Method of producing carbon fiber bundle composite and method of producing carbon fiber composite sheet

Abstract

This method of producing a carbon fiber bundle composite comprises: mixing carbon fiber cotton made of short carbon fiber and a bundling solution containing an uncured thermosetting resin to obtain a carbon fiber bundle containing the bundling solution; and causing the bundling solution to contain a curing agent, thus making it possible to thermally cure the bundling solution. This method of producing a carbon fiber composite sheet comprises: mixing carbon fiber cotton made of short carbon fiber and a bundling solution containing an uncured thermosetting resin to obtain a carbon fiber bundle containing the bundling solution; causing the bundling solution to contain a curing agent, thus making it possible to thermally cure the bundling solution; and causing a plurality of the aforementioned carbon fiber bundles, after the bundling solution was made to contain the curing agent, to be fixed to one another.

Description

Method for manufacturing carbon fiber bundle composite and method for manufacturing carbon fiber composite sheet

[0001] The present invention mainly relates to a method for producing a carbon-fiber bundle composite and a method for producing a carbon-fiber composite sheet.

[0002] CFRP (Carbon Fiber Reinforced Plastic), a fiber-reinforced plastic that uses carbon fiber as a reinforcing material, is a lightweight material with excellent mechanical properties that makes it suitable for parts of automobiles, ships, railway vehicles, manned aircraft, unmanned aircraft, and other transportation equipment, and its importance has been increasing in recent years.

[0003] It has been proposed to produce carbon fiber reinforced thermoplastics by adding carbon fiber pellets to a thermoplastic resin. The carbon fiber pellets are produced by mixing short carbon fibers with a solution or suspension of a sizing agent to form carbon fiber aggregates, pelletizing the aggregates using a disc pelletizer, and then drying the pellets (Patent Document 1).

[0004] It is known that needle-shaped self-assembled carbon fiber bundles can be obtained by dispersing short carbon fibers in water, adding a small amount of chloroform, and shaking the mixture vigorously, and that polyetherimide can be supported on the needle-shaped self-assembled carbon fiber bundles by dissolving polyetherimide in the chloroform in advance (Non-Patent Document 1).

[0005] Special Publication No. 10-503812

[0006] JR Baxter, GR Palmese, NJ Alvarez, Applied Materials Today 20 (2020) 100786

[0007] An object of the present invention is to provide a novel method for producing a prepreg material using short carbon fibers, which can use either virgin carbon fibers or recycled carbon fibers as a raw material. Another object of the present invention is to provide a method for producing a prepreg material using short carbon fibers, which includes a step of aggregating short carbon fibers using a bundling liquid, but does not require a step of removing the solvent in the bundling liquid. Problems that can be solved by each embodiment of the present invention may be explicitly or implicitly stated in this specification.

[0008] According to one aspect of the present invention, there is provided a method for producing a carbon fiber bundle composite, the method including: mixing carbon fiber batting made of short carbon fibers with a bundling liquid containing an uncured thermosetting resin to obtain a carbon fiber bundle containing the bundling liquid; and adding a curing agent to the bundling liquid to make the bundling liquid thermosetting.

[0009] According to another aspect of the present invention, there is provided a method for producing a carbon fiber bundle composite, the method comprising: forming a carbon fiber bundle containing a bundling liquid by aggregating discontinuous carbon fibers with the bundling liquid, wherein the bundling liquid is a resin composition containing an uncured thermosetting resin and a curing agent.

[0010] According to yet another aspect of the present invention, there is provided a method for manufacturing a carbon fiber composite sheet, including: mixing carbon fiber batting made of short carbon fibers with a bundling liquid containing an uncured thermosetting resin to obtain carbon fiber bundles containing the bundling liquid; adding a curing agent to the bundling liquid to make the bundling liquid thermally curable; and bonding a plurality of the carbon fiber bundles to each other after the curing agent has been added to the bundling liquid.

[0011] According to yet another aspect of the present invention, there is provided a method for producing a carbon fiber composite sheet, the method including: forming a carbon fiber bundle containing a bundling liquid by aggregating discontinuous carbon fibers with the bundling liquid; and fixing a plurality of the carbon fiber bundles to each other, wherein the bundling liquid is a resin composition containing an uncured thermosetting resin and a curing agent.

[0012] According to yet another aspect of the present invention, there is provided a method for producing a carbon fiber composite sheet, comprising: bringing a plurality of carbon fiber bundles into contact with each other and thickening a bundling liquid contained in each of the plurality of carbon fiber bundles to fix the plurality of carbon fiber bundles to each other, wherein discontinuous carbon fibers in each of the plurality of carbon fiber bundles are aggregated by the bundling liquid, and the bundling liquid is a resin composition containing an uncured thermosetting resin, a curing agent, and a thickener.

[0013] According to yet another aspect of the present invention, there is provided a method for producing a carbon fiber composite sheet, the method including: depositing a plurality of carbon fiber bundles to form a carbon fiber bundle layer; compressing the carbon fiber bundle layer; and thereafter thickening a bundling liquid contained in each of the plurality of carbon fiber bundles, wherein discontinuous carbon fibers in each of the plurality of carbon fiber bundles are aggregated by the bundling liquid, and the bundling liquid is a resin composition containing an uncured thermosetting resin, a curing agent, and a thickener.

[0014] According to one embodiment of the present invention, there is provided a novel method for producing a prepreg material using short carbon fibers, which can use either virgin carbon fibers or recycled carbon fibers as a raw material. According to one embodiment of the present invention, there is provided a method for producing a prepreg material using short carbon fibers, which includes a step of aggregating short carbon fibers using a bundling liquid, but does not require a step of removing the solvent in the bundling liquid.

[0015] Fig. 1 is a schematic diagram illustrating the relationship between the bundle length of a carbon fiber bundle composite and the fiber length of the carbon fibers constituting the carbon fiber bundle composite. Fig. 2 is a photograph of a carbon fiber bundle composite having a seed-like appearance. Fig. 3 is a conceptual diagram showing a manufacturing apparatus for a carbon fiber composite sheet. Fig. 4 is a photograph showing a carbon fiber bundle composite placed in a zippered plastic bag and crushed. Fig. 5 is a photograph showing a carbon fiber composite sheet.

[0016] When viscosity values ​​are referred to in this specification, unless otherwise specified, they refer to values ​​measured using a rotational viscometer (for example, HAAKE MARS 40 manufactured by Thermo Fisher Scientific) under the following conditions: measurement mode: constant stress, stress value: 300 Pa, frequency: 1.59 Hz, plate diameter: 25 mm, plate type: parallel plate, plate gap: 0.5 mm.

[0017] Carbon fibers are typically produced as continuous fibers long enough to be wound onto a spool. Short carbon fibers are produced by cutting continuous carbon fibers. Short carbon fibers can also be called chopped or discontinuous carbon fibers.

[0018] Hereinafter, embodiments of the present invention will be described in detail.

[0019] 1. Manufacturing Method of Carbon Fiber Bundle Composite One aspect of the present invention relates to a manufacturing method of a carbon fiber bundle composite (hereinafter sometimes abbreviated as "CBC"). This manufacturing method includes mixing carbon fiber wadding made of short carbon fibers with a bundling liquid containing an uncured thermosetting resin to obtain a carbon fiber bundle containing the bundling liquid, and adding a curing agent to the bundling liquid to make the bundling liquid heat-curable. Below, manufacturing methods according to embodiments will be described, divided into a method using virgin carbon fiber as a starting material and a method using recycled carbon fiber as a starting material.

[0020] 1.1 Method Using Virgin Carbon Fiber as a Starting Material The method using virgin carbon fiber as a starting material typically includes the following steps (i) to (iii): (i) Chopping step, (ii) Defibration step, and (iii) Bundling step. Each step is described in detail below.

[0021] (i) Chopping Step In the chopping step, a continuous carbon fiber bundle made of virgin carbon fibers is cut at predetermined intervals in the fiber direction using, for example, a rotary cutter to form chopped carbon fiber bundles.

[0022] The bundle size of the continuous carbon fiber bundle (the number of carbon fiber filaments constituting the bundle) is, for example, 10K or more, and may be 12K or more, 15K or more, 24K or more, 36K or more, 48K or more, or 50K or more. There is no particular upper limit, but it is, for example, 100K or less. Here, "K" is a symbol representing 1000, so for example, 1K means 1000 and 10K means 10000.

[0023] The larger the bundle size of the continuous carbon fiber bundle, the greater the number of carbon fiber filaments contained in one piece of chopped carbon fiber bundle, and therefore the higher the production efficiency of CBC. In addition, the production cost of the continuous carbon fiber bundle also decreases with increasing bundle size. Therefore, the bundle size of the continuous carbon fiber bundle is preferably 24K or more, more preferably 36K or more, and even more preferably 48K or more.

[0024] The diameter of the carbon fiber filament is generally within the range of 5 μm to 15 μm in the case of PAN-based carbon fibers made from polyacrylonitrile fibers.

[0025] The fiber length of the chopped carbon fiber bundles is set to the length required for the carbon fibers that make up the CBC to be manufactured, since there is no step of intentionally cutting the carbon fibers after the chopping step.

[0026] The fiber length of the chopped carbon fiber bundle is not limited, but may be, for example, 3 mm or more, 5 mm or more, or 10 mm or more, or may be, for example, 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less. The longer the fiber length of the chopped carbon fiber bundle, the better the mechanical properties of the CFRP product molded using the produced CBC. On the other hand, the shorter the fiber length of the chopped carbon fiber bundle, the more suitable the produced CBC is for molding CFRP products having complex shapes. This is because the shorter the carbon fibers contained in the CBC, the more easily it tends to flow in a molding die.

[0027] The bundle length of a CBC formed from multiple short carbon fibers having the same fiber length is typically longer than the fiber length of the short carbon fibers, as shown in Figure 1. In one example, the bundle length of the CBC may be more than two times, or even more than three times, the fiber length of the short carbon fibers.

[0028] (ii) Defibration Step In the defibration step, the chopped carbon fiber bundles obtained in the chopping step are loosened to obtain carbon fiber wads. Defibration is preferably carried out so that all of the carbon fibers contained in the chopped carbon fiber bundles are in the form of monofilaments, but this is not essential. That is, the carbon fiber wads obtained in the defibration step may include ultrafine carbon fiber bundles consisting of a small number of filaments, for example, less than 100.

[0029] In the fiber-opening step, a general fiber-opening machine can be used, but there is no limitation thereto.

[0030] For example, chopped carbon fiber bundles can be defibrated by adding only the chopped carbon fiber bundles to a stirring mixer such as a Henschel mixer and stirring them in a dry state. This method has the advantage that the resulting carbon fiber floss can be advanced to the next bundling step without being removed from the stirring mixer.

[0031] For example, chopped carbon fiber bundles can be defibrated by immersing them in an organic solvent, such as acetone, that can dissolve the sizing agent contained in the chopped carbon fiber bundles, and then irradiating them with ultrasonic waves. After the sizing agent is washed away, flocculent carbon fibers remain.

[0032] (iii) Bundling Step In the bundling step, the carbon fiber wad obtained in the defibrating step is mixed with a bundling liquid. The bundling liquid is a resin composition. The carbon fiber filaments or ultrafine carbon fiber bundles that make up the carbon fiber wad are aggregated by capillary force based on the surface tension of the bundling liquid to form carbon fiber bundles containing the bundling liquid.

[0033] <Viscosity of bundling liquid> The lower the viscosity of the bundling liquid, the easier it is to mix it with the carbon fiber floss, and the shorter the time required to form the carbon fiber bundles. From this viewpoint, the viscosity of the bundling liquid when mixed with the carbon fiber floss in the bundling step is preferably 10 Pa s or less, more preferably 5 Pa s or less, and even more preferably 1 Pa s or less. There is no particular lower limit to this viscosity, but it may be, for example, 0.001 Pa s or more.

[0034] In the bundling step, the bundling liquid may be heated before use. For example, a bundling liquid having a viscosity of 10 Pa s at 25° C. may be used without heating when the bundling step is performed in a room at 25° C., but when the bundling step is performed in a room at 15° C., it is preferable to heat the bundling liquid until the viscosity becomes 10 Pa s or less before use.

[0035] To prevent a bundling liquid having a viscosity of 10 Pa·s at 50°C from hardening during heating to 50°C and mixing with carbon fiber floss, a curing agent that does not harden at 50°C or below can be selected as the curing agent contained in the bundling liquid. For a bundling liquid having a viscosity of 10 Pa·s at 40°C, selecting a curing agent that does not harden at 40°C or below will prevent the bundling liquid from hardening during heating to 40°C and mixing with carbon fiber floss. Thus, the lower the temperature at which the viscosity of the bundling liquid reaches 10 Pa·s, the wider the options for selecting a curing agent that will not harden the bundling liquid when mixed with carbon fiber floss at that temperature. Therefore, the viscosity of the bundling liquid is preferably 10 Pa·s or less at 50°C, more preferably 10 Pa·s or less at 40°C, and even more preferably 10 Pa·s or less at 30°C.

[0036] The viscosity of the bundling liquid can be adjusted by reactive diluents, as described below.

[0037] <Thermosetting Resin> The bundling liquid contains an uncured thermosetting resin. There are no limitations on the type of thermosetting resin, but suitable examples include epoxy resin, vinyl ester resin, unsaturated polyester resin, and resol-type phenolic resin. Examples of thermosetting resins that can be contained in the bundling liquid further include various (meth)acrylates (compounds having an acryloyl group or a methacryloyl group) other than epoxy vinyl ester, and diallyl phthalate. The various (meth)acrylates include alkyl (meth)acrylates that may have a substituent on the alkyl group, and urethane acrylate. The bundling liquid may contain two or more types of thermosetting resins.

[0038] <Epoxy Resin> There is no limitation on the type of epoxy resin that can be contained in the bundling liquid, and various types of epoxy resins can be used, including bisphenol-type epoxy resins, naphthalene-type epoxy resins, biphenyl-type epoxy resins, novolac-type epoxy resins, glycidylamine-type epoxy resins, epoxy resins having an oxazolidone ring structure, alicyclic epoxy resins, and aliphatic epoxy resins. When epoxy resins with different viscosities are used in combination, the viscosity of the bundling liquid generally decreases as the amount of the resin with a lower viscosity is increased.

[0039] Epoxy resins that can be particularly preferably contained in the bundling liquid are bisphenol-type epoxy resins such as bisphenol A-type epoxy resins (epoxy resins whose main component is diglycidyl ether of bisphenol A) and bisphenol F-type epoxy resins (epoxy resins whose main component is diglycidyl ether of bisphenol F). Among commercially available bisphenol-type epoxy resins, there are varieties having a viscosity of 5 Pa s or less at 25°C, which can be preferably used.

[0040] For example, 50 wt % or more, 60 wt % or more, 65 wt % or more, 70 wt % or more, or 75 wt % or more of the total epoxy resin contained in the bundling liquid may be bisphenol-type epoxy resin. Bisphenol-type epoxy resin, particularly bisphenol A-type epoxy resin, has the advantages of relatively small shrinkage during curing and relatively good heat resistance of the cured product. Therefore, the bundling liquid preferably contains at least one of bisphenol A-type and bisphenol F-type epoxy resin, and more preferably contains bisphenol A-type epoxy resin.

[0041] <Vinyl ester resin> The type of vinyl ester resin that can be contained in the bundling liquid is not particularly limited, but typical examples include bisphenol vinyl ester resins and novolac vinyl ester resins. Bisphenol vinyl ester resins are produced by esterifying a bisphenol epoxy resin, such as a bisphenol A epoxy resin, with an unsaturated monocarboxylic acid, typically acrylic acid or methacrylic acid. Novolac vinyl ester resins are produced by esterifying a novolac epoxy resin, such as a phenol novolac epoxy resin, with an unsaturated monocarboxylic acid, typically acrylic acid or methacrylic acid.

[0042] <Unsaturated Polyester Resin> The type of unsaturated polyester resin that can be contained in the bundling liquid is not particularly limited. The unsaturated polyester resin is mainly composed of a polymer containing a structure obtained by polycondensation of a dicarboxylic acid, such as fumaric acid, maleic acid, and / or maleic anhydride, with a diol.

[0043] <Reactive Diluent> As described above, the viscosity of the bundling liquid can be adjusted by a reactive diluent. The compound contained in the bundling liquid as the reactive diluent may be only one type, or two or more types.

[0044] When the bundling liquid contains an epoxy resin, a known reactive diluent for epoxy resins can be used as appropriate. Examples of reactive diluents for epoxy resins that have a high viscosity-reducing effect include, but are not limited to, monoepoxide compounds having only one epoxy group in the molecule.

[0045] Methylhexahydrophthalic anhydride and tetrahydromethylphthalic anhydride can be considered as reactive diluents because they function as both thickeners or hardeners for epoxy resins and as diluents.

[0046] When at least one of a vinyl ester resin and an unsaturated polyester resin is blended into the bundling liquid, a compound having one or two ethylenically unsaturated groups in the molecule and having a low viscosity of preferably 1 Pa·s or less at 25°C can be blended together as a reactive diluent. Examples of such compounds include styrene-based compounds such as styrene, methylstyrene, halogenated styrene, and divinylbenzene; methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, benzyl (meth)acrylate, methylbenzyl (meth)acrylate, phenoxyethyl (meth)acrylate, methylphenoxyethyl (meth)acrylate, morpholine (meth)acrylate, phenylphenoxyethyl acrylate, phenylbenzyl (meth)acrylate, phenyl methacrylate, and divinyl acrylate. Examples of the hydroxyl group-containing (meth)acrylate include monofunctional (meth)acrylates such as clopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, and dicyclopentanyl methacrylate; hydroxyl group-containing (meth)acrylates such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate; and bifunctional (meth)acrylates such as ethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, bisphenol di(meth)acrylate, and 1,4-cyclohexanedimethanol di(meth)acrylate.

[0047] <Thickener> In a preferred example, a thickener may be added to the bundling liquid to increase the viscosity of the bundling liquid after the bundling process, for example, to 50 Pa·s or more at 25°C. By thickening the bundling liquid in this manner, the shape of the CBC formed in the bundling process is stabilized. For example, when a large amount of CBCs are packed into a large-capacity container for transportation or storage, strong pressure due to their own weight is applied to the CBCs near the bottom of the container. In this case, if the bundling liquid is thickened, the CBCs are less likely to collapse, stick together, or exude liquid components from the CBCs.

[0048] When a thickener is used, it is preferable to blend the thickener into the bundling liquid immediately before mixing the bundling liquid with the carbon fiber cotton. In one embodiment, the bundling liquid may be divided into two liquids: a first liquid that does not contain a thickener and a second liquid that contains a thickener. In this embodiment, the first liquid and the carbon fiber cotton may be mixed, and then the second liquid may be mixed with the mixture obtained thereby, or the order may be reversed, and the second liquid and the carbon fiber cotton may be mixed, and then the first liquid may be mixed with the mixture obtained thereby. In this embodiment, the first liquid and the second liquid are mixed during the bundling process to complete the bundling liquid.

[0049] The type and amount of the thickener to be blended are determined so that the viscosity of the bundling liquid does not increase during the bundling process, causing a hindrance to the formation of carbon fiber bundles. When the viscosity of the bundling liquid is 10 Pa s or less, the bundling process can be completed within one hour. Therefore, in a preferred example, the type and amount of the thickener to be blended in the bundling liquid can be determined so that the viscosity does not exceed 10 Pa s when the bundling liquid is kept at the temperature at which it is mixed with the carbon fiber cotton for one hour.

[0050] The viscosity of the bundling liquid containing the thickener after thickening may be 100 Pa·s or more, further 200 Pa·s or more, further 1000 Pa·s or more, further 2000 Pa·s or more, or even 5000 Pa·s or more at 25°C, and may be a value outside the range measurable using a rotational viscometer.

[0051] When the CBC formed in the bundling step is used to produce a carbon fiber composite sheet described later, the viscosity of the bundling liquid containing the thickener after thickening can be, for example, in the range of 200 Pa s to 100,000 Pa s. When a bundling liquid with an appropriately low viscosity after thickening is used, a flexible carbon fiber composite sheet can be obtained.

[0052] In one example, after the bundling step, the carbon fiber bundles formed in the bundling step may be held at a temperature higher than room temperature (a temperature in a range of 20°C ± 15°C), i.e., a temperature exceeding 35°C, to promote thickening of the bundling liquid contained in the carbon fiber bundles. The holding temperature and holding time are set within a range in which hardening of the bundling liquid due to the action of a curing agent does not occur. In one example, the carbon fiber bundles formed in the stirring tank of the stirring mixer can be thickened while being stirred in the stirring tank without being removed from the stirring tank. In another example, the carbon fiber bundles formed in the stirring tank of the stirring mixer may be transferred to a disc pelletizer and rolled to thicken the bundling liquid contained in the carbon fiber bundles.

[0053] Examples of thickeners that can be used when blending an epoxy resin into the bundling liquid include polyisocyanates, carboxylic acid anhydrides, and amines. Suitable examples of polyisocyanates include diisocyanates with an aromatic ring in their molecular structure, such as bis(4-isocyanatophenyl)methane and toluene diisocyanate, and aliphatic diisocyanates, such as isophorone diisocyanate and hexamethylene diisocyanate. Polymeric MDI (crude MDI) is also a suitable example of polyisocyanates. Low-viscosity polyisocyanates (preferably 1 Pa·s or less at 25°C) can also function as a reactive diluent for a while after being added to the bundling liquid.

[0054] The polyisocyanate may be blended into the bundling liquid together with a polyol. Examples of polyols include ethylene glycol, polyethylene glycol, isosorbide, neopentyl glycol, cyclohexanediol, cyclohexanedimethanol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, and 1,6-hexanediol. A polyol with a low viscosity (preferably 1 Pa·s or less at 25°C) can also function as a reactive diluent for a while after being added to the bundling liquid.

[0055] Suitable examples of carboxylic acid anhydrides include methylhexahydrophthalic anhydride and tetrahydromethylphthalic anhydride, which, as mentioned above, also function as reactive diluents for a period of time after being added to the bundling liquid.

[0056] Suitable examples of the amine include isophoronediamine, bis(4-aminocyclohexyl)methane and 1,3-bis(aminomethyl)cyclohexane.

[0057] The bundling liquid containing the epoxy resin may contain styrene and maleic anhydride as thickeners together with a radical polymerization initiator. In this case, the action of the radical polymerization initiator copolymerizes styrene and maleic anhydride, thereby thickening the bundling liquid. Styrene before polymerization can also function as a reactive diluent.

[0058] Preferred thickeners that can be used when compounding the vinyl ester resin and / or unsaturated polyester resin in the bundling liquid include polyisocyanates, alkaline earth metal hydroxides such as magnesium hydroxide and calcium hydroxide, and alkaline earth metal oxides such as magnesium oxide and calcium oxide.

[0059] Suitable examples of polyisocyanates are the same as those suitable for use as thickeners when epoxy resins are blended into the bundling liquid. A polyisocyanate having a low viscosity (preferably 1 Pa s or less at 25°C) can also function as a reactive diluent for a while after being added to the bundling liquid.

[0060] The polyisocyanate may be blended into the bundling liquid together with a polyol. Examples of the polyol are the same as the examples of the polyol that can be blended together with the polyisocyanate when the epoxy resin is blended into the bundling liquid. A low-viscosity polyol (preferably 1 Pa·s or less at 25°C) can also function as a reaction diluent for a while after being added to the bundling liquid.

[0061] Examples of thickeners that can be preferably used when blending a resole-type phenolic resin into the bundling liquid include alkaline earth metal hydroxides such as magnesium hydroxide and calcium hydroxide, alkaline earth metal oxides such as magnesium oxide and calcium oxide, and polyisocyanates.

[0062] Even when the thermosetting resin blended into the bundling liquid does not fall into any of the following categories: epoxy resin, vinyl ester resin, unsaturated polyester resin, and resol-type phenolic resin, polyisocyanate can be suitably used as a thickener. When the thermosetting resin does not contain a hydroxyl group-containing component, polyisocyanate can be blended into the bundling liquid together with a polyol. Suitable examples of polyisocyanate and polyol are the same as the suitable examples of polyisocyanate and polyol that can be used when an epoxy resin is blended into the bundling liquid.

[0063] <Curing Agent> The bundling liquid is made thermosetting by containing a curing agent that is appropriate for the thermosetting resin that is blended.

[0064] When the bundling liquid contains an epoxy resin, a curing agent for the epoxy resin (hereinafter also referred to as an "epoxy curing agent") is added to the bundling liquid.

[0065] Typical examples of epoxy hardeners include dicyandiamides, phenols including novolacs, amines, carboxylic acid anhydrides, thiols and imidazoles.

[0066] Particularly preferred epoxy curing agents are latent curing agents, i.e., curing agents that are solids with low solubility in epoxy resins at room temperature, but melt or dissolve in epoxy resins when heated to a predetermined temperature, thereby exhibiting a curing effect.

[0067] Imidazoles, dicyandiamide and boron trifluoride-amine complexes are typical examples of latent hardeners.

[0068] Imidazoles are compounds having an imidazole ring, and include substituted imidazoles in which the hydrogen atoms of imidazole are substituted with substituents, as well as imidazolium salts and imidazole complexes.

[0069] Examples of the substituted imidazole preferred as the latent curing agent include substituted imidazoles having an aromatic ring, which may be a heteroaromatic ring, in the molecule, such as 2,4-diamino-6-[2'-methylimidazolyl-(1')]-ethyl-s-triazine, 2-phenyl-4-methylimidazole, 2-phenyl-4-methyl-5-hydroxymethylimidazole, 2-phenyl-4-benzyl-5-hydroxymethylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole, 2-para-toluyl-4-methyl-5-hydroxymethylimidazole, 2-para-toluyl-4,5-dihydroxymethylimidazole, 2-meta-toluyl-4-methyl-5-hydroxymethylimidazole, 2-meta-toluyl-4,5-dihydroxymethylimidazole and 1-cyanoethyl-2-phenylimidazole.

[0070] Imidazolium salts such as 1-cyanoethyl-2-ethyl-4-methylimidazolium trimellitate, 1-cyanoethyl-2-undecylimidazolium trimellitate, and 1-cyanoethyl-2-phenylimidazolium trimellitate are also suitable examples of the imidazole-based latent curing agent.

[0071] Isocyanuric acid adducts of various substituted imidazoles including 2-phenylimidazole, 2-methylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole and 2-phenyl-4-methyl-5-hydroxymethylimidazole, in particular, isocyanuric acid adducts of substituted imidazoles having a triazine ring such as 2,4-diamino-6-(2'-methylimidazolyl-(1'))-ethyl-s-triazine, 1-(4,6-diamino-s-triazin-2-yl)ethyl-2-undecylimidazole and 2,4-diamino-6-[2-(2-ethyl-4-methyl-1-imidazolyl)ethyl]-s-triazine are particularly preferred imidazole-based latent curing agents.

[0072] Amine adducts are also suitable examples of latent curing agents. Amine adducts are polymerized by reacting imidazole and / or tertiary amine with epoxy resin and / or isocyanate, and have relatively low solubility in epoxy resins.

[0073] The latent curing agent may be used alone or in combination of two or more kinds.

[0074] When dicyandiamide is used as an epoxy curing agent, it is preferable to use a urea derivative such as 4,4'-methylenebis(phenyldimethylurea) or 2,4-bis(3,3-dimethylureido)toluene in combination as a curing accelerator.

[0075] When the bundling liquid contains a vinyl ester resin and / or an unsaturated polyester resin, a radical polymerization initiator is added to the bundling liquid as a curing agent. The radical polymerization initiators may be used singly or in combination of two or more. Typical examples of radical polymerization initiators are organic peroxides such as ketone peroxides, hydroperoxides, diacyl peroxides, dialkyl peroxides, peroxyketals, alkyl peresters, and percarbonates.

[0076] Suitable examples of organic peroxides include 1,1-di(t-butylperoxy)cyclohexane, t-butylperoxyisopropyl carbonate, t-amylperoxyisopropyl carbonate, methyl ethyl ketone peroxide, t-butylperoxybenzoate, benzoyl peroxide, dicumyl peroxide, acetylacetone peroxide and cumene hydroperoxide.

[0077] The radical polymerization initiator preferably has a 10-hour half-life temperature in the range of 70 to 120°C, more preferably in the range of 80 to 110°C, and even more preferably in the range of 90 to 100°C. The 10-hour half-life temperature is the temperature at which the half-life of a polymerization initiator dissolved in benzene when thermally decomposed at a constant temperature is 10 hours, and for measurement, a benzene solution containing the polymerization initiator at a concentration of 0.2 mol / L is used. For organic peroxides whose 10-hour half-life temperature falls within the above preferred range, reference can be made, for example, to International Publication No. 2019 / 017254.

[0078] When the bundling liquid contains a radical polymerization initiator, it is preferable to also contain a radical polymerization inhibitor in the bundling liquid. Radical polymerization inhibitors are well known to those skilled in the art, and suitable examples include catechol, hydroquinone, benzoquinone, and nitroso compounds.

[0079] When a resol type phenolic resin is blended into the bundling liquid, examples of components that can be contained in the bundling liquid as a curing agent include organic acids such as benzenesulfonic acid, paratoluenesulfonic acid, xylenesulfonic acid, and phenolsulfonic acid; inorganic acids such as phosphoric acid, hydrochloric acid, and sulfuric acid; amines such as triethylamine, tri-n-propylamine, diethylamine, n-propylamine, n-butylamine, aniline, and benzylamine; and reaction products of isocyanates with primary amines and / or secondary amines.

[0080] <Optional Components> The components that can be contained in the bundling liquid are not limited to those described above. Examples of components that can be contained in the bundling liquid further include low-profile agents, antioxidants, internal mold release agents, colorants, modifiers (e.g., rubber, elastomers, or thermoplastic resins), flame retardants, fillers, and antibacterial agents.

[0081] Examples of low-profile additives include polyethylene, polystyrene, styrene-based thermoplastic elastomers, crosslinked polystyrene, polyvinyl acetate-polystyrene block copolymers, polyvinyl acetate, polymethyl methacrylate, and saturated polyester resins. Examples of fillers include inorganic fillers, which may be oxides, hydroxides, carbonates, sulfates, silicates, phosphates, or clay minerals. Other examples of fillers include organic fillers, which may be cured thermosetting resins such as phenolic resins and resorcinol resins.

[0082] Flame retardants that can be contained in the bundling liquid are as follows: Preferred flame retardants include phosphorus-containing flame retardants. Examples of phosphorus-containing flame retardants include non-halogen phosphate esters such as trimethyl phosphate, triethyl phosphate, tributyl phosphate, trioctyl phosphate, tributoxyethyl phosphate, triphenyl phosphate, tricresyl phosphate, cresyl diphenyl phosphate, octyl diphenyl phosphate, and aromatic polyphosphates.

[0083] Other examples of phosphorus-containing flame retardants include halogenated phosphate esters such as tris(chloroethyl)phosphate, tris(dichloropropyl)phosphate, tris(chloropropyl)phosphate, bis(2,3-dibromopropyl)2,3-dichloropropylphosphate, tris(2,3-dibromopropyl)phosphate, bis(chloropropyl)octylphosphate, halogenated alkyl polyphosphates, and halogenated alkyl polyphosphonates.

[0084] Further examples of phosphorus-containing flame retardants include metal phosphinates, which include not only metal salts of phosphinic acids having no organic group, but also metal salts of organic phosphinic acids such as diphenylphosphinic acid, monophenylphosphinic acid, dialkylphosphinic acid, monoalkylphosphinic acid, and alkylphenylphosphinic acid, as well as metal salts of diphosphinic acids such as methane(dimethylphosphinic acid) and benzene-1,4-di(methylphosphinic acid).

[0085] Examples of dialkylphosphinic acids include dimethylphosphinic acid, ethylmethylphosphinic acid, diethylphosphinic acid, and methyl-n-propylphosphinic acid. Examples of monoalkylphosphinic acids include methylphosphinic acid, ethylphosphinic acid, and n-propylphosphinic acid. Examples of alkylphenylphosphinic acids include methylphenylphosphinic acid. The phosphinic acid metal salts may be, but are not limited to, aluminum salts, zinc salts, calcium salts, magnesium salts, and the like.

[0086] Further examples of phosphorus-containing flame retardants include red phosphorus, ammonium polyphosphate, melamine phosphate, guanidine phosphate, and guanylurea phosphate.

[0087] In addition to the phosphorus-containing flame retardant, the bundling liquid may contain a phosphorus-free flame retardant, such as a melamine compound such as melamine cyanurate, a triazine compound, a guanidine compound, a nitrogen-based flame retardant such as ammonium carbonate, a hydrated metal such as aluminum hydroxide or magnesium hydroxide, or an organic metal salt-based flame retardant such as ferrocene or an acetylacetone metal complex.

[0088] In a preferred embodiment, all materials blended into the bundling solution are selected to be halogen-free, thereby making the final CBC halogen-free.

[0089] Another example of an optional component that can be contained in the bundling liquid is an uncured thermosetting resin powder that is solid and insoluble in the bundling liquid at the temperature when the bundling liquid is mixed with the carbon fiber cotton. For example, when a liquid epoxy resin is contained in the bundling liquid, an uncured epoxy resin powder can be used as the uncured thermosetting resin powder. For example, when a liquid phenol resin is contained in the bundling liquid, an uncured phenol resin powder can be used as the uncured thermosetting resin powder.

[0090] <Mixing of Carbon Fiber Floss and Bundling Liquid> There is no limitation on the method for mixing the carbon fiber floss and the bundling liquid, but stirring is preferred for efficient mixing in a short time.

[0091] For the mixing, a powder agitator mixer known as a Henschel mixer can be preferably used. The agitator mixer may be a type equipped with only an agitator blade (stirring impeller), or may be one equipped with a chopper. In a jacketed agitator mixer, the temperature of the agitation tank can be controlled by flowing a heat transfer medium inside the jacket. For mixing the carbon fiber floss and the bundling liquid, not only an agitator mixer but also a tumbler mixer without a stirring mechanism can be used. In one example, the bundling liquid or a liquid component to be contained in the bundling liquid can be supplied into the mixing tank of the mixer by spraying.

[0092] The temperature T at which the viscosity of the bundling liquid falls within the aforementioned preferred range Ａ is the temperature T of the room where the bundling process is performed. Ｂ When the temperature is higher than 1000°C, before mixing the bundling liquid and the carbon fiber floss in the stirring tank of the stirring mixer, at least one of the bundling liquid, the carbon fiber floss, and the stirring tank, preferably all of them, is heated to a temperature T Ａ In this case, either the bundling liquid or the carbon fiber cotton may be heated to a temperature T Ａ It may be heated to

[0093] The timing for adding the curing agent to the bundling liquid may be before mixing the bundling liquid with the carbon fiber cotton, or may be simultaneously with mixing the bundling liquid with the carbon fiber cotton. All of the curing agents may be added to the bundling liquid at the same time, or some of the curing agents may be added to the bundling liquid at different times. When two or more types of curing agents are used, the timing for adding them to the bundling liquid may be changed depending on the type.

[0094] A curing agent that is liquid or soluble in the bundling liquid at the temperature when the bundling liquid is mixed with the carbon fiber cotton is preferably added to the bundling liquid before mixing the bundling liquid with the carbon fiber cotton. A curing agent that is solid and insoluble in the bundling liquid at the temperature when the bundling liquid is mixed with the carbon fiber cotton may be added to the bundling liquid before mixing the bundling liquid with the carbon fiber cotton, or may be added to the bundling liquid simultaneously with mixing the bundling liquid with the carbon fiber cotton. Such a curing agent is a powder or fine powder, and its maximum particle size is, for example, 150 μm or less, preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 20 μm or less. To add the curing agent to the bundling liquid simultaneously with mixing the bundling liquid with the carbon fiber cotton, the carbon fiber cotton and the curing agent may be first mixed, and then the bundling liquid containing components other than the curing agent may be added to the resulting mixture.

[0095] There is no particular limitation on the timing of adding components other than the curing agent to the bundling liquid. Like the curing agent, components that are liquid or soluble in the bundling liquid at the temperature when the bundling liquid is mixed with the carbon fibers are preferably added to the bundling liquid before mixing the bundling liquid with the carbon fiber cotton. However, adding such components to the bundling liquid during the bundling process is also acceptable, as needed. Components that are solid and insoluble in the bundling liquid at the temperature when the bundling liquid is mixed with the carbon fiber cotton may be added to the bundling liquid before mixing the bundling liquid with the carbon fiber cotton, or may be added to the bundling liquid simultaneously with mixing the bundling liquid with the carbon fiber cotton. Such components are usually powder or fine powder, and their maximum particle size is, for example, 150 μm or less, preferably 100 μm or less, more preferably 50 μm or less, and even more preferably 20 μm or less.

[0096] The ratio of the weight of the carbon fiber wadding to the total weight of the bundling liquid and the carbon fiber wadding to be mixed with each other can be 20% or more and less than 30%, 30% or more and less than 40%, 40% or more and less than 50%, 50% or more and less than 60%, 60% or more and less than 70%, or 70% or more and less than 80%. This ratio is approximately equal to the fiber weight content in the final product, CBC. In reality, carbon fiber wadding made from virgin carbon fiber contains a sizing agent, so the fiber weight content of the CBC is determined by multiplying the above ratio by the weight ratio of carbon fiber in the carbon fiber wadding.

[0097] When the same carbon fiber floss and bundling liquid are used as raw materials, the smaller the above ratio, the thicker the CBC formed and the longer the bundle length tends to be. When the same carbon fiber floss is used, the higher the viscosity of the bundling liquid, the longer the bundle length of the CBC formed tends to be. According to observations made in experiments conducted by the inventors, these two tendencies seem to be weakened when the bundling liquid contains powder and / or fine powder consisting of components that are solid at the temperature when mixed with the carbon fiber floss and are not dissolved in the bundling liquid.

[0098] 1.2. Method using recycled carbon fiber as starting material In the CBC manufacturing method described in 1.1 above, all or part of the virgin carbon fiber as starting material can be replaced with recycled carbon fiber.

[0099] A suitable example of recycled carbon fiber is carbon fiber recovered from waste SMC (sheet molding compound), such as scraps of SMC, or waste CFRP derived from SMC (CFRP obtained by curing SMC). Almost all of the carbon fibers contained in SMC have the same fiber length, which is usually within the range of 3 mm to 60 mm. SMC may contain a small amount of carbon fibers longer than the intended fiber length, which occurs due to miscutting of the carbon fiber bundles during the manufacturing process. The proportion of such carbon fibers among all the carbon fibers contained in SMC is usually less than 1% by weight.

[0100] Methods for recovering carbon fibers from waste SMC or SMC-derived waste CFRP can be broadly divided into thermal decomposition and chemical decomposition. In a preferred example of the thermal decomposition method, waste SMC or SMC-derived waste CFRP is dry distilled at a temperature of preferably 600°C or higher, and then further heated in an oxidizing atmosphere to, for example, 550°C or higher, preferably 600°C or higher. This two-stage process completely pyrolyzes the matrix resin, leaving behind carbon fiber batting composed almost entirely of recycled carbon fibers with identical fiber lengths. This recycled carbon fiber is thermally degraded and has lower strength than virgin carbon fiber, but still has sufficient strength for use as a reinforcing material for FRP. A typical example of a heating method used in the thermal decomposition method is an electric furnace, but microwave heating and induction heating can also be used. In one example of the thermal decomposition method, waste CFRP can be treated in superheated steam.

[0101] Examples of chemical decomposition methods include atmospheric pressure dissolution, supercritical fluid method (a method in which a matrix resin is decomposed using a subcritical or supercritical fluid), semiconductor thermal activation method, and electrolytic oxidation method. The matrix resin must be sufficiently removed so that a cotton-like recycled carbon fiber can be obtained. Resin residue (residual carbon) that cannot be completely removed by chemical decomposition method may be removed by heat treatment in an oxidizing atmosphere.

[0102] Instead of subjecting the resin residue that could not be completely removed by one-stage thermal decomposition or chemical decomposition to thermal decomposition in an oxidizing atmosphere, it is also possible to obtain recycled carbon fibers in a flocculent state by mechanically breaking the bonds between the carbon fibers caused by the resin residue. This operation can be carried out using, for example, a stirring mixer with rotating blades, such as a Henschel mixer.

[0103] Another method for recovering carbon fibers from waste SMC is to wash out the uncured matrix resin using a solvent, which may be a subcritical or supercritical fluid. This method allows for the production of recycled carbon fibers that have the same strength as virgin carbon fibers and are not thermally deteriorated.

[0104] Recycled carbon fibers recovered from SMC-derived waste CFRP or waste SMC are typically short fibers with a fiber length in the range of 3 mm to 60 mm, and do not require further cutting for use in the production of CBCs, and sizing agents have also been removed. When such recycled carbon fibers alone are used as the starting material, unlike when virgin carbon fibers are used as the starting material, the (i) chopping step and (ii) defibrating step are not required, and only the aforementioned (iii) bundling step is required.

[0105] Recycled carbon fibers can also be used, such as carbon fibers recovered from waste UD prepregs or waste CFRP derived from UD prepregs (CFRP obtained by curing UD prepregs) by the aforementioned pyrolysis or chemical decomposition method. The recycled carbon fibers obtained from waste UD prepregs or waste CFRP derived from UD prepregs are cut to an appropriate length before being used to manufacture CBCs. The length of the cut recycled carbon fibers is, for example, 3 mm or more, 5 mm or more, or 10 mm or more, and may also be, for example, 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less. The longer the fiber length of the recycled carbon fibers, the better the mechanical properties of CFRP products molded using the resulting CBCs. On the other hand, the shorter the fiber length of the recycled carbon fibers, the more suitable the resulting CBCs are for molding CFRP products with complex shapes. This is because the shorter the carbon fibers contained in a CBC, the more easily it flows within a molding die.

[0106] In some cases, recycled carbon fibers in the form of cotton are mixed with fibers other than carbon fibers. For example, recycled carbon fiber cotton recovered from CFRP that uses carbon fiber cloth containing stitches made of glass fibers as a reinforcing material may contain glass fibers.

[0107] When recycled carbon fiber is used as a starting material, it is possible to remove fibers other than carbon fiber mixed in the carbon fiber batting before use, but this is not essential. In other words, in the manufacturing method according to the embodiment, recycled carbon fiber batting mixed with fibers other than carbon fiber may be used as a starting material for CBC.

[0108] 2. Carbon Fiber Bundle Composite Another embodiment of the present invention is a CBC produced by the production method described in Section 1 above. This CBC comprises a plurality of short carbon fibers and a bundling liquid, and the plurality of short carbon fibers are held in a bundled state by the bundling liquid. Due to the production method, the positions of the tips of the plurality of short carbon fibers at each end of the CBC are irregular. The bundling liquid is a resin composition containing an uncured thermosetting resin and a curing agent.

[0109] The number of short carbon fibers (filament count) forming the bundle, i.e., the number of short carbon fibers contained in one CBC, can be, for example, 1,000 to 10,000. At least 99% by weight, and preferably all, of the short carbon fibers forming the bundle in one CBC can have a fiber length of 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less. The shorter the fiber lengths of all the short carbon fibers contained in the CBC, e.g., 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less, the more suitable the CBC is for molding CFRP products having complex shapes. This is because the shorter the carbon fibers contained in the CBC, the more easily it tends to flow in a molding die.

[0110] Because carbon fibers with too short a fiber length are less effective as a reinforcing material, the majority of the short carbon fibers forming the bundles in a single CBC preferably have a fiber length of 3 mm or more, more preferably 5 mm or more, and even more preferably 10 mm or more, by weight. In a preferred example, the CBC does not contain any carbon fibers with a fiber length of less than 3 mm, or if it does contain any, the amount is less than 5 wt% of the total carbon fibers constituting the CBC. In another preferred example, the CBC does not contain any carbon fibers with a fiber length of less than 5 mm, or if it does contain any, the amount is less than 5 wt% of the total carbon fibers constituting the CBC. In yet another preferred example, the CBC does not contain any carbon fibers with a fiber length of less than 10 mm, or if it does contain any, the amount is less than 5 wt% of the total carbon fibers constituting the CBC.

[0111] In a CBC in which the majority of the short carbon fibers constituting the bundle have a fiber length of L (mm) or more by weight, the bundle length usually exceeds L (mm).

[0112] The shape of the CBC may be seed-shaped (spindle-shaped), needle-shaped, or wire-shaped. Figure 2 shows a photograph of the appearance of a seed-shaped (spindle-shaped) CBC.

[0113] In one embodiment, the CBC contains only short carbon fibers having similar fiber lengths, thereby reducing the variation in quality between production lots. In this case, the difference between the maximum and minimum fiber lengths of the short carbon fibers constituting the bundle is preferably within 5 mm, more preferably within 4 mm, and even more preferably within 3 mm.

[0114] There is no particular limitation on the filament diameter of the short carbon fibers forming the bundle, and it may be, for example, within the range of filament diameters normally possessed by PAN-based carbon fibers, that is, within the range of 5 μm to 15 μm.

[0115] The bundling liquid contained in a CBC is the same bundling liquid used to form the CBC. Therefore, the types and preferred examples of the uncured thermosetting resin that the CBC may contain are the same as those in the bundling liquid (for example, preferred examples of the uncured thermosetting resin that the CBC may contain include epoxy resins, vinyl ester resins, unsaturated polyester resins, and resol-type phenolic resins, just like the preferred examples of the uncured thermosetting resin that may be contained in the bundling liquid). The types and preferred examples of components other than the uncured thermosetting resin contained in the CBC are also the same as those in the bundling liquid used to form the CBC. However, not all of the components contained in the bundling liquid are necessarily contained in the CBC in the same state as they were when the CBC was formed. In particular, compounds involved in the thickening reaction typically change into different compounds over time after the CBC is formed.

[0116] The fiber weight content of the CBC can be, for example, 20 wt% or more but less than 30 wt%, 30 wt% or more but less than 40 wt%, 40 wt% or more but less than 50 wt%, 50 wt% or more but less than 60 wt%, 60 wt% or more but less than 70 wt%, or 70 wt% or more but less than 80 wt%. From the viewpoint of improving the mechanical properties of CFRP products molded using the CBC, the fiber content is preferably 30 wt% or more, more preferably 40 wt% or more, and even more preferably 50 wt% or more. On the other hand, the lower the fiber content, the easier the CBC flows in a molding die, making it more suitable for molding CFRP products with complex shapes. From this viewpoint, the fiber content is preferably less than 70 wt%, and more preferably less than 60 wt%.

[0117] The carbon fibers contained in the CBC may all be virgin carbon fibers, or some may be virgin carbon fibers and the remainder may be recycled carbon fibers, or all may be recycled carbon fibers. The carbon fibers contained in the CBC may all be non-thermally degraded carbon fibers, or some may be non-thermally degraded carbon fibers and the remainder may be thermally degraded carbon fibers, or all may be thermally degraded carbon fibers. A typical example of non-thermally degraded carbon fibers is virgin carbon fibers. A typical example of thermally degraded carbon fibers is carbon fibers recovered from waste CFRP, which have been thermally degraded in the process of pyrolyzing and removing the matrix resin.

[0118] The CBC may contain fibers other than carbon fiber. For example, when recycled carbon fiber containing glass fiber is used as the starting material, a CBC is obtained that contains a plurality of short carbon fibers bundled together with the glass fiber, and a resin composition containing an uncured thermosetting resin and a curing agent. When the CBC contains fibers other than carbon fiber, the content of such fibers is preferably less than 10 wt %, more preferably less than 5 wt %, and even more preferably less than 1 wt %, based on the total amount of carbon fiber contained in the CBC.

[0119] In this specification, the fiber weight content of a CBC containing fibers other than carbon fibers means the ratio of the total weight of carbon fibers contained in the CBC to the weight of the CBC.

[0120] CBC can be used, for example, as an intermediate material when producing CFRP products by press molding. CBC can also be used to produce carbon fiber composite sheets, as described in 3. below.

[0121] 3. Carbon Fiber Composite Sheet Using CBC, a carbon fiber composite sheet, which is a sheet-shaped thermosetting molding material, can be manufactured, for example, by sequentially performing the following first to third steps. First step: Provide a first protective film and a second protective film. Second step: Sprinkle CBC on the first protective film to deposit a CBC layer. Third step: Cover the CBC layer with a second protective film, and then bond the CBC contained in the CBC layer to each other.

[0122] The materials for the first protective film and the second protective film can be appropriately selected from polyolefins such as polyethylene and polypropylene, polyvinylidene chloride, vinyl chloride resins, polyamides, etc. Either or both of the first protective film and the second protective film may be a multilayer film. At least one of the first protective film and the second protective film may be a release paper commonly used in the production of carbon fiber prepregs.

[0123] In the third step, the CBC layer may be crushed using, for example, a press. The press may be, for example, a double-belt press or a roll press. If the CBC is hard, the CBC layer may be heated before or during crushing to reduce the viscosity of the bundling liquid contained in the CBC. In this case, the temperature and heating time are adjusted so that the bundling liquid does not gel and lose its fluidity. When the viscosity of the bundling liquid increases upon cooling, the CBCs in the crushed CBC layer stick to each other to form a sheet.

[0124] In a preferred embodiment of the manufacturing method, a thickener is blended into the bundling liquid containing CBC. Before the effect of the thickener is fully realized, the first and second steps are added, and a second protective film is placed on the CBC layer, followed by crushing the CBC layer using, for example, a press. When the bundling liquid is then sufficiently thickened, the CBCs in the crushed CBC layer bond together to form a sheet. When the bundling liquid is thickened, the CBC layer may be maintained at a temperature higher than room temperature. The temperature and time are set within a range that does not cause hardening of the bundling liquid due to the action of the curing agent.

[0125] Figure 3 shows a conceptual diagram of a manufacturing apparatus that can be used to manufacture a carbon fiber composite sheet using the above procedure. This manufacturing apparatus has a section for depositing a CBC layer by spraying a CBC onto a first protective film unwound from a roll, a section for covering the CBC layer with a second protective film unwound from a roll, a section for pressurizing the CBC layer, and a section for winding up the carbon fiber composite sheet. In one example, a section for heating the CBC layer may be provided upstream of the section for pressurizing the CBC layer. In another example, the section for pressurizing the CBC layer may also serve as the section for heating the CBC layer.

[0126] The orientation of the CBC in the CBC layer may be random or biased in one direction. For example, the slower the running speed of the first protective film, the more random the orientation of the CBC. When the running speed of the first protective film is increased, the CBC tends to be oriented along the running direction.

[0127] In one example, the fiber weight content of a carbon fiber composite sheet can be adjusted by supplementing the components of the bundling solution during production of the carbon fiber composite sheet. This can be achieved by coating the surface of a first protective film with a paste containing some or all of the components of the bundling solution prior to depositing the CBC layer. Alternatively, or in addition, the surface of a second protective film can be coated with the same paste prior to application of the CBC layer.

[0128] The carbon fiber composite sheet produced by the above procedure is, simply put, a sheet made of multiple CBCs bonded together.

[0129] At least 99 wt %, preferably all, of the carbon fibers contained in the carbon fiber composite sheet may have a fiber length of 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less. In a preferred example, the carbon fiber composite sheet does not contain carbon fibers with a fiber length of less than 3 mm, or if it contains any, the amount is less than 5 wt % of the total carbon fibers contained in the carbon fiber composite sheet. In another preferred example, the carbon fiber composite sheet does not contain carbon fibers with a fiber length of less than 5 mm, or if it contains any, the amount is less than 5 wt % of the total carbon fibers contained in the carbon fiber composite sheet. In yet another preferred example, the carbon fiber composite sheet does not contain carbon fibers with a fiber length of less than 10 mm, or if it contains any, the amount is less than 5 wt % of the total carbon fibers contained in the carbon fiber composite sheet.

[0130] The carbon fiber composite sheet contains a resin composition derived from the bundling liquid contained in the CBC. The components that can be contained in the resin composition and preferred examples thereof are the same as those of the bundling liquid contained in the CBC. The fiber weight content in the carbon fiber composite sheet can be, for example, 20 wt% or more and less than 30 wt%, 30 wt% or more and less than 40 wt%, 40 wt% or more and less than 50 wt%, 50 wt% or more and less than 60 wt%, 60 wt% or more and less than 70 wt%, or 70 wt% or more and less than 80 wt%.

[0131] From the viewpoint of improving the mechanical properties of CFRP products molded using the carbon fiber composite sheet, the fiber weight content is preferably 30 wt% or more, more preferably 40 wt% or more, and even more preferably 50 wt% or more. On the other hand, the lower the fiber weight content, the easier the carbon fiber composite sheet flows in the molding die, making it more suitable for molding CFRP products with complex shapes. From this viewpoint, the fiber weight content is preferably less than 70 wt%, and more preferably less than 60 wt%.

[0132] In this specification, the fiber weight content in a carbon fiber composite sheet containing fibers other than carbon fibers means the ratio of the total weight of carbon fibers contained in the carbon fiber composite sheet to the weight of the carbon fiber composite sheet.

[0133] The basis weight of the carbon fiber composite sheet can be appropriately designed depending on the application. The basis weight is, for example, 300 g / m ２ More than 500g / m ２ Less than 500 g / m ２ More than 1000g / m ２ Less than 1000 g / m ２ More than 2000g / m ２ Less than 2000 g / m ２ More than 4000g / m ２ Less than 4000 g / m ２ More than 6000g / m ２ Less than 6000 g / m ２ More than 8000g / m ２ Less than or equal to 8000 g / m ２ More than 10000g / m ２ It may be less than.

[0134] The thickness of the carbon fiber composite sheet can be designed to be, for example, 0.5 mm or more and less than 1.5 mm, 1.5 mm or more and less than 3 mm, or 3 mm or more and 5 mm or less, but is not limited thereto.

[0135] A carbon fiber composite sheet is a prepreg material used in molding CFRP. When a CFRP product is manufactured using a carbon fiber composite sheet, a press molding method is preferably used as the molding method, but this is not limited thereto, and molding methods other than press molding, such as autoclave molding, can also be used.

[0136] 4. Summary of Embodiments Embodiments of the present invention include, but are not limited to, the following.

[0137] [Embodiment 1] A method for producing a carbon fiber bundle composite, comprising: mixing carbon fiber wadding made of short carbon fibers with a bundling liquid containing an uncured thermosetting resin to obtain carbon fiber bundles containing the bundling liquid; and adding a curing agent to the bundling liquid to make the bundling liquid thermosetting. [Embodiment 2] The production method according to embodiment 1, wherein the uncured thermosetting resin comprises one or more selected from the group consisting of epoxy resin, vinyl ester resin, unsaturated polyester resin, and resol-type phenolic resin. [Embodiment 3] The production method according to embodiment 1 or 2, wherein the viscosity of the bundling liquid when mixed with the carbon fiber wadding is 10 Pa s or less, preferably 5 Pa s or less, more preferably 1 Pa s or less, and may be 0.001 Pa s or more. [Embodiment 4] The production method according to any one of embodiments 1 to 3, wherein the bundling liquid has a viscosity of 10 Pa s or less at 50°C, preferably 40°C, and more preferably 30°C. [Embodiment 5] The production method according to any one of Embodiments 1 to 4, wherein a reactive diluent is blended into the bundling liquid. [Embodiment 6] The production method according to any one of Embodiments 1 to 4, wherein an uncured epoxy resin and a monoepoxide compound are blended into the bundling liquid. [Embodiment 7] The production method according to any one of Embodiments 1 to 4 and 6, wherein an uncured epoxy resin and at least one of methylhexahydrophthalic anhydride and tetrahydromethylphthalic anhydride are blended into the bundling liquid. [Embodiment 8] The production method according to any one of Embodiments 1 to 4, 6 and 7, wherein the bundling liquid is blended with at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin, and a compound having one or two ethylenically unsaturated groups in its molecule and having a viscosity of 1 Pa s or less at 25°C. [Embodiment 9] The production method according to any one of Embodiments 1 to 8, wherein a thickener is blended into the bundling liquid. [Embodiment 10] The manufacturing method according to any one of embodiments 1 to 8, wherein the bundling liquid is blended with an uncured epoxy resin and one or more selected from the group consisting of polyisocyanate, carboxylic acid anhydride, and amine.[Embodiment 11] The production method according to any one of Embodiments 1 to 8, wherein at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin, and a polyisocyanate are blended into the bundling liquid. [Embodiment 12] The production method according to any one of Embodiments 1 to 8, wherein an uncured resol-type phenolic resin and one or more components selected from an alkaline earth metal hydroxide, an alkaline earth metal oxide, and a polyisocyanate are blended into the bundling liquid. [Embodiment 13] The production method according to any one of Embodiments 9 to 12, comprising thickening the bundling liquid after obtaining the carbon fiber bundles and adding the curing agent to the bundling liquid. [Embodiment 14] The production method according to any one of Embodiments 9 to 13, wherein a stirring mixer is used for the mixing, and comprising thickening the bundling liquid while stirring the carbon fiber bundles formed in a stirring tank of the stirring mixer in the stirring tank. [Embodiment 15] The production method according to any one of Embodiments 9 to 13, comprising thickening the bundling liquid while rolling the carbon fiber bundles in a disc pelletizer. [Embodiment 16] The production method according to any one of Embodiments 1 to 15, wherein the bundling liquid contains an uncured epoxy resin, and the curing agent contains an epoxy curing agent. [Embodiment 17] The production method according to Embodiment 16, wherein the epoxy curing agent contains a latent curing agent. [Embodiment 18] The production method according to any one of Embodiments 1 to 17, wherein the bundling liquid contains at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin, and the curing agent contains a radical polymerization initiator. [Embodiment 19] The production method according to any one of Embodiments 1 to 18, wherein the bundling liquid contains an uncured resol-type phenolic resin, and the curing agent contains one or more selected from the group consisting of an organic acid, an inorganic acid, an amine, and a reaction product of at least one of a primary amine and a secondary amine with an isocyanate. [Embodiment 20] The manufacturing method according to any one of embodiments 1 to 19, further comprising adding a flame retardant to the bundling liquid. [Embodiment 21] The manufacturing method according to any one of embodiments 1 to 20, further comprising preparing at least a portion of the carbon fiber batting by defibrating chopped carbon fiber bundles.[Embodiment 22] The manufacturing method according to any one of embodiments 1 to 20, wherein the short carbon fibers include thermally deteriorated carbon fibers. [Embodiment 23] The manufacturing method according to any one of embodiments 1 to 20 and 22, wherein the carbon fiber batting is mixed with fibers other than carbon fibers, and the fibers other than carbon fibers may be glass fibers. [Embodiment 24] The manufacturing method according to any one of embodiments 1 to 23, wherein all or at least 99 wt % of the short carbon fibers have a fiber length of 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less. [Embodiment 25] The manufacturing method according to any one of embodiments 1 to 24, wherein the carbon fiber bundle has a bundle length of 3 mm or more, 5 mm or more, or 10 mm or more. [Embodiment 26] The manufacturing method according to any one of embodiments 1 to 24, wherein the carbon fiber bundle does not contain carbon fibers with a fiber length of less than 3 mm, or even if it contains such fibers, the content of such fibers is less than 5 wt % of the total carbon fibers in the carbon fiber bundle. [Embodiment 27] The production method according to any one of Embodiments 1 to 24, wherein the carbon fiber bundle does not contain carbon fibers with a fiber length of less than 5 mm, or if it contains any, the content of such fibers is less than 5 wt % of all carbon fibers in the carbon fiber bundle. [Embodiment 28] The production method according to any one of Embodiments 1 to 24, wherein the carbon fiber bundle does not contain carbon fibers with a fiber length of less than 10 mm, or if it contains any, the content of such fibers is less than 5 wt % of all carbon fibers in the carbon fiber bundle. [Embodiment 29] The production method according to any one of Embodiments 1 to 28, wherein the fiber weight content of the carbon fiber bundle is 20 wt % or more and less than 30 wt %, 30 wt % or more and less than 40 wt %, 40 wt % or more and less than 50 wt %, 50 wt % or more and less than 60 wt %, 60 wt % or more and less than 70 wt %, or 70 wt % or more and 80 wt % or less.

[0138] [Embodiment 30] A method for producing a carbon fiber bundle composite, comprising aggregating discontinuous carbon fibers with a bundling liquid to form a carbon fiber bundle containing the bundling liquid, wherein the bundling liquid is a resin composition containing an uncured thermosetting resin and a curing agent. [Embodiment 31] The production method according to embodiment 30, wherein the uncured thermosetting resin contains one or more selected from the group consisting of an epoxy resin, a vinyl ester resin, an unsaturated polyester resin, a resol-type phenolic resin, a (meth)acrylate other than an epoxy vinyl ester, and a diallyl phthalate. [Embodiment 32] The production method according to embodiment 30 or 31, wherein the bundling liquid further contains a reactive diluent. [Embodiment 33] The production method according to any one of embodiments 30 to 32, wherein the bundling liquid further contains a thickener. [Embodiment 34] The production method according to embodiment 33, further comprising thickening the bundling liquid after forming the carbon fiber bundles. [Embodiment 35] The production method according to any one of embodiments 30 to 34, wherein the discontinuous carbon fibers contain recycled carbon fibers. [Embodiment 36] The production method according to embodiment 35, wherein the recycled carbon fibers include thermally deteriorated carbon fibers. [Embodiment 37] The production method according to any one of embodiments 30 to 36, wherein all or at least 99 wt % of the discontinuous carbon fibers have a fiber length of 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less. [Embodiment 38] The production method according to any one of embodiments 30 to 37, wherein the carbon fiber bundle has a bundle length of 3 mm or more, 5 mm or more, or 10 mm or more. [Embodiment 39] The production method according to any one of embodiments 30 to 37, wherein the carbon fiber bundle does not contain any carbon fibers with a fiber length of less than 3 mm, or even if it contains any, the content of such fibers is less than 5 wt % of the total carbon fibers in the carbon fiber bundle. [Embodiment 40] The production method according to any one of embodiments 30 to 37, wherein the carbon fiber bundle does not contain any carbon fibers with a fiber length of less than 5 mm, or even if it contains any, the content of such fibers is less than 5 wt % of the total carbon fibers in the carbon fiber bundle. [Embodiment 41] A manufacturing method according to any one of embodiments 30 to 37, wherein the carbon fiber bundle does not contain carbon fibers with a fiber length of less than 10 mm, or even if it does contain carbon fibers, the content of such fibers is less than 5 wt% of the total carbon fibers in the carbon fiber bundle.[Embodiment 42] The production method according to any one of embodiments 30 to 41, wherein the fiber weight content of the carbon fiber bundle is 20 wt% or more and less than 30 wt%, 30 wt% or more and less than 40 wt%, 40 wt% or more and less than 50 wt%, 50 wt% or more and less than 60 wt%, 60 wt% or more and less than 70 wt%, or 70 wt% or more and less than 80 wt%. [Embodiment 43] The production method according to embodiment 42, wherein the fiber weight content is 30 wt% or more and less than 70 wt%, preferably 40 wt% or more and less than 60 wt%. [Embodiment 44] A carbon fiber bundle composite produced by the production method according to any one of embodiments 1 to 43.

[0139] [Embodiment 45] A method for producing a carbon fiber composite sheet, comprising: mixing carbon fiber wadding made of short carbon fibers with a bundling liquid containing an uncured thermosetting resin to obtain carbon fiber bundles containing the bundling liquid; adding a curing agent to the bundling liquid to make the bundling liquid heat-curable; and bonding the carbon fiber bundles to each other after adding the curing agent to the bundling liquid. [Embodiment 46] The production method according to embodiment 45, wherein the uncured thermosetting resin comprises one or more selected from the group consisting of epoxy resin, vinyl ester resin, unsaturated polyester resin, and resol-type phenolic resin. [Embodiment 47] The production method according to embodiment 45 or 46, wherein the viscosity of the bundling liquid when mixed with the carbon fiber wadding is 10 Pa s or less, preferably 5 Pa s or less, more preferably 1 Pa s or less, and may be 0.001 Pa s or more. [Embodiment 48] The method according to any one of embodiments 45 to 47, wherein the bundling liquid has a viscosity of 10 Pa s or less at 50°C, preferably at 40°C, and more preferably at 30°C. [Embodiment 49] The method according to any one of embodiments 45 to 48, wherein a reactive diluent is blended into the bundling liquid. [Embodiment 50] The method according to any one of embodiments 45 to 48, wherein an uncured epoxy resin and a monoepoxide compound are blended into the bundling liquid. [Embodiment 51] The method according to any one of embodiments 45 to 48 and 50, wherein an uncured epoxy resin and at least one of methylhexahydrophthalic anhydride and tetrahydromethylphthalic anhydride are blended into the bundling liquid. [Embodiment 52] The method of any one of embodiments 45 to 48, 50, and 51, wherein the bundling liquid is blended with at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin, and a compound having one or two ethylenically unsaturated groups in its molecule and having a viscosity of 1 Pa s or less at 25° C. [Embodiment 53] The method of any one of embodiments 45 to 52, wherein the bundling liquid is blended with a thickener.[Embodiment 54] The production method according to any one of embodiments 45 to 52, wherein the bundling liquid contains an uncured epoxy resin and one or more selected from the group consisting of polyisocyanate, carboxylic acid anhydride, and amine. [Embodiment 55] The production method according to any one of embodiments 45 to 52, wherein the bundling liquid contains at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin, and a polyisocyanate. [Embodiment 56] The production method according to any one of embodiments 45 to 52, wherein the bundling liquid contains an uncured resol-type phenolic resin and one or more selected from the group consisting of an alkaline earth metal hydroxide, an alkaline earth metal oxide, and a polyisocyanate. [Embodiment 57] The production method according to any one of embodiments 53 to 56, comprising adding the curing agent to the bundling liquid and then thickening the bundling liquid while bringing the carbon fiber bundles into contact with each other. [Embodiment 58] The manufacturing method according to any one of embodiments 45 to 57, wherein the bundling liquid contains an uncured epoxy resin and the curing agent comprises an epoxy curing agent. [Embodiment 59] The manufacturing method according to embodiment 57, wherein the epoxy curing agent comprises a latent curing agent. [Embodiment 60] The manufacturing method according to any one of embodiments 45 to 59, wherein the bundling liquid contains at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin and the curing agent comprises a radical polymerization initiator. [Embodiment 61] The manufacturing method according to any one of embodiments 45 to 60, wherein the bundling liquid contains an uncured resol-type phenolic resin and the curing agent comprises at least one selected from the group consisting of an organic acid, an inorganic acid, an amine, and a reaction product of at least one of a primary amine and a secondary amine with an isocyanate. [Embodiment 62] The manufacturing method according to any one of embodiments 45 to 61, further comprising adding a flame retardant to the bundling liquid. [Embodiment 63] The manufacturing method according to any one of embodiments 45 to 62, comprising preparing at least a portion of the carbon fiber batting by defibrating chopped carbon fiber bundles. [Embodiment 64] The manufacturing method according to any one of embodiments 45 to 62, wherein the short carbon fibers comprise thermally deteriorated carbon fibers.[Embodiment 65] The manufacturing method according to any one of embodiments 45 to 62 and 64, wherein the carbon fiber batting is mixed with fibers other than carbon fibers, and the fibers other than carbon fibers may be glass fibers. [Embodiment 66] The manufacturing method according to any one of embodiments 45 to 65, wherein all or at least 99% by weight of the short carbon fibers have a fiber length of 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less. [Embodiment 67] The manufacturing method according to any one of embodiments 45 to 66, wherein the carbon fiber bundles have a bundle length of 3 mm or more, 5 mm or more, or 10 mm or more. [Embodiment 68] The manufacturing method according to any one of embodiments 45 to 66, wherein the carbon fiber composite sheet does not contain carbon fibers with a fiber length of less than 3 mm, or if it does contain such fibers, their content is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet. [Embodiment 69] The method of any one of embodiments 45 to 66, wherein the carbon fiber composite sheet does not contain carbon fibers with a fiber length of less than 5 mm, or if it contains any, the content of such fibers is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet. [Embodiment 70] The method of any one of embodiments 45 to 66, wherein the carbon fiber composite sheet does not contain carbon fibers with a fiber length of less than 10 mm, or if it contains any, the content of such fibers is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet. [Embodiment 71] The method of any one of embodiments 45 to 70, wherein the fiber weight content of the carbon fiber composite sheet is 20 wt% or more but less than 30 wt%, 30 wt% or more but less than 40 wt%, 40 wt% or more but less than 50 wt%, 50 wt% or more but less than 60 wt%, 60 wt% or more but less than 70 wt%, or 70 wt% or more but less than 80 wt%.

[0140] [Embodiment 72] A method for producing a carbon fiber composite sheet, comprising: forming a carbon fiber bundle containing a bundling liquid by aggregating discontinuous carbon fibers with the bundling liquid, and bonding a plurality of the carbon fiber bundles to each other, wherein the bundling liquid is a resin composition containing an uncured thermosetting resin and a curing agent. [Embodiment 73] A method for producing a carbon fiber composite sheet, comprising: bonding a plurality of carbon fiber bundles to each other by bringing the plurality of carbon fiber bundles into contact with each other and thickening the bundling liquid contained in each of the plurality of carbon fiber bundles, wherein discontinuous carbon fibers in each of the plurality of carbon fiber bundles have been aggregated by the bundling liquid, and the bundling liquid is a resin composition containing an uncured thermosetting resin, a curing agent, and a thickener. [Embodiment 74] A method for producing a carbon fiber composite sheet, comprising: depositing a plurality of carbon fiber bundles to form a carbon fiber bundle layer; compressing the carbon fiber bundle layer; and then thickening a bundling liquid contained in each of the plurality of carbon fiber bundles, wherein discontinuous carbon fibers in each of the plurality of carbon fiber bundles are aggregated by the bundling liquid, and the bundling liquid is a resin composition containing an uncured thermosetting resin, a curing agent, and a thickener. [Embodiment 75] The method according to any one of embodiments 72 to 74, wherein the uncured thermosetting resin comprises one or more selected from the group consisting of epoxy resin, vinyl ester resin, unsaturated polyester resin, resol-type phenolic resin, (meth)acrylate other than epoxy vinyl ester, and diallyl phthalate. [Embodiment 76] The method according to any one of embodiments 72 to 75, wherein the bundling liquid further contains a reactive diluent. [Embodiment 77] The method according to any one of embodiments 72 to 76, wherein the discontinuous carbon fibers comprise recycled carbon fibers. [Embodiment 78] The manufacturing method of embodiment 77, wherein the recycled carbon fibers include thermally deteriorated carbon fibers. [Embodiment 79] The manufacturing method of any of embodiments 72 to 78, wherein all or at least 99% by weight of the carbon fibers contained in the carbon fiber composite sheet have a fiber length of 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less.[Embodiment 80] The method according to any one of embodiments 72 to 79, wherein the plurality of carbon fiber bundles each have a bundle length of 3 mm or more, 5 mm or more, or 10 mm or more. [Embodiment 81] The method according to any one of embodiments 72 to 80, wherein the carbon fiber composite sheet does not contain carbon fibers with a fiber length of less than 3 mm, or even if it contains them, the content thereof is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet. [Embodiment 82] The method according to any one of embodiments 72 to 80, wherein the carbon fiber composite sheet does not contain carbon fibers with a fiber length of less than 5 mm, or even if it contains them, the content thereof is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet. [Embodiment 83] The method according to any one of embodiments 72 to 80, wherein the carbon fiber composite sheet does not contain carbon fibers with a fiber length of less than 10 mm, or even if it contains them, the content thereof is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet. [Embodiment 84] The manufacturing method according to any one of embodiments 72 to 83, wherein the fiber weight content of the carbon fiber composite sheet is 20 wt% or more and less than 30 wt%, 30 wt% or more and less than 40 wt%, 40 wt% or more and less than 50 wt%, 50 wt% or more and less than 60 wt%, 60 wt% or more and less than 70 wt%, or 70 wt% or more and less than 80 wt%. [Embodiment 85] The manufacturing method according to embodiment 84, wherein the fiber weight content is 30 wt% or more and less than 70 wt%, preferably 40 wt% or more and less than 60 wt%. [Embodiment 86] A carbon fiber composite sheet manufactured by the manufacturing method according to any one of embodiments 45 to 85. [Embodiment 87] A method for manufacturing a CFRP product, comprising heating and pressurizing the carbon fiber bundle composite according to embodiment 44 or the carbon fiber composite sheet according to embodiment 86 in a molding die to cure it.

[0141] 5. Experimental Results The results of the experiments conducted by the present inventors are described below.

[0142] The materials used in each experiment were as follows: Carbon fiber: Cotton-like carbon fiber obtained by drying chopped carbon fiber with a fiber length of 6 mm that had been sized with water at 120°C for 1 hour and then manually loosening it. Epoxy resin 1: jER (registered trademark) 807, a bisphenol F-type epoxy resin manufactured by Mitsubishi Chemical Corporation. Epoxy resin 2: TETRAD-X (registered trademark) manufactured by Mitsubishi Gas Chemical Company, Inc., containing N,N,N',N'-tetraglycidyl-m-xylylenediamine. Epoxy curing agent 1: Curesol 2MZA-PW manufactured by Shikoku Chemical Industries, Ltd., containing 2,4-diamino-6-[2'-methylimidazole-(1')]-ethyl-s-triazine. Epoxy curing agent 2: Amicure (registered trademark) PN-23J, an amine adduct-based latent curing agent for epoxy resins manufactured by Ajinomoto Fine-Techno Co., Inc. Thickener: HN-2200 from Resonaq Co., Ltd., containing 3 or 4-methyl-1,2,3,6-tetrahydrophthalic anhydride.

[0143] [Preparation of Bundling Liquid] The epoxy resin, epoxy curing agent, and thickener were mixed in the weight ratio shown in Table 1 to prepare a bundling liquid.

[0144]

[0145] Immediately after preparation, the viscosity of the bundling liquid was measured using a Brookfield type rotational viscometer (LVDV-1, spindle S63, 10 rpm) and found to be 4 Pa ​​s at 22° C. After leaving this bundling liquid at 22° C. for 6 days, the viscosity was again measured using a Brookfield type rotational viscometer (Brookfield digital viscometer HBDVE, spindle S07, 10 rpm) and found to be 470 Pa s at 22° C.

[0146] [Experiment 1] 30 g of carbon fiber and 16.2 g of bundling liquid were placed in a 500 mL plastic container (500 mL PP Hempei Bottle from AS ONE Corporation) and shaken by hand for approximately 5 minutes. By mixing with the bundling liquid, the carbon fibers formed bundles. When the contents of the container were observed after shaking, no carbon fibers were observed that were not involved in the formation of bundles. Furthermore, all bundles were seed-shaped (spindle-shaped), with bundle lengths in the range of 10 to 20 mm and maximum diameters in the range of 3 to 5 mm for almost all of them. The maximum diameter is the diameter of the bundle at the point where the bundle is thickest, and may be referred to as the diameter of the bundle at the center in the longitudinal direction (fiber direction) (the same applies hereinafter).

[0147] [Experiment 2] Carbon fibers and the bundling liquid were placed in a container and shaken in the same manner as in Experiment 1, except that the amount of the bundling liquid was increased to 20.0 g. When the contents of the container were observed after shaking, no carbon fibers not involved in the formation of bundles were observed, and all bundles were seed-shaped (spindle-shaped), and almost all of them had bundle lengths within the range of 15 to 25 mm and maximum diameters within the range of 5 to 10 mm.

[0148] [Experiment 3] The carbon fibers and the bundling liquid were placed in a container and shaken in the same manner as in Experiment 1, except that the amount of carbon fibers was reduced to 20 g and the amount of the bundling liquid was increased to 20.0 g. When the contents of the container were observed after shaking, no carbon fibers not involved in the formation of bundles were observed, and all bundles were seed-shaped (spindle-shaped), and almost all of them had bundle lengths within the range of 20 to 30 mm and maximum diameters within the range of 10 to 20 mm.

[0149] [Experiment 4] Using a press mold consisting of a 100 mm x 60 mm cavity with a flat bottom and a core, 15 g of the carbon fiber bundle composite obtained in Experiment 2 was molded into a CFRP plate with a length and width of 100 mm x 60 mm. The pressing time was 1 hour and 30 minutes, with the temperature being 150°C and the pressure being 8 MPa for the first 30 minutes, and the temperature being 180°C and the pressure being 8 MPa for the next hour. The obtained CFRP plate had a smooth surface, a thickness of 2.5 mm, and a density of 0.92 g / cm. ３ It was.

[0150] [Experiment 5] 15 g of freshly prepared carbon fiber bundle composites using the same procedure as in Experiment 2 were placed in a zippered plastic bag measuring 100 mm x 70 mm. The plastic bag was placed on a flat surface, and the bottom of a beaker was manually pressed against the carbon fiber composite inside the plastic bag to crush it, as shown in Figure 4. Immediately after crushing, the carbon fiber bundles were not strongly bonded to each other, and the contents of the plastic bag could not be handled as a free-standing sheet. However, after being crushed, the contents of the plastic bag were transformed into a free-standing sheet by leaving it at 22°C for 6 days. This sheet did not break even when bent, as shown in Figure 5. Furthermore, when the seed-shaped carbon fiber bundle composites were left at 22°C for 6 days, they became too hard to be crushed using the above method, and they could not be bonded together.

[0151] [Experiment 6] The carbon fiber composite sheet obtained in Experiment 5 was cut into a size of 90 mm x 50 mm and used to mold a CFRP plate measuring 100 mm x 60 mm. The press mold and molding conditions used were the same as those in Experiment 4. The area of ​​the carbon fiber composite sheet placed in the press mold was 75% of the area of ​​the bottom surface of the cavity of the press mold, which indicates that the carbon fiber composite sheet flowed during molding. The obtained CFRP plate had a smooth surface, a thickness of 1.2 mm, and a density of 1.58 g / cm. ３ It was.

[0152] The present invention has been described above with reference to specific embodiments. However, each embodiment is presented as an example and does not limit the scope of the present invention. Each embodiment described in this specification can be modified in various ways within the scope of the effects of the invention and can be combined with features described in other embodiments to the extent possible. This application is based on Japanese Patent Application No. 2022-137548, filed on August 31, 2022, and is incorporated by reference in its entirety.

[0153] The carbon fiber bundle composites and carbon fiber composite sheets obtained by the production methods according to the respective embodiments can be preferably used to produce various CFRP parts used in automobiles, motorcycles, bicycles, ships, railway vehicles, manned aircraft, unmanned aircraft, and other transportation equipment, as well as sporting goods, leisure goods, home appliances, agricultural machinery, building materials, and the like.

Claims

1. A method for producing a carbon fiber bundle composite, comprising: mixing carbon fiber cotton made of short carbon fibers with a bundling liquid containing an uncured thermosetting resin to obtain a carbon fiber bundle containing the bundling liquid; and adding a curing agent to the bundling liquid to make the bundling liquid thermally curable.

2. The method according to claim 1, wherein the uncured thermosetting resin comprises at least one selected from the group consisting of epoxy resins, vinyl ester resins, unsaturated polyester resins and resol-type phenolic resins.

3. The manufacturing method according to claim 1, wherein the viscosity of the bundling liquid when mixed with the carbon fiber cotton is 10 Pa·s or less, preferably 5 Pa·s or less, more preferably 1 Pa·s or less, and may be 0.001 Pa·s or more.

4. The method of claim 1, wherein the bundling liquid has a viscosity of 10 Pa·s or less at 50° C., preferably at 40° C., more preferably at 30° C.

5. The method of claim 1 , wherein the bundling solution includes a reactive diluent.

6. The method according to claim 1 , wherein the bundling liquid contains an uncured epoxy resin and a monoepoxide compound.

7. The method according to claim 1 , wherein the bundling liquid contains an uncured epoxy resin and at least one of methylhexahydrophthalic anhydride and tetrahydromethylphthalic anhydride.

8. 2. The method according to claim 1, wherein the bundling liquid contains at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin, and a compound having one or two ethylenically unsaturated groups in a molecule and having a viscosity of 1 Pa·s or less at 25°C.

9. The method of claim 1 , wherein the bundling liquid further comprises a thickening agent.

10. The method according to claim 1 , wherein the bundling liquid contains an uncured epoxy resin and at least one selected from the group consisting of polyisocyanate, carboxylic acid anhydride, and amine.

11. The method according to claim 1 , wherein the bundling liquid contains at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin, and a polyisocyanate.

12. The method according to claim 1, wherein the bundling liquid contains an uncured resol type phenolic resin and one or more components selected from the group consisting of an alkaline earth metal hydroxide, an alkaline earth metal oxide, and a polyisocyanate.

13. The method according to claim 9 , further comprising: thickening the bundling liquid after obtaining the carbon fiber bundles and adding the curing agent to the bundling liquid.

14. The method for producing a carbon fiber bundle according to claim 9, further comprising: mixing the carbon fiber bundles in a mixing tank of the mixing tank; and thickening the bundling liquid while stirring the carbon fiber bundles in the mixing tank.

15. The method of claim 9, comprising thickening the bundling solution while tumbling the carbon fiber bundles in a disc pelletizer.

16. The method of claim 1 , wherein the bundling solution is formulated with an uncured epoxy resin and the curing agent comprises an epoxy curing agent.

17. The method of claim 16 , wherein the epoxy hardener comprises a latent hardener.

18. The method according to claim 1 , wherein the bundling liquid contains at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin, and the curing agent contains a radical polymerization initiator.

19. 2. The method according to claim 1, wherein the bundling liquid contains an uncured resol-type phenolic resin, and the curing agent contains at least one selected from the group consisting of an organic acid, an inorganic acid, an amine, and a reaction product of at least one of a primary amine and a secondary amine with an isocyanate.

20. The method of claim 1 further comprising including a flame retardant in the bundling fluid.

21. The method of claim 1 , comprising providing at least a portion of the carbon fiber batting by defibrating chopped carbon fiber bundles.

22. The method of claim 1 , wherein the short carbon fibers comprise thermally degraded carbon fibers.

23. The method according to claim 1 , wherein the carbon fiber cotton contains fibers other than carbon fibers, and the fibers other than carbon fibers may be glass fibers.

24. The method according to claim 1 , wherein all or at least 99% by weight of the short carbon fibers have a fiber length of 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less.

25. The method of claim 1 , wherein the carbon fiber bundles have a bundle length of 3 mm or more, 5 mm or more, or 10 mm or more.

26. The method according to claim 1 , wherein the carbon fiber bundle does not contain any carbon fibers having a fiber length of less than 3 mm, or even if the carbon fiber bundle contains any carbon fibers, the content of the carbon fibers is less than 5 wt % of the total carbon fibers in the carbon fiber bundle.

27. The method according to claim 1 , wherein the carbon fiber bundle does not contain any carbon fibers having a fiber length of less than 5 mm, or even if the carbon fiber bundle contains any carbon fibers, the content of the carbon fibers is less than 5 wt % of the total carbon fibers in the carbon fiber bundle.

28. The method according to claim 1 , wherein the carbon fiber bundle does not contain any carbon fibers having a fiber length of less than 10 mm, or even if the carbon fiber bundle contains any carbon fibers, the content of the carbon fibers is less than 5 wt % of the total carbon fibers in the carbon fiber bundle.

29. 2. The method according to claim 1, wherein the carbon fiber bundle has a fiber weight content of 20 wt% or more and less than 30 wt%, 30 wt% or more and less than 40 wt%, 40 wt% or more and less than 50 wt%, 50 wt% or more and less than 60 wt%, 60 wt% or more and less than 70 wt%, or 70 wt% or more and less than 80 wt%.

30. 1. A method for producing a carbon fiber bundle composite, comprising: forming a carbon fiber bundle containing a bundling liquid by agglomerating discontinuous carbon fibers with the bundling liquid, the bundling liquid being a resin composition containing an uncured thermosetting resin and a curing agent.

31. The method according to claim 30, wherein the uncured thermosetting resin comprises at least one selected from the group consisting of an epoxy resin, a vinyl ester resin, an unsaturated polyester resin, a resol-type phenolic resin, a (meth)acrylate other than an epoxy vinyl ester, and a diallyl phthalate.

32. The method of claim 30, wherein the bundling liquid further comprises a reactive diluent.

33. The method of claim 30 , wherein the bundling liquid further comprises a thickening agent.

34. The method of claim 33, further comprising thickening the bundling liquid after forming the carbon fiber bundles.

35. The method of claim 30 , wherein the discontinuous carbon fibers comprise recycled carbon fibers.

36. The method of claim 35 , wherein the recycled carbon fibers comprise thermally degraded carbon fibers.

37. 31. The method of claim 30, wherein all or at least 99% by weight of the discontinuous carbon fibers have a fiber length of 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less.

38. The method of claim 30, wherein the carbon fiber bundles have a bundle length of 3 mm or more, 5 mm or more, or 10 mm or more.

39. The method according to claim 30, wherein the carbon fiber bundle does not contain any carbon fibers having a fiber length of less than 3 mm, or even if the carbon fiber bundle contains any carbon fibers, the content of the carbon fibers is less than 5 wt% of the total carbon fibers in the carbon fiber bundle.

40. The method according to claim 30, wherein the carbon fiber bundle does not contain any carbon fibers having a fiber length of less than 5 mm, or even if the carbon fiber bundle contains any carbon fibers, the content of the carbon fibers is less than 5 wt% of the total carbon fibers in the carbon fiber bundle.

41. The method according to claim 30, wherein the carbon fiber bundle does not contain any carbon fibers having a fiber length of less than 10 mm, or even if the carbon fiber bundle contains any carbon fibers, the content of the carbon fibers is less than 5 wt% of the total carbon fibers in the carbon fiber bundle.

42. 31. The method according to claim 30, wherein the fiber weight content of the carbon fiber bundle is 20 wt% or more and less than 30 wt%, 30 wt% or more and less than 40 wt%, 40 wt% or more and less than 50 wt%, 50 wt% or more and less than 60 wt%, 60 wt% or more and less than 70 wt%, or 70 wt% or more and less than 80 wt%.

43. 43. A method according to claim 42, wherein the fibre weight content is ≧30 wt% and less than 70 wt%, preferably ≧40 wt% and less than 60 wt%.

44. A carbon fiber bundle composite produced by the method according to any one of claims 1 to 43.

45. A method for producing a carbon fiber composite sheet, comprising: mixing carbon fiber cotton made of short carbon fibers with a bundling liquid containing an uncured thermosetting resin to obtain a carbon fiber bundle containing the bundling liquid; making the bundling liquid contain a curing agent to make the bundling liquid thermally curable; and bonding a plurality of the carbon fiber bundles to each other after the bundling liquid contains the curing agent.

46. The method according to claim 45, wherein the uncured thermosetting resin comprises at least one selected from the group consisting of an epoxy resin, a vinyl ester resin, an unsaturated polyester resin, and a resol-type phenolic resin.

47. The manufacturing method according to claim 45, wherein the viscosity of the bundling liquid when mixed with the carbon fiber cotton is 10 Pa·s or less, preferably 5 Pa·s or less, more preferably 1 Pa·s or less, and may be 0.001 Pa·s or more.

48. 46. ​​The method of claim 45, wherein the bundling liquid has a viscosity of less than or equal to 10 Pa.s at 50°C, preferably at 40°C, more preferably at 30°C.

49. The method of claim 45, wherein the bundling solution includes a reactive diluent.

50. The method of claim 45, wherein the bundling solution contains an uncured epoxy resin and a monoepoxide compound.

51. 46. ​​The method of claim 45, wherein the bundling solution comprises uncured epoxy resin and at least one of methylhexahydrophthalic anhydride and tetrahydromethylphthalic anhydride.

52. The method according to claim 45, wherein the bundling liquid contains at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin, and a compound having one or two ethylenically unsaturated groups in a molecule and having a viscosity of 1 Pa·s or less at 25°C.

53. The method of claim 45, wherein the bundling liquid is formulated with a thickening agent.

54. The method according to claim 45, wherein the bundling liquid contains an uncured epoxy resin and at least one selected from the group consisting of polyisocyanate, carboxylic acid anhydride, and amine.

55. The method according to claim 45, wherein the bundling liquid contains at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin, and a polyisocyanate.

56. The method according to claim 45, wherein the bundling liquid contains an uncured resol type phenolic resin and at least one selected from the group consisting of an alkaline earth metal hydroxide, an alkaline earth metal oxide, and a polyisocyanate.

57. The method for producing a carbon fiber bundle according to claim 53, further comprising: after the bundling liquid contains the curing agent, thickening the bundling liquid while contacting the plurality of carbon fiber bundles with each other.

58. 46. ​​The method of claim 45, wherein the bundling solution is formulated with an uncured epoxy resin and the curing agent comprises an epoxy curing agent.

59. 58. The method of claim 57, wherein the epoxy hardener comprises a latent hardener.

60. The method of claim 45, wherein the bundling liquid contains at least one of an uncured vinyl ester resin and an uncured unsaturated polyester resin, and the curing agent contains a radical polymerization initiator.

61. 46. ​​The method according to claim 45, wherein the bundling liquid contains an uncured resole phenolic resin, and the curing agent contains at least one selected from the group consisting of an organic acid, an inorganic acid, an amine, and a reaction product of at least one of a primary amine and a secondary amine with an isocyanate.

62. 46. ​​The method of claim 45, further comprising including a fire retardant in the bundling fluid.

63. 46. ​​The method of claim 45, comprising providing at least a portion of the carbon fiber batting by defibrating chopped carbon fiber bundles.

64. 46. ​​The method of claim 45, wherein the short length carbon fibers comprise thermally degraded carbon fibers.

65. The manufacturing method according to claim 45, wherein the carbon fiber cotton is mixed with fibers other than carbon fibers, and the fibers other than carbon fibers may be glass fibers.

66. The method of claim 45, wherein all or at least 99% by weight of the short carbon fibers have a fiber length of 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less.

67. 46. ​​The method of claim 45, wherein the carbon fiber bundles have a bundle length of 3 mm or more, 5 mm or more, or 10 mm or more.

68. The method according to claim 45, wherein the carbon fiber composite sheet does not contain any carbon fibers having a fiber length of less than 3 mm, or even if it does contain any carbon fibers, the content of such carbon fibers is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet.

69. The method according to claim 45, wherein the carbon fiber composite sheet does not contain any carbon fibers having a fiber length of less than 5 mm, or even if it does contain any carbon fibers, the content of such carbon fibers is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet.

70. The method according to claim 45, wherein the carbon fiber composite sheet does not contain any carbon fibers having a fiber length of less than 10 mm, or even if it does contain any carbon fibers, the content of such carbon fibers is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet.

71. 46. ​​The method of claim 45, wherein the carbon fiber composite sheet has a fiber weight content of 20 wt% or more and less than 30 wt%, 30 wt% or more and less than 40 wt%, 40 wt% or more and less than 50 wt%, 50 wt% or more and less than 60 wt%, 60 wt% or more and less than 70 wt%, or 70 wt% or more and less than 80 wt%.

72. 1. A method for producing a carbon fiber composite sheet, comprising: forming a carbon fiber bundle containing a bundling liquid by agglomerating discontinuous carbon fibers with the bundling liquid; and bonding a plurality of the carbon fiber bundles to each other, wherein the bundling liquid is a resin composition containing an uncured thermosetting resin and a curing agent.

73. 1. A method for producing a carbon fiber composite sheet, comprising: bringing a plurality of carbon fiber bundles into contact with each other and thickening a bundling liquid contained in each of the plurality of carbon fiber bundles, thereby bonding the plurality of carbon fiber bundles to each other, wherein discontinuous carbon fibers in each of the plurality of carbon fiber bundles are aggregated by the bundling liquid, and the bundling liquid is a resin composition containing an uncured thermosetting resin, a curing agent, and a thickener.

74. 1. A method for producing a carbon fiber composite sheet, comprising: depositing a plurality of carbon fiber bundles to form a carbon fiber bundle layer; compressing the carbon fiber bundle layer; and thereafter thickening a bundling liquid contained in each of the plurality of carbon fiber bundles, wherein discontinuous carbon fibers in each of the plurality of carbon fiber bundles are aggregated by the bundling liquid, and the bundling liquid is a resin composition containing an uncured thermosetting resin, a curing agent, and a thickener.

75. The uncured thermosetting resin comprises at least one selected from the group consisting of epoxy resins, vinyl ester resins, unsaturated polyester resins, resol-type phenolic resins, (meth)acrylates other than epoxy vinyl esters, and diallyl phthalates. The method according to any one of claims 72 to 74.

76. The method of any one of claims 72 to 74, wherein the bundling liquid further comprises a reactive diluent.

77. The method of any one of claims 72 to 74, wherein the discontinuous carbon fibers comprise recycled carbon fibers.

78. 78. The method of claim 77, wherein the recycled carbon fibers comprise thermally degraded carbon fibers.

79. The manufacturing method according to any one of claims 72 to 74, wherein all or at least 99% by weight of the carbon fibers contained in the carbon fiber composite sheet have a fiber length of 60 mm or less, 50 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less.

80. The manufacturing method according to any one of claims 72 to 74, wherein the plurality of carbon fiber bundles each have a bundle length of 3 mm or more, 5 mm or more, or 10 mm or more.

81. The carbon fiber composite sheet does not contain carbon fibers having a fiber length of less than 3 mm, or even if it contains carbon fibers, the content is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet. The manufacturing method according to any one of claims 72 to 74.

82. The carbon fiber composite sheet does not contain carbon fibers having a fiber length of less than 5 mm, or even if it contains carbon fibers, the content is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet. The manufacturing method according to any one of claims 72 to 74.

83. The carbon fiber composite sheet does not contain carbon fibers having a fiber length of less than 10 mm, or even if it contains carbon fibers, the content is less than 5 wt% of the total carbon fibers in the carbon fiber composite sheet. The manufacturing method according to any one of claims 72 to 74.

84. The carbon fiber composite sheet has a fiber weight content of 20 wt% or more and less than 30 wt%, 30 wt% or more and less than 40 wt%, 40 wt% or more and less than 50 wt%, 50 wt% or more and less than 60 wt%, 60 wt% or more and less than 70 wt%, or 70 wt% or more and less than 80 wt%. The manufacturing method according to any one of claims 72 to 74.

85. 85. A method according to claim 84, wherein the fibre weight content is ≧30 wt% and less than 70 wt%, preferably ≧40 wt% and less than 60 wt%.

86. A carbon fiber composite sheet manufactured by the manufacturing method according to any one of claims 45 to 74.

87. A method for producing a CFRP product, comprising curing the carbon fiber bundle composite according to claim 44 by applying heat and pressure in a mold.

88. A method for producing a CFRP product, comprising heating and pressurizing the carbon fiber composite sheet described in claim 86 in a molding mold to harden it.