Method for recovering fibrous filler and ε-caprolactam, and method for producing polyamide 6

Abstract

Provided is a method for recovering a highly pure fibrous filler and ε-caprolactam from a polyamide 6 resin composition, which contains the fibrous filler, only by solid-liquid separation. The present invention pertains to a method for recovering a fibrous filler and ε-caprolactam, said method comprising adding and contacting a polyamide 6 resin composition (A) containing at least the fibrous filler (D) with water (B) heated to 290-350°C inclusive and / or an aqueous polyamide 6 oligomer solution (B1) heated to 290-350°C inclusive to give a mixture (C) and then recovering the fibrous filler and ε-caprolactam from the mixture (C), characterized in that the following steps (a) to (c) are performed in this order. (a) A step for preparing the mixture (C) under such condition that, when the mass ratio of water to polyamide 6, etc. is X:1 and the reaction temperature is Y°C, then the product of X and Y is 2,000 or less. (b) A step for performing solid-liquid separation (I). (c) A step for washing the material on the filter.

Description

Method for recovering fibrous filler and ε-caprolactam, and method for producing polyamide 6

[0001] The present invention relates to a method for recovering a fibrous filler and ε-caprolactam by depolymerizing a fiber-reinforced polyamide resin composition, which achieves both the cyclical use of fossil resources and the reduction of greenhouse gas emissions. More specifically, the present invention relates to a recycling method for depolymerizing a fiber-reinforced polyamide 6 resin composition using a small amount of water having a high specific heat capacity and a high heat of vaporization, and recovering a high-purity fibrous filler and ε-caprolactam in high yields.

[0002] In recent years, interest in global environmental issues has grown, triggered by the issue of marine plastic pollution, and there is a growing awareness of the need to build a sustainable society. Global environmental issues include global warming, resource depletion, and water shortages, many of which are caused by the rapid increase in resource consumption and greenhouse gas emissions due to human activity since the Industrial Revolution. Therefore, in order to build a sustainable society, technologies related to the recycling of fossil resources such as plastics and the reduction of greenhouse gas emissions are becoming increasingly important.

[0003] As a plastic recycling technology, pyrolysis-to-oil / gasification technology, which involves pyrolyzing plastic waste to recover gas, oil, etc., has attracted attention, and numerous methods have been proposed. For example, Patent Document 1 discloses a method for producing hydrocarbons through a process including the pyrolysis and steam cracking of waste plastics. Although these methods have the advantage of being able to pyrolyze mixed waste plastics into oil, they require high-temperature cracking at 800°C or higher to convert the pyrolysis oil into secondary raw materials such as plastic monomers. Furthermore, if the waste plastics contain plastics containing chlorine, such as polyvinyl chloride, or sulfur, such as polyarylene sulfide, there is a risk of corrosion to the plant, and if plastics containing oxygen or nitrogen, such as polyamide, are mixed in, there is a risk of explosion.

[0004] As a method for recycling polyamide 6, which is used in large quantities in various fields as fibers, films, and engineering plastics, a method has been disclosed in which the raw material ε-caprolactam is obtained by blowing superheated steam into the polyamide 6 in the presence of a phosphoric acid catalyst (see, for example, Patent Document 2). Also, as a method for depolymerizing polyamide 6 without using a catalyst such as an acid or a base, a method has been disclosed in which polyamide 6 is brought into contact with superheated water at a temperature of 280°C to 320°C to recover lactam (see, for example, Patent Documents 3 and 4).

[0005] Special Table of Contents No. 2019-533041 Publication of Japanese Patent Application Publication No. Hei 8-217746 Publication of Special Publication No. Hei 10-510280 Publication of Special Publication of Publication No. Hei 10-510282

[0006] The ε-caprolactam recovery method disclosed in Patent Document 2 is a high-yield reaction, with a depolymerization yield of over 80% for polyamide 6, but the depolymerization reaction requires a long time. Furthermore, because it requires a large amount of superheated steam—approximately 10 times the amount of polyamide 6 fiber—this technology poses challenges in achieving both fossil resource recycling and reduced greenhouse gas emissions. Furthermore, because this method uses phosphoric acid as a catalyst, it is susceptible to impurities, such as catalyst deactivation due to additives contained in plastics or impurities attached to waste plastics. In fact, when the inventors conducted recovery experiments using polyamide 6 containing potassium salt under conditions identical to or similar to those of the method described in Patent Document 2, they found a significant decrease in yield. This is thought to be due to deactivation of the phosphoric acid catalyst by potassium salt. Furthermore, Patent Document 2 does not disclose a method for recovering high-purity fibrous fillers with low organic matter attachment.

[0007] On the other hand, the ε-caprolactam recovery methods disclosed in Patent Documents 3 and 4 use only water in the depolymerization reaction and do not use a catalyst such as phosphoric acid, thereby offering the advantage of not experiencing reaction deactivation due to additives or adhering impurities. However, the disclosed ε-caprolactam recovery methods use a large amount of water (approximately 10 times the amount of water used relative to polyamide 6) with a very high specific heat capacity (4.2 kJ / kg·K) and heat of vaporization (2,250 kJ / kg) for a long reaction time, requiring a large amount of energy for the depolymerization reaction and recovery of ε-caprolactam from a low-concentration aqueous ε-caprolactam solution. Furthermore, simply reducing the amount of water used under the same or similar conditions only resulted in a decrease in the ε-caprolactam recovery rate. This is thought to be because simply reducing the amount of water used shifts the thermodynamic equilibrium point between ε-caprolactam produced by depolymerization and the linear oligomers produced by hydrolytic ring-opening of ε-caprolactam toward the linear oligomers. Furthermore, Patent Documents 3 and 4 do not disclose the recovery of high-purity fibrous fillers with a small amount of organic matter attached thereto.

[0008] In order to solve the above problems, the present invention has the following configuration: 1. A method for recovering fibrous filler and ε-caprolactam by adding and contacting a polyamide 6 resin composition (A) containing at least a fibrous filler (D) with at least one of water (B) heated to 290°C or higher but not higher than 350°C or an aqueous polyamide 6 oligomer solution (B1) heated to 290°C or higher but not higher than 350°C to obtain a mixture (C) containing at least the fibrous filler (D), ε-caprolactam, a polyamide 6 oligomer, and water (B), and then recovering the fibrous filler (D) and ε-caprolactam from the mixture (C), the method comprising carrying out the following steps (a) to (c) in this order: 2. (a) a step of adding and contacting a polyamide 6 resin composition (A) containing a fibrous filler (D) with at least one of water (B) or an aqueous polyamide 6 oligomer solution (B1) under conditions where the mass ratio of water to polyamide 6, or the total of water to polyamide 6 and polyamide 6 oligomer, is X:1 and the reaction temperature is Y°C, such that the product of X and Y is 2,000 or less, to prepare a mixture (C); (b) a step of subjecting the mixture (C) to solid-liquid separation (I) in a temperature range not exceeding the boiling point of water at the operating pressure; and (c) a step of washing the filter cake containing the fibrous filler (D) obtained in the solid-liquid separation (I) with water not exceeding the boiling point of water at normal pressure to recover the fibrous filler (D). 10. The method for recovering fibrous filler and ε-caprolactam according to claim 1, comprising contacting a polyamide 6 resin composition (A) containing at least a fibrous filler (D) with water (B) heated to 290°C or higher but not exceeding 350°C, or with water (B) heated to 290°C or higher but not exceeding 350°C and an aqueous polyamide 6 oligomer solution (B1) heated to 290°C or higher but not exceeding 350°C, to obtain a mixture (C) containing at least the fibrous filler (D), ε-caprolactam, a polyamide 6 oligomer, and water (B), and then recovering the fibrous filler (D) and ε-caprolactam from the mixture (C), the method comprising carrying out the following steps (a) to (c) in this order:(a) a step of contacting a polyamide 6 resin composition (A) containing a fibrous filler (D) with water (B), or water (B) and an aqueous polyamide 6 oligomer solution (B1) under conditions where the mass ratio of water to polyamide 6, or the total of water to polyamide 6 and polyamide 6 oligomer is X:1 and the reaction temperature is Y°C, such that the product of X and Y is 2,000 or less, to prepare a mixture (C); (b) a step of subjecting the mixture (C) to solid-liquid separation (I) in a temperature range not exceeding the boiling point of water at the operating pressure; and (c) a step of washing the filter cake containing the fibrous filler (D) obtained in the solid-liquid separation (I) with water not exceeding the boiling point of water at normal pressure, to recover the fibrous filler (D). 3. 3. The method for recovering a fibrous filler and ε-caprolactam according to any one of items 1 or 2, characterized in that when the recovered fibrous filler (D) is heat-treated in an air atmosphere at 600°C for 3 hours, the mass loss of the fibrous filler (D) is 3.0 mass% or less. 4. The method for recovering a fibrous filler and ε-caprolactam according to any one of items 1 to 3, characterized in that the solid-liquid separation (I) is carried out in a temperature range not exceeding the boiling point of water at atmospheric pressure. 5. The method for recovering a fibrous filler and ε-caprolactam according to any one of items 1 to 4, characterized in that a mixture (C) containing at least the fibrous filler (D), ε-caprolactam, polyamide 6 oligomer, and water (B) is prepared, and then the solid-liquid separation (I) is subsequently carried out. 6. 6. The method for recovering a fibrous filler and ε-caprolactam according to any one of items 1 to 5, wherein the aqueous polyamide 6 oligomer solution (B1) is an extract obtained in a step of hot-water extraction of a polyamide 6 oligomer from polyamide 6, which is a product produced during the production of polyamide 6. 7. The method for recovering a fibrous filler and ε-caprolactam according to any one of items 1 to 6, wherein the polyamide 6 resin composition (A) containing at least a fibrous filler (D) is a waste resin molded product containing polyamide 6 having at least a fibrous filler (D). 8. A method for producing polyamide 6, comprising obtaining ε-caprolactam by the method of any one of items 1 to 7 and polymerizing polyamide 6 from the obtained ε-caprolactam.

[0009] The present invention provides a method for recovering ε-caprolactam and a fibrous filler by depolymerizing a polyamide 6 resin composition containing a fibrous filler, which method consumes little energy and is capable of recovering ε-caprolactam and a high-purity fibrous filler in high yield even when using a small amount of water having a high specific heat capacity.

[0010] The present invention provides a method for recovering the fibrous filler (D) and the ε-caprolactam from the mixture (C), which comprises contacting a polyamide 6 resin composition (A) containing at least a fibrous filler (D) with at least one of water (B) heated to 290°C or higher but not higher than 350°C and an aqueous polyamide 6 oligomer solution (B1) heated to 290°C or higher but not higher than 350°C, thereby obtaining a mixture (C) containing at least the fibrous filler (D), ε-caprolactam, a polyamide 6 oligomer, and water, and then recovering the fibrous filler (D) and ε-caprolactam from the mixture (C), the method comprising the steps of: (a) a step of adding and contacting a polyamide 6 resin composition (A) containing a fibrous filler (D) with at least one of water (B) or an aqueous polyamide 6 oligomer solution (B1) under the conditions that the mass ratio of water (B) to polyamide 6, or the total mass ratio of water to polyamide 6 and polyamide 6 oligomer is X:1, and the reaction temperature is Y ° C., and the product of X and Y is 2,000 or less, to prepare a mixture (C); (b) a step of subjecting the mixture (C) to solid-liquid separation (I) in a temperature range below the boiling point of water under operating pressure; (c) a step of washing the filter cake containing the fibrous filler (D) obtained by solid-liquid separation (I) with water below the boiling point of water under normal pressure to recover the fibrous filler (D). Hereinafter, the present invention will be described in more detail.

[0011] (1) Polyamide 6 Resin Composition (A) The polyamide 6 used in the present invention is a polyamide resin whose main raw material is 6-aminocaproic acid and / or ε-caprolactam. Other monomers may be copolymerized within the scope of the present invention. Here, "main raw material" means that, out of a total of 100 mol% of the monomer units constituting the polyamide resin, a total of 50 mol% or more of units derived from 6-aminocaproic acid or units derived from ε-caprolactam is contained. It is more preferable that the polyamide resin contains 70 mol% or more, and even more preferable that the polyamide resin contains 90 mol% or more of units derived from 6-aminocaproic acid or units derived from ε-caprolactam.

[0012] Examples of other copolymerizable monomers include amino acids such as 11-aminoundecanoic acid, 12-aminododecanoic acid, and para-aminomethylbenzoic acid; lactams such as ω-laurolactam; aliphatic diamines such as tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, 2-methylpentamethylenediamine, nonamethylenediamine, decamethylenediamine, undecamethylenediamine, dodecamethylenediamine, 2,2,4- / 2,4,4-trimethylhexamethylenediamine, and 5-methylnonamethylenediamine; aromatic diamines such as metaxylylenediamine and paraxylylenediamine; 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, 1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane, and bicyclohexane. Examples of suitable diamines include alicyclic diamines such as bis(4-aminocyclohexyl)methane, bis(3-methyl-4-aminocyclohexyl)methane, 2,2-bis(4-aminocyclohexyl)propane, 1,4-bis(3-aminopropyl)piperazine, and 1-(2-aminoethyl)piperazine; aliphatic dicarboxylic acids such as adipic acid, suberic acid, azelaic acid, sebacic acid, and dodecanedioic acid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, 2-chloroterephthalic acid, 2-methylterephthalic acid, 5-methylisophthalic acid, 5-sodiumsulfoisophthalic acid, and 2,6-naphthalenedicarboxylic acid; and alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, and 1,3-cyclopentanedicarboxylic acid. Two or more of these may be copolymerized.

[0013] Furthermore, these polyamides 6 may contain a polymerization degree regulator, an end group regulator, etc. Examples of polymerization degree regulators and end group regulators include acetic acid and benzoic acid.

[0014] Although there are no particular limitations on the degree of polymerization of the polyamide 6 of the present invention, it is preferable that the relative viscosity is in the range of 1.5 to 5.0 as measured in a 98% concentrated sulfuric acid solution with a resin concentration of 0.01 g / mL at 25° C. When the relative viscosity is in this preferred range, the reaction efficiency with a small amount of water tends to be high, and this is a preferable example.

[0015] The polyamide 6 resin composition (A) of the present invention contains a fibrous filler (D). Specific examples of the fibrous filler (D) include glass fiber, flat glass fiber, modified cross-section glass fiber, cut glass fiber, flat glass fiber, and carbon fiber, and two or more of these may be used in combination. Among these, glass fiber is preferred. The content of the fibrous filler (D) in the resin composition (A) is preferably 1 to 200 parts by mass per 100 parts by mass of the resin composition (A).

[0016] The resin composition (A) of the present invention may further contain fillers, thermoplastic resins other than polyamide 6, various additives, etc., within the scope of not impairing the object of the present invention.

[0017] The filler may be either an organic filler or an inorganic filler, and may be, for example, a non-fibrous filler, and two or more of these may be blended together. Examples of the non-fibrous filler include non-swelling silicates such as talc, wollastonite, zeolite, sericite, mica, kaolin, clay, pyrophyllite, bentonite, asbestos, alumina silicate, calcium silicate, and the like; swellable layered silicates such as Li-type fluorine taeniolite, Na-type fluorine taeniolite, Na-type tetrasilicic fluorine mica, and Li-type tetrasilicic fluorine mica, silicon oxide, magnesium oxide, alumina, silica, diatomaceous earth, zirconium oxide, titanium oxide, iron oxide, zinc oxide, calcium oxide, tin oxide, antimony oxide, and other metal oxides; calcium carbonate, magnesium carbonate, zinc carbonate, barium carbonate, dodecyl ether, and the like. Examples of suitable clay minerals include metal carbonates such as lomite and hydrotalcite, metal sulfates such as calcium sulfate and barium sulfate, metal hydroxides such as magnesium hydroxide, calcium hydroxide, aluminum hydroxide, and basic magnesium carbonate, smectite clay minerals such as montmorillonite, beidellite, nontronite, saponite, hectorite, and sauconite, and various clay minerals such as vermiculite, halloysite, kanemite, Kenyaite, zirconium phosphate, and titanium phosphate, glass beads, glass flakes, ceramic beads, boron nitride, aluminum nitride, silicon carbide, calcium phosphate, carbon black, and graphite. The swellable layered silicate may have exchangeable cations between layers exchanged with organic onium ions. Examples of organic onium ions include ammonium ions, phosphonium ions, and sulfonium ions.

[0018] Specific examples of various additives include phenolic compounds such as N,N'-hexamethylenebis(3,5-di-t-butyl-4-hydroxy-hydrocinnamide) and tetrakis[methylene-3-(3',5'-di-t-butyl-4'-hydroxyphenyl)propionate]methane, phosphorus compounds, sulfur compounds such as mercaptobenzimidazole compounds, dithiocarbamic acid compounds, and organic thioacid compounds, heat stabilizers such as amine compounds such as N,N'-di-2-naphthyl-p-phenylenediamine and 4,4'-bis(α,α-dimethylbenzyl)diphenylamine, and isocyanate compounds. Examples of additives include coupling agents such as polyamide 6, organic silane compounds, organic titanate compounds, organic borane compounds, and epoxy compounds; plasticizers such as polyalkylene oxide oligomer compounds, thioether compounds, ester compounds, and organic phosphorus compounds; nucleating agents such as organic phosphorus compounds and polyether ether ketone; metal soaps such as montanic acid waxes, lithium stearate, and aluminum stearate; release agents such as ethylenediamine-stearic acid-sebacic acid polycondensates and silicone compounds; color inhibitors such as hypophosphites; lubricants, ultraviolet inhibitors, colorants, flame retardants, and foaming agents. When these additives are contained, the content thereof is preferably 10 parts by mass or less, and more preferably 1 part by mass or less, per 100 parts by mass of polyamide 6.

[0019] Specific examples of thermoplastic resins other than polyamide 6 contained in the polyamide 6 resin composition (A) include polyamide resins other than polyamide 6, polyester resins, polyolefin resins, modified polyphenylene ether resins, polysulfone resins, polyketone resins, polyetherimide resins, polyarylate resins, polyethersulfone resins, polyetherketone resins, polythioetherketone resins, polyetheretherketone resins, polyimide resins, polyamideimide resins, and tetrafluoroethylene resins. Two or more of these may be blended. The amount of thermoplastic resin other than polyamide 6 blended here is preferably 30 parts by mass or less per 100 parts by mass of polyamide 6 in the polyamide 6 resin composition (A) of the present invention.

[0020] The polyamide 6 resin composition (A) containing at least a fibrous filler (D) of the present invention may be waste of a resin molded product containing at least a polyamide 6 having a fibrous filler (D). Examples of waste resin molded products include polyamide 6 products, industrial waste generated during the production of polyamide 6 products, and post-consumer waste of polyamide 6 products. Examples of polyamide 6 products containing a fibrous filler (D) include molded parts for housing construction materials, molded electrical and electronic parts, aircraft parts, industrial machinery parts, extrusion molded products, on-site polymerization molded products, and RIM molded products. Furthermore, product scraps, pellet scraps, lump scraps, and cutting chips generated during these production processes are also subject to waste.

[0021] (2) Polyamide 6 Oligomer The polyamide 6 oligomer of the present invention is a polyamide 6 oligomer whose main constituent is 6-aminocaproic acid and / or ε-caprolactam. The polyamide 6 oligomer may contain other monomers as long as the objective of the present invention is not impaired. Here, "main constituent" means that, out of a total of 100 mol% of the monomer units constituting the polyamide 6 oligomer, a total of 50 mol% or more of units derived from 6-aminocaproic acid or ε-caprolactam is contained. It is more preferable that the polyamide 6 oligomer contains 70 mol% or more, and even more preferable that the polyamide 6 oligomer contains 90 mol% or more of units derived from 6-aminocaproic acid or ε-caprolactam.

[0022] Other monomers contained in the polyamide 6 oligomer include, for example, amino acids such as 11-aminoundecanoic acid, 12-aminododecanoic acid, and para-aminomethylbenzoic acid; lactams such as ω-laurolactam; aliphatic diamines such as tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, 2-methylpentamethylenediamine, nonamethylenediamine, decamethylenediamine, undecamethylenediamine, dodecamethylenediamine, 2,2,4- / 2,4,4-trimethylhexamethylenediamine, and 5-methylnonamethylenediamine; aromatic diamines such as metaxylylenediamine and paraxylylenediamine; 1,3-bis(aminomethyl)cyclohexane, 1,4-bis(aminomethyl)cyclohexane, and 1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane; alicyclic diamines such as hexane, bis(4-aminocyclohexyl)methane, bis(3-methyl-4-aminocyclohexyl)methane, 2,2-bis(4-aminocyclohexyl)propane, 1,4-bis(3-aminopropyl)piperazine, and 1-(2-aminoethyl)piperazine; aliphatic dicarboxylic acids such as adipic acid, suberic acid, azelaic acid, sebacic acid, and dodecanedioic acid; aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, 2-chloroterephthalic acid, 2-methylterephthalic acid, 5-methylisophthalic acid, 5-sodiumsulfoisophthalic acid, and 2,6-naphthalenedicarboxylic acid; and alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, 1,3-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, and 1,3-cyclopentanedicarboxylic acid. Two or more of these may be used.

[0023] The number-average molecular weight of the polyamide 6 oligomer of the present invention is not particularly limited, but is preferably in the range of 100 to 5,000, more preferably in the range of 200 to 3,000, and particularly preferably in the range of 200 to 2,000. Having the molecular weight of the polyamide 6 oligomer in this preferred range increases its solubility in water, tending to facilitate the preparation of the aqueous polyamide 6 oligomer solution (B1) used in the present invention. The number-average molecular weight here was calculated by GPC analysis using 1,1,1,3,3,3-hexafluoro-2-propanol as the solvent. A GPC-HFIP805 column manufactured by Showa Denko K.K. was used, and PMMA was used as the standard.

[0024] (3) Method for Preparing Mixture (C) In the present invention, a mixture (C) containing at least a fibrous filler (D), ε-caprolactam, a polyamide 6 oligomer, and water is prepared by adding and contacting a polyamide 6 resin composition (A) containing at least a fibrous filler (D) with at least one of water (B) heated to 290°C or higher but not exceeding 350°C, or an aqueous polyamide 6 oligomer solution (B1) heated to 290°C or higher but not exceeding 350°C. In the present invention, the mixture (C) is prepared under the conditions that the mass ratio of water (B) to polyamide 6, or the total mass of water, polyamide 6, and polyamide 6 oligomer is X:1, and the reaction time is Y°C, and the product of X and Y is 2,000 or less.

[0025] There are no particular limitations on the water (B) used here, and any type of water may be used, such as tap water, ion-exchanged water, distilled water, or well water. However, from the viewpoint of suppressing side reactions due to the influence of coexisting salts, ion-exchanged water or distilled water is preferably used.

[0026] In the present invention, water heated to 290°C to 350°C and water in an aqueous polyamide 6 oligomer solution (B1) heated to 290°C to 350°C are reaction substrates. Water is neither liquid nor gaseous when heated to a pressure of 22.1 MPa and a temperature of 374.2°C. This point is called the critical point of water, and hot water at a temperature and pressure lower than the critical point is called subcritical water. The water (B) or aqueous polyamide 6 oligomer solution (B1) used in the present invention is at a temperature of 290°C to 350°C and corresponds to subcritical water. Despite being water, this subcritical water has the characteristics of (i) a low dielectric constant and (ii) a high ionic product. The dielectric constant and ionic product of subcritical water depend on the temperature and the partial pressure of water and can be controlled. The low dielectric constant makes it an excellent solvent for organic compounds, despite being water, and the high ionic product increases the hydrogen ion and hydroxide ion concentrations, resulting in excellent hydrolysis properties. The temperature of the water (B) or polyamide 6 oligomer aqueous solution (B1) of the present invention is preferably 300°C or higher and 340°C or lower, and more preferably 320°C or higher and 340°C or lower. This preferred range tends to suppress corrosion of the equipment during the reaction. Furthermore, a preferred example of the pressure of the water (B) or polyamide 6 oligomer aqueous solution (B1) is higher than the saturated vapor pressure. Water may be used in a liquid state, a gaseous state such as water vapor, or both. However, since the reaction proceeds more easily in a liquid state than in a gaseous state, the pressure of the water (B) or polyamide 6 oligomer aqueous solution (B1) is preferably higher than the saturated vapor pressure. There is no particular upper limit to the pressure of the water (B) or polyamide 6 oligomer aqueous solution (B1), but a pressure of 20 MPa or lower is an example. This pressure range tends to increase the ionic product of water, which is preferred. To achieve this pressure range for the water, a method of pressurizing the inside of a pressure vessel and sealing it can be used. To pressurize the inside of the pressure vessel, a gas may be enclosed in addition to the water (B) or the aqueous polyamide 6 oligomer solution (B1). Examples of such a gas include air, argon, and nitrogen. However, from the viewpoint of suppressing side reactions such as oxidation reactions, it is preferable to use nitrogen or argon.The degree of gas pressure is not particularly limited as long as it is set to the desired pressure, but may be 0.3 MPa or more.

[0027] The present invention is characterized in that when preparing the mixture (C), the mass ratio of water (B) to polyamide 6, the mass ratio of the total of water in the polyamide 6 oligomer aqueous solution (B1) to the total of polyamide 6 and polyamide 6 oligomer, or the mass ratio of the total of water, polyamide 6, and polyamide 6 oligomer in the polyamide 6 oligomer aqueous solution (B1) is X:1, and the reaction temperature is Y ° C., the product of X and Y is 2,000 or less. The product of X and Y is preferably 1,600 or less, more preferably 1,300 or less, and particularly preferably 1,200 or less. There is no particular restriction on the lower limit of the product of X and Y, but it is preferably 300 or more, more preferably 320 or more, and particularly preferably 340 or more. The present invention relates to an energy-saving method for recovering ε-caprolactam and high-purity fibrous filler from a polyamide 6 resin composition containing a fibrous filler, with the aim of achieving both fossil resource recycling and reduced greenhouse gas emissions. Because water has a specific heat capacity of 4.3 kJ / kg·K and a heat of vaporization of 2,250 kJ / kg, both of which are very high compared to other organic solvents, it is important to reduce the amount of water used. By ensuring that the product of X and Y falls within these ranges, both efficient ε-caprolactam production and energy savings can be achieved. Furthermore, when the residence time at a reaction temperature of Y°C is Z minutes, a preferred example of the condition is that the product of X, Y, and Z is 60,000 or less. A more preferred condition is 40,000 or less, even more preferred is 30,000 or less, and particularly preferred is 20,000 or less. Furthermore, there is no particular lower limit to the product of X, Y, and Z, but a condition of 5,000 or more is preferred, a condition of 8,000 or more is more preferred, and a condition of 9,000 or more is particularly preferred. Setting the product of X, Y, and Z within such a preferred range of conditions is preferred because it tends to increase the efficiency of ε-caprolactam production while saving energy. In the reaction of polyamide 6 with water, in addition to the production of ε-caprolactam, a side reaction of linear oligomers produced by the reaction of ε-caprolactam with water also occurs. Simply reducing the amount of water used results in the production of a large amount of linear oligomers, significantly reducing the production efficiency of ε-caprolactam.The present inventors have clarified the thermodynamic equilibrium points of the reaction of polyamide 6 with water to produce ε-caprolactam and the side reaction of producing linear oligomers, and as a result have found that by setting the product of X and Y, and the product of X, Y and Z within the above-mentioned ranges, the by-production of linear oligomers can be suppressed and the production efficiency of ε-caprolactam can be significantly improved, leading to the present invention.

[0028] In addition, a preferred embodiment of the mixture (C) is prepared by contacting a polyamide 6 resin composition (A) containing at least a fibrous filler (D) with water (B) heated to 290°C or higher and 350°C or lower, and further adding an aqueous polyamide 6 oligomer solution (B1) heated to 290°C or higher and 350°C or lower.

[0029] The structure of the polyamide 6 oligomer in the polyamide 6 oligomer aqueous solution (B1) is the same as that described in section (2) above. The composition of the polyamide 6 oligomer in the polyamide 6 oligomer aqueous solution (B1) is not particularly limited, but a preferred example is that the content of linear polyamide oligomers having a dimer to dodecamer is 90% by mass or more. A more preferred range is 93% by mass or more, with 95% by mass or more being particularly preferred. When the content of linear polyamide 6 oligomers having a dimer to dodecamer contained in the polyamide 6 oligomer falls within this preferred range, the solubility in water is increased, and the terminal carboxylic acid concentration of the polyamide 6 oligomer is increased, which tends to promote the reaction between polyamide 6 and water and increase the efficiency of ε-caprolactam production. The amount of linear dimer to dodecamer oligomers in the polyamide 6 oligomer was quantitatively analyzed by high-performance liquid chromatography using a formic acid aqueous solution and a formic acid acetonitrile solution as eluents.

[0030] The method for preparing the polyamide 6 oligomer used in the aqueous polyamide 6 oligomer solution (B1) is not particularly limited, and may be, for example, a polyamide 6 oligomer contained in the extract obtained by hot water extraction of the polyamide 6 oligomer from a polyamide 6 resin during the production of a typical fatty acid-based polyamide 6 resin, or a polyamide 6 oligomer prepared by the same method as the synthesis of a typical fatty acid-based polyamide 6 resin. Alternatively, a polyamide 6 oligomer obtained as a by-product during the production of ε-caprolactam by adding a resin composition (A) containing at least polyamide 6 and at least one of water (B) heated to 290°C or higher and 350°C or lower, or an aqueous polyamide 6 oligomer solution (B1) heated to 290°C or higher and 350°C or lower may be used. From the viewpoint of reducing industrial waste generated during the production of ε-caprolactam, it is preferable to use a polyamide 6 oligomer recovered as a by-product during the production of ε-caprolactam by adding a resin composition (A) containing at least polyamide 6 and at least one of water (B) heated to 290°C or higher and 350°C or lower, and an aqueous polyamide 6 oligomer solution (B1) heated to 290°C or higher and 350°C or lower.

[0031] The polyamide 6 oligomer aqueous solution (B1) is prepared by heating and mixing the polyamide 6 oligomer with water.There is no particular limitation on the water used here, and tap water, ion-exchanged water, distilled water, well water, etc. can be used, but ion-exchanged water or distilled water is preferred from the viewpoint of suppressing side reactions caused by the influence of coexisting salts.In addition, the concentration of the polyamide 6 oligomer in the polyamide 6 oligomer aqueous solution (B1) can be any concentration as long as the polyamide 6 oligomer dissolves in water when heated to 290 ° C or higher and 350 ° C or lower, but preferably 20 mass % or less, preferably 15 mass % or less, and more preferably 10 mass % or less.By having the concentration of the polyamide 6 oligomer in the polyamide 6 oligomer aqueous solution (B1) within these preferred ranges, the solubility in water during the preparation of the polyamide 6 oligomer aqueous solution (B1) is increased, and the polyamide 6 oligomer aqueous solution (B1) can be prepared at a lower temperature. In addition, an extract containing polyamide 6 oligomers obtained in the process of hot water extraction of polyamide 6 oligomers from polyamide 6, a product of polyamide 6 production, can also be used as the polyamide 6 oligomer aqueous solution (B1). Polyamide 6 resins obtained by polymerizing ε-caprolactam usually contain impurities such as unreacted monomers and polyamide 6 oligomers generated in the polymerization equilibrium reaction. Therefore, in order to remove these impurities, the pellets after polymerization are fed to a hot water extraction tower, and the unreacted monomers and polyamide 6 oligomers are extracted and removed by hot water extraction. Using the extract obtained in the process of hot water extraction of polyamide 6 oligomers from polyamide 6 during polyamide 6 production as the polyamide 6 oligomer aqueous solution (B1) of the present invention is also a preferred example from the perspective of reducing industrial waste.

[0032] The preparation of the mixture (C) of the present invention can be carried out using various known reaction methods, such as batch and continuous methods. Examples of batch methods include autoclaves equipped with a stirrer and a heating function, vertical and horizontal reactors, and vertical and horizontal reactors equipped with a compression mechanism such as a cylinder in addition to a stirrer and a heating function. Examples of continuous methods include extruders equipped with a heating function, tubular reactors, tubular reactors equipped with a mixing mechanism such as a baffle, line mixers, vertical and horizontal reactors, vertical and horizontal reactors equipped with a stirrer, and towers. The production atmosphere is preferably a non-oxidizing atmosphere, and is preferably carried out under an inert atmosphere such as nitrogen, helium, or argon. From the standpoints of economy and ease of handling, a nitrogen atmosphere is preferred.

[0033] (4) Solid-Liquid Separation (I) Step The method for recovering fibrous filler (D) and ε-caprolactam of the present invention is characterized by subjecting a mixture (C) containing at least fibrous filler (D), ε-caprolactam, polyamide 6 oligomer, and water to solid-liquid separation (I) in a temperature range below the boiling point of water at the operating pressure. By performing solid-liquid separation (I) of mixture (C) in a temperature range below the boiling point of water at the operating pressure, ε-caprolactam and polyamide 6 oligomer are separated into a liquid phase, and the fibrous filler (D) is separated into a solid phase. The solid-liquid separation (I) can be performed at any temperature range below the boiling point of water at the operating pressure. Since the mixture (C) of the present invention is prepared at a temperature of 290°C to 350°C, it is preferable to perform solid-liquid separation (I) at a temperature below the preparation temperature of the mixture (C). The solid-liquid separation (I) of the mixture (C) containing at least the fibrous filler (D), ε-caprolactam, polyamide 6 oligomer, and water is preferably carried out at 95°C or less, and even more preferably at 90°C or less, for example.

[0034] Furthermore, the present inventors have investigated the dissolution behavior and precipitation behavior of the polyamide 6 oligomer in water according to the present invention described in the above section (2), and have found that the temperature at which polyamide 6 oligomer dissolves in water is 100°C or higher, while the temperature at which polyamide 6 oligomer precipitates once dissolved in water is below 100°C. This difference between the temperature at which polyamide 6 oligomer dissolves in water and the precipitation temperature occurs because polyamide 6 oligomer is in a supercooled state in the temperature range below 100°C and above the temperature at which polyamide 6 oligomer precipitates. Generally, for dissolved polyamide 6 oligomer to precipitate, it first goes through the steps of generating crystalline nuclei of polyamide 6 oligomer, and then the generated crystalline nuclei grow to a size sufficient for precipitation, resulting in the precipitation of polyamide 6 oligomer. Therefore, when the aqueous solution of polyamide 6 oligomer dissolved in water is cooled, it is believed that at the stage of cooling to less than 100 ° C, which is the temperature at which polyamide 6 oligomer dissolves in water, crystalline nuclei of polyamide 6 oligomer are generated, and then the generated crystalline nuclei grow during the cooling process, and polyamide 6 oligomer precipitates.From the above results, the solid-liquid separation (I) of the mixture (C) containing at least fibrous filler, ε-caprolactam, polyamide 6 oligomer, and water is preferably carried out in a temperature range below the boiling point at normal pressure, more preferably at 95 ° C or less, and even more preferably at 90 ° C or less.In addition, since the solid-liquid separation (I) recovers polyamide 6 oligomer as a liquid phase component by solid-liquid separation, the lower limit temperature for carrying out solid-liquid separation (I) can be the temperature at which polyamide 6 oligomer is in a supercooled state, specifically, preferably 50 ° C or more, more preferably 60 ° C or more, and even more preferably 70 ° C or more. By carrying out the solid-liquid separation (I) within such a preferred temperature range, at least 97%, preferably 98% or more, and more preferably 99% or more of the ε-caprolactam and polyamide 6 oligomer can be recovered as liquid phase components, and the recovery loss of the ε-caprolactam and polyamide 6 oligomer due to the solid-liquid separation (I) tends to be reduced.

[0035] Further, the solid-liquid separation (I) of the mixture (C) containing at least fibrous filler (D), ε-caprolactam, polyamide 6 oligomer, and water of the present invention can be exemplified by a method in which a separately prepared mixture (C) is reheated to a temperature at which the polyamide 6 oligomer dissolves and cooled to a solid-liquid separation (I) temperature to perform solid-liquid separation (I), or a method in which a polyamide 6 resin composition (A) containing at least fibrous filler (D) is contacted with water (B) heated to 290 ° C. or higher and 350 ° C. to prepare a mixture (C) containing at least fibrous filler (D), ε-caprolactam, polyamide 6 oligomer, and water, and then cooled from the preparation temperature of the mixture (C) to a solid-liquid separation (I) temperature to perform solid-liquid separation (I). Preferably, a method in which the mixture (C) is prepared and then cooled to a temperature at which solid-liquid separation (I) is performed to perform solid-liquid separation (I) can be exemplified.

[0036] The method for performing solid-liquid separation (I) is not particularly limited and can be any known method, including pressure filtration or vacuum filtration using a filter, centrifugation or precipitation separation based on the difference in specific gravity between the solids and the solution, or a combination of these. A decanter separation method in which precipitation separation is performed before filtration is also preferred. Any filter that is stable under the conditions for solid-liquid separation (I) can be used; for example, a filter sieve or sintered plate can be suitably used. The mesh size or pore size of the filter can be adjusted over a wide range depending on the viscosity, pressure, and temperature of the mixture (C) subjected to the filtration, the size of the fibrous filler (D), and the purity (solid content) of the resulting filtrate. It is particularly effective to select the mesh size or pore size depending on the size of the fibrous filler (D) in the mixture (C) recovered as a solid phase component by solid-liquid separation (I).

[0037] According to the solid-liquid separation (I) here, most of the fibrous filler (D) in the mixture (C) can be separated as a filter cake, and preferably 95% by mass or more, more preferably 97% by mass or more, and even more preferably 99% by mass or more of the fibrous filler (D) contained in the mixture (C) can be recovered as a solid content. In addition, when the filter cake separated by the solid-liquid separation (I) contains water containing ε-caprolactam and polyamide 6 oligomer, the amount of ε-caprolactam and polyamide 6 oligomer remaining on the filter cake can be reduced by washing the filter cake with fresh water (step (c)). Here, the washing of the filter cake with fresh water is carried out in a temperature range below the boiling point at normal pressure, preferably 95 ° C or less, more preferably 90 ° C or less. In this way, by separating the fibrous filler (D) in the mixture (C) from the ε-caprolactam and polyamide 6 oligomer by the solid-liquid separation (I) and washing with water, a complicated process becomes unnecessary, and therefore, this method can be said to be preferable from the viewpoint of process cost and environmental load.

[0038] (5) Fibrous Filler Recovery Step In the present invention, the filter residue containing the fibrous filler (D) obtained in the solid-liquid separation (I) is washed with water below the boiling point of water at atmospheric pressure to recover the fibrous filler (D). By washing the filter residue containing the fibrous filler (D) with water below the boiling point of water at atmospheric pressure, a high-purity fibrous filler (D) with a low residual organic matter can be recovered. The temperature of the water required for washing is not particularly limited, as long as it is below the boiling point of water at atmospheric pressure. Furthermore, the recovered fibrous filler (D) is a high-purity fibrous filler (D) with a low residual organic matter content, such as ε-caprolactam and polyamide 6 oligomer. When the recovered fibrous filler (D) is subjected to heat treatment at 600 °C for 3 hours in an air atmosphere, the mass loss is preferably 3.0% by mass or less, and more preferably 2.0% by mass or less. The fact that the mass loss of the recovered fibrous filler (D) during the heat treatment is in such a preferred range means that the amount of organic matter adhering to the fibrous filler (D) is small, that is, it means that the recovery loss of ε-caprolactam and polyamide 6 oligomer in the solid-liquid separation (I) and water washing steps of the mixture (C) is small, which is preferable.

[0039] (6) Method for Recovering ε-Caprolactam There are no particular limitations on the method for recovering ε-caprolactam from the filtrate obtained by solid-liquid separation (I) of the present invention, and any method can be employed. For example, highly pure ε-caprolactam can be recovered by distilling the aqueous ε-caprolactam solution obtained by solid-liquid separation (I) to separate it from water and polyamide 6 oligomers. Furthermore, if water-insoluble components precipitate when the aqueous ε-caprolactam solution recovered by solid-liquid separation (I) is cooled, the components can be separated in advance by a known method such as solid-liquid separation and then subjected to distillation separation. Furthermore, methods for obtaining highly pure ε-caprolactam can be combined with purification methods such as precision distillation of recovered ε-caprolactam, vacuum distillation with the addition of a trace amount of sodium hydroxide, activated carbon treatment, ion exchange treatment, and recrystallization. These methods can efficiently remove impurities that are difficult to separate by distillation separation.

[0040] (7) Polyamide 6 and Molded Articles Thereof The method for recovering ε-caprolactam described in the present invention can obtain highly pure ε-caprolactam, which can be used as a polymerization raw material for polyamide 6. Polyamide 6 can be produced by a commonly known method of heat-melt polymerizing ε-caprolactam in the presence of a small amount of water. Furthermore, the polyamide 6 obtained in this manner can be melt-kneaded with a fibrous filler (D) and various additives as needed to produce a polyamide 6 resin composition, from which various molded articles such as sheets and films can be obtained by commonly known methods such as injection molding and extrusion molding.

[0041] Taking advantage of their excellent properties, the polyamide 6 of the present invention and its molded articles can be used in a variety of applications, such as electrical and electronic parts, building materials, various containers, daily necessities, household goods, and sanitary goods. In particular, they are particularly preferred for aircraft parts and electrical and electronic part applications, which require toughness and rigidity. Specifically, they are used in aircraft-related parts such as landing gear pods, winglets, spoilers, edges, rudders, elevators, failings, and ribs, and electrical and electronic parts such as generators, electric motors, transformers, current transformers, voltage regulators, rectifiers, resistors, inverters, relays, power contacts, switches, circuit breakers, switches, knife switches, multi-pole rods, motor cases, television housings, notebook computer housings and internal parts, CRT display housings and internal parts, printer housings and internal parts, mobile terminal housings and internal parts such as mobile phones, mobile personal computers, and handheld mobile phones, housings for ICs and LEDs, capacitor base plates, fuse holders, various gears, and various cases. Examples of electronic components include electrical components such as switches and cabinets, connectors, SMT compatible connectors, card connectors, jacks, coils, coil bobbins, sensors, LED lamps, sockets, resistors, relays, relay cases, reflectors, small switches, power supply components, coil bobbins, capacitors, variable capacitor cases, optical pickup chassis, oscillators, various terminal boards, transformers, plugs, printed circuit boards, tuners, speakers, microphones, headphones, small motors, magnetic head bases, power modules, Si power modules and SiC power modules, semiconductors, liquid crystal displays, FDD carriages, FDD chassis, motor brush holders, transformer materials, parabolic antennas, and computer-related components.

[0042] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0043] The following raw materials were used in each example. [Polyamide 6 (PA6-A)] Polyamide 6 resin ("Amilan" (registered trademark) CM1017, manufactured by Toray Industries, Inc.), ηr = 2.70, melting point 225°C, cyclic dimer-tetramer oligomer content 0.2% by mass. The solution viscosity ηr was measured at 25°C using a 0.01 g / mL solution of 98% concentrated sulfuric acid. The melting point was measured using a differential scanning calorimeter in a nitrogen gas atmosphere. The polyamide was cooled from a molten state to 30°C at a rate of 20°C / min, and then heated to the melting point + 40°C at a rate of 20°C / min. The melting point was determined as the temperature of the endothermic peak that appeared when the polyamide was cooled from a molten state to 30°C at a rate of 20°C / min, and then heated to the melting point + 40°C at a rate of 20°C / min. However, if two or more endothermic peaks were detected, the temperature of the endothermic peak with the greatest peak intensity was used as the melting point. Here, the amount of the cyclic dimer to tetramer oligomers was determined by crushing polyamide 6, collecting polyamide 6 powder that passed through a JIS standard sieve with a 24 mesh sieve but was impermeable to a 124 mesh sieve, and extracting 20 g of the polyamide 6 powder with 200 mL of methanol for 3 hours using a Soxhlet extractor, and quantitatively analyzing the cyclic oligomers contained in the extract using high performance liquid chromatography. The measurement conditions were as follows: <Measurement conditions> High performance liquid chromatography: Waters 600E Column: GL Sciences ODS-3 Detector: Waters 484 Tunable Absorbance Detector Detection wavelength: 254 nm Solvent: methanol / water (gradient analysis with a methanol / water composition of 20:80 → 80:20) Flow rate: 1 mL / min

[0044] [Fiber filler] Glass fiber (T-249 manufactured by Nippon Electric Glass Co., Ltd.)

[0045] Reference Example 1 Polyamide 6 having fibrous filler (PA6-B) Polyamide 6 (PA6-A) and glass fiber were blended so that the mass ratio of polyamide 6 to glass fiber was 70 / 30, and a twin-screw extruder (TEX30α manufactured by The Japan Steel Works, Ltd.) was used with a cylinder set temperature of 250°C and a screw rotation speed of 150 rpm, where polyamide 6 was fed from the main feeder and glass fiber was fed from the side feeder, and the extruded string was pelletized to prepare glass fiber reinforced polyamide 6 (PA6-B).

[0046] Reference Example 2 Waste of polyamide resin molded body (PA6-C) Polyamide 6 resin containing 30% by mass of glass fiber ("Amilan" (registered trademark) CM1011G30 manufactured by Toray Industries, Inc.) was molded into dumbbell pieces, which were then crushed into pellet-shaped waste of polyamide 6 resin molded body.

[0047] Reference Example 3: Production of Polyamide 6 Oligomer 20.0 g of polyamide 6 (PA6-A) and 60.0 g of deionized water were charged into a SUS316L autoclave equipped with a stirrer. The mass ratio of water to polyamide 6 (X:1) was 3:1. The reaction vessel was purged with nitrogen and sealed under a nitrogen pressure of 0.5 MPa. The reaction was carried out by holding the temperature at 320°C for 15 minutes while stirring at 200 rpm. After completion of the reaction, the mixture was cooled to room temperature and the reaction mixture was recovered. Since the reaction temperature Y°C was 320°C, the product of X and Y was 960. Since the residence time at the reaction temperature of 320°C was 15 minutes, the product of X, Y, and Z was 14,400. The ε-caprolactam yield calculated by high-performance liquid chromatography of the recovered reaction mixture was 78%. A 10-fold (mass) amount of methanol was added to the resulting reaction mixture, and the mixture was stirred to form a slurry. This was then filtered through a glass filter (average pore size: 10-16 μm) to obtain a solid fraction. A 5-fold amount of methanol was then added to the resulting solid fraction, and the mixture was stirred to form a slurry. This process was repeated three times, and the residue was then vacuum dried at 50°C for 12 hours to obtain a polyamide 6 oligomer. High-performance liquid chromatography analysis of the resulting polyamide 6 oligomer revealed that it contained 95.8% by mass of linear dimer to dodecamer oligomers. The high-performance liquid chromatography measurement conditions were as follows: Apparatus: Shimadzu LC-10Avp series Column: Mightysil RP-18GP150-4.6 Detector: Photodiode array detector (UV = 205 nm) Flow rate: 1 mL / min Column temperature: 40°C Mobile phase: 0.1% formic acid aqueous solution / 0.1% formic acid acetonitrile solution Polyamide 6 oligomer composition: The amount of linear dimer to dodecamer oligomer in polyamide 6 oligomer was calculated from the peak area ratio of each polyamide 6 oligomer.

[0048] Reference Example 4: Hot Water Extraction of Polyamide 6 A 70-liter autoclave was charged with 20 kg of ε-caprolactam, 4.32 g of benzoic acid, and 3.0 kg of ion-exchanged water, and the inside of the polymerization vessel was sealed. After thorough nitrogen replacement, the inside of the polymerization reactor was heated with stirring until the pressure inside the vessel reached 0.98 MPa. While maintaining this pressure inside the vessel, the temperature was continued to rise to 250°C. After reaching 250°C, the pressure was released over 40 minutes to atmospheric pressure. After 180 minutes at atmospheric pressure and 250°C, the polyamide 6 polymer was discharged, cooled, and cut into pellets. The pellets were extracted with 20 times the amount of hot water at 98°C, and an extract containing unreacted caprolactam and polyamide 6 oligomer was recovered. The total amount of unreacted caprolactam and polyamide 6 oligomer in the extract was 0.5% by mass, and the polyamide 6 oligomer was 0.1% by mass.

[0049] Evaluation Methods [ε-Caprolactam Yield (HPLC)] The ε-caprolactam yield (HPLC) of the present invention was calculated by high-performance liquid chromatography (HPLC). The measurement conditions are as follows: Apparatus: Shimadzu LC-10Avp Series Column: Mightysil RP-18GP150-4.6 Detector: Photodiode array detector (UV = 205 nm) Flow rate: 1 mL / min Column temperature: 40°C Mobile phase: 0.1% aqueous acetic acid / acetonitrile Sample: Approximately 0.1 g of the reaction mixture was diluted with approximately 10 g of deionized water, and components insoluble in the deionized water were separated and removed by filtration to prepare a sample for high-performance liquid chromatography analysis. Quantitation of ε-Caprolactam: The amount of ε-caprolactam relative to polyamide 6 was quantified using the absolute calibration curve method.

[0050] Example 1: 28.6 g of PA6-B prepared in Reference Example 1 and 60.0 g of deionized water were charged into an SUS316L autoclave equipped with a stirrer, a bottom stopper valve, and a glass filter (average mesh size: 10 μm) at the bottom. The mass ratio of water to polyamide 6 (X:1) was 3:1. The reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 0.5 MPa, and then the reaction was carried out by holding the temperature at 320°C for 15 minutes while stirring at 200 rpm. The pressure reached during the reaction was 10.4 MPa. Since the reaction temperature Y°C was 320°C, the product of X and Y was 960. Furthermore, since the residence time at the reaction temperature of 320°C was 15 minutes, the product of X, Y, and Z was 14,400. After completion of the reaction, the internal temperature was cooled to 90°C, and while maintaining the temperature at 90°C, the bottom stopper valve was opened to carry out solid-liquid separation (I). When the filtration rate decreased, solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, deionized water in an amount approximately three times (by mass) the filtered product was introduced into the autoclave, and the filtered product was rinsed three times at 90°C, recovering the filtrate and wet filtered product. High-performance liquid chromatography analysis of the filtrate obtained by solid-liquid separation (I) revealed that the filtrate contained 14.0 g of ε-caprolactam, representing a yield of 70.0% relative to the polyamide 6 in the PA6-B used as the raw material. The wet filtered product was then vacuum dried at 50°C for 12 hours, recovering 8.6 g of glass fiber. The recovered glass fiber was weighed into a 1.0 g crucible and treated in an electric furnace heated to 600°C under air for 3 hours. The amount of organic matter adhering to the recovered glass fibers was evaluated from the mass loss, and the mass loss was found to be 1.4% by mass, indicating that the recovered glass fibers were high-purity glass fibers with a low amount of organic matter adhering. The recovered filtrate was heated to 55°C under a reduced pressure of 30 mmHg to distill off water, yielding a concentrated aqueous solution of ε-caprolactam. Further distillation was performed at a reduced pressure of 5 mmHg and a heating temperature of 150 to 170°C to recover distilled ε-caprolactam. The concentration and distillation yield were 95.8%. The HPLC impurity content of the distilled ε-caprolactam was 0.48%, indicating that it was of a quality suitable for use as a raw material for polyamide 6 polymerization.From the above, it can be seen that the present invention is a low environmental load process that can recover high-purity glass fibers and ε-caprolactam without using a large amount of organic solvent.

[0051] Example 2: 28.6 g of the waste polyamide resin molded product PA6-C prepared in Reference Example 2 and 60.0 g of deionized water were charged into an SUS316L autoclave equipped with a stirrer, a bottom stopper valve, and a glass filter (average mesh size 10 μm) at the bottom. The mass ratio of water to polyamide 6 (X:1) was 3:1. The reaction vessel was purged with nitrogen and sealed under a nitrogen pressure of 0.5 MPa. The reaction was then carried out by holding the temperature at 340°C for 15 minutes while stirring at 200 rpm. Since the reaction temperature Y°C is 340°C, the product of X and Y is 1,020. Furthermore, since the residence time at the reaction temperature of 340°C is 15 minutes, the product of X, Y, and Z is 15,300. After completion of the reaction, the internal temperature was cooled to 80°C, and while maintaining the temperature at 80°C, the bottom stopper valve was opened to carry out solid-liquid separation (I). When the filtration rate decreased, solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, deionized water in an amount approximately three times (by mass) the filtered product was introduced into the autoclave, and the filtered product was rinsed three times at 80°C, recovering the filtrate and wet filtered product. High-performance liquid chromatography analysis of the filtrate obtained by solid-liquid separation (I) revealed that the filtrate contained 14.2 g of ε-caprolactam, representing a yield of 70.9% relative to the polyamide 6 in the PA6-C used as the raw material. The wet filtered product was then vacuum dried at 50°C for 12 hours, recovering 8.6 g of glass fiber. The recovered glass fiber was weighed into a 1.0 g crucible and treated in an electric furnace heated to 600°C under air for 3 hours. The amount of organic matter adhering to the recovered glass fibers was evaluated from the mass loss, and it was found that the mass loss was 1.5 mass %, indicating that the recovered glass fibers were high-purity glass fibers with a small amount of organic matter adhering.

[0052] Example 3: 25.1 g of PA6-B prepared in Reference Example 1 and 25.5 g of deionized water were weighed into an SUS316L autoclave equipped with a stirrer, a bottom stopper valve, and a glass filter (average mesh size 10 μm) at the bottom. 36.9 g of a 6.5 wt% aqueous polyamide 6 oligomer solution was then added. The combined mass ratio of water to polyamide 6 and polyamide 6 oligomer was X:1 = 3:1. The polyamide 6 oligomer used here was the polyamide 6 oligomer produced by the method described in Reference Example 3. The reaction vessel was purged with nitrogen and sealed under a nitrogen pressure of 0.5 MPa. The reaction was carried out at 320°C for 15 minutes while stirring at 200 rpm. Since the reaction temperature Y°C was 320°C, the product of X and Y was 960. Furthermore, since the residence time at the reaction temperature of 320°C was 15 minutes, the product of X, Y, and Z was 14,400. After the reaction was completed, the autoclave was cooled to an internal temperature of 90°C, and while maintaining this temperature, the bottom stop valve was opened to carry out solid-liquid separation (I). When the filtration rate decreased, solid-liquid separation (I) was carried out while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, deionized water in an amount approximately three times the mass of the filtered product was introduced into the autoclave, and the filtered product was rinsed three times at 90°C, and the filtrate and wet filtered product were recovered. High-performance liquid chromatography analysis of the obtained filtrate revealed that the filtrate contained 14.5 g of ε-caprolactam, a yield of 82.5% relative to the polyamide 6 in the PA6-B used as the raw material. The obtained wet filtered product was subjected to vacuum drying at 50°C for 12 hours, and 7.5 g of glass fiber was recovered. The recovered glass fiber was weighed out into a 1.0 g crucible and treated in an air atmosphere in an electric furnace heated to 600°C for 3 hours. The amount of organic matter adhering to the recovered glass fiber was evaluated from the mass loss. The mass loss was found to be 1.2 mass%, and the recovered glass fiber was found to be a high-purity glass fiber with a small amount of organic matter adhering.

[0053] Comparative Example 1: 28.6 g of PA6-B prepared in Reference Example 1 and 60.0 g of deionized water were charged into a SUS316L autoclave equipped with a stirrer. The mass ratio of water to polyamide 6 (X:1) was 3:1. The reaction vessel was purged with nitrogen and sealed under a nitrogen pressure of 0.5 MPa. The reaction was then held at 320°C for 15 minutes while stirring at 200 rpm, followed by cooling to prepare a reaction mixture. Since the reaction temperature Y°C was 320°C, the product of X and Y was 960. Furthermore, since the residence time at the reaction temperature of 320°C was 15 minutes, the product of X, Y, and Z was 14,400. High-performance liquid chromatography analysis of the resulting reaction mixture revealed that the reaction mixture contained 14.0 g of ε-caprolactam, representing a yield of 70.0% of the polyamide 6 based on the PA6-B used as the raw material. Furthermore, the reaction mixture was heated to 55°C under a reduced pressure of 30 mmHg to distillatively separate water, yielding a concentrated aqueous solution of ε-caprolactam. Further distillation was carried out at a reduced pressure of 5 mmHg and a heating temperature of 150 to 170°C to recover distilled ε-caprolactam. Attempts were made to recover glass fiber from the distillation residue, but prolonged high-temperature heating during distillation produced solvent-insoluble components, making it impossible to separate the organic residue by solid-liquid separation, and recovering high-purity glass fiber difficult. Comparison of Example 1 and Comparative Example 1 reveals that the reaction of a polyamide 6 resin composition containing a fibrous filler with water, followed by solid-liquid separation (I), allows the recovery of high-purity ε-caprolactam and high-purity glass fiber with a low amount of attached organic matter, and is a low-environmental-impact process that can reduce industrial waste.

[0054] Example 4: Polymerization of Polyamide 6. 10 g of ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water recovered by the method described in Example 1 were weighed into a test tube. The test tube was placed in an autoclave, and the autoclave was purged with nitrogen. The jacket temperature was set to 250°C and heating was initiated. After the internal pressure reached 1.0 MPa, the internal pressure was maintained at 1.0 MPa for 3 hours. The internal pressure was then released to atmospheric pressure over 1.5 hours, and heating was stopped when the internal temperature reached 228°C. After polymerization was complete, the polymer was recovered from the test tube and crushed. The crushed polymer was treated in 95°C hot water for 15 hours to extract and remove unreacted monomers and oligomers. The extracted polymer was vacuum dried at 80°C for 24 hours to obtain polyamide 6 resin with a melting point of 226°C and ηr = 2.73. The solution viscosity ηr was measured at 25° C. using a 0.01 g / mL solution of 98% concentrated sulfuric acid.

[0055] Example 5: 58.0 g of PA6-B prepared in Reference Example 1 and 122.0 g of deionized water were weighed into an SUS316L autoclave equipped with a stirrer, a bottom stopper valve, and a glass filter (average mesh size 10 μm) at the bottom. The mass ratio of water to polyamide 6 (X:1) was 3:1. The reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 5.0 MPa, and then the reaction was carried out by holding the temperature at 320°C for 15 minutes while stirring at 200 rpm. The pressure reached during the reaction was 19.7 MPa. Since the reaction temperature Y°C is 320°C, the product of X and Y is 960. Furthermore, since the residence time at the reaction temperature of 320°C is 15 minutes, the product of X, Y, and Z is 14,400. After completion of the reaction, the internal temperature was cooled to 90°C, and while maintaining the temperature at 90°C, the bottom stopper valve was opened to carry out solid-liquid separation (I). When the filtration rate decreased, solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, deionized water in an amount approximately three times (by mass) the filtered product was introduced into the autoclave, and the filtered product was rinsed three times at 90 ° C., recovering the filtrate and wet filtered product. High-performance liquid chromatography analysis of the filtrate obtained by solid-liquid separation (I) revealed that the filtrate contained 32.9 g of ε-caprolactam, representing a yield of 81.0% relative to the polyamide 6 in the PA6-B used as the raw material. The wet filtered product was then vacuum dried at 50 ° C. for 12 hours, recovering 17.4 g of glass fiber. The recovered glass fiber was weighed into a 1.0 g crucible and treated in an electric furnace heated to 600 ° C. under air for 3 hours. The amount of organic matter adhering to the recovered glass fibers was evaluated from the mass loss, and it was found that the mass loss was 1.3 mass%, and the recovered glass fibers were high-purity glass fibers with a small amount of organic matter adhering. Comparison with Example 1 shows that ε-caprolactam tends to be obtained in a high yield by setting the pressure during the reaction at a high pressure equal to or higher than the saturated vapor pressure.

[0056] Example 6: 43.0 g of PA6-B prepared in Reference Example 1 and 60.0 g of deionized water were weighed into an SUS316L autoclave equipped with a stirrer, a bottom stopper valve, and a glass filter (average mesh size 10 μm) at the bottom. The mass ratio of water to polyamide 6 (X:1) was 2:1. The reaction vessel was purged with nitrogen and sealed under a nitrogen pressure of 0.3 MPa. The reaction was then carried out by stirring at 200 rpm at 320°C for 15 minutes. Since the reaction temperature Y°C is 320°C, the product of X and Y is 640. Furthermore, since the residence time at the reaction temperature of 320°C is 15 minutes, the product of X, Y, and Z is 9,600. After the reaction was completed, the internal temperature was cooled to 90°C, and while maintaining the temperature at 90°C, the bottom stopper valve was opened to carry out solid-liquid separation (I). When the filtration rate decreased, solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, deionized water in an amount approximately three times (by mass) the volume of the filtered material was introduced into the autoclave, and the filtered material was rinsed three times at 90 ° C., and the filtrate and wet filtered material were recovered. High-performance liquid chromatography measurement of the filtrate obtained by solid-liquid separation (I) revealed that the filtrate contained 19.2 g of ε-caprolactam, and the yield relative to polyamide 6 in the PA6-B used as the raw material was 63.9%. The obtained wet filtered material was subjected to vacuum drying at 50 ° C. for 12 hours, and 12.9 g of glass fiber was recovered. The recovered glass fiber was weighed into a 1.0 g crucible and treated in an electric furnace heated to 600 ° C. under an air atmosphere for 3 hours. The amount of organic matter adhering to the recovered glass fibers was evaluated from the mass loss, and it was found that the mass loss was 1.4 mass%, indicating that the recovered glass fibers were high-purity glass fibers with a small amount of organic matter adhering.

[0057] Example 7: 75.0 g of PA6-B prepared in Reference Example 1 and 150.0 g of deionized water were weighed into an SUS316L autoclave equipped with a stirrer, a bottom stopper valve, and a glass filter (average mesh size 10 μm) at the bottom. The mass ratio of water to polyamide 6 (X:1) was 2:1. The reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 5.0 MPa, and then the reaction was carried out by holding the temperature at 320°C for 15 minutes while stirring at 200 rpm. The pressure reached during the reaction was 19.8 MPa. Since the reaction temperature Y°C was 320°C, the product of X and Y was 640. Furthermore, since the residence time at the reaction temperature of 320°C was 15 minutes, the product of X, Y, and Z was 9,600. After completion of the reaction, the internal temperature was cooled to 90°C, and while maintaining the temperature at 90°C, the bottom stopper valve was opened to carry out solid-liquid separation (I). When the filtration rate decreased, solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, deionized water in an amount approximately three times (by mass) the volume of the filtered material was introduced into the autoclave, and the filtered material was rinsed three times at 90°C, recovering the filtrate and wet filtered material. High-performance liquid chromatography (HPLC) analysis of the filtrate obtained from solid-liquid separation (I) revealed that the filtrate contained 39.3 g of ε-caprolactam, representing a yield of 74.9% relative to the polyamide 6 in the PA6-B used as the raw material. Comparison with Example 6 demonstrates that a high reaction pressure above the saturated vapor pressure tends to produce a high yield of ε-caprolactam. Furthermore, the resulting wet filtered material was subjected to vacuum drying at 50°C for 12 hours, recovering 22.5 g of glass fiber. The recovered glass fiber was weighed into a 1.0 g crucible and treated in an electric furnace heated to 600°C under air for 3 hours. The amount of organic matter adhering to the recovered glass fibers was evaluated from the mass loss, and it was found that the mass loss was 1.4 mass%, indicating that the recovered glass fibers were high-purity glass fibers with a small amount of organic matter adhering.

[0058] Example 8: This example describes a hot water extract from the PA6 production process, obtained by the method described in Reference Example 4, concentrated to a concentration of 6.5% by mass of unreacted caprolactam and polyamide 6 oligomer, and used as the depolymerization feedstock. 25.1 g of PA6-B prepared in Reference Example 1 and 25.5 g of deionized water were weighed into an SUS316L autoclave equipped with a stirrer, a bottom stop valve, and a glass filter (average mesh size 10 μm) at the bottom. 36.9 g of the 6.5% by mass aqueous solution of polyamide 6 oligomer prepared above was then added. The mass ratio of water to polyamide 6 and polyamide 6 oligomer combined was X:1 = 3.3:1. The reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 0.5 MPa, and then the reaction was carried out at 320°C for 15 minutes while stirring at 200 rpm. Since the reaction temperature Y°C is 320°C, the product of X and Y is 1,056. Furthermore, since the residence time at a reaction temperature of 320°C is 15 minutes, the product of X, Y, and Z is 15,840. After the reaction was completed, the internal temperature was cooled to 90°C, and while maintaining this temperature, the bottom plug valve was opened to perform solid-liquid separation (I). When the filtration rate decreased, solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, deionized water in an amount approximately three times the mass of the filtered material was introduced into the autoclave, and the filtered material was rinsed three times at 90°C, and the filtrate and wet filtered material were recovered. High-performance liquid chromatography analysis of the obtained filtrate revealed that the filtrate contained 14.0 g of ε-caprolactam, representing a yield of 79.7% relative to the polyamide 6 in the PA6-B used as the raw material. The wet filtered product was vacuum dried at 50° C. for 12 hours to recover 7.5 g of glass fiber. The recovered glass fiber was weighed into a 1.0 g crucible and treated in an air atmosphere in an electric furnace heated to 600° C. for 3 hours. The amount of organic matter adhering to the recovered glass fiber was evaluated from the mass loss. The mass loss was 1.2 mass%, and it was found that the recovered glass fiber was a high-purity glass fiber with a small amount of organic matter adhering.

[0059] Example 9: This example describes a case in which the hot water extract from the PA6 production process, obtained by the method described in Reference Example 4, was concentrated until the total concentration of unreacted caprolactam and polyamide 6 oligomer reached 6.5% by mass, and then used as the depolymerization feedstock. 25.5 g of PA6-B prepared in Reference Example 1 and 60.0 g of the 6.5% by mass aqueous solution of polyamide 6 oligomer prepared above were added to an SUS316L autoclave equipped with a stirrer, a bottom stop valve, and a glass filter (average mesh size 10 μm) at the bottom. The mass ratio of water to polyamide 6 and polyamide 6 oligomer combined was X:1 = 3.0:1. The reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 0.5 MPa, and then the reaction was carried out at 320°C for 15 minutes while stirring at 200 rpm. Since the reaction temperature Y°C is 320°C, the product of X and Y is 963. Furthermore, since the residence time at a reaction temperature of 320°C is 15 minutes, the product of X, Y, and Z is 14,454. After the reaction was completed, the internal temperature was cooled to 90°C, and while maintaining this temperature, the bottom plug valve was opened to perform solid-liquid separation (I). When the filtration rate decreased, solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, deionized water in an amount approximately three times the mass of the filtered product was introduced into the autoclave, and rinsing of the filtered product at 90°C was performed three times, and the filtrate and wet filtered product were recovered. High-performance liquid chromatography analysis of the obtained filtrate revealed that the filtrate contained 14.0 g of ε-caprolactam, representing a yield of 78.4% relative to the polyamide 6 in the PA6-B used as the raw material. The wet filtered product was vacuum dried at 50° C. for 12 hours to recover 7.6 g of glass fiber. The recovered glass fiber was weighed into a 1.0 g crucible and treated in an air atmosphere in an electric furnace heated to 600° C. for 3 hours. The amount of organic matter adhering to the recovered glass fiber was evaluated from the mass loss. The mass loss was 1.3 mass%, and it was found that the recovered glass fiber was a high-purity glass fiber with a small amount of organic matter adhering.

Claims

1. A method for recovering the fibrous filler (D) and the ε-caprolactam from the mixture (C), the method comprising: contacting a polyamide 6 resin composition (A) containing at least a fibrous filler (D) with at least one of water (B) heated to 290°C or higher but not higher than 350°C and an aqueous polyamide 6 oligomer solution (B1) heated to 290°C or higher but not higher than 350°C to obtain a mixture (C) containing at least the fibrous filler (D), ε-caprolactam, a polyamide 6 oligomer, and water (B); and recovering the fibrous filler (D) and the ε-caprolactam from the mixture (C), the method comprising the steps of: A method for recovering ε-caprolactam and a fibrous filler comprising the steps of: (a) a step of adding and contacting a polyamide 6 resin composition (A) containing a fibrous filler (D) with at least one of water (B) and an aqueous polyamide 6 oligomer solution (B1), under the following conditions: when the mass ratio of water to polyamide 6, or the total mass ratio of water to polyamide 6 and polyamide 6 oligomer is X:1 and the reaction temperature is Y°C, the product of X and Y is 2,000 or less, and when the residence time at the reaction temperature Y°C is Z minutes, the product of X, Y and Z is 60,000 or less, and the ε-caprolactam yield of the mixture (C) is 70% or more; (b) A step (I) of subjecting the mixture (C) to solid-liquid separation in a temperature range not exceeding the boiling point of water at the operating pressure. (c) A step of washing the filtered residue containing the fibrous filler (D) obtained in the solid-liquid separation (I) with water at a temperature not higher than the boiling point of water at normal pressure to recover the fibrous filler (D).

2. A method for recovering the fibrous filler (D) and the ε-caprolactam from the mixture (C), comprising the steps of: (a) contacting a polyamide 6 resin composition (A) containing at least a fibrous filler (D) with at least one of water (B) heated to 290°C or higher but not exceeding 350°C; or (B1) adding and contacting the polyamide 6 resin composition (A) with at least one of water (B) heated to 290°C or higher but not exceeding 350°C; and (b) obtaining a mixture (C) containing at least the fibrous filler (D), ε-caprolactam, a polyamide 6 oligomer, and water (B), and then recovering the fibrous filler (D) and ε-caprolactam from the mixture (C), the method comprising the steps of: A method for recovering ε-caprolactam and a fibrous filler comprising the steps of: (a) a step of adding and contacting a polyamide 6 resin composition (A) containing a fibrous filler (D) with at least one of water (B) or an aqueous polyamide 6 oligomer solution (B1) under the conditions that, when the mass ratio of water to polyamide 6, or the total of water to polyamide 6 and polyamide 6 oligomer is X:1 and the reaction temperature is Y°C, the product of X and Y is 2,000 or less, and when the residence time at the reaction temperature Y°C is Z minutes, the product of X, Y and Z is 30,000 or less, to prepare a mixture (C); (b) A step (I) of subjecting the mixture (C) to solid-liquid separation in a temperature range not exceeding the boiling point of water at the operating pressure. (c) A step of washing the filtered residue containing the fibrous filler (D) obtained in the solid-liquid separation (I) with water at a temperature not higher than the boiling point of water at normal pressure to recover the fibrous filler (D).

3. A method for recovering the fibrous filler (D) and the ε-caprolactam from the mixture (C), the method comprising contacting a polyamide 6 resin composition (A) containing at least a fibrous filler (D) with water (B) heated to 290°C or higher but not exceeding 350°C, or with water (B) heated to 290°C or higher but not exceeding 350°C, and further adding an aqueous polyamide 6 oligomer solution (B1) heated to 290°C or higher but not exceeding 350°C, to obtain a mixture (C) containing at least the fibrous filler (D), ε-caprolactam, a polyamide 6 oligomer, and the water (B), and then recovering the fibrous filler (D) and the ε-caprolactam from the mixture (C), the method comprising carrying out the following steps (a) to (c) in this order:

3. The method for recovering fibrous filler and ε-caprolactam according to claim 2. (a) a step of contacting a polyamide 6 resin composition (A) containing a fibrous filler (D) with water (B), or water (B) and an aqueous polyamide 6 oligomer solution (B1) under the conditions that the mass ratio of water to polyamide 6, or the total mass ratio of water to polyamide 6 and polyamide 6 oligomer is X:1 and the reaction temperature is Y°C, such that the product of X and Y is 2,000 or less, to prepare a mixture (C); (b) A step (I) of subjecting the mixture (C) to solid-liquid separation in a temperature range not exceeding the boiling point of water at the operating pressure. (c) A step of washing the filtered residue containing the fibrous filler (D) obtained in the solid-liquid separation (I) with water at a temperature not higher than the boiling point of water at normal pressure to recover the fibrous filler (D).

4. The recovered fibrous filler (D) is subjected to a heat treatment at 600°C for 3 hours in an air atmosphere, and the mass loss of the fibrous filler (D) is 3.0 mass% or less.

4. The method for recovering ε-caprolactam and a fibrous filler according to claim 1.

5. Solid-liquid separation (I) is carried out in a temperature range below the boiling point of water at atmospheric pressure.

5. The method for recovering fibrous filler and ε-caprolactam according to claim 1.

6. A mixture (C) containing at least a fibrous filler (D), ε-caprolactam, a polyamide 6 oligomer, and water is prepared, and then a solid-liquid separation (I) is carried out. The method for recovering fibrous filler and ε-caprolactam according to any one of claims 1 to 5.

7. The polyamide 6 oligomer aqueous solution (B1) is an extract obtained in a step of hot water extraction of polyamide 6 oligomer from polyamide 6, which is a product during the production of polyamide 6.

7. The method for recovering fibrous filler and ε-caprolactam according to claim 1.

8. The polyamide 6 resin composition (A) containing at least a fibrous filler (D) is a waste of a resin molding containing polyamide 6 having at least a fibrous filler (D). The method for recovering fibrous filler and ε-caprolactam according to any one of claims 1 to 7.

9. ε-caprolactam is obtained by any one of the methods of claims 1 to 8, and the ε-caprolactam is polymerized. A method for producing polyamide 6, characterized in that