WO2025234467A1 - Polyester decomposition method, method for producing dicarboxylic acid dialkyl ester obtained by said polyester decomposition method, method for producing dicarboxylic acid obtained by hydrolyzing said dicarboxylic acid dialkyl ester, and method for producing polyester using said dicarboxylic acid dialkyl ester and said dicarboxylic acid as starting material - Google Patents

Abstract

The present invention provides: a polyester decomposition method with which it is possible to decompose a polyester or a polyester contained in various polyester-containing materials into a monomer at a low temperature of 150°C or less by a simple process, and with which it is possible to sufficiently decompose a polyester even if moisture is contained; a method for producing a dicarboxylic acid dialkyl ester obtained by the polyester decomposition method; a method for producing a dicarboxylic acid obtained by hydrolyzing the dicarboxylic acid dialkyl ester; and a method for producing a polyester using the dicarboxylic acid dialkyl ester and the dicarboxylic acid as a starting material. Provided is a polyester decomposition method which includes a decomposition step for decomposing a polyester by mixing a polyester-containing material, potassium phosphate, a monohydric alcohol, and a carbonic acid diester with each other. With this polyester decomposition method, it is possible to decompose a polyester even at a low temperature of 150°C or less.

Description

A method for decomposing polyesters, a method for producing dialkyl dicarboxylates obtained by the method for decomposing polyesters, a method for producing dicarboxylic acids obtained by hydrolyzing the dialkyl dicarboxylates, and a method for producing polyesters using the dialkyl dicarboxylates and the dicarboxylic acids as raw materials

The present invention relates to a polyester decomposition method for decomposing polyesters, a method for producing dialkyl dicarboxylate esters obtained by the polyester decomposition method, a method for producing dicarboxylic acids obtained by hydrolyzing the dialkyl dicarboxylate esters, and a method for producing polyesters using the dialkyl dicarboxylates and the dicarboxylic acids as raw materials.

In recent years, concerns about environmental destruction, exemplified by marine pollution, have led to an urgent need to develop plastic recycling technology. Polyester is widely used as a material for bottles and fibers, with approximately 80 million tons of polyethylene terephthalate (PET) produced annually worldwide. Two recycling methods have been developed for polyester: a material recycling method that does not involve depolymerization, and a chemical recycling method that involves depolymerization and repolymerization. While the former method is easy to apply to polyester used in PET bottles and other products due to its high purity, it is difficult to apply to materials that contain polyester (polyester-containing materials), such as polyester fibers and films that contain polyester.

Known methods for depolymerizing polyester include those using water or supercritical alcohol (see Patent Documents 1 and 2). However, both require high temperatures of 300°C or higher. On the other hand, transesterification 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 temperatures ranging from room temperature (15-25°C) to approximately 50°C. Another method is known that uses dimethyl carbonate as an ethylene glycol scavenger to improve depolymerization efficiency (Non-Patent Document 2). However, these methods are limited to high-purity PET derived from PET bottles and other materials, and have not been applied to other polyester-containing materials. A known method for depolymerizing colored polyester fibers uses a base catalyst and excess ethylene glycol, but this requires high temperatures of approximately 200°C. Furthermore, to obtain high-purity monomers, it is necessary to decolorize the fibers using a high-boiling-point solvent (see Patent Documents 4-6) or decompose the dye using an oxidizing agent (see Patent Document 7).

Japanese Patent No. 5099416 Japanese Patent Application Publication No. 2001-39908 U.S. Pat. No. 10,252,976 Japanese Patent No. 4537288 Japanese Patent No. 5134563 Japanese Patent No. 6659919 Japanese Patent No. 6986813

Green Chem. 2021,23,511. Green Chem. 2021,23,9412.

However, there was a problem in that polyester, and polyesters contained in various materials, could not be decomposed into monomers at low temperatures below 150°C using a simple process.

Furthermore, when polyester or polyester-containing materials contain moisture, conventional polyester decomposition methods have the problem of not being able to sufficiently decompose the polyester.

In light of the above-mentioned circumstances, the present invention aims to provide a polyester decomposition method that can decompose polyesters and polyesters contained in various polyester-containing materials into monomers at low temperatures of 150°C or less and in a simple process, and that can sufficiently decompose them even if they contain moisture. It also aims to provide a method for producing dialkyl dicarboxylate esters obtained by this polyester decomposition method, a method for producing dicarboxylic acids obtained by hydrolyzing these dialkyl dicarboxylate esters, and a method for producing polyesters using these dialkyl dicarboxylates and these dicarboxylic acids as raw materials.

As a result of intensive research into the above-mentioned problems, the inventors of the present invention have discovered the following groundbreaking method for decomposing polyesters, a method for producing dialkyl dicarboxylate esters obtained by this polyester decomposition method, a method for producing dicarboxylic acids obtained by hydrolyzing these dialkyl dicarboxylate esters, and a method for producing polyesters using these dialkyl dicarboxylate esters and these dicarboxylic acids as raw materials.

The first aspect of the present invention, which aims to solve the above problems, is a polyester decomposition method for polyester contained in a polyester-containing material, characterized by including a decomposition step in which potassium phosphate, a monohydric alcohol, and a carbonic acid diester are mixed to decompose the polyester.

Here, "decomposition" is a concept that includes depolymerization.

According to this first aspect, polyester and polyester contained in various polyester-containing materials can be decomposed at low temperatures of 150°C or less using a simple process.

A second aspect of the present invention is the polyester decomposition method described in the first aspect, characterized in that the reaction temperature in the decomposition step is in the range of 40°C to 150°C.

According to this second aspect, polyester can be sufficiently decomposed even when the reaction temperature in the decomposition step is in the range of 40°C to 150°C.

A third aspect of the present invention is the polyester decomposition method according to the first or second aspect, characterized in that the monohydric alcohol is methanol.

According to the third aspect, polyester can be decomposed at a higher rate.

A fourth aspect of the present invention is the polyester decomposition method according to the first or second aspect, characterized in that the carbonate diester is dimethyl carbonate.

According to the fourth aspect, polyester can be decomposed at a higher rate.

A fifth aspect of the present invention is the method for decomposing polyester according to the first or second aspect, wherein the polyester-containing material contains water in an amount of more than 0 and not more than 1000 ppm by mass (1000×10 ⁻⁴ mass%).

According to the fifth aspect, even polyester-containing materials containing more than 0 and up to 1000 ppm by mass of water can be sufficiently decomposed.

A sixth aspect of the present invention is the polyester decomposition method according to the first or second aspect, characterized in that the polyester contained in the polyester-containing material is one or more selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, and polyethylene furanoate.

According to the sixth aspect, polyester-containing materials containing one or more polyesters selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, and polyethylene furanoate can be decomposed at a low temperature of 150°C or less using a simple process.

A seventh aspect of the present invention is the polyester decomposition method according to the first or second aspect, characterized in that the material other than polyester contained in the polyester-containing material is one or more selected from the group consisting of cotton, rayon, polyurethane, nylon, acrylic, polyethylene, polypropylene, carbon-based materials, dyes, and pigments.

According to the seventh aspect, it is possible to decompose polyester-containing materials that contain, in addition to polyester, one or more materials selected from the group consisting of cotton, rayon, polyurethane, nylon, acrylic, polyethylene, polypropylene, carbon-based materials, dyes, and pigments.

An eighth aspect of the present invention is a method for producing a dicarboxylic acid dialkyl ester, characterized in that the dicarboxylic acid dialkyl ester is obtained by the polyester decomposition method described in the first aspect.

According to the eighth aspect, dicarboxylic acid dialkyl esters can be easily obtained.

A ninth aspect of the present invention is a method for producing a dicarboxylic acid, characterized in that the dicarboxylic acid is obtained by decomposing the dicarboxylic acid dialkyl ester described in the eighth aspect.

According to the ninth aspect, dicarboxylic acids can be easily obtained.

A tenth aspect of the present invention is a method for producing a polyester, characterized in that the polyester is produced using as raw materials at least one of a dicarboxylic acid dialkyl ester obtained by the method for producing a dicarboxylic acid dialkyl ester described in the eighth aspect and a dicarboxylic acid obtained by the method for producing a dicarboxylic acid described in the ninth aspect.

According to the tenth aspect, polyester can be easily produced.

FIG. 1 shows the spectrum of the reaction mixture obtained by gas chromatography. FIG. 2 is a graph showing the relationship between the time of the polyethylene terephthalate decomposition reaction and the yield of dimethyl terephthalate. FIG. 3 is a graph showing the relationship between the temperature in the decomposition step and the yield of dimethyl terephthalate. FIG. 4 is a table showing the materials, conditions, DMT yields, etc. used in Examples 4-18.

Below, an embodiment of the polyester decomposition method according to the present invention is described. Note that the present invention is not limited to the following embodiment.

(Embodiment 1)
The polyester decomposition method according to the present invention includes a decomposition step of mixing a material containing polyethylene terephthalate (PET), which is a type of polyester (a polyethylene terephthalate-containing material), potassium phosphate (K ₃ PO ₄ ), a monohydric alcohol, and a carbonic acid diester, and decomposing the polyethylene terephthalate contained in the polyethylene terephthalate-containing material, as shown in the following formula, for example.

Unlike polyester films, conventional decomposition methods have made it difficult to decompose the polyester contained in polyester-containing materials at relatively low temperatures below 150°C.

However, with the polyester decomposition method of the present invention, decomposition can be carried out at low temperatures of 150°C or less. Moreover, the main decomposition product (depolymerized product) is dimethyl terephthalate (DMT), a raw material monomer for polyethylene terephthalate, and can be obtained in high purity through a simplified process without adding any complicated steps after decomposition.

The substances used in the polyester decomposition method of the present invention are described below.

<Polyester-containing materials>
The polyester-containing material is not particularly limited as long as it contains polyester. Examples of the polyester-containing material include polyester itself (100% polyester), a material containing polyester fibers (polyester fibers), and a material containing polyester and components other than polyester. The shape of the polyester-containing material is not particularly limited, and may be in the form of a block, film, or fiber.

The components other than polyester contained in the polyester-containing material are not particularly limited. Examples of components other than polyester include polyolefins such as polyethylene and polypropylene, cellulose, polyamides such as nylon, polyurethane, acrylic resin, inorganic components (inorganic compounds) such as aluminum, and colorants such as dyes and pigments.

Furthermore, the polyester-containing material used in the present invention may be of only one type, or may be of two or more types. According to the present invention, the polyester contained in these materials can be sufficiently decomposed.

<Polyester>
The polyester contained in the polyester-containing material (the polyester to be decomposed) is not particularly limited. The polyester may be, for example, an aromatic polyester having only aromatic groups (divalent groups having a structure in which one hydrogen atom is removed from each of two carbon atoms forming an 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 both an aliphatic group and no aromatic group), or a polyester having both aromatic groups and aliphatic groups in its main chain.

Aromatic polyesters 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 contained in aromatic heterocyclic groups include oxygen atoms and nitrogen atoms.

When carrying out the polyester depolymerization method of the present invention, from the viewpoint of making it easier to proceed with polyester depolymerization and of high versatility, the polyester is preferably an aromatic polyester. Specifically, 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), which are represented by the following chemical formula, are more preferred, with polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate being particularly preferred.

In the above chemical formula, n may be 10 or more and 1000 or less, and preferably 50 or more and 250 or less.

The polyester-containing material may contain only one type of polyester, or two or more types. If there are two or more types, the combination and ratio of these can be selected as desired depending on the purpose.

Next, the percentage of polyester content in the polyester-containing material ([polyester content (parts by mass) in the polyester-containing material] / [total mass (parts by mass) of the polyester-containing material] x 100) is not particularly limited, but may be 20% by mass or more and 100% by mass or less, preferably 40% by mass or more and 100% by mass or less, more preferably 55% by mass or more and 100% by mass or less, and particularly preferably 70% by mass or more and 100% by mass or less, or 70% by mass or more and less than 100% by mass. The higher the polyester content, the greater the amount of monomer, one of the raw materials for polyester, produced per unit mass from the polyester-containing material due to polyester decomposition.

<Potassium phosphate>
The form of potassium phosphate (K ₃ PO ₄ ) is not particularly limited, but a powder form having a large surface area per unit weight is preferred.

The weight (amount used) of potassium phosphate used in the present invention is not particularly limited, but is preferably 0.1 to 20 parts by mass, more preferably 0.2 to 10 parts by mass, and particularly preferably 0.3 to 3.0 parts by mass, per 100 parts by mass of polyester contained in the polyester-containing material. If the weight of potassium phosphate is at or above the lower limit (0.1 part by mass), the decomposition of the polyester proceeds at a higher rate. On the other hand, if the weight of potassium phosphate is at or below the upper limit (20 parts by mass), excessive use of potassium phosphate can be suppressed.

<Monohydric alcohol>
During decomposition, the monohydric alcohol undergoes a transesterification reaction with the polyester contained in the polyester-containing material. For example, during decomposition of polyethylene terephthalate, the polyethylene terephthalate reacts with the monohydric alcohol to produce dimethyl terephthalate, which is one of the raw materials used in producing polyester.

The monohydric alcohol used in the present invention is not particularly limited and may be either an aliphatic alcohol or an aromatic alcohol, but in terms of promoting polyester decomposition at a higher rate, an aliphatic alcohol (saturated aliphatic alcohol, unsaturated aliphatic alcohol) is preferred, and saturated aliphatic alcohol is even more preferred.

Saturated aliphatic alcohols include, for example, 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), with methanol being particularly preferred. When methanol is used as the monohydric alcohol, polyesters can be decomposed at a higher rate than with other monohydric alcohols.

The monohydric alcohol used to decompose the polyester may be one type only, or two or more types. If two or more types are used, the combination and ratio of these alcohols are not particularly limited and can be selected as desired depending on the purpose.

There are no particular restrictions on the weight (amount used) of the monohydric alcohol used in the present invention, but for example, when the polyester is polyethylene terephthalate (PET), the amount is preferably 30 to 200 parts by weight, and more preferably 50 to 160 parts by weight, per 100 parts by weight 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, and more preferably 20 to 140 parts by mass, per 100 parts by mass of PBT in the polyester-containing material.

Furthermore, when the polyester is polyethylene naphthalate (PEN), the amount is preferably 10 to 150 parts by weight, and more preferably 15 to 120 parts by weight, per 100 parts by weight of PEN in the polyester-containing material.

When the amount of monohydric alcohol used is at or above these lower limits (30 parts by mass for PET, 10 parts by mass for PBT, and 10 parts by mass for PEN), the effects achieved by using the monohydric alcohol are enhanced. On the other hand, when the amount of monohydric alcohol used is at or below these upper limits (200 parts by mass for PET, 180 parts by mass for PBT, and 150 parts by mass for PEN), excessive use of monohydric alcohol is suppressed.

<Carbonate diester>
The carbonate diester reacts with glycols generated from the polyester during decomposition of the polyester to produce cyclic compounds or chain compounds, and has the effect of shifting the equilibrium between the depolymerization reaction and polymerization reaction of the polyester during decomposition in favor of the depolymerization reaction, thereby improving the monomer production rate. In other words, the carbonate diester functions as a glycol scavenger.

The reaction products of a carbonate diester and a glycol can include cyclic compounds (e.g., ethylene carbonate) that are the reaction product of one molecule of a carbonate diester and one molecule of a glycol, chain compounds (e.g., hydroxyethyl methyl carbonate) that are the reaction product of one molecule of a carbonate diester and one molecule of a glycol, and chain compounds (e.g., dimethyl 2,5-dioxahexanedioate) that are the reaction product of one molecule of a carbonate diester and two molecules of a glycol. Whether a cyclic or chain compound is produced is primarily determined by the type (e.g., size) of the glycol.

Examples of carbonate diesters include dialkyl carbonates and diaryl carbonates. The two alkyl groups bonded to the oxygen atom in a dialkyl carbonate may be the same or different. Furthermore, the two aryl groups bonded to the oxygen atom in a diaryl carbonate may be the same or different.

Furthermore, the alkyl group in the dialkyl carbonate may be linear, branched, or cyclic, and if cyclic, may be either monocyclic or polycyclic.

In the dialkyl carbonate, the linear or branched alkyl group preferably has 1 to 8 carbon atoms. Examples of such alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, neopentyl, tert-pentyl, 1-methylbutyl, n-hexyl, 2-methylpentyl, 3-methylpentyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, n-heptyl, 2-methylhexyl, 3-methylhexyl, 2,2-dimethylpentyl, 2,3-dimethylpentyl, 2,4-dimethylpentyl, 3,3-dimethylpentyl, 3-ethylpentyl, 2,2,3-trimethylbutyl, n-octyl, and isooctyl.

Among these, the number of carbon atoms in the alkyl group is more preferably 1 to 4, and even more preferably 1 or 2. Particularly preferred dialkyl carbonates include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.

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

The carbonate diester used to decompose the polyester may be one type or two or more types. If two or more types are used, the combination and ratio of these are not particularly limited and can be selected as desired depending on the purpose.

In terms of promoting decomposition of the polyester at a higher rate, it is more preferable that the carbonate diester be one or more selected from the group consisting of dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and diphenyl carbonate, with dimethyl carbonate being particularly preferred.

The weight (amount used) of the carbonate diester used in the present invention is not particularly limited, but is preferably 100 to 10,000 parts by mass, and more preferably 100 to 2,000 parts by mass, per 100 parts by mass of polyester in the polyester-containing material. If the weight of the carbonate diester is at or above the lower limit (100 parts by mass), the effects obtained by using the carbonate diester will be greater. On the other hand, if the weight of the carbonate diester is at or below the upper limit (10,000 parts by mass), excessive use of the carbonate diester will be suppressed.

<Solvent>
In the decomposition of the polyester, a solvent that does not fall under any of the categories of base, monohydric alcohol, and carbonic acid diester may be used. In the present embodiment, the decomposition of the polyester proceeds efficiently even without using a solvent, but by using a solvent as necessary, the handleability of the blend of raw materials such as a reaction liquid may be improved, and the decomposition of the polyester may proceed more efficiently.

In this specification, the term "solvent" refers to a substance (liquid) that has at least one of the properties of dissolving a specified substance (solid or liquid) and dispersing the specified substance at a temperature between 15 and 25°C.

The solvent used in the present invention 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 to decompose the polyester may be one type or two or more types. If two or more types are used, the combination and ratio of the solvents can be selected as desired depending on the purpose.

When a solvent is used, the weight (amount used) of the solvent during polyester decomposition 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. If the weight of the solvent is at or above the lower limit (1 part by mass), the effects obtained by using the solvent will be greater. On the other hand, if the weight of the solvent is at or below the upper limit (100,000 parts by mass), excessive use of the solvent can be prevented.

<Other ingredients>
When decomposing the polyester, other components (substances) 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 they do not impair the effects of the present invention.

The other components can be selected arbitrarily depending on the purpose and are not particularly limited. The other components used to decompose the polyester may be one type only, or two or more types. If there are two or more types, the combination and ratio of these components are not particularly limited and can be selected arbitrarily depending on the purpose.

When decomposing polyester, the ratio of the total weight (parts by mass) of the polyester-containing material, base, monohydric alcohol, and carbonate diester to the total weight (parts by mass) of components other than the solvent (([weight (parts by mass) of polyester-containing material] + [weight (parts by mass) of base] + [weight (parts by mass) of monohydric alcohol] + [weight (parts by mass) of carbonate diester]) / [total weight (parts by mass) of components other than the solvent] x 100) is preferably 80% to 100% by mass, more preferably 90% to 100% by mass, and even more preferably 95% to 100% by mass, and may be, for example, either 97% to 100% by mass or 99% to 100% by mass. If this ratio is equal to or greater than the lower limit (80% by mass), the polyester can be decomposed more efficiently.

Here, the total weight (parts by mass) of components other than the solvent during decomposition of the polyester is synonymous with the combined weight (parts by mass) of the polyester-containing material, base, monohydric alcohol, carbonic acid diester, and other components during decomposition.

<Reaction conditions for decomposition step>
The polyester contained in the polyester-containing material can be decomposed by blending (mixing) the polyester-containing material, potassium phosphate, a monohydric alcohol, a carbonic acid diester, the polyester-containing material, and, if necessary, a solvent and other components.

The order in which these raw materials are mixed is not particularly limited, but it is preferable to prepare a raw material composition that is a blend of potassium phosphate, a monohydric alcohol, a carbonic acid diester, and, if necessary, a solvent and other components (for example, a blend of all raw materials other than the polyester-containing material), and then mix this raw material composition with the polyester-containing material. Using this blending order allows for a higher rate of decomposition of the polyester.

Furthermore, the reaction temperature during the decomposition of the polyester contained in the polyester-containing material (reaction temperature during the decomposition process) can be adjusted as appropriate, and may be carried out at room temperature or under heated conditions. There are no particular restrictions on the equipment used for heating, and equipment such as a heater can be used.

The reaction temperature during polyester decomposition (reaction temperature during the decomposition process) is preferably 20°C to 150°C. Furthermore, according to the decomposition method of the present invention, polyester can be sufficiently decomposed even at relatively low temperatures of 30°C to 100°C, and polyester can be decomposed quickly and sufficiently at temperatures of 40°C to 70°C in particular.

According to this embodiment, polyester and polyester contained in various polyester-containing materials can be decomposed into monomers at low temperatures of 150°C or less and in a simple process.

The decomposition of polyester may be carried out under normal pressure, reduced pressure, or increased pressure. The decomposition of polyester may also be carried out in air or in an inert gas atmosphere.

Furthermore, the reaction time during polyester decomposition is not particularly limited and can be adjusted appropriately taking into account other reaction conditions such as reaction temperature. The reaction time during polyester decomposition is not particularly limited as long as it is 0.1 to 24 hours, but is preferably 0.15 to 60 minutes, and more preferably 20 to 60 minutes.

In addition, immediately after blending the raw materials, some insoluble materials may remain in the blend. In such cases, they can be dissolved by stirring using a known method during the decomposition of the polyester. Specific stirring methods that can be used include, for example, stirring by rotating a magnetic stirrer or stirring blades, stirring using a ball mill, stirring by irradiating with ultrasonic waves, and stirring using a shaker.

<Post-processing conditions, removal conditions>
After the polyester decomposition step is completed, post-treatment is carried out by a known method, and the monomers that are the raw materials for the polyester can be extracted with high purity.

For example, after decomposing polyethylene terephthalate, the resulting reaction product can be subjected to distillation or other methods to obtain highly pure (e.g., 75% or higher) dimethyl terephthalate.

The other major decomposition product, the reaction product between glycol and carbonate diester, can also be extracted with high purity (e.g., 75% or more), just like dimethyl terephthalate, by adjusting the post-treatment and extraction conditions appropriately.

<Specific example of polyester decomposition method>
According to the polyester decomposition method of the present invention, in addition to the above-mentioned polyethylene terephthalate, for example, the following five types of polyester can be decomposed.

Next, an example of this embodiment will be described in detail.

[Example 1]
Potassium phosphate ( _{K3PO4 )} (Fujifilm Wako Pure Chemical Industries, hereinafter the same, 11 mg, 0.052 mmol, 11 mass% relative to the mass of polyethylene terephthalate described below) was added to a 4 mL vial, and methanol (Kishida Chemical Co., Ltd., hereinafter the same, 0.3 mL, 238 mass% relative to the mass of polyethylene terephthalate described below) and dimethyl carbonate (DMC) (Tokyo Chemical Industry Co., Ltd., hereinafter the same, 1.5 mL, 1605 mass% relative to the mass of polyethylene terephthalate described below) were further added and these blended components were uniformly dissolved to prepare a raw material composition. Then, powdered polyethylene terephthalate (100 mg) was added to the raw material composition in the eggplant flask.

Next, the mixture of raw material composition and polyethylene terephthalate was stirred at 90°C for 17 minutes using an aluminum block heater with a rotating stirrer in the recovery flask, thereby decomposing the polyethylene terephthalate. After filtration, mesitylene (manufactured by Fujifilm Wako Pure Chemical Industries) was added to the resulting solution as an internal standard, and the results of analysis by gas chromatography (apparatus: Shimadzu Corporation GC-2000; the same applies below) are shown in Figure 1.

These results indicate that polyethylene terephthalate decomposes and its depolymerized product, dimethyl terephthalate (DMT), is produced in a molar yield of 77.6%.

(Embodiment 2)
In the first embodiment, the polyester-containing material does not contain water, but the present invention is not limited to this. That is, the polyester decomposition method according to the present invention can also be used for polyester-containing materials that contain water.

<Polyester-containing materials>
The polyester-containing material in this embodiment is the polyester-containing material of embodiment 1 to which water has been added. That is, the polyester-containing material in this embodiment is polyester itself (100% polyester) or various polyester-containing materials that contain water. Note that the polyester-containing material of this embodiment naturally includes polyester-containing materials that do not contain water and to which water is added during or before the polyester decomposition process.

Here, the amount of moisture contained in the polyester-containing material is not particularly limited, but is preferably greater than 10 ppm by mass and less than or equal to 1000 ppm by mass, more preferably 10 ppm by mass to 700 ppm by mass, and particularly preferably 15 ppm by mass to 400 ppm by mass, per 100 parts by mass of the polyester-containing material. If the amount of moisture contained in the polyester-containing material exceeds the upper limit (1000 ppm by mass), the decomposition of the polyester will slow down.

Note that the substances used in the polyester decomposition method of this embodiment, other than the polyester-containing material, are the same as those in embodiment 1. Furthermore, the reaction conditions for the decomposition step are also the same as those in the polyester decomposition method of embodiment 1.

[Example 2]
A 100 mL recovery flask was charged with fibrous polyethylene terephthalate (10 g), and dimethyl carbonate (DMC) (Tokyo Chemical Industry Co., Ltd., 50 _mL , 535% by mass relative to the polyethylene terephthalate) and 1500 ppm or less of water were added. Potassium phosphate ( _K3PO4 ) (Fujifilm Wako Pure Chemical Industries, Ltd., 1.1 g, 5.2 mmol, 11% by mass relative to the polyethylene terephthalate described below) was suspended in methanol (Kishida Chemical Co., Ltd., 10 mL, 79.2% by mass relative to the polyethylene terephthalate) and stirred at 70°C for 30 minutes to prepare a solution (raw material composition). This solution was transferred to the 100 mL recovery flask containing the polyethylene terephthalate and DMC using a Teflon (registered trademark) tube.
Then, using a magnetic stirrer and an oil bath, the mixture of the raw material composition, 1500 ppm or less of water, and polyethylene terephthalate was heated and stirred at 70°C while rotating the stirrer in the recovery flask, thereby decomposing the polyethylene terephthalate.

2 shows a graph of the relationship between the time of the polyethylene terephthalate decomposition reaction and the yield of dimethyl terephthalate. The water concentration was determined by stirring a mixture of polyethylene terephthalate, dimethyl carbonate, and water at 70°C for 30 minutes and then measuring the water concentration contained in the dimethyl carbonate. The yield of dimethyl terephthalate was determined based on the dimethyl terephthalate concentration in the reaction solution, which was calculated from the peak intensity at 734 ^cm in the spectrum obtained with a probe spectrometer (React IR).

As can be seen from this graph, the lower the water content, the faster the decomposition reaction of polyethylene terephthalate progresses. In particular, when the water concentration is greater than 0 ppm and less than 572 ppm, the decomposition of polyethylene terephthalate is completed within 20 minutes at the latest, and when the water concentration is greater than 0 ppm and less than 980 ppm, the decomposition of polyethylene terephthalate is completed within 100 minutes at the latest.

[Example 3]
The reaction temperature in the decomposition step was changed, and the same experiment as in Example 2 was carried out to investigate the relationship between the reaction temperature in the decomposition step and the yield of dimethyl terephthalate. The results are shown in Figure 3.

As can be seen from this graph, in the temperature range of 66.1°C to 100°C, decomposition of polyethylene terephthalate is completed within 20 minutes at the latest, in the temperature range of 57.2°C to 100°C, decomposition of polyethylene terephthalate is completed within 30 minutes at the latest, and in the temperature range of 48.5°C to 100°C, decomposition of polyethylene terephthalate is completed within 70 minutes at the latest.

[Comparative Example 1]
Under the conditions described in Example 2, the reaction was carried out using potassium carbonate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 0.72 g, 5.2 mmol, 7.2 mass% relative to the mass of polyethylene terephthalate) as a catalyst instead of potassium phosphate, and with a water content of 980 ppm. Even after a reaction time of 300 minutes, the yield of dimethyl terephthalate was about 10%.

[Examples 4-7]
As shown in Example 4-7 of Figure 4, a lab cloth (10 g) made of 100% polyethylene terephthalate was placed in a 100 mL recovery flask, and moist dimethyl carbonate was added. Potassium phosphate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was suspended in methanol (manufactured by Kishida Chemical Co., Ltd.) and stirred at 70°C for 30 minutes to prepare a solution (raw material composition). This solution was then transferred to the 100 mL recovery flask containing polyethylene terephthalate and DMC using a Teflon (registered trademark) tube.

Then, under the conditions shown in Example 4-7 in Figure 4, a magnetic stirrer and oil bath were used to heat and stir the mixture of raw material composition, water, and lab cloth in a recovery flask while rotating the stirrer, thereby decomposing the polyethylene terephthalate.

As shown in Example 4-6 in Figure 4, it was found that polyethylene terephthalate could be decomposed even when the amount of potassium phosphate and methanol (catalyst amount) was changed.

Here, the catalyst amount in Figure 4 is the weight part (wt%) of potassium phosphate.

[Example 8]
As shown in Example 8 of FIG. 4 , a 3 L reaction filtration apparatus was used, and a lab coat cloth (458 g) made of 100% polyethylene terephthalate was placed in it, and the polyethylene terephthalate was decomposed in the same manner as in Example 4.

As a result, as can be seen from the DMT yield column for Example 8 in Figure 4, it was found that polyethylene terephthalate can be decomposed even when the scale of the equipment is changed.

[Examples 9-10]
As shown in Examples 9 and 10 of FIG. 4 , 10 g or 458 g of a colored apron (a 100% polyethylene terephthalate apron colored with a dye), which is a polyester-containing material, was placed in a container (a 100 mL recovery flask, a 3 L reaction filtration apparatus), and the polyethylene terephthalate contained therein was decomposed in the same manner as in Example 4.

As a result, as can be seen from the DMT yield column for Examples 9-10 in Figure 4, it was found that polyester could be decomposed even in colored aprons. It was also found that colored aprons could be decomposed even when the scale of the experimental equipment was changed.

[Examples 11 to 13]
As shown in Examples 11-13 of FIG. 4 , 10 g of each of the following polyester-containing materials was placed in a 100 mL recovery flask: a cotton-blended polyester-containing material (PET/cotton (mass ratio: 51/49)), a nylon-blended polyester-containing material (PET/Nylon (mass ratio: 67/33 (blended))), and a nylon-blended polyester-containing material (PET/Nylon (mass ratio: 67/33 (blended, acryl-coated))). The polyesters contained therein were decomposed in the same manner as in Example 4.

As a result, as can be seen from the DMT yield columns for Examples 11-13 in Figure 4, it was found that the polyester contained in cotton-blended polyester-containing materials, nylon-blended polyester-containing materials, and polyester-containing materials mixed with nylon could each be decomposed.

[Example 14]
As shown in Example 14 of FIG. 4 , 10 g of a polyethylene terephthalate adhesive film coated with an adhesive, which is a polyester-containing material, was placed in a 100 mL recovery flask, and the polyethylene terephthalate was decomposed in the same manner as in Example 4.

As can be seen from the DMT yield column in Figure 14, the polyester contained in polyethylene terephthalate adhesive film coated with adhesive can be decomposed.

[Example 15]
As shown in Example 15 of FIG. 4 , 12.1 g of a polyester-containing material blended with polyurethane (mass ratio: 82/18), which is a polyester-containing material, was placed in a 100 mL recovery flask, and the polyethylene terephthalate contained therein was decomposed in the same manner as in Example 4.

As a result, as can be seen from the DMT yield column for Example 15 in Figure 4, it was found that even in polyethylene terephthalate adhesive films coated with adhesive, the polyester contained therein could be decomposed.

[Examples 16-17]
As can be seen from the column for DMT yield of Examples 16 and 17 in FIG. 4 , 10 g each of 100% polybutylene terephthalate pellets and 100% polyethylene naphthalate film, which are polyester-containing materials, were placed in a 100 mL recovery flask, and the polybutylene terephthalate and polybutylene terephthalate were decomposed in the same manner as in Example 4.

As a result, as can be seen from the DMT yield column for Examples 16-17 in Figure 14, it was found that even pellets made of 100% polybutylene terephthalate and films made of 100% polyethylene naphthalate could decompose the polyester contained within them.

[Example 18]
As shown in Example 18 of Figure 4, a lab cloth (10 g) made of 100% polyethylene terephthalate was placed in a 100 mL recovery flask, and toluene (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and water were added. Potassium phosphate was suspended in methanol and stirred at 70°C for 30 minutes to prepare a solution (raw material composition), which was then transferred to the 100 mL recovery flask containing the polyethylene terephthalate and toluene using a Teflon (registered trademark) tube.

Then, under the conditions shown in Example 18 in Figure 4, a magnetic stirrer and oil bath were used to heat and stir the mixture of raw material composition, water, and lab cloth in a recovery flask while rotating the stirrer, thereby decomposing the polyethylene terephthalate.

As can be seen from the DMT yield column for Example 18 in Figure 4, it was found that polyethylene terephthalate can be decomposed even when toluene is used instead of DMC.

(Embodiment 3)
As described above, dialkyl dicarboxylates can be obtained by using the polyester decomposition method of the present invention. Specifically, by purifying the decomposition mixture obtained using the polyester decomposition method of the present invention, for example, dimethyl terephthalate, diethyl terephthalate, dibutyl terephthalate, etc. can be obtained from polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polytrimethylethylene terephthalate (PTT), etc.; dimethyl naphthalenedicarboxylate, diethyl naphthalenedicarboxylate, dibutyl naphthalenedicarboxylate, etc. can be obtained from polyethylene naphthalate (PEN), polybutylene naphthalate (PBN), etc.; and dimethyl furan dicarboxylate, diethyl dicarboxylate, dibutyl furan dicarboxylate, etc. can be obtained from polyethylene furanoate (PEF, also known as polyethylene furandicarboxylate). Note that the method for purifying dialkyl dicarboxylates from the decomposition mixture is not particularly limited, and known purification methods can be used.

As described above, the polyester decomposition method according to the present invention can decompose polyester at relatively low temperatures, making it possible to efficiently obtain dicarboxylic acid dialkyl esters with few impurities.

(Embodiment 4)
Dicarboxylic acids can be obtained by hydrolyzing the dialkyl dicarboxylate esters obtained in embodiment 3. Specifically, terephthalic acid can be obtained from dimethyl terephthalate, diethyl terephthalate, dibutyl terephthalate, or the like; naphthalenedicarboxylic acid can be obtained from dimethyl naphthalenedicarboxylate, diethyl naphthalenedicarboxylate, dibutyl naphthalenedicarboxylate, or the like; or furandicarboxylic acid can be obtained from dimethyl furandicarboxylate, diethyl dicarboxylate, dibutyl furandicarboxylate, or the like.

The decomposition method used for this hydrolysis is not particularly limited, and general hydrolysis methods can be used. For example, alkaline hydrolysis can be used to efficiently obtain dicarboxylic acids from dicarboxylic acid dialkyl esters.

(Embodiment 5)
A polyester can be synthesized again by using as a raw material at least one of the dicarboxylic acid dialkyl ester obtained in Embodiment 3 and the dicarboxylic acid obtained in Embodiment 4. That is, a new polyester can be synthesized from the dicarboxylic acid dialkyl ester and the dicarboxylic acid obtained by the present invention. The method for synthesizing the polyester is not particularly limited, and known methods can be used.

As mentioned above, the polyester decomposition method according to the present invention can decompose polyester at relatively low temperatures, making it possible to efficiently obtain dicarboxylic acid dialkyl esters with few impurities. As a result, the resynthesized polyester also has a high purity.

The polyester to be synthesized may be the same as or different from the polyester decomposed by the polyester decomposition method.

Claims (10)

A method for decomposing polyester contained in a polyester-containing material, comprising:
A polyester decomposition method, comprising a decomposition step of mixing the polyester-containing material with potassium phosphate, a monohydric alcohol, and a carbonic acid diester to decompose the polyester. The polyester decomposition method described in claim 1, characterized in that the reaction temperature in the decomposition step is in the range of 40°C to 150°C. The polyester decomposition method according to claim 1 or 2, characterized in that the monohydric alcohol is methanol. The polyester decomposition method according to claim 1 or 2, characterized in that the carbonate diester is dimethyl carbonate. The polyester decomposition method described in claim 1 or 2, characterized in that the polyester-containing material contains moisture greater than 0 and less than 1000 ppm by mass. The polyester decomposition method according to claim 1 or 2, characterized in that the polyester contained in the polyester-containing material is one or more selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, and polyethylene furanoate. The polyester decomposition method according to claim 1 or 2, characterized in that the material other than polyester contained in the polyester-containing material is one or more selected from the group consisting of cotton, rayon, polyurethane, nylon, acrylic, polyethylene, polypropylene, carbon-based materials, dyes, and pigments. A method for producing a dialkyl dicarboxylate, wherein the dialkyl dicarboxylate is obtained by the polyester decomposition method described in claim 1. A method for producing a dicarboxylic acid, characterized in that it is obtained by hydrolyzing the dicarboxylic acid dialkyl ester described in claim 8. A method for producing a polyester, characterized in that the polyester is produced using as a raw material at least one of a dicarboxylic acid dialkyl ester obtained by the method for producing a dicarboxylic acid dialkyl ester according to claim 8 and a dicarboxylic acid obtained by the method for producing a dicarboxylic acid according to claim 9.