Self-assembled carbon fiber bundles and manufacturing method therefor, and prepreg and manufacturing method therefor

Abstract

To provide a novel technique for readily manufacturing short carbon fiber bundles having smaller bundle sizes than continuous carbon fiber bundles used as a raw material; a technique for manufacturing carbon fiber bundles suitable for use as reinforcing materials for FRP from carbon fibers recovered from CFRP waste or prepreg scraps; a novel reinforcing material for thermosetting prepregs; and a novel thermosetting prepreg using SACFB as a reinforcing material and a method for manufacturing the same.SOLUTION: A method for manufacturing self-assembled carbon fiber bundles, comprising: mixing carbon fiber cotton with a bundling liquid to spontaneously bundle carbon fibers within the carbon fiber cotton and obtain a mixture; and removing a liquid component of the bundling liquid from the mixture. A method for manufacturing a prepreg, comprising impregnating a carbon fiber mat composed of a plurality of carbon fiber bundles, including the self-assembled carbon fiber bundles, with a liquid thermosetting resin composition.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a self-assembled carbon fiber bundle (hereinafter sometimes abbreviated as "SACFB") and a method for producing the same, and a prepreg using the SACFB as a reinforcing material and a method for producing the same. [Background technology]

[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 in automobiles, ships, railway vehicles, manned aircraft, unmanned aerial vehicles, and other transportation equipment, and its importance has been increasing in recent years.

[0003] One method for efficiently manufacturing CFRP products is compression molding prepregs, such as sheet molding compounds (SMCs). Prepregs are intermediate materials that consist of a matrix of uncured thermosetting resin compositions impregnated with fiber reinforcement. The fiber reinforcement material used in CF-SMC is a carbon fiber mat formed by cutting continuous carbon fiber bundles into chopped carbon fiber bundles and scattering the chopped carbon fiber bundles on a carrier film (for example, Patent Document 1). Continuous carbon fiber bundles are produced by baking acrylic fiber bundles made of mechanically bound fiber filaments, and are sized to maintain the bound state of the carbon fibers.

[0004] Although continuous carbon fiber bundles with a large bundle size, such as 48K, called large tow, have high production efficiency, some applications, including CF-SMC, require carbon fiber bundles with a small bundle size, such as 6K or less. Under these circumstances, a technique for dividing large tow into multiple tows has been developed (Patent Document 2).

[0005] It has been proposed to produce carbon fiber pellets for use in the production of fiber-reinforced thermoplastics (FRTP) by mixing chopped carbon fibers with a solution or suspension of a sizing agent, pelletizing the resulting agglomerates using a rotary disc pelletizer, and then drying the pellets (Patent Document 3).

[0006] It has been reported that when a small amount of chloroform is added to water containing a dispersion of short carbon fibers 3 to 5 mm in length and the water is shaken vigorously, the short carbon fibers aggregate and self-organize to form needle-like bundles. The group that reported this phenomenon states that carbon fibers recycled from waste can be bundled using this phenomenon and reused as raw materials for high-performance materials (Non-Patent Document 1).

[0007] [Patent Document 1] Japanese Patent Application Publication No. 1-163218 [Patent Document 2] U.S. Patent No. 6,385,828 [Patent Document 3] Special Publication No. 10-503812

[0008] [Non-Patent Document 1] JR Baxter, GR Palmese, NJ Alvarez, Applied Materials Today, 20 (2020) 100786 Summary of the Invention [Problem to be solved by the invention]

[0009] The present invention has been made with the object of solving at least one of the following problems. To provide a novel technique for easily producing short carbon fiber bundles having a bundle size smaller than that of continuous carbon fiber bundles using continuous carbon fiber bundles as a raw material. To provide a technique for producing carbon fiber bundles that can be easily used as reinforcing materials for FRP from carbon fibers recovered from CFRP waste or prepreg offcuts. To provide a novel reinforcement material for thermosetting prepregs. To provide a novel thermosetting prepreg using SACFB as a reinforcing material and a manufacturing method thereof. In this specification, problems that can be solved by each embodiment of the present invention may be explicitly or implicitly stated. [Means for solving the problem]

[0010] According to one aspect of the present invention, there is provided a prepreg comprising a thermosetting resin composition and self-assembled carbon fiber bundles impregnated with the thermosetting resin composition.

[0011] According to another aspect of the present invention, there is provided a prepreg comprising a carbon fiber mat formed of a plurality of carbon fiber bundles including self-assembled carbon fiber bundles and a thermosetting resin composition, wherein the carbon fiber mat is impregnated with the thermosetting resin composition.

[0012] According to yet another aspect of the present invention, there is provided a method for producing a prepreg, which comprises impregnating self-assembled carbon fiber bundles with a liquid thermosetting resin composition.

[0013] According to yet another aspect of the present invention, there is provided a method for producing a prepreg, the method comprising impregnating a carbon fiber mat composed of a plurality of carbon fiber bundles including self-assembled carbon fiber bundles with a liquid thermosetting resin composition.

[0014] According to yet another aspect of the present invention, there is provided a self-assembled carbon fiber bundle comprising a plurality of carbon fibers and an organic binder, wherein the plurality of carbon fibers have a fiber length of 60 mm or less, and the plurality of carbon fibers do not contain any carbon fibers with a fiber length of less than 5 mm, or if any, the content of such carbon fibers is less than 5 wt %.

[0015] According to yet another aspect of the present invention, there is provided a method for producing a self-assembled carbon fiber bundle, comprising: mixing carbon fiber cotton with a bundling liquid to obtain a mixture; and removing a liquid component of the bundling liquid from the mixture, wherein at least one of the carbon fiber cotton and the bundling liquid contains an organic binder; all carbon fibers contained in the carbon fiber cotton have a fiber length of 60 mm or less; and the carbon fiber cotton does not contain carbon fibers with a fiber length of less than 5 mm, or if it does contain carbon fibers, the content is less than 5 wt%. [Brief explanation of the drawings]

[0016] [Figure 1] Figures 1(a) to (d) are enlarged photographs showing the ends of the SACFB. [Figure 2] FIG. 2 is an enlarged photograph showing the end of a chopped carbon fiber bundle. [Figure 3] FIG. 3 is a conceptual diagram of a sheet prepreg manufacturing device. [Figure 4] Figure 4 shows a photograph of the SACFB obtained in Experiment 1. [Figure 5] FIG. 5 is a schematic diagram illustrating the relationship between the bundle length of the SACFB and the fiber length of the carbon fibers constituting the SACFB. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an embodiment of the present invention will be described in detail.

[0018] 1. Self-assembled carbon fiber bundles (SACFB) The SACFB is formed through a process in which a plurality of short carbon fibers are aggregated to form a bundle. Prior to forming the SACFB, the plurality of short carbon fibers may all be monofilaments. That is, the SACFB may be formed by aggregating a plurality of carbon fiber filaments to form a bundle. In another example, the SACFB may be formed by aggregating a plurality of fine carbon fiber bundles, each consisting of a small number of filaments, for example, less than 100. In yet another example, the SACFB may be formed by aggregating a plurality of carbon fiber filaments and a plurality of fine carbon fiber bundles.

[0019] At the end of the SACFB, the tips of the multiple carbon fibers constituting the bundle are not aligned, as shown in Figures 1(a) to 1(d). This is in contrast to the end of the chopped carbon fiber bundle obtained by cutting a continuous carbon fiber bundle, where the tips of the multiple carbon fibers are aligned, as shown in Figure 2. As shown in Figure 5, the bundle length of a SACFB is generally longer than the number average fiber length of the carbon fibers contained in the SACFB.

[0020] The number of carbon fibers contained in one SACFB (the bundle size of the SACFB) is preferably within the range of 1.5K or more and less than 4.5K. Here, "K" is the symbol for 1000. For example, 1K means 1000. 10K means 10000. 100K means 100000.

[0021] The shorter the fiber length, the less likely the carbon fibers are to be entangled when they are bundled by self-organization, and the easier it is to impregnate the SACFB with resin. Therefore, the fiber length of all short carbon fibers constituting the SACFB is preferably 60 mm or less, more preferably 40 mm or less, even more preferably 30 mm or less, and may be 20 mm or less.

[0022] On the other hand, carbon fibers with too short a fiber length have a low reinforcing effect when used in FRP. For this reason, the fiber length of the carbon fibers constituting the SACFB is preferably 5 mm or more, and more preferably 10 mm or more. SACFBs containing carbon fibers with a majority of fiber lengths of 5 mm or greater typically have bundle lengths greater than 5 mm. SACFBs containing carbon fibers with a majority of fiber lengths of 10 mm or greater typically have bundle lengths greater than 10 mm.

[0023] By limiting the amount of carbon fibers shorter than a certain length, the reinforcing effect of the SACFB when used as a reinforcing material for FRP can be improved. For example, it is preferable that the SACFB does not contain carbon fibers shorter than fiber length L1 (mm), or if it does contain such fibers, the amount is less than 5 wt% of the carbon fibers constituting the SACFB. Here, L1 can be 5, 6, 7, 8, 9, or 10. SACFBs with a larger L1 exhibit a higher reinforcing effect when used in FRP.

[0024] For example, by constructing the SACFB from only carbon fibers having the same fiber length, it is possible to reduce variation in the quality of the SACFB between production lots. The difference between the maximum and minimum fiber lengths of the multiple carbon fibers constituting the SACFB is preferably within 5 mm, more preferably within 4 mm, and even more preferably within 3 mm.

[0025] There are no particular restrictions on the filament diameter of the carbon fibers that make up the SACFB, as long as it is within the range of filament diameters that PAN-based carbon fibers normally have, for example, within the range of 5 μm to 15 μm.

[0026] SACFB contains an organic binder that bonds the short carbon fibers together. Suitable examples of organic binder materials are resins used for sizing common carbon fiber bundles available on the market. In other words, they can be called component resins of sizing agents. Examples of such resins include, but are not limited to, epoxy resins, unsaturated polyester resins, vinyl ester resins (also known as epoxy acrylate resins), polyurethane resins, and polyamide resins. These resins may be used alone or in combination of two or more. In addition to the above resins, a surfactant may be blended into the organic binder. The organic binder may be the same sizing agent used in common carbon fiber bundles.

[0027] The organic binder content of the SACFB can be 0.5 wt% or more and less than 1.5 wt%, 1.5 wt% or more and less than 3 wt%, 3 wt% or more and less than 5 wt%, 5 wt% or more and less than 7 wt%, or 7 wt% or more and less than 10 wt%.

[0028] The carbon fibers constituting the SACFB may all be non-thermally degraded carbon fibers, or may be partially non-thermally degraded carbon fibers and the remaining partially thermally degraded carbon fibers. The carbon fibers constituting the SACFB may all be thermally degraded carbon fibers. Non-thermally degraded carbon fibers are typically virgin carbon fibers. Thermally degraded carbon fibers are typically recycled carbon fibers recovered from CFRP waste, and are thermally degraded in the process of pyrolysis and removal of the matrix resin.

[0029] Although SACFB preferably contains only carbon fiber as a fiber component, it is acceptable to contain fibers other than carbon fiber if there is no particular problem. For example, recycled carbon fiber containing glass fiber is preferably used as a raw material for SACFB while still containing the glass fiber, from the viewpoint of reducing the burden on the environment. From the viewpoint of stabilizing the quality of the SACFB, the amount of fibers other than carbon fibers that the SACFB may contain is preferably less than 10 wt%, more preferably less than 5 wt%, and even more preferably less than 1 wt% of the total fiber components.

[0030] 2. Manufacturing method of self-assembled carbon fiber bundles (SACFB) Regarding the manufacturing method of SACFB, an embodiment using virgin carbon fiber as the raw material and an embodiment using recycled carbon fiber will be described.

[0031] 2.1. Use of virgin carbon fiber as raw material The SACFB made of virgin carbon fibers can be produced, for example, through the following steps (i) to (iii). (i) Chopping process (ii) Defibration process (iii) Bundling process However, as will be described later, the defibrating step (ii) may be omitted. Each step will be described in detail below.

[0032] (i) Chopping process In the chopping step, continuous carbon fiber bundles made of virgin carbon fibers, that is, new continuous carbon fiber bundles, are cut to a predetermined length using, for example, a rotary cutter to obtain chopped carbon fiber bundles.

[0033] 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.

[0034] The larger the bundle size of the continuous carbon fiber bundle, the more SACFBs can be obtained from one piece of chopped carbon fiber bundle, resulting in higher production efficiency. In addition, the larger the bundle size, the lower the production cost of the continuous carbon fiber bundle. 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.

[0035] The fiber length of the chopped carbon fiber bundle may be set according to the bundle length of the SACFB to be produced, and is not particularly limited, but is preferably 6 mm or more, and may be 7 mm or more, 8 mm or more, 9 mm or more, or 10 mm or more, and is preferably 60 mm or less, more preferably 40 mm or less, and even more preferably 30 mm or less, and may be 20 mm or less. Most of the carbon fibers contained in the chopped carbon fiber bundle participate in the formation of the SACFB while maintaining their length.

[0036] The bundle length of a SACFB formed from multiple short carbon fibers, most of which have the same fiber length, is approximately the same as or longer than the fiber length of the short carbon fibers. When substantially all of the short carbon fibers contained in the SACFB have a fiber length of L2 (mm), the bundle length of the SACFB is often within the range of L2 + 1 (mm) to L2 × 1.4 (mm). However, there are exceptions. In one example, chopped carbon fiber bundles having different fiber lengths may be mixed and used, but to reduce the variation in quality of SACFB between production lots, it is preferable to use only chopped carbon fiber bundles having the same fiber length.

[0037] (ii) Defibration process In the defibrating step, the chopped carbon bundles obtained in the chopping step are loosened to obtain carbon fiber batting. The carbon fiber batting may consist of only monofilaments, or may contain fine carbon fiber bundles consisting of a small number of filaments, for example, less than 100 filaments.

[0038] For the defibration, a general defibrator can be preferably used, but there is no limitation thereto. 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. In this case, the resulting carbon fiber floss can be used in the subsequent bundling process without being removed from the stirring mixer.

[0039] For example, chopped carbon fiber bundles can also 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 irradiating them with ultrasonic waves. After the sizing agent is washed away, flocculent carbon fibers remain. Since this method results in the loss of at least a portion of the sizing agent contained in the chopped carbon fiber bundles, it is desirable to supplement the organic binder in the subsequent bundling process.

[0040] (iii) Bundling process In the bundling process, the carbon fiber swath obtained in the defibration process is mixed with a bundling liquid to spontaneously bundle the short carbon fibers. After bundling, the liquid component in the bundling liquid is evaporated and removed.

[0041] A preferred liquid component contained in the bundling liquid is water. The strong surface tension of water strongly promotes bundling of short carbon fibers by capillary effect. In addition, water is non-flammable, so it is also preferred in that it does not require an explosion-proof stirring device.

[0042] The amount of the bundling liquid is, for example, 30 to 100 parts by weight per 100 parts by weight of the short carbon fibers, but is not limited thereto. The amount of the bundling liquid can be appropriately adjusted while observing the state of the mixture.

[0043] An organic binder may be added to the bundling liquid as a dispersoid or a solute. When dispersing the organic binder in the bundling liquid containing water as the main component, an organic solvent may be used as a dispersion aid.

[0044] When the amount of organic binder is sufficient, the bundle size distribution among the formed SACFBs tends to be narrow, which is due to the promotion of the formation of SACFBs with relatively small bundle sizes below 4.5 K.

[0045] When determining the amount of organic binder to be added to the bundling solution, it is desirable to consider the amount of sizing agent contained in the raw virgin carbon fiber bundles, because this sizing agent remains in the final product, SACFB, while retaining its ability as an organic binder.

[0046] There is no limitation on the method for mixing the carbon fiber cotton and the bundling liquid, but stirring is preferred for efficient mixing in a short time. For stirring, a powder stirring mixer known as a Henschel mixer can be preferably used. The stirring mixer may be a type equipped with only an agitator blade (stirring impeller), or may be one equipped with a chopper.

[0047] In one example, carbon fiber bundles produced by stirring and mixing carbon fiber cotton with a bundling liquid can be modified by processing them in a tumbling granulator before removing the liquid components in the bundling liquid. In another example, the defibration step can be omitted, and the chopped carbon fiber bundles can be mixed with the bundling liquid using a stirring mixer, thereby performing defibration and bundling by self-organization at the same time. In order to produce SACFB with stable quality, it is preferable to provide the defibration step.

[0048] 2.2. Use of recycled carbon fiber In the manufacturing method of SACFB described in 2.1 above, all or part of the virgin carbon fiber raw material can be replaced with recycled carbon fiber.

[0049] A suitable example of recycled carbon fiber is carbon fiber recovered from CFRP waste or SMC scraps generated from products molded from SMC. For example, the waste or scrap is dry distilled at a temperature of preferably 600°C or higher, and then heated to, for example, 550°C or higher, preferably 600°C or higher, in an oxidizing atmosphere to completely pyrolyze the matrix resin. This leaves cotton-like recycled carbon fiber with a fiber length of 60 mm or less. Recycled carbon fiber has been thermally degraded and is weaker than virgin carbon fiber, but it is still strong enough to be used as a reinforcing material for FRP.

[0050] Another method for recovering carbon fiber from CFRP waste is to decompose the matrix resin using a subcritical or supercritical fluid. The matrix resin is completely removed to obtain a flocculent recycled carbon fiber. Residual resin that cannot be completely removed by this method can be removed by heat treatment in an oxidizing atmosphere. The recycled carbon fiber obtained by this method using a subcritical or supercritical fluid is also thermally degraded, and therefore has lower strength than virgin carbon fiber. As yet another method for recovering carbon fibers from CFRP waste, a method of decomposing the matrix resin by microwave heating can also be used.

[0051] Another method for recovering carbon fiber from SMC scraps 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 fiber that has the same strength as virgin carbon fiber and is not thermally degraded.

[0052] CFRP molded from SMC or recycled carbon fibers recovered from SMC are short fibers with a fiber length of 60 mm or less, and do not require further cutting when used to manufacture SACFB. When using these recycled carbon fibers as raw materials with the sizing agent removed, the (ii) defibration step is not necessary, and only the aforementioned (iii) bundling step is required. An organic binder must be added to the bundling solution.

[0053] 3. Prepreg A prepreg can be produced by impregnating the SACFB with a liquid thermosetting resin composition. Impregnation can be carried out, for example, by immersing the SACFB in a liquid thermosetting resin composition.

[0054] The fiber content of prepregs using SACFB can be 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%, 70 wt% or more but less than 80 wt%, or 80 wt% or more but less than 90 wt%.

[0055] The higher the fiber content, the better the mechanical properties of the CFRP obtained by curing the prepreg.The lower the fiber content, the easier it is to flow during pressure molding, which increases the degree of freedom in designing the shape of CFRP products molded from prepregs.

[0056] Examples of the base resin of the liquid thermosetting resin composition include, but are not limited to, vinyl ester resin, unsaturated polyester resin, epoxy resin, polyimide resin, maleimide resin, and phenolic resin. The liquid thermosetting resin composition may contain one type of thermosetting resin or two or more types of thermosetting resins.

[0057] Liquid thermosetting resin compositions usually contain a curing agent in addition to a base resin. In addition, the liquid thermosetting resin composition may contain, as necessary, polymerization inhibitors, thickeners, reactive diluents, low-shrinkage agents, antioxidants, internal mold release agents, colorants, modifiers (e.g., rubber, elastomers, or thermoplastic resins), flame retardants, antibacterial agents, etc.

[0058] The liquid thermosetting resin composition is preferably solvent-free, i.e., not a varnish. If it is a varnish, a process of evaporating and removing the solvent is required after impregnating the SACFB. Reactive diluents are not included in the solvents referred to here.

[0059] One suitable example of a liquid thermosetting resin composition that is not a varnish is a vinyl ester resin-based composition that contains a vinyl ester resin, an unsaturated polyester resin, an ethylenically unsaturated monomer, a thickener, a polymerization initiator, and a polymerization inhibitor.

[0060] Suitable examples of vinyl ester resins include bisphenol A epoxy vinyl ester resins and novolac vinyl ester resins. Either one of these may be blended, or both may be blended. The weight ratio of the blended vinyl ester resin to the unsaturated polyester resin may be 1:9 to 9:1, 1:7 to 7:1, 1:4 to 4:1, or 1:2 to 2:1, etc.

[0061] The ethylenically unsaturated monomer is blended as a reactive diluent. At least one monofunctional ethylenically unsaturated monomer, at least one polyfunctional ethylenically unsaturated monomer, or both of these may be blended. A suitable example of the monofunctional ethylenically unsaturated monomer is styrene. Other examples include, but are not limited to, monofunctional (meth)acrylates such as 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, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, and dicyclopentanyl methacrylate. Examples of polyfunctional ethylenically unsaturated monomers include, but are not limited to, difunctional (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.

[0062] The thickener is a polyisocyanate. A polyisocyanate is an organic compound having two or more isocyanate groups (-NCO) per molecule. Suitable examples of polyisocyanates are diisocyanates such as 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, diphenylmethane diisocyanate, isophorone diisocyanate, and hexamethylene diisocyanate.

[0063] The polymerization initiator includes organic peroxides that are commonly used as curing agents for vinyl ester resins and unsaturated polyester resins, such as ketone peroxides, hydroperoxides, diacyl peroxides, dialkyl peroxides, peroxyketals, alkyl peresters, and percarbonates.

[0064] The polymerization inhibitor can be appropriately selected from various compounds generally known as polymerization inhibitors, and suitable examples include catechol, hydroquinone, and benzoquinone.

[0065] Another suitable example of a liquid thermosetting resin composition that is not a varnish is an epoxy resin-based composition, which contains an epoxy resin and an epoxy curing agent, and optionally a thickener.

[0066] There is no limitation on the type of epoxy resin, 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.

[0067] In a preferred example, a bisphenol-type epoxy resin such as bisphenol A-type epoxy resin or bisphenol F-type epoxy resin is blended. Some commercially available liquid bisphenol-type epoxy resins have a low viscosity of 5 Pa·s or less at 25°C. The bisphenol-type epoxy resin may account for 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 blended.

[0068] It is preferable to use a latent curing agent as the epoxy curing agent. A latent curing agent is a solid that has low solubility in epoxy resins at room temperature, but when heated, it melts or dissolves in the epoxy resin and functions as a curing agent. Various imidazoles, dicyandiamide and boron trifluoride-amine complexes are typical examples of latent hardeners. 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] Suitable examples of the substituted imidazole that functions as a 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 imidazole-based latent curing agents. Isocyanuric acid adducts of various substituted imidazoles such as 2-phenylimidazole, 2-methylimidazole, 2-phenyl-4,5-dihydroxymethylimidazole, and 2-phenyl-4-methyl-5-hydroxymethylimidazole, and 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. 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 resin.

[0071] The latent curing agent may be used alone or in combination of two or more. When dicyandiamide is used as the latent curing agent, urea derivatives such as 4,4'-methylenebis(phenyldimethylurea) and 2,4-bis(3,3-dimethylureido)toluene can be preferably used as curing accelerators. In addition to or instead of the latent hardeners, epoxy hardeners other than latent hardeners, such as carboxylic acid anhydrides, aromatic amines, and phenolic resins, can also be used.

[0072] Among carboxylic acid anhydrides, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, and methyl-5-norbornene-2,3-dicarboxylic anhydride (methyl-3,6-endomethylene-1,2,3,6-tetrahydrophthalic anhydride) all have a viscosity of less than 0.5 Pa s at 25°C, and therefore may be used for the purpose of lowering the viscosity of the composition. It is known that carboxylic acid anhydrides react with epoxy compounds at low temperatures to form bonds with the aid of the catalytic action of a tertiary amine, which may be glycidylamine. Therefore, when 20 parts by weight or less of carboxylic acid anhydride is blended with a tertiary amine per 100 parts by weight of epoxy compound, it acts as a thickener.

[0073] Amine compounds also function as thickeners when blended in an amount such that the active hydrogen per epoxy group is 0.1 to 0.5 equivalents. Examples of amine compounds that can be preferably used as thickeners include, but are not limited to, isophoronediamine, bis(4-aminocyclohexyl)methane, and 1,3-bis(aminomethyl)cyclohexane.

[0074] Polyisocyanates which may be diisocyanates, particularly diisocyanates having an aromatic ring in the molecular structure such as bis(4-isocyanatophenyl)methane and toluene diisocyanate, are suitable examples of thickeners. Polyisocyanates exhibit a stronger thickening effect when blended with polyols, examples of which include, but are not limited to, ethylene glycol, polyethylene glycol, isosorbide, neopentyl glycol, cyclohexanediol, cyclohexanedimethanol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, and 1,6-hexanediol.

[0075] The flame retardants that can be incorporated into the liquid thermosetting resin composition 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.

[0076] 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.

[0077] 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). 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. An example of an alkylphenylphosphinic acid is methylphenylphosphinic acid. The metal phosphinate may be, but is not limited to, aluminum phosphinate, zinc phosphinate, calcium phosphinate, magnesium phosphinate, and the like. Further examples of phosphorus-containing flame retardants include red phosphorus, ammonium polyphosphate, melamine phosphate, guanidine phosphate, and guanylurea phosphate.

[0078] In addition to the phosphorus-containing flame retardant, the liquid thermosetting resin composition may contain a phosphorus-free flame retardant. Examples of the phosphorus-free flame retardant include melamine compounds such as melamine cyanurate, triazine compounds, guanidine compounds, nitrogen-based flame retardants such as ammonium phosphate and ammonium carbonate, hydrated metals such as aluminum hydroxide and magnesium hydroxide, and organic metal salt-based flame retardants such as ferrocene and acetylacetone metal complexes. In a preferred embodiment, all materials, including the flame retardant, that are formulated into the liquid thermosetting resin composition are selected to be halogen-free, thereby producing a halogen-free, flame-retardant SACFB prepreg.

[0079] An example of a prepreg that can be produced using SACFB is a sheet prepreg. In a suitable example, the sheet prepreg can be produced through the following first to fourth steps.

[0080] First step: applying a liquid thermosetting resin composition to the surface of each of the first protective film and the second protective film. Second step: A step of depositing a plurality of short carbon fiber bundles containing SACFB onto the surface of the first protective film to which the liquid thermosetting resin composition has been applied to form a carbon fiber mat. Third step: A step of forming a laminate by bonding the second protective film to the first protective film with the carbon fiber mat sandwiched therebetween so that the surfaces coated with the liquid thermosetting resin composition face each other. Fourth step: A step of impregnating the carbon fiber mat with the liquid thermosetting resin composition by pressing the laminate to obtain a sheet prepreg.

[0081] The first protective film and the second protective film are synthetic resin films, and the material thereof can be appropriately selected from polyolefins such as polyethylene and polypropylene, polyvinylidene chloride, vinyl chloride resins, polyamides, etc. The first protective film and the second protective film may be multilayer films. The specifications of the first protective film and the second protective film may be the same or different.

[0082] In the second step, a carbon fiber mat is formed by depositing, for example by scattering, a plurality of short carbon fiber bundles containing SACFB on the surface of the first protective film to which the liquid thermosetting resin composition has been applied.

[0083] The amount of liquid thermosetting resin composition applied to the first protective film and the second protective film in the first step and the basis weight of the carbon fiber mat formed on the first protective film in the second step are adjusted taking into account the basis weight and fiber content of the sheet prepreg to be produced.

[0084] When a thickener is blended in the liquid thermosetting resin composition, the prepreg is aged after the fourth step until the viscosity of the liquid thermosetting resin composition becomes sufficiently high.

[0085] In the above procedure, the first protective film and the second protective film may be carrier films unwound from a roll. In a preferred embodiment, a sheet prepreg manufacturing apparatus, the conceptual diagram of which is shown in FIG. 3, can be used to continuously manufacture long sheet prepregs. The sheet prepreg manufacturing apparatus shown in Figure 3 has a section for applying a liquid thermosetting resin composition to a first protective film unwound from a roll, a section for scattering short carbon fiber bundles on the first protective film to deposit a carbon fiber mat, a section for applying a liquid thermosetting resin composition to a second protective film unwound from a roll, a section for bonding the second protective film to the first protective film to form a laminate, a section for pressurizing the laminate, and a section for winding up the laminate.

[0086] The weight per unit area of ​​the sheet prepreg using SACFB can be appropriately designed depending on the application. The weight per unit area is, for example, 300 g / m 2 More than 500g / m 2 Less than 500g / m 2 More than 1000g / m 2 Less than 1000g / m 2 More than 2000g / m 2 Less than 2000g / m 2 More than 4000g / m 2 Less than 4000g / m 2 More than 6000g / m 2 Less than 6000g / m 2 More than 8000g / m 2 Less than or equal to 8000g / m 2 More than 10000g / m 2 It may be less than. The thickness of the sheet prepreg 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.

[0087] When manufacturing CFRP products from prepregs using SACFB, the preferred molding method is press molding, but it is not limited to this. For example, molding methods other than press molding, such as autoclave molding, can also be used.

[0088] 4. Experimental Results The following are the results of experiments conducted by the present inventors.

[0089] [Experiment 1] A new continuous carbon fiber bundle consisting of carbon fibers with a filament diameter of 7 μm, a bundle size of 15K, and containing 1 wt% of a sizing agent was cut with a rotary cutter to obtain chopped carbon fiber bundles with a fiber length of 25 mm (approximately 1 inch). 1000 g of this chopped carbon fiber bundle was placed in the mixing vessel of a stirring mixer "SP Granulator" (SPG-25T, manufactured by Dalton Co., Ltd.) and stirred without adding any liquid to obtain carbon fiber cotton. Without removing the carbon fiber cotton, water was added to the mixing tank of the agitator mixer so that the weight ratio of carbon fiber cotton to water was 100:40, and the mixture was stirred for 7 minutes. The peripheral speed of the agitator blade tip was 4 m / s for the first minute, 8 m / s for the next 3 minutes, and 4 m / s for the next 3 minutes. After stirring, the mixture was observed and found to have spontaneously bundled short carbon fibers. The mixture was then dried at 110°C for 30 minutes using a vibrating hot air dryer to obtain SACFB. Since no organic binder was added to the water used as the bundling liquid, the only organic binder contained in this SACFB was the sizing agent contained in the raw continuous carbon fiber bundle. Since it is believed that the entire amount of this sizing agent remains, the organic binder content of the obtained SACFB is 1 wt%.

[0090] Figure 4 shows a photograph of the appearance of the SACFB fabricated in Experiment 1. The SACFB had a flat outer shape. A sample of 300 SACFBs was obtained and examined, and it was found that approximately 80% of the SACFBs had a bundle size of less than 15K. The bundle size distribution of SACFBs was broad, and when the frequency distribution of bundle sizes was expressed with a class width of 1K, no class accounted for more than 10%. When 50 of the 300 SACFBs were examined, the bundle length of the majority of them was within the range of 26 mm to 35 mm, that is, within the range of the carbon fiber length (25 mm) plus 1 mm to 1.4 times the carbon fiber length. The fabricated SACFB was immersed in liquid epoxy resin (jER (registered trademark) 807, manufactured by Mitsubishi Chemical) and left at room temperature for three days. As the resin penetrated between the filaments, the bundle loosened and its width (long diameter of the cross section) more than doubled.

[0091] [Experiment 2] The SACFB was fabricated in the same manner as in Experiment 1, except that an aqueous dispersion containing a 2.5 wt% concentration of a carbon fiber sizing agent containing epoxy resin as the main component was used as the bundling liquid. Combined with the sizing agent contained in the raw continuous carbon fiber bundle, the organic binder content of the resulting SACFB was 2 wt%. The appearance of the SACFB obtained in Experiment 2 was similar to that obtained in Experiment 1. The bundle size distribution of the SACFB was narrower than that in Experiment 1, and when sampled and examined in the same manner as in Experiment 1, 90% had a bundle size of less than 8K, and furthermore, the majority had a bundle size between 1.5K and less than 4.5K. Furthermore, when 50 of the 300 SACFBs were examined, more than 60% by number had bundle sizes within the range of 26 mm to 35 mm.

[0092] [Experiment 3] A liquid thermosetting resin composition containing vinyl ester resin, unsaturated polyester resin, styrene, polyisocyanate, and a radical polymerization initiator was impregnated into the SACFB prepared in Experiment 2 to prepare a sheet prepreg. The procedure is as follows:

[0093] Three 40 cm x 40 cm polyethylene films (polyethylene films A to C) were prepared, and a paste consisting of the liquid thermosetting resin composition described above was applied to a 30 cm x 30 cm area on one side of two of them (polyethylene films A and B). Next, the SACFB prepared in Experiment 2 was scattered on a piece of polyethylene film (polyethylene film C) to which no paste had been applied, and a carbon fiber mat was deposited. Next, one of the polyethylene films (polyethylene film A) having the above paste applied to one side thereof was placed on the carbon fiber mat, with the paste-applied side facing downwards. Next, the two films were turned upside down so that polyethylene film C, which had not been coated with paste, was on top and polyethylene film A was on the bottom. In this state, another polyethylene film B, one side of which had been coated with paste, was then placed in place of polyethylene film C. In this way, a laminate was obtained in which two polyethylene films A and B, each coated with paste, were bonded together with the sides coated with paste facing each other, with the carbon fiber mat sandwiched between them. Next, the laminate was pressed from both sides in the thickness direction to impregnate the carbon fiber mat with the paste. Thereafter, the mixture was left at 25°C for 7 days to thicken the paste. The weight of the obtained sheet prepreg was 2000 g / m 2 The fiber content was 53 wt%.

[0094] Two 26cm x 26cm prepreg pieces cut from the prepared sheet prepreg were stacked on top of each other and press-molded at a temperature of 140°C, a pressure of 8MPa, and a pressing time of 2 minutes to produce a CFRP plate measuring 30cm in length and width and 2mm in thickness. Test pieces measuring 8 mm wide and 60 mm long were cut from the CFRP plate and subjected to bending tests using a universal testing machine (Instron 4465, manufactured by Instron) at a crosshead speed of 2 mm / min and a span of 32 mm (16 times the thickness of the CFRP plate). The average measurement results for the six test pieces were a bending strength of 360 MPa and a bending modulus of 24 GPa.

[0095] [Experiment 4] A carbon fiber mat having a basis weight of 2000 g / m was prepared in the same manner as in Experiment 3, except that the same chopped carbon fiber bundles as those prepared in Experiment 1 were used instead of the SACFB, i.e., chopped carbon fiber bundles having a fiber length of 25 mm (about 1 inch) obtained by cutting continuous carbon fiber bundles having a bundle size of 15K with a rotary cutter. 2 A sheet prepreg with a CF content of 53 wt% was produced. A CFRP plate was fabricated from this sheet prepreg in the same manner as in Experiment 3, and the mechanical properties of the CFRP plate were evaluated, revealing a bending strength of 251 MPa and a bending modulus of elasticity of 23 GPa.

[0096] 5. Summary of embodiments Embodiments of the present invention include, but are not limited to, the following. [Embodiment 1] A prepreg comprising a thermosetting resin composition and self-assembled carbon fiber bundles impregnated with the thermosetting resin composition. [Embodiment 2] A prepreg according to embodiment 1, wherein all of the carbon fibers contained in the self-assembled carbon fiber bundles have a fiber length of 60 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less. [Embodiment 3] The prepreg according to embodiment 2, wherein at least some of the carbon fibers have a fiber length of 5 mm or more. [Embodiment 4] The prepreg according to embodiment 2, wherein at least some of the carbon fibers have a fiber length of 10 mm or more. [Embodiment 5] A prepreg according to any one of embodiments 2 to 4, wherein the carbon fibers do not contain carbon fibers with a fiber length of less than 5 mm, or even if they do contain carbon fibers, the content is less than 5 wt %. [Embodiment 6] A prepreg according to embodiment 2 or 4, wherein the carbon fibers do not contain carbon fibers with a fiber length of less than 10 mm, or even if they do contain carbon fibers, the content is less than 5 wt%. [Embodiment 7] A prepreg according to embodiment 4 or 6, wherein the difference between the maximum and minimum fiber lengths of the carbon fibers is within 5 mm, 4 mm, or 3 mm. [Embodiment 8] A prepreg according to any one of embodiments 1 to 7, wherein the bundle size of the self-assembled carbon fiber bundles is within the range of 1.5K or more and less than 4.5K. [Embodiment 9] The prepreg according to any one of embodiments 1 to 8, wherein the self-assembled carbon fiber bundles contain carbon fibers that have not been thermally deteriorated. [Embodiment 10] The prepreg according to any one of embodiments 1 to 8, wherein the self-assembled carbon fiber bundles contain thermally deteriorated carbon fibers. [Embodiment 11] A prepreg according to any one of embodiments 1 to 8, wherein all of the carbon fibers contained in the self-assembled carbon fiber bundles are thermally deteriorated carbon fibers. [Embodiment 12] A prepreg according to any one of embodiments 1 to 11, wherein the self-assembled carbon fiber bundles contain at least one resin selected from the group consisting of epoxy resin, unsaturated polyester resin, vinyl ester resin, polyurethane resin, and polyamide resin. [Embodiment 13] The prepreg according to any one of embodiments 1 to 11, wherein the self-assembled carbon fiber bundles contain an organic binder. [Embodiment 14] A prepreg according to any one of embodiments 1 to 13, in which at least one resin selected from the group consisting of vinyl ester resin, unsaturated polyester resin, epoxy resin, polyimide resin, maleimide resin, and phenolic resin is blended into the thermosetting resin composition. [Embodiment 15] The prepreg according to any one of embodiments 1 to 14, wherein a flame retardant is blended into the thermosetting resin composition. [Embodiment 16] A method for producing a carbon fiber reinforced plastic product, comprising curing the prepreg according to any one of embodiments 1 to 15. [Embodiment 17] A method for producing a prepreg according to any one of embodiments 1 to 15, comprising impregnating the self-assembled carbon fiber bundles with the liquid thermosetting resin composition.

[0097] [Embodiment 18] A method for producing a prepreg, comprising impregnating a carbon fiber mat consisting of a plurality of carbon fiber bundles, each of which includes one or more self-assembled carbon fiber bundles, with a liquid thermosetting resin composition. [Embodiment 19] A method for producing a prepreg according to embodiment 18, comprising: applying the liquid thermosetting resin composition to the surface of each of a first protective film and a second protective film; forming the carbon fiber mat on the surface of the first protective film on which the liquid thermosetting resin composition has been applied; bonding the first protective film and the second protective film together with the carbon fiber mat sandwiched therebetween so that the surfaces on which the liquid thermosetting resin composition has been applied face each other to form a laminate; and pressurizing the laminate to impregnate the carbon fiber mat with the liquid thermosetting resin composition. [Embodiment 20] A method for producing a prepreg according to embodiment 18 or 19, wherein all of the carbon fibers contained in at least one of the one or more self-assembled carbon fiber bundles have a fiber length of 60 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less. [Embodiment 21] A method for producing a prepreg according to embodiment 20, wherein at least one of the one or more self-assembled carbon fiber bundles has at least a portion of the carbon fibers having a fiber length of 5 mm or more. [Embodiment 22] A method for producing a prepreg according to embodiment 20, wherein at least one of the one or more self-assembled carbon fiber bundles has at least a portion of the carbon fibers having a fiber length of 10 mm or more. [Embodiment 23] A method for producing a prepreg according to any one of embodiments 20 to 22, wherein at least one of the one or more self-assembled carbon fiber bundles does not contain carbon fibers with a fiber length of less than 5 mm, or if it does contain carbon fibers, the content is less than 5 wt%. [Embodiment 24] A method for producing a prepreg according to embodiment 20 or 22, wherein at least one of the one or more self-assembled carbon fiber bundles does not contain carbon fibers with a fiber length of less than 10 mm, or if it does contain carbon fibers, the content is less than 5 wt%. [Embodiment 25] A method for producing a prepreg according to embodiment 22 or 24, wherein the difference between the maximum and minimum fiber lengths of the carbon fibers contained in at least one of the one or more self-assembled carbon fiber bundles is within 5 mm, within 4 mm, or within 3 mm. [Embodiment 26] A method for producing a prepreg according to any one of embodiments 20 to 25, wherein at least one of the one or more self-assembled carbon fiber bundles has a bundle size within the range of 1.5K or more and less than 4.5K. [Embodiment 27] A method for producing a prepreg according to any one of embodiments 20 to 26, wherein at least one of the one or more self-assembled carbon fiber bundles includes carbon fibers that have not been thermally deteriorated. [Embodiment 28] A method for producing a prepreg according to any one of embodiments 20 to 26, wherein at least one of the one or more self-assembled carbon fiber bundles includes thermally deteriorated carbon fiber. [Embodiment 29] A method for producing a prepreg according to any one of embodiments 20 to 26, wherein all of the carbon fibers contained in at least one of the one or more self-assembled carbon fiber bundles are thermally deteriorated carbon fibers. [Embodiment 30] A method for producing a prepreg according to any one of embodiments 18 to 29, wherein the self-assembled carbon fiber bundles contain at least one resin selected from the group consisting of epoxy resin, unsaturated polyester resin, vinyl ester resin, polyurethane resin, and polyamide resin. [Embodiment 31] A method for producing a prepreg according to any one of embodiments 18 to 29, wherein the self-assembled carbon fiber bundles contain an organic binder. [Embodiment 32] The method for producing a prepreg according to any one of Embodiments 18 to 31, wherein the liquid thermosetting resin composition is solvent-free. [Embodiment 33] A method for producing a prepreg according to any one of embodiments 18 to 32, in which at least one resin selected from the group consisting of vinyl ester resin, unsaturated polyester resin, epoxy resin, polyimide resin, maleimide resin, and phenolic resin is blended into the liquid thermosetting resin composition. [Embodiment 34] The method for producing a prepreg according to any one of Embodiments 18 to 33, wherein the liquid thermosetting resin composition has a viscosity increasing property. [Embodiment 35] A method for producing a prepreg according to any one of Embodiments 18 to 34, wherein a flame retardant is blended into the liquid thermosetting resin composition. [Embodiment 36] A method for producing a prepreg according to any one of embodiments 18 to 32, in which a vinyl ester resin, an unsaturated polyester resin, an ethylenically unsaturated monomer, a polyisocyanate, a polymerization initiator, and a polymerization inhibitor are blended into the liquid thermosetting resin composition. [Embodiment 37] A method for producing a prepreg according to any one of embodiments 18 to 32, wherein an epoxy resin and an epoxy curing agent are blended into the liquid thermosetting resin composition. [Embodiment 38] A prepreg manufactured by the manufacturing method according to any one of embodiments 18 to 37. [Embodiment 39] A method for producing a carbon fiber reinforced plastic product, comprising curing the prepreg of embodiment 38. [Embodiment 40] A method for producing a carbon fiber reinforced plastic product, comprising producing a prepreg using the production method according to any one of embodiments 18 to 37, and curing the prepreg.

[0098] [Embodiment 41] A self-assembled carbon fiber bundle consisting of a plurality of carbon fibers and an organic binder, wherein the fiber length of the plurality of carbon fibers is 60 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less. [Embodiment 42] A self-assembled carbon fiber bundle according to embodiment 41, wherein at least some of the plurality of carbon fibers have a fiber length of 5 mm or more. [Embodiment 43] A self-assembled carbon fiber bundle according to embodiment 41, wherein at least some of the plurality of carbon fibers have a fiber length of 10 mm or more. [Embodiment 44] A self-assembled carbon fiber bundle according to any one of embodiments 41 to 43, wherein the plurality of carbon fibers does not include carbon fibers with a fiber length of less than 5 mm, or if they do include carbon fibers, the content is less than 5 wt%. [Embodiment 45] A self-assembled carbon fiber bundle according to embodiment 41 or 43, in which the plurality of carbon fibers does not include carbon fibers with a fiber length of less than 10 mm, or if they do include carbon fibers, the content is less than 5 wt%. [Embodiment 46] A self-assembled carbon fiber bundle according to embodiment 43 or 45, in which the difference between the maximum and minimum fiber lengths of the plurality of carbon fibers is within 5 mm, within 4 mm, or within 3 mm. [Embodiment 47] A self-assembled carbon fiber bundle according to any one of embodiments 41 to 46, having a bundle size in the range of 1.5K or more and less than 4.5K. [Embodiment 48] A self-assembled carbon fiber bundle according to any one of embodiments 41 to 47, containing at least one resin selected from the group consisting of epoxy resins, unsaturated polyester resins, vinyl ester resins, polyurethane resins, and polyamide resins. [Embodiment 49] A self-assembled carbon fiber bundle according to any one of embodiments 41 to 48, wherein the content of the organic binder is 0.5 wt% or more and less than 1.5 wt%, 1.5 wt% or more and less than 3 wt%, 3 wt% or more and less than 5 wt%, 5 wt% or more and less than 7 wt%, or 7 wt% or more and less than 10 wt%. [Embodiment 50] A self-assembled carbon fiber bundle according to any one of embodiments 41 to 49, comprising carbon fibers that have not been thermally deteriorated. [Embodiment 51] A self-assembled carbon fiber bundle according to any one of embodiments 41 to 49, comprising thermally deteriorated carbon fibers. [Embodiment 52] A self-assembled carbon fiber bundle according to any one of embodiments 41 to 49, wherein all of the plurality of carbon fibers are thermally deteriorated carbon fibers.

[0099] [Embodiment 53] A method for producing a prepreg, comprising impregnating a carbon fiber mat consisting of a plurality of carbon fiber bundles, including the self-assembled carbon fiber bundles according to any one of embodiments 41 to 52, with a liquid thermosetting resin composition. [Embodiment 54] A method for producing a prepreg according to embodiment 53, comprising: applying the liquid thermosetting resin composition to the surface of each of a first protective film and a second protective film; forming the carbon fiber mat on the surface of the first protective film on which the liquid thermosetting resin composition has been applied; bonding the first protective film and the second protective film together with the carbon fiber mat sandwiched therebetween so that the surfaces on which the liquid thermosetting resin composition has been applied face each other to form a laminate; and pressing the laminate to impregnate the carbon fiber mat with the liquid thermosetting resin composition. [Embodiment 55] A method for producing a prepreg according to embodiment 53 or 54, wherein the liquid thermosetting resin composition is solvent-free. [Embodiment 56] A method for producing a prepreg according to any one of embodiments 53 to 55, in which at least one resin selected from the group consisting of vinyl ester resin, unsaturated polyester resin, epoxy resin, polyimide resin, maleimide resin, and phenolic resin is blended into the liquid thermosetting resin composition. [Embodiment 57] The method for producing a prepreg according to any one of embodiments 53 to 56, wherein the liquid thermosetting resin composition has a viscosity increasing property. [Embodiment 58] A method for producing a prepreg according to any one of embodiments 53 to 57, wherein a flame retardant is blended into the liquid thermosetting resin composition. [Embodiment 59] A method for producing a prepreg according to any one of embodiments 53 to 55, in which the liquid thermosetting resin composition is blended with a vinyl ester resin, an unsaturated polyester resin, an ethylenically unsaturated monomer, a polyisocyanate, a polymerization initiator, and a polymerization inhibitor. [Embodiment 60] A method for producing a prepreg according to any one of embodiments 53 to 55, wherein an epoxy resin and an epoxy curing agent are blended into the liquid thermosetting resin composition. [Embodiment 61] A prepreg manufactured by the manufacturing method according to any one of embodiments 53 to 60. [Embodiment 62] A method for producing a carbon fiber reinforced plastic product, comprising curing the prepreg of embodiment 61. [Embodiment 63] A method for producing a carbon fiber reinforced plastic product, comprising producing a prepreg using the production method according to any one of embodiments 53 to 60, and curing the prepreg.

[0100] [Embodiment 64] A method for producing a self-assembled carbon fiber bundle, comprising: mixing carbon fiber cotton with a bundling liquid to obtain a mixture; and removing the liquid component of the bundling liquid from the mixture, wherein at least one of the carbon fiber cotton and the bundling liquid contains an organic binder, and the fiber length of all carbon fibers contained in the carbon fiber cotton is 60 mm or less, 40 mm or less, 30 mm or less, or 20 mm or less. [Embodiment 65] A manufacturing method according to embodiment 64, wherein at least some of the carbon fibers contained in the carbon fiber cotton have a fiber length of 5 mm or more. [Embodiment 66] A manufacturing method according to embodiment 64, wherein at least some of the carbon fibers contained in the carbon fiber cotton have a fiber length of 10 mm or more. [Embodiment 67] A manufacturing method according to any one of embodiments 64 to 66, wherein the carbon fiber cotton does not contain carbon fibers with a fiber length of less than 5 mm, or if it does contain carbon fibers, the content is less than 5 wt%. [Embodiment 68] A manufacturing method according to embodiment 64 or 66, in which the carbon fiber cotton does not contain carbon fibers with a fiber length of less than 10 mm, or if it does contain carbon fibers, the content is less than 5 wt%. [Embodiment 69] A manufacturing method according to embodiment 66 or 68, in which the difference between the maximum and minimum fiber lengths of the carbon fibers contained in the carbon fiber cotton is within 5 mm, within 4 mm, or within 3 mm. [Embodiment 70] A manufacturing method according to any one of embodiments 64 to 69, wherein at least one of the carbon fiber cotton and the bundling liquid contains at least one resin selected from the group consisting of epoxy resin, unsaturated polyester resin, vinyl ester resin, polyurethane resin, and polyamide resin. [Embodiment 71] The manufacturing method according to any one of embodiments 64 to 70, wherein the bundling liquid contains water. [Embodiment 72] The manufacturing method according to any one of embodiments 64 to 71, wherein at least the bundling liquid contains the organic binder. [Embodiment 73] A manufacturing method according to any one of embodiments 64 to 72, wherein a stirring mixer is used for the mixing. [Embodiment 74] A manufacturing method according to any one of embodiments 64 to 73, comprising treating the mixture in a tumbling granulator before removing the liquid component. [Embodiment 75] A manufacturing method according to any one of embodiments 64 to 74, wherein the carbon fiber batting contains carbon fibers that have not been thermally deteriorated. [Embodiment 76] A manufacturing method according to any one of embodiments 64 to 74, wherein the carbon fiber cotton contains thermally deteriorated carbon fiber. [Embodiment 77] A manufacturing method according to any one of embodiments 64 to 74, wherein all of the carbon fibers contained in the carbon fiber cotton are thermally deteriorated carbon fibers. [Embodiment 78] A manufacturing method according to any one of embodiments 64 to 76, wherein at least a portion of the carbon fiber batting is made by defibrating chopped carbon fiber bundles. [Embodiment 79] A manufacturing method according to embodiment 78, in which a stirring mixer is used for the fiberization. [Embodiment 80] A manufacturing method according to embodiment 78 or 79, wherein the bundle size of the chopped carbon fiber bundle is 24K or more. [Embodiment 81] A manufacturing method according to any one of embodiments 78 to 80, wherein the chopped carbon fiber bundles are sized. [Embodiment 82] A self-assembled carbon fiber bundle produced by the production method according to any one of embodiments 64 to 81. [Embodiment 83] A method for producing a prepreg, comprising impregnating the self-assembled carbon fiber bundles of embodiment 82 with a liquid thermosetting resin composition. [Embodiment 84] A method for producing a prepreg, comprising impregnating a carbon fiber mat consisting of a plurality of carbon fiber bundles, including the self-assembled carbon fiber bundles of embodiment 82, with a liquid thermosetting resin composition. [Embodiment 85] A method for producing a prepreg, comprising producing self-assembled carbon fiber bundles using the production method of any one of embodiments 64 to 81, and impregnating the self-assembled carbon fiber bundles with a liquid thermosetting resin composition. [Embodiment 86] A method for producing a prepreg, comprising producing self-assembled carbon fiber bundles using the production method of any one of embodiments 64 to 81, forming a carbon fiber mat consisting of a plurality of carbon fiber bundles including the self-assembled carbon fiber bundles, and impregnating the carbon fiber mat with a liquid thermosetting resin composition. [Embodiment 87] A method for producing a prepreg according to embodiment 84 or 86, comprising: applying the liquid thermosetting resin composition to the surface of each of a first protective film and a second protective film; forming the carbon fiber mat on the surface of the first protective film on which the liquid thermosetting resin composition has been applied; bonding the first protective film and the second protective film together with the carbon fiber mat sandwiched therebetween so that the surfaces on which the liquid thermosetting resin composition has been applied face each other to form a laminate; and pressing the laminate to impregnate the carbon fiber mat with the liquid thermosetting resin composition. [Embodiment 88] A method for producing a prepreg according to any one of embodiments 83 to 87, wherein the liquid thermosetting resin composition is solvent-free. [Embodiment 89] A method for producing a prepreg according to any one of embodiments 83 to 88, in which at least one resin selected from the group consisting of vinyl ester resin, unsaturated polyester resin, epoxy resin, polyimide resin, maleimide resin, and phenolic resin is blended into the liquid thermosetting resin composition. [Embodiment 90] A method for producing a prepreg according to any one of embodiments 83 to 89, wherein the liquid thermosetting resin composition has viscosity increasing properties. [Embodiment 91] A method for producing a prepreg according to any one of embodiments 83 to 90, wherein a flame retardant is blended into the liquid thermosetting resin composition. [Embodiment 92] A method for producing a prepreg according to any one of embodiments 83 to 88, in which the liquid thermosetting resin composition is blended with a vinyl ester resin, an unsaturated polyester resin, an ethylenically unsaturated monomer, a polyisocyanate, a polymerization initiator, and a polymerization inhibitor. [Embodiment 93] A method for producing a prepreg according to any one of embodiments 83 to 88, in which an epoxy resin and an epoxy curing agent are blended into the liquid thermosetting resin composition. [Embodiment 94] A method for producing a carbon fiber reinforced plastic product, comprising producing a prepreg using the production method of any one of embodiments 83 to 93, and curing the prepreg.

[0101] Although the present invention has been described in detail using specific embodiments, it will be apparent to those skilled in the art that various modifications are possible within the scope of the invention. This application is based on Japanese Patent Application No. 2021-174789 filed on October 26, 2021, Japanese Patent Application No. 2021-101844 filed on June 18, 2021, Japanese Patent Application No. 2021-101845 filed on June 18, 2021, and Japanese Patent Application No. 2021-101846 filed on June 18, 2021, and is incorporated by reference in its entirety. [Industrial Applicability]

[0102] Prepregs using SACFB can be preferably used to manufacture various CFRP products used in automobiles, motorcycles, bicycles, ships, railway vehicles, manned aircraft, unmanned aircraft and other transportation equipment, sporting goods, leisure goods, home appliances, agricultural machinery, building materials, etc.

Claims

1. A method for producing a prepreg, comprising impregnating a carbon fiber mat made of a plurality of carbon fiber bundles including self-assembled carbon fiber bundles containing 0.5 wt % or more of an organic binder, the self-assembled carbon fiber bundles being produced through a process in which a plurality of carbon fibers are assembled to form bundles by the method (A) below, with a liquid thermosetting resin composition. (A) A method for producing a self-assembled carbon fiber bundle, comprising: mixing carbon fiber cotton with a bundling liquid to spontaneously bundle carbon fibers in the carbon fiber cotton and obtain a mixture; and removing a liquid component of the bundling liquid from the mixture, wherein at least one of the carbon fiber cotton and the bundling liquid contains an organic binder, and the fiber length of all carbon fibers contained in the carbon fiber cotton is 60 mm or less.

2. The method for producing a prepreg according to claim 1, wherein at least a portion of the carbon fibers contained in the carbon fiber batting has a fiber length of 5 mm or more.

3. The method for producing a prepreg according to claim 2, wherein at least a portion of the carbon fibers contained in the carbon fiber batting has a fiber length of 10 mm or more.

4. The method for producing a prepreg according to claim 3, wherein the carbon fiber batting does not contain carbon fibers having a fiber length of less than 5 mm, or if it does contain carbon fibers, the content is less than 5 wt%.

5. The method for producing a prepreg according to claim 4, wherein the carbon fiber batting does not contain carbon fibers having a fiber length of less than 10 mm, or if it does contain carbon fibers, the content is less than 5 wt%.

6. 2. The method for producing a prepreg according to claim 1, wherein at least one of the carbon fiber wadding and the bundling liquid contains at least one resin selected from the group consisting of epoxy resin, unsaturated polyester resin, vinyl ester resin, polyurethane resin, and polyamide resin.

7. The method for producing a prepreg according to claim 1 , wherein the carbon fiber batting comprises carbon fibers that have not been thermally deteriorated.

8. The method for producing a prepreg according to claim 1 , wherein the carbon fiber batting comprises thermally deteriorated carbon fiber.

9. 2. A method for producing a prepreg according to claim 1, comprising: applying the liquid thermosetting resin composition to each surface of a first protective film and a second protective film; forming the carbon fiber mat on the surface of the first protective film on which the liquid thermosetting resin composition has been applied; bonding the first protective film and the second protective film together with the carbon fiber mat sandwiched therebetween so that the surfaces on which the liquid thermosetting resin composition has been applied face each other to form a laminate; and pressurizing the laminate to impregnate the carbon fiber mat with the liquid thermosetting resin composition.

10. 2. The method for producing a prepreg according to claim 1, wherein at least one resin selected from the group consisting of vinyl ester resin, unsaturated polyester resin, epoxy resin, polyimide resin, maleimide resin and phenolic resin is blended into the liquid thermosetting resin composition.

11. The method for producing a prepreg according to claim 1 , wherein a flame retardant is blended in the liquid thermosetting resin composition.

12. 2. The method for producing a prepreg according to claim 1, wherein the liquid thermosetting resin composition is blended with a vinyl ester resin, an unsaturated polyester resin, an ethylenically unsaturated monomer, a polyisocyanate, a polymerization initiator, and a polymerization inhibitor.

13. The method for producing a prepreg according to claim 1 , wherein the liquid thermosetting resin composition is blended with an epoxy resin and an epoxy curing agent.

14. A method for producing a carbon fiber reinforced plastic product, comprising producing a prepreg by the prepreg production method according to any one of claims 1 to 13, and curing the produced prepreg.

15. A method for producing a prepreg, comprising impregnating self-assembled carbon fiber bundles containing 0.5 wt % or more of an organic binder, the self-assembled carbon fiber bundles being produced through a process in which a plurality of carbon fibers are assembled to form bundles by the following method (A), with a liquid thermosetting resin composition: (A) A method for producing a self-assembled carbon fiber bundle, comprising: mixing carbon fiber cotton with a bundling liquid to spontaneously bundle carbon fibers in the carbon fiber cotton and obtain a mixture; and removing a liquid component of the bundling liquid from the mixture, wherein at least one of the carbon fiber cotton and the bundling liquid contains an organic binder, and the fiber length of all carbon fibers contained in the carbon fiber cotton is 60 mm or less.

16. The method for producing a prepreg according to claim 15, wherein at least a portion of the carbon fibers contained in the carbon fiber batting have a fiber length of 5 mm or more.

17. The method for producing a prepreg according to claim 16, wherein at least a portion of the carbon fibers contained in the carbon fiber batting have a fiber length of 10 mm or more.

18. The method for producing a prepreg according to claim 17, wherein the carbon fiber batting does not contain carbon fibers having a fiber length of less than 5 mm, or if it does contain carbon fibers, the content is less than 5 wt%.

19. The method for producing a prepreg according to claim 18, wherein the carbon fiber batting does not contain carbon fibers having a fiber length of less than 10 mm, or if it does contain carbon fibers, the content is less than 5 wt%.

20. The method for producing a prepreg according to claim 15, wherein at least one of the carbon fiber wadding and the bundling liquid contains at least one resin selected from the group consisting of epoxy resin, unsaturated polyester resin, vinyl ester resin, polyurethane resin, and polyamide resin.

21. The method for producing a prepreg according to claim 15, wherein the carbon fiber batting comprises carbon fibers that have not been thermally deteriorated.

22. The method for producing a prepreg according to claim 15, wherein the carbon fiber batting comprises thermally degraded carbon fiber.

23. 16. The method for producing a prepreg according to claim 15, wherein at least one resin selected from the group consisting of vinyl ester resin, unsaturated polyester resin, epoxy resin, polyimide resin, maleimide resin and phenolic resin is blended into the liquid thermosetting resin composition.

24. The method for producing a prepreg according to claim 15, wherein a flame retardant is blended into the liquid thermosetting resin composition.

25. 16. The method for producing a prepreg according to claim 15, wherein the liquid thermosetting resin composition is blended with a vinyl ester resin, an unsaturated polyester resin, an ethylenically unsaturated monomer, a polyisocyanate, a polymerization initiator, and a polymerization inhibitor.

26. The method for producing a prepreg according to claim 15, wherein the liquid thermosetting resin composition is blended with an epoxy resin and an epoxy curing agent.

27. A method for producing a carbon fiber reinforced plastic product, comprising producing a prepreg by the prepreg production method according to any one of claims 15 to 26, and curing the produced prepreg.

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