Manufacturing method of carbon fiber granules

Abstract

To provide carbon fiber granules which are excellent in terms of stable supply through a feeder and in preventing scattering of fibers even when the carbon fibers have a relatively long fiber length, and to provide a method for producing the carbon fiber granules by a simple method. [Solution] A carbon fiber granule comprising carbon fibers having an average fiber diameter of 5 μm or more and 20 μm or less and an average fiber length of 0.10 mm or more and 1.50 mm or less, and a binder contained in an amount of 2.0 mass% or more and 12.0 mass% or less in terms of solid content relative to the mass of the carbon fibers, wherein the carbon fiber granule has an outer diameter of 1.0 mm or more and 10.0 mm or less, a length of 2 mm or more and 15 mm or less, and a compressive strength of 10 N or more and 20 N or less.

Description

[Technical field]

[0001] The present invention provides a carbon fiber granulation method that is excellent in stable supply through a feeder and prevention of powder scattering. Body It relates to a manufacturing method. [Background technology]

[0002] Carbon fibers are used as fillers for resin additives to impart wear resistance, conductivity, etc. to resins. Carbon fibers are classified into PAN-based carbon fibers made from polyacrylonitrile (PAN) and pitch-based carbon fibers made from coal tar pitch or petroleum pitch. Carbon fibers are available in two shapes: chopped carbon fibers that are cut short (3-6 mm) and milled carbon fibers that are finely ground (less than 3 mm). Milled carbon fibers have high fluidity when mixed with resin, so they are used as fillers for resin additives.

[0003] When milled carbon fiber is mixed with resin, the fibers scatter, making it difficult to mix the desired amounts of carbon fiber and resin. Therefore, it is necessary to form the carbon fiber into granules to prevent the carbon fiber from scattering.

[0004] Patent Document 1 describes a technology related to carbon fiber aggregates produced using a wet extrusion granulation method, and discloses that by using an epoxy-based sizing material, good shape retention can be achieved even for recycled carbon fibers when they are formed into carbon fiber aggregates. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2020-196882 A Summary of the Invention [Problem to be solved by the invention]

[0006] The longer the carbon fiber, the more dramatically the mechanical properties, electrical conductivity, heat resistance, corrosion resistance, and wear resistance of the resin can be improved. However, if the carbon fiber is too long, it becomes bulky and difficult to make granules. In addition, if the amount of binder is increased, the carbon fiber granules become hard and the dispersibility when mixed with resin becomes poor. On the other hand, if the amount of binder is reduced, the strength of the granules is low, the shape of the granules cannot be maintained, and the mixability with resin becomes poor. Furthermore, since the binder may affect the moldability, strength, wear resistance, electrical conductivity, etc. of the resin, carbon fiber granules with a low binder content are desired.

[0007] In order to obtain a stable amount of carbon fiber granules from the extruder, it is important to supply a constant amount of mixed powder to the extruder by a feeder. However, depending on the physical properties of the carbon fiber and the type and amount of the binder, the mixed powder may form an arch structure at the top of the hopper outlet and solidify, causing a bridge phenomenon in which the supply to the extruder stops. In addition, if a funnel flow occurs in which a flow path for the mixed powder is created only in a limited area above the hopper outlet, the mixed powder near the inner wall of the hopper is not replaced by new mixed powder, and the mixed powder near the inner wall of the hopper may form a solidified layer and deteriorate. If these phenomena occur, it is not possible to obtain a constant amount and quality of carbon fiber granules.

[0008] The present invention has been made in view of the above problems, and an object of the present invention is to provide a carbon fiber feeder which is excellent in terms of stable feeding through a feeder and in terms of preventing the fibers from scattering, even if the fibers have a relatively long fiber length. R, The object of the present invention is to provide a method for producing carbon fiber granules, which is simple and easy to produce. [Means for solving the problem]

[0015] The characteristic configuration of the method for producing carbon fiber granules according to the present invention is that the average fiber diameter is 5 μm or more and 20 μm or less, and the average fiber length is 0.68 mm or more 1.00 A step of mixing carbon fibers having a particle size of 0.01 mm or less and a moisture content of 3% by mass or more and 15% by mass or less with water and a binder to obtain a mixture; A step of extrusion granulating the mixture to obtain granules; and drying the granules by heating them to 130° C. or more and 150° C. or less. For the mixture The proportion of water is 15.2% by mass or more and 16.3% by mass or less, The binder is a nylon-based emulsion resin. If the average fiber diameter is greater than 20 μm, dispersibility with the resin is poor. If the average fiber length is greater than 1.00 mm, the fiber is bulky and cannot be compressed, making it impossible to form granules. If the moisture content of the carbon fiber is less than 3% by mass, it is not mixed sufficiently with water, making it impossible to form granules. When a powdered polyethylene resin is used as the binder, the granules tend to become brittle and the binder ratio is increased. In addition, as the binder ratio is increased, the amount of water added increases, and the load during drying increases. By using a nylon-based emulsion resin as the binder, the carbon fibers can be bonded with a small amount of binder, and strong carbon fiber granules can be obtained. In addition, the amount of water added can be reduced, and the load during drying can be reduced. Therefore, according to the above characteristic configuration, even if carbon fibers have a relatively long fiber length, it is possible to produce carbon fiber granules that are stable to feed through a feeder and have excellent fiber scattering prevention properties.

[0016] A further characteristic configuration of the method for producing carbon fiber granules according to the present invention is that the ratio of the carbon fibers to the mixture is 76.2% by mass or more and 81.6% by mass or less, the ratio of the water is 15.2% by mass or more and 16.3% by mass or less, and the ratio of the binder is 2.0% by mass or more and 8.6% by mass or less in terms of solid content, and the carbon fiber granules have an outer diameter of 1.0 mm or more and 10.0 mm or less, a length of 2 mm or more and 15 mm or less, and a compressive strength of 10 N or more and 20 N or less. If the compression strength is higher than 20N, the dispersibility when mixed with resin is poor, and if the compression strength is lower than 10N, the shape of the granules cannot be maintained, and the carbon fibers scatter, resulting in poor handling. If the outer diameter is larger than 10mm, the strength of the granules is insufficient and the shape of the granules cannot be maintained, and if the outer diameter is smaller than 1.0mm, the handling of the granules is poor. Furthermore, if the length is longer than 15mm, the strength of the granules is insufficient and the shape of the granules cannot be maintained, and if the length is shorter than 2mm, the carbon fibers cannot be bonded to each other, making it impossible to obtain granules. Therefore, according to the above characteristic configuration, it is possible to produce carbon fiber granules that are composed of carbon fibers having a relatively long fiber length and that are stable to feed in a feeder and have excellent fiber scattering prevention properties.

[0017] A further characteristic feature of the method for producing carbon fiber granules according to the present invention is that the carbon fibers are obtained by pulverizing a carbon fiber aggregate having a tensile strength of 100 MPa or more and 800 MPa or less, and are isotropic pitch-based carbon fibers having a crystal lattice spacing d(002) of 3.576 nm or more and 3.579 nm or less as measured by X-ray diffraction method. If the fiber strength of the carbon fiber is too high, the molding pressure during granulation will be exceeded and granules will not be formed. Isotropic pitch-based carbon fibers have small crystallites consisting of hexagonal carbon mesh planes and are arranged randomly, so they have relatively low mechanical strength and hardness. Therefore, according to the above characteristic configuration, it is possible to produce carbon fiber granules that are stable to supply through a feeder, have excellent fiber scattering prevention properties, and ensure dispersibility when mixed with a resin. In addition, the lattice spacing affects the moisture content of carbon fiber and the compressive strength of carbon fiber granules. If the crystal lattice spacing d(002) is higher than 3.579 nm, the moisture content of carbon fiber decreases, the mixture with water becomes insufficient, and granules cannot be formed. If the crystal lattice spacing d(002) is lower than 3.576 nm, the compressive strength of the granules decreases, they cannot maintain their shape, and handling becomes poor. Therefore, according to the above-mentioned characteristic configuration, even if carbon fibers have a relatively long fiber length, the decrease in compressive strength of the carbon fiber granules can be suppressed, and carbon fiber granules that are stable to supply through the feeder and have excellent fiber scattering prevention properties can be produced. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The carbon fiber granulation according to the embodiment of the present invention will be described below. Body The manufacturing method will be described.

[0020] The carbon fiber used in this embodiment may be either pitch-based or PAN-based. Pitch-based carbon fiber means carbon fiber with pitch as a carbon precursor. Among them, one that can be added to tetrafluoroethylene-based resin as a filler can be adopted, but from the viewpoint of sliding properties, isotropic pitch-based carbon fiber with isotropic pitch as a carbon precursor is preferred. In particular, isotropic pitch-based carbon fiber has small crystallites made of carbon hexagonal mesh planes and is arranged randomly, so that mechanical strength and hardness are relatively low. Therefore, isotropic pitch-based carbon fiber is more likely to be broken by sliding, and is more likely to form a good sliding film containing carbon fiber crystallites, so that the sliding properties are easily improved. Incidentally, isotropy includes the case where the material is optically isotropic and the molecules or molecular groups are randomly oriented. In addition, the carbon precursor means a series of carbonized intermediates in the stage before the pitch-based carbon fiber, which is the final carbon product of interest.

[0021] Pitch, which is a carbon precursor, is a solid at room temperature obtained by heat treatment and polymerization of liquid tar obtained during the dry distillation of wood, coal, etc., bitumen obtained from oil sands, oil obtained by dry distillation of oil shale, residual oil by distillation of crude oil, tar produced by cracking petroleum fractions, etc. Specifically, depending on the raw material, coal-based pitch, petroleum-based pitch, synthetic pitch obtained by polymerizing aromatic compounds such as naphthalene, etc. are listed. Chemically, pitch is a mixture of countless condensed polycyclic aromatic compounds. Examples of coal-based pitch obtained from coal as a raw material include pitch obtained by heat treatment of coal tar generated from a coke oven. The pitch in the present invention is not particularly limited, but isotropic pitch is preferable from the viewpoint of sliding characteristics (amount of wear, temperature near the sliding surface, etc.) and imparting electrical conductivity, and coal-based isotropic pitch (isotropic pitch obtained from coal as a raw material) is more preferable.

[0022] The carbon fiber bundle is obtained by collecting spun coal-based isotropic pitch-based carbon fibers, subjecting them to infusibility treatment in an oxidizing atmosphere, and then carbonizing them in an inert gas atmosphere, directly activating them in an activating gas atmosphere, or activating them after carbonization. The spinning method includes the centrifugal method in which the pitch is stretched by centrifugal force, and the vortex method in which the pitch is stretched by a swirling force by blowing a tornado-like hot air onto it, and the carbon fiber bundle can be produced by any of these methods.

[0023] Carbon fibers are classified into chopped (3 to 6 mm) and milled (less than 3 mm) according to the fiber length, and milled is preferably used from the viewpoint of mixability with resin. The milled carbon fibers used in this embodiment are obtained by crushing the carbon fiber aggregate. The crushing can be performed using a hammer mill, a cutter mill, a crusher using a shear screen method, a crusher using an impact screen method, a crusher using a grinding type crushing method, a crusher using a media stirring method such as a jet mill or a ball mill. The crusher may be used alone or in combination of two or more types. The crusher using the impact screen method is preferably used. By appropriately setting the crushing conditions in the crushing step, the fiber length of the obtained milled carbon fibers can be adjusted to a desired length. In addition, the distribution of the fiber length of the milled carbon fibers may be adjusted by a classifier or a sieve as necessary.

[0024] The water used in this embodiment may be tap water, ion-exchanged water, distilled water, ultrapure water, or the like.

[0025] In this embodiment, a mixture of carbon fibers, water, and a binder is forced to pass through a large number of holes in an extruder while being compressed, and extruded into a cylindrical shape to obtain carbon fiber granules. As the extruder, a basket type extruder, a roller type extruder, a screw type extruder, or the like can be used, but a roller type extruder is preferably used.

[0026] The die (screen die) of the extruder used in this embodiment can change the outer diameter of the cylindrical shape of the carbon fiber granules by changing the hole diameter. In this embodiment, the outer diameter of the carbon fiber granules is preferably 1.0 mm or more and 10.0 mm or less, and more preferably 2.0 mm or more and 4.0 mm or less. Specifically, the outer diameter of the carbon fiber granules is 1.0 mm, 2.0 mm, 3.0 mm, 4.0 mm, 5.0 mm, 6.0 mm, 7.0 mm, 8.0 mm, 9.0 mm, and 10.0 mm, and may be within a range between any two of these numerical values.

[0027] The length of the cylindrical shape of the carbon fiber granules can be changed by adjusting the position of the knife cutter of the extruder used in this embodiment. In this embodiment, the length of the carbon fiber granules is preferably 2 mm or more and 15 mm or less, and more preferably 5 mm or more and 8 mm or less. Specifically, the length of the carbon fiber granules is 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, and 15 mm, and may be within a range between any two of these values.

[0028] The binder used in this embodiment is used for the purpose of promoting the aggregation of carbon fibers during granulation, increasing the strength of the carbon fiber granules to maintain their shape, and ensuring good dispersibility when the carbon fiber granules are mixed with a resin. As the binder, olefin-based resins, polyimide-based resins, polyamide-based resins, phenol-based resins, polyether ketone-based resins, synthetic resin emulsions, etc. can be used, and among them, synthetic resin emulsions are preferably used. As the synthetic resin emulsions, vinyl chloride-based emulsion resins, vinyl acetate-based emulsion resins, acrylic-based emulsion resins, and nylon-based emulsion resins are included, and copolymer nylon resin emulsions are particularly preferably used. The binder may be used alone or in combination of two or more. In this embodiment, the content of the nylon-based emulsion resin is preferably 2.0% by mass or more and 12.0% by mass or less in terms of solid content relative to the mass of the carbon fiber, more preferably 2.0% by mass or more and 10.0% by mass or less, even more preferably 2.0% by mass or more and 8.0% by mass or less, and particularly preferably 2.0% by mass or more and 6.0% by mass or less. If the content of the binder relative to the mass of the carbon fiber is higher than 12.0% by mass in terms of solid content, poor formation occurs during extrusion granulation, and if it is lower than 2.0% by mass, the strength of the carbon fiber granules after drying is insufficient, and the carbon fibers fly off during transportation of the carbon fiber granules or mixing with a resin, resulting in poor handleability.

[0029] In this embodiment, the means for drying the carbon fiber granules is not particularly limited as long as it can remove water contained in the carbon fiber granules, and a general dryer can be used. In this embodiment, the temperature in the drying step is preferably 130° C. or higher and 150° C. or lower.

[0030] [Average fiber length] The average fiber length (Lv) of the carbon fibers is calculated by the following formula. Lv = (l1 × l1 + l2 × l2 + + l n ×l n ) / (l1+l2+···+l n ) Here, l1~l nindicates the length of the 1st to nth fibers, respectively. The average fiber length of the pitch-based carbon fibers was calculated by measuring the length of 6000 carbon fibers using a microscope VHX-7000 (manufactured by KEYENCE Corporation) and the attached image analysis software, and the average fiber length was calculated by the above formula. In this embodiment, the average fiber length of the carbon fibers is preferably 0.10 mm or more and 1.50 mm or less, more preferably 0.20 mm or more and 1.40 mm or less, and even more preferably 0.30 mm or more and 1.30 mm or less. In addition, the average fiber length is specifically 0.10 mm, 0.20 mm, 0.30 mm, 0.40 mm, 0.50 mm, 0.60 mm, 0.70 mm, 0.80 mm, 0.90 mm, 1.00 mm, 1.10 mm, 1.20 mm, 1.30 mm, 1.40 mm, and 1.50 mm, and may be within a range between any two of these numerical values. If the average fiber length is greater than 1.50 mm, the fibers are bulky and cannot be compressed to form granules.

[0031] [Average fiber diameter] The average fiber diameter of the carbon fibers was measured simultaneously with the fiber length using a microscope VHX-7000 (manufactured by KEYENCE Corporation) and the attached image analysis software. In this embodiment, the average fiber diameter of the carbon fibers is preferably 5 μm or more and 20 μm or less, more preferably 7 μm or more and 18 μm or less, even more preferably 10 μm or more and 15 μm or less, and particularly preferably 12 μm or more and 14 μm or less. If the average fiber diameter is more than 20 μm, the dispersibility when mixed with resin is poor.

[0032] [Moisture percentage] The moisture content of the carbon fiber is calculated by the following formula. Moisture content (mass%) = (R0-R1) / R0×100 About 10 g of carbon fiber was cut, and the mass was measured and designated as R0. After drying for 2 hours in a dryer at 120°C, the fiber was cooled to room temperature, and the mass was measured and designated as R1, and the mass was calculated according to the above formula. In this embodiment, the moisture content of the carbon fiber is preferably 3% by mass or more and 15% by mass or less, and more preferably 3% by mass or more and 9% by mass or less. If the moisture content of the carbon fiber is less than 3% by mass, the fiber is not mixed with water sufficiently, and granules cannot be formed.

[0033] [Tensile strength] The tensile strength of the carbon fiber collection body was measured by extracting carbon fibers from the carbon fiber collection body and measuring the tensile strength of the carbon fiber collection body according to the method described in JIS R7606:2000. In this embodiment, the tensile strength of the carbon fiber collection body is preferably 100 MPa or more and 800 MPa or less, and more preferably 300 MPa or more and 800 MPa or less. If the tensile strength of the carbon fiber collection body is stronger than 800 MPa, the molding pressure during granulation will be exceeded and granules will not be formed.

[0034] [Crystal lattice spacing d(002)] The crystal lattice spacing d(002) of the carbon fiber was measured by X-ray diffraction (XRD). Specifically, it was measured according to the method described in JIS R7651:2007. For the measurement, an Ultima IV (manufactured by Rigaku Corporation) was used, and CuKα rays were used as the X-ray source. The enclosed tube voltage was 40 kV, and the current was 40 mA. In this embodiment, the crystal lattice spacing d(002) of the carbon fiber is particularly preferably 3.576 nm or more and 3.579 nm or less. If the crystal lattice spacing d(002) is higher than 3.579 nm, the moisture content of the carbon fiber decreases, the mixture with water becomes insufficient, and granules are not formed. If the crystal lattice spacing d(002) is lower than 3.576 nm, the compressive strength of the granules decreases, the shape cannot be maintained, and the handling properties are poor.

[0035] [Compressive strength] The crushing strength of the carbon fiber granules was measured as a compressive strength (N) using a digital force gauge (manufactured by Imada Co., Ltd.). Specifically, the point at which the carbon fiber granules collapsed due to compression was measured as the compressive strength (N). Measurements were performed five times from each horizontal direction of the carbon fiber granules, and the average value was calculated and used as the measured value. In this embodiment, the compressive strength of the carbon fiber granules is preferably 10N or more and 20N or less. Specifically, the compressive strength is 10N, 11N, 12N, 13N, 14N, 15N, 16N, 17N, 18N, 19N, and 20N, and may be within a range between any two of these values. If the compressive strength is higher than 20N, it cannot be mixed with the resin and dispersibility is poor. If the compressive strength is lower than 10N, the shape of the granules cannot be maintained, and the carbon fibers scatter, resulting in poor handleability.

[0036] In this embodiment, when the entire mixture of carbon fiber, water and binder is 100 mass%, the carbon fiber is preferably 76.2 mass% to 81.6 mass%, the water is preferably 15.2 mass% to 16.3 mass%, and the binder is preferably 2.0 mass% to 8.6 mass% in terms of solid content. By having the carbon fiber, water and binder within the above ranges, the granulation property and the shape of the granules are good. EXAMPLES

[0037] The carbon fiber pelletization method of the present invention will be described below based on the examples and comparative examples shown in Table 1. Body The production method will be described below, but the present invention is not limited to the following examples.

[0038] [Table 1]

[0039] The granulation properties and shape of the carbon fiber granules based on the compounding conditions shown in Table 1 were evaluated according to the following criteria. (granulation) ○: Extruded from the die hole while maintaining a cylindrical shape. ×: Not extruded from the die hole while maintaining the cylindrical shape. (Granule shape) ○: The shape is visually recognizable as cylindrical. ×: The shape is not visually recognizable as a cylinder.

[0040] Example 1 As the carbon fiber, isotropic pitch-based carbon fiber (trade name: DonaCarbo Milled S-244 (DonaCarbo is a registered trademark), average fiber length about 0.68 mm, average fiber diameter about 13.0 μm, moisture content 6 mass%, crystal lattice spacing 3.577 nm, manufactured by Osaka Gas Chemicals Co., Ltd.) was used. First, 600 g of this carbon fiber was put into a mixer together with 38 g (15 g in terms of solid content) of nylon-based emulsion resin (trade name: Sepolsion PA200 (Sepolsion is a registered trademark), solid content ratio 40%, manufactured by Sumitomo Seika Chemicals Co., Ltd.) as a binder, and 96 g of water were stirred for 5 minutes to thoroughly mix the above materials. Next, the mixture obtained above was extruded and granulated by an extrusion granulator (trade name: Disc Pelleter, manufactured by Fuji Paudal Co., Ltd.) set to a screen diameter of 3 mm. The granules were dried in a dryer at 130°C for 3 hours to obtain cylindrical carbon fiber granules with a diameter of 3 mm and an average length of about 6 mm (No. 11 in Table 1). The granulation properties when the mixture was extruded and granulated and the shape of the granules after drying were good, and the compressive strength of the obtained carbon fiber granules was 16.5 N. The isotropic pitch-based carbon fiber was obtained by pulverizing a carbon fiber aggregate (product name: Dona Carbo Mat, manufactured by Osaka Gas Chemicals Co., Ltd.), and the tensile strength of the carbon fiber aggregate was 588 MPa.

[0041] Example 2 As shown in No. 8 of Table 1, carbon fiber granules were obtained in the same manner as in Example 1, except that the amount of nylon-based emulsion resin was 168 g (67 g in terms of solid content) and the amount of water was 19 g. The granulation property when the mixture was extruded and granulated, and the shape of the granules after drying were good, and the compressive strength of the carbon fiber granules obtained under the condition of No. 8 was 20 N.

[0042] Example 3 As shown in No. 10 of Table 1, carbon fiber granules were obtained in the same manner as in Example 1, except that the amount of nylon-based emulsion resin was 80 g (32 g in terms of solid content) and the amount of water was 72 g. The granulation property when the mixture was extruded and granulated, and the shape of the granules after drying were good, and the compressive strength of the carbon fiber granules obtained under the condition of No. 10 was 20 N.

[0043] Example 4 As shown in No. 16 of Table 1, carbon fiber granules were obtained in the same manner as in Example 1, except that isotropic pitch-based carbon fiber (trade name: DonaCarbo Milled S-246, average fiber length about 1.00 mm, average fiber diameter about 13.0 μm, moisture content 6 mass%, crystal lattice spacing 3.577 nm) was used as the carbon fiber. The granulation property when the mixture was extruded and granulated and the shape of the granules after drying were good, and the compressive strength of the obtained carbon fiber granules was 11.4 N. The isotropic pitch-based carbon fiber was obtained by pulverizing a carbon fiber aggregate (trade name: DonaCarbo Mat, manufactured by Osaka Gas Chemicals Co., Ltd.), and the tensile strength of the carbon fiber aggregate was 588 MPa.

[0044] Comparative Example 1 As shown in No. 17 of Table 1, carbon fiber granules were obtained in the same manner as in Example 1, except that isotropic pitch-based carbon fiber (trade name: DonaCarbo Milled S-247, average fiber length of about 1.55 mm, average fiber diameter of about 13.0 μm, moisture content of 6 mass%, crystal lattice spacing of 3.577 nm) was used as the carbon fiber. Since the carbon fiber had an average fiber length of about 1.55 mm and was bulky, it was not compressed and granules could not be obtained. The isotropic pitch-based carbon fiber was obtained by pulverizing a carbon fiber aggregate (trade name: DonaCarbo Mat, manufactured by Osaka Gas Chemicals Co., Ltd.), and the tensile strength of the carbon fiber aggregate was 588 MPa.

[0045] Comparative Example 2 Carbon fiber granules were obtained with the blend shown in No. 6 in Table 1. In No. 6, the blending amount of water was large, so the mixture became a fluid slurry, and granules could not be obtained.

[0046] Comparative Example 3 Carbon fiber granules were obtained with the blend shown in No. 7 in Table 1. The amount of water used in No. 7 was less than that used in No. 6, but the amount of water used was so large that the mixture became a fluid slurry, and granules could not be obtained.

[0047] Comparative Example 4 Carbon fiber granules were obtained with the blend shown in No. 9 in Table 1. Under the condition of No. 9, granules could not be obtained because water was not blended.

[0048] Comparative Example 5 A mixture was prepared using powdered polyethylene resin (product name: Hiwax 160H, manufactured by Mitsui Chemicals, Inc.) as a binder in the formulation shown in No. 1 of Table 1, and carbon fiber granules were obtained. Under the conditions of No. 1, the granules could not maintain a cylindrical shape. The amount of water blended was small, so the granules could not maintain a cylindrical shape.

[0049] Comparative Example 6 A mixture was prepared using powdered polyethylene resin (product name: Hiwax 160H, manufactured by Mitsui Chemicals, Inc.) as a binder in the formulation shown in No. 2 of Table 1, and carbon fiber granules were obtained. Under the conditions of No. 2, the granules could not maintain a cylindrical shape. The amount of water blended was small, so the granules could not maintain a cylindrical shape.

[0050] Comparative Example 7 A mixture was prepared using powdered polyethylene resin (product name: Hiwax 160H, manufactured by Mitsui Chemicals, Inc.) as a binder in the formulation shown in No. 3 of Table 1, and carbon fiber granules were obtained. The amount of water was greater than when nylon emulsion was used, so it took longer to dry and the load during drying was greater.

[0051] Comparative Example 8 A mixture was prepared using powdered polyethylene resin (product name: Hiwax 160H, manufactured by Mitsui Chemicals, Inc.) as a binder in the formulation shown in No. 4 of Table 1, and carbon fiber granules were obtained. The amount of water was greater than when nylon emulsion was used, so it took longer to dry and the load during drying was greater.

[0052] Comparative Example 9 A mixture was prepared using powdered polyethylene resin (product name: Hiwax 160H, manufactured by Mitsui Chemicals, Inc.) as a binder in the formulation shown in No. 5 of Table 1, and carbon fiber granules were obtained. Under the condition of No. 5, the amount of water was large, so the mixture became a fluid slurry, and the mixture could not maintain its cylindrical shape when coming out of the hole in the die of the disc pelletizer.

[0053] Comparative Example 10 Carbon fiber granules were obtained with the blend shown in No. 12 in Table 1. Under the condition of No. 12, the blending ratio (mass%) of the binder was low, so the granules were not visually cylindrical and had low compressive strength.

[0054] Comparative Example 11 Carbon fiber granules were obtained with the blend shown in No. 13 in Table 1. Under the condition of No. 13, the blending ratio (mass%) of the binder was low, so the granules were not visually cylindrical and had low compressive strength.

[0055] Comparative Example 12 Carbon fiber granules were obtained with the blend shown in No. 14 in Table 1. Under the condition of No. 14, the blending ratio (mass%) of the binder was high, so the shape was poor during extrusion granulation, and granules could not be obtained.

[0056] As described above, according to Examples 1 to 4 (Nos. 8, 10, 11, and 16 in Table 1), the granulation properties and the shape of the granules are good, and even when the carbon fibers have a relatively long fiber length, the carbon fiber granules can be obtained that are stable to be fed through a feeder, have excellent fiber scattering prevention properties, and ensure dispersibility when mixed with a resin.

[0057] [Another embodiment] In the above embodiment, when the carbon fiber, water, and binder are mixed, a second component may be mixed in. For example, molybdenum disulfide, polytetrafluoroethylene, amorphous carbon, glass fiber, graphite, or the like may be mixed in.

[0058] Furthermore, the configurations disclosed in the above embodiments (including other embodiments, the same applies below) can be applied in combination with configurations disclosed in other embodiments, provided that no contradiction arises. Furthermore, the embodiments disclosed in this specification are merely examples, and the embodiments of the present invention are not limited thereto, and can be appropriately modified within the scope that does not deviate from the purpose of the present invention.

Claims

1. A step of mixing carbon fibers having an average fiber diameter of 5 μm or more and 20 μm or less, an average fiber length of 0.68 mm or more and 1.00 mm or less, and a moisture content of 3 mass% or more and 15 mass% or less with water and a binder to obtain a mixture; A step of extrusion granulating the mixture to obtain granules; and drying the granules by heating the granules to 130° C. or more and 150° C. or less. The ratio of the water to the mixture is 15.2% by mass or more and 16.3% by mass or less, The method for producing carbon fiber granules, wherein the binder is a nylon-based emulsion resin.

2. 2. The method for producing carbon fiber granules according to claim 1, wherein a ratio of the carbon fibers to the mixture is 76.2% by mass or more and 81.6% by mass or less, a ratio of the water is 15.2% by mass or more and 16.3% by mass or less, a ratio of the binder is 2.0% by mass or more and 8.6% by mass or less in terms of solid content and 5.0% by mass or more and 21.4% by mass or less in a state containing water, and the carbon fiber granules have an outer diameter of 1.0 mm or more and 10.0 mm or less, a length of 2 mm or more and 15 mm or less, and a compressive strength of 10 N or more and 20 N or less.

3. 3. The method for producing carbon fiber granules according to claim 1 or 2, wherein the carbon fibers are isotropic pitch-based carbon fibers obtained by pulverizing a carbon fiber aggregate having a tensile strength of 100 MPa or more and 800 MPa or less, and have a crystal lattice spacing d(002) of 3.576 nm or more and 3.579 nm or less as measured by an X-ray diffraction method.

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