Method for forming multilayer coating film for can inner surface, multilayer coating film for can inner surface, coated metal sheet for cans, and can body

Abstract

Provided is a method for forming a multilayer coating film for can inner surfaces, that can form a multilayer coating film that exhibits an excellent crack resistance, excellent corrosion resistance by unprocessed parts, and excellent corrosion resistance by processed parts.　The method for forming a multilayer coating film for can inner surfaces comprises a step (1): a step for forming a first coating film by coating an object to be coated with a first coating composition containing a first polyester resin (A1), and a step (2): a step for forming a second coating film by coating the first coating film formed in step (1) with a second coating composition containing a second polyester resin (A2). The glass transition temperature TgA1 of the first polyester resin (A1) is higher than the glass transition temperature TgA2 of the second polyester resin (A2).

Description

Method for forming multilayer coating film for can inner surface, multilayer coating film for can inner surface, coated metal sheet for can and can body

[0001] The present invention relates to a method for forming a multilayer coating film for the inner surface of a can, a multilayer coating film for the inner surface of a can, a coated metal sheet for a can, and a can body.

[0002] As paints for the inner surface of cans, various paint compositions such as epoxy-based, polyvinyl chloride-based and polyester-based paints have been used from the viewpoints of coating film performance such as corrosion resistance and ease of painting.

[0003] In particular, coating compositions containing, as a base resin, an epoxy resin produced using raw materials containing bisphenol A and the like have been widely and generally used.

[0004] However, from the perspective of environmental impact, there is a need for coating compositions for the interior surfaces of cans that do not use raw materials containing bisphenol A (BPA), including raw materials that may contain residual levels of BPA.

[0005] Patent Document 1 discloses a coated metal sheet having at least two coating layers on a metal sheet, characterized in that a coating film (1) containing a polyester (A) having a glass transition temperature of −15° C. or more and 30° C. or less is provided on the metal sheet, and a coating film (2) containing a polyester (B) having a glass transition temperature of 35° C. or more and less than 60° C. is further provided on the coating film (1). It is described that this coated metal sheet can provide a coated metal sheet having excellent content resistance (corrosion resistance, hydrogen sulfide resistance), retort resistance, and processability, and is suitable for use on the inner surface of a food can, without using BPA-derived raw materials.

[0006] JP 2017-61088 A

[0007] Although the technology described in Patent Document 1 does not use raw materials derived from BPA, crack resistance is sometimes insufficient.

[0008] An object of the present invention is to provide a method for forming a multilayer coating film for the inner surface of a can, which can form a multilayer coating film that has excellent crack resistance, corrosion resistance in unprocessed parts, and corrosion resistance in processed parts.

[0009] As a result of extensive research into achieving the above object, the present inventors have discovered a method for forming a multi-layer coating film for the inner surface of a can, comprising: step (1): applying a first coating composition containing a first polyester resin (A1) to an object to be coated to form a first coating film; and step (2): applying a second coating composition containing a second polyester resin (A2) to the first coating film formed in step (1) to form a second coating film, wherein the glass transition temperature Tg Ａ１ is the glass transition temperature Tg of the second polyester resin (A2). Ａ２ It has been found that the above object can be achieved by a method for forming a multi-layer coating film for the inner surface of a can, which has a coating strength higher than that of the conventional method.

[0010] That is, the present invention relates to the following items <1> to <14>: <1> A method for forming a multilayer coating film for the inner surface of a can, comprising: step (1): applying a first coating composition containing a first polyester resin (A1) to an object to be coated to form a first coating film; and step (2): applying a second coating composition containing a second polyester resin (A2) to the first coating film formed in step 1 to form a second coating film, wherein the glass transition temperature Tg of the first polyester resin (A1) is 100°C. Ａ１ is the glass transition temperature Tg of the second polyester resin (A2). Ａ２ <2> A method for forming a multi-layer coating film for the inner surface of a can, wherein the glass transition temperature Tg of the first polyester resin (A1) is higher than or equal to Ａ１ The method for forming a multi-layer coating film according to <1>, wherein the glass transition temperature Tg of the second polyester resin (A2) is in the range of 30 to 90°C. Ａ２ The method for forming a multi-layer coating film according to <1> or <2>, wherein the glass transition temperature Tg of the first polyester resin (A1) is in the range of 0 to 30°C. Ａ１ and the glass transition temperature Tg of the second polyester resin (A2). Ａ２ and the following formula (1): 5°C≦Tg Ａ１ -Tg Ａ２<90°C The method for forming a multi-layer coating film according to any one of <1> to <3>, which satisfies the following formula (1): <5> The method for forming a multi-layer coating film according to any one of <1> to <4>, wherein the first coating composition further comprises a curing agent (B1), and the curing agent (B1) comprises a resol-type phenolic resin (B11). <6> The method for forming a multi-layer coating film for the inner surface of a can according to <5>, wherein the curing agent (B1) comprises a resol-type phenolic resin (B11) and an amino resin (B12). <7> The method for forming a multi-layer coating film for the inner surface of a can according to any one of <1> to <6>, wherein the second coating composition further comprises a curing agent (B2), and the curing agent (B2) comprises a resol-type phenolic resin (B21). <8> The method for forming a multi-layer coating film for the inner surface of a can according to <7>, wherein the curing agent (B2) comprises a resol-type phenolic resin (B21) and an amino resin (B22). <9> The method for forming a multilayer coating film according to any one of <1> to <8>, wherein the second coating composition further comprises a wax (C2). <10> The method for forming a multilayer coating film according to any one of <1> to <9>, wherein the dry film thickness of the first coating film is within the range of 5 to 15 μm. <11> The method for forming a multilayer coating film according to any one of <1> to <10>, wherein the dry film thickness of the second coating film is within the range of 3 to 15 μm. <12> A multilayer coating film formed by the method for forming a multilayer coating film according to any one of <1> to <11>. <13> A coated metal sheet for a can, which is coated with the multilayer coating film according to <12>. <14> A can body having the multilayer coating film according to <12> on its inner surface.

[0011] According to the present invention, a method for forming a multilayer coating film for the inner surface of a can can be provided, which can form a multilayer coating film that has excellent crack resistance, corrosion resistance in unprocessed parts, and corrosion resistance in processed parts.

[0012] Hereinafter, the embodiments of the present invention will be described in detail, but these are merely examples of desirable embodiments, and the present invention is not limited to these contents.

[0013] [Method for forming a multilayer coating film for the inner surface of a can] The method for forming a multilayer coating film of the present invention is a method for forming a multilayer coating film for the inner surface of a can, comprising: step (1): applying a first coating composition containing a first polyester resin (A1) to an object to be coated to form a first coating film; and step (2): applying a second coating composition containing a second polyester resin (A2) to the first coating film formed in step 1 to form a second coating film, Ａ１ is the glass transition temperature Tg of the second polyester resin (A2). Ａ２ This is a method for forming a multi-layer coating film for the inside of a can, which has a higher coating resistance than that of the conventional method.

[0014] The method for forming a multilayer coating film according to the present embodiment can provide a multilayer coating film suitable for the inner surface of a can. The multilayer coating film has excellent crack resistance, corrosion resistance in the unprocessed portion, and corrosion resistance in the processed portion, and is therefore particularly suitable for the inner surface of a can lid having a complex shape.

[0015] The first polyester resin (A1) may be used alone or in combination of two or more kinds, and the second polyester resin (A2) may be used alone or in combination of two or more kinds.

[0016] The glass transition temperature of the polyester resin can be measured, for example, using a differential scanning calorimeter "DSC-50Q" (trade name, manufactured by Shimadzu Corporation), by placing a sample in a measuring cup, vacuum suctioning the sample to completely remove the solvent, and then measuring the change in heat quantity in the range of -100°C to 150°C at a temperature increase rate of 3°C / min, and defining the first change point in the baseline on the low temperature side as the static glass transition temperature.

[0017] When the first coating composition contains two or more types of the first polyester resin (A1), the glass transition temperature Tg Ａ１ is a value calculated by the following formula: 1 / Tg Ａ１ =W Ａ１１ / Tg Ａ１１ +W Ａ１２ / Tg Ａ１２ +...W Ａ１ｎ / Tg Ａ１ｎ During the ceremony, W Ａ１１ , W Ａ１２、 ...WＡ１ｎ is the mass fraction of each polyester resin, and Tg Ａ１１ , Tg Ａ１２ ...Tg Ａ１ｎ is the glass transition temperature Tg (K) of each polyester resin.

[0018] In addition, when the second coating composition contains two or more types of the second polyester resin (A2), the glass transition temperature Tg Ａ２ is a value calculated by the following formula: 1 / Tg Ａ２ =W Ａ２１ / Tg Ａ２１ +W Ａ２２ / Tg Ａ２２ +...W Ａ２ｎ / Tg Ａ２ｎ During the ceremony, W Ａ２１ , W Ａ２２、 ...W Ａ２ｎ is the mass fraction of each polyester resin, and Tg Ａ２１ , Tg Ａ２２ ...Tg Ａ２ｎ is the glass transition temperature Tg (K) of each polyester resin.

[0019] The glass transition temperature Tg of the first polyester resin (A1) Ａ１ is the glass transition temperature Tg of the second polyester resin (A2). Ａ２ If it is not higher than 100%, the crack resistance of the multi-layer coating film formed, the corrosion resistance of the processed portion and / or the corrosion resistance of the unprocessed portion will be insufficient, which is not preferable.

[0020] The glass transition temperature Tg of the first polyester resin (A1) Ａ１ and the glass transition temperature Tg of the second polyester resin (A2). Ａ２ is determined from the viewpoint of the crack resistance, workability, and corrosion resistance of the unprocessed portion of the multilayer coating film to be formed, by satisfying the following formula (1): 5°C≦Tg Ａ１ -Tg Ａ２ It is preferable that the following formula (1) is satisfied: 20°C≦Tg Ａ１ -Tg Ａ２ It is more preferable that Tg ≦80°C is satisfied, and 30°C ≦ Tg Ａ１ -Tg Ａ２ It is more preferable that the temperature satisfies the condition of ≦70°C.

[0021] As a method for applying the first and second coating compositions to a substrate using the multilayer coating film of this embodiment, various known methods can be applied, such as roll coater coating, spray coating, dip coating, electrodeposition coating, etc. Among these, roll coater coating or spray coating is preferred, and roll coater coating is particularly preferred.

[0022] The dry film thickness of the first coating film is preferably in the range of 5 to 15 μm, more preferably in the range of 6 to 12 μm, and even more preferably in the range of 7 to 10 μm, from the viewpoint of the corrosion resistance and cracking resistance of the unprocessed portion of the formed multilayer coating film.

[0023] As used herein, dry film thickness is determined by measuring with an electromagnetic film thickness gauge.

[0024] As for the drying conditions for the first coating film, the maximum temperature that the material reaches is preferably within a range of 120 to 300° C., and more preferably within a range of 200 to 280° C. Furthermore, the drying time is preferably within a range of 10 seconds to 30 minutes, and more preferably within a range of 15 seconds to 10 minutes.

[0025] The dry film thickness of the second coating film is preferably in the range of 3 to 15 μm, more preferably in the range of 5 to 12 μm, and even more preferably in the range of 7 to 10 μm, from the viewpoints of the crack resistance of the formed multilayer coating film, the corrosion resistance and the cracking resistance of the processed part, etc.

[0026] It is also preferable that the thickness of the first coating film is greater than the thickness of the second coating film.

[0027] As for the drying conditions for the second coating film, the maximum temperature that the material reaches is preferably within the range of 120 to 300° C., and more preferably within the range of 200 to 280° C. Furthermore, the drying time is preferably within the range of 10 seconds to 30 minutes, and more preferably within the range of 15 seconds to 10 minutes.

[0028] [Substrate] Examples of the substrate to be coated in the present disclosure include untreated or surface-treated metal plates such as aluminum plates, steel plates, and tin plates, metal plates coated with various primer paints, and metal plates processed into can bodies or the like.

[0029] Examples of can forms on which a multilayer coating film can be formed by the multilayer coating film forming method of this embodiment include two-piece cans consisting of two parts, a lid and a body part integrated with the bottom, three-piece cans consisting of three parts, a lid, a bottom, and a body, and bottle-shaped cans.The coated metal sheet for cans obtained by forming a multilayer coating film by the multilayer coating film forming method of this embodiment can be used in each of the above parts with the multilayer coating film facing the inside of the can.A bottle-shaped can is a can in the shape of a bottle, with a screw-type cap on the can body.

[0030] [First Coating Composition] The first coating composition of the present disclosure is a coating composition containing a first polyester resin (A1).

[0031] [First Polyester Resin (A1)] The first polyester resin (A1) may be any of an oil-free polyester resin, an alkyd resin, or a modified product of these resins, such as a urethane-modified polyester resin or a urethane-modified alkyd resin, and among these, an oil-free polyester resin is preferably used. The oil-free polyester resin refers to a polyester resin that does not contain fatty acids.

[0032] The oil-free polyester resin is mainly an ester of a polybasic acid and a polyhydric alcohol.

[0033] The polybasic acid is primarily one or more dibasic acids selected from, for example, phthalic anhydride, isophthalic acid, terephthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, hexahydroterephthalic acid, succinic acid, fumaric acid, adipic acid, sebacic acid, maleic anhydride, etc. Trivalent or higher polybasic acids such as trimellitic anhydride, methylcyclohexene tricarboxylic acid, pyromellitic anhydride, etc. may also be used in place of or in addition to the dibasic acid. These polybasic acid components may be used alone or in combination of two or more.

[0034] If necessary, monobasic acids such as benzoic acid, crotonic acid, and pt-butylbenzoic acid can be used in combination with polybasic acids.

[0035] The polyhydric alcohol component is typically a dihydric alcohol such as ethylene glycol, diethylene glycol, 1,2-propylene glycol, 1,3-propanediol, 1,4-butanediol, neopentyl glycol, 3-methylpentanediol, 1,4-hexanediol, 1,6-hexanediol, 2-methyl-1,3-propanediol, 2-ethyl-2-butyl-1,3-propanediol, or 1,4-dimethylolcyclohexane. Alternatively, trihydric or higher polyhydric alcohols such as glycerin, trimethylolethane, trimethylolpropane, or pentaerythritol can be used instead of or in addition to the dihydric alcohol. These polyhydric alcohols can be used alone or in combination. The esterification reaction of the polybasic acid and polyhydric alcohol components can be carried out by known methods.

[0036] The oil-free polyester resin can also be obtained by carrying out a transesterification reaction using a lower alkyl ester of a polybasic acid (e.g., methyl ester, ethyl ester, etc.) instead of the polybasic acid in the esterification reaction. The transesterification reaction of both the lower alkyl ester of a polybasic acid and the polyhydric alcohol can be carried out by a known method.

[0037] The alkyd resin is a resin obtained by reacting the acid component and alcohol component of the oil-free polyester resin with an oil fatty acid by a known method, such as coconut oil fatty acid, soybean oil fatty acid, linseed oil fatty acid, safflower oil fatty acid, tall oil fatty acid, dehydrated castor oil fatty acid, tung oil fatty acid, etc.

[0038] The urethane-modified polyester resin is a resin obtained by reacting the oil-free polyester resin or a low-molecular-weight oil-free polyester resin obtained by reacting an acid component and an alcohol component during the production of the oil-free polyester resin, with a polyisocyanate compound by a known method.

[0039] The urethane-modified alkyd resin is a resin obtained by reacting the above alkyd resin or a low-molecular-weight alkyd resin obtained by reacting each component in the production of the above alkyd resin with a polyisocyanate compound by a known method.

[0040] Examples of polyisocyanate compounds used in producing the urethane-modified polyester resin and the urethane-modified alkyd resin include hexamethylene diisocyanate, isophorone diisocyanate, xylylene diisocyanate, tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 4,4'-methylenebis(cyclohexyl isocyanate), and 2,4,6-triisocyanatotoluene.

[0041] The glass transition temperature Tg of the first polyester resin (A1) Ａ１ From the viewpoint of the corrosion resistance and crack resistance of the unprocessed portion of the multilayer coating film to be formed and the corrosion resistance of the processed portion, the temperature is preferably in the range of 30 to 90°C, more preferably in the range of 40 to 80°C, and even more preferably in the range of 45 to 70°C.

[0042] The number average molecular weight of the first polyester resin (A1) is preferably within the range of 3,000 to 100,000, more preferably within the range of 8,000 to 50,000, and even more preferably within the range of 10,000 to 30,000, from the viewpoints of the corrosion resistance and crack resistance of the unprocessed portion of the multilayer coating film to be formed.

[0043] The hydroxyl value of the first polyester resin (A1) is preferably within the range of 0.5 to 40 mgKOH / g, more preferably within the range of 3 to 20 mgKOH / g, from the viewpoint of the corrosion resistance and crack resistance of the unprocessed portion of the multilayer coating film to be formed.

[0044] In this specification, the hydroxyl value means the number of milligrams of potassium hydroxide equivalent to the hydroxyl groups in 1 g of sample.

[0045] In this specification, the number average molecular weight is a value obtained by converting the number average molecular weight measured using a gel permeation chromatograph ("HLC8120GPC" manufactured by Tosoh Corporation) to the number average molecular weight of polystyrene. Four columns, "TSKgel G-4000HxL," "TSKgel G-3000HxL," "TSKgel G-2500HxL," and "TSKgel G-2000HxL" (all manufactured by Tosoh Corporation, trade names), were used, and the measurement was performed under the following conditions: mobile phase: tetrahydrofuran, measurement temperature: 40°C, flow rate: 1 mL / min, and detector: RI.

[0046] From the viewpoint of corrosion resistance and processability of the unprocessed portion of the multilayer coating film to be formed, the content of the first polyester resin (A1) is preferably in the range of 50 to 100 parts by mass, more preferably in the range of 65 to 97 parts by mass, and even more preferably in the range of 70 to 95 parts by mass, based on the total resin solid content in the first coating composition.

[0047] In this specification, the term "solid content" refers to non-volatile components such as resins, curing agents, and pigments that remain after drying for 1 hour at 110° C. The solid content can be determined, for example, by weighing a sample into a heat-resistant container such as an aluminum foil cup, spreading the sample on the bottom of the container, drying it for 1 hour at 110° C., and weighing the mass of the components remaining after drying.

[0048] In addition, in this specification, the "solid content concentration" means the mass ratio of the solid content in the composition. Therefore, for example, the solid content concentration of the composition can be calculated by weighing out the composition into a heat-resistant container such as an aluminum foil cup, spreading the composition on the bottom of the container, drying it at 110°C for 1 hour, weighing the mass of the components in the composition remaining after drying, and determining the ratio of the mass of the components remaining after drying to the total mass of the composition before drying.

[0049] In this specification, the term "total resin solid content" refers to the total solid mass of the resin and the curing agent.

[0050] [Curing Agent (B1)] The first coating composition of this embodiment preferably contains a curing agent (B1).

[0051] When the first polyester resin (A1) contains a reactive functional group-containing polyester resin, the curing agent (B1) is a compound capable of reacting with a reactive functional group in the reactive functional group-containing polyester resin and capable of forming a crosslinked structure by the reaction. It is preferred that the reactive functional group in the reactive functional group-containing polyester resin is a hydroxyl group, and that the curing agent is a compound reactive with the hydroxyl group.

[0052] Specific examples of the curing agent (B1) that can be suitably used include resol-type phenolic resin (B11), amino resin (B12), polyisocyanate compounds, etc. Among these, from the viewpoint of the corrosion resistance of the unprocessed portion of the multilayer coating film to be formed, it is preferable to contain resol-type phenolic resin (B11), and it is more preferable to contain resol-type phenolic resin (B11) and amino resin (B12).

[0053] The resol type phenolic resin (B11) is a resin obtained by condensation reaction of a phenol component and a formaldehyde component in the presence of an alkali catalyst.

[0054] Examples of the phenol component include difunctional phenols such as o-cresol, p-cresol, p-tert-butylphenol, p-ethylphenol, 2,3-xylenol, and 2,5-xylenol; trifunctional phenols such as m-cresol, phenol, m-ethylphenol, 3,5-xylenol, and m-methoxyphenol; and tetrafunctional phenols such as bisphenol A and bisphenol F. These can be used alone or in combination of two or more.

[0055] Of the above, it is preferable not to use bisphenol A in the first coating composition of this embodiment, from the viewpoint of legal restrictions on the environment, etc.

[0056] Examples of the formaldehyde component include formaldehyde, paraformaldehyde, and trioxane, and these can be used alone or in combination of two or more.

[0057] When the first coating composition of this embodiment contains the resol type phenolic resin (B11), the content of the resol type phenolic resin (B11) is preferably in the range of 50 to 100 mass %, more preferably in the range of 60 to 99 mass %, and even more preferably in the range of 70 to 98 mass %, based on the total solid content of the curing agent (B1), from the viewpoint of the corrosion resistance of the unprocessed portion of the formed multilayer coating film, etc.

[0058] The amino resin (B12) is a condensation product of an aldehyde such as formaldehyde, acetaldehyde, crotonaldehyde, or benzaldehyde with an amino- or amide-group-containing substance such as urea, melamine, or benzoguanamine, and may be alkyl-etherified with an alcohol, such as a monohydric alcohol such as methanol, ethanol, propanol, butanol, hexanol, benzyl alcohol, cyclohexanol, or ethoxyethanol.

[0059] Specific examples of the amino resin (B12) include benzoguanamine-formaldehyde resin (B121), melamine-formaldehyde resin (B122), and urea-formaldehyde resin.

[0060] As the amino resin (B12), benzoguanamine-formaldehyde resin (B121) can be preferably used from the viewpoint of crack resistance of the multi-layer coating film to be formed.

[0061] When the first coating composition of the present embodiment contains the benzoguanamine-formaldehyde resin (B121), the content of the benzoguanamine-formaldehyde resin (B121) is preferably in the range of 0.5 to 50 mass%, more preferably in the range of 1 to 20 mass%, and even more preferably in the range of 2 to 10 mass%, based on the total solid content of the curing agent (B1), from the viewpoint of hygiene and the like.

[0062] When the first coating composition of this embodiment contains the amino resin (B12), the content of the amino resin (B12) is preferably in the range of 0.5 to 50 mass %, more preferably in the range of 1 to 20 mass %, and even more preferably in the range of 2 to 10 mass %, based on the total solid content of the curing agent (B1), from the viewpoint of the corrosion resistance of the unprocessed portion of the multilayer coating film to be formed, etc.

[0063] When the first coating composition of this embodiment contains the curing agent (B1), the content of the curing agent (B1) is preferably in the range of 1 to 50 mass %, more preferably in the range of 5 to 40 mass %, and even more preferably in the range of 10 to 30 mass %, based on the total resin solid content in the first coating composition, from the viewpoint of the corrosion resistance and crack resistance of the unprocessed part of the formed multilayer coating film.

[0064] The first coating composition of the present embodiment may further contain, as necessary, a solvent, a resin other than the first polyester resin (A1), a catalyst, a pigment, an anti-aggregating agent, an anti-foaming agent, a leveling agent, and the like.

[0065] Examples of the solvent include alkyl alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, isobutanol, amyl alcohol, n-hexanol, octanol, and 2-ethylhexyl alcohol; glycol ethers such as methyl cellosolve, ethyl cellosolve, propyl cellosolve, butyl cellosolve, hexyl cellosolve, methyl carbitol, ethyl carbitol, and butyl carbitol; or glycol ether esters such as methyl cellosolve acetate and ethyl cellosolve acetate; dioxane, dimethylformamide, tetrahydrofuran, methyl ethyl ketone, methyl isobutyl ketone, and diacetone alcohol. These may be used alone or in combination of two or more.

[0066] Examples of resins other than the first polyester resin (A1) include ethylene-polymerizable unsaturated carboxylic acid copolymers, ethylene-polymerizable unsaturated carboxylic acid copolymer ionomers, and the like.

[0067] The above-mentioned ethylene-polymerizable unsaturated carboxylic acid copolymer and ethylene-polymerizable unsaturated carboxylic acid copolymer ionomer can be preferably used from the viewpoint of improving the processability of the multilayer coating film to be formed.

[0068] As the catalyst, for example, a sulfonic acid compound or an amine neutralized product of a sulfonic acid compound is preferably used. Representative examples of sulfonic acid compounds include p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, and dinonylnaphthalenedisulfonic acid. The amine in the amine neutralized product of a sulfonic acid compound may be any of a primary amine, a secondary amine, and a tertiary amine. Of these, from the viewpoint of corrosion resistance of the unprocessed portion of the multilayer coating film to be formed, an amine neutralized product of p-toluenesulfonic acid and / or an amine neutralized product of dodecylbenzenesulfonic acid is preferred.

[0069] When the first coating composition of this embodiment contains the above-mentioned catalyst, the content of the catalyst is preferably in the range of 0.1 to 5 parts by mass, and more preferably in the range of 0.2 to 2 parts by mass, based on 100 parts by mass of the total resin solids in the first coating composition, from the viewpoint of corrosion resistance of the unprocessed part of the formed multilayer coating film, etc.

[0070] As the pigment, color pigments (for example, titanium oxide), extender pigments, etc., known in the paint field can be used.

[0071] [Second Coating Composition] The second coating composition of the present disclosure is a coating composition containing a second polyester resin (A2).

[0072] [Second Polyester Resin (A2)] As the second polyester resin (A2), for example, the polyester resins exemplified for the first polyester resin (A1) can be used.

[0073] The glass transition temperature Tg of the second polyester resin (A2) Ａ２ From the viewpoint of the corrosion resistance and crack resistance of the unprocessed portion of the multilayer coating film to be formed, and the corrosion resistance and lubricity of the processed portion, the temperature is preferably in the range of 0 to 30°C, more preferably in the range of 5 to 20°C, and even more preferably in the range of 8 to 17°C.

[0074] The number average molecular weight of the second polyester resin (A2) is preferably in the range of 3,000 to 100,000, more preferably in the range of 8,000 to 50,000, and even more preferably in the range of 10,000 to 30,000, from the viewpoints of the corrosion resistance and crack resistance of the unprocessed portion of the multilayer coating film to be formed.

[0075] The hydroxyl value of the second polyester resin (A2) is preferably within the range of 0.5 to 40 mgKOH / g, more preferably within the range of 3 to 20 mgKOH / g, from the viewpoint of the corrosion resistance and crack resistance of the unprocessed portion of the multilayer coating film to be formed.

[0076] From the viewpoint of corrosion resistance and processability of the unprocessed portion of the multilayer coating film to be formed, the content of the second polyester resin (A2) is preferably in the range of 50 to 100 parts by mass, more preferably in the range of 65 to 97 parts by mass, and even more preferably in the range of 70 to 95 parts by mass, based on the total resin solid content in the second coating composition.

[0077] [Curing Agent (B2)] The second coating composition of this embodiment preferably contains a curing agent (B2).

[0078] When the second polyester resin (A2) contains a reactive functional group-containing polyester resin, the curing agent (B2) is a compound capable of reacting with the reactive functional group in the reactive functional group-containing polyester resin and capable of forming a crosslinked structure by the reaction. It is preferred that the reactive functional group in the reactive functional group-containing polyester resin is a hydroxyl group, and that the curing agent is a compound reactive with the hydroxyl group.

[0079] Specific examples of the curing agent (B2) that can be suitably used include resol-type phenolic resin (B21), amino resin (B22), polyisocyanate compound, etc. Among these, from the viewpoint of the corrosion resistance of the unprocessed portion of the multilayer coating film to be formed, it is preferable to contain resol-type phenolic resin (B21), and it is more preferable to contain resol-type phenolic resin (B21) and amino resin (B22).

[0080] As the resol type phenolic resin (B21), for example, the resol resins exemplified in the resol type phenolic resin (B11) can be used.

[0081] When the second coating composition of this embodiment contains the resol type phenolic resin (B21), the content of the resol type phenolic resin (B21) is preferably in the range of 50 to 100 mass %, more preferably in the range of 60 to 99 mass %, and even more preferably in the range of 70 to 98 mass %, based on the total solid content of the curing agent (B2), from the viewpoint of the corrosion resistance of the unprocessed portion of the multilayer coating film to be formed, etc.

[0082] The amino resin (B22) is a condensation product of an aldehyde such as formaldehyde, acetaldehyde, crotonaldehyde, or benzaldehyde with an amino- or amide-group-containing substance such as urea, melamine, or benzoguanamine, and may be alkyl-etherified with an alcohol, such as a monohydric alcohol such as methanol, ethanol, propanol, butanol, hexanol, benzyl alcohol, cyclohexanol, or ethoxyethanol.

[0083] Specific examples of the amino resin (B22) include benzoguanamine-formaldehyde resin (B221), melamine-formaldehyde resin (B222), and urea-formaldehyde resin.

[0084] As the amino resin (B22), benzoguanamine-formaldehyde resin (B221) can be preferably used from the viewpoint of crack resistance of the multi-layer coating film to be formed.

[0085] When the second coating composition of the present embodiment contains the benzoguanamine-formaldehyde resin (B221), the content of the benzoguanamine-formaldehyde resin (B221) is preferably in the range of 0.5 to 50 mass%, more preferably in the range of 1 to 20 mass%, and even more preferably in the range of 2 to 10 mass%, based on the total solid content of the curing agent (B2), from the viewpoint of hygiene, etc.

[0086] When the second coating composition of this embodiment contains the amino resin (B22), the content of the amino resin (B22) is preferably in the range of 0.5 to 50 mass %, more preferably in the range of 1 to 20 mass %, and even more preferably in the range of 2 to 10 mass %, based on the total solid content of the curing agent (B2), from the viewpoint of the corrosion resistance of the unprocessed portion of the multilayer coating film to be formed, etc.

[0087] When the second coating composition of this embodiment contains the curing agent (B2), the content of the curing agent (B2) is preferably in the range of 1 to 50 mass %, more preferably in the range of 5 to 40 mass %, and even more preferably in the range of 10 to 30 mass %, based on the total resin solid content in the second coating composition, from the viewpoint of the corrosion resistance and crack resistance of the unprocessed part of the formed multilayer coating film.

[0088] [Wax (C2)] The second coating composition of this embodiment preferably contains wax (C2) from the viewpoint of the lubricity of the multi-layer coating film to be formed.

[0089] Examples of the wax (C2) that can be used include natural waxes, synthetic waxes, glycerides, waxes, and oxides or acid-modified products thereof. Examples of natural waxes include hydrogenated fats and oils obtained by hydrogenating beef tallow or lard, animal and vegetable waxes such as lanolin wax, carnauba wax, candelilla wax, rice wax, Japan wax, jojoba wax, and shellac, and mineral waxes such as paraffin wax, microcrystalline wax, montan wax, and sericin wax. Examples of synthetic waxes include polyethylene wax, polypropylene wax, polytetrafluoroethylene wax, and silicone wax. Examples of waxes include beeswax, whale wax, and hydrogenated whale wax. These waxes can be used alone or in combination of two or more.

[0090] As the wax (C), polyethylene wax can be preferably used from the viewpoint of the lubricity of the multi-layer coating film to be formed.

[0091] When the second coating composition of this embodiment contains the wax (C), the content of the wax (C) is preferably in the range of 0.1 to 5 parts by mass, more preferably in the range of 0.2 to 2 parts by mass, and even more preferably in the range of 0.25 to 1 part by mass, based on 100 parts by mass of the total resin solids in the second coating composition, from the viewpoint of the lubricity of the formed multilayer coating film, etc.

[0092] The second coating composition of the present embodiment may further contain, as necessary, a solvent, a resin other than the second polyester resin (A2), a catalyst, a pigment, an anti-aggregating agent, an anti-foaming agent, a leveling agent, and the like.

[0093] Examples of the solvent include alkyl alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol, isobutanol, amyl alcohol, n-hexanol, octanol, and 2-ethylhexyl alcohol; glycol ethers such as methyl cellosolve, ethyl cellosolve, propyl cellosolve, butyl cellosolve, hexyl cellosolve, methyl carbitol, ethyl carbitol, and butyl carbitol; or glycol ether esters such as methyl cellosolve acetate and ethyl cellosolve acetate; dioxane, dimethylformamide, tetrahydrofuran, methyl ethyl ketone, methyl isobutyl ketone, and diacetone alcohol. These may be used alone or in combination of two or more.

[0094] Examples of resins other than the first polyester resin (A2) include ethylene-polymerizable unsaturated carboxylic acid copolymers, ethylene-polymerizable unsaturated carboxylic acid copolymer ionomers, and the like.

[0095] The above-mentioned ethylene-polymerizable unsaturated carboxylic acid copolymer and ethylene-polymerizable unsaturated carboxylic acid copolymer ionomer can be preferably used from the viewpoint of improving the processability of the multilayer coating film to be formed.

[0096] As the catalyst, for example, a sulfonic acid compound or an amine neutralized product of a sulfonic acid compound is preferably used. Representative examples of sulfonic acid compounds include p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, and dinonylnaphthalenedisulfonic acid. The amine in the amine neutralized product of a sulfonic acid compound may be any of a primary amine, a secondary amine, and a tertiary amine. Of these, from the viewpoint of corrosion resistance of the unprocessed portion of the multilayer coating film to be formed, an amine neutralized product of p-toluenesulfonic acid and / or an amine neutralized product of dodecylbenzenesulfonic acid is preferred.

[0097] When the second coating composition of this embodiment contains the catalyst, the content of the catalyst is preferably in the range of 0.1 to 5 parts by mass, and more preferably in the range of 0.2 to 2 parts by mass, based on 100 parts by mass of the total resin solids in the second coating composition, from the viewpoint of corrosion resistance of the unprocessed portion of the multilayer coating film to be formed, etc. As the pigment, color pigments (e.g., titanium oxide), extender pigments, etc. known in the paint field can be used.

[0098] The present invention will be explained in more detail below with reference to Production Examples, Examples, and Comparative Examples. However, the present invention is not limited thereto. In each example, "parts" and "%" are based on mass unless otherwise specified. Furthermore, the film thickness of the coating film is based on the cured coating film.

[0099] Preparation of Polyester Resin Solution Preparation Example 1 "Vylon GK-360" (trade name, manufactured by Toyobo Co., Ltd., polyester resin, glass transition temperature (hereinafter referred to as Tg) 56°C, number average molecular weight 16,000, hydroxyl value 7 mgKOH / g, solids concentration 100%) was diluted with a 50 / 50 mixed solvent of methyl ethyl ketone / cyclohexanone to obtain a polyester resin (A1-1) solution with a solids content of 30% in a stirring vessel.

[0100] Production Example 2 "Uralac SH978 S1E5-50" (trade name, manufactured by Covestro Corporation, polyester resin, Tg 20°C, number average molecular weight 10,000, hydroxyl value 12 mgKOH / g, solids concentration 50%) was diluted with a 50 / 50 mixed solvent of methyl ethyl ketone / cyclohexanone to obtain a polyester resin (A1-2) solution with a solids content of 30% in a stirring vessel.

[0101] Production Example 3 "ES-750" (trade name, manufactured by SK Chemicals Corporation, polyester resin, Tg 35°C, number average molecular weight 8000, hydroxyl value 47 mgKOH / g, solids concentration 100%) was diluted with a 50 / 50 mixed solvent of methyl ethyl ketone / cyclohexanone to obtain a polyester resin (A1-3) solution with a solids content of 30% in a stirring vessel.

[0102] Production Example 4 "Vylon GK-888" (trade name, manufactured by Toyobo Co., Ltd., polyester resin, Tg 88°C, number average molecular weight 15,000, hydroxyl value 5 mgKOH / g, solids concentration 100%) was diluted with a 50 / 50 mixed solvent of methyl ethyl ketone / cyclohexanone to obtain a polyester resin (A1-4) solution with a solids content of 30% in a stirring vessel.

[0103] Production Example 5 "DYNAPOL L914" (trade name, manufactured by Evonik Corporation, polyester resin, Tg 100°C, number average molecular weight 15,000, hydroxyl value 7 mgKOH / g, solids concentration 100%) was diluted with a 50 / 50 mixed solvent of methyl ethyl ketone / cyclohexanone to obtain a polyester resin (A1-5) solution with a solids content of 30% in a stirring vessel.

[0104] Production Example 6 "ES-680" (trade name, manufactured by SK Chemical Corporation, polyester resin, Tg 57°C, number average molecular weight 20,000, hydroxyl value 5 mgKOH / g, solids concentration 100%) was diluted with a 50 / 50 mixed solvent of methyl ethyl ketone / cyclohexanone to obtain a polyester resin (A1-6) solution with a solids content of 30% in a stirring vessel.

[0105] Production Example 7: "Vylon 300" (trade name, manufactured by Toyobo Co., Ltd., polyester resin, Tg 7°C, number average molecular weight 23,000, hydroxyl value 5 mgKOH / g, solids concentration 100%) was diluted with a 50 / 50 mixed solvent of methyl ethyl ketone / cyclohexanone to obtain a polyester resin (A1-7) solution with a solids content of 30% in a stirring vessel.

[0106] Production Example 8: "Vylon GK-680" (trade name, manufactured by Toyobo Co., Ltd., polyester resin, Tg 10°C, number average molecular weight 6000, hydroxyl value 21 mgKOH / g, solids concentration 100%) was diluted with a 50 / 50 mixed solvent of methyl ethyl ketone / cyclohexanone to obtain a polyester resin (A1-8) solution with a solids content of 30% in a stirring vessel.

[0107] Production Example 9: In a stirring vessel, "Vylon 516" (trade name, manufactured by Toyobo Co., Ltd., polyester resin, Tg -17°C, number average molecular weight 30,000, solid content 100%) was diluted with a 50 / 50 mixed solvent of methyl ethyl ketone / cyclohexanone to obtain a polyester resin (A1-9) solution with a solid content of 30%.

[0108] Production Example 10: "ES-410" (trade name, manufactured by SK Chemical Corporation, polyester resin, Tg 47°C, number average molecular weight 16,000, hydroxyl value 6 mgKOH / g, solids concentration 100%) was diluted with a 50 / 50 mixed solvent of methyl ethyl ketone / cyclohexanone to obtain a polyester resin (A1-10) solution with a solids content of 30% in a stirring vessel.

[0109] Preparation of First Coating Composition Preparation Example 11 267 parts (solid content 80 parts) of the polyester resin (A1-1) solution obtained in Preparation Example 1, 38 parts (solid content 19 parts) of "PR-55317" (trade name, manufactured by Sumitomo Bakelite Co., Ltd., resol-type phenolic resin, solid content 50%), 1 part (solid content 1 part) of "CYMEL 1123" (trade name, manufactured by Allnex Co., Ltd., benzoguanamine-formaldehyde resin, solid content 98%), and 2 parts (solid content 0.5 parts) of "NACURE 5225" (trade name, manufactured by KING INDUSTRIES Co., Ltd., amine-neutralized solution of dodecylbenzenesulfonic acid, dodecylbenzenesulfonic acid content 25%) were uniformly mixed, and the mixture was diluted and stirred with a mixed solvent of methyl ethyl ketone / cyclohexanone = 50 / 50 (weight ratio) so that the paint solid content concentration was 25%, to obtain a first coating composition (1-1).

[0110] Production Examples 12 to 24 Coating compositions (1-2) to (1-14) were obtained in the same manner as in Production Example 11, except that the formulations in Production Example 11 were as shown in Table 1 below.

[0111]

[0112] The components listed in the table are as follows: (Note 1) "CYMEL 303LF": Trade name, manufactured by Allnex, melamine-formaldehyde resin, solid content 99%

[0113] Preparation of Second Coating Composition Preparation Example 25 267 parts (solid content 80 parts) of the polyester resin (A1-8) solution obtained in Preparation Example 8, 38 parts (solid content 19 parts) of "PR-55317" (trade name, manufactured by Sumitomo Bakelite Co., Ltd., resol-type phenolic resin, solid content 50%), 1 part (solid content 1 part) of "CYMEL 1123" (trade name, manufactured by Allnex Corporation, benzoguanamine-formaldehyde resin, solid content 98%), 2.5 parts (solid content 0.5 parts) of "Lanco Gridd KX" (trade name, manufactured by Lubrizol Corporation, polyethylene wax, solid content 20%), and "NACURE 5225" (trade name, KING Two parts (0.5 parts solids) of an amine-neutralized solution of dodecylbenzenesulfonic acid (dodecylbenzenesulfonic acid content: 25%) manufactured by INDUSTRIES were uniformly mixed, and the mixture was diluted with a mixed solvent of methyl ethyl ketone / cyclohexanone = 50 / 50 (weight ratio) so that the paint solids concentration was 25%, followed by stirring, to obtain a second paint composition (2-1).

[0114] Production Examples 26 to 41 Coating compositions (2-2) to (2-17) were obtained in the same manner as in Production Example 25, except that the blending compositions in Production Example 25 were as shown in Table 2 below.

[0115]

[0116] Preparation of test coated plate Example 1 The first coating composition (1-1) obtained in Production Example 11 was roll coated onto a 0.27 mm thick #5182 aluminum plate so that the dry film thickness was 9 μm, and the conveyor-transported hot air drying oven was baked under baking conditions of a material reaching maximum temperature (PMT) of 255 ° C. and a passage time in the drying oven of 20 seconds, to obtain a first coating film. Next, the second coating composition (2-1) obtained in Production Example 25 was roll coated onto the first coating film so that the dry film thickness was 9 μm, and the conveyor-transported hot air drying oven was baked under baking conditions of a material reaching maximum temperature (PMT) of 255 ° C. and a passage time in the drying oven of 20 seconds, to obtain a test coated plate No. 1 in which a second coating film was coated on the first coating film.

[0117] Examples 2 to 36, Comparative Examples 1 to 8 Test substrates Nos. 2 to 44 were obtained in the same manner as in Example 1, except that the types of the first coating composition and second coating composition, and the dry film thicknesses of the first coating film and second coating film were as shown in Table 3 below.

[0118] Evaluation of Crack Resistance: Each coated test plate obtained above was cut into a 5 cm length in the rolling direction and a 4 cm length perpendicular to the rolling direction. The 5 cm x 4 cm cut test plate was then folded in half parallel to the short sides at a point closer to one of the short sides than the midline of the two short sides. The coated test plate was placed so that the larger area was on top and the smaller area was on the bottom. In a room at 20°C, two 0.26 mm thick aluminum plates were sandwiched between the folded portions of the coated test plate specimen and placed in a special fold-type DuPont impact tester. A 1 kg iron weight with a flat contact surface was dropped from a height of 50 cm to impact the folded portion. The condition of the coating film was observed with an optical microscope, and crack resistance was evaluated according to the following evaluation criteria: A, B, and C were acceptable. The evaluation results are shown in Table 3. A: No cracks are observed at all; B: No large cracks are observed, but a few small cracks are observed; C: A few large cracks are observed; D: Significantly large cracks are observed.

[0119] Evaluation of corrosion resistance of processed parts After the above crack resistance test, each coated test plate was immersed in a mixed aqueous solution containing 3% each of citric acid, malic acid, and sodium chloride, and stored at 40°C for 2 weeks.The state of the coating film in the processed part (folded part) was then observed with an optical microscope, and the corrosion resistance of the processed part was evaluated according to the following evaluation criteria. A, B, and C are acceptable.The evaluation results are shown in Table 3.A: No blister corrosion or burnishing was observed.B: No blister corrosion was observed, but burnishing was observed.C: Slight blister corrosion was observed.D: Significant blister corrosion was observed.

[0120] Evaluation of Workability After the crack resistance test, a voltage of 6.5 V was applied to the bent tip of each coated test plate for 6 seconds, and the current value (mA) over a 20 mm width at the bent tip was measured. The workability was evaluated according to the following criteria. If the coating film has poor workability, the coating film at the bent part will crack, exposing the underlying metal plate and increasing conductivity, resulting in a high current value. A, B, and C are acceptable. The evaluation results are shown in Table 3. A: Current value less than 0.1 mA, B: Current value 0.1 mA or more and less than 0.5 mA, C: Current value 0.5 mA or more and less than 2.0 mA, D: Current value 2.0 mA or more.

[0121] Evaluation of corrosion resistance of unprocessed portion Each of the coated test panels was immersed in a mixed aqueous solution containing 3% each of citric acid, malic acid, and sodium chloride, heated in an autoclave at 125°C for 30 minutes, and stored at 40°C for 2 weeks. The state of the coating was then visually observed, and the corrosion resistance of the unprocessed portion was evaluated according to the following evaluation criteria: A, B, and C are acceptable. The evaluation results are shown in Table 3. A: No corrosion or tarnishing was observed; B: No corrosion was observed, but tarnishing was observed; C: Slight corrosion was observed; D: Significant corrosion was observed.

[0122] Evaluation of Slipperiness A rectangular parallelepiped block with a mass of 1 kg, supported on its underside by three steel balls, was placed on each of the coated test panels, and one end of the block was pulled at a rate of 150 cm / min to determine the coefficient of dynamic friction (μ value). The coefficient of dynamic friction (μ value) was measured and the slipperiness was evaluated according to the following evaluation criteria. The smaller the coefficient of dynamic friction (μ value), the better the slipperiness. A, B, and C are acceptable. The evaluation results are shown in Table 3. A: μ value less than 0.06, B: μ value 0.06 or more and less than 0.1, C: μ value 0.1 or more and less than 0.15, D: μ value 0.15 or more.

[0123] Evaluation of popping resistance Each of the above test coated panels was cut into pieces measuring 10 cm x 10 cm. The number of poppings that had occurred in the coating film of each cut test coated panel was then observed under an optical microscope, and popping resistance was evaluated according to the following evaluation criteria. A, B, and C are acceptable. The evaluation results are shown in Table 3. A: No popping was observed. B: 1 to 4 poppings were observed. C: 5 to 9 poppings were observed. D: 10 or more poppings were observed.

[0124]

[0125]

[0126]

Claims

1. A method for forming a multi-layer coating film for the inner surface of a can, comprising: step (1): applying a first coating composition containing a first polyester resin (A1) to a substrate to form a first coating film; and step (2): applying a second coating composition containing a second polyester resin (A2) to the first coating film formed in step (1) to form a second coating film, wherein the glass transition temperature Tg of the first polyester resin (A1) is 100°C. Ａ１ is the glass transition temperature Tg of the second polyester resin (A2). Ａ２ A method for forming a multi-layer coating film on the inside of a can, which has a higher coating resistance than conventional coatings.

2. The glass transition temperature Tg of the first polyester resin (A1) Ａ１ 2. The method for forming a multi-layer coating film for the inner surface of a can according to claim 1, wherein the temperature is in the range of 30 to 90°C.

3. The glass transition temperature Tg of the second polyester resin (A2) Ａ２ The method for forming a multi-layer coating film for the inner surface of a can according to claim 1 or 2, wherein the temperature is in the range of 0 to 30°C.

4. The glass transition temperature Tg of the first polyester resin (A1) Ａ１ and the glass transition temperature Tg of the second polyester resin (A2). Ａ２ and the following formula (1): 5°C≦Tg Ａ１ -Tg Ａ２ The method for forming a multi-layer coating film for the inner surface of a can according to any one of claims 1 to 3, wherein the following formula (1) is satisfied: ≦90°C.

5. A method for forming a multi-layer coating film for the inner surface of a can according to any one of claims 1 to 4, wherein the first coating composition further comprises a curing agent (B1), and the curing agent (B1) comprises a resol-type phenolic resin (B11).

6. The method for forming a multi-layer coating film for the inner surface of a can according to claim 5, wherein the curing agent (B1) comprises a resol-type phenolic resin (B11) and an amino resin (B12).

7. A method for forming a multi-layer coating film for the inner surface of a can according to any one of claims 1 to 6, wherein the second coating composition further comprises a curing agent (B2), and the curing agent (B2) comprises a resol-type phenolic resin (B21).

8. The method for forming a multi-layer coating film for the inner surface of a can according to claim 7, wherein the curing agent (B2) comprises a resol-type phenolic resin (B21) and an amino resin (B22).

9. A method for forming a multi-layer coating film for the inner surface of a can according to any one of claims 1 to 8, wherein the second coating composition further contains a wax (C2).

10. A method for forming a multi-layer coating film for the inner surface of a can according to any one of claims 1 to 9, wherein the dry film thickness of the first coating film is within the range of 5 to 15 μm.

11. A method for forming a multi-layer coating film for the inner surface of a can according to any one of claims 1 to 10, wherein the dry film thickness of the second coating film is within the range of 3 to 15 μm.

12. A multi-layer coating film for the inner surface of a can, formed by the method for forming a multi-layer coating film according to any one of claims 1 to 11.

13. A coated metal sheet for cans, which is coated with the multi-layer coating film according to claim 12.

14. A can body having the multi-layer coating film according to claim 12 on the inner surface thereof.

Images