Method of producing thermoplastic resin

Abstract

Provided is a method for producing a thermoplastic resin by using ε-caprolactam obtained by performing depolymerization of a waste product of a polyamide 6 resin composition molded article by using only a small amount of water, etc., and using less energy. The present invention pertains to a method that is for producing a thermoplastic resin and that involves obtaining ε-caprolactam through steps (a) and (b) by using as a material a waste product (A) of a resin molded article at least containing polyamide 6, and polymerizing a material containing the obtained ε-caprolactam. (a) A step for adding the waste product (A) of the resin molded article to, so as to bring into contact with, water (B) heated to 290-350°C or a polyamide 6 oligomer aqueous solution (B1) heated to 290-350°C. (b) A step for separating the reaction mixture obtained in the step (a) into solid matter and an ε-caprolactam aqueous solution through solid-liquid separation (I).

Description

Thermoplastic resin manufacturing method

[0001] The present invention relates to a method for producing a thermoplastic resin from waste molded polyamide 6 resin compositions via ε-caprolactam, which achieves both resource recycling and a reduction in greenhouse gas emissions. The method involves depolymerizing waste molded polyamide 6 resin compositions using only a small amount of water, which has a high specific heat capacity and a high heat of vaporization, and producing a thermoplastic resin using the obtained high-purity ε-caprolactam.

[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 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. While 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 plant corrosion, 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 ε-caprolactam by blowing superheated steam 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 superheated 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 by 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 that the yield significantly decreased. This is thought to be due to deactivation of the phosphoric acid catalyst by the potassium salt.

[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.

[0008] In order to solve the above problems, the present invention has the following configuration: 1. A method for producing a thermoplastic resin, which comprises using waste resin moldings (A) containing at least polyamide 6 as a raw material, obtaining ε-caprolactam through the following steps (a) and (b), and polymerizing the raw material containing ε-caprolactam: (a) a step of contacting the waste resin moldings (A) with 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, and (b) a step of separating the reaction mixture obtained in step (a) into a solid and an aqueous solution containing ε-caprolactam by solid-liquid separation (I). 3. The method for producing a thermoplastic resin according to item 1, wherein the step (a) is a step of adding waste resin moldings (A) 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. 3. The method for producing a thermoplastic resin according to item 1 or 2, wherein the solid-liquid separation (I) includes a step (b1) of separating the reaction mixture by solid-liquid separation into a non-molten material and an aqueous solution containing at least ε-caprolactam and polyamide 6 oligomer, and a step (b2) of separating the filtrate obtained in step (b1) by solid-liquid separation into a polyamide 6 oligomer and an aqueous ε-caprolactam solution. 4. The method for producing a thermoplastic resin according to any one of items 1 to 3, characterized in that the polyamide 6 oligomer separated in step (b) or (b2) is mixed with water, and the mixture is heated to 290° C. or higher and 350° C. to obtain an aqueous polyamide 6 oligomer solution (B1), which is used in step (a). 5. The method for producing a thermoplastic resin according to any one of items 1 to 4, characterized in that the aqueous polyamide 6 oligomer solution (B1) is an extract obtained in a step of hot-water extraction of polyamide 6 oligomer from polyamide 6, which is a product of polyamide 6 production.6. The method for producing a thermoplastic resin according to any one of items 1 to 5, wherein in step (a), the contacting is carried out under conditions where the mass ratio of water to polyamide 6 in the resin molding waste (A) or the mass ratio of water to the total of polyamide 6 and polyamide oligomer in the resin molding waste (A) is X:1 and the reaction temperature is Y°C, such that the product of X and Y is 2,000 or less.

[0009] The present invention provides a method for producing a thermoplastic resin using high-purity ε-caprolactam obtained by depolymerizing waste polyamide 6 resin composition molded articles in an energy-saving manner using only a small amount of water or a polyamide 6 oligomer aqueous solution containing water as a solvent, which has a high specific heat capacity and a high heat of vaporization.

[0010] The present invention will be described in further detail below. (1) Waste of Resin Molded Products (A) The present invention relates to a method for producing a thermoplastic resin, which comprises obtaining ε-caprolactam from waste of resin molded products (A) containing at least polyamide 6 as a raw material, and then polymerizing the raw material containing ε-caprolactam. 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 also be copolymerized within the scope of the present invention, provided that the objectives of the present invention are not impaired. 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 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 ε-caprolactam.

[0011] Examples of other copolymerizable monomers include amino acids such as 11-aminoundecanoic acid, 12-aminododecanoic acid, and para-aminomethylbenzoic acid; lactams such as ω-lactam; 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.

[0012] 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.

[0013] 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 having 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.

[0014] The polyamide 6 of the present invention may contain a cyclic oligomer represented by the following formula (a). There are no particular restrictions on the amount of the cyclic oligomer represented by the following formula (a) contained in the polyamide 6, 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 represented by the following formula (a), m is an integer of 2 to 4. The cyclic oligomer represented by the following formula (a) melts and volatilizes, causing line blockages and the like. Therefore, by keeping the amount of the cyclic oligomer within a preferred range, line blockages due to melting and volatilization tend to be suppressed. Note that cyclic oligomers represented by the following formula (a) where m is 5 or more are not of interest in the present invention, taking into account the degree of volatilization.

[0015] The present invention uses waste resin moldings containing at least polyamide 6 as a raw material. The waste resin moldings used herein may be any waste resin moldings containing at least polyamide 6. Examples of waste resin moldings containing polyamide 6 include polyamide 6 products, industrial waste generated during the manufacturing process of polyamide 6 products, and post-consumer waste polyamide 6 products. Examples of polyamide 6 products include textile structures for clothing, such as used clothing, uniforms, sportswear, and underwear; industrial textile structures, such as curtains, carpets, ropes, nets, belts, and sheets; molded parts for housing construction materials; electrical and electronic molded parts; aircraft parts; industrial machinery 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.

[0016] The resin molding waste (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 of its easy availability, excellent dispersibility in polyamide 6, high reactivity with radicals, and improved 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.

[0017] The alkali metal halide is preferably blended in an amount of 0.01 to 1 part by mass per 100 parts by mass of polyamide 6 in the resin molded waste (A). By blending the alkali metal halide in this preferred range, side reactions other than hydrolysis in this process can be suppressed, and the lactam yield tends to be higher. The amount of 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.

[0018] The resin molded waste (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 molded waste (A).

[0019] The waste resin molding (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.

[0020] 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.

[0021] Specific examples of thermoplastic resins other than polyamide 6 contained in the resin molding waste (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. Furthermore, 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 resin molding waste (A) of the present invention.

[0022] (2) Thermoplastic Resin The present invention is a method for producing a thermoplastic resin, which comprises obtaining ε-caprolactam using waste resin moldings (A) containing at least polyamide 6 as a raw material, and polymerizing the raw material containing the ε-caprolactam.

[0023] The thermoplastic resin here may be any thermoplastic resin obtained by polymerizing a raw material containing ε-caprolactam, and may be, for example, polyamide 6 made mainly from ε-caprolactam, a copolymer made mainly from ε-caprolactam with a monomer other than ε-caprolactam, or a block copolymer with a polyalkylene glycol such as polyethylene glycol. Examples of monomers other than ε-caprolactam include amino acids such as 11-aminoundecanoic acid, 12-aminododecanoic acid, and para-aminomethylbenzoic acid; lactams such as ω-lyrolactam; 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 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.

[0024] Of the total 100 mol % of monomer units constituting the thermoplastic resin, the total content of units derived from ε-caprolactam is preferably 50 mol % or more, more preferably 70 mol % or more, and even more preferably 90 mol % or more.

[0025] Since the present invention relates to a method for depolymerizing waste resin moldings containing at least polyamide 6, it is preferable that the thermoplastic resin be polyamide 6 from the viewpoint of repeatedly recycling fossil resources.

[0026] Although there are no particular limitations on the degree of polymerization of the thermoplastic resin of the present invention, it is preferable that the relative viscosity measured at 25° C. in a 98% concentrated sulfuric acid solution having a resin concentration of 0.01 g / mL is in the range of 1.5 to 5.0. When the relative viscosity of the thermoplastic resin is in this preferred range, it tends to be possible to achieve all of the strength, rigidity, and toughness that are characteristic of thermoplastic resins obtained using ε-caprolactam as the main raw material, and therefore this range can be cited as a preferred range.

[0027] (3) Polyamide 6 Oligomer The polyamide 6 oligomer used in the present invention is a polyamide 6 oligomer whose main constituent is 6-aminocaproic acid and / or ε-caprolactam. It 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.

[0028] Examples of other monomers contained in the polyamide 6 oligomer 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 bis( Aliphatic diamines such as bis(3-methyl-4-aminocyclohexyl)methane, bis(3-methyl-4-aminocyclohexyl)methane, 2,2-bis(4-aminocyclohexyl)propane, bis(aminopropyl)piperazine, and aminoethylpiperazine; 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, 2,6-naphthalenedicarboxylic acid, hexahydroterephthalic acid, and hexahydroisophthalic 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.

[0029] 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.

[0030] 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 dimers to dodecamers contained 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 of linear polyamide 6 oligomers having dimers to dodecamers contained in the polyamide 6 oligomer ...5% by mass or more. When the content of linear polyamide 6 oligomers having dimers to dodecamers contained 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.

[0031] The method for preparing the polyamide 6 oligomer used in the present invention is not particularly limited. For example, polyamide 6 oligomers contained in the extract obtained by hot water extraction of polyamide 6 resins during the production of conventional fatty acid-based polyamide 6 resins, or polyamide 6 oligomers prepared by the same method as the synthesis of conventional fatty acid-based polyamide 6 resins, may be used. Furthermore, polyamide 6 oligomers obtained as a by-product during the production of ε-caprolactam by adding a resin composition containing at least polyamide 6 to at least one of water heated to 290°C to 350°C or an aqueous polyamide 6 oligomer solution may also be used. Furthermore, the polyamide 6 oligomer may be a polyamide 6 oligomer obtained as a by-product during the production of ε-caprolactam by contacting a waste resin molding (A) containing polyamide 6 with at least one of water heated to 290°C to 350°C or an aqueous polyamide 6 oligomer solution (B1) heated to 290°C to 350°C as a polyamide 6-containing resin composition. From the viewpoint of reducing industrial waste generated during the production of ε-caprolactam, it is preferable to use a polyamide 6 oligomer obtained as a by-product during the production of ε-caprolactam by contacting a waste (A) of resin molded articles containing polyamide 6 with water (B) heated to 290°C or higher and 350°C or lower, or a polyamide 6 oligomer recovered as a by-product during the production of ε-caprolactam by adding a waste (A) of resin molded articles containing at least polyamide 6 to at least one of water heated to 290°C or higher and 350°C or lower and an aqueous polyamide 6 oligomer solution.

[0032] (4) 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 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 salt, 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 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. Furthermore, an extract containing polyamide 6 oligomer obtained in the process of hot water extraction of polyamide 6 oligomer 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 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.

[0033] (5) Non-Melted Materials In the present invention, the term "non-melted materials" refers to any non-melted materials contained in the waste resin molding (A) containing at least polyamide 6. Specifically, non-melted materials are those present in a solid state in the reaction mixture when the waste resin molding (A) is depolymerized by adding at least one of water (B) heated to 290°C to 350°C or an aqueous polyamide 6 oligomer solution (B1). More specifically, non-melted materials include fillers such as fibrous fillers, non-fibrous fillers, and elastomers used in producing polyamide 6-containing resin moldings, as well as metal and rubber parts contained in the waste resin molding (A) containing polyamide 6. When the depolymerization of polyamide 6 is performed by adding at least one of water (B) heated to 290°C to 350°C or an aqueous polyamide 6 oligomer solution (B1), these non-melted materials remain in a solid state in the reaction mixture and can be separated from ε-caprolactam and polyamide 6 oligomer by solid-liquid separation.

[0034] (6) Step (a) The method for producing a thermoplastic resin composition of the present invention includes step (a) of adding and contacting waste resin moldings (A) 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. The water used in the water (B) or aqueous polyamide 6 oligomer solution (B1) is not particularly limited, 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 due to the influence of coexisting salts.

[0035] In step (a), the reaction substrate is water (B) heated to 290°C to 350°C or water in an aqueous polyamide 6 oligomer solution (B1) heated to 290°C to 350°C. Water reaches a state that is neither liquid nor gas 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 is subcritical water, and the aqueous polyamide 6 oligomer solution is subcritical water in which polyamide 6 oligomer is dissolved. 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 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) and the 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, when at least one of the water (B) and the polyamide 6 oligomer aqueous solution (B1) is added, the water pressure is preferably 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 water pressure is preferably higher than the saturated vapor pressure. The upper limit of the water pressure is not particularly limited, but 20 MPa or lower is an example. This pressure range is preferred because the ionic product of water described above tends to be higher. To achieve this pressure range for 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) and 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.

[0036] In addition, there is no particular limitation on the amount of water used in the water (B) and the aqueous polyamide 6 oligomer solution (B1), but 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 in the waste resin molding (A), the mass ratio of the total water in the 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 water to 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 preferably 1,600 or less, more preferably 1,300 or less, and particularly preferably 1,200 or less. In addition, there is no particular limitation 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 a method for recovering ε-caprolactam from waste resin moldings containing polyamide 6 with energy conservation, and producing a thermoplastic resin therefrom, 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, and by ensuring that the product of X and Y is within these preferred ranges, both the efficiency of ε-caprolactam production and energy conservation can be achieved.

[0037] Furthermore, assuming a residence time of Z minutes at a reaction temperature of Y°C, the product of X, Y, and Z is preferably 60,000 or less. More preferably, it is 40,000 or less, even more preferably 30,000 or less, and particularly preferably 20,000 or less. There is no particular lower limit to the product of X, Y, and Z, but a condition of 5,000 or more is preferred, with 8,000 or more being more preferred, and 9,000 or more being 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 energy-saving production efficiency of ε-caprolactam and thermoplastic resins. 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. Simply reducing the amount of water used results in the production of large amounts 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 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.

[0038] The reaction method for contacting the resin-molded waste (A) with at least one of water (B) and the aqueous polyamide 6 oligomer solution (B1) can be performed using various known reaction methods, such as batch and continuous systems. Examples of batch systems include autoclaves equipped with a stirrer and a heating function, vertical and horizontal reactors, and vertical and horizontal reactors equipped with a stirrer, a heating function, and a compression mechanism such as a cylinder. Examples of continuous systems 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 an inert atmosphere such as nitrogen, helium, or argon. A nitrogen atmosphere is preferred for economical efficiency and ease of handling.

[0039] (7) Step (b) The present invention includes a step (b) of separating the reaction mixture containing the polyamide 6 oligomer obtained in step (a) and an aqueous ε-caprolactam solution into a solid and an aqueous ε-caprolactam solution by solid-liquid separation (I). Here, the solid includes the non-melt material and precipitated polyamide 6 oligomer.

[0040] The temperature at which the solid-liquid separation (I) is carried out may be in any temperature range as long as it allows separation of the solid matter from the aqueous ε-caprolactam solution, but is preferably carried out in a temperature range below the boiling point of water at the operating pressure, and is preferably a temperature below the boiling point of water at normal pressure. When the waste material (A) of a resin molded product contains non-melt materials, the temperature at which the solid-liquid separation (I) is carried out is more preferably 95°C or lower, and even more preferably 90°C or lower. When the waste material (A) of a resin molded product does not contain non-melt materials, the temperature at which the solid-liquid separation (I) is carried out is 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 the solid-liquid separation (I) is carried out, but is preferably 10°C or higher, more preferably 15°C or higher, and even more preferably 20°C or higher. When the resin molding waste (A) contains non-melt material, the non-melt material exists as a solid, and by performing solid-liquid separation (I) at a temperature at which the polyamide 6 oligomer dissolves in water, most of the non-melt material can be separated as a solid component. When the resin molding waste (A) does not contain non-melt material, ε-caprolactam dissolves in water within the above temperature range, but polyamide 6 oligomer tends to be less soluble in water. Therefore, by performing solid-liquid separation (I) within the above preferred temperature range, most of the polyamide 6 oligomer can be separated as a solid component, making it possible to recover high-purity ε-caprolactam.

[0041] 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 reaction mixture subjected to the filtration, the size of the non-melt material, 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 non-melt material in the reaction mixture to be recovered as a solid phase component by solid-liquid separation (I).

[0042] In addition, when carrying out solid-liquid separation (I) with polyamide 6 oligomer as the filtered product, the filter material used must be able to separate polyamide 6 oligomer, and at least the solution containing ε-caprolactam and water can pass through.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 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 centrifugal separator, 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.

[0043] It is preferable that the mother liquor adhering to the solid fraction separated into solid and liquid 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 removed from the solid fraction.

[0044] In addition, there is no particular limitation to the atmosphere when carrying out solid-liquid separation (I), but when the polyamide 6 oligomer separated as solid content is oxidized and deteriorated due to the conditions such as contact time and temperature, it is preferable to carry out under non-oxidizing atmosphere.In addition, non-oxidizing atmosphere refers to the atmosphere in which the oxygen concentration of gas phase is 5% by volume or less, preferably 2% by volume or less, more preferably does not contain oxygen substantially, that is, inert gas atmosphere such as nitrogen, helium, argon, etc.Among these, it is particularly preferable to carry out under nitrogen atmosphere from the viewpoint of economic efficiency and ease of handling.

[0045] The solid-liquid separation (I) of the present invention can be carried out by reheating the reaction mixture obtained in step (a) to a temperature at which the solid-liquid separation (I) is carried out, or by cooling the reaction mixture after step (a) to a temperature at which the solid-liquid separation (I) is carried out. Preferably, the solid-liquid separation (I) can be carried out by cooling the reaction mixture after step (a) to a temperature at which the solid-liquid separation (I) is carried out.

[0046] Furthermore, when performing solid-liquid separation (I), 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 are preferably crystals of the same substance as the polyamide 6 oligomer, so polyamide 6 or polyamide 6 oligomers are preferably used as seed crystals. The use of these seed crystals promotes precipitation of the polyamide 6 oligomer, tending to increase the purity of the ε-caprolactam recovered by solid-liquid separation (I). Furthermore, although the wet-cake-like polyamide 6 oligomer recovered as a solid phase component by solid-liquid separation (I) contains water, it can be used in its wet-cake form as a raw material for the aqueous polyamide 6 oligomer solution (B1) without undergoing a drying process. Utilizing the polyamide 6 oligomer recovered by solid-liquid separation (I) as a raw material without undergoing a drying process in this way is preferable because it reduces the amount of industrial waste and also eliminates the energy required for the drying process.

[0047] When the waste (A) of the resin molded article containing polyamide 6 used in the present invention contains non-molten material, it is preferable to carry out the following steps: (b1) in which the reaction mixture obtained in step (a) is separated by solid-liquid separation into the non-molten material and an aqueous solution containing at least ε-caprolactam and polyamide 6 oligomer, and (b2) in which the filtrate obtained in step (b1) is separated by solid-liquid separation into polyamide 6 oligomer and an aqueous ε-caprolactam solution. By carrying out solid-liquid separation of the non-molten material in step (b1), the amount of polyamide 6 oligomer that is disposed of as industrial waste together with the non-molten material is reduced, and further, it is preferable to facilitate the utilization of the polyamide 6 oligomer as a raw material.

[0048] The temperature at which step (b1) is carried out may be any temperature range as long as it allows separation of the non-melt material into a solid phase and the polyamide 6 oligomer and ε-caprolactam into a liquid phase. Since the depolymerization reaction of the resin-molded waste (A) in step (a) is carried out at 290°C or higher and 350°C or lower, a temperature below these temperatures is preferred. Furthermore, the present inventors have investigated the dissolution and precipitation behavior of the polyamide 6 oligomer in water and found that, while the temperature at which polyamide 6 oligomer dissolves in water is 100°C or higher, the temperature at which polyamide 6 oligomer precipitates once melted and dissolved in water is below 100°C. This difference between the temperature at which polyamide 6 oligomer dissolves in water and the temperature at which it precipitates is due to the polyamide 6 oligomer being 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, first, polyamide 6 oligomer crystalline nuclei are generated, and the generated crystalline nuclei grow to a size large enough to precipitate, and polyamide 6 oligomer precipitates.Therefore, when cooling the aqueous solution of polyamide 6 oligomer dissolved in water, it is believed that when the temperature is cooled to less than 100 ° C, which is the temperature at which polyamide 6 oligomer dissolves in water, polyamide 6 oligomer crystalline nuclei are generated, and then the generated crystalline nuclei grow during the cooling process, and polyamide 6 oligomer precipitates.From the above results, it is more preferable to exemplify that step (b1) is 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, the lower limit temperature for carrying out step (b1) can be the temperature at which polyamide 6 oligomer is supercooled, and specifically, it is preferably 50 ° C or more, more preferably 60 ° C or more, and even more preferably 70 ° C or more. By carrying out step (b1) within such a preferred temperature range, 97% or more, preferably 98% or more, and more preferably 99% or more of ε-caprolactam and polyamide 6 oligomer can be recovered as liquid phase components, and recovery loss in step (b1) tends to be reduced.Furthermore, the step (b1) of the present invention can be exemplified by a method in which the reaction mixture prepared in the step (a) is reheated to a temperature at which the polyamide 6 oligomer dissolves, and then cooled to a temperature at which the step (b1) is carried out, thereby carrying out solid-liquid separation, or a method in which the reaction mixture is prepared in the step (a), and then cooled to a temperature at which the step (b1) is carried out, thereby carrying out solid-liquid separation. A preferred method is a method in which the reaction mixture is prepared in the step (a), and then cooled to a temperature at which the step (b1) is carried out, thereby carrying out solid-liquid separation.

[0049] The solid-liquid separation in step (b1) allows the non-melt material in the reaction mixture obtained in step (a) to be separated as a filter residue, and preferably 95% or more, more preferably 97% or more, and even more preferably 99% or more of the non-melt material contained in the mixture can be recovered as solids. Furthermore, if an aqueous solution containing ε-caprolactam and polyamide 6 oligomer adheres to the non-melt material separated in step (b1), washing the filter residue with fresh water can reduce the amount of ε-caprolactam and polyamide 6 oligomer remaining on the filter residue. The washing of the filter residue with fresh water is carried out in a temperature range below the boiling point at atmospheric pressure, preferably 95°C or less, more preferably 90°C or less. Separating the non-melt material from ε-caprolactam and polyamide 6 oligomer by solid-liquid separation and washing with water in step (b1) eliminates the need for complicated processes, making this a preferable method from the standpoints of process cost and environmental impact.

[0050] (8) 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.

[0051] (9) Method for Producing Thermoplastic Resin The present invention relates to a method for producing a thermoplastic resin by obtaining high-purity ε-caprolactam from waste resin moldings (A) containing polyamide 6 and polymerizing a raw material containing this ε-caprolactam. Thermoplastic resins can be produced by commonly known methods, such as by heat-melt polymerizing ε-caprolactam in the presence of a small amount of water, by heat-melt polymerizing ε-caprolactam and a copolymerizable component in the presence of a small amount of water, or by heat-melt polymerizing ε-caprolactam and a block copolymer component such as polyethylene glycol in the presence of a small amount of water. Furthermore, the thermoplastic resin obtained in this manner can be melt-kneaded, if necessary, with a fibrous filler or various additives to produce a thermoplastic resin composition, and various molded articles such as sheets and films can be obtained by commonly known methods such as injection molding or extrusion molding.

[0052] The thermoplastic resin and molded articles thereof of the present invention can be utilized to their advantage in various applications, such as electrical and electronic parts, building materials, various containers, daily necessities, household goods, and sanitary goods. In particular, they are particularly suitable for aircraft parts and electrical and electronic parts applications, which require toughness and rigidity. Specifically, they are suitable for aircraft-related parts such as landing gear pods, winglets, spoilers, edges, rudders, elevators, failings, and ribs. Examples of electrical and electronic parts include 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, IC and LED housings, capacitor base plates, fuse holders, various gears, and various cables. 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.

[0053] The present invention will be described below with reference to examples, but the present invention is not limited to these examples. The following raw materials were used in each example: Used unreinforced polyamide 6 fastener parts (the polyamide 6 content in the resin component is 99% by mass or more); Used glass fiber reinforced polyamide 6 mobile phone housings (the polyamide 6 content in the resin component is 99% by mass or more, and the glass fiber content in the resin composition is 45% by mass).

[0054] 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 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.

[0055] [Analysis of Polyamide 6 Oligomer (HPLC)] The polyamide 6 oligomer of the present invention was analyzed by high performance liquid chromatography. The measurement conditions are as follows: Apparatus: LC-10Avp series manufactured by Shimadzu Corporation 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 the polyamide 6 oligomer was calculated from the peak area ratio of each polyamide 6 oligomer.

[0056] [Analysis of Thermal Properties of Polyamide 6] The melting point of polyamide 6 here was determined using differential scanning calorimetry analysis by lowering the temperature of the polyamide from a molten state to 30° C. at a rate of 20° C. / min in a nitrogen gas atmosphere, and then heating it at a rate of 20° C. / min to the melting point plus 40° C. However, when two or more endothermic peaks were detected, the temperature of the endothermic peak with the greatest peak intensity was determined as the melting point.

[0057] [Measurement of Solution Viscosity of Polyamide 6] Here, the solution viscosity ηr of polyamide 6 was measured at 25° C. using a 0.01 g / mL solution of 98% concentrated sulfuric acid.

[0058] Reference Example 1: 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.

[0059] Example 1: Used unreinforced polyamide 6 fastener parts (with a polyamide 6 content of 99% by mass or more) were collected and placed in a crusher equipped with a 7 mm diameter screen to obtain crushed parts with an average particle size of 6 mm. Visually identifiable contaminants were then removed. 20.0 g of the crushed parts and 60.0 g of deionized water were placed in an SUS316L autoclave equipped with a stirrer, a bottom plug valve, and a glass filter (average mesh size 10 μm) at the bottom. Since the polyamide 6 content of the fastener parts was 99% by mass or more, 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 at 320°C for 15 minutes while stirring at 200 rpm. The ultimate pressure during the reaction was 10.5 MPa. Since the reaction temperature Y°C is 320°C, the product of X and Y is 960, and since the residence time at a reaction temperature of 320°C is 15 minutes, the product of X, Y, and Z is 14,400. After the reaction is completed, the internal temperature is cooled to 50°C, and while maintaining this temperature, the bottom plug 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 50°C, and the filtrate and wet filtered product were recovered.

[0060] The filtered product obtained from solid-liquid separation (I) was subjected to vacuum drying at 50°C for 12 hours, recovering 2.7 g of solids. High-performance liquid chromatography analysis of the resulting solids under the conditions described above revealed that the product was a polyamide 6 oligomer containing 96.5% by mass of linear dimers to dodecamers. Due to its high purity, this polyamide 6 oligomer can be further utilized as a raw material for depolymerization. High-performance liquid chromatography analysis of the filtrate obtained from solid-liquid separation (I) revealed that the filtrate contained 15.0 g of ε-caprolactam, a yield of 75.2% relative to the polyamide 6 in the crushed product used as the raw material. The recovered filtrate was then heated to 55°C under a reduced pressure of 30 mmHg to distillatively separate water, yielding a concentrated aqueous ε-caprolactam solution. The distilled ε-caprolactam was then recovered by further distillation at a reduced pressure of 5 mmHg and a heating temperature of 150-170°C. The concentration and distillation yield of ε-caprolactam was 95.8%. The distilled ε-caprolactam contained 0.48% HPLC impurities, indicating that it was of a quality suitable for use as a polymerization raw material for polyamide 6. Next, polymerization was carried out using the resulting ε-caprolactam as a raw material. 10 g of the recovered ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water 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 commenced. 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 a polyamide 6 resin having a melting point of 225° C. and an ηr of 2.69.

[0061] Example 2: Used glass fiber-reinforced polyamide 6 mobile phone housings (resin polyamide 6 content of 99% by mass or more, resin composition glass fiber content of 45% by mass) were collected and placed in a crusher equipped with a 7 mm diameter screen to obtain crushed pieces with an average particle size of 6 mm. Visually identifiable contaminants were removed. 36.4 g of the crushed pieces and 60.0 g of deionized water were placed in an SUS316L autoclave equipped with a stirrer, a bottom plug valve, and a glass filter (average mesh size 10 μm) at the bottom. Since the polyamide 6 content of the resin component in the mobile phone housing was 99% by mass or more and the glass fiber content of the resin composition was 45% by mass, the mass ratio of water to polyamide 6 (X:1) was 3:1. The reactor was purged with nitrogen and sealed under a nitrogen pressure of 0.5 MPa. The reaction was then carried out 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 a reaction temperature of 340°C is 15 minutes, the product of X, Y, and Z is 15,300. After the reaction is completed, the internal temperature is cooled to 50°C, and while maintaining this temperature, the bottom plug valve is opened to perform solid-liquid separation (I) of the non-melt material. 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 is introduced into the autoclave, and rinsing of the filtered product at 50°C is performed three times, and the filtrate and wet filtered product are recovered.

[0062] High-performance liquid chromatography (HPLC) analysis of the filtrate obtained from solid-liquid separation (I) revealed that the filtrate contained 14.4 g of ε-caprolactam, a yield of 71.9% relative to the polyamide 6 in the crushed material used as the raw material. Furthermore, the recovered filtrate was heated to 55°C under a reduced pressure of 30 mmHg to distill off water, yielding a concentrated ε-caprolactam aqueous solution. Further distillation was performed at a reduced pressure of 5 mmHg and a heating temperature of 150-170°C to recover distilled ε-caprolactam. 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 polymerization raw material for polyamide 6. Next, polymerization was carried out using the resulting ε-caprolactam as the raw material. 10 g of the recovered ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water 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 then 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 normal 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 225°C and ηr = 2.69.

[0063] Example 3: Used glass fiber-reinforced polyamide 6 mobile phone housings (resin polyamide 6 content of 99% by mass or more, resin composition glass fiber content of 45% by mass) were collected and placed in a crusher equipped with a 7 mm diameter screen to obtain crushed pieces with an average particle size of 6 mm. Visually identifiable contaminants were removed. 36.4 g of the crushed pieces and 60.0 g of deionized water were placed in an SUS316L autoclave equipped with a stirrer, a bottom plug valve, and a glass filter (average mesh size 10 μm) at the bottom. Since the polyamide 6 content of the resin component in the mobile phone housing was 99% by mass or more and the glass fiber content of the resin composition was 45% by mass, the mass ratio of water to polyamide 6 (X:1) was 3:1. The reactor was purged with nitrogen and sealed under a nitrogen pressure of 0.5 MPa. The reaction was then carried out 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, and 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 the reaction is completed, the internal temperature is cooled to 90°C, and while maintaining the temperature at 90°C, the bottom plug valve is opened to separate the non-melted glass fiber by solid-liquid separation (step (b1)). When the filtration rate decreased, solid-liquid separation was carried out while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, the filtered product was rinsed with deionized water heated to 90°C in an amount approximately three times (by mass) the filtered product, and the filtrate and wet filtered product were recovered.

[0064] The filtrate obtained in step (b1) was cooled to an internal temperature of 25°C and further subjected to solid-liquid separation using a glass filter with an average mesh size of 10 to 16 μm (step (b2)). The filtrate was rinsed three times with deionized water heated to 25°C, approximately three times the amount of the filtrate, and the filtrate and wet filtrate were recovered. High-performance liquid chromatography (HPLC) analysis of the filtrate obtained in step (b2) revealed that the filtrate contained 14.1 g of ε-caprolactam, representing a yield of 70.5% relative to the polyamide 6 in the crushed material used as the raw material. The recovered filtrate was then heated to 55°C under a reduced pressure of 30 mmHg to distillatively separate water, yielding a concentrated ε-caprolactam aqueous solution. The distilled ε-caprolactam was then recovered by distillation at a reduced pressure of 5 mmHg and a heating temperature of 150 to 170°C. 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 polymerization raw material for polyamide 6. Next, polymerization was carried out using the obtained ε-caprolactam as a raw material. 10 g of the recovered ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water 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 low-molecular-weight polymers. The extracted polymer was vacuum dried at 80° C. for 24 hours to obtain a polyamide 6 resin having a melting point of 225° C. and an ηr of 2.69.

[0065] The wet filter residue obtained in step (b1) was subjected to vacuum drying at 50°C for 12 hours, recovering 16.2 g of glass fibers. The recovered glass fibers were 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 fibers was evaluated based on the mass loss. The mass loss was 1.3% by mass, indicating that the recovered glass fibers were high-purity glass fibers with low amounts of organic matter adhering. The wet filter residue obtained in step (b2) was subjected to vacuum drying at 50°C for 12 hours, recovering 2.0 g of solids from solid-liquid separation (I). High-performance liquid chromatography analysis of the obtained solids under the conditions described above revealed that the polyamide 6 oligomer contained 97.6% 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.

[0066] Example 4: A used glass fiber-reinforced polyamide 6 mobile phone case (resin polyamide 6 content: 99% by weight or more, resin composition glass fiber content: 45% by weight) was collected and placed in a crusher equipped with a 7 mm diameter screen to obtain crushed pieces with an average particle size of 6 mm. Visually identifiable contaminants were then removed. 32.7 g of the crushed pieces and 27.8 g of deionized water were weighed into an SUS316L autoclave equipped with a stirrer, a bottom plug valve, and a glass filter (average mesh size: 10 μm) at the bottom. 34.5 g of a 5.8% by weight aqueous polyamide 6 oligomer solution prepared using the polyamide 6 oligomer recovered by the method described in Example 3 was then added. Since the polyamide 6 content of the resin component of the mobile phone case was 99% by weight or more and the glass fiber content of the resin composition was 45% by weight, the weight ratio (X:1) of water to polyamide 6 and polyamide 6 oligomer combined 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 vessel 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 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 the reaction is completed, the internal temperature is cooled to 90 ° C, and while maintaining the temperature at 90 ° C, the bottom stopper valve is opened to perform solid-liquid separation (step (b1)). When the filtration rate decreased, solid-liquid separation was carried out while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, the filtered product was rinsed with deionized water heated to 90 ° C in an amount approximately three times (by mass) the filtered product, and the filtrate and wet filtered product were recovered.

[0067] The filtrate obtained in step (b1) was cooled to an internal temperature of 30°C and subjected to solid-liquid separation using a glass filter with an average mesh size of 10 to 16 μm (step (b2)). The filtrate was rinsed three times with deionized water heated to 30°C, approximately three times the amount of the filtrate, and the filtrate and wet filtrate were recovered. High-performance liquid chromatography analysis of the filtrate recovered in step (b2) revealed that the filtrate contained 13.8 g of ε-caprolactam, representing a yield of 76.7% relative to the polyamide 6 in the crushed product used as the raw material. Comparison with Example 3 reveals that the use of an aqueous polyamide 6 oligomer solution during the reaction of polyamide 6 with water increases the yield of ε-caprolactam relative to polyamide 6. The recovered filtrate was then heated to 55°C under a reduced pressure of 30 mmHg to distillatively separate water, yielding a concentrated aqueous solution of ε-caprolactam. The distilled ε-caprolactam was then recovered by further distillation at a reduced pressure of 5 mmHg and a heating temperature of 150 to 170°C. The concentration and distillation yield of ε-caprolactam was 95.8%. The HPLC impurity content of the distilled ε-caprolactam was 0.48%, and the resulting product was of such quality that it could be used as a raw material for polyamide 6 polymerization.

[0068] Next, polymerization was carried out using the obtained ε-caprolactam as a raw material. 10 g of the recovered ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water 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 removed 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 polyamide 6 resin with a melting point of 225°C and ηr = 2.70. The wet filter residue obtained in step (b1) was subjected to vacuum drying at 50°C for 12 hours, recovering 14.7 g of glass fibers. The recovered glass fibers were 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 fibers was evaluated based on the mass loss. The mass loss was 1.4% by mass, indicating that the recovered glass fibers were high-purity glass fibers with low amounts of organic matter adhering. The wet filter residue obtained in step (b2) was subjected to vacuum drying at 50°C for 12 hours, recovering 2.0 g of solid matter from solid-liquid separation (I). High-performance liquid chromatography analysis of the obtained solid matter under the conditions described above revealed that the solid matter was a polyamide 6 oligomer containing 97.8% 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.

[0069] Comparative Example 1: Used non-reinforced polyamide 6 fastener parts (with a polyamide 6 content of 99% by mass or more) were collected and placed in a crusher equipped with a 7 mm diameter screen to obtain crushed parts with an average particle size of 6 mm. Visually identifiable contaminants were then removed. 20.0 g of the crushed parts and 60.0 g of deionized water were placed in an SUS316L autoclave equipped with a stirrer. Since the polyamide 6 content of the fastener parts was 99% by mass or more, 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, and the reaction mixture was then cooled and recovered. Since the reaction temperature Y°C is 320°C, the product of X and Y is 960. Furthermore, since the residence time at a reaction temperature of 320°C is 15 minutes, the product of X, Y, and Z is 14,400. High-performance liquid chromatography analysis of the resulting reaction mixture revealed that 15.0 g of ε-caprolactam was obtained, a yield of 75.2% relative to the polyamide 6 in the crushed material used as the raw material. Furthermore, the reaction mixture was heated to 55°C under a reduced pressure of 30 mmHg to distill off water, yielding a concentrated aqueous ε-caprolactam solution. Further distillation was performed at a reduced pressure of 5 mmHg and a heating temperature of 150-170°C to recover the distilled ε-caprolactam. Analysis of the distillation residue revealed that solvent-insoluble components were formed due to the long heating time at high temperatures during distillation. Further utilization was difficult, and the residue was disposed of as industrial waste. A comparison between Example 1 and Comparative Example 1 shows that by performing solid-liquid separation (I), the amount of polyamide components that will be disposed of as industrial waste among the six raw polyamide components can be significantly reduced, making this method an excellent method for recycling fossil resources.

[0070] Example 5: Used unreinforced polyamide 6 fastener parts (with a polyamide 6 content of 99% by mass or more) were collected and placed in a crusher equipped with a 7 mm diameter screen to obtain crushed parts with an average particle size of 6 mm. Visually identifiable contaminants were then removed. 45.0 g of the crushed parts and 135.0 g of deionized water were placed in an SUS316L autoclave equipped with a stirrer, a bottom plug valve, and a glass filter (average mesh size 10 μm) at the bottom. Since the polyamide 6 content of the fastener parts was 99% by mass or more, 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 at 320°C for 15 minutes while stirring at 200 rpm. The ultimate pressure during the reaction was 19.6 MPa. Furthermore, since the reaction temperature Y°C is 320°C, the product of X and Y is 960, and since the residence time at a reaction temperature of 320°C is 15 minutes, the product of X, Y, and Z is 14,400. After the reaction is completed, the internal temperature is cooled to 50°C, and while maintaining this temperature, the bottom plug 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 amount of the filtered product was introduced into the autoclave, and the filtered product was rinsed three times at 50°C, and the filtrate and wet filtered product were recovered.

[0071] The filtered product obtained by solid separation (I) was subjected to vacuum drying at 50°C for 12 hours, and 4.8 g of solid-liquid content was recovered. High-performance liquid chromatography analysis of the obtained solid content under the conditions described above revealed that it was a polyamide 6 oligomer containing 97.1% by mass of linear dimer to dodecamer. Due to 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 37.4 g of ε-caprolactam, a yield of 83.0% relative to the polyamide 6 in the crushed product used as the raw material. Comparison with Example 1 reveals that the yield of ε-caprolactam obtained tends to improve by increasing the reaction pressure above the saturated vapor pressure. The recovered filtrate was then heated to 55°C under a reduced pressure of 30 mmHg to distillatively separate water, yielding a concentrated ε-caprolactam aqueous solution. This was followed by further distillation at a reduced pressure of 5 mmHg and a heating temperature of 150-170°C to recover distilled ε-caprolactam. 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 polymerization raw material for polyamide 6. Next, polymerization was carried out using the resulting ε-caprolactam as a raw material. 10.0 g of the recovered ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water 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 reaching an internal pressure of 1.0 MPa, the internal pressure was maintained at 1.0 MPa for 3 hours. Thereafter, the internal pressure was reduced 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 225°C and ηr = 2.69.

[0072] Example 6: Used glass fiber-reinforced polyamide 6 mobile phone housings (resin polyamide 6 content of 99% by mass or more, resin composition glass fiber content of 45% by mass) were collected and placed in a crusher equipped with a 7 mm diameter screen to obtain crushed pieces with an average particle size of 6 mm. Visually identifiable contaminants were removed. 68.0 g of the crushed pieces and 112.0 g of deionized water were placed in an SUS316L autoclave equipped with a stirrer, a bottom plug valve, and a glass filter (average mesh size 10 μm) at the bottom. Since the polyamide 6 content of the resin component in the mobile phone housing was 99% by mass or more and the glass fiber content of the resin composition was 45% by mass, the mass ratio of water to polyamide 6 (X:1) was 3:1. The reactor was purged with nitrogen and sealed under a nitrogen pressure of 5.0 MPa. The reaction was then carried out by stirring at 200 rpm at 320°C for 15 minutes. The ultimate pressure during the reaction was 19.8 MPa. Furthermore, 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 90°C, and while maintaining this temperature, the bottom plug valve was opened to separate the non-melted glass fiber by solid-liquid separation (step (b1)). When the filtration rate decreased, solid-liquid separation was carried out while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, the filtered product was rinsed with deionized water heated to 90°C in an amount approximately three times (by mass) the filtered product, and the filtrate and wet filtered product were recovered.

[0073] The filtrate obtained in step (b1) was cooled to an internal temperature of 25°C and further subjected to solid-liquid separation using a glass filter with an average mesh size of 10 to 16 μm (step (b2)). The filtrate was rinsed three times with deionized water heated to 25°C, approximately three times the amount of the filtrate, to recover the filtrate and wet filtrate. High-performance liquid chromatography (HPLC) analysis of the filtrate obtained in step (b2) revealed that the filtrate contained 29.9 g of ε-caprolactam, representing a yield of 80.0% relative to the polyamide 6 in the crushed material used as the raw material. The recovered filtrate was then heated to 55°C under a reduced pressure of 30 mmHg to distillatively separate water, yielding a concentrated aqueous ε-caprolactam solution. The distilled ε-caprolactam was then recovered by distillation at a reduced pressure of 5 mmHg and a heating temperature of 150 to 170°C. 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 polymerization raw material for polyamide 6. Next, polymerization was carried out using the obtained ε-caprolactam as a raw material. 10.0 g of the recovered ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water 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 low-molecular-weight polymers. The extracted polymer was vacuum dried at 80°C for 24 hours to obtain a polyamide 6 resin with a melting point of 225°C and ηr = 2.69. The wet filtered product obtained in step (b1) was vacuum dried at 50°C for 12 hours to recover 30.6 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 based on the mass loss. The mass loss was 1.2 mass%, indicating that the recovered glass fiber was a high-purity glass fiber with a low amount of organic matter adhering.Furthermore, the wet filtered product obtained in step (b2) was subjected to vacuum drying at 50°C for 12 hours, and 3.6 g of solids from solid-liquid separation (I) were recovered. The obtained solids were analyzed by high-performance liquid chromatography under the conditions described above, and were found to be polyamide 6 oligomers containing 97.4% by mass of linear dimer- to dodecamer oligomers. Because this polyamide 6 oligomer has a high purity, it can be further utilized as a raw material for depolymerization.

[0074] Example 7: Used unreinforced polyamide 6 fastener parts (with a polyamide 6 content of 99% by mass or more) were collected and placed in a crusher equipped with a 7 mm diameter screen to obtain crushed parts with an average particle size of 6 mm. Visually identifiable contaminants were then removed. 30.0 g of the crushed parts and 60.0 g of deionized water were placed in an SUS316L autoclave equipped with a stirrer, a bottom plug valve, and a glass filter (average mesh size 10 μm) at the bottom. Since the polyamide 6 content of the fastener parts was 99% by mass or more, 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 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 640, and since the residence time at a reaction temperature of 320°C is 15 minutes, the product of X, Y, and Z is 9,600. After the reaction is completed, the internal temperature is cooled to 50°C, and while maintaining this temperature, the bottom plug 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 amount of the filtered product was introduced into the autoclave, and the filtered product was rinsed three times at 50°C, and the filtrate and wet filtered product were recovered.

[0075] The residue obtained from solid-liquid separation (I) was vacuum dried at 50°C for 12 hours, recovering 6.5 g of solids. High-performance liquid chromatography (HPLC) analysis of the resulting solids under the conditions described above revealed that the resulting solids were polyamide 6 oligomers containing 97.5% by mass of linear dimers to dodecamers. Due to its high purity, this polyamide 6 oligomer can be further utilized as a raw material for depolymerization. High-performance liquid chromatography analysis of the filtrate obtained from solid-liquid separation (I) revealed that the filtrate contained 19.5 g of ε-caprolactam, a yield of 65.0% relative to the polyamide 6 in the crushed material used as the raw material. The recovered filtrate was then heated to 55°C under a reduced pressure of 30 mmHg to distillatively separate water, yielding a concentrated aqueous ε-caprolactam solution. This was then further distilled at a reduced pressure of 5 mmHg and a heating temperature of 150-170°C to recover the distilled ε-caprolactam. 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 polymerization raw material for polyamide 6. Next, polymerization was carried out using the obtained ε-caprolactam as a raw material. 10.0 g of the recovered ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water 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 hot water at 95°C 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 225°C and ηr = 2.69.

[0076] Example 8: Used unreinforced polyamide 6 fastener parts (with a polyamide 6 content of 99% by mass or more) were collected and placed in a crusher equipped with a 7 mm diameter screen to obtain crushed parts with an average particle size of 6 mm. Visually identifiable contaminants were then removed. 60.0 g of the crushed parts and 120.0 g of deionized water were placed in an SUS316L autoclave equipped with a stirrer, a bottom plug valve, and a glass filter (average mesh size 10 μm) at the bottom. Since the polyamide 6 content of the fastener parts was 99% by mass or more, 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 at 320°C for 15 minutes while stirring at 200 rpm. The ultimate pressure during the reaction was 19.6 MPa. Furthermore, since the reaction temperature Y°C is 320°C, the product of X and Y is 640, and 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 is completed, the internal temperature is cooled to 50°C, and while maintaining this temperature, the bottom plug 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 amount of the filtered product was introduced into the autoclave, and the filtered product was rinsed three times at 50°C, and the filtrate and wet filtered product were recovered.

[0077] The residue obtained by solid-liquid separation (I) was subjected to vacuum drying at 50°C for 12 hours, and 9.2 g of solids were recovered. High-performance liquid chromatography analysis of the obtained solids under the conditions described above revealed that the solids were polyamide 6 oligomers containing 97.1% by mass of linear dimers to dodecamers. Due to 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 45.0 g of ε-caprolactam, a yield of 75.0% relative to the polyamide 6 in the crushed product used as the raw material. Comparison with Example 7 reveals that the yield of ε-caprolactam tends to improve when the reaction pressure is increased above the saturated vapor pressure. The recovered filtrate was then heated to 55°C under a reduced pressure of 30 mmHg to distillatively separate water, yielding a concentrated ε-caprolactam aqueous solution. This was followed by further distillation at a reduced pressure of 5 mmHg and a heating temperature of 150-170°C to recover distilled ε-caprolactam. 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 polymerization raw material for polyamide 6. Next, polymerization was carried out using the resulting ε-caprolactam as a raw material. 10.0 g of the recovered ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water 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. Thereafter, the internal pressure was reduced 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 225°C and ηr = 2.69.

[0078] Example 9: Used glass fiber-reinforced polyamide 6 mobile phone housings (resin with a polyamide 6 content of 99% by weight or more, resin composition with a glass fiber content of 45% by weight) were collected and placed in a crusher equipped with a 7 mm diameter screen to obtain crushed pieces with an average particle size of 6 mm. Visually detectable contaminants were then removed. 60.0 g of the crushed pieces and 48.9 g of deionized water were weighed into an SUS316L autoclave equipped with a stirrer, a bottom plug valve, and a glass filter (average mesh size 10 μm) at the bottom. 67.7 g of a 6.5% by weight aqueous polyamide 6 oligomer solution prepared using the polyamide 6 oligomer recovered by the method described in Example 3 was then added. Since the polyamide 6 content of the resin component of the mobile phone housing was 99% by weight or more and the glass fiber content of the resin composition was 45% by weight, the weight ratio (X:1) of water to polyamide 6 and polyamide 6 oligomer combined 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 at 320 ° C for 15 minutes while stirring at 200 rpm. At this time, the pressure reached during the reaction was 19.6 MPa. Furthermore, since the reaction temperature Y ° C was 320 ° C, the product of X and Y was 960, and 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 90 ° C, and while maintaining it at 90 ° C, the bottom stopper valve was opened to perform solid-liquid separation (step (b1)). When the filtration rate decreased, solid-liquid separation was carried out while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, the filtered product was rinsed with deionized water heated to 90 ° C in an amount approximately three times (by mass) the filtered product, and the filtrate and wet filtered product were recovered.

[0079] The filtrate obtained in step (b1) was cooled to an internal temperature of 50°C and subjected to solid-liquid separation using a glass filter with an average mesh size of 10 to 16 μm (step (b2)). The filtrate was rinsed three times with deionized water heated to 50°C in an amount approximately three times the volume of the filtrate, and the filtrate and wet filtrate were recovered. High-performance liquid chromatography analysis of the filtrate recovered in step (b2) revealed that the filtrate contained 27.1 g of ε-caprolactam, representing a yield of 82.0% relative to the polyamide 6 in the crushed product used as the raw material. Comparison with Example 4 reveals that the yield of ε-caprolactam obtained tends to improve when the reaction pressure is increased to a pressure equal to or higher than the saturated vapor pressure. The recovered filtrate was then heated to 55°C under a reduced pressure of 30 mmHg to distillatively separate water, yielding a concentrated ε-caprolactam aqueous solution. This was followed by further distillation at a reduced pressure of 5 mmHg and a heating temperature of 150-170°C to recover distilled ε-caprolactam. 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 polymerization raw material for polyamide 6. Next, polymerization was carried out using the resulting ε-caprolactam as a raw material. 10.0 g of the recovered ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water 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. Thereafter, the internal pressure was reduced 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 removed 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 225°C and ηr = 2.70. The wet filtered product obtained in step (b1) was vacuum dried at 50°C for 12 hours to recover 27.1 g of glass fibers.The recovered glass fibers were 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 fibers was evaluated based on the mass loss. The mass loss was 1.4% by mass, indicating that the recovered glass fibers were high-purity glass fibers with a low amount of organic matter adhering. Furthermore, the wet filter residue obtained in step (b2) was subjected to vacuum drying at 50 °C for 12 hours, and 6.3 g of solid matter from solid-liquid separation (I) was recovered. High-performance liquid chromatography analysis of the obtained solid matter under the conditions described above revealed that it was a polyamide 6 oligomer containing 97.9% 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.

[0080] Example 10: This example describes the hot water extract from the PA6 production process, obtained by the method described in Reference Example 1, concentrated to a concentration of 5.8% by mass of unreacted ε-caprolactam and polyamide 6 oligomer, and used as a depolymerization feedstock. Used glass fiber-reinforced polyamide 6 mobile phone housings (resin containing at least 99% by mass of polyamide 6, and 45% by mass of glass fiber in the resin composition) were collected and placed in a crusher equipped with a 7 mm diameter screen to obtain crushed pieces with an average particle size of 6 mm. Visually identifiable contaminants were then removed. 32.7 g of the crushed pieces and 27.8 g of deionized water were weighed into a SUS316L autoclave equipped with a stirrer, a bottom plug valve, and a glass filter (average mesh size 10 μm) at the bottom. 34.5 g of the concentrated solution, adjusted to a concentration of 5.8% by mass of ε-caprolactam and polyamide 6 oligomer, was then added. Since the polyamide 6 content in the resin component of the mobile phone housing is 99% by mass or more and the glass fiber content in the resin composition is 45% by mass, the total mass ratio (X:1) of water to polyamide 6 and polyamide 6 oligomer is 3.3:1. 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 and maintaining the temperature at 320°C for 15 minutes. Since the reaction temperature Y°C is 320°C, the product of X and Y is 1,050. Furthermore, since the residence time at the reaction temperature of 320°C is 15 minutes, the product of X, Y, and Z is 15,740. After the reaction was completed, the internal temperature was cooled to 90°C, and while maintaining the temperature at 90°C, the bottom stop valve was opened to carry out solid-liquid separation (step (b1)). When the filtration rate decreased, solid-liquid separation was carried out by introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, the filtered product was rinsed with deionized water heated to 90° C. in an amount (mass) that was about three times the amount of the filtered product, and the filtrate and the wet filtered product were recovered.

[0081] The filtrate obtained in step (b1) was cooled to an internal temperature of 50°C and subjected to solid-liquid separation using a glass filter with an average mesh size of 10 to 16 μm (step (b2)). The filtrate was rinsed three times with deionized water heated to 50°C in an amount approximately three times the amount of the filtrate, and the filtrate and wet filtrate were recovered. High-performance liquid chromatography analysis of the filtrate recovered in step (b2) revealed that the filtrate contained 13.6 g of ε-caprolactam, representing a yield of 75.6% relative to the polyamide 6 in the crushed product used as the raw material. Comparison with Example 4 reveals that the extract from the polyamide 6 polymerization step can also be used without problem as a polyamide 6 oligomer aqueous solution. The recovered filtrate was then heated to 55°C under a reduced pressure of 30 mmHg to distillatively separate water, yielding a concentrated ε-caprolactam aqueous solution. This was followed by further distillation at a reduced pressure of 5 mmHg and a heating temperature of 150-170°C to recover distilled ε-caprolactam. 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 polymerization raw material for polyamide 6. Next, polymerization was carried out using the resulting ε-caprolactam as a raw material. 10.0 g of the recovered ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water 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. Thereafter, the internal pressure was reduced 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 removed 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 225°C and ηr = 2.70. The wet filtered product obtained in step (b1) was vacuum dried at 50°C for 12 hours to recover 14.7 g of glass fibers.The recovered glass fibers were 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 fibers was evaluated based on the mass loss. The mass loss was 1.3 mass%, indicating that the recovered glass fibers were high-purity glass fibers with a low amount of organic matter adhering. Furthermore, the wet filter residue obtained in step (b2) was subjected to vacuum drying at 50 °C for 12 hours, and 1.8 g of solid matter from solid-liquid separation (I) was recovered. High-performance liquid chromatography analysis of the obtained solid matter under the conditions described above revealed that it was a polyamide 6 oligomer containing 97.4 mass% linear dimer-12-mer oligomers. Due to its high purity, this polyamide 6 oligomer can be further utilized as a raw material for depolymerization.

[0082] Example 11: This example describes the use of a hot water extract from a PA6 production process, obtained by the method described in Reference Example 1, concentrated to a combined concentration of unreacted ε-caprolactam and polyamide 6 oligomer of 6.5% by mass. The resulting concentrate was used as a depolymerization feedstock. Used glass fiber-reinforced polyamide 6 mobile phone housings (resin containing at least 99% polyamide 6 by mass, and the resin composition containing 45% glass fiber) were collected and placed in a crusher equipped with a 7 mm screen to obtain crushed pieces with an average particle size of 6 mm. Visually detectable contaminants were then removed. 36.0 g of the crushed pieces and 65.0 g of the concentrated solution, adjusted to a concentration of ε-caprolactam and polyamide 6 oligomer of 6.5% by mass, were weighed into an SUS316L autoclave equipped with a stirrer, a bottom plug valve, and a glass filter (average mesh size: 10 μm) at the bottom. Since the polyamide 6 content in the resin component of the mobile phone housing is 99% by mass or more and the glass fiber content in the resin composition is 45% by mass, the total mass ratio (X:1) of water to polyamide 6 and polyamide 6 oligomer is 2.9:1. 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 and maintaining the temperature at 320°C for 15 minutes. Since the reaction temperature Y°C is 320°C, the product of X and Y is 941. Furthermore, since the residence time at the reaction temperature of 320°C is 15 minutes, the product of X, Y, and Z is 14,112. After the reaction was completed, the internal temperature was cooled to 90°C, and while maintaining the temperature at 90°C, the bottom stop valve was opened to carry out solid-liquid separation (step (b1)). When the filtration rate decreased, solid-liquid separation was carried out by introducing nitrogen into the autoclave at 0.3 MPa. The filtered product was then rinsed with deionized water heated to 90°C in an amount approximately three times the amount (by mass) of the filtered product, and the filtrate and wet product were recovered. The filtrate obtained in step (b1) was cooled to an internal temperature of 50°C and subjected to solid-liquid separation using a glass filter with an average mesh size of 10 to 16 μm (step (b2)). The filtered product was then rinsed three times with deionized water heated to 50°C in an amount approximately three times the amount of the filtered product, and the filtrate and wet product were recovered.The filtrate recovered in the (b2) step was subjected to high performance liquid chromatography measurement, and the amount of ε-caprolactam contained in the filtrate was 14.9 g, and the yield based on the polyamide 6 in the crushed product used as the raw material was 75.3%.

[0083] The recovered filtrate was then heated to 55°C under a reduced pressure of 30 mmHg to distillatively separate water, yielding a concentrated ε-caprolactam aqueous solution. This was followed by further distillation at a reduced pressure of 5 mmHg and a heating temperature of 150-170°C to recover distilled ε-caprolactam. The concentration and distillation yield of ε-caprolactam was 95.7%. Furthermore, the HPLC impurity content of the distilled ε-caprolactam was 0.39%, indicating that it was of a quality suitable for use as a polymerization raw material for polyamide 6. Next, polymerization was carried out using the resulting ε-caprolactam as a raw material. 10.0 g of the recovered ε-caprolactam, 2.2 mg of benzoic acid, and 10.0 g of ion-exchanged water 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. Thereafter, the internal pressure was reduced 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 removed 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 225 ° C. and ηr = 2.70. The wet filter residue obtained in step (b1) was vacuum dried at 50 ° C. for 12 hours to recover 16.3 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 an air atmosphere for 3 hours. The amount of organic matter adhering to the recovered glass fiber was evaluated based on the mass loss. The mass loss was 1.3% by mass, and it was found that the recovered glass fiber was a high-purity glass fiber with a low amount of organic matter attached. The wet filtered product obtained in step (b2) was vacuum dried at 50°C for 12 hours to recover 1.7 g of solid matter from solid-liquid separation (I). The obtained solid matter was analyzed by high-performance liquid chromatography under the conditions described above, and was found to be a polyamide 6 oligomer containing 97.9% by mass of linear dimer- to dodecamer oligomers. Because this polyamide 6 oligomer has a high purity, it can be further utilized as a raw material for depolymerization.

Claims

1. Using waste resin moldings (A) containing at least polyamide 6 as a raw material, ε-caprolactam is obtained by the following steps (a) and (b), and the raw material containing ε-caprolactam is polymerized. A method for producing a thermoplastic resin, comprising: (a) a step of adding and contacting a resin molding waste (A) with at least one of liquid 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, wherein the contacting is carried out under the following conditions: when the mass ratio of water to polyamide 6 in the resin molding waste (A) or the mass ratio of water to the sum of polyamide 6 and polyamide oligomer in the resin molding waste (A) is X:1, 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 65% or higher. (b) a step of separating the reaction mixture obtained in the step (a) into a solid and an aqueous solution containing ε-caprolactam by solid-liquid separation (I)

2. Using waste resin moldings (A) containing at least polyamide 6 as a raw material, ε-caprolactam is obtained by the following steps (a) and (b), and the raw material containing ε-caprolactam is polymerized. A method for producing a thermoplastic resin, comprising: (a) a step of adding and contacting a resin molding waste (A) with at least one of liquid 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, wherein the mass ratio of water to polyamide 6 in the resin molding waste (A) or the mass ratio of water to the sum of polyamide 6 and polyamide oligomer in the resin molding waste (A) is X:1, the reaction temperature is Y°C, the product of X and Y is 2,000 or less, and the product of X, Y and Z is 30,000 or less, when the residence time at the reaction temperature Y°C is Z minutes. (b) a step of separating the reaction mixture obtained in the step (a) into a solid and an aqueous solution containing ε-caprolactam by solid-liquid separation (I)

3. The step (a) is a step of adding a resin molding waste (A) 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. The method for producing a thermoplastic resin according to claim 1 or 2.

4. The solid-liquid separation (I) includes a step (b1) of separating the reaction mixture into a non-melt and an aqueous solution containing at least ε-caprolactam and polyamide 6 oligomer by solid-liquid separation, and a step (b2) of separating the filtrate obtained in the step (b1) into a polyamide 6 oligomer and an aqueous ε-caprolactam solution by solid-liquid separation. The method for producing a thermoplastic resin according to any one of claims 1 to 3.

5. The polyamide 6 oligomer separated in the step (b) or (b2) is mixed with water, and the mixture is heated to 290°C or higher and 350°C or lower to obtain an aqueous polyamide 6 oligomer solution (B1), which is used in the step (a). The method for producing a thermoplastic resin 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 producing a thermoplastic resin according to any one of claims 1 to 5.