Fiber recycling methods and ionic liquids

Abstract

To provide a safe and easy method for recycling textiles. [Solution] A method for recycling fibers (wherein the fibers are fibers other than carbon fibers), comprising the steps of: contacting an ionic liquid with a precursor of fiber-reinforced plastic containing fibers and uncured resin; and dissolving the uncured resin in the ionic liquid to isolate the fibers.

Description

Technical Field

[0001] The present invention relates to a fiber recycling method and an ionic liquid.

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 with growing demand in the future. Along with the increase in 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, 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 have been studied, but there is room for improvement from the viewpoints of safety and convenience. Patent Document 1 discloses a method of separating a resin component as a base material from reinforcing materials such as glass fibers and aramid fibers using a treatment solution containing oxidizing active species obtained by electrolyzing sulfuric acid.

[0003] Fiber-reinforced plastics are obtained by curing an uncured resin impregnated in an uncured product (prepreg). Since prepregs that may have reduced curability due to reasons such as insufficient quality control during storage, and offcuts of prepreg generated in the manufacturing process of fiber-reinforced plastics are discarded, 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 resin (CFRP) prepreg, and separating and recovering carbon fibers.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[0005] The method disclosed in Patent Document 1 requires specialized equipment for industrial electrolysis and also incurs power generation costs. 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 textiles. [Means for solving the problem]

[0007] The present invention relates to a method for recycling fibers (where the fibers are other than carbon fibers), comprising the steps of: contacting an ionic liquid with a precursor of fiber-reinforced plastic containing fibers and an uncured resin; and dissolving the uncured resin in the ionic liquid to isolate the fibers; and to an ionic liquid (where the fibers are other than carbon fibers) used to isolate fibers by contacting an precursor of fiber-reinforced plastic containing fibers and an uncured resin and dissolving the uncured resin. [Effects of the Invention]

[0008] According to the present invention, a safe and simple method for recycling textiles can be provided. [Modes for carrying out the invention]

[0009] [Methods for recycling textiles] The present invention provides a method for recycling fibers (hereinafter also referred to as "the method of the present invention") which includes the steps of: contacting an ionic liquid with a precursor of fiber-reinforced plastic containing fibers and an uncured resin; and dissolving the uncured resin in the ionic liquid to isolate the fibers. However, the fibers are fibers other than carbon fibers.

[0010] [Ionic liquid] The method of the present invention uses an ionic liquid. In the present 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).

[0011] Examples of cations that constitute ionic liquids include quaternary ammonium cations, amidinium cations, pyridinium cations, pyrrolidinium cations, piperidinium cations, phosphonium cations, and sulfonium cations.

[0012] 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.

[0013] Examples of amidinium cations include imidazolium cation, imidazolinium cation, and tetrahydropyrimidinium cation.

[0014] 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.

[0015] 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.

[0016] Examples of the tetrahydropyrimidinium cation include a diazabicycloundecenium cation, 1,5-diazabicyclo[4.3.0]-5-nonenium cation, and the like.

[0017] 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.

[0018] 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.

[0019] Examples of the sulfonic acid include alkylsulfonic acids (methanesulfonic acid, ethanesulfonic acid, etc.), alkylbenzenesulfonic acids (p-toluenesulfonic acid, etc.), sulfosuccinic acid diesters (sulfosuccinic acid di-2-ethylhexyl ester, etc.), polyvinylsulfonic acids (polystyrenesulfonic acid, etc.), and the like.

[0020] Examples of the dialkylphosphoric acid include dimethylphosphoric acid, diethylphosphoric acid, dibutylphosphoric acid, dihexylphosphoric acid, and the like.

[0021] Among the above anions, an anion composed of acetic acid, thiocyanic acid (or isothiocyanic acid), methanesulfonic acid, dimethylphosphoric acid or a dicyanoamine compound is preferred from the viewpoint of viscosity reduction after resin dissolution.

[0022] 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 resin.

[0023] The ionic liquid is preferably a salt containing no fluorine atom, and more preferably a salt containing no halogen atom.

[0024] Examples of the ionic liquid include 1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium dicyanamide, 1-ethyl-3-methylimidazolium thiocyanate, 1-ethyl-3-methylimidazolium methanesulfonate, 1-ethyl-3-methylimidazolium dimethyl phosphate, N,N-didecyl-N,N-dimethylammonium thiocyanate, 1-butyl-3-methylimidazolium acetate, 1-butyl-3-methylimidazolium thiocyanate, 1-butyl-3-methylimidazolium methanesulfonate, 1-butyl-3-methylimidazolium dimethyl phosphate, and the like. Among these, 1-ethyl-3-methylimidazolium acetate, 1-ethyl-3-methylimidazolium dicyanamide, 1-ethyl-3-methylimidazolium thiocyanate, 1-ethyl-3-methylimidazolium methanesulfonate, 1-ethyl-3-methylimidazolium dimethyl phosphate, 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 dicyanamide, 1-ethyl-3-methylimidazolium thiocyanate, 1-ethyl-3-methylimidazolium methanesulfonate, 1-ethyl-3-methylimidazolium dimethyl phosphate, and N,N-didecyl-N,N-dimethylammonium thiocyanate are more preferred.

[0025] [Precursor to fiber-reinforced plastic] In the present invention, the precursor of fiber-reinforced plastic (hereinafter also simply referred to as "precursor") includes fibers other than carbon fibers and uncured resin. Examples of such precursors include fiber-reinforced plastic prepregs (uncured material) and prepregs in which the curing reaction has partially progressed (semi-cured material).

[0026] The fibers are not particularly limited as long as they are fibers used in fiber-reinforced plastics other than carbon fibers, and include glass fibers, aramid fibers, Zylon® fibers, polyethylene fibers, boron fibers, ceramic fibers, metal fibers, mineral fibers, rock fibers, slug fibers, silicon carbide fibers, etc. Basalt fibers are an example of rock fibers. The 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 fibers are not particularly limited.

[0027] The proportion of fibers in the precursor is preferably 60-90% by weight, and more preferably 65-85% by weight.

[0028] The uncured resin is not particularly limited as long as it is a resin used in fiber-reinforced plastics, and examples include thermosetting resins and photocuring resins such as epoxy resins, phenolic resins, cyanate ester resins, unsaturated polyester resins, vinyl ester resins, polyimide resins, maleimide resins, urethane resins, and urethane acrylic resins. Examples of epoxy resins include 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.), glycidylamine type epoxy compounds [4,4'-methylenebis(N,N-diglycidylaniline), 4-(oxiran-2-ylmethoxy)-N,N-bis(oxiran-2-ylmethyl)aniline, etc.], high-bromine epoxy resins, low-bromine epoxy resins, and other epoxy monomers and epoxy resins. Among these, epoxy resins are preferred, and bisphenol F type epoxy resins are more preferred.

[0029] The precursor may contain components other than fibers and uncured resin. Examples include cured or semi-cured resin (compounds produced by the reaction of all or part of the reactive functional groups in the resin molecule), matrix resins other than uncured resin, curing agents, and fiber sizing agents. Examples of cured or semi-cured resins include products obtained by the reaction of uncured resins with each other, and products obtained by the reaction of uncured resins with a curing agent. Examples of matrix resins other than uncured resins include thermoplastic resins and other materials used as matrix resins for precursors. Examples include 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 resins. For example, amines [diaminodiphenylsulfone, dicyandiamide, etc.] and curing catalysts (2-methylimidazole, 2-ethyl-4-methylimidazole, etc.) are used. Examples of sizing agents include polyethyleneimine, polyether (polyethylene glycol, etc.), and polyurethane. If the precursor contains the above components, the processing conditions in the recycling method should be appropriately modified so that the fibers can be isolated. To simplify the explanation, the following description will primarily focus on the case where the precursor consists only of fibers and uncured resin.

[0030] [Contact process] In the contact process, an ionic liquid is brought into contact with the precursor of the 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.

[0031] 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.

[0032] The temperature of the ionic liquid in the contact process should be such that the ionic liquid does not solidify and the 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 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.

[0033] The amount of ionic liquid used in the contact process is not particularly limited, as long as it is sufficient to remove the uncured resin adhering to the surface of the fiber.

[0034] The contact time is not particularly limited, as long as it is sufficient time to remove the uncured resin adhering to the surface of the fibers.

[0035] The method of the present invention may include a step of adjusting the dimensions of the precursor before the contact step. In the process of adjusting the dimensions, for example, the precursor is cut to a certain size (e.g., 5cm x 5cm or smaller). By adjusting the dimensions of the precursor, it becomes easier to obtain fibers of a certain length.

[0036] In addition, the processing conditions for the precursor may be adjusted as needed, for example, to promote the dissolution of the resin. For example, the ionic liquid in which the precursor is immersed may be stirred.

[0037] [Process for isolating fibers] In the process of isolating the fibers, the uncured resin contained in the precursor, which has been brought into contact with an ionic liquid, is dissolved in the ionic liquid to isolate the fibers.

[0038] When dissolving the resin, it is sufficient that the resin adhering to the surface of the fibers is removed from the fiber surface. To facilitate the isolation of the fibers, the resin contained in the precursor may be completely dissolved in the ionic liquid. If the precursor also contains components other than uncured resin and fibers (e.g., cured or semi-cured resin), these components may also be dissolved in the ionic liquid. Whether the resin has dissolved can be confirmed by methods such as observing the presence or absence of residual resin on the surface of the freed fibers under a microscope.

[0039] When uncured resin dissolves in an ionic liquid, a mixture of free fibers and the dissolved resin in the ionic liquid (an ionic liquid solution of the resin) is obtained. If only a portion of the uncured resin has dissolved, the mixture will contain further uncured resin. In the isolation step, the free fibers are isolated from this mixture. The isolation method is not particularly limited and may include filtration or centrifugation.

[0040] The isolated fibers may be washed with a washing solvent. The washing solvent can be selected as appropriate, and alcohols, ketones, hydrocarbons, ionic liquids, etc., can be used. Even if small amounts of resin or other substances remain on the surface of the fibers, the washing process can be removed. The isolated fibers may also be dried.

[0041] The method of the present invention may include a step of recovering the uncured resin from a mixture of the uncured resin and the ionic liquid (an ionic liquid solution of the resin, or a mixture of the ionic liquid solution of the resin and the uncured resin) obtained after the isolation of the free fibers. As a method for recovering the uncured resin, for example, one method is to add a poor solvent to a mixture of the uncured resin and an ionic liquid to precipitate the resin and then separate it. Poor solvents can be selected as appropriate, and examples include methanol and water. The method for separating the resin can be selected as appropriate, and examples include filtration and centrifugation.

[0042] Furthermore, if the precursor contains components other than uncured resin and 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 resin.

[0043] 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.

[0044] An ionic liquid (where the fibers are other than carbon fibers) used to isolate fibers by contacting a precursor of fiber-reinforced plastic containing fibers and uncured resin, thereby dissolving the uncured resin, is also an embodiment of the present invention.

[0045] This specification discloses the following:

[0046] (1) This disclosure is a method for recycling fibers (wherein the fibers are fibers other than carbon fibers), comprising the steps of: contacting an ionic liquid with a precursor of fiber-reinforced plastic containing fibers and an uncured resin; and dissolving the uncured resin in the ionic liquid to isolate the fibers.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] Disclosure (5) further includes a step of recovering the uncured resin from the mixture of the uncured resin and the ionic liquid, according to any of Disclosures (1) to (4).

[0051] Disclosure (6) is a method according to any one of Disclosures (1) to (5), wherein the uncured resin is an epoxy resin.

[0052] (7) The present disclosure is an ionic liquid (where the fibers are fibers other than carbon fibers) used to isolate fibers by contacting a precursor of fiber-reinforced plastic containing fibers and an uncured resin and dissolving the uncured resin. [Examples]

[0053] The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples.

[0054] Each material was manufactured as follows:

[0055] (Prepreg A) Glass fibers (15 μm in diameter) were immersed in a sizing solution prepared by diluting 50 parts by weight of an aqueous solution of butadiene-maleic acid copolymer ammonia salt (manufactured by Sanyo Chemical Industries, Ltd.: Acrobinder BG-7) with 50 parts by weight of water. Then, 600 of these glass fibers were bundled together, and the resulting strands were cut and dried using a standard chop-cutting method to obtain chopped strands 5 mm in length. The resin (sizing agent) content of the chopped strands was 0.8% by weight. 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 prepare a solution of the epoxy resin composition. The chopped strands were immersed in this solution, removed, and dried in a fume hood for 24 hours. Then, the mixture was left to stand for 1 month in an environment of 25°C and 60% humidity to prepare prepreg A.

[0056] (Prepreg B) An epoxy resin composition was prepared by diluting 10.9 parts by weight of trifunctional epoxy resin (Printec Co., Ltd.: VG3101M80, epoxy equivalent 205-215 g / eq.), 15.1 parts by weight of high-bromine epoxy resin (DIC Corporation: EPICLON153-60M, epoxy equivalent 390-410 g / eq.), 29.9 parts by weight of low-bromine epoxy resin (DIC Corporation: EPICLON1121N-80M, epoxy equivalent 475-515 g / eq.), 3.3 parts by weight of dicyandiamide (Tokyo Chemical Industries, Ltd.), and 0.06 parts by weight of 2-ethyl-4-methylimidazole (Tokyo Chemical Industries, Ltd.) with 20 parts by weight of methyl ethyl ketone. 2 The material was immersed in the solution, removed, dried in an 80°C incubator (ESPEC PH-102) for 2 minutes, and then left to stand for 1 month in an environment of 25°C and 60% humidity to produce prepreg B.

[0057] (Prepreg C) A urethane acrylic resin composition was prepared by mixing 4.8 parts by weight of bisphenol A ethylene oxide adduct (manufactured by Sanyo Chemical Industries, Ltd.: Newpol BPE-20, hydroxyl value 344), 12.0 parts by weight of bisphenol A ethylene oxide adduct (manufactured by Sanyo Chemical Industries, Ltd.: Newpol BPE-60, hydroxyl value 228), 38.5 parts by weight of 2-hydroxyethyl methacrylate (manufactured by Tokyo Chemical Industries, Ltd.), 1.4 parts by weight of polymerization initiator (manufactured by Kayaku Akzo Co., Ltd.: Trigonox 122-C80), and 43.3 parts by weight of 4,4'- or 2,4'-diphenylmethane diisocyanate (manufactured by Tokyo Chemical Industries, Ltd.). A chopped strand mat of basalt fiber (thickness 0.2 mm, basis weight 200 g / m²) was added to this urethane acrylic resin composition. 2 The material was immersed in the solution, removed, and left to stand for 24 hours in a 50°C incubator (ESPEC PH-102). After that, it was left to stand for one month in an environment of 25°C and 60% humidity to produce prepreg C. The resin (sizing agent) content of the basalt fiber chopped strand mat was 0.8% by weight.

[0058] [Measurement of resin composition content in prepreg] The weight loss rates of the fibrous substrates (chopped strands in prepreg A, aramid fiber nonwoven fabric in prepreg B, and basalt fiber chopped strand mat in prepreg C) 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 resin composition content in each prepreg was calculated using the following formula (1) based on the weight loss rates described above. The resin composition content of prepreg A was 36.5%, prepreg B was 29.3%, and prepreg C was 25.2%. Resin composition content in prepreg (%) = [(AB) / C] × 100 (1) A: Weight loss in prepreg B: Weight loss in the fiber base material contained in the prepreg. C: Prepreg weight A represents the total weight loss of the resin composition and sizing agent contained in the prepreg. B represents the weight loss of the sizing agent contained in the fiber substrate. Therefore, (AB) represents the weight loss of the resin composition contained in the prepreg, and by determining (AB), the content of the resin composition contained in the prepreg can be calculated. In the case of prepreg B, since the aramid fiber nonwoven fabric does not contain a sizing agent, A represents the weight loss of the resin composition contained in prepreg B, and B is 0.

[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 dicyanamide: EMI-DCN) 147 parts by weight (1.0 mol) of 1-ethyl-3-methylimidazolium chloride and 89 parts by weight (1.0 mol) of sodium dicyanamide were dissolved in 200 parts by weight of methanol and stirred for 10 minutes. The mixture was then heated to 60°C under a reduced pressure of 1.0 kPa to remove the solvent, and after returning to room temperature, 1600 parts by weight of dehydrated acetone was added to precipitate sodium chloride. After removing the precipitated sodium chloride by filtration, the mixture was heated to 70°C under a reduced pressure of 1.0 kPa to remove the solvent, yielding pale yellow 1-ethyl-3-methylimidazolium dicyanamide.

[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] (1-ethyl-3-methylimidazolium dimethyl phosphate: EMI-DMP) 96 parts by weight (1.0 mole) of 1-ethylimidazole and 140 parts by weight (1.0 mole) of trimethylphosphate were placed in an Erlenmeyer flask and reacted in an 80°C oil bath with heating and stirring for 24 hours to obtain pale yellow 1-ethyl-3-methylimidazolium dimethyl phosphate.

[0065] (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.

[0066] The solvents used in the comparative examples are commercially available products. (solvent) Toluene [manufactured by Tokyo Chemical Industry Co., Ltd.] Dimethylacetamide (DMAc) [Manufactured by Tokyo Chemical Industry Co., Ltd.]

[0067] <Examples 1-11, 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. The immersion temperature was 25°C for Examples 1-9 and Comparative Examples 1-3, 50°C for Example 10, and 10°C for Example 11. The mixture was filtered to isolate the fibers (glass fibers, aramid fiber nonwovens, or basalt 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.

[0068] [Fiber regeneration rate] The fiber regeneration rate was calculated based on the following formula (2). The results are shown in Table 1. Fiber regeneration rate (%) = [(CD) / A] × 100 (2) C: Prepreg weight D: Weight of isolated 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 resin composition and sizing agent contained in the prepreg that were not attached to the isolated fibers (removed from the fibers). The closer the value in equation (2) is to 100%, the more resin composition and sizing agent have been removed from the fibers; therefore, the value in equation (2) can be said to represent the fiber regeneration rate.

[0069] [Recovery rate of resin composition and sizing agent] The recovery rates of the 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 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).

[0070] [Table 1]

Claims

1. A step of contacting an ionic liquid with a fiber-reinforced plastic precursor containing fibers and uncured resin, The process includes dissolving the uncured resin in the ionic liquid to isolate the fibers, A method for recycling fibers (wherein the fibers are fibers other than carbon fibers).

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 resin from the mixture of the uncured resin and the ionic liquid.

6. The method according to claim 1 or 2, wherein the uncured resin is an epoxy resin.

7. An ionic liquid used to isolate fibers by contacting a fiber-reinforced plastic precursor containing fibers and uncured resin, thereby dissolving the uncured resin (where the fibers are other than carbon fibers).

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