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

Abstract

This invention provides a method for producing diamines and dicarboxylic acids, and a method for recycling polyamides, which, even when implemented industrially, can reduce GHG emissions in the distillation process, recover diamines and dicarboxylic acids in high yield, and use the obtained monomers as polymerization raw materials to obtain polyamides with physical properties and quality equivalent to those of polyamides derived from petrochemical raw materials. [Solution] The method comprises the steps of (a) swelling a raw material containing polyamide with (b) an acidic aqueous solution containing an inorganic acid, heating to depolymerize, and obtaining a diamine and a dicarboxylic acid. 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, that is, 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, electrical and electronic parts, and other industrial products. On the other hand, in recent years, in the plastic industry, there is a demand for responding to a resource recycling society, and the establishment of recycling technology for polyamide is also required.

[0003] There are generally 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 is not effectively utilized as a resource. 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 being promoted.

[0005] Regarding the aforementioned chemical recycling technologies, Patent Document 2 proposes a technique for producing monomers by decomposing polyamides by ammoniaxation using a Lewis acid catalyst. Patent Document 3 proposes a method for separating polyamides and glass fibers from molded articles of glass fiber-containing polyamide resin compositions using an aqueous phosphoric acid solution, and further decomposing the polyamides into monomers. Non-Patent Document 1 proposes the depolymerization of polyamide 66 using microwaves. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 6839266 [Patent Document 2] Patent No. 3571723 [Patent Document 3] Japanese Patent Publication No. 2000-80199 [Non-patent literature]

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

[0008] However, the method described in Patent Document 2 has the problem of low monomer yield. Furthermore, the method described in Patent Document 3, in a specific example using polyamide 66, requires 200 minutes for the dissolution of the polyamide and glass fibers. Moreover, Patent Document 3 does not provide a specific description of the depolymerization method, and furthermore, since the dissolution of the polyamide and glass fibers takes 200 minutes, if further depolymerization of the polyamide is to be performed after the dissolution of the molded product of the polyamide resin composition, the total time will be even longer, which is a problem as it lacks practicality. Furthermore, the method described in Non-Patent Document 1 achieves rapid and high-yield depolymerization of polyamides by using hydrochloric acid at a high concentration of 17% by mass or more and maintaining a high temperature of 170°C or higher using microwaves.

[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, even when implemented industrially, can reduce GHG emissions in the distillation process, recover diamines and dicarboxylic acids in high yield, and produce polyamides with physical properties and quality equivalent to those of polyamides derived from petrochemical raw materials as polymerization raw materials. [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 swelling a raw material containing polyamide in a predetermined aqueous solution, and then heating and depolymerizing it to produce diamines and dicarboxylic acids, thereby completing the present invention. In other words, the present invention is as follows:

[0011] [1] (a) A raw material containing polyamide is swollen with an aqueous acid solution containing an inorganic acid, and then heated to depolymerize it, thereby obtaining a diamine and a dicarboxylic acid. A method for producing diamines and dicarboxylic acids. [2] (A) A step of mixing (a) a raw material containing polyamide, (b) an inorganic acid, and (c) water to obtain a mixture, (B) A step of immersing the mixture for 0.1 hours or more and 100 hours or less, thereby swelling the raw material containing (a) polyamide by 10% by mass or more, (C) A step of heating and depolymerizing the raw material containing the swollen (a) polyamide obtained in step (B) above to obtain a diamine and a dicarboxylic acid, Having, A method for producing diamines and dicarboxylic acids as described in [1] above. [3] After step (B) and before step (C), (D) A step of partially removing the aqueous layer from a mixture comprising the swollen (a) polyamide-containing raw material, (b) inorganic acid, and (c) water. A method for producing diamines and dicarboxylic acids as described in [2] above. [4] In step (D) above, The ratio of the number of moles of the amide group of the polyamide to the number of moles of the proton of the inorganic acid (b) is, The aqueous layer is removed so that the ratio of the amide group of the polyamide to the proton of the inorganic acid (b) is 1:0.8 to 1:5.5. The method for producing diamines and dicarboxylic acids as described in [3] above. [5] After step (C) above, (E) A separation step to remove components other than the diamine and the dicarboxylic acid from the reaction solution obtained in step (C) above, in order to obtain the diamine and the dicarboxylic acid, (F) A purification step in which the diamine and the dicarboxylic acid obtained in step (E) are isolated and purified, Having, A method for producing diamines and dicarboxylic acids according to any one of [2] to [4] above. [6] In step (B) above, temperature control is performed, and in the temperature control, a heat source derived from waste heat from equipment used in steps other than step (B) above is used. The method for producing diamine and dicarboxylic acid according to any one of the above [2] to [5]. [7] The heating in the step (C) is including heating up to the target heating temperature and heating at the target temperature, where the heating rate during the heating-up is 25 °C / min or less, The method for producing diamine and dicarboxylic acid according to any one of the above [2] to [6]. [8] The pKa of the inorganic acid (b) is 0 or less, The method for producing diamine and dicarboxylic acid according to any one of the above [1] to [7]. [9] The inorganic acid (b) is hydrochloric acid, The method for producing diamine and dicarboxylic acid according to any one of the above [1] to [8].

[10] The raw material containing the polyamide (a) is including substances derived from pre-consumer products or post-consumer products, The method for producing diamine and dicarboxylic acid according to any one of the above [1] to [9].

[11] Among the constituent components of the polyamide, the mass fraction of polyamide 66 is more than 50% by mass, The method for producing diamine and dicarboxylic acid according to any one of the above [1] to

[10] .

[12] In the step (E), when removing components other than the diamine and the dicarboxylic acid, a step of removing components other than the diamine and the dicarboxylic acid as insoluble matter from the reaction solution in a state where a part or the whole amount of the dicarboxylic acid obtained in the step (C) is dissolved in the reaction solution is included, The method for producing diamine and dicarboxylic acid according to any one of the above [5] to

[11] .

[13] In the step (A), the concentration of the inorganic acid (b) in the aqueous solution is 9% by mass or more and 25% by mass or less, A method for producing diamines and dicarboxylic acids according to any one of [2] to

[12] above.

[14] The above step (C) is carried out using a reaction vessel having an inner surface made of glass lining, zirconium, or tantalum. A method for producing diamines and dicarboxylic acids according to any one of [2] to

[13] above.

[15] In step (E) above, when removing components other than the diamine and the dicarboxylic acid, thermal filtration and centrifugation are performed. A method for producing diamines and dicarboxylic acids according to any one of [5] to

[14] above.

[16] In step (F) above, the dicarboxylic acid is purified by crystallization. A method for producing diamines and dicarboxylic acids according to any one of [5] to

[15] above.

[17] In step (F) above, the diamine is purified by distillation. A method for producing diamines and dicarboxylic acids according to any one of [5] to

[16] above.

[18] In step (C) above, heating is performed by microwave. A method for producing diamines and dicarboxylic acids according to any one of [2] to

[17] above.

[19] The microwave frequency during heating by microwave is 0.8 to 6 GHz. A method for producing diamines and dicarboxylic acids as described in

[18] above.

[20] A method for recycling polyamides, comprising a polymerization step of polymerizing the diamine and the dicarboxylic acid obtained by the method for producing the diamine and dicarboxylic acid described in any one of [1] to

[19] above to obtain a polyamide. [Effects of the Invention]

[0012] According to the present invention, even when implemented industrially, it is possible to reduce GHG emissions in the distillation process, recover diamines and dicarboxylic acids in high yield, and obtain polyamides with physical properties and quality equivalent to those of polyamides derived from petrochemical raw materials by using the obtained monomers as polymerization raw materials. The present invention provides a method for producing diamines and dicarboxylic acids, and a method for recycling polyamides. [Brief explanation of the drawing]

[0013] [Figure 1] This figure shows the flow diagram of the polyamide recycling method of the present invention. [Modes 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] [Method for producing diamines and dicarboxylic acids] The method for producing diamines and dicarboxylic acids according to this embodiment includes the steps of (a) swelling a raw material containing polyamide with (b) an acidic aqueous solution containing an inorganic acid, heating to depolymerize, and obtaining diamines and dicarboxylic acids.

[0016] The method for producing diamines and dicarboxylic acids in this embodiment is: (A): A step of mixing (a) a raw material containing polyamide, (b) an inorganic acid, and (c) water to obtain a mixture (hereinafter sometimes referred to as step (A)), (B): A step of immersing the mixture for 0.1 hours or more and 100 hours or less, thereby swelling the raw material containing (a) polyamide by 10% by mass or more (hereinafter sometimes referred to as step (B)), (C): A step in which the raw material containing the swollen (a) polyamide obtained in step (B) is heated and depolymerized to obtain a diamine and a dicarboxylic acid (hereinafter sometimes referred to as step (C)), A preferred form is one that has [this feature].

[0017] Furthermore, in this embodiment, the polyamide can be recycled by polymerizing the obtained diamine and dicarboxylic acid.

[0018] According to the method for producing diamines and dicarboxylic acids and the method for recycling polyamides of this embodiment, even when implemented industrially, it is possible to reduce GHG emissions in the distillation process, recover diamines and dicarboxylic acids in high yield, and obtain high-quality polyamides with a quality equivalent to that of polyamides derived from petrochemical raw materials.

[0019] ((A) process) In a preferred embodiment of the method for producing diamines and dicarboxylic acids of this embodiment, in step (A), (a) a raw material containing polyamide, (b) an inorganic acid, and (c) water are mixed to obtain a mixture.

[0020] <(a) Raw materials containing polyamide> In the method for producing diamines and dicarboxylic acids of this embodiment, (a) a raw material containing polyamide is used. (a) The raw material containing polyamide is not limited to any material containing polyamide, but waste containing polyamide can be used. Here, "waste" is not defined as waste as defined by law (the Waste Management and Public Cleansing Law), but rather as a general term for things that are no longer used as products, things that cannot be used as products, etc. (a) Waste containing polyamide includes not only molded articles of polyamide and polyamide resin compositions, but also polyamide and polyamide resin compositions as products. Specifically, these include 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. Furthermore, (a) raw materials containing polyamide may include those derived from pre-consumer or post-consumer products. Hereafter, these will be collectively referred to as (a) waste containing polyamide.

[0021] Figure 1 shows a flowchart illustrating a polyamide recycling method using the diamine and dicarbon production method of this embodiment.

[0022] [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.

[0023] 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 means a component that accounts for more than 50% by mass of the total polymer components (100% by mass). In particular, it is preferable that the polyamide contained in the raw material containing (a) polyamide has a mass fraction of polyamide 66 of more than 50% by mass among the constituent components of the polyamide. Polyamide 66 is a polyamide obtained by the condensation polymerization of hexamethylenediamine and adipic acid. Due to its excellent heat resistance, mechanical strength, and creep properties, it is suitably used as a material for functional components in automobiles, machinery, and electrical products, or as a high-strength fiber.

[0024] [Polyamide resin composition] A polyamide resin composition is a resin composition that includes the polyamide and, as necessary, inorganic fillers such as glass fibers, lubricants, and other additives such as heat stabilizers, flame retardants, pigments, and dyes.

[0025] <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.

[0026] <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.

[0027] <Other additives> The polyamide resin composition and its molded articles may contain other additives in addition to the polyamide, inorganic filler, and lubricant described above. Examples of other additives include antioxidants, ultraviolet absorbers, heat stabilizers, photodegradation inhibitors, plasticizers, mold release agents, nucleating agents, flame retardants, colorants such as pigments and dyes, and other thermoplastic resins.

[0028] [Molded products] The molded articles of polyamide or polyamide resin compositions used in this embodiment are manufactured by various known methods, such as injection molding. The molded articles used in this embodiment may also be fibers of polyamide or polyamide resin compositions.

[0029] Polyamide is manufactured in 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, as they are less affected by environmental contaminants and decomposition products due to degradation.

[0030] As shown in Figure 1, in this embodiment, a pretreatment step may be performed on the raw material containing (a) polyamide as a pre-step before step (A). In the pretreatment step, (a) the raw material containing polyamide is subjected to one or more steps selected from the group consisting of crushing, washing, and separation of foreign matter to obtain crude polyamide. This increases the surface area, which tends to make the premixing step (A) and the swelling step (B) described later more efficient. 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 raw material containing (a) polyamide is high and there are no noticeable impurities, the washing and separation of foreign matter steps can be simplified or omitted. By performing the pretreatment step followed by the swelling step (B) and the depolymerization step (C) described later, monomerization can be carried out with high efficiency.

[0031] In this embodiment, step (A) is performed, in which (a) a raw material containing polyamide, (b) an acid containing an inorganic acid, and (c) water are mixed in a reaction vessel or pre-mixing vessel. The pre-treatment step described above may be included before step (A). In step (A), by bringing the raw material containing polyamide into contact with an aqueous solution containing an acid whose concentration has been adjusted by first mixing (b) an inorganic acid and water, the swelling step of step (B) and the aqueous layer removal step of step (D), which will be described later, can be carried out efficiently.

[0032] In step (A) described above, the concentration of the inorganic acid (b) in the aqueous solution is preferably 9% by mass or more and 25% by mass or less. (A) The concentration of the inorganic acid (b) in the aqueous solution in step (A) is preferably 9% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, from the viewpoint of efficiently swelling the polyamide. Furthermore, the concentration of the inorganic acid (b) in the aqueous solution in step (A) is preferably 25% by mass or less, more preferably 23% by mass or less, and even more preferably 20% by mass or less, from the viewpoint of suppressing corrosion of the reaction apparatus.

[0033] ((B) Process: Swelling process) In a preferred embodiment of the method for producing diamines and dicarboxylic acids of this embodiment, step (B) is performed to swell the mixture obtained in step (A). Specifically, the mixture obtained in step (A) above is swelled at a predetermined temperature for a predetermined time while maintaining a mixed state. In the swelling step of step (B), the immersion time of the mixture is preferably 0.1 hours or more and 100 hours or less, more preferably 0.1 hours or more and 50 hours or less from the viewpoint of productivity, and even more preferably 0.1 hours or more and 10 hours or less. Here, the starting point for calculating the immersion time of the mixture is the time when (a) the raw material containing polyamide and (b) the acidic aqueous solution containing inorganic acid first come into contact. In addition, the temperature in the swelling step of step (B) is preferably lower than that of the depolymerization step of step (C) described later, more preferably 80°C or less from the viewpoint of handling, and even more preferably 60°C or less. A swing operation format may be adopted in which two or more premixing containers or reaction containers are set up in parallel, one is used for the swelling step and the other for the depolymerization step simultaneously, the temperature of the container used for the swelling step is raised when the depolymerization step is finished, and the contents are discharged from the reaction container that has finished the depolymerization step. The heating method is not particularly limited, but examples include steam and electric heaters. Furthermore, by utilizing waste heat from processes other than the swelling process, swelling can be achieved without using additional energy in the overall process. In the swelling step of step (B), it is preferable to swell the raw material containing the polyamide (a) by 10% by mass or more, from the viewpoint of concentrating the acid in the swollen polyamide. "Swelling by 10% by mass or more" means that the mass ratio of the swollen polyamide to the component obtained by vacuum drying the swollen polyamide is 1.1 (110% by mass) or more. Swelling by 10% by mass or more is more preferable, and swelling by 20% by mass or more is even more preferable. This amount of swelling can be controlled within the above numerical range by, for example, adjusting the acid concentration or adjusting the swelling temperature.

[0034] <Solvent> In the swelling process, (b) an acidic aqueous solution containing an inorganic acid is used. That is, (c) 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 derived from the recovered polyamide in subsequent processes. However, if an appropriate process can be established to remove impurities derived from the recovered polyamide in subsequent processes, organic solvents such as ethylene glycol and methanol may also be used. Furthermore, in the depolymerization process of step (C) described later, it is not necessary for the raw material containing (a) polyamide to be completely dissolved in the solvent as a polymer; it is sufficient if it is partially dissolved and decomposed into monomers during the depolymerization process. Since complete dissolution as a polymer in the solvent is not required during the depolymerization process, there is also the advantage that the thickening during the depolymerization process is small, and a large amount of raw material containing (a) polyamide can be added to the solvent.

[0035] <Inorganic acid> In this embodiment, (b) inorganic acid is used in steps (A) and (B). (b) Inorganic acids are thought to act as aids in the swelling of polyamides. (b) Examples of inorganic 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 depolymerization rate during depolymerization, acids with a pKa of 0 or less are preferred.

[0036] Furthermore, hydrochloric acid is preferred as the inorganic acid (b) from the viewpoint of reaction efficiency, reduction of impurities, and reduction of waste that is difficult to recycle.

[0037] In the swelling process, the amount of inorganic acid (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 inorganic acid is used, the amount of polyamide added in the swelling process will inevitably be relatively small, so from the viewpoint of productivity, it is preferable that the amount of inorganic acid is not too high. In particular, increasing the amount of inorganic acid increases the amount of water in the system, which increases the load on the water removal process during the subsequent diamine purification process, leading to increased costs and increased GHG emissions. Also, if a large excess of inorganic acid is used, a large amount of salt is generated in the neutralization process after the depolymerization process, leading to increased load and costs on the salt treatment process. For these reasons as well, it is preferable that the amount of inorganic acid is not too high. From the above viewpoint, the ratio of moles of amide groups in the polyamide to protons of the inorganic acid is preferably amide groups of polyamide : (b) protons of inorganic acid = 1:0.8 to 1:5.5, more preferably 1:1 to 1:5.5, even more preferably 1:1.1 to 1:3, and even more preferably 1:1.15 to 1:2.

[0038] (B) The concentration of the inorganic acid in the aqueous solution containing the inorganic acid in the swelling step of step (b) is preferably greater than 2.3 mol / L. In the swelling process, if the concentration of the inorganic acid in the aqueous solution is greater than 2.3 mol / L, the raw material containing (a) polyamide can be swelled by 10% by mass or more. There is no particular upper limit to the concentration of inorganic acid in the aqueous solution during the swelling process. However, since the inorganic acid from the swelling process is also used in subsequent processes, a concentration of 8.0 mol / L or less is preferable, for example, from the viewpoint of the reaction rate in the depolymerization process of step (C) described later.

[0039] In the swelling step of step (B), even if the temperature is below that of the depolymerization step of step (C) described later, if the raw material containing (a) polyamide contains impurities that can react with inorganic acids, the reaction may be carried out. For example, if the raw material containing (a) polyamide contains metals such as aluminum, hydrogen gas may be generated from the inorganic acid, water, and aluminum. Also, if carbonates such as sodium carbonate are present, carbon dioxide gas may be generated. In addition, carbonate ester compounds such as polycarbonate may also generate carbon dioxide gas. These gases increase the pressure in the reactor, so they may be removed in advance from the viewpoint of the safety of the operation of the apparatus.

[0040] (B) In the swelling step, the aqueous solution may be stirred as needed. At this time, considering the temperature rise due to the stirring power, the number of stirs and stirring time may be adjusted so that the aqueous solution temperature stays within a preferred temperature range. The number of stirs should be such that it does not destroy the raw material containing the swollen (a) polyamide. The stirring method is not particularly limited and may be either top drive or bottom drive.

[0041] In this embodiment, temperature control can be performed in step (B). For this temperature control, a heat source derived from waste heat from equipment used in processes other than step (B) can be used. (B) In the process, the temperature is preferably controlled to 0°C to 70°C from the viewpoint of achieving a swelling rate of 10% by mass or more.

[0042] ((D) process: water layer removal process) In this embodiment, as shown in Figure 1, after the swelling step of step (B) described above, and before the depolymerization step of step (C) described later, step (D) may be performed: a step of partially removing the aqueous layer from a mixture containing swollen (a) raw materials including polyamide, (b) inorganic acid, and (c) water (hereinafter, this may be referred to as step (D) or aqueous layer removal step). "Partial removal" means removing a portion of the aqueous layer in the reaction vessel while leaving some behind. By partially removing the aqueous layer, it is possible to reduce the amount of water per monomer produced while maintaining a high rate of amide bond decomposition. This reduces the energy required for heating in step (C) described later, and tends to further reduce GHG emissions. In the aqueous layer removal process, the aqueous layer may be selectively removed from the top, middle, or bottom of the reactor. To prevent solid matter from being carried out of the reactor during removal, a mesh or filter may be installed at the end or inside of valves, hoses, or tubes.

[0043] In this embodiment, in step (D), from the viewpoint described in step (B): swelling step above, it is preferable to remove the aqueous layer so that the ratio of moles of amide groups of the polyamide to protons of the inorganic acid (b) is amide groups of the polyamide : protons of the inorganic acid (b) = 1:0.8 to 1:5.5. More preferably, amide groups of the polyamide : protons of the inorganic acid (b) = 1:1 to 1:5.5, even more preferably 1:1.1 to 1:3, and even more preferably 1:1.15 to 1:2.

[0044] ((C) process: depolymerization process) As shown in Figure 1, in this embodiment, a depolymerization step is performed in which the raw material containing the swollen (a) polyamide obtained in the above-described (B) step: swelling step is heated and depolymerized. Furthermore, before step (C): depolymerization step, the above-described step (D): aqueous layer removal step may be performed. In the depolymerization step of step (C), it is preferable to heat the raw material containing (a) polyamide that has undergone the swelling step of step (B) and an aqueous solution containing (b) an inorganic acid, thereby depolymerizing 85% by mass or more of the total polyamide contained in the raw material containing (a) polyamide, and decomposing it into diamine and dicarboxylic acid.

[0045] Breaking the amide bonds in polyamides requires supplying the necessary energy from an external source, and therefore, heating is necessary during the depolymerization process. The heating method is not limited to the following, but examples include steam and electric heaters. Furthermore, depolymerization can be performed with low energy by using microwaves. The frequency of the microwaves used during heating will be described later. Furthermore, the addition of inorganic salts is effective in promoting depolymerization. Specific inorganic salts will be discussed later. Furthermore, the addition of inorganic acids is necessary to promote depolymerization. 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.

[0046] <Inorganic salts> (C) Step: In the depolymerization step, an inorganic salt may be added to the aqueous solution in order to increase the solubility of the raw material containing (a) polyamide in water. Using an aqueous solution containing inorganic salts tends to weaken the hydrogen bonds between the polymer chains of polyamides, thereby increasing their solubility. Inorganic salts are a general term for salts composed solely of inorganic components. Examples include metal salts, which are compounds in which a hydrogen atom of an acid is replaced with a metal ion. The aforementioned metal salts are not limited to the following, but from the viewpoint of suitability for depolymerization, examples include calcium, zinc, and lithium halides. Specifically, examples include calcium chloride, zinc chloride, zinc bromide, chromium bromide, iron bromide, lithium chloride, lithium bromide, and cobalt chloride. From the viewpoint of availability and safety, calcium chloride, zinc chloride, and lithium chloride are preferred.

[0047] <Inorganic acid> In the depolymerization step, a mixture is used in which a raw material containing a swollen (a) polyamide is mixed with an aqueous solution containing an inorganic acid (b). Inorganic acids are thought to act as catalysts for the hydrolysis of polyamides. As mentioned above, inorganic acids are not limited to the following, but include, for example, 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 depolymerization rate during depolymerization, acids with a pKa of 0 or less are preferred. Furthermore, hydrochloric acid is preferred as the inorganic acid from the viewpoint of reaction efficiency, reduction of impurities, and reduction of waste that is difficult to recycle.

[0048] (C) Step: The amount of inorganic acid used in the depolymerization step (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 step will inevitably be relatively small, so from the viewpoint of productivity, it is preferable that the amount of inorganic acid is not too high. In particular, increasing the amount of inorganic acid increases the amount of water in the system, which increases the load on the water removal step during the subsequent diamine purification, leading to increased costs and increased GHG emissions. Also, if a large excess of inorganic acid is used, a large amount of salt is generated in the neutralization step after the depolymerization step, leading to increased load and costs on the salt treatment process. For these reasons as well, it is preferable that the amount of inorganic acid is not too high. From the above viewpoint, the ratio of moles of amide groups to protons of the acid in the polyamide is preferably amide groups of polyamide : (b) protons of inorganic acid = 1:0.8 to 1:5.5, more preferably 1:1 to 1:5.5, even more preferably 1:1.1 to 1:3, and even more preferably 1:1.15 to 1:2.

[0049] (C) Step: In the depolymerization step, the concentration of inorganic acid, particularly hydrochloric acid, in the aqueous solution, as analyzed by the Arrhenius equation, increases with increasing hydrochloric acid concentration, which reduces the activation energy Ea and makes the reaction more likely to proceed. However, at the same time, as the number of water molecules decreases, the frequency factor A decreases, making the reaction less likely to proceed. Therefore, there is an optimal range for hydrochloric acid concentration, preferably 1.4 mol / L or higher and 8.0 mol / L or lower. More preferably, the concentration is 3.0 mol / L to 8.0 mol / L, and even more preferably, 3.5 mol / L to 7.0 mol / L.

[0050] <Heating> In step (C): depolymerization step, the raw material containing the swollen (a) polyamide is heated in an aqueous solution containing an inorganic acid. 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. For depolymerization, any known heating method such as steam or electric heater can be applied, but a microwave method is preferred from the viewpoint of shortening the heating time and reducing GHG emissions.

[0051] Furthermore, in step (C): depolymerization step, the heating may include raising the temperature to the target heating temperature and heating at the target temperature. The rate of heating during this step 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 time of step (C): depolymerization step, 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.

[0052] <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.

[0053] <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 from data such as that described in "Properties of Tantalum for Applications in the Chemical Process Industry: FJ Hunkeler (USA) ASTM STP849, 1984, pp. 28-49".

[0054] <Depolymerization rate of polyamides in the depolymerization process> In the depolymerization step, it is preferable to depolymerize 85% or more of the amide groups in the total polyamide contained in the raw material containing (a) polyamide. On the other hand, from the viewpoint of recovering decomposition products, it is preferable that the decomposition products, dicarboxylic acids, diamines, and their derivatives, constitute 80% by mass or more of the total polyamide contained in the crude polyamide. The remaining polyamide percentage is preferably such that (a) the amide groups in the total polyamide contained in the raw material containing polyamide are 10% or less, more preferably 7% or less, even more preferably 5% or less, and even more preferably 1% or less. The amount of the decomposition product is preferably 85% by mass or more, more preferably 95% by mass, and even more preferably 98% by mass, of the total polyamide contained in the raw material containing (a) 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.

[0055] ((E) process: separation process) As shown in Figure 1, in this embodiment, after step (C): depolymerization step described above, step (E): separation step may be performed. In the separation step, components other than the diamine and the dicarboxylic acid are removed from the reaction solution obtained in step (C): depolymerization step to obtain the diamine and the dicarboxylic acid. (a) The raw materials 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 step (C): depolymerization, and therefore need to be removed. The separation method is not particularly limited, but examples include sedimentation separation of insoluble components, centrifugation, and filtration.

[0056] As for specific methods for separating components other than diamines and dicarboxylic acids, known methods can be used depending on the substance to be removed, such as filtration using filters, ion exchange membranes, and ion exchange resins. 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. Therefore, 55°C or higher is preferred, 60°C or higher is more preferred, and 65°C or higher is even more preferred.

[0057] In this embodiment, when removing components other than diamine and dicarboxylic acid in step (E), in order to improve the recovery rate of diamine and dicarboxylic acid, it is preferable to separate the insoluble components when some or all of the dicarboxylic acid is dissolved in the reaction solution, for example, in a temperature range in which they are dissolved, and it is more preferable to separate the insoluble components in a temperature range in which all of them are dissolved.

[0058] In addition, in step (E): separation, it is not necessary to remove the entire amount of insoluble matter. In step (F): purification, which involves the isolation and purification of diamines and dicarboxylic acids as described later, it is also possible to remove any remaining solid matter again. 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.

[0059] ((F) process: purification process) As shown in Figure 1, in this embodiment, after step (E): separation step, step (F): purification step may be performed. (F) Step: In the purification step, the diamine and dicarboxylic acid obtained in the separation step described above are isolated and purified, respectively. 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.

[0060] <Isolation and purification process of diamines> (F) Step: In the method for isolating and purifying diamines in the purification step, the diamine is dissolved in the solution after the separation step described above as a diamine salt with a dicarboxylic acid (e.g., adipic acid) or an inorganic acid (e.g., hydrochloric acid). A method for separating the diamine from the 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 pKa of the alkali used in the neutralization treatment should 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. One method for purifying the liberated diamine to a purity suitable for use in polyamide polymerization is distillation. Distillation also allows for the removal of residues derived from the crude polyamide. If the reaction solution after the separation step described above contains a compound with a carboxylic acid, the diamine and dicarboxylic acid will polymerize during the heating of the distillation, leading to a decrease in the yield of the diamine and scaling of the equipment. Therefore, it is preferable to remove the dicarboxylic acid before distillation by methods such as crystallization, physical adsorption using activated carbon or ion exchange resin, or membrane separation. Other methods for isolating and purifying diamines include extraction using solvents that form an organic phase, and membrane separation.

[0061] <Isolation and purification process of dicarboxylic acids> (F) Step: In the method for isolating and purifying dicarboxylic acid in the purification step, it is preferable to perform purification by crystallization after the separation step described above. As an example of purification by crystallization, one method involves recrystallizing the dicarboxylic acid from the reaction solution obtained in the separation step described above, 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, any 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 using conventional petroleum-derived raw materials in that it may contain residual metal compounds 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, so 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, and membrane separation. The dicarboxylic acid purified in this way can be recovered as crystalline dicarboxylic acid by removing residual water through drying, or it can be mixed with a diamine without drying and used as a dicarboxylic acid-diamine salt.

[0062] [Methods for recycling polyamide] 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.

[0063] (Polymerization process) In the polymerization process, a method 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") is commonly used. However, polymerization is not limited to this method, and can be carried out by known methods such as solid-phase polymerization and solution polymerization.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] From the viewpoint of preventing the viscosity from becoming too high when additives are added and the molten mixture is made from the polyamide obtained by the polyamide recycling method described above, and ensuring good handling of the molten mixture, it is preferable that the Si element content be 1 ppm by mass or more relative to the total amount of polyamide. Furthermore, from the viewpoint of suppressing a decrease in the molecular weight of the polyamide, it is preferable that the Si content be 100 ppm by mass or less. More preferably, the concentration is 1 to 50 ppm by mass, and even more preferably, 1 to 30 ppm by mass.

[0068] 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]

[0069] 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.

[0070] [Raw materials containing (a) polyamide used in chemical recycling] (polyamide) A: Polyamide 66 (manufactured by Asahi Kasei Corporation, model number: Leona 1300)

[0071] The aforementioned polyamide 66 (manufactured by Asahi Kasei Corporation, model number: Leona 1300) was used as crude polyamide in pellet form.

[0072] [(A) Premixing step, (B) Swelling step, and (C) Depolymerization step: Solvents and acids used in these steps] (acid) The following inorganic acids were used as acids. Hydrochloric acid (special grade, purity 35-37%), manufactured by Kanto Chemical Co., Ltd. (Distilled water) Distilled water, manufactured by Takasugi Pharmaceutical Co., Ltd.

[0073] [Bases used in the purification process for isolating and purifying diamines] (base) The following sodium hydroxide was used as a base to neutralize the acid during the purification process. Sodium hydroxide TCI

[0074] [Examples 1-5], [Comparative Examples 1-6] Diamines and dicarboxylic acids were produced as described below.

[0075] (Premixing process ((A) process) 35% hydrochloric acid and distilled water were mixed to achieve the specified molar concentration (acid concentration in the aqueous layer: mol / L) for each example and comparative example, as shown in Table 1 below, to prepare hydrochloric acid aqueous solutions. A 20 mL glass pressure-resistant test tube (Reaction Vial G30, manufactured by Anton Paar) was used to prepare a mixture by adding a stirring bar and (a) a raw material containing polyamide (a predetermined amount of polyamide was charged, and then a pre-prepared aqueous hydrochloric acid solution was added and mixed to obtain the mixture).

[0076] (Swelling process (process (B)) The test tube containing the mixture obtained in the premixing step (step (A)) was covered, and the mixture was left to stand for a predetermined time in a water bath set to the specified temperature for each example and comparative example, as shown in Table 1 below, to swell the polyamide and obtain swollen polyamide.

[0077] <Swelling rate measurement> The swollen polyamide was wiped of any moisture adhering to its surface with a cloth. The swollen polyamide was then weighed using an electronic balance and recorded as Wa grams. It was placed in a vacuum dryer heated to 100°C, vacuumed, and held for 10 hours. After 10 hours, it was removed, weighed using an electronic balance, and recorded as Wb grams. Hexafluoroisopropanol (HFIP) was added to the sample in an amount 50 times the sample volume and heated and stirred at 60°C for 1 hour to dissolve the polyamide. The insoluble and soluble components were separated by suction filtration. For the soluble component, HFIP was removed using a rotary evaporator, and further vacuum drying was performed to determine the mass of the soluble component, Wc grams. The swelling rate (%) was defined as "100 × (Wa - Wb + Wc) / Wc".

[0078] (Aqueous layer removal process ((D) process) After the swelling process described above, the lid of the test tube was opened, and while weighing the test tube with an electronic balance, the hydrochloric acid phase (aqueous layer) was removed using a polypropylene dropper, taking care not to touch the solid material (swollen polyamide) portion, so that the amount to be weighed out was the predetermined amount, as shown in Table 1.

[0079] (Depolymerization process ((C) process) The above test tubes were stirred at a predetermined temperature for a desired time using a microwave synthesis reactor (Monowave 450, manufactured by Anton Paar). Temperature measurements were performed by inserting a sheath tube into the lid of the test tube and attaching a ruby ​​thermometer (an internal thermometer) to it. The heating process to reach 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 the predetermined reaction temperature and for the predetermined reaction time in each example and comparative example shown in Table 1 below, the apparatus was allowed to cool naturally, and the pressure-resistant test tube was removed when the temperature dropped to 55°C.

[0080] The results of the reaction in the depolymerization process, after a predetermined time, are shown in Table 1 below.

[0081] The terms in Table 1 are explained below. Hydrochloric acid correction amount (35% hydrochloric acid): The amount of 35% hydrochloric acid added to make the number of moles of acid in the reaction system the same for both the non-swelling and swelling-affected cases. System moisture content: The amount of moisture introduced from distilled water and 35% hydrochloric acid minus (amount of water removed from the aqueous layer after swelling × mass %) of water in the aqueous layer after swelling. That is, (distilled water) + (35% hydrochloric acid × 0.65) - (amount of water removed from the aqueous layer) × (mass %) of hydrochloric acid after swelling: converted from mol / kg in the neutralization titration). Moles of water in the system: The amount of water in the system divided by the molecular weight of water, which is 18. Amount of acid in the system: The initial mass of acid contained in the reaction vessel immediately before heating. Number of moles of acid in the system: The initial number of moles of acid contained in the reaction vessel immediately before heating. Number of moles of amide bonds in the system: The number of moles of amide bonds in the polyamide bond contained in the reaction vessel immediately before heating. That is, (amount of polyamide) / 113.16 Acidic hydrogen / amide bond ratio: The value obtained by dividing the above-mentioned (number of moles of acid in the system) × (valence of the acid) by the number of moles of amide bonds in the system. That is, (number of moles of acid in the system) × (valence of the acid) / (number of moles of amide bonds in the system)

[0082] [Measurement of depolymerization rate] (Evaluation of the amounts of hexamethylenediamine (HMD) and its derivatives relative to the polyamide before the depolymerization process, and evaluation of the amounts of adipic acid (ADA) and its derivatives relative to the polyamide before the depolymerization process) The liquid phase of the reaction solution after the depolymerization step was sampled and measured by NMR spectroscopy to calculate the depolymerization rate of the polyamide. The solutions obtained in the examples and comparative examples after the depolymerization process were added to a 5 mm diameter NMR tube. Then, a special NMR sample tube N-502B (Nippon Precision Science Co., Ltd.) filled with benzene d-6 was inserted into the 5 mm diameter NMR test tube, and measurements were taken using a JEOL NMR spectrometer (ECZ-500) with 1H as the observation nucleus, at a measurement temperature of 25°C, and with 1024 integration cycles. The integral values ​​of the hydrogen atoms of the methylene group adjacent to the hexamethylenediamine hydrochloride and the hydrogen atoms of the methylene group adjacent to the amide bond were used to calculate (integral value of hexamethylenediamine) / (integral value of hexamethylenediamine + integral value of the amide bond), and this was used as the depolymerization rate of the polyamide.

[0083] [Method for calculating the heat of vaporization of water / monomers] The value of "heat of vaporization of water / monomer" was calculated following the steps below. The amount of water present in the reaction vessel was calculated (abc grams) by subtracting the amount of water removed from the hydrochloric acid and the amount of water in the hydrochloric acid phase (b grams), and the amount of water consumed by the hydrolysis of the amide bond (c grams), from the amount of water derived from distilled water and hydrochloric acid (amount of water before removal: a grams). Multiplying this by the heat of vaporization of water (2.44 kJ / g), the amount of heat required to remove the water present in the reaction vessel (A) was calculated as "A = (abc) × 2.44 kJ". Next, following the theory of depolymerization of condensed polymers (for example, the formula described in "Organic Synthesis Chemistry, Vol. 24, No. 2 (1966), pp. 106-119"), the square of the amide bond decomposition rate was taken as the monomer yield (d). This value was then multiplied by the amount of hexamethylenediamine obtained when 1 g of polyamide completely decomposes (0.513 g) and the mass of the polyamide (e g) to calculate the amount of hexamethylenediamine present in the system (B = 0.513 × d × e g). Next, the value obtained by dividing A by B (A / B) was defined as "heat of vaporization of water / monomer".

[0084] [Quantitative determination of acid concentration in the removed water layer] To quantify the hydrochloric acid concentration of the hydrochloric acid used for swelling and the aqueous layer removed after swelling, a neutralization titration was performed according to the following procedure. <Dilution of the aqueous layer> The removed aqueous layer was weighed using a 3,000g balance, and 27,000g of distilled water was added to dilute it to a 10-fold increase in mass. <Preparation of sodium hydroxide aqueous solution> 0.4000 g of NaOH was weighed into an Erlenmeyer flask, and 99.6000 g of distilled water was added and stirred to prepare a 0.1000 mol / kg NaOH aqueous solution. <Titration operation> 10 g of the diluted aqueous layer was added to a conical beaker, and 3 drops of 1 w / v% phenolphthalein aqueous solution (manufactured by Fujifilm Wako Pure Chemical Industries) were added to it. The mixture was then stirred with a stirring bar and a magnetic stirrer. Next, the NaOH aqueous solution was gradually added while stirring the contents of the container, and the addition was stopped when the entire mixture turned reddish-purple. The concentration of the aqueous layer was quantified from the mass and molality of the added NaOH aqueous solution and the dilution ratio of the aqueous layer. Before swelling: 4.72 mol / kg After swelling: 4.34 mol / kg (Examples 1, 2, 3, 4, 5) It was found that hydrochloric acid was concentrated and incorporated into the swollen polyamide phase, resulting in a decrease in the acid concentration of the hydrochloric acid phase after swelling. Furthermore, the concentration of hydrochloric acid concentrated in the polyamide phase was calculated from the mass balance before and after swelling, and was as follows. Hydrochloric acid content in swollen polyamide: 6.16 mol / kg (Examples 1, 2, 3, 4, 5)

[0085] [Separation process] 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.

[0086] [Refining process] (Purification of dicarboxylic acids) The dicarboxylic acid crystals obtained in the separation step were dissolved in water equal to or greater than the mass of the crystals, and the mixture was heated to 80°C to dissolve them in water. After dissolution, the mixture was allowed to stand and cool, and the dicarboxylic acid was recrystallized. The precipitated dicarboxylic acid crystals were recovered by filtration. (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 sodium hydroxide in an amount greater than or equal 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.

[0087] [Monomer Analysis] The 1H-NMR spectra of the purified diamine and dicarboxylic acid were measured to confirm they were the target products. Furthermore, trace amounts of impurities were detected using ICP-AES semi-quantitative analysis. The analytical instrument used was a Hitachi (SII) SPS3520UV-DD ICP emission spectrometer.

[0088] [Methods for polymerizing polyamides] The polymerization reaction of polyamide was carried out using the "thermal fusion polymerization method" as follows. Furthermore, if the amount of adipic acid and hexamethylenediamine recovered as described above was insufficient for polymerization, the premixing and purification steps were repeated until the required amount was obtained. As described above, 50 g of the equimolar salts of adipic acid and hexamethylenediamine were dissolved in 50 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 allowed to continue 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), then water-cooled and cut to discharge it in pellet form. The pellets were then dried at 100°C under a nitrogen atmosphere for 12 hours to obtain polyamide.

[0089] [Analysis of polyamides] The obtained polyamide was analyzed using 1H-NMR to confirm that it was polyamide 66. Trace amounts of impurities were detected using the ICP-AES semi-quantitative method. The analytical instrument used was a Hitachi (SII) SPS3520UV-DD ICP emission spectrometer. Furthermore, the weight-average molecular weight Mw and the number-average molecular weight Mn ratio Mw / Mn of the obtained polyamide were Mw = 35,000 and Mw / Mn = 2.0, respectively. The analysis was performed using gel permeation chromatography with hexafluoro-2-propanol as the eluent.

[0090] In each example, depolymerization was achieved with a depolymerization rate of 85% or higher. Furthermore, it was confirmed that swelling accelerated the reaction, allowing for the depolymerization of polyamide components from waste materials containing various polyamides in high yield, thereby obtaining hexamethylenediamine and adipic acid. Moreover, polyamide 66 could be polymerized using the purified hexamethylenediamine and adipic acid.

[0091] [Table 1]

[0092] As shown in Table 1, it was found that using swollen polyamide resulted in a higher depolymerization rate, and that removing the aqueous layer, in addition to the increased depolymerization rate, reduced GHG emissions. This is because an improved depolymerization rate increases monomer yield, lowering the heat of vaporization of water / monomer ratio, and thus reducing GHG emissions. Furthermore, it was found that removing the aqueous layer further lowered the heat of vaporization of water / monomer ratio. [Industrial applicability]

[0093] The present invention provides a method for producing diamines and dicarboxylic acids, and a method for recycling polyamides, which have industrial applicability as efficient recycling methods for polyamide resins, polyamide fibers, polyamide resin compositions, and molded products used in automobile parts and various industrial parts.

Claims

1. (a) A raw material containing polyamide is swollen with (b) an aqueous acid solution containing an inorganic acid, heated to depolymerize, and a diamine and a dicarboxylic acid are obtained. A method for producing diamines and dicarboxylic acids.

2. (A) A step of mixing (a) a raw material containing polyamide, (b) an inorganic acid, and (c) water to obtain a mixture, (B) A step of immersing the mixture for 0.1 hours or more and 100 hours or less, thereby swelling the raw material containing the polyamide (a) by 10% by mass or more, (C) A step of heating and depolymerizing the raw material containing the swollen (a) polyamide obtained in step (B) above to obtain a diamine and a dicarboxylic acid, Having, A method for producing diamines and dicarboxylic acids according to claim 1.

3. After step (B) and before step (C), (D) A step of partially removing the aqueous layer from a mixture comprising the swollen (a) polyamide-containing raw material, (b) an inorganic acid, and (c) water. A method for producing diamines and dicarboxylic acids according to claim 2.

4. In step (D) above, The ratio of the number of moles of the amide group of the polyamide to the number of moles of the proton of the inorganic acid (b) is, The aqueous layer is removed so that the ratio of the amide group of the polyamide to the proton of the inorganic acid (b) is 1:0.8 to 1:5.

5. A method for producing diamines and dicarboxylic acids according to claim 3.

5. After step (C) above, (E) A separation step to remove components other than the diamine and the dicarboxylic acid from the reaction solution obtained in step (C) above, in order to obtain the diamine and the dicarboxylic acid, (F) A purification step in which the diamine and the dicarboxylic acid obtained in step (E) are isolated and purified, Having, A method for producing diamines and dicarboxylic acids according to claim 2 or 3.

6. In step (B) above, temperature control is performed, and in the temperature control, a heat source derived from waste heat from equipment used in steps other than step (B) above is used. A method for producing diamines and dicarboxylic acids according to claim 2.

7. The heating in step (C) above is This includes raising the temperature to the target heating temperature and heating at the target temperature. The heating rate during the aforementioned heating is 25°C / min or less. A method for producing diamines and dicarboxylic acids according to claim 2 or 3.

8. (b) The pKa of the inorganic acid is 0 or less. A method for producing diamines and dicarboxylic acids according to claim 1 or 2.

9. (b) The inorganic acid is hydrochloric acid, A method for producing diamines and dicarboxylic acids according to claim 1 or 2.

10. The raw material containing the polyamide described above (a) Including those derived from pre-consumer or post-consumer products, A method for producing diamines and dicarboxylic acids according to claim 1 or 2.

11. Among the constituent components of the polyamide, the mass fraction of polyamide 66 is greater than 50% by mass. A method for producing diamines and dicarboxylic acids according to claim 1 or 2.

12. In step (E), when removing components other than the diamine and the dicarboxylic acid, the step includes removing components other than the diamine and the dicarboxylic acid as insoluble matter from the reaction solution while a portion or all of the dicarboxylic acid obtained in step (C) is dissolved in the reaction solution. A method for producing diamines and dicarboxylic acids according to claim 5.

13. In step (A) above, the concentration of the inorganic acid (b) in the aqueous solution is 9% by mass or more and 25% by mass or less. A method for producing diamines and dicarboxylic acids according to claim 2 or 3.

14. The above step (C) is carried out using a reaction vessel having an inner surface made of glass lining, zirconium, or tantalum. A method for producing diamines and dicarboxylic acids according to claim 2 or 3.

15. In step (E) above, when removing components other than the diamine and the dicarboxylic acid, thermal filtration and centrifugation are performed. A method for producing diamines and dicarboxylic acids according to claim 5.

16. In step (F) above, the dicarboxylic acid is purified by crystallization. A method for producing diamines and dicarboxylic acids according to claim 5.

17. In step (F) above, the diamine is purified by distillation. A method for producing diamines and dicarboxylic acids according to claim 5.

18. In step (C) above, heating is performed by microwaves. A method for producing diamines and dicarboxylic acids according to claim 2 or 3.

19. The microwave frequency during heating by microwave is 0.8 to 6 GHz. A method for producing diamines and dicarboxylic acids according to claim 18.

20. The method comprises a polymerization step of polymerizing the diamine and dicarboxylic acid obtained by the method for producing the diamine and dicarboxylic acid according to claim 1 or 2 to obtain a polyamide. Methods for recycling polyamide.

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