Method for preparing monomer composition for synthesizing reprocessed plastic, monomer composition for synthesizing reprocessed plastic, reprocessed plastic using same, and molded article using same

Abstract

The present invention relates to a method for preparing a monomer composition for synthesizing a recycled plastic, a monomer composition for synthesizing a recycled plastic, a recycled plastic using the monomer composition, and a molded article using the recycled plastic. The method comprises the steps of: subjecting a polycarbonate-based resin to a depolymerization reaction; adjusting the pH of the depolymerization reaction product with the pH of 13 or above to 8-12; adjusting the pH of the depolymerization reaction product adjusted to a pH of 8 to 12 to less than 4; and adding a crystallization solvent to the depolymerization reaction product adjusted to a pH of less than 4 and recovering crystals of the aromatic diol compound formed.

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2023-0098843 filed in the Korean Intellectual Property Office on July 28, 2023, the disclosure of which is incorporated by reference in its entirety into this specification.

[0003] The present invention relates to a method for preparing a monomer composition for synthesizing recycled plastics, which improves the efficiency of the recycling process while allowing aromatic diol compounds to have good yields and optical properties, wherein the aromatic diol compounds are recycled by regeneration through chemical decomposition of polycarbonate resins; and the present invention relates to a monomer composition for synthesizing recycled plastics, recycled plastics and molded products using the same. Background Art

[0004] Polycarbonate is a thermoplastic polymer, and is a plastic having excellent properties such as excellent transparency, outstanding ductility, and relatively low production cost.

[0005] Although polycarbonate is widely used in various applications, environmental and health issues in the waste disposal process continue to rise.

[0006] Currently, a physical recycling method is performed, but in this case, a problem of accompanying quality deterioration occurs, and therefore, research on chemical recycling of polycarbonate is being conducted.

[0007] The chemical decomposition of polycarbonate refers to obtaining an aromatic diol compound (eg, bisphenol A (BPA)) as a monomer by decomposition of polycarbonate, and then the aromatic diol compound is used again in polymerization to obtain high-purity polycarbonate.

[0008] For this chemical decomposition, thermal decomposition, hydrolysis and alcohol decomposition are generally known. Among these, the most common method is alcohol decomposition using an alkali catalyst, but in the case of methanol decomposition, there is a problem of using methanol that is harmful to the human body, and in the case of ethanol, there is a problem that high temperature and high pressure conditions are required and the yield is not high.

[0009] Furthermore, although an alcohol decomposition method using an organic catalyst is known, it is economically disadvantageous. Summary of the invention

[0010] Technical issues

[0011] An object of the present invention is to provide a method for preparing a monomer composition for synthesizing recycled plastics, which improves the efficiency of the recycling process while allowing aromatic diol compounds to have good yields and optical properties, the aromatic diol compounds being recycled by regeneration utilizing chemical decomposition of polycarbonate-based resins.

[0012] Another object of the present invention is to provide a monomer composition for synthesizing recycled plastics, recycled plastics and molded products using the method for preparing the monomer composition for synthesizing recycled plastics.

[0013] Technical Solution

[0014] In order to achieve the above-mentioned object, a method for preparing a monomer composition for synthesizing recycled plastics is provided herein, the method comprising the steps of: subjecting a polycarbonate-based resin to a depolymerization reaction; adjusting the pH of a depolymerization reaction product having a pH of 13 or more to 8 to 12; adjusting the pH of the depolymerization reaction product having a pH adjusted to 8 to 12 to less than 4; and adding a crystallization solvent to the depolymerization reaction product having a pH adjusted to less than 4 and recovering the formed aromatic diol compound crystals.

[0015] Also provided herein is a monomer composition for synthesizing recycled plastics, comprising an aromatic diol compound obtained by the method for preparing the monomer composition for synthesizing recycled plastics.

[0016] Further provided herein is a recycled plastic comprising a reaction product of a monomer composition for synthesizing the recycled plastic and a comonomer.

[0017] Further provided herein is a molded article comprising the recycled plastic.

[0018] Hereinafter, a method for preparing a monomer composition for synthesizing recycled plastic, a monomer composition for synthesizing recycled plastic, recycled plastic, and a molded product using the recycled plastic according to a specific embodiment of the present invention will be described in more detail.

[0019] Unless explicitly stated herein, the technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the present invention.

[0020] As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0021] As used herein, "pH" refers to hydrogen ion concentration (pH), which is a numerical value that indicates the acidity and basicity of a material. pH can be determined by taking the reciprocal of the logarithmic dissociation concentration of hydrogen ions, and is used as a measure of the strength of the acid and base of a material.

[0022] It should be understood that the terms "comprises", "includes", "has", etc. are used herein to illustrate the presence of described features, regions, integers, steps, actions, elements and / or components, but do not exclude the presence or addition of one or more other features, regions, integers, steps, actions, elements, components and / or groups.

[0023] In addition, the terms including ordinal numbers such as "first", "second", etc. are only used for the purpose of distinguishing one component from another component and are not limited by the ordinal numbers. For example, without departing from the scope of the present invention, the first component may be referred to as the second component, or similarly, the second component may be referred to as the first component.

[0024] 1. Method for preparing a monomer composition for synthesizing recycled plastics

[0025] According to one embodiment of the present invention, there may be provided a method for preparing a monomer composition for synthesizing recycled plastics, the method comprising the steps of: subjecting a polycarbonate-based resin to a depolymerization reaction; adjusting the pH of a depolymerization reaction product having a pH of 13 or more to a pH of 8 to 12; adjusting the pH of the depolymerization reaction product having a pH of 8 to 12 to less than 4; and adding a crystallization solvent to the depolymerization reaction product having a pH adjusted to less than 4 and recovering the formed aromatic diol compound crystals.

[0026] The present inventors have experimentally discovered that in a process for regenerating a polycarbonate resin by chemical decomposition in the same manner as in a method for preparing a monomer composition for synthesizing recycled plastics according to one embodiment, the pH of the depolymerized polycarbonate resin is gradually adjusted in two steps, and the organic solvent can be separated and removed to a sufficient level in the neutralization step, thereby immediately recovering high-purity aromatic diol compound crystals in a high yield by recrystallization without removing the organic solvent by a separate distillation process, and have completed the present invention.

[0027] Specifically, generally, since a strong acid aqueous solution is used in the neutralization step to reduce the pH to a neutral or acidic level at one time, an organic solvent such as ethanol or diethyl carbonate cannot be fully separated or removed and remains, and therefore, a distillation step is required to remove such an organic solvent. In addition, due to the solubility between diethyl carbonate and ethanol, there is a limitation that it is difficult to recrystallize and precipitate bisphenol A as an aromatic diol compound.

[0028] On the other hand, in the present invention, a multi-step neutralization process is performed, wherein the pH is reduced to 8 to 12 at one time in the neutralization step, and then the pH is sequentially reduced to less than 4. Therefore, by adding an acidic aqueous solution during the process of reducing the pH to 8 to 12 and then removing the separated water layer, impurities such as salts dissolved in water can be easily removed, and the color characteristics of the monomer composition for recycled plastic synthesis can be improved. In addition, while increasing the solubility of ethanol in water, the effect of removing the ethanol remaining in the organic layer can be effectively achieved without the need for a distillation step. In addition, by separating the water layer from the organic solvent layer in multiple steps, even if a small amount of recrystallization solvent is used, the effect of fully carrying out the recrystallization and precipitation of bisphenol A as an aromatic diol compound is achieved.

[0029] Specifically, the method for preparing a monomer composition for synthesizing recycled plastics according to one embodiment may include a step of subjecting a polycarbonate-based resin to a depolymerization reaction.

[0030] Polycarbonate resin is intended to include homopolymers and copolymers that comprise polycarbonate repeating units, and refers jointly to the reaction product obtained by the polymerization reaction or copolymerization reaction of the monomer that comprises aromatic diol compound and carbonate precursor.When it comprises a type of carbonate repeating unit obtained by only using an aromatic diol compound and a type of carbonate precursor, homopolymers can be synthesized.In addition, when using an aromatic diol compound and two or more types of carbonate precursors as monomers, or when using two or more types of aromatic diol compounds and a type of carbonate precursors, or when also using one or more types of other glycols to comprise two or more types of carbonates except an aromatic diol compound and a type of carbonate precursor, copolymers can be synthesized.According to molecular weight range, homopolymers or copolymers can comprise all low molecular weight compounds, oligomers and polymers.

[0031] The polycarbonate-based resin may be used regardless of various forms and types, such as a new polycarbonate-based resin produced by synthesis, a recycled polycarbonate-based resin produced by a recycling process, or polycarbonate-based resin waste.

[0032] However, if necessary, before the polycarbonate resin is subjected to depolymerization reaction, a pretreatment step of the polycarbonate resin is performed, so that the efficiency of the process of recovering aromatic diol compounds and carbonate precursors from the polycarbonate resin can be improved. Examples of pretreatment steps may include washing, drying, grinding, glycol decomposition, etc. There is no restriction on the specific method of each pretreatment step, and the various methods widely used in the process of recovering aromatic diol compounds and carbonate precursors by the depolymerization of the polycarbonate resin can be applied without restriction.

[0033] In the process of the depolymerization reaction of polycarbonate resin, the depolymerization reaction can be carried out under acidic, neutral or alkaline conditions, and specifically, the depolymerization reaction can be carried out under alkaline (containing alkali) conditions. There is no particular limitation on the type of alkali, and its examples include sodium hydroxide (NaOH) or potassium hydroxide (KOH). Alkali is a base catalyst that acts as a catalyst, and has economic advantages relative to organic catalysts used mainly under mild conditions. More specifically, in the process of the depolymerization reaction of polycarbonate resin, the depolymerization reaction can be carried out at a pH of more than 13 or in the range of 13 to 14.

[0034] In the process of depolymerization of polycarbonate resin, the depolymerization reaction can be carried out by reacting a base in an amount of 0.5 mol or less, or 0.4 mol or less, or 0.3 mol or less, or 0.1 mol or more, or 0.2 mol or more, or 0.1 mol to 0.5 mol, or 0.1 mol to 0.4 mol, or 0.1 mol to 0.3 mol, or 0.2 mol to 0.5 mol, or 0.2 mol to 0.4 mol, or 0.2 mol to 0.3 mol relative to 1 mol of polycarbonate resin. When the polycarbonate resin is reacted with a base in an amount greater than 0.5 mol relative to 1 mol of the polycarbonate resin during the depolymerization of the polycarbonate resin, there is a limitation in that impurities increase due to the influence of an increase in the amount of alkali metal salt generated, so that the purity of the target recycled material is reduced, and the economic efficiency of the catalytic reaction is reduced.

[0035] In addition, the depolymerization reaction of the polycarbonate resin can be carried out in the presence of a solvent containing ethanol. The present invention can stably obtain bisphenol A as a high-purity monomer by decomposing the polycarbonate resin with a solvent containing ethanol, and has the advantage of further obtaining diethyl carbonate with high added value as a reaction byproduct.

[0036] The content of ethanol can be 1 mol to 5 mol, or 1 mol to 4 mol, or 1 mol to 3.75 mol relative to 1 mol of the polycarbonate resin. Since ethanol has good solubility in bisphenol A, ethanol within the above range should be substantially contained. When the content of ethanol is excessively reduced to less than 1 mol relative to 1 mol of the polycarbonate resin, the alcohol decomposition of the polycarbonate resin is difficult to be fully carried out. On the other hand, when the content of ethanol is excessively increased relative to 1 mol of the polycarbonate resin, it is difficult to fully remove ethanol in the neutralization step described later, and due to the inclusion of a separate distillation step, the economic efficiency of the process may be reduced.

[0037] The solvent for the depolymerization reaction of the polycarbonate-based resin may further include at least one organic solvent selected from the group consisting of tetrahydrofuran, toluene, dichloromethane, chloroform, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, and dipropyl carbonate, in addition to ethanol.

[0038] The organic solvent may include tetrahydrofuran, toluene, dichloromethane, chloroform, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, or a mixture of two or more thereof.

[0039] More preferably, dichloromethane may be used as the organic solvent. When dichloromethane is used as the organic solvent to be mixed with ethanol, there are advantages that the solubility in polycarbonate can be improved and the reactivity can be enhanced.

[0040] The content of the organic solvent may be 10 to 20 mol, or 10 to 15 mol, relative to 1 mol of the polycarbonate-based resin. In addition, the content of the organic solvent may be 2.2 to 5 mol relative to 1 mol of ethanol. By mixing the polycarbonate-based resin, ethanol, and the organic solvent within the above range, there is an advantage that the depolymerization reaction of the polymer can be performed at a desired level.

[0041] Meanwhile, the temperature at which the depolymerization reaction of the polycarbonate-based resin is performed is not particularly limited, but for example, the reaction may be performed at 20 to 100° C. or 70 to 90° C. Also, the depolymerization reaction of the polycarbonate-based resin may be performed for 1 to 30 hours.

[0042] Specifically, the conditions are mild process conditions relative to conventional pressurized / high temperature processes, and by performing stirring under the above conditions, the process can be performed in a mild process compared to the pressurized / high temperature process.

[0043] That is, according to the present invention, since the type and mixing amount of the mixed solvent and the type and content of the alkali catalyst are adjusted without using an organic catalyst, there are the following advantages: a high-purity aromatic diol compound (e.g., bisphenol A) can be obtained under mild conditions without using a pressurized / high-temperature process, and an ethanol solvent is used, so diethyl carbonate can be obtained as a by-product.

[0044] Meanwhile, during the depolymerization reaction of the polycarbonate resin, an antioxidant may be added to the reaction solution. Due to the addition of the antioxidant, the aromatic diol compound recovered by regeneration through chemical decomposition of the polycarbonate resin may satisfy the color coordinate b* value at a color level comparable to that of a commercially available agent or a reagent used for PC polymerization.

[0045] The specific examples of antioxidants are not particularly limited, and various antioxidants widely used in the prior art can be applied without restriction. However, an example of antioxidants includes sodium thiosulfate, sodium sulfite, isoascorbic acid, butylated hydroxytoluene, butylated hydroxyanisole, alpha-tocopherol, tocopherol acetate, L-ascorbic acid and salts thereof, L-ascorbyl palmitate, L-ascorbyl stearate, tripentyl gallate, propyl gallate or disodium ethylenediaminetetraacetic acid (EDTA), sodium pyrophosphate, sodium metaphosphate or a mixture of two or more thereof.

[0046] There is no particular limitation on the specific amount of the antioxidant added, but as an example, the antioxidant can be added in the range of 0.1 wt % to 5 wt % or 0.1 wt % to 1 wt % based on the total weight of the reaction solution at a level that does not affect the physical properties of the monomer composition for synthesizing recycled plastics.

[0047] Meanwhile, the method for preparing a monomer composition for synthesizing recycled plastic according to one embodiment may include a step of adjusting the pH of the depolymerization reaction product having a pH of 13 or more or 13 to 14 to 8 to 12, or 8 to 11, or 8 to 10, or 8 to 9.

[0048] Specifically, the step of adjusting the pH of the depolymerization reaction product to 8 to 12 may include the step of adding an acidic aqueous solution so that the pH of the depolymerization reaction product is 8 to 12. By adjusting the pH of the depolymerization reaction product to 8 to 12, water-soluble salt impurities can be effectively removed and color characteristics can be improved. In addition, the solubility of unreacted ethanol remaining in the depolymerization reaction product in water can be increased, and the amount of residual ethanol can be minimized.

[0049] Specifically, a layer separated into an organic solvent layer and a water layer may be formed in the step of adding an acidic aqueous solution so that the pH of the depolymerization reaction product is 8 to 12. More specifically, the layer separated into an organic solvent layer and a water layer may form a layer separated into a water layer containing impurities and an organic solvent layer containing an aromatic diol compound.

[0050] Since the aromatic diol compound has hydrophobicity, it can be contained in the organic solvent layer between the water layer and the organic solvent layer, and various water-soluble impurities can be contained in the water layer. Impurities are materials with hydrophilicity, and examples thereof can include salt compounds, ionic compounds, acid compounds, etc. In addition, the water layer containing impurities can further contain unreacted ethanol.

[0051] The acidic aqueous solution is a mixed solution of acid and water, wherein the acid may be a strong acid such as hydrochloric acid (HCl). Since the depolymerization reaction is carried out under a strong alkaline condition of pH 13 or above, the resulting aromatic diol compound exists in the form of a salt by reacting with the base, and thus has hydrophilicity. However, by adding the acidic aqueous solution, the salt of the aromatic diol compound contained in the depolymerization reaction product can be converted into an aromatic diol compound, thereby making it hydrophobic.

[0052] Meanwhile, the method may further include the step of removing the aqueous layer in the layer separated into the organic solvent layer and the aqueous layer after the step of adjusting the pH of the depolymerization reaction product having a pH of 13 or more to 8 to 12. Since various water-soluble impurities are separated from the aqueous layer in the layer separated into the aqueous layer and the organic solvent layer by adding the acidic aqueous solution, impurities can be easily removed from the aromatic diol compound as the main product only by a simple process of removing the aqueous layer.

[0053] There is no particular limitation on the specific conditions for removing the water layer from the organic layer. For the specific removal device and method, various known purification techniques can be applied without limitation. However, as an example, a drainage device can be used.

[0054] Meanwhile, the method for preparing a monomer composition for synthesizing recycled plastics according to one embodiment may include a step of adjusting the pH of the depolymerization reaction product adjusted to pH 8 to 12 to less than 4, or 1 to 3, or 1 to 2.

[0055] Specifically, the step of adjusting the pH of the depolymerization reaction product adjusted to pH 8 to 12 to less than 4 may include the step of adding an acidic aqueous solution to make the pH of the depolymerization reaction product less than 4. By adding the acidic aqueous solution to make the pH of the depolymerization reaction product less than 4, the salt of the aromatic diol compound remaining in the depolymerization reaction product adjusted to pH 8 to 12 can be converted into an aromatic diol compound, thereby improving the yield of the aromatic diol compound.

[0056] Specifically, in the step of adding the acidic aqueous solution so that the pH of the depolymerization reaction product is less than 4, a layer separated into an organic solvent layer and a water layer may be formed. More specifically, the layer separated into an organic solvent layer and a water layer may form a layer separated into a water layer containing impurities and an organic solvent layer containing an aromatic diol compound.

[0057] Since the aromatic diol compound has hydrophobicity, it can be contained in the organic solvent layer between water and the organic solvent, and various water-soluble impurities can be contained in the water layer. The impurities are materials having hydrophilicity, and examples thereof may include salt compounds, ionic compounds, acid compounds, and the like.

[0058] The acidic aqueous solution is a mixed solution of acid and water, wherein the acid may be a strong acid such as hydrochloric acid (HCl). By adding the acidic aqueous solution to the depolymerization reaction product whose pH is adjusted to 8 to 12, the salt of the aromatic diol compound remaining in the depolymerization reaction product may be converted into the aromatic diol compound, thereby making it hydrophobic.

[0059] Meanwhile, the method may further include a step of removing a water layer in the layer separated into the organic solvent layer and the water layer, after the step of adjusting the pH of the depolymerization reaction product adjusted to pH 8 to 12 to less than 4. Since various water-soluble impurities are separated from the water layer in the layer separated into the water layer and the organic solvent layer by adding the acidic aqueous solution, impurities can be easily removed from the aromatic diol compound as a main product only by a simple process of removing the water layer.

[0060] There is no particular limitation on the specific conditions for removing the water layer from the organic layer. For the specific removal device and method, various known purification techniques can be applied without limitation. However, as an example, a drainage device can be used.

[0061] Meanwhile, the pH difference according to the following Equation 1 may be 4 to 9, or 5 to 8, or 6 to 7.

[0062] [Equation 1]

[0063] pH difference = (pH of the depolymerization reaction product adjusted to pH 8 to 12) - (pH of the depolymerization reaction product adjusted to pH less than 4).

[0064] Since the pH difference according to Equation 1 satisfies 4 or more, or 4 to 10, or 5 to 8, or 6 to 7, the effect of removing salt impurities and residual ethanol produced by adjusting the pH to 8 to 12 and the effect of improving the yield of the aromatic diol compound produced by adjusting the pH to less than 4 can be further maximized.

[0065] In addition, the pH difference according to the following Equation 2 may be 4 to 11, or 4 to 10, or 4 to 9, or 4 to 8, or 4 to 7, or 5 to 6.

[0066] [Equation 2]

[0067] pH difference=(pH of the depolymerization reaction product)-(pH of the depolymerization reaction product adjusted to pH 8 to 12).

[0068] Since the pH difference according to Equation 2 satisfies 1 to 6, or 2 to 6, or 3 to 6, or 4 to 6, or 5 to 6, the effect of removing salt impurities and residual ethanol produced by adjusting the pH to 8 to 12 can be further maximized.

[0069] Specifically, according to the following Equation 3, the ratio of residual ethanol contained in the depolymerization reaction product adjusted to less than pH 4 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%.

[0070] [Equation 3]

[0071] Residual ethanol ratio (%) = {( 1 Ethanol peak area in H NMR) / ( 1 Ethanol peak area in H NMR + 1 Peak area of ​​aromatic diol compounds in H NMR)}×100.

[0072] Specifically, in Equation 3, 1 The ethanol peak in H NMR is targeted at the peak at 3.63 ppm (quartet, 2H), 1 The peak of the aromatic diol compound (bisphenol A (BPA)) in H NMR is targeted to be a peak at 6.67 ppm (4H).

[0073] Since the residual amount of ethanol is reduced in this way, ethanol is sufficiently removed, and the aromatic diol compound can be precipitated with high purity and high yield without a separate distillation process, thereby improving the economic efficiency of the process.

[0074] On the other hand, in the case where the residual amount of ethanol contained in the depolymerization reaction product whose pH is adjusted to less than 4 according to Equation 3 increases to more than 1%, even if a large amount of a crystallization solvent is added to such an extent that the precipitation of the aromatic diol compound can be promoted by removing ethanol only through a distillation process, the yield of the aromatic diol compound obtained by precipitation is reduced, which limits the efficiency of the process.

[0075] Meanwhile, the method for preparing a monomer composition for synthesizing recycled plastics according to one embodiment may include the steps of adding a crystallization solvent to a depolymerization reaction product whose pH is adjusted to less than 4 and recovering the formed aromatic diol compound crystals.

[0076] Since the crystallization solvent is added to precipitate the aromatic diol compound in this manner, the aromatic diol compound, which is the main synthesis target material in the present invention, can be secured with high purity and high yield.

[0077] The step of adding a crystallization solvent to the depolymerization reaction product whose pH is adjusted to less than 4 and recovering the formed aromatic diol compound crystals may include the steps of adding water to the depolymerization reaction product whose pH is adjusted to less than 4 to precipitate the aromatic diol compound crystals, wherein the water is added in an amount of 100 mol or less, or 50 mol to 100 mol, or 50 mol to 90 mol, or 50 mol to 80 mol, or 50 mol to 70 mol, relative to 1 mol of the polycarbonate resin.

[0078] When the amount of water added during the extraction of the aromatic diol compound is excessively reduced, the aromatic diol compound may not be fully extracted, and thus the yield may be reduced. On the other hand, when the amount of water added during the extraction of the aromatic diol compound is excessively increased, excessive water is required, which may reduce the efficiency of the extraction process.

[0079] Meanwhile, according to a method for preparing a monomer composition for synthesizing recycled plastics according to one embodiment, after the step of adding a crystallization solvent to the depolymerization reaction product whose pH is adjusted to less than 4 and recovering the formed aromatic diol compound crystals, a step of purifying the aromatic diol compound may be further included.

[0080] Specifically, the step of purifying the aromatic diol compound may include a step of washing the aromatic diol compound. In addition, the step of purifying the aromatic diol compound may include a step of dissolving the aromatic diol compound and a step of subjecting the aromatic diol compound to adsorption purification.

[0081] There is no particular limitation on the order of the washing step and the adsorption purification step, and it does not matter whether they are performed in any order, but as an example, they can be performed in the order of the washing step and the adsorption purification step. The washing step and the adsorption purification step can be repeated at least one or more times. As for the addition of specific washing, adsorption devices and methods, various known purification techniques can be applied without limitation.

[0082] In the washing step of the aromatic diol compound, since various impurities remain during the recovery process of obtaining the aromatic diol compound, washing may be performed to sufficiently remove these impurities and ensure a high-purity aromatic diol compound.

[0083] Specifically, the washing step may include a step of washing 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 washing container when washing with a solvent is performed. In order to maintain a high temperature deviating from room temperature, various heating devices may be applied without limitation.

[0084] The solvent used in the washing step may include one of water, alcohol and an organic solvent. As the organic solvent, tetrahydrofuran, toluene, dichloromethane, chloroform, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, dipropyl carbonate, or a mixture of two or more thereof may be used.

[0085] The solvent used in the washing step may be used in a weight ratio of 1 part by weight to 30 parts by weight, or 1 part by weight to 10 parts by weight, based on 1 part by weight of the polycarbonate-based resin used in the depolymerization reaction.

[0086] More specifically, the solvent in the step of washing with a solvent at a temperature of 10° C. or higher and 30° C. or lower may be an organic solvent. Methylene chloride is preferably used as the organic solvent.

[0087] Meanwhile, the step of dissolving the aromatic diol compound and the step of purifying by adsorption may include the step of dissolving the aromatic diol compound and the step of purifying by adsorption of the aromatic diol compound.

[0088] The step of dissolving the aromatic diol compound may include adding a solvent to the aromatic diol compound. Examples of the solvent include ethanol, which may be added in a ratio of 1 to 20 mol, or 1 to 10 mol, or 5 to 10 mol, based on 1 mol of the polycarbonate resin. By adding a solvent to the aromatic diol compound, the aromatic diol compound crystals may be redissolved in the solvent.

[0089] The step of adsorption purification of the aromatic diol compound may include the steps of adding an adsorbent to the aromatic diol compound for adsorption purification and then removing the adsorbent. In the step of adding an adsorbent to the aromatic diol compound for adsorption purification and then removing the adsorbent, the adsorbent may be brought into contact with the aromatic diol compound.

[0090] Examples of adsorbents that can be used include activated carbon, charcoal, or a mixture thereof. Activated carbon is a black carbon material having micropores produced by subjecting a raw material to a carbonization process at about 500° C. and an activated carbon process at about 900° C. Examples of activated carbon are not particularly limited, but for example, various activated carbons can be applied without limitation according to the type of raw material, such as plant-based activated carbon, coal-based activated carbon, petroleum-based activated carbon, and waste-based activated carbon.

[0091] In a more specific example, the plant-based activated carbon may include coconut activated carbon, wood activated carbon and sawdust activated carbon. In addition, the coal-based activated carbon may include lignite activated carbon, bituminous coal activated carbon and anthracite activated carbon. In addition, the petroleum-based activated carbon may include petroleum coke activated carbon and oil carbon activated carbon. In addition, the waste-based activated carbon may include synthetic resin activated carbon and pulp activated carbon.

[0092] The adsorbent may include at least one activated carbon selected from the group consisting of plant-based activated carbon, coal-based activated carbon, petroleum-based activated carbon, and waste-based activated carbon. That is, 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 thereof.

[0093] More specifically, the adsorbent may include at least one activated carbon selected from the group consisting of coconut activated carbon, lignite activated carbon, anthracite activated carbon, and bituminous coal activated carbon. That is, the adsorbent may include coconut activated carbon, lignite activated carbon, anthracite activated carbon, bituminous coal activated carbon, or a mixture of two or more thereof.

[0094] There is no particular limitation on the adsorption purification conditions by the adsorbent, and various known adsorption purification conditions can be used without limitation. However, in one example, the amount of the 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 stirring adsorption or a laboratory adsorption tower.

[0095] Meanwhile, after the dissolution step and the adsorption purification step of the aromatic diol compound, a recrystallization step of the aromatic diol compound may be further included. In the recrystallization step of the aromatic diol compound, various impurities contained in the aromatic diol compound may be sufficiently removed to ensure a high-purity aromatic diol compound.

[0096] Specifically, the recrystallization step may include a step of 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, and thus, the crystals or impurities interposed between the crystals can be dissolved to the greatest extent with the solvent, and further, since the dissolved aromatic diol compound has a poor solubility relative to the impurities, when the temperature is subsequently lowered, the aromatic diol compound can be easily precipitated into aromatic diol compound crystals by the difference in solubility.

[0097] 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 relative to 1 mole of the polycarbonate resin. When water is used in too little amount, the temperature for dissolving the aromatic diol compound contained in the solution in which the aromatic diol compound is dissolved becomes too high, thereby deteriorating the process efficiency and making it difficult to remove impurities by recrystallization. On the other hand, when water is used in too much amount, the solubility of the aromatic diol compound contained in the solution in which the aromatic diol compound is dissolved becomes too high, and therefore, the yield of the aromatic diol compound recovered after recrystallization is reduced, and due to the use of a large amount of solvent, the process efficiency may decrease.

[0098] If necessary, after performing the recrystallization step of dissolving the aromatic diol compound, a step of removing residual impurities by filtration or adsorption may be further performed.

[0099] In addition, if necessary, after the recrystallization step, the method may further include a drying step. The residual solvent may be removed by drying, and there is no particular limitation on the specific drying conditions, but for example, the drying may be performed at a temperature of 10° C. to 100° C., or 10° C. to 50° C. For the specific drying equipment and method used in the drying, various known drying techniques may be applied without limitation.

[0100] 2. Monomer composition for synthesizing recycled plastics

[0101] According to another embodiment of the present invention, a monomer composition for synthesizing recycled plastics may be provided, the monomer composition comprising an aromatic diol compound obtained by the method for preparing a monomer composition for synthesizing recycled plastics according to one embodiment.

[0102] The monomer composition for synthesizing recycled plastic according to another embodiment can be obtained by the method for preparing the monomer composition for synthesizing recycled plastic according to one embodiment. The details of the method for preparing the monomer composition for synthesizing recycled plastic according to another embodiment include all the contents described above in one embodiment.

[0103] That is, the monomer composition for synthesizing recycled plastic according to another embodiment corresponds to the resultant obtained through various processes of filtration, purification, washing and drying to ensure that only the aromatic diol compound as the main recovery target material has high purity after the depolymerization reaction of the polycarbonate resin.

[0104] In addition, specific examples of the aromatic diol compound 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, 1,1-bis(4-hydroxyphenyl)cyclohexane (bisphenol Z), 2,2-bis(4-hydroxyphenyl)butane, 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 a mixture of two or more thereof, etc. Preferably, the aromatic diol compound of the monomer composition for synthesizing recycled plastics according to one embodiment may be 2,2-bis(4-hydroxyphenyl)propane (bisphenol A).

[0105] The aromatic diol compound is characterized in that it is obtained by the method for preparing a monomer composition for synthesizing recycled plastics according to one embodiment. That is, the aromatic diol compound refers to a compound recovered from a polycarbonate resin used to recover the monomer composition for synthesizing recycled plastics. Therefore, the aromatic diol compound of the present invention does not include the case of adding a new aromatic diol compound from the outside, except for recovering from a polycarbonate resin to prepare a monomer composition for synthesizing recycled plastics according to one embodiment.

[0106] Specifically, "recovered from a polycarbonate resin" means obtained by a depolymerization reaction of a polycarbonate resin. The depolymerization reaction can be carried out under acidic, neutral or alkaline conditions, and in particular, the depolymerization reaction can be carried out under alkaline (containing alkali) conditions. Specifically, the depolymerization reaction can be preferably carried out in the presence of an ethanol solvent, as will be described later.

[0107] 3. Recycled plastics

[0108] According to another embodiment of the present invention, a recycled plastic may be provided, comprising a reaction product of a monomer composition for synthesizing recycled plastic according to another embodiment and a comonomer.

[0109] Details of the monomer composition for synthesizing recycled plastic according to another embodiment include all of the contents described above in another embodiment.

[0110] There is no particular limitation on examples corresponding to recycled plastics, and various plastics synthesized from 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 limitation, and a more specific example may be a polycarbonate resin.

[0111] Polycarbonate resin is intended to include homopolymers and copolymers that comprise polycarbonate repeating units, and refers jointly to the reaction product obtained by the polymerization reaction or copolymerization reaction of the monomer that comprises aromatic diol compound and carbonate precursor.When it comprises a type of carbonate repeating unit obtained by only using an aromatic diol compound and a type of carbonate precursor, homopolymers can be synthesized.In addition, when using an aromatic diol compound and two or more types of carbonate precursors as monomers, or when using two or more types of aromatic diol compounds and a type of carbonate precursors, or when also using one or more types of other glycols to comprise two or more types of carbonates except an aromatic diol compound and a type of carbonate precursor, copolymers can be synthesized.According to molecular weight range, homopolymers or copolymers can comprise all low molecular weight compounds, oligomers and polymers.

[0112] More specifically, in the recycled plastics comprising the reaction product of the monomer composition for synthesizing recycled plastics according to one embodiment and a comonomer, a carbonate precursor can be used as a comonomer. Specific examples of carbonate precursors include phosgene, triphosgene, diphosgene, bromophosgene, dimethyl carbonate, diethyl carbonate, dibutyl carbonate, dicyclohexyl carbonate, diphenyl carbonate, ditolyl carbonate, bis(chlorophenyl) carbonate, m-tolyl carbonate, dinaphthyl carbonate, bis(diphenyl) carbonate or bishaloformates.

[0113] There is no particular limitation on the example of the reaction process of the monomer composition for synthesizing recycled plastic and the comonomer for synthesizing the polycarbonate-based resin, and various known methods for preparing polycarbonate can be applied without limitation.

[0114] However, in one example of the polycarbonate preparation method, a polycarbonate preparation method including the step of polymerizing a composition containing a monomer composition for synthesizing recycled plastics and a comonomer can be used. In this case, the polymerization can be performed by interfacial polymerization, and during the interfacial polymerization, the polymerization reaction can be performed at normal pressure and low temperature, and the molecular weight can be easily adjusted.

[0115] The polymerization temperature may be 0° C. to 40° C., and the reaction time may be 10 minutes to 5 hours. In addition, the pH during the reaction may be maintained at 9 or above or 11 or above.

[0116] There is no particular limitation on the solvent that can be used for the polymerization as long as it is a solvent used for the polymerization of polycarbonate in the art, and as examples, halogenated hydrocarbons such as methylene chloride and chlorobenzene can be used.

[0117] In addition, the polymerization may be carried out in the presence of an acid binder. As the acid binder, an alkali metal hydroxide such as sodium hydroxide or potassium hydroxide, or an amine compound such as pyridine may be used.

[0118] In addition, in order to regulate the molecular weight of polycarbonate in the process of polymerization, polymerization can be carried out in the presence of a molecular weight regulator. Alkylphenols with carbon atoms of 1 to 20 can be used as molecular weight regulators, and its specific examples include p-tert-butylphenol, p-cumylphenol, decylphenol, dodecylphenol, tetradecylphenol, hexadecylphenol, octadecylphenol, eicosylphenol, docosylphenol or triacontylphenol. Molecular weight regulators can be added before polymerization initiation, during polymerization initiation or after polymerization initiation. Based on the aromatic diol compound of 100 weight parts, the consumption of molecular weight regulator can be 0.01 weight part to 10 weight parts or 0.1 weight part to 6 weight parts, and the molecular weight desired can be obtained within this range.

[0119] Furthermore, in order to promote the polymerization reaction, a reaction accelerator such as a tertiary amine compound, a quaternary ammonium compound or a quaternary phosphonium compound including triethylamine, tetra-n-butylammonium bromide or tetra-n-butylphosphonium bromide may be further used.

[0120] 4. Molded products

[0121] According to another embodiment of the present invention, a molded article comprising the recycled plastic of another embodiment may be provided. The details of the recycled plastic include all the contents described above in another embodiment.

[0122] The molded product can be obtained by applying the recycled plastic to various known plastic molding methods without limitation. As examples of the molding method, injection molding, foam injection molding, blow molding or extrusion molding can be mentioned.

[0123] There is no particular limitation on examples of molded products, and it can be applied to various molded products using plastics without limitation. Examples of molded products include automobile parts, electric and electronic products, communication products, daily necessities, building materials, optical components, exterior materials, and the like.

[0124] If necessary, the molded article may further include one or more additives selected from the group consisting of antioxidants, plasticizers, antistatic agents, nucleating agents, flame retardants, lubricants, impact enhancers, optical brighteners, ultraviolet absorbers, pigments and dyes in addition to the recycled plastic of another embodiment.

[0125] An example of the method for producing the molded article may include the steps of well mixing the recycled plastic of another embodiment with an additive using a mixer, extrusion-molding the mixture with an extruder to produce pellets, drying the pellets, and then injecting them with an injection molding machine.

[0126] Beneficial Effects

[0127] According to the present invention, there can be provided a method for preparing a monomer composition for synthesizing recycled plastics, a monomer composition for synthesizing recycled plastics, recycled plastics, and a molded product using the recycled plastics, wherein the method improves the efficiency of the recycling process while allowing the aromatic diol compound to have good yield and optical properties, wherein the aromatic diol compound is recycled by regeneration through chemical decomposition of a polycarbonate resin. DETAILED DESCRIPTION

[0128] Hereinafter, the present invention will be explained in detail with reference to the following examples. However, these examples are for illustrative purposes only, and the scope of the present invention is not limited thereto.

[0129] <Examples and Comparative Examples: Preparation of Regenerated Bisphenol A Monomer Composition>

[0130] Example 1

[0131] (1. Decomposition step) 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 the mixture was stirred. Then, 1.2 mol of waste polycarbonate (PC) was added thereto, and the mixture was stirred at 80° C. to perform a PC depolymerization reaction.

[0132] (2-1. First pH Adjustment) The depolymerization reaction product (pH 14) containing bisphenol A was cooled to 30° C. or less, and then a 10% HCl aqueous solution was added to the depolymerization reaction product so that the pH was adjusted to 8, and the aqueous layer was removed.

[0133] (2-2. Second pH Adjustment) A 10% hydrochloric acid (HCl) aqueous solution was added to the depolymerization reaction product whose pH was adjusted to 8 so that the pH was adjusted to 2, and the aqueous layer was removed.

[0134] (3. Precipitation Step) About 1500 g (about 83 mol) of water was added to the depolymerization reaction product obtained in the pH adjustment step, and the mixture was stirred to precipitate bisphenol A (BPA) crystals, and vacuum filtered to recover bisphenol A (BPA).

[0135] (4. Purification step - filtration) Thereafter, bisphenol A was washed with 300 g of dichloromethane (MC) at 20 to 30° C. and vacuum filtered.

[0136] (5-1. Additional purification step - re-dissolution step) Thereafter, bisphenol A was added to 500 g of ethanol and re-dissolved.

[0137] (5-2. Additional Purification Step - Adsorption Step) Thereafter, lignite activated carbon was added as an adsorbent at a ratio of 30 wt % relative to the waste polycarbonate, purified by an adsorption tower, and then filtered to remove the lignite activated carbon.

[0138] (5-3. Additional Purification Step - Recrystallization Step) Thereafter, bisphenol A was recrystallized by adding 750 g of water, and the resulting slurry was vacuum filtered at 20 to 30° C. to recover bisphenol A (BPA).

[0139] (6. Drying Step) Thereafter, the resultant was vacuum dried in a convection oven at 40° C. to prepare a recycled bisphenol A monomer composition in which recycled bisphenol A (BPA) had been recovered.

[0140] Example 2

[0141] A regenerated bisphenol A monomer composition was prepared in the same manner as in Example 1, except that the pH was adjusted to 9 instead of 8 in (2-1. First pH adjustment step) of Example 1.

[0142] Comparative Example 1

[0143] (1. Decomposition step) 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 the mixture was stirred. Then, 1.2 mol of waste polycarbonate (PC) was added thereto, and the mixture was stirred at 80° C. to perform a PC depolymerization reaction.

[0144] (2. pH Adjustment) The depolymerization reaction product (pH 14) containing bisphenol A was cooled to 30° C. or less, and then a 10% HCl aqueous solution was added to the depolymerization reaction product so that the pH was adjusted to 2, and the aqueous layer was removed.

[0145] (3. Precipitation Step) About 8000 g (about 444 mol) of water was added to the depolymerization reaction product obtained in the pH adjustment step, and the mixture was stirred to precipitate bisphenol A (BPA) crystals, and vacuum filtered to recover bisphenol A (BPA).

[0146] (4. Purification step - filtration) Thereafter, bisphenol A was washed with 300 g of dichloromethane (MC) at 20 to 30° C. and vacuum filtered.

[0147] (5-1. Additional purification step - re-dissolution step) Thereafter, bisphenol A was added to 500 g of ethanol and re-dissolved.

[0148] (5-2. Additional Purification Step - Adsorption Step) Thereafter, lignite activated carbon was added as an adsorbent at a ratio of 30 wt % relative to the waste polycarbonate, purified by an adsorption tower, and then filtered to remove the lignite activated carbon.

[0149] (5-3. Additional Purification Step - Recrystallization Step) Thereafter, bisphenol A was recrystallized by adding 750 g of water, and the resulting slurry was vacuum filtered at 20 to 30° C. to recover bisphenol A (BPA).

[0150] (6. Drying Step) Thereafter, the resultant was vacuum dried in a convection oven at 40° C. to prepare a recycled bisphenol A monomer composition in which recycled bisphenol A (BPA) had been recovered.

[0151] Comparative Example 2

[0152] A recycled bisphenol A monomer composition was prepared in the same manner as in Comparative Example 1, except that the pH was adjusted to 8 instead of 2 in (2. pH adjustment step) of Comparative Example 1.

[0153] <Experimental example>

[0154] The physical properties were measured by the following methods, and the results are shown in Table 1 below.

[0155] 1. Purity

[0156] The regenerated bisphenol A monomer composition was dissolved in acetonitrile (ACN) solvent at 1 wt % under normal pressure and 20-30° C., and then the regenerated bisphenol A monomer composition was dissolved in acetonitrile (ACN) solvent at 1 wt %. The purity of bisphenol A (BPA) was analyzed by UPLC (ultra-high performance liquid chromatography) using a Waters HPLC system using a BEH C18 1.7 μm column (2.1*50 mm).

[0157] 2. Color coordinates (L*, a*, b*)

[0158] The color coordinates of the recycled bisphenol A monomer composition were analyzed using a HunterLab UltraScan PRO spectrophotometer in reflectance mode.

[0159] 3. Yield

[0160] The weight of BPA generated when the polycarbonate used in the reaction was 100% decomposed was measured, and the weight of the obtained BPA was measured so that the yield of BPA was calculated as shown in Equation A below.

[0161] [Equation A]

[0162] Yield (%) = (W1 / W0) × 100 (%)

[0163] Wherein, in equation A, W0 is the mass of BPA obtained when 100% decomposed, and W1 is the mass of BPA actually obtained. Specifically, when about 100g of polycarbonate is decomposed, the mass of BPA obtained when 100% decomposed theoretically is 89g. If the mass of BPA actually obtained is 80g, the yield is 80 / 89*100=90%.

[0164] 4. Residual ethanol ratio

[0165] The depolymerization reaction product obtained in the pH adjustment step of (3. Precipitation step) was diluted to a concentration of 10 mg / ml with methanol-d4 to prepare a sample, and the 1 The area ratio of the ethanol peak between the ethanol peak and the bisphenol A (BPA) peak was measured using an H NMR spectrum detected by a H NMR apparatus and was used as the residual ethanol ratio.

[0166] Specifically, ethanol was targeted at a peak at 3.63 ppm (quartet, 2H), and bisphenol A (BPA) was targeted at a peak at 6.67 ppm (4H), and the residual ethanol content was calculated by Equation B below.

[0167] [Equation B]

[0168] Residual ethanol ratio (%) = {( 1 Ethanol peak area in H NMR) / ( 1 Ethanol peak area in H NMR + 1 BPA peak area in H NMR)}×100

[0169] [Table 1]

[0170] Experimental Example Measurement Results

[0171]

[0172] As shown in Table 1, the regenerated bisphenol A monomer compositions obtained in Examples 1 and 2 showed a high purity of 99.4% to 99.5%. In addition, the regenerated bisphenol A monomer compositions obtained in Examples 1 and 2 showed color coordinates L* of 97.6 to 98.03, a* of -0.12 to 0.06, and b* of 0.59 to 0.75, indicating excellent optical properties. In addition, the regenerated bisphenol A monomer compositions obtained in Examples 1 and 2 were measured to have a high BPA yield of 98.6% to 98.8%. In addition, the regenerated bisphenol A monomer compositions obtained in Examples 1 and 2 were measured to have a low residual ethanol content of 0.5% to 0.7%.

[0173] On the other hand, 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. In addition, the recycled bisphenol A monomer compositions obtained in Comparative Examples 1 and 2 showed color coordinates L* of 95.67 to 97.4, a* of 0.06 to 0.16, and b* of 1.47 to 1.72, indicating inferior optical properties compared to the Examples.

[0174] Specifically, although a larger amount of water was added than that in the examples, the yield of precipitated BPA of the regenerated bisphenol A monomer compositions obtained in Comparative Examples 1 and 2 was 18.2% to 21.1%, which was significantly lower than that in the examples. In addition, the regenerated bisphenol A monomer compositions obtained in Comparative Examples 1 and 2 were measured to have a residual ethanol content of 1.6% to 2.2%, which was higher than that in the examples.

Claims

1. A method for preparing a monomer composition for synthesizing recycled plastics, the method comprising the following steps: Depolymerizing a polycarbonate resin; adjusting the pH of the depolymerization reaction product having a pH of 13 or more to a pH of 8 to 12; adjusting the pH of the depolymerization reaction product adjusted to a pH of 8 to 12 to less than 4; as well as A crystallization solvent is added to the depolymerization reaction product adjusted to a pH of less than 4 and the formed aromatic diol compound crystals are recovered.

2. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 1, wherein: The pH difference is 4 to 11 according to equation 1 below: [Equation 1] pH difference = (pH of the depolymerization reaction product adjusted to pH 8 to 12) - (pH of the depolymerization reaction product adjusted to pH less than 4).

3. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 1, wherein: The pH difference is 1 to 6 according to equation 2 below: [Equation 2] pH difference=(pH of the depolymerization reaction product)-(pH of the depolymerization reaction product adjusted to pH 8 to 12).

4. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 1, wherein: Adjusting the pH of the depolymerization reaction product to 8 to 12 comprises: An acidic aqueous solution is added to make the pH of the depolymerization reaction product be between 8 and 12.

5. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 1, wherein: After adjusting the pH of the depolymerization reaction product having a pH of 13 or more to a pH of 8 to 12, The method further includes removing the water layer from the layer separated into the organic solvent layer and the water layer.

6. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 1, wherein: Adjusting the pH of the depolymerization reaction product having a pH of 8 to 12 to less than 4 comprises: An acidic aqueous solution is added so that the pH of the depolymerization reaction product adjusted to a pH of 8 to 12 is less than 4.

7. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 1, wherein: After the pH of the depolymerization reaction product adjusted to a pH of 8 to 12 is adjusted to less than 4, The method further includes removing a water layer from the layers separated into the organic solvent layer and the water layer.

8. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 1, wherein: According to the following equation 3, the ratio of residual ethanol contained in the depolymerization reaction product adjusted to a pH less than 4 is less than 1%: [Equation 3] Residual ethanol ratio (%) = {( 1 Ethanol peak area in H NMR) / ( 1 Ethanol peak area in H NMR + 1 Peak area of ​​aromatic diol compounds in H NMR)}×100.

9. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 1, wherein: The depolymerization reaction of the polycarbonate-based resin is performed in the presence of a solvent including ethanol.

10. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 9, wherein: The amount of the ethanol is 1 mol to 5 mol relative to 1 mol of the polycarbonate resin.

11. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 9, wherein: The amount of the ethanol is 1 mol to 4 mol relative to 1 mol of the polycarbonate resin.

12. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 1, wherein: The depolymerization reaction of the polycarbonate-based resin is performed by reacting a base in an amount of 0.5 mol or less relative to 1 mol of the polycarbonate-based resin.

13. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 1, wherein: Adding a crystallization solvent to the depolymerization reaction product adjusted to a pH of less than 4 and recovering the formed aromatic diol compound crystals comprises: To the depolymerization reaction product adjusted to a pH of less than 4, 100 moles or less of water relative to 1 mole of the polycarbonate-based resin is added, and aromatic diol compound crystals are precipitated.

14. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 1, wherein: After adding a crystallization solvent to the depolymerization reaction product having a pH adjusted to less than 4 and recovering the formed aromatic diol compound crystals, the method further comprises, Purification of aromatic diol compounds.

15. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 14, wherein: The purified aromatic diol compound comprises: The aromatic diol compound is washed.

16. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 14, wherein: The purified aromatic diol compound comprises: The aromatic diol compound is dissolved and purified by adsorption.

17. The method for preparing a monomer composition for synthesizing recycled plastics according to claim 16, wherein: After the dissolution step and the adsorption purification step of the aromatic diol compound, The method further includes recrystallizing the aromatic diol compound.

18. A monomer composition for synthesizing recycled plastics, comprising an aromatic diol compound obtained by the method for preparing a monomer composition for synthesizing recycled plastics according to claim 1.

19. A recycled plastic, comprising a reaction product of the monomer composition for synthesizing recycled plastic according to claim 18 and a comonomer.

20. A molded article comprising the recycled plastic according to claim 19.

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