Molded body for depolymerizing polystyrene by electric heating and method for depolymerizing polystyrene using same

Abstract

The present invention relates to a method for manufacturing a molded body depolymerized by heat generated during current flow and, more specifically, to a method for manufacturing a molded body by pelletizing a mixture, in which a conductive resistive heating member is dispersed in a polystyrene solution so as to allow polystyrene to be depolymerized by heat generated during current flow, and to a method for depolymerizing a molded body manufactured by the manufacturing method via electrical heating.

Description

Molded body for depolymerizing polystyrene by electric heating method and method for depolymerizing polystyrene using the same

[0001] The present invention relates to a molded body for depolymerizing polystyrene by an electric heating method and a method for depolymerizing polystyrene using the same. More specifically, the invention relates to a method for manufacturing a molded body by pelletizing a mixture in which a conductive resistance heating element is dispersed in a solution in which polystyrene is dissolved so as to be depolymerized by heat generated when an electric current is applied, and a method for depolymerizing the molded body manufactured by the above method by an electric heating method.

[0002] Plastics, manufactured by molding with heat and pressure, are primarily a type of carbon-chain-based polymer material. Due to their structural diversity, they possess mechanical and chemical properties required for products needed in both industrial and daily life sectors. For example, polystyrene is a polymer produced by the polymerization of styrene monomers. It is utilized in various industries, such as as packaging or insulation materials due to its excellent shock absorption and thermal insulation properties, or as electronic components and circuit boards due to its superior electrical insulation. As industries develop and the demand for increased user convenience grows, the demand for polystyrene is surging. However, given that polystyrene is typically manufactured using petroleum resources, research and development are underway for the recycling of used polystyrene from the perspective of carbon emission reduction and resource circularity.

[0003] Chemical recycling, which is a type of polystyrene recycling method described above, is a method of depolymerizing polystyrene to recover it in the form of styrene monomers, which are raw materials, so that it can be reused in the manufacture of polystyrene; thus, it can be an optimal recycling method from the perspective of resource circulation.

[0004] Conventional methods for depolymerizing polystyrene involve introducing polystyrene polymer and catalysts into a reactor and supplying a heat source to carry out the depolymerization reaction. However, given that the heat source supplied in these depolymerization methods is typically thermal energy generated from the combustion of fossil fuels, and in light of the need to curb carbon emissions, there have been ongoing attempts to replace this method by generating heat using electricity derived from renewable energy sources such as solar or wind power. Representative electric heating methods known include Joule heating or resistance heating, in which heat is generated by the resistance of an object through which an electric current flows.

[0005] Non-patent literature 1 (Esun Selvam et al. Recycling polyolefin plastic waste at short contact times via rapid joule heating. Nature Communications. 2024, 15, 5662) discloses a method for manufacturing a target material to be depolymerized using a depolymerization technology based on the Joule heating method. Specifically, the method for manufacturing the target material to be depolymerized discloses a method in which a zeolite, which is a depolymerization catalyst, is impregnated onto a conductive substrate, carbon fiber paper (CFP), and a plastic is coated thereon. However, according to the above manufacturing method, a multi-step process of impregnating the carbon fiber paper with zeolite and then coating it with plastic is required, which necessitates a large amount of equipment and time, and there is a problem in that skilled technology is required to ensure uniform coating of the plastic.

[0006] Accordingly, in the field of depolymerization technology using an electric heating method that uses electricity generated from renewable energy as a reaction heat source, moving away from conventional combustion methods, it is necessary to secure a method for manufacturing various target materials for depolymerization so that polystyrene and conductive materials can be uniformly mixed and dispersed within the target material to be depolymerized, the heat transfer area is large so that the styrene yield as a result of depolymerization is excellent, and the material can be easily manufactured without complex devices or processes, thereby facilitating application and technology diffusion.

[0007] The present invention aims to provide a method for manufacturing a molded body and a method for depolymerizing polystyrene using the same, wherein polystyrene and a conductive material can be uniformly mixed and dispersed, the heat transfer area is large so that the styrene yield resulting from depolymerization is excellent, and the molded body can be easily manufactured without complex devices or processes.

[0008] To solve the above problem, the method for manufacturing a molded body that is depolymerized by an electric heating method according to the present invention may be characterized by comprising: (a) a step of obtaining a mixture by introducing polystyrene and a conductive resistive heating member into a solvent; (b) a step of obtaining a lump comprising polystyrene and a conductive resistive heating member from the mixture of step (a); and (c) a step of manufacturing a molded body by pressure molding the lump of step (b) after pulverizing it.

[0009] The solvent in step (a) above may be one or more of THF, NMP, acetone, alcohol, dichloromethane, toluene, benzene, cyclohexane, hexane, xylene, and DMSO.

[0010] The conductive resistive heating element of step (a) above may be one or more of the following: a carbonaceous material including carbon black, carbon nanofibers, activated carbon, and graphite; a metallic material including a single metal or an alloy composed of two or more metals; and a conductive polymer including polyaniline, polypyrrole, and polyacetylene.

[0011] The pressure molding in step (c) above can be performed at a pressure in the range of 0.2 to 40 MPa.

[0012] The content of the conductive resistive heating element in the molded body manufactured in step (c) above may be in the range of 25 to 80 wt%.

[0013] The present invention may provide a molded body that is depolymerized by an electric heating method, manufactured from the above manufacturing method, and in addition to polystyrene, a polymer of a different type may be used in addition to polystyrene.

[0014] Meanwhile, a method for depolymerizing polystyrene within a molded body comprising polystyrene and a conductive resistive heating member according to the present invention may be characterized by comprising: (i) a step of obtaining a mixture by introducing polystyrene and a conductive resistive heating member into a solvent; (ii) a step of obtaining a lump comprising polystyrene and a conductive resistive heating member from the mixture of step (i); (iii) a step of manufacturing a molded body by pulverizing the lump of step (ii) and then press-molding it; and (iv) a step of depolymerizing the polystyrene within the molded body using heat generated by applying an electric current into the molded body of step (iii) and obtaining a reaction product.

[0015] In step (iii) above, pressure molding can be performed at a pressure of 0.2 to 40 MPa.

[0016] The molded body produced by the method of the present invention has a polystyrene and a conductive resistive heating element uniformly dispersed, so the heat transfer area of ​​the conductive resistive heating element is large, the styrene yield as a result of depolymerization is excellent, and it can be easily manufactured without complex equipment or processes.

[0017] FIG. 1 is a schematic diagram showing an example of the molding process of a molded body of the present invention.

[0018] FIG. 2 shows (A) a graph of TGA analysis results according to the carbon black content at the time of manufacturing the molded body of the present invention, (B) a graph of the thickness and resistance measurements of the disc-shaped molded body, and (C) a photograph of the molded body prototype.

[0019] Figure 3 is (A) a schematic diagram of a Joule-heating depolymerization system, (B) and (C) photographs of a molded body prototype, and (D) a photograph of a molded body prototype placed between copper foam electrodes.

[0020] Figure 4 is a graph showing (A) temperature and power over time according to pressure, (B) ratio of gaseous and liquid phases in the product, and (C) selectivity of the liquid phase product during depolymerization of the molded body of the present invention.

[0021] Figure 5 is a graph showing the temperature and power over time according to the carbon black content (A-1, A-2) during the depolymerization of the molded body of the present invention, (B) the ratio of gaseous and liquid phases in the product, and (C) the selectivity of the liquid phase product.

[0022] Figure 6 is a graph showing the selectivity of styrene monomer according to power consumption under different carbon black content and pressure conditions during the depolymerization of the molded body of the present invention.

[0023] FIG. 7 is a graph showing (A) temperature and power over time, (B) ratio of gaseous and liquid phases in the product, and (C) selectivity of the liquid phase product according to the amount of current during depolymerization of the molded body of the present invention.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by a skilled expert in the art to which this invention pertains. In general, the nomenclature used herein is well known and commonly used in the art.

[0025] Throughout this specification, when a part is described as “comprising” a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

[0026] Hereinafter, we will examine the method for manufacturing a molded body that is depolymerized by electric heating according to the present invention.

[0027] A method for manufacturing a molded body that is depolymerized by an electric heating method according to the present invention may be characterized by comprising: (a) a step of obtaining a mixture by introducing polystyrene and a conductive resistive heating member into a solvent; (b) a step of obtaining a lump comprising polystyrene and a conductive resistive heating member from the mixture of step (a); and (c) a step of manufacturing a molded body by pressure molding the lump of step (b) after pulverizing it.

[0028] The above step (a) is a step of obtaining a mixture by adding polystyrene and a conductive resistive heating element to a solvent.

[0029] The above solvent may be used without limitation as long as it is capable of dissolving the polystyrene to be depolymerized, and for example, one or more of tetrahydrofuran (THF), N-methyl-2-pyrrolidone (NMP), acetone, alcohol, dichloromethane, toluene, benzene, cyclohexane, hexane, xylene, and dimethyl sulfoxide (DMSO) may be used.

[0030] The above polystyrene is a polymer made by polymerizing styrene monomers. It is a polymer that is lightweight, hard, and transparent, and is widely used in food containers, cups, tray packaging boxes, protective padding, etc. The above polystyrene may consist of polystyrene alone, or it may be in a mixed form or copolymer containing other polymers such as acrylonitrile, butanediene, methacrylate, isoprene, maleic anhydride, polyethylene, polypropylene, polyphenylene oxide, and polysilicon.

[0031] The above-mentioned conductive resistive heating member is capable of generating heat through Joule heating when current is applied, and is not limited to any material that can maintain a dispersed state in a solution in which plastic is dissolved in the solvent, such as carbonaceous materials such as carbon black, carbon nanofibers, and activated carbon; metallic materials including a single metal or an alloy composed of two or more metals; and conductive polymers including polyaniline, polypyrrole, and polyacetylene may be used.

[0032] In step (a) above, stirring may be performed for uniform mixing, and heating may be involved during stirring to facilitate the dissolution of polystyrene.

[0033] The above step (b) is a step of obtaining a lump comprising polystyrene and a conductive resistive heating member from the mixture of step (a).

[0034] In other words, the above step (b) is a step of separating and obtaining a solid mass in which polystyrene and a conductive resistive heating element are dispersed from a large amount of solvent.

[0035] As an example of the present invention, in order to obtain a lump comprising polystyrene and a conductive resistive heating member in step (b), a polar solvent such as water or alcohol may be added to the mixture of step (a) to induce phase inversion, or a portion of the solvent may be removed by heating. In the case of inducing phase inversion, the solvent can be easily reused, which has the advantage of reducing process costs.

[0036] Next, step (c) above is a step of manufacturing a molded body by pulverizing the lump from step (b) and then pressure-molding it.

[0037] The lump of step (b) above is pulverized into a powder form through a known grinding technique such as milling, and the powder is supplied to a subsequent pressure molding process.

[0038] The pressure molding in step (c) above can be performed at a pressure in the range of 0.2 to 40 MPa, preferably in the range of 2 to 30 MPa, and more preferably in the range of 2 to 10 MPa. If the pressure is less than 0.2 MPa, a network between conductive particles is not formed, making it difficult to exhibit conductivity; if the pressure exceeds 40 MPa, the molded body is excessively compressed, preventing the polystyrene depolymerization component from flowing out smoothly, which may lead to a rapid decrease in conversion rate and styrene selectivity, as well as an increase in dimers and by-products. Furthermore, if the polystyrene depolymerization component does not flow out smoothly, it leads to an increase in gaseous products that are difficult to liquefy due to the excessive cracking reaction of styrene monomers.

[0039] In addition, the shape of the molded body manufactured in step (c) above is not particularly limited, and if necessary, a cavity may be formed in at least a part of the molded body.

[0040] FIG. 1 is a schematic diagram showing an example of the molding process of a molded body of the present invention, briefly illustrating the step of forming a cavity so that a thermometer for measuring temperature can be installed during the pressure molding step using powder.

[0041] Meanwhile, the content of the conductive resistance heating element in the molded body manufactured in step (c) above may be in the range of 25 to 80 wt%, preferably 25 to 50 wt%, and more preferably 30 to 40 wt%. If the content is less than 25 wt%, the resistance of the molded body is excessively high, so current does not flow and it is difficult to generate line heating. If the content exceeds 80 wt%, the amount of plastic that can be processed per unit time decreases, so it may take a long time to process the plastic.

[0042] Meanwhile, the present invention can provide a method for depolymerizing a plastic component within a molded body comprising polystyrene and a conductive resistive heating member.

[0043] A method for depolymerizing a plastic component within a molded body may be characterized by comprising: (i) a step of obtaining a mixture by introducing polystyrene and a conductive resistive heating element into a solvent; (ii) a step of obtaining a lump containing polystyrene and a conductive resistive heating element from the mixture of step (i); (iii) a step of manufacturing a molded body by pulverizing the lump of step (ii) and then pressure-molding it; and (iv) a step of depolymerizing the polystyrene within the molded body using heat generated by applying an electric current into the molded body of step (iii) and obtaining a reaction product.

[0044] Since the above steps (i) to (iii) are identical to those described above in the method for manufacturing the molded body, they will be omitted from the description.

[0045] The above step (iv) is a step of depolymerizing polystyrene in the molded body using heat generated by applying an electric current into the molded body of step (iii) and obtaining a reaction product.

[0046] The current applied in step (iv) above may vary depending on the type or content of the conductive resistive heating element introduced, but for example, when the conductive resistive heating element is carbon black, the applied current may be in the range of 18 to 100 A, preferably in the range of 20 to 60 A, and if the current is less than 18 A, there is a problem that the amount of heat required for depolymerization is insufficient and the conversion rate is low.

[0047] Figure 3 is (A) a schematic diagram of a Joule-heating depolymerization system, (B) and (C) photographs of a molded body prototype, and (D) a photograph of a molded body prototype placed between copper foam electrodes.

[0048] Referring to the schematic diagram of the Joule-heating depolymerization system in Fig. 3(A) above, a molded body is filled between copper foam electrodes in the center of a glass conduit, and an external power source is electrically connected to the electrodes. When current is applied from the external power source, the molded body receives a reaction heat source through Joule heating and undergoes depolymerization. Subsequently, the reaction product moves to a trap where the gaseous and liquid components are separated, and the composition and selectivity of the separated gaseous and liquid components can be analyzed by an analysis device (not shown), such as a chromatograph.

[0049] A carrier gas may be flowed to facilitate the movement of the reaction product, and the carrier gas may be an inert gas such as air, nitrogen, helium, or argon, or a low-carbon hydrocarbon gas such as methane, ethane, or propane.

[0050] Hereinafter, preferred embodiments of the present invention will be examined. For reference, the following embodiments are provided to illustrate one or more preferred embodiments of the present invention, but the present invention is not limited to such embodiments. A number of modifications falling within the scope of the present invention may be made to the following embodiments.

[0051] <Examples 1–7 and Comparative Examples 1–3: Method for manufacturing a molded article by dispersing carbon black in a solution in which polystyrene is dissolved in a solvent>

[0052] A homogeneous solution was prepared by stirring 200 g of polystyrene (Sigma Aldrich, 182427, Mw ~280,000) in 1000 mL of THF, a solvent. Subsequently, 50 to 300 g of carbon black (Thermofischer, 045527) was added and stirred to obtain a mixture. Additionally, 5000 mL of H2O was added to the mixture to obtain a mass containing polystyrene and a conductive resistive heating element, which was dried in an oven at 100 °C. Afterward, the dried sample was powdered and then pressure-molded under the pressure listed in Table 1 below to produce a molded body.

[0053] The carbon black content in the manufactured molded body is shown in Tables 1 and 2 below, and the graph of TGA analysis results according to carbon black content, the graph of thickness and resistance measurement values ​​of the disc-shaped molded body, and a photograph of the molded body prototype are shown in Figure 2.

[0054] Referring to Fig. 2, the molded body containing polystyrene and carbon black decomposed when it reached a temperature of 300 to 400 ℃.

[0055] <Comparative Example 4: Method for manufacturing a molded article in which polystyrene and carbon black are mechanically mixed without a solvent>

[0056] 50 g of solvent-free powdered polystyrene and 25 g of carbon black were ball-milled and mixed, and then melt-mixed in a flask at 250 ℃. Afterward, the melt-mixed sample was finely ground and then pressure-molded at a pressure of 10 MPa to produce a molded body.

[0057] <Experimental Example: Method for depolymerization of PS components in molded bodies upon current application, prepared by the methods of Examples 1–7 and Comparative Examples 1–4>

[0058] The PS depolymerization system using Joule heating consists of a power supply capable of providing a constant amount of current, a reaction section where the reaction actually takes place, and a collection section that collects the products generated during the depolymerization reaction at low temperatures. The power supply used was the EX100-36 model from ODA Technology, capable of supplying a maximum of 100 V and 36 A. This power supply was connected to two circular copper electrodes with a diameter of 3.9 cm. The copper electrodes have 4 mm holes for introducing nitrogen gas, and O-rings were placed at the ends of the electrodes to prevent leakage of the product. The two copper electrodes were mounted in a quartz reactor with an inner diameter of 4 cm and two 0.6 cm inlets. One of the 0.6 cm inlets was fitted with a thermocouple via a Tor fitting to measure the sample temperature, while the other serves to direct the product out to the collection section. Copper foam and a conductive plastic sample were filled between the two copper electrodes to ensure that the introduced nitrogen flows evenly across the entire surface area.

[0059] Fourteen copper foams, each 0.5 cm thick, were stacked on both sides of a conductive plastic sample and filled into a quartz chamber, and an electric current was applied through copper electrodes. During the reaction, the total volume decreased by a certain amount due to the release of the PS sample; to ensure the current could be continuously applied even as this volume reduction occurred, a spring was installed to push the copper electrodes toward the sample. The depolymerization reaction of PS was induced by Joule heating while applying a fixed amount of current set to a maximum voltage of 80 V for 5 minutes via a power supply. The product generated by Joule heating moved to a collection unit through an outlet and was collected in two tubes immersed in a cooling bath at -76°C. The collected liquid products were analyzed using offline gas chromatography. As a result, the conversion rate, selectivity, and yield of the styrene monomer during PS depolymerization within the molded body are shown in Tables 1 and 2 and Figures 4 through 7.

[0060]

[0061] Classification Pressure (MPa) CB Content (wt%) Current (A) Liquid Product Yield (%) Dimer Benzene Toluene EB Cumene Stya-MS Example 1 23330 10.07 1.05 3.28 0.40 42.85 3.69 Example 2 103330 7.33 0.52 3.09 0.56 36.39 3.80 Example 3303330 9.43 0.15 2.89 0.82 31.09 4.34 Comparative Example 1 503330 6.99 0.05 0.99 0.17 18.18 2.39 Comparative Example 2 1020 300.72 0.27 0.29 0.00 17.061 .68 Example 4 1060306.85 1.065.28 1.49035.085.95 Comparative Example 3 1033170000000 Example 5 1033187.130.473.080.59033.083.78 Example 6 1033205.410.43 3.260.66030.693.93 Example 7 1033358.030.813.370.45044.023.81 Comparative Example 4 103330n / an / an / an / an / an / an / a<Term>CB: Carbon black, EB: Ethylbenzene, Sty: Styrene, a-MS: α-methylstyrene, n / a: Not applicable

[0062] Referring to Tables 1 and 2 above, Examples 1 to 3, in which the pressure applied during the molding step increased from 2 MPa to 30 MPa, showed a conversion rate of at least 50% or more upon depolymerization, whereas Comparative Example 1, with a pressure of 50 MPa, showed a sharp decrease in conversion rate to 28% and a styrene yield of only 18%.

[0063] In addition, Example 4, in which the carbon black content in the molded body was 33 wt%, showed a conversion rate of 55% upon depolymerization, whereas Comparative Example 2, in which the content was reduced to 20 wt%, showed a significantly reduced conversion rate of 21% and a styrene yield of only 17%.

[0064] Meanwhile, in Examples 5 to 7, where the amount of current applied to the molded body was reduced from 35A to 18A, the conversion rate was at least 47% and at most 60%, whereas when the amount of current was reduced to 17A, the conversion rate decreased sharply to 4%, and the selectivity and yield of the styrene monomer were 0%, indicating that no monomer was produced.

[0065] In addition, Comparative Example 4, which was prepared without using a solvent, exhibits a very high resistance compared to the sample uniformly mixed using a solvent, as the conductive network is not properly formed due to the low dispersion of CB.

[0066] Therefore, Comparative Example 4 did not exhibit conductivity at voltages of 80V or lower. To overcome this, Comparative Example 4 has the disadvantage of requiring a larger amount of CB to obtain a resistance similar to that of the sample synthesized using a solvent. Additionally, to overcome the high resistance, a high-voltage power supply is required, which consequently leads to the problem of requiring more energy and equipment costs.

[0067] Although the present invention has been described above with reference to embodiments described in the specification or illustrated in the attached drawings, this is merely illustrative, and those skilled in the art will understand that various modifications and equivalent alternative embodiments are possible therefrom. Accordingly, the technical scope of protection of the present invention should be determined by the following claims.

Claims

1. (a) A step of obtaining a mixture by introducing polystyrene and a conductive resistive heating element into a solvent for dissolving polystyrene; (b) obtaining a lump comprising polystyrene and a conductive resistive heating element from the mixture of step (a); and (c) a step of manufacturing a molded body by pressure molding after pulverizing the lump from step (b) above; characterized by including, A method for manufacturing a molded body that is depolymerized by an electric heating method.

2. In Paragraph 1, The solvent of step (a) above is characterized as being one or more of THF, NMP, acetone, alcohol, dichloromethane, toluene, benzene, cyclohexane, hexane, xylene, and DMSO. A method for manufacturing a molded body that is depolymerized by an electric heating method.

3. In Paragraph 1, The conductive resistive heating member of step (a) above is characterized by being one or more of: a carbonaceous material including carbon black, carbon nanofibers, activated carbon, and graphite; a metallic material including a single metal or an alloy composed of two or more metals; and a conductive polymer including polyaniline, polypyrrole, and polyacetylene. A method for manufacturing a molded body that is depolymerized by an electric heating method.

4. In Paragraph 1, The pressure molding of step (c) above is characterized by being performed in a pressure range of 0.2 to 40 MPa. A method for manufacturing a molded body that is depolymerized by an electric heating method.

5. In Paragraph 1, Characterized by the content of the conductive resistive heating element in the molded body manufactured in step (c) above being in the range of 25 to 80 wt%, A method for manufacturing a molded body that is depolymerized by an electric heating method.

6. A molded body depolymerized by an electric heating method, characterized by being manufactured by the method of any one of claims 1 to 5.

7. In Paragraph 6, A molded body depolymerized by an electric heating method, characterized by further including a polymer of a different type from polystyrene in addition to the above-mentioned polystyrene.

8. (i) A step of obtaining a mixture by adding polystyrene and a conductive resistive heating element to a solvent; (ii) A step of obtaining a lump comprising polystyrene and a conductive resistive heating element from the mixture of step (i) above; (iii) a step of manufacturing a molded body by pulverizing the lump from step (ii) above and then press-molding it; and (iv) a step of depolymerizing polystyrene within the molded body using heat generated by applying an electric current into the molded body of step (iii) above and obtaining a reaction product; characterized by including, A method for depolymerizing polystyrene in a molded body comprising polystyrene and a conductive resistive heating element.

9. In Paragraph 8, Characterized by pressurizing at a pressure of 0.2 to 40 MPa in step (iii) above, A method for depolymerizing polystyrene comprising polystyrene and a conductive resistive heating element.

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