Carbon fiber recycling methods

Abstract

To provide a safe and simple method for recycling carbon fiber. [Solution] A method for recycling carbon fibers, comprising the steps of: contacting a precursor of carbon fiber reinforced plastic containing carbon fibers and uncured epoxy resin with an ionic liquid that does not contain fluorine atoms; and dissolving the uncured epoxy resin in the ionic liquid to isolate the carbon fibers.

Description

Technical Field

[0001] The present invention relates to a method for recycling carbon fibers.

Background Art

[0002] Fiber-reinforced plastics are used in a wide range of applications including aircraft, wind power generators, and hydrogen storage tanks, and are materials whose demand will continue to increase in the future. Along with the growing demand for fiber-reinforced plastics, a safe and convenient recycling method for fiber-reinforced plastics is required. As methods for separating reinforcing fibers from used fiber-reinforced plastics, methods of decomposing the resin by heat treatment or treatment with acid or alkali to separate it from the reinforcing fibers, and methods of heating and dissolving the resin in an organic solvent to separate it from the reinforcing fibers, among others, have been investigated, but there is room for improvement from the viewpoints of safety and convenience. In the method disclosed in Patent Document 1, a solvent containing a mixture of an ionic salt and an organic compound is used to attempt separation and recovery of reinforcing fibers at a treatment temperature lower than that of conventional methods.

[0003] Fiber-reinforced plastics are obtained by curing an uncured resin impregnated in an uncured material (prepreg). Prepregs that may have reduced curability due to reasons such as insufficient quality control during storage, and prepreg offcuts generated in the manufacturing process of fiber-reinforced plastics are discarded, so a safe and convenient recycling method for prepregs is also required. For example, Patent Document 2 discloses a method of dissolving an epoxy resin prepolymer in a solvent containing a specific aprotic organic solvent, thereby removing the epoxy resin prepolymer from a carbon fiber reinforced plastic (CFRP) prepreg, and separating and recovering carbon fibers.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

[0005] The method disclosed in Patent Document 1 requires heating to melt the ionic salt and organic compound, so when applying this method to the recycling of prepregs, there is room for improvement in terms of simplicity. The method disclosed in Patent Document 2 uses organic solvents such as NMP (N-methylpyrrolidone) and DMAc (N,N-dimethylacetamide). Since these organic solvents are highly likely to have adverse effects on the human body, there is room for improvement in this method from a safety standpoint.

[0006] The object of this invention is to provide a safe and simple method for recycling carbon fibers. [Means for solving the problem]

[0007] The present invention relates to a method for recycling carbon fibers, comprising the steps of: contacting a precursor of carbon fiber reinforced plastic containing carbon fibers and uncured epoxy resin with a fluorine atom-free ionic liquid; and dissolving the uncured epoxy resin in the ionic liquid to isolate the carbon fibers; and relating to a fluorine atom-free ionic liquid used to isolate carbon fibers by contacting a precursor of carbon fiber reinforced plastic containing carbon fibers and uncured epoxy resin and dissolving the uncured epoxy resin. [Effects of the Invention]

[0008] According to the present invention, a safe and simple method for recycling carbon fibers can be provided. [Brief explanation of the drawing]

[0009] [Figure 1]Figure 1 shows the infrared spectroscopic spectra of prepreg A used in Examples 1-5, the carbon fibers isolated in Example 3, and the carbon fibers isolated in Comparative Example 3. [Figure 2] Figure 2 shows a micrograph of prepreg A used in Examples 1 to 5. [Figure 3] Figure 3 is a micrograph of the carbon fibers isolated in Example 1. [Modes for carrying out the invention]

[0010] [Methods for recycling carbon fiber] The carbon fiber recycling method of the present invention (hereinafter also referred to as "the method of the present invention") includes the steps of: contacting a precursor of carbon fiber reinforced plastic containing carbon fibers and uncured epoxy resin with an ionic liquid that does not contain fluorine atoms; and dissolving the uncured epoxy resin in the ionic liquid to isolate the carbon fibers.

[0011] [Ionic liquid] The method of the present invention uses an ionic liquid that does not contain fluorine atoms.

[0012] In this invention, an ionic liquid means a salt composed of a cation and anion that is in liquid form. Ionic liquids are preferably liquid at 1 atmosphere and room temperature (25°C).

[0013] Examples of cations that constitute ionic liquids include quaternary ammonium cations, amidinium cations, pyridinium cations, pyrrolidinium cations, piperidinium cations, phosphonium cations, and sulfonium cations.

[0014] Quaternary ammonium cations include N,N-didecyl-N,N-dimethylammonium, N,N,N,N-tetramethylammonium, N,N,N,N-tetraethylammonium, N,N,N,N-tetrapropylammonium, N,N,N,N-tetraisopropylammonium, N,N,N,N-tetrabutylammonium, N,N,N,N-tetraisobutylammonium, N,N,N,N-triethyl-N-methylammonium, N-methyl-N,N,N-tripropylammonium, N,N,N-tributyl-N-methylammonium, N,N,N-triamyl-N-methylammonium, N,N,N-trihexyl-N-methylammonium, N-methyl-N,N,N-trioctylammonium, N,N,N-tridecyl-N-methylammonium, and N,N-diethyl-N,N-dimethylammonium Examples of cations include nium, N,N-dimethyl-N,N-dipropylammonium, N,N-dibutyl-N,N-dimethylammonium, N,N-diamyl-N,N-dimethylammonium, N,N-dihexyl-N,N-dimethylammonium, N,N-dicyclohexyl-N,N-dimethylammonium, N,N-dimethyl-N,N-dioctylammonium, N,N-didodecyl-N,N-dimethylammonium, N-ethyl-N,N,N-trimethylammonium, N-butyl-N,N,N-trimethylammonium, N,N,N-trimethyl-N-octylammonium, N-cecil-N,N,N-trimethylammonium, N,N-diethyl-N-methyl-N-methoxyethylammonium, N,N-diethyl-N-methyl-N-(2-methoxyethoxymethyl)ammonium, and cholinium. Among these, ammonium cations having long-chain alkyl groups such as N,N-didecyl-N,N-dimethylammonium cation and N,N-dimethyl-N,N-dioctylammonium cation are preferred, and N,N-didecyl-N,N-dimethylammonium cation is more preferred.

[0015] Examples of amidinium cations include imidazolium cation, imidazolinium cation, and tetrahydropyrimidinium cation.

[0016] Examples of imidazolium cations include 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1,3-dimethylimidazolium, 1,3-diethylimidazolium, 1,2,3-trimethylimidazolium, 1,2,3,4-tetramethylimidazolium, 1,3-dimethyl-2-ethylimidazolium, 1,2-dimethyl-3-ethylimidazolium, 1,2,3-triethylimidazolium, 1,2,3,4-tetraethylimidazolium, 1,3-dimethyl-2-phenylimidazolium, 1,3-dimethyl-2-benzylimidazolium, 1-benzyl-2,3-dimethylimidazolium, 4-cyano-1,2,3-trimethylimidazolium, 3-cyanomethyl-1,2-dimethylimidazolium, and 2-cyanomethyl-1,2-dimethylimidazolium. Examples of cations include methyl-1,3-dimethylimidazolium, 4-acetyl-1,2,3-trimethylimidazolium, 3-acetylmethyl-1,2-dimethylimidazolium, 4-methylcarboxymethyl-1,2,3-trimethylimidazolium, 3-methylcarboxymethyl-1,2-dimethylimidazolium, 4-methoxy-1,2,3-trimethylimidazolium, 3-methoxymethyl-1,2-dimethylimidazolium, 4-formyl-1,2,3-trimethylimidazolium, 3-formylmethyl-1,2-dimethylimidazolium, 3-hydroxyethyl-1,2-dimethylimidazolium, 4-hydroxymethyl-1,2,3-trimethylimidazolium, and 2-hydroxyethyl-1,3-dimethylimidazolium. Among these, 1-ethyl-3-methylimidazolium cation and 1-butyl-3-methylimidazolium cation are preferred, and 1-ethyl-3-methylimidazolium cation is more preferred.

[0017] Examples of the imidazolinium cation include cations such as 1-ethyl-3-methylimidazolinium, 1-ethyl-2,3-dimethylimidazolinium, 1,2,3,4-tetramethylimidazolinium, 2-ethyl-1,3,4-trimethylimidazolinium, 1,3-dimethyl-2,4-diethylimidazolinium, and 1,2-dimethyl-3,4-diethylimidazolinium.

[0018] Examples of the tetrahydropyrimidinium cation include a diazabicycloundecenium cation, 1,5-diazabicyclo[4.3.0]-5-nonenecation and the like.

[0019] Examples of the anion constituting the ionic liquid include those composed of carboxylic acid, sulfonic acid, thiocyanic acid (or isothiocyanic acid), dialkylphosphoric acid, dicyanamide and the like.

[0020] Examples of the carboxylic acid include aliphatic carboxylic acids (monocarboxylic acids such as acetic acid, propionic acid, butyric acid, and valeric acid; dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, fumaric acid, and maleic acid; polyvalent carboxylic acids such as citric acid and the like), aromatic or araliphatic carboxylic acids (monocarboxylic acids such as benzoic acid and cinnamic acid; dicarboxylic acids such as ortho-, iso- and terephthalic acid; polyvalent carboxylic acids such as trimellitic acid and pyromellitic acid and the like), alicyclic carboxylic acids (monocarboxylic acids such as cyclohexanecarboxylic acid; dicarboxylic acids such as camphoric acid and the like) and the like.

[0021] Examples of the sulfonic acid include alkylsulfonic acids (methanesulfonic acid, ethanesulfonic acid and the like), alkylbenzenesulfonic acids (p-toluenesulfonic acid and the like), sulfosuccinic acid diesters (sulfosuccinic acid-di-2-ethylhexyl ester and the like), polyvinylsulfonic acids (polystyrenesulfonic acid and the like) and the like.

[0022] Examples of the dialkylphosphoric acid include dimethylphosphoric acid, diethylphosphoric acid, dibutylphosphoric acid, dihexylphosphoric acid and the like.

[0023] Of the above anions, anions composed of acetic acid, orthophthalic acid, thiocyanic acid (or isothiocyanic acid), or methanesulfonic acid are preferred from the viewpoint of reducing viscosity after resin dissolution.

[0024] The ionic liquid preferably contains at least one cation selected from the group consisting of quaternary ammonium cations and amidinium cations, as this improves the solubility of the epoxy resin.

[0025] The ionic liquid is preferably a salt that does not contain halogen atoms.

[0026] Examples of ionic liquids include 1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium phthalate, 1-ethyl-3-methylimidazolium thiocyanate, 1-ethyl-3-methylimidazolium methanesulfonate, N,N-didecyl-N,N-dimethylammonium thiocyanate, 1-butyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium thiocyanate, 1-butyl-3-methylimidazolium methanesulfonate, and 1-butyl-3-methylimidazolium dimethyl phosphate. Among these, 1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium phthalate, 1-ethyl-3-methylimidazolium thiocyanate, 1-ethyl-3-methylimidazolium methanesulfonate, N,N-didecyl-N,N-dimethylammonium thiocyanate, and 1-butyl-3-methylimidazolium dimethyl phosphate are preferred, and 1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium phthalate, 1-ethyl-3-methylimidazolium thiocyanate, 1-ethyl-3-methylimidazolium methanesulfonate, and N,N-didecyl-N,N-dimethylammonium thiocyanate are more preferred.

[0027] [Precursor to carbon fiber reinforced plastic] In the present invention, the precursor of carbon fiber reinforced plastic (hereinafter also simply referred to as "precursor") includes carbon fibers and uncured epoxy resin. Examples of such precursors include carbon fiber reinforced plastic prepregs (uncured material) and prepregs in which the curing reaction has partially progressed (semi-cured material).

[0028] The carbon fibers are not particularly limited as long as they are fibers used in carbon fiber reinforced plastics, and examples include PAN-based carbon fibers and pitch-based carbon fibers. The carbon fibers may be included in a state treated with a sizing agent (fiber bundle) or in a state processed into a nonwoven fabric, etc. The length and shape of the carbon fibers are not particularly limited.

[0029] The proportion of carbon fibers in the precursor is preferably 60-90% by weight, and more preferably 65-85% by weight.

[0030] The uncured epoxy resin is not particularly limited as long as it is a resin used as the matrix resin for carbon fiber reinforced plastics, and examples include epoxy monomers and epoxy resins such as aromatic epoxy compounds (bisphenol A type epoxy resin, bisphenol F type epoxy resin, novolac type epoxy resin, etc.), aliphatic epoxy compounds (ethylene glycol diglycidyl ether, alkyl diol diglycidyl ether, etc.), and glycidylamine type epoxy compounds [4,4'-methylenebis(N,N-diglycidylaniline), 4-(oxiran-2-ylmethoxy)-N,N-bis(oxiran-2-ylmethyl)aniline, etc.]. Among these, bisphenol F type epoxy resin is preferred.

[0031] The precursor may contain components other than carbon fibers and uncured epoxy resin. Examples include cured or semi-cured epoxy resin (compounds produced by the reaction of all or part of the epoxy groups in the epoxy resin molecule), a matrix resin other than epoxy resin, a curing agent, and a carbon fiber sizing agent. Examples of cured or semi-cured epoxy resins include products obtained by the reaction of uncured epoxy resins with each other, products obtained by the reaction of uncured epoxy resins with resins other than epoxy resins, and products obtained by the reaction of uncured epoxy resins with a curing agent. Examples of matrix resins other than epoxy resins include thermosetting resins other than epoxy resins, photocurable resins, thermoplastic resins, etc., which are used as matrix resins for precursors. Examples include phenolic resins, cyanate ester resins, polyimide resins, polyamide-imide resins, polyphenylene ether resins, polyethersulfone resins, polyetherimide resins, and polyetheretherketone resins. Examples of curing agents include those known as curing agents for epoxy resins. For example, amines [diaminodiphenylsulfone, dicyandiamide, 2,4-diamino-6-[2-(2-methyl-1-imidazolyl)ethyl]-1,3,5-triazine, etc.] and curing catalysts (2-methylimidazole, 2-ethyl-4-methylimidazole, etc.). Examples of sizing agents for carbon fibers include polyethyleneimine, polyether (polyethylene glycol, etc.), polyurethane, and epoxy resin having a bisphenol A skeleton. If the precursor contains the above components, the processing conditions in the recycling method should be appropriately modified so that the carbon fibers can be isolated. To simplify the explanation, the following description will primarily focus on the case where the precursor consists only of carbon fibers and uncured epoxy resin.

[0032] [Contact process] In the contact process, an ionic liquid is brought into contact with a precursor of carbon fiber reinforced plastic. The method for bringing the ionic liquid into contact with the precursor is not particularly limited and includes methods such as immersing the precursor in the ionic liquid, spraying the ionic liquid onto the precursor, and washing the precursor with the ionic liquid.

[0033] When immersing a precursor in an ionic liquid, the entire precursor may be immersed, or only a portion of the precursor may be immersed. If only a portion of the precursor is immersed in the ionic liquid, the immersed portion may be changed during the process. Ultimately, it is preferable to immerse all parts of the precursor in the ionic liquid.

[0034] The temperature of the ionic liquid in the contact process should be such that the ionic liquid does not solidify and the epoxy resin dissolves. For example, it may be at room temperature or at a higher temperature. The temperature may be adjusted as needed, such as by heating, if it is desired to accelerate the dissolution of the epoxy resin. From the viewpoint of safety and simplicity, the temperature of the ionic liquid is preferably 50°C or lower, and more preferably at room temperature.

[0035] The amount of ionic liquid used in the contact process is not particularly limited, as long as it is sufficient to remove the uncured epoxy resin adhering to the surface of the carbon fibers from the carbon fiber surface.

[0036] The contact time is not particularly limited as long as it is sufficient time to remove the uncured epoxy resin adhering to the surface of the carbon fiber from the surface of the carbon fiber.

[0037] The method of the present invention may include a step of adjusting the dimensions of the precursor before the contact step. In the dimensional adjustment process, for example, the precursor is cut to a specific size (e.g., 5cm x 5cm or smaller). By adjusting the dimensions of the precursor, it becomes easier to obtain carbon fibers of a specific length.

[0038] In addition, the processing conditions for the precursor may be adjusted as needed, for example, to promote the dissolution of the epoxy resin. For instance, the ionic liquid in which the precursor is immersed may be stirred.

[0039] [Process for isolating carbon fibers] In the process of isolating carbon fibers, the uncured epoxy resin contained in the precursor is dissolved in the ionic liquid after contact with the ionic liquid, thereby isolating the carbon fibers.

[0040] When dissolving the epoxy resin, it is sufficient that the epoxy resin adhering to the surface of the carbon fibers is removed from the surface of the carbon fibers. To facilitate the isolation of the carbon fibers, the epoxy resin contained in the precursor may be completely dissolved in the ionic liquid. If the precursor also contains components other than uncured epoxy resin and carbon fibers (e.g., cured or semi-cured epoxy resin), these components may also be dissolved in the ionic liquid. Whether or not the epoxy resin has dissolved can be confirmed by methods such as observing the presence or absence of residual epoxy resin on the surface of the freed carbon fibers under a microscope.

[0041] When uncured epoxy resin is dissolved in an ionic liquid, a mixture of free carbon fibers and the dissolved epoxy resin (an ionic liquid solution of epoxy resin) is obtained. If only a portion of the uncured epoxy resin is dissolved, the mixture will further contain uncured epoxy resin. In the isolation step, the free carbon fibers are isolated from this mixture. The isolation method is not particularly limited and may include filtration or centrifugation.

[0042] The isolated carbon fibers may be washed with a washing solvent. The washing solvent can be selected as appropriate, and water, alcohol, ketones, hydrocarbons, ionic liquids, etc., can be used. The washing solvent may also contain a surfactant such as sodium dodecyl sulfate. Furthermore, even if a small amount of epoxy resin or the like remains on the surface of the carbon fibers, it can be removed by the washing process. Alternatively, the isolated carbon fibers may be dried.

[0043] The method of the present invention may include a step of recovering uncured epoxy resin from a mixture of uncured epoxy resin and ionic liquid (an ionic liquid solution of epoxy resin, or a mixture of an ionic liquid solution of epoxy resin and uncured epoxy resin) obtained after isolating free carbon fibers. As a method for recovering the uncured epoxy resin, for example, one method involves adding a poor solvent to a mixture of the uncured epoxy resin and an ionic liquid to precipitate the epoxy resin and then separating it. Poor solvents can be selected as appropriate, and examples include methanol and water. The method for separating the epoxy resin can be selected as appropriate, and examples include filtration and centrifugation.

[0044] Furthermore, if the precursor contains components other than uncured epoxy resin and carbon fibers, and these components are dissolved in the mixture, these components may be recovered as needed. The recovery method can be selected appropriately; for example, they may be precipitated together with the epoxy resin.

[0045] The ionic liquid may be recovered from the filtrate obtained after filtration. One method for recovering the ionic liquid is distillation of the filtrate. The method of the present invention may include a step of recovering the ionic liquid.

[0046] One embodiment of the present invention is a fluorine-free ionic liquid used to isolate carbon fibers by contacting a precursor of carbon fiber-reinforced plastic containing carbon fibers and uncured epoxy resin, thereby dissolving the uncured epoxy resin.

[0047] This specification discloses the following:

[0048] (1) This disclosure is a method for recycling carbon fibers, comprising the steps of: contacting a precursor of carbon fiber reinforced plastic containing carbon fibers and an uncured epoxy resin with an ionic liquid that does not contain fluorine atoms; and dissolving the uncured epoxy resin in the ionic liquid to isolate the carbon fibers.

[0049] Disclosure (2) is the method according to Disclosure (1), wherein the ionic liquid comprises at least one cation selected from the group consisting of quaternary ammonium cations and amidinium cations.

[0050] Disclosure (3) is the method according to Disclosure (1) or (2), wherein the temperature of the ionic liquid in the contact step is 50°C or lower.

[0051] Disclosure (4) is the method according to any one of Disclosures (1) to (3), wherein the ionic liquid is a salt that does not contain halogen atoms.

[0052] Disclosure (5) further includes a step of recovering the uncured epoxy resin from the mixture of the uncured epoxy resin and the ionic liquid, according to any of Disclosures (1) to (4).

[0053] (6) The present disclosure is a fluorine-free ionic liquid used to isolate carbon fibers by contacting a precursor of carbon fiber-reinforced plastic, which includes carbon fibers and an uncured epoxy resin, and dissolving the uncured epoxy resin. [Examples]

[0054] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0055] Each material was manufactured as follows: (Carbon fiber bundle) Three parts by weight of polyethyleneimine (BASF Japan Ltd.: LuPasol®, molecular weight 1,300) and seven parts by weight of polyethylene glycol (Sanyo Chemical Industries, Ltd.: PEG-600, number average molecular weight 600, bifunctional) were dissolved in 1540 parts by weight of water to prepare a sizing aqueous solution with a concentration of 0.65% by weight. Next, untreated carbon fibers (fineness 800tex, 12,000 filaments) were immersed in the sizing aqueous solution and dried for 3 minutes in a 180°C incubator (ESPEC Corporation: PH-102) to prepare carbon fiber bundles with 0.35% by weight of sizing agent attached, based on the weight of the carbon fiber bundles.

[0056] (Prepreg A) 31 parts by weight of 4,4'-methylenebis(N,N-diglycidylaniline) (manufactured by Tokyo Chemical Industries, Ltd., epoxy equivalent 106 g / eq.), 12 parts by weight of 4-(oxiran-2-ylmethoxy)-N,N-bis(oxiran-2-ylmethyl)aniline (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., epoxy equivalent 32 g / eq.), 18 parts by weight of bisphenol F type epoxy resin (manufactured by Nippon Steel Chemical & Material Co., Ltd.: YDF-170, epoxy equivalent 160-180 g / eq.), and 10 parts by weight of polyethersulfone (manufactured by Sumitomo Chemical Co., Ltd.: Sumika Excel PES4800P) were heated and mixed at 150°C, and then 28 parts by weight of diaminodiphenylsulfone (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were mixed to obtain an epoxy resin composition. This was diluted with 127 parts by weight of methylene chloride, 103 parts by weight of chloroform, and 10 parts by weight of methanol to obtain a solution of the epoxy resin composition. Carbon fiber bundles coated with the sizing agent were immersed in this solution, removed, and dried in a fume hood for 24 hours to allow the epoxy resin composition to adhere to the carbon fiber bundles. Prepreg A was then prepared by leaving it to stand for one month in an environment of 25°C and 60% humidity.

[0057] (Prepreg B) Carbon fiber bundles coated with a sizing agent were immersed in an epoxy resin composition prepared by mixing 67 parts by weight of bisphenol F type epoxy resin (manufactured by Nippon Steel Chemical & Material Co., Ltd.: YDF-170, epoxy equivalent 160-180 g / eq.), 25 parts by weight of bisphenol A type epoxy resin (manufactured by Kaneka Corporation: Kaneace MX-154, epoxy equivalent 301 g / eq.), 5 parts by weight of dicyandiamide (manufactured by Tokyo Chemical Industry Co., Ltd.), and 3 parts by weight of 2,4-diamino-6-[2-(2-methyl-1-imidazolyl)ethyl]-1,3,5-triazine (manufactured by Tokyo Chemical Industry Co., Ltd.), thereby adhering the epoxy resin composition to the carbon fiber bundles. The mixture was then left to stand for one month at 25°C and 60% humidity to prepare prepreg B.

[0058] [Measurement of epoxy resin composition content in prepreg] The weight loss rates of carbon fiber bundles and each prepreg were evaluated under the following conditions using a thermogravimetric-differential thermal analyzer (Hitachi High-Tech Science Corporation, NEXTA® STA200). Sample pan: Aluminum Temperature range: 30℃~500℃ Heating rate: 10℃ / min Gas environment: Air The epoxy resin composition content in each prepreg was calculated using the following formula (1) based on the weight loss rates described above. The epoxy resin composition content of prepreg A was 33.8%, and the epoxy resin composition content of prepreg B was 22.7%. Epoxy resin composition content in prepreg (%) = [(AB) / C] × 100 (1) A: Weight loss in prepreg B: Weight loss in the carbon fiber bundle contained in the prepreg. C: Prepreg weight A represents the total weight loss of the epoxy resin composition and sizing agent contained in the prepreg. B represents the weight loss of the sizing agent contained in the carbon fiber bundle. Therefore, (AB) represents the weight loss of the epoxy resin composition contained in the prepreg, and by determining (AB), the content of the epoxy resin composition contained in the prepreg can be calculated.

[0059] (1-ethyl-3-methylimidazolium methyl carbonate) A solution of 135 parts by weight (1.5 moles) of dimethyl carbonate dissolved in 192 parts by weight of methanol was added to a stirred autoclave, and 96 parts by weight (1.0 mole) of 1-ethylimidazole was added dropwise using a dropper funnel. The mixture was then stirred at 130°C for 40 hours to obtain a solution of 1-ethyl-3-methylimidazolium methyl carbonate.

[0060] (1-ethyl-3-methylimidazolium acetate:EMI-acetic acid) 423 parts by weight of the above 1-ethyl-3-methylimidazolium methyl carbonate solution were added to 60 parts by weight (1.0 molar part) of acetic acid, and carbon dioxide gas was generated. After the generation of carbon dioxide gas subsided, the solution was heated to 110°C under a reduced pressure of 1.0 kPa to remove the solvent and obtain pale yellow 1-ethyl-3-methylimidazolium acetic acid.

[0061] (1-ethyl-3-methylimidazolium phthalate:EMI-phthalic acid) 423 parts by weight of the above 1-ethyl-3-methylimidazolium methyl carbonate solution was mixed with 166 parts by weight (1.0 molar part) of phthalic acid, and carbon dioxide gas was generated. After the generation of carbon dioxide gas subsided, the mixture was heated to 110°C under a reduced pressure of 1.0 kPa to remove the solvent, yielding pale yellow 1-ethyl-3-methylimidazolium phthalic acid.

[0062] (1-Ethyl-3-methylimidazolium thiocyanate: EMI-SCN) A solution of 76 parts by weight (1.0 molar) of ammonium thiocyanate dissolved in 76 parts by weight of deionized water was added to 423 parts by weight of the above 1-ethyl-3-methylimidazolium methyl carbonate solution, and carbon dioxide gas was generated. After removing the precipitated white ammonium carbonate by filtration, the solvent was removed by heating to 110°C under a reduced pressure of 1.0 kPa while passing nitrogen gas at a rate of 0.1 mL / min, yielding pale yellow 1-ethyl-3-methylimidazolium thiocyanate.

[0063] (1-Ethyl-3-methylimidazolium methanesulfonate: EMI-MSA) 423 parts by weight of the above 1-ethyl-3-methylimidazolium methyl carbonate solution were added to 96 parts by weight (1.0 molar part) of methanesulfonic acid. After the generation of carbon dioxide gas subsided, the mixture was heated to 110°C under a reduced pressure of 1.0 kPa to remove the solvent and obtain pale yellow 1-ethyl-3-methylimidazolium methanesulfonate.

[0064] (N,N-Didecyl-N,N-Dimethylammonium thiocyanate: DDA-SCN) 311 parts by weight (1.0 mol) of didecylmethylamine, 90 parts by weight (1.0 mol) of dimethyl carbonate, and 64 parts by weight of methanol were charged into a stirred autoclave and reacted at 110°C for 12 hours to obtain didecyldimethylammonium methyl carbonate salt. Next, a solution of 76 parts by weight (1.0 mol) of ammonium thiocyanate dissolved in 152 parts by weight of methanol was added to generate carbon dioxide. The mixture was then stirred at 90°C for 4 hours to decompose and remove ammonium carbonate. Next, the mixture was heated to 110°C under a reduced pressure of 1.0 kPa while passing nitrogen gas at a rate of 0.1 mL / min to remove the solvent and obtain pale yellow N,N-didecyl-N,N-dimethylammonium thiocyanate.

[0065] The solvents used in the comparative examples are commercially available products. (solvent) EMI-TFSI: 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [Manufactured by Tokyo Chemical Industry Co., Ltd.] EMI-BF4: 1-Butyl-3-methylimidazolium tetrafluoroborate [Manufactured by Tokyo Chemical Industry Co., Ltd.] Acetonitrile [manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.]

[0066] <Examples 1-6, Comparative Examples 1-3> 99 parts by weight of the solvents listed in Table 1 (all liquid at 25°C) and 1 part by weight of prepreg were placed in a reaction vessel, and the mixture was left to stand for 48 hours with the prepreg constantly immersed in the solvent (25°C). The mixture was filtered to isolate the carbon fibers, washed and filtered with 10 parts by weight of the solvent in which the prepreg was immersed, washed with 100 parts by weight of methanol, and dried. The filtrate (referred to as "first filtrate") was collected. The first filtrate was gradually added to 10,000 parts by weight of methanol, the precipitate was filtered off, washed with 100 parts by weight of methanol, and then dried. The filtrate (referred to as the "second filtrate") was collected. The solvents listed in Table 1 were recovered by distillation of the second filtrate.

[0067] [Carbon fiber recycling rate] The carbon fiber recycling rate was calculated based on the following formula (2). The results are shown in Table 1. Carbon fiber recycling rate (%) = [(CD) / A] × 100 (2) C: Prepreg weight D: Weight of isolated carbon fibers A: Weight loss in prepreg In equation (2), A and C are the same as in equation (1), respectively. (CD) represents the total weight of the epoxy resin composition and sizing agent contained in the prepreg that were not attached to the isolated carbon fibers (removed from the carbon fibers). The closer the value of equation (2) is to 100%, the more epoxy resin composition and sizing agent have been removed from the carbon fibers; therefore, the value of equation (2) can be said to indicate the carbon fiber regeneration rate.

[0068] [Recovery rate of epoxy resin composition and sizing agent] The recovery rates of the epoxy resin composition and sizing agent from the first filtrate were calculated based on the following formula (3). The results are shown in Table 1. Recovery rate of epoxy resin composition and sizing agent (%) = (E / A) × 100 (3) E: Weight of precipitate recovered from the first filtrate A: Weight loss in prepreg A in equation (3) is the same as A in equation (1).

[0069] [IR measurement] Infrared spectroscopy was performed on the carbon fibers isolated in Example 3 using an infrared spectrometer (IRAffinity-1S, manufactured by Shimadzu Corporation). The same infrared spectroscopy was performed on prepreg A and the carbon fibers isolated in Comparative Example 3. The results are shown in Figure 1.

[0070] [Observation using a microscope] A scanning electron microscope (S-4800, Hitachi High-Tech Corporation, acceleration voltage 1.0kV) was used to observe each prepreg and the carbon fibers isolated in each example. To prevent charge buildup, the observation samples were treated with platinum deposition using a platinum vapor deposition unit (JFC-1600, JEOL Ltd.). In each example, it was confirmed that little to no resin was attached to the surface of the isolated carbon fibers. Figure 2 is a micrograph of prepreg A used in Examples 1-5, and Figure 3 is a micrograph of the carbon fibers isolated in Example 1.

[0071] [Table 1]

Claims

1. A step of contacting a carbon fiber reinforced plastic precursor containing carbon fibers and uncured epoxy resin with an ionic liquid that does not contain fluorine atoms, The process includes dissolving the uncured epoxy resin in the ionic liquid to isolate the carbon fibers. Methods for recycling carbon fiber.

2. The method according to claim 1, wherein the ionic liquid contains at least one cation selected from the group consisting of quaternary ammonium cations and amidinium cations.

3. The method according to claim 1 or 2, wherein in the contact step, the temperature of the ionic liquid is 50°C or lower.

4. The method according to claim 1 or 2, wherein the ionic liquid is a salt that does not contain halogen atoms.

5. The method according to claim 1 or 2, further comprising the step of recovering the uncured epoxy resin from the mixture of the uncured epoxy resin and the ionic liquid.

6. A fluorine-free ionic liquid used to isolate carbon fibers by contacting a precursor of carbon fiber-reinforced plastic containing carbon fibers and uncured epoxy resin, thereby dissolving the uncured epoxy resin.

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