JP7798147B2 - Carbon fiber bundle manufacturing method and SMC manufacturing method - Google Patents

Description

The present invention relates to a method for producing a carbon fiber bundle and a method for producing an SMC .

Conventionally, a known method for producing carbon fiber bundles involves 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 to 400°C (hereinafter also referred to as the "flame-resistant process"), followed by a carbonization process in an inert atmosphere of 1000°C or higher (hereinafter also referred to as the "carbonization process"; the "flame-resistant process" and "carbonization process" are collectively referred to as the "firing process") to obtain carbon fiber bundles.

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. When the running direction is changed, the filaments may wrap around the roll or adjacent fiber bundles may interfere with each other, which can cause a deterioration in the mechanical properties and quality of the final carbon fiber bundle. 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 made up of a large number of filaments to remain unfolded and remain together. Various studies have been conducted to date on how to maintain this togetherness.

As a means of entangling precursor fiber bundles, methods have been proposed for maintaining the bundled state by performing an entanglement process (a process in which the filaments that make up the precursor fiber bundle are entangled within the fiber bundle). For example, Patent Document 1 proposes a method of entangling the precursor fiber bundles by continuously running the fiber bundles through a single-stage or multi-stage entangler with a flat rectangular cross section that sprays air, thereby maintaining the bundled state.

The 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 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 is an intermediate material advantageous for producing molded products of various shapes, such as those with locally varying thicknesses or ribs and bosses, because the reinforcing fibers and resin composition flow together to fill the cavity. However, the carbon fiber bundles in SMC have the drawback of being short and randomly oriented, which limits their mechanical strength. Furthermore, mechanical strength varies significantly from place to place. Furthermore, Non-Patent Document 1 indicates that the strength and elastic modulus of SMC molded products decrease as the number of filaments in the carbon fiber bundle increases.

As a result, consideration is being given to finely dividing thick fiber bundles for use. Patent Document 2 discloses an apparatus that splits a carbon fiber bundle with 48,000 (48K) filaments into thin carbon fiber bundles (split tow) with thousands of filaments by passing the bundle through a crowned spreading bar and a grooved splitting bar.

Patent Document 3 also discloses a method for dividing a carbon fiber bundle in which a continuous carbon fiber bundle is continuously run while tension is applied to the bundle, and the carbon fiber bundle is widened using a widening tool placed midway through the run. At the same time, or after 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.

However, in carbon fiber bundles obtained by performing the entanglement process described in Patent Document 1, the filaments that make up the fiber bundle are strongly entangled, making it impossible to finely divide the thick fiber bundle as described in Patent Document 2, nor to widen the fiber bundle as described in Patent Document 3. As a result, it is difficult to obtain high-performance SMC molded products. 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, and the quality of the SMC molded product is reduced.

Japanese Patent Application Laid-Open No. 2002-294517 U.S. Patent No. 6,385,828 Japanese Patent Application Laid-Open No. 2006-219780

N. Tsuchiyama, “The Mechanical Properties of Carbon Fiber SMC”, Proceedings of the Fourth International Conference on Composite Materials (ICCM-IV), 1982, p. 497-503

In view of these problems with the prior art, the present invention aims to provide a method for producing carbon fiber bundles that are suitable for obtaining high-performance SMC, i.e., a method for splitting carbon fiber bundles into fine carbon fiber bundles (split fiber bundles) with 10,000 or fewer filaments by cutting them to lengths of 20 to 50 mm.

In order to achieve the above object, the present invention has the following features.
[1] A method for producing a carbon fiber bundle, in which an acrylic fiber bundle having a hook drop value of 15 cm or more measured in an untwisted state is supplied to a baking step to produce a carbon fiber bundle, and a twist of 1 turn/m or more and 10 turns/m or less is applied to the acrylic fiber bundle before supplying it to the baking step.
[2] The method for producing a carbon fiber bundle according to [1], wherein the method for twisting the acrylic fiber bundle at 1 turn/m or more and 10 turns/m or less 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 twist the carbon fiber precursor acrylic fiber bundle at 1 turn/m or more and 10 turns/m or less.
[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 wound around the bobbin in an untwisted state is unwound from the bobbin while the bobbin winding is rotated to twist the acrylic fiber bundle by 1 turn/m or more and 10 turns/m or less.

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 with minimal entanglement of the filaments. In addition, the carbon fiber bundle obtained by the manufacturing method of the present invention is extremely easy to widen and split in the SMC manufacturing process, allowing for the stable production of high-performance SMC molded products.

1 is an example of an apparatus for processing carbon fiber bundles produced according to the present invention into SMC. 1 is an example of a rope pinch cutter used to cut carbon fiber bundles into short lengths when the carbon fiber bundles produced by the present invention are processed into SMC.

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.

An acrylonitrile-based polymer is a polymer obtained by polymerizing acrylonitrile as the main monomer. An acrylonitrile-based polymer may be a homopolymer obtained solely from acrylonitrile, or an acrylonitrile-based copolymer obtained by combining other monomers with acrylonitrile, the main component.

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 solution, and the quality of the 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 the individual fibers can be avoided during processes such as drying the fibers or stretching with heated rollers 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 solution, and prevents the precipitation and coagulation of the copolymer from becoming too high, enabling stable production of precursor fiber.

When a copolymer is used, 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 their alkali metal salts or ammonium salts, acrylamide, etc., which have the effect of accelerating the flame retardation reaction, as this will accelerate the flame retardation.

Vinyl monomers copolymerizable with acrylonitrile are more preferably carboxyl group-containing vinyl monomers such as acrylic acid, methacrylic acid, and itaconic acid. The content of carboxyl group-containing vinyl monomer units 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.

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 perspective of improving productivity, as they have a fast coagulation rate, with dimethylacetamide being more preferred.

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. Specifically, it is preferable to prepare the spinning dope so that the polymer concentration in the spinning dope is at least 17% by mass, more preferably at least 19% by mass. Since the spinning dope needs to have appropriate viscosity and fluidity, it is preferable that the polymer concentration does not exceed 25% by mass.

The spinning method can be any known method, such as a wet spinning method in which the spinning dope is directly spun into a coagulation bath, a dry spinning method in which it is coagulated in air, or a dry-wet spinning method in which it is spun into air and then coagulated in a bath. However, wet spinning or dry-wet spinning is preferred for obtaining carbon fiber bundles with higher performance.

Spinning and shaping using the wet spinning method or dry-wet spinning method can be carried out by spinning the spinning dope through a nozzle with a circular cross-section into a coagulation bath. From the perspective of ease of solvent recovery, it is preferable to use an aqueous solution containing the solvent used in the spinning dope as the coagulation bath.

The spinning method can be any known method, such as a wet spinning method in which the spinning dope is directly spun into a coagulation bath, a dry spinning method in which it is coagulated in air, or a dry-wet spinning method in which it is spun into air and then coagulated in a bath. However, wet spinning or dry-wet spinning is preferred for obtaining carbon fiber bundles with higher performance.

Spinning and shaping using the wet spinning method or dry-wet spinning method can be carried out by spinning the spinning dope through a nozzle with a circular cross-section into a coagulation bath. From the perspective of ease of solvent recovery, it is preferable to use an aqueous solution containing the solvent used in the spinning dope as the coagulation bath.

When using an aqueous solution containing a solvent 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.

The polymer or copolymer is dissolved in a solvent, and the resulting spinning solution is extruded into a coagulation bath to form a coagulation yarn. The resulting coagulated yarn can then be stretched in the coagulation bath or a stretching bath. Alternatively, the yarn may be partially stretched in the air and then stretched in a bath. Water washing can be performed before, during, or after stretching to obtain a precursor fiber bundle in a water-swollen state.

Bath drawing is typically carried out in a water bath at 50°C to 98°C in one or more separate stages, and the coagulated yarn is drawn so that the total draw ratio of the air-drawing and bath-drawing is 2 to 10 times, which is preferred from the standpoint of the performance of the resulting carbon fiber bundle.

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 also 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.

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 exceeding 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 heated roller at a temperature of 100° C. to 200° C. In this case, the number of heated rollers may be one or more.

The densely dried fiber bundle is preferably subjected to a pressurized steam drawing treatment, which 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.

In the pressurized steam stretching process, it is preferable to control the temperature of the heating roller immediately before the pressurized steam stretching device to between 120°C and 190°C, and the fluctuation rate of the steam pressure during the pressurized steam stretching to 0.5% or less. 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 stretch ratio in small sections and the resulting fluctuations in the total fiber bundle fineness in small sections. If the temperature of the heating roller is less than 120°C, the temperature of the fiber bundle undergoing the pressurized steam stretching process will not rise sufficiently, and stretchability will likely decrease.

Precursor fiber bundles obtained by drying and densifying, or precursor fiber bundles obtained by drying and densifying followed by pressurized steam stretching, are preferably subjected to an entanglement process. In the entanglement process, the precursor fiber bundles are guided through a grooved guide or grooved roll, the width of which is kept constant, and the bundles are passed through an entanglement device with a flat rectangular cross section that sprays air in an untwisted state, thereby entangling the bundles.

There are no particular restrictions on the materials used for the groove guide, grooved roll, and intertwining device, 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.

The entangled precursor fiber bundle is wound onto a bobbin using a winder, or fed into a can and stored.

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. A hook drop value of 15 cm or more will ensure sufficient mechanical properties in the final SMC molded product, and a value of 20 cm or more is more preferable, and 25 cm or more is even more preferable.

From the viewpoint of productivity when converted into carbon fiber bundles, the acrylic fiber bundles used in the present invention preferably have a total fineness of 10,000 dtex or more and 100,000 dtex or less; from the viewpoint of productivity in the SMC production process, it is more preferable that it be 30,000 dtex or more and 100,000 dtex or less, and even more preferable that it be 50,000 dtex or more and 100,000 dtex or less.

The packaging of the acrylic fiber bundle is not particularly limited, but from the standpoint of productivity, it is preferable that the acrylic fiber bundle be wound around a bobbin in an untwisted state, or that the acrylic fiber bundle be placed in an untwisted state in a container with an opening. One packaging package may contain one acrylic fiber bundle, or two or more bundles.

In one aspect of the present invention, the acrylic fiber bundle is pulled out of the packaging and transferred to a baking process, where it is subjected to flame retardant treatment, carbonization treatment, and optionally graphitization treatment, surface treatment, and sizing treatment to become a carbon fiber bundle.

When the untwisted acrylic fiber bundle is pulled out from the packaging and fed to the baking step, 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 the twist is 1 turn/m or more, the fiber bundle can pass through the baking step in a coherent state without coming 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 step can be carried out evenly, and the mechanical properties of the obtained carbon fiber bundle can be improved.

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.

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 flameproofing process conditions include heating under tension in an oxidizing atmosphere at 200°C to 300°C until the density reaches preferably 1.28 g/ ^cm3 to 1.42 g/ ^cm3 , more preferably 1.29 g/ ^cm3 to 1.40 g/ ^cm3 . A density of 1.28 g/ ^cm3 or higher can prevent adhesion between single fibers during the subsequent carbonization process, allowing for trouble-free production in the carbonization process. Furthermore, a density of 1.42 g/cm3 or ^lower prevents the flameproofing process from being too long, making it economical. Known oxidizing atmospheres such as air, oxygen, and nitrogen dioxide can be used as the oxidizing atmosphere, but air is preferred from an economical standpoint.

The flame-retardant fiber bundle is continuously introduced into the carbonization process. In the carbonization process, the flame-retardant fiber bundle is carbonized in an inert atmosphere to obtain a carbon fiber bundle. Carbonization is carried out in an inert atmosphere with a maximum temperature of 1000°C or higher. The inert atmosphere can be nitrogen, argon, helium, etc., but from an economical standpoint, nitrogen is preferably used.

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, gradually increasing the fiber temperature at a rate of 300°C/min or less is preferable to ensure good mechanical properties in 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, it is preferable to stretch the carbon structure under tension to promote regular orientation. To control the temperature gradient and stretching (tension) below 900°C, it is more preferable to establish a pre-process (pre-carbonization process) separate from the final carbonization process.

At treatment temperatures of 900°C or higher, the remaining nitrogen atoms are released and the graphite structure develops, causing the fiber as a whole to shrink. Even when heat-treating 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.

The carbon fiber bundles obtained in this manner may be subjected to a graphitization treatment if necessary. Graphitization treatment increases the elastic modulus of the carbon fiber bundles.

As for graphitization conditions, it is preferable to perform the graphitization in an inert atmosphere with a maximum temperature of 2000°C or higher, while elongating 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, when attempting to obtain a carbon fiber bundle having a predetermined elastic modulus, a lower elongation rate condition requires a higher treatment temperature. On the other hand, when the elongation rate is 15% or lower, there is little difference between the surface layer and the interior in the effect of promoting the growth of the carbon structure due to elongation, and a uniform carbon fiber bundle can be formed, resulting in 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.

There are no restrictions on the surface treatment method, but electrolytic oxidation in an electrolyte solution is preferred. Electrolytic oxidation generates oxygen on the surface of the carbon fiber bundle, introducing oxygen-containing functional groups onto the surface and performing a surface modification treatment.

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 or higher and 15% by mass or lower, and the quantity of electricity is preferably 100 coulombs/g or lower.
The carbon fiber bundles obtained in this manner may be appropriately provided 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, it is possible to fully exhibit functions such as toughness. On the other hand, if the amount of sizing agent attached is 2% by mass or less, it is possible to prevent the carbon fiber bundles 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.

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 that can be suitably used for the carbon fiber bundle obtained by the present invention will be described with reference to Figure 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 a resin composition 11 to the film in a desired thickness, a group of rollers 24 that applies pressure to impregnate the carbon fiber bundle 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.

As shown in Figures 1 and 2, the roving cutter 31 consists of a rubber roller 32 and a cutter roller 33. Furthermore, the cutter roller has blades 34 spaced apart on its circumferential surface by the desired cutting length.

The present invention will be explained in more detail below using examples. However, the present invention is not limited to these examples. The various measurement methods and evaluation methods used in these examples are as follows.

[Example 1]
<Hook drop evaluation of acrylic fiber bundle>
A hook with a 50 g weight attached is hooked onto a carbon fiber precursor acrylic fiber bundle that is hanging vertically with a 1 kg weight attached, and the fiber 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 distances is calculated and used as the hook drop value (HD value) of the acrylic fiber bundle. The measurement of the hook drop value is used to confirm the level of entanglement of the precursor fiber bundle; the higher the value, i.e., the longer the descent distance of the hook hooked onto the acrylic fiber bundle, the less entangled the acrylic fiber bundle is.

<Production of carbon fiber precursor acrylic fiber bundle>
The acrylic fiber bundle used was prepared by 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 mass %, heated and dissolved to prepare a spinning dope, and discharged from a spinning nozzle with a pore size (diameter) of 45 μm and 24,000 holes into a coagulation bath filled with a 67 mass % dimethylacetamide aqueous solution at 38°C to form a coagulated fiber. The coagulated fiber was then desolvated in a water washing tank and stretched three times to form a precursor fiber bundle in a water-swollen state.

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 to which the oil agent has been applied is then dried and densified with a roller at 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 obtained 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.

<Production 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 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 in the range of 220°C to 260°C for 70 minutes to obtain a flame-resistant fiber bundle. Subsequently, the flame-resistant fiber bundle is baked 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 obtain a carbon fiber bundle.

<Determination of Passability of 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".

<Determination of Dividability of Carbon Fiber Bundle>
The carbon fiber bundle obtained as described above is supplied to the SMC production process shown in FIG. 1, cut using a roving cutter shown in FIG. 2, and if it is divided into fine fiber bundles consisting of 10,000 filaments or less, it is judged as "OK", and if it is not divided, it 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.

[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.
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.

As is clear from each example, acrylic fiber bundles with an appropriate twist can be used to obtain carbon fiber without causing problems during the baking process, even if the filaments are weakly entangled. Furthermore, when the carbon fiber bundles are cut during the SMC manufacturing process, they are divided into extremely fine fiber bundles.

[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.
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.

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 have no twist or a low twist and are weakly entangled, the filaments get caught on the roll during the baking process, and ultimately the fiber bundle winds around itself, resulting in no carbon fiber being obtained.

Furthermore, 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 bundles are supplied to the baking process in an extremely dense state, which causes problems such as heat accumulation and breakage of the carbon fiber precursor acrylic fiber bundles during the flame-proofing process, making it impossible to obtain carbon fiber.

Furthermore, as is clear from Comparative Examples 13, 14, 15, 17, 18, and 19, when the carbon fiber precursor acrylic fiber bundle is appropriately twisted and the filaments are tightly entangled, carbon fiber can be obtained without causing any problems during the firing process. However, when the carbon fiber bundle is cut during the SMC manufacturing process, it cannot be divided into fine filaments.

The carbon fiber manufacturing method of the present invention is suitable for obtaining high-performance SMC, something that could not be achieved with conventional technology. In other words, it produces carbon fiber that can be divided into thousands of fine filaments when the carbon fiber bundle is cut.

REFERENCE SIGNS LIST 1 Carbon fiber bundle 4 Short cut carbon fiber bundle 11 Resin composition 12 Doctor blade 21 Unwinder 22 Mesh belt 24 Roller group 25 Winder 31 Roving cutter 32 Rubber roller 33 Cutter roller 34 Blade

Claims (2)

A method for producing a carbon fiber bundle, comprising: twisting an acrylic fiber bundle having a hook drop value of 15 cm or more measured in an untwisted state with a twist of 1 turn/m or more and 10 turns/m or less, and then supplying the twisted acrylic fiber bundle to a baking step to produce a carbon fiber bundle; and cutting the carbon fiber bundle to a length of 20 to 50 mm . A method for producing 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 twisted acrylic fiber bundle to a baking step to produce a carbon fiber bundle, and cutting the carbon fiber bundle to lengths of 20 to 50 mm.