Methods for producing diamines and dicarboxylic acids, and methods for recycling polyamides.

Abstract

The present invention provides a method for producing diamines and dicarboxylic acids that effectively reduces discoloration and yields diamines and dicarboxylic acids with excellent color tone, even when polyamide-containing waste is used as a raw material for depolymerization. [Solution] The method includes a depolymerization step in which a raw material containing polyamide is heated in an aqueous solution containing an acid or a base to depolymerize it and obtain a diamine and a dicarboxylic acid. The depolymerization step is carried out in the presence of 0.0001 to 10 parts by mass of a reducing agent per 100 parts by mass of polyamide in the raw material. The reducing agent has a standard oxidation-reduction potential of -1.0V to 0.3V at 25°C in the aqueous solution. A method for producing diamines and dicarboxylic acids.

Description

Technical Field

[0001] The present invention relates to a method for producing diamine and dicarboxylic acid, and a method for recycling polyamide.

Background Art

[0002] Polyamide has excellent mechanical properties, namely mechanical strength, rigidity, impact resistance, etc., and also excellent heat resistance and chemical resistance. Therefore, conventionally, it has been used in various industrial fields such as clothing, industrial materials, automobiles, electric and electronic parts, and other industrial products. On the other hand, in recent years, the plastic industry is also required to respond to a resource recycling society, and the establishment of recycling technology for polyamide is also required.

[0003] Generally, there are three types of recycling: material recycling, chemical recycling, and thermal recycling. At present, in the automotive applications that account for the majority of polyamide uses, most of the polyamide in waste automobiles is burned as thermal recycling and not effectively utilized as resources. It is required to effectively utilize the polyamide by material recycling or chemical recycling. Also, from the perspective of reducing GHG (greenhouse gas) emissions, material recycling and chemical recycling are required for polyamide.

[0004] However, polyamide resin compositions and molded products for automotive applications contain, in addition to polyamide, various additives such as inorganic fillers such as glass fibers, heat stabilizers, pigments, dyes, etc. (see, for example, Patent Document 1). Therefore, in material recycling, there is a problem that it is difficult to maintain practically sufficient mechanical properties after recycling. In view of such problems, chemical recycling that depolymerizes polyamide to decompose it into diamine and dicarboxylic acid, which are monomers, and then polymerizes these monomers again for recycling is regarded as promising and research and development are underway.

[0005] As a technology related to the aforementioned chemical recycling, Patent Document 2 discloses a technique for hydrolyzing polyamide by mixing a mixture containing polyamide with an inorganic acid or methanesulfonic acid having a pKa lower than 0 and not containing nitrogen in an aqueous medium containing water, and then heating the mixture. Furthermore, Patent Document 3 discloses a technique for hydrolyzing the polyamide by mixing a polyamide composition with a hydroxide, oxide, or carbonate containing alkali metal ions or alkaline earth metal ions and heating the mixture to 230°C or higher. Furthermore, Non-Patent Document 1 discloses a technique for depolymerizing polyamide 66 using microwaves. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 6839266 [Patent Document 2] International Publication No. 2024 / 002975 [Patent Document 3] International Publication No. 2024 / 090534 [Non-patent literature]

[0007] [Non-Patent Document 1] Urska Cesarek et al. “Chemical Recycling of Alipahatic Polyamides by Microwave-Assisted Hydrolysis for Efficient Monomer Recovery” ACS Sustainable Chem. Eng.2020,8,16274-16282. [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] However, in the depolymerization of general polyamide-containing waste, such as polyamide 66 airbags, the conventional method described above has the problem that the reaction solution at the end of the process turns brown, and the adipic acid obtained by subsequent crystallization also turns brown.

[0009] Therefore, in view of the problems of the prior art described above, the present invention aims to provide a method for producing diamines and dicarboxylic acids, and a method for recycling polyamides, which can effectively reduce discoloration and produce diamines and dicarboxylic acids with excellent color tone, even when polyamide-containing waste is depolymerized. [Means for solving the problem]

[0010] The inventors of the present invention have conducted extensive research to solve the problems of the prior art described above, and have found that the above problems can be solved by depolymerizing a polyamide composition in the presence of a reducing agent whose standard oxidation-reduction potential at 25°C in an aqueous solution is within a predetermined numerical range, thereby completing the present invention. In other words, the present invention is as follows:

[0011] [1] The process includes a depolymerization step in which a raw material containing polyamide is heated in an aqueous solution containing an acid or base to depolymerize it and obtain diamines and dicarboxylic acids. The depolymerization step is carried out in the presence of 0.0001 to 10 parts by mass of a reducing agent per 100 parts by mass of polyamide in the raw material. The reducing agent has a standard oxidation-reduction potential of -1.0V to 0.3V at 25°C in the aqueous solution. A method for producing diamines and dicarboxylic acids. [2] The reducing agent is The solubility in water at 25°C is 0.01 g / 100 g-H2O or higher. A method for producing diamines and dicarboxylic acids as described in [1] above. [3] The reducing agent is One or more selected from the group consisting of phosphorous acid, metal salts of phosphorous acid, thiosulfuric acid, and metal salts of thiosulfuric acid The method for producing diamine and dicarboxylic acid according to the above [1] or [2]. [4] The concentration of the acid or base in the aqueous solution is 5% by mass or more and 25% by mass or less. The method for producing diamine and dicarboxylic acid according to any one of the above [1] to [3]. [5] The polyamide contained in the raw material contains more than 50% by mass of polyamide 66. The method for producing diamine and dicarboxylic acid according to any one of the above [1] to [4]. [6] The heating temperature in the depolymerization step is 90°C to 160°C under acidic decomposition conditions and 230°C to 400°C under basic decomposition conditions. The method for producing diamine and dicarboxylic acid according to any one of the above [1] to [5]. [7] From the reaction solution obtained by the depolymerization step, components other than the diamine, derivatives of the diamine, the dicarboxylic acid, and derivatives of the dicarboxylic acid are removed, and a separation step of obtaining the diamine, derivatives of the diamine, the dicarboxylic acid, and derivatives of the dicarboxylic acid is further provided. The method for producing diamine and dicarboxylic acid according to any one of the above [1] to [6]. [8] In the separation step When removing components other than the diamine, derivatives of the diamine, the dicarboxylic acid, and derivatives of the dicarboxylic acid, hot filtration and centrifugation are performed. The method for producing diamine and dicarboxylic acid according to the above [7]. [9] The separation step further includes a purification step of separately isolating and purifying the diamine and the dicarboxylic acid from the diamine, derivatives of the diamine, the dicarboxylic acid, and derivatives of the dicarboxylic acid obtained by the separation step. The method for producing diamine and dicarboxylic acid according to the above [7] or [8].

[10] In the purification step, the purification of the dicarboxylic acid is carried out by crystallization. The method for producing diamine and dicarboxylic acid according to the above [9].

[11] In the purification step, the purification of the diamine is carried out by distillation. The method for producing diamine and dicarboxylic acid according to the above [9] or

[10] .

[12] The raw material containing the polyamide contains 1% by mass or more and 50% by mass or less of components other than the polyamide. The method for producing diamine and dicarboxylic acid according to any one of the above [1] to

[11] .

[13] The raw material containing the polyamide is waste containing the polyamide. The method for producing diamine and dicarboxylic acid according to any one of the above [1] to

[12] .

[14] A method for recycling polyamide, further comprising a polymerization step of polymerizing the diamine and the dicarboxylic acid obtained by the method for producing diamine and dicarboxylic acid according to any one of the above [1] to

[13] to obtain a polyamide. [Advantages of the Invention]

[0012] According to the present invention, even when a polyamide-containing waste is used as a raw material for depolymerization, coloring can be effectively reduced, and a method for producing diamine and dicarboxylic acid, and a method for recycling polyamide, which can obtain diamine and dicarboxylic acid excellent in color tone, can be provided. [Brief Description of the Drawings]

[0013] [Figure 1] It is a diagram showing the flow of a method for recycling a polyamide resin. [Embodiments for Carrying Out the Invention]

[0014] The embodiments for carrying out the present invention (hereinafter referred to as "this embodiment") will be described in detail below. The following embodiments are illustrative 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.

[0015] [Methods for producing diamines and dicarboxylic acids, and methods for recycling polyamides] The method for producing diamines and dicarboxylic acids in this embodiment is: The process includes a depolymerization step in which a raw material containing polyamide (hereinafter sometimes simply referred to as "raw material") is heated in an aqueous solution containing an acid or base to depolymerize it and obtain a diamine and a dicarboxylic acid. The depolymerization step is carried out in the presence of 0.0001 to 10 parts by mass of a reducing agent per 100 parts by mass of polyamide in the raw material, wherein the reducing agent has a standard oxidation-reduction potential of -1.0V to 0.3V at 25°C in the aqueous solution.

[0016] Furthermore, in the polyamide recycling method of this embodiment, the above-described method for producing diamines and dicarboxylic acids uses, for example, waste containing polyamide as the raw material, depolymerizes the waste containing polyamide to obtain diamines and dicarboxylic acids, and recycles the polyamide by polymerizing these.

[0017] According to the method for producing diamines and dicarboxylic acids of this embodiment, and the method for recycling polyamides of this embodiment, even when waste containing polyamides, particularly post-consumer products containing polyamides, is used as raw material for depolymerization, monomers can be obtained with excellent color tones, and furthermore, the polyamides obtained by polymerizing these monomers have excellent color tones. In addition, since a purification step to remove discoloration is unnecessary, the yield of diamines and dicarboxylic acids can be increased.

[0018] (Raw materials containing polyamide) In the method for producing diamines and dicarboxylic acids of this embodiment, a raw material containing polyamide is used. The raw material may be waste containing polyamide. Here, "waste" is not limited to waste as defined by law (Waste Management and Public Cleansing Law), but includes things that are no longer used as products, things that cannot be used as products, etc., and is defined as a general term for these. Examples of waste containing polyamide include molded articles of polyamide or polyamide resin compositions, as well as polyamide or polyamide resin compositions as products (which may be spec-out products). Waste containing polyamide includes, but is not limited to, the following: unwanted in-process scraps and out-of-spec products discharged from the manufacturing process of polyamide fibers; unwanted in-process scraps and out-of-spec products discharged from the molding process using polyamide; unwanted in-process scraps and out-of-spec products discharged from processes such as weaving and sewing using polyamide fibers; airbags, carpets, and apparel products obtained from polyamide fibers; molded polyamide products that have been used and then discarded; and final products made from polyamide fibers that have been used and then discarded. Hereinafter, these will be collectively referred to as waste containing polyamide. In the method for producing diamines and dicarboxylic acids of this embodiment, the raw material containing polyamide used preferably has a content of components other than polyamide of 1% by mass or more and 50% by mass or less, from the viewpoint of reducing the load on the separation process between the polyamide and the dicarboxylic acids and diamines obtained by depolymerization.

[0019] Figure 1 shows flowcharts illustrating the methods for producing diamines and dicarboxylic acids according to this embodiment, and the method for recycling polyamides according to this embodiment.

[0020] <Polyamide> Polyamide refers to a polymer that has amide bonds (-NHCO-) ​​in its 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 sevacamide), polyamide 412 (polytetramethylene dodecamide), polyamide 610 (polyhexamethylene sevacamide), polyamide 612 (polyhexamethylene dodecamide), polyamide 1010 (polydecamethylene sevacamide), polyamide 1012 (polydecamethylene dodecamide), polyamide 6T (polyhexamethylene terephthalamide), polyamide 9T (polynonane methylene terephthalamide), polyamide 6I (polyhexamethylene isophthalamide), and copolymers or mixtures thereof.

[0021] In particular, the polyamide 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 as the main component, and polyamide 66, polyamide 66 / 6I, or a mixture of polyamide 66 and polyamide 6I is more preferred. Here, the main component refers to a component that accounts for more than 50% by mass of the total polymer components (100% by mass). Polyamide 66 is a polyamide obtained by the condensation polymerization of hexamethylenediamine and adipic acid, and is suitable for use as a material for functional parts of automobiles, machinery, and electrical products or as a high-strength fiber due to its excellent heat resistance, mechanical strength, and creep properties, and it is preferable that the raw material contains more than 50% by mass of polyamide 66.

[0022] <Polyamide resin composition> The polyamide resin composition is a resin composition that includes the polyamide and, if necessary, inorganic fillers such as glass fibers, lubricants, and other additives.

[0023] [Inorganic fillers] Polyamide resin compositions and their molded articles may contain inorganic fillers. As a result, polyamide resin compositions and their molded articles tend to have excellent mechanical strength and rigidity. 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, monocalcium phosphate, wollastonite, silica, zeolite, alumina, boehmite, aluminum hydroxide, titanium dioxide, silicon dioxide, magnesium oxide, calcium silicate, sodium aluminosilicate, magnesium silicate, Ketjenblack, acetylene black, furnace black, carbon nanotubes, graphite, brass, copper, silver, aluminum, nickel, iron, calcium fluoride, mica, montmorillonite, swollen fluoromica, apatite, and the like. These may be used individually or in combination of two or more types.

[0024] [Lubricant] The polyamide resin composition and its molded article may further contain a lubricant in addition to the polyamide resin and inorganic filler described above. As a result, the polyamide resin composition and its molded article tend to have excellent fluidity and appearance.

[0025] [Other additives] The polyamide resin composition and its molded article may contain other additives in addition to the polyamide, inorganic filler, and lubricant described above. Other additives include, for example, antioxidants, UV absorbers, heat stabilizers, photodegradation inhibitors, plasticizers, mold release agents, nucleating agents, flame retardants, colorants, and other thermoplastic resins.

[0026] <Molded products> The molded article of polyamide or polyamide resin composition used as the raw material in this embodiment is manufactured by molding by various known methods, such as injection molding. The molded article may also be made of polyamide or polyamide resin composition fibers.

[0027] Polyamide is manufactured into various forms such as fibers and films, and is used in a wide range of applications, including clothing, carpets, packaging films, automotive parts, and industrial components, with over 2 million tons used annually. Polyamide or textile waste suitable for chemical recycling is preferable if the polyamide content in the product is high, as this leads to higher recycling efficiency. For example, the base fabric of an airbag made of polyamide 66 is optimal, as it is approximately 90% polyamide 66. Polyamide 66 fibers can also be used in apparel, bags and other clothing items, outdoor equipment, and sportswear. Furthermore, one of the typical applications of unreinforced polyamide in molded products is cable ties. On the other hand, for automotive parts, glass fiber reinforced polyamide resin compositions are generally used from the standpoint of strength and physical properties, and the mass ratio of glass fibers in molded products is most often 30-40% by mass. Therefore, the polyamide component is about 60-70% by mass, resulting in low recycling efficiency of polyamide waste. From this perspective, suitable polyamide waste for chemical recycling includes the base fabric of airbags, cable ties, carpet pile, and, in the case of automotive parts, radiator tanks with a glass fiber ratio of 30% by mass. Furthermore, scraps generated within the factory during the manufacturing process are more preferable than used market-recovered products because they are less affected by environmental contaminants and decomposition products due to degradation.

[0028] (Pre-treatment process) As shown in Figure 1, in this embodiment, a pretreatment step may be performed on the raw materials, such as waste containing polyamide. In the pretreatment step, for example, the raw material, such as waste containing polyamide, is subjected to one or more processes selected from the group consisting of crushing, washing, and separation of foreign matter to obtain a polyamide resin composition. This results in a polyamide resin composition with a large surface area, which allows for more efficient depolymerization in the subsequent step. In this pretreatment step, impurities such as metal, stone, glass, and sand are removed. For washing and separation of foreign matter, for example, washing water can be added and specific gravity separation treatment can be used. However, if the purity of the polyamide in the waste containing polyamide is high and there are no noticeable impurities, the washing and separation of foreign matter steps can be simplified or omitted. By performing a pretreatment step followed by a depolymerization step described later, monomerization can be carried out with high efficiency.

[0029] (Depolymerization process) As shown in Figure 1, after an optional pretreatment step, a depolymerization step is performed in which the mixture is heated in an aqueous solution containing an acid or base to obtain diamines and dicarboxylic acids. The depolymerization step is carried out in the presence of 0.0001 to 10 parts by mass of a reducing agent having a standard oxidation-reduction potential of -1.0V to 0.3V in aqueous solution at 25°C, relative to 100 parts by mass of polyamide in the raw material, thereby obtaining monomers via the hydrolysis reaction of amide bonds in the polyamide.

[0030] Furthermore, cleaving the amide bonds in polyamides requires supplying the necessary energy from an external source; therefore, the depolymerization process is carried out under heating. The heating method is not particularly limited, but examples include steam and electric heaters. Furthermore, by using microwaves, depolymerization can be carried out with low energy.

[0031] Furthermore, in order to promote the depolymerization of polyamides, the depolymerization step in the method for producing diamines and dicarboxylic acids of this embodiment is carried out using an aqueous solution containing an acid or a base. Specific acids or bases will be described later. According to the depolymerization process, polyamides, polyamide resin compositions, and their molded articles can be monomerized in high yield with low energy consumption and can be chemically recycled.

[0032] <Solvent> In the depolymerization process, water is used as the solvent. This is because the diamines, dicarboxylic acids, and their derivatives produced by depolymerization are water-soluble, making it easier to physically remove water-insoluble components, such as inorganic fillers like glass fibers, carbon black, pigments, and additives, when removing impurities from the recovered polyamide in subsequent processes. There are no particular restrictions on the type of water used in the aqueous solution; any type of water may be used, such as tap water, deionized water, distilled water, or well water. From the viewpoint of suppressing side reactions caused by the influence of coexisting salts, deionized water or distilled water is preferably used. Furthermore, if an appropriate process can be established in a later step to remove impurities derived from the recovered polyamide, organic solvents such as ethylene glycol and methanol may be used in combination. Also, in the depolymerization step, it is not always necessary for the raw materials, such as waste containing polyamide, to be completely dissolved in the solvent as a polymer; it is sufficient if they are partially dissolved and decomposed into monomers during the depolymerization step. In the method for producing diamines and dicarboxylic acids of this embodiment, since the raw materials do not need to be completely dissolved in the solvent as a polymer during the depolymerization step, the thickening in the depolymerization step is small, and a large amount of waste containing polyamide can be added to the solvent.

[0033] <Reducing agent> In the method for producing diamines and dicarboxylic acids of this embodiment, a reducing agent having a standard oxidation-reduction potential of -1.0V to 0.3V at 25°C in an aqueous solution is present during the depolymerization step. The reducing agent has the effect of preventing discoloration of the reaction solution due to side reactions during the depolymerization process and preventing discoloration of the dicarboxylic acid after crystallization. The standard oxidation-reduction potential of the reducing agent under the aforementioned conditions is -1.0V or higher, which prevents excessive reduction of the product to an alcohol and maintains the dicarboxylic acid state while simultaneously preventing discoloration of the reaction solution. A standard oxidation-reduction potential of 0.3V or lower also prevents discoloration of the reaction solution. From the viewpoint of efficiently preventing discoloration of the monomer produced in this embodiment and the polymer produced from the monomer, the reducing agent shall have a standard oxidation-reduction potential of -1.0V to 0.3V. Preferably, it shall be -0.9V to 0.2V, and more preferably -0.8V to 0.2V.

[0034] The reducing agent can be any agent with a standard oxidation-reduction potential of -1.0V to 0.3V at 25°C in aqueous solution, and is not limited to the following, but examples include: sulfites such as sodium sulfite and sodium bisulfite; metal thiosulfates such as thiosulfate and sodium thiosulfate; L-ascorbic acids such as L-ascorbic acid and sodium L-ascorbate; nitrites such as sodium nitrite; phosphonates such as sodium phosphonate and potassium phosphonate; metal diphosphorites such as diphosphorous acid, sodium diphosphorous acid, and potassium diphosphorous acid; heavy metal salts such as iron salts and copper salts; aldehyde compounds such as oxalic acid, formic acid, formaldehyde, and benzaldehyde; phenols such as phenol, catechol, tert-butylcatechol, and pyrogallol; and hydrazine. From the viewpoint of efficiently preventing discoloration with a small amount of additive, metal phosphite salts such as sodium disophosphate and sodium thiosulfate, and metal thiosulfate salts are preferred. The standard oxidation-reduction potential value can be found in the "Chemical Handbook," edited by the Chemical Society of Japan, Basic Edition II, 1st edition. The standard oxidation-reduction potential, also known as the standard electrode potential, is the electrode potential using a hydrogen electrode, which is the unit activity of hydrogen ions in a solution at a hydrogen pressure of 1 atmosphere, as the reference electrode, and can be measured by cyclic voltammetry. In this embodiment, the standard oxidation-reduction potential is the value obtained in water at 25°C, pH 0, with a standard hydrogen electrode as the cathode, and can be specifically measured by cyclic voltammetry or the like. The standard oxidation-reduction potential of the reducing agent used in this embodiment can be controlled to the above numerical range by appropriately selecting the type of reducing agent.

[0035] From the viewpoint of efficiently carrying out the reaction in water, the reducing agent is preferably soluble in water at 25°C of 0.01 g / 100 g-H2O or more, more preferably soluble in water of 1 g / 100 g-H2O or more, and even more preferably soluble in water of 10 g / 100 g-H2O or more. The upper limit of the solubility of the reducing agent in water at 25°C is not particularly limited, but it can be, for example, 100g / 100g-H2O, and may be unconditionally miscible with water. The solubility of the reducing agent in water at 25°C can be controlled within the above numerical range by selecting the components of the reducing agent.

[0036] The amount of reducing agent used in the depolymerization process varies depending on the type of reducing agent, but is 0.0001 parts by mass or more, preferably 0.001 parts by mass or more, and more preferably 0.005 parts by mass or more, per 100 parts by mass of polyamide in the raw material. When the amount of reducing agent is 0.0001 parts by mass or more per 100 parts by mass of polyamide, the effect of preventing discoloration by the reducing agent tends to be sufficiently obtained. When it is 10 parts by mass or less, the antioxidant effect is sufficiently obtained, and the cost of the reducing agent itself can be prevented, which tends to simplify wastewater treatment and prevent cost increases.

[0037] <Oxygen removal in reaction systems> As a preliminary step to the depolymerization process, a step may be performed to remove oxygen from the reaction vessel in which the depolymerization process is carried out. The method for removing oxygen from the gas phase in the reaction vessel is not particularly limited, and known methods can be used. For example, methods include flowing an inert gas in an amount equal to or about 20 times the volume of the reaction vessel, repeatedly vacuuming and depressurizing the system and replacing it with an inert gas, and removing oxygen with an oxygen scavenger. However, from the viewpoint of ease of operation, the method of flowing an inert gas in an amount equal to or about 20 times the volume of the reaction vessel is preferred. The method for removing dissolved oxygen from the liquid phase in the reaction vessel is not particularly limited, and known methods can be used. For example, methods include bubbling with an inert gas, degassing under reduced pressure while applying ultrasonic vibration, freeze-degassing, passing through a degassing membrane, passing through a commercially available degassing module, and passing through a column of reduced copper and activated alumina. However, from the viewpoint of ease of operation, bubbling with an inert gas is preferred. Examples of inert gases that can be used include nitrogen gas, argon gas, and helium gas. When combining oxygen removal in the reaction system with the addition of a reducing agent, oxygen removal is preferable because it enhances the effect of preventing discoloration.

[0038] <base> The depolymerization step is carried out in an aqueous solution containing an acid or a base. In the depolymerization step when using the aforementioned base, a raw material containing polyamide, such as a polyamide resin composition, is added to an aqueous solution containing the base. The base is thought to function as a catalyst for the hydrolysis of polyamides. As a base, alkali (earth) metal compounds selected from hydroxides, oxides, and carbonates are preferred, although they are not limited to the following. Examples of alkali (earth) metal hydroxides include, but are not limited to, lithium hydroxide, sodium hydroxide, potassium hydroxide, magnesium hydroxide, calcium hydroxide, and barium hydroxide. Examples of alkali (earth) metal oxides include, but are not limited to, lithium oxide, sodium oxide, potassium oxide, magnesium oxide, calcium oxide, and barium oxide. The alkali (earth) metal carbonate can be either a normal salt or an acidic salt (bicarbonate). Examples of normal salts, though not limited to those listed below, include dilithium carbonate, disodium carbonate, dipotassium carbonate, magnesium carbonate, calcium carbonate, and barium carbonate. Examples of alkali metal bicarbonates include, but are not limited to, lithium bicarbonate, sodium bicarbonate, potassium bicarbonate, magnesium carbonate, calcium carbonate, and barium carbonate. In the aforementioned depolymerization step, two or more alkali (earth) metal hydroxides, alkali (earth) metal oxides, and alkali (earth) metal carbonates may be blended and used. As alkali (earth) metals, sodium, potassium, and calcium are preferred from the viewpoint of protecting mineral resources, and sodium and potassium are more preferred, and sodium is even more preferred, from the viewpoint of increasing the production rate of diamines and dicarboxylic acids. Furthermore, to increase the hydrolysis reactivity with polyamides, hydroxides, oxides, and carbonate salts are preferred, and hydroxides and oxides are more preferred.

[0039] The amount of base used in the depolymerization step (the number of moles of hydroxide ions released) is preferably in excess of the number of moles of amide groups in the polyamide. On the other hand, if a large excess of base is used, the amount of polyamide added in the depolymerization step will inevitably be relatively small, so from the viewpoint of productivity, it is preferable that the amount of base is not too high. In particular, increasing the amount of base increases the amount of water in the system, which increases the load on the water removal step during the subsequent diamine purification, and tends to lead to increased costs and increased GHG emissions. Also, if a large excess of base is used, a large amount of salt is generated in the neutralization step after the depolymerization step, which tends to increase the load and cost of the salt treatment process. From these viewpoints, it is preferable that the amount of base is not too high. From the above viewpoint, the ratio of moles of amide groups to bases in the polyamide is preferably amide groups:base = 1:1 to 1:5.5, more preferably 1:1.25 to 1:1.3, and even more preferably 1:1.15 to 1:2.

[0040] From the viewpoint of promoting depolymerization, the concentration of the base in the aqueous solution during the depolymerization step is preferably 5% by mass or more, more preferably 8% by mass or more, and even more preferably 10% by mass or more. Furthermore, from the viewpoint of suppressing the deaminement reaction of the diamine during depolymerization, it is preferably 25% by mass or less, more preferably 22% by mass or less, and even more preferably 20% by mass or less.

[0041] <acid> In the depolymerization step when using the aforementioned acid, a raw material containing polyamide, such as a polyamide resin composition, is added to an aqueous solution containing the acid. The acid is thought to act as a catalyst for the hydrolysis of the polyamide. Examples of acids 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 Lewis acids such as Sc(OTf)3, Yb(OTf)3, Nb2O5, and CeO2. These may be used individually or in combination, but in order to obtain a high decomposition rate during depolymerization, acids with a pKa of 0 or less are preferred.

[0042] Furthermore, hydrochloric acid is preferred as the acid from the viewpoint of reaction efficiency, reduction of impurities, and reduction of waste that is difficult to recycle.

[0043] The amount of acid used in the depolymerization process (the number of moles of protons released) is preferably in excess of the number of moles of amide groups in the polyamide. On the other hand, if a large excess of acid is used, the amount of polyamide added in the depolymerization process will inevitably be relatively small, so from the viewpoint of productivity, it is preferable that the amount of acid is not too high. In particular, increasing the amount of acid increases the amount of water in the system, which increases the load on the water removal process during the subsequent diamine purification process, and tends to lead to increased costs and increased GHG emissions. Also, if a large excess of acid is used, a large amount of salt is generated in the neutralization process after the depolymerization process, which tends to lead to increased load and costs in the salt treatment process. For these reasons as well, it is preferable that the amount of acid is not too high. From the above viewpoint, the ratio of moles of amide groups to acid protons in the polyamide is preferably amide groups of polyamide to acid protons = 1:1 to 1:5.5, more preferably 1:1.25 to 1:1.3, and even more preferably 1:1.15 to 1:2.

[0044] Analysis of the Arrhenius equation shows that in the depolymerization process, as the concentration of the acid in the aqueous solution increases, the activation energy Ea decreases, making the reaction more likely to proceed. However, at the same time, as the amount of water decreases, the frequency factor A decreases, making the reaction less likely to proceed. Therefore, there is an optimal range for the hydrochloric acid concentration, preferably 5% by mass or more, and preferably 25% by mass or less. More preferably, the amount is 10% to 25% by mass, and even more preferably, 12% to 22% by mass.

[0045] <Heating: When using acid> In the depolymerization step, a raw material containing polyamide, such as a polyamide resin composition, is placed in an aqueous solution containing acid and heated. The temperature inside the system during heating is preferably 90°C to 160°C, more preferably 95°C to 150°C, and even more preferably 100°C to 140°C, from the viewpoint of suppressing side reactions and ensuring corrosion resistance of the reaction vessel. Any known heating method, such as steam, electric heater, or microwave heating, can be applied for depolymerization.

[0046] <Heating: When using a base> In the depolymerization step, a raw material containing polyamide, such as a polyamide resin composition, is placed in an aqueous solution containing a base and heated. The temperature inside the system during heating is preferably 230°C to 400°C, more preferably 250°C to 350°C, and even more preferably 270°C to 300°C, from the viewpoint of suppressing side reactions and ensuring corrosion resistance of the reaction vessel. Any known heating method, such as steam, electric heater, or microwave heating, can be applied for depolymerization.

[0047] Furthermore, in the depolymerization step, the heating rate during heating is preferably 25°C / min or less, more preferably 20°C / min or less, and even more preferably 15°C / min or less, from the viewpoint of optimizing equipment efficiency by balancing the energy input during heating with the energy required to maintain the predetermined temperature after reaching it. Furthermore, from the viewpoint of shortening the depolymerization process time, the heating rate during the heating process is preferably 1°C / min or more, more preferably 2°C / min or more, and even more preferably 3°C / min or more.

[0048] <Microwave irradiation> When the heating is performed using microwaves, the microwave irradiation output should be appropriately selected to reach the above-mentioned temperature. There is no particular upper limit to the irradiation output. The microwave frequency is not particularly limited, but is preferably 0.8 to 6 GHz. From the viewpoint of making it easier to deliver the microwave into the system of the depolymerization process, 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.

[0049] <Corrosion resistance> In the depolymerization process, it is preferable to use a reaction vessel that exhibits minimal corrosion and has industrial practicality. Specifically, it is preferable to use a reaction vessel with an inner surface made of materials such as glass lining, zirconium, and tantalum. Corrosion resistance can be determined, for example, from the data described in "Properties of Tantalum for Applications in the Chemical Process Industry: FJ Hunkeler (USA) ASTM STP849, 1984, pp. 28-49".

[0050] <Depolymerization rate of polyamide in the depolymerization process> In the depolymerization step, 80% by mass or more of the total polyamide contained in the polyamide-containing raw material, such as a polyamide resin composition, is depolymerized. In other words, the decomposition products, dicarboxylic acids, diamines, and their derivatives, shall constitute 80% by mass or more of the total polyamide contained in the polyamide resin composition. The remaining polyamide percentage is preferably 10% by mass or less of the total polyamide contained in the raw material containing polyamide, more preferably 7% by mass or less, even more preferably 5% by mass or less, and even more preferably 1% by mass or less. The amount of the decomposition product is preferably 85% by mass or more, more preferably 90% by mass, and even more preferably 95% by mass, of the total polyamide contained in the raw material containing polyamide. The remaining polyamide content can be controlled within the above numerical range by adjusting the concentration of the inorganic acid, the reaction time, and the temperature. The amount of the decomposition product can be controlled within the above numerical range by adjusting the concentration of the inorganic acid, the reaction time, and the temperature.

[0051] (separation process) As shown in Figure 1, in this embodiment, a separation step may be performed after the depolymerization step described above. In the separation step, components other than the diamine and its derivatives, and the dicarboxylic acid and its derivatives, are removed from the reaction solution obtained in the depolymerization step to obtain the diamine and its derivatives, and the dicarboxylic acid and its derivatives. Waste containing polyamide may contain glass fibers, carbon fibers, other inorganic fillers, various water-repellent coatings, and impurities derived from dirt such as sand. These remain as insoluble solids in the reaction solution after the depolymerization process, and most of them are removed in a subsequent separation process. The separation method is not particularly limited, but examples include sedimentation separation of insoluble components, centrifugation, and filtration.

[0052] As a method for separating components other than diamines and their derivatives, and dicarboxylic acids and their derivatives, known methods can be used depending on the substance to be removed, such as filtration using a filter, using an ion exchange membrane, or using an ion exchange resin. However, since recovered polyamide generally contains metals, glass, glass fibers, sand, etc., it is preferable from the viewpoint of process efficiency and energy saving to first remove these foreign substances by thermal filtration or the like immediately after the depolymerization step while the reaction solution temperature is still high. The temperature for thermal filtration should preferably be such that diamines and dicarboxylic acids do not precipitate. For this reason, 55°C or higher is preferred, 60°C or higher is more preferred, and 65°C or higher is even more preferred.

[0053] In this embodiment, in order to improve the recovery rate of diamine and dicarboxylic acid, it is preferable to separate the insoluble matter in a temperature range in which some or all of the dicarboxylic acid and dicarboxylic acid derivatives are dissolved in the reaction solution, and it is more preferable to separate the insoluble matter in a temperature range in which all of them are dissolved.

[0054] It should be noted that the entire amount of insoluble matter does not necessarily need to be removed during the separation process. Any remaining solid matter can be removed again during the purification process, which involves the isolation and purification of diamines and dicarboxylic acids, as described later. In this embodiment, it is also preferable to remove the insoluble solids simultaneously with the dicarboxylic acid precipitated from the reaction solution. In this case, in a subsequent step, it is possible to selectively dissolve the dicarboxylic acid in a solvent from the mixture of dicarboxylic acid and insoluble solids, and then remove the solids by separating only the insoluble components from the solution.

[0055] (purification process) As shown in Figure 1, in this embodiment, a purification step may be performed after the separation step. In the purification process, the diamine and the dicarboxylic acid are isolated and purified from the diamine and its derivatives obtained by the separation process described above, as well as the dicarboxylic acid and its derivatives. In the purification process for isolating and purifying the diamine and dicarboxylic acid, known methods can be used and are not particularly limited, but examples include the following methods.

[0056] <Isolation and purification process of diamines> In the isolation and purification method of diamines in the purification process, the order of operations differs depending on whether the reaction conditions in the depolymerization step are acidic or basic. When the depolymerization process is carried out under acidic conditions, the diamine is dissolved in the solution after the separation process as a diamine salt with a dicarboxylic acid (e.g., adipic acid) or an inorganic acid (e.g., hydrochloric acid). One method for separating the diamine from a solution containing the diamine salt is neutralization. By adjusting the pH to a range of 7 to 14, it is possible to liberate the diamine from the diamine salt. The alkali used in the neutralization process only needs to be greater than the pKa of the diamine to be separated as the target product. Industrially, suitable alkalis include, for example, sodium hydroxide, calcium hydroxide, and potassium hydroxide. When the depolymerization process is carried out under basic conditions, the diamine is released as a diamine into the liquid from the separation process described above. One method for purifying the released diamine to a purity suitable for use in the polymerization of polyamide is, for example, purification by distillation. Purification by distillation can also remove residues contained in the raw materials containing polyamide. If the reaction solution after the separation process described above contains a compound with a carboxylic acid, the diamine and carboxylic acid will polymerize during the heating of the distillation, leading to a decrease in the yield of diamine and scaling of the equipment. Therefore, it is preferable to remove the carboxylic acid before distillation by methods such as crystallization, physical adsorption using activated carbon or ion exchange resin, or membrane separation. Other methods for purifying diamines include extraction using solvents that form an organic phase, and membrane separation.

[0057] <Isolation and purification process of dicarboxylic acids> In the method for isolating and purifying dicarboxylic acids in the purification process, it is preferable to perform purification by crystallization after the separation step described above. In the isolation and purification method of dicarboxylic acids in the purification process, the order of operations differs depending on whether the reaction conditions in the depolymerization step are acidic or basic. When the depolymerization process is carried out under acidic conditions, the dicarboxylic acid is present in a liberated state. When the depolymerization process is carried out under basic conditions, the dicarboxylic acid is dissolved in the solution after the separation process as a dicarboxylate salt with a diamine (e.g., hexamethylenediamine) or a base (e.g., sodium hydroxide). One method for separating the dicarboxylic acid from a solution containing the dicarboxylate salt is neutralization. By adjusting the pH to a range of 0 to 7, it is possible to liberate the dicarboxylic acid from the dicarboxylate salt. The acid used in the neutralization process only needs to have a pKa lower than that of the dicarboxylic acid to be separated as the target product, and industrially, hydrochloric acid is preferably used as such. For example, purification by crystallization can be performed by precipitating the dicarboxylic acid from the reaction solution obtained in the separation step described above, crystallizing it by recrystallization, obtaining crude dicarboxylic acid crystals by solid-liquid separation, further dissolving the obtained crude dicarboxylic acid crystals in pure water, crystallizing and solid-liquid separating them, and drying to obtain purified dicarboxylic acid. For crystallization, the crude dicarboxylic acid crystals may be dissolved by stirring or heating the solution, or the solution may be allowed to mature for an appropriate amount of time to promote crystal growth. For drying, suitable conditions should be selected, provided they are below the melting point of the dicarboxylic acid. The dicarboxylic acid obtained in the separation process described above differs from dicarboxylic acid produced by conventional methods in that it contains residual metal and organic compounds derived from additives and pigments as impurities. These impurities can cause discoloration of the dicarboxylic acid and act as polymerization inhibitors during repolymerization into polyamide; therefore, it is preferable to remove them by crystallization in this isolation and purification process. For example, suitable methods include washing the dicarboxylic acid obtained by crystallization with inorganic acids such as nitric acid, sulfuric acid, or hydrochloric acid, physical adsorption using ion exchange resins or activated carbon, or membrane separation. The dicarboxylic acid purified in this way 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.

[0058] (Manufacturing of polyamides) The polyamide recycling method of this embodiment includes a polymerization step, as shown in Figure 1, in which the diamine and dicarboxylic acid obtained by the diamine and dicarboxylic acid production method of this embodiment described above are polymerized to obtain polyamide. This allows for the recycling of polyamide. The polymerization step can be carried out using known methods and is not particularly limited, but examples include the methods described below.

[0059] In the polymerization step, a method is commonly used in which a dicarboxylic acid-diamine salt, an aqueous solution of a mixture of dicarboxylic acid and diamine, or a suspension of these in water is heated and polymerized while maintaining a molten state (hereinafter also referred to as "thermal fusion polymerization"). However, polymerization is not limited to this method, and can be carried out by known methods such as solid-phase polymerization or solution polymerization.

[0060] Specific methods for producing polyamides using purified diamines and dicarboxylic acids include various methods, as illustrated below. (1) A method of polymerization by heating a dicarboxylic acid-diamine salt, an aqueous solution of a mixture of a dicarboxylic acid and a diamine, or a suspension of these in water, while maintaining a molten state (hereinafter also referred to as "thermal fusion polymerization"). (2) A method for increasing the degree of polymerization of polyamide obtained by thermal fusion polymerization while maintaining a solid state at a temperature below the melting point (hereinafter also referred to as "thermal fusion polymerization / solid-phase polymerization"). (3) A method of polymerizing a dicarboxylic acid-diamine salt, or a mixture of a dicarboxylic acid and a diamine, while maintaining a solid state (hereinafter also referred to as "solid-phase polymerization"). (4) A method of polymerization using a dicarboxylic acid halide component and a diamine component equivalent to the dicarboxylic acid (hereinafter also referred to as the "solution method"). Among these methods, a manufacturing method including thermal fusion polymerization is preferred, and when producing polyamide by thermal fusion polymerization, it is preferable to maintain the molten state until polymerization is complete. In order to maintain the molten state, it is necessary to manufacture the polyamide under polymerization conditions suitable for the polyamide composition. For example, the polymerization pressure in thermal fusion polymerization should be 14 to 25 kg / cm². 2 The pressure inside the tank is controlled to (gauge pressure) and heating continues while the pressure inside the tank is brought to atmospheric pressure (gauge pressure is 0 kg / cm²). 2 Methods include gradually lowering the blood pressure over a period of 30 minutes or more until it reaches the target level.

[0061] The polymerization method of the polyamide is not particularly limited and may be either batch or continuous. The polymerization apparatus used for the production of polyamides is not particularly limited, and known apparatuses can be used, such as autoclave reactors, tumbler reactors, and extruder reactors such as kneaders.

[0062] The following describes a method for producing polyamides using a batch-type thermal fusion polymerization method, but the method of producing polyamides is not limited to this. First, an aqueous solution containing approximately 40-60% by mass of the raw material components of polyamide (dicarboxylic acid, diamine, and, if necessary, lactam and / or aminocarboxylic acid) is concentrated to approximately 65-90% by mass in a concentration tank operated at a temperature of 110-180°C and a pressure of approximately 0.035-0.6 MPa (gauge pressure) to obtain a concentrated solution. Next, the concentrated solution is transferred to an autoclave and heated until the pressure in the autoclave reaches approximately 1.2 to 2.2 MPa (gauge pressure). Subsequently, in an autoclave, the pressure is maintained at approximately 1.2 to 2.2 MPa (gauge pressure) while removing water and / or gas components. When the temperature reaches approximately 220 to 260°C, the pressure is reduced to atmospheric pressure (gauge pressure: 0 MPa). By reducing the pressure inside the autoclave to atmospheric pressure and then reducing the pressure as needed, the by-product water can be effectively removed. Subsequently, the autoclave is pressurized with an inert gas such as nitrogen, and the molten polyamide is extruded from the autoclave as strands. The extruded strands are then cooled and cut to obtain polyamide pellets.

[0063] As shown in Figure 1, various additives may be added to the polyamide obtained by the polymerization process described above, depending on the desired physical properties. By melt-kneading such polyamide, the desired recycled polyamide is finally obtained. [Examples]

[0064] 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 in any way by the following examples and comparative examples. Using the raw materials listed below, purified monomers and polymers were obtained in the examples and comparative examples using the process described below, and evaluated by the method described below.

[0065] [Raw materials containing polyamide used in chemical recycling] (Polyamide fiber fabric waste) Airbags were recovered from scrapped cars. It is stamped with PA66 and its main component is polyamide 66.

[0066] [Pretreatment process for raw materials containing polyamide] (Shredding of airbags) Airbag scraps and discarded airbags were shredded into appropriate sizes. The preferred shred size is approximately 1 cm to 10 cm on each side, but since this can vary depending on the equipment used and the processing volume, there are no particular restrictions on the shred size of airbag scraps and waste airbags.

[0067] [Solvents used in the depolymerization process] (acid) The following inorganic acids were used as acids. • Hydrochloric acid (special grade, purity 35-37%) manufactured by Kanto Chemical Co., Ltd.

[0068] (base) The following inorganic bases were used as the bases. Sodium hydroxide, manufactured by Tokyo Chemical Industry Co., Ltd.

[0069] (Reducing agent) The following reducing agents were used. Sodium disophosphate monohydrate (Wako Grade 1), manufactured by Fujifilm Wako Pure Chemical Industries, standard oxidation-reduction potential at 25°C: -0.507V, solubility in water at 25°C: 91g / 100g-H2O Sodium thiosulfate, manufactured by Fujifilm Wako Pure Chemical Industries, standard oxidation-reduction potential at 25°C: 0.08V, solubility in water at 25°C: 41g / 100g-H2O

[0070] [Bases used in the purification process for isolating and purifying diamines] (base) The following bases were used as bases to neutralize the acid in the purification process. • Made of sodium hydroxide TCI

[0071] [Acids used in the purification process for isolating and purifying dicarboxylic acids] The following acids were used as acids to neutralize the base during the purification process. • Hydrochloric acid (special grade, purity 35-37%) manufactured by Kanto Chemical Co., Ltd.

[0072] [Examples 1-6], [Comparative Examples 1-2] According to the conditions described in the upper section of Table 1, depolymerization of the polyamide-containing raw materials was carried out to isolate adipic acid and hexamethylenediamine. The color tone (Hazen unit color number in aqueous ammonia solution) of the purified adipic acid (ADA) after crystallization and the amide bond decomposition rate are described in the lower section.

[0073] (Depolymerization process) <Depolymerization by acid: Examples 1-4, Example 6, and Comparative Example 1> A stirring bar, raw materials containing polyamide, 35% hydrochloric acid, a reducing agent, and distilled water were added to a pressure-resistant test tube to the concentrations shown in Table 1, and the mixture was stirred at a predetermined temperature for the desired time using a microwave synthesis reactor (Monowave 450, manufactured by Anton Paar). The heating process to the predetermined temperature was fixed at a heating time of 16 minutes, and the rotation speed of the stirring bar was set to 600 rpm. After depolymerization was carried out at a predetermined temperature for the desired time, compressed air was blown into the container within the apparatus to rapidly cool it, and the pressure-resistant test tube was removed when the temperature dropped to 50°C. 1H NMR analysis of the recovered reaction mixture revealed that the decomposition rate of amide bonds in the polyamide was 100%.

[0074] <Depolymerization with bases: Example 5 and Comparative Example 2> In a SUS316L autoclave equipped with a stirrer, the raw materials containing polyamide, sodium hydroxide, a reducing agent, and water were charged in the quantities listed in Table 1. The reaction vessel was purged with nitrogen, sealed under a nitrogen pressurization of 0.5 MPa, and the reaction was carried out at 280°C for 15 minutes while stirring at 200 rpm. During the reaction, the pressure in the system was 6.5 MPa. After the reaction was complete, the reaction mixture was cooled to room temperature and recovered. The total time spent mixing the polyamide-containing raw materials with the sodium hydroxide aqueous solution in the reaction vessel at a temperature of 230°C or higher was 40 minutes. ¹H NMR analysis of the recovered reaction mixture revealed a 98% degradation rate of amide bonds in the polyamide.

[0075] (Measurement of amide bond degradation rate) <Evaluation of the amount of hexamethylenediamine (HMD) and its derivatives relative to the polyamide before the depolymerization process, and evaluation of the amount of adipic acid (ADA) and its derivatives relative to the polyamide before the depolymerization process> The reaction solution after the depolymerization step was sampled and measured by NMR spectroscopy to calculate the decomposition rate of the polyamide. After adding the hydrolyzed solutions obtained in the examples and comparative examples to a 5mm diameter NMR tube, a special NMR sample tube N-502B (Nippon Precision Science Co., Ltd.) filled with benzene d-6 was inserted into the 5mm diameter NMR test tube, and measurements were taken using a JEOL NMR spectrometer (ECZ-500) with 1H as the observed nucleus, at a measurement temperature of 25°C, and with 1024 integration cycles. The integral values ​​of hexamethylenediamine and amide bonds were used to calculate (integral value of hexamethylenediamine) / (integral value of hexamethylenediamine + integral value of amide bonds), and this was defined as the amide bond decomposition rate of the polyamide.

[0076] (separation process) <Depolymerization under acidic conditions: Examples 1-4, Example 6, and Comparative Example 1> The solution after the depolymerization process was heated and filtered at 85°C to remove insoluble solid components (including glass fibers, silicone coating, and metal fragments). Next, the filtrate was cooled to room temperature, causing the dicarboxylic acid to precipitate as crystals. The dicarboxylic acid crystals and the filtrate were separated by filtration.

[0077] <Depolymerization under basic conditions: Example 5 and Comparative Example 2> The solution after the depolymerization process was heated and filtered at 85°C to remove insoluble solid components (including glass fibers, silicone coating, and metal fragments). Next, 35% hydrochloric acid was added to the filtrate until the pH was below 1, and then the mixture was cooled to room temperature, causing the dicarboxylic acid to precipitate as crystals. The dicarboxylic acid crystals and the filtrate were separated by filtration.

[0078] (purification process) <Purification of dicarboxylic acids> The dicarboxylic acid crystals obtained in the separation step were added to water equal to or greater than the mass of the crystals, and dissolved in the water by heating to 80°C. After dissolution, the mixture was allowed to stand and cool to recrystallize the dicarboxylic acid. The precipitated dicarboxylic acid crystals were recovered by filtration.

[0079] <Purification of diamines> Sodium hydroxide was gradually added to the filtrate obtained in the separation step, and it was confirmed that the precipitate of salt was formed by adding more than an equimolar amount of sodium hydroxide relative to the hydrochloric acid. The reaction solution after neutralization was distilled using a Kugellohr. After heating at 100°C and 300 mbar, the temperature was gradually increased and the pressure reduced to 110°C and 160 mbar, and it was held for 3 hours. Then the temperature was further increased and the pressure reduced to 140°C and 80 mbar, and it was held for about 1 hour to finally recover the target diamine.

[0080] (Monomer analysis) The 1H NMR of the purified diamine and dicarboxylic acid was measured to confirm that they were the target products.

[0081] (Monomer color tone: Hazen unit color count (APHA value)) The Hazen unit color number (APHA value) is a color scale in which the color of a solution containing 1 mg of platinum in the form of hexachloroplatinum ions and 2 mg of cobalt(II) chloride hexahydrate in 1 L is defined as 1. <Equipment used>: • Spectrophotometer Shimadzu UV-1900i <Reagents used>: • 4.8% ammonia aqueous solution (prepared by diluting a 28% ammonia aqueous solution with pure water) • APHA color number standard solution No. 500 (manufactured by Kishida Chemical Co., Ltd.)

[0082] <Preparation of samples for calibration curve> APHA color standard solution No. 500 was weighed into a 100 mL volumetric flask using a 20 mL volumetric pipette, and the solution was made up with pure water and thoroughly mixed to prepare APHA 100 standard solution. The APHA100 standard solution was weighed into a 100 mL volumetric flask using a 20 mL volumetric pipette, and the solution was made up with pure water and thoroughly mixed to prepare the APHA20 standard solution. The APHA100 standard solution was weighed into a 100 mL volumetric flask using a 10 mL volumetric pipette, and the solution was made up with pure water and thoroughly mixed to prepare the APHA10 standard solution. The APHA100 standard solution was weighed into a 100 mL volumetric flask using a 5 mL volumetric pipette, and the APHA5 standard solution was prepared by making up the volume with pure water and mixing thoroughly.

[0083] <Creating a calibration curve> The various standard solutions prepared as described above were measured using a spectrophotometer in a 50 mm quartz cell, and the transmittance at 454 nm was determined for each. A calibration curve was created by plotting transmittance and APHA values ​​on the x and y axes, respectively.

[0084] <Preparation of the measurement solution> 12.0 g of crystallized adipic acid was weighed into a conical beaker, and 100 mL of 4.8% aqueous ammonia solution was added using a graduated cylinder. The adipic acid was then completely dissolved to prepare the measurement solution.

[0085] <Preparation of measurement samples> The measurement solutions prepared as described above were measured using a spectrophotometer in a 50 mm quartz cell, and the transmittance at 454 nm was determined. The APHA value was then determined from the calibration curve.

[0086] [Methods for polymerizing polyamides] (Polymerization using adipic acid and hexamethylenediamine obtained in Example 1) The polymerization reaction of polyamide was carried out using the "thermal fusion polymerization method" as follows. As described above, 30 g of the recovered adipic acid and hexamethylenediamine equimolar salts were dissolved in 30 g of distilled water to prepare a homogeneous aqueous solution of 50% by mass of the raw material monomers. This aqueous solution was placed in a 0.5 L autoclave and purged with nitrogen. The solution was concentrated by gradually removing water vapor while stirring at a temperature of 110-150°C until the solution concentration reached 70% by mass. Then, the internal temperature was raised to 220°C. At this time, the autoclave pressure was increased to 1.8 MPa. The reaction was then carried out for 1 hour, while gradually removing water vapor and maintaining the pressure at 1.8 MPa until the internal temperature reached 245°C. Next, the pressure was reduced over a period of one hour. Subsequently, the autoclave was maintained under reduced pressure of 650 torr (86.66 kPa) using a vacuum device for 10 minutes. At this time, the final internal temperature of polymerization was 265°C. Subsequently, the material was pressurized with nitrogen and formed into strands from the lower spindle (nozzle). After water cooling and cutting, it was discharged in pellet form and dried at 100°C under a nitrogen atmosphere for 12 hours to obtain polyamide 1. The Mw was 35000 and the Mw / Mn ratio was 2.0. The appearance was whitish and semi-transparent.

[0087] (Polymerization using adipic acid and hexamethylenediamine obtained in Comparative Example 2) As described above, 150 g of the recovered adipic acid and hexamethylenediamine equimolar salt was dissolved in 150 g of distilled water to prepare a homogeneous aqueous solution of 50% by mass of the raw material monomers. This aqueous solution was placed in a 0.5 L autoclave and purged with nitrogen. The solution was concentrated by gradually removing water vapor while stirring at a temperature of 110-150°C until the solution concentration reached 70% by mass. Then, the internal temperature was raised to 220°C. At this time, the autoclave pressure was increased to 1.8 MPa. The reaction was then carried out for 1 hour, while gradually removing water vapor and maintaining the pressure at 1.8 MPa until the internal temperature reached 245°C. Next, the pressure was reduced over a period of one hour. Subsequently, the autoclave was maintained under reduced pressure of 650 torr (86.66 kPa) using a vacuum device for 10 minutes. At this time, the final internal temperature of polymerization was 265°C. Subsequently, the material was pressurized with nitrogen and formed into strands from the lower spindle (nozzle). After water cooling and cutting, it was discharged in pellet form and dried at 100°C under a nitrogen atmosphere for 12 hours to obtain polyamide 2. The Mw was 30600 and the Mw / Mn ratio was 2.0. The appearance was a brownish, translucent material.

[0088] [Analysis of polyamides] The 1H NMR spectra of the obtained polyamides 1 and 2 were measured to confirm that they were polyamide 66. When this polyamide was compared to the polyamide resin LEONA1300S (manufactured by Asahi Kasei Corporation), polyamide 1 had a similar color tone to LEONA1300S, while polyamide 2 was browner than LEONA1300S. From this, it was found that the polyamide obtained in Example 1 has excellent color characteristics.

[0089] [Table 1]

[0090] In each example, depolymerization was possible with a decomposition rate equivalent to that of the case without the addition of a reducing agent. Furthermore, when a reducing agent was added, the degree of discoloration of the reaction solution was less compared to the case without the addition of a reducing agent. In addition, the ammonia aqueous solutions of adipic acid obtained by crystallization all had APHA values ​​below 10, indicating excellent color tone. Moreover, polyamide 66 obtained using purified hexamethylenediamine and adipic acid also exhibited excellent color tone. [Industrial applicability]

[0091] The present invention provides a method for producing diamines and dicarboxylic acids, and a method for recycling polyamides using monomers obtained by this method, which yields recycled polyamides with excellent color tone. Therefore, this method has industrial applicability as a recycling method that provides excellent appearance quality for polyamide resins, polyamide fibers, polyamide resin compositions, and molded products used in automotive parts, various industrial parts, and apparel such as textiles and fabrics.

Claims

1. The process includes a depolymerization step in which a raw material containing polyamide is heated in an aqueous solution containing an acid or base to depolymerize it and obtain diamines and dicarboxylic acids. The depolymerization step is carried out in the presence of 0.0001 to 10 parts by mass of a reducing agent per 100 parts by mass of polyamide in the raw material. The reducing agent has a standard oxidation-reduction potential of -1.0 V to 0.3 V at 25°C in the aqueous solution. A method for producing diamines and dicarboxylic acids.

2. The reducing agent is The solubility in water at 25°C is 0.01 g / 100 g - H 2 It is 0 or greater. A method for producing diamines and dicarboxylic acids according to claim 1.

3. The reducing agent is It is one or more selected from the group consisting of diphosphorous acid, metal diphosphorous acid salts, thiosulfate, and metal thiosulfate salts. A method for producing diamines and dicarboxylic acids according to claim 1.

4. The concentration of the acid or base in the aqueous solution is 5% by mass or more and 25% by mass or less. A method for producing diamines and dicarboxylic acids according to claim 1.

5. The polyamide contained in the raw material contains more than 50% by mass of polyamide 66. A method for producing diamines and dicarboxylic acids according to claim 1.

6. The heating temperature in the depolymerization step is 90°C to 160°C under acidic decomposition conditions, and 230°C to 400°C under basic decomposition conditions. A method for producing diamines and dicarboxylic acids according to claim 1.

7. The method further comprises a separation step of removing components other than the diamine and the diamine derivatives and the dicarboxylic acid and the dicarboxylic acid derivatives from the reaction solution obtained by the depolymerization step, thereby obtaining the diamine and the diamine derivatives and the dicarboxylic acid and the dicarboxylic acid derivatives. A method for producing diamines and dicarboxylic acids according to claim 1.

8. In the separation step, When removing components other than the diamine and its derivatives, and the dicarboxylic acid and its derivatives, thermal filtration and centrifugation are performed. A method for producing diamines and dicarboxylic acids according to claim 7.

9. The method further comprises 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 by the separation step, respectively. A method for producing diamines and dicarboxylic acids according to claim 7.

10. In the purification step, the dicarboxylic acid is purified by crystallization. A method for producing diamines and dicarboxylic acids according to claim 9.

11. In the purification step, the diamine is purified by distillation. A method for producing diamines and dicarboxylic acids according to claim 9.

12. The raw material containing the polyamide contains 1% by mass or more and 50% by mass or less of components other than the polyamide. A method for producing diamines and dicarboxylic acids according to claim 1.

13. The raw material containing the polyamide is waste containing polyamide. A method for producing diamines and dicarboxylic acids according to claim 1.

14. The method further comprises a polymerization step of polymerizing the diamine and dicarboxylic acid obtained by the method for producing a diamine and dicarboxylic acid according to any one of claims 1 to 13 to obtain a polyamide. Methods for recycling polyamide.

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