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

Abstract

The present invention provides a recovery method with less energy consumption, the recovery method being capable of recovering ε-caprolactam and a polyamide 6 oligomer with high yield by depolymerization with use of a little water and solid-liquid separation only. The present invention provides a method for recovering ε-caprolactam and a polyamide 6 oligomer, the method being characterized by a process in which: a resin composition (A) that contains at least a polyamide 6 is added with at least one of water (B) that is heated to a temperature of 290°C to 350°C and an aqueous polyamide 6 oligomer solution (B1) that is heated to a temperature of 290°C to 350°C so as to be in contact with each other, thereby obtaining a reaction mixture (C) that contains at least ε-caprolactam, the polyamide 6 oligomer and water; and subsequently, the reaction mixture (C) is subjected to solid-liquid separation (I) in a temperature range that is not more than the boiling point of water at the operating pressure, thereby separating and recovering the polyamide 6 oligomer in a solid phase and ε-caprolactam in a liquid phase.

Description

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

[0001] The present invention relates to a method for recovering ε-caprolactam and polyamide 6 oligomers by depolymerizing a polyamide 6 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 depolymerization method using a small amount of water, which has a high specific heat capacity and a high heat of vaporization, and a method for recovering high-purity ε-caprolactam and polyamide 6 oligomers by solid-liquid separation.

[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 for obtaining the raw material ε-caprolactam by blowing superheated steam into the polyamide 6 in the presence of a phosphoric acid catalyst has been disclosed (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 for recovering lactam by contacting polyamide 6 with heated water at a temperature of 280°C to 320°C has been disclosed (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 adhering 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 polyamide 6 oligomers or its utilization.

[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), which has a very high specific heat capacity (4.2 kJ / kg·K) and heat of vaporization (2,250 kJ / kg), and carry out the reaction for a long period of time. This requires a large amount of energy for the depolymerization reaction and the recovery of ε-caprolactam from the low-concentration ε-caprolactam aqueous 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 a method for recovering polyamide 6 oligomer and its utilization.

[0008] In order to solve the above problems, the present invention has the following configuration: 1. A method for recovering ε-caprolactam and polyamide 6 oligomer, comprising adding and contacting a resin composition (A) containing at least polyamide 6 with at least one of water (B) heated to 290°C or more and 350°C or less and an aqueous polyamide 6 oligomer solution (B1) heated to 290°C or more and 350°C or less to obtain a reaction mixture (C) containing at least ε-caprolactam, polyamide 6 oligomer, and water, and subjecting the reaction mixture (C) to solid-liquid separation (I) in a temperature range not exceeding the boiling point of water at the operating pressure, thereby separating and recovering the polyamide 6 oligomer into a solid phase and the aqueous ε-caprolactam solution into a liquid phase. Item 1. A method for recovering ε-caprolactam and polyamide 6 oligomers, comprising adding a resin composition (A) containing at least polyamide 6 to water (B) heated to 290°C or higher and 350°C or lower, or to water (B) heated to 290°C or higher and 350°C or lower, and further adding and contacting an aqueous polyamide 6 oligomer solution (B1) heated to 290°C or higher and 350°C or lower, to obtain a reaction mixture (C) containing at least ε-caprolactam, polyamide 6 oligomers, and water. 3. A method for recovering ε-caprolactam and polyamide 6 oligomers according to item 1 or 2, wherein the polyamide 6 oligomer recovered by solid-liquid separation (I) is used as the aqueous polyamide 6 oligomer solution (B1). 4. A method for recovering ε-caprolactam and polyamide 6 oligomers according to any one of items 1 to 3, wherein the polyamide 6 oligomers contain 90% by mass or more of linear di- to dodecamer oligomers. 5. 5. The method for recovering ε-caprolactam and polyamide 6 oligomers according to any one of items 1 to 4, characterized in that the operation pressure of the solid-liquid separation (I) is normal pressure. 6. The method for recovering ε-caprolactam and polyamide 6 oligomers according to any one of items 1 to 5, characterized in that the aqueous polyamide 6 oligomer solution (B1) is an extract obtained in a step of hot water extraction of polyamide 6 oligomers from polyamide 6, which is a product of polyamide 6 production.7. A method for recovering ε-caprolactam and polyamide 6 oligomer according to any one of items 1 to 6, characterized in that a reaction mixture (C) containing at least ε-caprolactam, polyamide 6 oligomer, and water is prepared, followed by solid-liquid separation (I). 8. A method for recovering ε-caprolactam and polyamide 6 oligomer according to any one of items 1 to 7, characterized in that the resin composition (A) containing at least polyamide 6 is a waste resin molded article containing at least polyamide 6. 9. A method for producing polyamide 6, comprising obtaining ε-caprolactam by the method of any one of items 1 to 8, and polymerizing polyamide 6 from the obtained resin composition.

[0009] The present invention provides a method for recovering ε-caprolactam and polyamide 6 oligomers by depolymerizing a polyamide 6 resin composition, which method consumes little energy and enables recovery of ε-caprolactam and polyamide 6 oligomers in high yields by only performing depolymerization and solid-liquid separation using a small amount of water.

[0010] The present invention is characterized by the steps of: contacting a resin composition (A) containing at least polyamide 6 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 to prepare a reaction mixture (C); and subjecting the reaction mixture (C) to solid-liquid separation (I) in a temperature range not exceeding the boiling point of water under the operating pressure to separate and recover the polyamide 6 oligomer into a solid phase and an aqueous ε-caprolactam solution into a liquid phase. The present invention will be described in further detail below.

[0011] (1) Resin Composition (A) In the present invention, polyamide 6 refers to 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] There are no particular restrictions on the amount of cyclic oligomer shown in Chemical Formula 1 below contained in the polyamide 6 of the present invention, but it is preferably 2.0% by mass or less, more preferably 1.8% by mass or less, and even more preferably 1.5% by mass or less. In the cyclic oligomer shown in Formula (a) below, m is an integer of 2 to 4. The cyclic oligomer shown in Formula (a) below melts and volatilizes, causing line blockages, etc., so having the amount of cyclic oligomer within a preferred range tends to suppress line blockages caused by melting and volatilization. Note that cyclic oligomers shown in Formula (a) below where m is 5 or more are not of interest in the present invention, taking into account the degree of volatilization.

[0016]

[0017] The resin composition (A) of the present invention may further contain an alkali metal halide, provided that the object of the present invention is not impaired. Examples of alkali metal halides include lithium iodide, sodium iodide, potassium iodide, lithium bromide, sodium bromide, potassium bromide, lithium chloride, sodium chloride, and potassium chloride, and two or more of these may be used in combination. Among these, potassium iodide is preferred because it is easily available, has excellent dispersibility in polyamide 6, is more reactive with radicals, and improves retention stability at high temperatures. Furthermore, these alkali metal halides are more preferably used in combination with Group 11 metal halides such as copper(I) iodide, copper(I) bromide, and copper(I) chloride, as this further improves retention stability at high temperatures.

[0018] The amount of the alkali metal halide blended is preferably 0.01 to 1 part by mass per 100 parts by mass of polyamide 6. Blending the alkali metal halide within this preferred range can suppress side reactions other than hydrolysis in this process, and tends to increase the ε-caprolactam yield. The amount of the alkali metal halide blended is more preferably 0.02 to 0.5 parts by mass, and even more preferably 0.03 to 0.4 parts by mass.

[0019] The resin composition (A) of the present invention may contain a fibrous filler. The fibrous filler may be any filler having a fibrous shape. Specific examples include glass fibers, polyacrylonitrile (PAN)-based or pitch-based carbon fibers, stainless steel fibers, metal fibers such as aluminum fibers and brass fibers, organic fibers such as polyester fibers and aromatic polyamide fibers, gypsum fibers, ceramic fibers, asbestos fibers, zirconia fibers, alumina fibers, silica fibers, titanium oxide fibers, silicon carbide fibers, rock wool, potassium titanate whiskers, silicon nitride whiskers, wollastonite, and alumina silicate, as well as glass fibers, carbon fibers, aromatic polyamide fibers, and polyester fibers coated with one or more metals selected from the group consisting of nickel, copper, cobalt, silver, aluminum, iron, and alloys thereof. Two or more of these may be contained. The content of the fibrous filler is preferably 1 to 200 parts by mass per 100 parts by mass of the resin composition (A).

[0020] The resin composition (A) of the present invention may further contain fillers other than fibrous fillers, thermoplastic resins other than polyamide 6, various additives, etc., within the scope of not impairing the object of the present invention. The fillers other than fibrous fillers may be either organic fillers or inorganic fillers, and examples thereof include non-fibrous fillers, and two or more of these may be blended. Examples of non-fibrous fillers 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.

[0021] 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.

[0022] Specific examples of thermoplastic resins other than polyamide 6 contained in the 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, tetrafluoroethylene resins, and polyphenylene sulfide resins. Two or more of these may be blended. The amount of the 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 thermoplastic resin (A) of the present invention.

[0023] The polyamide 6-containing resin composition (A) of the present invention may be waste from a resin molded article containing at least polyamide 6. Examples of waste from resin molded articles containing polyamide 6 include polyamide 6 products, industrial waste generated during the production of polyamide 6 products, and post-consumer waste from polyamide 6 products. Examples of polyamide 6 products include textile fabrics for clothing, such as used clothing, uniforms, sportswear, and underwear; industrial textile fabrics, such as curtains, carpets, ropes, nets, belts, and sheets; molded parts for housing construction materials; electrical and electronic molded parts; film products; extrusion molded products; in-situ polymerization molded products; and RIM molded products. Furthermore, waste also includes product scraps, pellet scraps, block scraps, and cutting chips generated during these production processes.

[0024] (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.

[0025] 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. Aliphatic diamines such as methyl methacrylate, 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.

[0026] 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 polyamide oligomer aqueous 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.

[0027] Furthermore, the composition of the polyamide 6 oligomer used in the present invention is not particularly limited, but a preferred example is that the content of linear polyamide 6 oligomers having a dimer to dodecamer in the polyamide 6 oligomer is 90% by mass or more. A more preferred example is that the amount of linear polyamide 6 oligomers is 93% by mass or more, and a particularly preferred example is that the amount of linear polyamide 6 oligomers is 95% by mass or more. When the content of linear polyamide 6 oligomers having a dimer to dodecamer in the polyamide 6 oligomer is 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 polyamide 6 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.

[0028] The method for preparing the polyamide 6 oligomer used in the present invention is not particularly limited, and may be, for example, a polyamide 6 oligomer contained in the extract obtained by hot water extraction of 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. Furthermore, a polyamide 6 oligomer obtained as a by-product during the production of ε-caprolactam by contacting a resin composition (A) containing at least polyamide 6 with 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 also 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 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.

[0029] (3) Polyamide 6 oligomer aqueous solution (B1) The polyamide 6 oligomer aqueous solution (B1) used in the present invention is prepared by heating and mixing the polyamide 6 oligomer and water.There is no particular limitation to the water used in preparing the polyamide 6 oligomer aqueous solution (B1), and tap water, ion-exchanged water, distilled water, well water, etc. can be used, but from the viewpoint of suppressing side reactions caused by the influence of coexisting salts, ion-exchanged water or distilled water is preferably used.In addition, the concentration of polyamide 6 oligomer in the polyamide 6 oligomer aqueous solution (B1) used in the present invention can be any concentration as long as the polyamide 6 oligomer is dissolved in water when heated to 290 ℃ or more and 350 ℃ or less, but preferably 20% by mass or less can be exemplified, more preferably 15% by mass or less, and even more preferably 10% by mass or less can be exemplified. When the concentration of polyamide 6 oligomer is within these preferred ranges, the solubility in water during preparation of the polyamide 6 oligomer aqueous solution (B1) is increased, making it possible to prepare the polyamide 6 oligomer aqueous solution (B1) at a lower temperature. Alternatively, the polyamide 6 oligomer aqueous solution (B1) can be obtained by hot water extraction of polyamide 6 oligomer from polyamide 6, a product of polyamide 6 production. Polyamide 6 resins obtained by polymerizing ε-caprolactam typically contain impurities such as unreacted monomers and polyamide 6 oligomers resulting from the polymerization equilibrium reaction. Therefore, in order to remove these impurities, the polymerized pellets are fed to a hot water extraction tower, where the unreacted monomers and polyamide 6 oligomers are extracted and removed by hot water extraction. Using the extract obtained in the hot water extraction of polyamide 6 oligomer 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.

[0030] (4) Method for Preparing Reaction Mixture (C) The reaction mixture (C) of the present invention, containing at least ε-caprolactam, polyamide 6 oligomer, and water, is prepared by adding and contacting a resin composition (A) containing at least polyamide 6 with at least one of water (B) heated to 290°C to 350°C or an aqueous polyamide 6 oligomer solution (B1) heated to 290°C to 350°C. Here, the water heated to 290°C to 350°C and the water in the aqueous polyamide 6 oligomer solution (B1) heated to 290°C to 350°C are the 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 used in the present invention and the water in the aqueous polyamide 6 oligomer solution are 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. The high ionic product increases the hydrogen ion and hydroxide ion concentrations, resulting in excellent hydrolysis. The temperature of the water (B) and 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 reaction equipment. Furthermore, a preferred example of the water pressure is higher than the saturated vapor pressure. Water may be used in either a liquid state or a gaseous state such as steam. However, because the reaction proceeds more easily in a liquid state than in a gaseous state, it is preferable that the water pressure be higher than the saturated vapor pressure. There is no particular upper limit to the water pressure, but a pressure of 20 MPa or lower is an example. This pressure range is preferred because it tends to increase the ionic product of water described above. To bring the water into this pressure range, the inside of a pressure vessel is pressurized and sealed.To pressurize the inside of the pressure vessel, a gas may be enclosed in addition to the water (B) and the aqueous polyamide 6 oligomer solution (B1). Examples of such a gas include air, argon, nitrogen, etc., but nitrogen and argon are preferred from the viewpoint of suppressing side reactions such as oxidation reactions. The degree of gas pressurization is not particularly limited as long as it is set to the desired pressure, but can be 0.3 MPa or more.

[0031] Although there is no particular restriction on the amount of water used in the ε-caprolactam recovery method of the present invention, a preferred example is to adjust the amount of water used so that the product of X and Y is 2,000 or less when the mass ratio of water (B) to polyamide 6, the mass ratio of the total of water in water (B) and the aqueous polyamide 6 oligomer 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 aqueous polyamide 6 oligomer solution (B1) is X:1 and the reaction temperature is Y°C. The product of X and Y is more preferably 1,600 or less, even more preferably 1,300 or less, and particularly preferably 1,200 or less. Furthermore, 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 polyamide 6 oligomers from a polyamide 6 resin composition, with the aim of achieving both the recycling of fossil resources and the reduction of 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 preferred ranges, both the efficient production of ε-caprolactam and energy savings can be achieved. Furthermore, assuming a residence time of Z minutes at a reaction temperature of Y°C, a preferred example of the condition is that the product of X, Y, and Z is 60,000 or less. More preferred examples include a condition of 40,000 or less, even more preferred a condition of 30,000 or less, and particularly preferred a condition of 20,000 or less. While there is no particular lower limit for the product of X, Y, and Z, a condition of 5,000 or more is preferred, with a condition of 8,000 or more being more preferred, and a condition of 9,000 or more being particularly preferred. By setting the product of X, Y and Z within such a preferable range, the production efficiency of ε-caprolactam tends to be increased while saving energy, which is preferable.In the reaction of polyamide 6 with water, in addition to the production of ε-caprolactam, a side reaction of ε-caprolactam with water to produce linear oligomers proceeds, and simply reducing the amount of water used results in the production of a large amount of linear oligomers, resulting in a significant decrease in the production efficiency of ε-caprolactam.The present inventors have clarified the thermodynamic equilibrium point of the reaction of polyamide 6 with water to produce ε-caprolactam and the side reaction of the production of linear oligomers, and have found that by setting the product of X and Y, and the product of X, Y, and Z within the above ranges, the by-production of linear oligomers can be suppressed and the production efficiency of ε-caprolactam can be significantly improved, thereby achieving the present invention.

[0032] The reaction mixture (C) of the present invention can be produced using various known reaction methods, such as batch and continuous methods. Examples of batch methods include autoclaves, vertical and horizontal reactors, and vertical and horizontal reactors equipped with a stirrer and a heating function, as well as a compression mechanism such as a cylinder. Examples of continuous methods include extruders, 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 an inert atmosphere such as nitrogen, helium, or argon. From the standpoints of economy and ease of handling, a nitrogen atmosphere is preferred.

[0033] (5) Solid-Liquid Separation (I) In the method for recovering ε-caprolactam and polyamide 6 oligomer of the present invention, a reaction mixture (C) containing at least ε-caprolactam, polyamide 6 oligomer, and water is subjected to solid-liquid separation (I) in a temperature range below the boiling point of water at the operating pressure, thereby separating and recovering the polyamide 6 oligomer into a solid phase and an aqueous ε-caprolactam solution into a liquid phase. In the present invention, a reaction mixture (C) containing at least ε-caprolactam, polyamide 6 oligomer, and water is subjected to solid-liquid separation (I) in a temperature range below the boiling point of water at the operating pressure. The temperature at which solid-liquid separation (I) is carried out is preferably below the boiling point of water at atmospheric pressure, more preferably 80°C or lower, even more preferably 60°C or lower, and particularly preferably 50°C or lower. There are no particular restrictions on the lower limit temperature at which solid-liquid separation (I) is carried out, but it is preferably 10°C or higher, more preferably 15°C or higher, and even more preferably 20°C or higher. In this preferred temperature range, the ε-caprolactam in the reaction mixture (C) is dissolved in water, but the polyamide 6 oligomer tends to be less soluble in water. Therefore, by carrying out the solid-liquid separation (I) in the above preferred temperature range, it becomes possible to separate most of the polyamide 6 oligomer as a solid phase component from the reaction mixture (C) containing at least the ε-caprolactam, the polyamide 6 oligomer, and water.

[0034] In addition, the filter material used when carrying out solid-liquid separation (I) must be able to separate polyamide 6 oligomer, and at least be able to pass through the solution containing ε-caprolactam and water.Usually, the sieve with a pore size smaller than 200 mesh (opening 0.074 mm) or the filter material with a pore size of 70 μm to 0.01 μm, preferably 40 μm to 0.05 μm, more preferably 20 μm to 0.1 μm, more preferably 5 μm to 0.1 μm, and even more preferably 1 μm to 0.1 μm can be exemplified.By using the filter material with a pore size within the above range, the polyamide 6 oligomer passing through the filter material tends to decrease, and when removing water from the filtrate and recovering ε-caprolactam, the purity of ε-caprolactam tends to increase.In addition, the recovery rate of the polyamide 6 oligomer recovered by solid-liquid separation (I) also tends to increase, so this is preferred. On the other hand, if the concentration is below the above-mentioned preferred range, the filtration efficiency tends to deteriorate. Examples of the filter include a method using a filter such as a sieve, a method using a centrifuge, a method using a centrifugal filter, a method using a vibrating screen, a method using a pressure filter, and a method using a suction filter, but are not limited to these. It is preferable that the mother liquor adhering to the solid fraction separated into solids and liquids by the solid-liquid separation (I) of the present invention is washed with water heated to the temperature at which the solid-liquid separation (I) was carried out, so that the mother liquor is substantially not adhered to the solid fraction.

[0035] In addition, there is no particular limitation on the atmosphere when carrying out solid-liquid separation (I), but if the polyamide 6 oligomer is oxidized and deteriorated due to the conditions of contact time, temperature, etc., it is preferable to carry out the solid-liquid separation in a non-oxidizing atmosphere. Note that the non-oxidizing atmosphere refers to an atmosphere in which the oxygen concentration in the gas phase is 5% by volume or less, preferably 2% by volume or less, and more preferably an atmosphere that does not substantially contain oxygen, that is, an inert gas atmosphere such as nitrogen, helium, or argon, and among these, it is preferable to carry out the solid-liquid separation in a nitrogen atmosphere from the viewpoint of economy and ease of handling.

[0036] The solid-liquid separation (I) of the reaction mixture (C) containing at least ε-caprolactam, polyamide 6 oligomer, and water of the present invention can be carried out by reheating a separately prepared reaction mixture (C) to the temperature at which solid-liquid separation (I) is carried out, or by preparing the reaction mixture (C) using the method for preparing the reaction mixture (C) required by the present invention, followed by cooling from the preparation temperature of the reaction mixture (C) to the temperature at which solid-liquid separation (I) is carried out. A preferred method is to cool the reaction mixture (C) to the temperature at which solid-liquid separation (I) is carried out after preparation. Furthermore, when carrying out solid-liquid separation (I) after preparation of the reaction mixture (C), seed crystals may be added to facilitate precipitation of the polyamide 6 oligomer. Seed crystals can be added at the start of cooling or during cooling. The seed crystals used here may be any crystals that can serve as nuclei for crystallizing the polyamide 6 oligomer. Since the seed crystals are preferably crystals of the same substance as polyamide 6 oligomers, it is preferable to use polyamide 6 or polyamide 6 oligomers as seed crystals. The use of these seed crystals is preferable because it promotes the precipitation of polyamide 6 oligomers and tends to increase the purity of ε-caprolactam recovered in the liquid phase by solid-liquid separation (I).

[0037] Furthermore, the polyamide 6 oligomer in the form of a wet cake containing water recovered as a solid phase component by the solid-liquid separation (I) can be used in the form of a wet cake as a raw material for the aqueous polyamide 6 oligomer solution (B1) without undergoing a drying treatment. By utilizing the polyamide 6 oligomer recovered by the solid-liquid separation (I) as a raw material without undergoing a drying step, the amount of industrial waste can be reduced and the energy required for the drying step can be saved, which is preferable.

[0038] (6) Method for Recovering ε-Caprolactam There is no particular limitation 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 separating water by distilling the aqueous ε-caprolactam solution obtained by solid-liquid separation (I). 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.

[0039] (7) Polyamide 6 and Molded Articles Thereof The method for producing ε-caprolactam described in the present invention can produce highly pure ε-caprolactam, which can therefore 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 thus obtained can be melt-kneaded with a fibrous filler or various additives, as necessary, 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 or extrusion molding.

[0040] 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, including 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; 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 for mobile phones, mobile personal computers, handheld mobile phones, and the like; 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.

[0041] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0042] 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

[0043] Reference Example 1: 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 was X:1 = 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.

[0044] [Reference Example 2] Waste of polyamide 6 resin molded body (PA6-B) Unreinforced polyamide 6 fastener parts (polyamide 6 content of 99% by mass or more) were collected and crushed to obtain waste of polyamide 6 molded body. As a result of the above-mentioned high performance liquid chromatography analysis, the amount of cyclic dimer to tetramer oligomer in polyamide 6 in the waste of polyamide 6 molded body was 0.4% by mass.

[0045] 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.

[0046] Reference Example 3: 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 polymerization reactor was heated with stirring until the internal pressure reached 0.98 MPa. While maintaining this internal pressure, the temperature was continued to 250°C. After reaching 250°C, the pressure was released to atmospheric pressure over 40 minutes. 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.

[0047] Example 1: 20.0 g of PA6-A and 60.0 g of deionized water were charged into 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 was X:1 = 3:1. The reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 0.5 MPa, and the reaction was carried out by holding the mixture at 320°C for 15 minutes while stirring at 200 rpm. The pressure reached during the reaction was 10.5 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 40°C, and while maintaining the temperature at 40°C, the bottom stop 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 was introduced into the autoclave in an amount approximately three times (by mass) the volume of the filtered material, and the filtered material was rinsed three times at 40°C, recovering the filtrate and wet filtered material. The resulting filtered material was vacuum dried at 50°C for 12 hours, recovering 2.8 g of solids obtained by solid-liquid separation (I). High-performance liquid chromatography analysis of the resulting solids under the conditions described in [Reference Example 1] revealed that the solids were polyamide 6 oligomers containing 96.4% by mass of linear dimers to dodecamers. Due to their high purity, these polyamide 6 oligomers can be further utilized as raw materials for depolymerization. Furthermore, compared to Reference Example 1, which uses a large amount of organic solvent, this method is able to recover high-purity polyamide 6 oligomers without using organic solvents, demonstrating its low environmental impact. Furthermore, the filtrate obtained by solid-liquid separation (I) was subjected to high-performance liquid chromatography measurement, and the result showed that the filtrate contained 15.0 g of ε-caprolactam, a yield of 75.1% based on the PA6-A used as the raw material. Furthermore, the recovered filtrate was heated to 55°C under a reduced pressure of 30 mmHg to separate water by distillation, thereby obtaining a concentrated aqueous ε-caprolactam solution. Further distillation was carried out under a reduced pressure of 5 mmHg at a heating temperature of 150 to 170°C to recover distilled ε-caprolactam. The concentration and distillation yield of ε-caprolactam was 95.8%.The HPLC impurity content of the distilled ε-caprolactam was 0.48%, and it was of a quality suitable for use as a raw material for polyamide 6 polymerization.

[0048] Example 2: 20.0 g of PA6-B and 60.0 g of deionized water were charged into 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 (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 stirring at 200 rpm at 340°C for 15 minutes. Since the reaction temperature Y°C was 340°C, the product of X and Y was 1,020. Furthermore, since the residence time at the reaction temperature of 340°C was 15 minutes, the product of X, Y, and Z was 15,300. After the reaction was completed, the internal temperature was cooled to 30°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 amount (by mass) of the filtered product was introduced into the autoclave, and the filtered product was rinsed three times at 30°C, recovering the filtrate and wet filtered product. The resulting filtered product was vacuum dried at 50°C for 12 hours, and 1.7 g of solids were recovered from solid-liquid separation (I). High-performance liquid chromatography analysis of the resulting solids under the conditions described in [Reference Example 1] revealed that the solids were polyamide 6 oligomers containing 97.3 mass% linear dimer to dodecamer oligomers. Due to their high purity, this polyamide 6 oligomer can be further utilized as a depolymerization raw material. Furthermore, high-performance liquid chromatography analysis of the filtrate obtained from solid-liquid separation (I) revealed that the filtrate contained 14.9 g of ε-caprolactam, representing a yield of 74.4% relative to the polyamide 6 content of PA6-B used as the raw material.

[0049] Example 3: 17.6 g of PA6-A 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 a 6.5 wt% aqueous polyamide 6 oligomer solution was then added. The combined mass ratio (X:1) of water to polyamide 6 and polyamide 6 oligomer was 3:1. The polyamide 6 oligomer used here was the polyamide 6 oligomer prepared by the method described in Reference Example 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 reaction 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 internal temperature was cooled to 25°C, and while maintaining this temperature, the bottom stop 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 was introduced into the autoclave in an amount approximately three times (by mass) the volume of the filtered material, and the filtered material was rinsed three times at 25°C, recovering the filtrate and wet filtered material. The resulting filtered material was vacuum dried at 50°C for 12 hours, and 2.9 g of solid material was recovered from solid-liquid separation (I). High-performance liquid chromatography analysis of the resulting solid material under the conditions described in [Reference Example 1] revealed that the resulting solid material was a polyamide 6 oligomer containing 96.8% by mass of linear dimer-12-mer oligomers. Due to its high purity, this polyamide 6 oligomer can be further utilized as a raw material for depolymerization. Furthermore, the filtrate obtained by the solid-liquid separation (I) was subjected to high performance liquid chromatography measurement, and as a result, the amount of ε-caprolactam contained in the filtrate was 15.3 g, and the yield based on the PA6-A used as the raw material was 86.9%.

[0050] Example 4: 20.0 g of PA6-A and 60.0 g of deionized water were charged into 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 (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 stirring at 200 rpm at 320°C for 15 minutes. 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 internal temperature was first cooled to 90°C, and 0.5 g of polyamide 6 oligomer produced by the method described in Reference Example 1 was added while maintaining the temperature at 90°C. The mixture was then cooled to 40°C, and while maintained at 40°C, the bottom stop 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 was introduced into the autoclave in an amount approximately three times (by mass) the volume of the filtered material, and the filtered material was rinsed three times at 40°C, recovering the filtrate and wet filtered material. The resulting filtered material was vacuum dried at 50°C for 12 hours, recovering 4.3 g of solid matter obtained in solid-liquid separation (I). Since 0.5 g of polyamide 6 oligomer was added at 90°C, the actual recovered solid matter was 3.8 g. High-performance liquid chromatography analysis of the resulting solid matter under the conditions described in [Reference Example 1] revealed that the resulting solid matter was polyamide 6 oligomer containing 97.3 mass% linear dimer to dodecamer. Due to its high purity, this polyamide 6 oligomer can be further utilized as a raw material for depolymerization. Furthermore, a comparison with Example 1 shows that the amount of polyamide 6 oligomer recovered by solid-liquid separation (I) was increased by adding a small amount of polyamide 6 oligomer as seed crystals before performing solid-liquid separation (I). Furthermore, high-performance liquid chromatography measurement of the filtrate obtained by solid-liquid separation (I) revealed that the filtrate contained 15.0 g of ε-caprolactam, a yield of 75.1% based on the PA6-A used as the raw material.

[0051] Comparative Example 1: 20.0 g of PA6-A and 60.0 g of deionized water were charged into 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 (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 stirring at 200 rpm at 320°C for 15 minutes. 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 (HPLC) analysis of the resulting reaction mixture revealed that the reaction mixture contained 15.0 g of ε-caprolactam, representing a yield of 75.1% relative to the PA6-A used as the raw material. 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 150-170°C to recover distilled ε-caprolactam and distillation residue. The concentration and distillation yield of ε-caprolactam was 95.8%. Furthermore, 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. However, HPLC analysis of the recovered distillation residue was attempted, but solvent-insoluble components were found, making it impossible to obtain high-purity polyamide 6 oligomer from the distillation residue.

[0052] Example 5: 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 completed, 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 low polymers. The extracted polymer was vacuum dried at 80°C for 24 hours to obtain a polyamide 6 resin with a melting point of 226°C and ηr = 2.73.

[0053] Example 6: 45.0 g of PA6-A and 135.0 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 5.0 MPa. The reaction was carried out by stirring at 200 rpm at 320°C for 15 minutes. 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 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 40°C, and while maintaining this temperature, 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 was introduced into the autoclave in an amount approximately three times (by mass) the filtered product, and the filtered product was rinsed three times at 40°C, recovering the filtrate and wet filtered product. The filtered product was then vacuum dried at 50°C for 12 hours, recovering 4.7 g of solids obtained from solid-liquid separation (I). High-performance liquid chromatography analysis of the resulting solids under the conditions described in [Reference Example 1] revealed that the solids were polyamide 6 oligomers containing 98.1% by mass of linear dimer- to dodecamer-containing oligomers. Due to their high purity, these polyamide 6 oligomers can be further utilized as raw materials for depolymerization. Furthermore, high-performance liquid chromatography analysis of the filtrate obtained from solid-liquid separation (I) revealed that the filtrate contained 36.9 g of ε-caprolactam, representing a yield of 82.0% of the polyamide 6 in the PA6-A used as the raw material. Comparison with Example 1 reveals that by setting the pressure during the reaction to a pressure equal to or higher than the saturated vapor pressure, ε-caprolactam tends to be obtained in a high yield.

[0054] Example 7: 30.0 g of PA6-A and 60.0 g of deionized water were charged into 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 (X:1) was 2:1. The reaction vessel was purged with nitrogen and sealed under a nitrogen pressure of 0.5 MPa. The reaction was then carried out by stirring at 200 rpm at 320°C for 15 minutes. 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 the reaction was completed, the internal temperature was cooled to 40°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 (mass) approximately three times the volume of the filtered product was introduced into the autoclave, and the filtered product was rinsed three times at 40°C, recovering the filtrate and wet filtered product. The resulting filtered product was vacuum dried at 50°C for 12 hours, and 8.2 g of solids were recovered from solid-liquid separation (I). High-performance liquid chromatography analysis of the resulting solids under the conditions described in [Reference Example 1] revealed that the solids were polyamide 6 oligomers containing 97.1% by mass of linear dimer- to dodecamer-containing oligomers. Due to their high purity, these polyamide 6 oligomers can be further utilized as raw materials for depolymerization. Furthermore, high-performance liquid chromatography analysis of the filtrate obtained from solid-liquid separation (I) revealed that the filtrate contained 19.2 g of ε-caprolactam, representing a yield of 64.0% of the polyamide 6 in the PA6-A used as the raw material.

[0055] Example 8: 60.0 g of PA6-A and 120.0 g of deionized water were charged into a 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 5.0 MPa. The reaction was carried out by stirring at 200 rpm at 320°C for 15 minutes. The pressure reached during the reaction was 19.5 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 the reaction was completed, the internal temperature was cooled to 40°C, and while maintaining this temperature at 40°C, the bottom stopper valve was opened to carry out solid-liquid separation (I). When the filtration rate decreased, solid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, deionized water was introduced into the autoclave in an amount approximately three times (by mass) the filtered product, and the filtered product was rinsed three times at 40°C, recovering the filtrate and wet filtered product. The filtered product was then vacuum dried at 50°C for 12 hours, and 8.5 g of solids were recovered from solid-liquid separation (I). High-performance liquid chromatography analysis of the resulting solids under the conditions described in [Reference Example 1] revealed that the solids were polyamide 6 oligomers containing 97.5% by mass of linear dimer- to dodecamer-containing oligomers. Due to their high purity, these polyamide 6 oligomers can be further utilized as raw materials for depolymerization. Furthermore, high-performance liquid chromatography analysis of the filtrate obtained from solid-liquid separation (I) revealed that the filtrate contained 44.8 g of ε-caprolactam, representing a yield of 74.7% relative to the polyamide 6 content of PA6-A used as the raw material. Comparison with Example 7 reveals that the yield of ε-caprolactam produced tends to improve by increasing the pressure during the reaction to a pressure equal to or higher than the saturated vapor pressure.

[0056] Example 9: This example describes the hot water extract from the PA6 production process, obtained by the method described in Reference Example 3, concentrated to a concentration of unreacted caprolactam and polyamide 6 oligomer of 6.5% by mass and used as the depolymerization feedstock. 17.6 g of PA6-A 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 concentrate, adjusted to a caprolactam and polyamide 6 oligomer concentration of 6.5% by mass, was then added. The mass ratio (X:1) of water to polyamide 6 and polyamide 6 oligomer was 3.3:1. The reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 0.5 MPa, and 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 1062. Furthermore, since the residence time at a reaction temperature of 320°C is 15 minutes, the product of X, Y, and Z is 15,930. After the reaction is completed, the internal temperature is cooled to 40°C, and while maintaining this temperature, the bottom stopper valve is opened to perform solid-liquid separation (I). When the filtration rate decreases, solid-liquid separation (I) is 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 40°C, and the filtrate and wet filtered product were recovered. The obtained filtered product was subjected to vacuum drying at 50°C for 12 hours, and 2.6 g of solids obtained by solid-liquid separation (I) was recovered. The solid obtained was analyzed by high-performance liquid chromatography under the conditions described in [Reference Example 1]. It was found to be a polyamide 6 oligomer containing 96.5% by mass of linear dimer to dodecamer oligomers. Because of its high purity, this polyamide 6 oligomer can be further utilized as a raw material for depolymerization. Furthermore, high-performance liquid chromatography analysis of the filtrate obtained by solid-liquid separation (I) revealed that the filtrate contained 15.0 g of ε-caprolactam, a yield of 85.2% relative to the PA6-A used as the raw material.

[0057] Example 10: This example describes the hot water extract from the PA6 production process, obtained by the method described in Reference Example 3, concentrated to a total concentration of unreacted caprolactam and polyamide 6 oligomer of 6.5% by mass and used as the depolymerization feedstock. 20.0 g of PA6-A and 65.0 g of the concentrate, adjusted to a caprolactam and polyamide 6 oligomer concentration of 6.5% by mass, 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 (X:1) of water to the total of polyamide 6 and polyamide 6 oligomer was 2.9:1. 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 933. Furthermore, since the residence time at a reaction temperature of 320 ° C. is 15 minutes, the product of X, Y, and Z is 13,995. After the reaction is completed, the internal temperature is cooled to 40 ° C., and while maintaining the temperature at 40 ° C., the bottom stopper valve is opened to perform solid-liquid separation (I). When the filtration rate decreases, solid-liquid separation (I) is 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 40 ° C., and the filtrate and wet filtered product were recovered. The obtained filtered product was subjected to vacuum drying at 50 ° C. for 12 hours, and 2.4 g of solids obtained by solid-liquid separation (I) was recovered. The solid obtained was analyzed by high-performance liquid chromatography under the conditions described in [Reference Example 1]. It was found to be a polyamide 6 oligomer containing 96.5% by mass of linear dimer to dodecamer oligomers. Because of its high purity, this polyamide 6 oligomer can be further utilized as a raw material for depolymerization. Furthermore, high-performance liquid chromatography analysis of the filtrate obtained by solid-liquid separation (I) revealed that the filtrate contained 15.7 g of ε-caprolactam, a yield of 78.5% relative to the PA6-A used as the raw material.

Claims

1. A method for recovering ε-caprolactam and polyamide 6 oligomer, comprising adding and contacting a resin composition (A) containing at least polyamide 6 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 reaction mixture (C) containing at least ε-caprolactam, polyamide 6 oligomer, and water, and subjecting the reaction mixture (C) to solid-liquid separation (I) in a temperature range not higher than the boiling point of water under an operating pressure, thereby separating and recovering the polyamide 6 oligomer into a solid phase and the aqueous ε-caprolactam solution into a liquid phase, The reaction mixture (C) is prepared under the conditions that the product of X and Y is 300 or more and 2,000 or less, where the mass ratio of water to polyamide 6 or the mass ratio of water to the sum of polyamide 6 and polyamide 6 oligomer is X:1 and the reaction temperature is Y°C. A method for recovering ε-caprolactam and polyamide 6 oligomers, comprising:

2. A resin composition (A) containing at least polyamide 6 is added to water (B) heated to 290°C or higher and 350°C or lower, or to 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 is further added and contacted to obtain a reaction mixture (C) containing at least ε-caprolactam, polyamide 6 oligomer, and water.

2. The method for recovering ε-caprolactam and polyamide 6 oligomers according to claim 1.

3. The polyamide 6 oligomer recovered by the solid-liquid separation (I) is used as an aqueous polyamide 6 oligomer solution (B1).

3. The method for recovering ε-caprolactam and polyamide 6 oligomer according to claim 1 or 2.

4. The polyamide 6 oligomer contains 90% by mass or more of linear dimer to dodecamer oligomers. The method for recovering ε-caprolactam and polyamide 6 oligomer according to any one of claims 1 to 3.

5. The operating pressure of the solid-liquid separation (I) is normal pressure. The method for recovering ε-caprolactam and polyamide 6 oligomer according to any one of claims 1 to 4.

6. 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. The method for recovering ε-caprolactam and polyamide 6 oligomer according to any one of claims 1 to 5.

7. After preparing a reaction mixture (C) containing at least ε-caprolactam, polyamide 6 oligomer, and water, solid-liquid separation (I) is subsequently carried out. The method for recovering ε-caprolactam and polyamide 6 oligomer according to any one of claims 1 to 6.

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

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