Mechanochemical depolymerization method for polyester

Abstract

Provided is a method for mechanochemical depolymerization of polyester, which comprises pulverizing polyester in the presence of a decomposing agent to generate heat, depolymerizing the polyester into a dibasic acid and an alkylene glycol, and vaporizing and separating the alkylene glycol using the heat.

Description

[Technical Field]

[0001] The present disclosure relates to a method for mechanochemical depolymerization of polyester. [Background technology]

[0002] Polyesters, such as polyethylene terephthalate (PET), have excellent chemical stability and are used in fibers, fabrics, clothing, films, sheets, and drinking water bottles.

[0003] As the use of polyester has increased rapidly, various methods for recovering and reusing waste polyester have been investigated. One of these methods is the so-called chemical recycling method, in which waste polyester such as polyester is depolymerized to convert it into monomers, which are then recovered, and the monomers are used as raw materials for repolymerization to produce recycled polyester such as polyethylene terephthalate.

[0004] Chemical recycling of polyester is expected to be a means of realizing resource reuse, as it is possible to separate impurities and the quality of the raw material is not much different from that of virgin polyester.

[0005] The methods for depolymerizing polyester into monomers can be broadly divided into three main methods: hydrolysis, which uses water as a solvent; alcoholysis, which uses alcohol as a solvent; and glycolysis, which uses glycol as a solvent.

[0006] An example of the hydrolysis method is a method in which molten polyethylene terephthalate is reacted with water and then with ammonium hydroxide to decompose it into terephthalic acid and ethylene glycol (see Patent Document 1). This method has the advantage of not using glycol or alcohol for the reaction, but the reaction is carried out under high-pressure conditions, so a special high-pressure reactor is required.

[0007] The alcoholysis decomposition method is a method for depolymerizing polyesters by heating them in an alcohol solvent (with the addition of a catalyst as needed) (see Patent Documents 2 and 3). This method has the advantage that, for example, when polyethylene terephthalate is depolymerized using methanol as the solvent, the useful and easy-to-handle monomer dimethyl terephthalate (DMT) is directly produced by the depolymerization reaction, and the depolymerization reaction is relatively fast. However, the alcohol used as the solvent has a low boiling point, and pressure is required to drive the reaction (for example, by reacting in supercritical or subcritical methanol), requiring a special high-pressure reactor.

[0008] The glycolysis method involves depolymerizing polyester by heating it in an excess alkylene glycol solvent together with a depolymerization catalyst such as sodium carbonate to produce bis(β-hydroxyalkyl) terephthalate and ethylene glycol (see Patent Documents 4 and 5). For example, when ethylene glycol is used as the solvent, bis(β-hydroxyethyl) terephthalate (BHET) is produced by the depolymerization reaction, and dimethyl terephthalate (DMT) can be recovered by adding methanol in the presence of a transesterification catalyst to carry out a transesterification reaction. The glycolysis method can be carried out at atmospheric pressure, but the relatively long reaction time requires a reduction in the reaction time, and there is a problem that the glycol solvent deteriorates when heated for a long period of time.

[0009] Such known methods for depolymerizing polyesters require a reaction time of at least several hours, which makes it difficult to treat large amounts of polyester waste. Furthermore, the reaction must be carried out under high temperature or pressure conditions, which requires special equipment that can withstand such reaction conditions, and therefore, the methods have not yet been commercialized. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 2003-527363 (Publication Date: 2003.09.16) [Patent Document 2] Japanese Patent Application Publication No. 1999-100336 (Publication Date: April 13, 1999) [Patent Document 3] Japanese Patent Application Publication No. 2003-300916 (Publication Date: October 21, 2003) [Patent Document 4] Japanese Patent Application Publication No. 2002-167468 (Publication Date: 2002.06.11) [Patent Document 5] Japanese Patent Application Publication No. 2004-300115 (Publication Date: October 28, 2004) Summary of the Invention [Problem to be solved by the invention]

[0011] One aspect of the present disclosure provides a method for mechanochemical depolymerization of polyester, which is environmentally friendly and can reduce production costs by shortening the process and reducing thermal energy.

[0012] Another aspect can provide a composition for polymerization of recycled polyester obtained by a method for mechanochemical depolymerization of polyester.

[0013] In yet another aspect, there can be provided recycled polyester produced using a dibasic acid and an alkylene glycol obtained by a method for mechanochemical depolymerization of polyester. [Means for solving the problem]

[0014] According to one aspect, a method for mechanochemical depolymerization of polyester involves pulverizing polyester in the presence of a decomposing agent to generate heat, depolymerizing the polyester into a dibasic acid and an alkylene glycol, and vaporizing and separating the alkylene glycol using the heat.

[0015] The depolymerization may be carried out in a comminution reactor containing a rotating disk.

[0016] The grinding reactor may further include blades attached to the disc.

[0017] The grinding reactor may further include baffles on the interior wall.

[0018] The grinding reactor may further include a vent for discharging vaporized alkylene glycol.

[0019] The heat can include heat of grinding generated by grinding the polyester, frictional heat generated by friction of the polyester with the grinding reactor, or both.

[0020] The depolymerization may be a solvent-free reaction carried out without the addition of a solvent.

[0021] The depolymerization may be carried out without applying any additional external heat other than the heat.

[0022] The method for mechanochemical depolymerization of polyester may not further include the step of grinding, crushing, or shredding the polyester before or after depolymerization.

[0023] The depolymerization may be carried out by rotating the disk at 500 rpm to 5000 rpm.

[0024] The temperature inside the grinding reactor may reach 130°C to 300°C due to heat.

[0025] The depolymerization may be carried out for 5 minutes to 120 minutes.

[0026] The disintegrating agent can include an alkali, an acid, a salt thereof, a monoalcohol, a polyalcohol, or a mixture thereof.

[0027] The alkali may include sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, or mixtures thereof.

[0028] The acid can include hydrochloric acid, nitric acid, sulfuric acid, carbonic acid, phosphoric acid, acetic acid, hypochlorous acid (HClO), or mixtures thereof.

[0029] The salts can include carbonates, bicarbonates, phosphates, sulfates, sulfites, nitrates, silicates, hypochlorites, formates, acetates, citrates, oxalates, or mixtures thereof.

[0030] The monoalcohol may include methanol, ethanol, propanol, butanol, or mixtures thereof.

[0031] The polyhydric alcohol can include ethylene glycol, n-propylene glycol, isopropylene glycol, diethylene glycol, polyethylene glycol, triethylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, glycerin, benzyl alcohol, polypropylene glycol, pentaerythritol, trimethylolpropane, or mixtures thereof.

[0032] The disintegrant may be in solid or aqueous form.

[0033] The decomposing agent may be added in an amount of 0.75 mol to 3.0 mol per 1.0 mol of the dibasic acid contained in the polyester.

[0034] In the mechanochemical depolymerization of polyester, the alkylene glycol is separated and the remaining dibasic acid is obtained in the form of a solid salt.

[0035] The method for mechanochemical depolymerization of polyester may further include a step of additionally separating the remaining alkylene glycol from the solid dibasic acid salt.

[0036] The mechanochemical depolymerization method for polyester involves dissolving the obtained solid dibasic acid salt in water, neutralizing the aqueous solution of the dibasic acid salt with an acid to precipitate dibasic acid crystals, and then performing solid-liquid separation of the dibasic acid crystals from the precipitated product.

[0037] The mechanochemical depolymerization method of polyester can remove impurities from aqueous solutions of dibasic acid salts.

[0038] The mechanochemical depolymerization process of polyester can recrystallize the diacid crystals.

[0039] Another aspect of the present invention provides a composition for polymerizing recycled polyester, which comprises a dibasic acid and an alkylene glycol obtained by mechanochemical depolymerization of polyester.

[0040] According to yet another aspect, recycled polyester is produced using a dibasic acid and an alkylene glycol obtained by mechanochemical depolymerization of polyester.

[0041] The color of the recycled polyester may have an L value of 45 or more. [Effects of the Invention]

[0042] According to one aspect of the mechanochemical depolymerization method for polyester, additional processes such as washing, crushing, or processing are not required before waste collected from industries or households is introduced into a chemical process. This method is applicable regardless of the type of waste, does not require the introduction of a large amount of additional solvent, does not require a separate heat source for the reaction, has a simple device configuration, requires low investment, and is environmentally advantageous because it does not require the treatment and disposal of solvents. [Brief explanation of the drawings]

[0043] [Figure 1] 1 is a process flow chart illustrating a method for mechanochemical depolymerization of polyester. DETAILED DESCRIPTION OF THE INVENTION

[0044] The advantages and features of the technology described below, and methods for achieving them, will become clearer with reference to the embodiments described below in detail together with the accompanying drawings. However, it can be said that the realized form is not limited to the embodiments disclosed below. Unless otherwise defined, all terms (including technical and scientific terms) used in this specification can be used with meanings that are commonly understood by those with ordinary skill in the art. Furthermore, terms defined in commonly used dictionaries should not be interpreted as ideal or excessive unless clearly defined otherwise.

[0045] Throughout this specification, when a part "comprises" a certain element, this means that it can further include other elements rather than excluding other elements, unless otherwise specified. Also, the singular form includes the plural form unless the language specifically states otherwise.

[0046] 1 is a process flow chart showing a method for mechanochemical depolymerization of polyester according to one aspect. The method for mechanochemical depolymerization of polyester will be described with reference to FIG.

[0047] The mechanochemical depolymerization method for polyester involves crushing polyester in the presence of a decomposing agent to generate heat and depolymerize the polyester (S1).

[0048] The polyester may be a polyester obtained by polymerizing a dibasic acid and an alkylene glycol, such as polyethylene terephthalate (PET), polybutylene terephthalate, or polyethylene naphthalate, or may be polycaprolactone obtained by polymerizing caprolactone. By depolymerizing these polyesters, the dibasic acid, the alkylene glycol, or additionally, caprolactone can be recovered as a monomer.

[0049] Examples of alkylene glycols obtained by depolymerizing polyester include ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol, neopentyl glycol, polytetramethylene glycol, 1,4-cyclohexanediol, and 1,4-benzenediol. For example, when the polyester is polyethylene terephthalate, ethylene glycol can be recovered as the alkylene glycol, and when the polyester is polybutylene terephthalate, butylene glycol can be recovered as the monomer.

[0050] Examples of dibasic acids obtained by depolymerizing polyesters include aromatic dicarboxylic acids such as terephthalic acid, orthophthalic acid, isophthalic acid, dibromoisophthalic acid, sodium sulfoisophthalate, phenylenedioxydicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 4,4'-diphenylketonedicarboxylic acid, 4,4'-diphenoxyethanedicarboxylic acid, 4,4'-diphenylsulfonedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, trimellitic acid, and pyromellitic acid. Other dicarboxylic acids include alicyclic dicarboxylic acids such as hexahydroterephthalic acid and hexahydroisophthalic acid, and aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecadicarboxylic acid, and todecadicarboxylic acid.

[0051] As a raw material for polyester depolymerization, polyester-containing molded products, particularly waste products, can be used. The waste may be residues or defective products generated during the production of molded products other than waste generated after using polyester-containing molded products. Examples of waste include used PET bottles, cups, strings, packaging packs, flash from molding these, sprues, sheets left after cutting off cups after vacuum molding, fibers, fabrics, clothing, films, or sheets.

[0052] In this case, the polyester content in the polyester waste may be 60% by weight to 100% by weight based on the total weight of the polyester waste. If the polyester content is less than 60% by weight, the amount of by-reaction products and unrecyclable separated and purified waste from the raw materials obtained in the depolymerization step will be excessively large, making it essentially uneconomical.

[0053] When polyester waste is used as a feedstock for depolymerization, the waste may optionally be subjected to a pretreatment step prior to the depolymerization step in which the waste is washed to remove contaminants, such as dyes, contents, or soil, adhering to the waste.

[0054] In addition, in the mechanochemical depolymerization method for polyester, depolymerization occurs simultaneously with pulverization, so the recovered polyester waste can be depolymerized as is or in relatively large pieces, but alternatively, the waste can be mechanically cut, pulverized, or processed to an appropriate size. The pulverization treatment can be carried out using any known suitable means, and for example, the polyester waste can be pulverized using a hammer mill or the like to pulverize it into small pieces of 2 mm to 8 mm in size, and then provided to the depolymerization reaction.

[0055] If necessary, the polyester can be immersed in a solvent to extract and remove dyes, or components lighter than the solvent can be separated, and only components of a certain size can be collected by blowing them off or using a sieve and then provided to the depolymerization reaction.

[0056] However, in this method of mechanochemical depolymerization of polyester, polyester is crushed and simultaneously depolymerized using the heat generated thereby. Therefore, waste collected from industries or households does not need to be subjected to additional washing, crushing, shredding, cutting, or processing (flake formation, popcorn formation) steps in order to input the waste into a chemical process. Therefore, waste in various forms can be used as a feedstock.

[0057] The decomposing agent may include an alkali, an acid, a salt thereof, a monoalcohol, a polyalcohol, or a mixture thereof. As will be described later, when the depolymerization reaction is carried out in the absence of a solvent, the decomposing agent may be added in the form of a solid or an aqueous solution.

[0058] For example, the alkali may include hydroxides of alkali metals or alkaline earth metals, ammonia, or mixtures thereof. Here, the alkali metal may be a monovalent metal such as lithium, sodium, potassium, rubidium, or cesium, among which relatively inexpensive sodium or potassium may be used. The alkaline earth metal may be beryllium, magnesium, calcium, strontium, barium, or radium. Examples of alkali metal hydroxides include sodium hydroxide, potassium hydroxide, and lithium hydroxide. Among these, sodium hydroxide exhibits excellent reaction speed and reaction rate when used in combination with ethylene glycol, etc.

[0059] The acid may be an organic or inorganic acid and may include, for example, hydrochloric acid, nitric acid, sulfuric acid, carbonic acid, phosphoric acid, acetic acid, hypochlorous acid (HClO), or mixtures thereof.

[0060] The salts can include inorganic acid salts such as carbonates, bicarbonates, phosphates, sulfates, sulfites, nitrates, silicates, hypochlorites, etc., organic acid salts such as formates, acetates, citrates, oxalates, etc., or mixtures thereof, such as sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, tripotassium phosphate (hydrate), sodium hypochlorite, or mixtures thereof.

[0061] The polyhydric alcohol may include ethylene glycol, n-propylene glycol, isopropylene glycol, diethylene glycol, polyethylene glycol, triethylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, glycerin, benzyl alcohol, polypropylene glycol, pentaerythritol, trimethylolpropane, or a mixture thereof. Among these polyhydric alcohols, ethylene glycol, n-propylene glycol, isopropylene glycol, diethylene glycol, polyethylene glycol, triethylene glycol, or glycerin, which have a high boiling point and relatively high reactivity, may be used.

[0062] The monoalcohol may include methanol, ethanol, propanol, butanol, or a mixture thereof, and examples thereof include alkyl ether compounds of polyhydric alcohols such as the diols or triols listed above, such as diethylene glycol monomethyl ether, benzyl alcohol, or 2-ethylhexanol.

[0063] Since the decomposing agent reacts with both the polyester and the dye, the amount of the decomposing agent added may be 0.75 mol to 3.0 mol per 1.0 mol of the dibasic acid contained in the polyester. For example, in the case of a monovalent decomposing agent (e.g., sodium hydroxide), it is appropriate to add 1.5 mol to 3.0 mol, and in the case of a divalent decomposing agent (e.g., calcium carbonate), it is appropriate to add 0.75 mol to 1.5 mol.

[0064] For example, when decomposing polyethylene terephthalate using sodium hydroxide as a decomposing agent, the reaction formula is PET (polyethylene terephthalate) + 2NaOH → TPA salt (disodium terephthalate) + EG (ethylene glycol), so 2 moles of NaOH (420 g) are required per mole of TPA to decompose 1 kg of PET.

[0065] In addition, when the polyester feedstock is fiber, the amount of dye added during processing varies from 0.5% to 20% by weight (based on the weight of fabric (OWF)) of the weight of the polyester fiber, so the amount of decomposing agent added is preferably 2.0 mol or more per 1.0 mol of dibasic acid contained in the polyester so that the dye and the polyester fiber can be completely decomposed.

[0066] The mechanochemical depolymerization method of polyester involves crushing polyester in the presence of a decomposing agent and simultaneously depolymerizing the polyester using the heat generated thereby, so that a high-pressure reactor is not required, and the apparatus can be configured as a batch or continuous reactor. For example, after the raw materials are introduced, the inside of the reactor can be purged with nitrogen (N2) to prevent side reactions, and the reaction can be carried out in a nitrogen (N2) atmosphere throughout the entire process.

[0067] The depolymerization may be carried out in a grinding reactor containing a device capable of grinding the polyester. For example, the grinding reactor may contain a rotating disk at the bottom. The disk grinds the polyester as it rotates, causing friction between the polyester and the inner wall of the grinding reactor.

[0068] To more efficiently grind the polyester, the disc may have blades attached to its surface, for example, in the form of a mixer. The grinding reactor may further include a baffle on its inner wall to increase frictional heat generated by friction between the polyester and the inner wall of the grinding reactor.

[0069] The grinding heat generated inside the grinding reactor is generated by grinding the polyester, and frictional heat is generated by friction between the polyester and the inner wall of the grinding reactor. The generated heat causes the polyester to react with the decomposing agent and depolymerize. For example, the heat may include grinding heat, frictional heat, or both.

[0070] As described above, the mechanochemical depolymerization method of polyester involves pulverizing polyester in the presence of a decomposing agent and simultaneously depolymerizing the polyester using the heat generated thereby, so that it can proceed as a solvent-free reaction without adding a large amount of solvent and can be carried out without adding additional heat from the outside other than the heat generated internally. For example, the mechanochemical depolymerization method of polyester may be carried out without adding a solvent such as water or ethylene glycol.

[0071] In addition, the mechanochemical depolymerization method of polyester does not require separate heat control, the introduction of large amounts of solvent to increase reaction uniformity, or a separate heat source for the reaction, and is environmentally friendly because it requires simple equipment configuration, low investment costs, and no separate solvent treatment or disposal.In addition, the dibasic acid, which is the depolymerization product, can be obtained as a salt in the form of a paste, powder, or pellet, which reduces the volume and makes storage and transportation easier.

[0072] For example, depolymerization may be performed by rotating the disk at 500 rpm to 5000 rpm, e.g., 700 rpm to 2500 rpm, or 1000 rpm to 1200 rpm. If the disk rotation speed during depolymerization is less than 500 rpm, grinding may be ineffective and heat generation due to grinding and friction may be insufficient, while if the rotation speed exceeds 5000 rpm, heat generation due to grinding and friction may be excessive.

[0073] For example, the temperature inside the pulverization reactor due to the generated heat may be 130°C to 300°C, for example, 170°C to 280°C, or 200°C to 250°C. If the temperature inside the pulverization reactor is less than 130°C, the depolymerization efficiency may decrease, and if it exceeds 300°C, depolymerization by-products may be produced.

[0074] The depolymerization may be carried out for 5 to 120 minutes, 5 to 60 minutes, or 10 to 40 minutes. The depolymerization reaction time can be increased within the range of 120 minutes as the concentration of impurities including dyes in the polyester increases, the amount of polyester processed increases, and the temperature inside the grinding reactor decreases.

[0075] Meanwhile, in the mechanochemical depolymerization method of polyester, the alkylene glycol produced during the depolymerization process can be vaporized using the heat generated, and then separated and recovered.

[0076] The mechanochemical depolymerization method for polyester can be carried out as a solvent-free reaction by pulverizing polyester in the presence of a decomposing agent and simultaneously depolymerizing the polyester using the heat generated by the pulverization. The temperature inside the pulverization reactor can reach up to 300°C due to the heat of pulverization and / or friction generated during the depolymerization process, so the alkylene glycol produced can be vaporized during depolymerization and then separated and recovered.

[0077] However, the present invention is not limited thereto, and additional heat may be supplied from an external source to vaporize, separate, and recover the alkylene glycol. That is, while the depolymerization of polyester can be sufficiently achieved by the heat of pulverization and friction generated within the pulverization reactor, the heat of pulverization and friction generated within the pulverization reactor may not be sufficient for the vaporization of the alkylene glycol. In this case, external heat may also be required.

[0078] In contrast, conventional alkaline hydrolysis depolymerization methods involve adding a large amount of depolymerization solvent and depolymerizing polyester in a high-temperature reactor. This requires a separate process of solid-liquid separation into liquid alkylene glycol and solid dibasic acid salt from the depolymerized product obtained during depolymerization. Furthermore, the depolymerization solvent and polyester must be heated to the reaction temperature, consuming a lot of energy. While the used depolymerization solvent can be separated from the product and reused, this requires a separate purification process, which incurs significant costs. Furthermore, side reactants are added depending on the type of depolymerization solvent used, making it difficult to control the amount of impurities.

[0079] In the mechanochemical depolymerization method for polyester, alkylene glycol can be recovered during the depolymerization process in an amount of 0 wt% or more or 100 wt% or less, for example, 30 wt% to 95 wt% or 50 wt% to 98 wt%, based on the total weight of alkylene glycol. When the content of alkylene glycol recovered during depolymerization is 0 wt%, the alkylene glycol is discharged mixed with the dibasic acid, requiring a separate process for separation before dissolution. When the content is 100 wt%, the alkylene glycol is not mixed with the generated dibasic acid, allowing the dibasic acid to be immediately dissolved and the next process to proceed.

[0080] The grinding reactor may further include an outlet for discharging the vaporized alkylene glycol. The vaporized alkylene glycol may be collected under atmospheric pressure or by liquefying it in a condenser using a gas flow by introducing nitrogen (N).

[0081] After the depolymerization reaction, the alkylene glycol is separated and the remaining dibasic acid can be obtained in the form of a solid salt.

[0082] However, depending on the rotation speed of the discs in the milling reactor, the alkylene glycol may not be recovered at all or only partially. In this case, the depolymerization product may contain a mixture of the dibasic acid salt and the remaining alkylene glycol. The mechanochemical depolymerization method for polyester may optionally further include a step of separating the alkylene glycol and the dibasic acid salt from the depolymerization product and then recovering the alkylene glycol. For example, the method for separating the alkylene glycol and the dibasic acid salt produced in the depolymerization reaction is not particularly limited in the present invention, and an appropriate method can be selected depending on the target compound. For example, the alkylene glycol and the dibasic acid salt may be separated by distillation concentration. As a means for distillation concentration, any conventional distillation concentration apparatus, such as a vacuum continuous distillation apparatus or a vacuum batch distillation apparatus, can be used. Subsequently, a solid dibasic acid salt can be obtained by a drying process.

[0083] Depending on the type of decomposing agent used during depolymerization, the method can be divided into hydrolysis using alkali, alcoholysis decomposition using alcohol, and glycolysis using glycol, and the type of dibasic acid obtained varies depending on the method.

[0084] When polyester is depolymerized using the glycolysis method, the resulting product varies depending on the type of decomposition agent used in the depolymerization reaction. For example, when polyethylene terephthalate is depolymerized using ethylene glycol as a decomposition agent, bis(β-hydroxyethyl) terephthalate (BHET) can be obtained as the monomer, while when propylene glycol is used as a decomposition agent, bis(β-hydroxyethyl isopropyl) terephthalate (BHEPT) can be obtained as the monomer.

[0085] Furthermore, the oligomer ester compounds obtained when benzyl alcohol and tripotassium phosphate are used as decomposing agents have very good solubility in chloroform, etc., and can therefore be efficiently recovered by solvent extraction. Alternatively, the dibasic acid or oligomer ester compounds can be recovered by means of filtration, distillation, or the like.

[0086] The dibasic acid or oligomer ester compound (e.g., BHET) thus obtained can also be recovered as the dibasic acid or oligomer methyl ester (e.g., DMT) by transesterification with methanol.

[0087] For example, a slurry in which solid DMT is dispersed in a mixture of methanol and alkylene glycol can be obtained by subjecting the depolymerization reaction concentrate to a transesterification reaction at 65°C to 85°C for 0.5 to 5 hours in the presence of a transesterification catalyst (such as an alkali metal compound).The DMT-containing cake can be separated using a solid-liquid separator or the like, and purified by distillation to recover purified DMT.

[0088] On the other hand, when polyester is depolymerized using hydrolysis, dibasic acid salts such as dibasic acids or dialkali metal salts of oligomers are produced, and the products vary depending on the type of alkali metal used in the depolymerization reaction. For example, when polyethylene terephthalate is depolymerized using sodium hydroxide as a decomposing agent, sodium terephthalate is produced along with ethylene glycol.

[0089] Among these, dibasic acid salts (e.g., sodium terephthalate) do not dissolve in alkylene glycol and form solid crystals, so they can be easily separated from the solvent by filtration methods such as solid-liquid separation. Furthermore, alcohols adhering to the obtained powdery crystals can be removed by washing with alcohols such as methanol or ethanol.

[0090] Next, the dibasic acid salt crystals are dissolved in water (S2), and then an acid is added to cause a neutralization reaction, resulting in the precipitation of the dibasic acid (e.g., terephthalic acid) (S4). The precipitated dibasic acid can be recovered by separating it from the water using a method such as centrifugation (S5).

[0091] The amount of water added during the recovery step may be 3 to 10 weight percent per weight of the dibasic acid salt. The amount of acid added to separate the dibasic acid may be equimolar or greater than the amount of alkali metal contained in the dibasic acid salt. The acid used may be an inorganic acid, such as a strong acid with a pH of 2, such as hydrochloric acid, sulfuric acid, or phosphoric acid, or an organic acid, such as formic acid, acetic acid, or oxalic acid. Among these, inorganic acids, particularly hydrochloric acid or sulfuric acid, are suitable because they can reduce impurities in the resulting monomer. The neutralization reaction temperature may be 20°C to 85°C, and the neutralization reaction is usually completed within 10 minutes to 5 hours.

[0092] Meanwhile, the mechanochemical depolymerization method of polyester may optionally further include a step of removing impurities contained in the polyester molded product or waste from the aqueous solution of the dibasic acid salt (S3).

[0093] The means for removing impurities is not particularly limited, and any appropriate method, device, etc. can be used. For example, solids such as unreacted resins other than polyester (polyethylene, polypropylene, polyvinyl chloride, etc.) and decomposing agents such as insoluble alkali metals can be removed using a mesh or the like. Furthermore, dyes, fillers, etc. cannot be removed using a mesh or the like, so they can be removed by centrifugation or filtration using an adsorbent such as activated carbon.

[0094] The resulting dibasic acid crystals exhibit a particle size distribution characteristic of several μm to several hundred μm, and therefore a step of selectively recrystallizing the dibasic acid crystals to a commercially available particle size (100 μm or more) may be further included (S6). The recrystallization method is not particularly limited, and may be performed, for example, by mixing the dibasic acid crystals with water and then performing the recrystallization under high temperature and high pressure conditions.

[0095] The dibasic acid and alkylene glycol obtained by the mechanochemical depolymerization method of polyester can be recycled as monomers for polymerizing recycled polyester. Specifically, recycled polyester can be produced by esterifying and polycondensing the recycled polyester polymerization composition containing the dibasic acid and alkylene glycol obtained by the mechanochemical depolymerization method of polyester.

[0096] Esterification is the reaction of the recovered dibasic acid monomer with an alkylene glycol, for example, terephthalic acid and ethylene glycol. This reaction may be carried out without a catalyst, or in the presence of a catalyst such as an alkaline earth metal compound (e.g., magnesium, calcium), or a metal compound (e.g., titanium, zinc, manganese), which is well known as a transesterification catalyst.

[0097] The product of the esterification process can then be polycondensed to produce a recycled polyester resin, which can be either solution polymerization or solid-state polymerization.

[0098] Furthermore, after the solid-state polymerization, a compound that promotes water treatment and / or crystallization can be added as needed to adjust the quality, and a polycondensation catalyst or stabilizer may be added at the start or during the polycondensation step.

[0099] Here, the water treatment may be carried out by contacting the recycled polyester resin produced in the form of solid particles with, for example, water, water vapor, a water vapor-containing inert gas, water vapor-containing air, etc. Examples of compounds that promote crystallization include polyhexamethylene terephthalate, inorganic compounds, higher aliphatic compounds, polyether compounds, and polyolefin thermoplastic resins such as polypropylene and polyethylene, and these compounds may be added in an amount of 1 ppm to 100 ppm to the recycled polyester resin.

[0100] The polycondensation catalyst may be a compound of germanium, antimony, titanium, aluminum, etc. The amount of the polycondensation catalyst added may be 2 ppm to 800 ppm, for example, 4 ppm to 500 ppm, in terms of the weight of the catalytic metal element, relative to the total weight of the dibasic acid component.

[0101] The recycled polyester may also contain, as a stabilizer, phosphoric acid esters such as trimethyl phosphate, triethyl phosphate, triphenyl phosphate, and triethylphosphonoacetate, phosphorous acid esters such as triphenyl phosphite and trisdodecyl phosphite, and phosphorus compounds such as methyl acid phosphate, dibutyl phosphate, monobutyl phosphate, phosphoric acid, phosphorous acid, hypophosphorous acid, and polyphosphoric acid. The amount of stabilizer added may be 1000 ppm or less, for example, 500 ppm or less, or 300 ppm or less, in terms of the weight of phosphorus element in the stabilizer, relative to the total weight of the recycled polyester.

[0102] Meanwhile, in the method for mechanochemical depolymerization of polyester according to one embodiment, the polyester is pulverized and simultaneously depolymerized by the heat generated thereby. Therefore, the color of the recycled polyester produced using the dibasic acid and alkylene glycol obtained thereby may have an L value of 45 or more, for example, 60 or more, or 65 to 95.

[0103] The L, a, and b color system is commonly used internationally as a standard for evaluating the color of polyester. These color numbers are one of the color systems used to standardize color measurement and describe perceptible colors and color differences. In this system, L is the lightness factor, and a and b are color measurement numbers. Of these, the L value is a numerical factor indicating brightness and is very important in the manufacture of fibers, fabrics, and clothing. A positive b value indicates yellow discoloration, a negative value indicates blue discoloration, a positive a value indicates red discoloration, and a negative value indicates green discoloration.

[0104] The L, a, and b values ​​are defined in the Korean Industrial Standards (KS) related to color measurement as KS A 0061, 0063, 0064, 0065, 0066, 0067, 0084, 0085, 0089, 0114, etc. For example, the L, a, and b values ​​can be determined by removing moisture from 50g of polyester resin to be measured in air, placing it in a Colorimeter model SA-2000, measuring the color 10 times, and averaging the result to determine the standard value.

[0105] The L value of recycled polyester is related to the purity of the recycled dibasic acid and alkylene glycol. For example, the greater the amount of impurities or quenchers, such as titanium dioxide (TiO2), the lower the L value. If the L value is less than 45, the impurity content is excessive, resulting in coloration after polymerization and fiberization. Also, excessive side reactions during polymerization can prevent the material from being used as a high-value-added material. Since its primary uses are the same as mechanically recycled polyester, this may be pointless. On the other hand, if the L value exceeds 95, this is a physical property that is difficult to achieve even with existing virgin materials. To achieve this value through recycling requires the addition of bleaching and purification processes or increased residence time, which increases unit production volume and process costs, making it less economical.

[0106] Specific examples of the present invention will be presented below. However, the examples described below are merely for the purpose of specifically illustrating or explaining the invention, and should not be construed as limiting the scope of the invention.

[0107] [Manufacturing example: mechanochemical depolymerization of polyester] Example 1 1 kg of waste polyethylene terephthalate (waste PET) and 416.3 g of sodium hydroxide (NaOH) are charged into a grinding reactor.

[0108] After the raw materials are added, the atmosphere inside the grinding reactor is replaced with nitrogen (N2) to prevent side reactions, and the reaction is carried out in a nitrogen (N2) atmosphere throughout the entire process.

[0109] The discs in the grinding reactor rotate at 1,200 rpm, generating heat through friction and grinding, which depolymerizes the waste PET into terephthalic acid (TPA) salt and ethylene glycol (EG). At the same time, the EG is vaporized and recovered using the 240°C heat generated by friction and grinding. The vaporized EG is recovered by liquefying it in a condenser with the gas flow, under atmospheric pressure conditions, with nitrogen (N2) introduced during the process.

[0110] After the EG is separated, the TPA salt powder is obtained by discharging it from the grinding reactor.

[0111] The TPA salt is dissolved in water, followed by purification and precipitation to recover terephthalic acid (TPA).

[0112] Example 2 1 kg of waste polyethylene terephthalate (waste PET) is placed in a grinding reactor.

[0113] The disk of the grinding reactor is rotated at 500 rpm, and 832.6 g of an aqueous solution of sodium hydroxide (NaOH 50% by weight) is uniformly injected into the reactor by a spray method.

[0114] After the raw materials are added, the atmosphere inside the grinding reactor is replaced with nitrogen (N2) to prevent side reactions, and the reaction is carried out in a nitrogen (N2) atmosphere throughout the entire process.

[0115] The discs in the grinding reactor rotate at 1,200 rpm, generating heat through friction and grinding, which depolymerizes the waste PET into terephthalic acid (TPA) salt and ethylene glycol (EG). At the same time, the 240°C heat generated by friction and grinding is used to vaporize and recover EG and water (H2O). The vaporized EG is recovered by liquefying it in a condenser with the gas flow, under atmospheric pressure, with nitrogen (N2) introduced during the process.

[0116] After the EG is separated, the TPA salt powder is obtained by discharging it from the grinding reactor.

[0117] The TPA salt is dissolved in water, followed by purification and precipitation to recover terephthalic acid (TPA).

[0118] (Comparative Example 1) Waste polyethylene terephthalate (waste PET) is crushed into pieces less than 10 mm in size using a crusher.

[0119] 1 kg of crushed waste polyethylene terephthalate (PET) and 416.3 g of sodium hydroxide (NaOH) are dissolved in 6 kg of ethylene glycol (EG) and fed to a depolymerization reactor, where depolymerization is carried out at 180°C for 60 minutes while stirring with an agitator.

[0120] The depolymerization products, terephthalic acid (TPA) salt and ethylene glycol (EG), are separated into solid and liquid in a decanter, and the liquid ethylene glycol (EG) is recovered by distillation to remove impurities, while the solid terephthalic acid (TPA) salt is dissolved in water to remove impurities.

[0121] Sodium hydroxide (NaOH) is added to the terephthalic acid (TPA) salt aqueous solution from which impurities have been removed to reduce the pH to 3.0 or less, and terephthalic acid (TPA) is recovered.

[0122] [Experimental example: Properties of terephthalic acid] The physical properties of the terephthalic acid produced in Examples 1 and 2 and Comparative Example 1 were measured, and the results are shown in Table 1.

[0123] 1)△Y (delta Y) A sample of recycled terephthalic acid (rTPA) was divided into small portions and dissolved in 2.8% NH4OH (aqueous ammonia solution) for at least one hour. After complete dissolution, the solution was passed through glass filter paper (pore size: 0.4 μm). The filter paper was dried at 50°C for one hour and then allowed to cool. The cooled filter paper was placed in the measurement position using a color spectrometer and the Y value was measured (A). A blank test solution (a solution containing only NH4OH without the rTPA sample) was passed through the filter paper separately, dried, and the Y value was measured as a reference (B). Calculation was then performed using the formula △Y=AB.

[0124] 2) Alkali transmittance (%) The rTPA sample was collected and divided into small portions in a beaker. The rTPA was then stirred in a 2N KOH solution at 300-500 rpm for 1 hour until it was completely dissolved. To measure the transmittance of the solution, first, the 2N KOH solution alone was placed in an empty cell of a UV-VIS spectrometer to correct the transmittance to 100%. The rTPA completely dissolved in the 2N KOH was then collected and the transmittance was measured three times at 40 nm to calculate the average transmittance.

[0125] 3) Heat resistance transmittance (%) The rTPA sample was divided into small portions, placed in a heat-resistant test tube, and inserted into a block heater. The tube was then heated to 280°C for 3 hours. After the test tube was cooled, the heat-treated sample was collected and divided into small portions in a 100ml beaker. The rTPA was then added to a 2N KOH solution and stirred at 300-500 rpm for 1 hour until the rTPA was completely dissolved. The dissolved sample was then transferred to a centrifuge tube and centrifuged at 3000 rpm for 15 minutes. The supernatant was then removed. To measure the transmittance of the solution, the 2N KOH solution was first placed in an empty UV-VIS spectrometer cell and the transmittance was corrected to 100%. The rTPA completely dissolved in 2N KOH was then collected and the transmittance was measured three times at 40nm to calculate the average transmittance.

[0126] 4) Acid value The rTPA sample was divided into small portions (Ag) in a beaker, and pyridine and purified water (DIW) were added to the beaker. The mixture was stirred at 300-500 rpm for 1 hour to completely dissolve the sample. Then, pyridine, purified water, and three drops of phenolphthalein indicator were added to a blank beaker. The solution was then stirred and titrated using an automatic burette (Dosimat) equipped with a 0.5N (Factor = 1) NaOH standard solution. The endpoint was the point at which the color changed from colorless to purple. The amount of 0.5N NaOH consumed at this point was measured (b ml). The same procedure was repeated for the beaker containing the dissolved rTPA sample. The amount of 0.5N NaOH consumed at the endpoint was measured (a ml). The acid value (mg KOH / g) was calculated as follows: (ab) ml × F × 28.06 ÷ Ag.

[0127] [Table 1]

[0128] Referring to Table 1, Examples 1 and 2 show similar properties to Comparative Example 1. This is because the Examples do not use a solvent and recover EG decomposed by depolymerization during the depolymerization process, which is more economical than the Comparative Example.

[0129] Although the preferred embodiment of the present invention has been described above, the present invention is not limited thereto, and various modifications can be made within the scope of the claims, the description of the invention, and the accompanying drawings, which also fall within the scope of the present invention. [Industrial Applicability]

[0130] The present disclosure relates to a method for mechanochemical depolymerization of polyester, which does not require additional processes such as washing, crushing, or processing of waste collected from industries or households before feeding the waste into a chemical process, and is applicable regardless of the form of the waste. It does not require the addition of a large amount of additional solvent or a separate heat source for the reaction, has a simple apparatus configuration, requires low investment, and is environmentally advantageous because it does not require solvent treatment or disposal.

Claims

1. grinding the polyester in the presence of a decomposing agent to generate heat and depolymerize the polyester into a dibasic acid and an alkylene glycol; using the heat to vaporize and separate the alkylene glycol; A method for mechanochemical depolymerization of polyester.

2. 2. The method for mechanochemical depolymerization of polyester according to claim 1, wherein the depolymerization is carried out in a grinding reactor comprising a rotating disk.

3. 3. The method for mechanochemical depolymerization of polyester according to claim 2, wherein the grinding reactor further comprises blades attached to the disc.

4. The method for mechanochemical depolymerization of polyester according to claim 2 , wherein the grinding reactor further comprises a baffle on the inner wall.

5. 2. The method for mechanochemical depolymerization of polyester according to claim 1, wherein the heat includes heat of comminution generated by comminution of the polyester, frictional heat generated by friction of the polyester with the comminution reactor, or both.

6. 2. The method for mechanochemical depolymerization of polyester according to claim 1, wherein the vaporization and separation of the alkylene glycol is carried out by additionally supplying heat from an external source.

7. 2. The method for mechanochemical depolymerization of polyester according to claim 1, wherein the depolymerization is a solvent-free reaction carried out without adding a solvent.

8. 2. The method for mechanochemical depolymerization of polyester according to claim 1, wherein the method does not further comprise a step of pulverizing, crushing, or cutting the polyester before or after the depolymerization.

9. 3. The method for mechanochemical depolymerization of polyester according to claim 2, wherein the depolymerization is carried out by rotating the disc at 500 rpm to 5000 rpm.

10. 3. The method for mechanochemical depolymerization of polyester according to claim 2, wherein the heat causes the temperature inside the grinding reactor to be 130°C to 300°C.

11. 2. The method for mechanochemical depolymerization of polyester according to claim 1, wherein the depolymerization is carried out for 5 minutes to 120 minutes.

12. 2. The method for mechanochemical depolymerization of polyester according to claim 1, wherein the decomposition agent comprises an alkali, an acid, a salt thereof, a monoalcohol, a polyalcohol, or a mixture thereof.

13. the alkali comprises sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, or a mixture thereof; the acid comprises hydrochloric acid, nitric acid, sulfuric acid, carbonic acid, phosphoric acid, acetic acid, hypochlorous acid (HClO), or a mixture thereof; the salts include carbonates, bicarbonates, phosphates, sulfates, sulfites, nitrates, silicates, hypochlorites, formates, acetates, citrates, oxalates, or mixtures thereof; the monoalcohol comprises methanol, ethanol, propanol, butanol, or a mixture thereof; 13. The method for mechanochemical depolymerization of polyester according to claim 12, wherein the polyhydric alcohol comprises ethylene glycol, n-propylene glycol, isopropylene glycol, diethylene glycol, polyethylene glycol, triethylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, glycerin, benzyl alcohol, polypropylene glycol, pentaerythritol, trimethylolpropane, or a mixture thereof.

14. The method for mechanochemical depolymerization of polyester according to claim 1 , wherein the decomposition agent is in a solid state or an aqueous solution state.

15. 2. The method for mechanochemical depolymerization of polyester according to claim 1, wherein the decomposing agent is added in an amount of 0.75 mol to 3.0 mol per 1.0 mol of the dibasic acid contained in the polyester.

16. 2. The method for mechanochemical depolymerization of polyester according to claim 1, wherein the alkylene glycol is separated and the remaining dibasic acid is obtained in the form of a solid salt.

17. 17. The method for mechanochemical depolymerization of polyester according to claim 16, further comprising the step of additionally separating residual alkylene glycol from the solid dibasic acid salt.

18. The method for mechanochemical depolymerization of polyester comprises: The solid dibasic acid salt is dissolved in water, An aqueous solution of a dibasic acid salt is neutralized with an acid to precipitate dibasic acid crystals; The method for mechanochemical depolymerization of polyester according to claim 16, wherein the dibasic acid crystals are subjected to solid-liquid separation from the precipitated product.

19. 20. The method for mechanochemical depolymerization of polyester according to claim 18, wherein the method for mechanochemical depolymerization of polyester removes impurities from the aqueous solution of the dibasic acid salt.

20. The method for mechanochemical depolymerization of polyester according to claim 18, wherein the dibasic acid crystals are recrystallized.

21. A composition for polymerizing recycled polyester, comprising a dibasic acid and an alkylene glycol, obtained by the method for mechanochemical depolymerization of polyester according to claim 1.

22. A regenerated polyester produced using a dibasic acid and an alkylene glycol obtained by the method for mechanochemical depolymerization of polyester according to claim 1.

23. 23. The recycled polyester of claim 22, wherein the color of the recycled polyester has an L value of 45 or greater.

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