A method for depolymerizing polyester, including a method for decolorizing polyester

Abstract

To provide a method for decolorizing polyester, which comprises adding a decolorizing agent to polyester and then contacting the polyester with superheated steam.

Description

Technical Field

[0001] The present invention relates to a method for decolorizing polyester, particularly colored polyester into which a large amount of dye is introduced during processing, and a depolymerization method including the same.

Background Art

[0002] Polyester represented by polyethylene terephthalate (PET) is used in fibers, fabrics, clothing, films, sheets, beverage bottles, etc. because of its excellent chemical stability.

[0003] As the amount of polyester used has increased rapidly, various methods for collecting and recycling its waste have been studied. One of them is the so-called chemical recycle method, in which waste such as polyester is depolymerized into monomers, recovered, and then repolymerized using these monomers as raw materials to produce recycled polyester such as polyethylene terephthalate.

[0004] This chemical recycle is expected as a means to achieve resource recycling because it enables separation of impurities and the quality as a raw material is not significantly different from virgin.

[0005] The methods for depolymerizing polyester into monomers can be roughly classified into three methods: a hydrolysis method using water as a solvent, an alcoholysis method using alcohol as a solvent, and a glycolysis method using glycol as a solvent.

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

[0007] The alcoholysis method is a method of depolymerizing a polyester by heating it in an alcohol solvent (optionally adding a catalyst) (see Patent Documents 2 and 3). For example, when depolymerizing polyethylene terephthalate using methanol as a solvent, dimethyl terephthalate (DMT), which is a useful and easily handled 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, reacting in methanol in a supercritical or subcritical state), and a special high-pressure reactor is required.

[0008] The glycolysis method is a method of depolymerizing a polyester by heating it in an excess of an 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 and carrying out a transesterification reaction. The glycolysis method can be carried out at normal pressure, but the reaction time is relatively long and shortening of the reaction time is required, and there is a problem that the glycol of the solvent is heated for a long time and deteriorates.

[0009] Also, such known methods for depolymerizing polyesters need to be reacted for at least several hours or more, which is difficult in the mass treatment of polyester waste, and need to be reacted under high-temperature or high-pressure conditions, so a special apparatus that can withstand such reaction conditions is required.

[0010] On the one hand, during the depolymerization of polyester, not all polyester raw materials are transparent like beverage bottles. In particular, in the case of colored polyester feedstocks such as fibers, textiles, and clothing, the removal of the hue of the dyes introduced during processing has a significant impact on the quality of the final product.

[0011] In particular, among the polyester feedstocks, waste fibers dyed in the processing step, such as waste colored polyester fibers, the amount of dye input increases sharply according to the hue and fineness (smallness of fiber fineness) of the fibers. Therefore, the uses of colored polyester feedstocks are limited. For the creation of added value, the most reasonable and environmentally friendly method is to depolymerize them into raw materials and then repolymerize and use them. However, the process cost is high and not economical.

[0012] Such dyes of colored polyester are removed by solvent treatment and adsorption process (activated carbon) for decolorization during the depolymerization process. However, the price of the solvent used is high, the recovery cost is high, the effective life decreases sharply as the dye content increases, and generally the process cost is high and there is no substantial economic efficiency. Therefore, usually, for the depolymerization of polyester, it is common to use flakes of polyester bottles with high recovery purity.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0014] According to one embodiment of the present invention, there is provided a method for decolorizing polyester that can remove dyes of colored polyester at low cost and with high efficiency.

[0015] According to another embodiment, there is provided a method for depolymerizing polyester including a step of decolorizing polyester.

[0016] According to another embodiment, there is provided a composition for producing recycled polyester produced by a method for depolymerizing polyester.

[0017] According to still another embodiment, there is provided a recycled polyester produced using the composition for producing recycled polyester.

Means for Solving the Problems

[0018] According to one embodiment of the present invention, there is provided a method for decolorizing polyester including a depolymerization step of adding a decolorizing agent containing an alkali, an acid, salts thereof, a monoalcohol, a polyhydric alcohol, or a mixture thereof to polyester and then bringing superheated steam into contact with the polyester.

[0019] The temperature of the superheated steam can be 250°C to 450°C.

[0020] The superheated steam can be added at an injection pressure of 0.1 bar to 2 bar.

[0021] The decolorization step can be carried out for 1 minute to 120 minutes.

[0022] The alkali may include sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia, or a mixture thereof.

[0023] The acid may include hydrochloric acid, nitric acid, sulfuric acid, carbonic acid, phosphoric acid, acetic acid, hypochlorous acid (HClO), or a mixture thereof.

[0024] The salt may include carbonate, bicarbonate, phosphate, sulfate, sulfite, nitrate, silicate, hypochlorite, formate, acetate, citrate, oxalate, or a mixture thereof.

[0025] The monoalcohol may include methanol, ethanol, propanol, butanol, or a mixture thereof.

[0026] 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.

[0027] The decolorizing agent can be added in an amount of 0.05 mol to 1.5 mol per 1.0 mol of the dibasic acid contained in the polyester.

[0028] According to another embodiment, after adding a decolorizing agent containing an alkali, an acid, a salt thereof, a monoalcohol, a polyhydric alcohol, or a mixture thereof to the polyester, a step of decolorizing the polyester by bringing the polyester into contact with superheated steam, a step of depolymerizing the decolorized polyester to obtain a depolymerization product containing alkylene glycol and dibasic acid salt, a step of solid-liquid separating the liquid alkylene glycol and the solid dibasic acid salt from the depolymerization product, a step of dissolving the solid dibasic acid salt in water, a step of neutralizing the aqueous solution of the dibasic acid salt with an acid to precipitate crystals of the dibasic acid, and a step of solid-liquid separating the crystals of the dibasic acid from the precipitation product are provided, which is a method for depolymerizing polyester.

[0029] The method for depolymerizing polyester may further include the step of removing impurities from the aqueous solution of dibasic acid salt.

[0030] The method for depolymerizing polyester may further include the step of recrystallizing the crystals of dibasic acid.

[0031] According to still another embodiment, there is provided a composition for recycled polyester polymerization, which contains dibasic acid and alkylene glycol and is obtained by the method for depolymerizing polyester according to one embodiment.

[0032] According to still another embodiment, there is provided a recycled polyester produced using the composition for polyester polymerization according to one embodiment.

[0033] The hue of the recycled polyester may have an L value of 60 or more.

Advantages of the Invention

[0034] The method for decolorizing polyester according to the present invention can remove the dye of colored polyester at low cost and with high efficiency.

Brief Description of the Drawings

[0035]

Figure 1

Figure 2

Modes for Carrying Out the Invention

[0036] The advantages, features of the technology to be described below, and the methods for achieving them will become clear by referring to the embodiments described in detail later together with the attached drawings. However, the forms to be implemented are not limited to the embodiments disclosed below. Unless otherwise defined, all terms (including technical and scientific terms) used in this specification are used in a meaning commonly understood by those having ordinary knowledge in the relevant technical field. Also, terms defined in commonly used dictionaries are not interpreted ideally or excessively unless specifically defined otherwise.

[0037] Throughout this specification, when a part "includes" a certain component, unless otherwise stated to the contrary, this does not exclude other components, but means that other components may further be included. Also, the singular form includes the plural form unless otherwise specified in the context.

[0038] The method for decolorizing polyester according to an embodiment of the present invention includes the step of contacting the polyester with superheated steam after adding a decolorizing agent to the polyester.

[0039] The polyester can be a polyester formed by polymerizing a dibasic acid and an alkylene glycol, for example, polyethylene terephthalate (PET), polybutylene terephthalate, polyethylene naphthalate, or it can be polycaprolactone formed by polymerizing caprolactone.

[0040] As a raw material for polyester decolorization, molded articles containing polyester, particularly waste, can be used. The waste can be residues, defective products, etc. generated during the production of molded articles other than the waste generated after using molded articles containing polyester. For example, used PET bottles, cups, strings, packaging packs, etc., or burs, sprues during molding them, sheets after cutting cups after vacuum molding, fibers, fabrics, clothes, films, sheets, etc.

[0041] In particular, the polyester decolorization method according to an embodiment of the present invention can economically and efficiently decolorize colored polyesters such as fibers, fabrics, and clothes containing a large amount of dyes, as described below.

[0042] At this time, the content of polyester in the polyester waste can be 60% by weight to 100% by weight based on the total weight of the polyester waste. When the content of polyester is less than 60% by weight, the amount of by-products and waste from separation and purification that cannot be recycled is too large for the raw and auxiliary materials obtained by the depolymerization process, and there may be no substantial economic efficiency.

[0043] The polyester decolorization method according to an embodiment of the present invention decolorizes the polyester in a dry manner by directly contacting it with superheated steam.

[0044] Unlike the conventional wet method in which the polyester is immersed in a decolorizing solvent, since the superheated steam directly reacts with the polyester, the decolorization efficiency is high, and because relatively inexpensive superheated steam is used, it is economical. Also, by adding superheated steam to the polyester, the efficiency of depolymerization can be improved when the polyester is depolymerized after decolorization.

[0045] Moreover, since the decolorization reaction is carried out by directly adding superheated steam to the polyester, a high-pressure reactor is not required, and the apparatus can be configured with a batch or continuous reactor.

[0046] As an example, the contact between the polyester and the superheated steam can be carried out by directly injecting high-temperature superheated steam at 250°C to 450°C with an injection pressure of 0.1 bar to 2 bar into the polyester. For example, the temperature of the superheated steam can be 280°C to 380°C.

[0047] When the temperature of superheated steam is less than 250 °C, the decolorization time of polyester becomes excessively long and the economy deteriorates. When it exceeds 450 °C, since the decolorization reaction is not a reaction carried out in an inert atmosphere, carbonization may occur. When the pressure of superheated steam is less than 0.1 bar, the contact effectiveness of direct reaction steam is insufficient, and the substantial decolorization time may not have a significant effect compared to general wet reactions. When it exceeds 2 bar, due to the directly contacting area and the fact that it is a thermoplastic material, melting precedes decomposition and large lumps are formed, so the time for stepwise decolorization from the surface to the inner surface can be long.

[0048] The decolorization reaction is carried out for 1 minute to 60 minutes, or 10 minutes to 40 minutes. The decolorization reaction time can be increased within the range of 60 minutes as the concentration of impurities including the dye in the polyester is higher, the throughput of the polyester is larger, and the temperature of the superheated steam is lower.

[0049] On the other hand, in the polyester decolorization method, a decolorizing agent can be added to the polyester before bringing the polyester into contact with superheated steam. When treating with high-temperature superheated steam alone, before the dye is decomposed, the polyester, which is a thermoplastic resin, may dissolve and be decolorized only on the surface and not inside the dissolved polyester. When the polyester is treated with a decolorizing agent first, it can be decolorized before the polyester dissolves.

[0050] The decolorizing agent can include an alkali, an acid, salts thereof, a monoalcohol, a polyhydric alcohol, or a mixture thereof.

[0051] As an example, the alkali can include hydroxides of alkali metals or alkaline earth metals, ammonia, or mixtures thereof. Here, the alkali metal can be a monovalent metal such as lithium, sodium, potassium, rubidium, cesium, etc., and among them, relatively inexpensive sodium or potassium can be used. The alkaline earth metals can be beryllium, magnesium, calcium, strontium, barium, radium, etc. Examples of the hydroxides of alkali metals include sodium hydroxide, potassium hydroxide, or lithium hydroxide. Among them, sodium hydroxide is excellent in terms of reaction rate and reaction yield when used in combination with ethylene glycol or the like.

[0052] The acid can be an organic acid or an inorganic acid, and can include, for example, hydrochloric acid, nitric acid, sulfuric acid, carbonic acid, phosphoric acid, acetic acid, hypochlorous acid (HClO), or mixtures thereof.

[0053] The salt can include inorganic acid salts such as carbonates, bicarbonates, phosphates, sulfates, sulfites, nitrates, silicates, hypochlorites, organic acid salts such as formates, acetates, citrates, oxalates, or mixtures thereof, and can include, for example, sodium carbonate, sodium bicarbonate, potassium carbonate, potassium bicarbonate, tripotassium phosphate (hydrate), sodium hypochlorite, or mixtures thereof.

[0054] The polyhydric alcohol can include ethylene glycol, n-propylene glycol, isopropylene glycol, diethylene glycol, polyethylene glycol, triethylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, glycerin, benzyl alcohol, polypropylene glycol, pentaerythritol, trimethylolpropane, or mixtures thereof. Among these polyhydric alcohols, ethylene glycol, n-propylene glycol, isopropylene glycol, diethylene glycol, polyethylene glycol, triethylene glycol, glycerin, which have a high boiling point and relatively high reactivity, can be used.

[0055] The monoalcohol can include methanol, ethanol, propanol, butanol, or a mixture thereof, and alkyl ether compounds of polyhydric alcohols such as the diols and triols exemplified above, for example, diethylene glycol monomethyl ether, benzyl alcohol, 2-ethylhexanol, and the like.

[0056] Since the decolorizing agent must react with the dye attached to the polyester, the amount of the decolorizing agent added can be 0.05 mol to 1.5 mol per 1.0 mol of the dibasic acid contained in the polyester. For example, in the case of a monovalent decolorizing agent (such as sodium hydroxide), it is suitable to add 0.1 mol to 1.5 mol, and in the case of a divalent decolorizing agent (such as calcium carbonate), it is suitable to add 0.05 mol to 0.75 mol.

[0057] Also, when the polyester feedstock is a fiber, the amount of the dye added during processing varies from 0.5 wt% to 20 wt% (on the weight of fabric (o.w.f.) basis) of the weight of the polyester fiber. Therefore, the amount of the decolorizing agent added is preferably 1.0 mol or more per 1.0 mol of the dibasic acid contained in the polyester so that all the decomposition of the dye can be carried out.

[0058] The method for depolymerizing polyester according to another embodiment includes a step of decolorizing the polyester and a step of depolymerizing the decolorized polyester.

[0059] The step of decolorizing the polyester is as described above, so the description thereof is omitted.

[0060] The polyester can be a polyester obtained by polymerizing a dibasic acid and an alkylene glycol, such as polyethylene terephthalate (PET), polybutylene terephthalate, polyethylene naphthalate, or it can be polycaprolactone obtained by polymerizing caprolactone. By depolymerizing these polyesters, the dibasic acid, alkylene glycol, or caprolactone can be recovered as monomers.

[0061] That is, examples of the alkylene glycol obtained by depolymerizing the 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, and the like. 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.

[0062] Examples of the dibasic acids obtained by depolymerizing polyesters include aromatic dicarboxylic acids such as terephthalic acid, phthalic acid (ortho form), isophthalic acid, dibromoisophthalic acid, sodium sulfoisophthalate, phenylenedioxydicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 4,4'-diphenylketonedicarboxylic acid, 4,4'-diphenoxyethanedicarboxylic acid, 4,4'-diphenylsulfonedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, trimellitic acid, and pyromellitic acid. Further, other dicarboxylic acids such as alicyclic dicarboxylic acids such as hexahydroterephthalic acid and hexahydroisophthalic acid, and aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, undecanedicarboxylic acid, and dodecanedicarboxylic acid are also included.

[0063] As methods for depolymerizing polyesters, all of a hydrolysis method using water as a solvent, an alcoholysis method using alcohol as a solvent, and a glycolysis method using glycol as a solvent can be used.

[0064] The hydrolysis method can decompose, for example, polyethylene terephthalate melt by reacting it with water and then reacting it with ammonium hydroxide to decompose it into terephthalic acid and ethylene glycol.

[0065] The alcoholysis method can depolymerize polyesters, for example, by heating in an alcohol solvent (adding a catalyst if necessary). This method has an advantage that, for example, when depolymerizing polyethylene terephthalate using methanol as a solvent, dimethyl terephthalate (DMT), which is a useful and easy-to-handle monomer, is directly produced by the depolymerization reaction, and the depolymerization reaction is relatively fast.

[0066] The glycolysis method can depolymerize polyester by heating it with a depolymerization catalyst such as sodium carbonate in an excess of alkylene glycol solvent to produce bis(β-hydroxyalkyl) terephthalate and ethylene glycol. 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 and performing a transesterification reaction.

[0067] Specifically, the depolymerization step may include the steps of producing a reaction solution, immersing the polyester in the reaction solution, and heating the reaction solution to depolymerize it.

[0068] The reaction solution can be produced by adding and dissolving a reaction catalyst in a solvent. Here, the reaction catalyst can include an alkali, an acid, or a salt thereof, and the solvent can include water, a monoalcohol, a polyhydric alcohol, or a mixture thereof. Since the same materials as those used in the decolorization step can be used for the alkali, acid, these salts, monoalcohol, or polyhydric alcohol, their description is omitted.

[0069] The addition amount of the reaction catalyst to the solvent can be 0.01 mol to 100 mol, for example, 0.1 mol to 10 mol, or 0.8 mol to 1.2 mol per 1 mol of the dibasic acid constituting the polyester.

[0070] As an example, when polyethylene terephthalate is decomposed using sodium hydroxide as the reaction catalyst, the reaction formula is PET (polyethylene terephthalate) + 2NaOH → TPA salt (disodium terephthalate) + EG (ethylene glycol). Therefore, to decompose 1 kg of PET, 2 moles of NaOH (420 g) are required per 1 mole of TPA.

[0071] Next, immerse it in the reaction solution in which the polyester was produced.

[0072] On the one hand, when using polyester waste as a feedstock for depolymerization, an additional pretreatment step can be performed to selectively wash the waste before the depolymerization step to remove contaminants attached to the waste, such as contents or soil.

[0073] Also, in order to carry out the reaction more efficiently, waste and the like can be mechanically cut, crushed, and processed. The crushing treatment can be carried out using known suitable means. For example, by crushing using a hammer mill or the like, the polyester waste can be crushed into chips with a size of 2 mm to 8 mm and then subjected to a depolymerization reaction.

[0074] Also, if necessary, the crushed chips can be immersed in a solvent to separate components lighter than the solvent, and the crushed material can be blown away by wind or only a certain size can be recovered using a sieve and then subjected to a depolymerization reaction.

[0075] Next, alkylene glycol and dibasic acid salt are separated and recovered from the depolymerization product obtained in the depolymerization step.

[0076] As an example, when using alkylene glycol, which is a constituent component of polyester, and other alcohols as a reaction solvent, although the alkylene glycol produced by depolymerization and the reaction solvent are temporarily mixed, the alkylene glycol produced by depolymerization can be recovered by separating the two.

[0077] The method for separating the alkylene glycol produced in the depolymerization reaction and the reaction solvent is not particularly limited, and an appropriate method can be selected according to the target compound. For example, it can be separated by a distillation concentration method. As means for distillation concentration, all conventional distillation concentration apparatuses, such as a vacuum continuous distillation apparatus and a vacuum batch distillation apparatus, can be used.

[0078] When the alkylene glycol produced by the depolymerization reaction is of the same type as the reaction solvent, the produced alkylene glycol does not need to be separately separated.

[0079] On the other hand, in the depolymerization stage, depending on the type of the reaction solvent, the hydrolysis method using a hydroxy group (OH group), the alcoholysis method using an alcohol, and the glycolysis method using a glycol can be selectively applied, and the types of dibasic acids obtained thereby are different.

[0080] When depolymerizing a polyester using the glycolysis method, the obtained dibasic acid salt can vary depending on the type of the reaction solvent used in the depolymerization reaction. For example, when depolymerizing polyethylene terephthalate using ethylene glycol as the reaction solvent, bis(β-hydroxyethyl) terephthalate (BHET) can be obtained as a monomer, and when depolymerizing using propylene glycol as the reaction solvent, bis(β-hydroxyethyl isopropyl) terephthalate (BHEPT) can be mainly obtained as a monomer.

[0081] Also, when using benzyl alcohol and tripotassium phosphate as the reaction solvent, the obtained ester compound of the oligomer has very good solubility in chloroform or the like, so it can be efficiently recovered using solvent extraction. In addition, the dibasic acid or the ester compound of the oligomer can also be recovered using means such as filtration or distillation.

[0082] Thus, the obtained dibasic acid or the ester compound of the oligomer (for example, BHET) can also be recovered as the methyl ester of the dibasic acid or the oligomer (for example, DMT) by subjecting it to a transesterification reaction with methanol.

[0083] The transesterification reaction can be appropriately carried out by using known methods. For example, by subjecting the depolymerization reaction concentrate and methanol to a transesterification reaction at 65°C to 85°C for 0.5 hour to 5 hours in the presence of a transesterification catalyst (such as an alkali metal compound), a slurry in which solid DMT is dispersed in a mixed solution of methanol and alkylene glycol or the like can be obtained. Further, a cake containing DMT can be separated by a solid-liquid separation device or the like, and purified DMT can be recovered by performing distillation purification.

[0084] On the other hand, when depolymerizing a polyester using the glycolysis method, dibasic acid salts such as dibasic acids or dialkali metal salts of oligomers may be produced, and this product may vary depending on the type of alkali metal used in the depolymerization reaction. For example, in the case of polyethylene terephthalate, when depolymerizing using sodium hydroxide as a reaction solvent, sodium terephthalate is produced together with ethylene glycol.

[0085] Among them, the dibasic acid salt (for example, sodium terephthalate) does not dissolve in alkylene glycol and forms solid-phase crystals, so it can be easily separated from the solvent by a filtration method such as solid-liquid separation. Further, the alcohols adhering to the obtained powder-type crystals can be removed by washing with an alcohol such as methanol or ethanol.

[0086] Next, after dissolving the dibasic acid salt crystal in water and mixing an acid for a neutralization reaction, a dibasic acid (for example, terephthalic acid) can be obtained. Since the dibasic acid also precipitates as crystals in water, it can be recovered by solid-liquid separation from water by a method such as centrifugation.

[0087] The water content added during the recovery process can be 3 to 10 weights relative to 1 weight of the dibasic acid salt. The acid content supplied for separating the dibasic acid can be equimolar or more relative to the alkali metal contained in the dibasic acid salt. At this time, as the type of acid used, inorganic acids such as hydrochloric acid, sulfuric acid, and phosphoric acid, which are strong acids with a pH of 2, or organic acids such as formic acid, acetic acid, and oxalic acid can be used. Among them, inorganic acids, particularly hydrochloric acid or sulfuric acid, are preferred because they reduce impurities in the resulting monomer. The temperature of the neutralization reaction is 65°C to 85°C, and the neutralization reaction usually ends within 0.5 to 5 hours.

[0088] On the other hand, the polyester depolymerization method can optionally further include a step of removing impurities contained in polyester molded articles, waste, etc. with an aqueous solution of a dibasic acid salt.

[0089] The means for removing impurities is not particularly limited, and any appropriate techniques, apparatuses, etc. can be used. For example, solid components such as resins other than polyester remaining in the unreacted state (such as polyethylene, polypropylene, polyvinyl chloride, etc.), reaction catalysts such as alkali metals that do not dissolve, etc. can be removed using a mesh or the like. Also, dyes, fillers, etc. cannot be removed by a mesh or the like, so they can be removed by centrifugation, filtration treatment with an adsorbent such as activated carbon, etc.

[0090] Since the obtained dibasic acid crystals exhibit a particle size distribution characteristic of several μm to several tens of μm, the method can further include a step of selectively recrystallizing the dibasic acid crystals and recrystallizing them to a particle size (100 μm or more) that enables commercialization. The method of recrystallization is not particularly limited. For example, after mixing the dibasic acid crystals with water, it can be carried out under high temperature and high pressure conditions.

[0091] The dibasic acids and alkylene glycols obtained by the method for depolymerizing polyester can be recycled as monomers for polymerizing recycled polyester. Specifically, esterification and polycondensation can be carried out on a composition for polymerizing recycled polyester containing dibasic acids and alkylene glycols obtained by the method for depolymerizing polyester, and recycled polyester can be produced.

[0092] Esterification is to react the recovered dibasic acid and alkylene glycol monomers, 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 catalysts such as alkaline earth metal compounds such as magnesium and calcium, and metal compounds such as titanium, zinc, and manganese, which are well-known transesterification catalysts.

[0093] Thereafter, the product of the esterification step can be polycondensed to produce a recycled polyester resin. At this time, either melt polymerization or solid-phase polymerization can be used.

[0094] In addition, after solid-phase polymerization, if necessary for quality adjustment, water treatment and / or addition of a compound that promotes crystallization can be carried out, and a polycondensation catalyst or stabilizer can also be added at the start or during the polycondensation step.

[0095] Here, the water treatment can be carried out by bringing the recycled polyester resin produced in solid particle form into contact with, for example, water, steam, an inert gas containing steam, air containing steam, etc. Examples of the compound that promotes crystallization include polyolefin-based thermoplastic resins such as polyhexamethylene terephthalate, inorganic compounds, higher aliphatic compounds, polyether-based compounds, polypropylene, and polyethylene. These compounds can be added to the recycled polyester resin at 1 ppm to 100 ppm.

[0096] As the polycondensation catalyst, compounds such as germanium, antimony, titanium, and aluminum can be used. The addition amount of the polycondensation catalyst is 2 ppm to 800 ppm, for example, 4 ppm to 500 ppm, as the weight of the catalyst metal element with respect to the total weight of the dibasic acid component.

[0097] In addition, the recycled polyester can contain, as a stabilizer, phosphate esters such as trimethyl phosphate, triethyl phosphate, triphenyl phosphate, and triethyl phosphonoacetate, phosphite esters such as triphenyl phosphite and trisdodecyl phosphite, and phosphorus compounds such as methyl acid phosphate, dibutyl phosphate, monobutyl phosphate, phosphoric acid, phosphorous acid, hypophosphorous acid, and polyphosphoric acid. The addition amount of the stabilizer is 1000 ppm or less, for example, 500 ppm or less, or 300 ppm or less, as the weight of the phosphorus element in the stabilizer with respect to the total weight of the recycled polyester.

[0098] On the other hand, as described above, the polyester depolymerization method according to one embodiment decolors using superheated steam, and solves the problem that a decolorizing agent is added and the polyester is dissolved and decolorized only on the surface and not inside the dissolved polyester. Thus, the hue of the recycled polyester produced using the dibasic acid and alkylene glycol obtained thereby has an L value of 60 or more, and can be, for example, 65 to 95.

[0099] The L, a, and b color systems are internationally commonly used as standards for evaluating the hue of polyester. Such color values are one of the color systems for standardizing color measurement and describe recognizable hues and differences in hues. In this system, L is the lightness factor, and a and b are color measurement numbers. Among these, the L value means a numerical factor indicating brightness and is a very important value in the production of fibers, fabrics, or 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.

[0100] The values of L, a, and b are defined in Korean Industrial Standards (KS) related to color measurement, such as KS A 0061, 0063, 0064, 0065, 0066, 0067, 0084, 0085, 0089, 0114, etc. As an example, the values of L, a, and b can be determined by putting 50 g of the polyester resin to be measured in a Colorimeter model SA - 2000 after removing moisture in the air and measuring the hue 10 times, and taking the average value as the standard value.

[0101] The L value of the recycled polyester is related to the purity of the recycled dibasic acid and alkylene glycol. As an example, inside, the more the amount of impurities or a matting agent such as titanium dioxide (TiO2) increases, the lower the L value becomes. When the L value is less than 60, due to the excessive amount of impurities, the hue after polymerization and fibrillation may have color, or many side reactions may occur during polymerization, and it may not be possible to be used as a high - value - added material. As the main use, since it is applied to the same use as mechanical recycled polyester, it may be meaningless. In addition, when the L value exceeds 95, it is a physical property that is difficult to reach even with conventional virgin materials. To achieve this value by recycling, the economic efficiency decreases due to an increase in unit production volume and process cost, such as adding a decolorization and purification process or increasing the residence time. <Embodiments of the Invention>

[0102] Hereinafter, specific embodiments of the present invention will be presented. However, the examples described below are only for specifically exemplifying or explaining the present invention, and the scope of the present invention is not limited thereby.

[0103] (Examples 1 to 3) 10 kg of polyethylene terephthalate colored waste fiber supply raw material (PET content 90% by weight) was impregnated with a 30% by weight aqueous solution of sodium hydroxide (NaOH), and the impregnation amount (based on on the weight of fabric (o.w.f.)) was made to be 50% by weight, 100% by weight, and 300% by weight, respectively.

[0104] At this time, the content of sodium hydroxide impregnated in polyethylene terephthalate is 0.18 mol, 0.36 mol, and 1.08 mol, respectively, per 1 mol of terephthalic acid.

[0105] This was put into a superheated steam system, and superheated steam at 350 °C was injected at a pressure of 1.5 bar for 10 minutes for decolorization treatment.

[0106] After adding 2.2 mol of sodium hydroxide (NaOH) to the decolorized polyethylene terephthalate, it was depolymerized at 180 °C for 5 hours.

[0107] After that, the decomposed depolymerization product was washed, and then sulfuric acid (H2SO4) was added to reduce the pH to 3.0 or less, and terephthalic acid (TPA) and ethylene glycol (EG) were recovered.

[0108] (Comparative Example 1) 10 kg of a polyethylene terephthalate colored waste fiber supply raw material (PET content 90% by weight) was put into a superheated steam system, and superheated steam at 450 °C was injected at a pressure of 1.5 bar for 10 minutes for decolorization treatment.

[0109] As a result, it was confirmed that a part of the decolorized polyethylene terephthalate was carbonized.

[0110] (Comparative Example 2) 10 kg of a polyethylene terephthalate colored waste fiber supply raw material (PET content 90% by weight) was impregnated with a 30% by weight aqueous solution of sodium hydroxide (NaOH), and the impregnation amount (on the weight of fabric (o.w.f.) basis) was adjusted to 50% by weight.

[0111] At this time, the content of sodium hydroxide impregnated in polyethylene terephthalate is 0.18 mol per 1 mol of terephthalic acid.

[0112] This was put into a system with saturated steam at 13.5 bar having a temperature of 196 °C for 10 minutes for decolorization treatment.

[0113] As a result, it was confirmed that a part of the decolorized polyethylene terephthalate was decolorized. However, due to the high pressure of 13.5 bar, the waste fibers flapped inside the system, making it unstable. The reaction temperature decreased due to the condensed water vapor, resulting in a decrease in efficiency.

[0114] When the pressure of the water vapor is increased to a maximum of 50 bar, the temperature can be raised to 265 °C. However, the target high-pressure water vapor production device is expensive and has the disadvantage of low efficiency compared to the cost.

[0115] After adding 2.2 moles of sodium hydroxide (NaOH) to the decolorized polyethylene terephthalate, it was depolymerized at 180 °C for 5 hours.

[0116] After that, the decomposed depolymerization product was washed, sulfuric acid (H2SO4) was added to reduce the pH to 3.0 or less, and terephthalic acid (TPA) and ethylene glycol (EG) were recovered.

[0117] [Experimental Example: Results of Decolorization Treatment] In the examples and comparative examples, the hue of the decolorized polyethylene terephthalate (PET) was measured using the L, a, and b color systems, and the results are shown in Table 1.

[0118] The values of L, a, and b were determined by putting 50 g of the decolorized polyethylene terephthalate in a calorimeter (colorimeter; Colorimeter) model SA-2000 after removing moisture in the air, measuring the hue 10 times, and taking the average value as the standard value.

[0119] In addition, photographs of the polyethylene terephthalate before and after the decolorization treatment in Example 3 and Comparative Example 1 are shown in FIGS. 1 and 2, respectively. In FIGS. 1 and 2, the left photograph is the photograph before the decolorization treatment, and the right photograph is the photograph after the decolorization treatment.

[0120]

Table 1

[0121] Referring to Table 1, Figure 1 and Figure 2, it was confirmed that part of the decolorized polyethylene terephthalate, as the result of Comparative Example 1, was carbonized. It was found that Examples 1 to 3 had superior hue compared to Comparative Example 2. This is because in the case of Comparative Example 2, the dye of the colored waste fiber was not completely decomposed by using normal steam at a low temperature of 196°C instead of superheated steam.

[0122] As described above, the preferred embodiments of the present invention have been described in detail. However, the scope of the present invention is not limited thereto, and various modifications and improvements by those skilled in the art using the basic concept of the present invention defined in the following claims also belong to the scope of the present invention.

Industrial Applicability

[0123] The present invention relates to a method for decolorizing polyester, particularly colored polyester into which a large amount of dye is introduced during processing, and a depolymerization method including the same, and can remove the dye of colored polyester at low cost and with high efficiency.

Claims

1. adding a decolorizing agent containing an alkali to the polyester, and then bringing the polyester into contact with superheated steam to decolorize the polyester; adding a reaction catalyst containing an alkali to depolymerize the decolorized polyester to obtain a depolymerization product containing alkylene glycol and dibasic acid salt; solid-liquid separating the liquid alkylene glycol and the solid dibasic acid salt from the depolymerization product; dissolving the solid dibasic acid salt in water; neutralizing the aqueous solution of the dibasic acid salt with an acid to precipitate crystals of the dibasic acid in water; solid-liquid separating the crystals of the dibasic acid from the dibasic acid in this water, and including: The method for depolymerizing polyester, wherein the temperature of the superheated steam is 250°C to 450°C.

2. The method for depolymerizing polyester according to claim 1, further comprising the step of removing impurities from the aqueous solution of the dibasic acid salt.

3. The method for depolymerizing polyester according to claim 1, further comprising the step of recrystallizing the crystals of the dibasic acid.

4. A composition for regenerated polyester polymerization, comprising the dibasic acid and alkylene glycol obtained by the method for depolymerizing polyester according to claim 1.

5. A method for producing regenerated polyester, produced using the composition for regenerated polyester polymerization according to claim 4.

6. The method for producing regenerated polyester according to claim 5, wherein the hue of the regenerated polyester has an L value of 60 or more.

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