Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J11/00—Recovery or working-up of waste materials
  • C08J11/04—Recovery or working-up of waste materials of polymers
  • C08J11/10—Recovery or working-up of waste materials of polymers by chemically breaking down the molecular chains of polymers or breaking of crosslinks, e.g. devulcanisation
  • C08J11/14—Recovery or working-up of waste materials of polymers by chemically breaking down the molecular chains of polymers or breaking of crosslinks, e.g. devulcanisation by treatment with steam or water
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07D—HETEROCYCLIC COMPOUNDS
  • C07D223/00—Heterocyclic compounds containing seven-membered rings having one nitrogen atom as the only ring hetero atom
  • C07D223/02—Heterocyclic compounds containing seven-membered rings having one nitrogen atom as the only ring hetero atom not condensed with other rings
  • C07D223/06—Heterocyclic compounds containing seven-membered rings having one nitrogen atom as the only ring hetero atom not condensed with other rings with hetero atoms or with carbon atoms having three bonds to hetero atoms with at the most one bond to halogen, e.g. ester or nitrile radicals, directly attached to ring carbon atoms
  • C07D223/08—Oxygen atoms
  • C07D223/10—Oxygen atoms attached in position 2
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07D—HETEROCYCLIC COMPOUNDS
  • C07D201/00—Preparation, separation, purification or stabilisation of unsubstituted lactams
  • C07D201/02—Preparation of lactams
  • C07D201/12—Preparation of lactams by depolymerising polyamides
• • C—CHEMISTRY; METALLURGY
  • C07—ORGANIC CHEMISTRY
  • C07D—HETEROCYCLIC COMPOUNDS
  • C07D201/00—Preparation, separation, purification or stabilisation of unsubstituted lactams
  • C07D201/16—Separation or purification
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
  • C08G69/00—Macromolecular compounds obtained by reactions forming a carboxylic amide link in the main chain of the macromolecule
  • C08G69/02—Polyamides derived from amino-carboxylic acids or from polyamines and polycarboxylic acids
  • C08G69/04—Preparatory processes
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J11/00—Recovery or working-up of waste materials
  • C08J11/04—Recovery or working-up of waste materials of polymers
  • C08J11/10—Recovery or working-up of waste materials of polymers by chemically breaking down the molecular chains of polymers or breaking of crosslinks, e.g. devulcanisation
  • C08J11/18—Recovery or working-up of waste materials of polymers by chemically breaking down the molecular chains of polymers or breaking of crosslinks, e.g. devulcanisation by treatment with organic material
  • C08J11/28—Recovery or working-up of waste materials of polymers by chemically breaking down the molecular chains of polymers or breaking of crosslinks, e.g. devulcanisation by treatment with organic material by treatment with organic compounds containing nitrogen, sulfur or phosphorus
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
  • C08G69/00—Macromolecular compounds obtained by reactions forming a carboxylic amide link in the main chain of the macromolecule
  • C08G69/02—Polyamides derived from amino-carboxylic acids or from polyamines and polycarboxylic acids
  • C08G69/08—Polyamides derived from amino-carboxylic acids or from polyamines and polycarboxylic acids derived from amino-carboxylic acids
  • C08G69/14—Lactams
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08G—MACROMOLECULAR COMPOUNDS OBTAINED OTHERWISE THAN BY REACTIONS ONLY INVOLVING UNSATURATED CARBON-TO-CARBON BONDS
  • C08G69/00—Macromolecular compounds obtained by reactions forming a carboxylic amide link in the main chain of the macromolecule
  • C08G69/46—Post-polymerisation treatment
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2377/00—Characterised by the use of polyamides obtained by reactions forming a carboxylic amide link in the main chain; Derivatives of such polymers
  • C08J2377/02—Polyamides derived from omega-amino carboxylic acids or from lactams thereof
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02W—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO WASTEWATER TREATMENT OR WASTE MANAGEMENT
  • Y02W30/00—Technologies for solid waste management
  • Y02W30/50—Reuse, recycling or recovery technologies
  • Y02W30/62—Plastics recycling; Rubber recycling

Definitions

• the present invention is a method for recovering ⁇ -caprolactam and polyamide 6 oligomers by depolymerization of a polyamide 6 resin composition, which achieves both the cyclical use of fossil resources and the reduction of greenhouse gas emissions. and a depolymerization method using a small amount of water having a high heat of vaporization, and a method for recovering high-purity ⁇ -caprolactam and polyamide 6 oligomer by solid-liquid separation.
• Patent Literature 1 discloses a method of producing hydrocarbons by a process including pyrolysis and steam cracking of waste plastics. Although these methods have the advantage of being able to convert mixed waste plastics into pyrolysis oil, they require cracking at a high temperature of 800°C or higher in order to convert the pyrolysis oil into secondary raw materials such as plastic monomers.
• the ⁇ -caprolactam recovery method disclosed in Patent Document 2 is a high-yield reaction with a depolymerization yield of polyamide 6 of 80% or more, the depolymerization reaction takes a long time. Furthermore, since a large amount of superheated steam, about 10 times that of polyamide 6 fiber, is required, this technology remains a problem in achieving both the recycling of fossil resources and the reduction of greenhouse gas emissions. In addition, since this method uses phosphoric acid as a catalyst, the reaction is susceptible to impurities, such as catalyst deactivation due to additives contained in plastics and adhering impurities in waste plastics.
• Patent Document 2 does not disclose a method for recovering the polyamide 6 oligomer and its utilization.
• the ⁇ -caprolactam recovery methods disclosed in Patent Documents 3 and 4 use only water for the depolymerization reaction, and do not use a catalyst such as phosphoric acid. There is an advantage that reaction deactivation due to does not occur.
• a large amount of water which has a specific heat capacity of 4.2 kJ/kg K and a heat of vaporization of 2,250 kJ/kg, is about 10 times that of polyamide 6. , a large amount of energy is required for the depolymerization reaction and recovery of ⁇ -caprolactam from a low-concentration ⁇ -caprolactam aqueous solution.
• Patent Documents 3 and 4 do not disclose a method for recovering polyamide 6 oligomer and its utilization.
• the present invention has the following configurations. 1. At least one of a resin composition (A) containing at least polyamide 6 and 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.
• a method for recovering ⁇ -caprolactam and polyamide 6 oligomer which comprises performing I), separating the polyamide 6 oligomer into a solid phase and recovering the ⁇ -caprolactam aqueous solution into a liquid phase. 2.
• a resin composition (A) containing at least polyamide 6 is added to water (B) heated to 290 ° C. or higher and 350 ° C. or lower, or water (B) heated to 290 ° C. or higher and 350 ° C.
• Item 1 characterized in that a heated aqueous polyamide 6 oligomer solution (B1) is further added and contacted to obtain a reaction mixture (C) containing at least ⁇ -caprolactam, polyamide 6 oligomer, and water. method for recovery of ⁇ -caprolactam and polyamide 6 oligomers. 3. 3. The method for recovering ⁇ -caprolactam and polyamide 6 oligomer according to item 1 or 2, wherein the polyamide 6 oligomer recovered by solid-liquid separation (I) is used as an aqueous polyamide 6 oligomer solution (B1). 4. 4. 4.
• the aqueous solution of polyamide 6 oligomer (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, according to items 1 to 5.
• the present invention provides a method for recovering ⁇ -caprolactam and polyamide 6 oligomer by depolymerization of a polyamide 6 resin composition, in which ⁇ -caprolactam and polyamide 6 oligomer are recovered in high yields only by depolymerization and solid-liquid separation using a small amount of water. It is possible to provide a recovery method with less energy consumption that can recover .
• the present invention provides a resin composition (A) containing at least polyamide 6, and at least 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.
• A resin composition
• B water
• B1 aqueous polyamide 6 oligomer solution
• B1 aqueous polyamide 6 oligomer solution
• One is added and contacted to prepare a reaction mixture (C)
• the reaction mixture (C) is subjected to solid-liquid separation (I) in a temperature range below the boiling point of water at the operating pressure
• polyamide 6 oligomer is a solid phase, ⁇ - characterized by separating and recovering the aqueous caprolactam solution into liquid phases;
• Resin composition (A) Polyamide 6 in the present invention is a polyamide resin mainly composed of 6-aminocaproic acid and/or ⁇ -caprolactam. Other monomers may be copolymerized to the extent that the object of the present invention is not impaired.
• “main raw material” means that a total of 50 mol% or more of 6-aminocaproic acid-derived units or ⁇ -caprolactam-derived units is contained in a total of 100 mol% of the monomer units constituting the polyamide resin. means It is more preferable to contain 6-aminocaproic acid-derived units or ⁇ -caprolactam-derived units in an amount of 70 mol% or more, more preferably 90 mol% or more.
• Other monomers to be copolymerized include, for example, amino acids such as 11-aminoundecanoic acid, 12-aminododecanoic acid and para-aminomethylbenzoic acid, lactams such as ⁇ -laurolactam, tetramethylenediamine, pentamethylenediamine, Hexamethylenediamine, 2-methylpentamethylenediamine, nonamethylenediamine, decamethylenediamine, undecamethylenediamine, dodecamethylenediamine, 2,2,4-/2,4,4-trimethylhexamethylenediamine, 5-methylnona Aliphatic diamines such as methylenediamine, 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,
• polyamides 6 may be added with polymerization degree modifiers, terminal group modifiers, and the like.
• polymerization degree modifiers and terminal group modifiers include acetic acid and benzoic acid.
• the degree of polymerization of the polyamide 6 of the present invention is not particularly limited, but the relative viscosity measured at 25 ° C. in a 98% concentrated sulfuric acid solution with a resin concentration of 0.01 g / mL is in the range of 1.5 to 5.0 Preferably.
• the relative viscosity is in such a preferable range, the reaction efficiency with a small amount of water tends to be high, so it can be exemplified preferably.
• the amount of the cyclic oligomer represented by Chemical Formula 1 below contained in the polyamide 6 of the present invention is not particularly limited, it is preferably 2.0% by mass or less, more preferably 1.8% by mass or less. More preferably, it is 5% by mass or less.
• m is an integer of 2-4.
• the cyclic oligomer represented by the following formula (a) melts and volatilizes, causing line clogging and the like. Therefore, when the amount of the cyclic oligomer is within a preferable range, line clogging due to melting and volatilization tends to be suppressed. Note that the cyclic oligomer represented by the following formula (a) where m is 5 or more is not considered in the present invention in consideration of the degree of volatilization.
• the resin composition (A) of the present invention can further contain an alkali metal halide within a range that does not impair the object of the present invention.
• alkali metal halides include alkali metal halides such as 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 can be used in combination.
• potassium iodide is preferable from the viewpoint of easy availability, excellent dispersibility in polyamide 6, higher reactivity with radicals, and improved retention stability at high temperatures.
• alkali metal halides can be used in combination with Group 11 metal halides such as copper (I) iodide, copper (I) bromide, and copper (I) chloride to further improve the retention stability at high temperatures. It is more preferably used for
• the blending amount of these alkali metal halides is preferably 0.01 to 1 part by mass per 100 parts by mass of polyamide 6.
• the content of the alkali metal halide is more preferably 0.02 to 0.5 parts by mass, more preferably 0.03 to 0.4 parts by mass.
• the resin composition (A) of the present invention may contain a fibrous filler.
• a fibrous filler herein may be any filler having a fibrous shape. Specifically, glass fibers, polyacrylonitrile (PAN)-based and pitch-based carbon fibers, stainless steel fibers, metal fibers such as aluminum fibers and brass fibers, polyester fibers, organic fibers such as aromatic polyamide fibers, gypsum fibers, ceramics Fibers, asbestos fibers, zirconia fibers, alumina fibers, silica fibers, titanium oxide fibers, silicon carbide fibers, rock wool, potassium titanate whiskers, silicon nitride whiskers, wollastonite, fibrous and whisker fillers such as alumina silicate, nickel , copper, cobalt, silver, aluminum, iron, and alloys thereof. You may contain 2 or more types of these.
• the content of the fibrous filler is preferably 1 to 200 parts by mass with respect to 100 parts by mass of the resin composition (A).
• the resin composition (A) of the present invention may further contain fillers other than fibrous fillers, thermoplastic resins other than polyamide 6, and various additives within a range that does not impair the object of the present invention.
• Fillers other than fibrous fillers may be either organic fillers or inorganic fillers, and examples include non-fibrous fillers, and two or more of these fillers may be blended.
• non-fibrous fillers examples include talc, wollastonite, zeolite, sericite, mica, kaolin, clay, pyrophyllite, bentonite, asbestos, alumina silicate, non-swelling silicates such as calcium silicate, Li-type fluorine Swellable layered silicates such as teniolite, Na-type fluoroteniolite, Na-type tetrasilicon fluoromica, swelling mica of Li-type tetrasilicon fluoromica, silicon oxide, magnesium oxide, alumina, silica, diatomaceous earth, zirconium oxide, titanium oxide, Metal oxides such as iron oxide, zinc oxide, calcium oxide, tin oxide and antimony oxide, metal carbonates such as calcium carbonate, magnesium carbonate, zinc carbonate, barium carbonate, dolomite and hydrotalcite, calcium sulfate and barium sulfate metal hydroxides such as metal sulfates, magnesium hydroxide, calcium hydroxide, aluminum
• Organic onium ions include, for example, ammonium ions, phosphonium ions, and sulfonium ions.
• additives include N,N'-hexamethylenebis(3,5-di-t-butyl-4-hydroxy-hydrocinnamide), tetrakis[methylene-3-(3',5'-di- Phenolic compounds such as t-butyl-4'-hydroxyphenyl)propionate]methane, phosphorus compounds, mercaptobenzimidazole compounds, dithiocarbamic acid compounds, sulfur compounds such as organic thioacid compounds, N,N'- Thermal stabilizers such as amine compounds such as di-2-naphthyl-p-phenylenediamine and 4,4'-bis( ⁇ , ⁇ -dimethylbenzyl)diphenylamine, isocyanate compounds, organic silane compounds, organic titanate compounds , organic borane compounds, coupling agents such as epoxy compounds, polyalkylene oxide oligomer compounds, thioether compounds, ester compounds, plasticizers such as organic phosphorus compounds, crystal nuclei such as
• thermoplastic resins other than polyamide 6 contained in the resin composition (A) include polyamide resins other than polyamide 6, polyester resins, polyolefin resins, modified polyphenylene ether resins, polysulfone resins, polyketone resins, and polyetherimide resins. , polyarylate resin, polyether sulfone resin, polyether ketone resin, polythioether ketone resin, polyether ether ketone resin, polyimide resin, polyamideimide resin, tetrafluoropolyethylene resin, polyphenylene sulfide resin, and the like. You may mix
• the amount of the thermoplastic resin other than polyamide 6 is preferably 30 parts by mass or less per 100 parts by mass of polyamide 6 in the thermoplastic resin (A) of the present invention.
• the resin composition (A) containing polyamide 6 of the present invention may be waste of resin moldings containing at least polyamide 6.
• the waste of resin moldings containing polyamide 6 includes polyamide 6 products, industrial waste generated in the process of manufacturing polyamide 6 products, and waste after use of polyamide 6 products.
• Polyamide 6 products include textile structures for clothing such as used clothes, uniforms, sportswear and innerwear; industrial textile structures such as curtains, carpets, ropes, nets, belts and sheets; Examples include electronic molded parts, film products, extrusion molded products, in-situ polymerization molded products, RIM molded products, and the like. Furthermore, product scraps, pellet scraps, lump-like scraps, and cutting scraps generated during the cutting process are also subject to waste.
• the polyamide 6 oligomer in the present invention is a polyamide 6 oligomer mainly composed of 6-aminocaproic acid and/or ⁇ -caprolactam.
• the polyamide 6 oligomer may contain other monomers as long as the object of the present invention is not impaired.
• "mainly constituting” means that the unit derived from 6-aminocaproic acid or the unit derived from ⁇ -caprolactam in the total 100 mol% of the monomer units constituting the polyamide 6 oligomer is 50 mol% or more in total. means to contain It is more preferable to contain 6-aminocaproic acid-derived units or ⁇ -caprolactam-derived units in an amount of 70 mol% or more, more preferably 90 mol% or more.
• polyamide 6 oligomer examples include, for example, amino acids such as 11-aminoundecanoic acid, 12-aminododecanoic acid, and para-aminomethylbenzoic acid, lactams such as ⁇ -laurolactam, tetramethylenediamine, and pentamethylene.
• Diamine Diamine, hexamethylenediamine, 2-methylpentamethylenediamine, nonamethylenediamine, decamethylenediamine, undecamethylenediamine, dodecamethylenediamine, 2,2,4-/2,4,4-trimethylhexamethylenediamine, 5- Aliphatic diamines such as 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, bis(4-aminocyclohexyl)methane, bis(3-methyl-4-aminocyclohexyl)methane, 2,2-bis(4-aminocyclohexyl)propane, Alicyclic diamines such as 1,4-bis(3-aminopropyl
• the number average molecular weight of the polyamide 6 oligomer of the present invention is not particularly limited, but the number average molecular weight is preferably in the range of 100 to 5000, more preferably in the range of 200 to 3000, and in the range of 200 to 2000. One is particularly preferable and can be exemplified. When the molecular weight of the polyamide 6 oligomer is within such a preferable range, the solubility in water tends to be high, and the polyamide oligomer aqueous solution (B1) used in the present invention tends to be easily prepared.
• the number average molecular weight here was calculated by GPC analysis using 1,1,1,3,3,3-hexafluoro-2-propanol as a solvent. Showa Denko GPC-HFIP805 was used as the column, and PMMA was used as the standard substance.
• composition of the polyamide 6 oligomer used in the present invention is not particularly limited, but the content of the linear polyamide 6 oligomer of 2 to 12-mer contained in the polyamide 6 oligomer is preferably 90% by mass or more. can. More preferably, the amount of the linear 2-12-mer oligomer is 93% by mass or more, and particularly preferably 95% by mass or more.
• the solubility in water is increased, and the terminal carboxylic acid concentration of the polyamide 6 oligomer is As the temperature increases, the reaction between polyamide 6 and water is accelerated, and the efficiency of ⁇ -caprolactam production tends to increase.
• the amount of linear 2- to 12-mer oligomers in the polyamide 6 oligomer was quantitatively analyzed by high-performance liquid chromatography using an aqueous formic acid solution and an acetonitrile formic acid solution as eluents.
• the method for preparing the polyamide 6 oligomer used in the present invention is not particularly limited.
• a polyamide 6 oligomer prepared by the same method as that for synthesizing the fatty acid-based polyamide 6 resin may be used.
• a polyamide 6 oligomer obtained as a by-product may be used when producing ⁇ -caprolactam by contacting one of them.
• 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.
• Water used for preparing the aqueous polyamide 6 oligomer aqueous solution (B1) is not particularly limited, and tap water, ion-exchanged water, distilled water, well water, etc. can be used. From the viewpoint of , ion-exchanged water and distilled water are preferably used.
• the concentration of the polyamide 6 oligomer in the polyamide 6 oligomer aqueous solution (B1) used in the present invention may be any concentration as long as the polyamide 6 oligomer is dissolved in water when heated to 290 ° C. or higher and 350 ° C. or lower. However, it can be exemplified that it is preferably 20% by mass or less, more preferably 15% by mass or less, and even more preferably 10% by mass or less. When the concentration of the polyamide 6 oligomer is within these preferred ranges, the solubility in water when preparing the aqueous polyamide 6 oligomer solution (B1) is increased, and the aqueous polyamide 6 oligomer solution (B1) can be prepared at a lower temperature.
• an extract containing a polyamide 6 oligomer obtained in a step of hot-water extracting a polyamide 6 oligomer from polyamide 6, which is a product of polyamide 6 production can be used as the polyamide 6 oligomer aqueous solution (B1).
• a polyamide 6 resin obtained by polymerizing ⁇ -caprolactam contains, as impurities, unreacted monomers produced in the polymerization equilibrium reaction and polyamide 6 oligomers. Therefore, in order to remove these, pellets after polymerization are supplied to a hot water extraction tower, and unreacted monomers and polyamide 6 oligomers are extracted and removed by hot water extraction.
• Using the extract obtained in the step of hot water extraction of the polyamide 6 oligomer from the polyamide 6 during the production of polyamide 6 as the aqueous polyamide 6 oligomer aqueous solution (B1) of the present invention is also a preferable form from the viewpoint of reducing industrial waste. I can give an example.
• reaction mixture (C) containing at least ⁇ -caprolactam, a polyamide 6 oligomer, and water of the present invention is prepared by mixing a resin composition (A) containing at least polyamide 6 with a temperature of 290°C or higher. It is prepared by adding at least one of water (B) heated to 350° C. or lower and polyamide 6 oligomer aqueous solution (B1) heated to 290° C. or higher and 350° C. or lower and bringing them into contact.
• reaction substrates When water is raised to a pressure of 22.1 MPa and a temperature of 374.2° C., it becomes neither liquid nor gas. This point is called the critical point of water, and hot water at a temperature and pressure lower than the critical point is called subcritical water.
• the water used in the present invention and the water in the polyamide 6 oligomer aqueous solution are subcritical water. Although this subcritical water is water, it has the characteristics of (i) a low dielectric constant and (ii) a high ionic product. It depends and can be controlled.
• the low dielectric constant makes it an excellent solvent for organic compounds, even though it is water, and the high ion product increases the concentration of hydrogen ions and hydroxide ions, resulting in an excellent hydrolytic action.
• 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, more preferably 320° C. or higher and 340° C. or lower. By being in such a preferable range, there is a tendency that corrosion of equipment during the reaction can be suppressed.
• the pressure of water it can preferably be exemplified to be higher than the saturated vapor pressure. Water may be in a liquid state, in a gaseous state such as water vapor, or both. is preferably high.
• the upper limit of water pressure is not particularly limited, but can be exemplified as 20 MPa or less.
• Such a pressure range tends to increase the ionic product of water, which is preferable.
• a method of pressurizing and sealing the inside of a pressure vessel is used to bring the water into such a pressure range.
• a gas may be sealed in addition to the water (B) or the aqueous polyamide 6 oligomer solution (B1).
• gases include air, argon, and nitrogen. From the viewpoint of suppressing side reactions such as reactions, it is preferable to use nitrogen or argon.
• the degree of gas pressurization is not particularly limited because it is set to a target pressure, but 0.3 MPa or more can be mentioned.
• the amount of water used in the method for recovering ⁇ -caprolactam of the present invention is not particularly limited, but the mass ratio of water (B) and polyamide 6, the sum of water (B) and water in the aqueous polyamide 6 oligomer aqueous solution (B1), and , the total mass ratio of polyamide 6 and polyamide 6 oligomer, or the total mass ratio of water, polyamide 6 and polyamide 6 oligomer in the aqueous polyamide 6 oligomer aqueous solution (B1) is X: 1, and the reaction temperature is Y ° C. It is preferable to adjust the amount of water used so that the product of X and Y is 2,000 or less.
• the product of X and Y is 1,600 or less, more preferably 1,300 or less, and particularly preferably 1,200 or less.
• the lower limit of the product of X and Y is not particularly limited, but is preferably 300 or more, more preferably 320 or more, and particularly preferably 340 or more.
• the present invention relates to an energy-saving method for recovering ⁇ -caprolactam and polyamide 6 oligomer from a polyamide 6 resin composition for the purpose of achieving both recycling of fossil resources and reduction of greenhouse gas emissions.
• Water has a specific heat capacity of 4.3 kJ/kg K and a heat of vaporization of 2,250 kJ/kg, which are extremely high compared to other organic solvents.
• the residence time at the reaction temperature Y° C. is Z minutes
• the condition that the product of X, Y and Z is 60,000 or less can be preferably exemplified. More preferably 40,000 or less, still more preferably 30,000 or less, and particularly preferably 20,000 or less.
• the lower limit of the product of X, Y and Z is not particularly limited, but it is preferably 5,000 or more, more preferably 8,000 or more, and particularly preferably 9,000 or more.
• the production efficiency of ⁇ -caprolactam tends to be increased while saving energy, which is preferable.
• a side reaction of linear oligomer formation due to the reaction of ⁇ -caprolactam and water proceeds as a side reaction, and if the amount of water used is simply reduced Due to the large amount of linear oligomers produced, the production efficiency of ⁇ -caprolactam is greatly reduced.
• the present inventors have clarified the thermodynamic equilibrium points of the ⁇ -caprolactam formation reaction by the reaction of polyamide 6 and water and the side reaction of linear oligomer formation. The present inventors have found that by setting the product of and Z within the above range, the by-production of linear oligomers can be suppressed and the production efficiency of ⁇ -caprolactam can be greatly improved, leading to the present invention.
• reaction mixture (C) of the present invention various known reaction methods such as a batch method and a continuous method can be employed.
• it is a continuous type it is an extruder equipped with a heating function, a tubular reactor, a tubular reactor equipped with a mixing mechanism such as a baffle, a line mixer, a vertical or horizontal reactor, and a vertical reactor equipped with a stirrer.
• the atmosphere in the production is desirably a non-oxidizing atmosphere, preferably an inert atmosphere such as nitrogen, helium, or argon, and a nitrogen atmosphere is preferable from the viewpoints of economy and ease of handling.
• Solid-liquid separation (I) In the method for recovering ⁇ -caprolactam and polyamide 6 oligomer of the present invention, a reaction mixture (C) containing at least ⁇ -caprolactam, polyamide 6 oligomer and water is subjected to solid-liquid separation (I ), the polyamide 6 oligomer is separated into a solid phase and the ⁇ -caprolactam aqueous solution is separated into a liquid phase and recovered.
• the reaction mixture (C) containing at least ⁇ -caprolactam, polyamide 6 oligomer and water is subjected to solid-liquid separation (I) in a temperature range below the boiling point of water under operating pressure.
• the temperature at which the solid-liquid separation (I) is performed is preferably carried out in a temperature range below the boiling point of water at normal pressure, more preferably 80° C. or lower, further preferably 60° C. or lower, and 50° C. or lower. It is especially preferable to be exemplified.
• the lower limit temperature for solid-liquid separation (I) is not particularly limited, it is preferably 10°C or higher, more preferably 15°C or higher, and even more preferably 20°C or higher. In such a preferred temperature range, ⁇ -caprolactam in the reaction mixture (C) is soluble in water, but polyamide 6 oligomer tends to be less soluble in water.
• the filter medium used for solid-liquid separation (I) must be able to separate polyamide 6 oligomers and pass a solution containing at least ⁇ -caprolactam and water.
• a filter medium having a thickness in the range of 1 ⁇ m, more preferably in the range of 5 ⁇ m to 0.1 ⁇ m, and even more preferably in the range of 1 ⁇ m to 0.1 ⁇ m can be exemplified.
• the polyamide 6 oligomer that permeates the filter medium tends to decrease, and the purity of ⁇ -caprolactam is high when water is removed from the filtrate and ⁇ -caprolactam is recovered. tend to become Moreover, the recovery rate of the polyamide 6 oligomer recovered by the solid-liquid separation (I) tends to be high, which is preferable. On the other hand, when it is less than the preferable range, the filtration efficiency tends to be deteriorated.
• the filter examples include a method using a filter such as a sieve, a method using a centrifugal separator, a method using a centrifugal filter, a method using a vibrating screen, a method using a pressure filter, and a method using a suction filter. Examples include, but are not limited to.
• the mother liquor adhering to the solid content 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 performed, and substantially the mother liquor is removed. It is preferable to avoid sticking.
• the non-oxidizing atmosphere is an atmosphere in which the oxygen concentration in the gas phase is 5% by volume or less, preferably 2% by volume or less, and more preferably an atmosphere substantially free of oxygen, that is, an inert gas atmosphere such as nitrogen, helium, or argon. Among these, it is preferable to carry out under a nitrogen atmosphere from the viewpoint of economy and ease of handling.
• the separately prepared reaction mixture (C) is heated to the temperature at which solid-liquid separation (I) is performed.
• the reaction mixture (C) is continuously heated from the preparation temperature to the solid-liquid A method of performing solid-liquid separation (I) by cooling to the temperature at which separation (I) is performed can be mentioned.
• a preferred method is a method of cooling the reaction mixture (C) to a temperature at which the solid-liquid separation (I) is performed after preparation, followed by the solid-liquid separation (I).
• seed crystals may be added to facilitate the precipitation of the polyamide 6 oligomer. Seed crystals can be added at the start of cooling or during cooling. Any seed crystal may be used as long as it serves as a nucleus for crystallizing the polyamide 6 oligomer. It is preferable to use polyamide 6 or polyamide 6 oligomer as the seed crystal, since it is preferably a crystal of the same substance as the polyamide 6 oligomer. The use of these seed crystals is preferable because the precipitation of the polyamide 6 oligomer is promoted and the purity of the ⁇ -caprolactam recovered in the liquid phase by the solid-liquid separation (I) tends to be high.
• the wet cake-like polyamide 6 oligomer containing water recovered as a solid phase component by solid-liquid separation (I) is used as a raw material for an aqueous polyamide 6 oligomer aqueous solution (B1) in a wet cake-like state without performing a drying treatment.
• aqueous polyamide 6 oligomer aqueous solution (B1) in a wet cake-like state without performing a drying treatment.
• the polyamide 6 oligomer recovered by solid-liquid separation (I) as a raw material without going through the drying process in this way, the amount of industrial waste can be reduced and the energy required for the drying process can be omitted.
• Recovery method of ⁇ -caprolactam There is no particular limitation on the recovery method of ⁇ -caprolactam from the filtrate obtained in the solid-liquid separation (I) of the present invention, and any method can be employed. For example, by subjecting the ⁇ -caprolactam aqueous solution obtained by the solid-liquid separation (I) to a distillation operation, the water is separated, and ⁇ -caprolactam with high purity can be recovered. Further, if the water-insoluble component is precipitated by cooling the ⁇ -caprolactam aqueous solution recovered by the solid-liquid separation (I), it can be separated in advance by a known method such as solid-liquid separation and subjected to distillation separation. .
• Methods for obtaining ⁇ -caprolactam of even higher purity include a method of precision distillation of recovered ⁇ -caprolactam, a method of vacuum distillation with the addition of a trace amount of sodium hydroxide, a method of activated carbon treatment, an ion exchange treatment method, and a recycling method. It can be combined with a purification method such as a method of crystallization. These methods can efficiently remove impurities that are difficult to separate by distillation.
• Polyamide 6 and Molded Articles Thereof According to the method for producing ⁇ -caprolactam described in the present invention, ⁇ -caprolactam with high purity can be obtained, so that it can be used as a raw material for polymerization of polyamide 6.
• Polyamide 6 can be produced by a generally known method of thermally melt-polymerizing ⁇ -caprolactam in the presence of a small amount of water. Further, the polyamide 6 obtained in this way is melt-kneaded with fibrous fillers and various additives as necessary to produce a polyamide 6 resin composition, and injection molding, extrusion molding, etc.
• Various molded articles such as sheets and films can be obtained by the method.
• the polyamide 6 of the present invention and its molded articles can be used for various purposes such as electric/electronic parts, building materials, various containers, daily necessities, household goods and sanitary goods, taking advantage of their excellent properties.
• it is particularly preferably used for aircraft parts, electrical and electronic parts that require toughness and rigidity.
• aircraft-related parts such as landing gear pods, winglets, spoilers, edges, rudders, elevators, failings, and ribs
• electrical and electronic parts such as generators, motors, transformers, and current transformers.
• the above solution viscosity ⁇ r was measured at 25° C. using a 0.01 g/mL solution of 98% concentrated sulfuric acid.
• the melting point is measured using a differential scanning calorimeter, in a nitrogen gas atmosphere, the polyamide is cooled from the molten state to 30 ° C.
• the amount of the above cyclic dimer to tetramer oligomer is obtained by pulverizing polyamide 6, passing through a JIS standard sieve of 24 mesh, collecting polyamide 6 powder that is impermeable to 124 mesh, and adding 20 g of the polyamide 6 powder to 200 mL of methanol.
• High-performance liquid chromatography analysis of the obtained polyamide 6 oligomer revealed that the linear 2- to 12-mer oligomer content was 95.8% by mass.
• the pellets were extracted with 20 times the amount of hot water at 98° C. to recover an extract containing unreacted caprolactam and polyamide 6 oligomer.
• the total amount of unreacted caprolactam and polyamide 6 oligomer in the extract was 0.5% by mass, and polyamide 6 oligomer was 0.1% by mass.
• Example 1 20.0 g of PA6-A and 60.0 g of deionized water were charged into a SUS316L autoclave equipped with a stirrer, a bottom plug valve and a glass filter (average opening 10 ⁇ m) at the bottom.
• the reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 0.5 MPa, and then reacted 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 the reaction temperature of 320 ° C.
• the obtained solid content was analyzed by high-performance liquid chromatography under the conditions described in [Reference Example 1]. As a result, it was found to be a polyamide 6 oligomer containing 96.4% by mass of linear 2-12-mers. Since this polyamide 6 oligomer has a high purity, it can be further utilized as a raw material for depolymerization. In addition, compared to Reference Example 1, which uses a large amount of organic solvent, this method can recover a high-purity polyamide 6 oligomer without using an organic solvent, so it is a low environmental load process. I understand.
• the filtrate obtained by solid-liquid separation (I) was measured by high performance liquid chromatography, and the amount of ⁇ -caprolactam contained in the filtrate was 15.0 g. It was 75.1%. Further, the recovered filtrate is heated to 55°C under reduced pressure of 30 mmHg to distill and separate water to obtain a concentrated ⁇ -caprolactam aqueous solution, which is further distilled under reduced pressure of 5 mmHg at a heating temperature of 150 to 170°C. ⁇ -caprolactam was recovered. The concentration and distillation yield of ⁇ -caprolactam was 95.8%. The HPLC impurity content of the distillate ⁇ -caprolactam was 0.48%, and the quality was such that it could be used as a raw material for polymerization of polyamide 6.
• Example 2 20.0 g of PA6-B and 60.0 g of deionized water were charged into a SUS316L autoclave equipped with a stirrer, a bottom plug valve and a glass filter (average opening 10 ⁇ m) at the bottom.
• the mass ratio (X:1) of water to polyamide 6 is 3:1.
• the reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 0.5 MPa, and then reacted 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.
• solid-liquid separation (I) 15 minutes, the product of X, Y and Z is 15,300. is.
• the internal temperature was cooled to 30°C, and the bottom plug valve was opened while maintaining the temperature at 30°C to perform solid-liquid separation (I).
• solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Further, about 3 times the amount (mass) of deionized water is introduced into the autoclave to rinse and wash the filtered product at 30 ° C. three times, and the filtrate and the wet filtered product are recovered. bottom.
• the filtered residue was vacuum-dried at 50° C. for 12 hours, and 1.7 g of solid content obtained in solid-liquid separation (I) was recovered.
• the resulting solid content was analyzed by high-performance liquid chromatography under the conditions described in [Reference Example 1]. As a result, it was found to be a polyamide 6 oligomer containing 97.3% by mass of a linear 2- to 12-mer oligomer. . Since this polyamide 6 oligomer has a high purity, it can be further utilized as a raw material for depolymerization. Further, the filtrate obtained by solid-liquid separation (I) was measured by high-performance liquid chromatography, and the amount of ⁇ -caprolactam contained in the filtrate was 14.9 g. Yield was 74.4%.
• Example 3 17.6 g of PA6-A and 25.5 g of deionized water are weighed into a SUS316L autoclave equipped with a stirrer, a bottom plug valve and a glass filter (average opening of 10 ⁇ m) at the bottom, and a polyamide having a concentration of 6.5% by mass. 36.9 g of an aqueous solution of 6 oligomer was added. The total mass ratio (X:1) of water to polyamide 6 and polyamide 6 oligomer is 3:1.
• the polyamide 6 oligomer used here was prepared by the method described in Reference Example 1. The reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 0.5 MPa, and then reacted at 320° C.
• Example 4 20.0 g of PA6-A and 60.0 g of deionized water were charged into a SUS316L autoclave equipped with a stirrer, a bottom plug valve and a glass filter (average opening 10 ⁇ m) at the bottom.
• the mass ratio (X:1) of water to polyamide 6 is 3:1.
• the reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 0.5 MPa, and then reacted 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, and since the residence time at the reaction temperature of 320 ° C. is 15 minutes, the product of X, Y and Z is 14,400.
• the inner temperature was first cooled to 90°C, and 0.5 g of polyamide 6 oligomer produced by the method described in Reference Example 1 was added while maintaining the temperature at 90°C. After that, the mixture was cooled to 40°C, and the bottom plug valve was opened to perform solid-liquid separation (I) while maintaining the temperature at 40°C. At the stage when the filtration rate decreased, solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, about 3 times the amount (mass) of deionized water is introduced into the autoclave, and the filtrate is rinsed and washed three times at 40°C, and the filtrate and the wet filtrate are recovered. bottom.
• the filtered residue was vacuum-dried at 50° C. for 12 hours, and 4.3 g of solid content obtained in solid-liquid separation (I) was recovered. Since 0.5 g of polyamide 6 oligomer was added at 90° C., the substantially recovered solid content was 3.8 g.
• the obtained solid content was analyzed by high-performance liquid chromatography under the conditions described in [Reference Example 1], and as a result, it was found to be a polyamide 6 oligomer containing 97.3% by mass of linear 2-12-mers. Since this polyamide 6 oligomer has a high purity, it can be further utilized as a raw material for depolymerization.
• Example 2 Compared with Example 1, by adding a small amount of polyamide 6 oligomer as a seed crystal before performing solid-liquid separation (I), the amount of polyamide 6 oligomer recovered by solid-liquid separation (I) increased. I know there is. Further, the filtrate obtained by solid-liquid separation (I) was measured by high performance liquid chromatography, and the amount of ⁇ -caprolactam contained in the filtrate was 15.0 g. It was 75.1%.
• the amount of ⁇ -caprolactam contained in the reaction mixture was 15.0 g, and the yield based on PA6-A used as a raw material was 75.1%. .
• the above reaction mixture is heated to 55° C. under reduced pressure of 30 mmHg to separate water by distillation to obtain a concentrated ⁇ -caprolactam aqueous solution. Caprolactam and distillation residues were recovered. The concentration and distillation yield of ⁇ -caprolactam was 95.8%.
• the HPLC impurity content of the distillate ⁇ -caprolactam was 0.48%, and the quality was such that it could be used as a raw material for polymerization of polyamide 6.
• an HPLC measurement of the collected distillation residue was attempted, but it was not possible to obtain a highly pure polyamide 6 oligomer from the distillation residue because there was a solvent-insoluble component.
• Example 6 45.0 g of PA6-A and 135.0 g of deionized water were charged into a SUS316L autoclave equipped with a stirrer, a bottom plug valve and a glass filter (average opening 10 ⁇ m) at the bottom.
• the mass ratio (X:1) of water to polyamide 6 is 3:1.
• the reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 5.0 MPa, and then reacted at 320° C. for 15 minutes while stirring at 200 rpm. The ultimate pressure during the reaction was 19.8 MPa.
• the reaction temperature Y ° C. is 320 ° C.
• the product of X and Y is 960, and since the residence time at the reaction temperature of 320 ° C.
• solid-liquid separation (I) 15 minutes, the product of X, Y and Z is 14,400. is.
• the internal temperature was cooled to 40°C, and the bottom plug valve was opened while maintaining the temperature at 40°C to perform solid-liquid separation (I).
• solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa.
• approximately 3 times the amount (mass) of deionized water is introduced into the autoclave to rinse and wash the filtered product at 40°C three times, and the filtrate and wet filtered product are recovered. bottom.
• the filtered residue was vacuum-dried at 50° C. for 12 hours, and 4.7 g of solid content obtained in solid-liquid separation (I) was recovered.
• the obtained solid content was analyzed by high-performance liquid chromatography under the conditions described in [Reference Example 1]. As a result, it was found to be a polyamide 6 oligomer containing 98.1% by mass of linear 2- to 12-mer oligomers. . Since this polyamide 6 oligomer has a high purity, it can be further utilized as a raw material for depolymerization. Further, the filtrate obtained by solid-liquid separation (I) was subjected to high-performance liquid chromatography measurement, and the amount of ⁇ -caprolactam contained in the filtrate was 36.9 g. Yield was 82.0%. By comparison with Example 1, it can be seen that ⁇ -caprolactam tends to be obtained in a high yield by increasing the reaction pressure to a pressure higher than the saturated vapor pressure.
• Example 7 30.0 g of PA6-A and 60.0 g of deionized water were charged into a SUS316L autoclave equipped with a stirrer, a bottom plug valve and a glass filter (average opening 10 ⁇ m) at the bottom.
• the weight ratio (X:1) of water to polyamide 6 is 2:1.
• the reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 0.5 MPa, and then reacted 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 the residence time at the reaction temperature of 320 ° C. is 15 minutes, so the product of X, Y and Z is 9,600.
• solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Furthermore, about 3 times the amount (mass) of the filtrate was introduced into the autoclave, and the filtrate was rinsed and washed three times at 40 ° C., and the filtrate and the wet filtrate were separated. Recovered. The filtered residue was vacuum-dried at 50° C. for 12 hours, and 8.2 g of solid content obtained in solid-liquid separation (I) was recovered.
• the obtained solid content was analyzed by high-performance liquid chromatography under the conditions described in [Reference Example 1]. . Since this polyamide 6 oligomer has a high purity, it can be further utilized as a raw material for depolymerization. Further, the filtrate obtained by solid-liquid separation (I) was subjected to high-performance liquid chromatography measurement, and the amount of ⁇ -caprolactam contained in the filtrate was 19.2 g. Yield was 64.0%.
• Example 8 60.0 g of PA6-A and 120.0 g of deionized water were charged into a SUS316L autoclave equipped with a stirrer, a bottom plug valve and a glass filter (average opening 10 ⁇ m) at the bottom.
• the weight ratio (X:1) of water to polyamide 6 is 2:1.
• the reaction vessel was purged with nitrogen, sealed under a nitrogen pressure of 5.0 MPa, and then reacted at 320° C. for 15 minutes while stirring at 200 rpm. The ultimate pressure during the reaction was 19.5 MPa.
• 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.
• the resulting solid content was analyzed by high-performance liquid chromatography under the conditions described in [Reference Example 1]. As a result, it was found to be a polyamide 6 oligomer containing 97.5% by mass of a linear 2- to 12-mer oligomer. . Since this polyamide 6 oligomer has a high purity, it can be further utilized as a raw material for depolymerization. Further, the filtrate obtained by solid-liquid separation (I) was subjected to high-performance liquid chromatography measurement, and the amount of ⁇ -caprolactam contained in the filtrate was 44.8 g. Yield was 74.7%. By comparison with Example 7, it can be seen that the yield of ⁇ -caprolactam produced tends to increase by increasing the pressure during the reaction to a pressure higher than the saturated vapor pressure.
• Example 9 the hot water extract in the PA6 production process obtained by the method described in Reference Example 3 was concentrated until the concentration of unreacted caprolactam and polyamide 6 oligomer reached 6.5% by mass, and was used as a depolymerization raw material.
• 17.6 g of PA6-A and 25.5 g of deionized water were weighed into a SUS316L autoclave equipped with a stirrer, a bottom plug valve and a glass filter (average opening 10 ⁇ m) at the bottom, and the above caprolactam and polyamide 6 oligomer concentrations were measured. 36.9 g of a concentrated liquid adjusted to 6.5% by mass was added.
• 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, sealed under a nitrogen pressure of 0.5 MPa, and then reacted at 320° C. for 15 minutes while stirring at 200 rpm. Since the reaction temperature Y ° C. is 320 ° C., the product of X and Y is 1062, and the residence time at the reaction temperature of 320 ° C. is 15 minutes, so the product of X, Y and Z is 15,930. . After completion of the reaction, the internal temperature was cooled to 40°C, and the bottom plug valve was opened while maintaining the temperature at 40°C to perform solid-liquid separation (I).
• solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa. Further, approximately 3 times the amount (mass) of deionized water was introduced into the autoclave to rinse and wash the filtered product three times at 40°C, and the filtrate and the wet filtered product were recovered. . The filtered residue was vacuum-dried at 50° C. for 12 hours, and 2.6 g of solid content obtained by solid-liquid separation (I) was recovered. The resulting solid content was analyzed by high-performance liquid chromatography under the conditions described in [Reference Example 1]. . Since this polyamide 6 oligomer has a high purity, it can be further utilized as a raw material for depolymerization. Further, the filtrate obtained by solid-liquid separation (I) was measured by high performance liquid chromatography, and the amount of ⁇ -caprolactam contained in the filtrate was 15.0 g. It was 85.2%.
• Example 10 the hot water extract in the PA6 production process obtained by the method described in Reference Example 3 was concentrated until the total concentration of unreacted caprolactam and polyamide 6 oligomer was 6.5% by mass, and used as a depolymerization raw material.
• a depolymerization raw material e.g., a SUS316L autoclave equipped with a stirrer, a bottom plug valve and a glass filter (average opening 10 ⁇ m) at the bottom, 20.0 g of PA6-A, the caprolactam and polyamide 6 oligomer concentration were adjusted to 6.5% by mass. 65.0 g of concentrate was added.
• 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, sealed under a nitrogen pressure of 0.5 MPa, and then reacted at 320° C. for 15 minutes while stirring at 200 rpm. Since the reaction temperature Y ° C. is 320 ° C., the product of X and Y is 933, and the residence time at the reaction temperature of 320 ° C. is 15 minutes, so the product of X, Y and Z is 13,995. .
• the internal temperature was cooled to 40°C, and the bottom plug valve was opened while maintaining the temperature at 40°C to perform solid-liquid separation (I). At the stage when the filtration rate decreased, solid-liquid separation (I) was performed while introducing nitrogen into the autoclave at 0.3 MPa.

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