Seamless can and coated metal sheet

Abstract

This invention relates to a seamless can having an inner surface coating with excellent coating coverage and corrosion resistance on at least the inner surface side, and a coated metal sheet capable of forming such a seamless can. The inner surface coating is characterized by containing a polyester resin, wherein when the total amount of polycarboxylic acid in the polyester resin is set to 100 mol%, the content of aromatic dicarboxylic acid is 70 mol% or more, and the gel fraction (A) of the inner surface coating, as shown in the following formula, is less than 90%. Gel fraction (A) = (W2a / W1a) × 100 (%) (1a) Wherein, W1a represents the mass of the inner surface coating isolated from the coated metal substrate cut from the seamless can, and W2a represents the mass of the isolated inner surface coating after immersion in MEK at room temperature for 60 minutes and drying.

Description

[Technical Field]

[0001] This invention relates to a seamless can and a coated metal sheet for forming the seamless can, and more specifically to a seamless can that effectively prevents metal exposure caused by harsh stamping and stretching processes, coating peeling caused by heat treatment, has excellent coating coverage and corrosion resistance, and excellent resistance to embrittlement over time and / or fragrance absorption, as well as a coated metal sheet that can be readily formed into such a seamless can. [Previous Technology]

[0002] Conventionally, organic resin coated metal sheets (thermoplastic resin coated metal sheets) are known as can materials, which are made by coating metal sheets such as aluminum with a plastic film (thermoplastic resin film) composed of thermoplastic resin. It is also well known that seamless cans for filling beverages are made by stamping or stamping and stretching such organic resin coated metal sheets, or that easy-open lids are made by press molding such organic resin coated metal sheets. For example, organic resin coated metal sheets having a thermoplastic resin film containing a crystalline polyester resin with polyethylene terephthalate units as the main body as the organic resin coating layer are used as can-making materials for seamless cans (hereinafter, sometimes referred to as stamped and stretched cans) formed by stamping and stretching (Patent Document 1, etc.). Such organic resin coated metal sheets can be stamped and stretched under dry conditions without the use of coolant (cooling, lubricant), which is environmentally advantageous compared to conventional cases where metal sheets are stamped and stretched using coolant.

[0003] This type of organic resin-coated metal sheet is manufactured by film lamination methods such as bonding a pre-formed thermoplastic resin film, such as thermoplastic polyester resin, to a metal sheet using thermal bonding, or by extrusion lamination of an extruded molten film, such as thermoplastic polyester resin, to a metal sheet. However, film lamination methods make it difficult to control the film thickness to a thin film, resulting in a tendency for the film thickness to become too thick, which poses economic problems.

[0004] It is also proposed that, instead of using an organic resin-coated metal sheet through film lamination, a coated metal sheet is formed by applying a coating to the metal sheet using a thin-film coating method, and a stamped and drawn can is manufactured under drying conditions. For example, Patent Document 2 below proposes a coated metal sheet for stamped and drawn cans, which is a double-sided coated metal sheet. After processing, the dry coating amount of the film on the inner surface of the can is 90 to 400 mg / 100 cm2, the glass transition temperature is 50 to 120°C, and the pencil hardness is H or higher under test conditions at 60°C, the elongation is 200 to 600%, and the dynamic coefficient of friction is in the range of 0.03 to 0.25. After processing, the dry coating amount of the film on the outer surface of the can is 15 to 150 mg / 100 cm2, the glass transition temperature is 50 to 120°C, and the pencil hardness is H or higher under test conditions at 60°C. [Prior Art Documents] [Patent Documents]

[0005] Patent Document 1: Japanese Patent Application Publication No. 2001-353812; Patent Document 2: Japanese Patent No. 3872998 [Summary of the Invention]

[0006] [The problem the invention aims to solve]

[0007] When using coated metal sheets to form seamless cans such as stamped and drawn cans under dry conditions, the coated metal sheets are subjected to severe processing and deformation while the temperature rise is caused by the heat generated during forming. At this time, if the coating film formed on the metal substrate of the coated metal sheet does not have sufficient processability, it cannot follow the processing of the metal substrate, and there is a concern that coating defects such as metal exposure may occur, leading to a decrease in coating coverage and cracking of the substrate. Therefore, it is required that the coating film formed on the metal substrate has excellent processability (can-making processability).

[0008] Furthermore, it is known that seamless cans, such as stamped and drawn cans formed from self-coated metal sheets under drying conditions, experience a significant decrease in the adhesion between the coating and the metal substrate (hereinafter sometimes referred to as "coating adhesion") after the can body is formed due to residual stress generated in the coating film from harsh processing such as stamping and drawing. This decrease in adhesion can adversely affect various properties, such as reduced corrosion resistance. This residual stress can be removed by heat treatment of the can body under specified conditions, thereby improving adhesion. However, during such heat treatment, as the residual stress generated by the harsh processing of the coating film is suddenly relieved, shrinkage force will act on the interface between the coating film and the metal substrate. Especially in areas where the can body is processed to be strictly thin-walled, the coating film may peel off from the metal substrate, resulting in metal exposure and a decrease in coating coverage. Therefore, it is required that the coating film will not peel off during such heat treatment (coating peel resistance).

[0009] On the other hand, polyester coatings formed from polyester coating compositions have a unique problem: their processability decreases over time during storage under specified conditions (embrittlement over time). While the exact cause of this embrittlement phenomenon is not yet fully understood, the inventors speculate that it is caused by a gradual decrease in enthalpy over time, leading to a reorientation of the polyester resin molecular chains (a shift towards equilibrium), thereby causing the coating to become embrittled and its processability to decrease. When the coated metal sheet described above is used for stamping and stretching under dry conditions in a state of embrittlement, coating defects such as metal exposure are easily generated during processing, resulting in insufficient coating coverage on the inner surface and making it difficult to obtain seamless cans such as stamped and stretched cans with sufficient corrosion resistance.

[0010] In addition, the inner surface coating formed on the inner surface side of the seamless can that is in contact with the contents, in addition to can-making processability, is also required not to impair the flavor or taste of the contents, that is, while inhibiting the dissolution of the coating components, it does not absorb (adsorb) the aroma components (taste components) of the contents (aroma absorption resistance).

[0011] Patent Document 2 above proposes a coated metal sheet that can withstand stamping and stretching processes even when a coating film is formed on the inner surface of the can, exhibiting good hardness and elongation even when exothermic at temperatures close to 60°C due to continuous stamping and stretching. It also proposes a stamped and stretched can formed from the coated metal sheet. However, it completely fails to address the issues of removing residual stress and improving adhesion after forming the can from this coated metal sheet using heat treatment, or the coating peeling caused by the heat treatment, and does not resolve these problems. Furthermore, it completely fails to address the issues of embrittlement of the coated metal sheet over time during storage, the decrease in processability caused by the embrittlement of the coating over time, or the need to exhibit good resistance to aroma absorption, and does not resolve these problems.

[0012] Therefore, the object of the present invention is to provide a seamless can, such as a stamped and drawn can, and a coated metal sheet capable of forming such a seamless can, which has the following coating: suppresses coating defects such as metal exposure during severe processing such as stamping and drawing, suppresses coating peeling caused by heat treatment after can forming, has high coating coverage even after heat treatment, has excellent corrosion resistance, and exhibits excellent resistance to embrittlement over time, suppressing the formation of coating defects even after severe processing such as stamping and drawing over time, and excellent resistance to aroma absorption, suppressing the absorption of aroma components contained in the contents. [Technical Means for Solving the Problem]

[0013] According to the present invention, a seamless can is provided, characterized in that: it is a seamless can having an inner surface coating at least on the inner surface side of the can, the aforementioned inner surface coating containing a polyester resin, wherein when the total amount of polycarboxylic acid component in the aforementioned polyester resin is set to 100 moles, the content of aromatic dicarboxylic acid is 70 moles or more, and the gel fraction (A) of the aforementioned inner surface coating as shown in the following formula (1a) is less than 90%. Gel fraction (A) = (W2a / W1a) × 100 (%) (1a) Wherein, W1a represents the mass of the aforementioned inner surface coating isolated from the coated metal substrate cut from the aforementioned seamless can, and W2a represents the mass of the isolated inner surface coating after being immersed in MEK (Methyl Ethyl Ketone) at room temperature for 60 minutes and then dried.

[0014] The seamless can of the present invention is preferably: [1] The gel fraction (B) of the aforementioned inner surface coating as shown in the following formula (2a) is 10% or more higher than the aforementioned gel fraction (A), gel fraction (B) = [(W4a-W5a) / (W3a-W5a)]×100(%) (2a) Wherein, W3a represents the mass of the coated metal substrate with the aforementioned inner surface coating cut out from the aforementioned seamless can, W4a represents the mass of the coated metal substrate after being immersed in MEK at 80°C for 60 minutes and then dried, and W5a represents the mass of the metal substrate after the aforementioned inner surface coating is removed from the coated metal substrate. [2] The aforementioned inner surface coating contains soluble phenolic resin and / or amino resin as a curing agent. [3] The terephthalic acid content of the aforementioned polyester resin is less than 21 mol%. [4] When the total amount of polycarboxylic acid components in the aforementioned polyester resin is set to 100 mol%, the terephthalic acid content is 60 mol% or more. [5] When the total amount of polycarboxylic acid components in the aforementioned polyester resin is set to 100 mol%, the isophthalic acid content is 2 mol% or more and less than 21 mol%. [6] The terephthalic acid content of the aforementioned polyester resin is 4 mol% or more. [7] The thickness of the central part of the can body is 20 to 75% of the thickness of the central part of the can bottom, and the thickness of the aforementioned inner surface coating in the central part of the can body is 20 to 75% of the thickness of the aforementioned inner surface coating in the central part of the can bottom. [8] The thickness ratio of the aforementioned inner surface coating to the metal substrate (the thickness of the aforementioned inner surface coating / the thickness of the metal substrate) is approximately the same at the bottom of the can and the can body. [9] The outer surface of the can is further provided with an outer surface coating, which contains polyester resin and amine resin as curing agent.

[10] The gel fraction (A) of the aforementioned outer surface coating as shown in the following formula (3a) is less than 90%. Gel fraction (A) = (W7a / W6a) × 100 (%) (3a) Wherein, W6a represents the mass of the aforementioned outer surface coating cut from the coated metal substrate cut from the aforementioned seamless can, and W7a represents the mass of the aforementioned outer surface coating after being immersed in MEK at room temperature for 60 minutes and then dried.

[11] The heat shrinkage rate of the coating on the inner surface of the central part of the can body, as shown in the following formula (5), is 30% or less. Heat shrinkage rate (%) = (ΔL1 / L0) × 100 (5) L0: the initial length of the coating in the height direction separated from the central part of the can body ΔL1: the maximum shrinkage length of the coating in the height direction of the part corresponding to L0 when a load of 5.20 × 105 N / m2 is applied per unit area and the temperature is raised from 30°C to 200°C at a heating rate of 5°C / min

[12] The heat shrinkage rate of the coating on the inner surface of the central part of the can body, as shown in the following formula (6), is 50% or less. Heat shrinkage rate (%) = (ΔL2 / L0) × 100 (6) L0: The initial length of the coating in the height direction separated from the center of the tank body ΔL2: The maximum shrinkage length of the coating in the height direction of the part corresponding to L0 when heated from 30℃ to 200℃ at a heating rate of 5℃ / min under no load

[13] The coverage of the aforementioned inner surface coating is less than 200mA when converted to ERV (Enamel Rater Value).

[0015] According to the present invention, a coated metal sheet is also provided, characterized in that: it is a seamless coated metal sheet for cans having an inner surface coating on the surface of the inner surface of the can after processing, wherein the inner surface coating contains polyester resin and soluble phenolic resin and / or amino resin as curing agent, wherein when the total amount of polycarboxylic acid component in the aforementioned polyester resin is set to 100 moles, the content of aromatic dicarboxylic acid is 70 moles or more, the gel fraction (A) of the aforementioned inner surface coating as shown in the following formula (1b) is less than 90%, and the gel fraction (B) of the aforementioned inner surface coating as shown in the following formula (2b) is 10% or more higher than the aforementioned gel fraction (A). Gel fraction (A) (%) = (W2b / W1b) × 100 (1b) Wherein, W1b represents the mass of the inner surface coating cut from the coated metal substrate from the aforementioned coated metal plate, and W2b represents the mass of the inner surface coating after immersion in MEK at room temperature for 60 minutes and drying. Gel fraction (B) (%) = [(W4b-W5b) / (W3b-W5b)] × 100 (2b) Wherein, W3b represents the mass of the coated metal substrate with the aforementioned inner surface coating cut from the aforementioned coated metal plate, W4b represents the mass of the coated metal substrate after immersion in MEK at 80°C for 60 minutes and drying, and W5b represents the mass of the metal substrate after the aforementioned inner surface coating is removed from the coated metal substrate.

[0016] The coated metal sheet of the present invention is preferably: [1] The surface that becomes the outer surface of the can after processing is further provided with an outer surface coating, the aforementioned outer surface coating containing polyester resin and an amine resin as a curing agent, the gel fraction (A) of the aforementioned outer surface coating as shown in the following formula (3b) is less than 90%, gel fraction (A) (%)=(W7b / W6b)×100 (3b) where W6b represents the mass of the aforementioned outer surface coating cut from the coated metal substrate, and W7b represents the mass of the aforementioned outer surface coating after being immersed in MEK at room temperature for 60 minutes and then dried. [2] The phthalic acid content of the aforementioned polyester resin contained in the aforementioned inner surface coating is less than 21 mol%, [3] The phthalic acid content of the aforementioned polyester resin contained in the aforementioned inner surface coating is 4 mol% or more. In addition, in this specification, room temperature refers to a temperature of 20℃±5℃. [Invention Effects]

[0017] In the seamless can and coated metal sheet of the present invention, by making the gel fraction (which represents the degree of crosslinking of the entire inner surface coating film (the mass ratio of coating film components that are gelled by crosslinking reaction and are insoluble in solvent), and more specifically, the gel fraction (A) of the inner surface coating film obtained by the above formulas (1a) and (1b) within the above range, even when performing severe processing such as stamping or stretching under dry conditions, there is no concern about the generation of coating film defects such as metal exposure or body cracking, and the residual stress of the coating film generated during processing can be reduced. Therefore, coating film peeling during heat treatment can be effectively suppressed, and the coating film coverage is also excellent after heat treatment. Furthermore, by removing residual stress through heat treatment, the coating film adhesion is improved, thus exhibiting excellent corrosion resistance. The above content can also be understood from the results of the experimental examples described later. It can be seen that the coated metal sheet of the present invention, when formed into a stamped and stretched can by stamping and stretching, and after heat treatment after forming, also has good coating coverage with an inner surface coating coverage of less than 200mA expressed in ERV, and has excellent can-making processability and coating peel resistance.

[0018] Furthermore, as mentioned above, it is speculated that the embrittlement of polyester coatings over time is caused by the reorientation (transition to an equilibrium state) of the molecular chains of the polyester resin due to enthalpy easing. The inventors have conducted intensive research on the embrittlement of polyester coatings over time and have found that the enthalpy easing phenomenon of the polyester resin, which is the cause, is significantly influenced by the constituent monomers of the polyester resin. This will be explained in detail below.

[0019] Typically, polyester resins used as the main component of the aforementioned polyester coatings contain a large amount of aromatic dicarboxylic acids such as terephthalic acid and isophthalic acid as polycarboxylic acid monomers to ensure the heat resistance and strength of the coating. The inventors have discovered that when using polyester resins containing a certain ratio or higher of isophthalic acid in these aromatic dicarboxylic acid components, enthalpy mitigation and subsequent embrittlement of the coating over time are easily caused. Furthermore, they have found that by using polyester resins with an isophthalic acid content of less than a certain ratio in the polycarboxylic acid components, enthalpy mitigation can be suppressed, thereby preventing embrittlement of the coating over time.

[0020] From this perspective, in this invention, by using a polyester resin in which the total amount of polycarboxylic acid component constituting the polyester resin is set to 100 moles and the content of phthalic acid component is less than 21 moles, the embrittlement of the coating over time can be effectively suppressed. Even when subjected to harsh processing under dry conditions, such as stamping and stretching after storage, where the coating thickness on the inner surface of the can body is 20 to 75% of the coating thickness on the inner surface of the can bottom, metal exposure is effectively prevented. After stamping and stretching, the coating has high coating coverage, providing seamless cans such as stamped and stretched cans with excellent corrosion resistance. The above can also be understood from the results of Experimental Examples 1 to 14 described later. The coated metal sheet used in this invention has excellent processability and excellent resistance to embrittlement over time, even after being stored at 40°C for one month and subjected to bending tests.

[0021] Furthermore, the inventors have conducted intensive research on the aroma absorption resistance of a polyester inner surface coating formed on the inner surface side in contact with the contents of a seamless can formed by stamping and stretching of a self-coated metal sheet. The results showed that the absorption behavior of aroma components is significantly influenced by the monomers constituting the polyester resin used as the main component of the coating. This will be explained in detail below. Polyester resins used as the main component of polyester coatings generally contain a large amount of aromatic dicarboxylic acids such as terephthalic acid and isophthalic acid as monomers to ensure the heat resistance and strength of the coating. It is known that when a coating is formed using a polyester resin containing a certain proportion or higher of isophthalic acid, it is less likely to absorb aroma components. While the exact reason for this is not yet fully understood, the inventors speculate as follows. In other words, it is speculated that when polyester resins containing a certain proportion or higher of isophthalic acid as a polycarboxylic acid component are used, the resulting coating film will be denser (higher density) compared to polyester resins with a low isophthalic acid content (such as polyester resins whose polycarboxylic acid component is composed only of terephthalic acid). This can inhibit the diffusion of aroma components into the coating film, thereby reducing the absorption of aroma components.

[0022] From this perspective, in the present invention, by using a polyester resin in which the total amount of polycarboxylic acid component constituting the polyester resin is set to 100 moles and the content of phthalic acid is 4 moles or more, especially 10 moles or more, to form a polyester inner surface coating, the coating structure can be densified (high-density), providing a stamped and drawn can with excellent aroma absorption resistance. Furthermore, the can-making process of the coating formed using the aforementioned polyester resin with a phthalic acid content of 4 moles or more as a polycarboxylic acid component is excellent; therefore, even when using coated metal sheets for stamping and drawing under dry conditions, coating defects such as metal exposure can be suppressed. Thus, a seamless can with good aroma absorption resistance, excellent inner surface coating coverage, and excellent corrosion resistance can be provided.

[0023] The above-mentioned effects of the seamless can of the present invention can also be understood from the results of the experimental examples described later. It can be seen that compared with the stamping and drawing can of Experimental Example 23, which has a polyester inner surface coating of polyester resin containing more than 4 moles of isophthalic acid, the limonene absorption rate of the stamping and drawing can of Experimental Example 15 is significantly reduced and the aroma absorption resistance is excellent.

Implementation Method

[0024] (Painted metal sheet)

[0025] In the coated metal sheet of the present invention, the first important feature is that the gel fraction, which represents the degree of crosslinking of the overall inner surface coating, is within a specified range. More specifically, the gel fraction (A) of the inner surface coating, determined by the above formula (1b) in the state of a single-layer coating, is ideally less than 90%. The reasons for this will be explained below. As mentioned above, seamless cans such as stamped and drawn cans formed from coated metal sheets under drying conditions have the following characteristics: after the can body is formed, due to the residual stress generated in the coating due to severe processing such as stamping and drawing, the adhesion between the coating and the metal substrate (hereinafter, sometimes referred to as "coating adhesion") decreases significantly, which has an adverse effect on various properties such as corrosion resistance. The residual stress can be removed by heat treatment of the can body under specified conditions, thereby improving the adhesion of the coating. However, during this heat treatment, as the residual stress generated by the harsh processing of the coating is suddenly relieved, shrinkage forces will act on the interface between the coating and the metal substrate. Especially in areas of the can body where the processing is strictly thin-walled, the coating may peel off from the metal substrate, leading to a decrease in coating coverage. To suppress this coating peeling phenomenon during heat treatment, it is necessary to reduce the residual stress of the coating after processing.

[0026] The inventors have conducted research on reducing the residual stress of the processed coating and thereby suppressing coating peeling during heat treatment. The results show that the gel fraction (the mass ratio of coating components that are gelled by cross-linking reaction and are insoluble in solvent), which represents the overall degree of cross-linking of the inner surface coating, or more specifically, the gel fraction (A) of the inner surface coating measured in the state of a single-layer coating as calculated by the above formula (1b), has a significant impact on the magnitude of the residual stress of the processed coating and the coating peeling phenomenon during heat treatment. Furthermore, the inventors have found a range of gel fraction (A) that does not produce coating peeling during heat treatment.

[0027] As described above, when using coated metal sheets to form seamless cans such as stamped and stretched at high speed under dry conditions, residual stress will be generated in the coating film after processing of the coated metal sheet. In particular, when the main resin such as polyester resin constituting the coating film is highly cross-linked by the curing agent, the residual stress is considered to increase. As will be described later, the gel fraction (A) of the coating film on the inner surface of the coated metal sheet is considered to represent the overall gel fraction of the inner surface coating film, that is, the degree of cross-linking of the coating film as a whole. When the gel fraction (A) of the coating film on the inner surface of the coated metal sheet is 90% or more, it indicates that the coating film is highly cross-linked as a whole, and therefore the residual stress after processing increases, making it difficult to prevent the coating film from peeling off during heat treatment. On the other hand, when the gel fraction (A) is adjusted to less than 90%, it is considered that the degree of cross-linking of the coating film as a whole is moderately controlled and not too high, and therefore the residual stress of the coating film after processing is reduced when the coated metal sheet is subjected to stamping and stretching processes. This reduces the shrinkage force at the interface between the coating and the metal substrate, which is relieved by residual stress during heat treatment, thus preventing coating peeling. In other words, the seamless can of this invention, even after heat treatment following seamless can forming, effectively prevents coating peeling, exhibits excellent coating coverage, and improves coating adhesion by removing residual stress through heat treatment, thus demonstrating excellent corrosion resistance.

[0028] As described above, in the inner surface coating of the coated metal plate used in this invention, the gel fraction (A) of the inner surface coating, determined by formula (1b) in the state of a single-layer coating, is ideally less than 90%, more preferably 40% or more and less than 90%, more preferably 45% to 88%, further preferably in the range of 50% to 85%, particularly preferably 55% or more and less than 85%, and most preferably in the range of 55% to 80%. When the gel fraction is higher than the above range, there is a concern that the coating may peel off during heat treatment. When it is lower than 40%, the degree of cross-linking of the coating becomes lower, and there is a concern that the heat resistance, boiling resistance, and resistance to embrittlement over time will decrease.

[0029] Furthermore, in the above-mentioned outer surface coating of the coated metal plate used in the present invention, from the viewpoint of the coating's peel resistance during heat treatment, the gel fraction (A) of the outer surface coating obtained by using the above formula (3b) is ideally less than 90%, more preferably 40% or more and less than 90%, more preferably 50 to 88%, further preferably in the range of 55 to 85%, even more preferably 60 to 84%, and most preferably in the range of 65 to 84%.

[0030] Furthermore, in the coated metal sheet of the present invention, the gel fraction (B) of the inner surface coating, determined by the above formula (2b) in the state of the coated metal sheet, is ideally 10% or more higher than the gel fraction (A) mentioned above. The reasons for this will be explained below. It is known that polyester coatings, depending on the specific combination of polyester resin and curing agent, can become coating structures where the curing agent is concentrated near the coating surface. It is presumed that this is due to the low compatibility between the polyester resin and the curing agent. In this type of coating structure, the degree of crosslinking on the coating surface is usually higher than the degree of crosslinking of the coating as a whole. In the present invention, as described above, from the viewpoint of residual stress after molding and coating peeling during heat treatment, the inner surface coating needs to have a gel fraction (A) of less than 90% to moderately suppress the degree of crosslinking of the coating as a whole. On the other hand, from the viewpoint of heat resistance, fragrance absorption resistance, etc., it is ideal for the coating surface to be highly crosslinked, that is, it is ideal to have a coating structure where the curing agent is concentrated near the coating surface as described above.

[0031] The gel fraction (B) of the coating film on the inner surface of the coated metal plate is determined by immersing the coated metal substrate in a solvent (MEK) and extracting the uncrosslinked components (uncured components) that are soluble in the solvent from the coating film. The result is calculated based on the mass of the coating film components that are insoluble in the solvent. When determining the gel fraction using this method, the uncrosslinked components are not extracted from the side of the metal substrate that is not in contact with the solvent, but rather from the surface of the coating film that is in contact with the solvent. Therefore, in the case of a coating film with a thickened layer of curing agent concentrated on the coating film surface as described above, and a coating film that is highly crosslinked only on the surface, the solvent cannot penetrate into the interior of the coating film due to the presence of the thickened layer of curing agent on the coating film surface, making it difficult to extract the uncrosslinked components present inside the coating film. Therefore, it can be said that the gel fraction (B) determined by the above method more strongly reflects the degree of crosslinking near the coating film surface than the overall degree of crosslinking of the coating film. In contrast, the gel fraction (A) mentioned above is obtained by immersing the uncrosslinked components in a solvent while the coating film is isolated from the coated metal substrate, based on the mass of coating film components that are insoluble in the solvent. In this method, uncrosslinked portions are extracted not only from the coating film surface but also from the metal plate side. Therefore, even in coatings with structures where the curing agent is concentrated near the coating surface, the solvent will penetrate into the interior and extract uncrosslinked components. Therefore, the gel fraction (A) measured using the above method can be said to reflect the overall degree of crosslinking of the coating film. Thus, in coatings where the curing agent is concentrated at the coating surface and only the coating surface is highly crosslinked, the values ​​of the gel fraction (A) and gel fraction (B) differ, with gel fraction (B) being higher in most cases. On the other hand, when the polyester resin and the curing agent have high compatibility, the curing agent is relatively uniformly distributed in the coating film, and the overall crosslinking degree of the coating film is uniform, the gel fraction (A) and the gel fraction (B) mentioned above are often relatively close.

[0032] As described above, in this invention, it is desirable that the degree of crosslinking on the coating surface is higher than that of the coating as a whole. Therefore, the coating on the inner surface of the coated metal sheet is ideally to have a higher gel fraction (B) representing the degree of crosslinking on the coating surface compared to the gel fraction (A) representing the degree of crosslinking of the coating as a whole. Specifically, it is desirable that the gel fraction (B) is 10% or more higher than the gel fraction (A), preferably 15% or more, and more preferably 20% or more. Furthermore, the range of the gel fraction (B) of the coating on the inner surface of the coated metal sheet is ideally 50% or more, preferably 50 to 99%, more preferably 60 to 98%, further preferably 70 to 97%, and even more preferably 80 to 97%, thereby providing a coated metal sheet that possesses the aforementioned properties. When the difference between the gel fraction (A) and the gel fraction (B) is 10% or less, or when the gel fraction (B) is lower than the above range, the heat resistance tends to decrease due to the lower degree of crosslinking on the coating surface. Therefore, when using high-speed forming for seamless cans such as stamping and stretching, the temperature rise is more significant, leading to a tendency for the coating to adhere to the mold during forming. Especially on the inner surface of the can, at the moment the can is removed from the forming punch after stamping and stretching, the can adheres to the forming punch, making separation difficult (poor demolding). This raises concerns about reduced productivity, such as can bending or breakage. Furthermore, there are concerns about reduced resistance to aroma absorption in the formed seamless cans.

[0033] In addition, the gel fraction (B) of the outer surface coating of the coated metal plate used in the present invention is calculated according to the following formula (4b). Ideally, the gel fraction (B) is 40% or more, more preferably 40 to 99%, more preferably 50 to 98%, further preferably 55 to 97%, and even more preferably 60 to 95%, but it is not limited thereto. Gel fraction (B) (%) = [(W9b-W10b) / (W8b-W10b)]×100 (4b) Wherein, W8b represents the mass of the coated metal substrate with the outer surface coating formed cut from the aforementioned coated metal plate, W9b represents the mass of the coated metal substrate after being immersed in MEK at 80°C for 60 minutes and then dried, and W10b represents the mass of the metal substrate after the outer surface coating is removed from the coated metal substrate. When the gel fraction (B) of the outer surface coating is lower than the above range, the degree of cross-linking of the coating surface becomes lower, resulting in a decrease in coating hardness. This raises concerns about coating peeling and other outer surface defects during the molding of seamless cans such as stamped and drawn cans.

[0034] In order to become a coated metal sheet with resistance to embrittlement over time, the coated metal sheet of the present invention is important to have the following second feature in addition to the first feature mentioned above: the inner surface coating of the inner surface side of the can contains polyester resin, and when the total amount of polycarboxylic acid component constituting the aforementioned polyester resin is set to 100 moles, the content of aromatic dicarboxylic acid component is 70 moles or more and the content of isophthalic acid component is less than 21 moles. The coated metal sheet of the present invention, by having the content of the phthalic acid component of the polyester resin contained in the inner surface coating film in the range of less than 21 mol%, preferably less than 20 mol%, more preferably less than 18 mol%, and even more preferably less than 16 mol%, as described above, suppresses the enthalpy reduction, i.e., the reorientation (transition to equilibrium state) of the molecular chains of the polyester resin, which is the cause of the embrittlement of the coating film over time. As a result, the processability of the coating film does not decrease after storage over time. Even if the stamped and drawn can is formed by harsh processing such as stamping and drawing under dry conditions, it goes without saying that the body cracks, and the coating defects such as metal exposure are effectively prevented. Seamless cans such as stamped and drawn cans with excellent coating coverage can be formed.

[0035] In addition, in order to make the coated metal plate of the present invention a coated metal plate with fragrance absorption resistance, it is important that, in addition to the first feature mentioned above, it also has the following third feature: at least the inner surface coating of the inner surface side of the can contains polyester resin, and when the total amount of polycarboxylic acid component in the aforementioned polyester resin is set to 100 moles, the content of aromatic dicarboxylic acid is 70 moles or more and the content of isophthalic acid is 4 moles or more. The coated metal sheet of the present invention, by setting all polycarboxylic acid components in the polyester resin contained in the inner surface coating to 100 mol%, and the isophthalic acid content to 4 mol% or more, preferably 10 mol% or more, more preferably 10 to 90 mol%, more preferably 16 to 80 mol%, further preferably 21 to 80 mol%, particularly preferably 21 to 70 mol%, and most preferably in the range of 21 to 51 mol%, as described above, can achieve a denser coating (high density), resulting in improved resistance to aroma absorption. Furthermore, from the viewpoint of can-making processability, it is also more ideal, suppressing the generation of coating defects during stamping, stretching, and other processing.

[0036] When the coated metal sheet of the present invention uses a polyester resin containing isophthalic acid as a polycarboxylic acid component within a range that does not affect embrittlement over time, it can simultaneously achieve high levels of resistance to embrittlement over time and resistance to aroma absorption, thereby improving canning processability. Ideally, the isophthalic acid content of the polyester resin, considering resistance to embrittlement over time and resistance to aroma absorption, is 2 mol% or more and less than 21 mol%, more preferably 4 mol% or more and less than 18 mol%, more preferably 4 mol% or more and less than 16 mol%, further preferably more than 7.2 mol% and less than 16 mol%, and most preferably 11 mol% or more and less than 16 mol.

[0037] In addition to the viewpoints of suppressing embrittlement over time and suppressing the absorption of aroma components, it is also important that, in terms of the hardness, heat resistance, and boiling resistance of the formed coating film, when the polycarboxylic acid component constituting the polyester resin in the inner surface coating film is set to 100 mol%, the content of aromatic dicarboxylic acid component is 70 mol% or more, preferably 80 mol% or more, and more preferably 90 mol% or more. When the content of aromatic dicarboxylic acid component is less than 70 mol%, there is a concern that the hardness, heat resistance, aroma absorption resistance, and boiling resistance of the coating film will decrease.

[0038] Regarding the excellent resistance to embrittlement over time and excellent processability after storage, it is evident that the ERV (Enamel Rating Value) of the bent portion after bending processing is less than 3 mA after the coated metal sheet of the present invention has been stored at 40°C for one month, demonstrating excellent processability. Furthermore, a detailed evaluation method for assessing embrittlement over time will be described later.

[0039] Ideally, the coated metal sheet of the present invention is a double-sided coated metal sheet that further has an outer surface coating on the surface that becomes the outer surface of the can after processing. More ideally, the aforementioned outer surface coating also contains polyester resin. Furthermore, in the polyester resin contained in the aforementioned outer surface coating, it is also ideal that, when the total amount of the polycarboxylic acid component constituting the polyester resin is set to 100 mol%, the content of the aromatic dicarboxylic acid component is 70 mol% or more, preferably 80 mol% or more, and even more preferably 90 mol% or more. In addition, from the viewpoint of can-making processability, the content of isophthalic acid is ideally 2 mol% or more, preferably 5 to 90 mol%, more preferably higher than 10 mol% and lower than 85 mol%, further preferably 15 to 80 mol%, particularly preferably 20 to 80 mol%, and most preferably in the range of 20 to 51 mol%, but is not limited thereto. This can suppress the formation of coating defects during molding. In addition, it is preferable that the amount of terephthalic acid is 10 mol% or more, more preferably 15 to 90 mol%, more preferably 19 mol% or more and less than 84 mol%, further preferably 25 to 80 mol%, especially preferably 30 to 80 mol%, and most preferably 40 to 80 mol%.

[0040] The glass transition temperature (Tg) of the aforementioned inner surface coating is preferably above 30°C, more preferably above 40°C, even more preferably above 50°C and below 120°C, further preferably above 55°C and below 110°C, particularly preferably 60 to 100°C, and most preferably in the range above 65°C and below 90°C. When the Tg is below the above range, the can body easily absorbs the odor components of the contents when filled with the contents after molding, raising concerns about poor resistance to aroma absorption, and the barrier properties of the coating decrease, raising concerns about poor corrosion resistance. On the other hand, when the Tg exceeds 120°C, the processability and elongation of the coating decrease, raising concerns about metal exposure due to molding, and the can-making processability deteriorates. At the same time, the residual stress of the coating increases, raising concerns about coating peeling during heat treatment.

[0041] The Tg of the aforementioned outer surface coating is preferably above 30°C, more preferably above 40°C, even more preferably above 50°C and below 120°C, further preferably above 55°C and below 110°C, particularly preferably between 60 and 100°C, and most preferably in the range above 65°C and below 90°C. When the Tg is below the above range, the hardness of the coating decreases, thus raising concerns about surface defects such as coating peeling. On the other hand, when the Tg exceeds 120°C, the processability and elongation of the coating decrease, raising concerns about metal exposure due to molding, and the can-making processability deteriorates.

[0042] As described above, by using a coated metal sheet having an inner surface coating on at least the surface that becomes the inner surface of the can to form a seamless can by stamping, stretching, or other methods, the entire can can be covered by the aforementioned inner surface coating from the bottom of the inner surface side to the main body. Furthermore, when using a double-sided coated metal sheet that also has an outer surface coating on the surface that becomes the outer surface of the can after forming, the entire can can be covered by the aforementioned outer surface coating from the bottom of the outer surface side to the main body.

[0043] The thickness of the inner surface coating, measured by dry film thickness, is preferably 0.2 to 20 µm, more preferably 1 to 16 µm, and even more preferably greater than 2 µm and less than 12 µm. Furthermore, the dry coating mass is preferably 3 to 300 mg / dm², more preferably 15 to 220 mg / dm², more preferably 15 to 150 mg / dm², and even more preferably greater than 25 mg / dm² and less than 150 mg / dm². When the film is thinner than the above ranges, metal exposure is more likely to occur during molding, resulting in poor coverage of the inner surface coating. On the other hand, when the film is thicker than the above ranges, the residual stress generated during processing increases, making coating peeling more likely during heat treatment after molding, and resulting in poor economic efficiency due to the film thickness exceeding the required amount. Furthermore, when the contents of the seamless can are highly corrosive acidic beverages, a relatively thick film is required to ensure corrosion resistance. This thickness is preferably greater than 5 µm and less than 16 µm, more preferably greater than 6 µm and less than 12 µm, and even more preferably in the range of 6.5 to 10 µm. Additionally, the dry coating quality is preferably greater than 70 mg / dm² and less than 150 mg / dm², more preferably greater than 85 mg / dm² and less than 150 mg / dm², and even more preferably in the range of 90 to 140 mg / dm². Thinner films beyond these ranges exhibit poor corrosion resistance, and films exceeding these ranges are prone to peeling during heat treatment after stamping and stretching. On the other hand, when the contents of the seamless container are relatively mild, low-acid beverages, a relatively thin film is sufficient to ensure corrosion resistance. Therefore, a film thickness of 1 µm or more and less than 6.5 µm is suitable, more preferably more than 2 µm and less than 6.5 µm, and even more preferably in the range of 2.5 to 6 µm. Furthermore, the quality of the dried coating is suitable of 15 mg / dm² or more and less than 90 mg / dm², more preferably more than 25 mg / dm² and less than 90 mg / dm², and even more preferably in the range of 30 to 85 mg / dm². Thinner films beyond these ranges exhibit poor corrosion resistance, while films exceeding these ranges require a thickness exceeding the required amount, resulting in poor economic efficiency.

[0044] Furthermore, the film thickness of the outer surface coating, measured by dry film thickness, is preferably 0.2 to 20 µm, more preferably 1 to 16 µm, more preferably greater than 2 µm and less than 12 µm, and even more preferably greater than 2 µm and less than 6.5 µm. Additionally, the dry coating mass is preferably 3 to 300 mg / dm², more preferably 15 to 220 mg / dm², more preferably greater than 25 mg / dm² and less than 150 mg / dm², and even more preferably greater than 25 mg / dm² and less than 90 mg / dm². When the film is thinner than the above ranges, metal exposure is more likely to occur during molding, resulting in poor coating coverage on the outer surface. On the other hand, when the film is thicker than the above ranges, the residual stress generated during processing increases, thus making coating peeling more likely to occur during heat treatment after stamping and stretching. In addition, regarding the film thickness of the inner and outer surface coatings of the coated metal sheet, it is preferable that the inner surface coating, which requires higher coverage, is thicker than the outer surface coating.

[0045] Examples of metal sheets used as the metal substrate for coated metal sheets include hot-rolled steel sheets, cold-rolled steel sheets, hot-dip galvanized steel sheets, electro-galvanized steel sheets, alloy-plated steel sheets, aluminum-zinc alloy-plated steel sheets, aluminum sheets, tin-plated steel sheets, stainless steel sheets, copper sheets, copper-plated steel sheets, tin-free steel sheets, nickel-plated steel sheets, ultra-thin tin-plated steel sheets, chromium-treated steel sheets, etc. Various surface treatments can be applied as needed, such as phosphate-chromate treatment, zirconium formation treatment, or coating treatment combining water-soluble resins like polyacrylic acid with zirconium salts like ammonium zirconium carbonate, but these are not limited to these. In this invention, aluminum sheets are preferred among the aforementioned metal sheets. Besides pure aluminum sheets, aluminum alloy sheets are preferred, specifically 3000 series, 5000 series, and 6000 series aluminum alloy sheets from "JIS H 4000". Aluminum alloy sheets are suitable in terms of strength and other aspects. As for the aluminum alloy sheet, in addition to surface-treated aluminum alloy sheets that have undergone the aforementioned surface treatments, untreated aluminum alloy sheets that have not undergone surface treatment are also preferable, in order to ensure excellent adhesion between the coating film formed by the aforementioned coating composition and the metal substrate. Regarding the thickness of the metal sheet, from the viewpoint of tank strength and formability, it can be from 0.1 to 1.00 mm, preferably from 0.15 to 0.40 mm, more preferably from 0.15 to 0.30 mm, and even more preferably from 0.20 to 0.28 mm. [Inner surface coating and outer surface coating]

[0046] The inner surface coating of the coated metal sheet and the seamless tank described later in this invention, preferably both the inner and outer surface coatings, contain polyester resin as the main agent, and preferably further contain a curing agent. In the aforementioned inner surface coating, the content of polyester resin, preferably the non-crystalline polyester resin described later, is preferably higher than 50% by mass, more preferably 60% by mass or more, further preferably 70% by mass or more, and even more preferably 80% by mass or more. Similarly, in the aforementioned outer surface coating, the content of polyester resin, preferably the non-crystalline polyester resin, is preferably higher than 50% by mass, more preferably 60% by mass or more, further preferably 70% by mass or more, and even more preferably 80% by mass or more. <Polyester Resin>

[0047] In the seamless tank and coated metal sheet of the present invention, polyester resin is used as the main agent (main component) constituting the inner surface coating and the outer surface coating. Here, the main agent refers to the resin component constituting the coating that has the highest content (mass ratio). In the present invention, the mass ratio of polyester resin in the resin components constituting the above-mentioned inner surface coating and outer surface coating is preferably higher than 50% by mass, more preferably 60% by mass or more, further preferably 70% by mass or more, and even more preferably 80% by mass or more.

[0048] As a polycarboxylic acid component constituting the polyester resin, when the total amount of the polycarboxylic acid component constituting the polyester resin is set to 100 mol%, as described above, the content of aromatic dicarboxylic acid is ideally 70 mol% or more, preferably 80 mol% or more, and more preferably 90 mol% or more. Examples of aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid. Furthermore, from the viewpoint of resistance to embrittlement over time, it is ideal for the polyester resin to contain less than 21 mol% of isophthalic acid, preferably less than 20 mol%, more preferably less than 18 mol%, and even more preferably less than 16 mol%. Furthermore, from the viewpoint of heat resistance and resistance to embrittlement over time, it is suitable for the polyester resin to contain 60 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more of terephthalic acid. Furthermore, from the viewpoint of aroma absorption resistance, the ideal content of isophthalic acid is 4 mmol or more, preferably 10 mmol or more, more preferably 10 to 90 mmol, even more preferably 16 to 80 mmol, further preferably 21 to 80 mmol, especially preferably 21 to 70 mmol, and most preferably 21 to 51 mmol. Moreover, it is suitable to contain terephthalic acid in an amount of 10 mmol or more, preferably 20 to 90%, more preferably 20 mmol or more and less than 84 mmol, further preferably 25 to 79 mmol, especially preferably 30 to 79 mmol, and most preferably 49 to 79 mmol. Furthermore, from the viewpoint of simultaneously achieving resistance to embrittlement over time, resistance to aroma absorption, and canning processability, it is ideal for the polyester resin to have an isophthalic acid content of 2 mol% or more and less than 21 mol%, more preferably 4 mol% or more and less than 18 mol%, more preferably 4 mol% or more and less than 16 mol%, further preferably more than 7.2 mol% and less than 16 mol%, and especially preferably 11 mol% or more and less than 16 mol%.

[0049] When using a blend of two or more polyester resins as the polyester resin, it is ideal that, when the total amount of all polycarboxylic acid components constituting the polyester resin blend is set to 100 mol%, the content of aromatic dicarboxylic acid is 70 mol% or more, preferably 80 mol% or more, and more preferably 90 mol% or more within the aforementioned range. Furthermore, from the viewpoint of resistance to embrittlement over time, it is ideal that the content of isophthalic acid is less than 21 mol%, preferably less than 20 mol%, more preferably less than 18 mol%, and even more preferably less than 16 mol%. Additionally, it is suitable that the polyester resin blend contains 60 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more. Furthermore, from the viewpoint of resistance to aroma absorption, the ideal content of isophthalic acid is 4 mol% or more, preferably 10 mol% or more, more preferably 10 to 90 mol%, more preferably 16 to 80 mol%, further preferably 21 to 80 mol%, especially preferably 21 to 70 mol%, and most preferably in the range of 21 to 51 mol%. Moreover, from the viewpoint of simultaneously achieving resistance to embrittlement over time and resistance to aroma absorption, ideally, the polyester resin blend contains 2 mol% or more and less than 21 mol%, preferably 4 mol% or more and less than 18 mol%, more preferably 4 mol% or more and less than 16 mol%, further preferably more than 7.2 mol% and less than 16 mol%, and especially preferably more than 11 mol% and less than 16 mol%.

[0050] As the remaining components in the polycarboxylic acid composition other than aromatic dicarboxylic acids, examples include aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanoic acid, and dimer acid; unsaturated dicarboxylic acids such as maleic acid (anhydride), fumaric acid, and terpene-maleic acid adducts; alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, tetrahydrophthalic acid, hexahydroisophthalic acid, and 1,2-cyclohexenedicarboxylic acid; trimellitic acid (anhydride), pyromellitic acid (anhydride), and methylcyclohexenetricarboxylic acid; and ternary or higher polycarboxylic acids, etc. One or more of these can be selected for use.

[0051] Furthermore, as a polycarboxylic acid component constituting the polyester resin, it can contain aliphatic dicarboxylic acids and alicyclic dicarboxylic acids in an amount less than 30 mol% of the aforementioned aromatic dicarboxylic acids. However, if the ratio of aliphatic dicarboxylic acid components and alicyclic dicarboxylic acid components increases, there is a concern that the coating film will have poor fragrance absorption resistance due to easy absorption of fragrance components. Therefore, the ideal ratio of aliphatic dicarboxylic acids and alicyclic dicarboxylic acids in the polycarboxylic acid component constituting the polyester resin is less than 30 mol%, more preferably less than 20 mol%, more preferably less than 10 mol%, further preferably less than 7 mol%, even more preferably less than 5 mol%, and most preferably less than 4 mol%. When using blends of two or more polyester resins as polyester resins, it is ideal that when the total amount of all polycarboxylic acid components constituting the polyester resin blend is set to 100 mol%, the ratio of aliphatic dicarboxylic acid to alicyclic dicarboxylic acid is less than 30 mol, preferably less than 20 mol, more preferably less than 10 mol, further preferably less than 7 mol, especially preferably less than 5 mol, and most preferably less than 4 mol.

[0052] Furthermore, as a polycarboxylic acid component constituting the polyester resin, the presence of a small amount of aliphatic dicarboxylic acids with 6 to 14 carbon atoms within a range that does not impair aroma absorption resistance improves the processability of the coating during seamless can molding, thereby further enhancing the coating coverage of the seamless can. Examples of aliphatic dicarboxylic acids with 6 to 14 carbon atoms include: adipic acid, pimelic acid, octanoic acid, azelaic acid, sebacic acid, and dodecanoic acid. Among aliphatic dicarboxylic acids with 6 to 14 carbon atoms, adipic acid and sebacic acid are particularly preferred, with sebacic acid being especially desirable. When the total amount of polycarboxylic acid components constituting the aforementioned polyester resin is set at 100 mol%, the content of aliphatic dicarboxylic acids with 6 to 14 carbon atoms is ideally 0.2 to 15 mol%, more preferably 0.3 to 10 mol%, even more preferably 0.5 mol% or more but less than 6 mol%, further preferably 1 to 5 mol%, and especially preferably 1 mol% or more but less than 4 mol%. When the content is less than the above range, it is difficult to obtain the effect of improving processability by containing the aforementioned aliphatic dicarboxylic acids with 6 to 14 carbon atoms. On the other hand, if the content is greater than the above range, there is a concern that the enthalpy of the polyester resin may be moderated, the coating film may easily become brittle over time, and there is also a concern that the resistance to fragrance absorption may decrease. Similarly, when using blends of two or more polyester resins as polyester resins, it is preferable that, when the total amount of all polycarboxylic acid components constituting the polyester resin blend is set to 100 moles, the blend contains an aliphatic dicarboxylic acid with 6 to 14 carbon atoms within the above range, preferably adipic acid or sebacic acid.

[0053] As a polyol component constituting the polyester resin, when the total amount of the polyol components constituting the polyester resin is set to 100 mol%, it is suitable to contain a polyol component with a molecular structure that does not easily generate steric hindrance of polymer chains at an amount of 20 mol% or more. Examples of such polyol components include ethylene glycol, propylene glycol, 1,4-butanediol, 2-methyl-1,3-propanediol, diethylene glycol, etc., but are not limited to these. It is suitable to contain at least 20 mol%, preferably 30 mol% or more, more preferably 40 mol% or more, further preferably 50 mol% or more, especially preferably 60 mol% or more, and most preferably 70 mol% or more, selected from at least one or a combination of two or more of these. In this way, when forming seamless cans from self-coated metal sheets, it is believed that stress relief caused by the rotation of polymer chains can be easily caused, which is expected to reduce the residual stress of the coating after molding, thereby suppressing coating peeling during heat treatment. Furthermore, from the viewpoint of aroma absorption resistance, when the total amount of polyol components constituting the polyester resin is set at 100 mol%, the aforementioned 1,4-butanediol is ideally less than 40 mol%, more preferably less than 30 mol%, further preferably less than 20 mol%, and even more preferably less than 10 mol%. When the 1,4-butanediol content is 40 mol% or higher, there is a concern that the aroma absorption resistance will deteriorate.

[0054] When using a blend of two or more polyester resins as the polyester resin, it is also suitable that the total amount of the polyol components constituting the polyester resin blend accounts for 20 mol% or more, preferably 30 mol% or more, more preferably 40 mol% or more, further preferably 50 mol% or more, especially preferably 60 mol% or more, and most preferably 70 mol% or more, of at least one or a combination of two or more selected from ethylene glycol, propylene glycol, 1,4-butanediol, 2-methyl-1,3-propanediol, and diethylene glycol. Furthermore, from the viewpoint of aroma absorption resistance, when the total amount of the polyol components constituting the polyester resin blend is set to 100 mol%, 1,4-butanediol is ideally less than 40 mol%, more preferably less than 30 mol%, further preferably less than 20 mol%, and most preferably less than 10 mol.

[0055] As a residual component of the polyol, one or more of the following may be used in combination: 1,2-butanediol, 1,3-butanediol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 2-ethyl-2-butyl-1,3-propanediol, 2,4-diethyl-1,5-pentanediol, 1-methyl-1,8-octanediol, 3-methyl-1,6-hexanediol, 4-methyl-1,7-heptanediol. Aliphatic diols such as diols, 4-methyl-1,8-octanediol, 4-propyl-1,8-octanediol, and 1,9-nonanediol; ether diols such as diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, and polybutanediol; alicyclic polyols such as 1,4-cyclohexanediol, 1,3-cyclohexanediol, and 1,2-cyclohexanediol; tricyclodecanediols; and hydrogenated bisphenols; as well as trimethylolpropane, trimethylolethane, neopentyl tetrol, and other polyols with three or more tertiary components.

[0056] Polyester resin can be manufactured by the following known methods: polycondensation of one or more of the above-mentioned polycarboxylic acid components with one or more of the polyol components, and depolymerization of the polycarboxylic acid components such as terephthalic acid, isophthalic acid, trimellitic anhydride, trimellitic acid, pyromellitic acid, etc. after polycondensation; and ring-opening addition of acid anhydrides such as phthalic anhydride, maleic anhydride, trimellitic anhydride, ethylene glycol bis(triptyl phthalate) etc. after polycondensation.

[0057] From the viewpoints of curability, retort resistance, and adhesion to the metal substrate, the ideal acid value of the polyester resin is 0.1 to 40 mgKOH / g, more preferably 0.5 to 25 mgKOH / g, even more preferably 1 to 10 mgKOH / g, further preferably 1.0 mgKOH / g or more and less than 10 mgKOH / g, particularly preferably 1.0 mgKOH / g to 8 mgKOH / g, and most preferably 1.0 mgKOH / g or more and less than 5 mgKOH / g. When the acid value is below the above range, there is a concern that the adhesion between the metal substrate and the coating film may decrease. On the other hand, when the acid value is above the above range, compared with the situation within the above range, the coating film is more prone to water absorption, which raises concerns about reduced retort resistance. Furthermore, due to the increased reaction points with the curing agent, the crosslinking density of the coating film increases, which raises concerns about decreased can-making processability and coating film peel resistance, resulting in metal exposure and reduced coating coverage. In addition, when the polyester resin is a blend of two or more polyester resins, the sum of the values ​​obtained by multiplying the acid value of each polyester resin by its mass fraction is taken as the average acid value (AVmix) of the blend, as long as the average acid value is within the above-mentioned acid value range.

[0058] The hydroxyl value of the polyester resin is not limited to the following, but from the viewpoints of can-making processability, coating peeling during heat treatment, and resistance to whitening in sterilization autoclave, it is ideal to be below 20 mgKOH / g, more preferably below 10 mgKOH / g, even more preferably from 1 to 10 mgKOH / g, and further preferably from 2 to 8 mgKOH / g.

[0059] The glass transition temperature (Tg) of the polyester resin is preferably above 30°C, more preferably above 40°C, even more preferably above 50°C and below 120°C, further preferably above 55°C and below 110°C, particularly preferably between 60 and 100°C, and most preferably above 65°C and below 90°C. If the Tg is below the above range, when filling the contents of a seamless can manufactured using the stamping and stretching processes described above, the increased mobility of the resin will cause the aroma components contained in the contents to easily diffuse into the coating film. Therefore, the amount of aroma components absorbed into the coating film will also increase, raising concerns about decreased resistance to aroma absorption, as well as decreased heat resistance, corrosion resistance, and boiling resistance. On the other hand, when Tg exceeds 120°C, the processability and elongation of the coating film decrease, resulting in poor can-making processability. There is a concern that metal exposure may occur due to molding, which in turn leads to poor coating coverage after molding. Furthermore, the increased residual stress during molding may cause the coating film to have poor peel resistance during heat treatment.

[0060] In this invention, two or more polyester resins with different Tg can also be used. By using polyester resins with different Tg, compared with the case of using only one polyester resin, a coating with excellent impact resistance can be formed, and the coating is not prone to defects even when subjected to external impact. In this case, the Tgmix of the polyester resin blend calculated by the following formula (7) only needs to be within the above-mentioned Tg range. 1 / Tgmix=(W1 / Tg1)+(W2 / Tg2)+…+(Wm / Tgm) (7) W1+W2+…+Wm=1 Wherein, Tgmix represents the glass transition temperature (K) of the polyester resin blend, and Tg1, Tg2, …, Tgm represent the glass transition temperature (K) of each polyester resin (polyester resin 1, polyester resin 2, … polyester resin m) monomer used. In addition, W1, W2, ..., Wm represent the mass fraction of each polyester resin (polyester resin 1, polyester resin 2, ..., polyester resin m).

[0061] Regarding the number average molecular weight (Mn) of the polyester resin, from the viewpoint of can-making processability, it is preferably 1,000 to 100,000, more preferably 3,000 to 50,000, further preferably 5,000 to 20,000, and even more preferably in the range of 10,000 to 20,000, but is not limited to this. If it is less than the above range, the coating film may become brittle and the can-making processability may be poor; if it is greater than the above range, there is a concern about the decrease in coating stability.

[0062] Furthermore, from the viewpoint of can-making processability, dent resistance, and coating properties, non-crystalline polyester resin is preferred as the polyester resin. Here, non-crystalline means that the melting point does not show a clearly defined crystalline component when measured using a DSC (Digital Scanning Calorimeter). Compared with crystalline polyester resin, non-crystalline polyester resin has excellent solubility in solvents, is easy to coat, and can form a coating film with excellent can-making processability and dent resistance. In addition, in this invention, the mass percentage of non-crystalline polyester resin in all polyester resin components contained in the aforementioned inner surface coating film and / or outer surface coating film is preferably higher than 40% by mass, more preferably higher than 50% by mass, further preferably 60% by mass or more, particularly preferably 70% by mass or more, and most preferably 80% by mass or more. <Curing Agent>

[0063] Ideally, the inner and outer surface coatings of the coated metal sheet and seamless tank of the present invention contain a curing agent. By reacting the curing agent with the functional groups of the polyester resin as the main component, such as carboxyl and hydroxyl groups, to form a cross-linked structure, the heat resistance and boiling resistance of the coating film can be significantly improved. Examples of such curing agents include: isocyanate compounds, soluble phenolic resins, amino resins, epoxy-containing compounds, acezoline-containing compounds, carbodiimide-containing compounds, β-hydroxyalkylamide compounds, etc. In particular, from the viewpoint of curability and hygiene, soluble phenolic resins and amino resins are suitable.

[0064] In the seamless metal can of the present invention, soluble phenolic resins and amino resins are more suitable for the coating composition forming the inner surface coating film (hereinafter, sometimes referred to as "coating composition for inner surface"). In particular, from the viewpoint of can-making processability and resistance to aroma absorption, soluble phenolic resins are preferred. In the coating composition forming the outer surface coating film (hereinafter, sometimes referred to as "coating composition for outer surface"), amino resins that can form a transparent coating film without coloring from a curing agent are preferred. On the other hand, since the coating film formed by the aforementioned soluble phenolic resin will turn yellow, care should be taken when using it in the coating composition forming the outer surface coating film. <<Soluble Phenolic Resin>>

[0065] As a soluble phenolic resin, one or more of the following phenolic compounds may be used: o-cresol, p-cresol, p-tert-butylphenol, p-ethylphenol, 2,3-xylenol, 2,5-xylenol, phenol, m-cresol, m-ethylphenol, 3,5-xylenol, m-methoxyphenol, etc., and the soluble phenolic resin may be formed by reacting such phenolic compounds with formaldehyde in the presence of an alkaline catalyst.

[0066] From the viewpoint of curability, the aforementioned phenolic compounds preferably contain more than 20% by mass, more preferably more than 30% by mass, more preferably more than 50% by mass, further preferably more than 60% by mass, and especially more preferably more than 80% by mass of a soluble phenolic resin that becomes trifunctional in reaction with formaldehyde as a starting material. Examples of phenolic compounds that become trifunctional in reaction with formalin include phenol, m-cresol, m-ethylphenol, 3,5-xylenol, and m-methoxyphenol, and one or more of these can be used. If the content of these trifunctional phenolic compounds is less than 20% by mass, sufficient curability cannot be obtained, and there is a concern that the curing degree of the coating film will decrease. Among these trifunctional phenolic compounds, from the perspective of curability, m-cresol is more preferred, and soluble phenolic resins containing m-cresol as a main component as a starting material are especially preferred (hereinafter, sometimes referred to as "m-cresol-based soluble phenolic resins"). This allows for sufficient curing of the coating film, which is ideal from the perspectives of heat resistance, corrosion resistance, and boiling resistance. Furthermore, here, the main component refers to the phenolic compound that constitutes the largest proportion (by mass) of the starting material. As a m-cresol-based soluble phenolic resin, it is preferable to contain more than 50% by mass, more preferably 60% by mass or more, and even more preferably 80% by mass or more of m-cresol as the starting material.

[0067] In addition to the aforementioned trifunctional phenolic compounds, when using a difunctional phenolic compound as a starting material in the reaction with formaldehyde, the content is preferably less than 70% by mass, more preferably less than 50% by mass, even more preferably less than 30% by mass, and further preferably less than 20% by mass. If it is 70% by mass or more, there is a concern about a decrease in curability. Examples of difunctional phenolic compounds include o-cresol, p-cresol, p-tert-butylphenol, p-ethylphenol, 2,3-xylenol, and 2,5-xylenol.

[0068] Furthermore, regarding the compatibility and curability of the soluble phenolic resin used in this invention with respect to polyester resin, it is preferable to use an alcohol with 1 to 12 carbon atoms that has been partially or completely alkylated (alkoxymethylated). The ratio of the hydroxymethyl groups to be alkylated is preferably 50% or more, more preferably 60% or more, and even more preferably 80% or more. If the alkyl etherification ratio is less than 50%, the compatibility with the polyester resin will be low, the coating film will become cloudy, or sufficient curability will not be achieved. The alcohol used for alkyl etherification is a monohydric alcohol with 1 to 8 carbon atoms, preferably 1 to 4 carbon atoms. Examples of preferred monohydric alcohols include methanol, ethanol, n-propanol, n-butanol, isobutanol, etc., and n-butanol is more preferred. Furthermore, the number of alkyl-etherified hydroxymethyl (alkoxymethyl) groups is preferably 0.3 or more on average for each alkoxymethyl group in the phenolic nucleus, more preferably 0.5 to 3. If it is less than 0.3, the curability with the polyester resin deteriorates. Additionally, the number-average molecular weight (Mn) of the aforementioned soluble phenolic resin is preferably 500 to 3,000, more preferably in the range of 800 to 2,500. If it is less than the above range, there is a tendency for the crosslinking density of the formed coating to increase, thus raising concerns about increased residual stress during molding and decreased peel resistance of the coating during heat treatment. On the other hand, if it is greater than the above range, curability deteriorates, resulting in concerns about poor heat resistance, corrosion resistance, and boiling resistance of the coating. <<Amino Resins>>

[0069] Examples of amino resins include hydroxymethylated amino resins obtained by reacting amino components such as melamine, urea, benzoguanamine, ethylguanidine, steroguanamine, spiroguanamine, and dicyandiamide with aldehyde components such as formaldehyde, paraformaldehyde, acetaldehyde, and benzaldehyde. Hydroxymethylated amino resins formed by partially or completely alkylating one or more of the hydroxymethyl groups of such resins with alcohols having 1 to 12 carbon atoms are also included in the above-mentioned amino resins. These can be used alone or in combination of two or more. From the perspectives of hygiene and can-making processability, hydroxymethylated amino resins containing benzoguanidine (benzoguanidine resin), hydroxymethylated amino resins containing melamine (melamine resin), and hydroxymethylated amino resins containing urea (urea resin) are preferred as amino resins. From the perspective of curing properties (reactivity with polyester resin), benzoguanidine resin and melamine resin are more preferred. From the perspective of coating peel resistance during heat treatment, benzoguanidine resin is the best.

[0070] As a benzoguanamine resin, it is preferably a benzoguanamine resin obtained by partially or completely alkylating one of the hydroxymethyl groups of the benzoguanamine resin with alcohols such as methanol, ethanol, n-butanol, and isobutanol, such as methyl etherified benzoguanamine resin, diethyl etherified benzoguanamine resin, butyl etherified benzoguanamine resin, or a mixed etherified benzoguanamine resin of methyl ether and butyl ether, a mixed etherified benzoguanamine resin of methyl ether and diethyl ether, or a mixed etherified benzoguanamine resin of diethyl ether and butyl ether. Among these, methyl etherified benzoguanamine resin and butyl etherified benzoguanamine resin are more preferred.

[0071] As a melamine resin, it is preferably a melamine resin obtained by partially or completely alkylating one of the hydroxymethyl groups of the melamine resin with alcohols such as methanol, ethanol, n-butanol, and isobutanol. Examples include methyl etherified melamine resin, diethyl etherified melamine resin, butyl etherified melamine resin, or a mixed etherified melamine resin of methyl ether and butyl ether, a mixed etherified melamine resin of methyl ether and diethyl ether, and a mixed etherified melamine resin of diethyl ether and butyl ether. Among these, methyl etherified melamine resin is more preferred, and fully etherified methyl etherified melamine resin is even more preferred.

[0072] Preferably, the urea resin is a urea resin obtained by partially or completely alkylating one of the hydroxymethyl groups of the urea resin with alcohols such as methanol, ethanol, n-butanol, and isobutanol, such as methyl etherified urea resin, diethyl etherified urea resin, butyl etherified urea resin, or a mixed etherified urea resin of methyl ether and butyl ether, a mixed etherified urea resin of methyl ether and diethyl ether, or a mixed etherified urea resin of diethyl ether and butyl ether.

[0073] Examples of functional groups possessed by the aforementioned melamine resin and benzoguanamine resin include imine groups (>NH), N-hydroxymethyl groups (>NCH2OH), and N-alkoxymethyl groups (>NCH2OR; R is alkyl). These functional groups function as reaction sites in crosslinking reactions with carboxyl groups (-COOH) and hydroxyl groups (-OH) contained in the polyester resin, which is the main agent, or in self-condensation reactions between amino resins (in addition, imine groups only contribute to self-condensation reactions). Furthermore, regarding the number of the aforementioned reaction sites (functional groups), if we compare the monomers of melamine resin and benzoguanamine resin, it is considered that melamine resin has more in its molecular structure. As a result, melamine resin has excellent curability; conversely, the crosslinking density of the formed coating film tends to be high, and depending on the formulation amount, there is a concern that the coating film may peel off during heat treatment. On the other hand, although benzoguanamine resin has poorer curing properties compared to melamine resin, the crosslinking density of the resulting coating is less likely to increase, making it arguably better than melamine resin in terms of peel resistance. Therefore, to achieve a balance between curing properties and peel resistance during heat treatment, melamine resin and benzoguanamine resin are sometimes used together, and more ideally, a mixed amine resin prepared by mixing them in a specified ratio is used. In this case, the mixing ratio (mass ratio) of melamine resin and benzoguanamine resin is ideally 90:10 to 5:95, more preferably 80:20 to 10:90, even more preferably 80:20 to 20:80, further preferably 75:25 to 25:75, and especially preferably 70:30 to 30:70.

[0074] The curing agent is ideally formulated in the range of 1 to 40 parts by weight, more preferably 1 to 30 parts by weight, and even more preferably 2 to 20 parts by weight, relative to 100 parts by weight of the polyester resin. When using a soluble phenolic resin as the curing agent, it is preferably formulated in the range of 1 to 40 parts by weight, more preferably 2 to 30 parts by weight, even more preferably 2 to 25 parts by weight, further preferably 2.5 to 20 parts by weight, and particularly preferably 3 to 15 parts by weight, relative to 100 parts by weight of the polyester resin (solid). Furthermore, when using an amino resin as the curing agent, it is ideal that the amount of melamine resin in the formulation is less than 10 parts by weight, and more preferably less than 5.5 parts by weight, relative to 100 parts by weight of the polyester resin. When melamine resin is used as a curing agent, it is preferably formulated at an amount of 0.1 parts by weight or more and less than 10 parts by weight relative to 100 parts by weight of polyester resin, more preferably 0.1 parts by weight or more and less than 5.5 parts by weight, more preferably 0.5 to 5.4 parts by weight, further preferably 0.5 to 5 parts by weight, especially preferably 0.5 to 4 parts by weight, and most preferably 1 part by weight or more and less than 4 parts by weight. When benzoguanamine resin is used as a curing agent, it is preferably formulated at an amount of 4 to 40 parts by weight relative to 100 parts by weight of polyester resin, more preferably 5 to 30 parts by weight, more preferably 6 to 28 parts by weight, further preferably 8 to 25 parts by weight, and especially preferably 8 to 24 parts by weight. When using the aforementioned mixed amine resin of melamine resin and guanidine resin as a curing agent, it is preferable to use 2 to 25 parts by weight relative to 100 parts by weight of polyester resin, more preferably 2 to 20 parts by weight, even more preferably 2.5 to 15 parts by weight, and even more preferably 3 parts by weight or more but less than 10 parts by weight.

[0075] When the curing agent dosage is less than the above range, sufficient curing properties cannot be obtained, the degree of cross-linking of the coating film becomes lower, and it is difficult to control the gel fraction (A) and gel fraction (B) of the coating film within the aforementioned range, raising concerns about decreased heat resistance. Therefore, when high-speed molding seamless cans, the temperature rise is more significant, raising concerns that the coating film may easily adhere to the mold during molding. Especially on the inner surface of the can, at the time the can is removed from the punch after molding, there are concerns about decreased productivity due to poor demolding properties, leading to can bending or breakage. On the outer surface of the can, there are concerns about surface defects such as coating peeling during stretching processes. In addition, there are concerns about decreased resistance to boiling. On the other hand, when the curing agent dosage is greater than the above range, although it also depends on the type of curing agent used, it is difficult to control the gel fraction (A) of the coating film within the aforementioned range, making it impossible to suppress coating peeling during heat treatment after molding, raising concerns about decreased coating coverage. <Curing Catalyst>

[0076] In the internal and external surface coating compositions used in this invention, a curing catalyst is preferably formulated to promote the crosslinking reaction between the polyester resin and the curing agent. As the curing catalyst, conventionally known curing catalysts can be used, such as organic sulfonic acid and phosphoric acid catalysts like p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenedisulfonic acid, phosphoric acid, alkyl phosphoric acid, or amine neutralizations thereof. Among the above-mentioned curing catalysts, organic sulfonic acid catalysts are preferred, and dodecylbenzenesulfonic acid and its amine neutralizations are particularly preferred.

[0077] Ideally, the curing catalyst, relative to 100 parts by weight of the polyester resin, is 0.01 to 3 parts by weight in solids, more preferably 0.02 to 1.0 parts by weight, even more preferably 0.02 parts by weight or more and less than 0.5 parts by weight, further preferably 0.03 parts by weight or more and less than 0.3 parts by weight, particularly preferably 0.04 parts by weight or more and less than 0.2 parts by weight, and most preferably 0.04 to 0.1 parts by weight. Furthermore, when using an amine neutralization of the aforementioned acid catalyst (e.g., an amine neutralization of dodecylbenzenesulfonic acid) as the curing catalyst, the content of the acid catalyst other than the amine is sufficient as long as it falls within the aforementioned range. If the content of the curing catalyst is less than the aforementioned range, there is a concern that the effect of promoting the curing reaction cannot be fully obtained. On the other hand, if the content of the curing catalyst is greater than the aforementioned range, the above-mentioned effect cannot be expected, and there is a concern that the water resistance of the coating film will decrease, resulting in deterioration of corrosion resistance, boiling resistance, etc. Furthermore, acid catalysts can locally exist on the surface of the metal substrate under acid-base interactions, raising concerns about reduced adhesion between the coating and the metal substrate, and potential coating peeling during canning. (Coating composition)

[0078] The coating composition forming the coating film of the coated metal sheet of the present invention contains at least the aforementioned polyester resin as the main agent and the aforementioned curing agent, and preferably further contains the aforementioned curing catalyst (acid catalyst). Furthermore, in the present invention, the component with the highest content (mass ratio) among the solid components (non-volatile components other than water, solvents, and other volatile substances) forming the coating film in the coating composition is defined as the main agent (main component). Furthermore, in the coating composition used in the present invention, the content of the aforementioned polyester resin, preferably a non-crystalline polyester resin, which serves as the main agent, is preferably higher than 50% by mass, more preferably 60% by mass or more, further preferably 70% by mass or more, and even more preferably 80% by mass or more among all the resin components contained in the coating composition. In the present invention, solvent-based coating compositions and water-based coating compositions can be examples of coating compositions that can be used for coating film formation. In the present invention, from the viewpoint of coatability, solvent-based coating compositions are preferred.

[0079] When the coating composition is a solvent-based coating composition, it contains the aforementioned polyester resin, curing agent, and an organic solvent as a solvent. Furthermore, in this embodiment, a solvent-based coating composition is defined as a coating formed by dissolving the main resin, curing agent, etc., in a known organic solvent, and wherein the organic solvent in the coating composition accounts for 40% or more by mass. As the aforementioned organic solvent, considering solubility, evaporation rate, etc., one or more are selected from toluene, xylene, aromatic hydrocarbon compounds, ethyl acetate, butyl acetate, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, isophorone, methyl celecoxib, butyl celecoxib, ethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether acetate, ethylene glycol monoethyl acetate, methanol, ethanol, butanol, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, solvent oil, etc.

[0080] When the coating composition is a water-based coating composition, it contains a conventionally known water-dispersible or water-soluble polyester resin and a curing agent, as well as an aqueous medium as a solvent. The aqueous medium can be water, as in conventional water-based coating compositions, or a mixture of water and organic solvents such as alcohols, polyols, or their derivatives. When using an organic solvent, it is preferable to contain it in an amount of 1 to 45% by mass relative to the total aqueous medium in the water-based coating composition, and more preferably in an amount of 5 to 30% by mass. By containing the solvent within the above range, the film-forming performance is improved. As such organic solvents, those with amphiphilic properties are preferred, for example: methanol, ethanol, isopropyl alcohol, n-butanol, ethylene glycol, methyl ethyl ketone, butyl celazol, carbitol, butyl carbitol, propylene glycol monopropyl ether, propylene glycol ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, 3-methyl-3-methoxybutanol, etc.

[0081] The coating composition may contain a lubricant as needed. Ideally, the amount of lubricant should be 0.1 to 20 parts by weight relative to 100 parts by weight of polyester resin, more preferably 0.2 to 10 parts by weight, and even more preferably 0.5 to 5 parts by weight. By adding a lubricant, damage to the coating film during molding can be suppressed, and the smoothness of the coating film during molding can be improved.

[0082] Examples of lubricants that can be added to coating compositions include: fatty acid ester waxes (esterifications of polyol compounds and fatty acids), silicone waxes, fluorinated waxes such as polytetrafluoroethylene, polyolefin waxes such as polyethylene, paraffin wax, lanolin, lignite wax, microcrystalline wax, carnauba wax, silicone compounds, petrolatum, etc. One or more of these lubricants may be used.

[0083] In addition to the above-mentioned components, leveling agents, pigments, defoamers, colorants, etc., that are conventionally formulated in coating compositions may also be added to the coating composition according to conventionally known formulations. Furthermore, without compromising the purpose of this invention, other resin components may be included together with the polyester resin, such as: polyvinyl acetate, ethylene-vinyl acetate copolymer, polyolefin resins, epoxy resins, polyurethane resins, acrylic resins, polyvinyl chloride resins, polyvinyl chloride-vinyl acetate copolymer resins, polyvinyl alcohol, ethylene-vinyl alcohol copolymers, polyvinylpyrrolidone, polyvinyl ether, polyacrylamide, acrylamide compounds, polyethyleneimine, starch, gum arabic, methylcellulose, and other resins.

[0084] The coating composition preferably contains polyester resin in an amount of 5 to 55% by weight, based on solids. When the resin solids content is less than the above range, an appropriate coating amount cannot be ensured, and the coating coverage deteriorates. On the other hand, when the resin solids content is more than the above range, workability and coatability are poor. (Method for manufacturing coated metal sheets)

[0085] The coated metal sheet of the present invention uses the aforementioned polyester resin as the main agent and applies an inner surface coating composition containing the polyester resin, preferably also containing a soluble phenolic resin and / or an amino resin as a curing agent, to at least the inner surface of the metal sheet in such a way as to achieve the aforementioned film thickness. Preferably, an outer surface coating composition containing the aforementioned polyester resin as the main agent and preferably further containing an amino resin as a curing agent is applied to the outer surface of the metal sheet in such a way as to achieve the aforementioned film thickness.

[0086] The baking conditions of the coating composition are appropriately adjusted according to the type of polyester resin, curing agent, metal matrix, coating amount, etc. However, in order to obtain sufficient curing, the coating composition is preferably cured by heating at a baking temperature of 150°C to 350°C, preferably above 200°C and below 320°C for more than 5 seconds, preferably 5 seconds to 30 minutes, and most preferably 5 seconds to 180 seconds. When the baking temperature is below the above range, there is a concern that sufficient curing may not be obtained. On the other hand, when the baking temperature is above the above range, there is a concern that the polyester resin may be thermally decomposed due to overheating. When the baking time is shorter than the above range, there is a concern that sufficient curing may not be obtained, and when the baking time is longer than the above range, the economy and productivity are poor.

[0087] In addition, after baking, the inner surface coating and the outer surface coating ideally have a gel fraction (A) of less than 90% as described above, and a gel fraction (B) that is more than 10% higher than the gel fraction (A).

[0088] As a coating method, the coating can be applied to at least the inner surface of the metal plate, preferably both sides, by using known coating methods such as roller coating, spray coating, or dip coating, and then baked by heating means such as a coil oven.

[0089] The coated metal sheet of the present invention may also form a coating film made of other coating components (solvent-based coating components or water-based coating components) on the inner surface coating film formed on the inner surface side of the can after processing such as stamping and stretching, and / or on the outer surface coating film formed on the outer surface side of the can after processing, but it is preferable not to form such a coating film as needed from an economic point of view. Ideally, the outermost layer of the coated metal sheet used in the present invention that becomes the inner surface side of the can is a coating film formed of a coating component, preferably an inner surface coating film formed by the aforementioned inner surface coating component, or a layer formed by the wax lubricant described later on the aforementioned inner surface coating film. Similarly, ideally, the outermost layer of the coated metal sheet used in the present invention that becomes the outer surface side of the can is a coating film formed of a coating component, preferably an outer surface coating film formed by the aforementioned outer surface coating component, or a layer formed by the wax lubricant described later on the aforementioned outer surface coating film. Furthermore, in the coated metal sheet of the present invention, the inner surface coating and the outer surface coating formed by the aforementioned coating composition are preferably formed in such a manner that the inner surface coating and / or the outer surface coating directly contact the metal sheet, which serves as the metal substrate, to ensure excellent adhesion to the metal substrate. (Seamless can)

[0090] The first feature of the seamless can such as the stamped and drawn can of the present invention is that, similar to the aforementioned coated metal sheet, the gel fraction (A) of the inner surface coating as shown in the above formula (1a) is less than 90%, preferably 40% or more and less than 90%, more preferably 45% to 88%, further preferably in the range of 50% to 85%, particularly preferably 55% or more and less than 85%, and most preferably in the range of 55% to 80%. Furthermore, the gel fraction (B) of the inner surface coating obtained using the above formula (2a) is preferably 10% or more higher than the gel fraction (A), preferably 15% or more, and more preferably 20% or more. Additionally, the gel fraction (B) is preferably 50% or more, preferably 50% to 99%, more preferably 60% to 98%, further preferably 70% to 97%, and most preferably in the range of 80% to 97%. Furthermore, the gel fraction (A) of the outer surface coating of the seamless can of the present invention and the gel fraction (B) shown in the following formula (4a) are ideally within the same range as the outer surface coating of the aforementioned coated metal plate. Gel fraction (B) = [(W12a-W13a) / (W11a-W13a)]×100(%) (4a) Wherein, W11a represents the mass of the coated metal substrate with the aforementioned outer surface coating cut from the aforementioned seamless can, W12a represents the mass of the coated metal substrate after being immersed in MEK at 80°C for 60 minutes and then dried, and W13a represents the mass of the metal substrate after the aforementioned outer surface coating is removed from the coated metal substrate.

[0091] Furthermore, the aforementioned gel fractions (A) and (B) of the coating of the seamless can can be measured, for example, from the inner surface coating and / or the outer surface coating located in the center of the bottom of the seamless can, but are not limited thereto. In addition, in this specification, the center of the bottom of the seamless can is a portion with relatively less processing accompanying the molding of the seamless can, and has a thickness similar to that of the coated metal sheet used for molding.

[0092] In addition, the seamless can of the present invention, for the same reasons as those for coated metal plates, also has the following second feature in terms of resistance to embrittlement over time, in addition to the first feature mentioned above: the inner surface coating on the inner surface side contains polyester resin, and when the total amount of polycarboxylic acid component in the aforementioned polyester resin is set to 100 moles, the content of aromatic dicarboxylic acid is 70 moles or more and the content of isophthalic acid is less than 21 moles. The seamless can of the present invention also suppresses the enthalpy degradation of the polyester resin, which is a cause of embrittlement of the coating over time, by ensuring that the amount of terephthalic acid contained in the inner surface coating is less than 21 mol%, preferably less than 20 mol%, more preferably less than 18 mol%, and even more preferably less than 16 mol%. As a result, the processability of the coating does not decrease over time. Therefore, even after storage for a period of time, when subjected to harsh processing such as stamping and stretching under dry conditions, a seamless can with excellent coating coverage is formed, not to mention the body cracking, and with few coating defects such as metal exposure. In addition, from the viewpoint of heat resistance and embrittlement over time, it is suitable to contain terephthalic acid in an amount of 60 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more.

[0093] Furthermore, from the viewpoint of the aroma absorption resistance of the seamless can, in addition to the first feature described above, the inner surface coating located on the inner surface side of the can also have the following third feature: it contains polyester resin, preferably further containing a curing agent, wherein when the total amount of polycarboxylic acid component in the aforementioned polyester resin is set to 100 mol%, the content of aromatic dicarboxylic acid is 70 mol% or more and the content of isophthalic acid is 4 mol% or more. The seamless can of the present invention achieves the densification (high density) of the coating by making the amount of isophthalic acid in the aforementioned polyester resin 4 mol% or more, preferably 10 mol% or more, preferably 10 to 90 mol%, more preferably 16 to 80 mol%, further preferably 21 to 80 mol%, particularly preferably 21 to 70 mol%, and most preferably 21 to 51 mol%, as described above, thereby suppressing the diffusion of aroma components into the coating.

[0094] Furthermore, from the viewpoint of simultaneously achieving resistance to embrittlement over time and resistance to aroma absorption, the phthalic acid content in the aforementioned polyester resin is ideally 2 mol% or more and less than 21 mol%, more preferably 4 mol% or more and less than 18 mol%, more preferably 4 mol% or more and less than 16 mol%, further preferably more than 7.2 mol% and less than 16 mol%, and even more preferably 11 mol% or more and less than 16 mol%. Additionally, for the same reasons stated for coated metal sheets, it is also important that 70 mol% or more, more preferably 80 mol% or more, and more preferably 90 mol% or more of the polycarboxylic acid component in the polyester resin constituting the coating is an aromatic dicarboxylic acid.

[0095] Furthermore, the aforementioned seamless can preferably have an outer surface coating on its outer surface, the aforementioned outer surface coating also containing polyester resin, and more preferably further containing a curing agent. In addition, of the polyester resin contained in the aforementioned outer surface coating, it is also ideal that, when the total amount of the polycarboxylic acid component constituting the polyester resin is set to 100 mol%, the content of the aromatic dicarboxylic acid component is 70 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more, but is not limited to this.

[0096] In addition, the aforementioned seamless can is preferably such that at least the bottom and body of the can on the inner surface side are continuously covered by the aforementioned inner surface coating, and more preferably the bottom and body of the can on the outer surface side are continuously covered by the aforementioned outer surface coating.

[0097] In addition, as described for coated metal sheets, the glass transition temperature (Tg) of the aforementioned inner surface coating and the aforementioned outer surface coating of the present invention is preferably 30°C or higher, preferably higher than 40°C, more preferably higher than 50°C and lower than 120°C, further preferably higher than 55°C and lower than 110°C, especially preferably 60 to 100°C, and most preferably in the range of higher than 65°C and lower than 90°C.

[0098] Furthermore, ideally, the thickness of the coating on the aforementioned inner surface of the central portion of the can body is 20% to 75% of the thickness of the coating on the aforementioned inner surface of the central portion of the can bottom. That is, the seamless can of the present invention is, as described below, formed by conventionally known methods using the aforementioned coated metal sheet, such as a stamped and drawn can (DI can), a stamped can (DR can), a deep-drawn can (DRD can), a thinned stamped-re-stamped can (DTR can), or a stretched stamped and drawn can, etc., with a stamped and drawn can being particularly preferred in the present invention. Since the aforementioned seamless cans are formed from self-coated metal sheets through stamping and stretching processes, the thickness of the coating on the inner surface of the can body is thinned to be the same as that of the metal substrate. The thickness of the coating on the inner surface of the central portion of the can body (the thinnest part in the height direction) is preferably 20% to 75% of the thickness of the coating on the inner surface of the central portion of the can bottom. This is particularly true for stamped and stretched cans, where it is preferably 20% to 60%, more preferably 20% to 50%, further preferably 25% to 45%, especially preferably 30% to 45%, and most preferably 30% to 40%. Similarly, the thickness of the coating on the outer surface of the central portion of the can body (the thinnest part in the height direction) is preferably 20% to 75% of the thickness of the coating on the outer surface of the central portion of the can bottom, preferably 20% to 60%, more preferably 20% to 50%, further preferably 25% to 45%, especially preferably 30% to 45%, and most preferably 30% to 40%.

[0099] Furthermore, in the seamless can of the present invention, it is preferable that the thickness of the coating on the inner surface of the thinnest wall portion of the can body is 80% or less, more preferably 70% or less, more preferably 60% or less, and even more preferably 55% or less, of the thickness of the coating on the inner surface of the thickest portion of the can body (the portion not thinned). Similarly, it is preferable that the thickness of the coating on the outer surface of the thinnest wall portion of the can body is 80% or less, more preferably 70% or less, more preferably 60% or less, and even more preferably 55% or less, of the thickness of the coating on the outer surface of the thickest portion of the can body (the portion not thinned).

[0100] The thickness of the metal substrate in the central part of the bottom of the seamless can is preferably 0.10 to 0.50 mm, more preferably 0.15 to 0.40 mm, even more preferably 0.15 to 0.30 mm, and further preferably 0.20 to 0.28 mm. The type of metal substrate is the same as that of the coated metal sheet used for molding. Alternatively, the film thickness of the coating on the inner surface of the central part of the bottom of the seamless can is the same as that described for the coated metal sheet used for molding, preferably 0.2 to 20 µm, more preferably 1 to 16 µm, and more preferably greater than 2 µm and less than 12 µm. Furthermore, the dry coating quality is preferably 3 to 300 mg / dm², more preferably 15 to 220 mg / dm², more preferably 15 to 150 mg / dm², and further preferably greater than 25 mg / dm² and less than 150 mg / dm².

[0101] When the contents filled into a seamless can are highly corrosive acidic beverages, the film thickness of the coating on the inner surface of the central part of the can bottom is preferably greater than 5 µm and less than 16 µm, more preferably greater than 6 µm and less than 12 µm, and even more preferably in the range of 6.5 to 10 µm. Furthermore, as for the quality of the dried coating, it is preferably greater than 70 mg / dm² and less than 150 mg / dm², more preferably greater than 85 mg / dm² and less than 150 mg / dm², and even more preferably in the range of 90 to 140 mg / dm². On the other hand, when the contents filled into a seamless can are relatively less corrosive, low-acid beverages, the film thickness of the coating on the inner surface of the central part of the can bottom is preferably more than 1 µm and less than 6.5 µm, more preferably greater than 2 µm and less than 6.5 µm, and even more preferably in the range of 2.5 to 6 µm. Furthermore, the quality of the dried coating film is preferably 15 mg / dm² or more and less than 90 mg / dm², more preferably greater than 25 mg / dm² and less than 90 mg / dm², and even more preferably in the range of 30 to 85 mg / dm². Additionally, the film thickness of the coating film on the outer surface of the central part of the can bottom, measured by dried film thickness, is preferably 0.2 to 20 µm, more preferably 1 to 16 µm, more preferably greater than 2 µm and less than 12 µm, and even more preferably greater than 2 µm and less than 6.5 µm. Furthermore, the quality of the dried coating film is preferably 3 to 300 mg / dm², more preferably 15 to 220 mg / dm², more preferably 25 to 150 mg / dm², and even more preferably greater than 25 mg / dm² and less than 90 mg / dm². (Manufacturing method of seamless can (stamped and drawn can))

[0102] The seamless can of the present invention can be manufactured using the coated metal sheet of the present invention described above, by a conventionally known molding method. In particular, the coated metal sheet of the present invention has excellent resistance to embrittlement over time. Therefore, even after storage in the state of the coated metal sheet, the coating film of the coated metal sheet maintains excellent elongation, processability, and bonding properties. Therefore, even during harsh processing such as stamping and stretching, there will be no body cracking or coating peeling at the can mouth end, and a stamped and drawn can with excellent coating coverage and no exposed metal can be formed. In addition, the coated metal sheet of the present invention has excellent formability and lubricity. Therefore, it goes without saying that a coolant is needed. Even without a coolant, a stamped and drawn can can be formed when molding is carried out under dry conditions. The manufacturing method of the stamped and drawn can will be described in detail below.

[0103] Preferably, a wax-based lubricant is applied to the coating surface of the coated metal sheet before stamping and stretching, thereby enabling efficient stamping and stretching under dry conditions. The wax-based lubricant is not limited to the following, but may include, for example, fatty acid ester waxes, silicone waxes, white petrolatum, rice bran wax, beeswax, wood wax, lignite wax and other mineral-derived waxes, Fischer-Tropsch wax, polyolefin waxes such as polyethylene or polypropylene, paraffin wax (solid paraffin wax, liquid paraffin wax), lanolin wax, microcrystalline wax, carnauba wax, etc., among which paraffin wax and white petrolatum are preferred, and paraffin wax is even more preferred. One or more of these wax-based lubricants may be used. Among the aforementioned wax lubricants, ideally, heat treatment (heating) at a temperature of 100 to 350°C, more preferably 150 to 250°C, and even more preferably around 200°C for a short period of time (e.g., 0.1 to 600 seconds, more preferably 1 to 300 seconds, and even more preferably 10 to 180 seconds) will volatilize more than 50% by mass of the wax lubricant (high-temperature volatile wax lubricant). In this way, the wax lubricant can be easily volatilized and removed in subsequent steps after the stamping and stretching of the can, and there is no concern about ink rejection due to the wax lubricant when printing on the outer surface of the can body, thus exhibiting better adaptability to outer surface printing. As a high-temperature volatile wax lubricant, it is ideal that at least 50% by mass, more preferably at least 60% by mass, even more preferably at least 80% by mass, and further preferably at least 90% by mass, can be volatilized away by heat treatment at a temperature of 150 to 250°C, preferably around 200°C for a short time (10 to 180 seconds). Ideally, it should have a melting point of 20 to 100°C, preferably 25 to 80°C. Regarding the coating amount of the wax lubricant, from the viewpoint of moldability and production efficiency, it is ideal to apply 1 to 1000 mg / m² to each side of the coated metal sheet, preferably 2 to 500 mg / m², more preferably 5 to 200 mg / m², further preferably 10 to 100 mg / m², and most preferably in the range of 20 to 80 mg / m².

[0104] For coated metal sheets with wax-based lubricants, a cupping press is used to stamp the blank, and a stamped cup is formed by stamping. In this invention, the stamping ratio RD, as defined by the following formula (8), is ideally in the range of 1.1 to 2.6, especially in the range of 1.4 to 2.6, on an overall basis (up to the state of stamped and drawn cans). If the stamping ratio is greater than the above range, the stamping wrinkles will become larger, and there is a concern that the coating film will crack, resulting in metal exposure. RD=D / d (8) Wherein, D represents the blank diameter, and d represents the can body diameter.

[0105] Then, the aforementioned stamped cup is subjected to re-stamping—one or more stages of stretching (stamping and stretching)—to thin the wall of the can body. In this case, the temperature of the punch used for forming is preferably adjusted to 10 to 100°C, more preferably 10 to 80°C, and even more preferably 15 to 70°C. If the punch temperature is below the above range, the wax-based lubricant applied to the coated metal sheet cannot adequately exhibit lubricity, raising concerns about poor demolding at the point when the can is removed from the punch, and also reducing the elongation of the inner surface coating, raising concerns about reduced coating coverage after forming. On the other hand, if the punch temperature is above the above range, there is a concern about poor demolding at the point when the can is removed from the forming punch. In this invention, the elongation R shown in the following formula (9) is ideally 25 to 80%, more preferably 40 to 80%, even more preferably 50 to 80%, further preferably 55 to 75%, particularly preferably 55 to 70%, and most preferably in the range of above 60% and below 70%. If the elongation is below the above range, the wall thickness cannot be sufficiently reduced, which is not economically satisfactory. On the other hand, if the elongation is above the above range, there is a concern about metal exposure. R(%)=(tp-tw) / tp×100 (9) Wherein, tp represents the thickness of the original coated metal sheet, and tw represents the thickness of the central portion of the side wall of the stamped and drawn can.

[0106] Furthermore, in the stamped and drawn can of the present invention, the thickness of the central portion of the can body (the central portion in the height direction, the thinnest wall portion) is preferably 20 to 75% of the thickness of the central portion of the can bottom, more preferably 20 to 60%, even more preferably 20 to 50%, further preferably 25 to 45%, particularly preferably 30 to 45%, and most preferably 30 to 40%. Similarly, the thickness of the metal substrate of the stamped and drawn can is preferably 20 to 75% of the thickness of the metal substrate in the central portion of the can body, more preferably 20 to 60%, even more preferably 20 to 50%, further preferably 25 to 45%, particularly preferably 30 to 45%, and most preferably 30 to 40%. Additionally, when a self-coated metal sheet is stamped and drawn to form a stamped and drawn can, the thickness of the coating on the can body is thinned to be the same as that of the metal substrate. Therefore, as described above, the coating thickness in the central part of the can body is preferably 20% to 75% of the coating thickness in the central part of the can bottom that is not substantially thinned during can manufacturing, more preferably 20% to 60%, even more preferably 20% to 50%, further preferably 25% to 45%, particularly preferably 30% to 45%, and most preferably 30% to 40%. Furthermore, in the stamped and drawn can of the present invention, the thickness of the thinnest part of the can body is preferably less than 80% of the thickness of the thickest part of the can body (the least thinned part), preferably less than 70%, more preferably less than 60%, and even more preferably less than 55%.

[0107] Furthermore, as described above, when a coated metal sheet having an inner surface coating is stamped and stretched to form a stamped and drawn can, the thickness of the inner surface coating in the can body is thinned to be the same as that of the metal substrate in the can body. Therefore, in the stamped and drawn can of the present invention, the thickness ratio of the aforementioned inner surface coating to the metal substrate in the can body is almost equal to the thickness ratio of the aforementioned inner surface coating to the metal substrate in the can bottom. That is, the stamped and drawn can of the present invention is characterized in that the aforementioned thickness ratio of the inner surface coating to the metal substrate (= the thickness of the aforementioned inner surface coating / the thickness of the metal substrate) is substantially the same in the can bottom and the can body. In addition, "substantially the same" here means that it includes manufacturing errors, for example, it means that the thickness of the aforementioned inner surface coating / the thickness of the metal substrate in the can body is 0.9 to 1.1 times that in the can bottom. The same applies to the outer surface coating. Furthermore, the stamped and drawn can of the present invention is characterized in that the thickness ratio of the aforementioned inner surface coating to the metal substrate (= thickness of the aforementioned inner surface coating / thickness of the metal substrate) of the can body is substantially the same throughout the entire can body, regardless of its position. The same applies to the outer surface coating.

[0108] Furthermore, the processing speed (punch movement speed) of the aforementioned one- or multiple-stage stretching process is ideally 2000 mm / sec or higher, more preferably 3000 mm / sec or higher, even more preferably 4000 mm / sec or higher, further preferably 5000 mm / sec or higher, and especially preferably 6000 mm / sec or higher. By setting the processing speed during stretching to the above-mentioned speeds, the heat generated during processing increases, resulting in a high-temperature state, thereby improving the processability (elongation) of the coating. As a result, metal exposure during forming is suppressed, and the coverage of the coating on the inner and outer surfaces after forming can be further improved. In addition, by forming at high temperatures, stress is easily relieved during forming, thus reducing the residual stress of the coating after forming, and is also better at suppressing coating peeling during heat treatment. After the stamping and stretching process, the bottom dome forming and the edge trimming of the opening are performed as needed according to conventional methods.

[0109] According to the present invention, it is preferable to perform a heat treatment step on the resulting seamless can after the aforementioned coated metal sheet has undergone stamping and stretching processes. As described above, since the gel fraction (A) of the coating film on the inner surface of the coated metal sheet and seamless can of the present invention is controlled to be less than 90%, the peeling of the coating film is effectively prevented even when heated by the heat treatment step. By performing at least one heat treatment on the seamless can such as the stamped and stretched can after forming, the residual stress of the coating film generated by processing can be removed. By removing the residual stress of the coating film, the adhesion between the processed coating film and the metal substrate (coating adhesion) can be improved. As a result, the corrosion resistance of the coating film is significantly improved, for example, when filling the seamless can with highly corrosive contents, corrosion under the coating film can be suppressed. The temperature of the heat treatment needs to be a temperature higher than the glass transition temperature of the coating film, preferably 100 to 300°C, and more preferably a temperature range of 150 to 250°C. There is no particular limitation on the heat treatment time, but it is preferable to heat for 0.1 to 600 seconds, more preferably 1 to 300 seconds, and even more preferably 10 to 180 seconds. In addition, when the wax lubricant used in the processing is the high-temperature volatile wax lubricant mentioned above, it can be evaporated and removed from the coating surface by the heat treatment.

[0110] If the residual stress of the coating of a seamless can such as a stamped and drawn can is not removed by heat treatment, and the coating of the central part of the can body (the central part in the height direction) with a large processing degree is separated from the metal substrate and heated, the size will change significantly in the direction of releasing the residual stress (mainly the height direction of the can). Therefore, by measuring the amount of dimensional change (thermal shrinkage rate) of the separated coating caused by heating, it can be set as a standard for whether the residual stress has been removed by heat treatment. The thermal shrinkage rate (with load) of the coating of the inner surface of the central part of the can body separated from the seamless can body as shown in the following formula (5) is ideally 30% or less, preferably 20% or less, more preferably 15% or less, and even more preferably 10% or less. In addition, the thermal shrinkage rate (without load) shown in the following formula (6) is ideally 50% or less, preferably 45% or less, more preferably 40% or less, and even more preferably 30% or less. When the thermal shrinkage rate is within the above-mentioned range, the coating adhesion is improved, resulting in excellent corrosion resistance. When the thermal shrinkage rate is greater than the above-mentioned range, residual stress is not sufficiently removed, raising concerns about decreased corrosion resistance due to insufficient coating adhesion, and concerns about coating peeling when the can is impacted and dented. Furthermore, when the outer surface of the can has the aforementioned outer surface coating, ideally, the thermal shrinkage rate of the outer surface coating in the central part of the can body should also be within the above-mentioned range. Additionally, the dimensional change (shrinkage) of the coating due to heating can be measured using a thermomechanical analysis (TMA) device.

[0111] Heat shrinkage rate (with load) = (ΔL1 / L0)×100(%) (5) Where, L0 is the initial length of the coating in the height direction separated from the center of the can (measuring part), and ΔL1 is the maximum shrinkage amount (maximum shrinkage length) of the coating in the height direction of the part corresponding to L0 when a load of 5.20×105N / m2 is applied per unit area and the temperature is raised from 30℃ to 200℃ at a heating rate of 5℃ / min.

[0112] Heat shrinkage rate (without load) = (ΔL2 / L0)×100(%) (6) Where, L0 is the initial length (measuring part) of the coating separated from the center of the can body in the height direction, and ΔL2 is the maximum shrinkage amount (maximum shrinkage length) of the coating corresponding to L0 in the height direction when the temperature is raised from 30℃ to 200℃ at a heating rate of 5℃ / min under no load.

[0113] After heat treatment, rapid cooling or evaporation is performed, and as needed, a printed layer is formed on the can body using conventionally known methods through printing and baking steps. A decorative varnish layer for protecting the printed layer is then formed on the printed layer. One or more necking processes are performed as needed, and flanging is then performed to produce a roll-sealed can. Alternatively, after forming a seamless can such as a stamped and drawn can, its upper part can be deformed into a bottle shape, or the bottom can be cut off and other can ends can be attached to form a bottle shape. The capacity of the stamped and drawn can of the present invention is preferably 150 mL or more, preferably 150 to 2200 mL, more preferably 180 to 1200 mL, and even more preferably 300 to 700 mL.

[0114] The inner surface coating of the coated metal sheet of the present invention has excellent can-making processability, so it can suppress the generation of metal exposure even during harsh processes such as stamping and stretching, and the coating will not peel off during heat treatment after forming. Therefore, by adjusting the forming conditions such as the stretching speed, a seamless can with excellent coating coverage, which is less than 200mA when the inner surface coating is converted to Enamel Rating Value, can be obtained. Here, the coverage of the inner surface coating obtained by Enamel Rating Value conversion refers to the value obtained by measuring the Enamel Rating Value when the seamless can is filled with 1% by mass of brine (which is used as electrolyte) to the vicinity of the can opening. The metal exposure part is formed on the outer surface side of the bottom of the can and connected to the anode. On the other hand, the cathode is immersed in the brine filled in the can and the current value is obtained after applying a DC voltage of 6.3V for 4 seconds at room temperature (about 20°C). In this type of measurement, the more current flows, the more it indicates that there are defects in the inner surface coating of the insulator, and the larger the area of ​​exposed metal on the inner surface of the can.

[0115] Ideally, the coverage of the inner surface coating of the seamless can, calculated using ERV, is less than 200 mA, more preferably less than 100 mA, and even more preferably less than 50 mA. Furthermore, when expressed as ERV per unit area (cm²), ideally, it is less than 0.70 mA / cm², more preferably less than 0.35 mA / cm², and even more preferably less than 0.18 mA / cm². Here, ERV per unit area refers to the value obtained by dividing the ERV of the stamped and drawn can, measured using the above method, by the evaluation area (the area of ​​the inner surface of the can body and bottom in contact with the aforementioned brine). Additionally, regarding the inner surface of the seamless can, after molding, a repair coating or similar material can be sprayed onto the inner surface as needed to form other coatings on the inner surface coating. Since the inner surface coating also has high coverage after molding, as described above, spraying is unnecessary, and from an economic perspective, it is preferable not to perform spraying. That is, the outermost layer of the inner surface of the seamless can is preferably a coating film formed of a paint composition, more preferably an inner surface coating film formed of the aforementioned inner surface paint composition, or a layer formed of a wax-based lubricant applied to the aforementioned inner surface coating film, and even more preferably the aforementioned inner surface coating film. Regarding the outer surface of the can, at the bottom of the can where a printing layer is substantially not formed, the aforementioned outer surface coating film may be located on the outermost layer. However, to improve the can's transportability, a coating film formed of other paint compositions may be further formed on the aforementioned outer surface coating film formed on the surface of the bottom outer surface. That is, the outermost layer of the bottom outer surface of the can is ideally a coating film formed of a paint composition, more preferably the aforementioned outer surface coating film, or a layer formed of a wax-based lubricant applied to the aforementioned outer surface coating film, and even more preferably the aforementioned outer surface coating film is located on the outermost layer of the bottom of the can.

[0116] The coated metal sheet of the present invention is preferably used for stamping and drawing cans that undergo rigorous forming processes. Therefore, it is also preferably applicable to applications other than stamping and drawing cans, such as seamless cans or can lids manufactured using conventional methods, including stamped cans (DR cans), deep-drawn cans (DRD cans), DTR cans, and stretch-stamped cans. The can lid can be of a conventional shape, such as an easy-open lid with serrations for forming an opening for dispensing contents and a pull ring for opening. It can be either a fully open type or a partially open type (with a retaining pull ring type). [Example]

[0117] The present invention will be specifically illustrated by the following experimental examples. Furthermore, descriptions using only parts indicate parts by mass.

[0118] The various test items of polyester resins A to S are determined according to the following methods. Furthermore, all polyester resins used are non-crystalline polyester resins. Table 1 shows the characteristics of the polyester resins (monomer composition, Tg, acid value, number-average molecular weight). (Determination of number-average molecular weight) Determined using a gel permeation chromatography (GPC) system with a calibration curve for standard polystyrene. (Determination of glass transfer temperature) Determined using a differential scanning calorimeter (DSC) at a heating rate of 10°C / min. (Determination of acid value) 1 g of the polyester resin solids was dissolved in 10 ml of chloroform, titrated with 0.1 N KOH ethanol solution, and the resin acid value (mgKOH / g) was determined. Phenolphthalein was used as the indicator. (Determination of monomer composition) 30 mg of the polyester resin solids was dissolved in 0.6 mL of dichloroform, and 1H-NMR was performed. The monomer composition ratio was determined based on the peak intensity. (Examples 1 to 23) [Preparation of coating composition for inner surfaces]

[0119] Use polyester resin A (acid value: 4 mgKOH / g, hydroxyl value: 5) mgKOH / g, Tg: 84℃, Mn=18,000, polycarboxylic acid composition: terephthalic acid / trimethoxylic acid = 99.8 / 0.2 mol%, polyol composition: ethylene glycol / propylene glycol = 27.4 / 72.6 mol%), etc., are used as the main agent, soluble phenolic resin (using n-butanol to alkylate hydroxymethyl to form cresol-type soluble phenolic resin, the ratio of alkylated hydroxymethyl is 90 mol%, Mn=1,200), benzoguanamine resin A (butylated benzoguanamine resin, partially etherified type containing imine and hydroxymethyl groups), benzoguanamine resin B (methylated benzoguanamine resin, partially etherified type containing imine and hydroxymethyl groups), or melamine resin (methylated melamine resin, fully etherified type) are used as the curing agent, and dodecylbenzenesulfonic acid is used as the curing catalyst (acid catalyst). Additionally, "Dodecylbenzenesulfonic acid (soft type) (mixture)" manufactured by Tokyo Chemical Industry Co., Ltd. was used as the dodecylbenzenesulfonic acid. Polyester resin was dissolved in a mixed solvent of methyl ethyl ketone / solvent oil = 50 / 50 (mass ratio) to obtain a polyester resin solution with a solid content of 30% by mass. The curing agent was diluted with methyl ethyl ketone to obtain a curing agent solution with a solid content of 30% by mass. Dodecylbenzenesulfonic acid was neutralized with 2-dimethylaminoethanol and then dissolved in isopropanol to obtain a dodecylbenzenesulfonic acid solution with a solid content of 30% by mass. Then, the polyester resin solution, curing agent solution, and acid catalyst solution were placed in a container at a specified solid content ratio and stirred for 10 minutes to prepare a solvent-based coating composition with a solid content concentration of 30% by mass. Tables 2 and 3 show the formulation composition of the coating composition for the inner surface (type of polyester resin, type of curing agent, solid content ratio of polyester resin / curing agent / acid catalyst). [Preparation of the coating composition for the outer surface]

[0120] Polyester resin was used as the main agent, melamine resin (methyl etherified melamine resin, all-ether type), benzoguanamine resin A (butyl etherified benzoguanamine resin, partially etherified type containing imine and hydroxymethyl groups) as the curing agent, and the above-mentioned dodecylbenzenesulfonic acid as the curing catalyst (acid catalyst). The polyester resin was dissolved in a mixed solvent of methyl ethyl ketone / solvent oil = 50 / 50 (mass ratio) to obtain a polyester resin solution with 30% by mass of solids. The curing agent was diluted with methyl ethyl ketone to obtain a curing agent solution with 30% by mass of solids. The dodecylbenzenesulfonic acid was neutralized with 2-dimethylaminoethanol and then dissolved in isopropanol to obtain a dodecylbenzenesulfonic acid solution with 30% by mass of solids. Then, the polyester resin solution, curing agent solution, and acid catalyst solution were placed in a container in a specified solids ratio and stirred for 10 minutes to prepare a solvent-based coating composition with a solids concentration of 30% by mass. Tables 2 and 3 show the formulation composition of the coatings used on the exterior surface (type of polyester resin, type of curing agent, and solids ratio of polyester resin / curing agent / acid catalyst). [Fabrication of Coated Metal Sheets]

[0121] The material is manufactured as follows: Using a phosphate-chromate surface-treated aluminum sheet (3104 alloy, sheet thickness: 0.27 mm) as the metal sheet, firstly, on the outer surface side after forming, the above-mentioned outer surface coating composition is applied using a bar coater to achieve a dried coating film mass of 40 mg / dm² (approximately 3 µm) after baking, and then dried at 120°C for 60 seconds. Then, on the opposite inner surface side, the above-mentioned inner surface coating composition is applied using a bar coater to achieve a dried coating film mass of 88 mg / dm² (approximately 6.4 µm) after baking, and then dried at 120°C for 60 seconds, followed by baking at 250°C (oven temperature) for 30 seconds. [Manufacturing of Stamped and Drawn Cans]

[0122] On both sides of the coated metal sheet produced by the above method, paraffin wax (which can be evaporated by heat treatment at around 200°C for a short time and is used as a high-temperature volatile wax lubricant, with more than 50% by mass being removed) is applied (coating amount: approximately 50 mg / m2 per side). The sheet is then stamped into a circle with a diameter of 142 mm to produce a shallow stamped cup. Then, for the shallow stamped cup, a punch with an outer diameter of Φ66 mm (with temperature control) is used to perform re-stamping, stretching (3 stages), and convexity processing under dry conditions. Subsequently, a heat treatment was performed at 201°C for 75 seconds in an oven to obtain a stamped and drawn can [can diameter: 66 mm, height: approximately 130 mm, capacity: approximately 370 ml, total stamping ratio: 2.15, draw ratio: 61%, thickness of the can body center section / thickness of the can bottom center section × 100 = approximately 40%, thickness of the metal substrate in the can body center section / thickness of the metal substrate in the can bottom center section × 100 = approximately 40%, thickness of the inner surface coating of the can body center section / thickness of the inner surface coating of the can bottom center section × 100 = approximately 39%, coating mass (film thickness) of the inner surface coating of the can bottom center section: 86 mg / dm2 (approximately 6.3 µm), thickness of the inner surface coating of the can bottom center section / thickness of the metal substrate in the can bottom center section = approximately 0.024, thickness of the inner surface coating of the can body center section / thickness of the metal substrate in the can body center section = approximately 0.023]. In addition, through the heat treatment at 201°C for 75 seconds, at least 90% by mass of the paraffin wax is evaporated and removed.

[0123] The coating film properties obtained from the inner surface coating composition and the outer surface coating composition used in each experimental example were tested according to the following test method. [Gel fraction (A)]

[0124] Using the inner or outer surface coating composition used in each experimental example, the single-release coating samples for testing were prepared as follows. Using the same coating conditions (coating type, dried coating quality, drying and baking conditions) as the inner or outer surface coatings in the coated metal plates of each experimental example, the inner or outer surface coating composition was applied to a phosphate-chromate surface-treated aluminum plate (3104 alloy, plate thickness: 0.27 mm) using a bar coater. After drying at 120°C, it was baked at 250°C for 30 seconds to prepare the coated metal plate. A 5 cm × 5 cm test piece was cut from the coated metal plate and immersed in a diluted hydrochloric acid solution to dissolve the aluminum plate (metal substrate). Then, the film-like single-release coating was removed, thoroughly washed with distilled water, and dried to serve as the single-release coating sample for testing. After determining the mass (W1) of the sample, the sample was immersed in 30 ml of MEK (methyl ethyl ketone) at room temperature (20°C) for 1 hour. Then, the solvent-insoluble portion (gelled portion) of the sample was removed and placed in an aluminum petri dish. It was dried at 120°C for 30 minutes and then cooled to room temperature. The total mass (W2') of the aluminum petri dish and the dried sample was measured, and the mass (W0) of the aluminum petri dish was subtracted to calculate the mass of the dried sample's solvent-insoluble portion (gelled portion) (W2 = W2' - W0). The gel fraction (A) (%) of the inner and outer surface coatings of the coated metal plate was calculated using the following formula (1). The results are shown in Table 2. Gel fraction (A) (%) = 100 × W2 / W1 (1) Alternatively, when obtaining a sample for testing from a coated metal plate or a stamped stretch can with coatings formed on both sides, after cutting out the test piece, the coating on the side not to be tested is removed by sanding with sandpaper, and then the metal substrate is dissolved using the methods described above, thereby obtaining a single-layer coating sample for testing. Alternatively, a test piece can be cut from the bottom of a coated metal plate or a stamped stretch can, immersed in boiling hydrogen peroxide for several minutes, thoroughly rinsed with distilled water, and then the film-like coating is peeled off from the metal substrate and dried; this can also be used as a single-layer coating sample for testing. [Gel fraction (A) of the coating on the inner surface of the can bottom]

[0125] For the stamped drawing can of Experimental Example 1, which was formed in the manner described in the "Preparation of Stamped Drawing Can" section above and subjected to heat treatment at 201°C for 75 seconds, the gel fraction (A) of the coating film on the inner surface of the can bottom was measured. The method for preparing the single-release coating sample for measurement is as follows. From the bottom of the heat-treated stamped drawing can, the bottom of the can was cut out with the center of the bottom as the center and with a size of 30 mm in the 0° direction and 30 mm in the 90° direction relative to the rolling direction of the metal substrate. After immersing the cut sample in boiling hydrogen peroxide for 2 to 3 minutes and thoroughly rinsing it with distilled water, the coating film on the inner surface side of the can was peeled off from the metal substrate and dried to obtain the single-release coating sample for measurement. The gel fraction (A) was measured in the same manner as the measurement method described in the "Gel Fraction (A)" section above. The results are shown below. The gel fraction (A) of the coating on the inner surface of the bottom of the stamped and drawn can in Experimental Example 1: 61% [Gel fraction (B)]

[0126] Using the inner or outer surface coating composition used in each experimental example, the test samples were prepared as follows. Using the same coating conditions (coating type, dried coating quality, drying and baking conditions) as the inner and outer surface coatings in the coated metal plates of each experimental example, the inner or outer surface coating composition was applied to a phosphate-chromate surface-treated aluminum plate (3104 alloy, plate thickness: 0.27 mm). After drying at 120°C, it was baked at 250°C for 30 seconds to prepare the coated metal plate. A 5 cm × 5 cm test piece was cut from the coated metal plate and used as the test sample to determine the mass of the test piece (W3). Using 400 ml of MEK (methyl ethyl ketone), the test piece was immersed in boiling MEK (reflux at 80°C) for 1 hour, and MEK extraction was performed at boiling point for 1 hour. After cleaning the extracted test piece with MEK, it was dried at 120°C for 30 minutes, and the mass of the extracted test piece was measured (W4). Then, the coating was peeled off and removed by decomposition with concentrated sulfuric acid, and the test piece was cleaned and dried, and the mass of the test piece was measured (W5). The gel fraction (B) (%) of the coating on the inner and outer surfaces of the coated metal plate was calculated using the following formula (2). The results are shown in Table 2. Gel fraction (B) (%) = 100 × (W4-W5) / (W3-W5) (2) In addition, when obtaining the test sample from a coated metal plate with coating on both sides, a stamped stretch can (bottom of the can, etc.), after cutting out the test piece, the coating on the side not to be measured can be removed by sanding with sandpaper, and the gel fraction (B) can be measured using the above method.

[0127] The coated metal sheets and stamped drawing cans obtained using each experimental example were evaluated according to the following test methods. [Evaluation of embrittlement resistance over time]

[0128] As an assessment of resistance to embrittlement over time, initial workability and workability over time are evaluated. The workability is evaluated according to the following bending test method.

[0129] Using the inner surface coating composition used in the experimental examples, evaluation coated metal sheets were prepared as follows. Evaluation coated metal sheets (3104 alloy, thickness: 0.27 mm) were coated using a bar coater under the same coating conditions (coating type, dried coating quality, drying and baking conditions) as in each experimental example. The sheets were dried at 120°C for 60 seconds and then baked at 250°C for 30 seconds. The coated metal sheets were cut into 3.5 × 3 cm pieces with the rolling direction of the aluminum sheet as the long side. The sheets were then bent parallel to the short side with the inner surface coating composition facing outwards. Two aluminum sheets (3104 alloy, thickness: 0.27 mm) were inserted inside the bent portion as spacers. A 3 kg weight was dropped from a height of 40 cm at room temperature (20°C) to perform the bending process. The 2 cm wide portion of the bent end was brought into contact with a sponge soaked in a 1% sodium chloride aqueous solution, and the current value (ERV) was measured after a voltage of 6.3V was applied for 4 seconds at room temperature.

[0130] Initial processability: The coated metal sheet for evaluation was evaluated within 2 days of manufacture. Processability over time: After being stored in a thermostat at 40°C for 1 month after manufacture, the coated metal sheet for evaluation was evaluated in the same manner as the initial processability. The evaluation criteria for both initial processability and processability over time are as follows. The results are shown in Table 2. ◎: Less than 0.5mA ○: 0.5mA or more but less than 1.0mA Δ: 1.0mA or more but less than 3mA ×: 3mA or more [Aroma absorption resistance evaluation (aroma component absorption test)]

[0131] The aroma absorption resistance assessment was conducted using a stamped and drawn can formed as described in the "Preparation of Stamped and Drawn Can" section above, and subjected to a heat treatment at 201°C for 75 seconds. The following steps were performed: A specified number of test pieces, each 2.5 cm × 5 cm in size, were cut, centered at a position 8.0 cm above the bottom of the can. The coating on the outer surface was sanded off with sandpaper, and the pieces were then cleaned and dried. A 5% ethanol aqueous solution containing 2 ppm limonene was prepared as a model flavor test solution. The model flavor test solution was placed in a gasketed bottle (Duran bottle), immersing the specified number of test pieces in the solution and sealing the bottle. The bottle was then stored at 30°C for 2 weeks. The test pieces were removed from the bottle, washed with water to remove water droplets, immersed in 50 mL of diethyl ether, sealed, and stored at room temperature for 24 hours. The extract was concentrated using a concentration device and analyzed by GC-MS (gas chromatography-mass spectrometry). The absorption amount was determined by using a calibration curve based on the peak of limonene obtained from GC-MS analysis. The ratio of the amount of limonene added to the absorption amount was calculated using the following formula (10) as the limonene absorption rate (%). Limonene absorption rate (%) = (Limonene absorption amount / Limonene added amount) × 100 (10) The evaluation criteria for Experiment Examples 1, 3, 4, 7, 8, 9, 10, and 12 are as follows. In addition, the number of test pieces immersed in the test solution was set to 8 for evaluation. The results are shown in Table 2. ◎: Limonene absorption rate is less than 4% ○: Limonene absorption rate is 4% or more and less than 6% Δ: Limonene absorption rate is 6% or more and less than 9% ×: Limonene absorption rate is 9% or more The evaluation criteria for Experiment Examples 15 to 23 are as follows. In addition, the number of test pieces immersed in the test solution was set to 1 for evaluation. The results are shown in Table 3. ◎: Limonene absorption rate less than 0.9% ○: Limonene absorption rate greater than 0.9% and less than 1.1% Δ: Limonene absorption rate greater than 1.1% and less than 1.3% ×: Limonene absorption rate greater than 1.3% [Internal Surface Coating Coverability Assessment (ERV Assessment)]

[0132] The evaluation of the coating coverage of the inner surface is performed on stamped and drawn cans obtained by performing stamping and drawing and doweling processes as described in the above-mentioned "Manufacturing of Stamped and Drawn Cans" section (referred to as "without heat treatment" in the table), and stamped and drawn cans after being heat-treated in an oven at 201°C for 75 seconds (referred to as "with heat treatment" in the table), as follows. In addition, the average processing speed during the drawing process (the average movement speed of the punch during the drawing process) is set to approximately 5500 mm / sec. A metal exposure part is formed on the outer surface side of the bottom of the stamped and drawn can. The can body is connected to the anode of the enamel scoring. On the other hand, 360 mL of 1% saline solution is injected into the can. The cathode of the enamel scoring is immersed in the saline solution filled with the can. The current value (ERV) is measured after applying a voltage of 6.3V for 4 seconds at room temperature (20°C). The evaluation criteria are as follows. ◎: Current value less than 50mA (less than 0.18mA / cm²) ○: Current value greater than 50mA and less than 200mA (greater than 0.18mA / cm² and less than 0.70mA / cm²) Δ: Current value greater than 200mA and less than 700mA (greater than 0.70mA / cm² and less than 2.50mA / cm²) ×: Current value greater than 700mA (greater than 2.50mA / cm²) [Coating Peel Resistance Assessment]

[0133] The coating peel resistance assessment is conducted on stamped and drawn cans manufactured according to the method described in the "Manufacturing of Stamped and Drawn Cans" section above, and subjected to a heat treatment at 201°C for 75 seconds. The presence and evaluation of coating peeling on the inner surface of the can body are observed. The evaluation criteria are as follows: ○: No coating peeling detected. Δ: Slight coating peeling detected in areas where the can body sidewalls are heavily thinned. ×: Large-scale coating peeling detected in areas where the can body sidewalls are heavily thinned. (Heat shrinkage rate assessment)

[0134] The assessment of thermal shrinkage rate was performed using the coating on the inner surface of the central portion of the can body of Experimental Example 1 and Experimental Example 15 (without heat treatment) obtained by performing the stamping and drawing process and the dovetailing process as described in the "Preparation of Stamped and Drawn Cans" section above, and the coating on the inner surface of the central portion ... Then, the inner surface coating of the inner side of the can is taken out, thoroughly cleaned with distilled water and dried, and the resulting film coating is cut into 4 mm wide (circumferential direction of the can body) and 20 mm long (height direction of the can body) to obtain the sample for testing.

[0135] The sample to be measured is clamped in a thermomechanical analysis apparatus with the clamp spacing (equivalent to the initial length of the measuring section of the coating in the height direction) being 5 mm. The displacement of the sample is measured under the following conditions to evaluate the thermal shrinkage rate in the height direction of the can under both loaded and unloaded conditions. Apparatus: Seiko Instruments TMA / SS6100 Heating rate: 5℃ / min Temperature range: 30 to 200℃ Measurement mode: Tensile mode Load under test: 5 mN (5.20 × 10⁵ N / m²) or unloaded Clamp spacing: 5 mm

[0136] The clamp spacing before measurement (equivalent to the initial length of the measuring part of the coating) is set as L0. The maximum value of the shrinkage in the height direction of the part corresponding to L0 when a load of 5.20×105N / m2 is applied per unit area and the temperature is increased from 30℃ to 200℃ at a heating rate of 5℃ / min is set as ΔL1. The value calculated by the formula shown in the following equation (5) is set as the thermal shrinkage rate (with load). In addition, the displacement is set as a positive value for shrinkage and a negative value for expansion or elongation. The results are shown below. Heat shrinkage rate (with load) = (ΔL1 / L0) × 100 (%) (5) Heat shrinkage rate of the inner surface coating of the stamped and drawn can (without heat treatment) in Experimental Example 1 (without load): 83% Heat shrinkage rate of the inner surface coating of the stamped and drawn can (with heat treatment) in Experimental Example 1 (with load): 2% Heat shrinkage rate of the inner surface coating of the stamped and drawn can (without heat treatment) in Experimental Example 15 (without load): 68% Heat shrinkage rate of the inner surface coating of the stamped and drawn can (with heat treatment) in Experimental Example 15 (with load): 8%

[0137] In addition, the clamp spacing before measurement (equivalent to the initial length of the measuring part of the coating) is set as L0, and the maximum value of the shrinkage in the height direction of the part corresponding to L0 when the temperature is increased from 30°C to 200°C at a heating rate of 5°C / min under no load (maximum shrinkage length) is set as ΔL2. The value calculated by the formula shown in the following equation (6) is set as the heat shrinkage rate (no load). In addition, the displacement amount is set as a positive value for shrinkage and a negative value for expansion or elongation. The results are shown below. Heat shrinkage rate (without load) = (ΔL2 / L0)×100(%) (6) Heat shrinkage rate of inner surface coating of stamped and drawn can (without heat treatment) in Experiment 1 (without load): 79% Heat shrinkage rate of inner surface coating of stamped and drawn can (with heat treatment) in Experiment 1 (without load): 19% Heat shrinkage rate of inner surface coating of stamped and drawn can (without heat treatment) in Experiment 15 (without load): 69% Heat shrinkage rate of inner surface coating of stamped and drawn can (with heat treatment) in Experiment 15 (without load): 30% (Corrosion resistance assessment)

[0138] The corrosion resistance assessment was conducted on the inner surface coating of the central portion of the can body of the stamped and drawn cans (without heat treatment) of Example 1 and Example 15, which were obtained by stamping and drawing and dovetailing processes as described in the "Preparation of Stamped and Drawn Cans" section above, and the stamped and drawn cans (with heat treatment) of Example 1 and Example 15, which were then heat-treated at 201°C for 75 seconds in an oven. The assessment was carried out as follows: Using the above-mentioned stamped and drawn cans, test pieces were cut with a circumference of 40 mm and a height of 40 mm centered on the central portion of the can body (the thinnest wall portion). Cross-shaped cuts with a length of 4 cm were made on the test pieces, which were then immersed in an acidic model solution containing salt and treated at 37°C for 2 weeks to assess the corrosion status. In addition, the model solution used in the test was prepared by adding citric acid to adjust the pH value to 2.5 with a salt content of 0.2%. The evaluation criteria were based on the periphery of the cross-shaped scratch. Corrosion widths under the coating exceeding 1.5 mm on each side were marked with ×, exceeding 0.5 mm but less than 1.5 mm with ○, and less than 0.5 mm with ◎. The results are shown below. Corrosion status of the stamped drawing can (without heat treatment) in Example 1: × Corrosion status of the stamped drawing can (with heat treatment) in Example 11: ◎ Corrosion status of the stamped drawing can (without heat treatment) in Example 15: × Corrosion status of the stamped drawing can (with heat treatment) in Example 15: ◎

[0139] Table 1 shows the characteristics (monomer composition, Tg, etc.) of the polyester resin used in each experimental example, and Tables 2 and 3 show the formulation composition of the coating composition for the inner surface and the coating composition for the outer surface, the coating characteristics (gel fraction) of the inner surface coating, and the evaluation results.

[0140] [Table 1]

[0141] [Table 2] Experimental Example 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Inner surface coating composition Polyester resin type Tg (°C) Isophthalic acid content (mol%) A 84 0 100 B 81 7.6 100 C 79 0 100 100 100 D 74 0 100 E 70 0 100 F 83 4.8 100 G 82 7.2 100 H 79 11.8 100 I 78 15.8 100 J 79 21.6 100 100 K 56 79.2 100 curing agent Soluble phenolic resin 10 10 10 10 10 6 10 10 10 10 10 10 Benzylguanidine resin A 15 melamine resin 6 acid catalyst Dodecylbenzenesulfonic acid 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Coating characteristics Gel fraction (A) (%) 62 63 64 64 59 56 68 69 70 72 72 62 92 93 Gel fraction (B)(%) 90 89 88 90 88 80 90 89 91 90 75 91 93 97 Difference (%) between gel fractions (A) and (B) 28 26 twenty four 26 29 twenty four twenty two 20 twenty one 18 3 29 1 4 Outer surface coating composition Polyester resin type Tg (°C) Isophthalic acid content (mol%) L 84 0 100 100 100 100 100 100 100 100 M 79 twenty four 100 100 100 100 100 100 curing agent Benzylguanidine resin A 3 3 3 3 3 3 3 15 3 3 25 7.8 melamine resin 3 3 3 3 3 3 3 3 3 3 5.5 acid catalyst Dodecylbenzenesulfonic acid 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.5 Coating characteristics Gel fraction (A) (%) 80 80 80 80 80 80 84 79 81 80 80 91 90 15 Gel fraction (B)(%) 82 82 82 82 82 82 81 77 78 82 82 97 90 5 Difference (%) between gel fractions (A) and (B) 2 2 2 2 2 2 3 2 3 2 2 6 0 10 Evaluate Resistance to embrittlement over time Initial processing properties ◎ ◎ ◎ ◎ ◎ ◎ ◎ ◎ ◎ ◎ ◎ ○ - - Processability over time ◎ ◎ ◎ ◎ ○ ◎ ◎ ◎ ◎ ◎ ◎ × - - Fragrance absorption resistance Δ - Δ Δ - - ○ ○ ◎ ◎ - ◎ - - Coating peel resistance inner surface ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ × × outer surface ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ ○ × × ○ Internal surface coating coating capability (ERV) No heat treatment ○ ○ ◎ ◎ ◎ ◎ - - - - - ◎ - - Heat treatment ○ ○ ◎ ◎ ◎ ◎ - - - - - ◎ - -

[0142] [Table 3] Experimental Example 15 16 17 18 19 20 twenty one twenty two twenty three Inner surface coating composition Polyester resin type Tg (°C) Isophthalic acid content (mol%) J 79 21.6 100 100 N 65 50.6 100 O 80 16.1 100 P 82 10.8 100 Q 58 71.3 100 R 72 19.2 100 S 61 21.8 100 A 84 0 100 curing agent Soluble phenolic resin 10 10 10 10 10 10 10 10 Benzylguanidine resin B 15 acid catalyst Dodecylbenzenesulfonic acid 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Coating characteristics Gel fraction (A) (%) 62 63 63 71 85 62 - 72 62 Gel fraction (B)(%) 91 88 90 89 92 91 80 77 90 Difference (%) between gel fractions (A) and (B) 29 25 27 18 7 29 - 5 28 Outer surface coating composition Polyester resin type Tg (°C) Isophthalic acid content (mol%) M 79 twenty four 100 100 100 100 100 100 100 100 100 curing agent Benzylguanidine resin A 3 3 3 3 3 3 15 25 melamine resin 3 3 3 3 3 3 3 acid catalyst Dodecylbenzenesulfonic acid 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Coating characteristics Gel fraction (A) (%) 84 84 84 84 84 84 81 79 91 Gel fraction (B)(%) 81 81 81 81 81 81 78 77 97 Difference (%) between gel fractions (A) and (B) 3 3 3 3 3 3 3 2 6 Evaluate Fragrance absorption resistance ◎ ◎ ○ Δ ○ ○ Δ ○ × Coating peel resistance inner surface ○ ○ ○ ○ Δ ○ ○ ○ ○ outer surface ○ ○ ○ ○ ○ ○ ○ ○ × Internal surface coating coating capability (ERV) No heat treatment ◎ ◎ ◎ ◎ ◎ ◎ ◎ ◎ ○ Heat treatment ◎ ◎ ◎ ◎ Δ ◎ ◎ ◎ ○ -: Not evaluated

[0143] The seamless cans, such as stamped and drawn cans, of the present invention are formed from a coated metal sheet having excellent coating adhesion and corrosion resistance, as well as excellent resistance to embrittlement over time and / or excellent resistance to aroma absorption, thus preventing metal exposure during heat treatment after forming. Therefore, they are preferably used for beverage containers containing aromatic components. In addition, the coated metal sheet for seamless cans of the present invention has excellent coating peel resistance, which does not cause coating peeling during heat treatment after forming, and has an inner surface coating with excellent resistance to embrittlement over time and / or excellent resistance to aroma absorption, thus making it preferably suitable for the manufacture of seamless cans.

Claims

1. A seamless can, characterized in that: it is a seamless can having an inner surface coating at least on the inner surface side of the can, the inner surface coating containing a non-crystalline polyester resin, wherein when the total amount of polycarboxylic acid component in the aforementioned non-crystalline polyester resin is set to 100 moles, the content of aromatic dicarboxylic acid is 70 moles or more, and the gel fraction (A) of the aforementioned inner surface coating as shown in the following formula (1a) is less than 90%, gel fraction (A) = (W2a / W1a) × 100 (%) (1a) where W1a represents the mass of the aforementioned inner surface coating cut from the aforementioned seamless can and isolated from the coated metal substrate, and W2a represents the mass of the isolated inner surface coating after being immersed in methyl ethyl ketone at a temperature of 20°C ± 5°C for 60 minutes and then removed and dried.

2. The seamless can of claim 1, wherein the gel fraction (B) of the aforementioned inner surface coating as shown in the following formula (2a) is 10% or more higher than the aforementioned gel fraction (A), gel fraction (B) = [(W4a-W5a) / (W3a-W5a)]×100(%) (2a) where W3a represents the mass of the coated metal substrate with the aforementioned inner surface coating cut from the aforementioned seamless can, W4a represents the mass of the coated metal substrate after being immersed in methyl ethyl ketone at 80°C for 60 minutes and then dried, and W5a represents the mass of the metal substrate after the aforementioned inner surface coating is removed from the coated metal substrate.

3. The seamless can as claimed in claim 1 or 2, wherein the aforementioned inner surface coating contains a soluble phenolic resin and / or an amino resin as a curing agent.

4. The seamless tank as requested in item 1 or 2, wherein the phthalic acid content of the aforementioned non-crystalline polyester resin is less than 21 moles.

5. For the seamless tank of claim 4, wherein when the total amount of polycarboxylic acid component in the aforementioned non-crystalline polyester resin is set to 100 moles, the content of terephthalic acid is 60 moles or more.

6. For the seamless tank of claim 4, wherein when the total amount of polycarboxylic acid component in the aforementioned non-crystalline polyester resin is set to 100 moles, the content of isophthalic acid is 2 moles or more and less than 21 moles.

7. The seamless tank as requested in item 1 or 2, wherein the phthalic acid content of the aforementioned non-crystalline polyester resin is 4 mol% or more.

8. The seamless can as requested in item 1 or 2, wherein the thickness of the central portion of the can body is 20 to 75% of the thickness of the central portion of the can bottom, and the thickness of the coating on the aforementioned inner surface of the central portion of the can body is 20 to 75% of the thickness of the coating on the aforementioned inner surface of the central portion of the can bottom.

9. The seamless can as claimed in claim 1 or 2, wherein the thickness ratio of the aforementioned inner surface coating to the metal substrate (thickness of the aforementioned inner surface coating / thickness of the metal substrate) is substantially the same at the bottom of the can and the body of the can.

10. The seamless can of claim 1 or 2, wherein the outer surface of the can is further provided with an outer surface coating containing an amorphous polyester resin and an amine resin as a curing agent.

11. The seamless tank of claim 10, wherein the gel fraction (A) of the aforementioned outer surface coating as shown in the following formula (3a) is less than 90%, gel fraction (A) = (W7a / W6a) × 100 (%) (3a) where W6a represents the mass of the aforementioned outer surface coating cut from the coated metal substrate from the aforementioned seamless tank, and W7a represents the mass of the aforementioned outer surface coating after being immersed in methyl ethyl ketone at a temperature of 20°C ± 5°C for 60 minutes and then dried.

12. For the seamless cans of claim 1 or 2, the heat shrinkage rate of the coating on the inner surface of the central part of the can body as shown in the following formula (5) is 30% or less, heat shrinkage rate (%) = (ΔL1 / L0) × 100 (5) L0: the initial length of the coating separated from the central part of the can body in the height direction ΔL1: the maximum shrinkage length of the coating in the height direction of the part corresponding to L0 when a load of 5.20 × 105 N / m2 is applied per unit area and the temperature is raised from 30°C to 200°C at a heating rate of 5°C / min.

13. For seamless cans as requested in item 1 or 2, wherein the heat shrinkage rate of the coating on the inner surface of the central part of the can body as shown in the following formula (6) is 50% or less, heat shrinkage rate (%) = (ΔL2 / L0) × 100 (6) L0: the initial length of the coating in the height direction separated from the central part of the can body ΔL2: the maximum shrinkage length of the coating in the height direction of the part corresponding to L0 when heated from 30°C to 200°C at a heating rate of 5°C / min under no load.

14. For the seamless can of claim 1 or 2, wherein the coverage of the aforementioned inner surface coating is less than 200 mA when converted to the enamel rating value, the coverage of the aforementioned inner surface coating is obtained by the following measurement method: forming a metal exposed part on the outer surface side of the bottom of the aforementioned seamless can, connecting the can body to the anode of the enamel rating, and injecting 360 mL of 1% saline solution into the can, immersing the cathode of the enamel rating in the saline solution filled in the can, and measuring the enamel rating value after applying a voltage of 6.3V at 20°C for 4 seconds.

15. A coated metal sheet, characterized in that: it is a seamless coated metal sheet for cans having an inner surface coating on at least the inner surface of the can after processing, wherein the inner surface coating contains a non-crystalline polyester resin and a soluble phenolic resin and / or an amino resin as a curing agent, wherein when the total amount of polycarboxylic acid component in the aforementioned non-crystalline polyester resin is set to 100 mol%, the content of aromatic dicarboxylic acid is 70 mol% or more, the gel fraction (A) of the aforementioned inner surface coating as shown in formula (1b) is less than 90%, and the gel fraction (B) of the aforementioned inner surface coating as shown in formula (2b) is 10% or more higher than the aforementioned gel fraction (A), and the gel fraction (A) (%) = (W2b / W1b) × 100 (1b) In the formula, W1b represents the mass of the inner surface coating cut from the coated metal substrate from the aforementioned coated metal plate, and W2b represents the mass of the inner surface coating after being immersed in methyl ethyl ketone at 20℃±5℃ for 60 minutes and then dried. Gel fraction (B)(%)=[(W4b-W5b) / (W3b-W5b)]×100 (2b) In the formula, W3b represents the mass of the coated metal substrate with the aforementioned inner surface coating cut from the aforementioned coated metal plate, W4b represents the mass of the coated metal substrate after being immersed in methyl ethyl ketone at 80℃ for 60 minutes and then dried, and W5b represents the mass of the metal substrate after the aforementioned inner surface coating is removed from the coated metal substrate.

16. The coated metal sheet of claim 15, wherein the surface that becomes the outer surface of the can after processing further has an outer surface coating, the outer surface coating containing a non-crystalline polyester resin and an amine resin as a curing agent, wherein the gel fraction (A) of the outer surface coating shown in the following formula (3b) is less than 90%, gel fraction (A) (%) = (W7b / W6b) × 100 (3b) where W6b represents the mass of the aforementioned outer surface coating isolated from the coated metal substrate cut from the aforementioned coated metal sheet, and W7b represents the mass of the isolated outer surface coating after being immersed in methyl ethyl ketone at a temperature of 20°C ± 5°C for 60 minutes and then dried.

17. The coated metal sheet of claim 15 or 16, wherein the phthalic acid content of the aforementioned non-crystalline polyester resin contained in the aforementioned inner surface coating is less than 21 moles.

18. The coated metal sheet of claim 15 or 16, wherein the phthalic acid content of the aforementioned non-crystalline polyester resin contained in the aforementioned inner surface coating is 4 mol% or more.