Method for producing carbon fiber granulates

Abstract

To provide carbon fiber granulates which, even when using carbon fibers with comparatively long fiber lengths, achieve stable supply in a feeder and superior resistance to fiber dispersion, and a simple method for producing carbon fiber granulates.SOLUTION: The present invention provides carbon fiber granulates 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. The carbon fiber granulates 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.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to carbon fiber granulates excellent in stable supply in a feeder and prevention of powder scattering, and a method for producing the same.

Background Art

[0002] Carbon fibers are used as fillers for resin addition for the purpose of imparting abrasion resistance, conductivity, etc. to the resin. Carbon fibers include PAN-based carbon fibers using polyacrylonitrile (PAN) as a raw material and pitch-based carbon fibers using coal tar pitch or petroleum-based pitch as a raw material. Further, as the shape of carbon fibers, there are chopped carbon fibers cut short (3 to 6 mm) and milled carbon fibers finely pulverized (less than 3 mm). Since milled carbon fibers have high fluidity when mixed with resin, they are used as fillers for resin addition.

[0003] When milled carbon fiber alone is mixed with resin, the fibers scatter, and it is difficult to mix a desired amount of carbon fiber and resin. Therefore, it is necessary to form the carbon fiber into granulates to prevent the scattering of the carbon fiber.

[0004] Patent Document 1 discloses a technique related to a carbon fiber aggregate produced by using a wet extrusion granulation method. By using an epoxy sizing material, even for recycled carbon fibers, when forming a carbon fiber aggregate, it is possible to exhibit good shape retention ability.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The longer the carbon fiber, the more significantly the mechanical properties, 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 form granulated bodies. Also, when the blending amount of the binder is increased, the carbon fiber granulated body becomes hard, and its dispersibility deteriorates when mixed with the resin. On the other hand, when the blending amount of the binder is decreased, the strength of the granulated body is low, it cannot maintain its shape as a granulated body, and its miscibility with the resin deteriorates. Furthermore, since the binder may affect the moldability, strength, wear resistance, conductivity, etc. of the resin, a carbon fiber granulated body with a low binder content is desired.

[0007] To obtain a stable amount of carbon fiber granulated bodies from an extruder, it is important to supply a certain amount of mixed powder to the extruder by a feeder. However, depending on the physical properties of the carbon fiber and the type and blending amount of the binder, a bridging phenomenon may occur where the mixed powder forms an arch structure and solidifies above the discharge port of the hopper, stopping the supply to the extruder. Also, when funnel flow occurs where only a limited area above the discharge port of the hopper forms a flow path for the mixed powder, the mixed powder near the inner wall of the hopper may not be replaced by new mixed powder, and thus the mixed powder near the inner wall of the hopper may form a sticking layer and deteriorate. When these phenomena occur, it is impossible to obtain carbon fiber granulated bodies of a certain amount and quality.

[0008] The present invention has been made in view of the above problems, and its object is to provide a carbon fiber granulated body that is excellent in stable supply by a feeder and prevention of fiber scattering even for carbon fibers with a relatively long fiber length, and a manufacturing method of the carbon fiber granulated body that is simple.

Means for Solving the Problems

[0009] The characteristic configuration of the carbon fiber granulated body according to the present invention for achieving the above object is a carbon fiber granulated body 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 in a solid content conversion of 2.0 mass% or more and 12.0 mass% or less based on the mass of the carbon fiber, It is characterized in that the outer diameter is 1.0 mm or more and 10.0 mm or less, the length is 2 mm or more and 15 mm or less, and the compression strength is 10 N or more and 20 N or less.

[0010] When the average fiber diameter is larger than 20 μm, the dispersibility with the resin is poor. When the average fiber length is longer than 1.50 mm, it is bulky and cannot be compressed, so granulated bodies cannot be formed. Also, when the ratio of the binder to the mass of the carbon fiber is more than 12.0% by mass in terms of solid content, the shape becomes defective during extrusion granulation, and when it is less than 2.0% by mass, the strength of the granulated body after drying is weak, and the carbon fibers scatter, resulting in poor handleability. Further, when the compression strength is higher than 20 N, the dispersibility when mixed with the resin is poor, and when the compression strength is lower than 10 N, the shape of the granulated body cannot be maintained, and the carbon fibers scatter, resulting in poor handleability. Also, when the outer diameter is larger than 10 mm, the strength of the granulated body becomes insufficient, and the shape of the granulated body cannot be maintained. When the outer diameter is smaller than 1.0 mm, the handleability of the granulated body deteriorates. Furthermore, when the length is longer than 15 mm, the strength of the granulated body becomes insufficient, and the shape of the granulated body cannot be maintained. When the length is shorter than 2 mm, the carbon fibers cannot adhere to each other, and granulated bodies cannot be obtained. Therefore, the carbon fiber granulated body having the above characteristic configuration is composed of carbon fibers with a relatively long fiber length, and is excellent in stable supply by a feeder and prevention of fiber scattering.

[0011] A further characteristic configuration of the carbon fiber granulated body according to the present invention is that the carbon fiber is an isotropic pitch-based carbon fiber having a crystal lattice plane spacing d(002) of 3.576 nm or more and 3.579 nm or less by X-ray diffraction method.

[0012] If the fiber strength of the carbon fiber is too strong, the forming pressure during granulation is exceeded and granulated bodies cannot be formed. Isotropic pitch-based carbon fibers have small crystallites composed of carbon hexagonal nets and are randomly arranged, so their mechanical strength and hardness are relatively low. Therefore, according to the above characteristic configuration, it is possible to provide a carbon fiber granulated body that is excellent in stable supply by a feeder and prevention of fiber scattering and ensures dispersibility when mixed with a resin. In addition, the lattice plane spacing affects the moisture content of the carbon fiber and the compressive strength of the carbon fiber granulated body. When the crystal lattice plane spacing d(002) is higher than 3.579 nm, the moisture content of the carbon fiber decreases, the mixing with water becomes insufficient, and granulated bodies cannot be formed. When the crystal lattice plane spacing d(002) is lower than 3.576 nm, the compressive strength of the granulated body decreases, the shape cannot be maintained, and the handleability is poor. Therefore, the carbon fiber granulated body having the above-described characteristic configuration can suppress a decrease in the compressive strength of the carbon fiber granulated body even with relatively long carbon fibers, and is excellent in stable supply by a feeder and fiber scattering prevention.

[0013] A further characteristic configuration of the carbon fiber granulated body according to the present invention lies in that the binder is a nylon-based emulsion resin.

[0014] When a powder-based polyethylene resin is used as the binder, the granulated body tends to be brittle and the blending ratio of the binder also increases. Further, as the blending ratio of the binder increases, the blending amount of water increases and the load during drying increases. By using a nylon-based emulsion resin as the binder, carbon fibers can be adhered with a small amount of binder, and a strong carbon fiber granulated body can be obtained. Further, the blending amount of water can be reduced and the load during drying can be reduced. Therefore, according to the above-described characteristic configuration, it is possible to provide a carbon fiber granulated body that is excellent in stable supply by a feeder and fiber scattering prevention even with relatively long carbon fibers.

[0015] A characteristic configuration of the method for manufacturing a carbon fiber granulated body according to the present invention includes a step of mixing water and a binder with carbon fibers having an average fiber diameter of 5 μm or more and 20 μm or less, an average fiber length of 0.10 mm or more and 1.50 mm or less, and a moisture content of 3% by mass or more and 9% by mass or less to obtain a mixture, a step of extruding and granulating the mixture to obtain a granulated body, and a step of drying the granulated body by heating the temperature to 130°C or more and 150°C or less.

[0016] A further characteristic configuration of the method for manufacturing carbon fiber granulates according to the present invention is that the ratio of the carbon fiber 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, the ratio of the binder is 2.0% by mass or more and 8.6% by mass or less in terms of solid content, the outer diameter of the carbon fiber granulates is 1.0 mm or more and 10.0 mm or less, the length is 2 mm or more and 15 mm or less, and the compressive strength is 10 N or more and 20 N or less.

[0017] A further characteristic configuration of the method for manufacturing carbon fiber granulates according to the present invention is that the carbon fiber is obtained by pulverizing a carbon fiber batt having a tensile strength of 100 MPa or more and 800 MPa or less, and is an isotropic pitch-based carbon fiber having a crystal lattice plane spacing d(002) of 3.576 nm or more and 3.579 nm or less by X-ray diffraction method.

[0018] A further characteristic configuration of the method for manufacturing carbon fiber granulates according to the present invention is that the binder is a nylon-based emulsion resin.

Embodiments for Carrying Out the Invention

[0019] Hereinafter, carbon fiber granulates according to embodiments of the present invention and a method for manufacturing the same will be described.

[0020] The carbon fibers used in this embodiment may be either pitch-based or PAN-based. Pitch-based carbon fibers mean carbon fibers using pitch as a carbon precursor. Among them, those that can be added as a filler to a tetrafluoroethylene-based resin can be adopted. From the viewpoint of sliding characteristics and the like, isotropic pitch-based carbon fibers using isotropic pitch as a carbon precursor are preferable. In particular, isotropic pitch-based carbon fibers have small crystallites composed of carbon hexagonal net planes and are randomly arranged, so their mechanical strength and hardness are relatively low. Therefore, isotropic pitch-based carbon fibers are more likely to undergo appropriate fracture due to sliding and are likely to form a high-quality sliding film containing the crystallites of carbon fibers, so it is easy to improve the sliding characteristics. Note that the term "isotropic" optically means isotropic and also includes cases where molecules or groups of molecules are disordered in orientation. Also, the carbon precursor means a series of carbonized intermediates in the stage before the pitch-based carbon fibers, which are the target final carbon products.

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

[0022] The carbon fiber aggregated mat is obtained by aggregating spun coal-based isotropic pitch-based carbon fibers, performing an infusibilization treatment in an oxidizing atmosphere, and then performing a carbonization treatment in an inert gas atmosphere, directly performing an activation treatment in an activating gas atmosphere, or performing an activation treatment after carbonization treatment. Further, as the spinning method, there are a centrifugal method in which pitch is stretched by centrifugal force and a vortex method in which a spiral hot air is blown onto the pitch and stretched by a swirling force. In any method, a carbon fiber aggregated mat can be manufactured.

[0023] Carbon fibers are classified into chopped (3 to 6 mm) and mild (less than 3 mm) according to the fiber length. From the viewpoint of the miscibility with the resin, mild is preferably used. Further, the carbon fiber mild used in the present embodiment is obtained by pulverizing a carbon fiber aggregated mat. The pulverization can be performed using a hammer mill, a cutter mill, a pulverizer using a shear-type screen method, a pulverizer using an impact-type screen method, a pulverizer using a grinding-type pulverization method, a jet mill, a pulverizer using a medium stirring method such as a ball mill, etc. The pulverizer may be used alone or in combination of two or more. Preferably used is a pulverizer using an impact-type screen method. By appropriately setting the pulverization conditions in the pulverization process, the fiber length of the obtained carbon fiber mild can be adjusted to the desired length. Further, if necessary, the fiber length distribution of the carbon fiber mild may be adjusted by a classifier or a sieve.

[0024] As the water used in the present embodiment, tap water, ion-exchanged water, distilled water, ultrapure water, etc. can be used.

[0025] In the present embodiment, a carbon fiber granule is obtained by forcibly passing a mixture composed of carbon fibers, water, and a binder through a large number of holes provided in an extruder while compressing and extruding it into a cylindrical shape. As the extruder, a basket-type extruder, a roller-type extruder, a screw-type extruder, etc. can be used, but preferably used is a roller-type extruder.

[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 granulate by changing the hole diameter. In this embodiment, the outer diameter of the carbon fiber granulate 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 granulate 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, 10.0 mm, and it may also be within the range between any two of these numerical values.

[0027] The length of the cylindrical shape of the carbon fiber granulate 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 granulate 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 granulate 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, 15 mm, and it may also be within the range between any two of these numerical 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 granulates to maintain their shape, and ensuring good dispersibility when the carbon fiber granulates and the resin are mixed. As the binder, olefin resins, polyimide resins, polyamide resins, phenolic resins, polyether ketone resins, synthetic resin emulsions, etc. can be used, and among them, synthetic resin emulsions are preferably used. As the synthetic resin emulsion, there are vinyl chloride emulsion resins, vinyl acetate emulsion resins, acrylic emulsion resins, nylon emulsion resins, and a copolymer nylon resin emulsion is particularly preferably used. Note that the binder may be used alone or in combination of two or more. In this embodiment, the content of the nylon emulsion resin is preferably 2.0% by mass or more and 12.0% by mass or less, more preferably 2.0% by mass or more and 10.0% by mass or less, still 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 in terms of solid content based on the mass of the carbon fibers. When the content of the binder with respect to the mass of the carbon fibers is higher than 12.0% by mass in terms of solid content, defective formation occurs during extrusion granulation, and when it is lower than 2.0% by mass, the strength of the carbon fiber granulates after drying is insufficient, and carbon fibers scatter during transportation of the carbon fiber granulates or during mixing with the resin, resulting in poor handleability.

[0029] In this embodiment, the means for drying the carbon fiber granulates is not particularly limited as long as it can remove the water contained in the carbon fiber granulates, and a general dryer can be used. In this embodiment, the temperature in the drying process 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 obtained by the following formula. Lv=(l 1 ×l 1 +l 2 ×l 2 +···+l n ×l n ) / (l 1 +l 2+ ··· + l n ) Here, l 1 ~ l n represents the lengths of the 1st to nth fibers respectively. The average fiber length of the pitch-based carbon fiber was measured for 6,000 carbon fibers using a microscope VHX-7000 (manufactured by KEYENCE CORPORATION) and the attached image analysis software, and calculated by the above formula. In this embodiment, the average fiber length of the carbon fiber 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. Specifically, the average fiber length is 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, 1.50 mm, and may also be within the range between any two of these numerical values. When the average fiber length is greater than 1.50 mm, the fibers are bulky and cannot be compressed, so granulated bodies cannot be formed.

[0031] 〔Average fiber diameter〕 The average fiber diameter of the carbon fiber was measured simultaneously with the fiber length using a microscope VHX-7000 (manufactured by KEYENCE CORPORATION) and the attached image analysis software. Also, in this embodiment, the average fiber diameter of the carbon fiber 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. When the average fiber diameter is greater than 20 μm, the dispersibility when mixed with the resin is poor.

[0032] 〔Moisture content〕 The moisture content of the carbon fiber is determined by the following formula. Moisture content (mass%) = (R 0 - R 1 ) / R 0 × 100 Cut about 10 g of carbon fiber, measure the mass and set it as R 0 . After drying in a dryer at 120 °C for 2 hours, cool it to room temperature and measure the mass as R 1and calculated by the above formula. In the present 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. When the moisture content of the carbon fiber is less than 3% by mass, the mixing with water becomes insufficient and granulated bodies cannot be formed.

[0033] 〔Tensile strength〕 The tensile strength of the carbon fiber batt was measured by extracting carbon fibers from the carbon fiber batt and following the method described in JIS R7606:2000, and was taken as the tensile strength of the carbon fiber batt. In the present embodiment, the tensile strength of the carbon fiber batt is preferably 100 MPa or more and 800 MPa or less, and more preferably 300 MPa or more and 800 MPa or less. When the tensile strength of the carbon fiber batt is stronger than 800 MPa, the forming pressure during granulation is exceeded and granulated bodies cannot be formed.

[0034] 〔Interplanar spacing d(002) of crystal lattice〕 The interplanar spacing d(002) of the carbon fiber was measured by X-ray diffraction method (XRD). Specifically, it was measured according to the method described in JIS R7651:2007. Ultima IV (manufactured by Rigaku Corporation) was used for the measurement, and CuKα ray was used as the X-ray source for the measurement. The enclosed tube voltage was 40 kV and the current was 40 mA. In the present embodiment, the interplanar spacing d(002) of the carbon fiber is particularly preferably 3.576 nm or more and 3.579 nm or less. When the interplanar spacing d(002) is higher than 3.579 nm, the moisture content of the carbon fiber decreases, the mixing with water becomes insufficient, and granulated bodies cannot be formed. When the interplanar spacing d(002) is lower than 3.576 nm, the compressive strength of the granulated body decreases, the shape cannot be maintained, and the handleability is poor.

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

[0036] In this embodiment, when the total mixture of carbon fiber, water, and binder is 100% by mass, the carbon fiber is preferably 76.2% by mass or more and 81.6% by weight or less, the water is preferably 15.2% by mass or more and 16.3% by mass or less, and the binder is preferably 2.0% by mass or more and 8.6% by mass or less in terms of solid content. When the carbon fiber, water, and binder are within the above ranges, the granulation property and the shape of the granule are good.

Examples

[0037] Hereinafter, the carbon fiber granule and the method for manufacturing the carbon fiber granule of the present invention will be described based on the examples and comparative examples described in Table 1, but the present invention is not limited to the following examples.

[0038]

Table 1

[0039] The granulation property and the shape of the carbon fiber granule based on the compounding conditions shown in Table 1 were evaluated according to the following criteria. (Granulation property) ○: Extruded while maintaining a cylindrical shape from the die hole. ×: Not extruded while maintaining a cylindrical shape from the die hole. (Shape of granule) ○: A shape that can be visually recognized as cylindrical. ×: Not a shape that can be visually recognized as cylindrical.

[0040] (Example 1) As the carbon fiber, isotropic pitch-based carbon fiber (product name: Donacarb Mild S-244 (Donacarb is a registered trademark), average fiber length of about 0.68 mm, average fiber diameter of about 13.0 μm, moisture content of 6% by mass, crystal lattice plane spacing of 3.577 nm, manufactured by Osaka Gas Chemical 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 a nylon-based emulsion resin (product name: Sepoljyon PA200 (Sepoljyon is a registered trademark), solid content ratio of 40%, manufactured by Sumitomo Seika Chemical Co., Ltd.) as a binder and 96 g of water, and stirred for 5 minutes to thoroughly mix the above materials. Next, the mixture obtained above was extruded and granulated using an extrusion granulation device (product name: Disk Pelletter, manufactured by Fuji Paudal Co., Ltd.) set with a screen diameter of 3 mm. This granulated product was dried in a dryer at 130 °C for 3 hours to obtain a carbon fiber granulated body having a cylindrical shape with a diameter of 3 mm and an average length of about 6 mm (No. 11 in Table 1). The granulation property when the mixture was extruded and granulated and the shape of the granulated body after drying were good, and the compressive strength of the obtained carbon fiber granulated body was 16.5 N. Incidentally, the isotropic pitch-based carbon fiber is obtained by pulverizing a carbon fiber batt (product name: Donacarb Mat, manufactured by Osaka Gas Chemical Co., Ltd.), and the tensile strength of the carbon fiber batt was 588 MPa.

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

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

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

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

[0045] (Comparative Example 2) Carbon fiber granulates were obtained with the formulation shown in No. 6 of Table 1. In No. 6, the amount of water blended was large, and the mixture became a fluid slurry, and granulates could not be obtained.

[0046] (Comparative Example 3) Carbon fiber granulates were obtained with the formulation shown in No. 7 of Table 1. The conditions of No. 7 were less than the amount of water used in No. 6. However, since the amount of water was large, the mixture became a fluid slurry and granulates could not be obtained.

[0047] (Comparative Example 4) Carbon fiber granulates were obtained with the formulation shown in No. 9 of Table 1. Under the conditions of No. 9, since no water was added, granulates could not be obtained.

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

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

[0050] (Comparative Example 7) Using powdered polyethylene resin (trade name: Hiwax 160H, manufactured by Mitsui Chemicals, Inc.) as a binder, a mixture was prepared with the formulation shown in No. 3 of Table 1, and carbon fiber granulates were obtained. The amount of water added was larger than when nylon emulsion was used, drying took a long time, and the load during drying increased.

[0051] (Comparative Example 8) Using the formulation shown in No. 4 of Table 1, a mixture was prepared using powdered polyethylene resin (trade name: Hiwax 160H, manufactured by Mitsui Chemicals, Inc.) as a binder to obtain carbon fiber granulates. The amount of water was larger compared to the case where nylon emulsion was used, resulting in a longer drying time and a greater drying load.

[0052] (Comparative Example 9) Using the formulation shown in No. 5 of Table 1, a mixture was prepared using powdered polyethylene resin (trade name: Hiwax 160H, manufactured by Mitsui Chemicals, Inc.) as a binder to obtain carbon fiber granulates. Under the conditions of No. 5, since the amount of water was large, the mixture became a fluid slurry, and the mixture could not maintain a cylindrical shape and come out from the die holes of the disk pelleter.

[0053] (Comparative Example 10) Using the formulation shown in No. 12 of Table 1, carbon fiber granulates were obtained. Under the conditions of No. 12, since the blending ratio (mass%) of the binder was small, the granulates were not in a shape that could be visually recognized as cylindrical, and the compressive strength was also low.

[0054] (Comparative Example 11) Using the formulation shown in No. 13 of Table 1, carbon fiber granulates were obtained. Under the conditions of No. 13, since the blending ratio (mass%) of the binder was small, the granulates were not in a shape that could be visually recognized as cylindrical, and the compressive strength was also low.

[0055] (Comparative Example 12) Using the formulation shown in No. 14 of Table 1, carbon fiber granulates were obtained. Under the conditions of No. 14, since the blending ratio (mass%) of the binder was large, defective shapes occurred during extrusion granulation, and granulates could not be obtained.

[0056] As described above, according to Examples 1 to 4 (No. 8, 10, 11, 16 in Table 1), granulation properties and the shape of the granulates are good, and even for carbon fibers with relatively long fiber lengths, carbon fiber granulates can be obtained that are excellent in stable supply by a feeder and fiber scattering prevention, and ensure dispersibility when mixed with a resin.

[0057] 〔Alternative Embodiment〕 In the above embodiment, when mixing carbon fiber, water, and a binder, a second component can be mixed. For example, molybdenum disulfide, polytetrafluoroethylene, amorphous carbon, glass fiber, graphite, etc. may be mixed.

[0058] Furthermore, the configurations disclosed in the above embodiments (including alternative embodiments, the same applies hereinafter) can be applied in combination with the configurations disclosed in other embodiments as long as there is no contradiction. Also, the embodiments disclosed in this specification are illustrative, and the embodiments of the present invention are not limited thereto, and can be appropriately modified within the scope not departing from the object of the present invention.

Claims

1. 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% by mass or more and 12.0% by mass or less in terms of solid content based on the mass of the carbon fibers, The carbon fiber granule having 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.

2. The carbon fiber granule according to claim 1, wherein the carbon fiber is an isotropic pitch-based carbon fiber having a crystal lattice plane spacing d(002) of 3.576 nm or more and 3.579 nm or less by X-ray diffraction method.

3. The carbon fiber granule according to claim 1 or 2, wherein the binder is a nylon-based emulsion resin.

4. A step of mixing water and a binder with carbon fibers having an average fiber diameter of 5 μm or more and 20 μm or less, an average fiber length of 0.10 mm or more and 1.50 mm or less, and a moisture content of 3% by mass or more and 15% by mass or less to obtain a mixture; A step of extruding and granulating the mixture to obtain granules; A method for producing a carbon fiber granule, comprising a step of heating the granule to 130°C or more and 150°C or less and drying it.

5. The proportion of the carbon fiber in the mixture is 76.2% by mass or more and 81.6% by mass or less, the proportion of the water is 15.2% by mass or more and 16.3% by mass or less, the proportion 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. The outer diameter of the carbon fiber granule is 1.0 mm or more and 10.0 mm or less, the length is 2 mm or more and 15 mm or less, and the compressive strength is 10 N or more and 20 N or less. The method for producing a carbon fiber granule according to claim 4.

6. The method for producing a carbon fiber granule according to claim 4 or 5, wherein the carbon fiber is obtained by pulverizing a carbon fiber flock having a tensile strength of 100 MPa or more and 800 MPa or less, and is an isotropic pitch-based carbon fiber having a crystal lattice plane spacing d(002) of 3.576 nm or more and 3.579 nm or less by X-ray diffraction method.

7. The method for producing a carbon fiber granule according to claim 4 or 5, wherein the binder is a nylon-based emulsion resin.