Method for dehydrating polyester-containing material

Abstract

A method for dehydrating a polyester-containing material according to the present invention includes a dehydration step of causing a polyester-containing material and a carbonic acid diester to contact one another, and heating the same. The carbonic acid diester is preferably a dialkyl carbonate, and the alkyl group in the dialkyl carbonate preferably has 1 to 12 carbon atoms. In addition, the heating temperature in the dehydration step is preferably 50°C or higher and lower than the boiling point of the carbonic acid diester. Furthermore, the polyester contained in the polyester-containing material is preferably one or more selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate.

Description

Method for dehydrating polyester-containing materials

[0001] The present invention relates to a method for dewatering polyester-containing materials.

[0002] In recent years, concerns about environmental destruction, such as marine pollution, have led to an urgent need for the development of plastic recycling technologies. Polyesters are widely used as materials for bottles and fibers, and polyethylene terephthalate (PET), in particular, is produced at approximately 80 million tons per year worldwide. Two recycling methods for polyester have been developed: a material recycling method that does not involve depolymerization, and a chemical recycling method that involves depolymerization and repolymerization. While the former method is easily applicable to polyesters used in PET bottles and the like due to their high purity, it is difficult to apply this method to materials containing polyester (polyester-containing materials), such as polyester fibers and films containing polyester.

[0003] Known methods for depolymerizing polyester include those using water or supercritical alcohol (see Patent Documents 1 and 2). However, both require high-temperature conditions of 300°C or higher. On the other hand, transesterification methods using a base catalyst and alcohol can achieve depolymerization at relatively low temperatures. Methods using methanol, including halogenated solvents and potassium carbonate (see Non-Patent Document 1) or alkali metal alkoxides (see Patent Document 3), can achieve depolymerization at low temperatures of room temperature to 50°C. Furthermore, a method is known in which depolymerization is made more efficient by using dimethyl carbonate as an ethylene glycol scavenger (see Non-Patent Document 2). However, these methods are limited to high-purity PET derived from PET bottles and the like, and other polyester-containing materials are not applicable to these methods. A method for depolymerizing colored polyester fibers using a base catalyst and excess ethylene glycol is known, but this requires high temperatures of around 200°C. Furthermore, to obtain high-purity monomers, it is necessary to decolorize the fibers with a high-boiling-point solvent (see Patent Documents 4 to 6) or decompose the dye with an oxidizing agent (see Patent Document 7).

[0004] From the viewpoint of solving these problems, Patent Document 8 discloses a catalyst composition containing a base catalyst, a monohydric alcohol, and a carbonate diester or a tetraalkoxysilane as a glycol scavenger, and a method for depolymerizing polyester using the same.

[0005] Japanese Patent No. 5099416, Japanese Patent Laid-Open No. 2001-39908, U.S. Patent No. 10252976, Japanese Patent No. 4537288, Japanese Patent No. 5134563, Japanese Patent No. 6659919, Japanese Patent No. 6986813, Japanese Patent Laid-Open No. 2022-126617

[0006] Green Chem. 2021,23,511. Green Chem. 2021,23,9412.

[0007] However, the depolymerization reaction of polyester described in Patent Document 8 has a problem in that moisture contained in the polyester-containing material inhibits the depolymerization reaction. Therefore, from the viewpoint of accelerating the depolymerization reaction, it is desired to establish a method for simply reducing the moisture contained in the polyester-containing material.

[0008] The present invention has been made in view of such demands, and an object of the present invention is to provide a method for dehydrating a polyester-containing material and a method for depolymerizing polyester, which can easily reduce the moisture contained in the polyester-containing material.

[0009] As a result of extensive research, the present inventors have found that the moisture content of a polyester-containing material can be reduced by a simple method of contacting the polyester-containing material with a carbonic acid diester and heating the material, thereby solving the above-mentioned problems. Specifically, the present invention provides the following aspects [1] to

[20] . [1] A method for dehydrating a polyester-containing material, comprising a dehydration step of contacting the polyester-containing material with a carbonic acid diester and heating the material. [2] The method for dehydrating a polyester-containing material according to [1] above, wherein the carbonic acid diester is a dialkyl carbonate, and the alkyl group in the dialkyl carbonate has 1 to 12 carbon atoms. [3] The method for dehydrating a polyester-containing material according to [1] or [2] above, wherein the heating temperature in the step is 50°C or higher and lower than the boiling point of the carbonic acid diester. [4] The method for dehydrating a polyester-containing material according to any one of [1] to [3] above, wherein the polyester contained in the polyester-containing material is one or more selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. [5] The method for dehydrating a polyester-containing material according to any one of [1] to [4] above, wherein the non-polyester material contained in the polyester-containing material is at least one selected from the group consisting of cotton, rayon, polyurethane, polyamide, acrylic resin, polyethylene, polypropylene, carbon-based materials, dyes, and pigments. [6] The method for dehydrating a polyester-containing material according to any one of [1] to [5] above, wherein the polyester-containing material is at least one selected from the group consisting of a material containing polyester fibers, a film containing polyester and components other than polyester, and a mass containing polyester and components other than polyester. [7] The method for dehydrating a polyester-containing material according to any one of [1] to [6] above, wherein the dehydration step is carried out in an open system and carbon dioxide produced in the dehydration step is discharged to the outside. [8] A method for depolymerizing a polyester, comprising a depolymerization step of adding a base to a reaction system after the dehydration step of the polyester-containing material, thereby depolymerizing the polyester. [9] A method for depolymerizing a polyester, wherein the dehydration step is carried out using the method for dehydrating a polyester-containing material according to any one of [1] to [7] above.

[10] The method for depolymerizing a polyester according to [8] or [9] above, wherein the base is one or more selected from alkali metal carbonates, alkali metal phosphates, alkali metal hydroxides, alkali metal alkoxides, and nitrogen-containing organic bases.

[11] The method for depolymerizing a polyester according to any one of [8] to

[10] above, wherein a carbonate diester is further added to the reaction system in the depolymerization step.

[12] The method for depolymerizing a polyester according to

[11] above, wherein the carbonate diester is a dialkyl carbonate, and the alkyl group in the dialkyl carbonate has 1 to 12 carbon atoms.

[13] The method for depolymerizing a polyester according to any one of [8] to

[12] above, wherein a monohydric alcohol is further added to the reaction system in the depolymerization step.

[14] The method for depolymerizing a polyester according to

[13] above, wherein the monohydric alcohol is an alcohol having 1 to 6 carbon atoms.

[15] The method for depolymerizing a polyester according to any one of [8] to

[14] above, wherein the reaction temperature in the depolymerization step is in the range of 20°C to 100°C.

[16] The method for depolymerizing a polyester according to any one of [8] to

[15] above, wherein the material other than polyester contained in the polyester-containing material is one or more selected from the group consisting of cotton, rayon, polyurethane, polyamide, acrylic resin, polyethylene, polypropylene, carbon-based materials, dyes, and pigments.

[17] The method for depolymerizing a polyester according to any one of [8] to

[16] above, wherein the polyester-containing material is at least one selected from the group consisting of a material containing polyester fibers, a film containing polyester and a component other than polyester, and a mass containing polyester and a component other than polyester.

[18] A method for producing a dicarboxylic acid diester, comprising a step of carrying out the method for depolymerizing a polyester according to any one of [8] to

[17] above.

[19] A method for producing a dicarboxylic acid, comprising a step of hydrolyzing a dicarboxylic acid diester obtained by the production method according to

[18] above.

[20] A method for producing a polyester, comprising producing a polyester using as a raw material at least one of the dicarboxylic acid diester obtained by the method for producing a polyester according to the above-mentioned

[18] and the dicarboxylic acid obtained by the method for producing a polyester according to the above-mentioned

[19] .

[0010] According to the present invention, it is possible to provide a method for dehydrating a polyester-containing material and a method for depolymerizing polyester, which can easily reduce the moisture contained in the polyester-containing material.

[0011] Hereinafter, an embodiment of the method for dehydrating a polyester-containing material of the present invention will be described, but the present invention is not limited to the following embodiment.

[0012] <<Method for Dehydrating Polyester-Containing Material>> A method for dehydrating a polyester-containing material according to one embodiment of the present invention includes a dehydration step in which the polyester-containing material is brought into contact with a carbonate diester and heated. When the polyester-containing material is brought into contact with the carbonate diester and heated, the moisture contained in the polyester-containing material reacts with the carbonate diester, ultimately producing carbon dioxide and a monohydric alcohol. Taking the case where the carbonate diester is dimethyl carbonate (DMC) as an example, as shown in Chemical Reaction 1 below, the moisture contained in the polyester-containing material and dimethyl carbonate come into contact and react to produce monomethyl carbonate and one molecule of methanol, followed by carbon dioxide and two molecules of methanol. When the carbon dioxide volatilizes and is discharged to the outside of the system, the equilibrium in Chemical Reaction 1 shifts to the right. As a result, the reaction between the moisture contained in the polyester-containing material and dimethyl carbonate is accelerated. In addition, the heating and swelling of the polyester-containing material can also increase the probability of contact between the moisture in the polyester-containing material and the carbonate diester. It is presumed that these factors, combined, accelerate dehydration from the polyester-containing material and reduce the moisture content in the polyester-containing material.

[0013] Thus, according to the method for dehydrating a polyester-containing material according to one embodiment of the present invention, the moisture contained in the polyester-containing material can be reduced by "(1) using a carbonic acid diester" and "(2) heating," and the polyester-containing material can be dehydrated easily.

[0014] <Polyester-Containing Material> In this embodiment, the polyester-containing material is not particularly limited as long as it contains polyester, and may also contain components other than polyester. Examples include materials containing polyester fibers, and films, packaging containers, bottles, and lumps containing polyester. Among these, materials containing polyester fibers are materials that easily absorb moisture and water due to capillary action, etc., and according to one aspect of the present invention, the moisture contained in such materials can also be effectively reduced. The component other than polyester may be a resin other than polyester or a non-resin component. Note that according to one aspect of the present invention, even if the component other than polyester contained in the polyester-containing material is a water-containing substance, moisture can be effectively removed from the component, and the moisture content of the entire polyester-containing material can be effectively reduced.

[0015] Specific examples of the polyester-containing material include a material containing polyester fibers (polyester fibers), a film containing polyester and a component other than polyester, and a lump containing polyester and a component other than polyester.

[0016] Examples of materials containing polyester fibers include polyester fibers (fibers containing no components other than polyester), woven polyester fiber fabrics, mixtures containing polyester fibers and components other than polyester fibers, and woven fabrics of such mixtures. Examples of mixtures containing polyester fibers and components other than polyester fibers include mixed fibers of polyester fibers and fibers other than polyester fibers and woven fabrics of such mixed fibers, composite mixtures containing polyester fibers and non-resin components (e.g., colored fibers containing polyester fibers and a colorant) and woven fabrics of such composite mixtures, and composite mixed fibers containing polyester fibers, fibers other than polyester fibers, and non-resin components (e.g., colored mixed fibers containing mixed fibers and a colorant) and woven fabrics of such composite mixtures. Here, in this specification, "non-resin component" refers to a component that does not fall into either polyester or a resin other than polyester.

[0017] Examples of films containing polyester and components other than polyester include a monolayer film containing polyester and a component other than polyester, and a laminate film that is a laminate of a film made of polyester (polyester film) and a film containing a component other than polyester. Examples of monolayer films containing polyester and a component other than polyester include a monolayer film that contains both polyester and a resin other than polyester and does not contain a non-resin component, a monolayer film that contains both polyester and a non-resin component and does not contain a resin other than polyester, and a monolayer film that contains all of polyester, a resin other than polyester, and a non-resin component.

[0018] Examples of laminate films that are laminates of a film made of polyester and a film containing a component other than polyester include a laminate film that is a laminate of a film made of polyester and a film that contains a resin other than polyester and that does not contain polyester or non-resin components, a laminate film that is a laminate of a film made of polyester and a film that contains both a resin other than polyester and polyester and that does not contain non-resin components, a laminate film that is a laminate of a film made of polyester and a film that contains both a resin other than polyester and non-resin components and that does not contain polyester, a laminate film that is a laminate of a film made of polyester and a film that contains both polyester and non-resin components and that does not contain resin other than polyester, a laminate film that is a laminate of a film made of polyester and a film that contains all of polyester, a resin other than polyester, and a non-resin component, a multilayer film that is a laminate of one or more of the above laminate films and one or more of the above monolayer films, a multilayer film that is a laminate of two or more of the above laminate films, and a multilayer film that is a laminate of two or more of the above monolayer films.

[0019] Examples of lumps containing polyester and components other than polyester (polyester lumps) include lumps containing both polyester and resins other than polyester but no non-resin components, lumps containing both polyester and non-resin components but no resins other than polyester, and lumps containing all of polyester, resins other than polyester, and non-resin components.

[0020] The resin other than polyester can be selected arbitrarily depending on the purpose and is not particularly limited. For example, in terms of high versatility and high applicability of the present invention, examples of resin other than polyester include polyolefins such as polyethylene and polypropylene, cellulose, polyamides such as nylon, polyurethane, and acrylic resins. These resins may be, for example, in the form of a film (resin film) or a fiber (resin fiber). Examples of fibers other than polyester fibers include fibers of resins other than the above polyesters, cotton, rayon, etc.

[0021] The non-resin component can be selected arbitrarily depending on the purpose and is not particularly limited. For example, examples of the non-resin component that are highly versatile and highly valuable for the application of the present invention include carbon-based materials, inorganic components (inorganic compounds) such as aluminum, and colorants such as dyes and pigments.

[0022] That is, the material other than polyester contained in the polyester-containing material is preferably one or more selected from the group consisting of cotton, rayon, polyurethane, polyamide, acrylic resin, polyethylene, polypropylene, carbon-based materials, dyes, and pigments.

[0023] The fiber diameters of the polyester fibers and the fibers other than polyester fibers in the material containing polyester fibers are not particularly limited and may be, for example, 0.1 μm to 200 μm or 1 μm to 50 μm. The thickness of the film containing polyester and a component other than polyester is not particularly limited and may be, for example, 0.5 μm to 1000 μm or 1 μm to 500 μm. Here, when the film is the above-mentioned laminate film, the film thickness refers to the thickness of the entire laminate film.

[0024] More specifically, examples of materials containing the above-mentioned mixed fibers include fabrics for various clothing, fibers that are materials for fabrics, etc. Fabrics may be unused or used, and may be uncut or cut pieces. Specific examples of uncut fabrics include unsold clothing and used clothing, and specific examples of cut pieces include cut pieces generated during the manufacture of various clothing, cut pieces from various clothing after use, etc. Fibers that are materials for fabrics may be, for example, small pieces of cut pieces generated during the manufacture of fabrics, cut pieces from used fabrics, etc.

[0025] The film containing polyester and components other than polyester may be unused or used, and may be uncut or cut. More specifically, examples include packaging film, small pieces of cut material generated during the production of packaging film, and small pieces of cut material from packaging film after use.

[0026] More specifically, examples of the aggregates containing polyester and components other than polyester include pellets and flakes containing polyester and components other than polyester. The maximum diameter of the aggregates is not particularly limited and may be, for example, 0.1 mm to 10 mm. Here, the "maximum diameter of the aggregate" refers to the maximum length of a line segment connecting two different points on the surface of the aggregate.

[0027] The polyester-containing material may be one kind or two or more kinds, and when two or more kinds are used, the combination and ratio thereof can be selected arbitrarily depending on the purpose. That is, the polyester-containing material may be one or two or more kinds selected from the group consisting of materials containing polyester fibers and films, packaging containers, bottles, and lumps containing polyester.

[0028] When the polyester-containing material is a material containing polyester fibers, the material containing mixed fibers is preferably one or more selected from the group consisting of polyester fibers, woven polyester fiber fabrics, polyester mixed fibers, woven polyester mixed fiber fabrics, mixtures containing polyester fibers and components other than polyester fibers, and woven fabrics of such mixtures.When the polyester-containing material is a film containing polyester and components other than polyester, the film containing polyester and components other than polyester is preferably either or both of a monolayer film containing polyester and components other than polyester, and a laminate film which is a laminate of a film made of polyester and a film containing components other than polyester.

[0029] In the polyester-containing material, the ratio of the polyester content (parts by mass) to the total mass (parts by mass) of the polyester-containing material ([content (parts by mass) of polyester contained in polyester-containing material] / [total mass (parts by mass) of polyester-containing material] × 100) (polyester content) can be selected arbitrarily depending on the purpose and is not particularly limited. In particular, the polyester content is preferably 20% by mass or more and 90% by weight or less, more preferably 25% by mass or more and 90% by weight or less, even more preferably 30% by mass or more and 80% by weight or less, and particularly preferably 35% by mass or more and 75% by weight or less. When carrying out the polyester depolymerization method described below, the higher the polyester content, the greater the amount of monomer (dicarboxylic acid diester described below) generated per unit mass from the polyester-containing material due to polyester decomposition.

[0030] [Polyester] The polyester in the polyester-containing material is not particularly limited, and may be an aromatic polyester having only aromatic groups (divalent groups having a structure in which one hydrogen atom is removed from each of the two carbon atoms forming the aromatic ring skeleton in an aromatic compound) in its main chain having an ester bond, an aliphatic polyester having no aromatic groups in its main chain (having an aliphatic group but not an aromatic group), or a polyester having both aromatic and aliphatic groups in its main chain. The aromatic polyester may have only divalent aromatic hydrocarbon groups (arylene groups) as aromatic groups, only divalent aromatic heterocyclic groups (heteroarylene groups), or both divalent aromatic hydrocarbon groups and divalent aromatic heterocyclic groups. Examples of heteroatoms in the aromatic heterocyclic groups include oxygen atoms and nitrogen atoms. Here, when the polyester depolymerization method described below is carried out following the method for dehydrating a polyester-containing material, from the viewpoint of facilitating the depolymerization of the polyester and from the viewpoint of high versatility, the polyester is preferably an aromatic polyester, more preferably polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), polybutylene naphthalate (PBN), or polyethylene furanoate (PEF, also known as polyethylene furan dicarboxylate), and even more preferably polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate. The polyester may be one type or two or more types, and when two or more types are used, the combination and ratio thereof can be selected as desired depending on the purpose.

[0031] <Carbonate diester> In this embodiment, the carbonate diester functions as a dehydrating agent. Examples of the carbonate diester include dialkyl carbonate and diaryl carbonate. The two alkyl groups bonded to the oxygen atom in the dialkyl carbonate may be the same as or different from each other. Furthermore, the two aryl groups bonded to the oxygen atom in the diaryl carbonate may be the same as or different from each other.

[0032] The alkyl group in the dialkyl carbonate may be linear, branched, or cyclic, and when cyclic, may be monocyclic or polycyclic. The number of carbon atoms in the alkyl group in the dialkyl carbonate is preferably 1 to 12. Preferred examples of such alkyl groups include linear and branched alkyl groups, such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group, an n-pentyl group, an isopentyl group, a neopentyl group, a tert-pentyl group, a 1-methylbutyl group, an n-hexyl group, a 2-methylpentyl group, a 3-methylpentyl group, a 2,2-dimethylbutyl group, a 2,3-dimethylbutyl group, an n-heptyl group, a 2-methylhexyl group, a 3-methylhexyl group, a 2,2-dimethylpentyl group, a 2,3-dimethylpentyl group, a 2,4-dimethylpentyl group, a 3,3-dimethylpentyl group, a 3-ethylpentyl group, a 2,2,3-trimethylbutyl group, an n-octyl group, and an isooctyl group. Among these, the number of carbon atoms in the alkyl group in the dialkyl carbonate is more preferably 1 to 4, and even more preferably 1 or 2. Such more preferred dialkyl carbonates include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

[0033] The aryl group in the diaryl carbonate may be either monocyclic or polycyclic. The number of carbon atoms in the aryl group in the diaryl carbonate is preferably 6 to 10, and examples of such aryl groups include phenyl group, 1-naphthyl group, 2-naphthyl group, o-tolyl group, m-tolyl group, p-tolyl group, 2,3-xylyl group (2,3-dimethylphenyl group), 2,4-xylyl group (2,4-dimethylphenyl group), 2,5-xylyl group (2,5-dimethylphenyl group), 2,6-xylyl group (2,6-dimethylphenyl group), 3,4-xylyl group (3,4-dimethylphenyl group), and 3,5-xylyl group (3,5-dimethylphenyl group). A more preferred diaryl carbonate is, for example, diphenyl carbonate.

[0034] The carbonic acid diester may be used alone or in combination of two or more kinds. When two or more kinds are used, the combination and ratio thereof are not particularly limited and can be selected arbitrarily depending on the purpose.

[0035] Here, when the method for dehydrating a polyester-containing material is followed by the method for depolymerizing a polyester described below, from the viewpoint of proceeding with the depolymerization of the polyester at a higher rate, the carbonate diester is more preferably one or more selected from the group consisting of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and diphenyl carbonate, and is particularly preferably dimethyl carbonate. When the method for dehydrating a polyester-containing material is followed by the method for depolymerizing a polyester described below, there is an advantage in that the unreacted carbonate diester used as a dehydrating agent that has not reacted with water can be subsequently used in the method for depolymerizing a polyester.

[0036] <Reaction Conditions for Dehydration Reaction> Dehydration of a polyester-containing material can be carried out by contacting the polyester-containing material with a carbonic acid diester and heating the resulting mixture. If necessary, the polyester-containing material may be contacted with a component other than the carbonic acid diester. In this embodiment, the dehydration process for the polyester-containing material is preferably carried out in an open system, and carbon dioxide generated in the dehydration process is preferably discharged to the outside of the system. This makes it easier to shift the equilibrium to the right side in Chemical Reaction Formula 1, thereby further promoting dehydration.

[0037] The heating temperature during dehydration of the polyester-containing material is not particularly limited, but is preferably 50°C or higher and lower than the boiling point of the carbonate diester. A heating temperature of 50°C or higher facilitates the reaction between the polyester-containing material and the carbonate diester, facilitating dehydration. Furthermore, a heating temperature lower than the boiling point of the carbonate diester allows the carbonate diester to remain in contact with the polyester-containing material without evaporating, facilitating sustained dehydration. Here, from the viewpoint of further facilitating dehydration, it is more preferable to perform heating and reflux at a temperature equal to or higher than the boiling point of the monohydric alcohol produced by the reaction of the carbonate diester with water. Note that, when performing heating and reflux, even if the heating temperature is higher than the boiling point of the carbonate diester, the release of the carbonate diester outside the reaction system due to evaporation of the carbonate diester can be prevented. Therefore, when performing heating and reflux, the upper limit of the heating temperature is preferably "the boiling point of the carbonate diester + 15°C," more preferably "the boiling point of the carbonate diester + 10°C."

[0038] The amount of carbonate diester used during dehydration of the polyester-containing material is preferably 100 to 10,000 parts by mass, more preferably 100 to 5,000 parts by mass, and may be, for example, 200 to 2,000 parts by mass, per 100 parts by mass of the polyester in the polyester-containing material. When the amount of carbonate diester used is equal to or greater than the lower limit, dehydration is facilitated. When the amount of carbonate diester used is equal to or less than the upper limit, excessive use of carbonate diester during dehydration of the polyester-containing material is suppressed. However, when the polyester depolymerization method described below is carried out following the polyester dehydration method, an amount of carbonate diester exceeding the upper limit may be used, and the excess carbonate diester may be utilized in the polyester depolymerization method.

[0039] The dehydration time of the polyester-containing material is appropriately set depending on the moisture content of the polyester-containing material, the heating temperature, the reaction scale, etc., and is not particularly limited, but is preferably 0.5 to 10 hours, more preferably 1 to 5 hours.

[0040] <Moisture Content of Polyester-Containing Material After Dehydration> The moisture content of the polyester-containing material after the dehydration method for the polyester-containing material is preferably 1,000 ppm by mass or less, more preferably 600 ppm by mass or less, even more preferably 450 ppm by mass or less, and still more preferably 100 ppm by mass or less, from the viewpoint of smoothly progressing depolymerization of the polyester by the polyester depolymerization method described below. In the present embodiment, the moisture content of the polyester-containing material means the moisture content obtained by measuring a dialkyl carbonate that has been brought into contact with the polyester-containing material using a Karl Fischer moisture meter.

[0041] <<Method for Depolymerizing Polyester>> A method for depolymerizing polyester according to one embodiment of the present invention includes a depolymerization step of adding a base to a reaction system in which a method for dehydrating a polyester-containing material has been performed, thereby depolymerizing the polyester. The dehydration method may be a chemical or physical method. Chemical methods include the above-described method for dehydrating a polyester-containing material (i.e., a dehydration method including a dehydration step in which a polyester-containing material is contacted with a carbonic acid diester and heated), as well as a method in which a substance that reacts with water is added to the system to dehydrate the material. Examples of such substances include alkali metals, acids such as sulfuric acid, phosphorus pentoxide, sodium sulfate, magnesium sulfate, zinc chloride, and acid anhydrides such as acetic anhydride. Physical methods include a method in which a substance that adsorbs water is added to the system to dehydrate the material. Examples of such substances include crystalline sieves, silica, alumina, and zeolite. These methods may be performed alone or in combination of two or more. Since the polyester-containing material after the method for dehydrating a polyester-containing material has reduced moisture, the depolymerization reaction of the polyester can proceed smoothly. In one embodiment of the present invention, it is preferable to carry out the above-described method for dehydrating a polyester-containing material (i.e., a dehydration method including a dehydration step in which a polyester-containing material is contacted with a carbonate diester and heated). In this case, the monohydric alcohol produced by the above-described method for dehydrating a polyester-containing material and the remaining carbonate diester can be used directly in the depolymerization reaction of the polyester, and therefore the carbonate diester and the monohydric alcohol can be effectively utilized without recovery. Therefore, it is possible to enjoy the advantage of being able to reduce the reagent costs required for depolymerization of the polyester while omitting the operation of recovering the carbonate diester and the monohydric alcohol.

[0042] <Polyester> In the method for depolymerizing polyester, the polyester to be depolymerized is included in the polyester-containing material whose moisture content has been reduced by the dehydration method described above. Details of the polyester-containing material are as described above.

[0043] By depolymerizing polyesters, for example, terephthalic acid diesters are produced from polyethylene terephthalate, polybutylene terephthalate, and polytrimethylene terephthalate, naphthalenedicarboxylic acid diesters (2,6-naphthalenedicarboxylic acid diesters) are produced from polyethylene naphthalate and polybutylene naphthalate, and furandicarboxylic acid diesters (2,5-furandicarboxylic acid diesters) are produced from polyethylene furanoate. For example, when methanol is used as the monohydric alcohol, dimethyl dicarboxylate is produced as the dicarboxylic acid diester. The polyester-containing material may contain only one type of polyester, or two or more types. When two or more types are used, the combination and ratio thereof are not particularly limited and can be selected as desired depending on the purpose.

[0044] <Carbonate diester> The carbonate diester may be the carbonate diester remaining after the dehydration method for the polyester-containing material described above. However, if the carbonate diester is insufficient when carrying out the polyester depolymerization method, the carbonate diester may be appropriately replenished as needed. Details of the carbonate diester are as described above.

[0045] During depolymerization of the polyester, the carbonate diester reacts with glycol generated from the polyester to produce a cyclic compound or a chain compound, which shifts the equilibrium between the depolymerization reaction and polymerization reaction of the polyester in favor of the depolymerization reaction, thereby improving the production rate of the target monomer. The carbonate diester functions as a glycol scavenger. That is, the carbonate diester functions as a dehydrating agent in the method for dehydrating a polyester-containing material, and as a glycol scavenger during depolymerization of the polyester. The reaction product of a carbonate diester and a glycol may be a cyclic compound which is a reaction product of one molecule of a carbonate diester and one molecule of a glycol (e.g., the cyclic compound (1) shown below), a chain compound which is a reaction product of one molecule of a carbonate diester and one molecule of a glycol (e.g., the first chain compound (2) shown below), a chain compound which is a reaction product of one molecule of a carbonate diester and two molecules of a glycol (e.g., the second chain compound (3) shown below), or a chain compound which is a reaction product of two molecules of a carbonate diester and one molecule of a glycol (e.g., the third chain compound (4) shown below). Whether a cyclic compound or a chain compound is produced is determined mainly by the type (e.g., size) of the glycol. For example, the reaction product of a carbonate diester and ethylene glycol is mainly a cyclic compound (1) (more specifically, ethylene carbonate) which is a reaction product of one molecule of a carbonate diester and one molecule of ethylene glycol.

[0046] (In the formula, X １ ~X ５ are each independently a divalent organic group, and R １ ~R ３ are each independently a monovalent organic group.

[0047] Above X １ ~X ５ The divalent organic group represented by the formula (I) may be, for example, an aromatic group, an aliphatic group, or a group having both an aromatic group and an aliphatic group, and is preferably a divalent hydrocarbon group. Preferred divalent organic groups include, for example, an ethylene group (—CH ２CH ２ -), 1,3-propylene group (trimethylene group, -CH ２ CH ２ CH ２ -), 1,4-butylene group (tetramethylene group, -CH ２ CH ２ CH ２ CH ２ -), and alkylene groups having 2 to 4 carbon atoms are more preferred. １ ~R ３ The monovalent organic group represented by the formula (I) may be, for example, either an aromatic group or an aliphatic group, but is preferably an aliphatic group (a saturated aliphatic group or an unsaturated aliphatic group), and more preferably an aliphatic hydrocarbon group. Preferred monovalent organic groups include, for example, monovalent saturated aliphatic groups (i.e., alkyl groups) such as a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, an isobutyl group, a sec-butyl group, and a tert-butyl group, and is more preferably a monovalent saturated aliphatic group (i.e., alkyl group) having 1 to 4 carbon atoms.

[0048] In the polyester depolymerization step, the amount of carbonate diester used is preferably 100 to 10,000 parts by mass, more preferably 100 to 2,000 parts by mass, and may be, for example, 200 to 1,000 parts by mass, relative to 100 parts by mass of the polyester in the polyester-containing material. When the amount of carbonate diester used is equal to or greater than the lower limit, the effects obtained by using the carbonate diester are enhanced. When the amount of carbonate diester used is equal to or less than the upper limit, excessive use of the carbonate diester is suppressed.

[0049] <Base> The base is not particularly limited and may be either an inorganic base or an organic base, but alkali metal carbonates or alkali metal hydroxides, alkali metal phosphates (hereinafter also referred to as "alkali metal phosphates"), or alkali metal alkoxides that react with monohydric alcohols to produce alkali metal alkoxides are preferred. Generally, transesterification reactions proceed by an alkoxide anion acting as a nucleophile attacking the carbonyl group of an ester, passing through a quaternary carbon intermediate, and then liberating the other alkoxide anion. Since the alkoxide anion is produced by the reaction of the corresponding alcohol with a base, any base that can deprotonate the alcohol can be used.

[0050] Examples of inorganic bases include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide, alkali metal oxides such as lithium oxide, sodium oxide, and potassium oxide, alkaline earth metal hydroxides such as calcium hydroxide and magnesium hydroxide, alkali metal phosphates such as lithium phosphate, sodium phosphate, and potassium phosphate, and alkaline earth metal oxides such as calcium oxide and magnesium oxide. Examples of organic bases include alkali metal alkoxides such as lithium methoxide, lithium ethoxide, lithium tert-butoxide, sodium methoxide, sodium ethoxide, sodium tert-butoxide, potassium methoxide, potassium ethoxide, and potassium tert-butoxide, calcium dimethoxide, calcium diethoxide, calcium di-tert-butoxide, magnesium dimethoxide, magnesium diethoxide, and magnesium di-tert-butoxide. Examples of the alkali metal alkoxide include alkaline earth metal alkoxides such as 1,5,7-triazabicyclo[4.4.0]dec-5-ene (abbreviation: TBD), 1,8-diazabicyclo[5.4.0]undec-7-ene (abbreviation: DBU), 1,3-dimesitylimidazol-2-ylidene, and 1,3-dicyclohexylimidazol-2-ylidene; and nitrogen-containing organic bases (organic bases having a nitrogen atom) such as lithium methoxide, sodium methoxide, or potassium methoxide, with sodium methoxide being more preferred, in terms of achieving a higher rate of depolymerization of the polyester. The nitrogen-containing organic base is preferably 1,5,7-triazabicyclo[4.4.0]dec-5-ene, in terms of achieving a higher rate of depolymerization of the polyester.

[0051] The base used in the depolymerization step may be one type or two or more types. When two or more types are used, the combination and ratio thereof are not particularly limited and can be selected arbitrarily depending on the purpose. The base is particularly preferably an alkali metal alkoxide, in that depolymerization of the polyester proceeds at a particularly high rate. The amount of base used in the polyester depolymerization step is preferably 0.1 to 10 parts by mass, more preferably 0.5 to 6 parts by mass, even more preferably 0.5 to 3 parts by mass, and particularly preferably 3 to 6 parts by mass, per 100 parts by mass of the polyester in the polyester-containing material. When the amount of base used is equal to or greater than the lower limit, depolymerization of the polyester proceeds at a higher rate. When the amount of base used is equal to or less than the upper limit, excessive use of the base is suppressed. That is, when depolymerizing the polyester, it is preferable that the amount of base used is a catalytic amount (the base is a catalyst).

[0052] <Monohydric alcohol> The monohydric alcohol may be a monohydric alcohol produced during the dehydration method for a polyester-containing material. However, if the monohydric alcohol is insufficient during the depolymerization method for a polyester, the monohydric alcohol may be appropriately replenished as needed.

[0053] During the depolymerization of polyester, the monohydric alcohol undergoes a transesterification reaction with the polyester in the polyester-containing material. That is, during the depolymerization of polyester, the reaction between the polyester and the monohydric alcohol produces a glycol corresponding to one of the monomers used in the production of the polyester, and a dicarboxylic acid diester corresponding to the other monomer or its derivative. The monohydric alcohol is not particularly limited. The reaction efficiency of the depolymerization reaction according to the present invention is controlled by the capture of the free dihydric alcohol by the excess diester carbonate. Therefore, as described below, based on the fact that the reaction using methanol and dimethyl carbonate proceeds efficiently, it can be easily assumed that the reaction will proceed efficiently with other combinations of monohydric alcohol and dialkyl carbonate. The monohydric alcohol may be, for example, either an aliphatic alcohol or an aromatic alcohol. However, in terms of achieving a higher rate of depolymerization of polyester, an aliphatic alcohol (saturated aliphatic alcohol, unsaturated aliphatic alcohol) is preferred, and a saturated aliphatic alcohol is more preferred. Furthermore, the monohydric alcohol is preferably an alcohol having 1 to 6 carbon atoms. The type of monohydric alcohol produced during the dehydration method for polyester-containing materials described above is appropriately adjusted depending on the type of carbonic acid diester used as a dehydrating agent. From the viewpoint of achieving a higher rate of depolymerization of polyester, the carbonic acid diester used as a dehydrating agent is preferably a dialkyl carbonate. Furthermore, the number of carbon atoms in the alkyl group in the dialkyl carbonate is preferably 1 to 12, more preferably 1 to 4, and even more preferably 1 or 2.

[0054] Examples of saturated aliphatic alcohols include alcohols having 1 to 4 carbon atoms, such as methanol, ethanol, 1-propanol (n-propyl alcohol), 2-propanol (isopropyl alcohol), 1-butanol (n-butyl alcohol), 2-methyl-1-propanol (isobutyl alcohol), 2-butanol (sec-butyl alcohol), and 2-methyl-2-propanol (tert-butyl alcohol). The monohydric alcohol used for depolymerization of polyester may be one type only, or two or more types. When two or more types are used, the combination and ratio thereof are not particularly limited and can be selected as desired depending on the purpose. Note that methanol is particularly preferred as the monohydric alcohol, as it allows depolymerization of polyester to proceed at a particularly high rate.

[0055] The amount of monohydric alcohol used during depolymerization of polyester is not particularly limited. For example, when the polyester is polyethylene terephthalate (PET), the amount is preferably 30 to 500 parts by mass, more preferably 50 to 200 parts by mass, per 100 parts by mass of PET in the polyester-containing material. Furthermore, when the polyester is polybutylene terephthalate (PBT), the amount of monohydric alcohol used is preferably 10 to 180 parts by mass, more preferably 20 to 140 parts by mass, per 100 parts by mass of PBT in the polyester-containing material. When the amount of monohydric alcohol used is equal to or greater than these lower limits, the effects obtained by using the monohydric alcohol are enhanced. On the other hand, when the amount of monohydric alcohol used is equal to or less than these upper limits, excessive use of the monohydric alcohol is suppressed.

[0056] <Solvent> In the polyester depolymerization step, a solvent that does not fall into any of the categories of base, monohydric alcohol, and carbonate diester may be used. In this embodiment, depolymerization of the polyester proceeds efficiently even without the use of a solvent. However, using a solvent as needed may improve the handleability of the blend of raw materials, such as the reaction liquid, and may result in more efficient depolymerization of the polyester. In this specification, unless otherwise specified, the term "solvent" encompasses both a component that is liquid at room temperature and serves as a dispersant for dissolving a solute, and a component that is liquid at room temperature and serves as a dispersion medium for dispersing a dispersoid. Furthermore, "room temperature" refers to a temperature that is not particularly cooled or heated, i.e., an ordinary temperature, such as a temperature of 15 to 25°C. Here, the solvent is preferably an organic solvent. Examples of organic solvents include aromatic hydrocarbons such as toluene, ethers such as tetrahydrofuran, alkanes such as n-hexane, halogenated hydrocarbons such as chloroform and dichloromethane, amides such as dimethylformamide, and sulfoxides such as dimethyl sulfoxide. The solvent used for depolymerization of polyester may be one kind or two or more kinds. When two or more kinds are used, the combination and ratio thereof can be selected arbitrarily depending on the purpose.

[0057] When a solvent is used during depolymerization of the polyester, the amount used is preferably 1 to 100,000 parts by mass, and more preferably 1 to 10,000 parts by mass, per 100 parts by mass of the polyester-containing material. When the amount of solvent used is equal to or greater than the lower limit, the effects obtained by using the solvent are enhanced. When the amount of solvent used is equal to or less than the upper limit, excessive use of the solvent is suppressed.

[0058] <Other Components> When depolymerizing the polyester, other components that do not fall under any of the polyester-containing material, base, monohydric alcohol, carbonic acid diester, and solvent may be used, as long as the effects of the present invention are not impaired. The other components can be selected arbitrarily depending on the purpose and are not particularly limited. The other components used in depolymerizing the polyester may be one type only or two or more types. When two or more types are used, the combination and ratio thereof are not particularly limited and can be selected arbitrarily depending on the purpose. During depolymerization of the polyester, the ratio of the total amount (parts by mass) of the polyester-containing material, base, monohydric alcohol, and carbonate diester to the total amount (parts by mass) of components other than the solvent (([amount (parts by mass) of polyester-containing material] + [amount (parts by mass) of base] + [amount (parts by mass) of monohydric alcohol] + [amount (parts by mass) of carbonate diester]) / [total amount (parts by mass) of components other than the solvent] × 100) is preferably 80% by mass to 100% by mass, more preferably 90% by mass to 100% by mass, and even more preferably 95% by mass to 100% by mass, and may be, for example, any one of 97% by mass to 100% by mass and 99% by mass to 100% by mass. When the ratio is equal to or greater than the lower limit, the polyester can be depolymerized more efficiently. Here, the total amount (parts by mass) of components other than the solvent used during depolymerization of the polyester is synonymous with the total amount (parts by mass) of the polyester-containing material, base, monohydric alcohol, carbonate diester, and other components used during depolymerization.

[0059] <Reaction conditions for depolymerization reaction> Depolymerization of polyester in a polyester-containing material can be carried out by contacting a base, a monohydric alcohol, a carbonic acid diester, the polyester-containing material, and, if necessary, a solvent and, if necessary, other components. The order of contacting these raw materials is not particularly limited, but after carrying out the above-mentioned method for dehydrating a polyester-containing material, a base can be added to the system while utilizing the monohydric alcohol and carbonic acid diester present in the system, and, if necessary, a solvent and, if necessary, other components can be contacted. In this case, if the monohydric alcohol and the carbonic acid diester are insufficient, they can be appropriately replenished.

[0060] The reaction temperature during depolymerization of the polyester can be adjusted appropriately taking into account the type of raw material used, etc., and the depolymerization may be carried out at room temperature (normal temperature) or under heated conditions. The equipment used for heating is not particularly limited, and equipment such as a heater can be used. The reaction temperature during depolymerization of the polyester is preferably 20°C or higher, more preferably 40°C or higher, and even more preferably 60°C or higher. The reaction temperature during depolymerization of the polyester is preferably less than 180°C, more preferably 150°C or lower, even more preferably 100°C or lower, and particularly preferably 70°C or lower. In one embodiment, the reaction temperature during depolymerization of the polyester is, for example, preferably 20°C or higher and lower than 180°C, more preferably 40°C or higher and 150°C or lower, more preferably 50°C or higher and 120°C or lower, and particularly preferably 60°C or higher and 100°C or lower. For example, by adjusting reaction conditions other than the reaction temperature, it is possible to depolymerize the polyester at a sufficiently high rate even at a reaction temperature of 20°C or higher and 70°C or lower. As described above, according to the present embodiment, it has been conventionally difficult to depolymerize polyester at relatively low temperatures, but it is now possible to depolymerize polyester at low temperatures, such as 150° C. or lower. This makes it possible to reduce the amount of by-products produced during depolymerization, unlike when polyester is depolymerized at high temperatures, and also reduces the coloration of dicarboxylic acid diesters (monomers), which are the main depolymerized products.

[0061] The depolymerization of the polyester may be carried out under normal pressure, reduced pressure, or increased pressure. The depolymerization of the polyester may be carried out in air or in an inert gas atmosphere. The reaction time during depolymerization of the polyester is not particularly limited and can be adjusted appropriately taking into account other reaction conditions such as the reaction temperature. The reaction time during depolymerization of the polyester is not particularly limited as long as it is 0.5 to 24 hours, but is preferably 0.5 to 12 hours, and more preferably 1 to 8 hours. In this embodiment, the end of depolymerization can be determined, for example, when the polyester disappears. Therefore, the time required for the polyester to disappear can be used as the depolymerization reaction time. The time required for the polyester to disappear can also be determined, for example, by the time required for the mass loss of the polyester-containing material to stop.

[0062] In the blend immediately after the contact of the raw materials, the unreacted polyester-containing material does not dissolve and remains insoluble in the other liquid components. In the blend during polyester depolymerization, typically, the unreacted polyester-containing material and the polyester-containing material during or after the polyester reaction do not dissolve and remain insoluble in the other liquid components. Meanwhile, the glycol and dicarboxylic acid diester, which are the reaction products of polyester depolymerization, typically dissolve in the liquid components. Such blends can be stirred by known methods during polyester depolymerization, such as by rotating a magnetic stirrer or impeller, or by using a ball mill. During polyester depolymerization, polyester-containing materials with low specific gravity may float near the liquid surface. In such cases, the contact area between the insoluble polyester-containing material and the liquid component can be increased by, for example, pushing the polyester-containing material near the liquid surface to submerge it, or by using a narrow-mouthed reaction vessel. As a result, the amounts of raw materials such as the base, monohydric alcohol, and carbonate diester used can be reduced compared to when other reaction vessels are used, and the polyester can be depolymerized more efficiently. Furthermore, when a narrow-mouthed reaction vessel is used, if one with a wide bottom surface is used, the reaction vessel can be enlarged in capacity while ensuring the contact area between the polyester-containing material and the liquid component.

[0063] <Post-Treatment Conditions and Extraction Conditions After Depolymerization Reaction> After the completion of the above-described polyester depolymerization process, post-treatment can be performed using known techniques, allowing one of the main depolymerized products, a dicarboxylic acid diester, to be extracted with high purity. Therefore, according to one aspect of the present invention, a method for producing a dicarboxylic acid diester (preferably a dicarboxylic acid dialkyl ester) is provided, which includes a step of carrying out the above-described polyester depolymerization method. For example, after depolymerization of the polyester, the resulting reaction product is subjected to solid-liquid separation procedures such as filtration, centrifugation, and decantation to recover components other than the polyester. The liquid obtained by the solid-liquid separation procedure is then subjected to distillation (concentration) of volatile components, and the resulting solid is washed with methanol and water to obtain a high-purity dicarboxylic acid diester. The resulting dicarboxylic acid diester may then be further purified by crystallization, distillation, or the like, as necessary. The other main depolymerized product, a reaction product of glycol and a carbonate diester, can also be extracted in the same manner as the dicarboxylic acid diester by appropriately adjusting the post-treatment and extraction conditions. According to this embodiment, as explained above, depolymerization is possible at low temperatures, such as below 180°C. Therefore, unlike depolymerization at high temperatures, the impurity content and discoloration of the dicarboxylic acid diester can be reduced. Therefore, a highly pure dicarboxylic acid diester (monomer) with reduced discoloration can be obtained by the simplified process of washing with methanol and water as described above, without adding any complicated steps. Furthermore, even if the polyester-containing material originally contains a colorant, the simplified process of washing with methanol and water as described above can also reduce discoloration of the dicarboxylic acid diester (monomer) derived from this colorant. In contrast, when depolymerization is performed at high temperatures using conventional methods, obtaining a highly pure monomer requires complex processes, such as a decolorization treatment using a high-boiling point solvent and a depolymerization treatment of the dye using an oxidizing agent.

[0064] Here, the dicarboxylic acid diester (preferably a dicarboxylic acid dialkyl ester) obtained by depolymerization of the polyester may be hydrolyzed to obtain a dicarboxylic acid. Therefore, according to one aspect of the present invention, a method for producing a dicarboxylic acid is provided, comprising a step of hydrolyzing the dicarboxylic acid diester obtained by carrying out the above-described method for depolymerizing a polyester. Furthermore, both the "dicarboxylic acid diester obtained by depolymerization of a polyester" and the "dicarboxylic acid obtained by hydrolyzing the dicarboxylic acid diester obtained by depolymerization of a polyester" can be used as a raw material (monomer) for a polyester. Therefore, according to one aspect of the present invention, a method for producing a polyester is provided, using at least one of the "dicarboxylic acid diester obtained by depolymerization of a polyester" and the "dicarboxylic acid obtained by hydrolyzing the dicarboxylic acid diester obtained by depolymerization of a polyester" as a raw material (monomer). When the "dicarboxylic acid diester obtained by depolymerization of a polyester" is used as a raw material (monomer), the polyester can be produced by a known method, such as a transesterification method involving a reaction with a polyhydric alcohol such as glycol. Furthermore, when "a dicarboxylic acid obtained by hydrolyzing a dicarboxylic acid diester obtained by depolymerization of a polyester" is used as a raw material (monomer), a polyester can be produced by a known method, such as a direct esterification method involving a reaction with a polyhydric alcohol such as glycol.

[0065] The present invention will be described in more detail below with reference to specific examples. However, the present invention is not limited to the examples shown below. Note that the unit "ppm" shown below is always based on mass ratio.

[0066] In the present examples, the "moisture content" refers to the moisture content obtained by measuring dimethyl carbonate brought into contact with the polyester-containing material using a Karl Fischer moisture meter (manufactured by Metrohm, product name "899 Coulometer").

[0067] Comparative Example 1 A white examination gown (100% by mass of polyethylene terephthalate fibers (PET fibers) in all fibers) was cut into a size of approximately 2 cm x 2 cm, and 50 g of the cut pieces were placed in a 500 mL separable flask. 25 μL of water was then placed in the separable flask. Dimethyl carbonate (DMC) (manufactured by Sankyo Chemical Co., Ltd., 250 mL) was then added to the separable flask, and the water content of the dimethyl carbonate that had been brought into contact with the polyester-containing material (initial water content in Table 1) was measured. After measuring the initial water content, potassium carbonate (K ２ CO ３ ) (Fujifilm Wako Pure Chemical Industries, Ltd., 3.6 g, 0.026 mol) and methanol (Kishida Chemical Co., Ltd., 50 mL) were added to a separable flask, a three-neck separable cover was attached to the top of the separable flask, and a stirring blade was inserted into the central opening of the separable cover. Stirring was then carried out at 70°C for 18 hours using the stirring blade and an oil bath. The contents of the separable flask were then filtered while still hot at 70°C without cooling, and insoluble components (mainly PET components) were recovered. The filtrate was then concentrated and washed with water to obtain crude ethylene carbonate (crude EC) and crude dimethyl terephthalate (crude DMT). The undissolved material after washing (filtered) was crude dimethyl terephthalate (crude DMT), and the concentrated filtrate was crude ethylene carbonate (crude EC).

[0068] Comparative Example 2 The same procedure as in Comparative Example 1 was carried out, except that the procedure of adding 25 μL of water to the separable flask was omitted.

[0069] Example 1 The same procedure as in Comparative Example 2 was carried out, except that after measuring the water content in the separable flask, refluxing was carried out at 100°C for 2 hours. In Example 1, the water content of the dimethyl carbonate contacted with the polyester-containing material (water content before reaction in Table 1) was measured after refluxing at 100°C for 2 hours. The refluxing was carried out by directly connecting the lower end of a Dimroth reflux tube to one of the three-necked ports of the separable cover, attaching a three-way stopcock to the upper end of the Dimroth reflux tube, and allowing nitrogen gas to flow from the top of the Dimroth reflux tube through the three-way stopcock while attaching a bubbler to the three-way stopcock to allow the nitrogen gas to escape. The coolant temperature of the Dimroth reflux tube was 5°C, and the nitrogen flow rate was approximately 0.1 L / min.

[0070] The results are shown in Table 1. In Comparative Examples 1 and 2, the water content before the start of the reaction was not measured. However, since no dehydration operation was performed in Comparative Examples 1 and 2, the water content before the start of the reaction is considered to be almost the same as the initial water content.

[0071]

[0072] The results shown in Example 1 demonstrate that by bringing PET fibers into contact with dimethyl carbonate and heating (refluxing), the moisture content of the PET fibers can be reduced, allowing the depolymerization reaction to proceed smoothly.

[0073] The dehydration method of the present invention can easily reduce the moisture content of a polyester-containing material. Then, by subjecting the polyester-containing material after the dehydration method of the present invention to a depolymerization step, the depolymerization of the polyester in the polyester-containing material can be promoted.

Claims

1. A method for dehydrating a polyester-containing material, comprising a dehydration step of contacting a polyester-containing material with a carbonic acid diester and heating the material.

2. The method for dehydrating a polyester-containing material according to claim 1, wherein the carbonic acid diester is a dialkyl carbonate, and the alkyl group in the dialkyl carbonate has 1 to 12 carbon atoms.

3. The method for dehydrating a polyester-containing material according to claim 1 or 2, wherein the heating temperature in the step is 50°C or higher and lower than the boiling point of the carbonic acid diester.

4. A method for dehydrating a polyester-containing material according to any one of claims 1 to 3, wherein the polyester contained in the polyester-containing material is one or more selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate.

5. A method for dehydrating a polyester-containing material according to any one of claims 1 to 4, wherein the material other than polyester contained in the polyester-containing material is one or more selected from the group consisting of cotton, rayon, polyurethane, polyamide, acrylic resin, polyethylene, polypropylene, carbon-based materials, dyes, and pigments.

6. A method for dehydrating a polyester-containing material according to any one of claims 1 to 5, wherein the polyester-containing material is at least one material selected from the group consisting of a material containing polyester fibers, a film containing polyester and a component other than polyester, and a mass containing polyester and a component other than polyester.

7. The method for dehydrating a polyester-containing material according to any one of claims 1 to 6, wherein the dehydration step is carried out in an open system and carbon dioxide produced in the dehydration step is discharged outside the system.

8. A method for depolymerizing polyester, comprising a depolymerization step of adding a base to a reaction system that has undergone a dehydration step of a polyester-containing material to depolymerize the polyester.

9. The method for depolymerizing polyester according to claim 8, wherein the dehydration step is carried out by the method for dehydrating a polyester-containing material according to claim 1.

10. The method for depolymerizing a polyester according to claim 8 or 9, wherein the base is at least one selected from the group consisting of alkali metal carbonates, alkali metal phosphates, alkali metal hydroxides, alkali metal alkoxides, and nitrogen-containing organic bases.

11. The method for depolymerizing a polyester according to any one of claims 8 to 10, wherein a carbonic acid diester is further added to the reaction system in the depolymerization step.

12. The method for depolymerizing a polyester according to claim 11, wherein the carbonic acid diester is a dialkyl carbonate, and the alkyl group in the dialkyl carbonate has 1 to 12 carbon atoms.

13. The method for depolymerizing polyester according to any one of claims 8 to 12, wherein a monohydric alcohol is further added to the reaction system in the depolymerization step.

14. The method for depolymerizing polyester according to claim 13, wherein the monohydric alcohol is an alcohol having 1 to 6 carbon atoms.

15. The method for depolymerizing polyester according to any one of claims 8 to 14, wherein the reaction temperature in the depolymerization step is in the range of 20°C to 100°C.

16. The method for depolymerizing polyester according to any one of claims 8 to 15, wherein the material other than polyester contained in the polyester-containing material is at least one selected from the group consisting of cotton, rayon, polyurethane, polyamide, acrylic resin, polyethylene, polypropylene, carbon-based materials, dyes, and pigments.

17. The method for depolymerizing polyester according to any one of claims 8 to 16, wherein the polyester-containing material is at least one selected from the group consisting of a material containing polyester fibers, a film containing polyester and a component other than polyester, and a lump containing polyester and a component other than polyester.

18. A method for producing a dicarboxylic acid diester, comprising the step of carrying out the method for depolymerizing a polyester according to any one of claims 8 to 17.

19. A method for producing a dicarboxylic acid, comprising a step of hydrolyzing the dicarboxylic acid diester obtained by the production method according to claim 18.

20. A method for producing a polyester, comprising producing a polyester using as a raw material at least one of the dicarboxylic acid diester obtained by the method according to claim 18 and the dicarboxylic acid obtained by the method according to claim 19.

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