Method for depolymerizing polyester

Abstract

A method for depolymerizing polyester is provided, which includes a depolymerization step of adding a decomposition agent to the polyester and then contacting the polyester with superheated steam.

Description

[Technical field]

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

[0002] Polyesters, such as polyethylene terephthalate (PET), are used in fibers, fabrics, clothing, films, sheets, beverage bottles, etc. because of their excellent chemical stability.

[0003] As the amount of polyester used has increased rapidly, various methods for recovering and reusing the 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 a monomer, which is recovered, and the monomer is used as a raw material for repolymerization to produce a recycled polyester such as polyethylene terephthalate.

[0004] This chemical recycling makes it possible to separate impurities, and the quality of the raw material is not significantly different from that of virgin material, so it is expected to be a means of realizing resource reuse.

[0005] 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] The hydrolysis method includes, for example, a method in which polyethylene terephthalate melt 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 requires a special high-pressure reactor because the reaction is carried out under high-pressure conditions.

[0007] The alcoholysis method is a method for depolymerizing polyester by heating in an alcohol solvent (with the addition of a catalyst as necessary) (see Patent Documents 2 and 3). This method has the advantage that, for example, when polyethylene terephthalate is depolymerized using methanol as a solvent, dimethyl terephthalate (DMT), a useful and easy-to-handle monomer, 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 pressurization is required to carry out the reaction (for example, reaction in methanol in a supercritical or subcritical state), requiring a special high-pressure reactor.

[0008] The glycolysis method is a method in which polyester is depolymerized 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 normal pressure, but the reaction time is relatively long, so it is necessary to shorten the reaction time, and there is a problem that the glycol solvent deteriorates when heated for a long time.

[0009] In addition, such known methods for depolymerizing polyester require a reaction for at least several hours, which makes it difficult to treat a large amount of polyester waste. Furthermore, since the reaction must be carried out under high temperature or high pressure conditions, a special device capable of withstanding such reaction conditions is required.

[0010] Meanwhile, not all polyester raw materials are transparent like beverage bottles during depolymerization of polyester, and especially in the case of colored polyester feedstocks such as fibers, fabrics, and clothing, the removal of the color of the dyes added during processing has a significant impact on the quality of the final product.

[0011] In particular, among polyester feedstocks, waste fibers dyed in the processing process, such as colored polyester waste fibers, require a sharp increase in the amount of dye input depending on the color and fineness of the fibers. Therefore, the uses of colored polyester feedstocks are limited, and the most rational and environmentally friendly method for creating added value is to decompose them into raw materials through depolymerization and then repolymerize them for use, but this is not economical due to the high process costs.

[0012] The dyes in colored polyester are removed by solvent treatment and adsorption (activated carbon) for decolorization during the depolymerization process, but the solvent used is expensive, so the recovery cost is high, and the useful life of the dye decreases rapidly as the dye content increases, so the overall process cost is high and it is essentially not economical.For this reason, polyester depolymerization generally uses polyester bottle flakes, which have a high recovery purity. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] JP 2003-527363 A (Publication Date: September 16, 2003) [Patent Document 2] JP 1999-100336 A (Publication Date: April 13, 1999) [Patent Document 3] JP 2003-300916 A (Publication Date: October 21, 2003) [Patent Document 4] JP 2002-167468 A (Publication Date: June 11, 2002) [Patent Document 5] JP 2004-300115 A (Publication Date: October 28, 2004) Summary of the Invention [Problem to be solved by the invention]

[0014] According to one embodiment, there is provided a method for depolymerizing polyester, which can remove dyes from colored polyester inexpensively and efficiently, while simultaneously depolymerizing the colored polyester within a short period of time, has high depolymerization efficiency, and is economical because it is not necessary to heat the entire composition for depolymerization up to the depolymerization temperature.

[0015] According to another embodiment, there is provided a composition for polymerization of recycled polyester produced by the method for depolymerizing polyester.

[0016] According to another embodiment, there is provided a recycled polyester produced using the composition for polymerizing recycled polyester. [Means for solving the problem]

[0017] According to one embodiment, there is provided a method for depolymerizing polyester, comprising a depolymerization step of adding a decomposition agent including an alkali, an acid, a salt thereof, a monoalcohol, a polyhydric alcohol, or a mixture thereof to a polyester, and then contacting the polyester with superheated steam.

[0018] The temperature of the superheated steam may be between 250°C and 450°C.

[0019] The superheated steam may be applied at an atomization pressure of 0.1 bar to 2 bar.

[0020] The depolymerization step can be carried out for 1 minute to 120 minutes.

[0021] The alkali may include sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, or mixtures thereof.

[0022] The acid may include hydrochloric acid, nitric acid, sulfuric acid, carbonic acid, phosphoric acid, acetic acid, hypochlorous acid (HClO), or mixtures thereof.

[0023] The salts may include carbonates, bicarbonates, phosphates, sulfates, sulfites, nitrates, silicates, hypochlorites, formates, acetates, citrates, oxalates, or mixtures thereof.

[0024] The monoalcohol may include methanol, ethanol, propanol, butanol, or mixtures thereof.

[0025] 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 mixtures thereof.

[0026] The decomposing agent can be added in an amount of 0.75 to 3.0 mol per 1.0 mol of the dibasic acid contained in the polyester.

[0027] The method for depolymerizing a polyester may further include a step of performing solid-liquid separation into liquid alkylene glycol and solid dibasic acid salt from the depolymerized product obtained in the depolymerization step, a step of dissolving the solid dibasic acid salt in water, a step of neutralizing an aqueous solution of the dibasic acid salt with an acid to precipitate dibasic acid crystals, and a step of performing solid-liquid separation of the dibasic acid crystals from the precipitated product.

[0028] The method for depolymerizing a polyester may further include removing impurities from the aqueous solution of the dibasic acid salt.

[0029] The method for depolymerizing a polyester may further include a step of recrystallizing the diacid crystals.

[0030] According to another embodiment, there is provided a composition for polymerization of recycled polyester, which contains a dibasic acid and an alkylene glycol, and is obtained by the method for depolymerizing a polyester according to one embodiment.

[0031] According to another embodiment, there is provided a recycled polyester produced using the polyester polymerization composition according to one embodiment.

[0032] The color of the recycled polyester can have an L value of 60 or more. Effect of the Invention

[0033] The method for depolymerizing polyester according to one embodiment can remove dyes from colored polyester inexpensively and efficiently, while depolymerizing the colored polyester within a short period of time, and is economical because it has high depolymerization efficiency and does not require heating the entire composition for depolymerization to the depolymerization temperature. [Brief description of the drawings]

[0034] [Figure 1] Photograph showing colored polyethylene terephthalate. [Diagram 2] Photograph showing bleached polyethylene terephthalate. [Diagram 3] This is a photograph showing the depolymerization product being washed with water to remove impurities. [Figure 4] Photograph of terephthalic acid recovered after water washing and acid precipitation. [Diagram 5] Photographs of colored polyethylene terephthalate fibers and recovered terephthalic acid before and after depolymerization. [Figure 6] 1 is a comparative photograph of polyester chips repolymerized using recovered terephthalic acid as a raw material and virgin polyester chips. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] The advantages and features of the technology described below, as well as the methods of achieving them, will become clear with reference to the embodiments described in detail below with the accompanying drawings. However, the specific form is not limited to the embodiments disclosed below. Unless otherwise defined, all terms (including technical and scientific terms) used in this specification are used in the sense commonly understood by those having ordinary knowledge in the relevant technical field. In addition, terms defined in a commonly used dictionary are not ideally or excessively analyzed unless clearly and specifically defined.

[0036] Throughout this specification, when a part "comprises" a certain element, this means that the part may further include other elements, not excluding other elements, unless otherwise specified. In addition, the singular form includes the plural form unless otherwise specified in the context.

[0037] A method for depolymerizing a polyester according to one embodiment includes a depolymerization step of adding a decomposition agent to a polyester and then contacting the polyester with superheated steam.

[0038] 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 a polycaprolactone obtained by polymerizing caprolactone. By depolymerizing these polyesters, the dibasic acid, the alkylene glycol, or the caprolactone can be recovered as a monomer.

[0039] 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, 1,4-benzenediol, 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, butylene glycol can be recovered as the monomer.

[0040] Examples of dibasic acids obtained by depolymerizing polyesters include aromatic dicarboxylic acids such as terephthalic acid, phthalic acid (ortho), 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, pyromellitic acid, etc. Examples of other dicarboxylic acids include aliphatic cyclic 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, undecadicarbonic acid, and dodecadicarboxylic acid.

[0041] As the raw material for polyester depolymerization, molded articles containing polyester, particularly waste, can be used. The waste can be residues and defective products generated during the manufacture of molded articles other than waste generated after the use of molded articles containing polyester. For example, it can be used PET bottles, cups, strings, packaging packs, etc., or bars, sprues, sheets after cutting cups after vacuum molding, fibers, textiles, clothes, films, sheets, etc.

[0042] In particular, the method for depolymerizing polyester according to an embodiment can economically and efficiently depolymerize colored polyester, such as fibers, fabrics, and clothing, that contain a large amount of dye, as described below.

[0043] At this time, 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-reactants and non-recyclable separated and refined waste is too large compared to the raw materials and auxiliary materials obtained by the depolymerization process, making it substantially uneconomical.

[0044] When polyester waste is used as a feedstock for depolymerization, it may optionally be subjected to a pretreatment step in which the waste is washed prior to the depolymerization step to remove contaminants, such as dyes, contents, or soil, adhering to the waste.

[0045] In addition, in the method for depolymerizing polyester according to one embodiment, the reaction rate is fast, so that relatively large pieces of the collected polyester waste can be directly provided to the depolymerization reaction, but in order to proceed with the reaction more efficiently, the waste can be mechanically cut, crushed, and processed. The crushing treatment can be performed using a known suitable means, and for example, the polyester waste can be crushed into small pieces of 2 mm to 8 mm in size by crushing using a hammer mill or the like, and then provided to the depolymerization reaction.

[0046] If necessary, the crushed chips can be immersed in a solvent to extract and remove dyes, or components lighter than the solvent can be separated. The crushed chips can be blown away by wind or sieved to collect only pieces of a certain size and then provided to the depolymerization reaction.

[0047] However, when using the method for depolymerizing polyester according to an embodiment, the depolymerization reaction is not significantly affected even if the foreign matter is not completely removed, so pretreatment processes such as separation, washing, and pulverization are not necessarily required.

[0048] In one embodiment of the method for depolymerizing polyester, polyester is directly contacted with superheated steam to perform dry depolymerization.

[0049] Unlike the conventional wet method in which polyester is immersed in a depolymerization composition, the method has high depolymerization efficiency because superheated steam reacts directly with polyester, and is economical because it is not necessary to raise the temperature of the entire depolymerization composition including the solvent to the depolymerization temperature. In addition, the method is advantageous for depolymerizing colored polyester containing a dye because depolymerization and decolorization are performed simultaneously by adding superheated steam to the polyester.

[0050] In addition, since the depolymerization reaction is carried out by directly adding superheated steam to the polyester, a high-pressure reactor is not required, and a batch or continuous reactor can be configured as the apparatus.

[0051] For example, the contact of the polyester with the superheated steam may be carried out by directly spraying high-temperature superheated steam at 250°C to 450°C onto the polyester at a spray pressure of 0.1 bar to 2 bar, and the temperature of the superheated steam may be, for example, 280°C to 380°C.

[0052] If the temperature of the superheated steam is below 250℃, the depolymerization time of polyester becomes too long, making it less economical, and if it exceeds 450℃, carbonization occurs because the depolymerization reaction does not proceed in an inert atmosphere. If the pressure of the superheated steam is below 0.1bar, the contact effectiveness of the direct reaction steam is insufficient, and the actual decomposition time is not as effective as a general wet reaction, and if it exceeds 2bar, since the material is a thermoplastic material in the direct contact area, melting precedes decomposition and forms large lumps, which increases the time for gradual decomposition from the surface to the inside.

[0053] The depolymerization reaction may be carried out for 1 to 120 minutes, or 10 to 60 minutes. The depolymerization reaction time may be increased within the range of 120 minutes as the concentration of impurities including dyes in the polyester increases, the amount of polyester treated increases, and the temperature of the superheated steam decreases.

[0054] Meanwhile, in the method of depolymerizing polyester, a decomposing agent can be added to polyester before the polyester is brought into contact with superheated steam. When high-temperature superheated steam is used alone, polyester, which is a thermoplastic resin, dissolves before the polyester is decomposed into monomers, and the polyester is not decomposed inside and may be partially peroxidized and carbonized. However, when the polyester is treated with a decomposing agent first, the polyester can be decomposed into monomers before it dissolves.

[0055] The disintegrating agent may include an alkali, an acid, a salt thereof, a monoalcohol, a polyhydric alcohol, or a mixture thereof.

[0056] 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 sodium or potassium, which is relatively inexpensive, may be used. The alkaline earth metal may be beryllium, magnesium, calcium, strontium, barium, radium, or the like. Examples of the hydroxides of alkali metals include sodium hydroxide, potassium hydroxide, or lithium hydroxide. Among these, sodium hydroxide has excellent reaction speed and reaction rate when used in combination with ethylene glycol, etc.

[0057] 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.

[0058] 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, and can include, for example, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, tripotassium phosphate (hydrate), sodium hypochlorite, or mixtures thereof.

[0059] 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, and glycerin, which have a high boiling point and relatively high reactivity, may be used.

[0060] 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 and triols exemplified above, such as diethylene glycol monomethyl ether, benzyl alcohol, and 2-ethylhexanol.

[0061] Since the decomposer reacts with both the polyester and the dye, the amount of the decomposer added can be 0.75 mol to 3.0 mol per 1.0 mol of dibasic acid contained in the polyester. For example, in the case of a monovalent decomposer (e.g., sodium hydroxide), it is appropriate to add 1.5 mol to 3.0 mol, and in the case of a divalent decomposer (e.g., calcium carbonate), it is appropriate to add 0.75 mol to 1.5 mol.

[0062] As an 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.

[0063] In addition, when the polyester feed material is a fiber, the amount of dye added during processing varies from 0.5% to 20% by weight of the polyester fiber (based on the weight of fabric (owf)), 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 both the dye and the polyester fiber are decomposed.

[0064] Next, the alkylene glycol and the dibasic acid salt are separated and recovered from the depolymerized product obtained in the depolymerization step.

[0065] As an example, when an alcohol different from alkylene glycol, which is a constituent component of polyester, is used as a decomposing agent, the alkylene glycol produced by depolymerization and the decomposing agent are mixed together once, but the alkylene glycol produced by depolymerization can be recovered by separating the two.

[0066] The method for separating the alkylene glycol and the decomposition agent produced by 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, they can be separated by distillation concentration. As a means for distillation concentration, any conventional distillation concentration apparatus, such as a reduced pressure continuous distillation apparatus or a reduced pressure arrangement distillation apparatus, can be used.

[0067] When the alkylene glycol produced in the depolymerization reaction is the same type as the decomposition agent, the produced alkylene glycol does not need to be separated separately.

[0068] Meanwhile, the depolymerization step can be selectively performed using a hydrolysis method using water, an alcoholysis method using alcohol, or a glycolysis method using glycol depending on the type of decomposition agent, and the type of dibasic acid obtained varies depending on the method.

[0069] When depolymerizing polyester using the glycolysis method, the dibasic acid salt obtained can vary depending on the type of decomposition agent used in the depolymerization reaction. For example, when depolymerizing polyethylene terephthalate using ethylene glycol as a decomposition agent, bis(β-hydroxyethyl) terephthalate (BHET) can be obtained as a monomer, and when depolymerizing using propylene glycol as a decomposition agent, bis(β-hydroxyethyl isopropyl) terephthalate (BHEPT) can be obtained as a monomer.

[0070] In addition, the ester compound of the oligomer obtained when benzyl alcohol and tripotassium phosphate are used as decomposing agents has very good solubility in chloroform, etc., and can therefore be efficiently recovered by solvent extraction. In addition, the ester compound of the dibasic acid or oligomer can also be recovered by means of filtration, distillation, etc.

[0071] The diacid or oligomer ester compounds thus obtained (e.g., BHET) can also be recovered as diacid or oligomer methyl esters (e.g., DMT) by transesterification with methanol.

[0072] 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 an ester exchange reaction at 65° C. to 85° C. for 0.5 to 5 hours in the presence of an ester exchange catalyst (such as an alkali metal compound). In addition, purified DMT can be recovered by separating a cake containing DMT using a solid-liquid separator or the like and purifying it by distillation.

[0073] On the other hand, when polyester is depolymerized using the glycolysis method, a dibasic acid salt such as a dibasic acid or a dialkali metal salt of an oligomer may be produced, and this product may 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 decomposition agent, sodium terephthalate is produced together with ethylene glycol.

[0074] Among these, dibasic acid salts (e.g., sodium terephthalate) do not dissolve in alkylene glycol and form solid-phase crystals, so they can be easily separated from the solvent by filtration methods such as solid-liquid separation. In addition, alcohols adhering to the obtained powder-type crystals can be removed by washing with alcohols such as methanol and ethanol.

[0075] Next, the crystals of the dibasic acid salt are dissolved in water, and then an acid is added to cause a neutralization reaction, to obtain a dibasic acid (e.g., terephthalic acid). The dibasic acid is also precipitated as crystals in the water, and can be recovered by separating the crystals from the water using a method such as centrifugation.

[0076] The content of water added during the recovery step may be 3 to 10 weights per weight of the dibasic acid salt. The content of the acid supplied to separate the dibasic acid may be equimolar or more with the alkali metal contained in the dibasic acid salt, and the type of acid used may be an inorganic acid such as hydrochloric acid, sulfuric acid, or phosphoric acid, which is a strong acid with a pH of 2, or an organic acid such as formic acid, acetic acid, or oxalic acid, among which inorganic acids, particularly hydrochloric acid or sulfuric acid, are suitable for reducing impurities in the monomer produced. The neutralization reaction temperature may be 65°C to 85°C, and the neutralization reaction can usually be completed within 0.5 to 5 hours.

[0077] Meanwhile, the method for depolymerizing polyester may further include a step of removing impurities contained in the polyester molded product or waste with an aqueous solution of a dibasic acid salt.

[0078] The means for removing impurities is not particularly limited, and all appropriate techniques, devices, etc. can be used. For example, solids such as resins other than polyester (polyethylene, polypropylene, polyvinyl chloride, etc.) remaining in an unreacted state and decomposing agents such as insoluble alkali metals can be removed using a mesh, etc. In addition, dyes, fillers, etc. cannot be removed using a mesh, etc., so they can be removed by centrifugation, filtration using an adsorbent such as activated carbon, etc.

[0079] Since the obtained dibasic acid crystals show a particle size distribution characteristic of several μm to several tens of μm, a step of selectively recrystallizing the dibasic acid crystals to a commercially available particle size (100 μm or more) may be further included. The recrystallization method is not particularly limited, and for example, the dibasic acid crystals are mixed with water and then subjected to high temperature and high pressure conditions.

[0080] The dibasic acid and alkylene glycol obtained by the method for depolymerizing polyester can be recycled as monomers for polymerizing recycled polyester. Specifically, the composition for polymerization of recycled polyester containing the dibasic acid and alkylene glycol obtained by the method for depolymerizing polyester can be esterified and polycondensed to produce recycled polyester.

[0081] Esterification is a process in which the recovered dibasic acid monomer is reacted with an alkylene glycol, for example, terephthalic acid and ethylene glycol. This 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, for example, magnesium or calcium, or a metal compound, for example, titanium, zinc, or manganese, which are well known as transesterification catalysts.

[0082] The product of the esterification process can then be polycondensed to produce a recycled polyester resin, which can be either solution or solid-state polymerization.

[0083] In addition, after the solid-state polymerization, a compound for promoting water treatment and / or crystallization can be added as necessary for quality adjustment, and a polycondensation catalyst or stabilizer can also be added at the start or during the polycondensation step.

[0084] Here, the water treatment is 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, polyolefin thermoplastic resins such as polypropylene and polyethylene, and these compounds can be added to the recycled polyester resin at 1 ppm to 100 ppm.

[0085] 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.

[0086] The recycled polyester may also contain, as a stabilizer, phosphoric acid esters such as trimethyl phosphate, triethyl phosphate, triphenyl phosphate, triethyl phosphonoacetate, etc., phosphorous acid esters such as triphenyl phosphate, trisdodecyl phosphate, etc., and phosphorus compounds such as methyl acid phosphate, dibutyl phosphate, monobutyl phosphate, phosphoric acid, phosphorous acid, hypophosphorous acid, polyphosphoric acid, etc. The amount of the 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.

[0087] Meanwhile, as described above, in the method for depolymerizing polyester according to one embodiment, decolorization is performed simultaneously with depolymerization by using superheated steam, and a decomposer is added to dissolve the polyester and decolorize only the surface, and the inside of the dissolved polyester is not decolorized and is partially carbonized due to overoxidation. Thus, the color of the recycled polyester produced using the dibasic acid and alkylene glycol obtained by the above-mentioned method may have an L value of 60 or more, for example, 65 to 95.

[0088] The L, a, and b hue system is commonly used internationally as a standard for evaluating the hue of polyester. Such hue numbers are one of the hue systems for standardizing hue measurement, and describe the recognizable hue and color difference. In this system, L is the lightness factor, and a and b are hue measurement numbers. Among them, the L value is a numerical factor that indicates brightness, and is a very important value in the manufacture of fibers, textiles, and clothing. Also, a positive b value means yellow discoloration, a negative value means blue discoloration, a positive a value means red discoloration, and a negative value means green discoloration.

[0089] The L, a, and b values ​​are defined in the Korean Industrial Standards (KS) related to color measurement, such 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 measuring 50g of polyester resin to be measured in air to remove moisture, then placing it in a Colorimeter model SA-2000 to measure the hue 10 times, and the average value can be determined as the standard value.

[0090] The L value of recycled polyester is related to the purity of the dibasic acid and alkylene glycol that are recycled. For example, the more impurities or quenchers such as titanium dioxide (TiO2) are present, the lower the L value. If the L value is less than 60, the amount of impurities is too high, and the color after polymerization and fiberization will be colored, or many side reactions will occur during polymerization, making it impossible to use as a high-value-added material, and it may be meaningless since its main uses are the same as mechanically recycled polyester. In addition, if the L value exceeds 95, it is a physical property that is difficult to achieve even with existing virgin materials, and in order to achieve this value through recycling, it will be less economical due to the increase in unit production volume and process costs, such as adding decolorization and purification processes or increasing residence time.

[0091] <Mode for carrying out the invention> Specific examples of the present invention are presented below. However, the examples described below are merely for the purpose of specifically illustrating or explaining the present invention, and the scope of the present invention should not be limited thereby.

[0092] (Examples 1 to 3) 10 kg of polyethylene terephthalate colored waste fiber feedstock (PET content 90 wt%) was decolorized and then impregnated with a 50 wt% aqueous solution of sodium hydroxide (NaOH) to achieve pick-up amounts (based on the weight of fabric (owf)) of 350 wt%, 400 wt%, and 500 wt%, respectively.

[0093] At this time, the contents of sodium hydroxide impregnated in the polyethylene terephthalate were 2.1 mol, 2.4 mol, and 3.0 mol per mol of terephthalic acid, respectively.

[0094] This was put into a superheated steam system and treated for 30 minutes by injecting 350°C superheated steam at a pressure of 1.5 bar.

[0095] The decomposed depolymerized product was then washed, and sulfuric acid (H2SO4) was added to reduce the pH from 11.2 to 2.41, and terephthalic acid (TPA) and ethylene glycol (EG) were recovered.

[0096] (Examples 4 and 5) 10 kg of polyethylene terephthalate colored waste fiber feedstock (PET content 90 wt%) was decolorized and then impregnated with a 50 wt% aqueous solution of sodium hydroxide (NaOH) to achieve impregnation amounts (based on the weight of fabric (owf)) of 200 wt% and 550 wt%, respectively.

[0097] At this time, the content of sodium hydroxide impregnated in the polyethylene terephthalate was 1.0 mol and 3.3 mol per mol of terephthalic acid, respectively.

[0098] This was put into a superheated steam system and treated for 60 minutes by injecting 350°C superheated steam at a pressure of 1.5 bar.

[0099] The decomposed depolymerized product was then washed and reduced to a pH of 3.0 or less by adding sulfuric acid (H2SO4), and terephthalic acid (TPA) and ethylene glycol (EG) were recovered.

[0100] Comparative Example 1 10 kg of polyethylene terephthalate colored waste fiber feedstock (PET content 90 wt%) was fed into the superheated steam system and treated for 30 minutes by injecting 350°C superheated steam at a pressure of 1.5 bar.

[0101] The decomposed depolymerized product was then washed and reduced to a pH of 3.0 or less by adding sulfuric acid (H2SO4), and terephthalic acid (TPA) and ethylene glycol (EG) were recovered.

[0102] Comparative Example 2 10 kg of polyethylene terephthalate colored waste fiber feedstock (PET content 90 wt%) was impregnated with a 50 wt% aqueous solution of sodium hydroxide (NaOH) to give an impregnation amount (based on the weight of fabric (owf)) of 350 wt%.

[0103] At this time, the content of sodium hydroxide impregnated in the polyethylene terephthalate was 2.1 moles per mole of terephthalic acid.

[0104] This was depolymerized for 180 minutes by injecting 13.5 bar saturated steam having a temperature of 196°C into the system.

[0105] The decomposed depolymerized product was then washed and reduced to a pH of 3.0 or less by adding sulfuric acid (H2SO4), and terephthalic acid (TPA) and ethylene glycol (EG) were recovered.

[0106] [Experimental Example 1: Observation of depolymerization reaction] The depolymerization process in Example 1 was observed, and photographs taken at each stage are shown in Figs. 1 to 6.

[0107] FIG. 1 is a photograph showing colored polyethylene terephthalate, FIG. 2 is a photograph showing decolorized polyethylene terephthalate, FIG. 3 is a photograph showing the depolymerized product being washed with water to remove impurities, FIG. 4 is a photograph of terephthalic acid recovered after washing with water and acid precipitation, FIG. 5 is a photograph of colored polyethylene terephthalate fibers before and after depolymerization and the recovered terephthalic acid, and FIG. 6 is a comparative photograph of polyester chips repolymerized using the recovered terephthalic acid as a raw material and virgin polyester chips.

[0108] [Experimental Example 2: Depolymerization reaction efficiency measurement] In the examples and comparative examples, the remaining amount of undecomposed polyethylene terephthalate (PET) was confirmed to measure the decomposition rate of the PET feedstock, and the yield of terephthalic acid (TPA) was determined by NMR and HPLC analysis, and the results are shown in Table 1. Here, the remainder excluding terephthalic acid was determined to be by-products, unreacted substances, and impurities.

[0109] [Table 1]

[0110] Referring to Table 1, it can be seen that Examples 1 to 5 have superior PET decomposition rates and TPA yields compared to Comparative Examples 1 and 2. This is because, in the case of Comparative Example 1, a decomposition agent was not used, and thus a neutral aqueous depolymerization treatment was performed, resulting in a large amount of unreacted and by-reacted substances, and thus a low yield of terephthalic acid. In the case of Comparative Example 2, it can be seen that, by using general steam, not superheated steam, the depolymerization treatment time was more than six times longer, resulting in a low PET decomposition rate and TPA yield.

[0111] In addition, in Example 4, the content of the decomposition agent was 1.0 mol, which does not reach the required 2.0 mol, and it is expected that PET will not be completely decomposed. However, the decomposition rate is higher than expected based on the equivalent ratio because thermal decomposition occurred in parallel at a high temperature of 350°C. In contrast, it can be confirmed that the TPA yield was low as expected due to unreacted materials and by-reactants.

[0112] In Example 5, the content of the decomposing agent was 3.3 mol, so the depolymerization step exceeded the appropriate level, and the amount of by-products generated increased, resulting in a low yield and purity of the recovered terephthalic acid. This resulted in a decrease in the L value due to side reactions during the repolymerization of the final polyester after purification by crystallization.

[0113] Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements made by those skilled in the art using the basic concept of the present invention defined in the following claims also fall within the scope of the present invention. [Industrial Applicability]

[0114] The present invention relates to a method for rapid depolymerization of polyester, which can inexpensively and efficiently remove dyes from colored polyester and simultaneously depolymerize the colored polyester within a short period of time, has high depolymerization efficiency, and is economical because it is not necessary to raise the entire composition for depolymerization to the depolymerization temperature.

Claims

1. After adding an alkali-containing decomposition agent to polyester, a depolymerization step of contacting the polyester with superheated steam, The alkali includes sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, or a mixture thereof; Method for depolymerizing polyester.

2. The method for depolymerizing polyester according to claim 1, wherein the temperature of the superheated steam is 250°C to 450°C.

3. 2. The method for depolymerizing polyester according to claim 1, wherein the superheated steam is added at an atomization pressure of 0.1 bar to 2 bar.

4. The method for depolymerizing polyester according to claim 1, wherein the depolymerization step is carried out for 1 minute to 120 minutes.

5. The method for depolymerizing a 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.

6. The method for depolymerizing polyester comprises: performing solid-liquid separation into liquid alkylene glycol and solid dibasic acid salt from the depolymerization product obtained in the depolymerization step; dissolving the solid dibasic acid salt in water; Neutralizing the aqueous solution of the dibasic acid salt with an acid to precipitate dibasic acid crystals in water; and A step of subjecting the dibasic acid crystals to solid-liquid separation from the dibasic acid crystals in water. The method for depolymerizing a polyester according to claim 1, further comprising:

7. The method for depolymerizing a polyester according to claim 6, further comprising removing impurities from the aqueous solution of the dibasic acid salt.

8. The method for depolymerizing a polyester according to claim 6, further comprising the step of recrystallizing the dibasic acid crystals.

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