Metal can coating composition and metal can coated therewith

Abstract

To provide a metal can coating composition free from PFAS.SOLUTION: A metal can coating composition that does not contain fluorine.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a coating composition for metal cans and a metal can coated with the coating composition. [Background technology]

[0002] Traditionally, organic fluorine compounds such as polytetrafluoroethylene (hereinafter also referred to as "PTFE") have been used in can coating compositions to impart abrasion resistance, lubricity, and other properties. Among these organic fluorine compounds, perfluoroalkyl and polyfluoroalkyl compounds are collectively referred to as "PFAS." Among PFAS, perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are particularly persistent, highly bioaccumulative, and highly mobile, and are known to persist widely on a global scale. Continued release of PFAS into the environment raises concerns about their further accumulation, potentially posing adverse effects on human health, the habitats and growth of plants and animals, and other factors through the environment and food chain. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-022004 [Patent Document 2] Japanese Patent Publication No. 2023-010488 [Patent Document 3] International Publication No. 2021 / 061677 [Patent Document 4] Japanese Patent Publication No. 2024-035828 Summary of the Invention [Problem to be solved by the invention]

[0004] Japanese Patent Publication No. 2023-022004 (Patent Document 1), Japanese Patent Publication No. 2023-010488 (Patent Document 2), International Publication No. 2021 / 061677 (Patent Document 3), and Japanese Patent Publication No. 2024-035828 (Patent Document 4) all point out the adverse effects of PFASs, including PFOS and PFOA, and teach the possibility of alternative materials. However, these documents make no mention of PFASs contained in coating compositions for metal cans. Therefore, a coating composition for metal cans that does not contain PFASs has not yet been obtained, and its development is eagerly awaited.

[0005] In view of the above circumstances, an object of the present invention is to provide a coating composition for metal cans that does not contain PFAS but has the necessary abrasion resistance, lubricity, and the like, and to provide a metal can coated with the coating composition. [Means for solving the problem]

[0006] The present inventors have conducted extensive research to solve the above problems and have arrived at the present invention. The inventors have searched for materials that can replace the organic fluorine compounds, such as PTFE, contained in conventional coating compositions for metal cans. That is, the inventors have conceived the idea of ​​eliminating organic fluorine compounds, such as PTFE, from coating compositions for metal cans by using alternative materials. As a result, they have discovered several compounds that can replace the above organic fluorine compounds and impart properties such as abrasion resistance and lubricity to coating compositions for metal cans, and have completed the present invention.

[0007] The present invention relates to a coating composition for metal cans, as described below, and to metal cans coated with the coating composition. [1] A fluorine-free coating composition for metal cans. [2] A coating composition for metal cans that does not contain organic fluorine compounds. [3] The coating composition for metal cans according to [1] or [2], which contains boron nitride. [4] The coating composition for metal cans according to [1] or [2], which contains wax. [5] The coating composition for metal cans according to [4], wherein the wax is at least one selected from the group consisting of beeswax, lanolin, spermaceti, privet wax, carnauba wax, amide wax, candelilla wax, rice wax, paraffin wax, microcrystalline wax, montan wax, polyethylene wax, polyolefin wax, and polyester wax. [6] The coating composition for metal cans according to any one of [1] to [5], which is imparted with lubricity and abrasion resistance. [7] A metal can coated with the coating composition for metal cans according to any one of [1] to [6]. [8] A metal can in which the coating composition for metal cans according to any one of [1] to [6] is applied to at least one selected from the group consisting of a can lid, a can body, a rim, a cap, and an OSS. [Effects of the Invention]

[0008] According to the present invention, there are provided a coating composition for metal cans that does not contain PFAS but has the necessary abrasion resistance, lubricity, etc., and a metal can coated with the coating composition. DETAILED DESCRIPTION OF THE INVENTION

[0009] An embodiment of the present invention (hereinafter also referred to as "this embodiment") will be described in further detail below, but the present invention is not limited thereto. In this specification, the notation in the form "A to B" means the upper and lower limits of a range (i.e., A or more and B or less), and when no unit is specified for A and only a unit is specified for B, the unit of A and the unit of B are the same. In this specification, "polyolefin wax" refers to a wax obtained by mixing two or more polyolefins (for example, a wax obtained by mixing polyethylene and polypropylene), and is distinguished from polyethylene wax and the like which contains only polyethylene as a polyolefin.

[0010] As used herein, "PFAS" refers to the general term for perfluoroalkyl and polyfluoroalkyl compounds, as described above. In particular, in this specification, "PFAS" refers to substances subject to regulation in the PFAS regulation proposal officially published by the European Chemicals Agency on March 22, 2023.

[0011] [Coating composition for metal cans] The metal can coating composition according to this embodiment does not contain fluorine or organic fluorine compounds. Because the metal can coating composition having these characteristics does not contain fluorine or organic fluorine compounds, the release of PFAS into the environment through the metal can to which it is applied is prevented. The metal can coating composition can be applied to any part of a metal can, such as the lid, can body, rim (bottom), or cap. The metal can coating composition can also be used as an OSS (Outside Sideseam Stripe: weld repair coating), etc.

[0012] As described above, the coating composition for metal cans does not contain fluorine. In particular, in the coating composition for metal cans, the fluorine is not present in the form of a fluorine atom, a fluorine atom contained in a compound, or a fluorine molecule. Furthermore, as described above, the coating composition for metal cans does not contain an organic fluorine compound. The "organic fluorine compound" not contained in the coating composition for metal cans refers to a fluorine compound containing at least one fully fluorinated methyl (CF3-) or methylene (-CF2-) carbon atom, with some exceptions, and specifically refers to PFAS as defined herein.

[0013] As used herein, "free of fluorine" or "free of organic fluorine compounds" is defined as follows: That is, it means that no fluorine is detected when elemental analysis is performed on the surface of a metal can to which the above-mentioned metal can coating composition has been applied at a magnification of 5000 times using a scanning electron microscope equipped with an energy dispersive X-ray analysis (EDS) function (for example, trade name: "JSM-IT200" manufactured by JEOL Ltd.).

[0014] <Boron nitride> The coating composition for metal cans preferably contains boron nitride, which allows the coating composition for metal cans to have various properties such as sufficient abrasion resistance and lubricity, making it suitable as a substitute for PTFE.

[0015] The boron nitride may be hexagonal boron nitride (hBN) or cubic boron nitride (cBN). The boron nitride may be a mixture of hBN and cBN. In the coating composition for metal cans, the boron nitride is preferably contained in an amount of 0.1 to 10 mass %. The boron nitride is more preferably contained in an amount of 0.2 to 5.0 mass %. The boron nitride is even more preferably contained in an amount of 0.2 to 2.0 mass %. The boron nitride may be in the form of a powder or a molded body. When the boron nitride is in the form of a powder, its average particle size may be 1 to 10 μm, preferably 3 to 7 μm, and more preferably 4 to 6 μm.

[0016] <Wax> The coating composition for metal cans preferably contains wax. In this case, the coating composition for metal cans can have various properties such as sufficient abrasion resistance and lubricity, and therefore can be suitable as a substitute for PTFE.

[0017] The wax is preferably at least one selected from the group consisting of beeswax, lanolin, spermaceti, privet wax, carnauba wax, amide wax, candelilla wax, rice wax, paraffin wax, microcrystalline wax, montan wax, polyethylene wax, and polyolefin wax. The wax is more preferably at least one selected from the group consisting of carnauba wax, amide wax, paraffin wax, microcrystalline wax, polyethylene wax, polyolefin wax, and polyester wax. The wax may be used alone or in combination of two or more selected from the group.

[0018] In the metal can coating composition, the wax is preferably contained in an amount of 0.1 to 10 mass %. The wax is more preferably contained in an amount of 0.2 to 5.0 mass %. The wax is even more preferably contained in an amount of 0.2 to 2.0 mass %. The wax may be in the form of a powder or a molded product. When the wax is in the form of a powder, its average particle size may be 1 to 10 μm, preferably 3 to 7 μm, and more preferably 4 to 6 μm.

[0019] <Lubricity and wear resistance> The coating composition for metal cans is preferably imparted with lubricity and abrasion resistance, and thus the coating composition for metal cans has lubricity and abrasion resistance without containing fluorine or an organic fluorine compound, and therefore can be suitable as a substitute for PTFE.

[0020] <Other ingredients> The coating composition for metal cans may contain, in addition to the boron nitride and wax described above, known components used in coatings without any particular limitation. Examples of components other than the boron nitride and wax that may be contained in the coating composition for metal cans include components used as lubricity-imparting agents other than the boron nitride and wax described above, and components used as curing accelerators.

[0021] (Component used as a lubricity imparting agent) Examples of components used as lubricity-imparting agents other than the boron nitride and wax include silicone-based components. Examples of the silicone-based components include dimethylpolysiloxane and modified products thereof. When a modified product of dimethylpolysiloxane is used, the modified product may be, for example, ethylene oxide, propylene oxide, epoxy, amine, or the like. The amount of these components used as lubricity-imparting agents to be added is not particularly limited, and may be any amount.

[0022] (Component used as a curing accelerator) Examples of components used as curing accelerators include acid catalysts such as monobutyl phosphate, dibutyl phosphate, diisopropyl phosphate, monooctyl phosphate, monodecyl phosphate, didecyl phosphate, metaphosphoric acid, orthophosphoric acid, and orthophosphate esters. The amount of these components used as curing accelerators to be added is not particularly limited, and may be any amount.

[0023] <Main ingredient> The metal can coating composition preferably contains a total of 0.1 to 10 mass% of the above-mentioned boron nitride or wax, a component used as a lubricity imparting agent, and a component used as a curing accelerator (hereinafter collectively referred to as "functional components"). The metal can coating composition more preferably contains 1 to 6 mass%, and even more preferably 2 to 5 mass%, of the functional components. The metal can coating composition may further contain the main components described below in addition to the functional components.

[0024] That is, the metal can coating composition can contain, as main components, a polyester resin, an amino resin, an epoxy resin, and a crosslinking agent.

[0025] (polyester resin) The metal can coating composition may contain a polyester resin. Any polyester resin known for use in coatings may be used as the polyester resin, without any particular limitations. The polyester resin may be obtained from a commercial source, or may be prepared by selecting any polycarboxylic acid and any polyhydric alcohol and synthesizing them through a polycondensation reaction.

[0026] The polycarboxylic acids and polyhydric alcohols used as raw materials for the polyester resin are not limited. The polycarboxylic acids and polyhydric alcohols may be either branched or linear, but linear is preferred. Examples of polycarboxylic acids include aromatic dicarboxylic acids, alicyclic dicarboxylic acids, and aliphatic dicarboxylic acids. Examples of aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, orthophthalic acid, 2,6-naphthalenedicarboxylic acid, 5-sodium sulfoisophthalic acid, and phthalic anhydride. Examples of alicyclic dicarboxylic acids include tetrahydrophthalic anhydride, hexahydrophthalic anhydride, and 1,4-cyclohexanedicarboxylic acid. Examples of aliphatic dicarboxylic acids include succinic acid (anhydride), fumaric acid, maleic acid (anhydride), adipic acid, sebacic acid, azelaic acid, and himic acid. Examples of trivalent or higher polycarboxylic acids include trimellitic acid (anhydride) and pyromellitic acid (anhydride).

[0027] Examples of polyhydric alcohols used as raw materials for the polyester resin include aliphatic dihydric alcohols such as ethylene glycol, diethylene glycol, propylene glycol, neopentyl glycol, 1,2-propanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2,4-trimethyl-1,3-pentanediol, 2-ethyl-1,3-hexanediol, 2-diethyl-1,3-propanediol, 1,9-nonanediol, 2-methyl-1,8-octanediol, 1,4-cyclohexanedimethanol, 3-methyl-1,5-pentanediol, bisphenol A or bisphenol F with ethylene oxide or propylene oxide, xylene glycol, and hydrogenated bisphenol A. Examples of trihydric or higher polyhydric alcohols include trimethylolethane, trimethylolpropane, glycerin, and pentaerythritol.

[0028] The number-average molecular weight of the polyester resin is preferably 2000 to 8000. When the number-average molecular weight of the polyester resin is within the above-mentioned range, the glass transition temperature (Tg) of the coating film layer (hereinafter also referred to as "cured coating film layer") formed by applying the coating composition for metal cans is appropriate, which is preferable. Furthermore, when the number-average molecular weight of the polyester resin is within the above-mentioned range, the viscosity of the coating composition for metal cans does not become too high, even when the solid content concentration is 60% or more, and the coating property is good. The number-average molecular weight of the polyester resin is preferably 2500 to 6500, more preferably 3000 to 6000.

[0029] The Tg of the polyester resin is preferably 15 to 80°C. If the Tg of the polyester resin is within the above range, the Tg of the cured coating film layer formed by applying the coating composition for metal cans is appropriate, which is preferable. The Tg of the polyester resin is preferably 20 to 75°C, more preferably 20 to 70°C. The Tg of the polyester resin is measured by dynamic mechanical analysis (DMA) in a nitrogen atmosphere at a temperature range of -80 to 450°C with a heating rate of 10°C / min.

[0030] The polyester resins may be used singly or in combination of two or more, but preferably in combination of two or more. When two or more polyester resins are used in combination, it is preferable that two or more polyester resins with different number average molecular weights are used. For example, a polyester resin having a number average molecular weight of 2000 to 8000 (hereinafter also referred to as "polyester resin A") and a polyester resin having a number average molecular weight of 2000 to 6000 (hereinafter also referred to as "polyester resin B") may be used in combination. The number average molecular weight of the polyester resin A is preferably 3000 to 8000, more preferably 4000 to 8000, and even more preferably 5000 to 8000. The number average molecular weight of the polyester resin B is preferably 2000 to 5000, and more preferably 2000 to 4000.

[0031] The polyester resin A preferably has a number average molecular weight of 2000 to 8000, a glass transition temperature (Tg) of 20 to 70°C, and a hydroxyl value of 10 to 30 KOHmg / g, and more preferably has a number average molecular weight of 3000 to 8000, a glass transition temperature (Tg) of 30 to 60°C, and a hydroxyl value of 12 to 25 KOHmg / g. When the number average molecular weight, Tg, and hydroxyl value of the polyester resin A are within the preferred ranges, coatability during application is maintained, and a cured coating layer with good processability is more easily obtained.

[0032] The polyester resin B preferably has a number average molecular weight of 2000 to 6000 and a hydroxyl value of 35 to 55 KOHmg / g, and more preferably a number average molecular weight of 2000 to 4000 and a hydroxyl value of 40 to 50 KOHmg / g. When the number average molecular weight, Tg, and hydroxyl value of the polyester resin B are within the preferred ranges, coatability during application is maintained, and a cured coating layer with good processability is more easily obtained. The Tg of the polyester resin B is preferably 30 to 60°C, more preferably 40 to 60°C, and even more preferably 50 to 55°C.

[0033] The combined use of two polyester resins, polyester resin A and polyester resin B, provides a cured coating layer with both flexibility that can withstand processing and high hardness that provides good abrasion resistance. Polyester resin A has a relatively high number-average molecular weight, giving it a flexible skeleton, and a low hydroxyl value, which allows it to react appropriately with crosslinkers and impart flexibility to the cured coating layer. Meanwhile, polyester resin B has a relatively low number-average molecular weight and a high hydroxyl value, which allows it to react well with crosslinkers and impart high hardness to the cured coating layer.

[0034] When polyester resin A and polyester resin B are used in combination as polyester resins, the amount of polyester resin B is preferably 15 to 80 parts by mass, more preferably 20 to 60 parts by mass, and even more preferably 20 to 50 parts by mass, per 100 parts by mass of polyester resin A (solid content equivalent). By keeping the ratio of polyester resin B to polyester resin A within the above-mentioned range, the reactivity with the crosslinking agent can be easily maintained within an appropriate range. Furthermore, the coatability of the metal can coating composition and the Tg of the cured coating layer can be easily adjusted.

[0035] When the polyester resin is used alone, it is preferable to use polyester resin A. In this case, the number average molecular weight of polyester resin A is preferably 2,000 to 6,000, and more preferably 3,000 to 6,000.

[0036] (amino resin) The metal can coating composition may contain an amino resin. The amino resin may be any amino resin known for use in coatings, without any particular limitation. Examples of amino resins include methylolated amino resins obtained by reacting an amino component, such as urea, melamine, benzoguanamine, acetoguanamine, steroguanamine, spiroguanamine, or dicyandiamide, with an aldehyde component, such as formaldehyde, paraformaldehyde, acetaldehyde, or benzaldehyde. Among the methylolated amino resins, melamine resins or benzoguanamine resins are preferred, with benzoguanamine resins being more preferred, from the viewpoints of the processability, water resistance, and gloss of the coating film. From the viewpoints of processability and reactivity with the polyester resin, the number-average molecular weight of the amino resin is preferably 300 to 1,000. The amino resin may be commercially available. One of the amino resins may be used alone, or two or more may be used in combination.

[0037] (epoxy resin) The metal can coating composition may contain an epoxy resin. Any known epoxy resin for coatings may be used as the epoxy resin, without any particular limitation. Examples of the epoxy resin include various known epoxy resins such as epi-bis type, novolak type, beta (β)-methyl epichlorohydrin type, cyclic oxirane type, glycidyl ether type, glycidyl ester type, polyglycol ether type, glycol ether type, epoxidized fatty acid ester type, polycarboxylic acid ester type, aminoglycidyl type, and resorcinol type. Among these, epi-bis type epoxy resins are preferred. The above epoxy resins may be used alone or in combination of two or more.

[0038] (Crosslinking agent) The metal can coating composition may contain a crosslinking agent. The crosslinking agent is not particularly limited as long as it reacts with the terminal hydroxyl group (-OH) or terminal carboxyl group (-COOH) of the polyester resin and functions to link the polyester resins together in a linear chain. By linking the polyester resins together with the crosslinking agent, a polymer with a higher molecular weight can be produced. This allows the formation of a cured coating layer with an appropriate Tg. Examples of the crosslinking agent include isocyanate-based crosslinking agents, carbodiimide-based crosslinking agents, oxazoline-based crosslinking agents, and aziridine-based crosslinking agents.

[0039] An isocyanate crosslinking agent is a crosslinking agent made of a compound having two or more isocyanate groups (-NCO) or blocked isocyanate groups. Examples of the isocyanate crosslinking agent include polyisocyanates containing two or more isocyanate groups and blocked isocyanates containing two or more blocked isocyanate groups.

[0040] Examples of polyisocyanates include aromatic polyisocyanates, aliphatic or alicyclic polyisocyanates, and modified products thereof. Examples of aromatic polyisocyanates include polymethylene polyphenyl polyisocyanate, diphenylmethane diisocyanate, carbodiimide-modified diphenylmethane diisocyanate, phenylene diisocyanate, and tolylene diisocyanate. Examples of aliphatic or alicyclic polyisocyanates include hexamethylene diisocyanate, lysine diisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane, cyclohexane diisocyanate, methylcyclohexane diisocyanate, isophorone diisocyanate, trimethylhexamethylene diisocyanate, dicyclohexylmethane diisocyanate, norbornene diisocyanate, dimer acid diisocyanate, xylylene diisocyanate, tetramethylxylylene diisocyanate, toluene diisocyanate, and trimers of these isocyanates. Examples of modified polyisocyanates include trimethylolpropane adduct modified products, isocyanurate modified products, biuret modified products, and allophanate modified products. The above polyisocyanates may be used alone or in combination of two or more.

[0041] Examples of blocked isocyanates include those obtained by blocking the isocyanate group of the polyisocyanate with a blocking agent. Examples of blocking agents for isocyanate groups include phenols such as phenol, thiophenol, methylthiophenol, ethylthiophenol, cresol, xylenol, resorcinol, nitrophenol, and chlorophenol; oximes such as acetoxime, methylethylketoxime, and cyclohexanoneoxime; alcohols such as methanol, ethanol, propanol, and butanol, halogen-substituted alcohols such as ethylene chlorohydrin and 1,3-dichloro-2-propanol; tertiary alcohols such as t-butanol and t-pentanol; and lactams such as epsilon (ε)-caprolactam, delta (δ)-valerolactam, gamma (γ)-butyrolactam, and beta (β)-propylolactam. Examples of blocking agents for isocyanate groups include aromatic amines, imides, active methylene compounds such as acetylacetone, acetoacetic ester, and malonic acid ethyl ester, mercaptans, imines, ureas, diaryl compounds, and sodium bisulfite. Blocked polyisocyanates can be obtained by subjecting the above-mentioned polyisocyanates and isocyanate blocking agents to an addition reaction using a known method. The above-mentioned blocked isocyanates may be used alone or in combination of two or more.

[0042] From the viewpoint of storage stability, the isocyanate-based crosslinking agent is preferably a blocked isocyanate, and more preferably a blocked diisocyanate containing two blocked isocyanate groups. Blocked isocyanates having a dissociation temperature of 110 to 150°C are particularly preferred. The dissociation temperature refers to the temperature at which the blocking agent dissociates from the isocyanate group. When the dissociation temperature is within the above-mentioned range, the reaction between the polyester resin and the blocked isocyanate is suppressed before and during application, and the blocking agent dissociates during baking after application, allowing the reaction between the polyester resin and the crosslinking agent to proceed smoothly. When a polyisocyanate in which the isocyanate groups are not blocked is used, the isocyanate is preferably blended into the coating composition for metal cans immediately before use.

[0043] The carbodiimide crosslinking agent is composed of a compound containing a carbodiimide group (-N=C=N-). The carbodiimide crosslinking agent preferably has two or more carbodiimide groups. Examples of the carbodiimide crosslinking agent include aromatic polycarbodiimides such as poly(4,4'-diphenylmethanecarbodiimide), poly(p-phenylenecarbodiimide), poly(m-phenylenecarbodiimide), poly(diisopropylphenylcarbodiimide), and poly(triisopropylphenylcarbodiimide); alicyclic polycarbodiimides such as poly(dicyclohexylmethanecarbodiimide); and aliphatic polycarbodiimides such as poly(diisopropylcarbodiimide).

[0044] The oxazoline crosslinking agent is a compound containing two or more oxazoline groups. Examples of the oxazoline crosslinking agent include aliphatic bisoxazoline compounds such as 2,2'-bis(2-oxazoline), 1,2-bis(2-oxazolin-2-yl)ethane, 1,4-bis(2-oxazolin-2-yl)butane, 1,8-bis(2-oxazolin-2-yl)butane, and 1,4-bis(2-oxazolin-2-yl)cyclohexane; 1,2-bis(2-oxazolin-2-yl)benzene; 1,3-bis(2-oxazoline-2-yl)benzene; and 1,4-bis(2-oxazolin-2-yl)cyclohexane. and aromatic bisoxazoline compounds such as 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, and 2-isopropenyl-5-ethyl-2-oxazoline, or polymers of two or more compounds selected from addition-polymerizable oxazolines such as 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, and 2-isopropenyl-5-ethyl-2-oxazoline. The above oxazoline-based crosslinking agents may be used alone or in combination of two or more.

[0045] The aziridine crosslinking agent is a compound containing an aziridine group. The aziridine crosslinking agent preferably has two or more aziridine groups. Examples of the aziridine crosslinking agent include diphenylmethane-4,4'-bis(1-aziridinecarboxamide), trimethylolpropane tri-beta(β)-aziridinylpropionate, tetramethylolmethane tri-beta(β)-aziridinylpropionate, toluene-2,4-bis(1-aziridinecarboxamide), triethylenemelamine, bisisophthaloyl-1-(2-methylaziridine), tris-1-(2-methylaziridine)phosphine, and trimethylolpropane tri-beta(β)-(2-methylaziridine)propionate. The above aziridine crosslinking agents may be used alone or in combination of two or more.

[0046] When an isocyanate-based crosslinking agent is used as the crosslinking agent, the carboxyl or hydroxyl group of the polyester resin reacts with the isocyanate group of the isocyanate-based crosslinking agent to form a urethane bond. Therefore, the cured coating layer may contain a polymer compound in which the polyester resin and the isocyanate-based crosslinking agent are crosslinked via a urethane bond. The polymer compound may be further crosslinked with both or either an amino resin and an epoxy resin.

[0047] When a carbodiimide-based crosslinking agent is used as the crosslinking agent, the carboxyl group of the polyester resin reacts with the carbodiimide group of the carbodiimide-based crosslinking agent to form an amide bond. Therefore, the cured coating layer may contain a polymer compound in which the polyester resin and the carbodiimide-based crosslinking agent are crosslinked via an amide bond. The polymer compound may be further crosslinked with both or either an amino resin and an epoxy resin.

[0048] When an oxazoline-based crosslinking agent is used as the crosslinking agent, the carboxyl groups of the polyester resin react with the oxazoline groups of the oxazoline-based crosslinking agent to form a bond represented by -CO-O-C2H4-NH-CO-. Therefore, the cured coating layer contains a polymer compound in which the polyester resin and the oxazoline-based crosslinking agent are crosslinked via the above bond. The polymer compound may be further crosslinked with both or either an amino resin and an epoxy resin.

[0049] When an aziridine-based crosslinking agent is used as the crosslinking agent, the carboxyl groups of the polyester resin react with the aziridine groups of the aziridine-based crosslinking agent to form a bond represented by -CO-O-C2H4-NH-. Therefore, the cured coating layer may contain a polymer compound in which the polyester resin and the aziridine-based crosslinking agent are crosslinked via the above bond. The polymer compound may be further crosslinked with both or either an amino resin and an epoxy resin.

[0050] The crosslinking agent may be used alone or in combination of two or more. As the crosslinking agent, an isocyanate-based crosslinking agent is preferred, and blocked isocyanate is more preferred.

[0051] The metal can coating composition may not contain a crosslinking agent. In this case, the polyester resins are directly bonded by ester bonds. This state can be achieved, for example, by repeating transesterification and depolymerization reactions between the polyester resins.

[0052] In the coating composition for metal cans, the content of the polyester resin may be 45 to 80% by mass, assuming that the total mass of all resin components (polyester resin, amino resin, epoxy resin, and crosslinking agent) is 100% by mass. When the content of the polyester resin is within the above range, the cured coating layer can have an appropriate hardness. Furthermore, the coating composition for metal cans has good coatability. The content of the polyester resin is preferably 50 to 70% by mass, more preferably 50 to 65% by mass, and even more preferably 50 to 60% by mass, based on the total mass of all solid components. When two or more polyester resins are contained, the content of the polyester resins mentioned above refers to the total content of the polyester resins. The same applies to the other components (amino resin, epoxy resin, and crosslinking agent).

[0053] In the coating composition for metal cans, the content of the amino resin is not particularly limited, but may be 20 to 50 mass %, preferably 25 to 50 mass %, and more preferably 30 to 50 mass %, when the total mass of all resin components is 100 mass %.

[0054] In the coating composition for metal cans, the content of the epoxy resin is not particularly limited, but may be 1 to 30 mass %, preferably 2 to 10 mass %, and more preferably 3 to 8 mass %, when the total mass of all resin components is 100 mass %.

[0055] In the metal can coating composition, the content of the crosslinking agent is not particularly limited, but may be 1 to 20 mass %, preferably 3 to 15 mass %, and more preferably 5 to 10 mass %, when the total mass of all resin components is 100 mass %. In addition, the ratio of the crosslinking agent to 100 mass parts of the polyester resin is preferably 1 to 30 mass parts, more preferably 5 to 20 mass parts, and even more preferably 10 to 20 mass parts.

[0056] <Secondary ingredients> The metal can coating composition may contain the following dilution solvents as sub-components in addition to the main components including the polyester resin, the amino resin, the epoxy resin, and the crosslinking agent.

[0057] (dilution solvent) The dilution solvent may be any solvent capable of dissolving the above-described main components. Examples of dilution solvents include aromatic hydrocarbon solvents, aliphatic hydrocarbon solvents, ester solvents, ether solvents, alcohol solvents, ketone solvents, and cellosolve solvents. Examples of aromatic hydrocarbon solvents include toluene, xylene, Solvesso 100, and Solvesso 150. Examples of aliphatic hydrocarbon solvents include hexane, heptane, octane, and decane. Examples of ester solvents include methyl acetate, ethyl acetate, isopropyl acetate, butyl acetate, amyl acetate, ethyl formate, butyl propionate, and cellosolve acetate. Examples of ether solvents include dioxane, diethyl ether, and tetrahydrofuran. Examples of alcohol solvents include methanol, ethanol, propanol, butanol, 2-ethylhexanol, and ethylene glycol. Examples of ketone solvents include acetone, methyl ethyl ketone, and cyclohexanone. Examples of cellosolve-based solvents include ethyl cellosolve, butyl cellosolve, butyl carbitol, etc. The dilution solvents may be used alone or in combination of two or more.

[0058] Specific preferred examples of the dilution solvent include aromatic hydrocarbon solvents, alcohol solvents, cellosolve solvents, and mixed solvents of two or more of these. More specific examples include mixed solvents of cyclohexanone, butyl cellosolve, Solvesso 100, Solvesso 150, and normal butanol.

[0059] <Fluorine-free> The coating composition for metal cans does not contain fluorine or fluorine compounds, even in the main components and subcomponents (dilution solvents) described above. As a result, the coating composition for metal cans does not contain fluorine or organic fluorine compounds, preventing the release of PFAS into the environment through the coating composition itself and the metal cans to which it is applied.

[0060] <Manufacturing method> The coating composition for metal cans is not particularly limited and can be obtained by a known method for producing this type of coating composition. The coating composition for metal cans can be prepared, for example, by dissolving the polyester resin, amino resin, epoxy resin, and crosslinking agent in a dilution solvent to obtain a solution, and then adding and mixing the required amount of the functional component to the solution.

[0061] The metal can coating composition preferably has a solids concentration of 60% or more. A solids concentration of 60% or more allows the metal can coating composition to provide a cured coating layer of appropriate thickness and reduce flammability. The solids concentration is preferably 60 to 75% by mass, more preferably 60 to 70% by mass, and even more preferably 60 to 65% by mass. Having a solids concentration within the above range allows the metal can coating composition to have good coatability. The "solids concentration" of the metal can coating composition refers to the ratio of the heating residue to the total mass (100% by mass) of the entire composition, as measured in accordance with JIS K5600-1-2. The heating residue can be determined, for example, by heating the metal can coating composition at 200°C for 10 minutes using a dryer.

[0062] <effect> Since the coating composition for metal cans does not contain fluorine or organic fluorine compounds, it can provide aluminum cans that do not accumulate PFAS in the environment.The coating composition for metal cans contains the boron nitride or wax described above, which allows it to have various properties such as sufficient abrasion resistance and lubricity.

[0063] [Metal cans] The metal can according to this embodiment is a metal can coated with the metal can coating composition described above. The metal can is preferably coated with the metal can coating composition described above on at least one selected from the group consisting of a can lid, a can body, a rim, a cap, and an OSS. The metal can can be obtained, for example, as follows. That is, the metal can is obtained by applying the metal can coating composition described above to a metal substrate such as a metal container or a metal lid by a known coating method such as roll coating, spray coating, or dip coating, and then baking the metal substrate using a heating means such as an oven to obtain a coated metal substrate such as a coated metal container or a coated metal lid. Alternatively, the metal can may be obtained by applying the metal can coating composition described above to a metal plate by a known coating method such as roll coating, spray coating, or dip coating, and then forming the coated metal plate, or the coated metal plate with the organic resin-coated coated metal plate further formed with the cured coating film layer, into a coated metal container or a coated metal lid.

[0064] A painted metal container can be obtained by forming a coating film made of the above-described metal can coating composition on both or at least one of the inner and outer surfaces of a metal container. Preferably, a coating film made of the above-described metal can coating composition is formed on at least the outer surface of the metal container. All conventionally known metal containers can be used as the metal container on which the coating film is formed. Examples of such metal containers include three-piece cans and seamless cans (two-piece cans) with side seams. A painted metal container can be obtained by molding the above-described painted metal sheet, or by forming a coating film made of the above-described metal can coating composition on a seamless can that has been molded through severe processing, such as a seamless can. The above-described metal can coating composition can also be suitably used as a finishing varnish, for example, to protect printing ink on the exterior surface.

[0065] Coated metal closures can be obtained by molding the above-described coated metal sheet using a conventionally known closure manufacturing method. For example, the coated metal closures are used for stay-on-tab type easy-open can closures and full-open type easy-open can closures. It is desirable that the coated metal closures have a coating film made of the above-described metal can coating composition formed on at least the outer surface of the metal closure.

[0066] In the metal can, the thickness of the cured coating layer is not particularly limited and may be a thickness that is usually used for coatings on the surface of a metal can. For example, when the cured coating layer is present as an outer top coating layer, the thickness may be 1 to 10 μm, preferably 2 to 7 μm, and more preferably 3 to 6 μm.

[0067] The Tg of the cured coating layer is preferably 48° C. or higher, more preferably 49° C. or higher, and even more preferably 50° C. or higher. The Tg of the cured coating layer may be, for example, 48 to 65° C., preferably 48 to 60° C., more preferably 48 to 55° C., and even more preferably 50 to 55° C.

[0068] <effect> By applying the coating composition for metal cans according to this embodiment in this way, it is possible to provide metal cans that have various properties such as more sufficient abrasion resistance and lubricity, and that do not allow PFAS to accumulate in the environment. [Example]

[0069] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. For example, the metal can coating composition according to the present invention can be applied to any part of a metal can, such as the can lid, cap, and bottom (rim), in addition to the can body described below. The metal can coating composition can also be used as an OSS coating. In the following description, metal can coating compositions of Samples 101 to 118 are examples. Samples 10A to 10B are comparative examples. In the following description, various evaluation tests, as described below, were carried out on coating films corresponding to the cured coating film layer formed by applying the metal can coating composition of each of the above-mentioned samples to the can body of an aluminum can. First, the details of the evaluation tests will be described.

[0070] [Evaluation test] <Elemental analysis> Using a scanning electron microscope (product name: "JSM-IT200", manufactured by JEOL Ltd.) equipped with an energy dispersive X-ray analysis (EDS) function, elemental analysis of the cured coating layer was performed on the surface of the cured coating layer on the side or bottom (rim) of the aluminum can (magnification: 5000 times).

[0071] <Pencil hardness> According to JIS K5600-5-4:1999, the maximum pencil hardness at which the cured coating film did not undergo plastic deformation (e.g., permanent indentations) or cohesive failure (e.g., scratches) when a pencil lead was pressed against the surface of the cured coating film layer was evaluated. For example, if a pencil with a hardness of 4H caused plastic deformation or cohesive failure in the cured coating film, but a pencil with a hardness of 3H did not, the pencil hardness of the cured coating film was evaluated as 3H. The higher the pencil hardness, the harder and more robust the cured coating film was evaluated to be.

[0072] <Adhesion> According to JIS K5400-5:1990, the coating film on the coated plate equivalent to the cured coating film layer was cut with a cutter knife at 1 mm intervals so as to penetrate the coating film and reach the substrate, forming a grid (10 squares x 10 squares). Cellophane tape (trade name: "Cellotape (registered trademark), manufactured by Nichiban Co., Ltd.) was completely adhered to the grid and then rapidly peeled off in a direction 90° from the grid. The state of the coating film at the grid from which the cellophane tape was peeled off was visually observed and evaluated based on the following criteria. A: No change B: 1 to 10 squares peeled off C: The squares around 50 peeled off. D: The grid has peeled off entirely.

[0073] <Retort resistance> After 30 minutes in a pressurized steam atmosphere at 130°C, the whitening state of the hard coating film was visually observed and evaluated according to the following criteria. A: No change B: Slight whitening or slight change in surface condition C: The surface has turned white or appears to have melted. D: The hard coating film has dissolved.

[0074] <Lubricity> A 1 kg weight supported by three hard balls was placed on a coated plate on which a coating similar to the cured coating layer had been formed, and the weight was then pulled at a speed of 120 cm / min to determine the coefficient of dynamic friction (μ value). The smaller the coefficient of dynamic friction (μ value), the better the lubricity was evaluated. The lubricity was evaluated based on the μ value indicated by the following criteria. If the evaluation was indicated as A or B, it is suggested that the evaluated sample has sufficient lubricity. A: μ value ≦ 0.040 B: 0.040<μ value<0.045 C: 0.045≦μ value

[0075] <Wear resistance> Using a friction and wear tester (product name: "Friction Player FPR-2000", manufactured by Rhesca Co., Ltd.), the number of rotations until the hard coating was damaged was counted. The friction player applied a load of 1000 g to the cured coating layer, and the rotation speed was 500 rpm. The abrasion resistance was evaluated based on the following criteria. If the evaluation was indicated as A or B, it is suggested that the evaluated sample has sufficient abrasion resistance. A: No scratches even at over 3000 rpm B: Scratches occurred between 3000 and 1000 revolutions C: Scratches occurred at less than 1000 revolutions

[0076] Examples and Comparative Examples <Sample preparation> (Sample 10A) The main components and subcomponents shown in Table 1 below were used as raw materials, and were added and mixed by a known method in the mass ratios shown in Table 1 below to prepare a coating composition for metal cans, Sample 10A. The coating composition for metal cans, Sample 10A, was applied to an aluminum plate (5052 material, plate thickness 0.23 mm, 10 × 20 cm) using a bar coater #8. 2 The coating composition for metal cans was then dried under the primary baking conditions of 200°C x 1 minute and the secondary baking conditions of 210°C x 2 minutes to form a cured coating film.

[0077] [Table 1]

[0078] (Sample 10B) A coating composition for metal cans of Sample 10B was prepared in the same manner as the coating composition for metal cans of Sample 10A, except that Seridust 9630F, which was included as a secondary component, was not included in the raw materials. Furthermore, the coating composition for metal cans was applied to an aluminum plate under the same conditions as for the cured coating film layer prepared using the coating composition for metal cans of Sample 10A, and a cured coating film layer corresponding to the coating composition for metal cans of Sample 10B was formed.

[0079] (Sample 101) A coating composition for metal cans of Sample 101 was prepared in the same manner as the coating composition for metal cans of Sample 10A, except that AC6041 (boron nitride) was added as a substitute material in place of Seridust 9630F contained as a secondary component in the mass ratio shown in Table 2. Furthermore, the above coating composition for metal cans was applied to an aluminum plate under the same conditions as for the cured coating film layer prepared using the coating composition for metal cans of Sample 10A, and a cured coating film layer corresponding to the coating composition for metal cans of Sample 101 was formed.

[0080] (Samples 102 to 118) Coating compositions for metal cans, Samples 102 to 118, were prepared in the same manner as the coating composition for metal cans, Sample 10A, except that instead of Seridust 9630F contained as a minor component, the alternative materials shown in Tables 2 and 3 were added in the mass ratios shown in Tables 2 and 3. Furthermore, the above coating compositions for metal cans were applied to an aluminum plate under the same conditions as for the cured coating layer prepared using the coating composition for metal cans, Samples 102 to 118, to form cured coating layers.

[0081] The various evaluation tests described above were carried out on the cured coating layers corresponding to the coating compositions for metal cans of Samples 10A to 10B and Samples 101 to 118 on the aluminum cans described above. The results are shown in Tables 2 and 3.

[0082] [Table 2]

[0083] [Table 3]

[0084] [Consideration] According to Tables 2 and 3, the cured coating layers produced from the metal can coating compositions of Samples 101 to 118 were superior in both abrasion resistance and lubricity to Sample 10B when a fluorine-free alternative to the fluorine-containing Seridust 9630F was contained in the metal can coating compositions at a predetermined mass ratio (hereinafter simply referred to as "ratio"). The cured coating layers produced from the metal can coating compositions of Samples 101 to 118 had abrasion resistance and lubricity comparable to Sample 10A when a fluorine-free alternative to the fluorine-containing Seridust 9630F was contained in the metal can coating compositions at a predetermined ratio. This suggests that the metal can coating compositions of Samples 101 to 118 have the necessary abrasion resistance, lubricity, etc., despite not containing PFAS.

[0085] Specifically, the metal can coating composition of Sample 101 is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS, when AC6041 is contained in a ratio greater than 0.1 or 0.5 or greater. The metal can coating composition of Sample 102 is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS, when NX5 is contained in a ratio greater than 0.1 or 0.5 or greater. The metal can coating composition of Sample 103 is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS, when Ceridast 9615A is contained in a ratio greater than 0.5 or 1.0 or greater. The metal can coating composition of Sample 104 is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS, when Ceridast 3030 is contained in a ratio greater than 0.2 or greater. The metal can coating composition of Sample 105, when containing Seridust 8330 at a ratio of 0.5 or more, is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS. The metal can coating composition of Sample 106, when containing Lanco 2510F at a ratio of 0.5 or more, is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS. The metal can coating composition of Sample 107, when containing Lanco 2520SF at a ratio of 0.2 or more, is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS. The metal can coating composition of Sample 108, when containing Lanco 2530SF at a ratio of 0.5 or more, is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS. It is suggested that the metal can coating composition of Sample 109, when it contains Lanco 2540SF in a ratio of more than 0.5 or 1.0 or greater, has the necessary abrasion resistance, lubricity, etc., despite not containing PFAS.

[0086] The metal can coating composition of Sample 110, containing VN-31 at a ratio greater than 0.5 or 1.0 or greater, is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS. The metal can coating composition of Sample 111, containing VN-33 at a ratio greater than 0.5, is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS. The metal can coating composition of Sample 112, containing VN-36 at a ratio greater than 0.5, is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS. The metal can coating composition of Sample 113, containing CE-10BE at a ratio greater than 0.5 or 1.0 or greater, is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS. The metal can coating composition of Sample 114, containing CERAFLOUR 1050 at a ratio greater than 0.2, is suggested to have the required abrasion resistance, lubricity, etc., despite not containing PFAS. The metal can coating composition of Sample 115, containing CERAFLOUR 1051 at a ratio of 0.2 or more, is suggested to have the necessary abrasion resistance, lubricity, etc., despite not containing PFAS. The metal can coating composition of Sample 116, containing CERAFLOUR 1052 at a ratio of 0.2 or more, is suggested to have the necessary abrasion resistance, lubricity, etc., despite not containing PFAS. The metal can coating composition of Sample 117, containing CERETAN VP 1055 at a ratio of 0.5 or more, is suggested to have the necessary abrasion resistance, lubricity, etc., despite not containing PFAS. The metal can coating composition of Sample 118, containing SE-8402 at a ratio of 0.5 or more, is suggested to have the necessary abrasion resistance, lubricity, etc., despite not containing PFAS.

[0087] Although the embodiments and examples of the present invention have been described above, it is intended from the beginning that the configurations of the above-described embodiments and examples may be combined as appropriate.

[0088] The embodiments and examples disclosed herein should be considered to be illustrative in all respects and not restrictive. The scope of the present invention is defined by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

Claims

1. A fluorine-free coating composition for metal cans.

2. A coating composition for metal cans that does not contain organic fluorine compounds.

3. The coating composition for metal cans according to claim 1 or 2, which contains boron nitride.

4. The coating composition for metal cans according to claim 1 or 2, which contains a wax.

5. 5. The coating composition for metal cans according to claim 4, wherein the wax is at least one selected from the group consisting of beeswax, lanolin, spermaceti, privet wax, carnauba wax, amide wax, candelilla wax, rice wax, paraffin wax, microcrystalline wax, montan wax, polyethylene wax, polyolefin wax and polyester wax.

6. 3. The coating composition for metal cans according to claim 1 or 2, which is endowed with lubricity and abrasion resistance.

7. A metal can coated with the coating composition for metal cans according to claim 1 or 2.

8. A metal can, wherein the coating composition for metal cans according to claim 1 or 2 is applied to at least one material selected from the group consisting of a can lid, a can body, a rim, a cap, and an OSS.

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