Method for producing monomer compositions for the synthesis of recycled plastics, and monomer compositions for the synthesis of recycled plastics using the same, recycled plastics, and molded articles.

Abstract

The present invention relates to a method for producing a monomer composition for synthesizing recycled plastics, the method including the steps of depolymerizing a polycarbonate resin; adjusting the pH of the depolymerized reaction product, which has a pH of 13 or higher, to 8 to 12; adjusting the pH of the depolymerized reaction product, whose pH has been adjusted to 8 to 12, to less than 4; and collecting aromatic diol compound crystals formed by adding a crystallization solvent to the depolymerized reaction product, whose pH has been adjusted to less than 4.

Description

[Technical Field]

[0001] (Cross-reference of related applications) This application claims priority under Korean Patent Application No. 10-2023-0098843 dated 28 July 2023, and all content disclosed in the said Korean Patent Application is incorporated herein as part of this specification.

[0002] The present invention relates to a method for producing a monomer composition for recycled plastic synthesis in which aromatic diol compounds recovered by the chemical decomposition and reuse of polycarbonate resins have good yield and optical properties, and the efficiency of the reuse process is improved, as well as a monomer composition for recycled plastic synthesis, recycled plastics, and molded articles using the same. [Background technology]

[0003] Polycarbonate is a thermoplastic polymer that is a plastic with excellent properties such as good transparency, ductility, and relatively low manufacturing costs.

[0004] Although polycarbonate is widely used in a variety of applications, concerns about the environment and health during waste disposal have been persistently raised.

[0005] Currently, physical recycling methods are being used, but this has led to problems such as quality degradation, and research into the chemical recycling of polycarbonate is underway.

[0006] The chemical decomposition of polycarbonate refers to the process of obtaining aromatic diol compounds (e.g., bisphenol A (BPA)) as monomers through the decomposition of polycarbonate, and then using these compounds again in polymerization to obtain high-purity polycarbonate.

[0007] Typical chemical decompositions include thermal decomposition, hydrolysis, and alcohol decomposition. Of these, the most common method is alcohol decomposition using a base catalyst. However, methanol decomposition has the problem of using methanol, which is harmful to the human body, and ethanol decomposition requires high temperature and high pressure conditions, resulting in low yields.

[0008] Furthermore, while methods for decomposing alcohol using organic catalysts are known, they currently have economic disadvantages. [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The present invention provides a method for producing monomer compositions for recycled plastics, in which aromatic diol compounds recovered by the chemical decomposition and reuse of polycarbonate resins have good yield and optical properties, and the efficiency of the reuse process is improved.

[0010] Furthermore, the present invention provides a monomer composition for the synthesis of recycled plastics, recycled plastics, and molded articles using the method for producing the monomer composition for the synthesis of recycled plastics described above. [Means for solving the problem]

[0011] To solve the above problems, this specification provides a method for producing a monomer composition for recycled plastic synthesis, comprising the steps of: depolymerizing a polycarbonate resin; adjusting the pH of the depolymerization reaction product, which has a pH of 13 or higher, to 8-12; adjusting the pH of the depolymerization reaction product adjusted to 8-12 to less than 4; and adding a crystallization solvent to the depolymerization reaction product adjusted to less than 4 to recover aromatic diol compound crystals formed.

[0012] This specification also provides monomer compositions for the synthesis of recycled plastics, comprising aromatic diol compounds obtained by the method for producing the monomer composition for the synthesis of recycled plastics described above.

[0013] This specification also provides recycled plastics comprising the monomer composition for the synthesis of recycled plastics and the reaction product of the comonmer.

[0014] This specification also provides molded articles containing the recycled plastic.

[0015] The following describes in more detail a method for producing a monomer composition for the synthesis of recycled plastics according to specific embodiments of the invention, as well as the monomer composition for the synthesis of recycled plastics, recycled plastics, and molded articles using the same.

[0016] In this specification, unless otherwise expressly stated, technical terms are used solely to refer to specific embodiments and are not intended to limit the invention.

[0017] As used herein, the singular form includes the plural form unless the wording explicitly indicates the opposite.

[0018] As used herein, "pH" refers to the hydrogen ion concentration (pH), a numerical value indicating the degree of acidity and alkalinity of a substance. It can be calculated by taking the reciprocal of the logarithm of the dissociation concentration of hydrogen ions and is used as a measure of the strength of the acidity and baseness of a substance.

[0019] As used herein, "includes" embodies a particular characteristic, domain, integer, stage, operation, element and / or component, and does not exclude the presence or addition of other particular characteristics, domains, integers, stages, operations, elements, components and / or groups.

[0020] Furthermore, in this specification, terms including ordinal numbers such as “first” and “second” are used for the purpose of distinguishing one component from another, and are not limited by such ordinal numbers. For example, within the scope of the rights of this invention, the first component may be named the second component, and similarly, the second component may be named the first component.

[0021] 1. Method for producing a monomer composition for recycled plastic synthesis According to an embodiment of the invention, a step of depolymerizing a polycarbonate resin; a step of adjusting the pH of the depolymerization reaction product having a pH of 13 or more to 8 to 12; a step of adjusting the pH of the depolymerization reaction product having the pH adjusted to 8 to 12 to less than 4; and a step of recovering aromatic diol compound crystals formed by adding a crystallization solvent to the depolymerization reaction product having the pH adjusted to less than 4; A method for producing a monomer composition for recycled plastic synthesis can be provided.

[0022] The inventors of the present invention, like the method for producing a monomer composition for recycled plastic synthesis according to the above embodiment, in the process of recycling a polycarbonate resin by chemical decomposition, gradually adjust the pH of the depolymerized polycarbonate resin in two steps, and in the neutralization step, the organic solvent can be separated and removed to a sufficient level, and high-purity aromatic diol compound crystals can be recovered in a high yield by recrystallization immediately without removing the organic solvent by a separate distillation process. The inventors have confirmed this through experiments and completed the invention.

[0023] In particular, conventionally, by rapidly reducing the pH to a neutral or acidic level using a strong acid aqueous solution in the neutralization step, organic solvents such as ethanol and diethyl carbonate cannot be sufficiently separated and removed and remain, and an essential distillation step for removing such organic solvents is involved. In addition, due to the solubility between diethyl carbonate and ethanol, there is a limit in that it is difficult to perform recrystallization precipitation for bisphenol A, which is an aromatic diol compound.

[0024] In contrast, in the present invention, a multi-stage neutralization process is carried out in which the pH is first lowered to 8-12 in the neutralization stage and then sequentially lowered to less than 4. By adding an acidic aqueous solution in the process of lowering the pH to 8-12 and then removing the separated aqueous layer, impurities that dissolve in water, such as salts, can be easily removed, and the color characteristics of the monomer composition for recycled plastic synthesis can be improved. The solubility of ethanol in water increases, and the ethanol remaining in the organic layer can be effectively removed without a distillation step. Also, through the process of separating the aqueous layer and the organic solvent layer in multiple stages, the effect that recrystallization precipitation of bisphenol A, which is an aromatic diol compound, can be sufficiently carried out even with a small amount of recrystallization solvent was achieved.

[0025] Specifically, the method for producing the monomer composition for recycled plastic synthesis according to the above embodiment may include a step of depolymerizing a polycarbonate resin.

[0026] The polycarbonate resin is meant to include all homopolymers or copolymers containing polycarbonate repeating units, and is a general term for reaction products obtained by a polymerization reaction or copolymerization reaction of monomers containing an aromatic diol compound and a carbonate precursor. When only one type of aromatic diol compound and one type of carbonate precursor are used to obtain a carbonate repeating unit, a homopolymer can be synthesized. Also, when using one type of aromatic diol compound and two or more types of carbonate precursors as the monomers, or two or more types of aromatic diol compounds and one type of carbonate precursor, or using one or more types of other diols in addition to one type of aromatic diol compound and one type of carbonate precursor and containing two or more types of carbonates, a copolymer can be synthesized. The homopolymer or copolymer can include all low molecular weight compounds, oligomers, and polymers depending on the molecular weight range.

[0027] The aforementioned polycarbonate resin can be applied regardless of its diverse form or type, such as newly synthesized polycarbonate resin, recycled polycarbonate resin produced through a recycling process, or polycarbonate resin waste.

[0028] However, if necessary, a pretreatment step of the polycarbonate resin can be performed before the depolymerization reaction of the polycarbonate resin to improve the efficiency of the process of recovering aromatic diol compounds and carbonate precursors from the polycarbonate resin. Examples of the aforementioned pretreatment steps include washing, drying, grinding, and glycol decomposition. The specific methods of each pretreatment step are not limited, and a wide variety of methods widely used in the process of recovering aromatic diol compounds and carbonate precursors by depolymerization of polycarbonate resins can be applied without restriction.

[0029] During the depolymerization reaction of the polycarbonate resin, the depolymerization reaction may be carried out under acidic, neutral, or basic conditions, and in particular, the depolymerization reaction can be carried out under basic (alkaline) conditions. The type of base is not broadly limited, and examples include sodium hydroxide (NaOH) or potassium hydroxide (KOH). The base is a basic catalyst that acts as a catalyst and has the advantage of being more economical than organic catalysts that are mainly used under mild conditions. More specifically, during the depolymerization reaction of the polycarbonate resin, the depolymerization reaction may be carried out at a pH of 13 or higher, or in the range of 13 to 14.

[0030] The depolymerization reaction of the polycarbonate resin can be carried out by reacting the base in an amount of 0.5 moles or less, 0.4 moles or less, 0.3 moles or less, 0.1 mole or more, 0.2 mole or more, 0.1 mole to 0.5 moles, 0.1 mole to 0.4 moles, 0.1 mole to 0.3 moles, 0.2 mole to 0.5 moles, 0.2 mole to 0.4 moles, or 0.2 mole to 0.3 moles relative to 1 mole of polycarbonate resin. However, when the base is reacted in an amount exceeding 0.5 moles relative to 1 mole of polycarbonate resin during the depolymerization reaction of the polycarbonate resin, there is a limitation that the amount of alkali salt generated increases, leading to an increase in impurities, a decrease in the purity of the target recovered material, and a decrease in the economic efficiency of the catalytic reaction.

[0031] Furthermore, the depolymerization reaction of the polycarbonate resin may be carried out in a solvent containing ethanol. The present invention has the advantage that a highly pure monomer, bisphenol A, can be stably obtained by decomposing a polycarbonate resin in a solvent containing ethanol, and that a high-value-added diethyl carbonate can be additionally obtained as a reaction byproduct.

[0032] The ethanol content may be 1 to 5 moles, 1 to 4 moles, or 1 to 3.75 moles per mole of polycarbonate resin. Since ethanol has good solubility in bisphenol A, it is essential that the ethanol be within the above range. If the ethanol content is excessively reduced to less than 1 mole per mole of polycarbonate resin, the alcohol decomposition of the polycarbonate resin will not be sufficiently carried out. On the other hand, if the ethanol content is excessively increased per mole of polycarbonate resin, the ethanol will not be sufficiently removed in the neutralization stage described later, and the economic efficiency of the process may decrease due to the need for a separate distillation step.

[0033] The solvent in which the depolymerization reaction of the polycarbonate resin is carried out may further include, in addition to ethanol, one or more organic solvents selected from the group consisting of tetrahydrofuran, toluene, methylene chloride, chloroform, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and dipropyl carbonate.

[0034] The organic solvent may include tetrahydrofuran, toluene, methylene chloride, chloroform, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, or a mixture of two or more of these.

[0035] More preferably, methylene chloride can be used as the organic solvent. When methylene chloride is used as the organic solvent to be mixed with ethanol, there is an advantage that the solubility of the polycarbonate is improved and the reactivity is enhanced.

[0036] The content of the organic solvent may be 10 to 20 moles or 10 to 15 moles per mole of polycarbonate resin. Alternatively, the content of the organic solvent may be 2.2 to 5 moles per mole of ethanol. Mixing the polycarbonate resin, ethanol, and organic solvent within this range has the advantage of carrying out a depolymerization reaction to the required level of polymer units.

[0037] On the other hand, the temperature at which the depolymerization reaction of the polycarbonate resin proceeds is not strictly limited, but for example, it can be carried out in the range of 20°C to 100°C or 70°C to 90°C. Furthermore, the time for which the depolymerization reaction of the polycarbonate resin proceeds can be 1 hour to 30 hours.

[0038] Specifically, the aforementioned conditions are milder process conditions compared to existing pressurized / high-temperature processes, and by stirring under these conditions, the process can be carried out in a milder process compared to a pressurized / high-temperature process.

[0039] In other words, the present invention has the advantage that, without using an organic catalyst, and by adjusting the type and amount of the mixed solvent, and the type and content of the base catalyst, high-purity aromatic diol compounds (e.g., bisphenol A) can be obtained under mild conditions without using a pressurized / high-temperature process, and because ethanol solvent is used, diethyl carbonate can be obtained as a by-product.

[0040] On the other hand, an antioxidant can be added to the reaction solution during the depolymerization reaction of the polycarbonate resin. By adding the antioxidant, the aromatic diol compounds recovered through the chemical decomposition and reuse of the polycarbonate resin can satisfy a low color coordinate b* value equivalent to that of reagents commercially sold or used for PC polymerization.

[0041] The specific examples of the aforementioned antioxidants are not particularly limited, and a wide variety of antioxidants that have been widely used in conventional arts can be applied without restriction. However, examples include sodium hyposulfite, sodium sulfite, erythorbic acid, dibutylhydroxytoluene, butylhydroxyanisole, α-tocopherol, tocopherol acetate, L-ascorbic acid and its salts, L-ascorbic acid palmitate, L-ascorbic acid stearate, triamyl gallate, propyl gallate, or disodium ethylenediaminetetraacetate (EDTA), sodium pyrophosphate, sodium metaphosphate, or mixtures of two or more of these.

[0042] The specific amount of antioxidant to be added is not significantly limited, but as an example, it can be added in a range of 0.1% to 5% by weight, or 0.1% to 1% by weight, based on the weight of the total reaction solution, at a level that does not affect the physical properties of the monomer composition for recycled plastic synthesis.

[0043] On the other hand, the method for producing the monomer composition for recycled plastic synthesis according to one embodiment may include a step of adjusting the pH of the depolymerization reaction product, which has a pH of 13 or higher, or 13-14, to 8-12, 8-11, 8-10, or 8-9.

[0044] Specifically, the step of adjusting the pH of the depolymerization reaction product to 8-12 may include the step of adding an acidic aqueous solution so that the pH of the depolymerization reaction product becomes 8-12. By adjusting the pH of the depolymerization reaction product to 8-12, water-soluble salt impurities can be effectively removed and the color properties can be improved. In addition, the solubility of unreacted ethanol remaining in the depolymerization reaction product in water can be increased, minimizing the amount of residual ethanol.

[0045] Specifically, in the step of adding an acidic aqueous solution so that the pH of the depolymerization reaction product is 8 to 12, a layer divided into an organic solvent layer and an aqueous layer can be formed. More specifically, the layer divided into an organic solvent layer and an aqueous layer can be divided into an aqueous layer containing impurities and an organic solvent layer containing an aromatic diol compound.

[0046] Since the aromatic diol compound is hydrophobic, it may be contained in the organic solvent layer of the water and organic solvent, and various water-soluble impurities may be contained in the aqueous layer. The impurities are hydrophilic substances and may include, for example, salt compounds, ionic compounds, or acid compounds. Furthermore, the aqueous layer containing the impurities may also contain unreacted ethanol.

[0047] The acidic aqueous solution is a solution obtained by mixing an acid and water, and the acid can be a strong acid, for example, hydrochloric acid (HCl). The depolymerization reaction is carried out under strongly basic conditions with a pH of 13 or higher, so that the resulting aromatic diol compound exists in the form of a salt after reaction with the base and is hydrophilic. However, by adding the acidic aqueous solution, the salt of the aromatic diol compound contained in the depolymerization reaction product can be converted back into the aromatic diol compound, which will then be induced to be hydrophobic.

[0048] On the other hand, after adjusting the pH of the depolymerization reaction product, which has a pH of 13 or higher, to 8-12, the process may further include a step of removing the aqueous layer from the layer that has been separated into an organic solvent layer and an aqueous layer. Since various water-soluble impurities are separated into the aqueous layer from the organic solvent layer by the addition of the acidic aqueous solution, even a simple step of removing the aqueous layer can easily remove impurities from the main product, the aromatic diol compound.

[0049] The specific conditions for removing the aqueous layer from the organic layer are not significantly limited, and a wide variety of existing known purification technologies can be applied without restriction to specific removal devices and methods. However, for example, a draining device can be used.

[0050] On the other hand, the method for producing the monomer composition for recycled plastic synthesis according to one embodiment may include a step of adjusting the pH of the depolymerization reaction product, which has been adjusted to a pH of 8 to 12, to less than 4, or 1 to 3, or 1 to 2.

[0051] Specifically, the step of adjusting the pH of the depolymerization reaction product, which has been adjusted to a pH of 8 to 12, to less than 4 may include the step of adding an acidic aqueous solution so that the pH of the depolymerization reaction product becomes less than 4. By adding an acidic aqueous solution so that the pH of the depolymerization reaction product becomes less than 4, the salts of the aromatic diol compounds remaining in the depolymerization reaction product, which has been adjusted to a pH of 8 to 12, can be converted back into aromatic diol compounds, thereby increasing the yield of aromatic diol compounds.

[0052] Specifically, in the step of adding an acidic aqueous solution so that the pH of the depolymerization reaction product is less than 4, a layer divided into an organic solvent layer and an aqueous layer can be formed. More specifically, the layer divided into an organic solvent layer and an aqueous layer can be divided into an aqueous layer containing impurities and an organic solvent layer containing an aromatic diol compound.

[0053] Since the aromatic diol compound is hydrophobic, it may be contained in the organic solvent layer of the water and organic solvent, and various water-soluble impurities may be contained in the aqueous layer. The impurities are hydrophilic substances and may include, for example, salt compounds, ionic compounds, acid compounds, etc.

[0054] The acidic aqueous solution is a solution obtained by mixing an acid and water, and the acid can be a strong acid, for example, hydrochloric acid (HCl). By adding the acidic aqueous solution to the depolymerization reaction product whose pH has been adjusted to 8-12, the salt of the aromatic diol compound remaining in the depolymerization reaction product can be converted back into the aromatic diol compound, thereby inducing it to be hydrophobic.

[0055] On the other hand, after adjusting the pH of the depolymerization reaction product, which has been adjusted to a pH of 8 to 12, to less than 4, the process may further include a step of removing the aqueous layer from the layers separated into an organic solvent layer and an aqueous layer. Since various water-soluble impurities are separated into the aqueous layer from the organic solvent layer by the addition of the acidic aqueous solution, even a simple step of removing the aqueous layer can easily remove impurities from the main product, the aromatic diol compound.

[0056] The specific conditions for removing the aqueous layer from the organic layer are not significantly limited, and a wide variety of existing known purification technologies can be applied without restriction to specific removal devices and methods. However, for example, a draining device can be used.

[0057] On the other hand, the pH difference value calculated using the following formula 1 may be 4-9, 5-8, or 6-7.

[0058] [Formula 1] pH difference = (pH of the depolymerization reaction product adjusted to pH 8-12) - (pH of the depolymerization reaction product adjusted to pH less than 4).

[0059] By satisfying the pH difference value of 4 or more, or 4 to 10, or 5 to 8, or 6 to 7 according to the above formula 1, the effect of removing salt impurities and residual ethanol by adjusting the pH to 8 to 12, and the effect of improving the yield of aromatic diol compounds by adjusting the pH to less than 4 can be further maximized.

[0060] Furthermore, the pH difference value calculated using formula 2 below may be 4-11, or 4-10, or 4-9, or 4-8, or 4-7, or 5-6.

[0061] [Formula 2] pH difference = (pH of the depolymerization reaction product) - (pH of the depolymerization reaction product adjusted to pH 8-12).

[0062] By satisfying the pH difference value of 1-6, 2-6, 3-6, 4-6, or 5-6 according to the above formula 2, the effect of removing salt impurities and residual ethanol by adjusting the pH to 8-12 can be further maximized.

[0063] Specifically, the residual ethanol content in the depolymerization reaction product, whose pH has been adjusted to less than 4 according to the following formula 3, may be less than 1%, or 0.1% to 0.9%, or 0.1% to 0.8%, or 0.1% to 0.7%, or 0.1% to 0.6%, or 0.1% to 0.5%.

[0064] [Formula 3] Residual ethanol ratio (%) = {( 1 (Ethanol peak area in 1H NMR) / ( 1 Peak area of ​​ethanol in 1H NMR +1 (Peak area of ​​aromatic diol compounds in 1H NMR) × 100.

[0065] Specifically, in the above formula 3, 1 The ethanol peak in the 1H NMR spectrum is the peak at 3.63 ppm (quartet, 2H). 1 For 1H NMR, the peak for aromatic diol compounds (bisphenol A (BPA)) is the peak at 6.67 ppm (4H).

[0066] By reducing the amount of residual ethanol in this way, ethanol is sufficiently removed, making it possible to precipitate aromatic diol compounds in high purity and high yield without performing a separate distillation process, thus improving the economic efficiency of the process.

[0067] In contrast, when the amount of residual ethanol in the depolymerization reaction product, whose pH has been adjusted to less than 4 according to formula 3, increases to 1% or more, the yield of the aromatic diol compound obtained by precipitation decreases even when a large amount of crystallization solvent is added, to the extent that precipitation of the aromatic diol compound can only be achieved by performing a distillation step to remove the ethanol, thus limiting the efficiency of the process.

[0068] On the other hand, the method for producing the monomer composition for recycled plastic synthesis according to one embodiment may include a step of adding a crystallization solvent to the depolymerization reaction product whose pH has been adjusted to less than 4 and recovering the aromatic diol compound crystals formed. In this way, by precipitating the aromatic diol compound with a crystallization solvent, the aromatic diol compound, which is the main target substance for synthesis in the present invention, can be secured in high purity and high yield.

[0069] The step of recovering aromatic diol compound crystals formed by adding a crystallization solvent to the depolymerization reaction product whose pH has been adjusted to less than 4 may include the step of adding 100 moles or less, or 50 moles to 100 moles, or 50 moles to 90 moles, or 50 moles to 80 moles, or 50 moles to 70 moles of water per mole of polycarbonate resin to the depolymerization reaction product whose pH has been adjusted to less than 4, to precipitate aromatic diol compound crystals.

[0070] If the amount of water added during the precipitation of the aromatic diol compound is excessively small, the aromatic diol compound may not be sufficiently extracted, resulting in a decrease in yield. Conversely, if the amount of water added during the precipitation of the aromatic diol compound is excessively large, an excess of water may be required, reducing the efficiency of the extraction process.

[0071] On the other hand, the method for producing the monomer composition for recycled plastic synthesis according to the above embodiment may further include a step of purifying the aromatic diol compound after the step of recovering aromatic diol compound crystals formed by adding a crystallization solvent to the depolymerization reaction product whose pH has been adjusted to less than 4.

[0072] Specifically, the purification step of the aromatic diol compound may include a washing step of the aromatic diol compound. Furthermore, the purification step of the aromatic diol compound may include a dissolution step and an adsorption purification step of the aromatic diol compound.

[0073] The order of the washing step and the adsorption purification step is not particularly limited and may be performed in any order, but for example, they may be performed in the order of washing step; adsorption purification step;. The washing step and the adsorption purification step may each be repeated at least once. A wide variety of existing known purification techniques can be applied without limitation to the specific washing, adsorption apparatus and methods.

[0074] In the washing step of the aromatic diol compound, various impurities remain during the recovery process to obtain the aromatic diol compound. Therefore, washing can be carried out to sufficiently remove these impurities and secure a high-purity aromatic diol compound.

[0075] Specifically, the cleaning step may include a step of cleaning with a solvent at a temperature of 10°C to 30°C, or 20°C to 30°C. The temperature condition refers to the temperature inside the cleaning container where cleaning with the solvent is performed, and a variety of heating devices can be applied without limitation to maintain a temperature above room temperature.

[0076] The solvent used in the washing step may include one of the following: water, alcohol, or organic solvent. The organic solvent may be tetrahydrofuran, toluene, methylene chloride, chloroform, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, or a mixture of two or more of these.

[0077] The solvent used in the washing step can be used in a weight ratio of 1 part by weight or more and 30 parts by weight or 1 part by weight or more and 10 parts by weight or less, based on 1 part by weight of the polycarbonate resin used in the depolymerization reaction.

[0078] More specifically, the solvent used in the step of washing with a solvent at a temperature of 10°C to 30°C may be an organic solvent. Preferably, methylene chloride can be used as the organic solvent.

[0079] On the other hand, the dissolution step and the adsorption purification step of the aromatic diol compound may include the dissolution step of the aromatic diol compound and the adsorption purification step of the aromatic diol compound.

[0080] The dissolution step of the aromatic diol compound may include the step of adding a solvent to the aromatic diol compound. An example of the solvent is ethanol, which may be added in a ratio of 1 mole to 20 moles, 1 mole to 10 moles, or 5 mole to 10 moles per mole of polycarbonate resin. By adding the solvent to the aromatic diol compound, the aromatic diol compound crystals can be redissolved in the solvent.

[0081] The adsorption purification step of the aromatic diol compound may include the step of adding an adsorbent to the aromatic diol compound and purifying it by adsorption, followed by the step of removing the adsorbent. In the step of adding an adsorbent to the aromatic diol compound and purifying it by adsorption, followed by the step of removing the adsorbent, the adsorbent can be brought into contact with the aromatic diol compound.

[0082] Examples of the adsorbent include activated carbon, charcoal, or mixtures thereof. The activated carbon is a black carbon material with fine pores produced by carbonizing the raw material at approximately 500°C and activating it at approximately 900°C. While the examples are not particularly limited, a variety of activated carbons, such as plant-based, coal-based, petroleum-based, and waste-based activated carbon, can be applied without restriction, depending on the type of raw material.

[0083] To give more specific examples, plant-based activated carbons include coconut activated carbon, wood activated carbon, and sawdust activated carbon. Coal-based activated carbons include lignite activated carbon, anthracite activated carbon, and anthracite activated carbon. Furthermore, petroleum-based activated carbons include petroleum coke activated carbon and oil carbon activated carbon. In addition, waste-based activated carbons include synthetic resin activated carbon and pulp activated carbon.

[0084] The adsorbent may include one or more activated carbons selected from the group consisting of plant-based activated carbon, coal-based activated carbon, petroleum-based activated carbon, and waste-based activated carbon. In other words, the adsorbent may include plant-based activated carbon, coal-based activated carbon, petroleum-based activated carbon, waste-based activated carbon, or a mixture of two or more of these.

[0085] More specifically, the adsorbent may include one or more activated carbons selected from the group consisting of coconut activated carbon, lignite activated carbon, anthracite activated carbon, and smoked activated carbon. In other words, the adsorbent may include coconut activated carbon, lignite activated carbon, anthracite activated carbon, smoked activated carbon, or a mixture of two or more of these.

[0086] The adsorption purification conditions using the adsorbent are not particularly limited, and a variety of conventionally known adsorption purification conditions can be used without restriction. However, for example, the amount of adsorbent added may be 10% to 50% by weight relative to the polycarbonate resin, the adsorption time may be 1 to 5 hours, and the adsorption method may be agitated adsorption or an adsorption tower for the lab.

[0087] On the other hand, the process may further include a recrystallization step of the aromatic diol compound after the dissolution and adsorption purification steps of the aromatic diol compound. In the recrystallization step of the aromatic diol compound, various impurities contained in the aromatic diol compound can be sufficiently removed to secure a high-purity aromatic diol compound.

[0088] Specifically, the recrystallization step may include adding water to the solution in which the aromatic diol compound is dissolved and recrystallizing it. By adding water to the solution in which the aromatic diol compound is dissolved and recrystallizing it, the solubility of the aromatic diol compound, or its salt, contained in the solution in which the aromatic diol compound is dissolved increases, allowing the crystals, or impurities sandwiched between the crystals, to be dissolved to the maximum extent in the solvent. Since the dissolved aromatic diol compound has lower solubility than the impurities, when the temperature is subsequently lowered, it can easily precipitate into the aromatic diol compound crystals due to the difference in solubility.

[0089] More specifically, in the step of adding water to the solution in which the aromatic diol compound is dissolved and recrystallizing it, 20 to 40 moles, or 25 to 35 moles, of water can be used per mole of polycarbonate resin. If too little water is used, the temperature required to dissolve the aromatic diol compound in the solution becomes excessively high, reducing the efficiency of the process and making it difficult to remove impurities by recrystallization. On the other hand, if too much water is used, the solubility of the aromatic diol compound in the solution becomes excessively high, reducing the yield of the aromatic diol compound recovered after recrystallization and potentially reducing the efficiency of the process due to the use of a large amount of solvent.

[0090] If necessary, after the recrystallization step of the aromatic diol compound, an additional step may be taken to remove any remaining impurities by filtration or adsorption.

[0091] Furthermore, if necessary, a drying step may be further included after the recrystallization step. The drying removes any remaining solvent, and the specific drying conditions are not significantly limited, but for example, drying can be carried out at temperatures of 10°C to 100°C or 10°C to 50°C. A wide variety of existing known drying techniques can be applied without limitation to the specific drying apparatus and methods used during the drying process.

[0092] 2. Monomer compositions for the synthesis of recycled plastics According to another embodiment of the invention, a monomer composition for the synthesis of recycled plastics can be provided, comprising an aromatic diol compound obtained by the method for producing a monomer composition for the synthesis of recycled plastics according to one embodiment.

[0093] The monomer composition for recycled plastic synthesis of the other embodiments may be obtained by the method for producing the monomer composition for recycled plastic synthesis of the first embodiment. The description of the method for producing the monomer composition for recycled plastic synthesis of the other embodiments includes all of the content described above in the first embodiment.

[0094] In other words, the monomer compositions for recycled plastic synthesis of the other embodiments described above correspond to products obtained after a depolymerization reaction of polycarbonate resins, through various filtration, purification, washing, and drying processes, in order to secure only aromatic diol compounds, which are the main target substances for recovery, in high purity.

[0095] Furthermore, specific examples of the aromatic diol compounds include bis(4-hydroxyphenyl)methane, bis(4-hydroxyphenyl)ether, bis(4-hydroxyphenyl)sulfone, bis(4-hydroxyphenyl)sulfoxide, bis(4-hydroxyphenyl)sulfide, bis(4-hydroxyphenyl)ketone, 1,1-bis(4-hydroxyphenyl)ethane, 2,2-bis(4-hydroxyphenyl)propane (bisphenol A), 2,2-bis(4-hydroxyphenyl)butane, and 1,1-bis(4-hydroxyphenyl)cyclohexane (bisphenol A). Examples include sphenol Z, 2,2-bis(4-hydroxy-3,5-dibromophenyl)propane, 2,2-bis(4-hydroxy-3,5-dichlorophenyl)propane, 2,2-bis(4-hydroxy-3-bromophenyl)propane, 2,2-bis(4-hydroxy-3-chlorophenyl)propane, 2,2-bis(4-hydroxy-3-methylphenyl)propane, 2,2-bis(4-hydroxy-3,5-dimethylphenyl)propane, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, or mixtures of two or more of these. Preferably, the aromatic diol compound in the monomer composition for recycled plastic synthesis of the above embodiment may be 2,2-bis(4-hydroxyphenyl)propane (bisphenol A).

[0096] The aromatic diol compound is characterized by being obtained by the method for producing the monomer composition for recycled plastic synthesis according to the first embodiment. In other words, the aromatic diol compound is characterized by being recovered from a polycarbonate resin used to recover the monomer composition for recycled plastic synthesis. Therefore, if a new aromatic diol compound is added from an external source, separate from recovery from a polycarbonate resin, in order to produce the monomer composition for recycled plastic synthesis according to the other embodiment, it is not included in the category of aromatic diol compounds of the present invention.

[0097] Specifically, "recovered from the polycarbonate resin" means obtained by a depolymerization reaction of the polycarbonate resin. The depolymerization reaction may be carried out under acidic, neutral, or basic conditions, and in particular, it can be carried out under basic (alkaline) conditions. In particular, the depolymerization reaction is preferably carried out in an ethanol solvent, as will be described later.

[0098] 3. Recycled plastics According to yet another embodiment of the invention, a recycled plastic can be provided comprising the monomer composition and comonomer reaction product of the other embodiment for the synthesis of recycled plastics.

[0099] The details relating to the monomer compositions for recycled plastic synthesis of the other embodiments described above include all of the details described above in the other embodiments.

[0100] Examples of recycled plastics are not limited in scope, and a wide variety of plastics synthesized using aromatic diol compounds such as bisphenol A and carbonate precursors such as dimethyl carbonate, diethyl carbonate, or ethyl methyl carbonate as monomers can be applied without restriction. A more specific example is polycarbonate resin.

[0101] The term "polycarbonate resin" encompasses all homopolymers or copolymers containing polycarbonate repeating units, and collectively refers to reaction products obtained by polymerization or copolymerization reactions of monomers containing aromatic diol compounds and carbonate precursors. A homopolymer can be synthesized when it contains one type of carbonate repeating unit obtained using only one type of aromatic diol compound and one type of carbonate precursor. Furthermore, a copolymer can be synthesized when the monomers include one type of aromatic diol compound and two or more types of carbonate precursors, or two or more types of aromatic diol compounds and one type of carbonate precursor, or when two or more types of carbonates are included using one or more other diols in addition to one type of aromatic diol compound and one type of carbonate precursor. The homopolymer or copolymer can include all low molecular weight compounds, oligomers, and polymers within a given molecular weight range.

[0102] More specifically, in the recycled plastic comprising the monomer composition for the synthesis of recycled plastics and the reaction product of the comonmer of the above embodiment, a carbonate precursor can be used as the comonmer. Specific examples of the carbonate precursor include phosgene, triphosgene, diphosgene, bromophosgene, dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dicyclohexyl carbonate, diphenyl carbonate, ditril carbonate, bis(chlorophenyl) carbonate, m-cresyl carbonate, dinaphthyl carbonate, bis(diphenyl) carbonate, or bishaloformate.

[0103] The examples of reaction steps for the monomer composition and comonmer for the synthesis of the aforementioned polycarbonate resin are not significantly limited, and a wide variety of conventionally known polycarbonate manufacturing methods can be applied without restriction.

[0104] However, as an example of the polycarbonate manufacturing method, a polycarbonate manufacturing method can be used that includes a step of polymerizing a composition containing monomers and comonomers for recycled plastic synthesis. In this case, the polymerization can be carried out by interfacial polymerization, and during interfacial polymerization, the polymerization reaction can be carried out at atmospheric pressure and low temperature, and the molecular weight can be easily adjusted.

[0105] The polymerization temperature may be between 0°C and 40°C, and the reaction time may be between 10 minutes and 5 hours. Furthermore, the pH during the reaction can be maintained at 9 or higher, or 11 or higher.

[0106] The solvent that can be used for the polymerization is not particularly limited as long as it is a solvent used in the industry for the polymerization of polycarbonates. For example, halogenated hydrocarbons such as methylene chloride and chlorobenzene can be used.

[0107] Furthermore, the polymerization can be carried out in the presence of an acid binder, and as the acid binder, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide or amine compounds such as pyridine can be used.

[0108] Furthermore, during polymerization, the polymerization can be carried out in the presence of a molecular weight modifier to adjust the molecular weight of the polycarbonate. As the molecular weight modifier, alkylphenols having 1 to 20 carbon atoms can be used, and specific examples include p-tert-butylphenol, p-cumylphenol, decylphenol, dodecylphenol, tetradecylphenol, hexadecylphenol, octadecylphenol, eicosylphenol, docosylphenol, or triacontylphenol. The molecular weight modifier can be added before, during, or after the start of polymerization. The molecular weight modifier can be used in amounts of 0.01 to 10 parts by weight, or 0.1 to 6 parts by weight, relative to 100 parts by weight of the aromatic diol compound, and a desired molecular weight can be obtained within this range.

[0109] Furthermore, to accelerate the polymerization reaction, reaction accelerators such as tertiary amine compounds (e.g., triethylamine, tetra-n-butylammonium bromide, tetra-n-butylphosphonium bromide), quaternary ammonium compounds, and quaternary phosphonium compounds may be used in addition.

[0110] 4. Molded products According to yet another embodiment of the invention, a molded article comprising the recycled plastic of the other embodiment can be provided. The details relating to the recycled plastic include all of the details described above in the other embodiment.

[0111] The molded product may be obtained by applying a variety of known plastic molding methods to the recycled plastic without limitation. Examples of such molding methods include injection molding, foam injection molding, blow molding, or extrusion molding.

[0112] The examples of the aforementioned molded products are not particularly limited, and the method can be applied without restriction to a wide variety of molded products using plastic. Examples of such molded products include automotive parts, electrical and electronic products, communication products, household goods, building materials, optical components, and exterior materials.

[0113] In addition to the recycled plastic of the other embodiments, the molded article may optionally contain one or more additives selected from the group consisting of antioxidants, plasticizers, antistatic agents, nucleating agents, flame retardants, lubricants, impact reinforcers, fluorescent whitening agents, ultraviolet absorbers, pigments, and dyes.

[0114] As an example of a method for manufacturing the molded product, the process may include the steps of thoroughly mixing the recycled plastic and additives of the other embodiment using a mixer, extruding them in an extruder to produce pellets, drying the pellets, and then injecting them in an injection molding machine. [Effects of the Invention]

[0115] According to the present invention, a method for producing a monomer composition for the synthesis of recycled plastics is provided, in which aromatic diol compounds recovered by the chemical decomposition and reuse of polycarbonate resins have good yield and optical properties, and the efficiency of the reuse process is improved. The present invention also provides a monomer composition for the synthesis of recycled plastics, recycled plastics, and molded articles using the same. [Modes for carrying out the invention]

[0116] The invention will be described in more detail by the following embodiments. However, the following embodiments are merely illustrative of the present invention, and the content of the present invention is not limited to the following embodiments.

[0117] <Examples and Comparative Examples: Production of Recycled Bisphenol A Monomer Composition> (Example 1) (1. Decomposition stage) 14 mol of methylene chloride (MC), 6 mol of ethanol (EtOH), and 0.25 mol of sodium hydroxide (NaOH) were added to a 3 L high-pressure reactor and stirred. Subsequently, 1.2 mol of waste polycarbonate (PC) was added and stirred at 80°C to allow the PC depolymerization reaction to proceed.

[0118] (2-1. First pH adjustment step) After cooling the depolymerization reaction product (pH 14) containing bisphenol A to below 30°C, a 10% HCl aqueous solution was added to the depolymerization reaction product to adjust the pH to 8, and the aqueous layer was removed.

[0119] (2-2. Second pH adjustment step) A 10% hydrochloric acid (HCl) aqueous solution was added to the depolymerization reaction product, which had been adjusted to pH 8, to adjust the pH to 2, and the aqueous layer was removed.

[0120] (3. Precipitation stage) Approximately 1500 g (approximately 83 mol) of water was added to the depolymerization reaction product obtained in the pH adjustment stage, and the mixture was stirred to precipitate bisphenol A (BPA) crystals. Bisphenol A (BPA) was then recovered by vacuum filtration.

[0121] (4. Purification step - Filtration) After this, bisphenol A was washed with 300 g of methylene chloride (MC) at 20-30°C and then vacuum filtered.

[0122] (5-1. Additional purification step - redissolution step) After this, bisphenol A was redissolved in 500 g of ethanol.

[0123] (5-2. Additional purification stage - adsorption stage) Subsequently, lignite activated carbon was added as an adsorbent at a ratio of 30% by weight relative to the waste polycarbonate, and after purification through an adsorption column, the lignite activated carbon was removed by filtration.

[0124] (5-3. Additional purification step - recrystallization step) After this, 750g of water was added to recrystallize bisphenol A, and the resulting slurry was vacuum filtered at 20-30°C to recover bisphenol A (BPA) crystals.

[0125] (6. Drying stage) Subsequently, the recycled bisphenol A monomer composition was produced by vacuum drying in a convection oven at 40°C, from which the recycled bisphenol A (BPA) was recovered.

[0126] (Example 2) A recycled bisphenol A monomer composition was produced in the same manner as in Example 1, except that the pH was adjusted from 8 to 9 in the first pH adjustment step (2-1) of Example 1.

[0127] (Comparative Example 1) (1. Decomposition stage) 14 mol of methylene chloride (MC), 6 mol of ethanol (EtOH), and 0.25 mol of sodium hydroxide (NaOH) were added to a 3 L high-pressure reactor and stirred. Subsequently, 1.2 mol of waste polycarbonate (PC) was added and stirred at 80°C to allow the PC depolymerization reaction to proceed.

[0128] (2. pH adjustment step) After cooling the depolymerization reaction product (pH 14) containing bisphenol A to 30°C or below, a 10% hydrochloric acid (HCl) aqueous solution was added to the depolymerization reaction product to adjust the pH to 2, and the aqueous layer was removed.

[0129] (3. Precipitation stage) Approximately 8,000 g of water (approximately 444 mol) was added to the depolymerization reaction product obtained in the pH adjustment stage, and the mixture was stirred to precipitate bisphenol A (BPA) crystals. Bisphenol A (BPA) was then recovered by vacuum filtration.

[0130] (4. Purification step - Filtration) After this, bisphenol A was washed with 300 g of methylene chloride (MC) at 20-30°C and then vacuum filtered.

[0131] (5-1. Additional purification step - redissolution step) After this, bisphenol A was redissolved in 500 g of ethanol.

[0132] (5-2. Additional purification stage - adsorption stage) Subsequently, lignite activated carbon was added as an adsorbent at a ratio of 30% by weight relative to the waste polycarbonate, and after purification through an adsorption column, the lignite activated carbon was removed by filtration.

[0133] (5-3. Additional purification step - recrystallization step) After this, 750g of water was added to recrystallize bisphenol A, and the resulting slurry was vacuum filtered at 20-30°C to recover bisphenol A (BPA) crystals.

[0134] (6. Drying stage) Subsequently, the recycled bisphenol A monomer composition was produced by vacuum drying in a convection oven at 40°C, from which the recycled bisphenol A (BPA) was recovered.

[0135] (Comparative Example 2) A recycled bisphenol A monomer composition was produced in the same manner as in Comparative Example 1, except that the pH was adjusted from 2 to 8 in step (2. pH adjustment) of Comparative Example 1.

[0136] <Example of experiment> The physical properties were measured using the method described below, and the results are shown in Table 1.

[0137] 1. Purity A recycled bisphenol A monomer composition was dissolved at 1 w% in acetonitrile (ACN) solvent under atmospheric pressure and 20-30°C. The purity of bisphenol A (BPA) was then analyzed using ULC (ultra-performance liquid chromatography) on a Waters HPLC system with an ACQUITY UPLC○RBEH C18 1.7 μm (2.1 × 50 mm column).

[0138] 2. Color coordinates (L*, a*, b*) The recycled bisphenol A monomer composition was analyzed in reflectance mode using a HunterLab UltraScan PRO Spectrophotometer.

[0139] 3. Yield The weight of BPA produced when the polycarbonate used in the reaction was 100% decomposed was measured, and the weight of the obtained BPA was measured to calculate the BPA yield using the following formula A.

[0140] [Formula A] Yield (%) = (W1 / W0) × 100 (%) In the above formula A, W0 is the mass of BPA obtained when 100% decomposition occurs, and W1 is the mass of BPA actually obtained. Specifically, if approximately 100g of polycarbonate is decomposed, theoretically, the mass of BPA obtained when 100% decomposition occurs is 89g. If the mass of BPA actually obtained is 80g, the yield is 80 / 89 × 100 = 90%.

[0141] 4. Residual ethanol ratio A sample is prepared by diluting the depolymerization reaction product obtained in the pH adjustment step (3. Precipitation step) with Methanol-d4 at a concentration of 10 mg / ml. 1An NMR spectrum detected by an \(^1H\) NMR apparatus was obtained, and the area ratio of the peak of ethanol to the peak of bisphenol A (BPA) was measured, and this was taken as the residual ethanol ratio.

[0142] Specifically, ethanol was targeted at the peak at 3.63 ppm (quartet, 2H), and bisphenol A (BPA) was targeted at the peak at 6.67 ppm (4H), and the content of residual ethanol was determined by the calculation of the following formula B.

[0143] [Formula B] Residual ethanol ratio (%) = {( 1 Peak area of ethanol in \(^1H\) NMR) / ( 1 Peak area of ethanol in \(^1H\) NMR + 1 Peak area of BPA in \(^1H\) NMR)} × 100.

[0144]

Table 1

[0145] As shown in Table 1 above, the recycled bisphenol A monomer compositions obtained in Examples 1 to 2 showed high purity of 99.4% to 99.5%. Further, the recycled bisphenol A monomer compositions obtained in Examples 1 to 2 showed excellent optical properties with color coordinates L* of 97.6 to 98.03, a* of -0.12 to 0.06, and b* of 0.59 to 0.75. Also, the recycled bisphenol A monomer compositions obtained in Examples 1 to 2 were measured to have a high BPA yield of 98.6% to 98.8%. Furthermore, the recycled bisphenol A monomer compositions obtained in Examples 1 to 2 were measured to have a low residual ethanol content of 0.5% to 0.7%.

[0146] In contrast, the recycled bisphenol A monomer compositions obtained in Comparative Examples 1 and 2 had a purity of 97.2% to 98.9%, which was lower than that of the examples. Furthermore, the recycled bisphenol A monomer compositions obtained in Comparative Examples 1 and 2 showed poorer optical properties compared to the examples, with color coordinates L* at 95.67 to 97.4, a* at 0.06 to 0.16, and b* at 1.47 to 1.72.

[0147] In particular, the recycled bisphenol A monomer compositions obtained in Comparative Examples 1 and 2 showed significantly lower BPA yields (18.2% to 21.1%) compared to the examples, despite the addition of a much larger amount of water. Furthermore, the recycled bisphenol A monomer compositions obtained in Comparative Examples 1 and 2 showed higher residual ethanol content (1.6% to 2.2%) compared to the examples.

Claims

1. The step of depolymerizing polycarbonate resin; A step of adjusting the pH of the depolymerization reaction product, which has a pH of 13 or higher, obtained in the above depolymerization reaction step, to 8 to 12; A step of adjusting the pH of the depolymerization reaction product, which has been adjusted to a pH of 8 to 12, to less than 4; and The step includes: adding a crystallization solvent to the depolymerization reaction product whose pH has been adjusted to less than 4 to recover the aromatic diol compound crystals formed; After the step of adjusting the pH of the depolymerization reaction product, which has a pH of 13 or higher, to 8 to 12, A method for producing a monomer composition for the synthesis of recycled plastics, further comprising the step of removing the aqueous layer from a layer that has been divided into an organic solvent layer and an aqueous layer.

2. A method for producing a monomer composition for the synthesis of recycled plastics according to claim 1, wherein the pH difference value according to the following formula 1 is 4 to 10: [Formula 1] pH difference = (pH of the depolymerization reaction product adjusted to pH 8-12) - (pH of the depolymerization reaction product adjusted to pH less than 4).

3. A method for producing a monomer composition for the synthesis of recycled plastics according to claim 1, wherein the pH difference value according to the following formula 2 is 1 to 6: [Formula 2] pH difference = (pH of the depolymerization reaction product with a pH of 13 or higher) - (pH of the depolymerization reaction product with a pH adjusted to 8-12).

4. The step of adjusting the pH of the depolymerization reaction product to 8 to 12 is as follows: A method for producing a monomer composition for the synthesis of recycled plastics according to claim 1, comprising the step of adding an acidic aqueous solution so that the pH of the depolymerization reaction product is 8 to 12.

5. The step of adjusting the pH of the depolymerization reaction product, which has been adjusted to a pH of 8 to 12, to less than 4 is as follows: A method for producing a monomer composition for the synthesis of recycled plastics according to claim 1, comprising the step of adding an acidic aqueous solution so that the pH of the depolymerization reaction product, whose pH has been adjusted to 8 to 12, becomes less than 4.

6. After adjusting the pH of the depolymerization reaction product, which has been adjusted to a pH of 8 to 12, to less than 4, A method for producing a monomer composition for the synthesis of recycled plastics according to claim 1, further comprising the step of removing the aqueous layer from a layer that has been divided into an organic solvent layer and an aqueous layer.

7. A method for producing a monomer composition for the synthesis of recycled plastics according to claim 1, wherein the residual ethanol content in the depolymerization reaction product, whose pH has been adjusted to less than 4 according to the following formula 3, is less than 1%: [Equation 3] Residual ethanol ratio (%) = { ( 1 (H NMR peak area of ​​ethanol) / ( 1 1H NMR peak area of ​​ethanol + 1 (Peak area of ​​aromatic diol compounds in 1H NMR) × 100.

8. The depolymerization reaction of the polycarbonate resin is A method for producing a monomer composition for the synthesis of recycled plastics according to claim 1, characterized by carrying out the process under a solvent containing ethanol.

9. The method for producing a monomer composition for the synthesis of recycled plastics according to claim 8, wherein the amount of ethanol is 1 mole to 5 moles relative to 1 mole of polycarbonate resin.

10. The method for producing a monomer composition for the synthesis of recycled plastics according to claim 8, wherein the amount of ethanol is 1 mole to 4 moles relative to 1 mole of polycarbonate resin.

11. The depolymerization reaction of the polycarbonate resin is A method for producing a monomer composition for the synthesis of recycled plastics according to claim 1, characterized by reacting a base in an amount of 0.5 moles or less per mole of polycarbonate resin.

12. The step of recovering aromatic diol compound crystals formed by adding a crystallization solvent to the depolymerization reaction product whose pH has been adjusted to less than 4 is as follows: A method for producing a monomer composition for the synthesis of recycled plastics according to claim 1, comprising the step of adding water at a ratio of 100 moles or less per mole of polycarbonate resin to the depolymerization reaction product whose pH has been adjusted to less than 4, thereby precipitating aromatic diol compound crystals.

13. After the step of adding a crystallization solvent to the depolymerization reaction product whose pH has been adjusted to less than 4 and recovering the aromatic diol compound crystals formed thereafter, A method for producing a monomer composition for the synthesis of recycled plastics according to claim 1, further comprising the step of purifying the aromatic diol compound.

14. The purification step of the aromatic diol compound is as follows: A method for producing a monomer composition for the synthesis of recycled plastics according to claim 13, comprising a washing step of the aromatic diol compound.

15. The purification step of the aromatic diol compound is as follows: A method for producing a monomer composition for the synthesis of recycled plastics according to claim 13, comprising the steps of dissolving the aromatic diol compound and adsorption purification.

16. After the dissolution step and the adsorption purification step of the aromatic diol compound, A method for producing a monomer composition for the synthesis of recycled plastics according to claim 15, further comprising the step of recrystallizing the aromatic diol compound.

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