Process for depolymerization of polymers comprising ester functional groups using mixed organic solvents

Abstract

The present invention relates to a method for depolymerizing a polymer containing an ester functional group, and more particularly, to a method for depolymerizing a polymer containing an ester functional group by mixing an aromatic compound containing an alkoxy functional group and a compound containing an alcohol functional group as a solvent for reaction and an extractant for impurity removal, wherein a polymer containing an ester functional group is decomposed by using an alkali hydroxide as a reactant and a monomer produced thereby is obtained with high purity.

Description

Technical Field

[0001] The present invention relates to a method for depolymerizing a polymer containing an ester functional group, and more particularly to a method in which an aromatic compound containing an alkoxy functional group and a compound containing an alcohol functional group are mixed and used as a reaction solvent and an extractant for removing impurities, wherein an alkali hydroxide is used as a reactant to decompose the polymer containing an ester functional group and obtain the monomer thus generated at a high purity. Background Art

[0002] Plastic is a cheap and durable material with a wide range of applications and can be used to produce a variety of products. As a result, the production and consumption of plastics in daily life have increased dramatically over the past few decades, which not only poses a threat of environmental hormones that may be exposed and accumulated in the body, but also causes serious environmental problems such as damage to natural ecosystems. Moreover, more than 50% of these plastics are single-use disposable products such as packaging, agricultural films, and disposable beverage containers, or short-life products that are discarded within 1 year after manufacturing.

[0003] Discarded plastics are becoming an increasingly serious environmental problem as much of it is dumped into designated landfills and natural habitats around the world where they are difficult to decompose naturally. Even plastics that are naturally or biodegradable may persist for decades depending on local environmental factors such as levels of UV exposure, temperature, and the presence of decomposing microorganisms.

[0004] In order to solve the above problems, people are actively conducting various research and efforts to minimize the accumulation of plastics or reduce environmental impact, starting from the chemical decomposition of existing petroleum-based plastics to the physical regeneration and reprocessing of plastics, including the development of new plastic materials with a shorter decomposition cycle in natural conditions.

[0005] Among the plastics discarded after consumption, polymers containing ester functional groups have the characteristic that the raw materials before synthesis can be recovered through relatively simple chemical reaction pathways such as decomposition or exchange of ester functional groups, and therefore have attracted attention as materials that can be repeatedly recycled through chemical recycling methods based on depolymerization. The most common examples of polymers containing ester functional groups include polyethylene terephthalate (PET) containers and polyester fibers for packaging. Polymers containing ester functional groups can generally be converted into monomers through chemical decomposition using reaction solvents and catalysts for ester exchange, and various chemical reaction pathways related to this are currently known. In theory, some of the monomers generated by decomposition can have the same properties as the raw materials input in the initial polymer synthesis. Depolymerization pathways used in industry to recycle polyethylene terephthalate (PET) include glycolysis, methanolysis, hydrolysis with the addition of acid or base catalysts, and alkaline decomposition.

[0006] Alcohol decomposition is a process that produces bis(2-hydroxyethyl) terephthalate (BHET) by adding an excess of ethylene glycol (EG), one of the raw monomer materials. Since ethylene glycol (EG), which is a part of the raw material of polyethylene terephthalate (PET), is used as a reactant, it has a high thermodynamic compatibility with the reaction product. In addition, since the produced monomer product, bis(2-hydroxyethyl) terephthalate (BHET), is an intermediate product of polymer polymerization, it can be used directly as a raw material for the production of polyethylene terephthalate (PET) by only changing part of the raw material supply section of the existing equipment. However, in order to increase the decomposition rate of the polymer, a metal catalyst and excess ethylene glycol (EG) are required, and the final yield of the monomer is limited due to the relatively wide molecular weight distribution due to the reaction balance between the monomer and the oligomer. In addition, the product is difficult to purify by high-temperature distillation because the boiling points of ethylene glycol (EG) as a solvent and the final product are high. In the recrystallization process using water, the recrystallization rate is slow, which may limit productivity and has a low product recovery rate. In order to increase the speed of the depolymerization reaction, metal acetate is used as a catalyst, and the most representative catalyst is zinc acetate.

[0007] Methanol decomposition is a reaction process that is widely used in commercial projects not only in multinational chemical companies but also in small and medium-sized plastic industries. It can be depolymerized using a variety of catalysts for transesterification reactions, or without a catalyst, using only methanol at high temperature and high pressure, such as supercritical pressure. However, since methanol is used as a reaction solvent, a high methanol vapor pressure is generated when high temperature reaction conditions are required, which requires a high-pressure reactor design and leak and explosion-proof facilities, and there may be a problem of excessive energy consumption per unit process for depolymerization and purification. As transesterification catalysts, zinc acetate, magnesium acetate, cobalt acetate, and lead dioxide are commonly used, and dimethyl terephthalate (DMT) can be obtained as the final monomer product. Dimethyl terephthalate (DMT) produced by methanol decomposition can be fed into a reactor for polyethylene terephthalate (PET) synthesis in a molten state at 150°C, and polyethylene terephthalate (PET) can be produced by adding ethylene glycol to carry out ester exchange reaction and polycondensation at a high temperature of 160 to 220°C.

[0008] The polyethylene terephthalate (PET) polymerization process using dimethyl terephthalate (DMT) as a raw material requires a separate process for producing bis(2-hydroxyethyl) terephthalate (BHET), and generates a large amount of steam methanol, so its process cost is high. The polyethylene terephthalate (PET) production facility based on dimethyl terephthalate (DMT) raw material was the main production method until the 1960s, before the development of a high-purity purification method for terephthalic acid (TPA). Nowadays, the two-step synthesis method of using terephthalic acid (TPA) and ethylene glycol (EG) as raw materials to directly perform an esterification reaction to initially synthesize bis(2-hydroxyethyl) terephthalate (BHET), then continuously removes water and performs polycondensation, occupies most of the polyethylene terephthalate (PET) material production processes. Therefore, if high-purity terephthalic acid (TPA) and ethylene glycol (EG) can be produced at a low cost by depolymerization of waste polyethylene terephthalate (PET), the recycled monomer can be directly applied to the production of polyethylene terephthalate (PET) without changing the equipment or operating conditions. The reaction pathway for producing terephthalic acid (TPA), a recycled monomer, by depolymerization of waste polyethylene terephthalate (PET) includes hydrolysis and alkali decomposition.

[0009] For hydrolysis, depolymerization can be carried out at high temperature in the presence of a high concentration of acid or base catalyst, and terephthalic acid (TPA) can be finally obtained as a depolymerization product. In the hydrolysis reaction using an acid catalyst, an extremely high concentration of sulfuric acid solution (87wt%) is required to achieve higher reactivity and yield, and it is currently known that it is difficult to ensure its economic efficiency due to corrosion of the reactor and generation of wastewater. The hydrolysis reaction using an alkali catalyst has the problem of low product initial concentration and difficulty in catalyst recovery due to its relatively slow reaction rate.

[0010] The alkali decomposition reaction using a high concentration of alkali hydroxide or a non-water-soluble solvent has an advantage of being able to produce terephthalic acid (TPA) at a high yield due to its high reaction speed, but it also has the problem of poor economic efficiency because it needs to consume excess alkali hydroxide as a reactant rather than a catalyst. In particular, since the product after depolymerization is obtained in the form of terephthalic acid alkali salt, acid treatment is required to obtain terephthalic acid, and there may be an excess of alkali hydroxide used to promote the reaction, so there is a problem of generating a large amount of acid and wastewater for neutralization.

[0011] As a prior art of alkali decomposition reaction of polyethylene terephthalate (PET), U.S. Patent Gazette No. 10087130 (published on October 2, 2018) relates to a method for producing and recovering monomers, namely terephthalic acid (TPA) and ethylene glycol (EG), obtained by depolymerizing polyethylene terephthalate (PET) using an excess of alkali hydroxide, and describes a method including a step of depolymerizing polyethylene terephthalate (PET) by adding (i) a non-polar solvent and (ii) a mixture consisting of an alcohol and a hydroxide, and a method in which a part or all of polyethylene terephthalate (PET) can be depolymerized in sufficient time even without external heating during the reaction. Although the patent No. 10087130 can induce depolymerization of polyethylene terephthalate (PET) using a small amount of energy, there is a problem that a solvent harmful to the environment and human body, such as methylene chloride, is used as a non-polar solvent. Furthermore, it is described that when polyethylene terephthalate (PET) is decomposed by an alkali decomposition reaction, it is necessary to add an equivalent amount of alkali hydroxide or more in order to completely decompose it into terephthalic acid (TPA) and ethylene glycol (EG) as depolymerization products, or that a long depolymerization reaction of up to 10 days is required.

[0012] Korean Patent Gazette No. 10-2223614 (published on March 5, 2021) relates to a method for depolymerizing a polymer containing an ester functional group, and describes a method for depolymerizing a polymer containing an ester functional group by contacting a mixture containing (1) a straight-chain primary alcohol that does not contain halogen; (2) a polar aprotic solvent that does not contain halogen and contains one or more functional groups selected from the group consisting of a ketone group, a nitrile group, and a furan group; and (3) a basic compound containing a hydroxyl group with the polymer containing an ester functional group, wherein a small amount of alcohol having a volume ratio of the primary alcohol to the polar aprotic solvent of 1:1 to 1:20 is used as a co-solvent. In the above technology, polyethylene terephthalate (PET) can be completely decomposed by using an excess of alkali hydroxide in a ratio of about 4 to 14 moles compared to the number of moles of repeating units of the polymer containing an ester functional group, but a portion of the co-solvent may be consumed due to aldol reaction and aldol condensation of a co-solvent containing a carbonyl group in a polar aprotic solvent. In addition, since foreign substances generated in the aldol reaction remain in the monomer generated by depolymerization, there are problems such as changes in the color of the product and difficulty in completely purifying impurities through a simple purification process. In addition, due to the use of an excess of alkali hydroxide and a small amount of alcohol, the unreacted alkali hydroxide may have limited solubility in the reaction mixture and remain in the form of a solid substance, so it may be recovered in the form of a filter cake together with the monomer product during the filtration process, resulting in unnecessary consumption of reactants and additional generation of a portion of wastewater.

[0013] In most other alkali-based depolymerizations of polyethylene terephthalate (PET), an excess of alkali hydroxide is usually used to increase the conversion rate of polyethylene terephthalate (PET) and the yield of the monomer terephthalic acid (TPA). Most of the waste plastics are not composed of uniform materials, or are discarded in a form containing a large amount of impurities, and generally cannot be completely removed through sorting and washing processes. In particular, impurities such as colored dyes or pigments are usually designed to not be lost under repeated external conditions such as sunlight, washing and friction, so it is generally difficult to remove them from the product using simple physical separation and purification technology.

[0014] As described above, waste plastics containing impurities usually remain in the product because it is difficult to easily remove foreign matter through depolymerization reaction and purification process. In order to improve the purity of the final product, a separate purification technology must be used, which may induce energy consumption and product loss, and it may be difficult to ensure the good economic efficiency of the comprehensive project for manufacturing recycled monomers. Therefore, waste plastics containing a large amount of impurities, especially fibers added with dyes or waste plastics applied with colored pigments, are materials that are difficult or impossible to reuse in industry and need to be treated by methods such as incineration or landfilling that release secondary pollution sources. Therefore, it has become one of the largest sources of pollution caused by waste plastics.

[0015] [Prior technical literature] [Patent Document] (Patent Document 1) U.S. Patent Gazette No. 10087130 (published on October 2, 2018) (Patent Document 2) Korean Patent Gazette No. 10-2223614 (published on March 5, 2021) Summary of the invention

[0016] Technical issues The object of the present invention is to provide an effective and economical depolymerization method which can improve the depolymerization performance of polymers containing ester functional groups and remove most of the organic and inorganic foreign matters during the manufacturing process of the monomers.

[0017] Technical Solution In order to solve the above-mentioned problems, the present invention provides a method for depolymerizing a polymer containing an ester functional group, characterized in that: the polymer containing an ester functional group is depolymerized by bringing a mixture containing (1) an aromatic compound having no proton donor functional group but having one or more alkoxy functional groups; (2) a compound having one or more alcohol functional groups; and (3) an alkali hydroxide into contact with the polymer containing an ester functional group.

[0018] In one embodiment of the present invention, the weight ratio of the aromatic compound having no proton donor functional group but having one or more alkoxy functional groups to the compound having one or more alcohol functional groups in the mixture may be 20:1 to 1:20.

[0019] In one embodiment of the present invention, the compound having an alcohol functional group may be a linear primary alcohol, and the aromatic compound having no proton donor functional group but having one or more alkoxy functional groups may be selected from methoxybenzene, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, 1,2,3-trimethoxybenzene, 1,2,4-trimethoxybenzene, 1,3,5-trimethoxybenzene, 1,2,3,4-tetramethoxybenzene. , 1,2,3,5-tetramethoxybenzene, 1,2,4,5-tetramethoxybenzene, 1-methoxy-2-methylbenzene, 1-methoxy-3-methylbenzene, 1-methoxy-4-methylbenzene, 1-methoxy-2-ethylbenzene, 1-methoxy-3-ethylbenzene, 1-methoxy-4-ethylbenzene, 1-methoxy-2-propylbenzene, 1-methoxy-3-propylbenzene, 1-methoxy-4-propylbenzene, ethoxybenzene, 1-ethoxy-2-methoxybenzene The present invention relates to a compound selected from the group consisting of 1-ethoxy-3-methoxybenzene, 1,2-diethoxybenzene, 1,3-diethoxybenzene, 1,4-diethoxybenzene, 1,2,4-triethoxybenzene, 1,3,5-triethoxybenzene, 1-ethoxy-2-methylbenzene, 1-ethoxy-3-methylbenzene, 1-ethoxy-4-methylbenzene, (1-methoxyethyl)benzene, (2-methoxyethyl)benzene, (2-methoxy)ethoxybenzene, 1-methoxy-2-(methoxymethoxy)benzene, 1-(diethoxymethyl)-4-methoxybenzene, propoxybenzene, 1,3-dimethyl-2-propoxybenzene, 1-methoxy-4-propoxybenzene, 1,3-dipropoxybenzene and phenoxyethanol, and the alkali hydroxide can be a substance selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, lithium hydroxide, magnesium hydroxide, ammonium hydroxide, tetraalkylammonium hydroxide and a combination thereof.

[0020] In one embodiment of the present invention, the temperature during the depolymerization may be 10°C to 100°C.

[0021] In addition, a first filtration step for solid-liquid separation of the reaction product after the depolymerization reaction may be additionally performed, and a second filtration step for dissolving the monomer product in water by adding water to the filter cake obtained in the first filtration and performing separation filtration may be additionally performed, and the water used in the second filtration process may contain an alkali hydroxide in an amount equal to or greater than the molar number of the monoalkyl terephthalate present in the monomer product.

[0022] Furthermore, in one embodiment of the present invention, the method may further include: adding an acid to an aqueous solution containing the monomers and precipitating the acid into a salt in order to recover the monomers of the depolymerized polymer.

[0023] Furthermore, the present invention may be characterized in that it further comprises the steps of separating a precipitate generated from the filtrate after the acid is added, and then recovering high-purity terephthalic acid through a drying process.

[0024] In addition, the present invention provides a composition for depolymerization of a polymer containing an ester functional group, characterized in that it contains: (1) an aromatic compound having no proton donor functional group but having one or more alkoxy functional groups; (2) a compound having one or more alcohol functional groups; and (3) an alkali hydroxide.

[0025] In the composition of the present invention, it may be characterized in that the weight ratio of the aromatic compound having no proton donor functional group but having one or more alkoxy functional groups to the compound having one or more alcohol functional groups may be 20:1 to 1:20.

[0026] Technical Effects The present invention provides a method for depolymerizing a polymer containing an ester functional group, which allows adding an alkali hydroxide as a reactant even without using high temperature and high pressure reaction conditions.

[0027] According to the present invention, by applying a reaction solvent mixed with an organic solvent, i.e., a liquid phase mixture composed of an alcohol and an aromatic compound having an alkoxy functional group, to the depolymerization of a polymer containing an ester functional group, the activation energy barrier of the reaction can be lowered and the reaction rate can be greatly increased. Furthermore, the polymer containing an ester functional group can be completely decomposed within 2 hours under low temperature conditions below the boiling point of the constituent solvent and the phthalate monomer can be converted at a high yield.

[0028] According to the present invention, the aromatic compound having an alkoxy functional group used can easily dissolve and separate substances such as dyes and pigments designed to have a strong interaction with the polymer during the monomerization process of the polymer, so that terephthalate monomers can be produced with high purity from colored waste plastics that are currently difficult to recycle.

[0029] The aromatic compound having an alkoxy functional group used in the present invention is a substance that can be synthesized on a large scale by nature or artificial means, and most of them are substances that can be biodegraded in a relatively short period of time in nature, so environmentally friendly working conditions can be provided. In addition, environmentally friendly substances that are harmless or less harmful to the human body or the environment are used as solvents in the depolymerization of polymers, and terephthalic acid, a raw material commonly used for reproducing the same material, can be directly produced by depolymerization of waste plastics, so that an environmentally friendly and economical method for depolymerizing polymers containing ester functional groups, a method for producing regenerated monomers, and a method for designing production processes for the respective purposes can be provided.

[0030] The depolymerization method of the polymer containing ester functional groups according to the present invention is simple in engineering, can achieve depolymerization under relatively low temperature conditions, and can obtain monomers with a high yield, so it can provide a very effective and efficient chemical recycling method for waste plastics. In addition, theoretically, it is possible to achieve industrial production of recycled monomers that can reuse materials indefinitely, so it can contribute to the realization of a plastic recycling economy, and it is possible to continuously combine depolymerization reactions, product purification, and repolymerization, so it is possible to construct a comprehensive chemical engineering that can mass-produce recycled polymers without making a separate secondary investment in existing polymer polymerization equipment or changing equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The schematic diagram illustrates the decomposition reaction pathway of polyethylene terephthalate (PET) by the depolymerization reaction of the present invention.

[0032] Figure 2 It is a schematic diagram illustrating the discharged solvent and terephthalic acid product after depolymerization of a colored polyester polymer using an ethanol-anisole mixed solvent. DETAILED DESCRIPTION

[0033] Unless otherwise defined, the meaning of all technical and scientific terms used in this specification is the same as that commonly understood by a skilled professional in the technical field to which the present invention belongs. Generally speaking, the nomenclature used in this specification is a general nomenclature well-known in the technical field.

[0034] Throughout the present specification, when a certain part or a certain constituent element is described, unless otherwise clearly stated to the contrary, it does not mean that other constituent elements are excluded but that other constituent elements may be included.

[0035] The method for depolymerizing a polymer containing an ester functional group of the present invention is characterized in that the depolymerization is carried out by contacting a mixture containing (1) an aromatic compound having no proton donor functional group but having one or more alkoxy functional groups; (2) a compound having one or more alcohol functional groups; and (3) an alkali hydroxide with a polymer containing an ester functional group. In the mixture, the aromatic compound having an alkoxy functional group in (1) may have an alkoxy functional group but not a proton donor functional group. Examples of the proton donor functional group include a hydroxyl functional group or a carboxylic acid functional group that can provide a proton (H + ) as an example. The compound having an alcohol functional group in (2) can also act as a solvent for dissolving the alkali hydroxide in (3).

[0036] In addition, in the process of obtaining the converted monomers, the present invention can recover and reuse the solvent used in the reaction and the unreacted products (polymers containing ester functional groups and alkali hydroxides) by simple physical separation methods such as filtration, and the solvent with alkoxy functional groups used in the depolymerization can make the strong interaction between the terephthalic acid functional groups and foreign substances ineffective during the separation process, thereby playing the role of an extractant for removing foreign substances. In this way, terephthalic acid with high purity and high yield can be obtained as the final monomer.

[0037] The method of the present invention can be applied to the depolymerization of a polymer containing an ester functional group, wherein the polymer containing an ester functional group can be in a form mixed with other polymers including polyethylene, high-density polyethylene, low-density polyethylene, polypropylene or a combination thereof but not limited thereto, and can be in a form containing a variety of organic / inorganic foreign substances.

[0038] In addition, the polymer containing an ester functional group may be a substance produced by a continuous polycondensation reaction based on dehydration using a dicarboxylic acid and a diol (glycol) as starting materials, or a substance having a plurality of ester bond structures at repeated or arbitrary positions, wherein the dicarboxylic acid may be selected from the group consisting of naphthalene dicarboxylic acid, biphenyl dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenoxy ethane dicarboxylic acid, succinic acid, adipic acid, sebacic acid, azelaic acid, decanedicarboxylic acid, cyclohexane dicarboxylic acid, trimellitic acid, pyromellitic acid, and combinations thereof. The diol can be selected from the group consisting of trimethylene glycol, 1,2-propylene glycol, tetramethylene glycol, neopentyl glycol, hexamethylene glycol, decamethylene glycol, dodecamethylene glycol, 1,4-cyclohexanedimethanol, diethylene glycol, triethylene glycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol, tripropylene glycol, tetrapropylene glycol, polypropylene glycol, di(tetramethylene) glycol, tri(tetramethylene) glycol, polytetramethylene glycol, pentaerythritol, 2,2-bis(4-β-hydroxyethoxyphenyl)propane, and combinations thereof.

[0039] As examples of the polymer containing ester functional groups, it can be selected from polyethylene terephthalate (PET), polyglycolide or polyglycolic acid (PGA), polylactic acid (PLA), polycaprolactone (PCL), polyhydroxyalkanoate (PHA), polyhydroxybutyrate (PHB), polyethylene adipate (PEA), polybutylene succinate (PBS), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), polybutylene terephthalate (PBT), polytrimethylene terephthalate (PTT), polyethylene naphthalate (PEN), Vectran and combinations thereof.

[0040] Preferably, the polymer containing an ester functional group is polyethylene terephthalate, and the starting materials for preparing the polymer may be terephthalic acid or its salt and ethylene glycol.

[0041] The polymer containing an ester functional group used in the present invention may be in an impure state, that is, may contain various impurities. For example, in addition to the polymer containing an ester functional group, it may contain bottle caps, adhesives, paper, residual liquid, dust, organic / inorganic materials for expressing colors (such as dyes or pigments), or a combination thereof, but is not limited thereto, and a mixture of various residues may be used as a raw material for depolymerization.

[0042] In the method for depolymerizing a polymer containing an ester functional group according to the present invention, the mass of the polymer raw material initially input can be adjusted to a ratio of 1 to 100% of the mass of a mixed solvent added for depolymerization, which is a mixture of an aromatic compound having no proton donor functional group but one or more alkoxy functional groups and a compound having one or more alcohol functional groups. However, if too little polymer raw material is input, it may be difficult to ensure its productivity and economy, while if too much is input, the reaction rate may be significantly reduced due to limited material transfer.

[0043] In the method for depolymerizing a polymer containing an ester functional group according to the present invention, the mixed solvent is formed by mixing the (1) aromatic compound having no proton donor functional group but having one or more alkoxy functional groups and the (2) compound having one or more alcohol functional groups. A portion of the compound having an alcohol functional group may participate in the alcohol decomposition reaction, but most of it acts as a reaction medium for the alkaline decomposition reaction. Although the compound having an alkoxy functional group does not directly participate in the depolymerization reaction as a reactant, it is used as a co-solvent that can promote the nucleophilic attack of the alkali oxide or other forms of anions derived therefrom on the ester bond and can exhibit the characteristic of increasing the depolymerization reaction rate.

[0044] The compound having no proton donor functional group but having an alkoxy functional group is an organic compound having at least one aromatic ring and at least one of the hydrogen atoms bonded to the carbon of the hydrocarbon constituting the aromatic ring is bonded to the alkoxy functional group or the carbon connected thereto, and the compound structure does not contain functional groups that can provide protons, such as hydroxyl functional groups and carboxylic acid functional groups. Specifically, it can be selected from methoxybenzene, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, 1,2,3-trimethoxybenzene, 1,2,4-trimethoxybenzene, 1,3,5-trimethoxybenzene, 1,2,3,4-tetramethoxybenzene, 1,2,3,5-tetramethoxybenzene, 1,2,4,5-tetramethoxybenzene, 1-methoxy-2-methylbenzene, 1-methoxy-3-methylbenzene, 1-methoxy-4-methylbenzene, 1-methoxy-2-ethylbenzene, 1-methoxy-3-ethylbenzene, 1 -Methoxy-4-ethylbenzene, 1-methoxy-2-propylbenzene, 1-methoxy-3-propylbenzene, 1-methoxy-4-propylbenzene, ethoxybenzene, 1-ethoxy-2-methoxybenzene, 1-ethoxy-3-methoxybenzene, 1,2-diethoxybenzene, 1,3-diethoxybenzene, 1,4-diethoxybenzene, 1,2,4-triethoxybenzene, 1,3,5-triethoxybenzene, 1-ethoxy-2-methylbenzene, 1-ethoxy-3- One or more compounds selected from the group consisting of methylbenzene, 1-ethoxy-4-methylbenzene, (1-methoxyethyl)benzene, (2-methoxyethyl)benzene, (2-methoxy)ethoxybenzene, 1-methoxy-2-(methoxymethoxy)benzene, 1-(diethoxymethyl)-4-methoxybenzene, propoxybenzene, 1,3-dimethyl-2-propoxybenzene, 1-methoxy-4-propoxybenzene, 1,3-dipropoxybenzene and phenoxyethanol.

[0045] The compound having an alcohol functional group may be a straight-chain primary alcohol, preferably an alcohol having a high solubility in alkali hydroxides such as methanol or ethanol, or a straight-chain primary alcohol having 3 or more carbon atoms, such as one or more alcohols selected from propanol and butanol.

[0046] One of the characteristics of the present invention is that, as a reaction solvent for performing depolymerization in the present invention, a mixed solvent containing at least one selected from the aromatic compounds having no proton donor functional group but having one or more alkoxy functional groups and at least one selected from the compounds having an alcohol functional group is used.

[0047] In the mixed solvent, the weight ratio of the aromatic compound having an alkoxy functional group to the alcohol compound is 1:20 to 20:1, preferably 2:1 to 1:2.

[0048] In the present invention, the alkali hydroxide added for depolymerization can be selected from the group consisting of alkali metal hydroxides, alkaline earth metal hydroxides, ammonium hydroxide, and combinations thereof. For example, it can be selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, lithium hydroxide, magnesium hydroxide, ammonium hydroxide, tetraalkylammonium hydroxide, and combinations thereof. Preferably, the alkali hydroxide can be sodium hydroxide, potassium hydroxide, or combinations thereof.

[0049] The alkali hydroxide may be used in an amount ranging from 0.1 to 100 times, preferably 2 to 50 times, the molar number of the repeating unit of the polymer containing the ester functional group.

[0050] The depolymerization of the polymer containing ester functional groups can be performed at a temperature range of 10°C to 100°C, preferably 30°C to 65°C, and can be performed through an alkaline decomposition reaction pathway in which an alkali hydroxide is involved as part of the reactants and thereby the polymer is quickly decomposed into monomers.

[0051] The depolymerization of the polymer containing ester functional groups can be performed in a heat-insulated reactor without inputting an additional heat source, only by the mixing heat and dissolution heat generated in the process of preparing the mixed solution for the depolymerization reaction. In addition, in some specific examples according to the present invention, the depolymerization can also be performed while supplying an external heat source.

[0052] In addition, the depolymerization method of the polymer of the present invention performs depolymerization below the boiling point of the applicable alcohol, so positive pressure may not be generated. However, when a portion of a low-boiling point alcohol solvent is used, the reaction may be performed at a lower absolute pressure of 1.0 atmosphere to 1.5 atmosphere.

[0053] In the method for depolymerizing a polymer containing an ester functional group according to the present invention, the events of the depolymerization reaction may vary depending on the amount and form of the applied polymer, but when the composition and conditions of the reactants are applied in order to quickly perform the depolymerization, the dominant polymer decomposition will occur within 2 hours of the initial reaction time by means of an extremely fast depolymerization reaction rate, and after 4 hours of the reaction, almost most of the initial polymer mass can be decomposed into a monomer form. When the depolymerization reaction time according to the present invention is adjusted, the depolymerization can be performed for 0.5 to 12 hours after the start of the reaction in order to achieve sufficient polymer decomposition, but in order to ensure economic efficiency and productivity, the depolymerization can be preferably performed within the range of 0.5 to 4 hours.

[0054] In the method for depolymerizing a polymer containing an ester functional group according to the present invention, a first filtering step for solid-liquid separation of the reaction product after the depolymerization reaction is completed may be additionally performed. The substances existing in the solid state in the reaction product may be, for example, unreacted polymers and terephthalic acid alkali salts generated by depolymerization, and the substances existing in the liquid state may be a reaction solvent in which an aromatic compound having no proton donor functional group but having one or more alkoxy functional groups and a compound having one or more alcohol functional groups are mixed with each other, unreacted alkali hydroxide dissolved in the reaction solvent, and foreign matter in the reaction solvent that is colored mainly due to the aromatic compound having an alkoxy functional group.

[0055] In the first filtration step, all or part of the colored hydrophobic organic foreign matter contained in the polymer before decomposition has a high affinity with the aromatic compound having an alkoxy functional group in the reaction solvent recovered through the filtrate and is discarded together with the filtrate, so the residual amount of the hydrophobic organic foreign matter in the filter cake is greatly reduced.

[0056] The solution containing colored organic foreign matter separated by the filtrate can be reused in the depolymerization reaction by additionally performing a purification process such as distillation or evaporation to separate the purified reaction solvent, and can be reused in the depolymerization reaction as a new reaction raw material in order to replenish the unreacted alkali hydroxide and reaction solvent consumed or lost in the depolymerization reaction and perform depolymerization after removing the foreign matter by adsorption or the like or without removing the foreign matter.

[0057] In the first filtering process, since the filter cake separated in a solid state is composed of a mixture of terephthalic acid alkali salt, monoalkyl terephthalate alkali salt and unreacted polymer produced by the depolymerization reaction, it is necessary to recover the terephthalate from the filter cake.

[0058] The intermediates for producing monomers, i.e., terephthalic acid alkali salts and monoalkyl terephthalate alkali salts, are soluble in water, and thus can be filtered in situ by adding water to the filter cake after the first filtration, or can be dissolved using a separate external filter and then passed through a filter to separate the terephthalic acid alkali salts and monoalkyl terephthalate alkali salts from the filter cake in a solid state, and then the terephthalate can be recovered by a second filtration. In the second filtration process, since the unreacted polymer substance is separated from the filter cake, it can also be re-introduced as a new reaction raw material for the depolymerization process.

[0059] In the case where the product produced by the depolymerization reaction contains monoalkyl terephthalate, it will also be present in the filtrate of the second filtration process. The alkyl group in the monoalkyl terephthalate can be replaced by an alkali salt again, thereby finally producing an alkali terephthalate. For this purpose, the process may further include a step of converting the alkali terephthalate alkali salt into the alkali terephthalate alkali salt by adding an alkali hydroxide in an amount of moles equal to the moles of the monoalkyl terephthalate to the water used in the second filtration process or the filtrate after filtration.

[0060] At this time, the amount of the alkali hydroxide used may exceed the molar number of the monoalkyl terephthalate alkali salt, and preferably 1.0 to 1.2 times of its equivalent can be added. However, since the decomposition reaction of the monoalkyl terephthalate alkali salt is fast and proceeds in an irreversible form, in order to reduce the waste of materials, it is best to add 1.0 times of its equivalent.

[0061] In the process of converting the monoalkyl terephthalate alkali salt into the terephthalate alkali salt, the aqueous solution added with the alkali hydroxide is fully mixed at a temperature above room temperature for 5 minutes to 30 minutes, so that all the monomer products generated during the depolymerization of the polymer are converted into the terephthalate alkali salt.

[0062] In the filtering process, the product obtained through the filtrate is an aqueous solution mainly composed of components that are easily soluble in water (such as alcohol and terephthalic acid alkali salts, etc.), and may contain a part of the colored organic foreign matter. In order to additionally separate the organic foreign matter from the filtrate, an aromatic compound having an alkoxy functional group can be added to the filtrate to induce the organic foreign matter to be distributed to the organic phase having an alkoxy functional group, thereby separating the colored organic foreign matter again. At this time, when the aromatic compound having an alkoxy functional group is further added to the filtrate, a thermodynamically unstable phase may be generated. In order to ensure that the foreign matter can be redistributed smoothly, the organic phase and the aqueous solution phase can be induced to separate after a short period of stirring within 30 minutes, and because the organic foreign matter is concentrated in the separated organic phase, the organic foreign matter can be additionally separated.

[0063] The present invention may further include a step of physically separating the terephthalic acid by adding an acid to an aqueous solution containing the product in the form of an alkali salt of terephthalic acid obtained in the separation process of the depolymerization reaction product to convert and precipitate terephthalic acid. After terephthalic acid is separated from the produced aqueous solution (e.g., by filtration), the purity of the terephthalic acid may be further improved by washing and drying processes.

[0064] As the acid added to convert the terephthalate into terephthalic acid, an organic acid or an inorganic acid can be used, but preferably, an acid that is easily ionized in an aqueous solution phase and provides protons (H + ) is an inorganic acid consisting of one or more selected from the group consisting of hydrochloric acid (HCl), nitric acid (HNO3), phosphoric acid (H3PO4) and sulfuric acid (H2SO4).

[0065] In addition, even when the monomer product is not filtered, a solid product with a relatively high density can be separated from the reaction mixture by applying other forms of physical separation methods such as centrifugation.

[0066] All or part of the final monomer produced by the method for depolymerizing a polymer containing ester functional groups according to the present invention has been purified to a level that can be re-introduced as a raw material for re-polymerization.

[0067] At this time, the monomer obtained by depolymerization of the polymer containing an ester functional group of the present invention can be used as a polymerization raw material for synthesizing a polymer, and the monomer may contain less than about 1% impurities (w / w), and the impurities may include different types of organic matter or catalysts such as isophthalic acid, phthalic acid, 4-methylbenzoic acid, and 4-formylbenzoic acid, or one or more of the metal foreign matter incidentally generated during the manufacturing process.

[0068] In addition, the present invention provides a method for producing a final monomer product with improved purity by further removing the colored foreign matter as described above.

[0069] As colored foreign substances contained in colored polymers containing ester functional groups, dyes or pigments are mostly designed to have strong interactions with terephthalic acid functional groups, and they form complexes with terephthalic acid functional groups during purification and recovery and move as a structure, so it may be difficult to remove them only by general purification methods. That is, the interaction (π-π interaction or hydrogen bonding force) between the compound having an alkoxy functional group and foreign substances such as dyes or pigments as described above is only effective in weakening the bonding force between foreign substances and terephthalic acid, but in order to more effectively and irreversibly separate foreign substances from the product, an adsorbent having a strong bonding force to the foreign substances may be more effective.

[0070] Therefore, the present invention provides a method for producing a monomer product by depolymerizing a colored polymer by depolymerizing a colored polymer compound including an ester functional group, further comprising a step of treating colored foreign matter using the adsorbent.

[0071] Since the irreversible separation of foreign matter using an adsorbent can be most effectively applied when the terephthalic acid functional groups are dissociated in the aqueous solution phase in the form of ions, the adsorbent can be applied optimally to the aqueous filtrate before the metal terephthalate salt is converted to terephthalic acid by adding an acid.

[0072] The adsorbent may include carbon adsorbents such as activated carbon, fly ash, ion exchange resin, natural clay, bentonite, zeolite, kaolin, chitosan, and metal-organic framework (MOF), and preferably activated carbon may be used.

[0073] The present invention also provides a depolymerization composition for depolymerizing a polymer containing an ester functional group, comprising (1) an aromatic compound having no proton donor functional group but having one or more alkoxy functional groups; (2) a compound having one or more alcohol functional groups; and (3) an alkali hydroxide.

[0074] The components of the depolymerization composition are as described above, and thus repeated description will be omitted.

[0075] Next, the detailed information of the process of the present invention will be described through examples, comparative examples and test examples. It should be noted that these descriptions are only representative examples related to the present invention, and the scope of application of the present invention is not limited thereby.

[0076] Raw material 1 (transparent waste polyethylene terephthalate (PET) flakes containing ester functional groups) As the waste polymer material containing an ester functional group, flakes produced by washing and cutting bottles of a transparent polyethylene terephthalate (PET) material discarded after use supplied by Saerom ENG, a recycling product manufacturer, were used. After washing and drying with excess ethanol and water, only flakes with an area of ​​less than 1 square centimeter were selected as raw material 1.

[0077] Raw material 2 (colored waste polyethylene terephthalate (PET) flakes containing ester functional groups) As the waste polymer material containing ester functional groups, flakes produced by washing and cutting bottles (including colored) of polyethylene terephthalate (PET) materials discarded after use supplied by the Korea Plastic Monomaterial Association were used, and after washing and drying with excess ethanol and water, only flakes with an area of ​​less than 1 square centimeter were selected as raw material 2. Among all the plastic scrap raw materials, the mass ratio of colored ones was about 56.8%, and the mass ratio of transparent ones was about 43.2%.

[0078] Raw material 3 (colored composite polymer containing ester functional groups) As the raw material of the depolymerized colored composite material, waste polymer materials in the form of hair, which are discarded as scraps after the material is manufactured, supplied by Joohyun Chemical, are used. A black filament-like polymer material containing an ester functional group, which is mixed with about 63.8% polyethylene terephthalate (PET), about 35.4% polytrimethylene terephthalate (PBT), and carbon black (0.8%) as a coloring material, is cut according to a weight ratio, and the material is cut into single strips not exceeding 2 cm in length to prepare Raw Material 3.

[0079] Raw material 4 (colored composite polymer containing ester functional groups) A waste polymer material in the form of hair discarded as scraps is cut according to a weight ratio of about 26.1% of polyethylene terephthalate (PET), about 73.1% of polytrimethylene terephthalate (PBT), and a black filamentous polymer material containing ester functional groups mixed with carbon black (0.8%) as a coloring material, and the material is cut into single strips not exceeding 2 cm in length as raw material 4.

[0080] Comparative Example 1: (a) Depolymerization of polymers containing ester functional groups About 15.0 g of raw material 1 (transparent polyethylene terephthalate (PET) flakes) was used as a raw material for depolymerization, and anhydrous ethanol was used as a reaction solvent. After dissolving by adding 12.49 g of sodium hydroxide (NaOH) (4 times the molar number of the repeating unit of the polyethylene terephthalate (PET) raw material) to 120 g of ethanol, it was put into a 500 mL three-necked flask in which a thermocouple for temperature measurement and a Teflon diaphragm for liquid phase sample collection were connected to the side inlet through a flange-type adapter, and a Teflon seal was connected to the middle inlet in a manner that a stirring rod equipped with an impeller that can be rotated by an external overhead stirrer passed through the center, and then placed in a thermostatic bath that can be maintained at a constant temperature by a proportional-integral-differential (PID) temperature controller, and the mixture was stirred at a speed of 300 rpm when the temperature inside the reactor reached 60°C. When the temperature inside the reactor became stable, about 15.0 g of the prepared polymer raw material was added to start the reaction.

[0081] (b) Quantitative analysis of depolymerization products 100 mg of liquid samples were collected every 30 minutes or 1 hour, diluted with tetrahydrofuran (THF) and a mobile phase solution (a solution of methanol and water in a volume ratio of 70:30), and then the concentration of each product was quantified using a high performance liquid chromatograph (High Performance Liquid Chromatography; HPLC, Younglin YL 3000 with C18 column (250×4.5mm) and UV detector (λ=254nm)) calibrated with standard samples in advance. When performing high performance liquid chromatography (HPLC) analysis, a mixed solution of methanol: water in a volume ratio of 70:30 was used as the mobile phase, and the total flow rate was maintained at 0.7 ml / min.

[0082] The conversion rate of polyethylene terephthalate (PET) obtained by the depolymerization reaction and the yields of the intermediate monomers, sodium mono-ethyl terephthalate (Na-MET) and sodium terephthalate (Na2-TPA), were calculated using the following general formula.

[0083] Polyethylene terephthalate (PET) conversion rate = (M o - M) / M o (Formula 1) Monoethyl terephthalate (MAT) alkali salt yield = (N MAT / N o ) × 100% (Formula 2) Terephthalic acid (TPA) alkali salt yield = (N TPA / N o ) × 100% (Formula 3) In the above formula, M o is the mass of the initial polymer input, M is the mass of the unreacted polymer, N o N is the number of moles of repeating units of the initial raw material polymer (polyethylene terephthalate (PET)), MAT and N TPA is the number of moles of monoalkyl terephthalate alkali salt (e.g., Na-MET) and terephthalic acid alkali salt (e.g., Na2-TPA) generated.

[0084] (c) Separation and purification of monomer products After reaching the target reaction time, the reactants were immediately moved to a vacuum filter and filtered for the first time. After the filtration was completed, the flask container containing the filtrate was replaced with an empty container and filtered for the second time. In the second filtration, unreacted polyethylene terephthalate (PET) was recovered as a solid substance (filter cake) by adding 200 ml of distilled water. The unreacted polyethylene terephthalate (PET) was placed in an evaporating dish and moved to a vacuum dryer maintained at 80°C, and then quantified after drying for 24 hours. After stirring the obtained second filtrate for 30 minutes, all monomers were converted into terephthalic acid by adding an equivalent amount of hydrochloric acid aqueous solution to the filtrate, and then stored at below 4°C for 12 hours, the precipitated crystals were washed and filtered, and then placed in an evaporating dish and moved to a vacuum dryer maintained at 80°C for 24 hours to obtain about 12.30g of a white product.

[0085] (d) Characteristic analysis of monomer products The obtained product was analyzed by hydrogen nuclear magnetic resonance spectroscopy ( 1 H-NMR) and electron impact mass spectrometry (EI-MS) showed that the chemical structure was consistent with that of terephthalic acid, and the purity of the product was determined by calibrated high performance liquid chromatography (HPLC). After a portion of the obtained product was made into a solid sample, the inherent color was observed using a spectrophotometer (manufacturer: Konica Minolta, model: CM-3600A). After collecting approximately 200 mg of the final terephthalic acid, a 13 mm diameter disc was produced using a pelletizing die (PikeEvacuable KBr Die Kit) and a hydraulic press (Pike CrushIR) (applicable pressure: 10 tons). For the produced disc sample, the color space coordinate value (L a b ) was measured. a The value represented by b is the coordinate value of the color space standardized by the International Commission on Illumination (CIE), L To represent lightness, a value from 0 (black) to 100 (white) is used. and b The values ​​are expressed according to the complementary color axes of green (-128) / red (127) and blue (-128) / yellow (127).

[0086] Comparative Example 2: Except that 180 g of ethanol was used as the solvent in the reaction, the depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Comparative Example 1. Approximately 12.39 g of terephthalic acid was finally obtained through the monomer production process.

[0087] Embodiment 1: Depolymerization reaction of polyethylene terephthalate (PET) and quantitative analysis of the product were performed in the same manner and procedure as in Comparative Example 1, except that 90 g of ethanol and 30 g of anisole were mixed instead of ethanol as the solvent fed into the reaction.

[0088] After the depolymerization reaction was completed, a trace amount of the reaction intermediate Na-MET was detected in the product. Then, a solution of sodium hydroxide (41 mg in this example) dissolved in 100 ml of distilled water was added to the filter cake, which was equivalent to the molar number of Na-MET detected in the "(c) Separation and purification of monomer products" (in this example, 0.013 moles of Na-MET were generated compared to the molar number of repeating units of the initial polymer). Then, 100 ml of distilled water was added and a second filtration was performed. Then, after stirring the produced filtrate for 30 minutes, an equivalent amount of hydrochloric acid aqueous solution was added to the filtrate to convert all monomers into terephthalic acid. The produced filtrate was recrystallized, washed and dried in the same manner as in Comparative Example 1, thereby obtaining about 12.37 g of a white product. Next, the product was analyzed in the same manner and steps as in Comparative Example 1.

[0089] Embodiment 2: Except that 90 g of ethanol and 90 g of anisole were mixed and used as the solvents put into the reaction, the depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product and characteristic analysis were carried out according to the same method and steps as in Example 1. Approximately 12.39 g of terephthalic acid was finally obtained through the monomer production process.

[0090] Embodiment 3: Except that 90 g of ethanol and 180 g of anisole were mixed as solvents put into the reaction, the depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product and characteristic analysis were carried out according to the same method and steps as in Example 1. Approximately 12.56 g of terephthalic acid was finally obtained through the monomer production process.

[0091] Comparative Example 3: The depolymerization reaction and the quantitative analysis of the depolymerization product were performed in the same manner and steps as in Comparative Example 1, except that only 120 g of anisole was used as the solvent fed into the reaction. Polyethylene terephthalate (PET) was hardly decomposed, and terephthalic acid was hardly obtained when the reactant was purified.

[0092] Embodiment 4: Except that 90 g of methanol instead of ethanol and 90 g of anisole were used as the solvent put into the reaction, the depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product and characteristic analysis were carried out according to the same method and steps as in Example 1. Approximately 12.48 g of terephthalic acid was finally obtained through the monomer production process.

[0093] Comparative Example 4: The depolymerization reaction and the quantitative analysis of the depolymerization product were performed in the same manner and steps as in Example 1, except that sodium hydroxide was not added to the reaction. Polyethylene terephthalate (PET) was hardly decomposed, and terephthalic acid was hardly obtained when the reactants were purified.

[0094] Embodiment 5: The depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Example 1, except that the amount of sodium hydroxide (NaOH) added to the reaction was increased to 1.56 g so as to reach a molar ratio of 0.5 to the number of moles of the repeating unit of the polymer. Approximately 5.18 g of terephthalic acid was finally obtained through the monomer production process.

[0095] Embodiment 6: The depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Example 1, except that the amount of sodium hydroxide (NaOH) added to the reaction was increased to 3.12 g so that the molar ratio to the number of moles of the repeating unit of the polymer reached 1.0. Approximately 9.96 g of terephthalic acid was finally obtained through the monomer production process.

[0096] Embodiment 7: The depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Example 1, except that the amount of sodium hydroxide (NaOH) added to the reaction was increased to 6.24 g so as to reach a molar ratio (equivalent) of 2.0 compared to the molar number of the repeating unit of the polymer. Approximately 12.44 g of terephthalic acid was finally obtained through the monomer production process.

[0097] Embodiment 8: The depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Example 1, except that 37.5 g of ethanol and 30 g of anisole were mixed as the solvents fed into the reaction and the amount of sodium hydroxide (NaOH) fed into the reaction was increased to 6.24 g so as to reach a molar ratio (equivalent) of 2.0 compared to the molar number of the repeating units of the polymer. Approximately 12.64 g of terephthalic acid was finally obtained through the monomer production process.

[0098] Embodiment 9: The depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Example 1, except that 37.5 g of ethanol and 30 g of anisole were mixed as the solvent fed into the reaction, and 8.74 g of potassium hydroxide (KOH) was used instead of sodium hydroxide (NaOH) as the alkali hydroxide fed into the reaction to achieve a molar ratio (equivalent) of 2.0 compared to the molar number of the repeating units of the polymer. Approximately 12.70 g of terephthalic acid was finally obtained through the monomer production process.

[0099] Embodiment 10: Except that the temperature of the depolymerization reaction was maintained at 30° C., the depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Example 2. The reaction was performed for more than 6 hours in total, and approximately 10.32 g of terephthalic acid was finally obtained through the monomer production process.

[0100] Embodiment 11: Except that the depolymerization reaction temperature was maintained at a temperature close to the boiling point of ethanol (78°C) and was performed under reflux conditions, the depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Example 2. Approximately 12.22 g of terephthalic acid was finally obtained through the monomer production process.

[0101] Embodiment 12: The depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Example 2, except that 90 g of ethanol and 90 g of 1,2-dimethoxybenzene (DMB) were mixed and used instead of anisole as the solvents put into the reaction. Approximately 12.34 g of terephthalic acid was finally obtained through the monomer production process.

[0102] Embodiment 13: The depolymerization reaction, quantitative analysis of the depolymerization product, purification and characteristic analysis of the monomer product were performed in the same manner and steps as in Example 2, except that 90 g of ethanol and 90 g of phenetoleor ethoxy benzene were mixed and used instead of anisole as the solvents put into the reaction. Approximately 12.07 g of terephthalic acid was finally obtained through the monomer production process.

[0103] Comparative Example 5: The depolymerization reaction, quantitative analysis of the depolymerization product, purification and property analysis of the monomer product were performed in the same manner and steps as in Example 2, except that 90 g of ethanol and 90 g of guaiacol 2-methoxyphenol were mixed and used instead of anisole as the solvents put into the reaction. Approximately 0.49 g of a trace solid component including terephthalic acid was finally obtained through the monomer production process.

[0104] Comparative Example 6: Except that about 15.0 g of raw material 2 (colored waste polyethylene terephthalate (PET) flakes) was used instead of raw material 1 (transparent polyethylene terephthalate (PET) flakes) as the raw material for depolymerization, the depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Comparative Example 2. Approximately 10.96 g of dark terephthalic acid was finally obtained through the monomer production process.

[0105] Embodiment 14: The depolymerization reaction and the quantitative analysis of the product were performed in the same manner and steps as in Example 2, except that about 15.0 g of Raw Material 2 (colored waste polyethylene terephthalate (PET) flake scraps) was used instead of Raw Material 1 (transparent polyethylene terephthalate (PET) flake scraps) as the raw material for depolymerization.

[0106] After the depolymerization reaction was completed, 10 g of anisole was added to the filtrate produced by performing the second filtration in the process of "(c) Separation and purification of monomer products" and stirred for 30 minutes. Next, anisole and the organic layer containing concentrated colored foreign matter were removed and an equivalent amount of hydrochloric acid aqueous solution or more was added to the aqueous solution phase to convert all monomers into terephthalic acid. The produced filtrate was recrystallized, washed and dried in the same manner as in Comparative Example 1, thereby obtaining about 12.14 g of a nearly white product. Next, the product was analyzed in the same manner and steps as in Comparative Example 1.

[0107] Comparative Example 7: Except that about 15.0 g of raw material 3 (colored composite polymer) was used instead of raw material 1 (transparent waste polyethylene terephthalate (PET) flakes) as the raw material for depolymerization, the depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Comparative Example 2. Approximately 11.91 g of dark terephthalic acid was finally obtained through the monomer production process.

[0108] Embodiment 15: The depolymerization reaction and the quantitative analysis of the product were performed in the same manner and steps as in Example 2, except that about 15.0 g of raw material 3 (colored composite polymer) was used instead of raw material 1 (transparent polyethylene terephthalate (PET) flakes) as the raw material for depolymerization.

[0109] After the depolymerization reaction was completed, 10 g of anisole was added to the filtrate produced by performing the second filtration in the process of "(c) Separation and purification of monomer products" and stirred for 30 minutes. Next, anisole and the organic layer containing concentrated colored foreign matter were removed and an equivalent amount of hydrochloric acid aqueous solution or more was added to the aqueous solution phase to convert all monomers into terephthalic acid. The produced filtrate was recrystallized, washed and dried in the same manner as in Comparative Example 1, thereby obtaining about 12.04 g of a nearly white product. Next, the product was analyzed in the same manner and steps as in Comparative Example 1.

[0110] Comparative Example 8: Except that about 15.0 g of raw material 4 (colored composite polymer) was used instead of raw material 1 (transparent waste polyethylene terephthalate (PET) flakes) as the raw material for depolymerization, the depolymerization reaction, quantitative analysis of the depolymerization product, purification of the monomer product, and characteristic analysis were performed in the same manner and steps as in Comparative Example 2. Approximately 10.86 g of dark terephthalic acid was finally obtained through the monomer production process.

[0111] Embodiment 16: The depolymerization reaction and the quantitative analysis of the product were performed in the same manner and steps as in Example 2, except that about 15.0 g of raw material 4 (colored composite polymer) was used instead of raw material 1 (transparent polyethylene terephthalate (PET) flakes) as the raw material for depolymerization.

[0112] After the depolymerization reaction was completed, 10 g of anisole was added to the filtrate produced by performing the second filtration in the process of "(c) Separation and purification of monomer products" and stirred for 30 minutes. Next, anisole and the organic layer containing concentrated colored foreign matter were removed and an equivalent amount of hydrochloric acid aqueous solution or more was added to the aqueous solution phase to convert all monomers into terephthalic acid. The produced filtrate was recrystallized, washed and dried in the same manner as in Comparative Example 1, thereby obtaining approximately 11.04 g of a nearly white product. Next, the product was analyzed in the same manner and steps as in Comparative Example 1.

[0113] Embodiment 17: The depolymerization reaction and quantitative analysis of the product were performed in the same manner and steps as in Example 14. 10 g of anisole was added to the filtrate produced by the second filtration in the process of "(c) separation and purification of monomer products" and stirred for 30 minutes. Next, the aqueous solution obtained by removing the anisole and concentrating the organic layer with colored foreign matter was passed through an activated carbon layer. An equivalent amount of hydrochloric acid aqueous solution was added to the aqueous solution phase after passing through the activated carbon layer to convert all monomers into terephthalic acid. The produced filtrate was recrystallized, washed and dried in the same manner as in Comparative Example 1, thereby obtaining approximately 12.06 g of a nearly pure white product. Next, the product was analyzed in the same manner and steps as in Comparative Example 1.

[0114] Embodiment 18: The depolymerization reaction and quantitative analysis of the product were performed in the same manner and steps as in Example 15. 10 g of anisole was added to the filtrate produced by the second filtration in the process of "(c) separation and purification of monomer products" and stirred for 30 minutes. Next, the aqueous solution obtained by removing the anisole and concentrating the organic layer with colored foreign matter was passed through an activated carbon layer. An equivalent amount of hydrochloric acid aqueous solution was added to the aqueous solution phase after passing through the activated carbon layer to convert all monomers into terephthalic acid. The produced filtrate was recrystallized, washed and dried in the same manner as in Comparative Example 1, thereby obtaining approximately 12.00 g of a nearly pure white product. Next, the product was analyzed in the same manner and steps as in Comparative Example 1.

[0115] Embodiment 19: The depolymerization reaction and quantitative analysis of the product were performed in the same manner and steps as in Example 16. 10 g of anisole was added to the filtrate produced by the second filtration in the process of "(c) separation and purification of monomer products" and stirred for 30 minutes. Next, the aqueous solution obtained by removing the anisole and concentrating the organic layer with colored foreign matter was passed through an activated carbon layer. An equivalent amount of hydrochloric acid aqueous solution was added to the aqueous solution phase after passing through the activated carbon layer to convert all monomers into terephthalic acid. The produced filtrate was recrystallized, washed and dried in the same manner as in Comparative Example 1 to obtain approximately 10.98 g of a nearly pure white product. Next, the product was analyzed in the same manner and steps as in Comparative Example 1.

[0116] In the prior patent document (Korean Patent No. 10-0983349), an example of an alkali decomposition reaction in which multiple types of alcohols with different carbon numbers are used as a single reaction solvent is proposed. Generally speaking, the solubility of alkali hydroxides in organic solvents is low, but as described in the prior document, when a large amount of alcohol is used as a reaction solvent, the excess alkali hydroxide used for polymer decomposition can be uniformly dissolved. However, if only alcohol is used as a reaction solvent, the depolymerization of polymers containing ester functional groups has only limited reactivity, and in order to increase the decomposition rate of the polymer or increase the conversion rate, the reaction temperature needs to be maintained above the boiling point of the alcohol used. Therefore, the internal pressure of the reactor may be maintained at a level higher than the atmospheric pressure during the depolymerization, so it is necessary to design a reaction equipment with drug resistance and airtightness.

[0117] In another prior patent document (Korean Patent No. 10-2223614), as a method for increasing the reaction rate or performing depolymerization under low temperature conditions, an alkali depolymerization method using a mixed reaction solvent composed of a polar aprotic solvent containing a keto group, a nitrile group or a furan group is disclosed. The polar aprotic solvent described in the corresponding prior document can play the role of a co-solvent that relieves the ester functional group and promotes the transesterification reaction through interaction with the polymer matrix, thereby increasing the decomposition rate. However, when the co-solvent having a keto group in the corresponding prior document is present and an excessive amount of alkali hydroxide is added as a reactant, reaction heat is generated or heating is applied from the outside, colored organic foreign matter is generated due to the aldol condensation reaction of the polar aprotic solvent molecules, which adversely affects the purity and quality of the terephthalic acid finally produced, and also causes the problem of the polar aprotic solvent being consumed. Furthermore, in order to obtain high-purity terephthalic acid, the monomer product generated by depolymerization, an additional purification process requiring high energy consumption may be required, and product loss may occur during the high-purification process, which may adversely affect the final yield of the product.

[0118] In addition, in the existing current documents, examples for improving the performance of the depolymerization reaction of the polymer containing an ester functional group or the yield of the monomer product are proposed, but most of them do not record methods or examples for improving the quality of the product. However, in the field of recycling of polymers using related technologies, raw materials containing organic and inorganic foreign matter are generally supplied, and because impurities remaining in the manufactured monomer (terephthalic acid) have a significant impact on the quality of the re-synthesized material, it may be necessary to perform a multi-stage purification process that consumes a high amount of energy.

[0119] The present invention can form a reaction solvent by mixing an aromatic compound having an alkoxy functional group and a compound having an alcohol functional group, thereby improving the performance of the depolymerization reaction and the quality of the final monomer product by means of a reaction and purification process that is completely different from the currently known method of producing terephthalic acid by alkali decomposition of polyethylene terephthalate (PET).

[0120]

Table 1

[0121] Table 1 is the result of comparative observation of the alkaline hydrolysis reaction (reaction temperature: 60°C) characteristics of polyethylene terephthalate (PET) according to the composition of the reaction solvent. When all the depolymerization products implemented in the present invention were analyzed, no compounds in the form of incomplete decomposition such as dimers or oligomers were observed in the product distribution. In addition, under all the conditions of the present invention, no monomers such as mono(hydroxyethyl)methyl terephthalate (HEMT) or bis(hydroxyethyl)terephthalate (BHET) were observed that formed ester bonds with ethylene glycol remaining in the terephthalic acid structure due to a low degree of decomposition or alkylene functional groups formed by the recombination of ethylene glycol released in the ester exchange reaction. Based on this, it can be predicted that in the depolymerization of polyethylene terephthalate (PET) using alkali hydroxides and alcohols as reaction solvents, it is in accordance with Figure 1 The two reaction pathways in the decomposition.

[0122] During the depolymerization of polymers containing ester functional groups, a base decomposition reaction occurs ( Figure 1 In the reaction pathway A in the reaction, the final monomer, terephthalic acid, is mainly produced in the form of an alkali salt from the early stage of the reaction. In some examples of the reaction, because alcohol is used as the reaction solvent, a partial alcohol decomposition reaction also occurs with the alkali hydroxide acting as a catalyst ( Figure 1 The reaction pathway B) was observed in Figure 2, whereby monoalkyl terephthalate base salts were observed as reaction intermediates.

[0123] In a prior document (Korean Patent No. 10-0983349), a method is disclosed for decomposing a polymer containing an ester functional group through an alcohol decomposition reaction pathway by adding an excess of alkali hydroxide and a primary alcohol to a reactant to thereby obtain a terephthalic acid product. Similar to the prior document, in Comparative Example 1, an excess of sodium hydroxide of 4 times the molar number of repeating units of the polymer containing an ester functional group is added as a reactant, and a large amount of alcohol equivalent to 8 times the weight of the polymer is added as a reaction solvent. In the early stage of the reaction, a rapid alkali decomposition reaction of polyethylene terephthalate (PET) occurs, so that its conversion rate exceeds about 84% within 2 hours of the reaction, but as part of the alkali hydroxide participates in the reaction, its concentration decreases and the reaction rate gradually slows down, so that it is still not completely decomposed after 6 hours of reaction.

[0124] In Comparative Example 2, in which the amount of alcohol added was increased by 1.5 times compared to Comparative Example 1, although the same molar number of alkali hydroxide was added to the reaction and the concentration was diluted, relatively rapid polymer decomposition was still observed. The alkali decomposition reaction and a part of the alcohol decomposition reaction occurred simultaneously within 2 hours of the reaction, and after 4 hours of the reaction, all products that may be generated when the polymer is depolymerized were converted into terephthalic acid alkali salt products generated when the alkali decomposition continued.

[0125] The results of Comparative Examples 1 and 2 confirm that the depolymerization reaction rate of the polymer containing an ester functional group can be increased by increasing the amount of the added alcohol solvent. However, in order to increase the depolymerization reaction rate, it is necessary to add a considerable excess of alcohol compared to the input polymer raw material. In this case, the volume of the reactor will become larger than the processing volume and more heat will be consumed in order to maintain the reaction temperature, so it may be necessary to invest higher investment costs in engineering equipment and higher operating costs.

[0126] Anisole is a type of co-solvent added to illustrate the effect of the present invention, and has the simplest structure among aromatic compounds with alkoxy functional groups. Example 1 is a case where a depolymerization reaction is performed after replacing a portion of the weight of the alcohol used in Comparative Example 1 with anisole. As a result of applying the co-solvent according to the present invention, most of the polymer decomposes at the beginning of the reaction, so that the conversion rate reaches 98.4% after 2 hours of reaction time, and almost all of the polyethylene terephthalate (PET) decomposes after 4 hours of reaction. By replacing only 1 / 3 of the ethanol in the composition with anisole on the basis of Comparative Example 1, which is performed with only ethanol, significantly higher reactivity is shown, and the monomer produced by depolymerization also shows a very high yield.

[0127] Examples 2 and 3 are the results of increasing the amount of anisole added. When the amount of anisole added is increased to be greater than the weight of ethanol, complete decomposition of polyethylene terephthalate (PET) can be rapidly induced from the initial stage of the reaction, and a terephthalic acid yield close to the ideal value can be achieved.

[0128] Comparative Example 3 is the result of a depolymerization test performed to confirm whether an alcohol decomposition reaction of polyethylene terephthalate (PET) occurs when anisole is used alone without using alcohol. The alkali hydroxide (here, sodium hydroxide) added as a reactant has a lower solubility in organic solvents than anisole, so it will remain in the reaction mixture in a solid phase when no alcohol is added, and may not be in uniform visual contact with the applied polymer. Referring to the results of Comparative Example 3, it can be confirmed that the depolymerization reactivity of the polymer containing ester functional groups is almost not observed from the initial stage of the reaction. Even when exposed to the same reaction conditions as the previous example and left for a long time (more than 4 hours), only a very small amount of monomer products were detected in the reaction mixture.

[0129] Example 4 is the result of maintaining the same reactant composition as Example 2, but using methanol as the type of alcohol added. In the early stage of the reaction, a high concentration of the reaction intermediate product, sodium monomethyl terephthalate (Na-MMT), was observed due to the faster reaction rate of the methanol decomposition reaction, but when exposed to the depolymerization reaction conditions for more than 2 hours, almost all of the polyethylene terephthalate (PET) was decomposed and converted into the final product of the alcohol decomposition reaction, namely the terephthalic acid base salt.

[0130]

Table 2

[0131] Table 2 shows the results of depolymerization of polyethylene terephthalate (PET) (reaction temperature: 60°C) according to the initial concentration of alkali hydroxide. The changes in the amount of products produced after exposure to depolymerization conditions for 2 hours were observed in the range of 0.0 mol to 4.0 mol of sodium hydroxide (NaOH) relative to the number of moles of polymer repeating units.

[0132] In Comparative Example 4, in which no alkali hydroxide was added, since there was neither an alkali reactant for alkali decomposition nor a material that could act as a catalyst for alcohol decomposition, as expected, almost no decomposition of polyethylene terephthalate (PET) occurred. The number of moles (equivalent ratio) required to be added in order to achieve complete alkali decomposition by depolymerization of the polymer was equivalent to 2.0 moles compared to the number of moles of repeating units. In Example 5 (0.5 mole of NaOH) and Example 6 (1.0 mole of NaOH), in which the amount added was less than the value, the concentration of sodium monoethyl terephthalate (Na-MET) equivalent to the reaction intermediate observed in the product after 2 hours of depolymerization reaction was significantly higher than that of sodium terephthalate (Na2-TPA). This indicates that in the early stage of depolymerization, a portion of the alcohol decomposition ( Figure 1The decomposition reaction of the polymer induced by reaction pathway A) occupies a more dominant position.

[0133] In Example 7, in which sodium hydroxide was added in an amount equivalent to that used for the alkaline decomposition reaction for producing terephthalic acid, depolymerization of polyethylene terephthalate (PET) induced by the alkaline decomposition reaction and a portion of the alcohol decomposition reaction occurred simultaneously ( Figure 1 The results show that the reaction pathways A and B in the reaction were consistent with those in the previous step, and similar molar amounts of Na-MET and Na2-TPA were observed in the distribution of the products generated after 2 h of reaction.

[0134] Furthermore, in Example 1 in which an excess amount of sodium hydroxide was added from the early stage of the reaction, alcohol decomposition occurred very quickly from the early stage of the reaction, and high-concentration Na2-TPA was directly produced by the depolymerization reaction.

[0135]

Table 3

[0136] Table 3 shows the results of the depolymerization reaction (reaction temperature: 60°C) when only the type of alkali hydroxide was changed and other reaction conditions were kept unchanged. In the corresponding example, a minimum amount of mixed solvent close to that which allows all polyethylene terephthalate (PET) flakes used as a raw material to be completely immersed in the reaction solvent was added to polyethylene terephthalate (PET), and 2.5 times the amount of alcohol and 2 times the weight of a compound having an alkoxy functional group (anisole) were added compared to the amount of polymer input. At the same time, the amount of metal hydroxide added was adjusted to the equivalent amount required for the production of terephthalic acid by alkali decomposition compared to the number of moles of repeating units of the polymer containing an ester functional group.

[0137] In Example 9 in which potassium was used as the cation of the alkali hydroxide, a slightly higher reaction rate of alcohol decomposition and an overall reaction rate of depolymerization in which polyethylene terephthalate (PET) was decomposed were observed compared to Example 8 in which sodium was used, but in both cases, almost most of the polyethylene terephthalate (PET) was decomposed within 2 hours after the start of the depolymerization reaction, and terephthalic acid alkali salt was produced at a higher yield.

[0138]

Table 4

[0139] Table 4 shows the results of observation on the characteristics of polyethylene terephthalate (PET) depolymerization reaction when depolymerization is performed using reactants having the same composition but the reaction temperature is changed.

[0140] Referring to the results of Example 10 in which the depolymerization was performed under relatively mild reaction conditions (30°C), it was observed that although the reaction lasted for 6 hours, the conversion rate of polyethylene terephthalate (PET) by depolymerization was less than 84%. In addition, according to the product distribution observed by quantitative analysis, it was confirmed that the yield of sodium terephthalate was 67.6%, and the yield of sodium monoethyl terephthalate (Na-MET), which may be generated through a reaction pathway of partial alcohol decomposition, was also as high as 16.4%.

[0141] In Examples 2 and 11, in which the reaction temperature was maintained at 60°C, which is close to the boiling point of ethanol, polyethylene terephthalate (PET) was completely or mostly decomposed despite the short reaction time of less than 2 hours, thereby confirming that depolymerization occurred with very high reactivity. In both cases, depolymerization occurred predominantly along the alcohol decomposition reaction pathway, and as a result, a high concentration of terephthalic acid alkali salt was obtained as a product.

[0142]

Table 5

[0143] Table 5 shows the results of depolymerization (reaction temperature: 60°C) of polymers containing ester functional groups when the type of co-solvent is changed. As shown in Example 12, when 1,2-dimethoxybenzene (1,2-DMB) in which two methoxy groups replace hydrogen atoms bonded to aromatic carbon atoms and alcohol are used as reaction solvents, very high depolymerization performance is exerted, and similar to Example 2 in which anisole is used as a co-solvent under the same conditions, a rapid depolymerization reaction of polyethylene terephthalate (PET) is exhibited from the initial stage of the reaction by means of a dominant alkaline decomposition reaction.

[0144] In Example 13, which used phenethyl ether as a co-solvent, high reactivity was also shown from the initial stage of the reaction, and the analysis results of the reaction products produced by locking the reaction system showed that the reaction system could provide high selectivity for terephthalic acid alkali salts. In addition, when a compound having two or more carbon atoms and an alkoxy group bonded to an aromatic ring was used as a co-solvent for the alkaline decomposition reaction, a significantly higher level of reaction performance was also observed compared to the depolymerization performed using only alcohol (Comparative Example 1 or Comparative Example 2).

[0145] In addition, as shown in Comparative Example 5, when guaiacol having a proton donor functional group in addition to an alkoxy functional group is used as a co-solvent, a completely different result is presented. When an aromatic compound having both an alkoxy functional group and a hydroxyl functional group such as guaiacol comes into contact with an alkali hydroxide, an acid-base reaction may occur. In fact, when sodium hydroxide is added during the preparation of the mixed solvent of Comparative Example 5, a white precipitate is generated very quickly. The precipitate generated at this time is sodium guaiacol, which results in the consumption of the sodium hydroxide put in for alkali decomposition from before the depolymerization reaction. When polyethylene terephthalate (PET) is put into the obtained mixed solvent and the occurrence of the reaction is observed, there is almost no depolymerization reaction by alkali decomposition reaction. The results as described above show that even in the case of containing alkoxy functional groups, aromatic compounds containing proton worker functional groups that may react with alkali hydroxides are difficult to use as co-solvents for alkali decomposition reactions of polymers containing ester functional groups.

[0146]

Table 6

[0147] Table 6 shows the performance comparison results of the alkali decomposition reaction (reaction temperature: 60° C.) performed on post-consumer waste colored waste polyethylene terephthalate (PET) and waste polyester raw materials according to an example of the prior patent document (KR 10-0983349) and the depolymerization method of the present invention.

[0148] Comparative Example 6 is the result of depolymerizing a raw material produced by crushing a colored polyethylene terephthalate (PET) bottle using only ethanol. Although the same depolymerization reaction conditions were applied, compared with the case of Comparative Example 2 in which the depolymerization was performed using a colorless polyethylene terephthalate (PET) raw material, although the conversion rate and the yield of the reaction were almost the same at the initial stage of the reaction, a significantly lower conversion rate and the yield of terephthalic acid alkali salt were observed when exposed to the reaction for a long time. This is generally because as the depolymerization proceeds, when foreign matter discharged to the outside of the polymer matrix, especially colored pigments or organic colorants, directly contacts the alkali hydroxide, the alkali hydroxide is consumed through various reaction pathways. This shows that in the initial stage of the reaction, there is no foreign matter exposure and it will not be affected too much, but as the depolymerization proceeds, the foreign matter released will have a direct and substantial impact on the reactivity of the depolymerization.

[0149] Example 14 is the result of depolymerization performed by adding an organic co-solvent, i.e., anisole, according to the present invention. Unlike the result of Comparative Example 6 in which only ethanol was applied without adding a co-solvent, the polymer decomposition occurred rapidly. From the initial stage of the reaction, the conversion rate and the yield of terephthalic acid alkali salt were maintained at more than 90%, and most of the polyethylene terephthalate (PET) was decomposed after 4 hours of reaction, so that a very high yield of terephthalic acid alkali salt was detected. As shown in Example 14, when a co-solvent according to the present invention, i.e., a compound having no proton functional group but an alkoxy functional group, is added, because the compound having an alkoxy functional group is hydrophobic, the solvent mixture used for the reaction will be hydrophobic, thereby significantly reducing the rate at which sodium hydroxide is consumed by impurities. In addition, most of the colored foreign matter will not remain in the solid component containing the product during the filtration process, but will be discharged together with the mixed solvent composed of ethanol and anisole. Therefore, even when the same purification and acid treatment process as in Comparative Example 6 is applied, high depolymerization performance can be maintained when depolymerization is performed according to Example 14, and the produced product is high-quality terephthalic acid that is relatively close to white. It can be easily confirmed with the naked eye that the amount of residual colored foreign matter in the produced terephthalic acid product is small.

[0150] Comparative Example 7 and Example 15 are the reaction performance comparison results of two cases in which depolymerization is performed using colored waste plastics with different chemical structures and forms and composed of a composite of multiple polymers containing ester functional groups (composition ratio: polyethylene terephthalate (PET) 63.8%, polybutylene terephthalate (PBT) 35.4%) as raw materials and the solvent composition is changed. Similar to the performance comparison of Comparative Example 6 and Example 14 as described above, in Example 15 in which depolymerization is performed after adding a co-solvent (anisole), a faster alkaline decomposition reaction is observed, and the yield of the terephthalic acid product obtained after the reaction is also high. In addition, in Example 15, most of the colored foreign matter (organic additives for dyeing, including carbon black) introduced into the initial polymer can be discharged / removed with the help of the co-solvent, so that a terephthalic acid product with a lighter color than that of Comparative Example 7 can be obtained.

[0151] Comparative Example 8 and Example 16 are the results of depolymerization using polymers in the form of composites similar to those of Comparative Example 7 and Example 15 described above, but using waste polymer raw materials with different compositions (composition ratio: polyethylene terephthalate (PET) 26.1%, polybutylene terephthalate (PBT) 73.1%). As in the two examples of performance comparison using different raw materials described above, a relatively faster alkali decomposition reaction was observed in the depolymerization reaction (Example 16) in which the co-solvent (anisole) was added to perform depolymerization, compared to the case in which the depolymerization was performed using only ethanol as the reaction solvent (Comparative Example 8), and the yield of the terephthalic acid product obtained after the reaction was also higher.

[0152]

Table 7

[0153] Table 7 lists the color space coordinate values ​​(L a b ). Reagent grade commercial terephthalic acid (Sigma-Aldric, Product No. 185361) and the final product obtained by depolymerization, purification and commercialization of the colored polymer were manufactured into disks and used as samples for the spectrophotometer for observing color characteristics.

[0154] The lightness values ​​(L ) is in the range of 87 to 89, which is consistent with the lightness value (L ) of 96.16. The terephthalic acid products produced by the alkaline decomposition reaction of the colored polymer (Examples 14 to 16) by replacing part of the ethanol solvent with the co-solvent (anisole) according to the present invention exhibited a relatively lighter color than the comparative example described above in which only ethanol was used, and the lightness value (L ) reached over 90.

[0155] The manufacturing process and morphology of the reaction solvent discharged during the depolymerization reaction of the colored polymer containing ester functional groups and the purification of the product according to the embodiment of the present invention and the final monomer product (made into a disc sample) are as follows: Figure 2 shown.

[0156] In the first filtration process performed to separate the terephthalic acid product from the reaction product after depolymerization, it was observed that a large amount of colored foreign matter was discharged together with the solvent suitable for the reaction ( Figure 2 This confirms that residual foreign matter in the final product can be effectively removed simply by replacing ethanol with a co-solvent ( Figure 2 (c) in the above process). Since the boiling point of foreign matters contained in the solvent discharged in the above process is usually higher than that of the solvent used, it can be purified by a simple method such as distillation or evaporation, and the purified solvent as described above can be reused in the depolymerization process of the letter.

[0157] As shown in Table 7, the color characteristics of the terephthalic acid produced in Examples 14 to 16 measured by an optical method showed that the lightness was relatively low and the absolute value of the saturation was relatively large compared with commercial terephthalic acid. This indicates that the method of the present invention of mixing a co-solvent with ethanol to form a reaction solvent can effectively remove impurities in the product, but cannot completely remove the impurities.

[0158] In order to improve this, an adsorbent is used to remove the residual impurities in the reactants, and a method of applying the existing purification process is modified. In the existing purification process, the filter cake (solid matter) obtained by the first filtration for separating the reaction solvent from the product generated after the depolymerization reaction contains most of the terephthalic acid alkali salt that can be converted into the final product, and in order to separate it from the unreacted polymer or other polymer materials (such as polyethylene (PE) or polypropylene (PP) etc.) added to the raw materials, a method can be adopted in which the terephthalic acid alkali salt is dissolved in water or an aqueous solution, and then the filtrate is recovered, and then the final product, i.e., terephthalic acid, is precipitated by adding an acid to the filtrate.

[0159] Examples 17 to 19 show the color characteristics of products produced by depolymerizing the polymer containing ester functional groups according to Examples 14 to 16, but additionally passing the polymer through an adsorbent layer before converting the polymer into terephthalic acid by adding an acid during the purification process. Activated carbon is used as an example of the adsorbent. The measured lightness (L ) reached a very high value of 97.6, and the coordinate values ​​(a and b ) also presents a value very close to neutral ( Figure 2(d) in the above). The extreme effect of impurity purification as described above is not the result achieved by applying the adsorbent alone. The result can be explained by the fact that although most of the co-solvent can be physically separated / removed during the filtration process of the depolymerization reaction according to the present invention, a trace amount of the co-solvent having an alkoxy functional group dissolved and present in the water-soluble filtrate can weaken the interaction between the organic foreign matter and the terephthalic acid alkali salt, thereby inducing more effective and irreversible foreign matter adsorption.

[0160] In the above content, specific parts of the present invention are described in detail, but people with general knowledge of the industry should understand that the specific technology is only a preferred embodiment, and the scope of the present invention is not limited thereto. Therefore, the substantive scope of the present invention should be defined by the attached claims and their equivalents.

Claims

1. A method for depolymerizing a polymer containing an ester functional group, characterized in that: The polymer containing ester functional groups is depolymerized by contacting a mixture comprising (1) an aromatic compound having no proton donor functional group but having one or more alkoxy functional groups, (2) a compound having one or more alcohol functional groups, and (3) an alkali hydroxide with the polymer.

2. The method for depolymerizing a polymer containing an ester functional group according to claim 1, characterized in that: In the mixture, the weight ratio of the aromatic compound having no proton donor functional group but having one or more alkoxy functional groups to the compound having one or more alcohol functional groups is 20:1 to 1:

20.

3. The method for depolymerizing a polymer containing an ester functional group according to claim 1, characterized in that: The compound having the alcohol functional group is a straight-chain primary alcohol.

4. The method for depolymerizing a polymer containing an ester functional group according to claim 1, characterized in that: The aromatic compound having no proton donor functional group but having one or more alkoxy functional groups is selected from methoxybenzene, 1,2-dimethoxybenzene, 1,3-dimethoxybenzene, 1,4-dimethoxybenzene, 1,2,3-trimethoxybenzene, 1,2,4-trimethoxybenzene, 1,3,5-trimethoxybenzene, 1,2,3,4-tetramethoxybenzene, 1,2,3,5-tetramethoxybenzene, 1,2,4,5-tetramethoxybenzene, 1-methoxy-2-methylbenzene, 1-methoxy-3-methylbenzene, 1-methoxy-4-methylbenzene, 1-methoxy-2-ethylbenzene, 1-methoxy-3-ethylbenzene, 1-methoxy-4-ethylbenzene, 1-methoxy-2-propylbenzene, 1-methoxy-3-propylbenzene, 1-methoxy-4-propylbenzene, ethyl One or more compounds selected from the group consisting of oxybenzene, 1-ethoxy-2-methoxybenzene, 1-ethoxy-3-methoxybenzene, 1,2-diethoxybenzene, 1,3-diethoxybenzene, 1,4-diethoxybenzene, 1,2,4-triethoxybenzene, 1,3,5-triethoxybenzene, 1-ethoxy-2-methylbenzene, 1-ethoxy-3-methylbenzene, 1-ethoxy-4-methylbenzene, (1-methoxyethyl)benzene, (2-methoxyethyl)benzene, (2-methoxy)ethoxybenzene, 1-methoxy-2-(methoxymethoxy)benzene, 1-(diethoxymethyl)-4-methoxybenzene, propoxybenzene, 1,3-dimethyl-2-propoxybenzene, 1-methoxy-4-propoxybenzene, 1,3-dipropoxybenzene and phenoxyethanol.

5. The method for depolymerizing a polymer containing an ester functional group according to claim 1, characterized in that: The alkali hydroxide is a substance selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide, lithium hydroxide, magnesium hydroxide, ammonium hydroxide, tetraalkylammonium hydroxide, and combinations thereof.

6. The method for depolymerizing a polymer containing an ester functional group according to claim 1, characterized in that: The temperature during the depolymerization is 10°C to 100°C.

7. The method for depolymerizing a polymer containing an ester functional group according to claim 1, characterized in that: A first filtration step is additionally performed to separate the reaction product after the depolymerization reaction into solid and liquid.

8. The method for depolymerizing a polymer containing an ester functional group according to claim 7, characterized in that: A second filtration step is additionally performed by adding water to the filter cake obtained in the first filtration to dissolve the monomer product in water and perform separation filtration.

9. The method for depolymerizing a polymer containing an ester functional group according to claim 8, characterized in that: The water used in the second filtration process also contains the same or more molar amount of alkali hydroxide as the monoalkyl terephthalate present in the monomer product.

10. The method for depolymerizing a polymer containing an ester functional group according to claim 8, characterized in that: The colored organic foreign matter is separated again by adding an aromatic compound having an alkoxy functional group to the filtrate obtained in the second filtration step.

11. The method for depolymerizing a polymer containing an ester functional group according to claim 8, characterized in that: Also includes: A step of adsorbing and removing colored foreign matter by bringing an aqueous solution phase containing colored organic foreign matter into contact with an adsorbent.

12. The method for depolymerizing a polymer containing an ester functional group according to claim 8, characterized in that: Also includes: A step of adding an acid to an aqueous solution containing the monomers to precipitate the monomers as salts in order to recover the monomers of the depolymerized polymer.

13. The method for depolymerizing a polymer containing an ester functional group according to claim 12, characterized in that: Also includes: After the acid is added, the precipitate generated from the filtrate is separated, followed by a step of recovering high-purity terephthalic acid through a drying process.

14. A composition for depolymerization of a polymer containing an ester functional group, characterized in that: Include: (1) Aromatic compounds that do not have a proton donor functional group but have one or more alkoxy functional groups; (2) Compounds having more than one alcohol functional group; and, (3) Alkali hydroxides.

15. The composition for depolymerization of a polymer containing an ester functional group according to claim 14, characterized in that: The weight ratio of the aromatic compound having no proton donor functional group but having one or more alkoxy functional groups to the compound having one or more alcohol functional groups is 20:1 to 1:20.

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