Monomer composition for synthesizing recycled plastics, its manufacturing method, and recycled plastics, molded articles, and plasticizer compositions using the same

Abstract

The present invention relates to a monomer composition for synthesizing recycled plastics, which is recovered from a (co)polymer synthesized from a monomer containing terephthalic acid and has a high acid value, a method for producing the same, and recycled plastics, molded articles, and plasticizer compositions each using the same.

Description

[Technical Field]

[0001] [CROSS-REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2021-0094470, dated July 19, 2021, Korean Patent Application No. 10-2021-0094471, dated July 19, 2021, Korean Patent Application No. 10-2021-0094472, dated July 19, 2021, and Korean Patent Application No. 10-2021-0094473, dated July 19, 2021, and all contents disclosed in the documents of said Korean patent applications are incorporated herein by reference.

[0002] The present invention relates to a monomer composition for synthesizing recycled plastics, which has a high acid value and ensures high-purity terephthalic acid when terephthalic acid is recovered by depolymerizing a (co)polymer synthesized from a monomer containing terephthalic acid; a method for producing the same; and recycled plastics, molded articles, and plasticizer compositions each using the same. [Background technology]

[0003] Polyethylene terephthalate (PET) is a thermoplastic (co)polymer that has excellent properties such as excellent transparency and heat insulation, and is a plastic that is widely used in wire coatings, household goods, toys, electrical insulators, radio and television cases, packaging materials, etc.

[0004] Although polyethylene terephthalate is widely used for various purposes, environmental and health concerns have been raised during waste disposal. Currently, physical recycling methods are being used, but this can lead to quality degradation, so research is underway into chemical recycling of polyethylene terephthalate.

[0005] Terephthalic acid is a valuable compound used as a raw material for a wide variety of products, including polyethylene terephthalate (PET), polyester fiber, and polyester film for packaging and containers.

[0006] The condensation polymerization of ethylene glycol with terephthalic acid to form polyethylene terephthalate (PET) is a reversible reaction process, and the polyethylene terephthalate (PET) can be depolymerized back to monomers or oligomers.

[0007] Various methods have been proposed for decomposing polyethylene terephthalate (PET) to recover the raw material monomers. The monomers used in the polycondensation reaction to regenerate recycled plastics can be obtained by alkaline decomposition of polyethylene terephthalate (PET) waste.

[0008] For example, the decomposition products of polyethylene terephthalate (PET) under basic conditions include ethylene glycol and the salt of terephthalic acid, which can be further neutralized with a strong acid to produce terephthalic acid.

[0009] However, terephthalic acid obtained by conventional methods typically contains 1-2% isophthalic acid as an impurity, and when it is reused as a raw material for manufacturing high-value-added plastics such as PBT / TPEE, there are limitations to its use as it can cause problems such as a decrease in the physical properties of the (co)polymer (low melting point, low tensile strength, low rigidity, etc.).

[0010] Therefore, it is necessary to develop a method that can significantly reduce the content of isophthalic acid, an impurity, in the process of decomposing (co)polymers synthesized from monomers containing terephthalic acid, such as polyethylene terephthalate (PET), to recover the raw monomers. Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention provides a monomer composition for synthesizing recycled plastics having a high acid value by ensuring high-purity terephthalic acid when terephthalic acid is recovered by depolymerizing a (co)polymer synthesized from a monomer containing terephthalic acid.

[0012] The present invention also provides a method for producing the monomer composition for synthesizing recycled plastics, as well as recycled plastics, molded articles, and plasticizer compositions using the monomer composition for synthesizing recycled plastics. [Means for solving the problem]

[0013] In order to solve the above problems, the present invention provides a monomer composition for synthesizing recycled plastics, which contains terephthalic acid and has an acid value of 670 mgKOH / g or more as measured by KS M ISO 2114, and is recovered from a (co)polymer synthesized from a monomer containing terephthalic acid.

[0014] The present invention also provides a method for producing a monomer composition for synthesizing recycled plastics, comprising: depolymerizing a (co)polymer synthesized from a monomer containing terephthalic acid to remove a diol component; and washing the depolymerization product from which the diol component has been removed, the washing step comprising washing with a protic polar solvent at a temperature of 20°C to 100°C, and washing with a washing solvent containing an aprotic polar solvent at a temperature of 20°C to 110°C.

[0015] The present invention also provides a recycled plastic comprising a reaction product of the monomer composition for synthesizing recycled plastic and a comonomer.

[0016] The present invention also provides a molded article containing the recycled plastic.

[0017] The present invention also provides a plasticizer composition comprising a reaction product of the monomer composition for synthesizing recycled plastics and an alcohol.

[0018] Hereinafter, a monomer composition for synthesizing recycled plastics according to specific embodiments of the invention, a method for producing the same, and recycled plastics, molded articles, and plasticizer compositions using the same will be described in more detail.

[0019] Unless expressly stated otherwise herein, terminology is for the purpose of referring to particular embodiments only and is not intended to be limiting of the invention.

[0020] As used herein, the singular forms "a," "an," and "the" include the plural forms as well, unless the context clearly indicates to the contrary.

[0021] As used herein, the meaning of "comprising" embodies certain properties, regions, integers, steps, operations, elements, and / or components, and does not exclude the presence or addition of other certain properties, regions, integers, steps, operations, elements, components, and / or groups.

[0022] In this specification, terms including ordinal numbers, such as "first" and "second," are used to distinguish one component from another, and are not limited by the ordinal numbers. For example, within the scope of the present invention, a first component may be called a second component, and similarly, a second component may be called a first component.

[0023] In this specification, the term "(co)polymer" refers to both a polymer and a copolymer, where the polymer refers to a homopolymer consisting of a single repeating unit, and the copolymer refers to a composite polymer containing two or more types of repeating units.

[0024] 1. Monomer composition for synthesizing recycled plastics According to one embodiment of the present invention, there is provided a monomer composition for synthesizing recycled plastics, which contains terephthalic acid and has an acid value of 670 mgKOH / g or more as measured by KS M ISO 2114, and is recovered from a (co)polymer synthesized from a monomer containing terephthalic acid.

[0025] The present inventors have experimentally confirmed that the purity of terephthalic acid in a monomer composition for synthesizing recycled plastics recovered from a (co)polymer synthesized from a monomer containing terephthalic acid, such as the monomer composition for synthesizing recycled plastics of the above embodiment, can be increased to a commercially usable level, thereby completing the invention.

[0026] Furthermore, the inventors have experimentally confirmed that although the monomer composition for synthesizing recycled plastics in one embodiment is recovered from a (co)polymer synthesized from a monomer containing terephthalic acid, the ratio of the other monomer, isophthalic acid, rather than terephthalic acid, which is the main target substance of the present invention, is extremely reduced to less than 0.85 mol % based on 100 mol % of the total monomer compounds contained in the monomer composition for synthesizing recycled plastics, and that when polyethylene terephthalate or high-value-added plastics (PBT, TPEE) are synthesized using this, excellent physical properties can be achieved, thereby completing the invention.

[0027] In particular, terephthalic acid recovered from a (co)polymer synthesized from a monomer containing terephthalic acid obtained by a conventional method usually contains 1 to 2% of isophthalic acid as an impurity, whereas the present invention is able to almost completely remove isophthalic acid by a secondary washing step, which is a characteristic of the method for producing a monomer composition for synthesizing recycled plastics, which will be described later.

[0028] Specifically, the monomer composition for synthesizing recycled plastics according to one embodiment may contain terephthalic acid, which is recovered from a (co)polymer synthesized from a monomer containing terephthalic acid used in recovering the monomer composition for synthesizing recycled plastics.

[0029] That is, terephthalic acid is also obtained as a result of recovery from a (co)polymer synthesized from a monomer containing terephthalic acid to obtain the monomer composition for synthesizing recycled plastics of the embodiment. Therefore, when new terephthalic acid is added externally, separately from the recovery from a (co)polymer synthesized from a monomer containing terephthalic acid to produce the monomer composition for synthesizing recycled plastics of the embodiment, it is not included in the scope of terephthalic acid of the present invention.

[0030] Specifically, the term "recovered from a (co)polymer synthesized from a monomer containing terephthalic acid" means that the compound is obtained by depolymerization of a (co)polymer synthesized from a monomer containing terephthalic acid. The depolymerization may be carried out under acidic, neutral, or basic conditions, and particularly under basic (alkaline) conditions.

[0031] For example, when the depolymerization reaction is carried out under the basic conditions, a terephthalic acid salt called Na2-TPA and ethylene glycol are primarily produced from the polyethylene terephthalate, and then Na2-TPA is converted to TPA by secondary neutralization with a strong acid, allowing terephthalic acid to be recovered.

[0032] That is, the terephthalic acid recovered from the (co)polymer synthesized from the monomer containing terephthalic acid may contain a base (alkali) decomposition product of the (co)polymer synthesized from the monomer containing terephthalic acid, an acid-neutralized product thereof, or a mixture thereof. Specifically, the base (alkali) decomposition product of the (co)polymer synthesized from the monomer containing terephthalic acid may contain Na-TPA, and the acid-neutralized product of the base (alkali) decomposition product of the (co)polymer synthesized from the monomer containing terephthalic acid may contain terephthalic acid.

[0033] The molar ratio of the terephthalic acid based on 100 mol% of the total monomer compounds contained in the monomer composition for synthesizing recycled plastics may be more than 99.15 mol%, or 99.5 mol% or more, or 99.9 mol% or more, or 99.95 mol% or more, or 99.99 mol% or more, or more than 99.15 mol% to 100 mol% or less, or 99.5 mol% to 100 mol%, or 99.9 mol% to 100 mol% or less, or 99.9 mol% to 100 mol% or less, or 99.95 mol% to 100 mol% or less, or 99.99 mol% to 100 mol% or less.

[0034] The method for measuring the molar ratio of terephthalic acid is not particularly limited, and examples thereof include 1 H NMR, ICP-MS analysis, HPLC analysis, etc. can be used without limitation. Specific methods, conditions, and devices for NMR, ICP-MS, and HPLC can be various conventional methods without limitation.

[0035] As an example of a method for measuring the molar ratio of terephthalic acid, 5 mg to 20 mg of a monomer composition for synthesizing recycled plastics is collected as a sample under normal pressure and at a temperature of 20°C to 30°C, dissolved in 1 ml of DMSO-d6 solvent, and then analyzed by an Agilent DD1 500 MHz NMR device. 1 H NMR spectra were obtained, and the peaks of all detected substances, such as terephthalic acid (TPA) and isophthalic acid (IPA), were individually assigned and integrated using analytical software (MestReC). The molar ratio (mol%) of terephthalic acid contained within 100 mol% of the total monomer compounds analyzed from the sample was calculated based on the peak integral value.

[0036] As described above, the ratio of terephthalic acid, which is the main synthetic target substance in the present invention, is significantly increased to over 99.15 mol% based on 100 mol% of the total monomer compounds contained in the monomer composition for synthesizing recycled plastics. By minimizing impurity monomers other than terephthalic acid (e.g., isophthalic acid), excellent physical properties can be achieved when polyethylene terephthalate or high-value-added plastics (PBT, TPEE) are synthesized using this.

[0037] In addition, the monomer composition for synthesizing recycled plastics according to one embodiment may further include isophthalic acid in a molar ratio of less than 0.85 mol % based on 100 mol % of the total monomer compounds contained in the monomer composition for synthesizing recycled plastics.

[0038] The isophthalic acid is recovered from a (co)polymer synthesized from a monomer containing terephthalic acid used in recovering the monomer composition for synthesizing recycled plastics.

[0039] In other words, isophthalic acid is also obtained as a result of recovery from a (co)polymer synthesized from a monomer containing terephthalic acid to obtain the monomer composition for synthesizing recycled plastics of the embodiment. Therefore, when new isophthalic acid is added externally, separately from the recovery from a (co)polymer synthesized from a monomer containing terephthalic acid to produce the monomer composition for synthesizing recycled plastics of the embodiment, it is not included in the scope of isophthalic acid of the present invention.

[0040] Specifically, the term "recovered from a (co)polymer synthesized from a monomer containing terephthalic acid" means that the compound is obtained by depolymerization of a (co)polymer synthesized from a monomer containing terephthalic acid. The depolymerization may be carried out under acidic, neutral, or basic conditions, and particularly under basic (alkaline) conditions.

[0041] The molar ratio of the isophthalic acid based on 100 mol% of the total monomer compounds contained in the monomer composition for synthesizing recycled plastics may be less than 0.85 mol%, or 0.5 mol% or less, or 0.1 mol% or less, or 0.05 mol% or less, or 0.01 mol% or less, or 0 mol% to less than 0.85 mol%, or 0 mol% to 0.5 mol%, or 0 mol% to 0.1 mol%, or 0 mol% to 0.05 mol% or 0 mol% to 0.01 mol%.

[0042] The method for measuring the molar ratio of isophthalic acid is not particularly limited, and examples thereof include 1 H NMR, ICP-MS analysis, HPLC analysis, etc. can be used without limitation. Specific methods, conditions, and devices for the NMR, ICP-MS, and HPLC can be various conventional methods without limitation.

[0043] As an example of a method for measuring the molar ratio of isophthalic acid, 5 mg to 20 mg of a monomer composition for synthesizing recycled plastics is collected as a sample under normal pressure and at a temperature of 20°C to 30°C, dissolved in 1 ml of DMSO-d6 solvent, and then analyzed by an Agilent DD1 500 MHz NMR device. 1 H NMR spectra were obtained, and the peaks of all detected substances, such as terephthalic acid (TPA) and isophthalic acid (IPA), were individually assigned and integrated using analytical software (MestReC). The molar ratio (mol%) of isophthalic acid contained within 100 mol% of the total monomer compounds analyzed from the sample was calculated based on the peak integral value.

[0044] In this way, the ratio of the impurity monomer isophthalic acid, rather than terephthalic acid, the main synthetic target substance in the present invention, is extremely reduced to less than 0.85 mol % based on 100 mol % of the total monomer compounds contained in the monomer composition for synthesizing recycled plastics. Therefore, when using this to synthesize polyethylene terephthalate or high-value-added plastics (PBT, TPEE), excellent physical properties can be achieved.

[0045] In addition, the monomer composition for synthesizing recycled plastics according to one embodiment is characterized in that it is recovered from a (co)polymer synthesized from a monomer containing terephthalic acid. That is, as a result of recovering from a (co)polymer synthesized from a monomer containing terephthalic acid to obtain the monomer composition for synthesizing recycled plastics according to one embodiment, a monomer composition for synthesizing recycled plastics containing both terephthalic acid and isophthalic acid is obtained.

[0046] Regarding the (co)polymer synthesized from the monomer containing terephthalic acid, the (co)polymer means both a polymer and a copolymer, and is a general term for a reaction product obtained by the (co)polymerization reaction of a monomer. The (co)polymer may include low molecular weight compounds, oligomers, and polymers depending on the molecular weight range.

[0047] The (co)polymer synthesized from the monomer containing terephthalic acid can include one or more (co)polymers selected from the group consisting of polyalkylene terephthalate, polyalkylene terephthalate copolymer, and thermoplastic polyester elastomer. That is, the (co)polymer synthesized from the monomer containing terephthalic acid can include one type of polyalkylene terephthalate, one type of polyalkylene terephthalate copolymer, one type of thermoplastic polyester elastomer, or a mixture of two or more types thereof.

[0048] The polyalkylene terephthalate copolymer refers to a copolymer obtained by further reacting an additional comonomer based on alkylene glycol and terephthalic acid, which are monomers for synthesizing polyalkylene terephthalate.

[0049] The (co)polymer synthesized from the monomer containing terephthalic acid may include a reaction product of terephthalic acid and a comonomer, i.e., the monomer containing terephthalic acid may further include a comonomer together with terephthalic acid.

[0050] The comonomer capable of reacting with terephthalic acid is not particularly limited, and specific examples thereof include aliphatic diols, polyalkylene oxides, fatty acids, fatty acid derivatives, and combinations thereof.

[0051] The aliphatic diol may be, for example, one or more of diols having a number average molecular weight (Mn) of 300 g / mol or less, i.e., ethylene glycol, propylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol (1,4-BG), 1,5-pentanediol, 1,6-hexanediol, and 1,4-cyclohexanedimethanol (1,4-CHDM). Specific examples include 1,4-butanediol, ethylene glycol, 1,4-cyclohexanedimethanol, or a mixture thereof.

[0052] The polyalkylene oxide is a unit constituting a soft segment and may be an aliphatic polyether. Examples of the polyalkylene oxide include one or more of polyoxyethylene glycol, polypropylene glycol, poly(tetramethylene ether) glycol (PTMEG), polyoxyhexamethylene glycol, a copolymer of ethylene oxide and propylene oxide, an ethylene oxide addition polymer of polypropylene oxide glycol, and a copolymer of ethylene oxide and tetrahydrofuran. A specific example is PTMEG, and in particular, PTMEG having a number average molecular weight (Mn) of 600 g / mol to 3,000 g / mol, 1,000 g / mol to 2,500 g / mol, or 1,500 g / mol to 2,200 g / mol may be used.

[0053] The fatty acid may be, for example, one or more aliphatic carboxylic acid compounds excluding terephthalic acid, and a specific example may be adipic acid. The fatty acid derivative is a compound derived from the above-mentioned fatty acid, and for example, may be one or more of fatty acid esters, fatty acid chlorides, fatty acid anhydrides, and fatty acid amides, and a specific example may be adipic acid esters.

[0054] To give a specific example, when 1,4-butanediol, an aliphatic diol, is used as a comonomer that can react with terephthalic acid, polybutylene terephthalate (PBT), a type of polyalkylene terephthalate, is obtained by the polymerization reaction of terephthalic acid and 1,4-butanediol.

[0055] In addition, when ethylene glycol, an aliphatic diol, is used as a comonomer that can react with terephthalic acid, polyethylene terephthalate (PET), a type of polyalkylene terephthalate, can be obtained by the polymerization reaction of terephthalic acid and ethylene glycol.

[0056] In addition, when the aliphatic diol 1,4-butanediol and the polyalkylene oxide PTMEG are used together as comonomers that can react with the terephthalic acid, a thermoplastic polyester elastomer (TPEE) can be obtained by the polymerization reaction of the terephthalic acid, 1,4-butanediol, and PTMEG.

[0057] In addition, when the aliphatic diol 1,4-butanediol and the fatty acid adipic acid are used together as comonomers that can react with the terephthalic acid, the polymerization reaction of terephthalic acid, 1,4-butanediol, and adipic acid produces polybutylene adipate terephthalate (PBAT), a type of polyalkylene terephthalate copolymer.

[0058] In addition, when the aliphatic diols ethylene glycol and 1,4-cyclohexanedimethanol are used together as comonomers that can react with terephthalic acid, a glycol-modified polyethylene terephthalate (PETG), a type of polyalkylene terephthalate copolymer, can be obtained through a polymerization reaction of terephthalic acid, ethylene glycol, and 1,4-cyclohexanedimethanol.

[0059] As a specific example, the polyalkylene terephthalate may include one or more (co)polymers selected from polybutylene terephthalate, polyethylene terephthalate, polycyclohexylene dimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, and polytrimethylene terephthalate.

[0060] As a specific example, the polyalkylene terephthalate copolymer may include one or more (co)polymers selected from polybutylene adipate terephthalate (PBAT) and glycol-modified polyethylene terephthalate (PETG).

[0061] Meanwhile, the acid value of the monomer composition for synthesizing recycled plastics, as measured by KS M ISO 2114, may be 670 mgKOH / g or more, or 671 mgKOH / g or more, or 672 mgKOH / g or more, or 673 mgKOH / g or more, or 674 mgKOH / g or more, or 680 mgKOH / g or less, or 670 mgKOH / g to 680 mgKOH / g, or 671 mgKOH / g to 680 mgKOH / g, or 672 mgKOH / g to 680 mgKOH / g, or 673 mgKOH / g to 680 mgKOH / g, or 674 mgKOH / g to 680 mgKOH / g.

[0062] The acid number corresponds to the mass of potassium hydroxide in mg required to neutralize 1 g of recycled terephthalic acid monomer composition under the conditions specified in KS M ISO 2114.

[0063] The acid value of the monomer composition for synthesizing recycled plastics is increased to 670 mgKOH / g or more, minimizing VOC impurities other than terephthalic acid (e.g., formic acid, glycolic acid, 4-CBA, etc.). When this is used to synthesize polyethylene terephthalate or high-value-added plastics (PBT, TPEE), excellent physical properties can be achieved.

[0064] If the acid value of the monomer composition for synthesizing recycled plastics is reduced to less than 670 mgKOH / g, the physical properties of the recovered terephthalic acid and the (co)polymer synthesized therefrom will be poor due to excessive VOC impurities (e.g., formic acid, glycolic acid, 4-CBA, etc.) remaining in the terephthalic acid.

[0065] The monomer composition for synthesizing recycled plastics according to one embodiment may further contain small amounts of other additives and solvents. The specific types of additives and solvents are not particularly limited, and various substances widely used in the process of recovering terephthalic acid by depolymerizing a (co)polymer synthesized from a monomer containing terephthalic acid may be used without limitation.

[0066] The monomer composition for synthesizing recycled plastics according to one embodiment may be obtained by a method for producing a monomer composition for synthesizing recycled plastics, which will be described later. That is, the monomer composition for synthesizing recycled plastics according to one embodiment corresponds to a resultant product obtained by depolymerizing a (co)polymer synthesized from a monomer containing terephthalic acid, followed by various filtration, purification, washing, and drying processes to obtain only terephthalic acid, which is the main synthesis target substance of the present invention, in high purity.

[0067] The monomer composition for synthesizing recycled plastics according to one embodiment can be used as a raw material for producing monomers used in synthesizing various recycled plastics (e.g., polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polybutylene adipate terephthalate (PBAT), glycol-modified polyethylene terephthalate (PETG), and thermoplastic polyester elastomer (TPEE)) described below, or can be used as a raw material for producing other additives (e.g., dioctyl terephthalate plasticizer) used in processing plastics (e.g., polyvinyl chloride (PVC)).

[0068] 2. Method for producing monomer composition for synthesizing recycled plastics According to another embodiment of the present invention, there is provided a method for producing a monomer composition for synthesizing recycled plastics, the method comprising: depolymerizing a (co)polymer synthesized from a monomer including terephthalic acid to remove a diol component; and washing the depolymerization reaction product from which the diol component has been removed, the washing step comprising washing with a protic polar solvent at a temperature of from 20°C to 100°C, and washing with a washing solvent including an aprotic polar solvent at a temperature of from 20°C to 110°C.

[0069] The present inventors have confirmed through experiments that, in the process of recovering terephthalic acid from a (co)polymer synthesized from a monomer containing terephthalic acid, as in the method for producing a monomer composition for synthesizing recycled plastics according to the other embodiment, a multi-stage washing process divided into two stages according to temperature ranges is applied, whereby the ratio of isophthalic acid, which is another monomer, rather than terephthalic acid, which is the main synthetic target substance of the present invention, is significantly reduced, and that when polyethylene terephthalate or high-value-added plastics (PBT, TPEE) are synthesized using this, excellent physical properties can be achieved, thereby completing the invention.

[0070] In particular, terephthalic acid recovered from (co)polymers synthesized from monomers containing terephthalic acid obtained by conventional methods usually contains 1 to 2% of isophthalic acid as an impurity, whereas the present invention has made it possible to almost completely remove isophthalic acid through the secondary washing step, which is a characteristic of the method for producing a monomer composition for synthesizing recycled plastics, as described below.

[0071] This is because, in the step of washing with a washing solvent containing an aprotic polar solvent at a temperature of 20°C or higher and 110°C or lower, the solubility of terephthalic acid is increased, so that impurities such as isophthalic acid attached to the crystals or between the crystals can be dissolved in the solvent as much as possible and removed.

[0072] Specifically, the method for producing a monomer composition for synthesizing recycled plastics according to another embodiment may include a step of depolymerizing a (co)polymer synthesized from a monomer containing terephthalic acid and removing a diol component.

[0073] The (co)polymer synthesized from the monomer containing terephthalic acid can be applied regardless of various forms and types, such as a (co)polymer synthesized from a new monomer containing terephthalic acid produced by synthesis, a (co)polymer synthesized from a monomer containing recycled terephthalic acid produced by a recycling process, or a (co)polymer waste synthesized from a monomer containing terephthalic acid.

[0074] However, if necessary, a pretreatment step of the (co)polymer synthesized from a monomer containing terephthalic acid may be carried out before the depolymerization reaction of the (co)polymer synthesized from a monomer containing terephthalic acid, thereby increasing the efficiency of the step of recovering terephthalic acid from the (co)polymer synthesized from a monomer containing terephthalic acid. Examples of the pretreatment step include washing, drying, pulverization, and glycolysis. The specific method of each pretreatment step is not limited, and various methods widely used in the step of recovering terephthalic acid by depolymerization of the (co)polymer synthesized from a monomer containing terephthalic acid may be applied without limitation.

[0075] The depolymerization reaction of the (co)polymer synthesized from the monomer containing terephthalic acid may be carried out under acidic, neutral, or basic conditions, and particularly under basic (alkaline) conditions. More specifically, the depolymerization reaction of the (co)polymer synthesized from the monomer containing terephthalic acid may be carried out in water, an alkylene glycol, or an alcohol solvent. A specific example of the alkylene glycol solvent is ethylene glycol, and a specific example of the alcohol solvent is ethanol.

[0076] The depolymerization reaction of the (co)polymer synthesized from the monomer containing terephthalic acid can be carried out under basic conditions. The type of the base is not particularly limited, and an example thereof is sodium hydroxide (NaOH).

[0077] The depolymerization reaction of the (co)polymer synthesized from the monomer containing terephthalic acid can be carried out by reacting a base in an amount of 2.3 mol or less, or 1 mol to 2.3 mol, or 1.5 mol to 2.3 mol, per mol of the (co)polymer synthesized from the monomer containing terephthalic acid.

[0078] During the depolymerization reaction of the (co)polymer synthesized from the monomer containing terephthalic acid, if more than 2.3 moles of base are reacted with 1 mole of the (co)polymer synthesized from the monomer containing terephthalic acid, the solubility of isophthalic acid salts decreases and the amount of alkali salt generated increases, resulting in an increase in the amount of impurities, isophthalic acid and sodium (Na), rather than terephthalic acid, the main synthetic target substance, which are difficult to remove sufficiently even by the washing step described below.

[0079] The temperature at which the depolymerization reaction of the (co)polymer synthesized from the monomer containing terephthalic acid is carried out is not particularly limited, and can be, for example, from 25° C. to 200° C., or from 130° C. to 180° C. The time for which the depolymerization reaction of the (co)polymer synthesized from the monomer containing terephthalic acid is carried out can be from 0 minute to 3 hours.

[0080] Meanwhile, the diol component can be removed after depolymerization of a (co)polymer synthesized from a monomer containing terephthalic acid. For example, the depolymerization product of polyethylene terephthalate (PET) contains ethylene glycol and terephthalic acid salt.

[0081] Since the main target substance for recovery in the present invention is terephthalic acid, other by-products can be removed by filtration. The filtered by-products can be recycled without separation and purification in the depolymerization reaction of the (co)polymer synthesized from the monomer containing terephthalic acid, or can be recycled after separation and purification by conventional distillation, extraction, and adsorption methods as needed.

[0082] Furthermore, since the main target substance for recovery in the present invention is terephthalic acid, in the case of terephthalic acid salts, they can be converted to terephthalic acid by an additional neutralization step with a strong acid, which will be described later.

[0083] As a specific example, the (co)polymer synthesized from the monomer containing terephthalic acid is depolymerized, and the resulting product is cooled to below 50°C, and then ethylene glycol is removed by vacuum filtration to obtain a terephthalate salt.

[0084] Meanwhile, the method for preparing a monomer composition for synthesizing recycled plastics according to another embodiment may include washing the depolymerization reaction product from which the diol component has been removed.

[0085] As described above, the depolymerization reaction product from which the diol component has been removed may contain terephthalic acid obtained by removing ethylene glycol and neutralizing terephthalic acid salt. However, since various impurities remain during the recovery process of obtaining terephthalic acid, washing may be performed to sufficiently remove these impurities and obtain high-purity terephthalic acid.

[0086] Specifically, the washing step may include washing with a protic polar solvent at a temperature of from 20° C. to 100° C., or from 20° C. to 50° C., or from 20° C. to 40° C., or from 20° C. to 30° C., and washing with a washing solvent containing an aprotic polar solvent at a temperature of from 20° C. to 110° C., or from 50° C. to 110° C., or from 70° C. to 110° C., or from 80° C. to 110° C. The temperature condition refers to the temperature inside a washing vessel where washing with the solvent is performed, and various heating mechanisms can be applied without limitation to maintain a high temperature above room temperature.

[0087] The washing step may be performed by first washing with a protic polar solvent at a temperature of from 20° C. to 100° C., followed by washing with a washing solvent containing an aprotic polar solvent at a temperature of from 20° C. to 110° C. Alternatively, the washing step may be performed by first washing with a washing solvent containing an aprotic polar solvent at a temperature of from 20° C. to 110° C., followed by washing with a protic polar solvent at a temperature of from 20° C. to 100° C.

[0088] More preferably, the washing step may be performed by first washing with a protic polar solvent at a temperature of 20° C. to 100° C., and then washing with a washing solvent containing an aprotic polar solvent at a temperature of 20° C. to 110° C. This can minimize corrosion of the reactor due to strong acid after the neutralization step.

[0089] The step of washing with a protic polar solvent at a temperature of 20°C to 100°C and the step of washing with a washing solvent containing an aprotic polar solvent at a temperature of 20°C to 110°C can each be repeated at least once.

[0090] In addition, if necessary, after the step of washing with a protic polar solvent at a temperature of 20°C to 100°C and the step of washing with a washing solvent containing an aprotic polar solvent at a temperature of 20°C to 110°C, a step of removing the remaining solvent by filtration may be further carried out.

[0091] The protic polar solvent may include water or an alcohol. Preferably, water is used as the protic polar solvent. If an organic solvent such as ethanol is used instead of water, salt (NaCl) may not be effectively removed, resulting in an increased amount of residual sodium (Na) in the composition.

[0092] The protic polar solvent means a solvent having an acidic proton and being polar, and is usually a solvent having a hydroxyl group or an amino group.

[0093] In the step of washing with a protic polar solvent at a temperature of 20°C to 100°C, the protic polar solvent can be used in an amount of 5 to 30 parts by weight, or 10 to 30 parts by weight, or 15 to 25 parts by weight, per part by weight of the (co)polymer synthesized from a monomer containing terephthalic acid.

[0094] Meanwhile, the washing solvent may include one of an aprotic polar solvent or a mixture of an aprotic polar solvent and a protic polar solvent, i.e., the washing solvent may include an aprotic polar solvent, or the washing solvent may include a mixture of an aprotic polar solvent and a protic polar solvent.

[0095] The aprotic polar solvent can be one of tetrahydrofuran or acetonitrile. The protic polar solvent can be one of water or alcohol. The aprotic polar solvent refers to a polar solvent that does not have an acidic proton, and is typically a solvent that does not have a hydroxyl group or an amino group. Such solvents act as proton acceptors but do not act as proton donors in hydrogen bonding.

[0096] The mixture of aprotic polar solvent and protic polar solvent is a solution in which an aprotic polar solvent and a protic polar solvent are mixed, and in this case, the weight ratio of the aprotic polar solvent may be 60% by weight or more and 90% by weight or less, and the weight ratio of the protic polar solvent may be 10% by weight or more and 40% by weight or less.

[0097] As described above, in the mixture of the aprotic polar solvent and the protic polar solvent, the aprotic polar solvent is contained in excess, and thus impurities that cannot be removed from water, which is a protic polar solvent, are dissolved and removed by the aprotic polar solvent, thereby increasing the purity of terephthalic acid and minimizing the amount of terephthalic acid lost together with impurities during washing with the protic polar solvent, thereby increasing the yield of terephthalic acid.

[0098] Conversely, when the protic polar solvent is contained in excess in a mixture of an aprotic polar solvent and a protic polar solvent, the yield of terephthalic acid decreases, and the purity of terephthalic acid also decreases due to an increase in impurities.

[0099] In the step of washing with a washing solvent containing an aprotic polar solvent at a temperature of 20°C to 110°C, the washing solvent can be used in an amount of 5 to 30 parts by weight, or 10 to 30 parts by weight, or 15 to 25 parts by weight, per part by weight of the (co)polymer synthesized from a monomer containing terephthalic acid.

[0100] The step of washing with a washing solvent containing an aprotic polar solvent may be performed at a temperature of 20°C to 110°C, or 50°C to 110°C, or 70°C to 110°C, or 80°C to 110°C.

[0101] If the temperature in the step of washing with the washing solvent containing the aprotic polar solvent is excessively increased to above 110°C, severe conditions are formed in order to maintain the extreme temperature conditions, and the process efficiency is reduced due to increased risks in terms of pressure and explosion risk.

[0102] The difference between the temperature in the step of washing with a washing solvent containing an aprotic polar solvent at a temperature of 20°C to 110°C and the temperature in the step of washing with a protic polar solvent at a temperature of 20°C to 100°C may be 30°C to 100°C, or 40°C to 90°C, or 50°C to 80°C.

[0103] The difference between the temperature in the step of washing with a washing solvent containing an aprotic polar solvent at a temperature of 20°C to 110°C and the temperature in the step of washing with a protic polar solvent at a temperature of 20°C to 100°C means the temperature in the step of washing with a washing solvent containing an aprotic polar solvent at a temperature of 20°C to 110°C minus the temperature in the step of washing with a protic polar solvent from the temperature of 20°C to 100°C.

[0104] If the temperature difference between the step of washing with a washing solvent containing an aprotic polar solvent at a temperature of 20°C to 110°C and the step of washing with a protic polar solvent at a temperature of 20°C to 100°C is too low, such as less than 30°C, it is difficult to sufficiently remove impurities such as isophthalic acid and sodium (Na) rather than the main target substance, terephthalic acid.

[0105] If the temperature difference between the step of washing with a washing solvent containing an aprotic polar solvent at a temperature of 20°C to 110°C and the step of washing with a protic polar solvent at a temperature of 20°C to 100°C is excessively increased to more than 100°C, severe conditions are formed in order to maintain the extreme temperature conditions, and the process efficiency is reduced due to increased risks in terms of pressure and explosion risk.

[0106] Meanwhile, the method for preparing a monomer composition for synthesizing recycled plastics according to another embodiment may further include a step of neutralizing the depolymerization reaction product from which the diol component has been removed with an acid before the step of washing the depolymerization reaction product from which the diol component has been removed.

[0107] For example, while the alkaline decomposition products of polyethylene terephthalate (PET) include ethylene glycol and terephthalic acid salts, the main target material for recovery in the present invention is terephthalic acid, and therefore, the terephthalic acid salts obtained by the alkaline decomposition can be converted to terephthalic acid by an additional neutralization step with a strong acid. That is, when the depolymerization reaction of a (co)polymer synthesized from a monomer containing terephthalic acid is alkaline decomposition, it can undergo a neutralization step with an acid.

[0108] The acid used in the neutralization reaction can be a strong acid, such as hydrochloric acid (HCl). The neutralization reaction using the strong acid can achieve a pH of 4 or less, or 2 or less, at the end of the neutralization reaction. The temperature during the neutralization reaction can be adjusted to 25°C or higher and 100°C or lower.

[0109] In addition, if necessary, after the neutralization step with an acid of the depolymerization reaction product from which the diol component has been removed, a step of removing remaining impurities by filtration may be further performed.

[0110] Meanwhile, the method for producing a monomer composition for synthesizing recycled plastics according to another embodiment may further include a step of purifying the depolymerization reaction product from which the diol component has been removed, after the step of depolymerizing a (co)polymer synthesized from a monomer containing terephthalic acid and removing the diol component.

[0111] The refining can remove remaining impurities, and the specific refining conditions are not particularly limited. Regarding the specific refining apparatus and method, various refining techniques that have been publicly known can be applied without any restrictions.

[0112] For example, the purification step of the depolymerization reaction product from which the diol component has been removed may include a step of dissolving and filtering the depolymerization reaction product from which the diol component has been removed, and a step of adsorbing the depolymerization reaction product using an adsorbent.

[0113] In the step of dissolving and filtering the depolymerization product from which the diol component has been removed, water may be used as a solvent for dissolving the depolymerization product from which the diol component has been removed, and the dissolution temperature may be 25° C. to 100° C. This allows for the removal of a (co)polymer synthesized from a monomer containing residual terephthalic acid that has not reacted in the depolymerization reaction.

[0114] In the adsorption step using an adsorbent, examples of the adsorbent that can be used include activated carbon, charcoal, celite, or a mixture thereof.

[0115] If necessary, the depolymerization reaction product from which the diol component has been removed may be further purified through steps such as extraction, washing, precipitation, recrystallization, and drying, without limitation.

[0116] When the step of purifying the depolymerization reaction product from which the diol components have been removed is performed, the step of washing the depolymerization reaction product from which the diol components have been removed may be performed after the step of purifying the depolymerization reaction product from which the diol components have been removed.

[0117] Furthermore, when the depolymerization reaction is alkaline decomposition as described above, the step of purifying the depolymerization reaction product from which the diol components have been removed may be followed by a step of neutralizing the depolymerization reaction product from which the diol components have been removed with an acid, and then a step of washing the depolymerization reaction product from which the diol components have been removed.

[0118] Meanwhile, the method for producing a monomer composition for synthesizing recycled plastics according to another embodiment may further include a drying step after washing the depolymerization reaction product from which the diol component has been removed. The drying may remove residual solvent, and the drying conditions are not particularly limited, but may be performed at a temperature of 100°C to 150°C. The drying apparatus and method used in the drying may be any of various conventionally known drying techniques.

[0119] 3. Recycled plastics According to yet another embodiment of the present invention, there is provided a recycled plastic comprising a reaction product of the monomer composition for synthesizing recycled plastics of the above embodiment and a comonomer. The content of the monomer composition for synthesizing recycled plastics includes all of the content described above in the above embodiment.

[0120] Examples of the recycled plastics are not particularly limited, and various plastics synthesized using terephthalic acid as a monomer can be applied without limitation. A more specific example is a (co)polymer synthesized from a monomer containing terephthalic acid.

[0121] The (co)polymer is synthesized from a monomer containing terephthalic acid, and the (co)polymer includes both polymers and copolymers, and is a general term for reaction products obtained by the (co)polymerization reaction of monomers. The (co)polymer may include low molecular weight compounds, oligomers, and polymers depending on the molecular weight range.

[0122] The (co)polymer synthesized from the monomer containing terephthalic acid can include one or more (co)polymers selected from the group consisting of polyalkylene terephthalate, polyalkylene terephthalate copolymer, and thermoplastic polyester elastomer. That is, the (co)polymer synthesized from the monomer containing terephthalic acid can include one type of polyalkylene terephthalate, one type of polyalkylene terephthalate copolymer, one type of thermoplastic polyester elastomer, or a mixture of two or more types thereof.

[0123] The polyalkylene terephthalate copolymer refers to a copolymer obtained by further reacting an additional comonomer based on alkylene glycol and terephthalic acid, which are monomers for synthesizing polyalkylene terephthalate.

[0124] The comonomer capable of reacting with the high-purity terephthalic acid contained in the monomer composition for synthesizing recycled plastics according to the embodiment is not particularly limited, and specific examples thereof include an aliphatic diol, a polyalkylene oxide, a fatty acid, a fatty acid derivative, or a combination thereof.

[0125] The aliphatic diol may be, for example, one or more of diols having a number average molecular weight (Mn) of 300 g / mol or less, i.e., ethylene glycol, propylene glycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol (1,4-BG), 1,5-pentanediol, 1,6-hexanediol, and 1,4-cyclohexanedimethanol (1,4-CHDM). Specific examples include 1,4-butanediol, ethylene glycol, 1,4-cyclohexanedimethanol, or a mixture thereof.

[0126] The polyalkylene oxide is a unit constituting a soft segment and may be an aliphatic polyether. Examples of the polyalkylene oxide include one or more of polyoxyethylene glycol, polypropylene glycol, poly(tetramethylene ether) glycol (PTMEG), polyoxyhexamethylene glycol, a copolymer of ethylene oxide and propylene oxide, an ethylene oxide addition polymer of polypropylene oxide glycol, and a copolymer of ethylene oxide and tetrahydrofuran. A specific example is PTMEG, and in particular, PTMEG having a number average molecular weight (Mn) of 600 g / mol to 3,000 g / mol, 1,000 g / mol to 2,500 g / mol, or 1,500 g / mol to 2,200 g / mol may be used.

[0127] The fatty acid may be, for example, one or more aliphatic carboxylic acid compounds excluding terephthalic acid, and a specific example may be adipic acid. The fatty acid derivative is a compound derived from the above-mentioned fatty acid, and for example, may be one or more of fatty acid esters, fatty acid chlorides, fatty acid anhydrides, and fatty acid amides, and a specific example may be adipic acid esters.

[0128] To explain this by citing a specific example, when 1,4-butanediol, an aliphatic diol, is used as a comonomer that can react with the high-purity terephthalic acid contained in the monomer composition for synthesizing recycled plastics according to one embodiment, polybutylene terephthalate (PBT), a type of polyalkylene terephthalate, can be obtained by a polymerization reaction between terephthalic acid and 1,4-butanediol.

[0129] In addition, when ethylene glycol, which is an aliphatic diol, is used as a comonomer that can react with the high-purity terephthalic acid contained in the monomer composition for synthesizing recycled plastics according to one embodiment, polyethylene terephthalate (PET), which is a type of polyalkylene terephthalate, can be obtained by a polymerization reaction between terephthalic acid and ethylene glycol.

[0130] In addition, when the aliphatic diol 1,4-butanediol and the polyalkylene oxide PTMEG are used together as comonomers that can react with the high-purity terephthalic acid contained in the monomer composition for synthesizing recycled plastics according to one embodiment, a thermoplastic polyester elastomer (TPEE) can be obtained by a polymerization reaction of terephthalic acid, 1,4-butanediol, and PTMEG.

[0131] In addition, when the aliphatic diol 1,4-butanediol and the fatty acid adipic acid are used together as comonomers that can react with the high-purity terephthalic acid contained in the monomer composition for synthesizing recycled plastics according to one embodiment, polybutylene adipate terephthalate (PBAT), a type of polyalkylene terephthalate copolymer, can be obtained by a polymerization reaction of terephthalic acid, 1,4-butanediol, and adipic acid.

[0132] In addition, when the aliphatic diols ethylene glycol and 1,4-cyclohexanedimethanol are used together as comonomers that can react with the high-purity terephthalic acid contained in the monomer composition for synthesizing recycled plastics according to one embodiment, a glycol-modified polyethylene terephthalate (PETG), which is a type of polyalkylene terephthalate copolymer, can be obtained by a polymerization reaction of terephthalic acid, ethylene glycol, and 1,4-cyclohexanedimethanol.

[0133] As a specific example, the polyalkylene terephthalate may include one or more (co)polymers selected from polybutylene terephthalate, polyethylene terephthalate, polycyclohexylene dimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, and polytrimethylene terephthalate.

[0134] As a specific example, the polyalkylene terephthalate copolymer may include one or more (co)polymers selected from polybutylene adipate terephthalate (PBAT) and glycol-modified polyethylene terephthalate (PETG).

[0135] The method for reacting the monomer composition for synthesizing recycled plastics and the comonomer is not particularly limited, and various conventionally known methods can be applied without limitation. However, specific examples of the reaction of the monomer composition for synthesizing recycled plastics and the comonomer include melt condensation polymerization and solid-state polymerization.

[0136] For example, to produce thermoplastic polyester elastomer (TPEE) from recycled plastics, aromatic dicarboxylic acids, aliphatic diols, and polyalkylene oxides are esterified in the presence of a titanium butoxide (TBT) catalyst at 180°C to 250°C for 30 to 210 minutes to produce bis(4-hydroxybutyl terephthalate) oligomers. The TBT catalyst is then reintroduced into the reactor and melt polycondensation is carried out at 200°C to 270°C for 20 to 240 minutes while gradually reducing the pressure from 760 torr to 0.3 torr. After the melt polycondensation reaction is complete, the mixture is discharged from the reactor under nitrogen pressure and pelletized into strands.

[0137] The pellets can then be subjected to solid-state polymerization in a solid-state polymerization reactor or a rotary vacuum dryer at a temperature ranging from 140°C to 200°C for 10 hours to 24 hours under high vacuum and in an inert gas stream such as nitrogen.

[0138] In addition, when producing polyalkylene terephthalate from the recycled plastic, it may be produced by melt-polymerizing an aromatic dicarboxylic acid and an aliphatic diol having a number average molecular weight (Mn) of 300 g / mol or less, followed by solid-state polymerization.

[0139] The polyalkylene terephthalate resin can be obtained by placing low molecular weight pellets obtained by melt polymerization into a solid-state polymerization reactor and reacting them under high vacuum and inert conditions, as described above in the solid-state polymerization of thermoplastic polyester elastomer (TPEE), to obtain a high molecular weight resin.

[0140] The specific method for producing polybutylene terephthalate, polyethylene terephthalate, polycyclohexylene dimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, polytrimethylene terephthalate, or polybutylene adipate terephthalate by reacting the monomer composition for synthesizing recycled plastics and the comonomer is not particularly limited, and various processes widely known in the field of conventional recycled plastic synthesis can be applied without limitation.

[0141] The physical properties of the recycled plastic may vary depending on the weight ratio of the monomer composition and comonomer for synthesizing the recycled plastic of the embodiment. The weight ratio of the monomer composition and comonomer for synthesizing the recycled plastic of the embodiment is not particularly limited. For example, based on thermoplastic polyester elastomer (TPEE), the recycled plastic may be synthesized at a weight ratio of TPA / (TPA+PTMG)=61.5.

[0142] 4. Molded products According to yet another embodiment of the present invention, there is provided a molded product containing the recycled plastic of the other embodiment. The content relating to the recycled plastic includes all of the content described above in the other embodiment.

[0143] The molded product may be obtained by applying the recycled plastic to various known plastic molding methods without limitation, and examples of the molding methods include injection molding, foam injection molding, blow molding, and extrusion molding.

[0144] The molded article is not particularly limited, and can be applied to various molded articles using plastic without limitation, such as automobiles, electrical and electronic products, communication products, and household goods.

[0145] 5. Plasticizer composition According to yet another embodiment of the present invention, there is provided a plasticizer composition including the reaction product of the monomer composition for synthesizing recycled plastics of the above embodiment and an alcohol. The content of the monomer composition for synthesizing recycled plastics includes all of the content described above in the above embodiment.

[0146] Generally, plasticizers are made by adding various additives such as fillers, stabilizers, pigments, and anti-fogging agents to resins such as polyvinyl chloride (PVC) to impart various processing properties, and are then used as materials for a variety of products, including electrical wires, pipes, flooring, wallpaper, sheets, artificial leather, tarpaulins, tapes, and food packaging materials, through processing methods such as extrusion molding, injection molding, and calendaring.

[0147] Typically, plasticizers are prepared by reacting alcohol with polycarboxylic acids such as phthalic acid and adipic acid to form the corresponding esters. In addition, in light of domestic and international regulations on phthalate-based plasticizers, which are harmful to the human body, research is ongoing into plasticizer compositions that can replace phthalate-based plasticizers, such as terephthalate-based, trimellitate-based, and other polymer-based plasticizers.

[0148] The reaction product of the monomer composition for synthesizing recycled plastics and the alcohol according to the embodiment may include a terephthalate-based compound. Specifically, the terephthalate-based compound may be obtained by a direct esterification reaction between terephthalic acid contained in the monomer composition for synthesizing recycled plastics and the alcohol.

[0149] The direct esterification reaction is prepared by adding terephthalic acid to alcohol, adding a catalyst, and reacting under a nitrogen atmosphere; removing unreacted alcohol and neutralizing unreacted acid; and dehydrating and filtering the resulting mixture by distillation under reduced pressure.

[0150] Examples of the terephthalate-based compound are not particularly limited, but include dioctyl terephthalate (DOTP), diisononyl terephthalate (DINTP), diisodecyl terephthalate (DIDTP), and di(2-propylheptyl) terephthalate (DPHTP).

[0151] The terephthalate-based compound can be prepared by a direct esterification reaction in which terephthalic acid reacts with any one alcohol selected from the group consisting of octanol, isononyl alcohol, isodecyl alcohol, and 2-propylheptyl alcohol.

[0152] The plasticizer composition can be applied to the production of electric wires, flooring materials, automobile interior materials, films, sheets, wallpapers, or tubes. [Effects of the Invention]

[0153] According to the present invention, it is possible to provide a monomer composition for synthesizing recycled plastics, which has a high acid value and ensures high-purity terephthalic acid when terephthalic acid is recovered by depolymerizing a (co)polymer synthesized from a monomer containing terephthalic acid, a method for producing the same, and recycled plastics, molded articles, and plasticizer compositions each using the same. DETAILED DESCRIPTION OF THE INVENTION

[0154] The present invention will be described in more detail below with reference to examples. However, the following examples are merely illustrative of the present invention and are not intended to limit the scope of the present invention.

[0155] <Example> Example 1 (1) Production of recycled terephthalic acid monomer composition 300g (1.56mol) of polyethylene terephthalate (PET) bottle scraps, 1647g (26.53mol) of ethylene glycol (EG), and 128g (3.2mol) of sodium hydroxide (NaOH) were added to a 3L stainless steel reactor and stirred in a closed system at 180°C for 2 hours to carry out the PET depolymerization reaction. The depolymerization product was cooled to below 50°C and vacuum filtered to obtain sodium terephthalate (Na2-TPA).

[0156] The filtrate containing the sodium terephthalate (Na2-TPA) was completely dissolved in 3000 g of water, and then the unreacted PET was removed again by vacuum filtration. Then, 15 g of charcoal and 30 g of celite were added to the filtrate and purified by adsorption for 1 hour, and the charcoal and celite were removed by filtration.

[0157] Thereafter, 500 to 550 ml of 6M HCl was added and neutralized at 20 to 30° C., and the slurry, whose pH was lowered to 2 or less, was again vacuum filtered to obtain terephthalic acid (TPA).

[0158] To remove NaCl generated during the neutralization process, the PET was first washed with 6000 g of water (20 times the mass of the PET) at 20-30°C and then vacuum filtered. The filtrate was then secondarily washed with 6000 g of tetrahydrofuran (THF) (20 times the mass of the PET) at 80°C for 1 hour.

[0159] After the first and second water washing processes, the product was dried in a convection oven at 100°C for 12 hours to prepare a recycled terephthalic acid monomer composition from which recycled terephthalic acid (TPA) was recovered.

[0160] (2) Manufacturing recycled plastics 200 g of the recycled terephthalic acid monomer composition obtained in Example 1(1), 200 g of 1,4-butylene glycol, and 125 g of poly(tetramethylene ether) glycol (PTMEG) with a number average molecular weight of 1,000 to 2,000 g / mol were placed in an ester interchange reactor, and 0.1 wt% of TBT catalyst was added. The reaction was carried out for 120 to 180 minutes while maintaining the temperature at 200 to 240°C. The reaction was terminated when the reaction rate (the amount of water effluent from the reaction converted into the reaction rate) reached 90% or more, yielding an oligomer.

[0161] The resulting oligomer was then transferred to a polycondensation reactor and added with 0.1 wt% TBT catalyst, 0.14-0.15 wt% hindered phenol antioxidant, and 0.15-0.2 wt% aromatic amine antioxidant or sulfur stabilizer. The temperature was maintained at 230-250°C, and the pressure was reduced from 760 torr to 0.3 torr for 30 minutes. The melt polycondensation reaction was then continued under high vacuum conditions of 0.3 torr or less until the desired torque was reached. The reaction was terminated and discharged under nitrogen pressure to form strands, which were then cooled and pelletized to produce thermoplastic polyester elastomer (TPEE) resin.

[0162] Example 2 As shown in Table 1 below, a recycled terephthalic acid monomer composition and a recycled plastic were prepared in the same manner as in Example 1, except that the second washing in (1) of Example 1 was performed at 100°C using a mixed solution of 4800 g of THF and 1200 g of water.

[0163] Example 3 As shown in Table 1 below, a recycled terephthalic acid monomer composition and a recycled plastic were prepared in the same manner as in Example 1, except that the second washing in Example 1(1) was performed using acetonitrile at a temperature of 100°C.

[0164] Example 4 As shown in Table 1 below, a recycled terephthalic acid monomer composition and a recycled plastic were prepared in the same manner as in Example 1 (1), except that the second washing was performed at 100°C using a mixed solution of 4800 g of acetonitrile and 1200 g of water.

[0165] Example 5 (1) Production of recycled terephthalic acid monomer composition A recycled terephthalic acid monomer composition was produced in the same manner as in Example 1 (1).

[0166] (2) Manufacturing recycled plastics 300 g of the recycled terephthalic acid monomer composition obtained in Example 1(1) and 300 g of 1,4-butylene glycol were placed in an esterification reactor and 0.1 wt% of TBT catalyst was added. The reaction was carried out for 120 to 180 minutes while maintaining the temperature at 200 to 240°C. The reaction was terminated when the reaction rate (the amount of water effluent from the reaction was converted into the reaction rate) reached 90% or more, thereby obtaining an oligomer.

[0167] The oligomer was then transferred to a polycondensation reactor and maintained at 230-260°C. The melt polycondensation reaction was carried out under high vacuum conditions of 0.3 torr or less, while the pressure was reduced from 760 torr to 0.3 torr for 30 minutes. The torque applied to the stirrer was then increased to the desired value. After the reaction was terminated, the mixture was discharged under nitrogen pressure to form strands, which were then cooled and pelletized to produce polybutylene terephthalate (PBT) resin.

[0168] <Comparative Example> Comparative Example 1 As shown in Table 1 below, a recycled terephthalic acid monomer composition and a recycled plastic were prepared in the same manner as in Example 1, except that the second washing in (1) of Example 1 was performed using water at a temperature of 25°C.

[0169] <Reference example> Reference example 1 As shown in Table 1 below, a recycled terephthalic acid monomer composition and a recycled plastic were prepared in the same manner as in Example 1, except that the depolymerization solvent in (1) of Example 1 was changed to water and the second washing was performed at a temperature of 25°C.

[0170] Reference example 2 As shown in Table 1 below, a recycled terephthalic acid monomer composition and a recycled plastic were prepared in the same manner as in Example 1 (1), except that the depolymerization solvent was changed to water and the secondary washing was performed using acetonitrile at a temperature of 25°C.

[0171] Reference example 3 As shown in Table 1 below, a recycled terephthalic acid monomer composition and a recycled plastic were prepared in the same manner as in Example 1 (1), except that the depolymerization solvent was changed to water and the secondary washing was performed using acetone at a temperature of 25°C.

[0172] Reference example 4 As shown in Table 1 below, a recycled terephthalic acid monomer composition and a recycled plastic were prepared in the same manner as in Example 1, except that the depolymerization solvent in (1) of Example 1 was changed to water and the second washing was performed using water at a temperature of 25°C.

[0173] <Experimental Example> The properties of the recycled terephthalic acid monomer compositions obtained in the above Examples, Comparative Examples, and Reference Examples were measured by the following methods, and the results are shown in Table 1.

[0174] 1. Terephthalic acid (TPA) and isophthalic acid (IPA) content 5 to 20 mg of the recycled terephthalic acid monomer composition was collected as a sample under normal pressure and 20 to 30°C conditions, dissolved in 1 ml of DMSO-d6 solvent, and then analyzed using an Agilent DD1 500 MHz NMR device. 1 H NMR spectra were obtained. Using analytical software (MestReC), peaks for all detected substances, including terephthalic acid (TPA) and isophthalic acid (IPA), were individually assigned and integrated. The molar ratio (mol%) of terephthalic acid and isophthalic acid contained within 100 mol% of the total monomer compounds analyzed from the sample was calculated based on the peak integration value.

[0175] 2.Acid value The recycled terephthalic acid monomer compositions obtained in the above Examples and Comparative Examples were measured according to KS M ISO 2114. The acid value corresponds to the mass of potassium hydroxide in mg required to neutralize 1 g of the recycled terephthalic acid monomer composition under the conditions specified in KS M ISO 2114.

[0176] [Table 1]

[0177] As shown in Table 1 above, among the monomers contained in the recycled terephthalic acid monomer compositions obtained in Examples 1 to 4, terephthalic acid was contained in 100 mol %, and the impurity isophthalic acid was completely removed, demonstrating the purification efficiency of high-purity terephthalic acid. On the other hand, among the monomers contained in the recycled terephthalic acid monomer composition obtained in Comparative Example 1, the impurity isophthalic acid was contained in an excessive amount of 1.6 compared to the Examples, and it was confirmed that the purification efficiency of terephthalic acid was significantly reduced.

[0178] In addition, it was confirmed that the recycled terephthalic acid monomer composition obtained in the Reference Example contained an impurity, isophthalic acid, in an amount of 0.1 mol % to 2.3 mol %, which is excessive compared to the Examples, and the purification efficiency of terephthalic acid was significantly reduced.

[0179] In addition, the acid value of the recycled terephthalic acid monomer composition obtained in the Examples was 674 mg KOH / g, which was high enough for commercial use. On the other hand, the acid value of the recycled terephthalic acid monomer composition obtained in Comparative Example 1 was 660 mg KOH / g, which was lower than that of the Examples.

[0180] Furthermore, it was confirmed that the acid value of the recycled terephthalic acid monomer composition obtained in the Reference Examples was 658 mg KOH / g to 663 mg KOH / g, which was lower than that of the Examples.

Claims

1. A method for producing a monomer composition for synthesizing recycled plastics, comprising: The monomer composition for synthesizing recycled plastics is recovered from a (co)polymer synthesized from a monomer containing terephthalic acid, The monomer composition for synthesizing recycled plastics contains terephthalic acid, The monomer composition for synthesizing recycled plastics has an acid value of 670 mg KOH / g or more as measured by KS M ISO 2114, The terephthalic acid contained in the monomer composition for synthesizing recycled plastics is recovered from a (co)polymer synthesized from a monomer containing the terephthalic acid, a step of depolymerizing a (co)polymer synthesized from a monomer containing terephthalic acid to remove a diol component; and washing the depolymerization reaction product from which the diol component has been removed, The washing step comprises: Washing with a protic polar solvent at a temperature of 20°C to 100°C; and washing the resulting monomer composition with a washing solvent containing an aprotic polar solvent at a temperature of 20°C or higher and 110°C or lower.

2. The monomer composition for synthesizing recycled plastics comprises:

2. The method for producing a monomer composition for synthesizing recycled plastics according to claim 1, further comprising isophthalic acid in a molar ratio of less than 0.85 mol% based on 100 mol% of the total monomer compounds contained in the monomer composition for synthesizing recycled plastics.

3. A method for producing a monomer composition for synthesizing recycled plastics as described in claim 1, wherein the terephthalic acid contained in the monomer composition for synthesizing recycled plastics has a molar ratio of more than 99.15 mol % based on 100 mol % of the total monomer compounds contained in the monomer composition for synthesizing recycled plastics.

4. A method for producing a monomer composition for synthesizing recycled plastics as described in claim 2, characterized in that the isophthalic acid is recovered from a (co)polymer synthesized from a monomer containing the terephthalic acid.

5. A method for producing a monomer composition for synthesizing recycled plastics as described in claim 1, wherein the (co)polymer synthesized from the monomer containing terephthalic acid includes one or more (co)polymers selected from the group consisting of polyalkylene terephthalate, polyalkylene terephthalate copolymer, and thermoplastic polyester elastomer.

6. 6. The method for producing a monomer composition for synthesizing recycled plastics according to claim 1, wherein the protic polar solvent comprises one of water and alcohol.

7. 6. The method for producing a monomer composition for synthesizing recycled plastics according to claim 1, wherein the aprotic polar solvent includes one of tetrahydrofuran and acetonitrile.

8. The washing solvent is 6. The method for producing a monomer composition for synthesizing recycled plastics according to claim 1, further comprising an aprotic polar solvent or a mixture of an aprotic polar solvent and a protic polar solvent.

9. washing with a protic polar solvent at a temperature of 20°C to 100°C, The method for producing a monomer composition for synthesizing recycled plastics according to any one of claims 1 to 5, wherein the protic polar solvent is used in an amount of 5 parts by weight or more and 30 parts by weight or less per part by weight of the (co)polymer synthesized from the monomer containing terephthalic acid.

10. washing with a washing solvent containing an aprotic polar solvent at a temperature of 20° C. to 110° C., The method for producing a monomer composition for synthesizing recycled plastics according to any one of claims 1 to 5, wherein the washing solvent is used in an amount of 5 parts by weight or more and 30 parts by weight or less per part by weight of the (co)polymer synthesized from the monomer containing terephthalic acid.

11. The washing step comprises: After the step of washing with a protic polar solvent at a temperature of 20°C to 100°C, The method for producing a monomer composition for synthesizing recycled plastics according to any one of claims 1 to 5, wherein the step of washing with a washing solvent containing an aprotic polar solvent is performed at a temperature of 20°C to 110°C.

12. The method for producing a monomer composition for synthesizing recycled plastics according to any one of claims 1 to 5, wherein the difference between the temperature in the step of washing with a washing solvent containing an aprotic polar solvent at a temperature of 20°C to 110°C and the temperature in the step of washing with a protic polar solvent at a temperature of 20°C to 100°C is 30°C to 100°C.

13. The depolymerization reaction of the (co)polymer synthesized from the monomer containing terephthalic acid is 6. The method for producing a monomer composition for synthesizing recycled plastics according to claim 1, wherein the base is reacted in an amount of 2.3 moles or less per mole of the (co)polymer synthesized from the monomer containing terephthalic acid.

14. The depolymerization reaction of the (co)polymer synthesized from the monomer containing terephthalic acid is 6. The method for producing a monomer composition for synthesizing recycled plastics according to claim 1, wherein the method is carried out in an alkylene glycol solvent.

15. Before the step of washing the depolymerization reaction product from which the diol component has been removed, The method for producing a monomer composition for synthesizing recycled plastics according to any one of claims 1 to 5, further comprising a step of neutralizing the depolymerization reaction product from which the diol component has been removed with an acid.

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