Method for recovering carbon fibers from carbon fiber reinforced resin

Abstract

To provide a method of recovering a carbon fiber from a carbon fiber-reinforced resin, which is, despite a disadvantage of heat-treatment performed to the carbon fiber-reinforced resin thereby, capable of recovering the carbon fiber-reinforced resin less deteriorated in the treatment.SOLUTION: A method of recovering a carbon fiber from a carbon fiber-reinforced resin of the present disclosure has a step of heating the carbon fiber-reinforced resin containing the carbon fiber and a matrix resin at a temperature lower than 400°C. in presence of an alkali metal compound in a state of not being immersed in a solvent.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a method for recovering carbon fibers from carbon fiber reinforced resin. [Background technology]

[0002] Carbon fiber reinforced resin, which contains carbon fiber and a matrix resin, has the excellent characteristics of being lightweight and having high strength and durability, and is therefore used in a variety of fields such as the sports industry, general industry, and aerospace industry.

[0003] The production volume of carbon fiber reinforced resin is increasing year by year, and the waste of used carbon fiber reinforced resin products is also increasing year by year. In addition, when manufacturing carbon fiber reinforced resin, a lot of process waste such as scraps and scraps is also generated. However, due to its excellent durability, it is difficult to process carbon fiber reinforced resin waste in a normal incinerator. For this reason, most of the waste is landfilled after crushing and pulverization, but there is a shortage of disposal sites for landfill processing. In addition, carbon fiber is expensive because it requires a large amount of energy to manufacture. Therefore, a technology is desired to process carbon fiber reinforced resin waste and recover carbon fiber.

[0004] Known methods for recovering carbon fibers from carbon fiber reinforced resin include, for example, a dissolution treatment method in which the carbon fiber reinforced resin is immersed in a treatment liquid containing an organic solvent to decompose and dissolve the matrix resin (see, for example, Patent Document 1), and a pyrolysis treatment method in which the carbon fiber reinforced resin is heated to pyrolyze the matrix resin (see, for example, Patent Document 2). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2005-255835 A [Patent Document 2] International Publication No. 2018 / 212016 Summary of the Invention [Problem to be solved by the invention]

[0006] In the dissolution treatment method, it is difficult to repeatedly use an organic solvent, and the cost tends to be high. Therefore, the present inventors have studied a pyrolysis treatment method, but the pyrolysis treatment method of the prior art requires high-temperature heating, for example, at 500° C. or higher, and the recovered carbon fibers tend to be significantly deteriorated.

[0007] An object of the present disclosure is to provide a method for recovering carbon fibers from carbon fiber reinforced resin, which involves subjecting carbon fiber reinforced resin to a heat treatment, yet is capable of recovering carbon fibers that are little deteriorated by the treatment. [Means for solving the problem]

[0008] As a result of investigations to solve the above problems, the present inventors have found that the above problems can be solved by the following recovery method. That is, the method for recovering carbon fibers from carbon fiber reinforced resin of the present disclosure includes a step of heat-treating a carbon fiber reinforced resin containing carbon fibers and a matrix resin in the presence of an alkali metal compound at a temperature of less than 400° C. without immersing the carbon fiber reinforced resin in a solvent. Effect of the Invention

[0009] According to the present disclosure, it is possible to provide a method for recovering carbon fibers from carbon fiber reinforced resin, which is a method for subjecting carbon fiber reinforced resin to a heat treatment, yet is capable of recovering carbon fibers that are little deteriorated by the treatment. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 shows the gas yield recovered in the recovery test of the example. [Diagram 2] FIG. 2 shows the residual rate of solid residue obtained in the recovery test. [Diagram 3] FIG. 3 shows the amount of solid residue on the sieve obtained in the recovery test. [Figure 4] Figure 4 is a photograph of the appearance of the solid residue obtained in the recovery test. [Diagram 5] Figure 5 is a photograph of the appearance of the solid residue obtained in the recovery test. [Figure 6] FIG. 6 is a photograph showing the appearance of the filtrate obtained in the recovery test. [Figure 7] FIG. 7 shows the results of GC-MS analysis of the filtrate obtained in the recovery test. [Figure 8] FIG. 8 is an SEM photograph of the solid residue obtained in the recovery test. [Figure 9] FIG. 9 shows the gas yields recovered in the recovery tests. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] In this specification, "A to B" indicating a range of values ​​includes the values ​​A and B before and after "to" as the lower and upper limits, or the upper and lower limits. For example, "1 to 10" means a range of values ​​from 1 to 10.

[0012] [Method for recovering carbon fibers from carbon fiber reinforced resin] The method for recovering carbon fibers from carbon fiber reinforced resin disclosed herein (hereinafter also referred to as "recovery method (R)") has a step of subjecting carbon fiber reinforced resin containing carbon fibers and a matrix resin to a heat treatment under specific conditions (hereinafter also referred to as "step (1)").

[0013] <Process (1)> In step (1), the carbon fiber reinforced resin is heat-treated at a temperature of less than 400°C in the presence of an alkali metal compound without being immersed in a solvent. This heat treatment can decompose the matrix resin contained in the carbon fiber reinforced resin. The product obtained by heat-treating the carbon fiber reinforced resin is hereinafter also referred to as the "heat-treated product".

[0014] <Carbon fiber reinforced resin> Carbon fiber reinforced resin is a fiber reinforced resin that contains carbon fiber and a matrix resin, and usually contains carbon fiber as a reinforcing material in the matrix resin. Carbon fiber reinforced resin is also generally called carbon fiber reinforced plastic (CFRP).

[0015] (Carbon Fiber) Examples of carbon fibers include polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers such as coal pitch or petroleum pitch, rayon-based carbon fibers, and vapor-grown carbon fibers, with PAN-based carbon fibers and pitch-based carbon fibers being preferred. The carbon fiber reinforced resin may contain one or more types of carbon fibers.

[0016] The carbon fibers preferably have an average diameter (average fiber diameter) of 0.1 to 30 μm, more preferably 1 to 20 μm, in single fibers. The diameter of the carbon fibers can be determined, for example, by a test method conforming to JIS R7607 "Carbon Fibers - Test Method for Diameter and Cross-Sectional Area of ​​Single Fibers". The carbon fibers may be long or short fibers. The average fiber length of the carbon fibers is not particularly limited, and is, for example, 100 mm or less. The average values ​​of the diameter and fiber length can be calculated, for example, based on 100 pieces arbitrarily selected from the carbon fibers obtained by the recovery method (R).

[0017] Examples of the form of carbon fiber include fiber bundles (filaments or tows) in which multiple single fibers are aligned in one direction, chopped yarns in which fiber bundles are cut to any length, milled yarns in which the fiber bundles are cut even finer than chopped yarns, unidirectional materials (UD materials), woven fabrics, knitted fabrics, nonwoven fabrics, and other carbon fiber substrates. Carbon fiber reinforced resins can contain one or more types of carbon fiber. Carbon fiber reinforced resins can also contain one or more carbon fiber substrates.

[0018] The carbon fiber content in the carbon fiber reinforced resin is preferably 20 to 80 mass%, more preferably 30 mass% or more, even more preferably 40 mass% or more, more preferably 75 mass% or less, and even more preferably 70 mass% or less. The carbon fiber content can be determined, for example, by the thermogravimetric analysis described in the Examples section below, or can be determined by a test method in accordance with JIS K7075 "Test method for fiber content and void ratio of carbon fiber reinforced plastics".

[0019] (Matrix resin) Examples of the matrix resin include resins formed from thermosetting resins and resins formed from thermoplastic resins. As the matrix resin, resins formed from thermosetting resins are preferred, and resins formed by thermal curing of thermosetting resins are more preferred. The carbon fiber reinforced resin can contain one or more types of matrix resins.

[0020] Examples of the thermosetting resin include epoxy resin, phenol resin, unsaturated polyester resin, vinyl ester resin, melamine resin, urea resin, cyanate ester resin, and thermosetting polyimide resin, and epoxy resin and phenol resin are preferred, and epoxy resin is more preferred, because decomposition proceeds well by heat treatment in the presence of an alkali metal compound. The matrix resin may be formed of one type of thermosetting resin, or may be formed of two or more types of thermosetting resin.

[0021] The thermosetting resin may be a resin that is cured by heating, or may be a resin that is cured by heating and the action of a curing agent. In the case of an epoxy resin, examples of the curing agent include an acid anhydride curing agent, an amine curing agent, and a phenol resin curing agent.

[0022] Examples of epoxy resins include bisphenol A type epoxy resins, bisphenol F type epoxy resins, bisphenol S type epoxy resins, alicyclic epoxy resins, linear aliphatic epoxy resins, phenol novolac type epoxy resins, cresol novolac type epoxy resins, bisphenol A novolac type epoxy resins, diglycidyl ethers of biphenol, diglycidyl ethers of naphthalenediol, diglycidyl ethers of phenols, diglycidyl ethers of alcohols, and alkyl substituted, halides, and hydrogenated products thereof.

[0023] Examples of the thermoplastic resin include polyolefin resin, acrylic resin, polystyrene resin, acrylonitrile-butadiene-styrene copolymer resin, polyester resin, polycarbonate resin, polyacetal resin, polyamide resin, polyether ketone resin, polyether ether ketone resin, and polyphenylene sulfide resin. The matrix resin may be formed from one type of thermoplastic resin, or may be formed from two or more types of thermoplastic resins.

[0024] In one embodiment, the matrix resin is preferably a resin in which a crosslinked structure is formed by thermally curing a thermosetting resin, and which contains functional groups such as ester bonds and amide bonds in the crosslinked structure. It is believed that the functional groups of the matrix resin having such a crosslinked structure are more easily decomposed by an alkali metal compound such as an alkali metal hydroxide, and the resin is converted into a low molecular weight resin even at low temperatures.

[0025] The content of the matrix resin in the carbon fiber reinforced resin is preferably 20 to 80 mass%, more preferably 25 mass% or more, even more preferably 30 mass% or more, more preferably 70 mass% or less, and even more preferably 60 mass% or less.

[0026] (Other ingredients) The carbon fiber reinforced resin may contain other components in addition to the carbon fiber and the matrix resin. Examples of the other components include additives such as antioxidants, heat stabilizers, weathering agents, release agents, lubricants, pigments, dyes, plasticizers, antistatic agents, and flame retardants. The carbon fiber reinforced resin may contain one or more additives.

[0027] (Carbon fiber reinforced resin form) The carbon fiber reinforced resin may be in the form of an intermediate material such as pellets, prepregs and laminates thereof, stampable sheets and laminates thereof, or in the form of a product. The carbon fiber reinforced resin may be waste from used products, or process waste such as scraps and scraps generated during the manufacturing process of intermediate materials or products. The prepreg is a sheet of thermosetting resin in a semi-cured state obtained by impregnating carbon fibers with a thermosetting resin. The stampable sheet is a sheet obtained by impregnating carbon fibers with a thermoplastic resin. As the carbon fiber reinforced resin, it is preferable to use waste from used products, or process waste generated during the manufacturing process of intermediate materials or products. By using the recovery method (R), the above waste materials, which were difficult to process in a normal incinerator, can be easily processed with energy saving and low cost.

[0028] In the case of a product, the entire product may be carbon fiber reinforced resin, or a part of the product may be carbon fiber reinforced resin. In other words, the target of the heat treatment in step (1) is sufficient as long as it contains at least carbon fiber reinforced resin, and may be, for example, a composite of carbon fiber reinforced resin and another member. Examples of the other member include fiber reinforced resin containing reinforcing fibers other than carbon fiber, resin molded products not containing reinforcing fibers, metals, and ceramics.

[0029] Examples of products include products in the sports industry, general industry, and aerospace industry. Specifically, in the sports industry, examples of materials used in products include golf club shafts, fishing rods, ski poles, racket frames for tennis and badminton, baseball bats, and hockey sticks. In the general industry, examples of materials used in products include structural materials, drive shafts, flywheels, and leaf springs in automobiles, ships, and railroad cars; various containers (e.g., high-pressure tanks, filtration tanks), rollers, cables, roofing materials, and windmill blades. In the aerospace industry, examples of materials used in aircraft (e.g., structural materials used in main wings, tails, or fuselages) and rocket parts.

[0030] The shape of the carbon fiber reinforced resin is not particularly limited, and examples of the shape of the carbon fiber reinforced resin include a plate shape, a sheet shape, a square pipe shape, a round pipe shape, an L-shaped cross section, a T-shaped cross section, a C-shaped cross section, an H-shaped cross section, and any other three-dimensional shape.

[0031] When the carbon fiber reinforced resin is a large intermediate material or product, the carbon fiber reinforced resin may be cut into an appropriate size by a cutter as necessary. When cutting a carbon fiber reinforced resin containing long fibers, it is preferable to cut the carbon fiber reinforced resin along the longitudinal direction of the long fibers contained in the carbon fiber reinforced resin, since the long fibers can be recovered as long as possible.

[0032] <Alkali metal compounds> The alkali metal compound is considered to act as a decomposition catalyst for the matrix resin during heat treatment. The alkali metal compound is a compound of an alkali metal such as lithium, sodium, potassium, rubidium, or cesium, and examples thereof include hydroxides, alcoholates, phenolates, inorganic acid salts (e.g., phosphates, carbonates, sulfates, and nitrates), and organic acid salts of alkali metals. Among these, hydroxides of alkali metals are preferred, sodium hydroxide and potassium hydroxide are more preferred, and sodium hydroxide is particularly preferred, in that the decomposition of the matrix resin can be favorably promoted at low temperatures. One or more types of alkali metal compounds can be used.

[0033] By using an alkali metal compound, the decomposition of the matrix resin proceeds well even at a low temperature of less than 400° C., particularly at 300° C. or lower, and in one embodiment, the matrix resin decomposes into a solvent-soluble non-gaseous compound together with the gaseous compound. In the recovery method (R), the heat treatment can be performed at the above-mentioned low temperature, so that carbon fibers with less deterioration and high strength and durability can be recovered compared to the conventional thermal decomposition treatment method that requires high-temperature heating of 500° C. or higher.

[0034] After step (1), the alkali metal compound usually remains together with the heat-treated carbon fiber reinforced resin, and therefore it is possible to recover and reuse the alkali metal compound, which is advantageous in terms of cost.

[0035] As described later, it is preferable to use particles of an alkali metal compound. The shape of the particles of the alkali metal compound can be, for example, spherical, flake-like, or irregular. The alkali metal compound can be, for example, powder-like or granular. When using particles of an alkali metal compound, the particle size is not particularly limited.

[0036] The amount of the alkali metal compound used is preferably 10 parts by mass or more, more preferably 50 parts by mass or more, even more preferably 100 parts by mass or more, and particularly preferably 150 parts by mass or more, relative to 100 parts by mass of the carbon fiber reinforced resin, from the viewpoint of good progress of decomposition of the matrix resin. The upper limit of the amount of the alkali metal compound used is not particularly limited, but from the viewpoint of cost, the amount of the alkali metal compound used is preferably 1000 parts by mass or less, more preferably 500 parts by mass or less, and even more preferably 300 parts by mass or less, relative to 100 parts by mass of the carbon fiber reinforced resin.

[0037] <Heat treatment conditions> In step (1), the carbon fiber reinforced resin is heat-treated without being immersed in a solvent. The term "heat treatment" in this disclosure means a process in which the carbon fiber reinforced resin is heated without being immersed in a solvent such as an organic solvent or water to reduce the molecular weight of the matrix resin, and is different from a dissolution treatment method in which the carbon fiber reinforced resin is heated while being immersed in a solvent, and a specific bond is cleaved by utilizing a reaction field in the solvent to reduce the molecular weight of the matrix resin. Examples of the organic solvent include alcohol solvents, ether solvents, ketone solvents, ester solvents, and amide solvents.

[0038] The heat treatment includes, for example, a method of applying a solution of an alkali metal compound to a carbon fiber reinforced resin and then heating the resin, or a method of immersing a carbon fiber reinforced resin in a solution of an alkali metal compound, then pulling it out, and then heating the resin. The heat treatment also includes a method of immersing a carbon fiber reinforced resin in a molten liquid of an alkali metal compound. That is, the solvent does not include a molten liquid of an alkali metal compound. These methods can effectively disperse and / or coat the alkali metal compound on the surface of the carbon fiber reinforced resin.

[0039] In step (1), it is preferable to heat treat the carbon fiber reinforced resin in the absence of a solvent (however, in the case of an alkali metal compound having deliquescent properties, such as sodium hydroxide, absorbed moisture may be present).

[0040] In step (1), the carbon fiber reinforced resin is heat-treated in the presence of an alkali metal compound. In the present disclosure, "in the presence of an alkali metal compound" means a state in which the alkali metal compound and the carbon fiber reinforced resin are in contact with each other at least in part. For example, particles of an alkali metal compound may be brought into contact with the carbon fiber reinforced resin, a melt of an alkali metal compound may be brought into contact with the carbon fiber reinforced resin, a solution of an alkali metal compound (e.g., an aqueous solution) may be applied to the carbon fiber reinforced resin, or the carbon fiber reinforced resin may be immersed in a solution of an alkali metal compound (e.g., an aqueous solution) and then pulled out.

[0041] Among these, it is preferable to bring particles of an alkali metal compound into contact with the carbon fiber reinforced resin and then heat the mixture, since a sufficient amount of the alkali metal compound can be brought into contact with the carbon fiber reinforced resin. For example, the particles may be disposed on the carbon fiber reinforced resin, or the carbon fiber reinforced resin may be embedded in the particles.

[0042] The heat treatment can be carried out in a conventionally known reactor. Examples of the heating method include convection heating using hot air, radiation heating using infrared rays, far infrared rays, microwaves, etc., conduction heating by contact with a hot plate, etc., and any combination of two or more of these.

[0043] The heating temperature of the carbon fiber reinforced resin in the heat treatment is less than 400°C, preferably 380°C or less, more preferably 350°C or less, even more preferably 330°C or less, still more preferably 300°C or less, particularly preferably 290°C or less, 280°C or less, or 260°C or less, and is preferably more than 200°C, more preferably 220°C or more, and even more preferably 240°C or more.

[0044] If the heating temperature is less than 400°C, the deterioration of the carbon fiber can be suppressed, and particularly if it is 300°C or less, the deterioration of the carbon fiber can be sufficiently suppressed, and therefore, carbon fiber with excellent strength (e.g., tensile strength) can be recovered. Furthermore, under such low temperature conditions, carbonization of the matrix resin is suppressed compared to the conventional thermal decomposition treatment method that requires high temperature conditions, so that the amount of carbonized material is reduced and the carbon fiber can be easily separated. If the heating temperature is more than 200°C, the decomposition of the matrix resin can be sufficiently promoted.

[0045] The heating time of the carbon fiber reinforced resin in the heat treatment is appropriately set according to the heating temperature. The heating time is preferably 1 minute or more, more preferably 5 minutes or more, even more preferably 15 minutes or more, even more preferably 25 minutes or more, and particularly preferably 50 minutes or more. If the heating time is equal to or more than the lower limit, the decomposition of the matrix resin can be sufficiently promoted. The upper limit of the heating time is not particularly limited, but is preferably 10 hours or less, more preferably 5 hours or less, and even more preferably 2 hours or less.

[0046] The heat treatment of the carbon fiber reinforced resin may be carried out under normal pressure, reduced pressure or increased pressure, but is preferably carried out under normal pressure.

[0047] The heat treatment of the carbon fiber reinforced resin is preferably carried out under a non-oxidizing gas atmosphere, since it can suppress oxidation deterioration of the carbon fiber surface and can recover carbon fibers having strength (e.g., tensile strength) comparable to that of virgin materials. The non-oxidizing gas atmosphere is an atmosphere that does not contain oxygen gas, or an atmosphere that does not substantially contain oxygen gas. The atmosphere that does not substantially contain oxygen gas does not include an atmosphere in which oxygen gas is intentionally added when the carbon fiber reinforced resin is heated, but includes an atmosphere in which oxygen gas is inevitably mixed in. Examples of the non-oxidizing gas include inert gases such as nitrogen gas and argon gas.

[0048] By heating the carbon fiber reinforced resin under the above conditions, the matrix resin is usually decomposed and at least a part of it is vaporized as a gaseous compound, and for example, a solid heat-treated product is obtained. The heat-treated product contains the carbon fibers and may further contain a non-gaseous compound.

[0049] In a preferred embodiment of step (1), from the standpoints of safety, cost, and the like, the amount of alkali metal compound used is 150 to 300 parts by mass per 100 parts by mass of the carbon fiber reinforced resin, the heating temperature is 220 to 330°C, and the heating time is 50 minutes to 2 hours. The heat-treated product obtained in step (1) is, if necessary, subjected to the following step (2).

[0050] <Gas recovery process> One embodiment of the recovery method (R) further includes a step of recovering the gas (gaseous compound) generated from the matrix resin by the heat treatment in step (1).

[0051] When the carbon fiber reinforced resin is heat-treated in step (1), usually at least a part of the matrix resin is decomposed into gaseous compounds. The gaseous compounds depend on the type of matrix resin, but examples of the gaseous compounds include gases that can be used as fuel, such as hydrogen (H2) and methane (CH4). In the decomposition of an epoxy resin in the presence of an alkali metal compound, hydrogen (H2) and methane (CH4) are usually generated. Therefore, the recovery method (R) can recover not only carbon fibers but also gases that can be used as fuel.

[0052] <Process (2)> When the matrix resin is sufficiently decomposed into gaseous compounds in step (1), the obtained heat-treated product can be used as it is as recycled carbon fibers. When the matrix resin is decomposed into gaseous compounds and non-gaseous compounds, it is preferable to separate the carbon fibers from the obtained heat-treated product.

[0053] That is, one embodiment of the recovery method (R) further includes a step (hereinafter also referred to as "step (2)") of separating the carbon fibers from the heat-treated product obtained by the heat treatment in the step (1).

[0054] The separation method is not particularly limited, and may be, for example, a method of washing the heat-treated product with a washing liquid. In one embodiment, the heat treatment in step (1) can decompose at least a part of the matrix resin into a solvent-soluble non-gaseous compound (e.g., phenols in the case of an epoxy resin), and therefore the non-gaseous compound contained in the heat-treated product can be dissolved in the washing liquid.

[0055] Examples of the cleaning method include immersion cleaning, ultrasonic cleaning, spray cleaning, shower cleaning, and jet cleaning, and two or more of these methods may be combined. In the case of immersion cleaning and ultrasonic cleaning, the heat-treated product is immersed in a cleaning liquid, and after stirring as necessary, the carbon fibers are separated from the cleaning liquid. Examples of the separation method include filtration, precipitation separation, and centrifugation. In the dissolution treatment method of the prior art, the carbon fiber reinforced resin is immersed in a dissolution treatment liquid, the resin is depolymerized and dissolved in the treatment liquid, and then the carbon fibers and the treatment liquid containing the resin are separated. In contrast, the recovery method (R) of one embodiment having the step (2) is different from the dissolution treatment method in that the carbon fiber reinforced resin is heated without being immersed in the dissolution treatment liquid, and then the heat-treated product containing the decomposed matrix resin is washed with the cleaning liquid.

[0056] Examples of the cleaning liquid include water and organic solvents. Examples of the organic solvent include alcohol solvents such as methanol, ethanol, propanol, benzyl alcohol, and ethylene glycol monomethyl ether; ether solvents such as dipropyl ether, diisopropyl ether, and dibutyl ether; ketone solvents such as acetone and methyl ethyl ketone; ester solvents such as ethyl acetate, propyl acetate, and γ-butyrolactone; and amide solvents such as N-methylformamide, N,N-dimethylformamide, N-methylacetamide, N,N-dimethylacetamide, and N-methylpyrrolidone. A mixture of water and an organic solvent may be used as the cleaning liquid, and one or more organic solvents may be used. As the cleaning liquid, a cleaning liquid containing at least water is preferable, and a cleaning liquid containing water at a rate of 50% by mass or more, 60% by mass or more, 70% by mass or more, or 80% by mass or more is more preferable. The cleaning liquid may be an acidic aqueous solution such as an aqueous solution of hydrochloric acid or phosphoric acid.

[0057] The cleaning conditions are not particularly limited. The temperature of the cleaning liquid may be room temperature, or may be a temperature at which the cleaning liquid maintains its liquid state, and may be heated, for example, to increase the amount of the solvent-soluble non-gaseous compound and the alkali metal compound dissolved. For example, when a cleaning liquid containing at least water is used, the temperature of the cleaning liquid is preferably 5 to 90°C, more preferably 10 to 80°C. The cleaning time is also not particularly limited.

[0058] The above-mentioned cleaning method makes it possible to easily separate the carbon fibers from the heat-treated material without deteriorating the carbon fibers, as compared with a method in which mechanical forces such as compressive stress, tensile stress, or shear stress are applied to the heat-treated material.

[0059] In addition to the heat-treated product, the alkali metal compound used in step (1) is usually also present in the reactor after the heat treatment in step (1). By washing the heat-treated product and the alkali metal compound with a washing liquid, the alkali metal compound can be dissolved in the washing liquid together with the non-gaseous compound. The solution thus obtained is also called a "resin solution". By treating the resin solution by a known method, the alkali metal compound can be recovered. In addition, after using the resin solution as fuel, the alkali metal compound can be recovered and reused. The so-called black liquor recovered in the paper pulp manufacturing process is used as fuel (incinerated), and the inorganic residue is recovered and reused. A similar use process to that of the black liquor is assumed for the resin solution.

[0060] After washing the carbon fibers, the carbon fibers may be dried using a dryer such as an oven.

[0061] Compared with conventional thermal decomposition treatment, the recovery method (R) can recover carbon fibers from carbon fiber reinforced resin at low temperatures, and the gas generated during the heat treatment can be used as fuel. In addition, since the alkali metal compound used as the decomposition catalyst for the matrix resin can be reused, the running costs of the recovery method (R) are also lower than those of conventional techniques. Furthermore, the recovery method (R) can recover reusable carbon fibers that have excellent strength and durability in the same form as when they were contained in the carbon fiber reinforced resin.

[0062] Using the above-described recovery method (R), high-quality recycled carbon fibers can be produced from carbon fiber reinforced resin containing carbon fibers and a matrix resin, in an energy-saving and low-cost manner.

[0063] [Method of manufacturing carbon fiber reinforced resin] The manufacturing method for carbon fiber reinforced resin of the present disclosure (hereinafter also referred to as "manufacturing method (P)") has a step of obtaining carbon fibers as recycled carbon fibers from carbon fiber reinforced resin containing carbon fibers and a matrix resin using the above-mentioned recovery method (R), and a step of manufacturing carbon fiber reinforced resin using the recycled carbon fibers and resin.

[0064] The recycled carbon fibers recovered by the recovery method (R) can be used as is. Furthermore, the recycled carbon fibers may be subjected to a conventionally known sizing treatment in order to improve the handling of the fibers. The sizing treatment is, for example, a treatment agent (sizing agent) for bundling carbon fibers is applied to the surface of the carbon fibers, and heated as necessary to bond the sizing agent to the carbon fibers. Examples of the sizing agent include urethane resins, epoxy resins, polyester resins, and polyamide resins. The amount of the sizing agent attached to the recycled carbon fibers is preferably 0.1 to 20 parts by mass, more preferably 0.5 to 10 parts by mass, and even more preferably 1 to 5 parts by mass, per 100 parts by mass of the recycled carbon fibers.

[0065] In order to increase the adhesion between the carbon fibers and the matrix resin, the recycled carbon fibers may be subjected to a surface treatment in which the carbon fiber surface is oxidized by anodic electrolytic oxidation or ozone oxidation to introduce oxygen-containing functional groups.

[0066] Examples of resins used to produce carbon fiber reinforced resins include the thermosetting resins and thermoplastic resins described in the (Matrix resin) section. Thermosetting resins are preferred, epoxy resins and phenolic resins are more preferred, and epoxy resins are even more preferred.

[0067] In addition to the carbon fiber obtained by the recovery method (R), various inorganic or organic fibers such as non-recycled carbon fiber (virgin material), aramid fiber, nylon fiber, polyester fiber, glass fiber, boron fiber, alumina fiber, and silicon nitride fiber can be used as necessary.

[0068] The carbon fiber reinforced resin obtained by the manufacturing method (P) may be in the form of an intermediate material such as pellets, prepregs and laminates thereof, stampable sheets and laminates thereof, or in the form of a product. In the case of a product, the entire product may be carbon fiber reinforced resin, or a part of the product may be carbon fiber reinforced resin. For example, it may be a composite of carbon fiber reinforced resin and other members. Examples of other members include fiber reinforced resin containing reinforcing fibers other than carbon fiber, resin molded products not containing reinforcing fibers, metals, and ceramics.

[0069] A conventionally known method can be used to produce carbon fiber reinforced resin using recycled carbon fibers. Several examples are described below, but the production method (P) is not limited to these examples.

[0070] Examples of methods for producing pellets include a method of impregnating carbon fibers such as fiber bundles (filaments or tows) in which multiple single fibers are aligned in one direction, chopped yarns in which fiber bundles are cut to any length, and milled fibers in which the fiber bundles are cut even finer than chopped yarns, with a resin that forms a matrix resin to form a composite.

[0071] Examples of the resin impregnation method include a method using a single screw extruder, a twin screw extruder, a press, a high-speed mixer, and an injection molding machine. When forming the composite, additives may be used together with the resin as necessary. Examples of additives include antioxidants, heat stabilizers, weatherproofing agents, release agents, lubricants, pigments, dyes, plasticizers, antistatic agents, and flame retardants.

[0072] Carbon fiber reinforced resin products can be obtained, for example, by molding the pellets. Examples of the molding method include injection molding, extrusion molding, press molding, blow molding, compression molding, injection compression molding, and foam injection molding.

[0073] Examples of methods for producing a prepreg include a method in which a thermosetting resin is applied onto a release sheet to create a resin film, and then the resin film and a carbon fiber substrate are superimposed and heated and pressurized to impregnate the carbon fiber substrate with the semi-cured thermosetting resin; a method in which a thermosetting resin is heated to reduce its viscosity, and then the carbon fiber substrate is impregnated with the resin; and a method in which a thermosetting resin is dissolved in a solvent to obtain a resin solution, and the carbon fiber substrate is immersed in the solution, then pulled out, and the solvent is evaporated using an oven or the like.

[0074] Carbon fiber reinforced resin products can be obtained by, for example, the autoclave method, the hand layup method, the filament winding method, the sheet winding method, the pultrusion molding method, the resin transfer molding method, and the sheet molding compound press molding method.

[0075] As one example, a carbon fiber reinforced resin product can be obtained by, for example, heating and pressurizing a prepreg, or by laminating a plurality of prepregs and then heating and pressurizing the laminate to sufficiently cure a semi-cured thermosetting resin.

[0076] The present disclosure relates to, for example, the following [1] to

[12] . [1] A method for recovering carbon fibers from carbon fiber reinforced resin, comprising a step of heat treating a carbon fiber reinforced resin containing carbon fibers and a matrix resin in the presence of an alkali metal compound at a temperature of less than 400°C without being immersed in a solvent. [2] The method according to [1] above, wherein the temperature of the heat treatment is greater than 200°C. [3] The method according to [1] or [2] above, wherein the heat treatment is carried out under a non-oxidizing gas atmosphere. [4] The method according to any one of the above [1] to [3], wherein the alkali metal compound is a hydroxide of an alkali metal. [5] The method according to any one of the above [1] to [4], wherein 10 to 1,000 parts by mass of the alkali metal compound is used per 100 parts by mass of the carbon fiber reinforced resin. [6] The method for recovering carbon fiber reinforced resin according to any one of the above [1] to [5], wherein the carbon fiber reinforced resin is at least one selected from waste materials from used products, process waste materials generated in the manufacturing process of intermediate materials, and process waste materials generated in the manufacturing process of products. [7] The method according to any one of the above [1] to [6], further comprising a step of recovering gas generated from the matrix resin by the heat treatment. [8] The method according to any one of [1] to [7] above, further comprising a step of separating the carbon fibers from the heat-treated product obtained by the heat treatment. [9] The method for recovering carbon fibers according to [8] above, wherein the heat-treated product is washed with a washing liquid in the step of separating the carbon fibers.

[10] The method according to [9] above, wherein the cleaning solution contains at least water.

[11] A method for producing recycled carbon fibers, comprising a step of obtaining carbon fibers as recycled carbon fibers from a carbon fiber reinforced resin containing carbon fibers and a matrix resin, by using the recovery method according to any one of the above [1] to

[10] .

[12] A method for producing carbon fiber reinforced resin, comprising: a step of obtaining carbon fibers as recycled carbon fibers from a carbon fiber reinforced resin containing carbon fibers and a matrix resin by using the recovery method according to any one of the above [1] to

[10] ; and a step of producing a carbon fiber reinforced resin using the recycled carbon fibers and a resin. EXAMPLES

[0077] The recovery method of the present disclosure will now be described in more detail based on test examples.

[0078] [Test piece] Scrap carbon fiber reinforced resin (structural material for filtration tanks, matrix resin formed from epoxy resin) manufactured domestically was cut into pieces measuring 1 cm x 1 cm to create test pieces.

[0079] Thermogravimetric analysis (TGA; air or nitrogen gas atmosphere, heating rate 10°C / min) was performed on the test specimen. The results of TGA showed that the weight loss occurred in two stages in both air and nitrogen gas atmospheres, which suggests that the weight loss occurs due to the decomposition of the matrix resin from around 300°C, and the weight loss occurs due to the decomposition of the carbon fiber from around 600°C. From the weight ratio around 600°C when the weight loss of the matrix resin ends, it was estimated that the carbon fiber content in the test specimen was about 65% by mass, and the matrix resin content was about 35% by mass.

[0080] [Recovery Test] 1 g of carbon fiber reinforced resin waste (the above test piece) and 0 g, 1 g, 2 g or 3 g of sodium hydroxide powder were placed in a stainless steel reactor, and nitrogen gas was passed through the reactor at 100 mL / min for 45 minutes to replace the atmosphere with nitrogen. Then, heat treatment was performed for 0 to 90 minutes at 200 ° C, 250 ° C, 300 ° C, 350 ° C, 400 ° C, 500 ° C or 600 ° C under a nitrogen gas atmosphere with a flow rate of 20 mL / min. After heating, the mixture was allowed to cool for 40 minutes, and 150 mL of distilled water was added to the reactor and stirred with a hot stirrer at a rotation speed of 600 rpm and a temperature of 70 ° C for 30 minutes. Then, the mixture in the reactor was filtered to obtain a solid residue and a filtrate.

[0081] The gas generated during heating was collected in a gas pack, and the yield and components were analyzed using a gas chromatograph (GC-8A, Shimadzu).

[0082] The solid residue was dried at 60°C for 12 hours or more in a dryer, and then the mass was measured using an electronic balance. The solid residue after the drying was ground in a mortar and then classified using a 106 μm mesh sieve to separate the solid residue into carbon fibers and carbide powder. The mass and surface condition of the separated carbon fibers were measured using an electronic balance and a scanning electron microscope (SEM) (JSM-6510A, manufactured by JEOL).

[0083] The filtrate was analyzed using a gas chromatograph mass spectrometer (GC-MS) (GCMS-QP2010Plus, manufactured by SHIMADZU).

[0084] [evaluation] Fig. 1 shows the gas yield when the heating time of the above heat treatment was fixed at 60 minutes and the amount of sodium hydroxide added and the heating temperature were changed, Fig. 2 shows the percentage of solid residue remaining after heating, and Fig. 3 shows the amount of solid residue (amount of carbon fiber) remaining on the sieve when the solid residue after heating was crushed in a mortar and then classified. The percentage of solid residue remaining was calculated by the mass of solid residue / mass of test piece (1g) x 100 (%). Note that "1:1" in the figure indicates the mass ratio of carbon fiber reinforced resin (test piece):sodium hydroxide.

[0085] As shown in Figure 1, the gas yield increases with increasing heating temperature and with increasing amount of sodium hydroxide added. Therefore, it is believed that sodium hydroxide promotes the decomposition and gasification of the matrix resin. When sodium hydroxide is added, the recovered gas is composed almost entirely of H2 and CH4. Below 400°C, when sodium hydroxide is not added, almost no usable gas is obtained, but when sodium hydroxide is added, usable gas is obtained.

[0086] As shown in Figure 2, when sodium hydroxide was not added, the residual solid residue rate decreased above 300°C and then remained almost constant. In contrast, when sodium hydroxide was added, the residual solid residue rate decreased at 250°C and, as the heating temperature increased, it remained constant at roughly the same rate as when sodium hydroxide was not added. This result shows that the addition of sodium hydroxide promotes decomposition of the matrix resin at a lower temperature.

[0087] As shown in Figure 3, when sodium hydroxide was not added, the amount of solid residue remaining on the sieve gradually decreased as the heating temperature increased. In contrast, when sodium hydroxide was added, the amount of solid residue remaining on the sieve decreased significantly at 250°C, and no significant decrease was observed between 250°C and 350°C. However, at temperatures above 400°C, most of the residue passed through the sieve and decreased significantly. When sodium hydroxide was added, the carbon fiber deteriorated at temperatures above 400°C and was ground into powder, which is thought to be why the amount of solid residue remaining on the sieve decreased significantly.

[0088] Figure 4 shows external photographs of the test piece before heating, the solid residue obtained when the mass ratio (test piece:sodium hydroxide) was 1:1 or 1:2 and the heating temperature was 250°C, and the solid residue obtained when the mass ratio was 1:2 and the heating temperature was 600°C. Most of the solid residue obtained when the mass ratio was 1:1 and the heating temperature was 250°C became fibrous, but some plate-shaped residue remained (Figure 4(b)). No plate-shaped residue was observed in the solid residue obtained when the mass ratio was 1:2 and the heating temperatures were 250°C and 600°C, and all of it became fibrous (Figures 4(c) and (d)).

[0089] Figure 5 shows photographs of the appearance of the solid residue obtained without the addition of sodium hydroxide at heating temperatures of 300°C, 400°C, 500°C, or 600°C, and the solid residue obtained with the above mass ratio of 1:2 at heating temperatures of 250°C or 350°C. Without the addition of sodium hydroxide, the carbon fibers remained almost entirely plate-like up to 500°C, but degradation of the carbon fibers was observed at 600°C.

[0090] Figure 6 shows the appearance of the filtrate obtained when the mass ratio was 1:2 and the heating temperatures were 250°C and 600°C, and Figure 7 shows the chromatograms analyzed by GC-MS. The filtrate obtained when the mass ratio was 1:2 and the heating temperature was 250°C was black (Figure 6(a)), while the filtrate obtained when the mass ratio was 1:2 and the heating temperature was 600°C was transparent (Figure 6(b)). Many peaks were detected in the chromatogram of the filtrate obtained when the mass ratio was 1:2 and the heating temperature was 600°C, and most of these peaks were assigned to phenols, which are resin components. It is believed that at a heating temperature of 250°C, the matrix resin was decomposed into water-soluble low molecular weight compounds (phenols, etc.), and the low molecular weight compounds were dissolved in distilled water.

[0091] Figure 8 shows SEM images of the solid residues obtained when the carbon fiber was treated at a mass ratio of 1:2 and heating temperatures of 250°C and 600°C. In the carbon fiber treated at a mass ratio of 1:2 and a heating temperature of 250°C, no resin was found remaining on the surface, and no deterioration of the carbon fiber was observed (Figure 8(a)). On the other hand, in the carbon fiber treated at a mass ratio of 1:2 and a heating temperature of 600°C, some areas where the fiber had been scraped off and become thinner, and some areas where the surface had become rough, were observed (Figure 8(b)). When sodium hydroxide was added and the fiber was treated at a heating temperature of 400°C or higher, the sodium hydroxide promoted the gasification of the resin, but it is believed that this also affected the carbon fiber, causing it to deteriorate.

[0092] Figure 9 shows the gas yield when the above mass ratio was fixed at 1:2, the heating temperature was fixed at 250°C, and the heating time was changed. The gas yield increased with increasing heating time, and became almost constant when the heating time was 60 minutes or more. The appearance of the solid residue at 0 minutes of heating time was the same as that of the test piece before heating. Plate-shaped residues were confirmed in the residues heated for 15 to 45 minutes, but no plate-shaped residues were confirmed in the residues heated for 60 minutes or more, and they were all fibrous.

Claims

1. A step of subjecting a carbon fiber reinforced resin containing carbon fibers and a matrix resin to a heat treatment at a temperature of less than 400° C. in the presence of alkali metal hydroxide particles without being immersed in a solvent. A method for recovering carbon fibers from a carbon fiber reinforced resin, comprising: The heat treatment step is carried out in the absence of a solvent. Recovery method.

2. The method according to claim 1 , wherein the temperature of the heat treatment is greater than 200° C.

3. The method according to claim 1 or 2, wherein the heat treatment is carried out in a non-oxidizing gas atmosphere.

4. The method according to any one of claims 1 to 3, wherein the hydroxide of the alkali metal is sodium hydroxide.

5. The recovery method according to any one of claims 1 to 4, wherein 10 to 1000 parts by mass of the alkali metal hydroxide is used per 100 parts by mass of the carbon fiber reinforced resin.

6. The carbon fiber reinforced resin is at least one selected from waste materials of used products, process waste materials generated in the manufacturing process of intermediate materials, and process waste materials generated in the manufacturing process of products. The recovery method according to any one of claims 1 to 5.

7. A step of recovering gas generated from the matrix resin by the heat treatment. The recovery method according to any one of claims 1 to 6, further comprising:

8. A step of separating the carbon fibers from the heat-treated product obtained by the heat treatment. The method according to any one of claims 1 to 7, further comprising:

9. The method according to claim 8 , wherein in the step of separating the carbon fibers, the heat-treated product is washed with a washing liquid.

10. The method according to claim 9 , wherein the cleaning solution comprises at least water.

11. A process for obtaining carbon fibers as recycled carbon fibers from a carbon fiber reinforced resin containing carbon fibers and a matrix resin by using the recovery method according to any one of claims 1 to 10. The method for producing recycled carbon fibers comprising the steps of:

12. A step of obtaining carbon fibers as recycled carbon fibers from a carbon fiber reinforced resin containing carbon fibers and a matrix resin by using the recovery method according to any one of claims 1 to 10; and A step of producing a carbon fiber reinforced resin using the recycled carbon fibers and a resin. The method for producing a carbon fiber reinforced resin comprising the steps of:

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