Process for mechanochemical depolymerization of polyesters

Abstract

The invention provides a method for mechanochemical depolymerization of polyester, which comprises the following steps of: crushing the polyester in the presence of a degradation agent to generate heat, depolymerizing the polyester into dibasic acid and alkylene glycol, and gasifying and separating the alkylene glycol by using the heat.

Description

Technical Field

[0001] The invention relates to a mechanochemical depolymerization method for polyester. Background Art

[0002] Polyesters, represented by polyethylene terephthalate (PET), have excellent chemical stability and are therefore used in fibers, fabrics, clothing, films, sheets, beverage bottles, and the like.

[0003] As the use of polyester has increased dramatically, various methods for recycling and reusing its waste have been studied. As one of them, a chemical recycling method is known, that is, waste polyesters such as polyesters are depolymerized and converted into monomers, which are then recycled and then repolymerized into recycled polyesters such as polyethylene terephthalate using the monomers as raw materials.

[0004] Chemical recycling of polyester can separate impurities, and as a raw material, its quality is not much different from that of virgin polyester. Therefore, it is highly anticipated as a solution that can achieve resource reuse.

[0005] Methods for depolymerizing polyester into monomers are roughly classified into three representative methods, namely, a hydrolysis method using water as a solvent, an alcoholysis method using an alcohol as a solvent, and a glycolysis method using a diol as a solvent.

[0006] The hydrolysis method may be, for example, a method of reacting a polyethylene terephthalate melt with water and then reacting with ammonium hydroxide to degrade it into terephthalic acid and ethylene glycol (see Patent Document 1). This method has the advantage of not requiring the use of diols or alcohols for the reaction, however, the reaction needs to be carried out under high pressure conditions, and therefore, a special high-pressure reactor is required.

[0007] The alcoholysis method is a method for depolymerizing polyester by, for example, heating in an alcohol solvent (adding a catalyst as needed) (see Patent Documents 2 and 3). In this method, for example, when methanol is used as a solvent to depolymerize polyethylene terephthalate, it has the advantages that dimethyl terephthalate (DMT), which is a useful and easy-to-handle monomer, can be directly generated by the depolymerization reaction, and the depolymerization reaction is relatively fast. However, the boiling point of the alcohol used as a solvent is low, and in order to react, it is necessary to pressurize (for example, react in methanol in a supercritical or subcritical state), and therefore, a special high-pressure reactor is required.

[0008] The glycolysis method is a method in which polyester and a depolymerization catalyst such as sodium carbonate are heated together in an excess of an alkylene glycol solvent to depolymerize and generate bis(β-hydroxyalkyl) terephthalate and ethylene glycol (see Patent Documents 4 and 5). For example, when ethylene glycol is used as a solvent, bis(β-hydroxyethyl) terephthalate (BHET) is generated by a depolymerization reaction, and methanol can be added in the presence of an ester exchange catalyst to perform an ester exchange reaction to recover dimethyl terephthalate (DMT). The glycolysis method can react at normal pressure, but the reaction time is long, so the reaction time needs to be shortened, and there is a problem that the diol as a solvent deteriorates due to long-term heating.

[0009] These known polyester depolymerization methods require at least several hours of reaction, so it is difficult to treat a large amount of polyester waste, and require reactions under high temperature or high pressure conditions, so special equipment that can withstand such reaction conditions is required, and therefore, commercialization is currently not possible.

[0010] Prior Art Literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Publication No. 2003-527363 (Publication Date: September 16, 2003)

[0013] Patent Document 2: Japanese Patent Publication No. 1999-100336 (Publication Date: April 13, 1999)

[0014] Patent Document 3: Japanese Patent Publication No. 2003-300916 (Publication Date: October 21, 2003)

[0015] Patent Document 4: Japanese Patent Publication No. 2002-167468 (Publication Date: 2002.06.11)

[0016] Patent Document 5: Japanese Patent Publication No. 2004-300115 (Publication Date: 2004.10.28) Summary of the invention

[0017] Technical issues

[0018] One embodiment of the present invention can provide a mechanochemical depolymerization method of polyester, which can shorten the process and save heat energy, thus being beneficial to the environment and reducing the preparation cost.

[0019] Another embodiment can provide a composition for polymerizing recycled polyester obtained by a mechanochemical depolymerization process of polyester.

[0020] Another embodiment can provide a recycled polyester made using a dibasic acid and an alkylene glycol obtained through a mechanochemical depolymerization method of polyester.

[0021] Technical Solution

[0022] In one embodiment of the mechanochemical depolymerization method of polyester, polyester is pulverized in the presence of a degradation agent to generate heat, and the polyester is depolymerized into dibasic acid and alkylene glycol, and the alkylene glycol is gasified and separated by heat.

[0023] Depolymerization can be carried out in a pulverizing reactor comprising a rotating disk.

[0024] The pulverizing reactor may further include a plurality of blades attached to the rotating disk.

[0025] The pulverizing reactor may further include a plurality of baffles located on the inner wall.

[0026] The pulverization reactor may further include a discharge port for discharging the vaporized alkylene glycol.

[0027] The heat may include pulverization heat generated by pulverization of the polyester, friction heat generated by friction between the polyester and a pulverization reactor, or both.

[0028] The depolymerization may be a solvent-free reaction performed without adding a solvent.

[0029] Depolymerization can be achieved without additional application of heat from the outside, in addition to the heat mentioned.

[0030] The mechanochemical depolymerization process of polyester does not include the step of pulverizing, crushing or cutting the polyester before or after depolymerization.

[0031] Depolymerization can be performed by rotating the turntable at 500 rpm to 5000 rpm.

[0032] The internal temperature of the pulverizing reactor can be raised to 130°C to 300°C by heat.

[0033] The depolymerization can be carried out for 5 minutes to 120 minutes.

[0034] The degradation agent may include a base, an acid, their salts, a monohydric alcohol, a polyhydric alcohol, or a mixture thereof.

[0035] The base may include sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, or mixtures thereof.

[0036] The acid may include hydrochloric acid, nitric acid, sulfuric acid, carbonic acid, phosphoric acid, acetic acid, hypochlorous acid (HClO), or mixtures thereof.

[0037] The salt may include carbonates, bicarbonates, phosphates, sulfates, sulfites, nitrates, silicates, hypochlorites, formates, acetates, citrates, oxalates, or mixtures thereof.

[0038] The monohydric alcohol may include methanol, ethanol, propanol, butanol, or mixtures thereof.

[0039] The polyol may include ethylene glycol, n-propylene glycol, isopropylene glycol, diethylene glycol, polyethylene glycol, triethylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, glycerol, benzyl alcohol, polypropylene glycol, pentaerythritol, trimethylolpropane, or mixtures thereof.

[0040] The degradation agent can be in a solid state or in an aqueous solution state.

[0041] The degradation agent may be added in an amount of 0.75 mol to 3.0 mol relative to 1.0 mol of the dibasic acid in the polyester.

[0042] In the mechanochemical depolymerization process of polyester, the dibasic acid remaining after separation of the alkylene glycol can be obtained in the form of a salt in a solid phase.

[0043] The mechanochemical depolymerization method of polyester may further comprise the step of further separating the residual alkylene glycol from the dibasic acid salt in the solid phase.

[0044] In the mechanochemical depolymerization method of polyester, the obtained solid phase dibasic acid salt can be dissolved in water, and the aqueous solution of the dibasic acid salt can be neutralized with an acid to precipitate dibasic acid crystals, and the dibasic acid crystals can be obtained from the precipitated product by solid-liquid separation.

[0045] In the mechanochemical depolymerization process of polyesters, impurities can be removed from aqueous solutions of dibasic acid salts.

[0046] In the mechanochemical depolymerization process of polyesters, dibasic acid crystals can be recrystallized.

[0047] Another embodiment of the composition for polymerizing recycled polyester includes a dibasic acid obtained by a mechanochemical depolymerization method of polyester and an alkylene glycol.

[0048] Another embodiment of the recycled polyester is made using a dibasic acid and an alkylene glycol obtained by a mechanochemical depolymerization process of the polyester.

[0049] The L value of the color of the recycled polyester may be 45 or more.

[0050] Beneficial Effects

[0051] According to one embodiment of the mechanochemical depolymerization method of polyester, there is no need to perform additional steps such as washing, crushing or processing before the waste collected from industry or ordinary households is put into the chemical process, and its application is not limited to the form of the waste. It does not require the input of a large amount of additional solvent, nor does it require an additional heat source required for the reaction. Therefore, the structure of the equipment is simple and the investment cost is low. There is no need to treat and discard the solvent, so it is also beneficial to the environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 is a process flow diagram showing a method for mechanochemical depolymerization of polyester. DETAILED DESCRIPTION

[0053] The advantages, features and methods for implementing the technologies described below will become clearer through the accompanying drawings and multiple implementation examples described below. However, the implementation form is not limited to the multiple implementation examples disclosed below. Unless otherwise defined, all terms (including technical terms and scientific terms) used in this specification can use the meanings commonly understood by those skilled in the art. In addition, unless otherwise defined, multiple terms defined in commonly used dictionaries will not be interpreted ideally or excessively.

[0054] Throughout this specification, unless otherwise expressly stated, when a part "includes" a certain constituent element, it does not exclude other constituent elements, but indicates that other constituent elements may be further included. In addition, unless otherwise defined, the singular expression also includes the plural.

[0055] Figure 1 1 is a process flow chart showing a method for mechanochemical depolymerization of polyester according to one embodiment. Figure 1 , the mechanochemical depolymerization method of polyester is explained.

[0056] In the mechanochemical depolymerization method of polyester, polyester is pulverized in the presence of a degradation agent to generate heat and depolymerize the polyester (S1).

[0057] The polyester may be a polyester polymerized from a dibasic acid and an alkylene glycol, such as polyethylene terephthalate (PET), polybutylene terephthalate, polyethylene naphthalate, or may be polycaprolactone polymerized from caprolactone. By depolymerizing these polyesters, the dibasic acid and the alkylene glycol can be recovered as monomers, or caprolactone can be further recovered as a monomer.

[0058] The alkylene glycol that can be obtained by depolymerizing the polyester may include, for example, 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, or 1,4-benzene glycol, etc. For example, when the polyester is polyethylene terephthalate, ethylene glycol can be recovered as the alkylene glycol, and when the polyester is polybutylene terephthalate, butanediol can be recovered as the monomer.

[0059] The dibasic acid obtainable by depolymerizing the polyester may include, for example, aromatic dicarboxylic acids such as terephthalic acid, orthophthalic acid, isophthalic acid, dibromoisophthalic acid, sodium sulfoisophthalate, benzenedioxydicarboxylic acid, 4,4′-diphenyldicarboxylic acid, 4,4′-diphenyletherdicarboxylic acid, 4,4′-benzophenonedicarboxylic acid, 4,4′-diphenoxyethanedicarboxylic acid, 4,4′-dicarboxybiphenylsulfone, 2,6-naphthalene dicarboxylic acid, trimellitic acid, or pyromellitic acid. Other dicarboxylic acids may include, for example, alicyclic dicarboxylic acids such as hexahydroterephthalic acid or hexahydroisophthalic acid, and aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedicarboxylic acid, or dodecanedicarboxylic acid.

[0060] As a raw material for depolymerization of polyester, a molded product containing polyester, especially waste, can be used. In addition to waste generated after using a molded product containing polyester, waste can also be residues or defective products generated during the process of manufacturing the molded product. For example, waste can be used PET bottles, cups, ropes, packaging bags, etc., or burrs and sprues formed during the molding process of these, sheets, fibers, fabrics, clothes, films or sheets cut from vacuum-formed cups, etc.

[0061] At this time, in the polyester waste, the content of polyester can be 60 wt% to 100 wt% relative to the total weight of the polyester waste. When the content of polyester is less than 60 wt%, compared with the raw and auxiliary materials obtained by the depolymerization process, there are too many side reaction products and unrecoverable separation and purification wastes, so it may not be economical.

[0062] When polyester waste is used as the feed material for depolymerization, a pretreatment step may be optionally performed before the depolymerization step, that is, the waste is washed and contaminants attached to the waste, such as dyes, contents, soil, etc., are removed.

[0063] In addition, in the mechanochemical depolymerization method of polyester, depolymerization occurs while pulverizing, so the recycled polyester waste can be depolymerized directly or in larger slices, or the waste can be selectively cut, pulverized or processed into an appropriate size by mechanical means. The pulverization process can be carried out by using a known appropriate method, for example, pulverization can be carried out using a hammer mill, etc., so that the polyester waste is pulverized into small pieces with a size of 2 mm to 8 mm, and then provided to the depolymerization reaction.

[0064] Furthermore, as required, the polyester may be immersed in a solvent to extract and remove dyes and the like, or components lighter than the solvent may be separated, or only components of a specified size may be recovered by blowing them off with wind or using a sieve to be subjected to a depolymerization reaction.

[0065] However, as described above, in the mechanochemical depolymerization method of polyester, the polyester is depolymerized by utilizing the heat generated while the polyester is pulverized. Therefore, there is no need to perform additional washing, pulverizing, crushing, cutting or processing (flake formation, popcorn formation) steps in order to feed the waste collected from industry or general households into the chemical process, and waste in various forms can be used as feedstock.

[0066] The degradation agent may include a base, an acid, a salt thereof, a monohydric alcohol, a polyhydric alcohol or a mixture thereof. In addition, as described below, when the depolymerization reaction is carried out in a solvent-free reaction, the degradation agent may be added in a solid state or an aqueous solution state.

[0067] For example, the alkali may include a hydroxide of an alkali metal or an alkaline earth metal, ammonia or a mixture thereof. The alkali metal may be a monovalent metal such as lithium, sodium, potassium, rubidium, cesium, etc., wherein relatively cheap sodium or potassium may be used. The alkaline earth metal may be beryllium, magnesium, calcium, strontium, barium, radium, etc. The hydroxide of the alkali metal may be, for example, sodium hydroxide, potassium hydroxide or lithium hydroxide. When sodium hydroxide is used in combination with ethylene glycol, etc., the reaction speed and reaction rate are excellent.

[0068] The acid may be an organic acid or an inorganic acid, for example, hydrochloric acid, nitric acid, sulfuric acid, carbonic acid, phosphoric acid, acetic acid, hypochlorous acid (HClO), or a mixture thereof.

[0069] The salt may include inorganic salts such as carbonates, bicarbonates, phosphates, sulfates, sulfites, nitrates, silicates, hypochlorites, organic salts such as formates, acetates, citrates, oxalates, or mixtures thereof, and may include, for example, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, tripotassium phosphate (hydrate), sodium hypochlorite, or mixtures thereof.

[0070] The polyol may include ethylene glycol, n-propylene glycol, isopropylene glycol, diethylene glycol, polyethylene glycol, triethylene glycol, dipropylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, glycerol, benzyl alcohol, polypropylene glycol, pentaerythritol, trimethylolpropane or a mixture thereof. Among these polyols, ethylene glycol, n-propylene glycol, isopropylene glycol, diethylene glycol, polyethylene glycol, triethylene glycol or glycerol having a high boiling point and high reactivity can be used.

[0071] The monohydric alcohol may include methanol, ethanol, propanol, butanol or a mixture thereof, and may be an alkyl ether compound of a polyhydric alcohol such as the above-mentioned diol or triol, for example, diethylene glycol monomethyl ether, benzyl alcohol or 2-ethylhexanol.

[0072] The degradation agent reacts with both the polyester and the dye. Therefore, the amount of the degradation agent added can be 0.75 mol to 3.0 mol relative to 1.0 mol of the dibasic acid in the polyester. For example, the appropriate amount of a monovalent degradation agent (e.g., sodium hydroxide) added is 1.5 mol to 3.0 mol, and the appropriate amount of a divalent degradation agent (e.g., calcium carbonate) added is 0.75 mol to 1.5 mol.

[0073] As an example, when sodium hydroxide is used as a degradation agent to degrade polyethylene terephthalate, the reaction formula is PET (polyethylene terephthalate) + 2NaOH → TPA salt (disodium terephthalate) + EG (ethylene glycol). Therefore, in order to degrade 1 kg of PET, 2 moles (420 g) of NaOH are required for every 1 mole of TPA.

[0074] In addition, when the polyester supply raw material is fiber, the amount of dye added during processing is 0.5% to 20% by weight of the weight of the polyester fiber (based on the weight of the fabric (owf)), and therefore, the amount of the degradation agent added is preferably 2.0 mol or more relative to 1.0 mol of the dibasic acid in the polyester in order to completely degrade the dye and the polyester fiber.

[0075] In the mechanochemical depolymerization method of polyester, the polyester is crushed in the presence of a degradation agent and the heat generated thereby is used to depolymerize the polyester. Therefore, a high-pressure reactor is not required, and a batch reactor or a continuous reactor can be used to construct the device. As an example, after the raw materials are added, the inside of the reactor can be replaced with nitrogen (N2), and the reaction can be carried out in a nitrogen (N2) atmosphere during the entire process to suppress side reactions.

[0076] Depolymerization can be carried out in a grinding reactor including a device capable of grinding the polyester. As an example, the grinding reactor can be provided with a rotating disk at its bottom. The rotating disk grinds the polyester by rotating and causes the polyester to rub against the inner wall of the grinding reactor.

[0077] The rotating disk may be in the form of a mixer with a plurality of blades attached to the surface, for example, so as to more efficiently pulverize the polyester. In addition, the pulverizing reactor may further include a plurality of baffles located on the inner wall so as to increase the friction heat generated by the friction between the polyester and the inner wall of the pulverizing reactor.

[0078] Inside the pulverizing reactor, pulverizing heat is generated by pulverizing the polyester, and friction heat is generated by friction between the polyester and the inner wall of the pulverizing reactor. By the heat generated in this way, the polyester reacts with the degradation agent to depolymerize. For example, the heat may include pulverizing heat, friction heat, or both.

[0079] As described above, in the mechanochemical depolymerization method of polyester, the polyester is pulverized in the presence of a degradation agent and the polyester is depolymerized by utilizing the heat generated thereby. Therefore, it is not necessary to add a large amount of solvent, but it can be carried out in a solvent-free reaction mode, and it can be achieved without applying heat from the outside except for the heat generated internally. As an example, the mechanochemical depolymerization method of polyester can be carried out without adding a solvent such as water or ethylene glycol.

[0080] In addition, in the mechanochemical depolymerization method of polyester, there is no need to separately control the heat or add a large amount of solvent for increasing the uniformity of the reaction, and no additional heat source is required for the reaction. Therefore, the structure of the equipment is simple and the investment cost is low. In addition, there is no need to additionally treat and discard the solvent, which is beneficial to the environment. In addition, the obtained depolymerization product, that is, the dibasic acid, can be a salt in the form of a paste, powder, or pellet, so the volume is reduced and it is easy to store and transport.

[0081] As an example, the deagglomeration can be performed by rotating the turntable at 500 rpm to 5000 rpm, for example, 700 rpm to 2500 rpm or 1000 rpm to 1200 rpm. When the deagglomeration is performed, if the rotation speed of the turntable is less than 500 rpm, the pulverization cannot be performed effectively, and thus sufficient heat cannot be generated by pulverization and friction. If the rotation speed of the turntable is greater than 5000 rpm, the heat generated by pulverization and friction may be excessive.

[0082] As an example, the heat generated above can make the internal temperature of the pulverizing reactor reach 130° C. to 300° C., for example, 170° C. to 280° C. or 200° C. to 250° C. When the internal temperature of the pulverizing reactor is less than 130° C., the depolymerization efficiency is reduced, and when it is greater than 300° C., depolymerization by-products are generated.

[0083] The depolymerization can be carried out for 5 minutes to 120 minutes, 5 minutes to 60 minutes or 10 minutes to 40 minutes. The higher the concentration of impurities including dyes in the polyester, the larger the amount of polyester to be processed, and the lower the temperature inside the pulverization reactor, the longer the depolymerization reaction time will be within the range of 120 minutes.

[0084] On the other hand, in the mechanochemical depolymerization method of polyester, the alkylene glycol produced can be gasified by utilizing the heat generated during the depolymerization process to separate and recover it.

[0085] In the mechanochemical depolymerization method of polyester, the polyester is pulverized in the presence of a degradation agent and the polyester is depolymerized by utilizing the heat generated thereby. Therefore, the depolymerization can be carried out in a solvent-free reaction manner. The pulverization heat and / or friction heat generated during the depolymerization process can make the temperature inside the pulverization reactor reach up to 300° C., so that the generated alkylene glycol can be gasified during the depolymerization to be separated and recovered.

[0086] However, the present invention is not limited thereto, and in order to gasify the alkylene glycol to separate and recover it, heat may be additionally supplied from the outside. That is, although the pulverization heat and friction heat generated inside the pulverization reactor are sufficient to depolymerize the polyester, the pulverization heat and friction heat generated inside the pulverization reactor alone may not be sufficient to gasify the alkylene glycol. In this case, heat may need to be supplied from the outside.

[0087] On the contrary, in the existing alkaline hydrolysis depolymerization method, a large amount of depolymerization solvent is added, and the polyester is depolymerized in a high-temperature reactor. In this case, an additional process is required, that is, to obtain a liquid phase of alkylene glycol and a solid phase of dibasic acid salt from the depolymerization product obtained during the depolymerization by solid-liquid separation. In addition, the depolymerization solvent and the polyester need to be raised to the reaction temperature, so a large amount of energy needs to be consumed. In addition, although the used depolymerization solvent can be separated from the product and reused, an additional purification process is required for this purpose, so a large amount of cost is incurred. In addition, depending on the type of depolymerization solvent used, more side reaction substances may be generated, so it is difficult to adjust the impurities.

[0088] In the mechanochemical depolymerization method of polyester, during the depolymerization process, alkylene glycol can be recovered in an amount of 0% by weight or more or 100% by weight or less, for example, 30% by weight to 95% by weight or 50% by weight to 98% by weight, relative to the total weight of alkylene glycol. When the content of alkylene glycol that can be recovered during depolymerization is 0% by weight, alkylene glycol is discharged mixed with dibasic acid, and therefore, an additional step of separation is required before dissolution. When the content of alkylene glycol that can be recovered during depolymerization is 100% by weight, alkylene glycol is not mixed in the generated dibasic acid, and therefore, the dibasic acid can be directly dissolved before proceeding to the next step.

[0089] In addition, the pulverizing reactor may further include an outlet for discharging the vaporized alkylene glycol. The vaporized alkylene glycol may be under normal pressure conditions, and nitrogen (N2) may be introduced, and the gas flow may be utilized to liquefy and recover the gas in a condenser.

[0090] After the depolymerization reaction, the dibasic acid remaining after separation of the alkylene glycol can be obtained in the form of a salt in a solid phase.

[0091] However, depending on the rotation speed of the turntable of the pulverizing reactor, the alkylene glycol may not be recovered at all, or only a part of the alkylene glycol may be recovered. At this time, the depolymerization product may contain a mixture of a dibasic acid salt and residual alkylene glycol. The mechanochemical depolymerization method of polyester may selectively further include a process of recovering the alkylene glycol after separating the alkylene glycol and the dibasic acid salt from the depolymerization product. As an example, the present invention is not particularly limited to the method for separating the alkylene glycol and the dibasic acid salt generated by the depolymerization reaction. An appropriate method can be selected according to the target compound, for example, separation can be carried out by distillation and concentration. Existing distillation and concentration devices can be used, for example, a vacuum continuous distillation device, a vacuum intermittent distillation device, etc. as a device for distillation and concentration. Afterwards, a solid phase dibasic acid salt can be obtained through a drying process.

[0092] In addition, depending on the type of degradation agent used during depolymerization, there are three methods: hydrolysis using an alkali, alcoholysis using an alcohol, and glycolysis using a diol, and different types of dibasic acids are obtained from these methods.

[0093] When polyester is depolymerized by glycolysis, the obtained products may be different depending on the type of degradation agent used in the depolymerization reaction. For example, when ethylene glycol is used as a degradation agent to depolymerize polyethylene terephthalate, bis(β-hydroxyethyl)terephthalate (BHET) may be obtained as a monomer, and when propylene glycol is used as a degradation agent for depolymerization, bis(β-hydroxyethylisopropyl)terephthalate (BHEPT) may be mainly obtained as a monomer.

[0094] In addition, when benzyl alcohol and tripotassium phosphate are used as degradation agents, the oligomer ester compounds obtained have good solubility in chloroform and the like, and can therefore be efficiently recovered by solvent extraction. The dibasic acid or oligomer ester compounds can also be recovered by other means such as filtration or distillation.

[0095] The ester compound of the dibasic acid or oligomer (for example, BHET) thus obtained may be subjected to an ester exchange reaction with methanol to recover the methyl ester of the dibasic acid or oligomer (for example, DMT).

[0096] The transesterification reaction can be appropriately carried out by a known method, for example, a depolymerization reaction concentrate is subjected to a transesterification reaction with methanol at 65° C. to 85° C. for 0.5 to 5 hours in the presence of a transesterification reaction catalyst (alkali metal compound, etc.), thereby obtaining a slurry in which DMT is dissolved in a mixed solution of methanol and alkylene glycol, etc. In addition, a filter cake containing DMT can be separated and purified by distillation using a solid-liquid separation device, etc., to recover purified DMT.

[0097] On the other hand, when polyester is depolymerized by hydrolysis, dibasic acid or dibasic acid salt such as dialkali metal salt of oligomer may be generated, and the product may differ depending on the type of alkali metal used in the depolymerization reaction. For example, when sodium hydroxide is used as a degradation agent to depolymerize polyethylene terephthalate, sodium terephthalate is generated together with ethylene glycol.

[0098] Among them, the dibasic acid salt (e.g., sodium terephthalate) is insoluble in alkylene glycol and forms solid phase crystals, so it can be easily separated from the solvent by filtering methods such as solid-liquid separation. In addition, washing with alcohol such as methanol or ethanol can be used to remove the alcohol attached to the obtained powdery crystals.

[0099] Next, after the crystals of the dibasic acid salt are dissolved in water (S2), an acid is mixed to perform a neutralization reaction, thereby precipitating and obtaining a dibasic acid (e.g., terephthalic acid) (S4). The precipitated dibasic acid can be separated from water by solid-liquid separation by centrifugation or the like, thereby being recovered (S5).

[0100] The amount of water added during the recovery process may be 3 to 10 parts by weight relative to 1 part by weight of the dibasic acid salt. The amount of acid supplied for separating the dibasic acid may be greater than or equal to the molar amount of the alkali metal in the dibasic acid salt. The acid used may be a strong acid having a pH of 2 such as hydrochloric acid, sulfuric acid, phosphoric acid, or an organic acid such as formic acid, acetic acid, or oxalic acid. Inorganic acids, especially hydrochloric acid or sulfuric acid, can reduce impurities in the generated monomer. The temperature of the neutralization reaction may be 20 to 85°C, and the neutralization reaction may be completed in 10 minutes to 5 hours.

[0101] On the other hand, the method for mechanochemical depolymerization of polyester may further optionally include a step (S3) of removing impurities in the polyester molded product or waste in an aqueous solution of a dibasic acid salt.

[0102] There is no particular limitation on the method for removing impurities, and appropriate techniques, devices, etc. can be used. For example, a sieve or the like can be used to remove solids such as unreacted resins (polyethylene, polypropylene, polyvinyl chloride, etc.) other than polyester, and degradation agents including undissolved alkali metals. In addition, dyes, fillers, etc. cannot be removed by a sieve, and therefore, they can be removed by centrifugal separation, filtration treatment using an adsorbent such as activated carbon, etc.

[0103] The obtained dibasic acid crystals have a particle size distribution characteristic of several μm to several hundred μm, and therefore, the process may further include: recrystallizing the dibasic acid crystals to a particle size (100 μm or more) that can be commercialized (S6). There is no particular limitation on the recrystallization method, and for example, the process may be carried out under high temperature and high pressure conditions after mixing the dibasic acid crystals with water.

[0104] The dibasic acid and alkylene glycol obtained by the mechanochemical depolymerization method of polyester can be reused as monomers for polymerizing recycled polyester. Specifically, the composition for polymerizing recycled polyester containing the dibasic acid and alkylene glycol obtained by the mechanochemical depolymerization method of polyester can be subjected to esterification and polycondensation to prepare recycled polyester.

[0105] Esterification is the reaction of the recovered monomer, i.e., dibasic acid, with an alkylene glycol, for example, terephthalic acid and ethylene glycol. The reaction can be carried out without a catalyst, but can also be carried out in the presence of a catalyst such as an alkaline earth metal compound of magnesium, calcium, etc., or a metal compound of titanium, zinc, manganese, etc., which is well known as an ester exchange catalyst.

[0106] Afterwards, the product of the esterification process can be subjected to polycondensation to prepare a recycled polyester resin. In this case, solution polymerization or solid phase polymerization can be used.

[0107] Furthermore, after the solid phase polymerization, a compound for promoting water treatment and / or crystallization may be added as necessary to adjust the quality, and a polycondensation catalyst or stabilizer may be added at the beginning or during the polycondensation step.

[0108] The water treatment can be carried out by bringing the recycled polyester resin produced in the form of solid particles into contact with water, water vapor, an inert gas containing water vapor, air containing water vapor, etc. The compound that promotes crystallization can be, for example, polyolefin thermoplastic resins such as polyhexamethylene terephthalate, inorganic compounds, higher aliphatic compounds, polyether compounds, polypropylene, polyethylene, etc., and 1 ppm to 100 ppm of these compounds can be added to the recycled polyester resin.

[0109] Germanium, antimony, titanium, aluminum and other compounds can be used as polycondensation catalysts. The amount of polycondensation catalyst added can be 2 ppm to 800 ppm, for example, 4 ppm to 500 ppm, based on the weight of the catalytic metal element relative to the total weight of the dibasic acid component.

[0110] In addition, the recycled polyester may contain phosphorus compounds such as trimethyl phosphate, triethyl phosphate, triphenyl phosphate, triethyl phosphoacetate, triphenyl phosphite, tridodecyl phosphite, methyl phosphate, dibutyl phosphate, monobutyl phosphate, phosphoric acid, phosphoric acid, hypophosphorous acid, polyphosphoric acid, etc. as stabilizers. The amount of the stabilizer added may be 1000 ppm or less, for example, 500 ppm or less, or 300 ppm or less, based on the weight of phosphorus in the stabilizer relative to the total weight of the recycled polyester.

[0111] On the other hand, as described above, in the mechanochemical depolymerization method of polyester in one embodiment, the polyester is depolymerized using the heat generated while the polyester is pulverized, so the L value of the color of the regenerated polyester made from the dibasic acid and alkylene glycol obtained can be 45 or more, for example, 60 or more, or 65 to 95.

[0112] The L, a and b color system is an internationally used standard for evaluating the color of polyester. This color value is one of the color systems used to standardize color measurement, which describes the perceived color and color difference. In this system, L is the lightness factor, and a and b are color measurement values. Among them, the L value is a numerical factor representing lightness, which is a very important value in the manufacture of fibers, fabrics or clothing. In addition, a positive b value indicates a yellow discoloration, a negative b value indicates a blue discoloration, a positive a value indicates a red discoloration, and a negative a value indicates a green discoloration.

[0113] L, a, and b values ​​are defined in KS A 0061, 0063, 0064, 0065, 0066, 0067, 0084, 0085, 0089, 0114, etc. of the Korean Industrial Standard (KS) regarding color measurement. As an example, moisture in 50 g of a polyester resin as a measurement object can be removed in the air, and then the resin is placed in a colorimeter SA-2000, and its color is measured 10 times. The average value is defined as L, a, and b values.

[0114] The L value of recycled polyester is related to the purity of the regenerated dibasic acid and alkylene glycol. For example, the more impurities or matting agents such as titanium dioxide (TiO2) there are, the lower the L value. When the L value is less than 45, the content of impurities is too large, so the color after polymerization or fiberization is colored, or more side reactions occur during polymerization, so it cannot be used as a high value-added material, and is mainly used for the same purposes as mechanically recycled polyester. On the other hand, when the L value is greater than 95, it has physical properties that are difficult to achieve even with existing virgin materials. Therefore, in order to achieve this value through regeneration, it is necessary to add decolorization and purification steps, or increase the residence time, etc., that is, increase the unit production volume and process costs, thereby reducing the economic efficiency.

[0115] Hereinafter, a plurality of specific embodiments of the present invention are proposed. However, the plurality of embodiments described below are only used to specifically exemplify or illustrate the present invention, and the scope of the present invention is not limited thereto.

[0116] [Preparation Example: Mechanochemical Depolymerization of Polyester]

[0117] (Example 1)

[0118] 1 kg of waste polyethylene terephthalate (waste PET) and 416.3 g of sodium hydroxide (NaOH) were introduced into a pulverization reactor.

[0119] After the raw materials are charged, the interior of the pulverization reactor is replaced with nitrogen (N2), and the reaction is carried out under a nitrogen (N2) atmosphere during the entire process to suppress side reactions.

[0120] The turntable of the pulverizing reactor is rotated at 1,200 rpm, heat is generated by friction and pulverization, and waste PET is depolymerized into terephthalic acid (TPA) salt and ethylene glycol (EG). At the same time, the EG is gasified and recovered by using the heat of 240°C generated by friction and pulverization. The gasified EG is under normal pressure conditions, and in the process, nitrogen (N2) is added and liquefied and recovered in the condenser by using the air flow.

[0121] After EG is separated, it is discharged from the pulverizing reactor and TPA salt powder is obtained.

[0122] After the TPA salt is dissolved in water, it is purified and precipitated to recover terephthalic acid (TPA).

[0123] (Example 2)

[0124] 1 kg of waste polyethylene terephthalate (waste PET) was introduced into the pulverization reactor.

[0125] The turntable of the pulverizing reactor was rotated at 500 rpm, and 832.6 g of an aqueous sodium hydroxide solution (NaOH: 50 wt%) was uniformly added in a spraying manner.

[0126] After the raw materials are charged, the interior of the pulverization reactor is replaced with nitrogen (N2), and the reaction is carried out under a nitrogen (N2) atmosphere during the entire process to suppress side reactions.

[0127] The turntable of the pulverizing reactor is rotated at 1,200 rpm, heat is generated by friction and pulverization, and waste PET is depolymerized into terephthalic acid (TPA) salt and ethylene glycol (EG). At the same time, EG and water (H2O) are gasified and recovered by using the heat of 240°C generated by friction and pulverization. The gasified EG is under normal pressure conditions, and in the process, nitrogen (N2) is added and liquefied and recovered in the condenser by using the air flow.

[0128] After EG is separated, it is discharged from the pulverizing reactor and TPA salt powder is obtained.

[0129] The TPA salt is dissolved in water, and then purified and precipitated to recover terephthalic acid (TPA).

[0130] (Comparative Example 1)

[0131] Waste polyethylene terephthalate (waste PET) was pulverized into a size of less than 10 mm using a pulverizer.

[0132] 1 kg of pulverized polyethylene terephthalate (PET) and 416.3 g of sodium hydroxide (NaOH) were dissolved in 6 kg of ethylene glycol (EG), supplied to a depolymerization reactor, and depolymerized at 180° C. for 60 minutes while being stirred with a stirrer.

[0133] The depolymerization products of terephthalic acid (TPA) salt and ethylene glycol (EG) are separated into solid and liquid forms by a decanter. The ethylene glycol (EG) as a liquid phase is distilled to remove impurities and recovered, while the terephthalic acid (TPA) salt as a solid phase is dissolved in water to remove impurities.

[0134] Sodium hydroxide (NaOH) is added to the aqueous terephthalic acid (TPA) salt solution from which impurities have been removed to reduce the pH thereof to 3.0 or less, and terephthalic acid (TPA) is recovered.

[0135] [Experimental Example: Physical Properties of Terephthalic Acid]

[0136] The physical properties of the terephthalic acid produced in Example 1, Example 2, and Comparative Example 1 were measured, and the results are shown in Table 1.

[0137] 1) ΔY (delta Y)

[0138] After the regenerated terephthalic acid (rTPA) sample is divided into small portions and placed in a beaker, it is dissolved in 2.8% NH4OH (ammonia solution) for more than 1 hour. After the solution is completely dissolved, the solution is passed through a glass filter paper (pore size: 0.4 μm), and the filter paper is dried at 50°C for 1 hour and then cooled. After the cooled filter paper is placed at the measurement position, the Y value (A) is measured using a color spectrometer. After a blank test solution (liquid containing only NH4OH but not rTPA sample) is passed through the filter paper and dried, the Y value (B) is measured as a reference, and calculation is performed using ΔY=AB.

[0139] 2) Alkali transmittance (%)

[0140] After taking small portions of the rTPA sample and placing them in a beaker, the rTPA was stirred in a 2N KOH aqueous solution at 300rpm-500rpm for 1 hour until it was completely dissolved. Then, in order to measure the transmittance of the solution, only 2N KOH solution was placed in the blank cell of the UV-VIS spectrophotometer to calibrate the transmittance to 100%. Then, the rTPA completely dissolved in 2N KOH was taken, and the transmittance was repeatedly measured three times at 40nm, and the transmittance average was calculated.

[0141] 3) Heat resistance transmittance (%)

[0142] After the rTPA sample is divided into small portions and placed in a forced heat-resistant test tube and inserted into a block heater, it is heated at 280°C for 3 hours. After the test tube is heated and cooled, the heat-treated sample is divided into small portions and placed in a 100 mL beaker. The rTPA is stirred for 1 hour in a 2N KOH aqueous solution until it is completely dissolved. After the dissolved sample is transferred to a centrifugal separation tube and centrifuged at 3000 rpm for 15 minutes, the supernatant is taken. In order to measure the transmittance of the solution, only 2N KOH solution is first placed in the blank cell of the UV-VIS spectrophotometer to calibrate the transmittance to 100%. After that, the rTPA completely dissolved in 2N KOH is taken, and the transmittance is repeatedly measured three times at 40 nm, and the transmittance average is calculated.

[0143] 4) Acid value

[0144] After the rTPA sample is divided into small portions (A g) and placed in a beaker and pyridine and pure water (DIW) are added to the beaker, the sample is stirred at 300 rpm to 500 rpm for 1 hour to completely dissolve the sample. After pyridine, pure water and three drops of phenolphthalein reagent are added to the prepared blank test beaker, the solution is stirred and titrated using an automatic titrator (Dosimat) equipped with a 0.5N (Factor = 1) NaOH standard solution. The end point is the moment when the color changes from colorless to purple, and the consumption of 0.5N NaOH at this time is measured (b mL). The same method is used for the beaker in which the rTPA sample is dissolved to measure the consumption of 0.5N NaOH at the end point (a mL), and the acid value (KOH mg / g) is calculated = (ab) mL × F × 28.06÷A g.

[0145] Table 1:

[0146] Forced heat resistance Alkali transmittance Acid value ΔY Example 1 80 83.6 673.3 33 Example 2 82.5 91.1 675.1 18.1 Comparative Example 1 82.2 81.9 672.8 46.5

[0147] Referring to Table 1, compared with Comparative Example 1, Example 1 and Example 2 show similar characteristics. Compared with the Comparative Example, the Example does not use a solvent and recovers the EG degraded by depolymerization in the depolymerization process, so it can be called a more economical method.

[0148] Preferred embodiments of the present invention have been described above, but the present invention is not limited thereto, and various modifications may be made within the scope of the claims, the specification, and the drawings, and these modifications also fall within the scope of the present invention.

[0149] Industrial Applicability

[0150] The present invention relates to a mechanochemical depolymerization method of polyester, which does not require additional steps such as cleaning, crushing or processing before waste collected from industry or general households is put into a chemical process, and its application is not limited to the form of the waste. It does not require the input of a large amount of additional solvent, nor does it require an additional heat source required for the reaction. Therefore, the equipment has a simple structure and low investment cost, and there is no need to treat and discard the solvent, so it is also beneficial to the environment.

Claims

1. A mechanochemical depolymerization method for polyester, characterized in that: The polyester is crushed in the presence of a degradation agent to generate heat and depolymerize the polyester into dibasic acid and alkylene glycol, The heat is used to vaporize and separate the alkylene glycol.

2. The mechanochemical depolymerization method of polyester according to claim 1, characterized in that: The depolymerization is carried out in a pulverizing reactor comprising a rotating disk.

3. The mechanochemical depolymerization method of polyester according to claim 2, characterized in that: The pulverizing reactor further includes a plurality of blades attached to the rotating disk.

4. The mechanochemical depolymerization method of polyester according to claim 2, characterized in that: The pulverizing reactor further includes a plurality of baffles located on the inner wall.

5. The mechanochemical depolymerization method of polyester according to claim 1, characterized in that: The heat includes pulverization heat generated by pulverization of the polyester, friction heat generated by friction between the polyester and the pulverization reactor, or both of them.

6. The mechanochemical depolymerization method of polyester according to claim 1, characterized in that: The alkylene glycol is separated by vaporization by additionally supplying heat from the outside.

7. The mechanochemical depolymerization method of polyester according to claim 1, characterized in that: The depolymerization is a solvent-free reaction performed without adding a solvent.

8. The mechanochemical depolymerization method of polyester according to claim 1, characterized in that: The method for mechanochemical depolymerization of polyester does not include the steps of pulverizing, crushing or cutting the polyester before or after the depolymerization.

9. The mechanochemical depolymerization method of polyester according to claim 2, characterized in that: The depolymerization is performed by rotating the turntable at 500 rpm to 5000 rpm.

10. The mechanochemical depolymerization method of polyester according to claim 2, characterized in that: The heat causes the internal temperature of the pulverizing reactor to reach 130°C to 300°C.

11. The mechanochemical depolymerization method of polyester according to claim 1, characterized in that: The depolymerization is carried out for 5 minutes to 120 minutes.

12. The mechanochemical depolymerization method of polyester according to claim 1, characterized in that: The degradation agent includes alkali, acid, their salts, monohydric alcohol, polyhydric alcohol or a mixture thereof.

13. The mechanochemical depolymerization method of polyester according to claim 12, characterized in that: The alkali includes sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia or a mixture thereof, The acid includes hydrochloric acid, nitric acid, sulfuric acid, carbonic acid, phosphoric acid, acetic acid, hypochlorous acid or a mixture thereof. The salts include carbonates, bicarbonates, phosphates, sulfates, sulfites, nitrates, silicates, hypochlorites, formates, acetates, citrates, oxalates or mixtures thereof, The monohydric alcohol includes methanol, ethanol, propanol, butanol or a mixture thereof. The polyol includes ethylene glycol, n-propylene glycol, isopropylene glycol, diethylene glycol, polyethylene glycol, triethylene glycol, dipropylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, glycerol, benzyl alcohol, polypropylene glycol, pentaerythritol, trimethylolpropane or a mixture thereof.

14. The mechanochemical depolymerization method of polyester according to claim 1, characterized in that: The degradation agent is in a solid state or an aqueous solution state.

15. The mechanochemical depolymerization method of polyester according to claim 1, characterized in that: The degradation agent is added in an amount of 0.75 mol to 3.0 mol relative to 1.0 mol of the dibasic acid in the polyester.

16. The mechanochemical depolymerization method of polyester according to claim 1, characterized in that: In the mechanochemical depolymerization process of the polyester, the dibasic acid remaining after separation of the alkylene glycol is obtained in the form of a salt in a solid phase.

17. The mechanochemical depolymerization method of polyester according to claim 16, characterized in that: The mechanochemical depolymerization method of the polyester further comprises the following steps: The residual alkylene glycol is further separated from the dibasic acid salt in the solid phase.

18. The mechanochemical depolymerization method of polyester according to claim 16, characterized in that: In the mechanochemical depolymerization process of the polyester, Dissolve the solid dibasic acid salt in water. The aqueous solution of the dibasic acid salt is neutralized with an acid to precipitate dibasic acid crystals. By solid-liquid separation, dibasic acid crystals are obtained from the precipitated product.

19. The mechanochemical depolymerization method of polyester according to claim 18, characterized in that: In the process for the mechanochemical depolymerization of the polyester, impurities are removed from an aqueous solution of the dibasic acid salt.

20. The mechanochemical depolymerization method of polyester according to claim 18, characterized in that: In the mechanochemical depolymerization process of the polyester, the dibasic acid crystals are recrystallized.

21. A composition for polymerizing recycled polyester, characterized in that: The invention comprises a dibasic acid and an alkylene glycol obtained by the mechanochemical depolymerization process of the polyester as claimed in claim 1.

22. A recycled polyester, characterized in that The method is prepared by using the dibasic acid obtained by the mechanochemical depolymerization process of the polyester as claimed in claim 1 and an alkylene glycol.

23. The regenerated polyester according to claim 22, characterized in that The L value of the color of the recycled polyester is 45 or more.

Images