JP7470450B1 - Manufacturing method of recycled reinforcing fiber - Google Patents

Abstract

[Problem] To provide a method for producing recycled reinforcing fibers that can efficiently recover reinforcing fibers from fiber-reinforced resin materials using a solvent method. [Solution] The method for producing raw reinforcing fibers according to the present invention includes a step of treating a fiber-reinforced resin material containing a resin and reinforcing fibers with a treatment liquid containing an oxidizing agent and a polymerization inhibitor, and dissolving at least a portion of the resin of the fiber-reinforced resin material in the treatment liquid. [Selected Figures] None

Description

The present invention relates to a method for producing recycled reinforcing fibers.

Fiber Reinforced Plastics (FRP), which uses fibers such as glass fiber as reinforcing materials, is a lightweight, high-strength, and highly elastic material, and is widely used in components for small ships, automobiles, railway cars, etc. Furthermore, with the aim of further reducing weight, increasing strength, and increasing elasticity, Carbon Fiber Reinforced Plastics (CFRP), which uses carbon fiber as a reinforcing material, has been developed and is used in components for aircraft, automobiles, etc.

In recent years, the amount of used fiber-reinforced plastics being discarded has been increasing, and the development of recycling technologies is being considered. Methods for recovering the reinforcing fibers of fiber-reinforced plastics mainly include the pyrolysis method, in which the resin components are thermally decomposed and removed by heat treatment to recover the reinforcing fibers, and the solvent method, in which the resin components are dissolved and removed using a solvent to recover the reinforcing fibers. Of these, the solvent method is advantageous from the perspective of resource recycling, as it is easy to recover the resin components.

Examples of the solvent method include the methods proposed in Patent Documents 1 and 2. Patent Document 1 proposes a method for producing carbon fibers, including a step of immersing a carbon fiber composite material in an acidic aqueous solution to dissolve at least a part of the resin content of the carbon fiber composite material to obtain a substantially fibrous material, and a step of immersing the substantially fibrous material in an alkaline aqueous solution to dissolve at least a part of the resin content of the substantially fibrous material to obtain a fibrous material. Patent Document 2 also proposes a carbon fiber recovery method that includes a step of dissolving a base material of a carbon fiber reinforced plastic material using a dissolving solution containing phosphoric acid, the phosphoric acid concentration of the dissolving solution being 110 mass% or more, and the dissolving step being performed at a temperature of the dissolving solution of 200°C or more and 300°C or less.

JP 2019-136932 A JP 2020-50704 A

However, conventional solvent methods are not efficient enough at removing resin components. For example, in the methods described in Patent Documents 1 and 2, the fiber-reinforced resin material is finely cut into chips of about several centimeters in length to allow for the penetration of the solvent-containing treatment liquid, and then the resin components are removed. If the fiber-reinforced resin material is finely cut in this way, the length of the recycled reinforcing fibers recovered from the fiber-reinforced resin material is inevitably shortened, making it difficult to maintain the performance of the reinforcing fibers contained in the fiber-reinforced resin material before recovery, and limiting the uses of the recovered reinforcing fibers. On the other hand, in order to efficiently remove resin components using the solvent method, long-term treatment under harsh conditions such as high temperatures is required.

The object of the present invention is therefore to provide a method for producing recycled reinforcing fibers that can efficiently recover reinforcing fibers from fiber-reinforced resin materials using a solvent method.

While studying the recovery of reinforcing fibers from fiber-reinforced resin materials, the inventors recognized the possibility that an epoxy resin having a specific chemical structure may repolymerize after being decomposed in the solvent method. They then discovered that the resin component can be efficiently dissolved in the solvent even under relatively mild conditions by using a polymerization inhibitor together with an oxidizing agent in the solvent method.
Based on the above findings, the present inventors conducted further studies and arrived at the present invention.

The gist of the present invention is as follows.
(1) A method for producing recycled reinforcing fibers, comprising the steps of treating a fiber-reinforced resin material containing a resin and reinforcing fibers with a treatment liquid containing an oxidizing agent and a polymerization inhibitor, and dissolving at least a portion of the resin of the fiber-reinforced resin material in the treatment liquid.
(2) The method for producing recycled reinforcing fibers according to (1), wherein the resin includes an epoxy resin.
(3) The method for producing recycled reinforcing fibers according to (1) or (2), wherein the epoxy resin includes an amine-cured epoxy resin.
(4) The method for producing recycled reinforcement fiber according to any one of (1) to (3), wherein the polymerization inhibitor includes at least one selected from the group consisting of nitrites and nitrite esters.
(5) The method for producing recycled reinforcement fiber according to any one of (1) to (4), wherein the polymerization inhibitor includes at least one selected from the group consisting of sodium nitrite, potassium nitrite, ethyl nitrite, and amyl nitrite.
(6) The method for producing recycled reinforcement fibers according to any one of (1) to (5), wherein the content of the polymerization inhibitor in the treatment liquid is 0.20% by mass or more and 5.0% by mass or less.
(7) The method for producing recycled reinforcing fiber according to any one of (1) to (6), wherein the oxidizing agent contains an acid having oxidizing power.
(8) The method for producing recycled reinforcing fiber according to (7), wherein the acid having oxidizing power includes at least one selected from the group consisting of nitric acid, a mixed acid of sulfuric acid and nitric acid, a mixed acid of nitric acid and hydrochloric acid (aqua regia), and a mixed solution of hydrogen peroxide and sulfuric acid.
(9) The method for producing recycled reinforcing fibers according to any one of (1) to (8), further comprising a step of treating the fiber-reinforced resin material with an acidic solution containing an acid prior to the step.
(10) The method for producing recycled reinforcing fibers according to (9), wherein the acid includes at least one acid selected from the group consisting of sulfuric acid, hydrochloric acid, phosphoric acid, and acetic acid.
(11) The method for producing recycled reinforcing fibers according to (9) or (10), wherein the concentration of the acid in the acidic solution is 0.5 mol/L or more.

The above configuration provides a method for producing recycled reinforcing fibers that can efficiently recover reinforcing fibers from fiber-reinforced resin materials using the solvent method.

Below, an example of a method for producing recycled reinforcing fibers according to a preferred embodiment of the present invention is described. The method for producing recycled reinforcing fibers according to the present invention includes a process (oxidation process) in which a fiber-reinforced resin material containing a resin and reinforcing fibers is treated with a treatment liquid containing an oxidizing agent and a polymerization inhibitor, and at least a portion of the resin of the fiber-reinforced resin material is dissolved in the treatment liquid.

The method for producing recycled reinforcing fibers according to this embodiment includes, prior to the oxidation step, a step of preparing a fiber-reinforced resin material (preparation step), a step of treating the fiber-reinforced resin material with an acidic solution containing an acid (acid treatment step), and a washing step of washing the fibrous material obtained from the fiber-reinforced resin material, and optionally includes a heating step of heating the fiber-reinforced material in a gas atmosphere after the washing step. Each step of the method for producing recycled reinforcing fibers according to this embodiment will be described in order below.

1. Preparation process First, prior to the acid treatment process, a fiber-reinforced resin material is prepared. The fiber-reinforced resin material is a resin material reinforced by embedding reinforcing fibers in a matrix resin (also simply referred to as "resin"). Such fiber-reinforced resin materials are not particularly limited, and examples thereof include carbon fiber reinforced plastics (CFRP), glass fiber reinforced plastics (GFRP), glass-mat reinforced thermoplastics (GMT), aramid fiber reinforced plastics (AFRP), Kevlar fiber reinforced plastics (KFRP), Dyneema fiber-reinforced plastics (DFRP), basalt fiber reinforced plastics, boron fiber reinforced plastics, and prepregs thereof. Among the above, carbon fiber reinforced plastics are used in relatively large quantities and the amount of energy consumed during the production of carbon fibers is large, so it is desirable to recover and reuse used carbon fiber reinforced plastics and/or the carbon fibers in the prepregs.

The reinforcing fibers in the fiber-reinforced resin material may be in the form of a fiber bundle (tow) in which multiple reinforcing fibers are aligned in one direction, a woven or nonwoven fabric in which bundles of reinforcing fibers are used as warp and weft threads, or each reinforcing fiber may be arranged in a random position and direction. The reinforcing fibers may be in chip form, and in this case, examples include chopped fibers in which fiber bundles are cut, and chip-shaped fabrics.

The resin in the fiber-reinforced resin material is not particularly limited, and may be, for example, a thermosetting resin or a thermoplastic resin. The thermosetting resin may be uncured or cured.

Thermosetting resins are not particularly limited, but examples include epoxy resins, unsaturated polyester resins, vinyl ester resins, phenolic resins, cyanate resins, polycarbonate resins, polyacetal resins, etc., and one of these can be used alone or two or more can be used in combination.

Thermoplastic resins are not particularly limited, but examples include polyolefins, polyesters, polycarbonates, acrylic resins, acrylonitrile-butadiene-styrene copolymers, polyether ketones, polyphenylene sulfides, etc., and one of these can be used alone or two or more can be used in combination.

The resin constituting the fiber-reinforced resin material preferably contains an epoxy resin. The epoxy resin is efficiently decomposed in the oxidation process using a polymerization inhibitor, which will be described later, and can be dissolved in the treatment liquid.

The epoxy resin is not particularly limited, but examples thereof include bisphenol A type epoxy resin, bisphenol F type epoxy resin, novolac type epoxy resin, aliphatic type epoxy resin, etc., and these can be used alone or in combination of two or more.

The content of epoxy resin in the resin in the fiber-reinforced resin material is not particularly limited, but is, for example, 50% by mass or more, preferably 70% by mass or more, more preferably 80% by mass or more, and more preferably the resin essentially consists of epoxy resin, and most preferably the resin consists of epoxy resin. This makes it even easier to remove the resin from the fiber-reinforced resin material and recover the reinforcing fibers when the oxidation step is carried out.

In addition, in this embodiment, the resin constituting the fiber-reinforced resin material preferably contains a resin having a basic structure. The resin having a basic structure can coordinate hydrogen ions (protons) in an acidic solution and swell in the acid treatment process described below. The oxidizing agent can sufficiently penetrate the swollen resin having a basic structure in the oxidation process described below, which promotes dissolution of the resin in the treatment solution.

The basic structure of the resin is not particularly limited, but may include amide bonds, imide bonds, azo groups, diazo groups, urea bonds, urethane bonds, peptide bonds, isocyanate groups, azide groups, etc., or chemical structures derived from or similar to these, and the resin having a basic structure may contain one or more of these. Among the above, the method according to this embodiment can be suitably used to swell and decompose resins having primary, secondary, or tertiary amide bonds as basic structures.

In addition, the resin having a basic structure preferably contains a chemical bond having basicity in its main chain structure. This allows the reaction with the chemical structure having basicity to be further swollen in the acid treatment step, and promotes the decomposition of the resin and subsequent dissolution in the treatment liquid in the oxidation step described below. Examples of resins having such a basic structure include amine-cured epoxy resins, urethane resins, polyimide resins, polyamides, melamine resins, aniline resins, urea resins, etc., and the resin can contain one or more of these as resins having a basic structure.

The resin having a basic structure may contain a basic structure in its side chain. Examples of such resins include those having the above-mentioned basic structure in the side chain of various resin components described below.

The content of the resin having a basic structure in the resin in the fiber-reinforced resin material is not particularly limited, but is, for example, 20% by mass or more, preferably 50% by mass or more, more preferably 70% by mass or more, and more preferably the resin essentially consists of a resin having a basic structure, and most preferably the resin consists of a resin having a basic structure. This makes it even easier to remove the resin from the fiber-reinforced resin material and recover the reinforcing fibers when the acid treatment process is performed.

The fiber-reinforced resin material may be in the form of a sheet, or may be in the form of cut chips. In particular, the method according to the present embodiment can remove the resin relatively efficiently, and therefore can be suitably applied to sheet-shaped fiber-reinforced resin materials from which it has been difficult to recover the reinforcing fibers in the past.

The size of the fiber-reinforced resin material is not particularly limited. However, in consideration of maintaining the direction of the reinforcing fibers in the fiber-reinforced resin material, the length of one piece of the fiber-reinforced resin material can be, for example, 100 mm or more, preferably 500 mm or more and 3000 mm or less. More specifically, as the fiber-reinforced resin material, for example, a fiber-reinforced resin material sheet with a thickness of about 300 mm, which is laminated with an area of 1000 mm x 500 mm, can be used. The relatively large fiber-reinforced resin material described above is difficult to penetrate with the treatment liquid, making it difficult to remove the resin and recover the reinforcing fibers. However, the method according to this embodiment can be applied to relatively large fiber-reinforced resin materials because it can remove the resin relatively efficiently.

2. Acid Treatment Step In this step, the prepared fiber reinforced resin material is treated with an acidic solution containing an acid. By treating the fiber reinforced resin material with an acidic solution in this way, at least a part of the components that are decomposable by acid contained in the resin of the fiber reinforced resin material are decomposed and dissolved in the acidic solution. Alternatively, at least a part of the components that are soluble in acid are dissolved in the acidic solution. As a result, in the oxidation step described later, the treatment liquid can more easily penetrate into the resin, and the resin can be more efficiently dissolved in the treatment liquid.

In addition, when the fiber-reinforced resin material contains a resin having a basic structure, the fiber-reinforced resin material is treated with an acidic solution containing an acid, which causes the hydrogen ions in the acidic solution to be coordinated with the resin component having a basic structure to form a salt, resulting in the resin component having a basic structure and the resin itself swelling. This makes it easier for the oxidizing agent to penetrate the resin in the oxidation process described below, accelerating the decomposition and elution of the resin in the oxidation process.

The acidic solution in this step contains at least an acid and optionally a solvent. The acid may be an inorganic acid, an organic acid, or a mixture of these. Examples of inorganic acids include sulfuric acid, hydrochloric acid, phosphoric acid, etc., and one of these may be used alone or two or more may be used in combination. Examples of phosphoric acid include orthophosphoric acid, metaphosphoric acid, hypophosphoric acid, phosphorous acid, hypophosphorous acid, pyrophosphoric acid, trimetaphosphoric acid, tetrametaphosphoric acid, pyrophosphorous acid, etc. Examples of organic acids include formic acid, acetic acid, citric acid, succinic acid, oxalic acid, etc.

Among the above, it is preferable that the acid contains at least one inorganic acid, particularly one selected from the group consisting of sulfuric acid, hydrochloric acid, phosphoric acid, and acetic acid, since this can favorably swell the resin component.

The acid dissociation constant pKa of the component with the highest molar concentration among the components contained in the acid is not particularly limited, but is preferably 5.0 or less, more preferably 1.5 or less. When the pKa of the component with the highest molar concentration among the components contained in the acid is sufficiently small in this way, hydrogen ions are easily released into the acidic solution, and as a result, the above-mentioned effect can be obtained even more effectively. Note that, when the component with the highest molar concentration among the components contained in the acid has multiple acid dissociation constants, it is preferable that the first stage, i.e., the smaller acid dissociation constant, is the above value.

The concentration of the acid contained in the acidic solution is not particularly limited, but is, for example, 0.5 mol/L or more, preferably 1.0 mol/L or more, and more preferably 3.5 mol/L or more. This makes it easier for hydrogen ions to be released into the acidic solution, and as a result, the above-mentioned effects can be obtained even more effectively. Note that the upper limit of the acid concentration is not limited as long as it can exist as an acidic solution, and varies depending on the type of acid.

The acidic solution usually contains a solvent. There are no particular limitations on the solvent, so long as it is miscible with the above-mentioned acid and is chemically stable against the acid. For example, water and/or various organic solvents can be used.

The organic solvent is not particularly limited, but examples thereof include alcohol-based solvents, ether-based solvents, ketone-based solvents, aromatic hydrocarbons, halogenated aromatic hydrocarbons, halogenated aliphatic hydrocarbons, etc., and one of these can be used alone or two or more can be used in combination.

Examples of the alcohol-based solvent include aliphatic alcohol-based solvents, aromatic alcohol-based solvents, glycol-based solvents, and other polyhydric alcohols such as glycerin.
Examples of aliphatic alcohols include acyclic aliphatic alcohols such as 1-butanol, 2-butanol, 2-methyl-1-propanol, 2-methyl-2-propanol, 1-pentanol, 2-pentanol, 3-pentanol, 2-methyl-1-butanol, 2-methyl-2-butanol, 3-methyl-1-butanol, 3-methyl-2-butanol, 2,2-dimethyl-1-propanol, 1-hexanol, 2-hexanol, 3-hexanol, 2-ethylhexanol, 2-methyl-1-pentanol, 4-methyl-2-pentanol, 2-ethyl-1-butanol, 1-heptanol, 2-heptanol, 3-heptanol, dodecanol, methanol, and ethanol; and alicyclic alcohols such as cyclohexanol, 1-methylcyclohexanol, 2-methylcyclohexanol, 3-methylcyclohexanol, and 4-methylcyclohexanol.

Examples of aromatic alcohol solvents include phenol, cresol, benzyl alcohol, and phenoxyethanol.
Examples of glycol solvents include ethylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, diethylene glycol, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, triethylene glycol, triethylene glycol monomethyl ether, triethylene glycol monoethyl ether, tetraethylene glycol, polyethylene glycol (molecular weight 200 to 400), 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 1,5-pentanediol, and dipropylene glycol.

Examples of ether solvents include aliphatic ethers such as dimethyl ether, diethyl ether, ethyl methyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, and dihexyl ether; cyclic ethers such as 1,3-dioxolane, 1,4-dioxane, tetrahydrofuran, and furan; and aromatic ethers such as anisole, phenetole, diphenyl ether, and benzofuran.

Examples of ketone solvents include acetone, methyl ethyl ketone, 2-pentanone, 3-pentanone, 2-hexanone, methyl isobutyl ketone, 2-heptanone, 4-butanone, diisobutyl ketone, cyclohexanone, methylcyclohexanone, phorone, isophorone, acetylacetone, acetophenone, diethyl ketone, and diacetone alcohol.

Examples of aromatic hydrocarbons include benzene, toluene, and xylene.
Examples of halogenated aromatic hydrocarbons include ortho-chlorophenol and ortho-dichlorobenzene.
Examples of halogenated aliphatic hydrocarbons include chloroform and methylene chloride.

Among the above, it is preferable that the solvent contains water, since it is easy to mix with the acid and dissociates hydrogen ions in the acid appropriately. The solvent may be a mixed solvent of water and an organic solvent miscible with water. Examples of organic solvents miscible with water include alcohol-based solvents and ketone-based solvents.

The amount of solvent contained in the acidic solution is not particularly limited, and can be the remainder of other components such as acid.

The fiber-reinforced resin material is treated with the acid solution described above. The temperature of the acid solution during treatment is not particularly limited and is, for example, 0°C or higher and 100°C or lower, preferably 50°C or higher and 100°C or lower.

The time for treatment with the acidic solution is not particularly limited, but is from 5 minutes to 1200 minutes, preferably from 10 minutes to 120 minutes, after the desired temperature is reached.

The treatment with the acidic solution may be carried out under normal pressure, under reduced pressure, or under pressure. When the treatment with the acidic solution is carried out under pressure, the treatment can be carried out, for example, in an atmosphere of 0.11 MPa or more and 7.0 MPa or less, particularly 0.11 MPa or more and 2.0 MPa or less. In consideration of safety and economy, it is preferable to carry out the treatment with the acidic solution under normal pressure.

The treatment of the fiber-reinforced resin material with the acidic solution is not particularly limited, and may be performed by immersing the fiber-reinforced resin material in the acidic solution, or by spraying the acidic solution onto the fiber-reinforced resin material with a spray or the like. Any means capable of contacting the acidic solution with the fiber-reinforced resin material may be used. The acidic solution may be stirred during the treatment with the acidic solution. The fiber-reinforced resin material may be fixed with a fixture so that the fiber bundles of the fiber-reinforced resin material are maintained.

In addition, this step may be omitted in the method for producing recycled reinforcing fibers of the present invention.

In this step, the fiber reinforced resin material is treated with a treatment liquid to dissolve at least a part of the resin of the fiber reinforced resin material in the treatment liquid. The treatment liquid in this step contains at least an oxidizing agent and a polymerization inhibitor, and optionally contains a solvent.

By containing a polymerization inhibitor in addition to an oxidizing agent in this way, the resin can be efficiently decomposed and dissolved in the treatment liquid. Although the details of the phenomenon that occurs during this treatment are unclear, the inventor speculates as follows.

First, the resin in the fiber-reinforced resin material can generally be broken down into smaller molecules by the cleavage of molecular chains in the resin by an oxidizing agent. Hydroxyl groups and carboxyl groups produced by the oxidation reaction and polar groups resulting from the oxidizing agent are then introduced into the resin, making it more soluble in the treatment liquid. The combination of the above causes the resin to dissolve in the treatment liquid.

On the other hand, the inventors noticed that even after the above-mentioned oxidation reaction, some of the resin in the reinforcing fiber resin material still remained after the treatment. As a result of the inventors' intensive research, they realized that in the oxidation reaction of the resin by the oxidizing agent, a repolymerization reaction may also occur in parallel with the above-mentioned decomposition reaction. They then discovered that by including a polymerization inhibitor in the treatment liquid together with the oxidizing agent, it is possible to dissolve the resin to a greater extent than when the polymerization inhibitor is not added, which led to the present invention.

Furthermore, fiber-reinforced resin materials treated with an acidic solution allow the oxidizing agent to easily penetrate the resin, and as a result, in this process, the resin is efficiently decomposed and eluted by treatment with the treatment liquid. In particular, when the resin contains a resin with a basic structure, the resin swells due to the acid treatment, making it easier for the oxidizing agent to penetrate the resin.

The oxidizing agent is not particularly limited, and examples thereof include nitric acid, hot concentrated sulfuric acid, a mixed acid of sulfuric acid and nitric acid, a mixed acid of nitric acid and hydrochloric acid (aqua regia), a mixed solution of hydrogen peroxide and sulfuric acid, acids having oxidizing power such as perchloric acid, chloric acid, hypochlorous acid, chlorous acid, perbromic acid, bromic acid, hypobromous acid, bromous acid, periodic acid, iodic acid, hypoiodous acid, and iodous acid, alkali (earth) metal salts thereof, oxygen-based oxidizing agents such as oxygen, ozone, hydrogen peroxide, and acetone peroxide (a reaction product of hydrogen peroxide and acetone), and halogen-based oxidizing agents such as chlorine, chlorine dioxide, bromine, fluorine, and iodine, and one of these may be used alone or in combination of two or more. Examples of alkali metal elements include lithium, sodium, potassium, rubidium, cesium, and francium, and examples of alkaline earth metals include calcium, strontium, barium, and radium.

Among the above, the oxidizing agent preferably contains an acid having oxidizing power, since it is relatively easy to handle and can stabilize the liquid properties of the treatment liquid. In particular, from the viewpoint of the efficiency of decomposing the resin component, the oxidizing agent more preferably contains one or more selected from the group consisting of nitric acid, a mixed acid of sulfuric acid and nitric acid, a mixed acid of nitric acid and hydrochloric acid (aqua regia), and a mixed solution of hydrogen peroxide and sulfuric acid, and particularly preferably contains nitric acid or a mixed solution of hydrogen peroxide and sulfuric acid.

The concentration of the oxidizing agent in the treatment liquid is not particularly limited and can be set appropriately depending on the amount of resin components in the fiber-reinforced resin material to be treated. However, for example, when the oxidizing agent is an acid having oxidizing power, the concentration of the oxidizing agent in the treatment liquid is, for example, 5% by mass or more and 80% by mass or less, preferably 20% by mass or more and 50% by mass or less.

As described above, the treatment liquid contains a polymerization inhibitor. The polymerization inhibitor is not particularly limited as long as it can suppress the repolymerization of the decomposition product of the resin. For example, nitrite salts such as alkali metal or alkaline earth metal salts of nitrous acid, nitrite esters, hydroquinone, oxoquinone, 4-tert-butylpyrocatechol, tert-butylhydroquinone, 1,4-benzoquinone, dibutylhydroxytoluene, 1,1-diphenyl-2-picrylhydrazyl free radical, mequinol, phenothiazine, etc. can be used alone or in combination of two or more of these. Examples of alkali metals include sodium, potassium, cesium, rubidium, etc. Examples of alkaline earth metals include beryllium, magnesium, calcium, strontium, barium, etc. Examples of nitrite esters include methyl nitrite, ethyl nitrite, amyl nitrite, isoamyl nitrite, isobutyl nitrite, isopropyl nitrite, t-butyl nitrite, n-butyl nitrite, n-propyl nitrite, etc.

Among the above, the polymerization inhibitor preferably contains one or more selected from the group consisting of nitrites and nitrite esters, more preferably one or more selected from the group consisting of sodium nitrite, potassium nitrite, ethyl nitrite and amyl nitrite. This allows the resin to be dissolved in the treatment solution more efficiently.

The concentration of the polymerization inhibitor in the treatment liquid is not particularly limited and can be set appropriately depending on the amount of resin in the fiber-reinforced resin material to be treated. However, for example, the concentration of the polymerization inhibitor in the treatment liquid is, for example, 0.010% by mass or more and 20% by mass or less, preferably 0.20% by mass or more and 5.0% by mass or less. This makes it possible to sufficiently suppress the repolymerization reaction of the low-molecular-weight resin without inhibiting the oxidation reaction of the resin by the oxidizing agent.

The treatment liquid also typically contains a solvent. There are no particular limitations on the solvent as long as it is stable to the oxidizing agent and polymerization inhibitor used and is miscible with these components. For example, water and various organic solvents listed as solvents for the treatment liquid described above can be used alone or in combination of two or more.

In particular, the solvent contained in the treatment liquid preferably contains water or an organic solvent contained in the solvent of the acidic solution described above. When the acidic solution contains multiple types of solvents, the treatment liquid preferably contains one or more of these multiple types of solvents. This more reliably prevents unintended side reactions from occurring and unintended shrinkage of the swollen fiber-reinforced resin material when the treatment liquid is brought into contact with the fiber-reinforced resin material.

In addition, from the viewpoints of ease of handling and promoting the oxidation reaction by the oxidizing agent, the treatment liquid preferably contains water or a mixed solvent of water and an organic solvent miscible with water, and more preferably contains water.

The amount of solvent contained in the treatment liquid is not particularly limited, and can be the remainder of other components such as the oxidizing agent.

The treatment liquid is preferably neutral or acidic, and more preferably acidic. This prevents the fiber-reinforced resin material treated with the acidic solution from being neutralized when the treatment liquid is brought into contact with the fiber-reinforced resin material, thereby preventing the resin from unintentionally solidifying and depositing.

Specifically, the pH of the treatment solution at 25°C is, for example, 5.0 or less, preferably 2.0 or less, and more preferably 1.5 or less.

The treatment liquid may contain an acid. As an acid capable of adjusting the pH of the treatment liquid, the acids listed in the acid treatment step can be mentioned. One of these acids can be used alone or two or more of them can be used in combination.

The fiber-reinforced resin material is treated using the treatment liquid as described above. The temperature of the treatment liquid during treatment is not particularly limited and is, for example, 0°C or higher and 100°C or lower, preferably 50°C or higher and 100°C or lower.

The time for treatment with the treatment solution is not particularly limited, but is from 5 minutes to 1200 minutes, preferably from 10 minutes to 120 minutes, after the desired temperature is reached.

The treatment with the treatment liquid may be carried out under normal pressure, reduced pressure, or pressurized. When the treatment with the treatment liquid is carried out under pressurized pressure, the treatment can be carried out, for example, in an atmosphere of 0.11 MPa or more and 7.0 MPa or less, particularly 0.11 MPa or more and 2.0 MPa or less. From the viewpoints of safety and economy, it is preferable to carry out the treatment with the treatment liquid under normal pressure.

The treatment of the fiber-reinforced resin material with the treatment liquid is not particularly limited, and may be performed by immersing the fiber-reinforced resin material in the treatment liquid, or by spraying the treatment liquid onto the fiber-reinforced resin material with a spray or the like. Any means capable of contacting the treatment liquid with the fiber-reinforced resin material may be used. The treatment liquid may be stirred during treatment with the treatment liquid. The fiber-reinforced resin material may be fixed with a fixing device so that the fiber bundles of the fiber-reinforced resin material are maintained.

4. Washing step Next, washing is performed as necessary. Washing can be performed by contacting a washing liquid with the fiber reinforced resin material. Specifically, washing can be performed by replacing the treatment liquid with the washing liquid in the above oxidation step. However, the temperature of the washing liquid and the washing time during washing can be appropriately set.

As the cleaning solution, water or various organic solvents listed as the solvents for the above-mentioned treatment solution can be used alone or in combination of two or more. In addition to the above-mentioned solvents, the following ester-based solvents and amide-based solvents may also be used as the organic solvent.

Examples of ester solvents include methyl formate, ethyl formate, propyl formate, butyl formate, isobutyl formate, pentyl formate, methyl acetate, ethyl acetate, propyl acetate, isopropyl acetate, butyl acetate, isobutyl acetate, pentyl acetate, isopentyl acetate, 3-methoxybutyl acetate, 2-ethylbutyl acetate, 2-ethylhexyl acetate, cyclohexyl acetate, benzyl acetate, methyl propionate, ethyl propionate, butyl propionate, and isopropionate. These include isopentyl, methyl lactate, ethyl lactate, butyl lactate, methyl butyrate, ethyl butyrate, butyl butyrate, isopentyl butyrate, isobutyl isobutyrate, ethyl isovalerate, isopentyl isovalerate, methyl benzoate, ethyl benzoate, propyl benzoate, butyl benzoate, gamma-butyrolactone, diethyl oxalate, dibutyl oxalate, diethyl malonate, methyl salicylate, ethylene glycol diacetate, tributyl borate, trimethyl phosphate, and triethyl phosphate.

Examples of amide solvents include formamide, N-methylformamide, N,N-dimethylformamide, N,N-diethylformamide, acetamide, N-methylacetamide, N,N-dimethylacetamide, 2-pyrrolidone, N-methyl-2-pyrrolidone, caprolactam, carbamic acid esters, etc.

The cleaning liquid may also contain a basic substance. By adjusting the liquid properties through neutralization with a basic substance, the remaining resin components and their reaction products in the fiber-reinforced resin material can be removed. This results in a fibrous material containing reinforcing fibers.

Examples of basic substances include inorganic basic substances such as hydroxides, carbonates, hydrogen carbonates, sulfates, sulfites, and nitrates of lithium, alkali metals, and alkaline earth metals, and amine compounds such as dimethylamine and diethylamine. One of these can be used alone or two or more can be used in combination. Examples of alkali metals include sodium, potassium, cesium, and rubidium. Examples of alkaline earth metals include beryllium, magnesium, calcium, strontium, and barium.

5. Heating Step After the cleaning step, the fibrous material may be heated in a gas atmosphere. This melts or pyrolyzes the resin remaining in the fibrous material, and the resin can be further removed. Specifically, in the oxidation step, the resin contained in the fiber reinforced resin material is decomposed or otherwise degraded into lower molecular weight molecules, and the surface area of the resin in contact with the outside air is increased due to the elution of the resin itself. By heating the fiber reinforced resin material in such a state in a gas atmosphere, the resin remaining in the fiber reinforced resin material is easily decomposed and/or melted, and as a result, recycled reinforcing fibers from which the resin has been sufficiently removed are obtained.

In general, when recycled reinforcing fibers are obtained by pyrolysis, they are treated at a temperature of 500°C or higher to remove the resin. In such cases, the reinforcing fibers themselves present in the fiber-reinforced resin material may be damaged by heat. Furthermore, when resin is removed by pyrolysis, resins such as curable resins are first hardened by heat and then decomposed. Such hardening of the resin actually hinders the decomposition and, therefore, removal of the resin.

In contrast, in this embodiment, at least a portion of the resin is decomposed in the oxidation process described above, so the resin is less likely to harden compared to when a thermal decomposition method is simply adopted. Furthermore, because at least a portion of the resin is decomposed in the oxidation process, the resin can be decomposed and removed even at relatively low temperatures, and damage to the reinforcing fibers due to heat can be suppressed.

The heating temperature is not particularly limited, but can be, for example, 150°C to 600°C, preferably 200°C to 350°C. This makes it possible to more reliably remove the resin while minimizing damage to the resulting recycled reinforcing fiber caused by heat, acid, oxidizing agents, etc.

The heating time is not particularly limited, and is from 30 to 1,500 minutes, preferably from 60 to 300 minutes, after the target temperature is reached. This makes it possible to more reliably remove the resin while suppressing damage to the resulting recycled reinforcing fiber caused by heat, acid, oxidizing agents, etc.

The gas present in the ambient atmosphere in this process is not particularly limited, but may include, for example, an inert gas such as nitrogen or a rare gas, air, and/or water vapor. Among the above, the gas present in the ambient atmosphere preferably includes air.

The heat treatment may be carried out under normal pressure, reduced pressure, or pressurized pressure. From the viewpoints of safety and economy, it is preferable to carry out the heat treatment under normal pressure.

Here, the fiber-reinforced resin material may be fixed while being heated as necessary. This allows the reinforcing fibers contained in the fiber-reinforced resin material to be dried while maintaining their shape and orientation.

The above-mentioned acid treatment step, oxidation step, cleaning step, and heating step can each be performed multiple times as necessary. The order of each step can also be changed as necessary. For example, the acid treatment step, oxidation step, and cleaning step can be repeated multiple times, followed by the heating step. For example, the acid treatment step can be performed multiple times, followed by the oxidation step, and then the cleaning step and heating step can be performed the required number of times. Alternatively, for example, the acid treatment step, oxidation step, cleaning step, and heating step can be performed in this order the required number of times.

6. Drying Step Instead of or in addition to the above heating step, the fibrous material may be dried in a gas atmosphere.

The drying temperature is not particularly limited, but can be, for example, 50°C or higher and lower than 150°C, and preferably 80°C or higher and 130°C or lower. The drying time is not particularly limited, and is 15 minutes or longer and 3000 minutes or shorter, and preferably 60 minutes or longer and 300 minutes or shorter, after the target temperature is reached.

Other conditions can be within the ranges described for the heating process above.

In this way, recycled reinforcing fibers can be obtained. In the method for producing recycled reinforcing fibers according to this embodiment, a fiber-reinforced resin material is treated with a treatment liquid containing an oxidizing agent and a polymerization inhibitor to obtain recycled reinforcing fibers.

As a result, this embodiment allows for efficient recovery of reinforcing fibers from fiber-reinforced resin materials, even though it is a solvent method. Furthermore, this embodiment allows for efficient removal of resin even under relatively mild temperature conditions.

Furthermore, according to this embodiment, since efficient resin removal is possible, unlike the conventional method, there is no need to cut the fiber-reinforced resin material into small pieces and then perform a resin dissolution process. In other words, even with relatively large fiber-reinforced resin materials, uniform and efficient resin removal is possible by adopting the method according to this embodiment.

The resin dissolved in each process can also be recovered and recycled.

The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

1. Production of Regenerated Reinforcement Fiber (Example 1)
(i) Preparation step First, a carbon fiber reinforced resin material was prepared as a sample. The carbon fiber reinforced resin material used was a sheet with a length of about 30 cm, a width of 5 cm, and a thickness of about 1 mm. The resin constituting the carbon fiber reinforced resin material was an amine-cured epoxy resin.

(ii) Acid Treatment Step Next, the carbon fiber reinforced resin material was immersed for 1 hour in a 40 mass % (5.3 mol/L) aqueous sulfuric acid solution that had been heated to 80° C. During the immersion, the temperature of the aqueous sulfuric acid solution was maintained at 80° C.

(iii) Oxidation step Next, 0.5% by mass of sodium nitrite was added to a 40% by mass (8.0 mol/L) aqueous nitric acid solution to obtain a treatment liquid. The concentration of nitric acid in the treatment liquid was 39.8% by mass, and the concentration of sodium nitrite was 0.50% by mass. The carbon fiber reinforced resin material was immersed in the treatment liquid heated to 80°C for 60 minutes. During the immersion, the temperature of the treatment liquid was maintained at 80°C.

(iv) Washing Step Next, the reaction product was neutralized with a 10% by mass aqueous solution of sodium hydrogen carbonate, and then the fibrous material (carbon fiber) obtained from the carbon fiber reinforced resin material was washed with purified water.

(v) Heating Step Next, the fibrous material obtained after washing was heated at 350° C. for 1 hour to obtain the recycled reinforcing fiber according to Example 1.

Example 2
Except for carrying out a drying step instead of the heating step, the recycled reinforcing fiber according to Example 2 was obtained in the same manner as in Example 1. In the drying step, the fibrous material obtained by washing was dried at 110° C. for 1 hour.

Example 3
The regenerated reinforcing fiber of Example 3 was obtained in the same manner as in Example 2, except that the neutralization of the reaction product with a 10% by mass aqueous solution of sodium bicarbonate in the washing step was omitted.

Example 4
The recycled reinforcing fiber of Example 4 was obtained in the same manner as in Example 3, except that the acid treatment step was omitted.

(Comparative Example 1)
The regenerated reinforcing fiber according to Comparative Example 1 was obtained in the same manner as in Example 2, except that sodium nitrite was not included in the treatment liquid in the oxidation step.

(Comparative Example 2)
Regenerated reinforcing fibers according to Comparative Example 2 were obtained in the same manner as in Comparative Example 1, except that neutralization of the reaction product with a 10% by mass aqueous solution of sodium bicarbonate in the washing step was omitted.

(Comparative Example 3)
The recycled reinforcing fiber of Comparative Example 3 was obtained in the same manner as in Comparative Example 2, except that the acid treatment step was omitted.

2. Evaluation of Recycled Reinforcing Fibers The amount of residual resin was evaluated by thermogravimetric analysis (TGA) for the recycled carbon fibers obtained in Examples 1 to 4 and Comparative Examples 1 to 3, and for the virgin carbon fibers used in the carbon fiber reinforced resin material as a reference example.

Specifically, the recycled carbon fibers according to Examples 1 to 4 and Comparative Examples 1 to 3 were first heated at a temperature increase rate of 10°C/min while flowing nitrogen at a flow rate of 200 ml/min, and then held at 200°C for 15 minutes to wait for the weight change due to convection of the sample to subside. Next, the sample was heated from 200°C to 600°C at a temperature increase rate of 10°C/min, and the weight loss of the recycled carbon fiber during this period was measured. The amount of resin in the untreated carbon fiber reinforced resin material was 50% as a volume content, and was approximately 37 to 43 mass% when the specific gravity of the epoxy resin was 1.1 to 1.4 g/ ^cm3 . The results are shown in Table 1 together with the experimental conditions.

As shown in Table 1, it was confirmed that in Examples 1 to 4, the resin was efficiently removed from the carbon fiber reinforced resin material. Specifically, the recycled carbon fibers of Examples 1 and 2 had significantly less residual resin than the recycled carbon fiber of Comparative Example 1, the recycled carbon fiber of Example 3 had significantly less residual resin than the recycled carbon fiber of Comparative Example 2, and the recycled carbon fiber of Example 4 had significantly less residual resin than the recycled carbon fiber of Comparative Example 3. This shows that adding a polymerization inhibitor together with an oxidizing agent in the oxidation process can efficiently remove resin from the carbon fiber reinforced resin material. In particular, in Examples 1 and 2, a weight loss rate close to that of the virgin carbon fiber used as a reference example was observed, and it was confirmed that almost all of the resin was removed.

Although the preferred embodiment of the present invention has been described in detail above, the present invention is not limited to such examples. It is clear that a person with ordinary knowledge in the technical field to which the present invention pertains can conceive of various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Claims (9)

The method includes a step of treating a fiber-reinforced resin material containing a resin and reinforcing fibers with a treatment liquid containing an oxidizing agent , a polymerization inhibitor , and water , and dissolving at least a portion of the resin of the fiber-reinforced resin material in the treatment liquid,
The oxidizing agent includes an acid having an oxidizing power,
The concentration of the oxidizing agent in the treatment liquid is 20% by mass or more,
A method for producing recycled reinforcement fibers , wherein the polymerization inhibitor comprises at least one selected from the group consisting of nitrites and nitrite esters . The method for producing recycled reinforcing fibers according to claim 1, wherein the resin includes an epoxy resin. The method for producing recycled reinforcing fibers according to claim 2, wherein the epoxy resin includes an amine-cured epoxy resin. The method for producing recycled reinforcing fibers according to claim 1, wherein the polymerization inhibitor comprises one or more selected from the group consisting of sodium nitrite, potassium nitrite, ethyl nitrite, and amyl nitrite. The method for producing recycled reinforcing fibers according to claim 1, wherein the content of the polymerization inhibitor in the treatment liquid is 0.20% by mass or more and 5.0% by mass or less. 2. The method for producing recycled reinforcement fiber according to claim 1 , wherein the oxidizing acid includes at least one selected from the group consisting of nitric acid, a mixed acid of sulfuric acid and nitric acid, a mixed acid of nitric acid and hydrochloric acid (aqua regia), and a mixed solution of hydrogen peroxide and sulfuric acid. The method for producing recycled reinforcing fibers according to claim 1 further comprises a step of treating the fiber-reinforced resin material with an acidic solution containing an acid prior to the above step. The method for producing recycled reinforcement fibers according to claim 7 , wherein the acid comprises at least one acid selected from the group consisting of sulfuric acid, hydrochloric acid, phosphoric acid, and acetic acid. The method for producing recycled reinforcing fibers according to claim 7 , wherein the concentration of the acid in the acidic solution is 0.5 mol/L or more.