Method for producing fibrous filler, and fibrous filler

Abstract

This method for producing a fibrous filler comprises immersing a fiber-reinforced resin composite material composed of at least reinforcing fibers and a thermosetting resin in a mixed liquid comprising an aprotic organic solvent to which a basic compound or an acidic compound has been added, raising the temperature of the mixed liquid to a temperature higher than the boiling point of the aprotic organic solvent at normal pressure, removing the resin component containing the thermosetting resin from the fiber-reinforced resin composite material, and recovering a fibrous filler comprising the reinforcing fibers. The method is characterized in that the mixed liquid does not contain an aliphatic alcohol.　Provided are a method for producing a fibrous filler from a fiber-reinforced resin composite material with little deterioration of the fibrous filler, and a fibrous filler obtained by decomposing a fiber-reinforced resin composite material.

Description

Method for producing fibrous filler, and fibrous filler

[0001] The present invention relates to a method for producing a fibrous filler, and to a fibrous filler. More specifically, the present invention relates to a method for producing a fibrous filler by removing a resin component from a fiber-reinforced resin composite material using a solvent and recovering the fibrous filler, and to a fibrous filler obtained by decomposing the fiber-reinforced resin composite material.

[0002] Fiber-reinforced resin composites, which consist of fibrous fillers such as glass fibers or carbon fibers and a matrix resin, are used in a wide range of fields, from general-purpose household electrical appliances to precision electronic devices and components. Most of the scraps and waste products generated during the manufacturing process of these fiber-reinforced resin composites are rarely reused, and most are disposed of by incineration or landfill. Therefore, from the standpoints of efficient resource utilization and prevention of environmental pollution, the development of recycling technologies is highly desirable.

[0003] As a conventional technique for recovering fibrous fillers from fiber-reinforced resin composites, for example, a method (thermal decomposition method) in which the resin component is thermally decomposed in a high-temperature combustion furnace to recover the fibrous fillers is known. However, there are issues with this method, such as degradation of the fibrous fillers during the thermal decomposition process and the generation of exhaust gas containing organic and inorganic hazardous compounds during the thermal decomposition of the matrix resin, preventing effective utilization of the fibrous fillers.

[0004] On the other hand, as a method other than the thermal decomposition method for recovering a fibrous filler, for example, Patent Document 1 discloses a treatment method in which a cured epoxy resin material is dissolved using a treatment liquid containing at least one catalyst selected from the group consisting of alkali metals, alkali metal compounds, phosphoric acid, phosphates, organic acids, and organic acid salts, and at least one organic solvent selected from the group consisting of amide solvents, alcohol solvents, ketone solvents, and ether solvents.

[0005] Furthermore, as a method other than the pyrolysis method for recovering fibrous fillers, a method has been disclosed in which fiber-reinforced plastics are brought into contact with and reacted with supercritical water or subcritical water in a reactor, and the fibers are separated and recovered (Patent Document 2).

[0006] As yet another method other than the pyrolysis method for recovering fibrous filler, a method for producing carbon fiber has been disclosed (Patent Document 3), which includes the steps of: (1) immersing an uncured prepreg, which is a composite material obtained by combining carbon fiber and a thermosetting resin before the thermosetting resin is cured, in an acidic aqueous solution containing nitric acid to elute a portion of the resin content of the uncured prepreg, thereby forming a fibrous portion and obtaining a product in which a resin residue remains (hereinafter referred to as a fibrous product); and (2) immersing the fibrous product obtained in step (1) in an alkaline aqueous solution to remove the resin residue from the fibrous product, thereby obtaining carbon fiber.

[0007] Furthermore, Patent Document 4 discloses a carbon fiber recovery method in which a resin portion is dissolved from carbon fiber reinforced plastic using a dissolving liquid to recover carbon fibers, wherein the dissolving liquid contains a mixed solvent obtained by mixing multiple types of organic solvents including at least one type of alcohol-based solvent, and a catalyst, and the distance between the HSP value of the mixed solvent and the HSP value of the resin portion of the carbon fiber reinforced plastic is set to a predetermined value or less.

[0008] JP 2001-172426 A JP 10-87872 A JP 2019-136932 A JP 2019-209540 A

[0009] The method described in Patent Document 1 is certainly a treatment performed in a temperature range where deterioration of carbon fibers is minimal. However, when sodium hydroxide is used, which is the least expensive of the alkali metal compounds described in Patent Document 1 and is also a material with superior solubility in epoxy resin cured products, there is a problem that the viscosity of the treatment solution becomes high.

[0010] The method described in Patent Document 2 is capable of separating and recovering fibers from fiber-reinforced plastics, but has the drawback of requiring expensive equipment due to the ultra-high temperature and high pressure reaction.

[0011] The method described in Patent Document 3 certainly makes it possible to recover carbon fibers from composite materials in which carbon fibers and thermosetting resins are combined, but it is a complicated process that involves treatment with an acidic aqueous solution and an alkaline aqueous solution, and there is room for improvement. Furthermore, there is also the problem that a very long reaction time is required when decomposing cured prepregs.

[0012] The method described in Patent Document 4 is a method that makes it possible to recover carbon fibers from carbon fiber reinforced plastics consisting of carbon fibers and a resin portion. However, it is necessary to calculate the HSP value of the mixed solvent according to the HSP value of the resin portion and to mix multiple types of organic solvents, including alcohol-based solvents. However, if the HSP value of the resin portion is unknown, there is a problem in that an appropriate mixed solvent cannot be prepared.

[0013] Therefore, the present invention aims to provide a method for producing a fibrous filler that processes a fiber-reinforced resin composite material composed of at least reinforcing fibers and a thermosetting resin using a simple processing method, without using expensive equipment or chemicals, and without the need for special prior preparation of a processing agent tailored to the resin that constitutes the fiber-reinforced resin composite material, and recovers the fibrous filler.

[0014] The present invention was achieved as a result of research aimed at solving the problems of the prior art described above. The present invention has the following features: [1] A method for producing a fibrous filler, comprising: immersing a fiber-reinforced resin composite material comprising at least reinforcing fibers and a thermosetting resin in a mixed solution of an aprotic organic solvent to which a basic compound or an acidic compound has been added; heating the mixed solution to a temperature above the boiling point of the aprotic organic solvent at atmospheric pressure; removing resin components including the thermosetting resin from the fiber-reinforced resin composite material; and recovering a fibrous filler comprising the reinforcing fibers, wherein the mixed solution does not contain an aliphatic alcohol. [2] A method for producing a fibrous filler as described in [1], wherein the mixed solution is heated to a temperature above the boiling point of the aprotic organic solvent at a pressure above atmospheric pressure of 3 MPa or less. [3] A method for producing a fibrous filler as described in [1] or [2], wherein an inorganic base is used as the basic or acidic compound. [4] A method for producing a fibrous filler according to any one of [1] to [3], wherein the basic compound or acidic compound is used in an amount of 0.1% by mass or more and 15% by mass or less relative to the aprotic organic solvent. [5] A method for producing a fibrous filler according to any one of [1] to [4], wherein the basic compound or acidic compound is dissolved in an aprotic organic solvent. [6] A method for producing a fibrous filler according to any one of [1] to [5], wherein the aprotic organic solvent is an amide compound. [7] A method for producing a fibrous filler according to any one of [1] to [6], wherein the thermosetting resin is an epoxy resin. [8] A method for producing a fibrous filler according to any one of [1] to [7], wherein the reinforcing fiber comprises carbon fiber. [9] A method for producing a fibrous filler according to any one of [1] to [8], wherein the resin content of the fibrous filler is 0.0001% by mass or more and 10% by mass or less.

[10] A method for producing a fibrous filler according to [7], further comprising a step of recovering phenols from the resin component.

[11] A method for producing a fibrous filler, comprising: immersing a fiber-reinforced resin composite material comprising at least reinforcing fibers and a thermosetting resin in a mixed solution of an aprotic organic solvent to which a basic compound or an acidic compound has been added; heating the mixed solution to a temperature above the boiling point of the aprotic organic solvent at atmospheric pressure; removing resin components including the thermosetting resin from the fiber-reinforced resin composite material; and recovering a fibrous filler comprising the reinforcing fibers; wherein at least a portion of the waste liquid obtained by separating the fibrous filler from the heated mixed solution is reused as a new mixed solution for producing a fibrous filler.

[12] A method for producing a fibrous filler according to

[11] , wherein a basic compound or an acidic compound is added to the waste liquid used as the new mixed solution.

[13] A reaction apparatus for use in the method for producing a fibrous filler according to any one of [1] to

[12] .

[14] A fibrous filler obtained by the method for producing a fibrous filler according to any one of [1] to

[12] , which satisfies the following conditions 1 to 3: Condition 1: The resin content is 0.0001% by mass or more and 10% by mass or less. Condition 2: The single fiber tensile strength retention of the reinforcing fibers in the fibrous filler is 85% or more and 100% or less. Condition 3: The surface oxygen concentration (O / C) of the reinforcing fibers in the fibrous filler is 0.01 or more and 0.13 or less, and the surface nitrogen concentration (N / C) is 0.0001 or more and 0.055 or less.

[15] A fibrous filler obtained by recycling a fiber-reinforced resin composite material, which satisfies the following conditions 1 to 3. Condition 1: The resin content is 0.0001% by mass or more and 10% by mass or less. Condition 2: The single fiber tensile strength retention of the reinforcing fibers in the fibrous filler is 85% or more and 100% or less. Condition 3: The surface oxygen concentration (O / C) of the reinforcing fibers in the fibrous filler is 0.01 or more and 0.13 or less, and the surface nitrogen concentration (N / C) is 0.0001 or more and 0.055 or less.

[16] The fibrous filler according to

[14] or

[15] , further satisfying the following condition: Condition 4: The resin on the surface of the reinforcing fibers in the fibrous filler contains 0.001 mass% or more and 3.0 mass% or less of an alkali metal element or an alkaline earth metal element.

[17] The fibrous filler according to any one of

[14] to

[16] , further satisfying the following condition: Condition 5: The weight average molecular weight of the resin extracted from the fibrous filler is more than 500 and 100,000 or less.

[0015] According to the present invention, it is possible to provide a method for producing a fibrous filler that treats a fiber-reinforced resin composite material using a simple processing method and recovers the fibrous filler, without using expensive equipment or chemicals, and without the need for special advance preparation of a treatment agent tailored to the resin that constitutes the fiber-reinforced resin composite material, and a fibrous filler that is obtained by decomposing a fiber-reinforced resin composite material and exhibits properties similar to those of virgin carbon fiber.

[0016] The method for producing a fibrous filler of the present invention involves immersing a fiber-reinforced resin composite material composed of at least reinforcing fibers and a thermosetting resin in a mixed liquid consisting of an aprotic organic solvent to which a basic compound or an acidic compound has been added, heating the mixed liquid to a temperature above the boiling point of the aprotic organic solvent at atmospheric pressure, removing resin components including the thermosetting resin from the fiber-reinforced resin composite material, and recovering a fibrous filler composed of the reinforcing fibers, wherein the mixed liquid does not contain an aliphatic alcohol.

[0017] The reaction apparatus of the present invention is a reaction apparatus used in the method for producing a fibrous filler of the present invention.

[0018] Furthermore, the fibrous filler obtained by the method for producing a fibrous filler of the present invention and the fibrous filler obtained by recycling a fiber-reinforced resin composite material satisfy the following conditions. Condition 1: The resin content is 0.0001% by mass or more and 10% by mass or less. Condition 2: The single fiber tensile strength retention rate of the reinforcing fibers in the fibrous filler is 85% or more and 100% or less. Condition 3: The surface oxygen concentration (O / C) of the reinforcing fibers in the fibrous filler is 0.01 or more and 0.13 or less, and the surface nitrogen concentration (N / C) is 0.0001 or more and 0.055 or less.

[0019] A preferred embodiment of the present invention will be described below.

[0020] (1) Reinforcing Fibers In the present invention, examples of reinforcing fibers include known reinforcing fibers such as glass fibers and carbon fibers. The shape of these reinforcing fibers includes continuous fibers, short fibers such as chopped strands, and whiskers.

[0021] There are no particular limitations on the carbon fibers that can be used in the present invention, and various known carbon fibers can be used, such as carbonaceous fibers and graphite fibers produced using polyacrylonitrile (PAN), pitch, rayon, lignin, hydrocarbon gas, etc. Among these, PAN-based carbon fibers are preferably used, as they are expected to have improved mechanical properties.

[0022] (2) Thermosetting Resin / Fiber-Reinforced Resin Composite Material In the present invention, a fiber-reinforced resin composite material refers to a composite material composed of a thermosetting resin reinforced with at least the reinforcing fibers. Examples of the thermosetting resin include epoxy resin, unsaturated polyester resin, vinyl ester resin, phenolic resin, melamine resin, and urea resin, and among these, epoxy resin and vinyl ester resin are preferred.

[0023] Furthermore, carbon fibers generally consume more energy during production than glass fibers and have greater recycling needs, so it is preferable to use a fiber-reinforced resin composite material using carbon fibers as the reinforcing fibers.

[0024] In the present invention, the fiber-reinforced resin composite material may contain additives such as stabilizers, release agents, ultraviolet absorbers, colorants, flame retardants, flame retardant assistants, lubricants, fluorescent brighteners, phosphorescent pigments, fluorescent dyes, flow modifiers, impact resistance modifiers, crystal nucleating agents, inorganic and organic antibacterial agents, photocatalytic antifouling agents, and infrared absorbers.

[0025] In the present invention, it is also possible to use crushed fiber-reinforced resin composite materials. As a crusher for crushing such fiber-reinforced resin composite materials, a shearing crusher, an impact crusher, a cutting crusher, or a compression crusher can be used. There is no problem with using any of the crushers, and they can also be combined.

[0026] (3) Aprotic Organic Solvent The solvent used in the present invention is preferably a high-boiling aprotic organic solvent. Specific examples include aromatic solvents such as toluene, xylene, naphthalene, 1-chloronaphthalene, biphenyl, tetralin, 1,4-diisopropylbenzene, and 1,3,5-triisopropylbenzene; ester solvents such as γ-butyrolactone and ε-caprolactone; amide solvents such as N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N,N-dimethylformamide, and N,N-dimethylamide; urea solvents such as 1,3-dimethyl-2-imidazolidinone; sulfoxide solvents such as dimethyl sulfoxide; sulfone solvents such as sulfolane; and mixtures or aqueous solutions thereof. Among these, amide solvents such as N-methyl-2-pyrrolidone and N-ethyl-2-pyrrolidone and urea solvents such as 1,3-dimethyl-2-imidazolidinone are preferred.

[0027] (4) Basic Compound Examples of basic compounds include inorganic bases such as sodium hydroxide, potassium hydroxide, lithium hydroxide, cesium hydroxide, calcium hydroxide, barium hydroxide, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, trisodium phosphate, disodium phosphate, and monosodium phosphate; organic bases such as 1,8-diazabicycloundecene, tetramethylethylenediamine, N,N-diisopropylethylamine, 4-dimethylaminopyridine, 1,4-diazabicyclo[2.2.2]octane, potassium tert-butoxide, sodium tert-butoxide, and potassium hexamethyldisilazide; and hydrates and mixtures thereof. Among these, inorganic bases are preferred, and strong inorganic bases such as sodium hydroxide and potassium hydroxide are more preferred because of their excellent efficiency in the resin decomposition reaction.

[0028] (5) Acidic Compounds Examples of acidic compounds include sulfuric acid, nitric acid, hydrogen chloride, hydrogen bromide, hydrogen iodide, perchloric acid, trifluoroacetic acid, benzenesulfonic acid, toluenesulfonic acid, and trifluoromethanesulfonic acid. Strong acids such as sulfuric acid and nitric acid are preferably used.

[0029] (6) Phenols The term "phenols" used in the present invention refers to compounds in which one or more hydroxyl groups are directly bonded to a benzene ring. Specific examples include monohydric phenols such as phenol, cresol, salicylic acid, naphthol, 2-methylphenol, 2-ethylphenol, 3-methylphenol, 3-ethylphenol, 4-methylphenol, 4-ethylphenol, 2-isopropylphenol, 3-isopropylphenol, 4-isopropylphenol, 2-tert-butylphenol, 3-tert-butylphenol, and 4-tert-butylphenol; dihydric phenols such as catechol, resorcinol, hydroquinone, bisphenol A, bisphenol F, bisphenol S, and bisphenol E; and trihydric phenols such as pyrogallol and phloroglucinol. Of these, phenol, 4-isopropylphenol, and bisphenol A are preferred.

[0030] (7) Aliphatic Alcohols Aliphatic alcohols are compounds in which at least one hydrogen atom of an aliphatic hydrocarbon is substituted with a hydroxyl group. Specific examples include aliphatic compounds having one hydroxyl group in the molecule, such as methanol, ethanol, hexanol, and ethylene glycol monoethyl ether, and aliphatic compounds having two or more hydroxyl groups, such as ethylene glycol, triethylene glycol, and glycerin.

[0031] (8) Treatment of Fiber-Reinforced Resin Composite Material In the present invention, the treatment of a fiber-reinforced resin composite material is a general term for a process of immersing a fiber-reinforced resin composite material in a mixed solution of an aprotic organic solvent to which a basic compound or an acidic compound has been added, heating the mixed solution to a temperature above the boiling point of the aprotic organic solvent at atmospheric pressure, removing resin components including the thermosetting resin from the fiber-reinforced resin composite material, and recovering a fibrous filler made of the reinforcing fibers. At temperatures below the boiling point of the aprotic organic solvent at atmospheric pressure, the ability to decompose the thermosetting resin tends to be poor.

[0032] The elevated temperature is a temperature above the boiling point of the aprotic organic solvent at normal pressure, and specifically, is more preferably 210° C. or higher, and even more preferably 230° C. or higher. There is no particular upper limit to the temperature, but it is possible to adopt a temperature of 400° C. or lower, more preferably 350° C. or lower, and even more preferably 300° C. or lower. By adopting such a temperature, the resin component in the fiber-reinforced resin composite material can be efficiently decomposed, and deterioration of the fibrous filler can be avoided.

[0033] The pressure during the temperature-raising treatment is preferably above normal pressure (atmospheric pressure), specifically above 0.1 MPa, and more preferably 0.2 MPa or higher. There is no particular upper limit to the pressure, but it is possible to adopt a pressure of 3 MPa or lower, more preferably 2.5 MPa or lower, and even more preferably 2.0 MPa or lower. By adopting such a pressure, the resin component in the fiber-reinforced resin composite material can be efficiently decomposed, and the pressure resistance of the reaction apparatus can be suppressed, which is preferable.

[0034] The time for the temperature-raising treatment cannot be generally defined because it depends on the type and amount of the fiber-reinforced resin composite material used, as well as the temperature and solvent, but is preferably 0.01 hours or more, more preferably 0.1 hours or more. By setting the time to this preferred value or more, the decomposition of the resin component in the fiber-reinforced resin composite material can be sufficiently advanced. On the other hand, there is no particular upper limit to the treatment time, but the decomposition can be sufficiently advanced within 24 hours, and preferably within 18 hours, and more preferably within 12 hours.

[0035] The concentration of the basic compound or acidic compound in the aprotic organic solvent during the temperature-raising treatment is preferably 0.1% by mass or more, more preferably 1% by mass or more, and even more preferably 2% by mass or more, relative to the aprotic organic solvent, and the upper limit thereof can be, for example, 15% by mass or less, preferably 12% by mass or less, and more preferably 10% by mass or less.

[0036] In addition, the basic compound or acidic compound may be dissolved or partially insoluble in the aprotic organic solvent, but a dissolved compound tends to have better decomposition ability for the matrix resin. Here, "dissolved" refers to a state in which the basic compound or acidic compound is in a homogenized liquid state in the aprotic organic solvent, with no solid visible, if the basic compound or acidic compound is solid, and a state in which the liquid is in a homogenized liquid state without liquid-liquid layer separation, if the basic compound or acidic compound is liquid. Whether the basic compound or acidic compound is dissolved in the aprotic organic solvent is determined by the state at the time when the predetermined elevated temperature is reached.

[0037] Furthermore, this treatment step is characterized in that the aprotic organic solvent does not contain an aliphatic alcohol, which is undesirable because the decomposition efficiency of the matrix resin decreases and the difficulty of recovering the aprotic organic solvent by distillation increases.

[0038] There are no particular limitations on the form of the reactor, but it is preferable to use one in which the parts that come into direct contact with the solvent are made of a corrosion-resistant material such as Hastelloy (an alloy consisting of Ni, Cr, Mo, etc.) or SUS, etc. Note that various known reaction methods such as batch and continuous methods can be used for the treatment.

[0039] Furthermore, the reaction apparatus for this treatment step is preferably equipped with a stirring mechanism for stirring the mixed solution consisting of an aprotic organic solvent to which a basic compound or an acidic compound has been added, and various stirring methods can be employed, such as a rotary stirring apparatus, a pump circulation stirring method in which the mixed solution is circulated by a pump, a bubbling method in which stirring is performed by gas bubbles, or a shaking method in which the apparatus is shaken.

[0040] The mass ratio of the fiber-reinforced resin composite material to the aprotic organic solvent during treatment cannot be generally defined because it depends on the type of fiber-reinforced resin composite material used and the reaction temperature. However, examples of the mass ratio of the fiber-reinforced resin composite material to the aprotic organic solvent include a ratio of 1:1000 to 1:1, and from the viewpoint of productivity, a ratio of 1:100 to 1:2 is more preferable.

[0041] (9) Reuse of Waste Liquid In a method for producing a fibrous filler, the method comprises immersing a fiber-reinforced resin composite material comprising at least reinforcing fibers and a thermosetting resin in a mixed solution of an aprotic organic solvent to which a basic or acidic compound has been added, heating the mixed solution to a temperature above the boiling point of the aprotic organic solvent at atmospheric pressure, removing resin components including the thermosetting resin from the fiber-reinforced resin composite material, and recovering a fibrous filler comprising the reinforcing fibers. At least a portion of the waste liquid from which the fibrous filler has been separated after heating can be reused as a new mixed solution for the production of a fibrous filler. Specifically, the waste liquid can be reused in whole or in part as is, or by adding a basic or acidic compound to the waste liquid, or by adding a basic or acidic compound and an aprotic organic solvent to the waste liquid.

[0042] (10) Recovery of fibrous filler In the method for producing a fibrous filler of the present invention, a fibrous filler consisting of reinforcing fibers can be separated and recovered from the reaction solution obtained by the above-mentioned step (8). There are no particular limitations on the method for separating and recovering the fibrous filler from this reaction solution, and any known method may be used, but a method of separating the fibrous filler and waste liquid by solid-liquid separation is simple and preferred. Here, the waste liquid refers to the liquid phase containing at least an aprotic organic solvent obtained by separating the fibrous filler from the reaction solution obtained by the above-mentioned step.

[0043] Examples of solid-liquid separation methods include a method using a filter, a method using a centrifuge to separate the fibrous filler from the liquid phase, and a combination of these methods. These methods are not limited to these, as long as they produce at least a liquid phase containing an aprotic organic solvent and a solid containing the fibrous filler. If necessary, a step of washing the fibrous filler with water or an organic solvent or a drying step may be added. Furthermore, it is also possible to separate and recover phenols from the waste liquid obtained by this step.

[0044] (11) Fibrous Filler The fibrous filler of the present invention is a fibrous filler obtained by the above-described method for producing a fibrous filler, and a fibrous filler obtained by recycling a fiber-reinforced resin composite material, and satisfies the following conditions 1 to 3. Condition 1: The resin content is 0.0001% by mass or more and 10% by mass or less. Condition 2: The single fiber tensile strength retention rate of the reinforcing fibers in the fibrous filler is 85% or more and 100% or less. Condition 3: The surface oxygen concentration (O / C) of the reinforcing fibers in the fibrous filler is 0.01 or more and 0.13 or less, and the surface nitrogen concentration (N / C) is 0.0001 or more and 0.055 or less.

[0045] In the present invention, the resin content in the fibrous filler is 10% by mass or less, more preferably 8% by mass or less, and even more preferably 5% by mass or less. The lower limit of the resin content in the fibrous filler in the present invention is 0.0001% by mass or more, and preferably 0.001% by mass or more or 0.01% by mass or more. Here, the resin content in the fibrous filler is the amount of resin component obtained by subtracting the total weight of the fibrous filler immediately after treatment from the total mass after further heating the fibrous filler immediately after treatment to remove any adhering resin. Within this range, the fibrous filler is easily opened to the single-fiber level when reused, and the fiber-reinforced resin composition obtained by mixing new resin into the fibrous filler tends to have improved mechanical properties such as strength and elastic modulus. Furthermore, the resin in the fibrous filler is a resin component derived from the matrix resin of the fiber-reinforced resin composite material being treated, and is also referred to as resin residue.

[0046] The preferred single fiber strength of the fibrous filler of the present invention is 3.6 GPa or more, more preferably 4.0 GPa or more, and even more preferably 4.4 GPa or more. There is no particular upper limit, but examples include 8.0 GPa or less, and preferably 7.5 GPa or less. The single fiber tensile strength retention of the fibrous filler obtained by treating the fiber-reinforced resin composite material of the present invention is 85% or more, preferably 86% or more, more preferably 88% or more, and even more preferably 90% or more. When the single fiber tensile strength retention is within this range, the fibrous filler has few defects. Therefore, when a fiber-reinforced resin composite material is produced using such a fibrous filler, the mechanical properties of the resulting composite material tend to be similar to those of a fiber-reinforced resin composite material produced using virgin fibrous filler, which is preferable. Here, the single fiber tensile strength retention is a value defined by the following formula. The higher the single fiber tensile strength retention of the present invention, the more preferable it is, with the upper limit being 100%.

[0047] Single fiber tensile strength retention (%) = [single fiber tensile strength (GPa) of fibrous filler obtained by treating fiber-reinforced resin composite material] / [single fiber tensile strength (GPa) of fibrous filler before being made into fiber-reinforced resin composite material] × 100. Furthermore, the fibrous filler obtained by treating the fiber-reinforced resin composite material of the present invention has a surface oxygen concentration (O / C) of 0.13 or less, preferably 0.12 or less, and more preferably 0.11 or less. The lower limit is 0.01 or more, preferably 0.02 or more, and more preferably 0.03 or more. Furthermore, the surface nitrogen concentration (N / C) is 0.055 or less, preferably 0.050 or less, and more preferably 0.040 or less. The lower limit is 0.0001 or more, preferably 0.001 or more, and more preferably 0.005 or more.

[0048] When the O / C and N / C ratios are within these ranges, modification of the surface layer of the fibrous filler is suppressed, and the quality tends to be close to that of the fibrous filler before it is made into a fiber-reinforced resin composite material.The use of such a fibrous filler tends to result in excellent physical properties when made into a fiber-reinforced resin composite material, and is therefore preferred.

[0049] The fibrous filler obtained by treating the fiber-reinforced resin composite material of the present invention preferably has a weight-average molecular weight (Mw) of more than 500 and not more than 100,000, more preferably 600 to 80,000, even more preferably 800 to 50,000, and even more preferably 900 to 20,000. Having a very small amount of resin with such a weight-average molecular weight attached to the surface of the fibrous filler tends to result in excellent physical properties when the fiber-reinforced resin composite material is produced using such a fibrous filler, which is preferable. It is believed that this is because the resin remaining on the surface of the fibrous filler contributes to the adhesion between the fibrous filler and the matrix resin constituting the fiber-reinforced resin composite material. Here, the weight-average molecular weight is a value measured by gel permeation chromatography (GPC) of the resin component extracted using an organic solvent.

[0050] Furthermore, the fibrous filler obtained by treating the fiber-reinforced resin composite material of the present invention preferably contains alkali metals or alkaline earth metals in the resin residue at 0.001% by mass to 3.0% by mass, more preferably 0.01% by mass to 2.5% by mass, and even more preferably 0.05% by mass to 2.0% by mass. Having a very small amount of resin residue with such elemental composition adhering to the surface of the fibrous filler tends to result in excellent physical properties when the fiber-reinforced resin composite material is produced using such a fibrous filler, which is preferable. It is believed that this resin residue contributes to the adhesion between the fibrous filler and the matrix resin that constitutes the fiber-reinforced resin composite material. The alkali metal or alkaline earth metal content of the resin adhering to the surface of the fibrous filler can be measured using scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDX).

[0051] The fibrous filler obtained by the present invention can be mixed with a new resin to obtain a fiber-reinforced resin composite material, which can be used in a variety of applications, including electronic parts, electrical equipment parts, household and office electrical appliance parts, machine-related parts, optical equipment and precision machinery-related parts, automobile and vehicle-related parts, and various aerospace applications.

[0052] The present invention will be described in more detail below with reference to examples, which are illustrative and not limiting.

[0053] <Thermosetting Resin> An epoxy resin was used as the thermosetting resin. The epoxy resin used was a two-component type consisting of a base resin and a curing agent as described below.

[0054] <Base Agent> Sumiepoxy (registered trademark) ELM434 (tetraglycidyldiaminodiphenylmethane, manufactured by Sumitomo Chemical Co., Ltd., hereinafter also referred to as ELM434) jER (registered trademark) 828 (diglycidyl ether of bisphenol A, manufactured by Mitsubishi Chemical Corporation, hereinafter also referred to as jER828) jER (registered trademark) 825 (diglycidyl ether of bisphenol A, manufactured by Mitsubishi Chemical Corporation, hereinafter also referred to as jER825) Epiclon (registered trademark) 830 (diglycidyl ether of bisphenol F, manufactured by DIC Corporation, hereinafter also referred to as EP830).

[0055] <Curing agent> 4,4'-diaminodiphenyl sulfone (Seikacure (registered trademark) S, manufactured by Seika Corporation, hereinafter also referred to as DDS) HN-2200 (a mixture of 1,2,3,6-tetrahydro-3-methylphthalic anhydride and 1,2,3,6-tetrahydro-4-methylphthalic anhydride, manufactured by RESONAC Corporation) Dicyandiamide (manufactured by Mitsubishi Chemical Corporation, hereinafter also referred to as DICY).

[0056] <Reinforcing fiber (CF-1)> Carbon fiber bundle (hereinafter also referred to as CF-1, "Torayca (registered trademark)" T700SC-12k-50C manufactured by Toray Industries, Inc.).

[0057] <Reinforced fiber (CF-2)> Using a copolymer consisting of 99.4 mol% acrylonitrile and 0.6 mol% methacrylic acid, an acrylic fiber bundle having a single fiber fineness of 1 denier (1.1 dtex) and 12,000 filaments was obtained by a dry and wet spinning method. The obtained acrylic fiber bundle was heated in air at a temperature of 240 to 280 ° C. at a draw ratio of 1.05 to convert it into a flame-resistant fiber. Next, the temperature was increased at a rate of 200 ° C. / min, and 10% stretching was performed in a temperature range of 300 to 900 ° C. in a nitrogen atmosphere, followed by heating to a temperature of 1,300 ° C. and calcination to obtain a carbon fiber bundle. This carbon fiber bundle was subjected to an electrolytic surface treatment of 3 coulombs per 1 g of carbon fiber in an aqueous solution using sulfuric acid as an electrolyte, and then dried in heated air at a temperature of 120 ° C. to obtain a reinforcing fiber (PAN-based carbon fiber). This is designated CF-2.

[0058] The surface oxygen concentration (O / C) of CF-2 was 0.07, the surface nitrogen concentration (N / C) was 0.007, the single fiber tensile strength was 4.6 GPa, the average fiber diameter was 7 μm, and the cross-sectional shape was approximately elliptical and substantially circular, with the ratio (diameter of the circle inscribed in the outline of the cross section) / (diameter of the circle circumscribed in the outline of the cross section) being 0.95.

[0059] <Basic compounds> Sodium hydroxide (hereinafter also referred to as NaOH) Potassium hydroxide (hereinafter also referred to as KOH) <Acidic compounds> Sulfuric acid (hereinafter also referred to as H ２ SO ４ ) <Aprotic organic solvent> N-methyl-2-pyrrolidone (boiling point at normal pressure = 202°C. hereinafter also referred to as NMP) 1,3-dimethyl-2-imidazolidinone (boiling point at normal pressure = 222°C. hereinafter also referred to as DMI) <Aliphatic alcohol> Diethylene glycol monobutyl ether (boiling point at normal pressure = 230°C. hereinafter also referred to as DEG) <Measurement of resin content of fiber reinforced resin composite material and fibrous filler> The resin content of a fiber reinforced resin composite material and a fibrous filler was measured by measuring the mass (M ０ ) and the smallest mass (M １) was calculated. Apparatus: TGA7 manufactured by PerkinElmer Measurement atmosphere: under nitrogen gas flow Heating program: (a) Program temperature 50°C is held for 1 minute (b) Program temperature is heated from 50°C to 550°C at a heating rate of 20°C / min, and held at 550°C for 30 minutes. Resin content: (M ０ -M １ ) / M ０ x100.

[0060] <Single Fiber Tensile Strength of Fiber> The single fiber tensile strength of fiber was determined as follows based on JIS R7606 (2000). A single yarn was sampled from the sample and fixed to a perforated mount using an adhesive. The fixed mount was attached to a tensile tester, and a tensile test was performed with a gauge length of 10 mm, a strain rate of 0.4 mm / min, and 50 samples. The single fiber tensile strength was determined using the following formula. The cross-sectional area of ​​the single fiber was calculated based on the fiber diameter (diameter of the circumscribed circle) determined from a microscope image, assuming the cross section to be a circle. When the sample contained resin, the single fiber was sampled from a location where the resin was small and where single yarn sampling was possible. Single fiber tensile strength (MPa) = (maximum tensile load (N)) / (cross-sectional area of ​​single fiber (mm ２ )).

[0061] <Single fiber tensile strength retention> The single fiber tensile strength retention was calculated using the following formula: Single fiber tensile strength retention (%) = [single fiber tensile strength (GPa) of carbon fiber obtained by treating fiber-reinforced resin composite material] / [single fiber tensile strength (GPa) of carbon fiber before being made into fiber-reinforced resin composite material] × 100.

[0062] <Measurement of Surface Oxygen Concentration (O / C) and Surface Nitrogen Concentration (N / C) of Carbon Fiber> The surface oxygen concentration (O / C) and surface nitrogen concentration (N / C) of the carbon fiber were measured by X-ray photoelectron spectroscopy.

[0063] First, the carbon fiber bundle was cut into 20 mm pieces and spread out on a copper sample support. Then, AlKα1,2 was used as the X-ray source, and the sample chamber was heated to 1 × 10 －８Torr. The binding energy value of the main C1s peak (peak top) is set to 284.6 eV as a correction value for peaks associated with charging during measurement. The C1s peak area is determined by drawing a straight baseline in the range of 282 to 296 eV. The O1s peak area is determined by drawing a straight baseline in the range of 528 to 540 eV, and the N1s peak area is determined by drawing a straight baseline in the range of 391 to 411 eV. Here, the surface oxygen concentration and surface nitrogen concentration can be calculated as an atomic ratio from the ratio of the O1s or N1s peak area to the C1s peak area using a sensitivity correction value specific to the instrument. The X-ray photoelectron spectroscopy instrument used was an ESCA-1600 manufactured by ULVAC-PHI, Inc.

[0064] <Measurement of Weight-Average Molecular Weight (Mw)> The weight-average molecular weight of the resin residue was calculated in polystyrene terms using gel permeation chromatography (GPC), a type of size exclusion chromatography (SEC). The GPC measurement conditions are shown below. Column name: TSKgel α-M, α-3000 (one of each) Eluent: NMP (50 mM LiCl) Detector: Differential refractive index detector (Tosoh Corporation, RI-8020) Column temperature: 40°C Flow rate: 0.5 mL / min Sample injection amount: 200 μL <SEM-EDX Measurement of Resin Residue> Trace amounts of resin residue present on the surface of the carbon fiber were subjected to elemental analysis using SEM-EDX, and the content of alkali metals or alkaline earth metals was quantified. For SEM-EDX, a Hitachi H-3000 scanning electron microscope equipped with a SEMEDX Type-H energy dispersive X-ray fluorescence analyzer was used.

[0065] Reference Example 1 ELM434 (70 parts by mass), jER828 (10 parts by mass), EP830 (20 parts by mass), and DDS (45.2 parts by mass) were mixed to prepare a resin raw material. CF-1, which was aligned in one direction and set in a mold, was impregnated with this resin raw material at 90°C, heated from 90°C to 180°C for 60 minutes, and then heated at 180°C for 2 hours to cure the resin, thereby obtaining a sheet-like fiber-reinforced resin composite material. This material, hereinafter referred to as CFRP-1, was shredded into strips approximately 40 mm long and used in the Examples and Comparative Examples. The resin content of CFRP-1 was 40% by mass.

[0066] Reference Example 2: 100 parts by mass of jER825, 80 parts by mass of HN-2200, and 3 parts by mass of DY080 (manufactured by Huntsman) as a curing accelerator were mixed to prepare a resin raw material. This resin raw material was impregnated at 60°C into CF-1, which was aligned in one direction and set in a mold, and heated from 60°C to 160°C over 40 minutes, and then heated at 160°C for 1 hour to cure the resin, thereby obtaining a sheet-like fiber-reinforced resin composite material. This was designated CFRP-2 and was used after being shredded into strips approximately 40 mm long. The resin content of CFRP-2 was 40% by mass.

[0067] Reference Example 3 A sheet-shaped fiber-reinforced resin composite material was obtained in the same manner as in Reference Example 1, except that CF-2 was used instead of CF-1. This material, hereinafter referred to as CFRP-3, was cut into strips approximately 40 mm long and used in the Examples and Comparative Examples. The resin content of CFRP-3 was 40% by mass.

[0068] Reference Example 4 A sheet-shaped fiber-reinforced resin composite material was obtained in the same manner as in Reference Example 2, except that CF-2 was used instead of CF-1. This material, hereinafter referred to as CFRP-4, was cut into strips approximately 40 mm long and used in the Examples and Comparative Examples. The resin content of CFRP-4 was 40% by mass.

[0069] Reference Example 5 100 parts by mass of jER828, 7 parts by mass of DICY, and 1 part by mass of "OMICURE (registered trademark)" U-24M (manufactured by Huntsman) as a curing accelerator were mixed to prepare a resin raw material. This resin raw material was impregnated at 60°C into CF-2, which had been aligned in one direction and set in a mold, and heated from 60°C to 150°C over 80 minutes, and then heated at 150°C for 1 hour to cure the resin, thereby obtaining a sheet-like fiber-reinforced resin composite material. This was designated CFRP-5 and was used after being shredded into strips approximately 40 mm long. The resin content of CFRP-5 was 40% by mass.

[0070] <Example 1> <Decomposition Step> NaOH (26 g), water (30 g), and NMP (190 g) were added to a windowed autoclave and heated to 160°C under a nitrogen stream to distill off the water. Then, NMP (304 g) was added to prepare an NMP solution with a NaOH concentration of 5% by mass. CFRP-1 (50 g) was added to the solution, and the temperature was raised to 240°C. At this time, the NaOH was dissolved in the NMP. The solution was then heated to 240°C, which is higher than the boiling point (202°C) of the NMP solution at atmospheric pressure, for 2 hours. The pressure at this time was 0.3 MPa. After the reaction, NMP (500 g) was added for dilution, and the contents were recovered. <Solid-Liquid Separation Step> The contents were filtered at 200°C to recover the solid portion containing carbon fibers. The solid portion was washed with NMP (500 g) and water (500 g), and dried to recover the carbon fibers. The content heated to 200°C had low viscosity and excellent filterability. Here, the fibrous filler recovered by treating the fiber-reinforced resin composite material (hereinafter also referred to as recycled carbon fiber) is referred to as rCF-1.

[0071] <Example 2> <Decomposition Step> NaOH (26 g), NMP (494 g), and CFRP-1 (50 g) were added to a windowed autoclave and heated to 240°C. At this time, residual NaOH was observed. The mixture was then heated for 2 hours at 240°C, which is higher than the boiling point (202°C) of the NMP solution at atmospheric pressure. At this time, the pressure was 0.3 MPa. After the reaction, the mixture was diluted with NMP (500 g) and the contents were recovered. Note that even if residual NaOH was observed, the NMP solution had a NaOH concentration of 5% by mass. <Solid-Liquid Separation Step> Carbon fibers were recovered using the same procedure as in the solid-liquid separation step of Example 1. The recycled carbon fibers recovered here are designated rCF-2.

[0072] <Example 3> <Decomposition step> A reaction was carried out in the same manner as in Example 2, except that the treatment was carried out for 5 hours at 240°C, which is higher than the boiling point (202°C) of the NMP solution at normal pressure, and the contents were recovered. <Solid-liquid separation step> Carbon fibers were recovered in the same manner as in the solid-liquid separation step of Example 1. The recycled carbon fibers recovered here are designated rCF-3.

[0073] <Example 4> <Decomposition step> A reaction was carried out in the same manner as in Example 1, except that CFRP-2 was used instead of CFRP-1, and the contents were recovered. <Solid-liquid separation step> Carbon fibers were recovered in the same manner as in the solid-liquid separation step of Example 1. The recycled carbon fibers recovered here are designated rCF-4.

[0074] <Example 5> <Decomposition Step> NaOH (10 g), water (30 g), and NMP (190 g) were added to a windowed autoclave and heated to 160°C under a nitrogen stream, and the water was distilled off. Then, NMP (465 g) was added to prepare an NMP solution with a NaOH concentration of 1.5% by mass. CFRP-1 (23 g) was added to the solution, and the temperature was raised to 240°C. At this time, the NaOH was dissolved in the NMP. The solution was then heated to 250°C, which is higher than the boiling point (202°C) of the NMP solution at atmospheric pressure, for 5 hours. The pressure at this time was 0.35 MPa. After the reaction, the contents were recovered. <Solid-Liquid Separation Step> Carbon fiber was recovered using the same procedure as in the solid-liquid separation step of Example 1. The recycled carbon fiber recovered here is designated rCF-5. Example 6 Decomposition Step NaOH (15.5 g), water (30 g), and NMP (190 g) were added to a windowed autoclave and heated to 160°C under a nitrogen stream, and the water was distilled off. Subsequently, NMP (310 g) was added to prepare an NMP solution with a NaOH concentration of 3% by mass. CFRP-2 (18.1 g) was added thereto, and the temperature was raised to 225°C. At this time, NaOH was dissolved in NMP. The mixture was then heated for 5 hours at 225°C, which is higher than the boiling point (202°C) of the NMP solution at atmospheric pressure. The pressure at this time was 0.20 MPa. After the reaction, the contents were recovered. Solid-Liquid Separation Step Carbon fibers were recovered using the same procedure as in the solid-liquid separation step of Example 1. The recycled carbon fibers recovered here are designated rCF-6. <Example 7> <Decomposition Step> KOH (26.3 g), DMI (500 g), and CFRP-1 (18.5 g) were added to a windowed autoclave and heated to 250°C. At this time, the KOH was dissolved. Then, the mixture was heated and treated at 250°C, which is higher than the boiling point (222°C) of the DMI solution at atmospheric pressure, for 5 hours. At this time, the pressure was 0.15 MPa. After the reaction, the contents were recovered. The concentration of KOH was 5% by mass. <Solid-Liquid Separation Step> Carbon fiber was recovered in the same procedure as in the solid-liquid separation step of Example 1. The recycled carbon fiber recovered here is referred to as rCF-7. <Example 8> <Decomposition Step> H ２ SO ４ (26.3 g), NMP (500 g), and CFRP-2 (18.5 g) were added to a windowed autoclave, and the temperature was raised to 250°C.２ SO ４ The NMP solution was then heated to 250°C, which is higher than the boiling point (202°C) of the NMP solution at atmospheric pressure, for 5 hours. The pressure was 0.35 MPa. After the reaction, the contents were recovered. ２ SO ４ The concentration of NaOH was 5% by mass. <Solid-Liquid Separation Step> Carbon fibers were recovered using the same procedure as in the solid-liquid separation step of Example 1. The recycled carbon fibers recovered here are designated as rCF-8. <Example 9> <Decomposition Step> NaOH (26.3 g), water (30 g), and NMP (190 g) were added to a windowed autoclave and heated to 160°C under a nitrogen stream to distill off the water. NMP (310 g) was then added to prepare an NMP solution with a NaOH concentration of 5.0% by mass. CFRP-1 (18.5 g) was added to this solution, and the temperature was raised to 250°C. At this time, NaOH had dissolved in the NMP. The solution was then heated for 2 hours at 250°C, which is higher than the boiling point (202°C) of the NMP solution at atmospheric pressure. The pressure at this time was 0.35 MPa. After the reaction, the contents were recovered. <Solid-Liquid Separation Step> The contents were filtered at 200°C, and solid-liquid separation was carried out into a solid portion containing carbon fiber and waste liquid (500 g) containing NMP. <Decomposition Step> The waste liquid (500 g) obtained in the solid-liquid separation step and CFRP-1 (18.5 g) were added to a windowed autoclave and heated to 250°C. The mixture was then heated and treated at 250°C, which is higher than the boiling point (202°C) of the NMP solution at atmospheric pressure, for 2 hours. The pressure at this time was 0.38 MPa. After the reaction, the contents were recovered. <Solid-Liquid Separation Step> Carbon fiber was recovered using the same procedure as in the solid-liquid separation step of Example 1. The recycled carbon fiber recovered here is referred to as rCF-9. The amount of resin residue in rCF-9 was 0.7% by mass.

[0075] <Example 10> <Decomposition Step> A reaction was carried out in the same manner as in Example 1, except that CFRP-3 was used instead of CFRP-1, and the contents were recovered. <Solid-Liquid Separation Step> Carbon fibers were recovered in the same manner as in the solid-liquid separation step of Example 1. The recycled carbon fibers recovered here are designated rCF-10. rCF-10 (0.5 g) was dispersed in NMP (300 g) and stirred at 100°C for 30 minutes. rCF-10 was filtered off, and the recovered NMP portion was concentrated and subjected to GPC measurement to measure the weight-average molecular weight of the resin residue.

[0076] <Example 11> <Decomposition step> A reaction was carried out in the same manner as in Example 7, except that CFRP-4 was used instead of CFRP-1, and the contents were recovered. <Solid-liquid separation step> Carbon fibers were recovered in the same manner as in the solid-liquid separation step of Example 10, and the weight-average molecular weight of the resin residue was measured. The recycled carbon fibers recovered here are designated rCF-11.

[0077] Example 12 Decomposition Step: A reaction was carried out in the same manner as in Example 5, except that CFRP-5 was used instead of CFRP-1, and the contents were recovered. Solid-Liquid Separation Step: Carbon fiber was recovered in the same manner as in the solid-liquid separation step of Example 10, and the weight-average molecular weight of the resin residue was measured. The recycled carbon fiber recovered here was designated rCF-12. Example 13 Decomposition Step: NaOH (26 g), water (30 g), and NMP (190 g) were added to a windowed autoclave and heated to 160°C under a nitrogen stream to distill off the water. NMP (304 g) was then added to prepare an NMP solution with a NaOH concentration of 5% by mass. CFRP-5 (50 g) was added to this, and the temperature was raised to 250°C. At this time, NaOH was dissolved in the NMP. The solution was then heated to 250°C, which is higher than the boiling point (202°C) of the NMP solution at atmospheric pressure, for 2 hours. The pressure at this time was 0.35 MPa. After the reaction, NMP (500 g) was added to dilute and recover the contents. <Solid-liquid separation step and recovery of phenols> The contents were filtered at 200°C, and a solid portion containing carbon fiber and a waste liquid containing NMP were recovered. The solid portion containing carbon fiber was washed with NMP (500 g) and water (500 g) and dried to recover the carbon fiber. In addition, bisphenol A was recovered from the waste liquid containing NMP by distillation. The recovery rate of bisphenol A was 25 mass% based on the bisphenol A structure contained in the CFRP-5 used as the raw material.

[0078] <Comparative Example 1> <Decomposition step> The reaction was carried out in the same manner as in Example 1, except that the reaction temperature was set to 180°C, which is lower than the boiling point (202°C) of the NMP solution at normal pressure, and the reaction was carried out for 5 hours, and the contents were recovered. <Solid-liquid separation step> Carbon fibers were recovered in the same manner as in the solid-liquid separation step of Example 1. The recycled carbon fibers recovered here are referred to as rCF-A.

[0079] <Comparative Example 2> <Decomposition step> The same treatment as in Example 3 was carried out, except that NaOH was not added, and the contents were recovered. <Solid-liquid separation step> Carbon fibers were recovered in the same procedure as in the solid-liquid separation step in Example 1. The recycled carbon fibers recovered here are referred to as rCF-B.

[0080] <Comparative Example 3> <Decomposition Step> NaOH (26 g), NMP (98.8 g), DEG (395.2 g), and CFRP-1 (50 g) were added to a windowed autoclave and heated to 240°C. At this time, the NaOH was dissolved. The mixture was then heated and treated for 2 hours at 240°C, which is higher than the boiling point (202°C) of the NMP solution at atmospheric pressure. At this time, the pressure was 0.2 MPa. The NaOH concentration was 5% by mass in the NMP / DEG solution. <Solid-Liquid Separation Step> Carbon fibers were recovered using the same procedure as in the solid-liquid separation step of Example 1. The recycled carbon fibers recovered here are referred to as rCF-C.

[0081] Comparative Example 4: The shredded CFRP-3 was placed in a porcelain crucible and heated in an electric furnace at 600°C in air for 40 minutes to remove the resin component by pyrolysis. The carbon fiber recovered by pyrolysis was designated rCF-D. Its properties are shown in Table 2.

[0082] Reference Example 6 The rCF-10 obtained in Example 10 was chopped to approximately 6 mm and melt-kneaded with PA6 using a small kneader (HAAKE MiniLab manufactured by Thermo Fisher Scientific). At this time, the temperature was 260°C and the screw rotation speed was 200 rpm. The mass ratio of rCF-10 to PA6 was 20 / 80 (rCF-10 / PA6). The obtained resin composition was pelletized, and the pellets were used to mold a 2 mm thick test piece in accordance with ISO 527-2-5A using a small injection molding machine (HAAKE MiniJet Pro manufactured by Thermo Fisher Scientific, cylinder temperature 250°C, mold temperature 80°C, injection pressure 450 bar). The molded article was subjected to a tensile test in accordance with ISO 527-2-5A to measure the tensile modulus, elongation at break, and tensile strength. Each property was measured three times and the arithmetic mean value was used. The results are shown in Table 3.

[0083] The same procedure as in Reference Example 6 was repeated, except that rCF-D was used instead of rCF-10. The resulting molded article was subjected to a tensile test in accordance with ISO 527-2-5A to measure the tensile modulus, elongation at break, and tensile breaking strength. The results are shown in Table 3.

[0084] By comparing Examples 1 to 4 with Comparative Example 1, it was found that carbon fibers with a low content of resin components can be recovered by using a basic compound and carrying out a reaction at a temperature above the boiling point of an aprotic organic solvent at normal pressure.

[0085] A comparison of Example 3 or 8 with Comparative Example 2 revealed that by carrying out the reaction using a basic compound or an acidic compound, carbon fibers with a low resin component content can be recovered.

[0086] A comparison of Examples 1 to 8 with Comparative Example 3 revealed that the resin decomposition efficiency was superior when the aprotic organic solvent did not contain an aliphatic alcohol.

[0087] A comparison of Examples 1 to 3 revealed that the reaction proceeded more efficiently when the basic compound was dissolved in an aprotic organic solvent.

[0088] A comparison between Examples 1 and 4 revealed that, regardless of the type of thermosetting resin contained in the fiber-reinforced resin composite material, a fiber-reinforced resin composite material can be treated by a simple operation using a fixed treatment liquid, and carbon fibers with a low resin component content can be recovered.

[0089] A comparison of Examples 5 to 8 revealed that by using a basic or acidic compound and carrying out the reaction at a temperature above the boiling point of the aprotic organic solvent at atmospheric pressure, carbon fibers with a low resin component content can be recovered under various temperature conditions and types of aprotic organic solvent.

[0090] The results of Examples 10 to 12 show that under the conditions of the present invention, there is little decrease in single fiber strength, and carbon fibers having properties similar to those of the carbon fibers before being made into a fiber-reinforced resin composite material can be obtained with little resin residue.

[0091] Comparison of Reference Examples 6 and 7 revealed that the resin composition produced using the carbon fiber produced by the method of the present invention has superior physical properties compared to the resin composition produced using carbon fiber from which the resin components have been removed by pyrolysis.

[0092]

[0093]

[0094]

[0095] The fibrous filler obtained by the present invention can be mixed with a resin by a commonly known method to obtain a fiber-reinforced resin composite material. Taking advantage of its excellent mechanical properties and moldability, this fiber-reinforced resin composite material can be used for various electrical and electronic parts, automobile parts, aircraft parts, etc.

Claims

1. A method for producing a fibrous filler, comprising: immersing a fiber-reinforced resin composite material composed of at least reinforcing fibers and a thermosetting resin in a mixed liquid consisting of an aprotic organic solvent to which a basic compound or an acidic compound has been added; heating the mixed liquid to a temperature above the boiling point of the aprotic organic solvent at atmospheric pressure; removing resin components including the thermosetting resin from the fiber-reinforced resin composite material; and recovering a fibrous filler consisting of the reinforcing fibers, wherein the mixed liquid does not contain aliphatic alcohol.

2. The method for producing a fibrous filler according to claim 1, wherein the mixed solution is heated to a temperature above the boiling point of the aprotic organic solvent at a pressure above atmospheric pressure but not exceeding 3 MPa.

3. The method for producing a fibrous filler according to claim 1, wherein an inorganic base is used as the basic compound or acidic compound.

4. The method for producing a fibrous filler according to claim 1, wherein the basic compound or acidic compound is used in an amount of 0.1% by mass to 15% by mass relative to the aprotic organic solvent.

5. The method for producing a fibrous filler according to claim 1, wherein the basic compound or acidic compound is dissolved in an aprotic organic solvent.

6. The method for producing a fibrous filler according to claim 1, wherein the aprotic organic solvent is an amide compound.

7. The method for producing a fibrous filler according to claim 1, wherein the thermosetting resin is an epoxy resin.

8. The method for producing a fibrous filler according to claim 1, wherein the reinforcing fibers include carbon fibers.

9. The method for producing a fibrous filler according to claim 1, wherein the resin content of the fibrous filler is 0.0001% by mass or more and 10% by mass or less.

10. The method for producing a fibrous filler according to claim 7, further comprising the step of recovering phenols from the resin component.

11. A method for producing a fibrous filler, which comprises immersing a fiber-reinforced resin composite material comprising at least reinforcing fibers and a thermosetting resin in a mixed liquid of an aprotic organic solvent to which a basic compound or an acidic compound has been added, heating the mixed liquid to a temperature above the boiling point of the aprotic organic solvent at atmospheric pressure, removing resin components including the thermosetting resin from the fiber-reinforced resin composite material, and recovering a fibrous filler comprising the reinforcing fibers, wherein at least a portion of the waste liquid obtained by separating the fibrous filler from the mixed liquid after heating is reused as a new mixed liquid for producing a fibrous filler.

12. The method for producing a fibrous filler according to claim 11, wherein a basic compound or an acidic compound is added to the waste liquid to be used as a new mixed liquid.

13. A reaction apparatus used in the method for producing a fibrous filler according to any one of claims 1 to 12.

14. A fibrous filler obtained by the method for producing a fibrous filler according to any one of claims 1 to 12, which satisfies the following conditions 1 to 3. Condition 1: The resin content is 0.0001% by mass or more and 10% by mass or less. Condition 2: The single fiber tensile strength retention rate of the reinforcing fibers in the fibrous filler is 85% or more and 100% or less. Condition 3: The surface oxygen concentration (O / C) of the reinforcing fibers in the fibrous filler is 0.01 or more and 0.13 or less, and the surface nitrogen concentration (N / C) is 0.0001 or more and 0.055 or less.

15. A fibrous filler obtained by recycling a fiber-reinforced resin composite material, which satisfies the following conditions 1 to 3. Condition 1: The resin content is 0.0001% by mass or more and 10% by mass or less. Condition 2: The single fiber tensile strength retention rate of the reinforcing fibers in the fibrous filler is 85% or more and 100% or less. Condition 3: The surface oxygen concentration (O / C) of the reinforcing fibers in the fibrous filler is 0.01 or more and 0.13 or less, and the surface nitrogen concentration (N / C) is 0.0001 or more and 0.055 or less.

16. The fibrous filler according to claim 14, further satisfying the following condition: Condition 4: Resin residue on the surface of the reinforcing fibers in the fibrous filler contains 0.001% by mass or more and 3.0% by mass or less of alkali metal elements or alkaline earth metal elements.

17. The fibrous filler according to claim 14, further satisfying the following condition: Condition 5: The weight average molecular weight of the resin residue extracted from the fibrous filler is in the range of more than 500 and not more than 100,000.

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