Uncoincident method

Abstract

The present invention provides a depolymerization method comprising a depolymerization step in which at least one item selected from the group consisting of a polyamide resin, a polyamide resin composition containing the polyamide resin, and an article molded from the polyamide resin or polyamide resin composition is heated in a solvent containing a polyhydric alcohol and from 1 mass% to 25 mass% of an inorganic salt relative to 100 mass% of the solvent and depolymerized. The amount of polyamide resin remaining after the depolymerization step is 10 mass% or less, and the amount of dicarboxylic acids, diamines, and lactams that are the degradation products of the polyamide resin, as well as derivatives of these degradation products, is 70 mass% or more relative to 100 mass% of the polyamide resin before the depolymerization step.

Description

[Technical Field]

[0001] The present invention relates to a method for depolymerization. [Background technology]

[0002] Polyamide resins have excellent mechanical properties, such as mechanical strength, rigidity, and impact resistance, as well as excellent heat resistance and chemical resistance, and therefore have traditionally been used in a variety of industrial fields, including clothing, industrial materials, automobiles, electrical and electronic parts, and other industrial products. Meanwhile, in recent years, the plastics industry has also been called upon to respond to the trend toward a resource-recycling society, and there is a demand for the establishment of recycling technology for polyamide resins.

[0003] There are generally three types of recycling: material recycling, chemical recycling, and thermal recycling. Currently, in the automotive application that accounts for the majority of polyamide resin use, most of the polyamide resin in scrapped automobiles is burned as thermal recycling, and is not effectively utilized as a resource, so there is a demand for effective utilization of the polyamide resin through material recycling and chemical recycling. Furthermore, from the perspective of reducing greenhouse gas (GHG) emissions, material recycling and chemical recycling of polyamide resins are required.

[0004] However, polyamide resin compositions and molded articles for automotive applications contain, in addition to polyamide resin, inorganic fillers such as glass fibers, various additives such as heat stabilizers, pigments, dyes, etc., and therefore, in material recycling, there is a problem in that it is difficult to maintain practically sufficient mechanical properties after recycling. For this reason, chemical recycling, in which polyamide resin is depolymerized to decompose it into monomers such as diamines and dicarboxylic acids, and these monomers are then repolymerized for recycling, is seen as promising, and research and development into this method is underway.

[0005] As a technology relating to the chemical recycling, Patent Document 1 proposes a technology for producing a monomer by decomposing a polyamide resin by ammonolysis using a Lewis acid catalyst. Also, Patent Document 2 proposes a method for separating a polyamide resin and glass fibers from a molded article of a glass fiber-containing polyamide resin composition using an aqueous phosphoric acid solution, and further decomposing the polyamide resin into monomers. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 3571723 [Patent Document 2] Japanese Patent Application Laid-Open No. 2000-80199 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the method described in Patent Document 1 has a problem in that the monomer yield is low. Furthermore, in the method described in Patent Document 2, in a specific example using polyamide 66, it takes as long as 200 minutes to dissolve the polyamide resin and glass fiber. Furthermore, Patent Document 2 does not provide a specific description of a depolymerization method, and furthermore, since it takes as long as 200 minutes to dissolve the polyamide resin and glass fiber, if it is assumed that the polyamide resin is further depolymerized after dissolving a molded article of the polyamide resin composition, the total time becomes even longer, resulting in a problem of lacking practicality.

[0008] As described above, the methods described in Patent Documents 1 and 2 have problems in that the monomer yield is low and the time required, including monomer recovery and depolymerization, is not practical. Furthermore, if the process is carried out under conditions of high temperature and pressure in order to shorten the time, the energy required for the process increases, which is also not practical from the perspective of reducing greenhouse gas (GHG) emissions.

[0009] In view of the above-mentioned problems of the conventional techniques, an object of the present invention is to provide a method for depolymerizing a polyamide resin, a polyamide resin composition, and a molded article, which method has a high monomer yield and excellent energy efficiency for depolymerization. [Means for solving the problem]

[0010] As a result of extensive research into solving the problems of the prior art described above, the present inventors have found that the above problems can be effectively solved by heating a polyamide resin, a polyamide resin composition, or a molded article thereof in a predetermined solvent to depolymerize it, thereby completing the present invention. That is, the present invention is as follows.

[0011] [1] For 100% by mass of solvent, A halogen salt of at least one metal selected from the group consisting of calcium, zinc, and lithium. Inorganic salts in an amount of 1% by mass or more and 25% by mass or less, and polyhydric alcohols 10% by mass or more and 95% by mass or less a depolymerization step of heating and depolymerizing at least one selected from the group consisting of a polyamide resin, a polyamide resin composition containing the polyamide resin, and a molded article thereof in the solvent containing the polyamide resin; The temperature in the system during the heating is 210°C or less, After the depolymerization step, the residual rate of the polyamide resin is 10% by mass or less, The depolymerization method, wherein the decomposition products of the polyamide resin, such as dicarboxylic acids, diamines, lactams, and derivatives thereof, are contained in an amount of 70% by mass or more relative to 100% by mass of the polyamide resin before the depolymerization step. 〔2〕 The inorganic salt is calcium chloride or zinc chloride. 〔1〕 The depolymerization method according to claim 1. 〔3〕 [1] The polyamide resin is polyamide 66. or [2] The depolymerization method according to claim 1. 〔4〕 The polyhydric alcohol is ethylene glycol. 〔3〕The depolymerization method according to any one of the above. 〔5〕 The solvent is a mixed solvent containing ethylene glycol in a mass ratio of 1 to 9, and methanol or water in a mass ratio of 1 to 9. 〔4〕 The depolymerization method according to any one of the above. [Effects of the Invention]

[0012] According to the present invention, there is provided a depolymerization method that can convert polyamide resins, polyamide resin compositions, and molded articles thereof into monomers with low energy and in high yield. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail. The following embodiments are merely examples for explaining the present invention, and are not intended to limit the present invention to the following content. The present invention can be implemented by modifying it as appropriate within the scope of its gist.

[0014] [Depolymerization method] A first form of the depolymerization method of the present embodiment includes a depolymerization step of heating and depolymerizing at least one selected from the group consisting of a polyamide resin, a polyamide resin composition containing the polyamide resin, and a molded article thereof in a solvent containing, relative to 100% by mass of the solvent, 1% by mass or more and 25% by mass or less of an inorganic salt and a polyhydric alcohol, wherein the residual rate of the polyamide resin after the depolymerization step is 10% by mass or less, and the decomposition products of the polyamide resin, i.e., dicarboxylic acids, diamines, and derivatives thereof, are 70% by mass or more relative to 100% by mass of the polyamide resin before the depolymerization step. Furthermore, a second form of the depolymerization method of the present embodiment includes a depolymerization step of heating and depolymerizing at least one selected from the group consisting of polyamide resins, polyamide resin compositions containing the polyamide resins, and molded articles thereof in a solvent containing 1% by mass or more and 25% by mass or less of an inorganic salt and 30% by mass or more and 99% by mass or less of water, relative to 100% by mass of the solvent, wherein the residual rate of polyamide resin after the depolymerization step is 10% by mass or less, and the decomposition products of the polyamide resin, i.e., dicarboxylic acids, diamines, lactams, and derivatives thereof, are 70% by mass or more, relative to 100% by mass of the polyamide resin before the depolymerization step. According to the depolymerization method of the present embodiment described above, polyamide resins, polyamide resin compositions, and molded articles thereof can be converted into monomers with low energy and in high yield, and can be recycled.

[0015] (Polyamide resin) The polyamide resin used in the depolymerization method of the present embodiment means a polymer having an amide bond (—NHCO—) in the main chain.

[0016] (Polyamide resin composition) The polyamide resin composition used in the depolymerization method of the present embodiment is a resin composition containing the polyamide resin described above and, if necessary, an inorganic filler such as glass fiber, a lubricant, and other additives such as a heat stabilizer, a flame retardant, a pigment, a dye, etc.

[0017] (Molded products of polyamide resins and polyamide resin compositions) The molded article used in the depolymerization method of the present embodiment is a molded article of the polyamide resin or polyamide resin composition, and can be produced by molding using various known methods, for example, injection molding, etc. The molded article used in the depolymerization method of the present embodiment may be a fiber of a polyamide resin or a polyamide resin composition.

[0018] The polyamide resin, polyamide resin composition, and materials constituting the molded articles thereof used in the depolymerization method of this embodiment will be specifically described below.

[0019] <Polyamide resin> Examples of polyamide resins include, but are not limited to, polyamide 4 (poly-α-pyrrolidone), polyamide 6 (polycaproamide), polyamide 11 (polyundecaneamide), polyamide 12 (polydodecanamide), polyamide 46 (polytetramethylene adipamide), polyamide 56 (polypentamethylene adipamide), polyamide 66 (polyhexamethylene adipamide), polyamide 410 (polytetramethylene sebacamide), polyamide 412 (polytetramethyl sebacamide), polyamide 413 (polytetramethyl sebacamide), polyamide 414 (polytetramethyl sebacamide), polyamide 415 (polytetramethyl sebacamide), polyamide 416 (polytetramethyl sebacamide), polyamide 417 (polytetramethyl sebacamide), polyamide 418 (polytetramethyl sebacamide), polyamide 419 (polytetramethyl sebacamide), polyamide 420 (polytetramethyl sebacamide), polyamide 421 (polytetramethyl sebacamide), polyamide 422 (polytetramethyl sebacamide), polyamide 423 (polytetramethyl sebacamide), polyamide 424 (polytetramethyl sebacamide), polyamide 425 (polytetramethyl sebacamide), polyamide 426 (polytetramethyl sebacamide), polyamide 427 (polytetramethyl sebacamide), polyamide 428 (polytetramethyl sebacamide), polyamide 429 (polytetramethyl sebacamide), polyamide 430 (polytetramethyl sebacamide), polyamide 431 (polytetramethyl sebacamide), polyamide 432 (polytetramethyl sebacamide), polyamide 433 (polytetramethyl sebacamide), polyamide 434 (polytetramethyl sebacamide), polyamide 435 (polytetramethyl sebacamide), polyamide 43 Examples of suitable polyamides include polyamide 610 (polyhexamethylene sebacamide), polyamide 612 (polyhexamethylene dodecamide), polyamide 1010 (polydecamethylene sebacamide), polyamide 1012 (polydecamethylene dodecamide), polyamide 6T (polyhexamethylene terephthalamide), polyamide 9T (polynonamethylene terephthalamide), polyamide 6I (polyhexamethylene isophthalamide), and copolymers or mixtures thereof.

[0020] In particular, the polyamide resin is preferably one or more selected from the group consisting of polyamide 66, polyamide 66 / 6I, polyamide 610, polyamide 612, polyamide 6I, and polyamide 6, and more preferably polyamide 66, polyamide 66 / 6I, or a mixture of polyamide 66 and polyamide 6I. Polyamide 66 is a polyamide obtained by condensation polymerization of hexamethylenediamine and adipic acid. It has excellent heat resistance, mechanical strength, and creep properties, making it suitable for use as a material for functional parts in automobiles, machinery, and electrical appliances.

[0021] <Inorganic filler> The polyamide resin composition and molded article used in the depolymerization method of the present embodiment may contain an inorganic filler, which tends to give the polyamide resin composition and molded article excellent mechanical strength and rigidity.

[0022] Examples of inorganic fillers include, but are not limited to, glass fibers, carbon fibers, calcium silicate fibers, potassium titanate, aluminum borate, glass flakes, glass beads, talc, kaolin, mica, hydrotalcite, calcium carbonate, zinc carbonate, zinc oxide, calcium hydrogen phosphate, wollastonite, silica, zeolite, alumina, boehmite, aluminum hydroxide, titanium oxide, silicon oxide, magnesium oxide, calcium silicate, sodium aluminosilicate, magnesium silicate, ketjen black, acetylene black, furnace black, carbon nanotubes, graphite, brass, copper, silver, aluminum, nickel, iron, calcium fluoride, mica, montmorillonite, swellable fluoromica, and apatite. These may be used alone or in combination of two or more.

[0023] <Lubricant> The polyamide resin composition and molded article used in the depolymerization method of the present embodiment may contain a lubricant in addition to the polyamide resin and inorganic filler, which tends to give the polyamide resin composition and molded article excellent fluidity and appearance.

[0024] <Other additives> The polyamide resin composition and molded article used in the depolymerization method of this embodiment may contain, in addition to the polyamide resin, inorganic filler, and lubricant component, other additives, such as antioxidants, ultraviolet absorbers, heat stabilizers, photodegradation inhibitors, plasticizers, mold release agents, nucleating agents, flame retardants, colorants, and other thermoplastic resins.

[0025] (Depolymerization process) A first mode of the depolymerization method of the present embodiment includes a step of heating and depolymerizing at least one selected from the group consisting of a polyamide resin, a polyamide resin composition, and a molded article thereof in a solvent containing, relative to 100% by mass of the solvent, 1% by mass to 25% by mass of an inorganic salt and a polyhydric alcohol. Furthermore, a second form of the depolymerization method of the present embodiment includes a step of heating and depolymerizing at least one selected from the group consisting of polyamide resins, polyamide resin compositions, and molded articles thereof in a solvent containing, relative to 100 mass% of the solvent, 1 mass% to 25 mass% of an inorganic salt and 30 mass% to 99 mass% of water.

[0026] <Solvent> The solvent used in the first mode of the depolymerization method of the present embodiment contains, relative to 100% by mass of the solvent, 1% by mass to 25% by mass of an inorganic salt and a polyhydric alcohol. The solvent used in the second form of the depolymerization method of the present embodiment contains, relative to 100 mass% of the solvent, 1 mass% or more and 25 mass% or less of an inorganic salt and 30 mass% or more and 99 mass% or less of water relative to 100 mass% of the solvent. In the second configuration of the depolymerization method of the present embodiment, the content of water in the solvent is 30% by mass or more and 99% by mass or less, preferably 35% by mass or more and 90% by mass or less, and more preferably 40% by mass or more and 80% by mass or less.

[0027] <Inorganic salts> The inorganic salt is a general term for salts composed only of inorganic components, and examples thereof include metal salts, which are compounds in which the hydrogen atoms of an acid are substituted with metal ions. The metal salt is not limited to the following, but examples thereof include halides of calcium, zinc, and lithium, which are suitable for depolymerization, and specific examples thereof include calcium chloride, zinc chloride, zinc bromide, chromium bromide, iron bromide, lithium chloride, lithium bromide, cobalt chloride, etc. From the viewpoints of availability and safety, calcium chloride, zinc chloride, and lithium chloride are preferred. The amount of the metal salt used in the depolymerization step is 1% by mass to 25% by mass, preferably 5% by mass to 20% by mass, and more preferably 10% by mass to 15% by mass, relative to 100% by mass of the solvent. In the depolymerization method of the present embodiment, by using a solvent containing an inorganic salt, the effect of weakening the hydrogen bonds between polymer chains of the polyamide and making depolymerization easier can be obtained. The amount of inorganic salt is set to 1% by mass or more in the solvent from the viewpoint of the amount necessary to weaken hydrogen bonds, and is set to 25% by mass or less in the solvent from the viewpoint of suppressing the generation of by-products during separation and purification of depolymerization.

[0028] <Polyhydric alcohol> Examples of polyhydric alcohols include, but are not limited to, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, butanediol, pentanediol, hexanediol, glycerin, diglycerin, triglycerin, threitol, erythritol, pentaerythritol, arabitol, ribitol, xylitol, sorbite, sorbitan, sorbitol, and mannitol. In particular, ethylene glycol is preferred from the viewpoint of solubility.

[0029] <Polyhydric alcohol, inorganic salt, water, and other ingredients> The solvent used in the depolymerization method of the present embodiment can contain, as constituent components, a polyhydric alcohol, an inorganic salt, and water, and may further contain other components as necessary. For example, a mixed solvent containing other solvents may be used. In consideration of the purification step after the depolymerization step, for example, water or methanol may be mixed as a poor solvent. When a poor solvent other than a polyhydric alcohol is contained in the solvent, the mixing ratio is preferably 1 part polyhydric alcohol to 1 to 9 parts poor solvent. For example, a mixed solvent containing 1 part ethylene glycol to 1 to 9 parts methanol or water by mass ratio is possible. A mixed solvent containing 1 part polyhydric alcohol to 1 to 3 parts poor solvent by mass ratio is more preferred, and a mixed solvent containing 1 part polyhydric alcohol to 1 to 2 parts poor solvent by mass ratio is even more preferred. When depolymerizing, the polyamide resin does not need to be completely dissolved at the start of heating, and dissolution and decomposition by depolymerization may proceed in parallel. That is, decomposition into monomers may proceed sequentially starting from the dissolved polyamide resin. Furthermore, from the viewpoint of promoting depolymerization, that is, hydrolysis of amide bonds, an acid such as formic acid, phosphoric acid, sulfuric acid, or hydrochloric acid may be further added to the solvent.

[0030] <Heating> In the depolymerization method of the present embodiment, at least one selected from the group consisting of a polyamide resin, a polyamide resin composition, and a molded article thereof is heated in the above-mentioned solvent. The temperature in the system during heating is preferably 210°C or lower, more preferably 70 to 200°C, and even more preferably 100 to 190°C, from the viewpoint of suppressing side reactions. As a heating method for depolymerization, any known method such as steam or an electric heater can be applied.

[0031] (Residual rate of polyamide resin after depolymerization process and quantification of products) In the depolymerization method of the present embodiment, after the depolymerization step, the residual rate of the polyamide resin is set to 10 mass% or less, and the decomposition products of the polyamide resin, i.e., dicarboxylic acids, diamines, lactams, and derivatives thereof, are set to 70 mass% or more relative to 100 mass% of the polyamide resin before the depolymerization step. The residual rate of the polyamide resin is more preferably 7% by mass or less, and even more preferably 5% by mass or less. The amount of the decomposition products is preferably 75% by mass or more, and more preferably 80% by mass, based on 100% by mass of the polyamide resin before the depolymerization step. The residual rate of the polyamide resin can be controlled within the above range by adjusting the reaction time and temperature. The amount of the decomposition product can be controlled within the above range by adjusting the reaction time and temperature.

[0032] The residual rate of the polyamide resin and the decomposition products, i.e., dicarboxylic acid, diamine, lactam, and derivatives thereof, in the reaction solution after the depolymerization step can be quantified by known methods such as GPC, NMR, liquid chromatography, gas chromatography, and FT-IR. For example, the method described in Non-Patent Document 1 below can be used. <Non-Patent Document 1> Ema Zagar et al. “Quantitative Determination of PA6 and / or PA66 Content in Polyamide-Containing Waste” ACS Sustainable Chem. Eng. 2020,8 11818-11826

[0033] The decomposition products can be purified by known methods, including, but not limited to, for example, adipic acid can be purified by precipitation and recrystallization, and hexamethylenediamine can be purified by distillation.

[0034] When depolymerizing a molded article, it is preferable to crush and pulverize the molded article in advance using a known method before depolymerizing the molded article, from the viewpoint of highly efficient monomerization. [Example]

[0035] The present invention will be described in more detail below with reference to specific examples and comparative examples, but the present invention is not limited to the following examples and comparative examples. In the examples and comparative examples, the physical properties of the polymers were measured and the characteristics were evaluated by the following methods.

[0036] [Polyamide resin, polyamide resin composition, and molded article used in the depolymerization step] (Polyamide resin) A: Polyamide 66 (manufactured by Asahi Kasei Corporation, model number: Leona 1300)

[0037] (Polyamide resin composition) B: Polyamide 66 resin composition (manufactured by Asahi Kasei Corporation, model number: Leona 14G33, glass fiber ratio 33%)

[0038] (Molded article of polyamide resin composition) C: A molded article of a polyamide 66 resin composition was produced as follows. The Leona 14G33 (manufactured by Asahi Kasei) was dried in a nitrogen stream to reduce the water content in the polyamide resin composition to 500 ppm or less, thereby obtaining pellets of the polyamide resin composition. Next, using the pellets of the polyamide resin composition with the adjusted moisture content, a multipurpose test piece (A-type, dumbbell-shaped tensile test piece) was molded in accordance with ISO3167 using an injection molding machine (SE-50D manufactured by Sumitomo Heavy Industries, Ltd.). The dimensions of the multipurpose test piece were: total length ≥ 170 mm, distance between tabs 109.3 ± 3.2 mm, length of parallel part 80 ± 2 mm, radius of shoulder 24 ± 1 mm, width of end 20 ± 0.2 mm, width of central parallel part 10 ± 0.2 mm, and thickness 4 ± 0.2 mm. The injection molding conditions were set as follows: injection speed: 100 mm / sec (controlled by injection speed), injection time: 1 second, cooling time: 14 seconds, mold temperature: 80°C, cylinder temperature: 290°C. The obtained multipurpose test piece was used as a molded article of the polyamide resin composition.

[0039] <Pulverization of molded product of polyamide resin composition> The molded product of the polyamide resin composition was crushed using a crusher "PFS-40" manufactured by Nippon Seam Co., Ltd. After crushing, the molded product became irregular resin pieces having lengths of 3 to 6 mm.

[0040] [Method for preparing solvent used in depolymerization step] Ethylene glycol was weighed out in the amounts shown in Tables 1 and 2 below into a 20 mL glass reaction tube, and predetermined amounts of methanol and water were also weighed out.

[0041] [Depolymerization process] (Examples 1 to 10), (Comparative Examples 1 to 4) In Examples 1 to 10 and Comparative Examples 1 to 4, the depolymerization step was carried out according to the solvent compositions and reaction conditions shown in Tables 1 and 2 below. For depolymerization, a glass reaction tube was sealed, wrapped around with an electric heater, and heated for a certain period of time while monitoring the temperature on a thermometer inside the reaction tube and adjusting the voltage with a variac to maintain a predetermined temperature. The heating temperature was set to a temperature below the boiling point of each solvent, and the heating time was set to until the pellets or resin were almost no longer visible to the naked eye. Even if resin remained, the heating was stopped after 150 minutes.

[0042] [Evaluation method] (Evaluation of the amount of remaining polyamide resin) The amount of remaining polyamide resin was measured as follows. After the depolymerization was completed, the reaction liquid was returned to room temperature, and the insoluble matter was collected by filtration and dried at room temperature for 6 hours. Next, the insoluble matter was added to 10 mL of hexafluoroisopropanol, stirred at room temperature for 24 hours, and filtered to remove the insoluble matter. The hexafluoroisopropanol was collected, and the solvent was removed using a rotary evaporator. The remaining residue was taken as the remaining amount of polyamide resin.

[0043] (Evaluation of the amount of hexamethylenediamine (HMD) and its derivatives in polyamide resin before depolymerization, and evaluation of the amount of adipic acid (ADA) and its derivatives in polyamide resin before depolymerization) The reaction solution after depolymerization was measured using liquid chromatography (Agilent 1100, manufactured by Agilent Technologies), and the amounts of HMD, ADA, and their derivatives relative to the polyamide resin before depolymerization were quantified from a calibration curve prepared using reference HMD (or a derivative such as HMD hydrochloride) and ADA.

[0044] Tables 1 and 2 show the depolymerization conditions for the polyamide resins, polyamide resin compositions, and molded articles of Examples 1 to 10 and Comparative Examples 1 to 4, as well as the quantitative results of the decomposition products due to depolymerization.

[0045] [Table 1]

[0046] [Table 2]

[0047] In Examples 1 to 10, depolymerization of any one selected from the group consisting of polyamide resins, polyamide resin compositions, and molded articles thereof was carried out in a solvent containing 1 to 25% by mass of a metal salt and a polyhydric alcohol. As a result, a residual rate of polyamide resin was 10% by mass or less in a short period of time, and a decomposition product containing a total of 70% by mass or more of dicarboxylic acids, diamines, and their derivatives was obtained, thereby achieving monomerization in high yield.

[0048] This application is based on a Japanese patent application (Patent Application No. 2021-209125) filed with the Japan Patent Office on December 23, 2021, the contents of which are incorporated herein by reference. [Industrial Applicability]

[0049] The depolymerization method of the present invention has industrial applicability as an efficient method for recycling polyamide resins, polyamide resin compositions, and molded articles used in automobile parts and various industrial parts.

Claims

1. a depolymerization step of heating and depolymerizing at least one selected from the group consisting of a polyamide resin, a polyamide resin composition containing the polyamide resin, and a molded article thereof in a solvent containing, relative to 100% by mass of the solvent, 1% by mass to 25% by mass of an inorganic salt which is a halogen salt of at least one metal selected from the group consisting of calcium, zinc, and lithium, and 10% by mass to 95% by mass of a polyhydric alcohol; The temperature in the system during the heating is 210°C or less, After the depolymerization step, the residual rate of the polyamide resin is 10% by mass or less, the decomposition products of the polyamide resin, i.e., dicarboxylic acid, diamine, lactam, and derivatives thereof, are contained in an amount of 70% by mass or more relative to 100% by mass of the polyamide resin before the depolymerization step; Depolymerization method.

2. The inorganic salt is calcium chloride or zinc chloride. The depolymerization method according to claim 1 .

3. The polyamide resin is polyamide 66. The depolymerization method according to claim 1 .

4. The polyhydric alcohol is ethylene glycol. The depolymerization method according to claim 1 .

5. The solvent is a mixed solvent containing 1 part ethylene glycol to 1 part methanol or water in a mass ratio of 1 to 9 parts, The depolymerization method according to claim 1 .

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