Method for producing diamine and dicarboxylic acid, method for recycling polyamide, and polyamide

Abstract

A method for producing a diamine and a dicarboxylic acid, the method comprising: a pre-treatment step for subjecting a polyamide-containing waste to at least one selected from the group consisting of pulverization, washing, and foreign matter separation to obtain a crude polyamide; a depolymerization step for putting the crude polyamide into an aqueous solution containing an acid in a closed-system reactor, increasing the temperature of the solution to a target temperature of 90-160°C, and thereafter, heating the solution at around the target temperature, to depolymerize and decompose at least 80 mass% of all the polyamides into a diamine and a dicarboxylic acid; a separation step for removing components other than the diamine, a diamine derivative, the dicarboxylic acid, and a dicarboxylic acid derivative from a reaction liquid obtained in the depolymerization step to obtain the diamine, the diamine derivative, the dicarboxylic acid, and the dicarboxylic acid derivative; and a purification step for isolating and purifying the diamine and the dicarboxylic acid from the diamine and the diamine derivative, and the dicarboxylic acid and the dicarboxylic acid derivative, respectively, obtained in the separation step.

Description

Method for producing diamine and dicarboxylic acid, method for recycling polyamide, and polyamide

[0001] The present invention relates to a method for producing diamines and dicarboxylic acids, a method for recycling polyamides, and polyamides.

[0002] Polyamides 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 required to respond to a resource-recycling society, and there is a demand for the establishment of recycling technologies for polyamides as well.

[0003] Recycling is generally divided into three types: material recycling, chemical recycling, and thermal recycling. Currently, in automobile applications, which account for the majority of polyamide uses, most of the polyamide in scrapped automobiles is burned for thermal recycling and is not effectively utilized as a resource, so there is a need to effectively utilize the polyamide through material recycling and chemical recycling. Furthermore, from the perspective of reducing GHG (greenhouse gas) emissions, there is a need for material recycling and chemical recycling of polyamide.

[0004] However, polyamide resin compositions and molded articles for automotive applications contain, in addition to polyamide, for example, inorganic fillers such as glass fibers, various additives such as heat stabilizers, pigments, dyes, etc. (see, for example, Patent Document 1), and therefore polyamides obtained by material recycling have the problem that it is difficult to maintain practically sufficient mechanical properties after recycling. Therefore, chemical recycling, in which polyamide 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 related to the chemical recycling, Patent Document 2 proposes a technology for producing a monomer by decomposing polyamide by ammonolysis using a Lewis acid catalyst. Patent Document 3 proposes a method for separating polyamide and glass fiber from a molded article of a glass fiber-containing polyamide resin composition using an aqueous phosphoric acid solution, and further decomposing the polyamide into monomers. Non-Patent Document 1 proposes depolymerization of polyamide 66 using microwaves.

[0006] Japanese Patent No. 6839266 Japanese Patent No. 3571723 Japanese Patent Laid-Open No. 2000-80199

[0007] Urska Cesarek et al. “Chemical Recycling of Aliphahatic Polyamides by Microwave-Assisted Hydrolysis for Efficient Monomar Recovery” ACS Sutainable Chem. Eng. 2020, 8, 16274-16282

[0008] However, the method described in Patent Document 2 suffers from the problem of low monomer yield. Furthermore, the method described in Patent Document 3, in a specific example using polyamide 66, requires as long as 200 minutes to dissolve the polyamide and glass fiber. Furthermore, Patent Document 3 does not provide a specific description of the depolymerization method. Furthermore, since it takes as long as 200 minutes to dissolve the polyamide and glass fiber, if further depolymerization of the polyamide is performed after dissolving the molded product of the polyamide resin composition, the total time becomes even longer, resulting in a problem of lacking practicality. Furthermore, the method described in Non-Patent Document 1 achieves high-yield depolymerization of polyamide in a short time by using a high-concentration hydrochloric acid of 17% by mass or more and heating to a high temperature of 170°C or higher using microwaves. Therefore, while microwave irradiation using a glass reaction vessel is possible at the laboratory level, when considering implementation at an industrial level, the use of high-concentration hydrochloric acid at a high temperature of 170°C or higher causes corrosion of the reaction vessel, which severely limits the materials that can be used for the reaction vessel. For example, as described in Non-Patent Document 2 (Corrosion Resistance of New Metallic Materials, Takamura Akira, Anticorrosion Technology, Vol. 16 (1967) No. 3, pp. 97-106), even titanium, which is generally said to have high corrosion resistance, can suffer from corrosion problems when exposed to high concentrations of hydrochloric acid of 5% or more at temperatures of 100°C or higher.

[0009] As described above, the methods described in Patent Documents 2 and 3 have problems in that the monomer yield is low and the time required, including monomer recovery and depolymerization, is not practical. Furthermore, the method described in Non-Patent Document 1 has a problem of corrosion of the reaction apparatus, making it industrially impractical. Furthermore, if the process is carried out under conditions of high temperature, high pressure, etc. in order to shorten the time, the energy required for the process increases, making it impractical from the viewpoint of reducing greenhouse gas (GHG) emissions.

[0010] In view of the above-mentioned problems of the prior art, the present invention aims to provide a method for producing diamines and dicarboxylic acids, a method for recycling polyamides, and polyamides obtained by the method, which can be carried out safely even in a high-pressure environment even when carried out industrially, with little corrosion of the reaction apparatus, and which enable the recovery of diamines and dicarboxylic acids in high yield and high purity, and which can produce polyamides as polymerization raw materials having physical properties and quality equivalent to those of polyamides derived from petrochemical raw materials.

[0011] As a result of extensive research into solving the problems of the prior art described above, the present inventors have found that the above-mentioned problems can be solved by heating polyamides, polyamide resin compositions, and molded articles thereof in a predetermined solvent within a predetermined temperature range to depolymerize them, separating and removing impurities derived from recycled raw materials and foreign substances used in the recycling process or generated as by-products, and purifying diamines and dicarboxylic acids, thereby completing the present invention.

[0012] [1] A method for producing diamines and dicarboxylic acids, comprising: a pretreatment step of subjecting polyamide-containing waste to one or more treatments selected from the group consisting of pulverization, washing, and separation of foreign matter to obtain a crude polyamide; a depolymerization step of charging the crude polyamide into an aqueous solution containing an acid in a closed reactor, heating the crude polyamide to a target temperature of 90°C to 160°C, and then heating at a temperature close to the target temperature to depolymerize 80% or more of the amide groups in all the polyamides contained in the crude polyamide and decompose them into diamines and dicarboxylic acids; a separation step of removing components other than the diamines and derivatives of the diamines, and the dicarboxylic acids and derivatives of the dicarboxylic acids, from the reaction solution obtained in the depolymerization step, to obtain the diamines and derivatives of the diamines, and the dicarboxylic acids and derivatives of the dicarboxylic acids; and a purification step of isolating and purifying the diamines and dicarboxylic acids from the diamines and derivatives of the diamines, and the dicarboxylic acids and derivatives of the dicarboxylic acids obtained in the separation step. [2] The method for producing a diamine and a dicarboxylic acid according to [1] above, wherein, in the depolymerization step, a space is left above the reactor that is not filled with the aqueous solution containing the crude polyamide, and microwaves are irradiated from above the reactor by a microwave transmitter through a waveguide and the unfilled space. [3] The method for producing a diamine and a dicarboxylic acid according to [1] or [2] above, wherein, in the depolymerization step, the temperature rise rate during the temperature rise is 25°C / min or less. [4] The method for producing a diamine and a dicarboxylic acid according to any one of [1] to [3] above, wherein, in the depolymerization step, the reactor has a device for monitoring the pressure inside the reactor and venting the pressure when the pressure exceeds a reference pressure. [5] The method for producing a diamine and a dicarboxylic acid according to any one of [1] to [4], wherein the separation step comprises a step of removing components other than the diamine and the diamine derivative, and the dicarboxylic acid and the dicarboxylic acid derivative as insoluble matters from the reaction solution, in a state in which the dicarboxylic acid and / or the dicarboxylic acid derivative and the diamine and the diamine derivative are partially or entirely dissolved in the reaction solution.[6] The method for producing a diamine and a dicarboxylic acid according to any one of [1] to [5] above, wherein the pKa of the acid is 0 or less. [7] The method for producing a diamine and a dicarboxylic acid according to any one of [1] to [6] above, wherein the amount of the acid is such that the ratio of the molar numbers of amide groups in the crude polyamide to protons in the acid is 1:1 to 1:5.5 (amide groups in crude polyamide: protons of acid). [8] The method for producing a diamine and a dicarboxylic acid according to any one of [1] to [7] above, wherein the acid is hydrochloric acid. [9] The method for producing a diamine and a dicarboxylic acid according to [8] above, wherein the concentration of the hydrochloric acid in the aqueous solution is 5% by mass or more and 25% by mass or less.

[10] The method for producing a diamine and a dicarboxylic acid according to any one of [1] to [9] above, wherein the main component of the polyamide in the polyamide-containing waste is polyamide 66.

[11] The method for producing a diamine and a dicarboxylic acid according to any one of [1] to

[10] above, wherein a reactor having an inner surface made of glass lining, zirconium, or tantalum is used in the depolymerization step.

[12] The method for producing a diamine and a dicarboxylic acid according to any one of [1] to

[11] above, wherein hot filtration and centrifugation are performed in the separation step to remove components other than the diamine, the diamine derivative, and the dicarboxylic acid and the dicarboxylic acid derivative.

[13] The method for producing a diamine and a dicarboxylic acid according to any one of [1] to

[12] above, wherein the dicarboxylic acid is purified by crystallization in the purification step.

[14] The method for producing a diamine and a dicarboxylic acid according to any one of [1] to

[13] above, wherein the diamine is purified by distillation in the purification step.

[15] The method for producing a diamine and a dicarboxylic acid according to any one of [2] to

[14] above, wherein a partition window separating the reactor and the microwave transmission device is provided midway through the waveguide.

[16] The method for producing a diamine and a dicarboxylic acid according to

[15] above, wherein the partition window is made of quartz glass.

[17] The method for producing a diamine and a dicarboxylic acid according to any one of [2] to

[16] above, wherein the microwave frequency during the heating with microwaves is 0.8 to 6 GHz.

[18] The method for producing a diamine and a dicarboxylic acid according to any one of [1] to

[17] above, wherein in the purification step, the content of Si element is 1 ppm by mass or more and 200 ppm by mass or less relative to the total amount of the dicarboxylic acid.

[19] A method for recycling a polyamide, comprising a polymerization step of obtaining a diamine and a dicarboxylic acid by the method for producing a diamine and a dicarboxylic acid according to any one of [1] to

[18] above, and polymerizing the diamine and the dicarboxylic acid to obtain a polyamide.

[20] A polyamide obtained by the method for recycling a polyamide according to

[19] above, which contains Si element in an amount of 1 ppm by mass or more and 100 ppm by mass or less relative to the total amount of the polyamide.

[0013] According to the present invention, there are provided a method for producing diamines and dicarboxylic acids, a method for recycling polyamides, and polyamides obtained by the method, which can be carried out safely even in a high-pressure environment, cause little corrosion to the reaction apparatus, and enable the recovery of diamines and dicarboxylic acids in high yields and with high purity, and which can use the resulting monomers as polymerization raw materials to obtain polyamides having physical properties and quality equivalent to those of polyamides derived from petrochemical raw materials.

[0014] FIG. 1 is a diagram showing a flow of a polyamide recycling method.

[0015] Hereinafter, a mode for carrying out the present invention (hereinafter referred to as "the present embodiment") will be described in detail. The following present embodiment is an example for explaining the present invention, and is not intended to limit the present invention to the following content. The present invention can be carried out by appropriately modifying it within the scope of its gist.

[0016] [Method for Producing Diamine and Dicarboxylic Acid] The method for producing a diamine and a dicarboxylic acid of this embodiment includes: a pretreatment step of subjecting a polyamide-containing waste to one or more treatments selected from the group consisting of pulverization, washing, and separation of foreign matter to obtain a crude polyamide; a depolymerization step of charging the crude polyamide into an aqueous solution containing an acid in a closed reactor, heating the crude polyamide to a target temperature of 90°C to 160°C, and then heating at a temperature close to the target temperature to depolymerize 80% or more of the amide groups in all the polyamides contained in the crude polyamide and decompose them into diamine and dicarboxylic acid; a separation step of removing components other than the diamine and the diamine derivative, and the dicarboxylic acid and the dicarboxylic acid derivative from the reaction solution obtained in the depolymerization step, to obtain the diamine and the diamine derivative, and the dicarboxylic acid and the dicarboxylic acid derivative; and a purification step of isolating and purifying the diamine and the dicarboxylic acid from the diamine and the diamine derivative, and the dicarboxylic acid and the dicarboxylic acid derivative obtained in the separation step.

[0017] In this embodiment, the polyamide is recycled by polymerizing the diamine and dicarboxylic acid obtained as described above.

[0018] According to the method for producing diamines and dicarboxylic acids and the method for recycling polyamides of the present embodiment, even when carried out industrially, the method can be carried out safely even under high-pressure environments, the degree of corrosion of the reaction apparatus is small, and it is possible to recover diamines and dicarboxylic acids with high yields and high purity, thereby producing high-quality polyamides.

[0019] (Pretreatment Step) In the method for producing a diamine and a dicarboxylic acid of the present embodiment, in the pretreatment step, a polyamide-containing waste is subjected to one or more treatments selected from the group consisting of pulverization, washing, and separation of foreign matter to obtain a crude polyamide.

[0020] <Polyamide-Containing Waste> In the diamine and dicarboxylic acid production method of this embodiment, polyamide-containing waste is used. Here, "waste" does not mean waste as defined by law (such as the Waste Disposal and Public Cleansing Act), but is defined as a general term for things that are no longer used as products or cannot be used as products. Polyamide-containing waste includes molded products of polyamide or polyamide resin compositions, as well as polyamide and polyamide resin compositions as products. Specifically, examples include unnecessary in-process offcuts and out-of-spec products discarded from polyamide fiber production processes, unnecessary in-process offcuts and out-of-spec products discarded from molding processes using polyamide, unnecessary in-process offcuts and out-of-spec products discarded from processes such as weaving and sewing using polyamide fibers, airbags, carpets, and apparel products obtained from polyamide fibers, molded polyamide products discarded after actual use, and final products made of polyamide fibers discarded after actual use. Hereinafter, these are collectively referred to as polyamide-containing waste. Figure 1 shows a flowchart specifically illustrating the polyamide recycling method of this embodiment.

[0021] [Polyamide] Polyamide refers to a polymer having an amide bond (—NHCO—) in the main chain. The polyamide is preferably a polyamide polymerized from a diamine and a dicarboxylic acid. Examples of polyamides include, but are not limited to, polyamide 46 (polytetramethylene adipamide), polyamide 56 (polypentamethylene adipamide), polyamide 66 (polyhexamethylene adipamide), polyamide 410 (polytetramethylene sebacamide), polyamide 412 (polytetramethylene dodecamide), 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.

[0022] In particular, the polyamide preferably contains one or more selected from the group consisting of polyamide 66, polyamide 66 / 6I, polyamide 610, polyamide 612, polyamide 6I, and polyamide 6 as the main component, and more preferably polyamide 66, polyamide 66 / 6I, or a mixture of polyamide 66 and polyamide 6I. Here, "main component" means a component that accounts for more than 50% by mass of 100% by mass of the total polymer components. Polyamide 66 is a polyamide obtained by condensation polymerization of hexamethylenediamine and adipic acid, and is suitable for use as a material for functional parts of automobiles, machinery, and electrical appliances or as a high-strength fiber due to its excellent heat resistance, mechanical strength, and creep properties.

[0023] [Polyamide Resin Composition] The polyamide resin composition is a resin composition containing the above-mentioned polyamide and, if necessary, inorganic fillers such as glass fibers, lubricants, and other additives such as heat stabilizers, flame retardants, pigments, and dyes.

[0024] <Inorganic Filler> The polyamide resin composition and molded articles thereof may contain an inorganic filler. This tends to provide the polyamide resin composition and molded articles thereof with excellent mechanical strength and rigidity. Examples of inorganic fillers include, but are not limited to, glass fiber, carbon fiber, calcium silicate fiber, 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.

[0025] <Lubricant> The polyamide resin composition and molded articles thereof may further contain a lubricant in addition to the polyamide resin and inorganic filler described above. This tends to give the polyamide resin composition and molded articles thereof excellent fluidity and appearance.

[0026] <Other Additives> The polyamide resin composition and molded articles thereof may contain other additives in addition to the polyamide, inorganic filler, and lubricant described above. Examples of other additives include, but are not limited to, antioxidants, ultraviolet absorbers, heat stabilizers, photodegradation inhibitors, plasticizers, mold release agents, nucleating agents, flame retardants, colorants, other thermoplastic resins, etc.

[0027] [Molded Article] The molded article of polyamide or polyamide resin composition used in this embodiment is produced by molding using various known methods, for example, injection molding, etc. The molded article of this embodiment may be a fiber of polyamide or polyamide resin composition.

[0028] Polyamides are manufactured in various forms, including fibers and films, and are used annually at a rate of over 2 million tons for a wide range of applications, including clothing, carpets, packaging films, automotive parts, and industrial parts. Polyamide or fiber waste suitable for chemical recycling is preferable because the higher the polyamide content in the product, the higher the recycling efficiency. For example, the base fabric of polyamide 66 airbags is optimal, being approximately 90% polyamide 66. Polyamide 66 fiber can also be used in apparel, bags, and other clothing items, outdoor gear, sportswear, and other applications. Furthermore, cable ties are a typical application of unreinforced polyamides in molded products. Meanwhile, glass fiber-reinforced polyamide resin compositions are generally used in automotive parts for their strength and physical properties, with the mass ratio of glass fiber in molded products typically ranging from 30 to 40% by mass. Therefore, the polyamide component is approximately 60 to 70% by mass, resulting in low recycling efficiency for polyamide waste. From this perspective, suitable polyamide waste for chemical recycling includes airbag fabrics, cable ties, carpet pile, and, in the case of automobile parts, radiator tanks with a glass fiber ratio of about 30% by mass. Furthermore, scraps generated in factories during the manufacturing process are more preferable than used market-recovered products because they are less susceptible to environmental foreign matter and decomposition products due to deterioration.

[0029] As shown in FIG. 1 , in this embodiment, a pretreatment step is performed on the polyamide-containing waste. In the pretreatment step, the polyamide-containing waste is subjected to one or more processes selected from the group consisting of pulverization, washing, and separation of foreign matter to obtain crude polyamide. This results in a crude polyamide with a large surface area, which allows the subsequent depolymerization step to be carried out more efficiently. In this pretreatment step, impurities such as metals, stones, glass, and sand are removed. For washing and separation of foreign matter, for example, gravity separation can be performed by adding washing water. However, if the polyamide in the polyamide-containing waste is highly pure and does not contain significant impurities, the washing and separation of foreign matter steps can be simplified or omitted. After the pretreatment step, the depolymerization step described below can be carried out to achieve highly efficient monomerization.

[0030] <Depolymerization Step> As shown in FIG. 1 , the depolymerization step is carried out after the pretreatment step. In the depolymerization step, the crude polyamide obtained in the pretreatment step is placed in an acid-containing aqueous solution in a closed reactor, heated to a target temperature of 90°C to 160°C, and then heated near the target temperature to depolymerize 80% or more of the amide groups in the crude polyamide, resulting in a reaction solution in which the amide groups are decomposed into diamines and dicarboxylic acids. Note that "near the target temperature" refers to a temperature range in which the depolymerization reaction occurs stably and continuously. Cleavage of the amide bonds in polyamide requires external supply of the energy required to cleave the amide bonds, which necessitates heating. The heating method is not particularly limited, but examples include steam, an electric heater, etc. Furthermore, the use of microwaves allows depolymerization to be carried out with low energy, allowing the depolymerization step to be carried out safely even in a high-pressure environment. Furthermore, the addition of an acid is required to promote depolymerization. Specific acids will be described later. According to the depolymerization step, polyamide, polyamide resin composition, and molded articles thereof can be converted into monomers with low energy and in high yield, and can be chemically recycled.

[0031] [Reactor] In the depolymerization step, a closed reactor is used as the reactor. Specifically, a closed pressure-resistant vessel is preferred because the pressure inside the reactor can increase due to factors such as the evaporation of water in the reaction system, the generation of outgassing due to the decomposition of organic matter other than polyamide contained in the crude polyamide used as a raw material, and the generation of hydrogen gas due to the reaction of metals that may be contained as foreign matter in the polyamide-containing waste with acid. When microwaves are used in the depolymerization step, in order to efficiently irradiate the microwaves into the system, it is preferable to install a microwave generator (magnetron) and a waveguide above the reactor, and to have an unfilled portion in the reactor that is not filled with the aqueous solution containing the crude polyamide, rather than filling the entire reactor with the aqueous solution containing the crude polyamide. In this state, microwaves can be irradiated from the microwave generator through the waveguide and the space in the unfilled portion, thereby enabling uniform irradiation of the microwaves and tending to result in more uniform heating within the system. It is preferable that a partition window be provided midway through the waveguide, separating the reactor from the microwave generator and allowing microwaves to pass through but physically blocking them. The partition window is preferably designed to withstand an increase in internal pressure within the reactor. Specifically, the partition window is preferably made of quartz glass. In addition, from the viewpoint of preventing an abnormal increase in internal pressure due to sudden gas generation and damage to the reactor, it is preferable that the reactor be provided with equipment for constantly monitoring the internal pressure and venting the pressure when it exceeds a reference pressure.

[0032] [Solvent] In the depolymerization step, an aqueous solution containing an acid is used. In other words, water is used as the solvent. This is because the diamines, dicarboxylic acids, and their derivatives produced by depolymerization are water-soluble, making it easy to physically remove water-insoluble components, such as inorganic fillers such as glass fiber, carbon black, pigments, and additives, when removing impurities derived from the recovered polyamide in a subsequent step. However, if an appropriate process for removing impurities derived from the recovered polyamide can be established in a subsequent step, organic solvents such as ethylene glycol and methanol may also be used in combination. Furthermore, in the depolymerization step, the crude polyamide, i.e., the polyamide-containing waste, does not necessarily need to be completely dissolved in the solvent as a polymer; it is sufficient for it to be partially dissolved and decomposed into monomers in the depolymerization step. Since the crude polyamide does not need to be completely dissolved in the solvent as a polymer during the depolymerization step, there is an advantage in that viscosity increase during the depolymerization step is small, allowing a large amount of crude polyamide to be added to the solvent.

[0033] [Inorganic Salt] In the depolymerization step, an inorganic salt may be added to the aqueous solution to increase the solubility of the crude polyamide, i.e., the polyamide-containing waste, in water. Using an aqueous solution containing an inorganic salt tends to weaken the hydrogen bonds between the polymer chains of the polyamide, thereby increasing solubility. The term "inorganic salt" refers to a salt composed solely 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. Examples of the metal salt include, but are not limited to, calcium, zinc, and lithium halides, which are suitable for depolymerization. Specific examples include calcium chloride, zinc chloride, zinc bromide, chromium bromide, iron bromide, lithium chloride, lithium bromide, and cobalt chloride. From the viewpoints of availability and safety, calcium chloride, zinc chloride, and lithium chloride are preferred.

[0034] [Acid] In the depolymerization step, the crude polyamide is added to an aqueous solution containing an acid. The acid is thought to act as a catalyst for the hydrolysis of the polyamide. Examples of the acid include, but are not limited to, organic acids such as methanesulfonic acid, trifluoromethanesulfonic acid, p-toluenesulfonic acid, and trifluoroacetic acid; inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid; and Sc(OTf) ３ , Yb(OTf) ３ , Nb ２ O ５ , CeO ２ These may be used alone or in combination, but in order to obtain a high depolymerization rate during depolymerization, an acid having a pKa of 0 or less is preferred.

[0035] The acid is preferably hydrochloric acid from the viewpoints of reaction efficiency, reducing impurities, and reducing waste that is difficult to regenerate.

[0036] The amount of acid used in the depolymerization step (the number of moles of protons released) is preferably in excess relative to the number of moles of amide groups in the polyamide. On the other hand, using a large excess of acid inevitably results in a relatively small amount of polyamide being added in the depolymerization step, so from the viewpoint of productivity, it is preferable that the amount of acid is not too large. In particular, using a large amount of acid increases the amount of water in the system, which increases the load on the water removal step during the subsequent diamine purification process, leading to increased costs and GHG emissions. Furthermore, using a large excess of acid generates a large amount of salt in the neutralization step after the depolymerization step, which increases the load and costs of the salt treatment process. For this reason, it is also preferable that the amount of acid is not too large. From the above viewpoints, the ratio of the number of moles of amide groups to acid protons in the crude polyamide is preferably 1:1 to 1:5.5, more preferably 1:1 to 1:3, and even more preferably 1:1.15 to 1:2.

[0037] Analysis of the acid concentration in the aqueous solution in the depolymerization step using the Arrhenius equation shows that as the hydrochloric acid concentration increases, the activation energy Ea decreases, making the reaction more likely to proceed, but at the same time, as the number of water molecules decreases, the frequency factor A decreases, making it more difficult for the reaction to proceed. Therefore, there is an optimum range for the hydrochloric acid concentration, and it is preferably 5% by mass or more and 25% by mass or less. It is more preferably 10% by mass to 25% by mass, and even more preferably 12% by mass to 22% by mass.

[0038] [Temperature Increase] In the depolymerization step, the crude polyamide is introduced into an aqueous solution containing an acid and heated to a target temperature of 90°C or higher and 160°C or lower. The temperature increase rate during the temperature increase is preferably 25°C / min or lower, more preferably 20°C / min or lower, and even more preferably 15°C / min or lower, from the viewpoint of achieving a good balance between the energy input during the temperature increase and the energy required to maintain the predetermined temperature after it has been reached, and optimizing equipment efficiency. Furthermore, in order to prevent an abnormal increase in internal pressure due to sudden gas generation, such as outgassing caused by decomposition of organic matter other than polyamide contained in the crude polyamide, i.e., polyamide-containing waste, or hydrogen gas generated by reaction of metals potentially contained as foreign matter with acid, and thereby damaging the reactor, the temperature increase rate is preferably kept below a certain value, as in the above-mentioned numerical range. In the case of microwave heating, if an abnormal increase in internal pressure occurs, heating will immediately stop if the power supply is stopped, so microwaves are preferred from a safety perspective. Furthermore, from the viewpoint of shortening the time required for the depolymerization step, the temperature rise rate during heating is preferably 1°C / min or more, more preferably 2°C / min or more, and even more preferably 3°C / min or more. The target temperature is 90°C or more and 160°C or less, preferably 95 to 150°C, more preferably 100 to 145°C, and even more preferably 110 to 140°C, from the viewpoint of suppressing side reactions and improving the corrosion resistance of the reactor. As a heating method for raising the temperature, microwaves are preferably used from the viewpoint of shortening the heating time and reducing GHG emissions. Any of known methods such as steam and electric heaters can also be used. These methods may be used alone or in combination.

[0039] [Heating] In the depolymerization step, the temperature is raised to the target temperature as described above, and then the system is heated at a temperature close to the target temperature. "Close to the target temperature" means that the temperature difference between the temperature in the system during heating and the target temperature is within 10°C, and the temperature during heating is preferably 90°C or higher and 160°C or lower, more preferably 95 to 150°C, even more preferably 100 to 145°C, and even more preferably 110 to 140°C. Any known heating method such as steam or an electric heater can be used, but a method using microwaves is preferred from the viewpoints of shortening the heating time and reducing GHG emissions.

[0040] [Microwave Irradiation] The microwave irradiation output during the heating may be appropriately selected so as to heat to the above-mentioned temperature. There is no particular upper limit to the irradiation output. The microwave frequency is not particularly limited, but is preferably, for example, 0.8 to 6 GHz. From the viewpoint of making it easier for the microwave to reach the system of the depolymerization step, the frequency is more preferably 0.8 to 2.5 GHz, even more preferably 0.8 to 1.5 GHz, even more preferably 0.8 to 1 GHz, and even more preferably 0.9 to 0.95 GHz.

[0041] [Corrosion Resistance] In the depolymerization step, it is preferable to use a reactor that is less susceptible to corrosion and is industrially practical. Specifically, it is preferable to use a reactor having an inner surface made of a material such as glass lining, zirconium, or tantalum. The corrosion resistance can be determined, for example, from the data described in "Properties of Tantalum for Applications in the Chemical Process Industry: F.J. Hunkeler (USA) ASTM STP849, 1984, pp. 28-49."

[0042] [Depolymerization Rate of Polyamide in Depolymerization Step] In the depolymerization step, in the diamine and dicarboxylic acid production method of this embodiment, from the viewpoint of achieving a high recovery rate, 80% or more of the amide groups in the total polyamide contained in the crude polyamide are depolymerized. On the other hand, from the viewpoint of recovering decomposition products, the decomposition products, i.e., dicarboxylic acids, diamines, and their derivatives, account for 80% or more by mass of the total polyamide contained in the crude polyamide. The residual rate of polyamide is preferably 10% or less of the amide groups in the total polyamide contained in the crude polyamide, more preferably 7% or less, even more preferably 5% or less, and even more preferably 1% or less. The depolymerization rate is preferably 85% or more of the amide groups in the total polyamide contained in the crude polyamide, more preferably 90% or more, and even more preferably 95% or more. The depolymerization rate of the polyamide, the amount of decomposition products, and the residual rate of polyamide can be controlled within the above-mentioned numerical ranges by adjusting the acid concentration, reaction time, and temperature.

[0043] <Separation Step> As shown in FIG. 1 , in this embodiment, a separation step is performed after the depolymerization step described above. In the separation step, components other than the diamine and diamine derivatives and the dicarboxylic acid and dicarboxylic acid derivatives are removed as insoluble matter from the reaction solution obtained by the depolymerization step, while a portion or all of the dicarboxylic acid and / or dicarboxylic acid derivatives remain dissolved in the reaction solution, thereby obtaining the diamine and diamine derivatives and the dicarboxylic acid and dicarboxylic acid derivatives. Polyamide-containing waste and crude polyamide obtained in the pretreatment step may contain impurities derived from contaminants such as glass fiber, carbon fiber, other inorganic fillers, various water-repellent coating agents, and sand. These remain as insoluble solids in the reaction solution after the depolymerization step, and most of them are removed in the separation step. The separation method is not particularly limited, but examples include sedimentation of insoluble matter, centrifugation, filtration, etc.

[0044] Specific methods for separating components other than the diamine and diamine derivatives, and the dicarboxylic acid and dicarboxylic acid derivatives can be used, for example, filtration using a filter, a method using an ion exchange membrane, an ion exchange resin, or other known methods depending on the target to be removed. However, since polyamide-containing waste generally contains metal, glass, glass fiber, sand, etc., it is preferable from the viewpoint of process efficiency and energy saving to remove these foreign substances by hot filtration while the temperature of the reaction solution is high immediately after the depolymerization step. The hot filtration temperature is preferably a temperature at which the diamine and diamine derivatives, and the dicarboxylic acid and dicarboxylic acid derivatives do not precipitate. Therefore, a temperature of 55°C or higher is preferred, more preferably 60°C or higher, and even more preferably 65°C or higher is preferred.

[0045] In the present embodiment, in order to improve the recovery rate of the diamine and the dicarboxylic acid, in the separation step, it is preferable to separate the insoluble components in a state in which a part or all of the dicarboxylic acid and / or the dicarboxylic acid derivative and a part or all of the diamine and the diamine derivative are dissolved in the reaction liquid obtained in the depolymerization step, for example, in a temperature range in which they are dissolved, and it is more preferable to separate the insoluble components in a state in which all of them are dissolved.

[0046] It is not necessary to remove all of the insoluble matter in the separation step, and it is also possible to remove a small amount of remaining solid matter again in the purification step described below, in which the diamine and dicarboxylic acid are isolated and purified. In this embodiment, it is also preferable to remove the insoluble solid matter simultaneously with the dicarboxylic acid precipitated from the reaction solution in the purification step described below. In this case, it is also possible to remove the solid matter by selectively dissolving the dicarboxylic acid in a solvent from a mixture of the dicarboxylic acid and the insoluble solid matter in a subsequent step, and separating only the insoluble matter from the solution.

[0047] <Purification Step> In this embodiment, as shown in Fig. 1, a purification step is carried out after the separation step. In the purification step, the diamine and dicarboxylic acid are isolated and purified from the diamine and diamine derivative, and the dicarboxylic acid and dicarboxylic acid derivative obtained in the separation step described above. In the purification step for isolating and purifying the diamine and dicarboxylic acid, respectively, a known method can be used, and is not particularly limited, but examples thereof include the following methods.

[0048] [Diamine Isolation and Purification Process] In the diamine isolation and purification process, the diamine is dissolved in the liquid after the separation process described above as a diamine salt with a dicarboxylic acid (e.g., adipic acid) or an acid (e.g., hydrochloric acid). Neutralization is one method for separating the diamine from the solution containing the diamine salt. Adjusting the pH to a range of 7 to 14 allows the diamine to be liberated from the diamine salt. The alkali used in the neutralization process should have a pKa greater than that of the diamine to be isolated. Examples of alkalis commonly used industrially include sodium hydroxide, calcium hydroxide, and potassium hydroxide. Distillation is one method for purifying the liberated diamine to a purity sufficient for use in polyamide polymerization. Distillation purification also makes it possible to remove residues contained in the crude polyamide. If the reaction liquid after the separation process contains a compound having a carboxylic acid, the diamine and the carboxylic acid may polymerize during heating during distillation, resulting in a decrease in diamine yield and scaling of the equipment. Therefore, it is preferable to remove the carboxylic acid before distillation by a method such as crystallization, physical adsorption with activated carbon or an ion exchange resin, membrane separation, etc. Other methods for purifying the diamine include extraction with a solvent that forms an organic phase and membrane separation.

[0049] [Dicarboxylic Acid Isolation and Purification Step] In the dicarboxylic acid isolation and purification method in the purification step, it is preferable to perform purification by crystallization after the above-mentioned separation step. Examples of purification by crystallization include precipitating dicarboxylic acid from the reaction solution obtained in the above-mentioned separation step, crystallizing by recrystallization, obtaining crude dicarboxylic acid crystals by solid-liquid separation, dissolving the obtained crude dicarboxylic acid crystals in pure water, crystallizing and solid-liquid separation, and drying to obtain purified dicarboxylic acid. During crystallization, the crude dicarboxylic acid crystals may be dissolved by stirring or heating the solution, or may be aged for an appropriate period of time to allow crystal growth. Appropriate drying conditions may be selected as long as the temperature is below the melting point of the dicarboxylic acid. Unlike dicarboxylic acids produced by conventional methods, the dicarboxylic acid obtained in the above-mentioned separation step contains residual impurities, such as metal compounds and organic compounds derived from additives and pigments. These impurities cause discoloration of the dicarboxylic acid and act as polymerization inhibitors when repolymerizing to polyamide, so they are preferably removed by crystallization in this isolation and purification step. For example, suitable methods include washing the dicarboxylic acid obtained by crystallization with an inorganic acid such as nitric acid, sulfuric acid, or hydrochloric acid, and then applying a method such as physical adsorption with an ion exchange resin or activated carbon, membrane separation, etc. The dicarboxylic acid purified in this manner can be recovered as crystalline dicarboxylic acid by drying off the remaining water, or it can be mixed with a diamine without drying and used as a dicarboxylic acid-diamine salt.

[0050] In this embodiment, as shown in FIG. 1 , it is preferable that first, dicarboxylic acid is separated from the reaction solution obtained in the separation step, and then, after the residual catalyst treatment, diamine is separated, and the dicarboxylic acid and diamine are each purified.

[0051] [Si Element Content] In the above-mentioned purification process, when producing a polyamide by polymerizing a dicarboxylic acid and a diamine, the content of Si element is preferably 1 mass ppm or more relative to the total amount of the dicarboxylic acid, from the viewpoints of optimizing the viscosity of the polyamide, stabilizing the polymerization, and improving the handleability of the resulting polyamide. Furthermore, from the viewpoint of preventing polymerization inhibition when producing a polyamide by polymerizing a dicarboxylic acid and a diamine, the content of Si element is preferably 200 mass ppm or less. The content of Si element is more preferably 1 to 150 mass ppm, even more preferably 1 to 120 mass ppm, and even more preferably 1 to 100 mass ppm. The Si element is derived from waste polyamide, and can be controlled to the above-mentioned numerical range by adjusting the purification process.

[0052] [Polyamide Recycling Method] The polyamide recycling method of this embodiment includes a polymerization step of polymerizing the diamine and dicarboxylic acid obtained by the above-described method for producing diamine and dicarboxylic acid of this embodiment to obtain a polyamide, as shown in Figure 1. This allows the polyamide to be recycled. For the polymerization step, a known method can be used, and is not particularly limited, but examples include the following methods.

[0053] (Polymerization Step) In the polymerization step, a method is generally used in which an aqueous solution of a dicarboxylic acid-diamine salt, or a mixture of a dicarboxylic acid and a diamine, or an aqueous suspension of these is heated and polymerized while maintaining the molten state (hereinafter also referred to as "thermal melt polymerization method"). However, the polymerization is not limited to this, and known methods such as solid-phase polymerization and solution polymerization can also be used.

[0054] Specific methods for producing polyamides using purified diamines and dicarboxylic acids include, for example, the following: (1) A method in which a dicarboxylic acid-diamine salt, or an aqueous solution of a mixture of dicarboxylic acid and diamine, or a suspension thereof, is heated and polymerized while maintaining the molten state (hereinafter also referred to as "thermal melt polymerization"). (2) A method in which the degree of polymerization of a polyamide obtained by thermal melt polymerization is increased while maintaining the solid state at a temperature below the melting point (hereinafter also referred to as "thermal melt polymerization / solid-state polymerization"). (3) A method in which a dicarboxylic acid-diamine salt, or a mixture of dicarboxylic acid and diamine, is polymerized while maintaining the solid state (hereinafter also referred to as "solid-state polymerization"). (4) A method in which a dicarboxylic acid halide component equivalent to the dicarboxylic acid and a diamine component are polymerized (hereinafter also referred to as "solution method"). Among these, a production method including a thermal melt polymerization is preferred. When producing polyamides by a thermal melt polymerization, it is preferable to maintain the molten state until the polymerization is complete. To maintain the molten state, it is necessary to produce the polyamide under polymerization conditions appropriate for the polyamide composition. For example, the polymerization pressure in the thermal melt polymerization method is 14 to 25 kg / cm ２ (gauge pressure), and while continuing to heat, the pressure inside the vessel was controlled to atmospheric pressure (gauge pressure 0 kg / cm ２ ) for 30 minutes or more until the blood pressure drops.

[0055] The polymerization mode of polyamide is not particularly limited, and may be a batch system or a continuous system. The polymerization apparatus used for producing polyamide is not particularly limited, and known apparatuses may be used, for example, autoclave-type reactors, tumbler-type reactors, and extruder-type reactors such as kneaders.

[0056] Below, a method for producing polyamide by batch-type melt polymerization is specifically described as a method for producing polyamide, but the method for producing polyamide is not limited thereto. First, for example, an aqueous solution containing about 40 to 60% by mass of polyamide raw material components (dicarboxylic acid, diamine, and, if necessary, lactam and / or aminocarboxylic acid) is concentrated to about 65 to 90% by mass in a concentration tank operated at a temperature of 110 to 180°C and a pressure of about 0.035 to 0.6 MPa (gauge pressure) to obtain a concentrated solution. The resulting concentrated solution is then transferred to an autoclave and heated until the pressure in the autoclave reaches about 1.2 to 2.2 MPa (gauge pressure). Thereafter, the pressure in the autoclave is maintained at about 1.2 to 2.2 MPa (gauge pressure) while removing water and / or gas components, and when the temperature reaches about 220 to 260°C, the pressure is reduced to atmospheric pressure (gauge pressure: 0 MPa). The pressure inside the autoclave is reduced to atmospheric pressure, and then reduced pressure as needed to effectively remove the by-product water. The autoclave is then pressurized with an inert gas such as nitrogen, and the polyamide melt is extruded from the autoclave as strands. The extruded strands are cooled and cut to obtain polyamide pellets.

[0057] The polyamide obtained by the above-mentioned polyamide recycling method preferably has a Si element content of 1 ppm by mass or more relative to the total amount of polyamide, from the viewpoint of preventing excessive viscosity when melt-kneading with the addition of additives and improving the handleability of the melt-kneaded product. Furthermore, from the viewpoint of suppressing a decrease in the molecular weight of the polyamide, the Si element content is preferably 100 ppm by mass or less. It is more preferably 1 to 50 ppm by mass, and even more preferably 1 to 30 ppm by mass.

[0058] The polyamide obtained by the above-described polymerization step may be blended with various additives depending on the desired physical properties, as shown in Figure 1. By melt-kneading such polyamide, the final target recycled polyamide is obtained.

[0059] 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 these examples and comparative examples. Purified monomers and polymers were obtained in the examples and comparative examples using the following raw materials and processes described below, and were evaluated by the following methods.

[0060] [Polyamide, polyamide resin composition, and polyamide-containing waste to be used in chemical recycling] (Polyamide) A: Polyamide 66 (manufactured by Asahi Kasei Corporation, model number: Leona 1300) (Polyamide resin composition) B: Polyamide 66 resin composition (manufactured by Asahi Kasei Corporation, model number: Leona 14G33, glass fiber ratio 33%) In the examples and comparative examples described below, these polyamides and polyamide resin compositions were also used as "polyamide-containing waste" and "crude polyamide", similar to the waste described below, mainly from the perspective of utilizing scrap materials and recycling inventory.

[0061] (Waste molded articles of polyamide resin composition) Radiator tank caps were collected from discarded automobiles. They were stamped with >PA66+GF30< and contained polyamide 66 as the main component with 30% by mass of glass fiber. (Cloth-like waste polyamide fiber) Airbags were collected from discarded automobiles. They were stamped with PA66 and contained polyamide 66 as the main component. Both colored airbags (with silicone coating) and plain airbags (without silicone coating) were used.

[0062] [Pretreatment step for crushing polyamide-containing waste to produce crude polyamide] (Crushing of molded polyamide resin composition products) Molded polyamide resin composition products were crushed using a Nippon Seam PFS-40 crusher. After crushing, irregularly shaped resin pieces measuring 3 to 6 mm in length were obtained. (Crushing of cloth-like polyamide fiber waste (airbags)) Airbag scraps and waste airbags were shredded to appropriate sizes. The shredded dimensions were preferably approximately 1 to 10 cm per side, but as this varied depending on the equipment used and the processing volume, no particular restrictions were placed on the shredded dimensions of the airbag scraps and waste airbags. (Handling of polyamide 66 and polyamide 66 resin composition) The polyamide 66 (manufactured by Asahi Kasei Corporation, model number: Leona 1300) and polyamide 66 resin composition (manufactured by Asahi Kasei Corporation, model number: Leona 14G33, glass fiber ratio 33%) were used as crude polyamide pellets.

[0063] [Solvents used in the depolymerization step] (Acids) The following inorganic and organic acids were used as acids: Hydrochloric acid (special grade, purity 35-37%) manufactured by Kanto Chemical Sulfuric acid (special grade, purity >96%) manufactured by Kanto Chemical Phosphoric acid (special grade, purity 85%) manufactured by Hayashi Pure Chemical Industries Methanesulfonic acid manufactured by TCI p-Toluenesulfonic acid monohydrate manufactured by TCI

[0064] [Alkali used in the purification step for isolating and purifying diamine] (Alkali) The following sodium hydroxide was used as the alkali for neutralizing the acid in the purification step. Sodium hydroxide (manufactured by TCI)

[0065] [Preparation of sample solutions used in the depolymerization step] Inorganic or organic acid, water, polyamide, polyamide resin composition, and crude polyamide were weighed into a 20 mL glass pressure test tube (Reaction Vial G30 manufactured by Anton Paar) according to Tables 1 to 3 below. In the case of the polyamide resin composition, the amount of resin component excluding the mass of the inorganic filler was defined as the amount of polyamide, and was combined with the amount of polyamide in other systems. Tables 1 to 3 show the compositions of sample solutions 1 to 18 used in each of the Examples and Comparative Examples.

[0066]

[0067]

[0068]

[0069] [Examples 1 to 37] [Comparative Examples 1 to 16] Using each of sample solutions 1 to 18 shown in Tables 1 to 3, a depolymerization step was carried out under the conditions shown in Tables 4 to 8 and 10. Table 9 shows the analytical results of hexamethylenediamine and adipic acid after purification and the analytical results of polyamide 66 after polymerization.

[0070] [Depolymerization Step] <Microwave Heating> A pressure-resistant test tube was charged with a stirrer, polyamide, polyamide resin composition, and crude polyamide in predetermined amounts, and 37% hydrochloric acid and distilled water were added to the desired mass percentages. The mixture was stirred at a predetermined temperature for a desired time using a microwave synthesis reactor (Monowave 450, manufactured by Anton Paar). The temperature-raising process to the predetermined temperature was fixed at a temperature-raising time of 16 minutes, and the stirrer rotation speed was set to 600 rpm. After depolymerization at the predetermined temperature for the desired time, the mixture was allowed to cool naturally within the reactor, and the pressure-resistant test tube was removed when the temperature had dropped to 70°C. When scaling up to obtain monomers for polymerization, as shown in Examples 28 to 32, depolymerization was carried out under the following conditions using Milestone General's StartSYNTH. A pressure-resistant vessel was charged with 30 g of a stirrer, polyamide, polyamide resin composition, and crude polyamide, and 37% hydrochloric acid and distilled water were added to obtain 11% hydrochloric acid. The mixture was stirred at 140°C for 1 hour. The temperature-raising process up to the predetermined temperature was fixed at a temperature-raising time of 20 minutes. After depolymerization, the apparatus was allowed to cool naturally, and the pressure vessel was removed when the temperature had dropped to 80°C. Furthermore, in Examples 28 to 32, the actual recovery rates of adipic acid and hexamethylenediamine actually recovered after purification were calculated using the following method. The masses of the pure polyamide components in the charged polyamide, polyamide resin composition, and crude polyamide were calculated, and the theoretical maximum recovery mass, assuming that 100% of the pure polyamide components were depolymerized, 100% of diamines and dicarboxylic acids were produced without secondary reactions after depolymerization, and 100% was recovered without any physical loss during purification, was defined as 100%. The actual recovery rate (%) was calculated as the ratio of the masses of adipic acid and hexamethylenediamine actually recovered to the theoretical maximum recovery mass.

[0071] <Normal heating> A stirrer, polyamide, polyamide resin composition, and crude polyamide were placed in a PTFE pressure vessel (a microwave / data decomposition vessel, temperature measurement type, manufactured by Toei Co., Ltd.) in predetermined amounts, and 37% hydrochloric acid and distilled water were added to the vessel to the desired mass %. The vessel was then stirred at a predetermined temperature for a desired time using an oil bath. The depolymerization time was measured from the time the internal temperature reached a predetermined temperature, and the heating time for each oil bath is shown in Tables 4 and 10.

[0072] The results of microwave heating and conventional heating, where the depolymerization time was fixed at 1 hour and the depolymerization temperature was changed, are shown in Table 4. Furthermore, the results of microwave heating and conventional heating, where the depolymerization time and depolymerization temperature were changed, are shown in Table 5.

[0073] <Measurement of Depolymerization Rate> [Evaluation of the Amount of Hexamethylenediamine (HMD) and Its Derivatives in the Polyamide Before the Depolymerization Step, and Evaluation of the Amount of Adipic Acid (ADA) and Its Derivatives in the Polyamide Before the Depolymerization Step] The reaction solution after the depolymerization step was sampled and measured by NMR, and the depolymerization rate (%) of the polyamide was calculated. The hydrolyzed solutions obtained in the Examples and Comparative Examples were added to a 5 mm diameter NMR tube, and then a special NMR sample tube N-502B (Nippon Precision Science Co., Ltd.) whose measurement section had been filled with benzene d-6 was inserted into the 5 mm diameter NMR test tube. Measurement was then performed using a JEOL NMR apparatus (ECZ-500) under the conditions of a measurement temperature of 25°C and an accumulation number of 1024, with 1H as the observation nucleus. From the integrals of hexamethylenediamine and amide bond, (integral of hexamethylenediamine) / (integral of hexamethylenediamine+integral of amide bond) was calculated, and this was taken as the depolymerization rate of polyamide.

[0074] [Separation Step] The solution after the depolymerization step was heated and filtered while hot at 85°C to remove insoluble solids (including glass fibers, silicone coating, metal pieces, etc.). The filtrate was then cooled to room temperature, whereby dicarboxylic acid crystals were precipitated. The dicarboxylic acid crystals and the filtrate were separated by filtration.

[0075] [Purification Step] (Dicarboxylic Acid Purification) The dicarboxylic acid crystals obtained in the separation step were dissolved in water in an amount equal to or greater than the mass of the crystals, heated to 80°C, and allowed to cool after dissolution, thereby recrystallizing the dicarboxylic acid. The precipitated dicarboxylic acid crystals were recovered by filtration. (Diamine Purification) Sodium hydroxide was gradually added to the filtrate obtained in the separation step, and sodium hydroxide was added in an amount equal to or greater than the molar amount of hydrochloric acid, and it was confirmed that a salt precipitated. The neutralized reaction solution was distilled using a Kugelrohr. After heating at 100°C and 300 mbar, the temperature was gradually increased and the pressure was reduced to 110°C and 160 mbar, and this was maintained for 3 hours. The temperature was further increased and the pressure was reduced to 140°C and 80 mbar, and this was maintained for approximately 1 hour, and the target diamine was finally recovered.

[0076] (Monomer analysis) NMR of the purified diamine and dicarboxylic acid was measured to confirm that they were the target products. Furthermore, trace impurities were confirmed by ICP-AES semi-quantitative analysis. The analytical equipment used was an ICP optical emission spectrometer SPS3520UV-DD manufactured by Hitachi (SII).

[0077] [Polyamide Polymerization Method] A polyamide polymerization reaction was carried out using the "hot melt polymerization method" as follows. 150 g of the equimolar salt of adipic acid and hexamethylenediamine recovered as described above was dissolved in 150 g of distilled water to prepare a homogeneous aqueous solution containing 50% by mass of the raw material monomers. This aqueous solution was charged into an autoclave with an internal volume of 0.5 L and purged with nitrogen. While stirring at a temperature of 110 to 150°C, the solution was concentrated by gradually removing steam to a solution concentration of 70% by mass. The internal temperature was then raised to 220°C. At this time, the autoclave was pressurized to 1.8 MPa. The reaction was continued for one hour, with steam gradually removed to maintain the pressure at 1.8 MPa, until the internal temperature reached 245°C. The pressure was then reduced over one hour. The autoclave was then maintained at a reduced pressure of 650 torr (86.66 kPa) for 10 minutes using a vacuum device. The final internal temperature of the polymerization was 265° C. The mixture was then pressurized with nitrogen and formed into strands from the lower spinneret (nozzle), which were then cooled with water, cut, and discharged in pellet form. The pellets were dried at 100° C. for 12 hours in a nitrogen atmosphere to obtain polyamide with Mw=35,000 and Mw / Mn=2.0.

[0078] (Analysis of Polyamide) The obtained polyamide was measured for NMR and confirmed to be polyamide 66. Trace amounts of impurities were confirmed by ICP-AES semi-quantitative analysis. The analytical device used was an ICP optical emission spectrometer SPS3520UV-DD manufactured by Hitachi (SII).

[0079] [Evaluation of Corrosion Resistance] Corrosion resistance was evaluated from the example of zirconium in the graph of hydrochloric acid concentration and temperature described in "Properties of Tantalum for Applications in the Chemical Process Industry: F.J. Hunkeler (USA) ASTM STP849, 1984, p. 28-49." Corrosion exceeding the 5 mpy (5 mil per year) line was evaluated as x, corrosion near the line was evaluated as △, and corrosion further away from the line was evaluated as ◯ and ⊚.

[0080]

[0081]

[0082]

[0083]

[0084]

[0085]

[0086]

[0087] In each example, by carrying out depolymerization at a depolymerization rate of 80% or more, the effect of achieving high actual monomer recovery efficiency after separation and purification was achieved. Specifically, as shown in Examples 28 to 32, the actual monomer recovery efficiency was 82% or more. In the comparative examples, even Comparative Example 7, which had the highest depolymerization rate, was 75%. Even if all 75% of the decomposed monomers were converted to adipic acid and hexamethylenediamine without any secondary reactions occurring and the entire amount was recovered without any physical loss during purification, the actual recovery rate would not exceed 75%, and therefore would not exceed the lowest actual recovery rate of 83% in the examples. Furthermore, in each example, it was confirmed that the degree of corrosion of the reactor was small, and high-purity hexamethylenediamine and adipic acid could be obtained in high yield from various polyamide-containing wastes. Furthermore, polyamide 66 could be polymerized using the purified hexamethylenediamine and adipic acid.

[0088] This application is based on a Japanese patent application (Patent Application No. 2023-165285) filed with the Japan Patent Office on September 27, 2023, the contents of which are incorporated herein by reference.

[0089] The method for producing diamines and dicarboxylic acids and the method for recycling polyamides of the present invention have industrial applicability as efficient methods for recycling polyamide resins, polyamide fibers, polyamide resin compositions, and molded articles used in automobile parts and various industrial parts.

Claims

1. A method for producing diamines and dicarboxylic acids, comprising: a pretreatment step of subjecting polyamide-containing waste to one or more treatments selected from the group consisting of pulverization, washing, and separation of foreign matter to obtain a crude polyamide; a depolymerization step of feeding the crude polyamide into an aqueous solution containing an acid in a closed reactor, heating the crude polyamide to a target temperature of 90°C to 160°C and then heating the crude polyamide at a temperature close to the target temperature to depolymerize 80% or more of the amide groups in all the polyamides contained in the crude polyamide and decompose them into diamines and dicarboxylic acids; a separation step of removing components other than the diamines and derivatives of the diamines, and the dicarboxylic acids and derivatives of the dicarboxylic acids from the reaction liquid obtained by the depolymerization step, to obtain the diamines and derivatives of the diamines, and the dicarboxylic acids and derivatives of the dicarboxylic acids; and a purification step of isolating and purifying the diamines and dicarboxylic acids from the diamines and derivatives of the diamines, and the dicarboxylic acids and derivatives of the dicarboxylic acids obtained by the separation step.

2. The method for producing a diamine and a dicarboxylic acid according to claim 1, wherein in the depolymerization step, a space is provided in the upper part of the reactor that is not filled with the aqueous solution containing the crude polyamide, and microwaves are irradiated from above the reactor through a waveguide and the unfilled space by a microwave transmitter.

3. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, wherein in the depolymerization step, the temperature increase rate during the temperature increase is 25° C. / min or less.

4. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, wherein in the depolymerization step, the reactor has a device for monitoring the pressure inside the reactor and venting the pressure when the pressure inside the reactor becomes higher than a reference pressure.

5. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, comprising a step of removing components other than the diamine and the diamine derivatives and the dicarboxylic acid and the dicarboxylic acid derivatives as insoluble matters from the reaction liquid in a state in which the dicarboxylic acid and / or the dicarboxylic acid derivatives and the diamine and the diamine derivatives are partially or entirely dissolved in the reaction liquid in the separation step.

6. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, wherein the pKa of the acid is 0 or less.

7. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, wherein the amount of the acid is in a ratio of the molar number of the amide groups of the crude polyamide to the protons of the acid, i.e., amide groups of the crude polyamide: protons of the acid=1:1 to 1:5.

5.

8. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, wherein the acid is hydrochloric acid.

9. The method for producing a diamine and a dicarboxylic acid according to claim 8, wherein the concentration of the hydrochloric acid in the aqueous solution is 5% by mass or more and 25% by mass or less.

10. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, wherein the main component of the polyamide in the polyamide-containing waste is polyamide 66.

11. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, wherein a reactor having an inner surface made of a material such as glass lining, zirconium or tantalum is used in the depolymerization step.

12. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, wherein in the separation step, when components other than the diamine, the diamine derivative, the dicarboxylic acid, and the dicarboxylic acid derivative are removed, hot filtration and centrifugation are carried out.

13. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, wherein in the purification step, purification of the dicarboxylic acid is carried out by crystallization.

14. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, wherein in the purification step, the diamine is purified by distillation.

15. The method for producing a diamine and a dicarboxylic acid according to claim 2, wherein the waveguide has a partition window separating the reactor from the microwave transmission device.

16. The method for producing a diamine and a dicarboxylic acid according to claim 15, wherein the partition window is made of quartz glass.

17. The method for producing a diamine and a dicarboxylic acid according to claim 2, wherein the frequency of the microwaves used during the heating is 0.8 to 6 GHz.

18. The method for producing a diamine and a dicarboxylic acid according to claim 1 or 2, wherein in the purification step, the content of Si element is adjusted to 1 ppm by mass or more and 200 ppm by mass or less based on the total amount of the dicarboxylic acid.

19. A method for recycling polyamide, comprising the steps of obtaining diamine and dicarboxylic acid by the method for producing diamine and dicarboxylic acid according to claim 1 or 2, and polymerizing the diamine and dicarboxylic acid to obtain a polyamide.

20. A polyamide obtained by the method for recycling polyamide according to claim 19, comprising: a polyamide containing 1 ppm by mass or more and 100 ppm by mass or less of silicon element relative to the total amount of the polyamide.

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