Seamless cans and coated metal sheets

Abstract

This invention relates to a seamless can having an inner surface coating at least on its inner surface side, and a coated metal sheet capable of being formed into such a seamless can. The seamless can is characterized in that the inner surface coating contains a polyester resin and a first-order phenolic resin and / or an amino resin as a curing agent, and the gel fraction (A) of the inner surface coating, as shown in the following formula, is 55% or more and less than 90%. This provides a seamless can with excellent resistance to boiling whitening, resistance to embrittlement over time, no coating peeling during heat treatment after forming, and excellent coating coverage, particularly for deep-drawn and thinned cans. Gel fraction (A) = (W2a / W1a) × 100 (%) where W1a represents the mass of the inner surface coating after separation from the coated metal substrate cut from the seamless can, and W2a represents the mass of the separated inner surface coating after immersion in MEK at room temperature for 60 minutes, followed by removal and drying.

Description

Technical Field

[0001] This invention relates to a seamless can made of a coated metal sheet and a coated metal sheet that can be formed into the seamless can. More specifically, it relates to a seamless can with excellent resistance to boiling and whitening, excellent resistance to embrittlement over time, effective prevention of metal exposure caused by harsh deep drawing and thinning processes, coating peeling caused by heat treatment, excellent coating coverage and corrosion resistance, and a coated metal sheet that can be formed into such a seamless can. Background Technology

[0002] Organic resin coated metal sheets (thermoplastic resin coated metal sheets), which are made of a plastic film (thermoplastic resin film) composed of thermoplastic resin and coated with metal sheets such as aluminum, have long been known as can materials. It is also known that these organic resin coated metal sheets can be deep-drawn or thinned to produce seamless cans for filling beverages, or that they can be pressed into easy-open lids. For example, organic resin coated metal sheets, which have a thermoplastic resin film formed from a crystalline polyester resin mainly composed of polyethylene terephthalate units as the organic resin coating layer, are used as can-making materials for seamless cans (hereinafter sometimes referred to as deep-drawn thinned cans) formed by deep-drawing thinning (Patent Document 1, etc.). Such organic resin coated metal sheets can be stretched and thinned under drying conditions without the use of coolant (coolant / lubricant), thus offering environmental advantages compared to conventional stretching and thinning of metal sheets using coolant.

[0003] Such organic resin coated metal sheets can be manufactured by film lamination methods, such as thermal bonding of a pre-formed thermoplastic resin film, such as thermoplastic polyester resin, to a metal sheet, or by extrusion lamination of a molten film of extruded thermoplastic polyester resin, such as thermoplastic polyester resin, to a metal sheet.

[0004] However, in terms of film lamination, it is difficult to control the film thickness to a thin film, so the film thickness is prone to become too thick, which sometimes becomes an economic problem.

[0005] It was also proposed that, instead of using an organic resin-coated metal sheet obtained by such a film lamination method, a coated metal sheet that forms a coating on the metal sheet by a coating method that can form a thin film should be used to manufacture a deep-drawing and thinning can under drying conditions.

[0006] For example, Patent Document 2 below discloses a coated metal sheet for deep-drawn and thinned cans, which is a double-sided coated metal sheet, and the dry coating amount of the film on the inner surface of the can after processing is 90-400 mg / 100 cm². 2The glass transition temperature is 50℃~120℃, and under test conditions at 60℃, the pencil hardness is above H, the elongation is within the range of 200%~600%, and the coefficient of dynamic friction is within the range of 0.03~0.25. After processing, the dry coating amount of the film on the outer surface of the can is 15~150mg / 100cm. 2 The glass transition temperature is 50℃~120℃, and the pencil hardness is H or higher under test conditions at 60℃.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2001-353812

[0010] Patent Document 2: Japanese Patent No. 3872998 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] It is known that in seamless cans such as deep-drawn and thinned cans formed from coated metal sheets under dry conditions, residual stress in the coating film generated by harsh processing such as deep drawing and thinning significantly reduces the adhesion between the coating film and the metal substrate (hereinafter sometimes referred to as "coating adhesion"). This reduction in adhesion adversely affects various properties, such as 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, sometimes as the residual stress in the coating film generated by harsh processing is relieved all at once, shrinkage forces act on the interface between the coating film and the metal substrate, especially in areas where the can body has been strictly thinned. This causes the coating film to peel off from the metal substrate, resulting in metal exposure and reduced coating coverage. Therefore, it is required that the coating film not peel off during such heat treatment (coating peel resistance).

[0013] In addition, depending on the type of contents filled into the seamless tank, sterilization treatment under high temperature and high humidity conditions, such as boiling treatment, is sometimes applied after filling. In such cases, the coating film is required to not whiten (resistance to boiling whitening).

[0014] On the other hand, polyester coatings formed from polyester-based coating compositions have a unique problem: reduced processability (time-induced embrittlement) due to prolonged storage under specified conditions. The exact cause of this time-induced embrittlement phenomenon has not been precisely elucidated, but the inventors speculate as follows: It is believed that due to enthalpy relaxation over time, the molecular chains of the polyester resin undergo reorientation (a shift towards equilibrium) within the coating, thus causing the coating to become embrittled and reducing processability. When the coating is in a state of time-induced embrittlement, and harsh processing such as deep drawing and thinning is performed on the coated metal sheet as described above under drying conditions, coating defects such as exposed metal are easily generated during processing. Insufficient coating coverage on the inner surface makes it difficult to obtain seamless cans such as deep-drawn and thinned cans with sufficient corrosion resistance. Therefore, it is required that the coating does not become embrittled over time (time-induced embrittlement resistance).

[0015] Patent document 2 proposes that by forming a polyester-based coating on the inner surface of a coated metal sheet, which maintains hardness, elongation, etc., even when generating heat of nearly 60°C due to continuous drawing and thinning processes, the coated metal sheet and the drawn and thinned can formed from the coated metal sheet can withstand the drawing and thinning process. However, it lacks insights into removing residual stress caused by heat treatment after forming the drawn and thinned can using such a coated metal sheet, improving adhesion, and addressing coating peeling caused by the heat treatment; the aforementioned problems remain unresolved. Furthermore, it also lacks insights into the resistance to boiling whitening and the resistance to embrittlement over time of the polyester-based coating formed on the coated metal sheet, thus failing to resolve the aforementioned problems.

[0016] Therefore, one object of the present invention is to provide a seamless can, such as a deep-drawn thinning can, which has a coating that suppresses the occurrence of coating peeling caused by heat treatment after can forming, has high coating coverage after heat treatment, and has excellent resistance to boiling whitening and resistance to embrittlement over time, and a coated metal sheet that can be formed into such a seamless can.

[0017] Technical solution

[0018] According to the present invention, a seamless can is provided, characterized in that it has an inner surface coating at least on the inner surface side of the can, the inner surface coating containing a polyester resin and a first-order phenolic resin and / or an amino resin as a curing agent, wherein the gel fraction (A) of the inner surface coating as shown in the following formula (1a) is 55% or more and less than 90%.

[0019] Gel fraction (A) = (W2a / W1a) × 100 (%) ... (1a)

[0020] In the formula, W1a represents the mass of the inner surface coating after being separated from the coated metal substrate cut from the seamless can, and W2a represents the mass of the separated inner surface coating after being immersed in MEK at room temperature for 60 minutes, and then removed and dried.

[0021] In the seamless tank of the present invention, it is appropriate that,

[0022] [1] The first-stage phenolic resin is a m-cresol first-stage phenolic resin;

[0023] [2] The first-order phenolic resin is formulated in an amount greater than 2 parts by mass and less than 10 parts by mass relative to 100 parts by mass of the polyester resin;

[0024] [3] The amino resin is a benzoguanamine resin, which is formulated in an amount of 8 or more but less than 25 parts by weight relative to 100 parts by weight of the polyester resin.

[0025] [4] The inner surface coating further contains an acid catalyst, and the content of the acid catalyst in the inner surface coating is less than 0.5 parts by weight relative to 100 parts by weight of polyester resin;

[0026] [5] The difference between the gel fraction (B) shown in the following formula (2a) of the inner surface coating and the gel fraction (A) is less than 10%;

[0027] Gel fraction (B) = [(W4a-W5a) / (W3a-W5a)] × 100 (%) ... (2a)

[0028] In the formula, W3a represents the mass of the coated metal substrate with the inner surface coating cut from the 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 inner surface coating is removed from the coated metal substrate.

[0029] [6] 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 inner surface coating of the central part of the can body is 20% to 75% of the thickness of the inner surface coating of the central part of the can bottom.

[0030] [7] The thickness ratio of the inner surface coating to the metal substrate (the thickness of the inner surface coating / the thickness of the metal substrate) is approximately the same at the bottom of the tank and the main body of the tank;

[0031] [8] The outer surface of the can is further provided with an outer surface coating, which contains polyester resin and amino resin as a curing agent;

[0032] [9] The gel fraction (A) of the outer surface coating as shown in the following formula (3a) is 40% or more and less than 90%.

[0033] Gel fraction (A) = (W7a / W6a) × 100 (%) ... (3a)

[0034] In the formula, W6a represents the mass of the outer surface coating after being separated from the coated metal substrate cut from the seamless can, and W7a represents the mass of the separated outer surface coating after being immersed in MEK at room temperature for 60 minutes, and then removed and dried.

[0035]

[10] 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.

[0036] Heat shrinkage rate (%) = (ΔL1 / L0) × 100)……(5)

[0037] L0: The initial length of the coating in the height direction after separation from the center of the can body;

[0038] ΔL1: Apply 5.20 × 10 to each unit area on one side. 5 N / m 2 The maximum shrinkage length in the height direction of the coating film corresponding to the portion of L0 when the load side is heated from 30℃ to 200℃ at a heating rate of 5℃ / min.

[0039]

[11] The heat shrinkage rate of the coating on the inner surface of the central part of the can body, as shown by the following formula (6), is 50% or less.

[0040] Thermal shrinkage rate (%) = (ΔL2 / L0) × 100……(6)

[0041] L0: The initial length of the coating in the height direction after separation from the center of the can body;

[0042] ΔL2: The maximum shrinkage length in the height direction of the coating film corresponding to the portion of L0 when heated from 30℃ to 200℃ at a heating rate of 5℃ / min under no load.

[0043]

[12] The coverage of the inner surface coating is less than 200 mA when converted to ERV;

[0044]

[13] The seamless can is a deep-drawn and thinned can.

[0045] According to the present invention, a seamless coated metal sheet for cans is also provided, characterized in that it has an inner surface coating at least on the surface that forms the inner surface of the can, the inner surface coating containing a polyester resin and a first-order phenolic resin and / or an amino resin as a curing agent, the gel fraction (A) of the inner surface coating as shown in formula (1b) below is 55% or more and less than 90%, and the difference between the gel fraction (B) of the inner surface coating as shown in formula (2b) below and the gel fraction (A) is less than 10%.

[0046] Gel fraction (A) = (W2b / W1b) × 100 (%) ... (1b)

[0047] In the formula, W1b represents the mass of the inner surface coating after separation from the coated metal plate, and W2b represents the mass of the separated inner surface coating after immersion in MEK at room temperature for 60 minutes, followed by removal and drying.

[0048] Gel fraction (B) = [(W4b-W5b) / (W3b-W5b)] × 100 (%) ... (2b)

[0049] In the formula, W3b represents the mass of the coated metal sheet with the inner surface coating, W4b represents the mass of the coated metal sheet after being immersed in MEK at 80°C for 60 minutes and then dried, and W5b represents the mass of the metal sheet after the inner surface coating is removed from the coated metal sheet.

[0050] In the seamless coated metal sheet for cans of the present invention, it is appropriate that the surface that becomes the outer surface of the can further have an outer surface coating, which contains a polyester resin and an amino resin as a curing agent, and the gel fraction (A) of the outer surface coating as shown in the following formula (3b) is 40% or more and less than 90%.

[0051] Gel fraction (A) = (W7b / W6b) × 100 (%) ... (3b)

[0052] In the formula, W6b represents the mass of the outer surface coating after separation from the coated metal plate, and W7b represents the mass of the separated outer surface coating after immersing it in MEK at room temperature for 60 minutes, and then removing and drying it.

[0053] Invention Effects

[0054] The inventors conducted in-depth research on the coating peel resistance during heat treatment of seamless cans such as deep-drawn and thinned cans made of coated metal sheets with polyester coatings, the resistance to boiling whitening after filling with contents, and the resistance to embrittlement over time. The results showed that the coating peel resistance, resistance to boiling whitening, and resistance to embrittlement over time are related to the gel fraction (A) of the polyester coating determined in the above formula (1) in the state of coating separation. Furthermore, they found that a suitable range of gel fraction (A) of the coating that can take into account both the coating and the coating that can exhibit such a gel fraction (A) was found.

[0055] In other words, even seamless cans obtained through harsh processes such as deep drawing do not exhibit coating peeling during heat treatment. Therefore, even after heat treatment following the forming of the seamless can, if the coverage of the inner surface coating, expressed in ERV, is less than 200 mA, metal exposure is effectively prevented, resulting in excellent coating coverage. Furthermore, residual stress is removed through heat treatment, thereby improving coating adhesion and thus exhibiting excellent corrosion resistance. Moreover, even after boiling treatment following filling, the inner surface coating exhibits excellent resistance to boiling whitening, and even after prolonged storage, the inner surface coating is prevented from becoming brittle, demonstrating excellent resistance to embrittlement over time.

[0056] Furthermore, the coating of the coated metal sheet of the present invention exhibits excellent elongation and processability. Even under harsh processing conditions such as deep drawing and thinning under drying conditions, it effectively prevents damage to the can body (sometimes referred to as body breakage in the present invention) and metal exposure. Therefore, it maintains high coating coverage even after deep drawing and thinning processes, resulting in excellent can-making processability. Moreover, by controlling the gel fraction of the coating formed on the inner surface of the can, residual stress generated by deep drawing and thinning processes can be reduced. This allows for the provision of seamless cans, such as deep-drawn and thinned cans, that effectively suppress coating peeling during heat treatment after can forming, maintain high coating coverage even after heat treatment, and exhibit excellent corrosion resistance. In addition, as described above, seamless cans, such as deep-drawn and thinned cans, formed using this coated metal sheet also exhibit excellent resistance to boiling whitening. Furthermore, the coated metal sheet of the present invention has excellent resistance to embrittlement over time, preventing coating embrittlement even after prolonged storage and maintaining good processability. Detailed Implementation

[0057] (Painted metal sheet)

[0058] In the coated metal sheet used in this invention, from the viewpoints of coating peel resistance during heat treatment, resistance to boiling and whitening, and resistance to embrittlement over time, an important feature is that the inner surface coating, which forms the inner surface side of the can, contains a polyester resin and a first-order phenolic resin and / or an amino resin as a curing agent. The gel fraction (A) of the inner surface coating, as shown in the above formula (1b) and measured in the separated coating state, is in the range of 55% or more and less than 90%. The reasons for this are explained below.

[0059] First, let's consider the issue from the perspective of coating peeling resistance during heat treatment. When using coated metal sheets for high-speed forming under dry conditions to create seamless cans such as deep-drawn thin-film cans, the coated metal sheet is subjected to severe processing / deformation as its temperature rises due to processing heat. At this time, the coating formed on the coated metal sheet is subjected to large deformation due to can-making processing, thus generating large residual stress in the coating during processing. It is argued that if the formed can is then heat-treated under these conditions, heated above the glass transition temperature of the polyester resin, then as the residual stress eases, shrinkage forces act on the interface between the coating and the metal substrate, causing coating peeling and exposing the metal. To suppress this coating peeling phenomenon during heat treatment, it is necessary to reduce the residual stress in the coating generated during can-making processing.

[0060] The inventors conducted in-depth research on reducing residual stress in the coating film generated during processing and suppressing coating film peeling during heat treatment caused therefrom. The results showed that the gel fraction (mass proportion of coating film components that become insoluble in solvent through gelation via cross-linking reaction), which represents the overall degree of cross-linking of the inner surface coating film, and more specifically, the gel fraction (A) of the inner surface coating film measured in the state of coating film separation as determined by the above formula (1b), has a significant impact on the magnitude of residual stress in the processed coating film and, consequently, on coating film peeling during heat treatment.

[0061] As described above, when coated metal sheets are used for high-speed forming under drying conditions, such as in deep-drawing and thinning processes, residual stress is generated in the coating film after the coated metal sheet is processed. It is believed that this residual stress increases, especially when the main resins, such as the polyester resin constituting the coating film, are highly cross-linked by the curing agent. Regarding the gel fraction (A) of the coating film on the inner surface of the coated metal sheet, it is considered to represent the overall gel fraction of the inner surface coating film, i.e., the degree of cross-linking (cross-linking) of the overall coating film, as described below. 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 overall coating film is highly cross-linked, thus increasing the residual stress after processing and making it difficult to prevent coating peeling during heat treatment. On the other hand, it is believed that when the gel fraction (A) is adjusted to be less than 90%, the overall degree of cross-linking of the coating film is moderately controlled without being excessively high, thus reducing the residual stress of the coating film after deep-drawing and thinning processes on the coated metal sheet. Therefore, as the residual stress during heat treatment is alleviated, the shrinkage force at the interface between the coating and the metal substrate is reduced, thus preventing coating peeling. In other words, by adjusting the gel fraction (A) to less than 90%, the coating coverage is excellent even after heat treatment following molding, and the residual stress is removed by heat treatment, thereby improving the coating adhesion. As a result, seamless cans such as deep-drawn and thinned cans with excellent corrosion resistance can be provided.

[0062] Next, from the viewpoint of the coating's resistance to boiling whitening, the following explanation will be given. It is understood that when polyester coatings are subjected to sterilization treatments such as boiling, the coating may sometimes whiten due to water absorption. However, when the overall cross-linking degree of the coating is relatively high, possessing a dense cross-linked structure, the whitening caused by boiling treatment can be suppressed by inhibiting the penetration of moisture into the coating. Therefore, the whitening phenomenon caused by boiling treatment is also greatly affected by the overall cross-linking degree of the coating, i.e., the gel fraction (A) of the inner surface coating mentioned above. When the gel fraction (A) is less than 55%, the coating does not have a dense cross-linked structure, making it difficult to suppress coating whitening. On the other hand, when the gel fraction (A) is 55% or higher, the coating exhibits a relatively dense cross-linked structure, thereby suppressing coating whitening. In other words, by adjusting the gel fraction (A) to 55% or higher, a deep-drawing and thinning can with excellent resistance to boiling whitening can be provided.

[0063] Next, the explanation will focus on the resistance of the coating to embrittlement over time. As mentioned above, the embrittlement of polyester coatings over time is presumed to occur due to the reorientation (transition to equilibrium state) of the polyester resin molecular chains caused by enthalpy relaxation. It is believed that this enthalpy relaxation is due to the molecular motion of the polyester resin; therefore, the more the polyester resin is cross-linked by a curing agent, the more the molecular motion is suppressed, and the less likely enthalpy relaxation will occur. Therefore, this embrittlement phenomenon is also greatly affected by the overall degree of cross-linking of the coating, i.e., the gel fraction (A) of the inner surface coating mentioned above. When the gel fraction (A) is less than 55%, the polyester resin is not sufficiently cross-linked, making it difficult to suppress the embrittlement of the coating over time. On the other hand, when the gel fraction (A) is 55% or more, the polyester resin of the entire coating is moderately cross-linked, thereby suppressing molecular motion and thus suppressing the embrittlement of the coating over time.

[0064] Based on the above, in the coating film on the inner surface of the coated metal sheet used in this invention, from the viewpoint of balancing coating peel resistance during heat treatment, resistance to boiling whitening, and resistance to embrittlement over time, it is important that the gel fraction (A) is in the range of 55% or more and less than 90%. Therefore, it is possible to provide seamless cans, particularly deep-drawn thin-film cans, that effectively prevent coating peeling even after heat treatment following the forming of seamless cans such as thin-film cans, exhibit excellent coating coverage, resistance to boiling whitening, and resistance to embrittlement over time.

[0065] Ideally, the gel fraction (A) is in the range of 55% or more and less than 90%, preferably in the range of 60% to 88%, more preferably in the range of more than 62% and less than 85%, further preferably in the range of 65% to 84%, particularly preferably in the range of 65% to 80%, and most preferably in the range of 68% or more and less than 78%. As mentioned above, when the gel fraction is higher than the above range, there is a possibility that the coating may peel off during heat treatment. Furthermore, when the gel fraction is lower than the above range, there is a possibility that the degree of cross-linking of the coating will decrease, resulting in reduced resistance to boiling and whitening, reduced resistance to embrittlement over time, and reduced heat resistance of the coating. It is believed that when the heat resistance of the coating decreases, during high-speed forming of a deep-drawing and thinning can, the coating becomes easier to soften due to the increase in temperature, and sometimes the coating may easily stick to the mold during forming. Especially on the inner surface of the can, at the point when the can is pulled out from the forming punch after molding, the can sticks to the forming punch, resulting in the phenomenon that the forming punch and the can are not easily separated (poor demolding). As a result, the can may be warped or the main body may be damaged, which may reduce productivity.

[0066] Ideally, the coated metal sheet used in this invention is a double-sided coated metal sheet with an outer surface coating on the side that becomes the outer surface of the can after processing. Ideally, the outer surface coating also contains polyester resin, and more ideally, it further contains a curing agent, preferably an amino resin.

[0067] Furthermore, in the outer surface coating, from the viewpoints of coating peel resistance, resistance to boiling and whitening, and resistance to embrittlement over time, it is ideal that the gel fraction (A) calculated according to the above formula (3b) is in the range of 40% or more and less than 90%, preferably in the range of 55% or more and less than 90%, more preferably in the range of more than 60% and less than 88%, further preferably in the range of 65% to 85%, particularly preferably in the range of 68% to 84%, and most preferably in the range of 70% to 84%.

[0068] Furthermore, in the coated metal sheet of the present invention, it is ideal that the difference between the gel fraction (B) of the inner surface coating measured in the state of the coated metal substrate, obtained by the above formula (2b), and the above gel fraction (A) is less than 10%. The reasons for this will be explained below.

[0069] It is known that in polyester coatings, a specific combination of polyester resin and curing agent results in a coating structure where the curing agent is enriched near the coating surface. It is believed that the low compatibility between the polyester resin (the main agent) and the curing agent leads to the localized presence of the curing agent on the coating surface during film formation, resulting in this coating structure. However, this structure results in a highly cross-linked coating surface due to the curing agent enrichment, while the concentration of the curing agent decreases within the coating. Consequently, a dense cross-linked structure cannot be formed overall, hindering excellent resistance to boiling and whitening. Therefore, from the perspective of boiling and whitening resistance, ideally, the polyester resin and curing agent should have high compatibility, and the coating should have a uniform distribution of the curing agent within the polyester resin, resulting in a dense cross-linked structure overall.

[0070] Regarding the gel fraction (B) of the coating film on the inner surface of the coated metal sheet, the coating is immersed in MEK (methyl ethyl ketone) as a solvent while the metal substrate is in the coated state. The uncrosslinked components (uncured components) soluble in the solvent are extracted from the coating film, and the gel fraction is determined based on the mass of the coating film components insoluble in the solvent. When the gel fraction is determined by this method, the uncrosslinked components are not extracted from the side of the metal substrate that is not in contact with the solvent, but selectively extracted from the coating surface that is in contact with the solvent. However, in the case of a coating film with a high degree of crosslinking only on the surface due to the enrichment of curing agent on the coating surface as described above, the solvent cannot penetrate into the interior of the coating film due to the presence of a dense layer of curing agent enriched on the coating surface, making it difficult to extract the uncrosslinked components present inside the coating film. Therefore, the gel fraction (B) determined by the above method reflects the degree of crosslinking near the coating surface more strongly than the overall degree of crosslinking of the coating film. In contrast, the gel fraction (A) mentioned above is determined by immersing the separated coating film in a solvent after it has been separated from the coated metal substrate, extracting the uncrosslinked components, and calculating based on the mass of the coating film components insoluble in the solvent. In this method, since uncrosslinked portions are extracted not only from the coating surface but also from the metal substrate side, even in coatings with a structure where the curing agent is enriched near the coating surface as described above, the solvent can penetrate the interior and extract the uncrosslinked components. Therefore, the gel fraction (A) measured by 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 enriched on 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) often being higher. On the other hand, in coatings where the curing agent is uniformly distributed and the overall degree of crosslinking is uniform, the gel fraction (A) and gel fraction (B) are often quite close in value.

[0071] As described above, in this invention, from the viewpoint of resistance to boiling whitening, it is ideal that the polyester resin and the curing agent have high compatibility, resulting in a coating structure in which the curing agent is uniformly distributed throughout the coating film, and the coating film as a whole has a dense cross-linked structure. Therefore, it is ideal that the gel fraction (A) and the gel fraction (B) are close in value; specifically, it is ideal that the difference between the gel fraction (A) and the gel fraction (B) is less than 10%. When the difference between the gel fraction (A) and the gel fraction (B) is 10% or more, it is considered that the coating structure has a curing agent enriched on the surface of the coating film, and as described above, it is sometimes difficult to exhibit resistance to boiling whitening. As for the gel fraction (B), it is ideal to be in the range of more than 45% and less than 99%, preferably in the range of more than 50% and less than 98%, more preferably in the range of more than 52% and less than 95%, further preferably in the range of 55% to 94%, particularly preferably in the range of 55% to 90%, and most preferably in the range of 58% or more and less than 88%.

[0072] Furthermore, in the above-mentioned outer surface coating of the coated metal plate used in this invention, it is not limited to this, but ideally, the difference between the gel fraction (A) and the gel fraction (B) is less than 10%.

[0073] Furthermore, the gel fraction (B) of the coating film on the outer surface of the coated metal sheet is calculated according to the following formula (4b). Ideally, the gel fraction (B) is in the range of more than 45% and less than 99%, preferably in the range of more than 50% and less than 98%, more preferably in the range of more than 52% and less than 95%, further preferably in the range of 55% to 94%, particularly preferably in the range of 55% to 90%, and most preferably in the range of more than 58% and less than 88%.

[0074] Gel fraction (B)% = [(W9b-W10b) / (W8b-W10b)]×100……(4b)

[0075] In the formula, W8b represents the mass of the coated metal sheet with the outer surface coating, W9b represents the mass of the coated metal sheet after it has been immersed in MEK at 80°C for 60 minutes and then dried, and W10b represents the mass of the metal sheet after the inner surface coating has been removed from the coated metal sheet.

[0076] When the gel fraction of the outer surface coating is lower than the above range, the degree of cross-linking of the coating becomes lower, thereby reducing the hardness of the coating and potentially causing coating wear and other outer surface defects during the forming of seamless cans such as deep drawing and thinning cans.

[0077] The glass transition temperature (Tg) of the inner surface coating is preferably in the range of 30°C or higher, more preferably in the range of 40°C or higher, more preferably in the range of 45°C or higher but below 120°C, further preferably in the range of 50°C to 110°C, particularly preferably in the range of 55°C or higher but below 100°C, and most preferably in the range of 55°C or higher but below 90°C. When the Tg is lower than the above range, the flavor components of the contents are more easily adsorbed when filling the molded can, potentially leading to decreased aroma adsorption resistance and reduced barrier properties and corrosion resistance of the coating. On the other hand, when the Tg exceeds 120°C, the processability and elongation of the coating decrease, potentially leading to metal exposure during molding, resulting in poor can-making processability and increased residual stress in the coating, thereby increasing the possibility of coating peeling during heat treatment.

[0078] Regarding the Tg of the aforementioned outer surface coating, it is preferable to be in the range of 30°C or higher, more preferably in the range of 40°C or higher, more preferably in the range of 45°C or higher but below 120°C, even more preferably in the range of 50°C to 110°C or lower, particularly preferably in the range of 55°C or higher but below 100°C, and most preferably in the range of 55°C or higher but below 90°C. When the Tg is lower than the above ranges, the hardness of the coating decreases, which may lead to surface defects such as coating wear. On the other hand, when the Tg exceeds 120°C, the processability and elongation of the coating decrease, and there is a possibility of metal exposure during molding, resulting in poor can-making processability.

[0079] By forming seamless cans, particularly drawn and thinned cans, using coated metal sheets having an inner surface coating on at least the surface becoming the inner surface of the can as described above, the entire can be covered by the continuous inner surface coating from the bottom of the inner surface side to the main body. Furthermore, when using double-sided coated metal sheets that also have an outer surface coating on the surface becoming the outer surface of the can after the drawing and thinning process, the entire can be covered by the continuous outer surface coating from the bottom of the outer surface side to the main body.

[0080] Suitablely, the film thickness of the inner surface coating, measured by dry film thickness, is in the range of 0.2 to 20 μm, preferably in the range of 1 to 16 μm, and more preferably in the range of greater than 2 μm and less than 12 μm. Furthermore, as for the quality of the dried coating, it is suitable to be in the range of 3 to 300 mg / dm³. 2 Within the range of 15–220 mg / dm³, preferably 15–220 mg / dm³ 2 More preferably, it is within the range of 15–150 mg / dm³. 2 Within the range, more preferably greater than 25 mg / dm 2 And it is 150 mg / dm 2Within the following range. In the case of films thinner than the above range, metal exposure is more likely to occur during molding, and the coating coverage on the inner surface deteriorates. On the other hand, in the case of films thicker than the above range, the residual stress generated during processing increases, so the coating is more likely to peel off during heat treatment after stretching and thinning molding, and thus requires a film of the above thickness, resulting in poor economic efficiency.

[0081] Furthermore, when the contents of the stretch-thinning can are highly corrosive acidic beverages, a thicker film is required to ensure corrosion resistance. Suitablely, this film thickness is in the range of greater than 5 μm and less than 16 μm, preferably greater than 6 μm and less than 12 μm, and more preferably in the range of 6.5 to 10 μm. Additionally, the dry coating quality is preferably greater than 70 mg / dm³. 2 And it is 150 mg / dm 2 The following range is preferred, greater than 85 mg / dm³. 2 And it is 150 mg / dm 2 The following range is preferred: 90–140 mg / dm³ 2 The range.

[0082] On the other hand, when the contents filled in the seamless container are low-acid beverages or similar substances with low corrosivity, corrosion resistance can be ensured even with a relatively thin film. Therefore, a film thickness of 1 μm or more and less than 6.5 μm is preferred, more preferably greater than 2 μm and less than 6.5 μm, and even more preferably 2.5 to 6 μm. Furthermore, a dry coating quality of 15 mg / dm³ is suitable. 2 Above and below 90 mg / dm 2 The range is preferably greater than 25 mg / dm³. 2 And less than 90 mg / dm 2 The range is more preferably 30–85 mg / dm³. 2 The range is defined as follows: Thinner films exhibit poor corrosion resistance, while films exceeding this range require thicker films, resulting in poor economic efficiency.

[0083] Furthermore, it is preferable that the film thickness of the outer surface coating, measured as dry film thickness, is in the range of 0.2 to 20 μm, preferably in the range of 1 to 16 μm, more preferably in the range of greater than 2 μm and less than 12 μm, and even more preferably in the range of greater than 2 μm and less than 6.5 μm. Furthermore, as for the dry coating mass, it is preferable to have a mass of 3 to 300 mg / dm³. 2 The preferred range is 15–220 mg / dm³. 2 The range is more preferably greater than 25 mg / dm 2 And it is 150 mg / dm 2The following range is further preferably greater than 25 mg / dm³. 2 And less than 90 mg / dm 2 The range is as follows. When the film is thinner than the range described above, metal exposure is more likely to occur during molding, and the coating coverage on the outer surface deteriorates. On the other hand, when the film is thicker than the range described above, the residual stress generated during processing increases, making coating peeling more likely to occur during heat treatment after stretching and thinning molding.

[0084] It should be noted that, 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, has a thicker film than the outer surface coating.

[0085] The metal sheet used as the metal substrate for coated metal sheets is not limited to this. Examples include: hot-rolled steel sheet, cold-rolled steel sheet, hot-dip galvanized steel sheet, electro-galvanized steel sheet, alloy steel sheet, aluminum-zinc alloy steel sheet, aluminum sheet, tin-plated steel sheet, stainless steel sheet, copper sheet, copper-plated steel sheet, tinfree steel sheet, nickel-plated steel sheet, ultra-thin tin-plated steel sheet, chromium-treated steel sheet, etc. Metal sheets that have undergone various surface treatments as needed, such as chromate phosphate treatment, zirconium-based chemical conversion treatment, coating-type treatment combining water-soluble resins such as polyacrylic acid and zirconium salts such as ammonium zirconium carbonate, etc.

[0086] In this invention, aluminum plates are preferred among the aforementioned metal plates. Besides pure aluminum plates, aluminum alloy plates can be appropriately used, specifically aluminum alloy plates from the 3000, 5000, and 6000 series of "JIS H 4000". Considering strength and other factors, aluminum alloy plates are suitable. As for aluminum alloy plates, besides surface-treated aluminum alloy plates that have undergone the various surface treatments described above, the coating film formed by the aforementioned coating composition exhibits excellent adhesion to the metal substrate; therefore, untreated aluminum alloy plates without surface treatment can be appropriately used.

[0087] From the perspective of tank strength and formability, the thickness of the metal plate is preferably in the range of 0.1 to 1.00 mm, more preferably in the range of 0.15 to 0.40 mm, even more preferably in the range of 0.15 to 0.30 mm, and even more preferably in the range of 0.20 to 0.28 mm.

[0088] [Inner surface coating and outer surface coating]

[0089] The coating on the inner surface of the coated metal sheet and the seamless can described later in this invention is preferably composed of a polyester resin as the main agent and a curing agent, both of which are included in the inner and outer surface coatings.

[0090] In the 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 particularly preferably 80% by mass or more.

[0091] Similarly, in the outer surface coating, the content of polyester resin, preferably 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 particularly preferably 80% by mass or more.

[0092] <Polyester Resin>

[0093] In the coated metal sheet and the seamless can described later in this 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 substance with the highest content (mass ratio) in the resin component constituting the coating. In this invention, the mass ratio of polyester resin in the resin component constituting the aforementioned 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 particularly preferably 80% by mass or more.

[0094] Examples of polycarboxylic acid components constituting polyester resins include: aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and 2,6-naphthalenedicarboxylic acid; aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanoic acid, and dimer acids; 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; and polycarboxylic acids with three or more ternary components such as trimellitic acid (anhydride), pyromellitic acid (anhydride), and methylcyclohexenetricarboxylic acid. One or more of these can be selected. Of the aforementioned polycarboxylic acids, it is preferable to use one or more selected from the group consisting of isophthalic acid, phthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, trimellitic acid, adipic acid, sebacic acid, fumaric acid, maleic acid, dimer acid, and 1,4-cyclohexanedicarboxylic acid.

[0095] In this invention, from the viewpoints of hardness, heat resistance, fragrance adsorption resistance, and resistance to boiling and whitening of the obtained coating film, it is preferable that, when the total amount of the polycarboxylic acid components constituting the polyester resin is set to 100 mol%, the coating contains at least one or more of the aromatic dicarboxylic acids selected from terephthalic acid, phthalic acid, isophthalic acid, and 2,6-naphthalenedicarboxylic acid in an amount of at least 70 mol%, preferably at least 80 mol%, and more preferably at least 90 mol%. Furthermore, terephthalic acid and isophthalic acid are particularly preferred among the aforementioned aromatic dicarboxylic acids. Preferably, when the total amount of the polycarboxylic acid components constituting the polyester resin is set to 100 mol%, the total content of terephthalic acid and isophthalic acid is preferably at least 70 mol%, preferably at least 80 mol%, and more preferably at least 90 mol%. When using a mixture of two or more polyester resins as the polyester resin, it is preferable that, when the total amount of all polycarboxylic acid components constituting the polyester resin mixture is set to 100 mol%, it contains at least one or more of the aromatic dicarboxylic acids selected from terephthalic acid, phthalic acid, isophthalic acid, and 2,6-naphthalenedicarboxylic acid in an amount of at least 70 mol%, preferably at least 80 mol%, and more preferably at least 90 mol%. Furthermore, when the total amount of all polycarboxylic acid components constituting the polyester resin mixture is set to 100 mol%, the combined content of terephthalic acid and isophthalic acid is preferably at least 70 mol%, more preferably at least 80 mol%, and even more preferably at least 90 mol%.

[0096] Furthermore, from the viewpoint of aroma absorption resistance and canning processability, it is ideal that the content of isophthalic acid is in the range of 2 mol% or more, preferably in the range of 5 to 90 mol%, more preferably in the range of more than 10 mol% and less than 85 mol%, further preferably in the range of 15 to 80 mol%, and particularly preferably in the range of 20 to 80 mol%. This allows for a denser coating film (high density), resulting in improved aroma absorption resistance and improved canning processability, thereby suppressing coating defects during molding and improving coating coverage. In addition, from the viewpoint of heat resistance, it is appropriate to contain terephthalic acid in an amount of 10 mol% or more, preferably in an amount of 15 to 90 mol%, more preferably in an amount of 19 mol% or more and less than 84 mol%, further preferably in an amount of 25 to 80 mol%, particularly preferably in an amount of 30 to 80 mol%, and most preferably in an amount of 40 to 76 mol%. Similarly, when using a mixture of two or more polyester resins as the polyester resin, ideally, when the total amount of all polycarboxylic acid components constituting the polyester resin mixture is set to 100 mol%, the content of isophthalic acid is in the range of 2 mol% or more, preferably in the range of 5 to 90 mol%, more preferably in the range of more than 10 mol% and less than 85 mol%, further preferably in the range of 15 to 80 mol%, and particularly preferably in the range of 20 to 80 mol%. Furthermore, it is suitable to contain terephthalic acid in an amount of 10 mol% or more, preferably in an amount of 15 to 90 mol%, more preferably in an amount of 19 mol% or more and less than 84 mol%, further preferably in an amount of 25 to 80 mol%, particularly preferably in an amount of 30 to 80 mol%, and most preferably in an amount of 40 to 76 mol%.

[0097] It should be noted that the polycarboxylic acid component constituting the polyester resin may contain components other than aromatic dicarboxylic acids, such as aliphatic dicarboxylic acids or alicyclic dicarboxylic acids, in a proportion less than 30 mol% of the aforementioned aromatic dicarboxylic acids. However, if the proportion of aliphatic dicarboxylic acids or alicyclic dicarboxylic acids increases, the coating film may easily adsorb the aroma components of the contents when filling the can, potentially resulting in poor aroma adsorption resistance of the coating film and reduced resistance to boiling and whitening. Therefore, ideally, the proportion of components other than aromatic dicarboxylic acids, such as aliphatic dicarboxylic acids, in the polycarboxylic acid component constituting the polyester resin is less than 30 mol%, preferably less than 20 mol%, more preferably less than 10 mol%, further preferably less than 7 mol%, particularly preferably less than 5 mol%, and most preferably less than 4 mol%.

[0098] When using a mixture of two or more polyester resins as the polyester resin, ideally, when the total amount of all polycarboxylic acid components constituting the polyester resin mixture is set to 100 mol%, the proportion of aliphatic dicarboxylic acid and alicyclic dicarboxylic acid is less than 20 mol%, preferably less than 15 mol%, more preferably less than 10 mol%, and particularly preferably less than 7 mol%.

[0099] As a polyol component constituting polyester resin, one or more of the following substances may be used: ethylene glycol, propylene glycol (1,2-propanediol), 1,3-propanediol, 1,4-butanediol, 1,2-butanediol, 1,3-butanediol, 2-methyl-1,3-propanediol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 2-ethyl-2-butyl-1,3-propanediol, 2,4-dimethyl-1,5-pentanediol, and 1-methyl-1,8-octanediol. Aliphatic diols such as 3-methyl-1,6-hexanediol, 4-methyl-1,7-heptanediol, 4-methyl-1,8-octanediol, 4-propyl-1,8-octanediol, and 1,9-nonanediol; ether alcohols such as diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol; alicyclic polyols such as 1,4-cyclohexanediethanol, 1,3-cyclohexanediethanol, 1,2-cyclohexanediethanol, tricyclodecanediols, and hydrogenated bisphenols; and polyols with three or more tertiary groups such as trimethylolpropane, trimethylolethane, and pentaerythritol. In this invention, among the above-mentioned polyol components, ethylene glycol, propylene glycol, neopentyl glycol, 1,4-butanediol, 1,4-cyclohexanediethanol, and diethylene glycol may be appropriately used as components constituting the polyester resin.

[0100] In particular, from the viewpoint of can-making processability and coating peel resistance during heat treatment, it is appropriate that, when the total amount of the polyol components constituting the polyester resin is set to 100 mol%, it preferably contains at least one or more of ethylene glycol, propylene glycol, 2-methyl-1,3-propanediol, and 1,4-butanediol in an amount of 20 mol% or more, preferably 30 mol% or more, more preferably 40 mol% or more, further preferably 50 mol% or more, and particularly preferably 60 mol% or more. Furthermore, from the viewpoint of aroma adsorption resistance, it is ideal that, when the total amount of the polyol components constituting the polyester resin is set to 100 mol%, the aforementioned 1,4-butanediol is less than 40 mol%, more preferably less than 30 mol%, further preferably less than 20 mol%, and particularly preferably less than 10 mol%. When the 1,4-butanediol content is 40 mol% or more, there is a possibility that the aroma adsorption resistance may deteriorate.

[0101] When a mixture of two or more polyester resins is used as the polyester resin, it is also appropriate that the mixture contains at least one or more of ethylene glycol, propylene glycol, 2-methyl-1,3-propanediol, and 1,4-butanediol in an amount of at least 20 mol%, preferably at least 30 mol%, more preferably at least 40 mol%, further preferably at least 50 mol%, and particularly preferably at least 60 mol%, of the total polyol components constituting the polyester resin mixture. Furthermore, when considering the resistance to aroma adsorption, it is ideal that the 1,4-butanediol content is less than 40 mol%, more preferably less than 30 mol%, further preferably less than 20 mol%, and particularly preferably less than 10 mol%.

[0102] Polyester resins can be manufactured by the following known methods: polycondensation of one or more of the aforementioned polycarboxylic acid components and one or more of the polyol components; followed by depolymerization of the polycarboxylic acid components, such as terephthalic acid, isophthalic acid, trimellitic anhydride, trimellitic acid, benzopyrene, etc.; furthermore, by ring-opening addition of acid anhydrides, such as phthalic anhydride, maleic anhydride, trimellitic anhydride, ethylene glycol bis(triptyl) dianhydride, etc., after polycondensation.

[0103] From the perspectives of curability, resistance to boiling and whitening, and adhesion to the metal substrate, ideally, the acid value of the polyester resin should be in the range of 0.1 mg KOH / g to 40 mg KOH / g, preferably in the range of 0.5 mg KOH / g to 25 mg KOH / g, more preferably in the range of 1 mg KOH / g to 15 mg KOH / g, even more preferably in the range of 1.0 mg KOH / g or more but less than 10 mg KOH / g, particularly preferably in the range of 1.0 mg KOH / g to 8 mg KOH / g, and most preferably in the range of 1.0 mg KOH / g or more but less than 5 mg KOH / g. When the acid value is lower than the above ranges, there is a possibility that the adhesion between the metal substrate and the coating film may decrease. On the other hand, when the acid value is higher than the above range, compared with the case within the above range, the coating becomes more hygroscopic, and there is a possibility of reduced resistance to boiling and whitening. Furthermore, there is an increase in the reaction points with the curing agent, which increases the crosslinking density of the coating, reduces can-making processability and coating peel resistance, and may result in metal exposure and reduced coating coverage.

[0104] It should be noted that when the polyester resin is a mixture of two or more polyester resins, the sum of the values ​​obtained by multiplying the acid values ​​of each polyester resin by its mass fraction is taken as the average acid value (AV) of the mixture. mix As long as its average acid value is within the above-mentioned acid value range, it is acceptable.

[0105] Regarding the hydroxyl value of the polyester resin, it is not limited to this. From the viewpoints of can-making processability, coating peeling during heat treatment, and resistance to boiling and whitening, it is ideal to be in the range of 20 mg KOH / g or less, preferably in the range of 10 mg KOH / g or less, more preferably in the range of 1 to 10 mg KOH / g, and even more preferably in the range of 2 to 8 mg KOH / g.

[0106] It should be noted that when the polyester resin is a mixture of two or more polyester resins, the sum of the values ​​obtained by multiplying the hydroxyl value of each polyester resin by its mass fraction is taken as the average hydroxyl value of the mixture, as long as the average hydroxyl value is within the above range.

[0107] Suitablely, the glass transition temperature (Tg) of the polyester resin is in the range of 30°C or higher, preferably in the range of 40°C or higher, more preferably in the range of 45°C or higher but below 120°C, further preferably in the range of 50°C to 100°C, particularly preferably in the range of 55°C or higher but below 100°C, and most preferably in the range of 55°C or higher but below 90°C. If the Tg becomes lower than the above range, when filling the contents into the container manufactured by the deep drawing and thinning process as described above, due to the increased mobility of the resin, the aroma components (flavor components) contained in the contents easily diffuse into the interior of the coating film, thus increasing the adsorption of aroma components. There is a possibility that the aroma adsorption resistance will deteriorate, and there is also a possibility that the heat resistance, corrosion resistance, and resistance to boiling whitening will also deteriorate. On the other hand, when the Tg exceeds 120°C, the processability and elongation of the coating film decrease, which may lead to poor can-making processability and the possibility of metal exposure due to molding. As a result, the coating film's coverage deteriorates after molding, and the residual stress increases, which may lead to a deterioration in the coating film's peel resistance during heat treatment.

[0108] In this invention, two or more polyester resins with different Tg can be mixed and used. By mixing polyester resins with different Tg, compared with the case of using only one polyester resin, it is sometimes possible to form a coating with excellent impact resistance, which is not easy to produce coating defects even when subjected to external impact.

[0109] In this case, as long as the Tg of the polyester resin mixture is calculated by the following formula (7), mix It can be within the Tg range mentioned above.

[0110] 1 / Tg mix =(W1 / Tg1)+(W2 / Tg2)+……+(Wm / Tgm)……(7)

[0111] W1 + W2 + ... + Wm = 1

[0112] In the formula, Tg mixThe glass transition temperature (K) of the polyester resin mixture is represented by Tg1, Tg2, ..., Tgm, where Tg1, Tg2, ..., Tgm represent the glass transition temperature (K) of each polyester resin monomer used (polyester resin 1, polyester resin 2, ..., polyester resin m). Furthermore, W1, W2, ..., Wm represent the mass fraction of each polyester resin (polyester resin 1, polyester resin 2, ..., polyester resin m).

[0113] The number-average molecular weight (Mn) of the polyester resin is not limited thereto. From the viewpoint of can-making processability, it is preferable to have a range of 1,000 to 100,000, more preferably 3,000 to 50,000, even more preferably 5,000 to 20,000, and particularly preferably 10,000 to 20,000. If the number-average molecular weight of the polyester resin is less than the above range, the coating film becomes brittle, and sometimes the can-making processability is poor. If the number-average molecular weight of the polyester resin is greater than the above range, there is a possibility of reduced coating stability.

[0114] Furthermore, from the viewpoints of can-making processability, dent resistance, and coating properties, non-crystalline polyester resins are 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 digital scanning calorimeter (DSC). Compared to crystalline polyester resins, non-crystalline polyester resins exhibit superior solubility in solvents, are easier to coat, and can form films with excellent processability and dent resistance. It should be noted that, in this invention, the mass percentage of non-crystalline polyester resin in all polyester resin components contained in the aforementioned inner surface coating and / or outer surface coating 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.

[0115] <Curing agent>

[0116] The inner surface coating of the coated metal sheet and seamless cans such as deep-drawn and thinned cans of the present invention is characterized by containing a methyl phenolic resin and / or an amino resin as a curing agent. Furthermore, in the outer surface coating, it is desirable to contain an amino resin as a curing agent.

[0117] In the coated metal sheet and the seamless can described later in this invention, in the coating composition forming the inner surface coating film (hereinafter, sometimes referred to as "coating composition for inner surface"), as described above, from a hygienic point of view, methyl phenolic resin and / or amino resin can be used as the curing agent. In particular, from the viewpoint of can-making processability and resistance to aroma adsorption, methyl phenolic resin can be used more appropriately. In the coating composition forming the outer surface coating film (hereinafter, sometimes referred to as "coating composition for outer surface"), amino resin capable of forming a colored, transparent coating film without originating from the curing agent can be appropriately used. On the other hand, the coating film formed by the methyl phenolic resin turns yellow, so this needs to be noted in the case of coating compositions used to form the outer surface coating film.

[0118] Type A phenolic resin

[0119] As a first-order phenolic resin, the following first-order phenolic resins can be used: for example, one or more of the following phenolic compounds, such as o-cresol, p-cresol, p-tert-butylphenol, p-ethylphenol, 2,3-xylenol, 2,5-xylenol, phenol, m-cresol, m-ethylphenol, 3,5-xylenol, and m-methoxyphenol, are reacted with formaldehyde in the presence of an alkaline catalyst.

[0120] From the viewpoint of curability, among the aforementioned phenolic compounds, the following methyl-type phenolic resins are preferred: those containing more than 20% by mass, preferably more than 30% by mass, more preferably more than 50% by mass, further preferably more than 60% by mass, and particularly preferably more than 80% by mass as starting materials of a phenolic compound that becomes trifunctional through reaction with formaldehyde. Examples of phenolic compounds that become trifunctional through reaction with formaldehyde include: phenol, m-cresol, m-ethylphenol, 3,5-xylenol, and m-methoxyphenol; one or more of these can be selected. If the content of these trifunctional phenolic compounds is less than 20% by mass, sufficient curability cannot be obtained, and there is a possibility of reduced curability of the coating film. Among these trifunctional phenolic compounds, from the viewpoint of curability, m-cresol is more preferred, and methyl-type phenolic resins containing m-cresol as the main component of the starting material (hereinafter, sometimes referred to as "m-cresol-based methyl-type phenolic resins") are particularly preferred. Therefore, from the perspective of achieving sufficient curing degree of the coating film, as well as its heat resistance, corrosion resistance, and resistance to boiling and whitening, this is ideal. It should be noted that the "main component" here refers to the component with the highest content (mass ratio) among the phenolic compounds used as starting materials. As a m-cresol-based methyl phenolic resin, it is preferable to use m-cresol containing more than 50% by mass, preferably more than 60% by mass, and more preferably more than 80% by mass as the starting material.

[0121] In addition to the aforementioned trifunctional phenolic compounds, it is preferable that the content of a difunctional phenolic compound, which reacts with formaldehyde to become difunctional, used as a starting material is less than 70% by mass, preferably less than 50% by mass, and more preferably less than 30% by mass. If it is 70% by mass or more, there is a possibility of reduced curability. Examples of difunctional phenolic compounds include o-cresol, p-cresol, p-tert-butylphenol, p-ethylphenol, 2,3-xylenol, and 2,5-xylenol.

[0122] Furthermore, as the methyl phenolic resin used in this invention, considering compatibility with polyester resin and curability, it is preferable to use a substance obtained by alkylating (alkoxymethylating) part or all of the hydroxymethyl group contained in an alcohol with 1 to 12 carbon atoms. The proportion of alkylated hydroxymethyl group is preferably 50% or more, more preferably 60% or more, and even more preferably 80% or more. If the proportion of alkylated hydroxymethyl group is less than 50%, the compatibility with polyester resin becomes low, the coating film becomes cloudy, or sufficient curability cannot be obtained. The alcohol used for alkylation is a monovalent alcohol with 1 to 8 carbon atoms, preferably 1 to 4 carbon atoms. Preferred monovalent alcohols include methanol, ethanol, n-propanol, n-butanol, isobutanol, etc., with n-butanol being more preferred.

[0123] Furthermore, it is preferable that the number of alkyl-etherified hydroxymethyl (alkoxymethyl) groups is at least 0.3 per alkoxymethyl group per phenolic nucleus, 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 methyl phenolic resin is preferably in the range of 500 to 3000, preferably in the range of 800 to 2500. If it is less than the above range, there is a tendency for the crosslinking density of the formed coating to be higher, thus the residual stress after molding is more likely to be greater, and there is a possibility of poor coating peel resistance. On the other hand, if it is greater than the above range, the curability deteriorates, resulting in poor heat resistance, corrosion resistance, and resistance to boiling whitening of the coating.

[0124] Amino Resins

[0125] Examples of amino resins include hydroxymethylated amino resins obtained by reacting amino components such as melamine, urea, benzoguanamine, methylguanamine, steroidalguanamine, spiroguanamine, and dicyandiamide with aldehyde components such as formaldehyde, paraformaldehyde, acetaldehyde, and benzaldehyde. Substances obtained by alkylating part or all of the hydroxymethyl group of such hydroxymethylated amino resins with alcohols having 1 to 12 carbon atoms are also included in the above-mentioned amino resins. They can be used alone or in combination of two or more.

[0126] From the perspectives of hygiene and can-making processability, hydroxymethylated amino resins using benzoguanidine (benzoguanidine resin), hydroxymethylated amino resins using melamine (melamine resin), and hydroxymethylated amino resins using urea (urea-formaldehyde resin) are preferred. From the perspective of curability (reactivity with polyester resin), benzoguanidine resin and melamine resin are more preferred. From the perspectives of coating peel resistance and resistance to boiling whitening during heat treatment, benzoguanidine resin is the most preferred.

[0127] As a benzoguanamine resin, it is preferred to obtain a benzoguanamine resin by partially or completely alkylating the hydroxymethyl group of the benzoguanamine resin with alcohols such as methanol, ethanol, n-butanol, and isobutanol. Examples include methylated benzoguanamine resin, diethylated benzoguanamine resin, butylated 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.

[0128] As a melamine resin, it is preferred to be a melamine resin obtained by alkylating part or all of the hydroxymethyl group of the melamine resin with alcohols such as methanol, ethanol, n-butanol, and isobutanol, such as methyl etherified melamine resin, diethyl etherified melamine resin, butyl etherified melamine resin, or mixed etherified melamine resin of methyl ether and butyl ether, mixed etherified melamine resin of methyl ether and diethyl ether, or mixed etherified melamine resin of diethyl ether and butyl ether.

[0129] As a urea-formaldehyde resin, it is preferred to be a urea-formaldehyde resin obtained by etherifying part or all of the hydroxymethyl group of the urea-formaldehyde resin with alcohols such as methanol, ethanol, n-butanol, and isobutanol, such as methyl etherified urea-formaldehyde resin, diethyl etherified urea-formaldehyde resin, butyl etherified urea-formaldehyde resin, or a mixed etherified urea-formaldehyde resin of methyl ether and butyl ether, a mixed etherified urea-formaldehyde resin of methyl ether and diethyl ether, or a mixed etherified urea-formaldehyde resin of diethyl ether and butyl ether.

[0130] The functional groups possessed by the aforementioned melamine resin and benzoguanamine resin include: imino (>NH), N-hydroxymethyl (>NCH2OH), and N-alkoxymethyl (>NCH2OR; R is alkyl). These functional groups function as reaction sites in crosslinking reactions with the carboxyl groups (-COOH) and hydroxyl groups (-OH) contained in the polyester resin as the main agent, or in the self-condensation reactions between amino resins (it should be noted that imino groups only contribute to self-condensation reactions). It should be noted that, regarding the number of the aforementioned reaction sites (functional groups), melamine resin has more in its molecular structure compared to benzoguanamine resin monomers. Therefore, melamine resin has excellent curing properties; however, the crosslinking density of the formed coating film tends to be high, and depending on the amount added, there is a possibility of coating peeling 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. From the viewpoint of coating peel resistance, it can be said to be more suitable than melamine resin. Therefore, in order to obtain a balance between curing properties and coating peel resistance during heat treatment, a mixed amino resin prepared by mixing melamine resin and benzoguanamine resin in a specified ratio can also be used. In this case, ideally, the mixing ratio (mass ratio) of melamine resin to benzoguanamine resin is set to 90:10 to 5:95, preferably 80:20 to 10:90, more preferably 80:20 to 20:80, further preferably 75:25 to 25:75, and particularly preferably 70:30 to 30:70.

[0131] Ideally, the curing agent is formulated in a range of 1 to 40 parts by weight, preferably 1 to 30 parts by weight, and more preferably 2 to 20 parts by weight relative to 100 parts by weight of the polyester resin.

[0132] When using a methyl phenolic resin as a curing agent, it is preferable that the resin is formulated in a range of 1 to 30 parts by mass relative to 100 parts by mass of the polyester resin (solid component) that forms the main agent, preferably 1.5 to 20 parts by mass, more preferably 2 to 15 parts by mass, further preferably 2 to 10 parts by mass, particularly preferably more than 2 parts by mass and less than 10 parts by mass, and most preferably 2.5 to 8.5 parts by mass.

[0133] Furthermore, when using amino resin as a curing agent, it is ideal that the amount of melamine resin is less than 10 parts by weight relative to 100 parts by weight of polyester resin, and preferably less than 5.5 parts by weight.

[0134] When melamine resin is used as a curing agent, it is preferable to use an amount of 0.1 parts by mass or more and less than 10 parts by mass relative to 100 parts by mass of polyester resin, preferably 0.1 parts by mass or more and less than 5.5 parts by mass, more preferably 0.5 to 5.4 parts by mass, further preferably 0.5 to 5 parts by mass, particularly preferably 0.5 to 4 parts by mass, and most preferably 1 part by mass or more and less than 4 parts by mass.

[0135] When benzoguanidine resin is used as a curing agent, it is preferred to use 4 to 40 parts by weight, more preferably 5 to 30 parts by weight, more preferably 6 to 28 parts by weight, further preferably 7 to 25 parts by weight, particularly preferably 8 parts by weight or more but less than 25 parts by weight, and most preferably 10 to 24 parts by weight relative to 100 parts by weight of polyester resin.

[0136] When the mixed amino resin of melamine resin and benzoguanamine resin is used as a curing agent, it is preferable to use 2 to 25 parts by weight, more preferably 2 to 20 parts by weight, more preferably 2.5 to 15 parts by weight, and even more preferably 3 parts by weight or more and less than 10 parts by weight relative to 100 parts by weight of polyester resin.

[0137] When the curing dosage is less than the range mentioned above, sufficient curing cannot be achieved, the degree of cross-linking of the coating film decreases, and there is a possibility that the gel fraction (A) of the coating film will be lower than the range mentioned above. Sometimes, the resistance to boiling whitening, resistance to embrittlement over time, and heat resistance will decrease. It should be noted that when the heat resistance of the coating film decreases, there is a possibility that the coating film may easily stick to the mold when high-speed molding into a seamless can, especially on the inner surface of the can, resulting in poor demolding, which may lead to can warping or damage to the main body, and thus reduce productivity. On the outer surface of the can, there is a possibility of surface defects such as coating wear.

[0138] On the other hand, if the curing dosage is higher than the above range, there is a possibility that the gel fraction (A) of the coating film is higher than the range, which may not be able to suppress coating peeling during heat treatment after the seamless can is formed, and there is a possibility that the coating coverage of the seamless can is reduced.

[0139] <Catalyst Solidification>

[0140] In the coating compositions for inner and outer surfaces used in this invention, a curing catalyst is preferably incorporated to promote the crosslinking reaction between the polyester resin and the curing agent.

[0141] 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 their amine neutralizations. Among the above-mentioned curing catalysts, organic sulfonic acid catalysts are preferred, and dodecylbenzenesulfonic acid and its amine neutralizations are particularly suitable.

[0142] Ideally, the curing catalyst, in terms of solid content, should be in the range of 0.01 to 3 parts by weight relative to 100 parts by weight of polyester resin, preferably in the range of 0.01 to 1.0 parts by weight, more preferably in the range of 0.01 parts by weight or more but less than 0.5 parts by weight, even more preferably in the range of 0.02 parts by weight or more but less than 0.3 parts by weight, and particularly preferably in the range of 0.02 parts by weight or more but less than 0.2 parts by weight. Furthermore, when using an amine neutralization product of the aforementioned acid catalyst (e.g., an amine neutralization product of dodecylbenzenesulfonic acid) as the curing catalyst, the content of the acid catalyst other than the amine is sufficient as long as it is within the above-mentioned range. If the content of the curing catalyst is less than the above-mentioned range, there is a possibility that the effect of promoting the curing reaction cannot be sufficiently obtained. On the other hand, if the content of the curing catalyst is more than the above-mentioned range, further effects cannot be expected, and the water resistance of the coating film decreases, resulting in the possibility of deterioration in corrosion resistance, resistance to boiling and whitening, etc. Furthermore, acid catalysts can be localized on the surface of a metal substrate through acid-base interactions, which may reduce the adhesion between the coating and the metal substrate and lead to problems such as coating peeling during can forming.

[0143] (Coating composition)

[0144] The coating composition for forming the coating film of the coated metal sheet of the present invention contains at least the aforementioned polyester resin and the aforementioned curing agent as main agents, and preferably contains the aforementioned curing catalyst (acid catalyst). It should be noted that, in the present invention, the component with the highest content (by mass percentage) among the solid components (non-volatile components excluding 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, as the main agent among all the resin components contained in the coating composition is preferably higher than 50% by mass, more preferably 60% by mass or more, further preferably 70% by mass or more, and particularly preferably 80% by mass or more.

[0145] In this invention, solvent-based coating compositions and water-based coating compositions are examples of coating compositions that can be used to form a coating film. From the viewpoint of coatability, solvent-based coating compositions are preferred in this invention.

[0146] 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. It should be noted that, in this embodiment, a solvent-based coating composition refers to a coating composition 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.

[0147] As the organic solvent, factors such as solubility and evaporation rate can be considered, and one or more of the following substances can be selected for use: toluene, xylene, aromatic hydrocarbon compounds, ethyl acetate, butyl acetate, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, isophorone, methyl cellosolve, butyl cellosolve, ethylene glycol monoethyl ether acetate, diethylene glycol monoethyl ether acetate, ethylene glycol monoethyl ether, methanol, ethanol, butanol, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, solvent naphtha, etc.

[0148] In the case of a water-based coating composition, an aqueous medium is contained as a solvent along with conventionally known water-dispersible or water-soluble polyester resin and curing agent.

[0149] As an aqueous medium, water or a mixture of water and organic solvents such as alcohols, polyols, or their derivatives can be used, similar to known aqueous coating compositions. When using organic solvents, it is preferable that the organic solvent is present in an amount of 1 to 45% by mass relative to the total aqueous medium in the aqueous coating composition, and particularly preferably in an amount of 5 to 30% by mass. Containing solvents within the above range improves film-forming performance.

[0150] As such organic solvents, amphiphilic organic solvents are preferred, such as methanol, ethanol, isopropanol, n-butanol, ethylene glycol, methyl ethyl ketone, butyl cellosolve, 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.

[0151] The coating composition may contain a lubricant as needed. In this case, the preferred amount of lubricant is 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.

[0152] By adding lubricant, scratches on the coating during molding can be suppressed, and the sliding properties of the coating can be improved.

[0153] Examples of lubricants that can be added to coating compositions include, for example, fatty acid ester waxes that are 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, as well as silicone compounds, petrolatum, etc. One of these lubricants may be used, or two or more may be used in combination.

[0154] 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 prescriptions.

[0155] Furthermore, without prejudice to 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 resin, epoxy resin, polyurethane resin, acrylic resin, polyvinyl chloride resin, polyvinyl chloride-vinyl acetate copolymer resin, polyvinyl alcohol, ethylene / vinyl alcohol copolymer, polyvinylpyrrolidone, polyvinyl ether, polyacrylamide, acrylamide compounds, polyethyleneimine, starch, gum arabic, methylcellulose, and other resins.

[0156] In coating compositions, it is preferable to contain polyester resin in an amount of 5 to 55% by mass, based on solids content. When the resin solids content is lower than this range, an adequate coating amount cannot be ensured, and the coating's coverage deteriorates. On the other hand, when the resin solids content is higher than this range, workability and coatability may sometimes be poor.

[0157] (Manufacturing method of coated metal sheet)

[0158] In this invention, as described above, an inner surface coating composition containing the polyester resin as a main agent and a methyl phenolic resin and / or amino resin as a curing agent is applied to at least the inner surface of the metal plate in such a way as the film thickness. Suitablely, an outer surface coating composition containing the main agent, i.e., the polyester resin, and a curing agent, and preferably containing an amino resin, is further applied to the outer surface of the metal plate in such a way as the film thickness.

[0159] The sintering conditions of the coating composition can be appropriately adjusted according to the type of polyester resin, curing agent, metal matrix, coating amount, etc. For the above-mentioned coating composition, to obtain sufficient curing, it is preferable to heat and cure at a sintering temperature of 150°C to 350°C, preferably above 200°C but below 320°C, for at least 5 seconds, preferably 5 seconds to 30 minutes, and particularly preferably 5 seconds to 180 seconds. If the sintering temperature is lower than the above range, there is a possibility that sufficient curing may not be obtained. On the other hand, if the sintering temperature is higher than the above range, there is a possibility that the polyester resin may thermally decompose due to excessive heating. If the sintering time is shorter than the above range, there is a possibility that sufficient curing may not be obtained; if the sintering time is longer than the above range, the economy and productivity are poor.

[0160] Furthermore, in the sintered inner surface coating, as mentioned above, it is important that the gel fraction (A) is 55% or more and less than 90%. More ideally, the difference between the gel fraction (A) and the gel fraction (B) of the inner surface coating is less than 10%. As a result, the residual stress of the processed coating is sufficiently reduced, which can effectively suppress coating peeling and can be formed into seamless cans such as deep-drawn and thinned cans with excellent resistance to boiling whitening and resistance to embrittlement over time.

[0161] It can be manufactured by applying a coating to at least the inner surface of the metal sheet, preferably both sides, using known coating methods such as roller coating, spray coating, or dip coating, and then sintering it using a heating unit such as a coil oven.

[0162] In the coated metal sheet of the present invention, a coating film formed of other coating compositions (solvent-based coating compositions or water-based coating compositions) may be formed as needed on the inner surface coating film formed on the surface that becomes the inner surface side of the can after molding and / or the outer surface coating film formed on the surface that becomes the outer surface side of the can after molding. However, it is preferable not to form such a coating film from an economic point of view.

[0163] Ideally, the outermost layer of the coated metal sheet used in this invention, which forms the inner surface of the can, is a coating film formed from a coating composition. Suitablely, this inner surface coating film is formed from the aforementioned inner surface coating composition, or a layer formed on the inner surface coating film by a wax-based lubricant described later. Similarly, ideally, the outermost layer of the coated metal sheet used in this invention, which forms the outer surface of the can, is a coating film formed from a coating composition. Suitablely, this outer surface coating film is formed from the aforementioned outer surface coating composition, or a layer formed on the outer surface coating film by a wax-based lubricant described later.

[0164] Furthermore, in the coated metal sheet of the present invention, the inner surface coating and the outer surface coating formed by the coating composition have excellent adhesion to the metal substrate. Therefore, it is appropriate to form the inner surface coating and / or the outer surface coating in direct contact with the metal sheet, which is the metal substrate.

[0165] (Seamless can)

[0166] In the seamless cans such as the deep-drawn and thinned cans of the present invention, the inner surface coating located on the inner surface side of the can has the same characteristics as the inner surface coating of the coated metal sheet, namely, it is characterized by containing polyester resin and a first-order phenolic resin and / or amino resin as a curing agent, and the gel fraction (A) of the inner surface coating as shown in the above formula (1a) is in the range of 55% or more and less than 90%, preferably in the range of 60% to 88%, more preferably in the range of more than 62% and less than 85%, further preferably in the range of 65% to 84%, particularly preferably in the range of 65% to 80%, and most preferably in the range of 68% or more and less than 78%.

[0167] Regarding the coated metal sheet of the present invention, as described above, in the seamless tank of the present invention, the gel fraction (A) of the inner surface coating containing polyester resin and methyl phenolic resin and / or amino resin as curing agent, as shown in the above formula (1a), can be within the above range, thereby taking into account resistance to boiling whitening, resistance to embrittlement over time and coating peeling resistance during heat treatment.

[0168] Furthermore, from the viewpoint of resistance to boiling and whitening, it is ideal that the difference between the gel fraction (A) shown in formula (1a) and the gel fraction (B) shown in formula (2a) of the inner surface coating is less than 10%.

[0169] Ideally, the gel fraction (B) is in the range of more than 45% and less than 99%, preferably in the range of more than 50% and less than 98%, more preferably in the range of more than 52% and less than 95%, further preferably in the range of 55% to 94%, particularly preferably in the range of 55% to 90%, and most preferably in the range of more than 58% and less than 88%.

[0170] Furthermore, ideally, the seamless can further has an outer surface coating on the outer surface side of the can, the outer surface coating also containing polyester resin, and preferably further containing amino resin as a curing agent.

[0171] Furthermore, ideally, the gel fraction (A) of the outer surface coating as shown in formula (3a) above is in the range of 40% or more and less than 90%, preferably in the range of 55% or more and less than 90%, more preferably in the range of more than 60% and less than 88%, further preferably in the range of 65% to 85%, particularly preferably in the range of 68% to 84%, and most preferably in the range of 70% to 84%. Furthermore, from the viewpoint of resistance to boiling and whitening, ideally, the difference between the gel fraction (A) of the outer surface coating as shown in formula (3a) above and the gel fraction (B) as shown in formula (4a) above is less than 10%. Gel fraction (B) = [(W12a-W13a) / (W11a-W13a)] × 100 (%) … (4a)

[0172] In the formula, W11a represents the mass of the coated metal substrate with the outer surface coating cut from the seamless tank, 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 outer surface coating is removed from the coated metal substrate.

[0173] Ideally, the gel fraction (B) is in the range of more than 45% and less than 99%, preferably in the range of more than 50% and less than 98%, more preferably in the range of more than 52% and less than 95%, further preferably in the range of 55% to 94%, particularly preferably in the range of 55% to 90%, and most preferably in the range of more than 58% and less than 88%.

[0174] It should be noted that the gel fractions (A) and (B) of the coating of the seamless can are not limited thereto, and can be determined, for example, based on the inner and / or outer coatings located in the central portion of the bottom of the seamless can. It should also be noted that, in this specification, the central portion of the bottom of the seamless can refers to a region with a relatively small degree of processing accompanying the seamless can forming, having a thickness approximately similar to that of the coated metal sheet used for forming.

[0175] Furthermore, the seamless can preferably have at least the bottom of the can and the main body of the can on the inner surface side continuously covered by the inner surface coating, and more preferably the bottom of the can and the main body of the can on the outer surface side continuously covered by the outer surface coating.

[0176] Furthermore, in the seamless can of the present invention, a second feature is that the thickness of the coating on the inner surface of the central portion of the can body is 20% to 75% of the thickness of the coating on the inner surface of the central portion of the can bottom.

[0177] The seamless can of the present invention is a deep-drawn thinning can (DI can), deep-drawn can (DR can), deep-drawn can (DRD can), DTR can, stretching and thinning can, etc., formed by conventionally known methods using the coated metal sheet as described below, but deep-drawn thinning can is particularly suitable in the present invention.

[0178] The aforementioned seamless cans are formed through processes such as deep drawing and thinning. Therefore, the thickness of the coating on the inner surface of the can body is reduced to the same thickness as the metal substrate through processing. Appropriately, the thickness of the coating on the inner surface of the central part of the can body (the part with the thinnest wall in the height direction) is in the range of 20% to 75% of the thickness of the coating on the inner surface of the central part of the can bottom, which is hardly thinned during can manufacturing. Especially in deep-drawn and thinned cans, it is appropriate, preferably 20% to 60%, more preferably 20% to 50%, further preferably 25% to 45%, particularly preferably 30% to 45%, and most preferably 30% to 40%. Regarding the thickness of the outer surface coating, it is appropriate that the thickness of the outer surface coating of the central part of the can body (the central part in the height direction, the thinnest part) is in the range of 20% to 75% of the thickness of the outer surface coating of the central part of the can bottom. Especially in deep-drawn and thinned cans, it is appropriate, preferably 20% to 60%, more preferably 20% to 50%, further preferably 25% to 45%, particularly preferably 30% to 45%, and most preferably 30% to 40%.

[0179] Furthermore, in the seamless can of the present invention, it is suitable that the thickness of the coating on the inner surface of the thinnest part of the can body is in the range of 80% or less of the thickness of the coating on the inner surface of the thickest part (the unthinned part) of the can body, particularly in deep-drawn thinning cans, preferably 70% or less, more preferably 60% or less, and even more preferably 55% or less. Similarly, for the outer surface coating, it is suitable that the thickness of the coating on the outer surface of the thinnest part of the can body is in the range of 80% or less of the thickness of the coating on the outer surface of the thickest part (the unthinned part) of the can body, particularly in deep-drawn thinning cans, preferably 70% or less, more preferably 60% or less, and even more preferably 55% or less.

[0180] 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, more preferably 0.15 to 0.30 mm, and even more preferably 0.20 to 0.28 mm. The type of metal substrate is the same as that of the coated metal sheet used for forming.

[0181] Furthermore, the thickness of the coating film on the inner surface of the center portion of the bottom of the seamless can is the same as the thickness described regarding the coated metal sheet used for forming, preferably in the range of 0.2 to 20 μm, preferably in the range of 1 to 16 μm, and more preferably in the range of greater than 2 μm and less than 12 μm. Furthermore, as for the quality of the dried coating film, it is suitable to be in the range of 3 to 300 mg / dm³. 2 Within the range of 15–220 mg / dm³, preferably 15–220 mg / dm³ 2 More preferably, it is within the range of 15–150 mg / dm³. 2 Within the range, more preferably greater than 25 mg / dm 2 And it is 150 mg / dm 2 Within the following range.

[0182] When the contents of a seamless can are a highly corrosive acidic beverage, it is appropriate that the coating thickness on the inner surface of the central part of the can bottom is greater than 5 μm and less than 16 μm, preferably greater than 6 μm and less than 12 μm, and more preferably in the range of 6.5 to 10 μm. Furthermore, as for the quality of the dried coating, it is appropriate to have a thickness greater than 70 mg / dm³. 2 The above is 150 mg / dm 2 The following range is preferred, greater than 85 mg / dm³. 2 And it is 150 mg / dm 2 The following range is preferred: 90–140 mg / dm³ 2 The range.

[0183] On the other hand, when the contents filled in the seamless can are low-acid beverages or similar substances with low corrosiveness, the film thickness of the coating on the inner surface of the central part of the can bottom is in the range of 1 μm or more and less than 6.5 μm, preferably greater than 2 μm and less than 6.5 μm, and more preferably in the range of 2.5 to 6 μm. Furthermore, as for the quality of the dried coating, it is appropriate to have a thickness of 15 mg / dm³. 2 Above and below 90 mg / dm 2 The range is preferably greater than 25 mg / dm³. 2 And less than 90 mg / dm 2 The range is more preferably 30–85 mg / dm³. 2 The range.

[0184] Furthermore, it is suitable that the film thickness of the coating on the outer surface of the central part of the can bottom, measured as dry film thickness, is in the range of 0.2 to 20 μm, preferably in the range of 1 to 16 μm, more preferably in the range of greater than 2 μm and less than 12 μm, and even more preferably in the range of greater than 2 μm and less than 6.5 μm. Furthermore, it is suitable that the dry coating weight is 3 to 300 mg / dm³. 2The preferred range is 15–220 mg / dm³. 2 The range is more preferably 25–150 mg / dm³. 2 The range is further preferably greater than 25 mg / dm³. 2 And less than 90 mg / dm 2 The range.

[0185] (Manufacturing method of seamless cans (deep-drawn and thinned cans))

[0186] The deep-drawn thinning can of the present invention can be manufactured using the coated metal sheet of the present invention through conventionally known forming methods. In particular, as described above, the coated metal sheet of the present invention can be formed into a seamless can such as a deep-drawn thinning can with excellent coating properties, without metal exposure, even during harsh processing such as stretching and thinning, without causing damage to the main body or peeling of the coating at the can opening. It should be noted that the coated metal sheet of the present invention is a metal sheet with excellent formability and lubricity; therefore, needless to say, it can be formed into a seamless can such as a deep-drawn thinning can when a coolant is used, and even when forming under dry conditions without a coolant, it can be formed. The manufacturing method of the deep-drawn thinning can will be described in detail below.

[0187] Before deep drawing and thinning, it is preferable to apply a wax-based lubricant to the coating surface of the coated metal sheet, thereby enabling efficient deep drawing and thinning under dry conditions. The wax-based lubricant is not limited to any particular type; examples include: fatty acid ester waxes, silicone waxes, white petrolatum, rice bran wax, beeswax, wood wax, lignite wax, and other mineral-derived waxes; Fischer-Tropsch waxes; polyolefin waxes such as polyethylene and polypropylene; paraffin waxes (solid paraffin, liquid paraffin); lanolin wax; microcrystalline wax; carnauba wax, etc. Paraffin waxes and white petrolatum are particularly suitable. One of these wax-based lubricants may be used, or a mixture of two or more may be used.

[0188] In the aforementioned wax-based lubricants, it is ideal to volatilize more than 50% by mass of the wax-based lubricant (high-temperature volatile wax-based lubricant) through heat treatment (heating) at a temperature of 100°C to 350°C, preferably 150°C to 250°C, more preferably around 200°C for a short time (e.g., 0.1 to 600 seconds, preferably 1 to 300 seconds, more preferably 10 to 180 seconds). As a result, in subsequent processes after deep drawing and thinning of the can, the wax-based lubricant can be easily volatilized and removed through heat treatment. When printing on the outer surface of the can body, there is no possibility of ink repulsion due to the wax-based lubricant, which is ideal in terms of adaptability to outer surface printing. Ideally, as a high-temperature volatile wax-based lubricant, it should be able to volatilize and remove 50% or more, preferably 60% or more, more preferably 80% or more, and even more preferably 90% or more by mass, of the lubricant through a short-term heat treatment (10 to 180 seconds) at a temperature of 150°C to 250°C, preferably around 200°C. Ideally, the lubricant should have a melting point of 20°C to 100°C, preferably 25°C to 80°C. From the viewpoint of moldability and production efficiency, the coating amount of the wax-based lubricant should ideally be 1 to 1000 mg / m² per single side of the coated metal sheet. 2 The preferred range is 2–500 mg / m³. 2 The range is more preferably 5–200 mg / m³. 2 The range is further preferably 10–100 mg / m³. 2 The range is preferably 20–80 mg / m³. 2 The range.

[0189] A coated metal sheet coated with a wax-based lubricant is drawn / pressed into a deep-drawn cup by a deep-drawing process. In this invention, ideally, the total drawing ratio RD defined by the following formula (8) (up to the deep-drawn thinning cup) is in the range of 1.1 to 2.6, particularly in the range of 1.4 to 2.6. If the drawing ratio is larger than the above range, the deep-drawing wrinkles become larger, and there is a possibility of coating cracking and metal exposure.

[0190] RD = D / d……(8)

[0191] In the formula, D represents the diameter of the billet, and d represents the diameter of the tank body.

[0192] Next, the drawn cup undergoes further drawing—one or more stages of thinning (drawing and thinning)—to thin the wall of the can body. In this invention, it is preferable to adjust the temperature of the forming punch to 10°C to 100°C, more preferably 10°C to 80°C, and even more preferably 15°C to 70°C. If the forming punch temperature is lower than the above range, the wax-based lubricant applied to the coated metal sheet cannot adequately exhibit lubricity, and there is a possibility of poor demolding at the time the can is withdrawn from the forming punch, as well as reduced elongation of the inner surface coating and reduced coating coverage after forming. On the other hand, if the punch temperature is higher than the above range, the coating tends to adhere to the forming punch at the time the can is withdrawn from the forming punch, potentially resulting in poor demolding. Furthermore, regarding other molds (mold heads, etc.) used for molding, from the viewpoint of stable and continuous molding, it is ideal to adjust the temperature to be 10°C to 100°C, preferably 10°C to 80°C, and more preferably 15°C to 70°C.

[0193] In this invention, ideally, the thinning rate R shown in formula (9) is in the range of 25% to 80%, preferably in the range of 40% to 80%, more preferably in the range of 50% to 80%, further preferably in the range of 55% to 75%, particularly preferably in the range of 55% to 70%, and most preferably in the range higher than 60% and lower than 70%. If the thinning rate is lower than the above range, sufficient thinning cannot be achieved, and the economic aspect is not sufficiently satisfied. On the other hand, if the thinning rate is higher than the above range, there is a possibility of metal exposure.

[0194] R(%)=(tp-tw) / tp×100……(9)

[0195] In the formula, tp represents the thickness of the original coated metal sheet, and tw represents the thickness of the central part of the can body of the deep-drawn and thinned can.

[0196] Furthermore, in the deep-drawn thinning can of the present invention, it is appropriate that the thickness of the central portion of the can body (the central portion in the height direction, the thinnest part) is 20% to 75%, preferably 20% to 60%, more preferably 20% to 50%, further preferably 25% to 45%, particularly preferably 30% to 45%, and most preferably 30% to 40% of the thickness of the central portion of the can bottom. Appropriately, the thickness of the metal substrate of the deep-drawn thinning can is also 20% to 75%, preferably 20% to 60%, more preferably 20% to 50%, further preferably 25% to 45%, particularly preferably 30% to 45%, and most preferably 30% to 40% of the thickness of the metal substrate of the central portion of the can bottom. Furthermore, when a deep-drawn thinning can is formed from a coated metal sheet through deep-drawing and thinning processing, the thickness of the coating film located in the can body is thinned in the same way as the metal substrate during processing. Therefore, as described above, it is appropriate that the thickness of the coating in the central part of the can body is 20% to 75% of the thickness of the coating in the central part of the can bottom where the can is hardly thinned during can making, preferably 20% to 60%, more preferably 20% to 50%, further preferably 25% to 45%, particularly preferably 30% to 45%, and most preferably 30% to 40%.

[0197] Furthermore, in the deep-drawn thinning can of the present invention, it is appropriate that the thickness of the thinnest part of the can body is 80% or less, preferably 70% or less, more preferably 60% or less, and even more preferably 55% or less of the thickness of the thickest part (the least thinned part) of the can body.

[0198] Furthermore, as described above, when a thinned-out can is formed from a coated metal sheet having at least an inner surface coating through a deep-drawing process, the thickness of the inner surface coating located in the can body is thinned in the same way as the metal substrate located in the can body. Therefore, in the thinned-out can of the present invention, the thickness ratio of the inner surface coating to the metal substrate in the can body and the thickness ratio of the inner surface coating to the metal substrate in the can bottom are substantially the same. That is, in the thinned-out can of the present invention, the characteristic is that the thickness ratio of the inner surface coating to the metal substrate (=thickness of the inner surface coating / thickness of the metal substrate) is substantially the same in the can bottom and the can body. It should be noted that "substantially the same" here means that manufacturing errors are also included within its range, for example, the thickness ratio of the inner surface coating to the metal substrate in the can body is in the range of 0.9 to 1.1 times that in the can bottom. It should be noted that the same applies to the outer surface coating.

[0199] Furthermore, in the deep-drawn thinning can of the present invention, the thickness ratio of the inner surface coating of the can body to the metal substrate (= thickness of the inner surface coating / thickness of the metal substrate) is substantially the same throughout the can body regardless of its position. It should be noted that the same applies to the outer surface coating.

[0200] It should be noted that the processing speed (punch movement speed) for the one-stage or multi-stage thinning process is ideally set to 2000 mm / sec or higher, preferably 3000 mm / sec or higher, more preferably 4000 mm / sec or higher, even more preferably 5000 mm / sec or higher, and particularly preferably 6000 mm / sec or higher. By setting the processing speed during the thinning process to the above-mentioned speed or higher, the processing heat generation increases, reaching a high-temperature state, thereby improving the processability (elongation) of the coating. As a result, metal exposure during forming can be suppressed, further improving the coverage of the inner and outer surface coatings after forming. Moreover, forming at high temperatures can further reduce the residual stress of the formed coating, which is also preferable in suppressing coating peeling during heat treatment.

[0201] After the deep drawing and thinning process, the bottom is bulged and the open end edge is trimmed according to the usual method, as desired.

[0202] According to the present invention, it is appropriate to apply a heat treatment process to the resulting seamless can, such as a drawn and thinned can, after the coated metal sheet has undergone drawing and thinning processes. As described above, in the coated metal sheet and drawn and thinned can of the present invention, the gel fraction (A) of the coating film is controlled to be as low as less than 90%, thus effectively preventing coating peeling even when heated during the heat treatment process.

[0203] As described above, by performing at least one stage of heat treatment on seamless cans such as deep-drawn and thinned cans after forming, residual stress in the coating film generated during processing can be removed. Removing this residual stress improves the adhesion between the processed coating film and the metal substrate (coating adhesion). As a result, the corrosion resistance of the coating film is significantly improved; for example, when the seamless can is filled with highly corrosive contents, undercoating corrosion can be suppressed. The heat treatment temperature needs to be higher than the glass transition temperature of the coating film, preferably 100°C to 300°C, more preferably in the range of 150°C to 250°C. The heat treatment time is not particularly limited, but preferably 0.1 to 600 seconds, more preferably 1 to 300 seconds, and more preferably 10 to 180 seconds. It should be noted that if the wax-based lubricant used during processing is the aforementioned high-temperature volatile wax-based lubricant, this heat treatment can cause it to evaporate and be removed from the coating surface.

[0204] When the residual stress in the coating of seamless cans such as deep-drawn and thinned cans is not removed by heat treatment, when the coating of the central part (the central part in the height direction) of the can body with a large machining degree is separated from the metal substrate and heated, the size changes significantly in the direction of releasing residual stress (mainly the height direction of the can). Therefore, by measuring the amount of dimensional change of the separated coating caused by heating (thermal shrinkage rate), it can be used as a reference value to determine whether the residual stress has been removed by heat treatment. Ideally, the thermal shrinkage rate (with load) of the coating on the inner surface of the central part of the can body after separation from the seamless can body, as shown in the following formula (5), is 30% or less, preferably 25% or less, and more preferably 20% or less. Furthermore, ideally, the thermal shrinkage rate (without load) shown in the following formula (6) is 50% or less, preferably 45% or less, and more preferably 40% or less. When the thermal shrinkage rate is within the above range, the coating adhesion can be improved, and excellent corrosion resistance can be exhibited. If the thermal shrinkage rate is greater than the above range, residual stress may not be sufficiently removed, resulting in insufficient coating adhesion. This could lead to reduced corrosion resistance and the possibility of coating peeling when the can is subjected to impacts or dents. Furthermore, when the outer surface coating is present on the outer surface of the can, ideally, the thermal shrinkage rate of the outer surface coating in the central part of the can body should also be within the above range.

[0205] It should be noted that the dimensional change (shrinkage) caused by heating of the separated coating film can be measured by thermomechanical analysis (TMA) and other instruments.

[0206] Heat shrinkage rate (with load) = (ΔL1 / L0) × 100 (%) ... (5)

[0207] In the formula, L0 is the initial length (measuring section) of the coating in the height direction after separation from the center of the can body, and ΔL1 is the amount of coating applied per unit area at a rate of 5.20 × 10⁻⁶ mm. 5 N / m 2 The maximum shrinkage (maximum shrinkage length) of the coating film in the height direction of the portion corresponding to L0 when the load side is heated from 30℃ to 200℃ at a heating rate of 5℃ / min.

[0208] Heat shrinkage rate (without load) = (ΔL2 / L0) × 100 (%)……(6)

[0209] In the formula, L0 is the initial length (measuring part) of the coating film in the height direction after being separated from the central part of the can body, and ΔL2 is the maximum shrinkage (maximum shrinkage length) of the coating film 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.

[0210] After heat treatment, and following rapid cooling or evaporation, a printed layer is formed on the can body using conventionally known methods, followed by a finishing varnish layer to protect the printed layer. Depending on the desired outcome, one or more necking processes are applied, followed by flange forming, to create a can for roll sealing. Furthermore, after forming seamless cans such as deep-drawn and thinned cans, the upper part can be deformed to create a bottle shape, or the bottom can be cut off and fitted with other can ends to create a bottle shape.

[0211] The capacity of the seamless can, such as the deep-drawn and thinned 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.

[0212] The inner surface coating of the coated metal sheet of the present invention has excellent can-making processability. Therefore, even during harsh processing such as deep drawing and thinning, metal exposure can be suppressed. Furthermore, even during heat treatment after forming, coating peeling does not occur. Therefore, when forming a deep drawing and thinning can, by adjusting forming conditions such as the thinning processing speed, seamless cans such as deep drawing and thinning cans with excellent coating coverage, having an inner surface coating coverage of less than 200mA converted by ERV (Enamel Rater Value), can be obtained.

[0213] Here, the coverage of the inner surface coating, calculated using ERV, is set as follows: In the obtained deep-drawing and thinning can, a 1% by mass salt solution (which will become the electrolyte) is filled to near the can opening. The ERV value obtained by measuring the ERV using an inner coating measuring instrument is set as follows: A metal exposure portion is formed on the outer surface of the can bottom and connected to the anode. Meanwhile, the cathode is immersed in the salt solution filled in the can. The current value is determined after applying a DC voltage of 6.3V for 4 seconds at room temperature (20°C). In this measurement, the greater the current flow, the more defects exist in the inner surface coating, which acts as an insulator, and the larger the area of ​​exposed metal on the inner surface of the can.

[0214] Ideally, the coverage of the coating on the inner surface of seamless cans, such as those for deep-drawn and thinned cans, calculated using ERV, is less than 200 mA, preferably less than 100 mA, and more preferably less than 50 mA. Furthermore, it is ideal that the coverage per unit area (cm²) is... 2 In the case of ERV representation, less than 0.70 mA / cm 2 Preferably less than 0.35 mA / cm 2 More preferably, it is less than 0.18 mA / cm. 2Here, ERV per unit area refers to the value obtained by dividing the ERV of the seamless tank, measured by the above method, by the evaluation area (the total area of ​​the inner surface of the tank body and bottom in contact with the brine).

[0215] It should be noted that, regarding the inner surface of seamless cans such as deep-drawn and thinned cans, after molding, it is possible to further spray-coat the inner surface with corrective coatings or other coatings as needed to form other coatings on the inner surface coating film. However, as mentioned above, the inner surface coating film already has high coverage after molding, so there is no need for spray coating. From an economic point of view, it is preferable not to perform spray coating. That is, preferably, the outermost layer of the inner surface of the deep-drawn and thinned can of the present invention is a coating film formed by a coating composition, appropriately the inner surface coating film formed by the aforementioned inner surface coating composition, or a layer formed by the aforementioned wax-based lubricant applied to the inner surface coating film, more preferably the inner surface coating film.

[0216] Furthermore, regarding the outer surface of the can, specifically the bottom of the can where at least a printing layer is not substantially formed, for purposes such as improving the can's conveyability, a coating film formed from other paint compositions may be further formed on the outer surface coating film formed on the outer surface side of the bottom. That is, ideally, the outermost layer of the outer surface of the can bottom is a coating film formed from a paint composition; appropriately, this outer surface coating film, or a layer formed from the wax-based lubricant applied to the outer surface coating film; more preferably, this outer surface coating film is located at the outermost layer of the can bottom.

[0217] The coated metal sheet of this invention can be used for stretch-thinned cans obtained through rigorous forming processes, and therefore can also be appropriately applied to applications other than deep-drawn and thinned cans, such as seamless cans like deep-drawn cans (DR cans), deep-drawn cans (DRD cans), DTR cans, and stretch-thinned cans, or can lids, obtained by conventional manufacturing methods. The shape of the can lid can be a conventionally known shape, such as an easy-open lid with a score for forming an opening for dispensing contents and a handle for opening, and can be either a full-opening type or a stay-on-tab type.

[0218] Example

[0219] The following examples and comparative examples illustrate the present invention in detail. It should be noted that the abbreviation "parts" refers to parts by mass.

[0220] The various test items for polyester resins A through D are performed according to the following methods. It should be noted that polyester resins A through D are all non-crystalline polyester resins.

[0221] (Determination of number-average molecular weight)

[0222] The determination was performed using a calibration curve of standard polystyrene via gel permeation chromatography (GPC).

[0223] (Determination of glass transition temperature)

[0224] The measurements were performed using a differential scanning calorimeter (DSC) at a heating rate of 10 °C / min.

[0225] (Determination of acid value)

[0226] 1g of solid polyester resin was dissolved in 10ml of chloroform and titrated with 0.1N KOH ethanol solution to determine the resin acid value (mgKOH / g). Phenolphthalein was used as the indicator.

[0227] (Determination of monomer composition)

[0228] Dissolve 30 mg of solid polyester resin in 0.6 mL of deuterated chloroform and proceed. 1 The monomer composition ratio is determined by H-NMR measurement and the peak intensity.

[0229] (Example 1)

[0230] [Preparation of coating compositions for internal surfaces]

[0231] As polyester resins, polyester resin A (acid value: 5 mg KOH / g, hydroxyl value: 6 mg KOH / g, Tg: 56℃, Mn = 16000, polycarboxylic acid composition: terephthalic acid / isophthalic acid / trimethicone = 19 / 80 / 1 mol%) and polyester resin D (acid value: 22 mg KOH / g, hydroxyl value: 6 mg KOH / g, Tg: 56℃, Mn = 16000, polyol composition: 1,4-cyclohexanediol / 2-methyl-1,3-propanediol = 36 / 64 mol%) and polyester resin D (acid value: 22 mg KOH / g, hydroxyl value: 6 mg KOH / g, Tg: 56℃, Mn = 16000, polycarboxylic acid composition: terephthalic acid / isophthalic acid / trimethicone = 19 / 80 / 1 mol%) were used. The substance is a mixture of terephthalic acid (98 / 2 mol%) and polycarboxylic acid (90:10 by mass) with hydroxyl value of 0 mg KOH / g, Tg of 82℃, Mn of 6000, and polycarboxylic acid composition of 98 / 2 mol% (terephthalic acid / trimethicone = 24 / 76 mol%). The curing agent is a methyl phenolic resin, and the curing catalyst (acid catalyst) is dodecylbenzenesulfonic acid.

[0232] It should be noted that, as the above-mentioned methyl phenolic resin, a m-cresol-based methyl phenolic resin obtained by alkylating hydroxymethyl with n-butanol (the proportion of alkylated hydroxymethyl is 90 mol%, Mn = 1200) is used; as the acid catalyst, "NACURE5925" (manufactured by King Industries, amine-neutralized dodecylbenzenesulfonic acid solution, active ingredient 25% by mass) is used.

[0233] Polyester resin A and polyester resin D were dissolved in a mixed solvent of methyl ethyl ketone / solvent naphtha = 50 / 50 (mass ratio) to obtain solutions of polyester resin A and polyester resin D with a solid content of 25% by mass. A n-butanol solution of methyl phenolic resin (50% by mass) was diluted with methyl ethyl ketone to obtain a methyl phenolic resin solution with a solid content of 25% by mass.

[0234] Next, 360 parts of polyester resin A solution (90 parts solids), 40 parts of polyester resin D solution (10 parts solids), 40 parts of methyl phenolic resin solution (10 parts solids), and 0.40 parts of acid catalyst solution (0.10 parts solids) were added to a glass container and stirred for 10 minutes to prepare a solvent-based coating composition [solids concentration: approximately 25% by mass, solids ratio: polyester resin A / polyester resin D / methyl phenolic resin / acid catalyst = 90 / 10 / 10 / 0.1 (mass ratio)].

[0235] [Preparation of coating compositions for exterior surfaces]

[0236] Polyester resin B (acid value: 2 mg KOH / g, hydroxyl value: 5 mg KOH / g, Tg: 75℃, Mn = 18000, polycarboxylic acid composition: terephthalic acid / isophthalic acid = 76 / 24 mol%), polyol composition: ethylene glycol / propylene glycol = 34 / 66 mol%) was used as the polyester resin; benzoguanamine resin A (butylated benzoguanamine resin, partially etherified type containing imino / hydroxymethyl groups) was used as the curing catalyst (acid catalyst) (manufactured by King Industries, amine-neutralized dodecylbenzenesulfonic acid solution, active ingredient 25% by mass) was used as the curing catalyst.

[0237] Polyester resin B was dissolved in a mixed solvent of methyl ethyl ketone and naphtha in a ratio of 50 / 50 (mass ratio) to obtain a polyester resin B solution with a solid content of 25% by mass. A benzoguanamine resin A n-butanol solution (with a solid content of 70% by mass) was dissolved in methyl ethyl ketone to obtain a benzoguanamine resin solution with a solid content of 25% by mass.

[0238] Next, 400 parts of polyester resin B solution (100 parts of solids), 60 parts of benzoguanamine resin solution (15 parts of solids), and 0.4 parts of acid catalyst solution (0.10 parts of solids) were added to a glass container and stirred for 10 minutes to prepare a solvent-based coating composition [solids concentration: approximately 25% by mass, solids ratio: polyester resin B / benzoguanamine resin A / acid catalyst (dodecylbenzenesulfonic acid) = 100 / 15 / 10 / 0.1 (mass ratio)].

[0239] [Production of coated metal sheets]

[0240] As a metal sheet, an aluminum sheet (3104 alloy, sheet thickness: 0.27mm) with chromate phosphate surface treatment was used. First, after forming, the outer surface was coated with a dry film of 40mg / dm³ after sintering. 2 The above-mentioned coating composition for the outer surface was applied using a doctor blade coater in a manner that resulted in a thickness of approximately 3 μm, and then dried at 120°C for 60 seconds. Then, on the inner surface side, which is the opposite side, a dried coating film with a mass of 88 mg / dm³ after sintering was applied. 2 The above-mentioned coating composition for the inner surface is applied using a doctor blade coater in a manner that produces a thickness of approximately 6.4 μm. After drying at 120°C for 60 seconds, it is sintered at 250°C (the oven temperature) for 30 seconds to produce the coating.

[0241] [Fabrication of Thin-Drawn Cans]

[0242] The coated metal sheet produced by the above method is coated on both sides (coating amount: approximately 50 mg / m² per side). 2 Paraffin wax, used as a high-temperature volatile wax-based lubricant (which can evaporate more than 50% by mass through short-term heat treatment at around 200°C), is stamped into a circle with a diameter of 142mm to produce a shallow-drawn cup. Then, using a punch with an outer diameter of Φ66mm (with temperature control), the shallow-drawn cup undergoes further drawing, thinning (three stages), and bulging processes under dry conditions. Then, a heat treatment was performed at 201°C for 75 seconds in an oven to obtain a deep-drawn and thinned can [can diameter: 66mm, height: approximately 130mm, capacity: approximately 370ml, total drawing ratio: 2.15, thinning rate: 61%, thickness of can body center section / thickness of can bottom center section × 100 = approximately 40%, thickness of metal substrate in can body center section / thickness of metal substrate in can bottom center section × 100 = approximately 40%, inner surface coating thickness of can body center section / inner surface coating thickness of can bottom center section × 100 = approximately 39%, inner surface coating mass (film thickness) of can bottom center section: 86mg / dm]. 2 (Approximately 6.3 μm), the coating thickness on the inner surface of the central part of the can bottom / the thickness of the metal substrate in the central part of the can bottom = approximately 0.024, and the coating thickness on the inner surface of the central part of the can body / the thickness of the metal substrate in the central part of the can body = approximately 0.023. It should be noted that after the above-mentioned heat treatment at 201°C for 75 seconds, at least 90% by mass of the above-mentioned paraffin wax is volatilized and removed.

[0243] (Examples 2-10, Comparative Examples 1-5)

[0244] As shown in Table 1, the formulation of the inner and outer surface coating compositions (type of polyester resin, type of curing agent, and solid component ratio of polyester resin / curing agent / acid catalyst) was changed. Otherwise, the coated metal sheet was manufactured in the same manner as in Example 1, and a deep-drawn thinning can was produced. It should be noted that, in addition to the polyester resin described above, polyester resin B and polyester resin C (acid value: 2 mg KOH / g, hydroxyl value: 5 mg KOH / g, Tg: 85°C, Mn = 18000, polycarboxylic acid composition: terephthalic acid = 100 mol%, polyol composition: ethylene glycol / propylene glycol = 28 / 72 mol%) were used. In addition to the aforementioned first-stage phenolic resin and benzoguanamine resin A, benzoguanamine resin B (a mixed etherified benzoguanamine resin of methyl ether and diethyl ether, fully etherified type), melamine resin (methyl etherified melamine resin, fully etherified type), and p-cresol-based first-stage phenolic resin (first-stage phenolic resin containing p-cresol as the main component of the initial raw material) are also used as curing agents.

[0245] The properties of the coating film obtained using the inner surface coating compositions used in the various embodiments and comparative examples were tested according to the following test methods.

[0246] [Gel fraction (A)]

[0247] Using the inner and outer surface coating compositions used in the various embodiments and comparative examples, separation coating samples for testing were prepared as follows. Each coating composition was applied to a chromate phosphate-treated aluminum plate (3104 alloy, plate thickness: 0.27 mm) using a doctor blade coater under the same coating conditions (coating type, dried coating quality, drying / sintering conditions) as the inner or outer surface coating compositions of the coated metal plates in the various embodiments and comparative examples. After drying at 120°C, the plate was sintered at 250°C for 30 seconds to produce a 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). Next, the film-like separation coating was removed, thoroughly washed with distilled water, and dried to prepare the separation 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 (approximately 20°C) for 1 hour. Then, the solvent-insoluble matter (MEK-insoluble matter) of the sample was removed onto an aluminum dish and dried at 120°C for 30 minutes. After cooling to room temperature, the total mass (W2') of the aluminum dish and the dried sample was measured. The mass (W0) of the aluminum dish (previously determined) was then subtracted from this total mass to obtain the mass (W2 = W2' - W0) of the dried sample. The gel fraction (A) (%) of the inner and outer surface coatings of the coated metal plate was determined using the following formula (1). The results are shown in Table 1.

[0248] Gel fraction (A)(%) = 100 × W2 / W1……(1)

[0249] It should be noted that when the sample for testing is obtained from a coated metal sheet or a thinning can with coatings on both sides, after cutting out the test piece, the coating on the side not to be tested can be removed by sanding or other means, and then the metal substrate can be dissolved using the method described above to obtain the separation coating sample for testing. Alternatively, the test piece can be cut from the bottom of the coated metal sheet or the thinning can, immersed in boiling hydrogen peroxide for a few minutes, thoroughly washed with distilled water, and then the film-like coating can be peeled off from the metal substrate and dried to obtain the separation coating sample for testing.

[0250] [Gel fraction (A) of the coating on the inner surface of the can bottom]

[0251] For the deep-drawn thinning cans of Examples 2 and 3, which were formed as described in the "Preparation of Deep-Drawn Thinning Cans" section above and subjected to a 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. It should be noted that the average processing speed during the thinning process (the average movement speed of the punch during the thinning process) was set to approximately 5500 mm / sec. The method for preparing the separated coating samples used for measurement is described below.

[0252] From the bottom of the heat-treated, deep-drawn, thinned can, the bottom of the can is cut out with the center of the bottom as the center, in a manner 30 mm in the 0° direction and 30 mm in the 90° direction relative to the rolling point of the metal substrate. The cut sample is immersed in boiling hydrogen peroxide for 2-3 minutes, thoroughly rinsed with distilled water, and then the coating on the inner surface side of the can is peeled off from the metal substrate and dried, thus obtaining the separation coating sample for testing.

[0253] The gel fraction (A) was determined in the same manner as described in the section on "Gel Fraction (A)" above. The results are shown below.

[0254] The gel fraction (A) of the coating film on the inner surface of the bottom of the deep-drawn and thinned can in Example 2 is 80%.

[0255] The gel fraction (A) of the coating film on the inner surface of the bottom of the deep-drawn and thinned can in Example 3 is 74%.

[0256] [Gel fraction (B)]

[0257] The test samples were prepared using the inner and outer surface coating compositions used in the various examples and comparative examples, as described below. Using the same coating conditions (coating type, dried film quality, drying / sintering conditions) as the inner and outer surface coating compositions of the coated metal sheets in the various examples and comparative examples, each coating composition was applied to a chromate phosphate-treated aluminum sheet (3104 alloy, sheet thickness: 0.27 mm) using a doctor blade coater. After drying at 120°C, the sheet was sintered at 250°C for 30 seconds to produce a coated metal sheet. Test pieces measuring 5 cm × 5 cm were cut from the coated metal sheet. After the mass of the test pieces was measured (W3), the test pieces were immersed in boiling MEK (methyl ethyl ketone) (reflux at 80°C) for 1 hour using 400 ml of MEK, and MEK extraction was performed at boiling point for 1 hour. After cleaning the extracted test pieces with MEK, they were dried at 120°C for 30 minutes, and the mass of the extracted test pieces (W4) was determined. The coating was further peeled / removed, cleaned / dried using concentrated sulfuric acid, and the mass of the test pieces (W5) was determined. The gel fraction (B) (%) of the inner and outer surface coatings of the coated metal sheet was calculated using the following formula (2). The results are shown in Table 1.

[0258] Gel fraction (B)(%) = 100 × (W4 - W5) / (W3 - W5)……(2)

[0259] It should be noted that when the sample for testing is obtained from a coated metal sheet with a coating on both sides or a deep-drawing and thinning can, the coating on the side not to be measured can be removed by sanding or other means after cutting out the test piece, and then the gel fraction (B) can be determined by the above method.

[0260] The coated metal sheets and thinned cans obtained in each embodiment and comparative example were evaluated according to the following test methods.

[0261] [Evaluation of Resistance to Boiling and Whitening - 1 (Coated Metal Sheet)]

[0262] The evaluation of resistance to boiling and whitening was conducted using coated metal sheets produced as described in the section "Preparation of Coated Metal Sheets" above, as follows.

[0263] Test pieces measuring 2.5cm × 10cm were cut from the coated metal sheet. The test pieces were placed upright in a glass beaker, and water was added until it reached halfway up the piece. The beaker was then placed in an autoclave and subjected to a boiling treatment at 125°C for 30 minutes. After boiling treatment, the test pieces were removed from the autoclave and allowed to cool at room temperature. Visual evaluation was then performed to check for any boiling whitening of the coating on both the inner and outer surfaces of the coated metal sheet. The results are shown in Table 1.

[0264] The evaluation criteria are as follows.

[0265] 〇: No evidence of bleaching during cooking has been confirmed.

[0266] Δ: Minimal confirmation of bleaching during cooking.

[0267] ×: Significantly confirmed that cooking whitening has occurred.

[0268] [Evaluation of Resistance to Boiling and Whitening - 2 (Deep Drawing and Thinning Tank)]

[0269] Regarding the evaluation of resistance to boiling and whitening -2 (deep-drawn thinning can), the deep-drawn thinning can, after being formed as described in the "Production of Deep-drawn Thinning Can" section above and subjected to a heat treatment at 201°C for 75 seconds, is as follows. It should be noted that the average processing speed during the thinning process (the average movement speed of the punch during the thinning process) is set to approximately 1000 mm / sec.

[0270] 300 mL of water was poured into the tank, which was then placed in an autoclave and subjected to a boiling treatment at 125°C for 30 minutes. After the boiling treatment, the deep drawing and thinning tank was removed from the autoclave, the water was drained, and the tank was allowed to cool at room temperature. Then, the presence of boiling whitening of the coating on the inner surface of the deep drawing and thinning tank was visually evaluated.

[0271] The evaluation criteria are as follows.

[0272] 〇: No evidence of bleaching during cooking has been confirmed.

[0273] Δ: Minimal confirmation of bleaching during cooking.

[0274] ×: Significantly confirmed that cooking whitening has occurred.

[0275] The results are shown below.

[0276] The resistance to boiling and whitening of the deep-drawn and thinned tank in Example 3: 0.

[0277] The resistance to boiling and whitening of the deep-drawn and thinned can of Example 9: 0.

[0278] The resistance to boiling and whitening of the deep-drawn and thinned can of Comparative Example 2: ×.

[0279] The resistance to boiling and whitening of the deep-drawn and thinned can of Comparative Example 3: ×.

[0280] [Evaluation of embrittlement over time]

[0281] As an evaluation of embrittlement over time, initial workability and workability over time are evaluated. Workability is evaluated according to the bending test method described below.

[0282] Evaluation coated metal sheets were prepared using the internal surface coating compositions used in the various embodiments and comparative examples, as described below. Using the same coating conditions (coating type, dried coating quality, drying / sintering conditions) as the internal surface coatings in the embodiments and comparative examples, each chromate phosphate-treated aluminum sheet (3104 alloy, sheet thickness: 0.27 mm) was coated with a doctor blade coater. After drying at 120°C for 60 seconds, it was sintered at 250°C for 30 seconds to produce the evaluation coated metal sheet. A 3.5 × 3 cm section was cut from the coated metal sheet with the rolling direction of the aluminum sheet as the long side. The sheet was then bent parallel to the short side with the side coated with the internal surface coating composition as the outer side. Two aluminum sheets (3104 alloy, sheet thickness: 0.27 mm) were sandwiched between the inside of the bent section as spacers. The bending process was performed by dropping a 3 kg hammer from a height of 40 cm at room temperature (approximately 20°C). The bent front end, with a width of 2 cm, was brought into contact with a sponge soaked in a 1% sodium chloride aqueous solution, and the current value (ERV) was measured after applying a voltage of 6.3V for 4 seconds at room temperature.

[0283] Initial processability: Evaluation shall be conducted within 2 days after the production of the coated metal sheet for evaluation.

[0284] Processability over time: After the coated metal sheet for evaluation is produced, it is stored in a thermostat at 37°C for 2 weeks and then evaluated in the same manner as the initial processing.

[0285] The evaluation criteria for initial processability and time-dependent processability are as follows. The results are shown in Table 1.

[0286] ◎: Less than 0.4mA.

[0287] 〇: Above 0.4mA and less than 1.0mA.

[0288] Δ: Above 1.0mA and less than 2.5mA.

[0289] ×: 2.5mA or more.

[0290] [Evaluation of Internal Surface Coating Coverage (ERV Evaluation)]

[0291] The evaluation of the coating coverage of the inner surface was conducted on deep-drawn thinning cans (referred to as "without heat treatment" in the table) as described in the "Fabrication of Deep-Drawn Thinning Cans" section above, up to the deep-drawn thinning and bulging processes, and on deep-drawn thinning cans after subsequent heat treatment in an oven at 201°C for 75 seconds (referred to as "with heat treatment" in the table). It should be noted that the average processing speed during the thinning process (the average movement speed of the punch during the thinning process) was set to approximately 5500 mm / sec.

[0292] A metal exposure portion is formed on the outer surface of the bottom of the deep-drawing and thinning can. The can is connected to the anode of the inner coating tester. Meanwhile, 360 mL of 1% saline solution is injected into the can. The cathode of the inner coating tester is immersed in the saline solution filled in the can. The current value (ERV) after applying a voltage of 6.3V for 4 seconds at room temperature (about 20°C) is measured.

[0293] The evaluation criteria are as follows. The results are shown in Table 1.

[0294] ◎: Current value less than 50mA (less than 0.18mA / cm² per unit area) 2 ).

[0295] 〇: Current value is above 50mA and less than 200mA (0.18mA / cm) 2 Above and less than 0.70 mA / cm 2 ).

[0296] Δ: Current value above 200mA and below 700mA (0.70mA / cm) 2 Above and less than 2.50 mA / cm 2 ).

[0297] ×: Current value is above 700mA (2.50mA / cm) 2 above).

[0298] [Coating Peel Resistance Evaluation]

[0299] Regarding the evaluation of coating peel resistance, the deep-drawn and thinned cans, after being formed as described in the "Manufacturing of Thinned Cans" section above and subjected to a heat treatment at 201°C for 75 seconds, were observed and evaluated for any peeling of the coating on the inner and outer surfaces of the can body. It should be noted that the average processing speed during the thinning process (the average movement speed of the punch during the thinning process) was set to approximately 1000 mm / sec.

[0300] The evaluation criteria are as follows. The results are shown in Table 1.

[0301] 〇: Coating peeling not confirmed.

[0302] Δ: Coating peeling was minimally detected in areas where the sidewall of the can body was strictly thinned during processing.

[0303] ×: Coating peeling was confirmed over a large area in the parts where the sidewall of the main body of the tank was strictly thinned.

[0304] [Evaluation of Heat Shrinkage Rate]

[0305] The evaluation of heat shrinkage rate was conducted using the coating film on the inner surface of the central part of the can body of Example 3 (without heat treatment) after the deep drawing and thinning process and the bulging process, as described in the "Production of Deep Drawing and Thinning Cans" section above, and the deep drawing and thinning can of Example 3 (with heat treatment) after heat treatment in an oven at 201°C for 75 seconds. It should be noted that the average processing speed during the thinning process (the average movement speed of the punch during the thinning process) was set to approximately 5500 mm / sec.

[0306] Using the aforementioned deep-drawing and thinning can, a sample measuring 10 mm circumferentially and 20 mm in height was cut from the center of the can body (the thinnest part) in the 0° direction relative to the rolled metal substrate. The coating on the outer surface of the can was removed by sanding, exposing the metal surface. The exposed metal surface was then immersed in a diluted hydrochloric acid solution to dissolve the metal substrate. Next, the film-like coating on the inner surface of the can was removed, thoroughly washed with distilled water, and dried. The resulting film-like coating was then cut into sections 4 mm wide (circumferentially) and 20 mm long (height), thus obtaining the sample for testing.

[0307] The sample was clamped in a thermomechanical analysis apparatus with a clamp spacing (equivalent to the initial length of the measuring section in the height direction of the coating) set to 5 mm. The displacement of the sample was measured under the following conditions to evaluate the thermal shrinkage rate in the height direction of the can under both loaded and unloaded conditions.

[0308] Device: TMA / SS6100 manufactured by Seiko Instruments Co., Ltd.

[0309] Heating rate: 5℃ / minute.

[0310] Temperature range: 30℃~200℃.

[0311] Measurement mode: Tension mode.

[0312] Load during measurement: 5 mN (5.20 × 10⁻⁶) 5 N / m 2 (or no load)

[0313] Clamp spacing: 5mm.

[0314] Set the clamp spacing before measurement (equivalent to the initial length of the measurement section of the coating) to L0, and apply a 5.20 × 10⁻⁶ ohmmeter to each unit area on one side. 5 N / m 2 The maximum shrinkage (maximum shrinkage length) in the height direction of the portion corresponding to L0 when the load side is heated from 30°C to 200°C at a heating rate of 5°C / min is set as ΔL1, and the value calculated by the formula shown in equation (5) below is taken as the thermal shrinkage rate (with load). It should be noted that, in terms of displacement, shrinkage is set as a positive value, and expansion or elongation is set as a negative value. The results are shown below.

[0315] Heat shrinkage rate (with load) = (ΔL1 / L0) × 100 (%) ... (5)

[0316] The thermal shrinkage rate (with load) of the inner surface coating of the deep-drawn and thinned can (without heat treatment) in Example 3 is 66%.

[0317] The thermal shrinkage rate (with load) of the inner surface coating of the deep-drawn and thinned can (with heat treatment) in Example 3 is 18%.

[0318] Furthermore, the clamp spacing before measurement (equivalent to the initial length of the measurement section of the coating) is set as L0, and the maximum shrinkage value (maximum shrinkage length) of the portion corresponding to L0 in the height direction when heated from 30°C to 200°C at a heating rate of 5°C / min under no-load conditions is set as ΔL2. The value calculated by the formula shown in equation (6) below is taken as the thermal shrinkage rate (no load). It should be noted that, in terms of displacement, shrinkage is set as a positive value, and expansion or elongation is set as a negative value. The results are shown below.

[0319] Heat shrinkage rate (without load) = (ΔL2 / L0) × 100 (%)……(6)

[0320] The thermal shrinkage rate (without load) of the inner surface coating of the deep-drawn and thinned can (without heat treatment) in Example 3 is 72%.

[0321] The thermal shrinkage rate (without load) of the inner surface coating of the deep-drawn and thinned can (with heat treatment) in Example 3 is 34%.

[0322] (Corrosion resistance evaluation)

[0323] The corrosion resistance evaluation was conducted on the coating film on the inner surface of the central part of the can body of the deep-drawn thinning can (without heat treatment) of Example 3, which was subjected to deep drawing and thinning and bulging processes as described in the "Production of Deep Drawing Thinning Cans" section above, and the deep-drawn thinning can of Example 3 (with heat treatment) after being heat-treated in an oven at 201°C for 75 seconds. It should be noted that the average processing speed during the thinning process (the average movement speed of the punch during the thinning process) was set to approximately 5500 mm / sec.

[0324] Using the aforementioned deep-drawing and thinning can, a test piece measuring 40 mm circumferentially and 40 mm in height was cut from the center of the can body (the thinnest wall section). A 4 cm long cross-shaped cut reaching the substrate was made on the test piece, which was then immersed in an acidic model solution containing salt. The corrosion status was evaluated after two weeks at 37°C. It should be noted that the model solution used in the experiment contained 0.2% salt, with citric acid added to adjust the pH to 2.5. Regarding the evaluation criteria, around the cross-shaped cut, samples with a maximum undercoat corrosion width of 1 mm or more on each side were marked ×, samples with a maximum undercoat corrosion width of 0.5 mm or more but less than 1 mm on each side were marked ○, and samples with a maximum undercoat corrosion width of less than 0.5 mm on each side were marked ◎. The results are shown below.

[0325] Corrosion status of the deep-drawn and thinned can (without heat treatment) in Example 3: ×.

[0326] Corrosion status of the deep-drawn and thinned can (with heat treatment) in Example 3: ◎.

[0327] Table 1 shows the formulation (type of polyester resin, type of curing agent, and proportion of solid components) of the inner and outer surface coating compositions of each embodiment and comparative example, the coating film characteristics (gel fraction (A) and gel fraction (B)) of the inner and outer surface coating films, and the evaluation results.

[0328] [Table 1]

[0329]

[0330]

[0331] -: No rating

[0332] Industrial availability

[0333] The seamless cans, such as the deep-drawn and thinned-out cans of the present invention, have a coating with excellent resistance to boiling whitening and embrittlement over time. The coating does not peel off during heat treatment, effectively preventing metal exposure. They also possess excellent coating coverage and corrosion resistance, making them suitable for use as contents for food and beverages requiring boiling and sterilization. Furthermore, the coated metal sheet for the seamless cans of the present invention has a coating with excellent resistance to boiling whitening, embrittlement over time, and coating resistance that does not peel off even during heat treatment after forming. Therefore, it is suitable for use in the manufacture of seamless cans, such as deep-drawn and thinned-out cans, for filling food and beverages requiring boiling and sterilization.

Claims

1. A seamless tank, characterized in that, The container has an inner surface coating at least on the inner surface side, the inner surface coating containing polyester resin and, as a curing agent, a form of phenolic resin and / or an amino resin. The gel fraction A of the inner surface coating, as shown in formula (1a) below, is 55% or more and less than 90%. Gel fraction A = (W2a / W1a) × 100%……(1a) In the formula, W1a represents the mass of the inner surface coating after being separated from the coated metal substrate cut from the seamless can, and W2a represents the mass of the separated inner surface coating after being immersed in MEK at room temperature for 60 minutes, removed, and dried. The difference between the gel fraction B and the gel fraction A shown in the following formula (2a) of the inner surface coating is less than 10%. Gel fraction B = [(W4a-W5a) / (W3a-W5a)] × 100%……(2a) In the formula, W3a represents the mass of the coated metal substrate with the inner surface coating cut from the 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 inner surface coating is removed from the coated metal substrate.

2. The seamless tank according to claim 1, wherein, The first-stage phenolic resin is a m-cresol-based first-stage phenolic resin.

3. The seamless tank according to claim 1 or 2, wherein, The alpha-phenolic resin is formulated in an amount greater than 2 parts by weight and less than 10 parts by weight relative to 100 parts by weight of the polyester resin.

4. The seamless tank according to claim 1 or 2, wherein, The amino resin is a benzoguanamine resin, and the benzoguanamine resin is compounded in an amount of 8 parts by weight or more and less than 25 parts by weight relative to 100 parts by weight of the polyester resin.

5. The seamless tank according to claim 1 or 2, wherein, The inner surface coating further contains an acid catalyst, and the content of the acid catalyst in the inner surface coating is less than 0.5 parts by weight relative to 100 parts by weight of polyester resin.

6. The seamless tank according to claim 1 or 2, characterized in that, 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 inner surface coating of the central part of the can body is 20% to 75% of the thickness of the inner surface coating of the central part of the can bottom.

7. The seamless tank according to claim 1 or 2, wherein, The thickness ratio of the inner surface coating to the metal substrate, i.e., the thickness of the inner surface coating / the thickness of the metal substrate, is approximately the same at the bottom of the can and the main body of the can.

8. The seamless tank according to claim 1 or 2, wherein, The outer surface of the can is further provided with an outer surface coating, which contains polyester resin and amino resin as a curing agent.

9. The seamless tank according to claim 8, wherein, The gel fraction A of the outer surface coating, as shown in formula (3a) below, is 40% or more and less than 90%. Gel fraction A = (W7a / W6a) × 100%……(3a) In the formula, W6a represents the mass of the outer surface coating after being separated from the coated metal substrate cut from the seamless can, and W7a represents the mass of the separated outer surface coating after being immersed in MEK at room temperature for 60 minutes, and then removed and dried.

10. The seamless tank according to claim 1 or 2, wherein, The heat shrinkage rate of the coating film on the inner surface of the central part of the can body, as shown in the following formula (5), is 30% or less. Thermal shrinkage rate = (ΔL1 / L0) × 100……(5), in % L0: The initial length of the coating in the height direction after separation from the center of the can body; ΔL1: Apply 5.20 × 10 to each unit area on one side. 5 N / m 2 The maximum shrinkage length in the height direction of the coating film corresponding to the portion L0 when the load side is heated from 30℃ to 200℃ at a heating rate of 5℃ / min.

11. The seamless tank according to claim 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. Thermal shrinkage rate = (ΔL2 / L0) × 100……(6), in % L0: The initial length of the coating in the height direction after separation from the center of the can body; ΔL2: The maximum shrinkage length in the height direction of the coating film corresponding to the portion of L0 when heated from 30℃ to 200℃ at a heating rate of 5℃ / min under no load.

12. The seamless tank according to claim 1 or 2, wherein, The coverage of the inner surface coating is less than 200 mA when converted to ERV.

13. The seamless tank according to claim 1 or 2, wherein, The seamless can is a deep-drawn and thinned can.

14. A seamless coated metal sheet for tanks, characterized in that, The inner surface of the container has an inner surface coating, at least on the surface that forms the inner surface of the container. The inner surface coating contains a polyester resin and a first-order phenolic resin and / or an amino resin as a curing agent. The gel fraction A of the inner surface coating, as shown in formula (1b) below, is 55% or more and less than 90%, and the difference between the gel fraction B of the inner surface coating, as shown in formula (2b) below, and the gel fraction A is less than 10%. Gel fraction A = (W2b / W1b) × 100%……(1b) In the formula, W1b represents the mass of the inner surface coating after separation from the coated metal plate, and W2b represents the mass of the separated inner surface coating after immersion in MEK at room temperature for 60 minutes, followed by removal and drying. Gel fraction B = [(W4b-W5b) / (W3b-W5b)] × 100%……(2b) In the formula, W3b represents the mass of the coated metal sheet with the inner surface coating, W4b represents the mass of the coated metal sheet after it has been immersed in MEK at 80°C for 60 minutes and then dried, and W5b represents the mass of the metal sheet after the inner surface coating has been removed from the coated metal sheet.

15. The seamless coated metal sheet for tanks according to claim 14, characterized in that, The surface that forms the outer surface of the can further has an outer surface coating, the outer surface coating containing a polyester resin and an amino resin as a curing agent, wherein the gel fraction A of the outer surface coating, as shown in formula (3b) below, is 40% or more and less than 90%. Gel fraction A = (W7b / W6b) × 100%……(3b) In the formula, W6b represents the mass of the outer surface coating after separation from the coated metal plate, and W7b represents the mass of the separated outer surface coating after immersing it in MEK at room temperature for 60 minutes, and then removing and drying it.

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