Method for decomposing polycarbonate

Abstract

The present invention provides a method for decomposing polycarbonate.SOLUTION: The method for decomposing polycarbonate includes providing a depolymerization solvent, adding a metal hydroxide to the depolymerization solvent to prepare a mixed solution, adding a polycarbonate material to the mixed solution to prepare a reaction solution and subjecting the polycarbonate material to a depolymerization reaction to form a bisphenol-A product, passing the reaction solution containing the bisphenol-A product through an ion exchange column filled with an ion exchange resin for ion exchange to obtain a purified liquid while metal ions are dissociated from the metal hydroxide, and subjecting the purified liquid to crystal purification to precipitate solid crystals of bisphenol-A from the purified liquid.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a chemical process, in particular to a process for decomposing polycarbonate. [Background technology]

[0002] In existing technologies, most patents proposed by companies such as LG (Patent Document 1) and Sabic (Patent Document 2) use low-carbon alcohols (e.g., methanol or ethanol) and cosolvents (e.g., toluene or dichloromethane) to depolymerize polycarbonate. The use of cosolvents improves depolymerization efficiency. However, the cosolvents used in existing technologies remain in the resulting bisphenol A (BPA) and can affect subsequent reactions, making subsequent processing difficult due to their toxic nature. Furthermore, with existing technologies, the alkali metal catalyst or metal ions remaining in the resulting BPA after the depolymerization process can easily cause product quality issues (e.g., poor color) in subsequent applications.

[0003] Therefore, the inventors believed that the above-mentioned defects could be improved, and after diligent research and application of scientific principles, they discovered an invention that is rationally designed and can effectively improve the above-mentioned drawbacks. Summary of the Invention [Problem to be solved by the invention]

[0004] The technical problem to be solved by the present invention is to provide a method for decomposing polycarbonate, which addresses the shortcomings of the prior art. [Means for solving the problem]

[0005] To solve the above technical problems, one technical solution adopted by the present invention is a method for decomposing polycarbonate, which includes a preparation step of providing a depolymerization solvent that is a monohydric alcohol solvent, an addition step of adding a metal hydroxide to the depolymerization solvent to prepare a mixed solution, a depolymerization step of adding a polycarbonate material (PC) to the mixed solution to prepare a reaction solution and depolymerizing the polycarbonate material to form a bisphenol A product, an ion exchange step of passing the reaction solution containing the bisphenol A product through an ion exchange column filled with an ion exchange resin to obtain a purified liquid and dissociate metal ions from the metal hydroxide, and a crystal purification step of performing crystal purification on the purified liquid to precipitate solid crystals of bisphenol A from the purified liquid.

[0006] Preferably, the monohydric alcohol solvent is phenol, the concentration of the metal hydroxide added is 100 ppm or more, and the depolymerization step further includes preparing the reaction solution by adding a predetermined amount of water to the mixed solution so as to control the water content of the reaction solution to 10 wt % or less.

[0007] Preferably, the preparation step further includes heating the depolymerization solvent to a first heating temperature of 50°C to 100°C, and the depolymerization step further includes heating the reaction solution to a second heating temperature of 110°C to 150°C.

[0008] Preferably, the depolymerization step further includes adding water to the reaction liquid when the water content in the reaction liquid is reduced to less than 0.5 wt % so as to control the water content in the reaction liquid to 0.5 wt % to 10 wt %.

[0009] Preferably, the ion exchange resin is a strongly acidic cation exchange resin.

[0010] Preferably, the ion exchange resin is a strongly acidic cation exchange resin containing sulfonic acid groups, and the total exchange capacity of the ion exchange resin is 1.0 eq / Lt (equivalents / liter) or more.

[0011] Preferably, in the ion exchange process, when the content of the metal element in the reaction liquid is the content of a first metal element and the content of the metal element in the purified liquid is the content of a second metal element, the content of the second metal element is 1 / 10 or less of the content of the first metal element.

[0012] Preferably, the content of the second metal element is 10 ppm or less.

[0013] Preferably, in the ion exchange step, the reaction solution containing the bisphenol A product, the depolymerization solvent, and the metal ions is pre-cooled to an ion exchange temperature of 65°C to 105°C before being introduced into the ion exchange column filled with the ion exchange resin.

[0014] Preferably, the ratio of the amount of the ion exchange resin used in the ion exchange column to the amount of the reaction liquid is 2,000 g to 3,400 g of the reaction liquid per 100 g of the ion exchange resin.

[0015] Preferably, the reaction liquid is passed through the ion exchange resin in an ion exchange column so as to be circulated, and the circulation flow rate of the reaction liquid is 30 g to 200 g per minute.

[0016] Preferably, the ion exchange step involves analyzing the purified liquid using an inductively coupled plasma optical emission spectrometer (ICP-OES, also called an ICP optical emission spectrometer) and controlling the content of metal elements in the purified liquid to 10 ppm or less.

[0017] Preferably, the crystal purification step includes cooling the purified liquid from an ion exchange temperature of 65°C to 105°C to a first crystallization temperature of 40°C to 60°C, maintaining the first reduced pressure at 50 Torr to 150 Torr to precipitate solid crystals of bisphenol A, separating the solid crystals of bisphenol A by filtration, and washing the solid crystals of bisphenol A with a washing solvent.

[0018] Preferably, the crystal purification step further comprises heating the bisphenol A solid crystals mixed with the wash solvent to a third heating temperature of 105°C to 115°C, then cooling to a second crystallization temperature of 40°C to 60°C, maintaining the reduced second reduced pressure at 50 Torr to 150 Torr, separating the bisphenol A solid crystals from the wash solvent by filtration, subjecting the bisphenol A solid crystals to a second wash with fresh wash solvent, and finally dephenolizing the bisphenol A crystals under reduced pressure to obtain a bisphenol A crystal product. [Effects of the Invention]

[0019] As an advantageous effect of an embodiment of the present invention, the polycarbonate decomposition method according to the present invention comprises the technical feature that "it comprises: a preparation step, which comprises providing a depolymerization solvent that is a monohydric alcohol-based solvent; an addition step, which comprises adding a metal hydroxide to the depolymerization solvent to prepare a mixed liquid; a depolymerization step, which comprises adding a polycarbonate material (PC) to the mixed liquid to prepare a reaction liquid and subjecting the polycarbonate material to a depolymerization reaction to form a bisphenol A product; an ion exchange step, which comprises passing the reaction liquid containing the bisphenol A product through an ion exchange column filled with an ion exchange resin to perform ion exchange, thereby obtaining a purified liquid and dissociating metal ions from the metal hydroxide; and a crystal purification step, which comprises subjecting the purified liquid to crystal purification to precipitate solid crystals of bisphenol A from the purified liquid." This allows the chemical reaction to proceed in a manner favorable to the depolymerization reaction, thereby improving the conversion rate of polycarbonate depolymerization and increasing the yield of the product bisphenol A (BPA). The method for decomposing polycarbonate of the present invention can efficiently avoid the problem of co-solvents (e.g., toluene and dichloromethane) remaining in the product (e.g., BPA) after using them in existing techniques.

[0020] Furthermore, in the polycarbonate decomposition method according to an embodiment of the present invention, the amount of metal ions remaining in the solid crystalline product of bisphenol A (e.g., the content of sodium element) is reduced by purifying the bisphenol A product through an ion exchange process and a crystal purification process. DETAILED DESCRIPTION OF THE INVENTION

[0021] In order to better understand the features and technical contents of the present invention, please refer to the following detailed description of the present invention, however, the detailed description provided is only for reference and explanation, and does not limit the scope of the claims of the present invention.

[0022] The following describes the implementation of the present invention through certain specific embodiments, and those skilled in the art can understand the advantages and effects of the present invention based on the contents disclosed in this specification. The present invention can be implemented or applied through other different specific embodiments, and various modifications and changes can be made to each detail in this specification based on different perspectives and applications, without departing from the concept of the present invention.

[0023] [How to disassemble polycarbonate] An embodiment of the present invention provides a method for decomposing polycarbonate (PC), and in particular, a method for decomposing polycarbonate using transesterification technology. The polycarbonate decomposition method according to an embodiment of the present invention efficiently avoids the problem of cosolvents (e.g., toluene or dichloromethane) remaining in the product (e.g., BPA) after using them in existing technologies. Furthermore, the ion exchange and crystal purification processes can reduce the amount of residual metal ions (e.g., sodium ions or potassium ions) in the product, thereby improving product quality (e.g., efficiently improving the color of the resin produced after repolymerization of the product BPA).

[0024] Specifically, the method for decomposing polycarbonate (PC) according to an embodiment of the present invention includes steps S110, S120, S130, S140, and S150. It should be noted that the order of the steps and the actual operation method in this embodiment can be adjusted as needed and are not limited to this embodiment.

[0025] In step S110, a preparation step is carried out. The preparation step includes providing a depolymerizing solvent, which is a monohydric alcohol-based solvent, and adding the solvent to a reaction tank. In the preparation step of this embodiment, a monohydric alcohol-based solvent is used as the single depolymerizing solvent.

[0026] In one embodiment of the present invention, the monohydric alcohol solvent may be at least one selected from the group consisting of methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, pentanol, hexanol, octyl alcohol, isooctyl alcohol, nonanol, and phenol (preferably a monoalcohol having a boiling point of 100°C or higher). For example, in this embodiment, the depolymerization solvent is phenol. That is, in the preparation process of this embodiment, phenol is used as the sole depolymerization solvent.

[0027] In this embodiment, the preparation step further includes heating the depolymerization solvent to a first heating temperature. Here, the first heating temperature is 50°C to 100°C, preferably 70°C to 90°C, and particularly preferably 75°C to 85°C. For example, the first heating temperature may be 80°C (when phenol is used as the depolymerization solvent), but the present invention is not limited thereto. The first heating temperature can be appropriately adjusted based on the liquid temperature range of the depolymerization solvent.

[0028] In step S120, an adding step is carried out, which includes adding metal hydroxide to the depolymerization solvent in the reaction tank to prepare a mixed solution.

[0029] In one embodiment of the present invention, the metal hydroxide may be at least one selected from the group consisting of alkali metal (Group 1 element) hydroxide, alkaline earth metal (Group 2 element) hydroxide, and transition metal hydroxide. For example, the alkali metal (Group 1 element) hydroxide may be, for example, sodium hydroxide (NaOH) or potassium hydroxide (KOH). The alkaline earth metal (Group 2 element) hydroxide may be, for example, magnesium hydroxide (Mg(OH)2) or calcium hydroxide (Ca(OH)2). The transition metal hydroxide may be, for example, manganese hydroxide (Mn(OH)2), but the present invention is not limited thereto. In this embodiment, the metal hydroxide is preferably at least one selected from the group consisting of sodium hydroxide (NaOH) and potassium hydroxide (KOH).

[0030] The concentration of the metal hydroxide added to the mixed solution is 100 ppm (parts per million) or more, preferably 100 ppm to 200,000 ppm, and particularly preferably 500 ppm to 10,000 ppm. Specifically, it may be 829 ppm to 8,719 ppm, but the present invention is not limited thereto.

[0031] More specifically, in the adding step according to an embodiment of the present invention, the aqueous solution containing the metal hydroxide is added to the depolymerization solvent, thereby mixing with the depolymerization solvent to prepare the mixed solution containing the depolymerization solvent, the metal hydroxide, and water. Here, the metal hydroxide converts into hydroxide ions (OH - ) and metal cation Mn + (e.g. Na + YaK + ), which then catalyzes the depolymerization of the added polycarbonate (PC).

[0032] Here, the weight percent concentration of the metal hydroxide in the aqueous solution is 15 wt% to 60 wt%, preferably 20 wt% to 45 wt%, and particularly preferably 25 wt% to 40 wt%. For example, in one embodiment of the present invention, the metal hydroxide added to the aqueous solution is sodium hydroxide (NaOH), and its weight percent concentration is 32 wt%, but the present invention is not limited to this.

[0033] The amount of the aqueous solution (containing a metal hydroxide) added to the depolymerization solvent is approximately 1 / 400 to 1 / 20 of the depolymerization solvent, and the concentration of the metal hydroxide in the depolymerization solvent can be controlled within the range (e.g., 100 ppm or more) by adjusting the amount of the aqueous solution added, but the present invention is not limited thereto. In the adding step according to an embodiment of the present invention, the metal hydroxide powder may be directly added to the depolymerization solvent, and then an appropriate amount of water may be separately added to prepare a mixed solution containing a specific concentration of metal hydroxide.

[0034] In step S130, a depolymerization step is carried out. The depolymerization step includes adding a polycarbonate material to a mixture in a reaction tank and selectively adding a predetermined amount of water to the mixture to prepare a reaction solution. Thus, the reaction solution contains water, a depolymerization solvent, a metal hydroxide, and a polycarbonate material.

[0035] In this embodiment, the water content in the reaction solution is controlled to 10 wt % or less. The polycarbonate material is polycarbonate (PC) particles to be depolymerized, and the polycarbonate to be depolymerized can be obtained by crushing recycled polycarbonate scraps, but the present invention is not limited thereto.

[0036] The depolymerization step further includes heating the reaction solution to a second heating temperature to cause a depolymerization reaction in the polycarbonate material to produce bisphenol A (BPA) and carbon dioxide (CO2) gas.

[0037] More specifically, in the depolymerization step, the reaction solution is heated from a first heating temperature (for example, 50°C to 100°C) to a second heating temperature, and the second heating temperature is 110°C to 150°C, preferably 110°C to 140°C, and particularly preferably 110°C to 135°C. For example, the second heating temperature may be 120°C, but the present invention is not limited thereto.

[0038] In the depolymerization step, after the reaction solution is heated to the second heating temperature, the reaction solution is continuously stirred for 1 to 20 hours. The stirring time is preferably 2 to 16 hours, and particularly preferably 3 to 10 hours, so that the depolymerization reaction can proceed sufficiently.

[0039] It is noteworthy that in the depolymerization step of the present embodiment, during the depolymerization reaction, the water content in the reaction liquid is controlled to 0.5 wt % to 10 wt %, preferably 1 wt % to 10 wt %, and particularly preferably 1.4 wt % to 9 wt %.

[0040] Thus, the polycarbonate material can achieve a high depolymerization conversion rate under the conditions of the metal hydroxide addition concentration (100 ppm or more) and water content (0.5 to 10 wt%).

[0041] In one embodiment of the present invention, the initial weight ratio of the depolymerization solvent to the polycarbonate material in the reaction solution (i.e., the ratio of the initial weight of the depolymerization solvent / the initial weight of the polycarbonate material) is 0.5 to 14, preferably 0.8 to 5.0, particularly preferably 1 to 3.5, and more particularly preferably 1.5 to 3.5.

[0042] Furthermore, in the depolymerization step (step S130), the water content in the reaction liquid is controlled to 0.5 wt% to 10 wt%, and as a method for controlling the water content, for example, a small amount of reaction liquid (for example, 1 ml to 10 ml of reaction liquid) is taken out from the reaction tank, and the water content in the reaction liquid is detected with a water detector.

[0043] It is worth noting that the depolymerization reaction may reduce the water content of the reaction solution (because water is consumed in the depolymerization reaction). In an embodiment of the present invention, the water content may be monitored during the depolymerization reaction. When the water content of the reaction solution decreases to less than 0.5 wt%, the depolymerization step may further include adding water to the reaction solution to control the water content of the reaction solution to 0.5 wt% to 10 wt%. In this way, this operating method helps to further depolymerize the polycarbonate material without decomposing the product bisphenol A (BPA), thereby improving the yield of the product bisphenol A.

[0044] It is further noted that in the depolymerization reaction, the intermediate product obtained by depolymerizing the polycarbonate material is diphenyl carbonate (DC), which is further depolymerized into phenol (R—OH) and carbon dioxide (CO2) in the depolymerization reaction.

[0045] Generally speaking, the depolymerization reaction proceeds in the following order of Chemical Reaction Mechanism 1 and Chemical Reaction Mechanism 2.

[0046] Chemical Reaction Mechanism 1: Polycarbonate (PC) material is first depolymerized to form the product bisphenol A (BPA) and the intermediate diphenyl carbonate (DC) in the presence of a depolymerization solvent (i.e., phenol, R-OH, where R is a phenyl group), a metal hydroxide (catalyst, M-OH, where M is a metal), and water (HO).

[0047] [Chemical reaction mechanism 1: PC → BPA + DC] [ka]

[0048] Chemical Reaction Mechanism 2: The intermediate diphenyl carbonate (DC) is further depolymerized in the depolymerization reaction (in the presence of water) to form phenol (R-OH) and carbon dioxide (CO2) gas.

[0049] [Chemical reaction mechanism 2: DC → R-OH + CO 2(g) ] [ka]

[0050] Here, the decomposition degree of bisphenol A (BPA), the product produced in chemical reaction mechanism 1, is efficiently reduced under the above conditions (water content is controlled to 0.5 wt% to 10 wt%) (only the intermediate diphenyl carbonate DC is decomposed), so the bisphenol A product is retained and the yield is improved.

[0051] Thus, the polycarbonate material ultimately forms the product bisphenol A (BPA) and the by-product phenol (R-OH) at the above water content and metal oxide concentration, and also produces carbon dioxide (CO2) gas. Here, the conversion rate of depolymerization of polycarbonate (PC) is 90% to 99% (i.e., the conversion rate at which PC is decomposed from a high molecular weight compound to a low molecular weight compound, and 90% to 99% of PC is depolymerized).

[0052] It is noteworthy that the intermediate (diphenyl carbonate) produced from polycarbonate (PC) can be decomposed into phenol (R-OH) during the reaction. Phenol (R-OH) is the same compound as the single depolymerization solvent (e.g., phenol) employed in embodiments of the present invention. Therefore, during the depolymerization reaction, the weight ratio of the depolymerization solvent (phenol) to polycarbonate (PC) (i.e., the phenol / polycarbonate weight ratio) continues to increase, which is beneficial for the progress of the depolymerization reaction and for recycling the depolymerization solvent. Furthermore, since the weight ratio of the depolymerization solvent / polycarbonate continues to increase during the depolymerization reaction, the initial weight of the depolymerization solvent can be reduced.

[0053] It is also worth noting that the inventors of the present application have found through experiments that a combination of a metal hydroxide (e.g., NaOH or KOH) content of 100 ppm or more and a water content of 0.5 to 10 wt% is advantageous for the rapid depolymerization of polycarbonate (PC) to form bisphenol A (BPA) and carbon dioxide. In this way, it is possible to achieve the depolymerization reaction of polycarbonate using a single depolymerization solvent. If the water content is less than 0.5 wt%, the concentration of the metal hydroxide will be excessive (this is because the metal hydroxide will dissociate in water and form an anion, hydroxide ion OH). - and metal cation M n+ The undesirable reaction of the product, bisphenol A (BPA), decomposing in a strongly alkaline environment can occur. On the other hand, if the water content exceeds 10 wt%, the concentration of metal hydroxides becomes too low, reducing the overall depolymerization efficiency.

[0054] Step S140 is to carry out an ion exchange step. The ion exchange step involves passing the reaction solution (i.e., crude reaction solution) containing the bisphenol A (BPA) product formed after the depolymerization reaction, the depolymerization solvent (e.g., phenol), and metal ions through an ion exchange column filled with an ion exchange resin, and removing the metal ions from the reaction solution by the ion exchange resin. Here, the metal ions are metal cations dissociated from metal hydroxides, such as sodium ions (Na + ) are mentioned.

[0055] In this embodiment, the ion exchange resin is a cation exchange resin, preferably a strongly acidic cation exchange resin, and particularly preferably a strongly acidic cation exchange resin containing sulfonic acid groups. In the ion exchange step, ion exchange and adsorption of metal ions in the reaction solution are performed to remove sodium ions or potassium ions in the crude reaction solution. The crude reaction solution is passed through the ion exchange resin to undergo an acid-alkali neutralization reaction between the sodium ions or potassium ions in the reaction solution and the strongly acidic cation exchange resin, thereby producing salt and water. As the salt is adsorbed onto the ion exchange resin, water, bisphenol A products, and the depolymerization solvent (phenol) pass through the ion exchange resin and are discharged from the ion exchange column, yielding a purified liquid.

[0056] Here, when the content of a metal element (e.g., sodium content) in the reaction liquid is defined as the content of a first metal element, and the content of a metal element (e.g., sodium content) in the purified liquid is defined as the content of a second metal element, the content of the second metal element is 1 / 10 or less of the content of the first metal element. More specifically, the content of the first metal element is 100 ppm or more (corresponding to the concentration of the metal hydroxide added), and the content of the second metal element is 10 ppm or less, preferably 5 ppm or less, and particularly preferably 2 ppm or less.

[0057] That is, the content of metal elements in the purified liquid formed in the ion exchange process is effectively reduced, which is advantageous for reducing the content of metal elements in the bisphenol A crystals obtained in the subsequent crystallization, thereby benefiting subsequent applications. For example, the color of the polymer obtained by repolymerizing bisphenol A can be effectively improved.

[0058] It is worth noting that in the ion exchange step, before the reaction solution (i.e., crude reaction solution) containing the bisphenol A (BPA) product, the depolymerization solvent (e.g., phenol), and metal ions is introduced into the ion exchange column filled with ion exchange resin, the reaction solution must be cooled to an ion exchange temperature of 65°C to 105°C in advance. Preferably, the ion exchange temperature is 80°C to 105°C.

[0059] In other words, the reaction liquid is cooled from the second heating temperature of 110°C to 140°C to an ion exchange temperature of 65°C to 105°C.

[0060] Thereby, the reaction solution is put into an ion exchange column filled with the ion exchange resin to perform ion exchange at an ion exchange temperature of 65° C. to 105° C. Within this temperature range, the ion exchange resin has an excellent adsorption effect on metal ions (sodium ions or potassium ions).

[0061] If the cooling temperature is not within the above range, adverse effects may occur. For example, if the cooling temperature is too low (e.g., below 60°C), the bisphenol A (BPA) product and the depolymerization solvent (e.g., phenol) may crystallize, causing deterioration in the fluidity of the liquid, which is unfavorable for the progress of the ion exchange and adsorption operations. The crystals also make it difficult for the bisphenol A product and the depolymerization solvent (e.g., phenol) to pass through the ion exchange resin.

[0062] On the other hand, if the cooled temperature is too high (for example, higher than 110°C), the heat resistance temperature of the ion exchange resin may be exceeded, causing the ion exchange resin to decompose, and undesired substances may precipitate from the ion exchange resin into the reaction liquid, affecting the effectiveness of the ion exchange.

[0063] To improve the efficiency of the ion exchange step, in an embodiment of the present invention, the ratio of the amount of ion exchange resin used in the ion exchange column to the reaction solution (i.e., crude reaction solution) is preferably 2,000 to 3,400 g of reaction solution per 100 g of ion exchange resin. Furthermore, the ratio may be 4,000 to 6,800 g of reaction solution per 200 g of ion exchange resin.

[0064] The reaction liquid is passed through the ion exchange resin in an ion exchange column so as to circulate. The circulation flow rate of the reaction liquid is 30 g to 200 g per minute, preferably 65 g to 135 g per minute. The circulation time of the reaction liquid is 1 hour to 6 hours, preferably 3 hours to 5 hours, so that the ion exchange resin performs ion exchange (acid-alkali neutralization) and adsorbs salts (e.g., NaOH) on the reaction liquid, and finally passes through the ion exchange column to discharge the purified liquid. During the ion exchange process, the operating temperature of the ion exchange may be controlled, for example, to the ion exchange temperature (e.g., 65°C to 105°C, preferably 80°C to 105°C).

[0065] In one embodiment, the ion exchange step achieves the purpose of the ion exchange by, for example, analyzing the purified liquid with an inductively coupled plasma optical emission spectrometer (ICP-OES, also called an ICP optical emission spectrometer) and controlling the content of the metal element (i.e., the content of the second metal element, for example, the sodium content) in the purified liquid to 10 ppm or less (preferably 5 ppm or less, particularly preferably 2 ppm or less).

[0066] The inductively coupled plasma optical emission spectroscopy (ICP-OES) uses high-temperature plasma to excite metal atoms (e.g., sodium atoms) in a sample, which then emit light of a specific wavelength when they return to their ground state. By measuring the light intensity of these specific wavelengths, the content of metal atoms in a sample can be quantitatively analyzed. For example, the specific wavelength of sodium atoms is approximately 589 nm. For the analytical method, 1 g of the purified liquid is sampled and microwave-digested using nitric acid to a constant volume, after which the sodium content can be analyzed using ICP-OES.

[0067] In order to improve the efficiency of ion exchange and adsorption, in one embodiment of the present invention, the total exchange capacity of the ion exchange resin is, for example, 1.0 equivalents / liter (eq / L) or more, preferably 1.5 equivalents / liter or more, and particularly preferably 1.7 equivalents / liter or more, where the total exchange capacity is the amount of ions that can be exchanged per unit volume of the ion exchange resin.

[0068] Step S150 is a crystal purification step, which includes performing crystal purification on the purified liquid formed in the ion exchange step to precipitate solid crystals of bisphenol A (BPA) from the purified liquid.

[0069] More specifically, the crystal purification step involves cooling the purified liquid from the ion exchange temperature (e.g., 65°C to 105°C, preferably 80°C to 105°C) to a first crystallization temperature, thereby precipitating solid crystals of bisphenol A (BPA). The first crystallization temperature is 40°C to 60°C, preferably 45°C to 55°C. Here, the crystal purification step involves cooling the purified liquid to the first crystallization temperature under reduced pressure, and controlling the first reduced pressure to 10 Torr (corresponding to mmHg) to 150 Torr, preferably 30 Torr to 120 Torr.

[0070] Then, in the crystal purification step, the solid crystals of bisphenol A (BPA) are separated from the purified liquid by filtration, and the solid crystals of bisphenol A are washed with a washing solvent (e.g., phenol). Here, the amount of the washing solvent in the first wash is 8% to 15% (by weight), preferably 8% to 12%, of the solid crystals of bisphenol A. This can further reduce the concentration of metal ions and metal hydroxides remaining in the solid crystals of bisphenol A.

[0071] Thereafter, in the crystal purification step, the solid crystals of bisphenol A washed and mixed with the washing solvent are heated to a third heating temperature (e.g., 105°C to 115°C) and then cooled to a second crystallization temperature. The second crystallization temperature is 40°C to 60°C, preferably 45°C to 55°C. In the crystal purification step, the solid crystals of bisphenol A are cooled to the second crystallization temperature under reduced pressure, and the second reduced pressure is maintained at 10 Torr (corresponding to mmHg) to 150 Torr, preferably 30 Torr to 120 Torr.

[0072] The crystal purification step then further involves separating the bisphenol A (BPA) solid crystals from the washing solvent by filtration, and then subjecting the bisphenol A solid crystals to a secondary wash with a new washing solvent (e.g., phenol). The amount of the second wash solvent is 4 wt% to 8 wt%, preferably 5 wt% to 7 wt%, of the bisphenol A solid crystals. This can further reduce the concentration of metal ions (e.g., sodium content) remaining in the bisphenol A solid crystals.

[0073] Finally, in the crystal purification step, the solid bisphenol A crystals obtained by the above-mentioned crystallization and filtration / washing steps twice are subjected to dephenolization under reduced pressure to obtain a bisphenol A crystal product with a purity of 99.5% or more (preferably 99.8% or more). However, the above-mentioned crystal purification step is merely an explanation of one embodiment of the present invention, and the present invention is not limited to this embodiment.

[0074] To solve the technical problems of existing technologies as described above, the present invention provides a method for decomposing polycarbonate. The method for decomposing polycarbonate uses a monohydric alcohol solvent (e.g., phenol) as a single depolymerization solvent, and the polycarbonate material is decomposed under conditions where a metal hydroxide (e.g., NaOH or KOH) is present at 100 ppm or more (preferably 100 to 200,000 ppm, and particularly preferably 500 to 10,000 ppm). Furthermore, during the depolymerization reaction, the water content of the reaction solution is controlled to 0.5 wt% to 10 wt%. This allows the reaction intermediate (diphenyl carbonate) to be further depolymerized to form phenol (the same as the depolymerization solvent) and carbon dioxide (which can be discharged as exhaust gas from the system or removed by carbon dioxide adsorption), allowing the system to proceed in a manner favorable to the depolymerization reaction and increasing the polycarbonate depolymerization conversion rate to 90% to 99%.

[0075] The method for decomposing polycarbonate of the present invention can efficiently avoid the problem of the co-solvent (e.g., toluene, dichloromethane, etc.) remaining in the product (e.g., BPA) after using the co-solvent in the existing technology.

[0076] Furthermore, in the polycarbonate decomposition method according to the embodiment of the present invention, the amount of residual metal ions in the bisphenol A product is reduced by purifying the bisphenol A product through an ion exchange process and a crystal purification process.

[0077] This makes the purified bisphenol A product advantageous for subsequent applications, for example, it can efficiently improve the color of polymers obtained by repolymerizing bisphenol A.

[0078] [Experimental data and measurement results] Hereinafter, the present invention will be described in detail with reference to <Example 1-1> to <Example 1-3> and <Comparative Example 1-1>, and the requirements for the metal hydroxide concentration and water content in the reaction solution of the present invention, and the technical effect of improving the depolymerization conversion rate were verified. However, these examples are provided for the purpose of making the present invention easier to understand, and the present invention is not limited to these examples.

[0079] <Example 1-1> 500 g of depolymerization solvent (phenol) was added to a reaction tank and heated to 80°C (i.e., the first heating temperature). 4 g of a metal hydroxide aqueous solution (32% NaOH) was added to the reaction tank and mixed with the depolymerization solvent to prepare a mixed solution. 200 g of polycarbonate (PC) particles and 35 g of water were added to the mixed solution in the reaction tank to prepare a reaction solution. The reaction solution was heated to 120°C (i.e., the second heating temperature) and stirred continuously for 5 hours to depolymerize the polycarbonate (PC) and produce bisphenol A (BPA), phenol, and carbon dioxide. The concentration of metal hydroxide (NaOH) in the reaction solution was 1,732 ppm, and the water content in the reaction solution was controlled at 4.7 wt%.

[0080] <Example 1-2> and <Example 1-3> are generally the same as <Example 1-1>, and the main difference between them is the control of the concentration and water content of the metal hydroxide.

[0081] The main difference between <Comparative Example 1-1> and the above <Example 1-1> to <Example 1-3> is that in <Comparative Example 1-1>, water was not added separately to the reaction solution, and the water content in the reaction solution was 0.18 wt% (not controlled), which was lower than 4.7 wt% in <Example 1-1> and lower than 0.5 wt% required by the present invention.

[0082] Next, measurements were performed on <Example 1-1> to <Example 1-3> and <Comparative Example 1-1> to obtain the conversion rate of depolymerization of polycarbonate (PC) and the yield (%) of bisphenol A (BPA) product. In the measurement results section of Table 1, the conversion rate of depolymerization of polycarbonate (PC) was measured by the method described below. First, 10 g of the crude reaction solution after the depolymerization reaction (i.e., the reaction solution after the depolymerization reaction) was taken out. 50 g of methanol was added to this 10 g of crude reaction solution, and the mixture was filtered. After filtration, the cake filter was washed with 50 g of methanol. The cake filter was then dried, and the weight (S) of the resulting solid was recorded.

[0083] Depolymerization conversion rate (%) = (100 - S / (10 × (PC / total weight of reaction solution))

[0084] In the above formula, S is the weight of the solid obtained after drying, 10 g is the weight of the crude reaction solution initially taken out, PC is the initial weight of the polycarbonate (PC) particles, and the total weight of the reaction solution is the total weight of the reaction solution.

[0085] The yield (%) of the bisphenol A (BPA) product was measured as follows: First, the crude reaction solution after the depolymerization reaction was collected and analyzed by high performance liquid chromatography (HPLC) to determine the concentration A of bisphenol A (BPA). Then, the yield (%) of the product bisphenol A (BPA) was calculated using the following formula:

[0086] Bisphenol A (BPA) yield (%) = A / (PC / total weight of reaction solution) x 100 In this formula, A is the concentration of bisphenol A (BPA), PC is the initial weight of polycarbonate (PC) particles, and total reaction weight is the total weight of the reaction solution.

[0087] Manufacturing conditions and measurement results [Table 1]

[0088] As is evident from the experimental results in Table 1, the production conditions for <Example 1-1> to <Example 1-3> satisfied the metal hydroxide concentration and water content requirements for the reaction solution of the present invention. The depolymerization conversion rate of polycarbonate (PC) was 90% or higher, and the yield of bisphenol A (BPA) product was 60% or higher. In <Comparative Example 1-1>, water was not added separately to the reaction solution, and the water content of the reaction solution was 0.18 wt% (uncontrolled), lower than the 4.7 wt% in <Example 1-1> and the 0.5 wt% required by the present invention. The depolymerization conversion rate of polycarbonate (PC) in <Comparative Example 1-1> was 53%, significantly lower than the results of <Example 1-1> to <Example 1-3>. The yield of bisphenol A (BPA) product in <Comparative Example 1-1> was 38%, significantly lower than the results of <Example 1-1> to <Example 1-3>.

[0089] Hereinafter, the effects of the additional ion exchange process and crystal purification process of the present invention on the quality of the solid crystal product of bisphenol A (BPA) will be verified through <Example 2-1> and <Comparative Example 2-1>.

[0090] <Example 2-1>: 4,000 g of depolymerization solvent (phenol) was added to a reaction tank and heated to 80°C (i.e., the first heating temperature). 32 g of a metal hydroxide aqueous solution (32% NaOH) was added to the reaction tank and mixed with the depolymerization solvent to prepare a mixed solution. 1,600 g of polycarbonate (PC) particles and 280 g of water were added to the mixed solution in the reaction tank to prepare a crude reaction solution. The crude reaction solution was heated to 120°C (i.e., the second heating temperature) and stirred continuously for 5 hours to depolymerize the polycarbonate to form bisphenol A (BPA), phenol, and carbon dioxide. The metal hydroxide (NaOH) concentration in the crude reaction solution was 1,732 ppm (i.e., the sodium content), and the water content in the crude reaction solution was controlled at 4.7 wt%.

[0091] The crude reaction solution was then cooled to 100°C (i.e., the ion exchange temperature) and passed through a column containing a strongly acidic cation exchange resin containing sulfonic acid groups to carry out an ion exchange process. In the ion exchange process, approximately 5,600 g of the crude reaction solution (containing 4,000 g of depolymerization solvent and 1,600 g of polycarbonate depolymerized product, with water consumed in the depolymerization reaction) was circulated through a column packed with 200 g of strongly acidic cation exchange resin containing sulfonic acid groups. After the ion exchange process was completed, the purified liquid was discharged. The liquid circulation flow rate in the ion exchange process was 100 g / min, and the adsorption was carried out for 4 hours, resulting in acid-alkali neutralization and adsorption of the sodium ions in the crude reaction solution, forming an ion-adsorbed reaction solution. The ion exchange temperature of the crude reaction solution was controlled at 100°C. After the ion exchange process, the sodium content was analyzed by ICP-OES and monitored to control the sodium content to less than 2 ppm, thereby confirming that the ion exchange process can be terminated. In Example 2-1, the sodium content under the above conditions was about 0.32 ppm.

[0092] After the ion resin was desorbed, the purified liquid was crystallized by reducing the pressure from 100°C to 50°C and cooling (maintaining the pressure at 100 Torr). The BPA solid crystals were filtered and washed with phenol, which accounted for 10 wt% of the solid crystals. The BPA solid crystals and phenol mixture were heated to 110°C, then reduced the pressure and cooled to 50°C for crystallization (maintaining the pressure at 100 Torr). The BPA solid crystals were filtered and then washed with phenol, which accounted for 6 wt% of the solid crystals, for dephenolization under reduced pressure. Finally, BPA solid crystals with a purity of 99.8% or more were obtained. However, the sodium content in the solid crystals could not be measured by ICP-OES (the test result was ND).

[0093] <Comparative Example 2-1> was generally similar to the depolymerization step in the first half of <Example 2-1> above, except that in <Comparative Example 2-1>, the depolymerization operation was carried out to the end, but the subsequent ion exchange step and crystal purification step were not carried out. That is, the sodium content in the crude reaction solution of <Comparative Example 2-1> was 1340 ppm, and the sodium content of the BPA product far exceeded the required 2 ppm.

[0094] Further analysis of the color of the BPA product obtained in <Example 2-1> and the BPA product obtained in <Comparative Example 2-1> revealed that the color of the resin in <Example 2-1> was 5alpha, and the color of the resin in <Comparative Example 2-1> was 232alpha. The color of the resin in <Comparative Example 2-1> was far inferior to that of <Example 2-1>.

[0095] Here, the color analysis method was to dissolve the BPA product in methanol, measure and analyze it using a colorimeter, and determine the alpha value of the hue.

[0096] According to the above experimental results, by further performing ion exchange and crystal purification on the BPA product of <Example 2-1> after the depolymerization reaction, the amount of residual sodium in the BPA product can be significantly reduced, and the color of BPA can be efficiently improved. Furthermore, when BPA is used for repolymerization into polymer resin (PC resin), an even better color can be obtained.

[0097] [Advantageous Effects of the Embodiments] As an advantageous effect of an embodiment of the present invention, the polycarbonate decomposition method according to the present invention comprises the technical feature that "it comprises: a preparation step, which comprises providing a depolymerization solvent that is a monohydric alcohol-based solvent; an addition step, which comprises adding a metal hydroxide to the depolymerization solvent to prepare a mixed liquid; a depolymerization step, which comprises adding a polycarbonate material (PC) to the mixed liquid to prepare a reaction liquid and subjecting the polycarbonate material to a depolymerization reaction to form a bisphenol A product; an ion exchange step, which comprises passing the reaction liquid containing the bisphenol A product through an ion exchange column filled with an ion exchange resin to perform ion exchange, thereby obtaining a purified liquid and dissociating metal ions from the metal hydroxide; and a crystal purification step, which comprises subjecting the purified liquid to crystal purification to precipitate solid crystals of bisphenol A from the purified liquid." This allows the chemical reaction to proceed in a manner favorable to the depolymerization reaction, thereby improving the conversion rate of polycarbonate depolymerization and increasing the yield of the product bisphenol A (BPA).

[0098] The method for decomposing polycarbonate of the present invention can efficiently avoid the problem of co-solvents (such as toluene and dichloromethane) remaining in the product (e.g., BPA) after using them in existing techniques.

[0099] Furthermore, the polycarbonate decomposition method according to the present invention purifies the bisphenol A product through the ion exchange and crystal purification processes, thereby reducing the amount of metal ions remaining in the solid crystalline product of bisphenol A, which is advantageous for subsequent applications. For example, the color of the polymer obtained by repolymerizing bisphenol A can be efficiently improved.

[0100] The above disclosure is merely a preferred embodiment of the present invention, and the scope of the claims of the present invention is not limited thereto. Therefore, any equivalent technical modifications made by utilizing the contents of the specification and drawings of the present invention are included in the scope of the claims of the present invention.

Claims

1. a preparing step, comprising providing a depolymerization solvent that is a monohydric alcohol-based solvent; an adding step including adding a metal hydroxide to the depolymerization solvent to prepare a mixed solution; a depolymerization step, which includes preparing a reaction solution by adding a polycarbonate material (PC) to the mixed solution, and performing a depolymerization reaction on the polycarbonate material to form a bisphenol A product; an ion exchange step, which includes passing the reaction liquid containing the bisphenol A product through an ion exchange column filled with an ion exchange resin to perform ion exchange, thereby obtaining a purified liquid and dissociating metal ions from the metal hydroxides; a crystal purification step in which the purified liquid is subjected to crystal purification to precipitate solid crystals of bisphenol A from the purified liquid.

2. 2. The method for decomposing polycarbonate according to claim 1, wherein the monohydric alcohol solvent is phenol, the concentration of the metal hydroxide added is 100 ppm or more, and the depolymerization step further comprises preparing the reaction solution by adding a predetermined amount of water to the mixed solution so as to control the water content of the reaction solution to 10 wt % or less.

3. the preparing step further includes heating the depolymerization solvent to a first heating temperature of 50°C to 100°C; 3. The method for decomposing polycarbonate according to claim 2, wherein the depolymerization step further comprises heating the reaction solution to a second heating temperature of 110°C to 150°C.

4. 4. The method for decomposing polycarbonate according to claim 3, wherein the depolymerization step further comprises adding water to the reaction solution when the water content in the reaction solution has decreased to less than 0.5 wt % so as to control the water content in the reaction solution to 0.5 wt % to 10 wt %.

5. 2. The method for decomposing polycarbonate according to claim 1, wherein the ion exchange resin is a strongly acidic cation exchange resin.

6. 2. The method for decomposing polycarbonate according to claim 1, wherein the ion exchange resin is a strongly acidic cation exchange resin containing sulfonic acid groups, and the total exchange capacity of the ion exchange resin is 1.0 eq / Lt (equivalents per liter) or more.

7. 2. The method for decomposing polycarbonate according to claim 1, wherein, in the ion exchange step, when the content of a metal element in the reaction liquid is the content of a first metal element and the content of a metal element in the purified liquid is the content of a second metal element, the content of the second metal element is 1 / 10 or less of the content of the first metal element.

8. 8. The method for decomposing polycarbonate according to claim 7, wherein the content of the second metal element is 10 ppm or less.

9. 2. The method for decomposing polycarbonate according to claim 1, wherein in the ion exchange step, the reaction solution containing the bisphenol A product, the depolymerization solvent, and the metal ions is pre-cooled to an ion exchange temperature of 65°C to 105°C before being introduced into the ion exchange column filled with the ion exchange resin.

10. 10. The method for decomposing polycarbonate according to claim 9, wherein the ratio of the amount of the ion exchange resin used in the ion exchange column to the amount of the reaction solution is 2,000 g to 3,400 g per 100 g of the ion exchange resin.

11. 10. The method for decomposing polycarbonate according to claim 9, wherein the reaction solution is passed through the ion exchange resin in the ion exchange column in a circulating manner, and the circulating flow rate of the reaction solution is 30 g to 200 g per minute.

12. 10. The method for decomposing polycarbonate according to claim 9, wherein the ion exchange step includes analyzing the purified liquid with an inductively coupled plasma optical emission spectroscopy (ICP-OES) and controlling the content of metal elements in the purified liquid to 10 ppm or less.

13. 2. The method for decomposing polycarbonate according to claim 1, wherein the crystal purification step comprises cooling the purified liquid from an ion exchange temperature of 65°C to 105°C to a first crystallization temperature of 40°C to 60°C, maintaining the first reduced pressure at 50 Torr to 150 Torr to precipitate solid crystals of bisphenol A, and then separating the solid crystals of bisphenol A by filtration and washing the solid crystals of bisphenol A with a washing solvent.

14. 14. The method for decomposing polycarbonate according to claim 13, wherein the crystal purification step further comprises heating the solid bisphenol A crystals mixed with the washing solvent to a third heating temperature of 105°C to 115°C, then cooling to a second crystallization temperature of 40°C to 60°C, maintaining the reduced second reduced pressure at 50 Torr to 150 Torr, separating the solid bisphenol A crystals from the washing solvent by filtration, subjecting the solid bisphenol A crystals to a second washing with fresh washing solvent, and finally dephenolizing the solid bisphenol A crystals under reduced pressure to obtain a bisphenol A crystal product.

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