Cans for pressing and cans, and painted metal plates for pressing and pressing cans

Abstract

The present invention relates to a drawn / ironed can which has an inner coating film at least on the can inner surface, wherein: the inner coating film contains a polyester resin and a curing agent; and the stress relaxation ratio of the inner coating film on the can bottom after the lapse of 10 minutes at 1% elongation under the test condition of 100°C is 50% or more. Consequently, the present invention effectively prevents metal exposure due to rough processing such as drawing and ironing as well as coating film separation due to a heat treatment after shaping.

Description

[Technical Field]

[0001] The present invention relates to a draw-and-iron can made of a painted metal sheet and a painted metal sheet for a draw-and-iron can, and more specifically, to a draw-and-iron can in which metal exposure due to harsh processing such as draw-and-iron processing and peeling of the paint film due to heat treatment after forming are effectively prevented, and to a painted metal sheet for a draw-and-iron can that enables such a draw-and-iron can to be formed with high productivity. [Background technology]

[0002] Organic resin-coated metal sheets, such as aluminum sheets coated with a thermoplastic resin film, have long been known as can materials. These organic resin-coated metal sheets are subjected to drawing or ironing processes to form seamless cans for filling beverages, etc., or they are press-molded to form can lids such as easy-open ends, which are also well known to be produced. For example, organic resin-coated metal sheets having an organic resin coating layer consisting of a thermoplastic resin film made of crystalline polyester resin mainly composed of ethylene terephthalate units are used as can-making materials for seamless cans (iron-molded cans) formed by drawing (Patent Document 1, etc.). Such organic resin-coated metal sheets can be iron-molded under dry conditions without the use of coolant, which offers environmental advantages compared to conventional methods of iron-molding using coolant from metal sheets.

[0003] Such organic resin-coated metal sheets are manufactured by film lamination methods, such as a method in which a pre-formed film of thermoplastic polyester resin or the like is bonded to a metal sheet by heat adhesion, or by an extrusion lamination method in which a molten thin film of extruded thermoplastic polyester resin or the like is bonded to a metal sheet. However, with the film lamination method, it is difficult to control the film thickness to a thin film due to the film formation process, so the film tends to be thick, which can be problematic from an economic standpoint.

[0004] Instead of such an organic resin-coated metal plate by a film lamination method, it has also been proposed to manufacture a drawn and ironed can under dry conditions from a coated metal plate on which a coating film is formed by a coating method capable of film formation with a thin film. For example, in Patent Document 2 below, there is a double-sided coated metal plate, and the dry coating amount of the film on the inner surface side of the can after processing is 90 to 400 mg / 100 cm 2 , the glass transition temperature is 50 to 120°C, and under the test conditions of 60°C, it is above pencil hardness H, the elongation rate is 200 to 600%, and the coefficient of kinetic friction is within the range of 0.03 to 0.25. The dry coating amount of the film on the outer surface side of the can after processing is 15 to 150 mg / 100 cm 2 , the glass transition temperature is 50 to 120°C, and under the test conditions of 60°C, a coated metal plate for a drawn and ironed can with a pencil hardness of H or more has been proposed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] In a drawn and ironed can formed under dry conditions from a coated metal plate, after the can body is formed, due to the residual stress in the coating film caused by severe drawing and ironing processing, the adhesion between the coating film and the metal substrate (hereinafter sometimes referred to as "coating film adhesion") may be significantly reduced. This residual stress can be removed by subjecting the can body to heat treatment under predetermined conditions. However, when such heat treatment is performed, as the residual stress of the coating film generated by severe processing is suddenly relaxed, a contraction force acts on the interface between the coating film and the metal substrate. Especially in the parts where the processing of the can body is severe and thinned, the coating film may peel off from the metal substrate, resulting in metal exposure and a decrease in the coating film coverage.

[0007] However, Patent Document 2, mentioned above, proposes a painted metal sheet that can withstand drawing and ironing by forming a coating on the inner surface of the painted metal sheet that will be the inner surface of the can, which can exhibit hardness and elongation even when heat is generated at nearly 60°C due to continuous drawing and ironing, and a drawn and ironed can formed from this painted metal sheet. However, there is no knowledge whatsoever regarding the removal of residual stress by heat treatment after forming the drawn and ironed can using this painted metal sheet, nor is there any knowledge regarding paint peeling caused by such heat treatment, and therefore it does not solve the problem.

[0008] Therefore, the object of the present invention is to provide a draw-and-iron can that has excellent coating peeling resistance, prevents coating peeling due to heat treatment after molding, suppresses metal exposure, maintains high coating coverage even after heat treatment, and has excellent corrosion resistance. Another object of the present invention is to provide a painted metal sheet for a drawing can that has a coating that does not cause the problems described above. [Means for solving the problem]

[0009] According to the present invention, a can is a draw-and-squeeze can having an inner coating film on at least the inner surface side, wherein the inner coating film is made of polyester resin 、 hardening agent and acid catalysts It contains, The curing agent contained in the inner coating film is a resol-type phenolic resin or an amino resin, and the amount of the acid catalyst contained in the inner coating film is 0.01 to 3 parts by mass per 100 parts by mass of polyester resin. A can is provided characterized in that the stress relaxation rate of the inner coating film at the bottom of the can after 10 minutes of 1% elongation under test conditions of 100°C is 50% or more, and the coverage of the inner coating film is less than 200 mA in terms of ERV.

[0010] In the squeezing can of the present invention, 1 .before The curing agent contained in the internal coating film is an m-cresol-based resol-type phenolic resin. 2 The content of the acid catalyst in the inner coating film is, per 100 parts by mass of polyester resin 0.02 parts by mass or more It must be less than 0.5 parts by mass. 3The polyester resin contained in the aforementioned inner coating film contains, when the total amount of polyhydric alcohol components constituting the polyester resin is set to 100 mol%, one or more selected from ethylene glycol, propylene glycol, 1,4-butanediol, 2-methyl-1,3-propanediol, and diethylene glycol in a combined amount of 20 mol% or more. 4 The can has an outer coating on its outer surface, the outer coating contains polyester resin and a hardening agent, and the stress relaxation rate of the outer coating at the bottom of the can is higher than 40%. 5 The thickness of the can body in the center should be 20-75% of the thickness of the can bottom in the center. 6 The thickness of the inner coating film in the center of the can body is 20-75% of the thickness of the inner coating film in the center of the can bottom. 7 The thickness ratio of the inner coating to the metal substrate (thickness of the inner coating / thickness of the metal substrate) is approximately the same in the bottom and body of the can. 8 The thermal shrinkage rate of the inner coating film in the central part of the can body, as expressed by the following formula, must be 30% or less. Thermal shrinkage rate (%) = (ΔL1 / L0) × 100 L0: Initial height of the paint film isolated from the center of the can body. ΔL1: 5.20 × 10⁻⁶ per unit area 5 N / m 2 The maximum shrinkage length in the height direction of the coating film in the L0 portion when the temperature is raised from 30°C to 200°C at a heating rate of 5°C / min while applying a load. This is preferable.

[0011] The present invention also provides a can made of a painted metal plate for cans having a coating on both sides, wherein the painted metal plate for cans has an inner coating on the side that becomes the inner surface of the can after the can-drawing process, and the inner coating is made of polyester resin and a resol-type phenolic resin as a curing agent. Abamata amino resin and an acid catalyst in an amount of 0.01 to 3 parts by mass per 100 parts by mass of the polyester resin.A can is provided in which the outer coating film on the surface that becomes the outer surface of the can after the drawing and ironing process contains polyester resin and amino resin as a curing agent, the stress relaxation rate of the inner coating film after 10 minutes of 1% elongation under test conditions of 100°C is 50% or more, the stress relaxation rate of the outer coating film after 10 minutes of 1% elongation under test conditions of 100°C is higher than 40%, and the coverage of the inner coating film of the can is less than 200 mA in terms of ERV.

[0012] This invention The above is used in the molding of cans by pressing and twisting. In painted metal sheets for pressed and ironed cans, it is preferable that the polyester resin contained in the inner coating film contains 20 mol% or more of one or more selected from ethylene glycol, propylene glycol, 1,4-butanediol, 2-methyl-1,3-propanediol, and diethylene glycol, when the total amount of polyhydric alcohol components constituting the polyester resin is set to 100 mol%.

[0013] Book In the above-mentioned squeezing can of the invention, 1. The thickness of the can body in the center is 20-75% of the thickness of the can bottom in the center. 2. The thickness of the inner coating in the center of the can body is 20-75% of the thickness of the inner coating in the center of the can bottom. 、 but It is suitable. [Effects of the Invention]

[0014] In view of the above-mentioned background, the inventors diligently investigated the peeling of the paint film caused by heat treatment of a pressed metal can made of painted metal sheet. As a result, they found that the likelihood of paint film peeling occurring during heat treatment is correlated with the stress relaxation behavior (stress relaxation rate) when the paint film is stretched under predetermined conditions. They also found a range of stress relaxation rates in which paint film peeling does not occur, and a paint film capable of exhibiting such a stress relaxation rate. In other words, in the ironed can of the present invention, since there is no peeling of the coating film during heat treatment, even after heat treatment is performed after the ironed can is formed, the coverage of the inner coating film, expressed in terms of ERV, is less than 200 mA, effectively preventing metal exposure and providing excellent corrosion resistance.

[0015] Furthermore, painted metal sheets capable of forming such drawn and ironed cans have excellent paint film elongation and processability. Even when subjected to harsh processing such as drawing or ironing under dry conditions, not only is fracture in the can body (sometimes referred to as "body fracture" in this invention) prevented, but metal exposure is also effectively prevented. As a result, the can has high paint film coverage even after drawing and ironing, and exhibits excellent can manufacturing processability. In other words, in this invention, by using a specific combination of polyester resin and curing agent as the polyester resin and curing agent constituting the paint film, and controlling the degree of hardening and crosslinking density of the paint film, excellent can manufacturing processability is achieved. Additionally, by controlling the stress relaxation rate and reducing residual stress generated during can manufacturing, it becomes possible to effectively suppress paint film peeling when heat treatment is applied after can body formation, making it possible to provide a drawn and ironed can with high paint film coverage and excellent corrosion resistance. [Modes for carrying out the invention]

[0016] (Painted metal sheet) As described above, the painted metal sheet used in forming the drawn-and-iron can of the present invention is a painted metal sheet having an inner coating on the surface that becomes the inner surface of the can after the drawing and ironing process. This inner coating contains a polyester resin and a hardening agent, and an important feature is that the stress relaxation rate of the inner coating after 10 minutes of 1% elongation under test conditions of 100°C is 50% or more. As will be clear from the measurement method described later, the stress relaxation rate in the present invention is calculated by applying a 1% elongation strain to the coating film under test conditions of 100°C using a thermomechanical analyzer or the like, measuring the change in stress over time while maintaining this strain, and using the following formula (1). Stress relaxation rate (%) = (δ1 - δ2) / δ1 × 100 ... (1) In the formula, δ1 is the stress when the isolated inner coating film is stretched by 1%, and δ2 is the stress after 10 minutes.

[0017] When forming cans at high speed using painted metal sheets under dry conditions, the painted metal sheets are subjected to harsh processing and deformation, accompanied by a temperature rise due to processing heat. In this process, the coating formed on the painted metal sheets is subjected to large deformations during can manufacturing, resulting in residual stress in the coating after processing. In particular, when the main resin component, such as polyester resin, that makes up the coating is highly crosslinked by a curing agent, the stress generated by processing is difficult to relieve, and residual stress tends to be large. If the can body is heat-treated after molding while the residual stress in the coating is still high, and heated above the glass transition temperature of the polyester resin, a shrinkage force acts at the interface between the coating and the substrate, which is thought to cause the coating to peel off and the metal to be exposed. A high stress relaxation rate in the coating of a painted metal sheet means that when the painted metal sheet is formed at high speed and the coating is processed while the sheet is at a high temperature due to processing heat, the stress generated by the processing is easily relieved immediately, meaning that the residual stress in the coating after processing is small. This makes it possible to reduce the shrinkage force that occurs at the interface between the coating and the metal substrate due to the relaxation of residual stress, and as a result the occurrence of coating delamination is prevented. This is also evident from the results of the examples described later, and it is difficult to suppress the occurrence of coating delamination by heat treatment when the stress relaxation rate of the inner coating is less than 50%.

[0018] As described above, the painted metal sheet used in forming the drawn-and-iron can of the present invention is a painted metal sheet having at least an inner coating on the surface that becomes the inner surface of the can after the drawing and ironing process, wherein this inner coating contains a polyester resin and a hardening agent, and it is desirable that the stress relaxation rate of the inner coating is in the range of 50% or more, preferably 50 to 95%, more preferably 54 to 90%, even more preferably 54 to 85%, particularly preferably 54 to 80%, and most preferably 60 to 80%. Furthermore, the surface that becomes the outer surface of the can after the drawing and ironing process has an outer coating film, and the outer coating film also contains polyester resin and a hardening agent, and it is desirable that the stress relaxation rate of the outer coating film is greater than 40%, preferably 45 to 95%, more preferably 50 to 90%, even more preferably 54 to 85%, particularly preferably 54 to 80%, and most preferably 60 to 80%.

[0019] By forming a can using a painted metal sheet having an internal coating on at least the inner surface of the can, as described above, it becomes possible to cover the entire can with a continuous internal coating from the bottom to the body on the inner surface side. Furthermore, if a double-sided coated metal sheet is used, which also has an outer coating on the surface that becomes the outer surface of the can after the drawing and ironing process, it becomes possible to cover the entire can with the same outer coating from the bottom to the body. Since the outer coating of the coated metal sheet of the present invention also has excellent retort resistance, it is possible to obtain a drawing and ironing can with excellent retort resistance on the outer surface. Generally, the bottom of a pressed metal can consists of a bottom section located in the center, a grounding section (rim) that slopes down from the periphery of this bottom section, and a chime section that slopes outward and upward from this grounding section and connects to the lower end of the body. However, in conventional pressed metal cans formed from unpainted metal sheets using coolant, it is necessary to paint the grounding section on the outside immediately after forming the can body to improve its transportability. Furthermore, if the chime section and bottom section are also painted to consider retort resistance, etc., it is necessary to paint the chime section and bottom section on the outside using different painting methods and equipment and different paint compositions after painting the inside and printed on the outside body. This results in a large number of steps, which can be problematic in terms of productivity and economics. On the other hand, when forming a can by drawing and ironing using a painted metal sheet having an outer coating on the outer surface of the can, as in the present invention, the entire can, from the bottom to the body, can be continuously covered with the same outer coating. Therefore, there is no need to paint the chime section, ground contact section, and bottom section that constitute the bottom of the can using different painting methods and equipment and different paint compositions, resulting in superior productivity and cost-effectiveness.

[0020] In the painted metal sheet of the present invention, it is preferable that the glass transition temperature (Tg) of the inner coating film is 30°C or higher, preferably higher than 40°C, more preferably higher than 50°C and 120°C or lower, even more preferably between 60°C and 110°C, particularly preferably higher than 65°C and 100°C or lower, and most preferably in the range of 68°C to 90°C. If the Tg is lower than the above range, when the contents are filled into the molded can, the flavor components of the contents are more likely to adhere, which may result in poor flavor adhesion resistance, as well as a decrease in the barrier properties of the coating film and a decrease in corrosion resistance. On the other hand, if the Tg exceeds 120°C, the processability of the coating film for can manufacturing decreases, which may result in metal exposure during molding, a decrease in the coverage of the inner coating film, and a decrease in corrosion resistance. Furthermore, the Tg of the outer coating film is preferably in the range of 30°C or higher, preferably higher than 40°C, more preferably higher than 50°C and 120°C or lower, even more preferably between 60°C and 110°C, particularly preferably higher than 65°C and 100°C or lower, and most preferably between 68°C and 90°C. If the Tg is lower than the above range, the hardness of the coating film will decrease, which may cause surface defects such as coating abrasion. On the other hand, if the Tg exceeds 120°C, the workability of the coating film for can manufacturing will decrease, which may cause metal exposure during molding, and the coating coverage on the outer surface will be poor.

[0021] Furthermore, the thickness of the inner coating film is preferably in the range of 0.2 to 20 μm, preferably 1 to 12 μm, and more preferably greater than 2 μm and 12 μm or less, in terms of dry film thickness. The dry film weight is preferably 3 to 300 mg / dm 2 Preferably 15-150 mg / dm 2 More preferably 25 mg / dm 2 Larger dose: 150 mg / dm 2 The following ranges are preferable. If the film is thinner than the above range, metal exposure is more likely to occur during molding, resulting in poor coverage of the inner coating. On the other hand, if the film is thicker than the above range, the internal stress generated during processing becomes larger, making it easier for the coating to peel off during heat treatment after drawing and ironing, and the resulting film is thicker than necessary, making it less economical. Furthermore, when the content filled in the drawn and ironed can is a strongly corrosive acidic beverage, it is necessary to make the film thickness relatively thick to ensure corrosion resistance, and it is preferably in the range of more than 6 μm and 1.2 μm or less, preferably 6.5 to 10 μm. Also, as the dry film weight, it is more than 85 mg / dm 2 and 150 mg / dm 2 or less, preferably in the range of 90 to 140 mg / dm 2 . If the film is thinner than the above range, the corrosion resistance is inferior, and if it exceeds the above range, film peeling is likely to occur during the heat treatment after the drawing and ironing forming

[0022] On the other hand, when the content filled in the drawn and ironed can is a low-acid beverage or the like with relatively weak corrosivity, it is possible to ensure corrosion resistance even with a relatively thin film, so it is preferably in the range of 1 μm or more and less than 6.5 μm, preferably more than 2 μm and less than 6.5 μm, more preferably 2.5 to 6 μm. Also, as the dry film weight, it is 15 mg / dm 2 or more and 90 mg / dm 2 or less, preferably more than 25 mg / dm 2 and less than 90 mg / dm 2 , more preferably in the range of 30 to 85 mg / dm 2 . If the film is thinner than the above range, the corrosion resistance is inferior, and if it exceeds the above range, the film will be thicker than necessary and the economic efficiency will be inferior Also, the film thickness of the outer surface coating is preferably in the range of 0.2 to 20 μm, preferably 1 to 12 μm, more preferably more than 2 μm and 10 μm or less, and still more preferably more than 2 μm and 6.5 μm or less in terms of dry film thickness. Also, as the dry film weight, it is 3 to 300 mg / dm 2 , preferably 15 to 150 mg / dm 2 , more preferably more than 25 mg / dm 2 and 140 mg / dm 2 or less, still more preferably more than 25 mg / dm 2 and 90 mg / dm 2It is preferable that the film thickness be within the range of less than the above range. If the film is thinner than the above range, metal exposure is more likely to occur during molding, resulting in poor coating performance on the outer surface. On the other hand, if the film is thicker than the above range, the internal stress generated during processing becomes larger, making it easier for the coating to peel off during heat treatment after drawing and ironing. Furthermore, regarding the film thickness of the inner and outer coatings of the painted metal plate, it is preferable that the inner coating, which requires higher coverage, be thicker than the outer coating.

[0023] The inner and outer coatings of the painted metal sheet and the ironing can of the present invention are composed of a polyester resin and a hardening agent as the main components. In the aforementioned inner coating film, the content of polyester resin, preferably the amorphous polyester resin described later, is preferably higher than 50% by mass, more preferably 60% by mass or more, even more preferably 70% by mass or more, and particularly preferably 80% by mass or more. Similarly, in the aforementioned outer coating film, the content of polyester resin, preferably amorphous polyester resin, is preferably higher than 50% by mass, more preferably 60% by mass or more, even more preferably 70% by mass or more, and particularly preferably 80% by mass or more.

[0024] (Polyester resin) In the drawing can and painted metal sheet of the present invention, polyester resin is used as the main component (main agent) constituting the inner and outer coating films, where the main agent is defined as the component with the highest content (mass ratio) among the resin components constituting the coating film. In the present invention, it is preferable that the mass percentage of polyester resin among the resin components constituting the inner and outer coating films is higher than 50% by mass, more preferably 60% by mass or more, even more preferably 70% by mass or more, and particularly preferably 80% by mass or more. It is preferable that the polyester resin used contains a polyhydric alcohol component (polyol component) that has a low-bulk molecular structure that does not easily cause steric hindrance to the polymer chain, in an amount of 20 mol% or more when the total amount of polyhydric alcohol components constituting the polyester resin is set at 100 mol%. Examples of such polyhydric alcohol components, although not limited to these, include ethylene glycol, propylene glycol, 1,4-butanediol, 2-methyl-1,3-propanediol, diethylene glycol, etc., and it is preferable to contain at least one or more selected from these in a total amount of 20 mol% or more. This is thought to facilitate stress relaxation due to rotation of the polymer chain when forming a pressed metal can from a painted metal sheet, and as a result, it is presumed that residual stress in the coating after molding will be reduced, leading to suppression of coating peeling during heat treatment. It should be noted that bulky polyols are not limited to these, but examples include alicyclic polyols such as 1,4-cyclohexanedimethanol and neopentyl glycol. It is presumed that such polyhydric alcohols are less susceptible to stress relaxation due to molecular chain rotation because of steric hindrance.

[0025] The glass transition temperature (Tg) of the polyester resin is preferably 30°C or higher, preferably higher than 40°C, more preferably higher than 50°C and 120°C or lower, even more preferably between 60°C and 110°C, particularly preferably higher than 65°C and 100°C or lower, and most preferably in the range of 68°C to 90°C. If the Tg is lower than the above range, when contents are filled into a container manufactured by the drawing process described above, the increased mobility of the resin makes it easier for flavor components (aroma components) contained in the contents to diffuse into the coating film. This can lead to an increase in the amount of flavor component adsorption, potentially resulting in poor flavor adsorption resistance, as well as poor heat resistance, corrosion resistance, and retort resistance. On the other hand, if the Tg exceeds 120°C, the processability and elongation of the coating film decrease, resulting in poor can manufacturing processability. This may lead to metal exposure during molding, and as a result, the coating performance after molding deteriorates, the stress relaxation rate decreases, and the coating film peeling resistance may deteriorate.

[0026] In the present invention, two or more polyester resins with different Tg values ​​can be blended and used. By blending polyester resins with different Tg values, it may be possible to form a coating that has superior impact resistance and is less prone to coating defects even when subjected to external impacts, compared to using only one type of polyester resin. In that case as well, the Tg of the polyester resin blend calculated by the following formula (2) mix It is sufficient if it falls within the above Tg range. 1 / Tg mix =(W1 / Tg1)+(W2 / Tg2)+…+(Wm / Tgm) ···(2) W1 + W2 + ... + Wm = 1 In the formula, Tg mix Tg1 represents the glass transition temperature (K) of the polyester resin blend, and Tg1, Tg2, ..., Tgm represent the glass transition temperature (K) of each individual polyester resin used (polyester resin 1, polyester resin 2, ..., polyester resin m). Also, W1, W2, ..., Wm represent the mass fraction of each polyester resin (polyester resin 1, polyester resin 2, ..., polyester resin m).

[0027] Examples of polycarboxylic acid components that make up polyester resins include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid, and 2,6-naphthalenedicarboxylic acid; aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedionic acid, and dimer acid; unsaturated dicarboxylic acids such as maleic 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 valencies such as trimellitic anhydride, pyromellitic anhydride, and methylcyclohexentricarboxylic acid. One or more of these can be selected and used. Among the polycarboxylic acids listed above, it is preferable to use one or more selected from the group consisting of isophthalic acid, orthophthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, trimellitic acid, adipic acid, sebacic acid, fumaric acid, maleic acid, dimer acid, and 1,4-cyclohexanedicarboxylic acid. In the present invention, from the viewpoint of the hardness, heat resistance, flavor sorption resistance, and retort resistance of the resulting coating film, when the total amount of polycarboxylic acid components constituting the polyester resin is set to 100 mol%, it is preferable that at least one or more selected from aromatic dicarboxylic acids such as terephthalic acid, orthophthalic acid, isophthalic acid, and 2,6-naphthalenedicarboxylic acid are included in a combined amount of 60 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more. Among the above aromatic dicarboxylic acids, terephthalic acid and isophthalic acid are particularly preferred, and when the total amount of polycarboxylic acid components constituting the polyester resin is set to 100 mol%, it is preferable that the combined content of terephthalic acid and isophthalic acid be 60 mol% or more, more preferably 70 mol% or more, even more preferably 80 mol% or more, and especially preferably 90 mol% or more.

[0028] As mentioned above, when the total amount of polyhydric alcohol components constituting the polyester resin is set to 100 mol%, it is preferable that at least one or more selected from ethylene glycol, propylene glycol, 1,4-butanediol, 2-methyl-1,3-propanediol, and diethylene glycol be included in a combined amount of 20 mol% or more, preferably 30 mol% or more, more preferably 40 mol% or more, even more preferably 50 mol% or more, particularly preferably 60 mol% or more, and most preferably 70 mol% or more. The remaining components after the polyhydric alcohol component include 1,2-butanediol, 1,3-butanediol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 2-ethyl-2-butyl-1,3-propanediol, 2,4-diethyl-1,5-pentanediol, 1-methyl-1,8-octanediol, 3-methyl-1,6-hexanediol, 4-methyl-1,7-heptanediol, 4-methyl-1,8-octanediol, 4-propyl-1,8-octanediol, 1,9-nonanediol, etc. It can be used as one or more of the following: aliphatic glycols, ether glycols such as diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol; alicyclic polyalcohols such as 1,4-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,2-cyclohexanedimethanol, tricyclodecane glycols, and hydrogenated bisphenols; and trivalent or higher polyalcohols such as trimethylolpropane, trimethylolethane, and pentaerythritol.

[0029] Polyester resins can be produced by known methods, such as polycondensation of one or more of the above-mentioned polycarboxylic acid components with one or more of the polyhydric alcohol components; depolymerization after polycondensation with a polycarboxylic acid component, such as terephthalic acid, isophthalic acid, trimellitic anhydride, trimellitic acid, pyromellitic acid, etc.; or ring-opening addition of an acid anhydride, such as phthalic anhydride, maleic anhydride, trimellitic anhydride, ethylene glycol bistrimellitate dianhydride, etc., after polycondensation. From the viewpoint of curability, retort whitening resistance, and adhesion to the metal substrate, the polyester resin is preferably in the range of 0.1 to 40 mgKOH / g, more preferably 0.5 to 25 mgKOH / g, more preferably 1 to 10 mgKOH / g, even more preferably higher than 2 mgKOH / g and 10 mgKOH / g or less, particularly preferably 2.5 to 8 mgKOH / g, and most preferably 3 to 7 mgKOH / g. If the acid value is lower than the above range, the adhesion between the metal substrate and the coating film may decrease. On the other hand, if the acid value is higher than the above range, the coating film may absorb water more easily than when it is within the above range, which may reduce its retort resistance. In addition, the crosslinking density of the coating film will increase due to the increase in reaction sites with the curing agent, which may reduce the processability of the can and the resistance to coating film peeling, leading to metal exposure and a decrease in the coverage of the coating film. Furthermore, in the case of a blend of two or more polyester resins, the sum of the values ​​obtained by multiplying the acid value and mass fraction of each polyester resin is used to determine the average acid value (AV) of the blend. mix ) and it is sufficient if the average acid value falls within the above-mentioned acid value range.

[0030] The hydroxyl value of the polyester resin is not limited to this, but is preferably 20 mg KOH / g or less, more preferably 10 mg KOH / g or less.

[0031] The number-average molecular weight (Mn) of the polyester resin is not limited to this, but from the viewpoint of can manufacturing processability, it is preferably in the range of 1,000 to 100,000, more preferably 3,000 to 50,000, and even more preferably 5,000 to 20,000. If it is smaller than the above range, the coating film may become brittle and the can manufacturing processability may be poor, and if it is larger than the above range, the paint stability may decrease.

[0032] Furthermore, as the polyester resin, amorphous polyester resin is preferable from the viewpoint of can-making processability, dent resistance, and paint-forming properties. Here, amorphous means that it does not show a clear melting point for crystalline components when measured by a scanning calorimeter (DSC). In the case of amorphous polyester resin, compared to crystalline polyester resin, it has superior solubility in solvents, is easy to form into paint, and can form a coating film with excellent can-making processability and dent resistance. The hydroxyl value of the polyester resin is not limited to this, but is preferably 20 mg KOH / g or less, more preferably 10 mg KOH / g or less.

[0033] (Hardening agent) The curing agent used in this invention is one that reacts with the functional groups of the polyester resin, such as carboxyl groups or hydroxyl groups, to form a crosslinked structure. Examples of such curing agents include isocyanate compounds, resol-type phenolic resins, amino resins, epoxy group-containing compounds, oxazoline group-containing compounds, carbodiimide group-containing compounds, and β-hydroxyalkylamide compounds. Resol-type phenolic resins and amino resins are particularly preferred from the viewpoint of curability and hygiene.

[0034] In the painted metal sheets and pressed metal cans of the present invention, resol-type phenolic resins and amino resins are suitable for the coating composition that forms the inner coating film (hereinafter sometimes referred to as the "inner coating composition"), and resol-type phenolic resins are particularly suitable from the viewpoint of can manufacturing processability. For the coating composition that forms the outer coating film (hereinafter sometimes referred to as the "outer coating composition"), amino resins that can form a transparent coating film without coloration derived from the curing agent are suitable. On the other hand, since the coating film formed by the aforementioned resol-type phenolic resin turns yellow, caution is required when using it in the coating composition that forms the outer coating film.

[0035] (Resol-type phenolic resin) As a resol-type phenolic resin, for example, one or more 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 can be used in combination, and a resol-type phenolic resin can be obtained by reacting these phenolic compounds with formaldehyde in the presence of an alkaline catalyst. From the viewpoint of curability, among the above phenol compounds, a resol-type phenol resin containing more than 50% by mass, preferably 60% by mass or more, and more preferably 80% by mass or more, of a phenol compound that becomes trifunctional upon reaction with formaldehyde as a starting material is preferred. Examples of phenol compounds that become trifunctional upon reaction with formalins include phenol, m-cresol, m-ethylphenol, 3,5-xylenol, and m-methoxyphenol, and one or more of these can be selected and used. If the amount of these trifunctional phenol compounds is 50% by mass or less, sufficient curability may not be obtained, and the degree of curing of the coating film may decrease. Among these trifunctional phenol compounds, m-cresol is more preferred from the viewpoint of curability, and a resol-type phenol resin containing more than 50% by mass of m-cresol as a starting material (hereinafter sometimes referred to as "m-cresol-based resol-type phenol resin") is particularly preferred. This allows for a sufficient degree of curing of the coating film, which is desirable from the viewpoint of heat resistance, corrosion resistance, retort resistance, etc. of the coating film. The m-cresol-based resol-type phenolic resin preferably contains more than 50% by mass, more preferably 60% by mass or more, and more preferably 80% by mass or more, of m-cresol as a starting material. In addition to the trifunctional phenolic compounds mentioned above, when using bifunctional phenolic compounds that react with formaldehyde as starting materials, the content is preferably less than 50% by mass, more preferably less than 40% by mass, and more preferably less than 20% by mass. If it is 50% by mass or more, the curability may decrease. Examples of bifunctional phenolic compounds include o-cresol, p-cresol, p-tert-butylphenol, p-ethylphenol, 2,3-xylenol, and 2,5-xylenol.

[0036] Furthermore, as the resol-type phenolic resin used in the present invention, from the viewpoint of compatibility with polyester resins and curability, it is preferable to use one in which some or all of the methylol groups contained are alkyl etherified (alkoxymethylated) with C1-C12 alcohols. The proportion of methylol groups to be alkyl etherified is preferably 50% or more, more preferably 60% or more, and even more preferably 80% or more. If the proportion of alkyl etherification is less than 50%, the compatibility with polyester resins will be low, resulting in cloudiness in the coating film or insufficient curability. The alcohol used for alkyl etherification is a monohydric alcohol having 1 to 8 carbon atoms, preferably 1 to 4 carbon atoms. Suitable monohydric alcohols include methanol, ethanol, n-propanol, n-butanol, and isobutanol, with n-butanol being more preferable. Furthermore, the number of alkyl etherified methylol groups (alkoxymethyl groups) is preferably 0.3 or more, more preferably 0.5 to 3, on average per phenol nucleus. If the number is less than 0.3, the curing properties with polyester resin will be poor. The number average molecular weight (Mn) of the resol-type phenol resin is preferably in the range of 500 to 3,000, more preferably 800 to 2,500. If it is smaller than the above range, the crosslinking density of the formed coating film tends to be high, making it difficult to relax stress during molding, which may result in poor coating film peel resistance. On the other hand, if it is larger than the above range, the curing properties will be poor, which may result in poor heat resistance, corrosion resistance, retort resistance, etc. of the coating film.

[0037] (Amino resin) Examples of amino resins include methylolated amino resins obtained by the reaction of amino components such as melamine, urea, benzoguanamine, acetoganaamine, steroguanamine, spiloganamine, and dicyandiamide with aldehyde components such as formaldehyde, paraformaldehyde, acetaldehyde, and benzaldehyde. The above amino resins also include those obtained by alkylating some or all of the methylol groups of the methylolated amino resin with an alcohol having 1 to 6 carbon atoms. These can be used individually or in combination of two or more. From the viewpoints of hygiene, processability for can manufacturing, and curability, methylolated amino resins using benzoguanamine (benzoguanamine resins) and methylolated amino resins using melamine (melamine resins) are preferred.

[0038] As the benzoguanamine resin, benzoguanamine resins in which some or all of the methylol groups of methylolated benzoguanamine resin are alkyl etherified with alcohols such as methanol, ethanol, n-butanol, and i-butanol are preferred, such as methyl etherified benzoguanamine resin, ethyl etherified benzoguanamine resin, butyl etherified benzoguanamine resin, or mixed etherified benzoguanamine resin of methyl ether and butyl ether, mixed etherified benzoguanamine resin of methyl ether and ethyl ether, or mixed etherified benzoguanamine resin of ethyl ether and butyl ether are preferred. Among these, methyl etherified benzoguanamine resin is more preferred, and a partially etherified type of methyl etherified benzoguanamine resin containing imino group and methylol group is particularly preferred.

[0039] As the melamine resin, melamine resins in which some or all of the methylol groups of methylolated melamine resin are alkyl etherified with alcohols such as methanol, ethanol, n-butanol, and i-butanol are preferred, such as methyl etherified melamine resin, ethyl 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 ethyl ether, or mixed etherified melamine resin of ethyl ether and butyl ether are preferred. Among these, methyl etherified melamine resin is more preferred, and fully etherified type methyl etherified melamine resin is particularly preferred.

[0040] The functional groups present in the melamine resin and benzoguanamine resin mentioned above include imino groups (>NH), N-methylol groups (>NCH2OH), and N-alkoxymethyl groups (>NCH2OR; R is an alkyl group). These functional groups act as reaction sites in crosslinking reactions with carboxyl groups (-COOH) and hydroxyl groups (-OH) contained in the polyester resin, which is the main component, or in self-condensation reactions between amino resins (note that imino groups contribute only to self-condensation reactions). Regarding the number of reaction sites (functional groups) mentioned above, when comparing the monomers of melamine resin and benzoguanamine resin, it is thought that melamine resin has more due to its molecular structure. As a result, while melamine resin has excellent curability, the crosslinking density of the formed coating film tends to be high, and depending on the blending amount, the stress relaxation rate may be low, potentially leading to coating film peeling during heat treatment. On the other hand, although benzoguanamine resin has inferior curability compared to melamine resin, the crosslinking density of the resulting coating film is less likely to be high, making it preferable to melamine resin from the viewpoint of coating film peeling resistance. Therefore, in order to balance curability and coating film peeling resistance, a mixed amino resin may be used, which is obtained by using melamine resin and benzoguanamine resin in combination and mixing them in a predetermined ratio. In that case, the blending ratio (mass ratio) of melamine resin and benzoguanamine resin should preferably be 95:5 to 5:95, more preferably 90:10 to 10:90, more preferably 80:20 to 15:85, and even more preferably 70:30 to 25:75.

[0041] The curing agent should preferably be blended in an amount of 1 to 40 parts by mass, preferably 1 to 30 parts by mass, and more preferably 2 to 20 parts by mass, per 100 parts by mass of polyester resin. When using a resol-type phenolic resin as a curing agent, it is preferable to blend it in an amount of 2 to 40 parts by mass, preferably 3 to 30 parts by mass, more preferably 3 to 25 parts by mass, even more preferably 3 to 20 parts by mass, and particularly preferably 4 to 15 parts by mass, per 100 parts by mass of the main polyester resin (solids). When using a melamine resin as a curing agent, it is preferable to blend it in an amount of 1 to 15 parts by mass, preferably 1 part by mass or more and less than 10 parts by mass, more preferably 2 to 5.5 parts by mass, and particularly preferably 2 to 5 parts by mass, per 100 parts by mass of the polyester resin. When using a benzoguanamine resin as a curing agent, it is preferable to blend it in an amount of 4 to 40 parts by mass, preferably 5 to 30 parts by mass, more preferably 6 to 28 parts by mass, even more preferably 8 to 25 parts by mass, and particularly preferably 10 to 24 parts by mass, per 100 parts by mass of the polyester resin. When using the aforementioned mixed amino resin of melamine resin and benzoguanamine resin as a curing agent, it is preferable to blend it in an amount of 2 to 25 parts by mass, preferably 2 to 20 parts by mass, 2.5 to 15 parts by mass, or 3 parts by mass or more but less than 10 parts by mass, per 100 parts by mass of polyester resin. If the amount of hardener is less than the range mentioned above, sufficient curing cannot be obtained, resulting in a lower degree of hardening of the coating and a tendency for reduced heat resistance. Therefore, when forming cans by drawing at high speed, the temperature rise becomes more pronounced, which may cause the coating to stick to the mold during forming. Particularly on the inner surface of the can, when removing the can from the forming punch after drawing, the can may stick to the forming punch, making it difficult to separate the forming punch and the can (poor stripping ability), which may lead to buckling or fracture of the can, thus reducing productivity. On the outer surface of the can, there is a risk of external defects such as coating abrasion. On the other hand, if the amount of hardener is greater than the range mentioned above, depending on the type of hardener used, the processability of the coating film may decrease, potentially leading to metal exposure during drawing and ironing. Furthermore, the increased crosslinking density of the coating film may make it difficult to relax stress during molding, potentially making it difficult to adjust the stress relaxation rate of the inner coating film to 50% or more, and consequently reducing the coverage of the coating film.

[0042] In the interior coating composition and exterior coating composition used in the present invention, it is preferable to incorporate a curing catalyst for the purpose of promoting the crosslinking reaction between the polyester resin and the curing agent. Conventional known curing catalysts can be used as curing catalysts, and for example, organic sulfonic acid-based and phosphoric acid-based acid catalysts such as p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenedisulfonic acid, phosphoric acid, alkyl phosphoric acid, or their amine neutralized products can be used. Among the above curing catalysts, it is preferable to use organic sulfonic acid-based acid catalysts, and dodecylbenzenesulfone and its amine neutralized product are particularly preferred.

[0043] The curing catalyst is preferably in the range of 0.01 to 3 parts by mass, preferably 0.02 to 1.0 parts by mass, more preferably 0.02 parts by mass or more and less than 0.5 parts by mass, even more preferably 0.03 parts by mass or more and less than 0.3 parts by mass, and particularly preferably 0.04 parts by mass or more and less than 0.2 parts by mass, as solid content per 100 parts by mass of polyester resin. Furthermore, when using an amine neutralized product of the above acid catalyst (for example, an amine neutralized product of dodecylbenzenesulfonic acid) as the curing catalyst, it is sufficient if the content of the acid catalyst excluding the amine is within the above range. If the content of the curing catalyst is less than the above range, the effect of promoting the curing reaction may not be sufficiently obtained, while if the content of the curing catalyst is more than the above range, no further effect can be expected, and the water resistance of the coating film may decrease, resulting in deterioration of corrosion resistance and retort resistance, etc. Furthermore, the acid catalyst may localize on the metal substrate surface due to acid-base interactions, potentially reducing the adhesion between the coating and the metal substrate, which could lead to problems such as the coating peeling off during can molding.

[0044] (Paint composition) The paint composition for forming a coating film on a painted metal plate according to the present invention contains at least the above-mentioned polyester resin and the above-mentioned curing agent as main components, and more preferably the above-mentioned curing catalyst (acid catalyst). In the present invention, the main component is defined as the component with the highest content (mass percentage) among the solid components (non-volatile components excluding volatile substances such as water and solvents) that form the coating film in the paint composition. Furthermore, in the paint composition used in the present invention, it is preferable that the content of the above-mentioned polyester resin, preferably amorphous polyester resin, which is the main component, is higher than 50% by mass, more preferably 60% by mass or more, even more preferably 70% by mass or more, and particularly preferably 80% by mass or more. In the present invention, examples of paint compositions that can be used to form a coating film include solvent-based paint compositions and aqueous paint compositions. In the present invention, solvent-based paint compositions are preferred from the viewpoint of paintability and other factors.

[0045] When the paint composition is a solvent-type paint composition, it contains the polyester resin, curing agent, and organic solvent as described above. In this embodiment, a solvent-type paint composition is defined as a paint composition in which the main resin, curing agent, etc. are dissolved in a known organic solvent and formed into a paint, and in which the mass proportion of the organic solvent in the paint composition is 40% by mass or more. As the aforementioned organic solvent, one or more are selected and used from 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 monoacetate, methanol, ethanol, butanol, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, solvent naphtha, etc., taking into consideration solubility, evaporation rate, etc.

[0046] If the paint composition is an aqueous paint composition, it contains a conventionally known water-dispersible or water-soluble polyester resin and curing agent, along with an aqueous medium as a solvent. As an aqueous medium, water or a mixture of water and an organic solvent such as an alcohol, polyhydric alcohol, or its derivative can be used, similar to known aqueous coating compositions. When using an organic solvent, it is preferable to include it in an amount of 1 to 45% by mass, and particularly preferably in an amount of 5 to 30% by mass, relative to the total aqueous medium in the aqueous coating composition. Including the solvent within the above range improves film-forming performance. Such organic solvents are preferably amphiphilic, and examples include methyl alcohol, ethyl alcohol, isopropyl alcohol, 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, and 3-methyl 3-methoxybutanol.

[0047] <Lubricant> The paint composition may contain a lubricant as needed. It is preferable to add 0.1 to 10 parts by mass of lubricant per 100 parts by mass of polyester resin. By adding a lubricant, damage to the coating during molding can be suppressed, and the lubricity of the coating during molding can be improved.

[0048] Examples of lubricants that can be added to the paint composition include fatty acid ester waxes, which are esters of polyol compounds and fatty acids; silicone waxes; fluorine-based waxes such as polytetrafluoroethylene; polyolefin waxes such as polyethylene; paraffin wax; lanolin; montan wax; microcrystalline wax; carnauba wax; and silicone compounds and petrolatum. These lubricants can be used individually or in mixtures of two or more.

[0049] <Other> In addition to the above-mentioned components, the paint composition may also contain leveling agents, pigments, defoamers, colorants, etc., which have been conventionally used in paint compositions, according to conventionally known formulations. Furthermore, to the extent that the objectives of the present invention are not impaired, other resin components may be included in addition to the polyester resin. For example, the resin may include 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 ethyl ether, polyacrylamide, acrylamide compounds, polyethyleneimine, starch, gum arabic, methylcellulose, and other resins.

[0050] In the paint composition, it is preferable that the polyester resin is contained in an amount of 5 to 55% by mass as solid content. If the resin solid content is less than the above range, it will not be possible to secure an appropriate amount of paint film, and the coverage of the paint film will be poor. On the other hand, if the resin solid content is more than the above range, the workability and coating properties may be poor.

[0051] (Method of manufacturing painted metal sheets) In the present invention, as described above, an interior coating composition containing a polyester resin and a curing agent, preferably a resol-type phenolic resin, as the main components is applied to at least the inner surface of the metal plate. Preferably, an exterior coating composition containing the aforementioned polyester resin and curing agent, preferably an amino resin, is further applied to the outer surface of the metal plate. The curing conditions for the paint composition are appropriately adjusted depending on the type of polyester resin, curing agent, metal substrate, coating amount, etc. However, to obtain sufficient curing, the above-mentioned paint composition is preferably heated and cured at a temperature of 150°C to 350°C, preferably higher than 200°C and 320°C or lower, for 5 seconds or more, preferably 5 seconds to 30 minutes, and particularly preferably 5 seconds to 180 seconds. If the curing temperature is lower than the above range, sufficient curing may not be obtained. On the other hand, if the curing temperature is higher than the above range, there is a risk that the polyester resin will decompose due to excessive heating. If the curing time is shorter than the above range, sufficient curing may not be obtained, and if the curing time is longer than the above range, it will be inferior in terms of economy and productivity.

[0052] Furthermore, in the inner and / or outer coating films after baking, it is preferable that the MEK extraction rate (MEK boiling point, 1 hour), which is an indicator of the degree of hardening, is in the range of 50% or less, preferably 1 to 40%, more preferably 2 to 30%, even more preferably 3 to 25%, and particularly preferably 3 to 20%. Having the MEK extraction rate within the above range allows for control of the degree of hardening of the coating film, which is preferable from the viewpoint of heat resistance, corrosion resistance, retort resistance, and coating film peeling resistance (stress relaxation rate). If the MEK extraction rate is higher than the range mentioned above, the degree of hardening of the coating film tends to decrease, and the heat resistance tends to decline. Therefore, when forming cans by drawing at high speed, the temperature rise becomes more pronounced, and the coating film may stick to the mold when formed. Particularly on the inner surface of the can, when removing the can body from the forming punch after drawing, the can body may stick to the forming punch, making it difficult to separate the forming punch and the can body (poor stripping performance), which may lead to the can body buckling or breaking, thus reducing productivity. On the outer surface of the can, there is a risk of external defects such as coating film abrasion, as well as poor resistance to retort whitening. On the other hand, if the MEK extraction rate is lower than 1%, it becomes difficult to keep the aforementioned stress relaxation rate within the aforementioned range, and there is a risk of paint delamination occurring.

[0053] As for the coating method, the can can be manufactured by coating at least the inner surface of the metal plate, preferably both sides, with known coating methods such as roll coater coating, spray coating, or dip coating, and then baking it using a heating means such as a coil oven.

[0054] The metal sheets used as the metal substrate for painted metal sheets are not limited to those mentioned above, but examples include hot-rolled steel sheets, cold-rolled steel sheets, hot-dip galvanized steel sheets, electro-galvanized steel sheets, alloy-plated steel sheets, aluminum-zinc alloy-plated steel sheets, aluminum sheets, tin-plated steel sheets, stainless steel sheets, copper sheets, copper-plated steel sheets, tin-free steel, nickel-plated steel sheets, ultra-thin tin-plated steel sheets, and chromium-treated steel sheets. If necessary, these can be subjected to various surface treatments, such as chromate phosphate treatment, zirconium-based chemical conversion treatment, or coating treatments combining water-soluble resins such as polyacrylic acid with zirconium salts such as zirconium ammonium carbonate. In the present invention, among the above metal plates, aluminum plates are preferred. As aluminum plates, in addition to pure aluminum plates, aluminum alloy plates, specifically aluminum alloy plates of the 3000, 5000, and 6000 series in "JIS H 4000", can be suitably used, and aluminum alloy plates are preferred in terms of strength and other factors. As aluminum alloy plates, in addition to surface-treated aluminum alloy plates that have undergone the various surface treatments described above, untreated aluminum alloy plates that have not undergone surface treatment can also be suitably used because the coating film made of the above-mentioned paint composition has excellent adhesion to the metal substrate. The thickness of the metal plate is preferably in the range of 0.1 to 1.00 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, from the viewpoint of can body strength and formability.

[0055] In the painted metal sheet of the present invention, a coating film made of another paint composition may be formed on the inner coating film formed on the surface that becomes the inner surface of the can after the drawing and ironing process, and / or on the outer coating film formed on the surface that becomes the outer surface of the can after the drawing and ironing process, if necessary. Furthermore, in the painted metal sheet of the present invention, the inner and outer coatings made of the aforementioned paint composition have excellent adhesion to the metal substrate, so it is preferable that the inner and / or outer coatings are formed in direct contact with the metal sheet which is the metal substrate.

[0056] (Squeezing and squeezing can) The ironed can of the present invention is formed by ironing from the above-mentioned painted metal sheet, and is an ironed can having an inner coating on at least the inner surface of the can, wherein the inner coating contains a polyester resin and a hardening agent, and an important feature is that the stress relaxation rate of the inner coating at the bottom of the can after 10 minutes of 1% elongation under test conditions of 100°C is 50% or more. The stress relaxation rate of the inner coating film is preferably 50% or more, more preferably 50-95%, more preferably 54-90%, even more preferably 54-85%, particularly preferably 54-80%, and most preferably 60-80%. Furthermore, it is preferable that the can further has an outer coating on the outer surface of the can, the outer coating contains a polyester resin and a hardening agent, and the stress relaxation rate of the outer coating at the bottom of the can after 10 minutes of 1% elongation under test conditions of 100°C is higher than 40%. The stress relaxation rate of the outer coating film is preferably in the range of over 40%, more preferably 45-95%, more preferably 50-90%, even more preferably 54-85%, particularly preferably 54-80%, and most preferably 60-80%. Furthermore, it is preferable that the can bottom and body on the inner surface side of the can be continuously covered with the inner coating, and it is even more preferable that the can bottom and body on the outer surface side of the can be continuously covered with the outer coating. The reason the stress relaxation rate is measured on the coating at the bottom of the drawn and ironed can is that the degree of processing at the bottom of the can is extremely small compared to the can body, and therefore it can be approximated to the coating on a painted metal sheet before molding, and has similar characteristics.

[0057] (Method of manufacturing a squeezed can) The ironed metal can of the present invention can be manufactured using the painted metal sheet described above by a conventionally known molding method. Because the coating on the painted metal sheet of the present invention has excellent stretchability, workability, and adhesion, the ironed metal can can be formed without cylinder breakage or peeling of the coating at the can mouth, even during harsh ironed metal drawing processes. Furthermore, because the painted metal sheet of the present invention has excellent formability and lubricity, the ironed metal can can be formed not only when using a coolant, but also when molding is performed under dry conditions without using a coolant.

[0058] Prior to drawing and ironing, it is preferable to apply a wax-based lubricant, such as paraffin wax, white petrolatum, palm oil, various natural waxes, or polyethylene wax, to the surface of the painted metal sheet, thereby enabling efficient drawing and ironing under dry conditions. The painted metal sheet coated with the wax-based lubricant is punched out as a blank using a cupping press, and a drawn cup is formed by the drawing method. In this invention, it is desirable that the drawing ratio RD, defined by the following formula (3), is in the range of 1.1 to 2.6, and particularly in the range of 1.4 to 2.6, for the total (up to the drawn and ironed can). If the drawing ratio is larger than the above range, the drawing wrinkles will become larger, and there is a risk that cracks will occur in the coating and metal exposure will occur. RD = D / d ... (3) In the formula, D represents the blank diameter and d represents the can body diameter.

[0059] Next, the can body is thinned by performing a re-drawing process (drawing and ironing process) on the drawing cup in one or several stages. In the present invention, it is desirable that the ironing ratio R, represented by the following formula (4), is in the range of 25 to 80%, preferably 40 to 80%, more preferably 50 to 80%, even more preferably 55 to 75%, particularly preferably 55 to 70%, and most preferably higher than 60% and 70% or less. If the ironing ratio is lower than the above range, the wall thickness cannot be sufficiently thinned, which is not sufficiently satisfactory in terms of economics. On the other hand, if the ironing ratio is higher than the above range, there is a risk of metal exposure. R(%)=(tp-tw) / tp ×100 ···(4) In the formula, tp represents the thickness of the original painted metal sheet, and tw represents the thickness of the center of the side wall of the can body of the drawn and ironed can.

[0060] Furthermore, in the ironed can of the present invention, it is preferable that the thickness of the central part of the can body (the central part in the height direction, the thinnest part) is 20-75%, preferably 20-60%, more preferably 20-50%, even more preferably 25-45%, particularly preferably 30-45%, and most preferably 30% or more and less than 40% of the thickness of the central part of the can bottom. Similarly, regarding the thickness of the metal substrate of the ironed can, it is preferable that the thickness of the metal substrate in the central part of the can body is 20-75%, preferably 20-60%, more preferably 20-50%, even more preferably 25-45%, particularly preferably 30-45%, and most preferably 30% or more and less than 40% of the thickness of the metal substrate in the central part of the can bottom. Also, when an ironed can is formed from a painted metal sheet by ironing, the thickness of the coating located on the can body becomes thin due to the processing, just like the metal substrate. Therefore, the thickness of the coating in the center of the can body is preferably 20-75%, more preferably 20-60%, more preferably 20-50%, even more preferably 25-45%, particularly preferably 30-45%, and most preferably 30% or more and less than 40% of the thickness of the coating in the center of the can bottom, which is hardly thinned during can manufacturing.

[0061] The thickness of the metal substrate in the center of the can bottom 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. Furthermore, the film thickness of the inner coating in the center of the can bottom is preferably in the range of 0.2 to 20 μm, preferably 1 to 12 μm, and more preferably greater than 2 μm and 12 μm or less, in terms of dry film thickness. The dry film weight is preferably 3 to 300 mg / dm 2 Preferably 15-150 mg / dm 2 More preferably 25 mg / dm 2 Larger dose: 150 mg / dm 2 The following ranges are preferable. Furthermore, if the contents to be filled into the squeezed can are highly corrosive acidic beverages, the range should be greater than 6 μm and less than or equal to 12 μm, preferably in the range of 6.5 to 10 μm. The dry coating weight should be 85 mg / dm 2 Larger dose: 150 mg / dm 2Preferably 90-140 mg / dm 2 It is preferable that the thickness be within this range. On the other hand, if the contents to be filled into the squeezed can are low-acid beverages with relatively low corrosiveness, it is preferable that the thickness be 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 between 2.5 and 6 μm. The dry coating weight is 15 mg / dm 2 More than 90mg / dm 2 Less than 25 mg / dm 2 Larger 90mg / dm 2 Less than, more preferably 30-85 mg / dm 2 It is preferable that the range be within this range.

[0062] Furthermore, the thickness of the outer coating film in the center of the bottom of the can is preferably in the range of 0.2 to 20 μm, preferably 1 to 10 μm, more preferably greater than 2 μm and 10 μm or less, and even more preferably greater than 2 μm and 6.5 μm or less, in terms of dry film thickness. The dry film weight is 3 to 300 mg / dm 2 Preferably 15-150 mg / dm 2 More preferably 25-140 mg / dm 2 More preferably 25 mg / dm 2 Larger 90mg / dm 2 It is preferable that the range be less than the specified value.

[0063] Furthermore, as described above, when a drawn and ironed can is formed from a painted metal sheet having an internal coating by drawing and ironing, the thickness of the internal coating located in the can body becomes thin due to the processing, similar to the thickness of the metal substrate located in the can body. Therefore, in the drawn and ironed can of the present invention, the thickness ratio of the internal coating to the metal substrate in the can body and the thickness ratio of the internal coating to the metal substrate in the can bottom are approximately the same. In other words, a characteristic of the drawn and ironed can of the present invention is that the thickness ratio of the internal coating to the metal substrate (= thickness of the internal coating / thickness of the metal substrate) is substantially the same in the can bottom and the can body. Here, "approximately the same" means that the manufacturing tolerance is within that range, for example, that the (thickness of the internal coating / thickness of the metal substrate) in the can body is within the range of 0.9 to 1.1 times the (thickness of the internal coating / thickness of the metal substrate) in the can bottom. The same applies to the external coating.

[0064] Furthermore, the processing speed (punch movement speed) for the one or more stages of ironing is preferably 1500 mm / sec or more, more preferably 3000 mm / sec or more, more preferably 4000 mm / sec or more, even more preferably 5000 mm / sec or more, and particularly preferably 6000 mm / sec or more. By setting the processing speed of the ironing to the above speed or higher, the heat generated during processing increases, and the high temperature improves the processability (stretchability) of the coating film. As a result, metal exposure during forming is suppressed, and the coverage of the inner and outer coating films after forming can be further improved. Moreover, forming at a high temperature makes it easier to relax stress during the forming process, which reduces residual stress in the coating film after forming and is also preferable in suppressing coating film peeling during heat treatment. After the drawing and ironing process, the bottom is domed and the opening edge is trimmed according to the standard method, if desired.

[0065] According to the present invention, the painted metal sheet is subjected to a drawing and ironing process, and the resulting drawn and ironed can is then subjected to a heat treatment process. As described above, in the painted metal sheet and drawn and ironed can of the present invention, the stress relaxation rate of the coating film is high, at 50% or more on the inner surface of the can and at over 40% on the outer surface of the can. Therefore, even when heated in the heat treatment process, peeling of the coating film is effectively prevented. By subjecting the molded can to at least one heat treatment, residual stress in the coating film generated by the processing can be removed. By removing this residual stress in the coating film, it is possible to improve the adhesion between the processed coating film and the metal substrate (coating film adhesion). As a result, the corrosion resistance of the coating film is significantly improved, and for example, when the can is filled with highly corrosive contents, the occurrence of subcoating corrosion can be suppressed. The temperature of the heat treatment must be higher than the glass transition temperature of the coating film, and a temperature range of 100 to 300°C, preferably 150 to 250°C, is preferred. The time of the heat treatment is not particularly limited, but it is preferable to heat for 0.1 to 600 seconds, preferably 1 to 300 seconds, and more preferably 20 to 180 seconds.

[0066] If residual stress in the coating of a pressed and ironed can is not removed by heat treatment, when the coating in the central part of the can body (the central part in the height direction), which has a high degree of processing, is isolated from the metal substrate and heated, the dimensions change significantly in the direction that releases the residual stress (mainly in the height direction of the can). Therefore, by measuring the amount of dimensional change (thermal shrinkage rate) of the isolated coating due to heating, it is possible to determine whether the residual stress has been removed by heat treatment. It is desirable that the thermal shrinkage rate (with load) of the inner coating in the central part of the can body isolated from the pressed and ironed can, represented by the following formula (5), be 30% or less, preferably 20% or less, more preferably 15% or less, and even more preferably 10% or less. It is also desirable that the thermal shrinkage rate (without load), represented by the following formula (6), be 50% or less, preferably 45% or less, more preferably 40% or less, and even more preferably 35% or less. When the thermal shrinkage rate is within the above range, the adhesion of the coating is improved, and excellent corrosion resistance can be achieved. If the thermal shrinkage rate is greater than the range described above, residual stress may not be sufficiently removed, potentially leading to insufficient adhesion of the coating and reduced corrosion resistance. Furthermore, the coating may peel off if the can is subjected to impact and dents. In addition, if the can has the above-mentioned outer coating on its outer surface, it is desirable that the thermal shrinkage rate of the outer coating in the center of the can body also be within the above range. Furthermore, the dimensional change (shrinkage) of the isolated coating film due to heating can be measured using a thermomechanical analyzer (TMA) or the like.

[0067] Thermal shrinkage rate (with load) = (ΔL1 / L0) × 100 (%) ... (5) In the formula, L0 is the initial height length of the coating isolated from the center of the can body (measurement area), and ΔL1 is 5.20 × 10 per unit area. 5 N / m 2 This is the maximum shrinkage amount (maximum shrinkage length) in the height direction of the coating film in the L0 portion when the temperature is raised from 30°C to 200°C at a heating rate of 5°C / min while applying a load.

[0068] Thermal shrinkage rate (without load) = (ΔL2 / L0) × 100 (%) ... (6) In the formula, L0 is the initial height length of the coating film isolated from the center of the can body, and ΔL2 is the maximum shrinkage (maximum shrinkage length) in the height direction of the coating film corresponding to L0 when the temperature is raised from 30°C to 200°C at a heating rate of 5°C / min under no load.

[0069] After heat treatment, the can is rapidly cooled or allowed to cool, and if necessary, a printed layer is formed on the can body by a printing and baking process using conventionally known methods, and a finishing varnish layer is formed on top of the printed layer to protect it. If desired, it can be subjected to one or more stages of neck-in processing and flange processing to create a can for seam sealing. Alternatively, after forming the drawn can, the top can be deformed into a bottle shape, or the bottom can be cut off and another can end attached to create a bottle shape. The capacity of the squeezing 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.

[0070] The painted metal sheet of the present invention has excellent can-making processability, allowing it to withstand harsh processing such as the manufacture of drawn cans, and also prevents paint peeling during post-forming heat treatment. By adjusting the forming conditions such as the drawing speed, it is possible to obtain a drawn can with excellent coating coverage, where the inner coating has a coverage of less than 200 mA in terms of ERV (Enamel Rater Value). Here, the coverage of the inner coating obtained by ERV conversion refers to the value measured by enamelizer when the obtained drawn can is filled with a 1% by mass saline solution, which serves as the electrolyte, up to near the mouth of the can. A metal exposed portion is formed on the outer surface of the bottom of the can and connected to the anode, while the cathode is immersed in the saline solution filling the can. The current value is measured after applying a DC voltage of 6.3 V for 4 seconds at room temperature (approximately 23°C). In such measurements, the more current that flows, the more defects are present in the insulating inner coating, indicating a large area of ​​metal exposure on the inner surface of the can. The coverage of the inner coating film, calculated in ERV terms, should preferably be less than 200 mA, more preferably less than 100 mA, and more preferably less than 50 mA.2 When expressed as ERV per unit, it is 0.70 mA / cm². 2 Less than 0.35 mA / cm², preferably 0.35 mA / cm² 2 Less than, more preferably 0.18 mA / cm² 2 It is desirable that it be less than [a certain value]. Here, ERV per unit area is the value obtained by dividing the ERV of the squeezed can measured by the method described above by the evaluation area (the area in contact between the inner surface of the can body and bottom and the salt solution described above). Furthermore, regarding the inner surface of the pressed and squeezed can, if necessary after molding, a corrective paint or other spray coating may be applied to the inner surface to form another coating on top of the inner coating. However, as mentioned above, since the inner coating maintains a high degree of coverage even after molding, spray coating is not necessary, and from an economic standpoint, it is preferable that it is not spray coated. Furthermore, regarding the outer surface of the can, at least in the bottom where a printing layer is not basically formed, the outer coating may be located on the surface layer. However, for the purpose of improving the transportability of the can, a coating made of another coating composition may be formed on the outer coating formed on the surface layer of the bottom outer surface.

[0071] The painted metal sheet of the present invention can be suitably applied to applications other than drawn and ironed cans, such as conventionally known methods of drawing cans (DR cans), deep-drawn cans (DRD cans), DTR cans, tensile drawing and ironed cans, or can lids. 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 tab for opening, and may be either a full-open type or a partial-open type (stay-on-tab type). [Examples]

[0072] The present invention will be specifically described below with reference to examples, comparative examples, and reference examples. Note that "parts" refers to parts by mass.

[0073] The following methods were used to measure the various parameters of polyester resins A through D. Note that polyester resins A through D are all amorphous polyester resins. (Measurement of number-average molecular weight) The measurements were performed using gel permeation chromatography (GPC) with a calibration curve for standard polystyrene. (Measurement of glass transition temperature) Measurements were taken using a differential scanning calorimeter (DSC) at a heating rate of 10°C / min. (Measurement of acid value) The resin acid value (mgKOH / g) was determined by dissolving 1 g of polyester resin solid in 10 ml of chloroform and titrating with a 0.1 N KOH ethanol solution. Phenolphthalein was used as an indicator. (Measurement of monomer composition) 30 mg of polyester resin solid was dissolved in 0.6 mL of deuterated chloroform, and 1H-NMR measurements were performed to determine the monomer composition ratio from the peak intensity. Trace amounts of components (less than 1 mol% of the total monomer components) were excluded to determine the composition ratio.

[0074] (Example 1) [Preparation of interior coating compositions] Polyester resin A (acid value: 2 mg KOH / g, Tg: 75℃, Mn=18,000, monomer composition: terephthalic acid component / isophthalic acid component / ethylene glycol component / propylene glycol component = 38 / 12 / 17 / 33 mol%) was used as the polyester resin, a resol-type phenolic resin was used as the curing agent, and dodecylbenzenesulfonic acid was used as the curing catalyst (acid catalyst). Specifically, the resol-type phenolic resin used was an m-cresol-type resolic resin in which the methylol group was alkyl etherified with n-butanol (percentage of alkyl etherified methylol group: 90 mol%, Mn=1,200), and "Dodecylbenzenesulfonic acid (soft type) (mixture)" manufactured by Tokyo Chemical Industry Co., Ltd. was used as the acid catalyst. Polyester resin A was dissolved in a mixed solvent of methyl ethyl ketone / solvent naphtha = 50 / 50 (mass ratio) to obtain a polyester resin A solution with a solid content of 30% by mass. An n-butanol solution of resol-type phenolic resin (solid content 50% by mass) was diluted with methyl ethyl ketone to obtain a resol-type phenolic resin solution with a solid content of 30% by mass. Dodecylbenzenesulfonic acid was amine-neutralized with 2-dimethylaminoethanol, and then dissolved in isopropanol to obtain a dodecylbenzenesulfonic acid solution with a solid content of 30% by mass. Next, 333 parts of polyester resin A solution (100 parts solids), 33.3 parts of resol-type phenolic resin solution (10 parts solids), and 0.33 parts of acid catalyst solution (0.10 parts solids) were placed in a glass container and stirred for 10 minutes to prepare a solvent-type coating composition [solids concentration: approximately 30% by mass, solids blending ratio: polyester resin A / resol-type phenolic resin / acid catalyst = 100 / 10 / 0.1 (mass ratio)].

[0075] [Preparation of exterior coating compositions] Polyester resin A was used as the polyester resin, melamine resin (methyl etherified melamine resin, full ether type, weight-average degree of polymerization 1.3) and benzoguanamine resin (methyl etherified benzoguanamine resin, partially etherified type containing imino and methylol groups, weight-average degree of polymerization 1.5) as the curing agent, and dodecylbenzenesulfonic acid was used as the curing catalyst (acid catalyst). Polyester resin A was dissolved in a mixed solvent of methyl ethyl ketone / solvent naphtha = 50 / 50 (mass ratio) to obtain a polyester resin A solution with a solid content of 30% by mass. Melamine resin and benzoguanamine resin were dissolved in methyl ethyl ketone to obtain melamine resin solution and benzoguanamine resin solution with a solid content of 30% by mass. Dodecylbenzenesulfonic acid was amine-neutralized with 2-dimethylaminoethanol, and then dissolved in isopropanol to obtain a dodecylbenzenesulfonic acid solution with a solid content of 30% by mass. Next, 333 parts of polyester resin A solution (100 parts solids), 10 parts of melamine resin B solution (3 parts solids), 10 parts of benzoguanamine resin solution (3 parts solids), and 0.33 parts of acid catalyst solution (0.10 parts solids) were placed in a glass container and stirred for 10 minutes to prepare a solvent-type paint composition [solids concentration: approximately 30% by mass, solids blending ratio: polyester resin A / melamine resin / benzoguanamine resin / acid catalyst (dodecylbenzenesulfonic acid) = 100 / 3 / 3 / 0.1 (mass ratio)].

[0076] [Creation of painted metal sheets] As a metal sheet, we used a chromate phosphate surface-treated aluminum sheet (3104 alloy, sheet thickness: 0.27 mm, chromium weight in surface treatment film: 20 mg / m²). 2 Using this method, first, the surface that will become the outer surface after molding is coated with a dry coating weight of 40 mg / dm² after baking. 2 The above exterior coating composition was applied using a bar coater to a thickness of approximately 3 μm and dried at 120°C for 60 seconds. Then, the opposite side, which would become the interior side, was coated with a dry coating weight of 88 mg / dm² after baking. 2 The above-mentioned interior coating composition was applied using a bar coater to a thickness of approximately 6.4 μm, dried at 120°C for 60 seconds, and then baked at 250°C (oven oven temperature) for 30 seconds to create the product.

[0077] [Making a squeezed and squeezed can] After applying paraffin wax to both sides of the painted metal sheet created using the method described above, a shallow-drawn cup was punched out in a circle with a diameter of 142 mm. Next, this shallow-drawn cup was subjected to re-drawing, ironing (3 stages), and doming under dry conditions using a punch with an outer diameter of Φ66 mm. After that, it was heat-treated in an oven at 201°C for 75 seconds to produce a drawn and ironed can [can diameter: 66 mm, height: approx. 130 mm, capacity: approx. 370 ml, total drawing ratio: 2.15, ironing rate: 61%, thickness of can body center / thickness of can bottom center × 100 = approx. 40%, thickness of metal substrate in can body center / thickness of metal substrate in can bottom center × 100 = approx. 40%, thickness of inner coating film in can body center / thickness of inner coating film in can bottom center × 100 = approx. 39%, weight (thickness) of inner coating film in can bottom center: 86 mg / dm 2The following values ​​were obtained: (approximately 6.2 μm), inner coating thickness at the center of the can bottom / thickness of the metal substrate at the center of the can bottom = approximately 0.024, inner coating thickness at the center of the can body / thickness of the metal substrate at the center of the can body = approximately 0.023. The average processing speed during ironing (average punch movement speed during ironing) was 5500 mm / sec.

[0078] (Examples 2, 3) As shown in Table 1, interior coating compositions were prepared by changing the type of polyester resin and the solid content ratio, and painted metal plates were made in the same manner as in Example 1, except that these were used. A pressed metal can was then made. In addition to the polyester resins mentioned above, polyester resin B (acid value: 2 mg KOH / g, Tg: 85℃, Mn=18,000, monomer composition: terephthalic acid component / ethylene glycol component / propylene glycol component = 50 / 14 / 36 mol%) and polyester resin C (acid value: 5 mg KOH / g, Tg: 55℃, Mn=16,000, monomer composition: terephthalic acid component / isophthalic acid component / 1,4-cyclohexanedimethanol component / 2-methyl-1,3-propanediol component = 10 / 40 / 21 / 29 mol%) were used as the polyester resins.

[0079] (Example 4) As shown in Table 1, polyester resin A and polyester resin D (acid value: 22 mg KOH / g, Tg: 82°C, Mn = 6,000, monomer composition: terephthalic acid component / trimellitic acid component / ethylene glycol component / propylene glycol component = 49 / 1 / 12 / 38 mol%) are mixed in a mass ratio of 90:10 (Tg mix :76℃, AV mix An interior coating composition was prepared using 4 mg KOH / g. As shown in Table 1, an exterior coating composition was prepared by using benzoguanamine resin (methyl etherified benzoguanamine resin, partially etherified type containing imino and methylol groups, weight-average degree of polymerization 1.5) as a curing agent and changing the solid content ratio. Painted metal plates were then prepared in the same manner as in Example 1, except that these compositions were used, and a pressed and ironed can was made.

[0080] (Example 5) As shown in Table 1, a mixture of polyester resin A and polyester resin D in a mass ratio of 90:10 was used as the polyester resin, and the aforementioned benzoguanamine resin (methyl etherified benzoguanamine resin, partially etherified type containing imino and methylol groups, weight-average degree of polymerization 1.5) was used as the curing agent. An interior coating composition was prepared in the same manner as in Example 1, except that the solid content ratio was changed. Painted metal plates were then made and a pressed can was produced.

[0081] (Example 6) Painted metal sheets were prepared in the same manner as in Example 1, except that the average processing speed during the ironing process (average punch movement speed during ironing) was set to 1000 mm / sec when making the ironed cans, and then ironed cans were made.

[0082] (Comparative Examples 1 and 2) As shown in Table 1, painted metal sheets were prepared and pressed metal cans were manufactured in the same manner as in Example 1, except that the type of polyester resin and the solid content ratio were changed to prepare the coating compositions for the interior and exterior surfaces.

[0083] (Reference example 1) A painted metal plate was prepared in the same manner as in Example 1, except that the type of polyester resin and the solid content ratio were changed to prepare an interior coating composition as shown in Table 1.

[0084] The coating properties obtained from the interior coating compositions and exterior coating compositions used in each example, comparative example, and reference example were tested according to the following test methods.

[0085] [Stress relaxation rate of the coating film] Using the interior coating compositions and exterior coating compositions used in each example, comparative example, and reference example, coating film samples for measurement were prepared as follows. The coating conditions (type of coating, dry coating film weight, drying and baking conditions) for the interior coating film on the painted metal plate in each example, comparative example, and reference example were the same as those for the non-glossy side of aluminum foil ("Nippaku Foil" manufactured by Mitsubishi Aluminum Co., Ltd., 12 μm thick) and applied using a bar coater. After drying at 120°C for 60 seconds, the foil was baked at 250°C for 30 seconds to form a coating film on the aluminum foil. After cooling to room temperature, the aluminum foil with the coating film formed on it was cut to a width of 50 mm and a length of 40 mm and immersed in a diluted hydrochloric acid aqueous solution to dissolve the aluminum foil. Next, the film-like coating film was removed, thoroughly washed with distilled water and dried, and the resulting film-like coating film was cut to a width of 4 mm and a length of 40 mm to obtain a sample for measurement.

[0086] The stress relaxation rate of the obtained sample was measured using a thermomechanical analyzer (TMA). First, the sample was chucking the thermomechanical analyzer so that there was no deflection, and then the stress relaxation rate of the obtained sample was measured using a thermomechanical analyzer (TMA). First, the sample was chucking the thermomechanical analyzer so that there was no deflection, and the distance between the chucks (corresponding to the initial length of the sample) was set to 10 mm. Next, the measurement atmosphere was heated, and after 10 minutes had elapsed since reaching 100°C, the sample was stretched by 1% of its initial length at a tensile speed of 1 mm / min and held in that state for 10 minutes. The stress relaxation rate was calculated from the stress at the time of 1% stretching (δ1) and the stress after holding for 10 minutes (δ2) using the following formula (1). The measurement conditions are as follows. Device: Seiko Instruments Inc. TMA / SS6100 Measurement ambient temperature: 100℃ Chuck spacing: 10mm Pulling speed: 1 mm / min Elongation rate (tensile stretch strain): 1% Stress relaxation rate (%) = (δ1 - δ2) / δ1 × 100 ... (1) In the formula, δ1 is the stress at 1% elongation, and δ2 is the stress after holding for 10 minutes. Note that the values ​​of δ1 and δ2 are corrected values ​​obtained by subtracting the stress value just before 1% elongation after heating to 100°C from the actual measured value.

[0087] When obtaining a sample for measurement from a painted metal plate with a coating on both sides, the painted metal plate can be cut out, immersed in boiling hydrogen peroxide solution for several minutes, thoroughly washed with distilled water, the film-like coating can be peeled off the metal substrate and dried, and the resulting film can be cut into 4mm wide and 40mm long pieces to obtain a sample for measurement.

[0088] [Stress relaxation rate of the coating on the inner surface of the can bottom] The stress relaxation rate of the coating film on the inner surface of the can bottom was measured for the cans of Example 4 and Comparative Example 1, which were molded as described in the "Preparation of Cans by Drawing and Ironing" section above and then heat-treated at 201°C for 75 seconds. The method for preparing the measurement samples is as follows. From the bottom of a heat-treated drawn can, a section of the can bottom was cut out so that it measured 35 mm in the 0° direction relative to the rolling grain of the metal substrate and 30 mm in the 90° direction, centered on the bottom of the can bottom. The cut-out sample was immersed in boiling hydrogen peroxide solution for 2-3 minutes and thoroughly washed with distilled water. The film-like coating on the inner surface of the can was then peeled off from the metal substrate and dried. The resulting film-like coating was then cut into 4 mm wide and 35 mm long pieces to obtain a sample for measurement. The stress relaxation rate was measured using the same method as described in the "Stress Relaxation Rate of Coating Films" section above. The results are shown below. Stress relaxation rate of the coating film on the inner bottom of the can in Example 4: 67% Stress relaxation rate of the inner surface coating of the can bottom of the can in Comparative Example 1: 45%

[0089] [Glass transition temperature of coating film (coating film Tg)] Using the interior and exterior coating compositions used in each example, comparative example, and reference example, coating samples for measurement were prepared as follows. The coating conditions (type of coating, dry coating weight, drying and baking conditions) for the interior or exterior coatings on the painted metal plates in each example, comparative example, and reference example were the same as those for the non-glossy side of aluminum foil ("Nippaku Foil" manufactured by Mitsubishi Aluminum Co., Ltd., 12 μm thick) when applied using a bar coater. After drying at 120°C for 60 seconds, the foil was baked at 250°C for 30 seconds to form a coating film on the aluminum foil. After cooling to room temperature, the aluminum foil with the coating film was immersed in a diluted hydrochloric acid aqueous solution to dissolve the aluminum foil. Next, the film-like coating was removed, thoroughly washed with distilled water, and dried to obtain a sample for measurement. The glass transition temperature of the obtained coating film was measured using a differential scanning calorimeter (DSC) under the following conditions. In the second run (heating), the glass transition temperature of the coating film (coating film Tg) was defined as the temperature at the intersection of the extrapolation glass transition onset temperature, i.e., the straight line extending from the low-temperature baseline to the high-temperature side, and the tangent line drawn at the point where the slope of the curve representing the stepwise change portion of the glass transition is maximized. Device: Seiko Instruments Inc. DSC6220 Sample quantity: 5 mg Heating rate: 10°C / min Temperature range: -80 to 200°C (heating, cooling, heating) Environmental conditions: Under a nitrogen atmosphere

[0090] When obtaining a sample for measurement from a painted metal plate or a drawing can with a coating on both sides, the coating on the side not to be measured can be removed by sanding with sandpaper to expose the metal surface. Then, the painted metal plate can be cut out, and the metal substrate (metal plate) can be dissolved by conventional methods such as immersion in a diluted hydrochloric acid solution to extract the film-like coating. The sample can then be obtained by thoroughly washing it with distilled water and drying it.

[0091] [MEK extraction rate] Using the interior and exterior coating compositions used in each example, comparative example, and reference example, coating samples for measurement were prepared as follows. Each phosphate chromate-based surface-treated aluminum plate (3104 alloy, plate thickness: 0.27 mm, chromium weight in surface treatment film: 20 mg / m²) was prepared so as to be the same as the coating conditions (paint type, dry coating film weight, drying and baking conditions) for the interior or exterior coating film on the painted metal plates of each example, comparative example, and reference example. 2 A painted metal plate was prepared by coating a metal plate with a bar coater, drying it at 120°C for 60 seconds, and then baking it at 250°C for 30 seconds. A 5cm x 5cm test piece was cut from the painted metal plate, and after measuring the mass of the test piece (W1), the test piece was immersed in 200ml of MEK (methyl ethyl ketone) in boiling MEK (80°C reflux) for 1 hour, and MEK extraction was performed at the boiling point for 1 hour. After extraction, the test piece was washed with MEK, dried at 120°C for 1 hour, and the mass of the extracted test piece (W2) was measured. Furthermore, the coating film was peeled off and removed by decomposition with concentrated sulfuric acid, washed and dried, and the mass of the test piece (W3) was measured. The MEK extraction rate (mass %) of the coating film on the painted metal plate is calculated using the following formula (7). The results are shown in Table 1. MEK extraction rate (%)=100×(W1-W2) / (W1-W3) ···(7) Furthermore, when obtaining a sample for measurement from a painted metal plate or a drawing can with a coating formed on both sides, the coating on the side not to be measured can be removed by sanding with sandpaper, and then the MEK extraction rate of the coating on the painted metal plate can be measured using the method described above.

[0092] The drawn cans and painted metal sheets obtained in each example, comparative example, and reference example were evaluated according to the following test method.

[0093] [Evaluation of internal coating film coverage (ERV evaluation)] The evaluation of the inner coating performance was performed as follows for the drawn and doming process performed as described in the "Preparation of drawn and doming cans" section above (indicated as "no heat treatment" in the table) and the drawn and doming process performed after heat treatment at 201°C for 75 seconds in an oven (indicated as "heat treated" in the table). A metal exposed portion was formed on the outer surface of the bottom of a squeezing can, and the can body was connected to the anode of an enamelizer. 360 mL of 1% saline solution was poured into the can, and the cathode of the enamelizer was immersed in the saline solution filling the can. The current value (ERV) was measured after applying a voltage of 6.3 V for 4 seconds at room temperature. The evaluation criteria are as follows: ◎: Current value less than 50mA (0.18mA / cm² per unit area) 2 less than) ○: Current value 50mA or more and less than 200mA (0.18mA / cm²) 2 More than 0.70mA / cm 2 less than) △: Current value between 200mA and less than 700mA (0.70mA / cm²) 2 More than 2.50mA / cm 2 less than) ×: Current value 700mA or more (2.50mA / cm²) 2 (End)

[0094] [Evaluation of coating peel resistance] The paint peeling resistance was evaluated by observing whether or not the paint peeled off the inner and outer surfaces of the can body of the can-shaped cans that had been molded as described in the "Preparation of Drawn Cans" section above and then heat-treated at 201°C for 75 seconds. The evaluation criteria are as follows: ○: No paint film peeling was observed. △: Very slight paint peeling is observed in areas where the can body sidewall has been severely thinned during processing. ×: Extensive paint peeling is observed in areas where the can body sidewalls have been severely thinned during processing.

[0095] [Evaluation of substrate adhesion] From the painted metal sheet prepared as described above, a strip-shaped test piece measuring 50 mm in height and 15 mm in width was cut out. A scratch reaching the metal substrate was made 35 mm from the tip of the strip on the side of the painted metal sheet that would face the outside of the can. By repeatedly bending the sheet starting from the pre-made scratch, only the metal substrate was cut, creating a section connected only by the inner coating. This section was then bent inwards, and a 180° peel test was performed using a tensile testing machine (Shimadzu Corporation's "Autograph AG-IS") at 23°C and a tensile speed of 5 mm / min to measure the peel strength (180° peel strength). The evaluation area, where the inner coating film had been peeled from the metal substrate, was visually observed. If the inner coating film of the evaluation area had completely peeled off from the metal substrate, it was evaluated as interfacial delamination of the coating film. If the adhesion between the coating film and the metal substrate was strong and the inner coating film fractured before peeling off from the metal substrate, it was evaluated as cohesive failure of the coating film. The evaluation criteria are as follows: ◎: Interfacial delamination with a peel strength of 2.0 N / 15 mm or higher, or cohesive failure. ○: Interfacial delamination with a peel strength of less than 2.0 N / 15 mm and 1.0 N / 15 mm or more. △: Delamination strength is less than 1.0 N / 15 mm in interfacial delamination.

[0096] (Evaluation of thermal shrinkage rate) The thermal shrinkage rate was evaluated as follows, using the inner coating film of the center of the can body of the can-shaped can of Example 4 (without heat treatment) which had undergone the drawing and doming process as described in the "Manufacturing of the drawn can" section above, and the can-shaped can of Example 4 (with heat treatment) which had undergone heat treatment in an oven at 201°C for 75 seconds. Using the aforementioned drawing can, a sample measuring 10 mm in the circumferential direction and 20 mm in the height direction of the can was cut from the center of the can body (the thinnest part) at a 0° angle to the metal substrate rolling line. The coating on the outer surface of the can was removed by sanding with sandpaper to expose the metal surface, and then the metal substrate was dissolved by immersion in a diluted hydrochloric acid solution. Next, the film-like coating on the inner surface of the can was removed, thoroughly washed with distilled water and dried, and the resulting film-like coating was cut to a width of 4 mm (circumferential direction of the can body) and a length of 20 mm (height direction of the can body) to obtain a sample for measurement.

[0097] The sample was chucked into a thermomechanical analyzer, and the distance between the chucks (corresponding to the initial length of the measurement area in the height direction of the coating film) was set to 5 mm. The displacement of the sample was measured under the following conditions, and the thermal shrinkage rate in the height direction of the can was evaluated under both loaded and unloaded conditions. Device: Seiko Instruments Inc. TMA / SS6100 Heating rate: 5°C / min Temperature range: 30~200℃ Measurement mode: Pull mode Measurement load: 5mN (5.20 × 10 5 N / m 2 ) or unloaded Chuck spacing: 5mm

[0098] The distance between the chucks before measurement (corresponding to the initial length of the measurement area of ​​the coating film) is L0, and the unit area is 5.20 × 10 5 N / m 2 When the temperature was raised from 30°C to 200°C at a heating rate of 5°C / min while applying a load, ΔL1 was defined as the maximum amount of shrinkage in the height direction of the portion corresponding to L0 (maximum shrinkage length), and the value calculated by the formula shown in equation (5) below was defined as the thermal shrinkage rate (with load). Note that for displacement, shrinkage was defined as a positive value, and expansion or elongation as a negative value. The results are shown below. Thermal shrinkage rate (with load) = (ΔL1 / L0) × 100 (%) ... (5) Thermal shrinkage rate of the inner coating film of the squeezed can (without heat treatment) in Example 4 (with load): 68% Thermal shrinkage rate of the inner coating film of the heat-treated can (with load) in Example 4: 8%

[0099] Furthermore, the distance between the chucks before measurement (corresponding to the initial length of the measurement area of ​​the coating) was defined as L0, and the maximum amount of shrinkage in the height direction of the portion corresponding to L0 (maximum shrinkage length) when the temperature was raised from 30°C to 200°C at a heating rate of 5°C / min under no load was defined as ΔL2. The value calculated by the formula shown in equation (6) below was defined as the thermal shrinkage rate (without load). Note that for displacement, shrinkage was defined as a positive value, and expansion or elongation as a negative value. The results are shown below. Thermal shrinkage rate (without load) = (ΔL2 / L0) × 100 (%) ... (6) Thermal shrinkage rate of the inner coating film of the drawing can (without heat treatment) in Example 4 (without load): 69% Thermal shrinkage rate of the inner coating film of the ironed can (heat-treated) in Example 4 (without load): 30%

[0100] (Corrosion resistance evaluation) The corrosion resistance was evaluated as follows on the inner surface coating of the central part of the can body of the drawn can of Example 4 (without heat treatment) which had undergone drawing and doming processes as described in the "Manufacturing of Drawn Cans" section above, and the drawn can of Example 4 (with heat treatment) which had undergone heat treatment in an oven at 201°C for 75 seconds. Using the aforementioned pressed and ironed can, a test piece measuring 40 mm in the circumferential direction and 40 mm in the height direction of the can was cut out, centered on the center of the can body (the thinnest part). A 4 cm long cross-cut was made into the test piece with a cutter, reaching the base material, and the piece was immersed in an acidic model solution containing sodium chloride and left at 37°C for two weeks to evaluate the corrosion state. The model solution used in the test consisted of 0.2% sodium chloride, to which citric acid was added to adjust the pH to 2.5. The evaluation criteria were as follows: if the maximum width of undercoat corrosion around the cross-cut area was 1.5 mm or more on one side, it was marked ×; if it was 0.5 mm or more but less than 1.5 mm, it was marked ○; and if it was less than 0.5 mm, it was marked ◎. The results are shown below. Corrosion state of the drawing can (without heat treatment) in Example 4: × Corrosion status of the drawing can (heat-treated) in Example 4: ◎

[0101] Table 1 shows the formulation composition (type of polyester resin, type of curing agent, solid content ratio), coating performance (coating film Tg, MEK extraction rate, stress relaxation rate), and evaluation results for each example, comparative example, and reference example of the interior and exterior coating compositions.

[0102] [Table 1] [Industrial applicability]

[0103] The pressed and shaped can of the present invention does not undergo paint peeling during heat treatment, effectively prevents metal exposure, has excellent corrosion resistance, and is suitable for use as a beverage container and the like. Furthermore, the painted metal sheet for pressed and shaped cans of the present invention has excellent can-making processability and productivity, and has excellent paint peeling resistance, preventing paint peeling even during heat treatment after molding, making it suitable for use in the manufacture of pressed and shaped cans.

Claims

1. A draw-and-sew can having an inner coating on at least the inner surface of the can, wherein the inner coating contains a polyester resin, a curing agent, and an acid catalyst, the curing agent contained in the inner coating is a resol-type phenolic resin or an amino resin, the amount of the acid catalyst contained in the inner coating is 0.01 to 3 parts by mass per 100 parts by mass of polyester resin, the stress relaxation rate of the inner coating at the bottom of the can after 10 minutes of 1% elongation under test conditions of 100°C is 50% or more, and the coverage of the inner coating is less than 200 mA in terms of ERV.

2. The squeezing can according to claim 1, wherein the resol-type phenolic resin is an m-cresol-based resol-type phenolic resin.

3. The squeezing can according to claim 1 or 2, wherein the content of the acid catalyst in the inner coating film is 0.02 parts by mass or more and less than 0.5 parts by mass per 100 parts by mass of polyester resin.

4. The squeezing can according to any one of claims 1 to 3, wherein the polyester resin contained in the inner coating film contains 20 mol% or more of one or more selected from ethylene glycol, propylene glycol, 1,4-butanediol, 2-methyl-1,3-propanediol, and diethylene glycol, when the total amount of polyhydric alcohol components constituting the polyester resin is 100 mol%.

5. A can-to-draw can according to any one of claims 1 to 4, further having an outer coating on the outer surface of the can, wherein the outer coating contains a polyester resin and a hardening agent, and the stress relaxation rate of the outer coating at the bottom of the can after 10 minutes of 1% elongation under test conditions of 100°C is higher than 40%.

6. A can made by squeezing and twisting, according to any one of claims 1 to 5, wherein 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.

7. A can made by drawing and squeezing according to any one of claims 1 to 6, wherein the thickness of the inner coating film in the center of the can body is 20 to 75% of the thickness of the inner coating film in the center of the can bottom.

8. The can-type can according to any one of claims 1 to 7, wherein the thickness ratio of the inner coating to the metal substrate (thickness of the inner coating / thickness of the metal substrate) is substantially the same at the bottom of the can and the body of the can.

9. A can made by drawing and squeezing according to any one of claims 1 to 8, wherein the thermal shrinkage rate of the inner coating film in the central part of the can body, as expressed by the following formula, is 30% or less. Heat shrinkage rate (%) = (ΔL) １ / L ０ )×100 L ０ : Initial length in the height direction of the coating film isolated from the center of the can body ΔL １ : 5.20 × 10 per unit area ５ N / m ２ When the temperature is raised from 30°C to 200°C at a heating rate of 5°C / min while applying a load, L ０ Maximum shrinkage length in the height direction of the coating film in the relevant area

10. A can made of a painted metal sheet for drawing and ironing having a coating on both sides, wherein the painted metal sheet for drawing and ironing has an inner coating on the side that becomes the inner surface of the can after drawing and ironing, containing a polyester resin and a resol-type phenolic resin or amino resin as a curing agent, and an acid catalyst in an amount of 0.01 to 3 parts by mass per 100 parts by mass of the polyester resin, and an outer coating on the side that becomes the outer surface of the can after drawing and ironing, containing a polyester resin and an amino resin as a curing agent, the stress relaxation rate of the inner coating after 10 minutes of 1% elongation under test conditions of 100°C is 50% or more, the stress relaxation rate of the outer coating after 10 minutes of 1% elongation under test conditions of 100°C is higher than 40%, and the coverage of the inner coating of the can is less than 200 mA in terms of ERV.

11. The squeezing can according to claim 10, wherein the polyester resin contained in the inner coating film contains 20 mol% or more of one or more selected from ethylene glycol, propylene glycol, 1,4-butanediol, 2-methyl-1,3-propanediol, and diethylene glycol, when the total amount of polyhydric alcohol components constituting the polyester resin is 100 mol%.

12. The can-type can according to claim 10, wherein 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.

13. The can-type can according to claim 10 or 11, wherein the thickness of the inner coating film in the center of the can body is 20 to 75% of the thickness of the inner coating film in the center of the can bottom.

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