns / m or more and 5 turns / m or less. If there is a twist of 10 turns / m or less, the fiber bundle can pass through the baking process in a coherent state without falling apart, and a high-quality carbon fiber bundle can be obtained. Furthermore, if the twist is 10 turns / m or less, the oxidation treatment in the flame-proofing process can be carried out 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 it is preferable from the viewpoint of production that the acrylic fiber bundle is housed in a can or wound on a bobbin. In the present invention, a method of twisting the acrylic fiber bundle is preferably carried out by rotating the packaging package when pulling out the acrylic fiber bundle from the packaging package, from the viewpoint of productivity. In addition, when the packaging 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-resistant step is a step of converting the precursor fiber bundle into a flame-resistant fiber bundle by heat treating the precursor fiber bundle in an oxidizing atmosphere. The conditions for the flameproofing process are that the density is preferably 1.28 g / cm under tension in an oxidizing atmosphere. 3 More than 1.42g / cm 3 or less, more preferably 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 reaches 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 step, and the carbonization step can be produced without any problems. 3 If the temperature is less than this, the flame-proofing step does not take too long and is therefore economical. As the oxidizing atmosphere, known oxidizing atmospheres such as air, oxygen, and nitrogen dioxide can be used, but air is preferred from the viewpoint of economy.

[0043] The flame-resistant fiber bundle is continuously introduced into the carbonization step. In the carbonization step, the flame-resistant 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 from an economical point of view, it is preferable to use nitrogen.

[0044] In the early stages of the carbonization process, i.e., at processing temperatures between 300°C and 400°C, scission and crosslinking reactions occur in the flame-retardant polyacrylonitrile copolymer, a component of the fiber. In this temperature range, a gradual increase in the fiber temperature at a heating rate of 300°C / min or less is preferred to ensure good mechanical properties of the final carbon fiber bundle. Furthermore, at processing temperatures between 400°C and 900°C, a graphite structure is gradually formed. During this carbon structure formation stage, stretching under tension is preferred to promote the ordered orientation of the carbon structure. To control the temperature gradient and stretching (tension) at temperatures below 900°C, it is more preferable to establish 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 bundles thus obtained may be subjected to graphitization treatment as needed, which increases the elastic modulus of the carbon fiber bundles.

[0047] As for the conditions for 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 high elastic modulus, the lower the elongation rate, the higher the treatment temperature required. On the other hand, if the elongation rate is 15% or less, the difference in the effect of promoting the growth of carbon structure due to elongation between the surface layer and the interior is small, resulting in the formation of a uniform carbon fiber bundle and the production of 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 surface treatment method, electrolytic oxidation in an electrolyte solution is preferred. Electrolytic oxidation is a surface modification treatment in which oxygen is generated 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 electrolytic oxidation, the temperature of the electrolytic solution is preferably room temperature or lower, the electrolyte concentration is 1% by mass to 15% by mass, and the quantity of electricity is preferably 100 coulombs / g or lower. The carbon fiber bundles obtained in this manner may be appropriately coated with a carbon fiber sizing agent. The amount of sizing agent attached to the carbon fiber bundle is preferably 0.6% by mass or more and 2% by mass or less, and more preferably 1% by mass or more and 1.6% by mass or less, relative to the total mass of the carbon fiber bundle and the sizing agent. If the amount of sizing agent attached is 0.6% by mass or more, when the SMC is molded into a carbon fiber reinforced composite material, the functional properties such as toughness can be fully exhibited. On the other hand, if the amount of sizing agent attached is 2% by mass or less, the carbon fiber bundle can be prevented from becoming hard. Furthermore, if the amount of sizing agent applied is within the above range, the carbon fiber bundles will have excellent bundling properties and abrasion resistance.

[0050] The carbon fiber bundles obtained by the present invention have excellent mechanical properties and are extremely easy to widen and split, making them particularly suitable for producing high-performance SMC molded products. An example of an SMC production apparatus that can be suitably used for the carbon fiber bundles obtained by the present invention will be described with reference to Figure 1. The SMC production apparatus comprises a creel (not shown) that rotatably holds multiple 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 resin composition 11 to the film in a desired thickness, a group of rollers 24 that applies 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 is complete.

[0051] 1 and 2, the roving cutter 31 is made up 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. [Example]

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

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

[0054] <Production of carbon fiber precursor acrylic fiber bundle> The acrylic fiber bundle used was prepared using the following method: An acrylonitrile copolymer (composition ratio: acrylonitrile / acrylamide / methacrylic acid = 96.5 / 2.7 / 0.8 (mass ratio)) was dispersed in dimethylacetamide at a ratio of 21% by mass, and the resulting solution was heated and dissolved to prepare a spinning dope. This was then extruded into a 38°C coagulation bath filled with a 67% by mass aqueous solution of dimethylacetamide through a spinning nozzle with a pore size (diameter) of 45 μm and 24,000 holes to form a coagulated fiber. The coagulated fiber was then desolvated in a water washing tank and stretched three times its original 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 the oil is applied thereto. The precursor fiber bundle with the oil applied is then dried and densified using a roller at a surface temperature of 150°C, and then stretched five times in steam at a pressure of 0.3 MPa to obtain a carbon fiber precursor acrylic fiber bundle. The resulting acrylic fiber bundle has 24,000 filaments and a single fiber fineness of 1.0 dTex. Thereafter, the acrylic fiber bundle is collected in a state of being 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 wound on a bobbin so as to be twisted at 1 turn / m, and is then supplied to a baking process. The twisted acrylic fiber bundle is flame-resistant by passing it through a flame-resistant furnace having a temperature gradient ranging from 220°C to 260°C for 70 minutes to produce a flame-resistant fiber bundle. The flame-resistant fiber bundle is then baked by passing it through a carbonization furnace having a temperature gradient ranging from 400°C to 1400°C in a nitrogen atmosphere for 3 minutes to produce a carbon fiber bundle.

[0057] <Determining passability of the firing process> When a carbon fiber bundle is obtained by supplying an acrylic fiber bundle to a baking process, it is judged as "OK", and when 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 if it can be divided into fine fiber bundles consisting of 10,000 filaments or less, it is evaluated as "OK," and if it cannot be divided, it is evaluated as "NO." Table 1 shows the total fineness, entanglement treatment, HD value, packaging, twisting method, number of twists, passability through the baking process, and splittability of the carbon fibers.

[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 number of twists of the acrylic fiber bundles were changed as shown in Table 1, and then each measurement and evaluation were carried out after passing through the SMC production process. Table 1 shows the total fineness, entanglement treatment, HD value, packaging, twisting method, number of twists, passability through the baking process, and splittability of the carbon fibers.

[0060] [Table 1]

[0061] As is clear from each example, when an acrylic fiber bundle is appropriately twisted, even if the filaments are weakly entangled, no problems occur during the baking process, and carbon fibers can be obtained. Furthermore, when the carbon fiber bundle is cut during the SMC manufacturing process, it is 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 number of twists of the acrylic fiber bundles were changed as shown in Table 1, and then each measurement and evaluation were carried out after passing through the SMC production process. Table 1 shows the total fineness, entanglement treatment, HD value, packaging, twisting method, number of twists, passability through the baking process, and splittability of the carbon fibers.

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

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

[0065] Furthermore, as is clear from Comparative Examples 13, 14, 15, 17, 18, and 19, when the filaments in a carbon fiber precursor acrylic fiber bundle are properly twisted and the entanglement is strong, 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 is suitable for obtaining high-performance SMC, which could not be achieved by conventional techniques, i.e., it is possible to obtain carbon fibers that can be divided into thousands of fine filaments when the carbon fiber bundle is cut. [Explanation of symbols]

[0067] 1 carbon fiber bundle 4 Short cut carbon fiber bundles 11 Resin composition 12 Doctor Blade 21 Unwinder 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 manufacturing a carbon fiber bundle, comprising: adding a twist of 1 turn / m or more and 10 turns / m or less to an acrylic fiber bundle having a hook drop value of 15 cm or more measured in an untwisted state, and then supplying the bundle to a baking process to manufacture a carbon fiber bundle; and cutting the carbon fiber bundle to a length of 20 to 50 mm.

2. A method for manufacturing an SMC using split fiber bundles obtained by adding a twist of 1 turn / m or more and 10 turns / m or less to an acrylic fiber bundle having a hook drop value of 15 cm or more measured in an untwisted state, and then supplying the carbon fiber bundle to a baking process to produce a carbon fiber bundle, and cutting the carbon fiber bundle to lengths of 20 to 50 mm.

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