Powder coating, method for producing the same, and coated metal material

Abstract

To provide a powder coating with fully excellent properties of both thermoplastic polyethylene terephthalate resin and thermosetting resin, and to effectively utilize waste polyethylene terephthalate resin.SOLUTION: Provided is a method for producing a powder coating, including: preparing recycled polyethylene terephthalate resin; obtaining recycled polyethylene terephthalate resin powder by pulverizing the recycled polyethylene terephthalate resin that has been frozen with liquid nitrogen; preparing a thermosetting resin coating raw material; and obtaining the powder coating by mixing the recycled polyethylene terephthalate resin powder with the thermosetting resin coating raw material while heating them.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a powder coating, a method for producing the same, and a coated metal material.

Background Art

[0002] Recycling of resources has attracted attention from the viewpoints of the soaring prices of petroleum products, attention to effective utilization, ocean pollution caused by waste, and carbon neutrality. In particular, in recent years, marine pollution by plastics has been regarded as a global problem, and the use of recycled plastics has attracted attention. Among used plastics, polyethylene terephthalate resin is generated in large quantities, and its reuse other than for beverage containers has been studied. Therefore, conventionally, the use of polyethylene terephthalate resin as a powder coating has been studied. As coating methods using a powder coating of polyethylene terephthalate resin, a fluid dipping method and an electrostatic powder method can be considered. Since polyethylene terephthalate resin is a thermoplastic resin with a high melting point of 250°C, among these coating methods, coating by the fluid dipping method, in which a heated object to be coated is placed in a fluid bath in which a powder coating of polyethylene terephthalate resin is suspended, is preferable. Further, when coating is performed by the electrostatic powder method, due to the relationship with the melting point of polyethylene terephthalate resin, the heating temperature has to be increased, and studies have been conducted on adding a modifier to lower the melting temperature of polyethylene terephthalate resin.

[0003] In addition, since the performance requirements of the coating film layer required by individual coating products vary, there is a need for paint design according to the coating products. In the case of powder coatings of polyethylene terephthalate resin, it has not been possible to meet the various coating performance requirements according to the coating products, and its practical application has not progressed. Therefore, the use of a powder coating that combines the excellent properties of polyethylene terephthalate resin and a thermosetting resin by mixing a thermosetting resin having characteristics different from those of the polyethylene terephthalate resin into the powder coating of polyethylene terephthalate resin has been studied. Patent Document 1 (Japanese Patent Application Laid-Open No. 2000-53892) discloses a composition for a powder coating having metal adhesion, in which 100 parts by mass of chips obtained from recovered polyethylene terephthalate resin pellets or polyethylene terephthalate resin molded articles are melt-mixed with 5 to 40 parts by mass of a polyester resin or a modified polyester resin. Patent Document 2 (Japanese Patent Application Laid-Open No. 10-287844) discloses a powder coating obtained by melt-mixing 100 parts by mass of chips obtained from recovered polyethylene terephthalate resin pellets or polyethylene terephthalate resin molded articles with 5 to 40 parts by mass of a modified polyester resin and then pulverizing the mixture.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the powder coating containing the recycled polyethylene terephthalate resin powder and the thermosetting resin paint that has been conventionally studied has been found to have degraded properties of the coating film layer compared to the powder coating of the thermosetting resin. More specifically, since the melting points of the thermoplastic polyethylene terephthalate resin powder and the thermosetting resin are significantly different, it has been difficult to form a coating film layer under the same coating conditions (coating equipment and heating temperature conditions). Although the use of a modifier was also studied to lower the melting temperature of the thermoplastic polyethylene terephthalate resin, there were also problems such as a decrease in quality due to the modifier.

[0006] Therefore, the present invention has been made in view of the above problems, and by pulverizing the recycled polyethylene terephthalate resin frozen with liquid nitrogen, a recycled polyethylene terephthalate resin powder with a small particle size is produced. Thereby, in the present invention, even if the recycled polyethylene terephthalate resin powder is mixed with the thermosetting resin paint raw material at a baking temperature below the melting point of the recycled polyethylene terephthalate resin, a coating film layer can be formed without impairing the quality performance. That is, an object of the present invention is to provide a powder coating that sufficiently combines the excellent properties of both the recycled polyethylene terephthalate resin and the thermosetting resin. In addition, an object of the present invention is also to effectively utilize waste polyethylene terephthalate resin and contribute to a recycling-oriented society.

Means for Solving the Problems

[0007] In order to solve the above problems, each embodiment of the present invention is as follows. [1] A step of preparing a recycled polyethylene terephthalate resin, A step of pulverizing the recycled polyethylene terephthalate resin frozen with liquid nitrogen to obtain a recycled polyethylene terephthalate resin powder, A step of preparing a thermosetting resin paint raw material, A step of mixing the recycled polyethylene terephthalate resin powder and the thermosetting resin paint raw material while heating to obtain a powder coating, A method for manufacturing a powder coating having the above steps. [2] The thermosetting resin constituting the raw material of the thermosetting resin coating is at least one resin selected from the group consisting of polyester resin, modified acrylic resin, epoxy resin, and fluororesin, and the method for manufacturing the powder coating according to [1] above. [3] The powder coating includes a composite resin of recycled polyethylene terephthalate resin and a thermosetting resin, and the method for manufacturing the powder coating according to [1] or [2] above. [4] Between the step of obtaining the recycled polyethylene terephthalate resin powder and the step of obtaining the powder coating, The method for manufacturing the powder coating according to [1] or [2] above, which has a step of classifying the recycled polyethylene terephthalate resin powder using a mesh. [5] A powder coating comprising recycled polyethylene terephthalate resin powder with an average particle size of 100 μm or less, a thermosetting resin coating, and. [6] The thermosetting resin constituting the thermosetting resin coating is at least one resin selected from the group consisting of polyester resin, modified acrylic resin, epoxy resin, and fluororesin, and the powder coating according to [5] above. [7] A powder coating comprising a composite resin of recycled polyethylene terephthalate resin and a thermosetting resin, and the powder coating according to [5] or [6] above. [8] A coated metal material having a metal material and a coating film layer containing a powder coating comprising recycled polyethylene terephthalate resin powder and a thermosetting resin. [9] The thermosetting resin is at least one resin selected from the group consisting of polyester resin, modified acrylic resin, epoxy resin, and fluororesin, and the coated metal material according to [8] above.

[10] The average particle size of the recycled polyethylene terephthalate resin powder is equal to or less than the average film thickness of the coating film layer, and the coated metal material according to [8] or [9] above.

[11] In other aspects, in [1] to

[10] above, "recycled polyethylene terephthalate resin" is replaced with "recycled plastic".

Advantages of the Invention

[0008] According to an aspect of the present invention, it is possible to provide a powder coating that sufficiently combines the excellent characteristics of both a recycled polyethylene terephthalate resin powder and a thermosetting resin. In addition, waste polyethylene terephthalate resin can be effectively utilized.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0010] (Powder Coating) The powder coating of the present invention contains a recycled polyethylene terephthalate (PET) resin powder with an average particle size of 100 μm or less and a thermosetting resin coating. This recycled polyethylene terephthalate resin powder is obtained by pulverizing a recycled polyethylene terephthalate resin frozen with liquid nitrogen. Therefore, the strength of the coating film layer of the powder coating can be increased without impairing the target performance required for the coating film layer of the powder coating. The powder coating of the present invention is one in which such a recycled polyethylene terephthalate resin powder and a thermosetting resin are uniformly dispersed. Therefore, it can fully possess the excellent characteristics of both the recycled polyethylene terephthalate resin powder and the thermosetting resin. Further, the recycled polyethylene terephthalate resin powder obtained by pulverizing the frozen recycled polyethylene terephthalate resin has a small average particle size that is equal to or less than the average film thickness of the coating film layer. Therefore, the coating film layer containing the recycled polyethylene terephthalate resin powder obtained after coating with the powder coating has a uniform and smooth film thickness and can have a thin film thickness. Furthermore, since waste polyethylene terephthalate resin can be effectively utilized as recycled polyethylene terephthalate resin, it can contribute to a recycling-oriented society. In addition, in other embodiments, it can also be a powder coating containing recycled plastic and a thermosetting resin coating.

[0011] Note that the "thermosetting resin coating raw material" includes a thermosetting resin, a curing agent for the thermosetting resin, a pigment, and optionally additives (such as a surface conditioner, an anti-foaming agent, an electrostatic charge regulator, an ultraviolet absorber, a flame retardant, a fluidity-imparting agent, etc.), and represents the raw material for manufacturing a thermosetting resin coating in which the resin and the curing agent can react by heating to form a resin network. The "thermosetting resin coating" represents a coating in which a network is formed by the reaction of the above-mentioned thermosetting resin and curing agent, and the raw materials necessary for forming a coating film layer to which a coloring pigment and other necessary functions (additives) are imparted are properly formulated.

[0012] The type of the thermosetting resin constituting the thermosetting resin coating is not particularly limited, but it is preferably at least one resin selected from the group consisting of polyester resins, modified acrylic resins, epoxy resins, and fluororesins.

[0013] The polyester resin is not particularly limited, but generally it is a polyester resin having a hydroxyl group at the terminal, such as a urethane-curable polyester resin cured with a blocked isocyanate as a curing agent, or a polyester resin having a carboxyl group at the terminal and using β-hydroxyalkylamide as a curing agent, which are widely used. Note that this polyester resin does not include recycled polyethylene terephthalate resin. The modified acrylic resin is not particularly limited, but a type having an acrylic resin with a glycidyl group and using the carboxyl group of an aliphatic dibasic acid as a curing agent is preferably used. The epoxy resin is not particularly limited, but a bisphenol A glycidyl ether type epoxy resin is mainly preferably used, and a curing agent is blended therewith. Examples of the curing agent include dicyandiamide, imidazole, imidazoline-based compounds, other acid anhydrides, dibasic acid dihydrazide, and the like. The fluororesin is not particularly limited, but it is a fluororesin mainly composed of a fluoroolefin vinyl ether having a crosslinkable reaction group such as a hydroxyl group, and a type using a blocked isocyanate as a curing agent is preferably used.

[0014] Preferably, the powder coating is a composite resin of a recycled polyethylene terephthalate resin and a thermosetting resin, in which the recycled polyethylene terephthalate resin and the thermosetting resin are fused to each other. The "composite resin of a recycled polyethylene terephthalate resin and a thermosetting resin" means that the recycled polyethylene terephthalate resin and the thermosetting resin are fused to each other, and each powder particle is composed of a recycled polyethylene terephthalate resin and a cured thermosetting resin. By including such a composite resin of a recycled polyethylene terephthalate resin and a thermosetting resin in the powder coating, the coating film layer obtained from the powder coating can be made into a smoother and more uniform film. The average particle size of the recycled polyethylene terephthalate resin powder is not particularly limited, but from the viewpoint of forming a uniform and smooth coating film layer, it is preferably 10 to 100 μm, more preferably 15 to 70 μm, and even more preferably 15 to 50 μm. The average particle size of the powder coating is not particularly limited, but in order to fully combine the excellent properties of the recycled polyethylene terephthalate resin powder and the thermosetting resin, and to obtain a smooth coating surface for electrostatic coating, the average particle size is preferably 10 to 60 μm, more preferably 15 to 50 μm, and even more preferably 15 to 45 μm. The average particle sizes of the recycled polyethylene terephthalate resin powder and the powder coating can be measured by the laser diffraction / scattering method. The average film thickness of the coating film layer can be measured using an electromagnetic film thickness gauge.

[0015] The blending amount of the recycled polyethylene terephthalate resin powder in the powder coating is not particularly limited. However, in order to impart excellent durability to the coating film layer containing the powder coating, the recycled polyethylene terephthalate resin powder is preferably 0.01 to 10% by mass, more preferably 0.05 to 5% by mass, and even more preferably 0.05 to 4% by mass. When the blending amount of the recycled polyethylene terephthalate resin powder is 10% by mass or less, adhesion to the thermosetting resin coating does not occur, the burden of melt-kneading during paint preparation is reduced, and workability can be improved. Further, when the blending amount of the recycled polyethylene terephthalate resin powder is 5% by mass or less, excellent gloss with a gloss retention rate of 70% or more can be maintained, and when it is 4% by mass or less, even more excellent gloss can be maintained. Note that the "gloss retention rate" will be described later in the examples. When the blending amount of the recycled polyethylene terephthalate resin powder is 0.01% by mass or more or 0.05% by mass or more, a predetermined amount of the recycled polyethylene terephthalate resin powder can be recycled, and the recyclability can be made excellent. The thermosetting resin coating is originally designed with quality performance suitable for the intended coating application. In the powder coating of the present invention, since the recycled polyethylene terephthalate resin powder is added thereto, from the viewpoint of the original target performance, the blending amount of the recycled polyethylene terephthalate resin powder is preferably 10% by mass or less.

[0016] (Method for manufacturing powder coating) The method for manufacturing the powder coating of the present invention includes a step of preparing a recycled polyethylene terephthalate resin, a step of pulverizing the recycled polyethylene terephthalate resin frozen with liquid nitrogen to obtain a recycled polyethylene terephthalate resin powder, a step of preparing a thermosetting resin coating raw material, and a step of obtaining a powder coating by mixing the recycled polyethylene terephthalate resin powder and the thermosetting resin coating raw material while heating. Hereinafter, each step of the method for manufacturing the powder coating of the present invention will be described in detail. Note that the following embodiments are merely examples, and various forms are possible within the scope of the present invention.

[0017] FIG. 1 is a flowchart showing an example of a method for manufacturing a powder coating of the present invention. As shown in FIG. 1, first, a recycled polyethylene terephthalate resin is prepared (S1). The "recycled polyethylene terephthalate resin" represents a polyethylene terephthalate resin obtained by reusing a used polyethylene terephthalate resin. As the recycled polyethylene terephthalate resin, a commercially available recycled polyethylene terephthalate resin may be used, or waste polyethylene terephthalate resin of a polyethylene terephthalate resin synthesized by a known method may be used.

[0018] Next, the recycled polyethylene terephthalate resin frozen with liquid nitrogen is pulverized to obtain a recycled polyethylene terephthalate resin powder (S2). Liquid nitrogen is a substance that liquefies at -196°C under atmospheric pressure and has a temperature at which it does not solidify below -196°C. The process of freezing the recycled polyethylene terephthalate resin with liquid nitrogen can be carried out by bringing the liquid nitrogen into contact with the recycled polyethylene terephthalate resin. The process of bringing the liquid nitrogen into contact with the recycled polyethylene terephthalate resin to freeze the recycled polyethylene terephthalate resin may be carried out before pulverization of the recycled polyethylene terephthalate resin, during pulverization of the recycled polyethylene terephthalate resin, or before and during pulverization of the recycled polyethylene terephthalate resin. Also, (i) the liquid nitrogen and the recycled polyethylene terephthalate resin may be brought into contact by supplying the liquid nitrogen and the recycled polyethylene terephthalate resin into a container, (ii) they may be brought into contact while being supplied in parallel flow, or (iii) they may be brought into contact while being supplied in countercurrent flow. FIG. 2 is a diagram showing an example of a freeze pulverizer used in step S2. As shown in FIG. 2, the freeze pulverizer 1 is composed of a liquid nitrogen tank 2, a freezer 5, a screw feeder 9 connected to the bottom of the freezer 5, and a pulverizer 6. Liquid nitrogen 3 is supplied from the liquid nitrogen tank 2 to the freezer 5 and the pulverizer 6. Recycled polyethylene terephthalate resin 8 is supplied from the top of the freezer 5. In the freezer 5, the liquid nitrogen 3 and the recycled polyethylene terephthalate resin 8 are mixed to become a frozen recycled polyethylene terephthalate resin 4 in a frozen state. The frozen recycled polyethylene terephthalate resin 4 is supplied from the bottom of the freezer 5 to the pulverizer 6 as the frozen recycled polyethylene terephthalate resin 4 by the screw feeder 9. By pulverizing the frozen recycled polyethylene terephthalate resin 4 in the pulverizer 6, a recycled polyethylene terephthalate resin powder 7 is obtained. Here, the frozen recycled polyethylene terephthalate resin 4 becomes brittle due to being frozen and is easily crushed.Therefore, in the crusher 6, it is possible to obtain a recycled polyethylene terephthalate resin powder 7 with a low load, uniform, and small particle size. Further, the cryogenically recycled polyethylene terephthalate resin 4 is at an extremely low temperature. By supplying liquid nitrogen to the crusher 6 during pulverization, it is possible to prevent the recycled polyethylene terephthalate resin from becoming high temperature due to the heat during pulverization, which could cause a dust explosion or deterioration of the properties of the recycled polyethylene terephthalate resin.

[0019] As shown in FIG. 1, as a raw material for a thermosetting resin paint, a raw material for a thermosetting resin paint necessary for applying a thermosetting resin paint such as a thermosetting resin, a curing agent, a pigment, and an additive is prepared (S3). Next, the recycled polyethylene terephthalate resin powder 7 and the thermosetting resin paint raw material are mixed while being heated to obtain a powder paint (S4). More specifically, the recycled polyethylene terephthalate resin is a thermoplastic resin having excellent properties. Also, the thermosetting resin can increase the strength of the coating film layer of the powder paint. Since the powder paint of the present invention is such that the recycled polyethylene terephthalate resin powder 7 having such properties is uniformly dispersed in the thermosetting resin paint, it can sufficiently possess the excellent properties of both the recycled polyethylene terephthalate resin powder 7 and the thermosetting resin. In addition, since the waste polyethylene terephthalate resin can be effectively utilized as a recycled polyethylene terephthalate resin, it can contribute to a recycling-oriented society. In particular, previously, a large amount of waste plastic was exported, but due to the Basel Convention implemented in 2021, exports have become difficult and are less than half of what they were 10 years ago. For this reason, it has become important to recycle waste plastic within the country. According to the method of the present invention, it is basically possible to apply the recycled polyethylene terephthalate resin, which is waste plastic, to all coatings using powder paint, and it is possible to greatly contribute to the recycling of the recycled polyethylene terephthalate resin.

[0020] Note that in step S4, the specific method for obtaining a powder paint by mixing the recycled polyethylene terephthalate resin powder 7 and the thermosetting resin paint raw material while heating is not particularly limited, and for example, the following methods can be mentioned. (i) Feed polyethylene terephthalate resin powder and a thermosetting resin paint raw material into a heated extrusion molding machine, and extrude an extruded molded body of the polyethylene terephthalate resin powder and the thermosetting resin paint raw material from the die of the extrusion molding machine. The heating temperature in the extrusion molding machine is preferably 60 to 140°C, more preferably 80 to 120°C. (ii) Roll the extruded molded body with a rolling roll to form a sheet, and cool the sheet on a conveyor. (iii) Roughly crush the sheet to obtain a first powder. (iv) Finely crush the first powder to obtain a second powder. (v) Classify the second powder to obtain a powder coating.

[0021] The type of the thermosetting resin constituting the thermosetting resin paint raw material is not particularly limited, but it is preferably at least one resin selected from the group consisting of a polyester resin, a modified acrylic resin, an epoxy resin, and a fluororesin.

[0022] Preferably, in step S4, the recycled polyethylene terephthalate resin and the thermosetting resin are fused to each other to form a composite resin of the recycled polyethylene terephthalate resin and the thermosetting resin. By including the composite resin of the recycled polyethylene terephthalate resin and the thermosetting resin in the powder coating in this way, the coating film layer obtained from the powder coating can be made into a smoother and more uniform film.

[0023] Preferably, there is a step of classifying the recycled polyethylene terephthalate resin powder using a mesh between the step of obtaining the recycled polyethylene terephthalate resin powder and the step of obtaining the powder coating. By having the step of classifying the recycled polyethylene terephthalate resin powder, a powder coating having a desired particle size distribution can be obtained. The mesh opening is not particularly limited, but in order to obtain a recycled polyethylene terephthalate resin powder with a small average particle size, it is preferably 75 μm or less, more preferably 60 μm or less, and even more preferably 50 μm or less. The average particle size of the recycled polyethylene terephthalate resin powder is not particularly limited, but from the viewpoint of obtaining a smooth surface after coating, it is preferably 10 to 100 μm, more preferably 15 to 70 μm, and even more preferably 15 to 50 μm. The average particle size of the powder coating is not particularly limited, but in order to fully combine the excellent properties of both the recycled polyethylene terephthalate resin powder and the thermosetting resin, and to obtain a smooth coating surface for electrostatic coating, the average particle size is preferably 10 to 60 μm, more preferably 15 to 50 μm, and even more preferably 15 to 45 μm. The mass ratio of the thermosetting resin coating raw material to the recycled polyethylene terephthalate resin powder in the powder coating is not particularly limited, but in order to impart excellent durability to the coating film layer containing the powder coating, the recycled polyethylene terephthalate resin powder is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 5 parts by mass, and even more preferably 0.05 to 4 parts by mass with respect to 100 parts by mass of the thermosetting resin coating raw material.

[0024] (Coated metal material) The coated metal material of the present invention has a metal material and a coating film layer containing a powder coating containing recycled polyethylene terephthalate resin powder and a thermosetting resin. The metal material is not particularly limited, and examples thereof include materials made of metals such as steel materials, plated steel materials, and aluminum materials. Further, the method for forming the coating film layer on the metal material is not particularly limited. For example, after spraying the powder coating of the present invention on the metal material by an electrostatic powder coating method, the powder coating is heated and baked dry in a drying furnace such as a mountain-type furnace, a flat-type furnace, or a batch furnace to form a coating film layer. By using the electrostatic powder coating method, a coating film layer with a thin and uniform film thickness can be formed. The conditions for baking and drying can be appropriately changed depending on the composition of the powder coating, the thickness of the coating film layer, etc. For example, it can be set at 170 to 200 °C for a time within 60 minutes, preferably 10 to 40 minutes. Since the recycled polyethylene terephthalate resin powder constituting the coating film layer has a high melting point, its shape does not change even in the above baking and drying process. However, since the particle system is reduced to about the same thickness as the coating film layer by cryogenic grinding, the thickness of the coating film layer can be reduced.

[0025] The film thickness of the coating film layer is preferably 20 to 300 μm, more preferably 20 to 250 μm, and even more preferably 20 to 120 μm. Further, the coated metal material of the present invention can have, for example, excellent moisture resistance, salt spray corrosion resistance, sunshine weather resistance, acid resistance, and impact resistance. Furthermore, in the coated metal material of the present invention, by containing a pigment in the coating film layer, excellent glossiness can be achieved. The type of the thermosetting resin is not particularly limited, but it is preferably at least one resin selected from the group consisting of a polyester resin, a modified acrylic resin, an epoxy resin, and a fluororesin. Since a coating film layer with a more uniform and thinner film thickness can be formed, the average particle diameter of the recycled polyethylene terephthalate resin powder is preferably equal to or less than the average film thickness of the coating film layer. The average film thickness of the coating film layer can be measured using an electromagnetic film thickness gauge. The mass ratio of the thermosetting resin and the recycled polyethylene terephthalate resin powder in the powder coating is not particularly limited, but in order to impart excellent fastness to the coating film layer containing the powder coating, the recycled polyethylene terephthalate resin powder is preferably 0.01 to 10 parts by mass, more preferably 0.05 to 5 parts by mass, and even more preferably 0.05 to 4 parts by mass with respect to 100 parts by mass of the thermosetting resin.

[0026] Specific examples of the coated metal material are not particularly limited, and examples thereof include fences (mesh fences, privacy fences) and guardrails (guardrails, guard pipes) coated with powder coatings.

Examples

[0027] (Examples 1 to 8) Using a cryogenic crusher with liquid nitrogen, the recycled polyethylene terephthalate resin that had been frozen and had the average particle sizes shown in Table 1 below was crushed to obtain a recycled polyethylene terephthalate resin powder (Steps S1 and S2 in Figure 1). This recycled polyethylene terephthalate resin powder and a thermosetting resin paint raw material composed of a thermosetting resin, a curing agent, a pigment, an additive, etc. shown in Table 1 below were mixed by a mixer, then melt-kneaded while heating, and then cooled and crushed to obtain a powder paint by a so-called dry method (Steps S3 and S4 in Figure 1). Note that the powder paint manufacturing machine is composed of a plurality of machines, and the following operations were performed in the mixer. (i) The recycled polyethylene terephthalate resin powder and the thermosetting resin, curing agent, pigment, additive, etc. were put into an extrusion molding machine heated to about 100 °C, and an extruded molded body of the recycled polyethylene terephthalate resin powder and the thermosetting resin paint raw material was extruded from the die of the extrusion molding machine. (ii) The extruded molded body was rolled with a rolling roll to form a sheet, and the sheet was cooled on a conveyor. (iii) The sheet was coarsely crushed to obtain a first powder. (iv) The first powder was finely crushed to obtain a second powder. Next, the powder paint obtained as described above was sprayed onto a plate-shaped metal material made of metal of a JIS G3302 Z27 galvanized steel sheet, 150 mm long, 70 mm wide, and 2.3 mm thick, using an electrostatic powder coating apparatus (trade name) manufactured by Parker Engineering Co., Ltd. After that, the metal material sprayed with the powder paint was baked in a baking furnace at 170 to 190 °C for 30 minutes to form a coating film layer, thereby obtaining a coated metal material.

[0028] (Comparative Examples 1 to 5) A coated metal material was obtained in the same manner as in Examples 1 to 8, except that only the thermosetting resin paint raw material shown in Table 1 below was used instead of the powder paint containing the recycled polyethylene terephthalate resin powder and the thermosetting resin.

[0029] (Comparative Examples 6 and 7) The thermosetting resin coating was not included at all, and a coated metal material was obtained using only the recycled polyethylene terephthalate resin shown in Table 1 below. In Comparative Example 6, recycled polyethylene terephthalate resin powder was obtained using a cryogenic grinder using liquid nitrogen in the same manner as in Examples 1 to 8. In Comparative Example 7, recycled polyethylene terephthalate resin powder was obtained by performing normal temperature mechanical grinding on the recycled polyethylene terephthalate resin. In both Comparative Examples 6 and 7, for the coating on the metal material (JIS G3302 Z27 galvanized steel sheet), after heating to a surface temperature of 300 °C, the metal material was immersed for 3 seconds by the flow immersion method and then water-cooled to form a coating film layer.

[0030]

Table 1

[0031] For the coated metal materials of Examples 1 to 8 and Comparative Examples 1 to 7 obtained as described above, the film thickness of the coating film layer was measured, and the gloss retention rate or gloss, moisture resistance, salt spray corrosion resistance, sunshine weather resistance, acid resistance, impact resistance, and recyclability were evaluated. The method for measuring the film thickness of the coating film layer and each of the above evaluation methods are shown below.

[0032] (Method for measuring the film thickness of the coating film layer) Using an apparatus LZ-373 (trade name) manufactured by Kett Science Laboratory Co., Ltd., the film thickness (μm) of the coating film layer was measured electromagnetically.

[0033] (Gloss retention rate or gloss) The gloss (%) was measured for the surface of the coating film layer using a PG-2M (trade name) device manufactured by Nippon Denshoku Industries Co., Ltd. The gloss retention rate was calculated according to the following formula and evaluated according to the following criteria. (Gloss retention rate) (-) = (Gloss of the surface of the coating film layer in Examples 1 to 8) / (Gloss of the surface of the coating film layer obtained from the thermosetting resin paint raw material) ○: The gloss retention rate is 0.8 or more △: The gloss retention rate is 0.3 or more and less than 0.8 ×: The gloss retention rate is less than 0.3

[0034] (Moisture resistance) A sample was obtained by sealing the back surface and end surface on the opposite side of the coating film layer in the painted metal material. Next, according to JIS K5600 7-2, the sample was exposed in a humidity resistance test apparatus at a relative humidity of 95% or more and a temperature of 50°C for 500 hours. After the test time ended, the sample was taken out of the humidity resistance test apparatus and left at room temperature for 2 hours. Thereafter, a crosshatch tape adhesion test was conducted on the coating film layer of the sample. The crosshatch tape adhesion test was conducted according to the test conditions of JIS K 5600-5-6 and evaluated as follows. The determination of the result of the crosshatch tape adhesion test was evaluated in 6 grades from 0 to 5 according to Table 1 of JIS K 5600-5-6. According to the above determination criteria, "5" is the worst and "0" is the best.

[0035] (Salt spray corrosion resistance) A sample was obtained by sealing the back surface and end surface on the opposite side of the coating film layer in the painted metal material. A 80 mm long crosscut was made in the coating film layer, and according to JIS K 5600 7.1, an NaCl aqueous solution with a concentration of 5% by mass was spray-sprayed on the coating film layer in a tank at 35°C. After 240 hours passed, the sample was taken out of the tank, and an evaluation test was conducted 2 hours later. A peeling test was conducted on the crosscut portion with cellophane tape, the maximum swelling width (mm) on one side and the maximum peeling width (mm) on one side from the cut portion were measured, and the larger value was adopted as the evaluation value. A qualified evaluation value was 3 mm or less.

[0036] (Sunshine Weather Resistance) The sunshine carbon arc lamp weather resistance test based on JIS D0205 5.4 was conducted on the painted metal material. The gloss retention rate represented by the following formula was measured. (Gloss Retention Rate)(%) = (Gloss after 500-hour test) / (Initial Gloss) × 100 The obtained gloss retention rate was evaluated according to the following criteria. ○: The gloss retention rate is 70% or more ×: The gloss retention rate is less than 70%

[0037] (Acid Resistance) For the painted metal material, samples were obtained by sealing the back surface and end surface on the opposite side of the coating film layer. After immersing the samples in hydrochloric acid with a concentration of 5 mass% at 23°C for 120 hours, the samples were taken out from the hydrochloric acid. Thereafter, regarding the coating film layer of the samples, whether there was no swelling or peeling of the coating film was visually observed and evaluated according to the following criteria. ○: There is no swelling or peeling of the coating film layer △: In a part where the immersed area of the coating film layer is less than 20%, one or more of swelling and peeling of the coating film occurred ×: In a part where the immersed area of the coating film layer is 20% or more, one or more of swelling and peeling of the coating film occurred

[0038] (Impact Resistance) The DuPont impact test based on JIS K5600 5-3.6 was conducted on the painted metal material. More specifically, using a weight of 500 g, it was dropped from a height of 50 cm onto the coating film layer side using a mold of Φ1 / 2, and evaluated according to the following criteria. ○: There was no peeling of the coating film layer △: In a part where the immersed area of the coating film layer is less than 20%, one or more of swelling and peeling of the coating film occurred ×: In a part where the immersed area of the coating film layer is 20% or more, one or more of swelling and peeling of the coating film occurred

[0039] (Recyclability) Examples using recycled polyethylene terephthalate resin were evaluated as having excellent recyclability and given a rating of "Good", while examples not using recycled polyethylene terephthalate resin were evaluated as having poor recyclability and given a rating of "Poor".

[0040] The measurement results of the coating layer thickness, and the evaluation results of gloss retention or gloss, moisture resistance, salt spray corrosion resistance, sunshine weather resistance, acid resistance, impact resistance, and recyclability are shown in Table 2 below.

[0041] [Table 2] As shown in Table 2 above, it was confirmed that Examples 1 to 8 had excellent properties in terms of appearance, moisture resistance, salt spray corrosion resistance, sunshine weather resistance, acid resistance, and impact resistance, which were comparable to those of Comparative Examples 1 to 5. In addition, it was confirmed that Examples 1 to 8 had excellent recyclability compared to Comparative Examples 1 to 5, since they used recycled polyethylene terephthalate resin. On the other hand, Comparative Examples 6 and 7, which used only recycled polyethylene terephthalate resin, had excellent recyclability, but were inferior in moisture resistance, sunshine weather resistance, and impact resistance compared to Examples 1 to 8. From the above, it was confirmed that the present invention fully combines the excellent properties of both the recycled polyethylene terephthalate resin powder and the thermosetting resin, and can effectively utilize waste polyethylene terephthalate resin. [Explanation of symbols]

[0042] 1. Frozen grinder 2 Liquid nitrogen tank 3. Liquid Nitrogen 4. Frozen recycled polyethylene terephthalate resin 5. Freezer 6. Crusher 7 Recycled polyethylene terephthalate resin powder 8 Recycled polyethylene terephthalate resin 9. Screw feeder

Claims

1. A step of preparing a recycled polyethylene terephthalate resin; A step of pulverizing the recycled polyethylene terephthalate resin frozen with liquid nitrogen to obtain a recycled polyethylene terephthalate resin powder; A step of preparing a thermosetting resin paint raw material; A step of mixing the recycled polyethylene terephthalate resin powder and the thermosetting resin paint raw material while heating to obtain a powder paint; A method for manufacturing a powder paint having the above steps.

2. The thermosetting resin constituting the thermosetting resin paint raw material is at least one resin selected from the group consisting of a polyester resin, a modified acrylic resin, an epoxy resin, and a fluororesin. The method for manufacturing a powder paint according to Claim 1.

3. The powder paint contains a composite resin of a recycled polyethylene terephthalate resin and a thermosetting resin. The method for manufacturing a powder paint according to Claim 1 or 2.

4. Between the step of obtaining the recycled polyethylene terephthalate resin powder and the step of obtaining the powder paint, The method for manufacturing a powder paint according to Claim 1 or 2, which includes a step of classifying the recycled polyethylene terephthalate resin powder using a mesh.

5. A recycled polyethylene terephthalate resin powder having an average particle size of 100 μm or less, A thermosetting resin paint, And a powder paint containing the above components.

6. The thermosetting resin constituting the thermosetting resin paint is at least one resin selected from the group consisting of a polyester resin, a modified acrylic resin, an epoxy resin, and a fluororesin. The powder paint according to Claim 5.

7. The powder paint contains a composite resin of a recycled polyethylene terephthalate resin and a thermosetting resin. The powder paint according to Claim 5 or 6.

8. A metallic material and a coating film layer containing a powder coating containing recycled polyethylene terephthalate resin powder and a thermosetting resin and having a coated metallic material.

9. The coated metallic material according to claim 8, wherein the thermosetting resin is at least one resin selected from the group consisting of a polyester resin, a modified acrylic resin, an epoxy resin, and a fluororesin.

10. The coated metallic material according to claim 8 or 9, wherein the average particle diameter of the recycled polyethylene terephthalate resin powder is equal to or less than the average film thickness of the coating film layer.

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