Paints for cans, painted boards, and coated cans

Abstract

To provide a paint suitable for food and beverage cans that can form a coating film with good abrasion resistance, workability, and curability. [Solution] A paint for cans that satisfies (i) and (ii) in the following nano-scratch test. <Nano-scratch test> For samples obtained by applying can paint to an aluminum can body and baking it at 180°C for 52 seconds and 200°C for 2 minutes, the surface of the coating was measured at 23°C using a nanoindenter. (i) The average coefficient of friction is 2 or less, and (ii) the maximum scratch mark is 120 nm or less.

Description

Technical Field

[0001] The present invention relates to a can coating material that can be used for forming a coating film on a coated can or the like.

Background Art

[0002] On the outer surface of beverage cans for storing soft drinks, coffee, or alcoholic beverages such as beer, and food cans for storing fish, meat, etc. (hereinafter both are collectively referred to as food and beverage cans), a coating film using a coating material for surface protection and aesthetics is formed. Also, the coating film is required to have resistance to scratches and abrasion during line movement and transportation when manufacturing food and beverage cans. In addition, in the can manufacturing process and the content filling process, the conveying speed of cans is increasing more and more, and there is a demand for a coating material that can form a coating film with excellent scratch resistance, abrasion resistance, and hardness and can withstand high-speed conveyance.

[0003] Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2023-075011) discloses an aqueous can coating composition containing an acrylic-modified epoxy resin, at least one wax out of three waxes with different melting points, a polyolefin aqueous dispersion, and a phenolic resin, and having excellent lubricity, abrasion resistance, and processability.

[0004] However, in the coating composition described in Patent Document 1, when a large amount of wax or polyolefin aqueous dispersion is blended to improve lubricity and abrasion resistance, the scratch resistance decreases due to the deterioration of the smoothness of the coating film surface. Therefore, it is difficult to achieve high levels of lubricity, abrasion resistance, and scratch resistance, and hardness is not described.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] This invention has been made in view of the above circumstances, and aims to provide a paint suitable for food and beverage cans that can form a coating film having good abrasion resistance, workability, and curability. [Means for solving the problem]

[0007] [1] A paint for cans that satisfies (i) and (ii) in the following nanoscratch test. <Nano-scratch test> [1] A sample obtained by applying can paint to an aluminum can body and baking it at 180°C for 52 seconds and 200°C for 2 minutes was measured at 23°C using a nanoindenter. (i) The average coefficient of friction is 2 or less, and (ii) the maximum scratch mark depth is 120 nm or less. [2] The can paint described in [1] that satisfies (iii) in the following indentation test. <Indentation Test> For samples obtained by applying can paint to an aluminum can body and baking it at 180°C for 52 seconds and 200°C for 2 minutes, the surface of the coating was measured at 23°C using a nanoindenter. (iii) Film hardness is 0.35 GPa or less. [3] The paint for cans according to [1], wherein the paint for cans comprises an acrylic resin, a polyester resin, and an amino resin, and the amino resin is present in an amount of 25 to 185 parts by mass per 100 parts by mass of the total of the acrylic resin and the polyester resin. [4] The amino resin comprises a melamine resin and a benzoguanamine resin, or comprises only a melamine resin. The paint for cans according to [3], wherein the mass ratio of the melamine resin to the benzoguanamine resin is melamine resin / benzoguanamine resin = 30 / 70 to 100 / 0. [5] The paint for cans according to [1], wherein the content of the fluorine atom-containing compound in the paint for cans is 2 parts by mass or less per 100 parts by mass of the total resin content of the paint for cans. [6] A painted plate comprising a metal plate and a coating film which is a cured product of any of the can paints described in [1] to [5]. [7] A coated can comprising a metal plate and a coating film which is a cured product of any of the can paints described in [1] to [5]. [Effects of the Invention]

[0008] According to the present invention with the above configuration, it is possible to provide a paint suitable for food and beverage cans that can form a coating film having good abrasion resistance, workability, and curability. [Modes for carrying out the invention]

[0009] The embodiments for carrying out the present invention will be described in detail below.

[0010] <Paint for cans> The can coating of the present invention is used to coat the surface (outer and inner) of cans. In particular, it is suitable as an exterior coating for cans to protect the outer surface and enhance the aesthetic appearance of food and beverage cans.

[0011] <Method for forming an evaluation coating film> The coating film for evaluating various physical properties in this invention is obtained by applying paint to an aluminum substrate, baking it at 180°C for 52 seconds, and then further curing it by baking at 200°C for 2 minutes.

[0012] <Nano-scratch test> The nano-scratch test in this invention is performed using a micro-area mechanical property evaluation device (nanoindenter). This test allows for the measurement of the average friction coefficient of the coating film and the depth of scratch marks on the coating film.

[0013] <Average friction coefficient of the coating film> The coefficient of friction obtained by the nanoscratch test is the value obtained by dividing the horizontal load applied to the indenter of the nanoindenter by the vertical load at that time, and in this invention, the value was measured using Hyzitron TI Premier (manufactured by Bruker). The method for calculating the average friction coefficient of the coating film is as follows. As the indenter of the nanoindenter, a conical indenter with a conical shape is used. The conical indenter is brought into contact with the measurement sample, and at 23°C, in conjunction with moving the indenter horizontally at 0.4 μm / s for 6 μm, a load of up to 100 μN is applied vertically at a load rate of 6.7 μN / s. At this time, the average friction coefficient was obtained by averaging the friction coefficient values between a horizontal movement distance of 1 μm and 5 μm. In the present invention, a conical indenter with a tip curvature radius of 1 μm or less is used.

[0014] The coating film surface of the coating film formed from the can coating of the present invention has an average friction coefficient of 2 or less by the nanoindentation method. The lower limit of the average friction coefficient is not particularly limited, but for example, 0.2 or more is preferable. If the average friction coefficient of the coating film by the nanoindentation method is 2 or less, the friction during scratching becomes small, and the wear resistance of the coating film is improved. If the wear resistance is poor, the coating film is likely to be damaged.

[0015] <Maximum scratch mark depth of the coating film> The maximum scratch mark depth by the nanoscratch method is a value representing the horizontal durability of the coating film measured using a nanoindenter, and in the present invention, it is measured using the same apparatus as that for measuring the average friction coefficient of the coating film. The deeper the scratch mark, the lower the durability against horizontal stimuli, and the coating film tends to be easily damaged. The scratch mark depth of the coating film was measured by tracing the same location of the coating film with a weak load that does not cause deformation after horizontally driving the conical indenter on the measurement sample under the following pressing conditions, and the maximum value was taken as the maximum scratch mark depth (nm). The pressing conditions were as follows: at 23°C, in conjunction with moving the indenter horizontally at 0.4 μm / s for 6 μm, a load of up to 100 μN was applied vertically at a load rate of 6. μN / s.

[0016] The coating surface of the coating film formed from the can coating of the present invention has a maximum scratch mark depth of 120 nm or less, preferably 90 nm or less, by the nanoindentation method. By setting the maximum scratch mark depth of the coating film to 120 nm or less, deformation against the horizontal load during scratching is suppressed, and the wear resistance of the coating film is improved.

[0017] The average coefficient of friction and the maximum scratch mark depth of the coating surface of the coating film formed from the can coating of the present invention can be mainly adjusted by the composition and type of the curing agent component constituting the can coating. Specifically, when the content of the curing agent component is within an appropriate range, the scratch resistance of the coating film is improved, and the average coefficient of friction and the maximum scratch mark depth of the coating surface become smaller. On the other hand, if the curing agent component is insufficient, curing becomes insufficient and no coating film is formed, and the average coefficient of friction and the maximum scratch mark depth of the coating surface become larger. If it is excessive, overcuring occurs and the coating film becomes brittle and easily damaged, and the maximum scratch mark depth becomes larger. Also, when using an amino resin as the curing agent component, adding a melamine resin with many reaction points can obtain an appropriate crosslinking density, and the average coefficient of friction and the maximum scratch mark depth of the coating surface tend to become smaller. Adding a benzoguanamine resin with a rigid structure tends to increase the average coefficient of friction and the maximum scratch mark depth of the coating surface.

[0018] <Indentation test> The indentation test of the present invention is measured using a micro-area mechanical property evaluation device (nanoindenter). By this test, the hardness of the coating surface by the nanoindentation method can be measured.

[0019] <Hardness of the coating film> The hardness by the nanoindentation method is the value of the indentation hardness measured using a micro-area mechanical property evaluation device (nanoindenter), and in the present invention, it is measured using the same device as that for measuring the average coefficient of friction of the coating surface. Because the nanoindenter only indents the very surface layer of the coating, it is unaffected by the substrate and can measure the mechanical properties of the thin coating film alone. In other measurement methods, the indentation load and displacement are too large, making it difficult to measure only the thin film layer due to the influence of the substrate. In this invention, it is possible to evaluate the hardness of the coating film alone. The method for measuring the indentation hardness (H) of the coating film is as follows, and it is calculated by the following (Equation 1). H = F max / Ac ... (Equation 1) A triangular pyramidal Berkovich indenter is used as the indenter for the nanoindenter. The Berkovich indenter is pressed into the sample under the indentation conditions described below, and the indentation depth h (nm) for each indentation load F (μN) is continuously measured to create a load-displacement curve. The maximum indentation load Fmax (μN) is determined from the created load-displacement curve. Then, the indentation hardness (H) is calculated by dividing the maximum indentation load Fmax (μN) by the contact projection area Ac (μ) between the indenter and the sample at that time. Here, Ac is the contact projection area obtained by correcting the indenter tip curvature using the standard method of the instrument with fused silica as the standard sample. The contact projection area Ac is calculated from the indentation depth h (nm), where Ac = 24.56h 2 (nm 2 )

[0020] The indentation conditions were as follows: at 23°C, the indenter was first pressed to a depth of 200 nm over 5 seconds (i.e., an indentation speed of 40 nm / s), then held at a depth of 200 nm for 2 seconds, and finally unloaded to 0 nm over 5 seconds. The maximum indentation load Fmax used in hardness calculation is the value of the load after holding for 2 seconds. In the present invention, the coating film preferably has a hardness of 0.35 GPa or less as measured by nanoindation, and more preferably 0.20 GPa or more and 0.30 GPa or less. Within the range of hardness of the coating film as measured by nanoindation, the desired hardness of the coating film, as well as performance such as workability and curability, can be effectively optimized. The hardness of the coating surface formed from the can coating of the present invention can be adjusted by the type and composition of the resin and curing agent components that make up the can coating. Specifically, using a large amount of curing agent tends to result in a higher indentation hardness. On the other hand, using less curing agent or a resin with a low glass transition temperature will result in the opposite trend.

[0021] <Composition of paint for cans> The composition of the paint for cans of the present invention is described below. Examples of paint components include acrylic resin, polyester resin, amino resin, organic solvent, water, and lubricating agent.

[0022] <Acrylic resin> The acrylic resin in this invention is a polymer of an ethylenically unsaturated monomer (hereinafter also simply referred to as monomer) that essentially contains a compound having a (meth)acryloyl group (the acryloyl group and the methacryloyl group are collectively referred to as a (meth)acryloyl group; the same applies hereinafter).

[0023] In the present invention, monomers such as (meth)acrylamides, alkyl (meth)acrylates, vinyl monomers, monomers having a hydroxyl group, and monomers having a carboxyl group are used. Examples of (meth)acrylamides include N-methoxymethyl(meth)acrylamide, N-ethoxymethyl(meth)acrylamide, Nn-propoxymethyl(meth)acrylamide, N-isopropoxymethyl(meth)acrylamide, Nn-butoxymethyl(meth)acrylamide, N-isobutoxymethyl(meth)acrylamide, N-pentyloxymethyl(meth)acrylamide, N-hexyloxymethyl(meth)acrylamide, N-heptyloxymethyl(meth)acrylamide, N-octyloxymethyl(meth)acrylamide, N-2-ethylhexyloxymethyl(meth)acrylamide, (meth)acrylamide, N,N-dimethyl(meth)acrylamide, N,N-diethyl(meth)acrylamide, N-isopropyl(meth)acrylamide, N-butyl(meth)acrylamide, isobutyl(meth)acrylamide, t-butyl(meth)acrylamide, t-octyl(meth)acrylamide, and diacetone(meth)acrylamide.

[0024] Among these, from the viewpoint of scratch resistance of the coating film, it is preferable to use N-alkoxymethyl(meth)acrylamide such as N-methoxymethyl(meth)acrylamide, N-ethoxymethyl(meth)acrylamide, Nn-propoxymethyl(meth)acrylamide, N-isopropoxymethyl(meth)acrylamide, Nn-butoxymethyl(meth)acrylamide, N-isobutoxymethyl(meth)acrylamide, N-pentyloxymethyl(meth)acrylamide, N-hexyloxymethyl(meth)acrylamide, N-heptyloxymethyl(meth)acrylamide, N-octyloxymethyl(meth)acrylamide, or N-2-ethylhexyloxymethyl(meth)acrylamide. N-alkoxymethyl(meth)acrylamide is a crosslinkable monomer and readily crosslinks with other resins under heating or acidic conditions, further improving the scratch resistance and workability of the coating film.

[0025] Alkyl (meth)acrylates are primarily used to adjust the glass transition temperature (also denoted as Tg) of acrylic resins. In this invention, the alkyl group in the alkyl (meth)acrylate may be substituted with substituents other than those containing a hydroxyl group or a carboxyl group. Specifically, for example, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, cyclohexyl (meth)acrylate, n-hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, no Examples include nyl (meth)acrylate, isononyl (meth)acrylate, decyl (meth)acrylate, undecyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, tetradecyl (meth)acrylate, caprolactone-modified (meth)acrylate, N,N-dimethylaminoethyl (meth)acrylate, N,N-diethylaminoethyl (meth)acrylate, and N,N-dibutylaminoethyl (meth)acrylate.

[0026] Vinyl monomers include, for example, styrene, p-methylstyrene, α-methylstyrene, and Examples include toluene, vinyl acetate, and vinyl propionate. Monomers containing hydroxyl groups impart hydroxyl groups to acrylic resins, and when they react with a curing agent, the crosslinking density of the coating film increases, thereby improving the workability and scratch resistance of the resulting coating film. Examples of hydroxyl group-containing monomers include hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate.

[0027] Monomers containing carboxyl groups impart appropriate hydrophilicity to acrylic resins, and neutralization with basic compounds allows the acrylic resin to be converted into an aqueous solution or aqueous dispersion. Examples of carboxyl group-containing monomers include acrylic acid, methacrylic acid, maleic acid, fumaric acid, and itaconic acid. Among these, (meth)acrylic acid is preferred from the viewpoint of copolymerization with other monomers. All monomers that can be used can be used individually or in combination of two or more.

[0028] Acrylic resins can be synthesized by methods such as solution polymerization, emulsion polymerization, suspension polymerization, and bulk polymerization. Among these, solution polymerization is preferred because the reaction can be easily controlled. In addition, radical polymerization initiators commonly used in the synthesis of acrylic resins can be used for the synthesis of acrylic resins. Examples of azo compounds that can be used as radical polymerization initiators include 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 1,1'-azobis(cyclohexane1-carbonitride), 2,2'-azobis(2,4-dimethylvaleronitrile), 2,2'-azobis(2,4-dimethyl-4-methoxyvaleronitrile), dimethyl2,2'-azobis(2-methylpropionate), 4,4'-azobis(4-cyanovaleric acid), 2,2'-azobis(2-hydroxymethylpropionitrile), and 2,2'-azobis[2-(2-imidazolin-2-yl)propane]. Examples of peroxide compounds include benzoyl peroxide, t-butyl peroxybenzoate, cumene hydroperoxide, diisopropyl peroxydicarbonate, t-butyl peroxy-2-ethylhexanoate, di-n-propyl peroxydicarbonate, di(2-ethoxyethyl) peroxydicarbonate, t-butyl peroxyneodecanoate, t-butyl peroxybivalate, di(3,5,5-trimethylhexanoyl) peroxide, dipropionyl peroxide, and diacetyl peroxide. It is preferable to use 0.1 to 10 parts by mass of the radical polymerization initiator per 100 parts by mass of the total monomers.

[0029] The weight-average molecular weight of the acrylic resin is preferably between 1,000 and 100,000. A molecular weight of 1,000 or more improves the hardness of the resulting coating film, while a molecular weight of 100,000 or less makes it easier to ensure paint stability when the coating is water-based. Furthermore, the glass transition temperature (Tg) of the acrylic resin is preferably between 0 and 60°C. Setting it above 0°C improves the hardness of the formed coating film, while setting it below 60°C makes it easier to ensure good adhesion to the material. In this invention, the glass transition temperature (Tg) of the acrylic resin is a calculated value derived from the Tg of each monomer homopolymer and the blending ratio of each monomer. In this invention, the value obtained using the FOX formula is used. The Tg of the monomer homopolymer is the value listed in the Polymer Handbook (published in 1975, second edition).

[0030] <Polyester resin> The polyester resin of the present invention can be synthesized by a polycondensation reaction (esterification reaction) between a polycarboxylic acid (including its anhydride) and a polyol. Polycarboxylic acids are divalent or higher carboxylic acids, including aromatic dicarboxylic acids, aliphatic dicarboxylic acids, alicyclic dicarboxylic acids, and polycarboxylic acids with three or more functionalities. Examples of aromatic dicarboxylic acids include terephthalic acid, isophthalic acid, orthophthalic acid, naphthalenedicarboxylic acid, 5-sodium sulfisoisophthalic acid, phthalic anhydride, and biphenyldicarboxylic acid. Examples of aliphatic dicarboxylic acids include succinic acid (anhydride), fumaric acid, maleic acid (anhydride), adipic acid, sebacic acid, azelaic acid, hymic acid, dodecanedionic acid, and dimer acid. Examples of alicyclic dicarboxylic acids include 1,3-cyclohexanedicarboxylic acid, 1,2-cyclohexanedicarboxylic acid, and their anhydrides. Examples of polycarboxylic acids with three or more functions include pyromellitic anhydride, trimellitic anhydride, and ethylene glycol bistrimellitate dianhydride. Polycarboxylic acids can be used individually or in combination of two or more types. In addition, in the present invention, monocarboxylic acids such as benzoic acid, para-t-butylbenzoic acid, 12-hydroxystearic acid, myristic acid, and crotonic acid can also be used as carboxylic acid components for the purpose of adjusting the molecular weight of the polyester resin.

[0031] Polyols are dihydric alcohols or polyhydric alcohols with three or more functionalities. Dihydric alcohols include, for example, aliphatic diols such as ethylene glycol, diethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, neopentyl glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2-n-butyl-2-ethyl-1,3-propanediol, 2,2-diethyl-1,3-propanediol, 1,9-nonanediol, 2-methyl-1,8-octanediol, 3-methyl-1,5-pentanediol, 2-methylidene-1,4-butanediol, 2-methylidene-1,5-pentanediol, or 2-methylidene-1,6-hexanediol; Alicyclic diols such as 1,2-cyclohexanediol, 1,3-cyclohexanediol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, 2-hydroxycyclohexyl-methanol, 3-hydroxycyclohexyl-methanol, 4-hydroxycyclohexyl-methanol, hydrogenated bisphenol A, hydrogenated bisphenol AP, hydrogenated bisphenol F, hydrogenated bisphenol S, and hydrogenated biphenol; Aromatic diols such as bisphenol A propylene oxide adducts (2-8 molar adducts), bisphenol F propylene oxide adducts (2-8 molar adducts), and bisphenol A or bisphenol F with added ethylene oxide; These are some examples. Examples of polyhydric alcohols with three or more functionalities include trimethylolethane, trimethylolpropane, glycerin, pentaerythritol, dipentaerythritol, mannitol, sorbitol, and α-methylglucoside. Polyols can be used individually or in combination of two or more types.

[0032] The weight-average molecular weight of the polyester resin is preferably between 1,000 and 50,000. A molecular weight of 1,000 or more improves the workability of the formed coating film, while a molecular weight of 50,000 or less makes it easier to adjust the viscosity to an appropriate level when used as a water-based paint. Furthermore, the acid value is preferably between 10 and 50 mgKOH / g. Keeping it within this range makes it easier to ensure paint stability when it is converted to a water-based solution.

[0033] Polyester resins can be synthesized by polycondensation (esterification) reactions of polycarboxylic acids and polyols, which can be carried out under either atmospheric pressure or reduced pressure. The molecular weight of the reaction product is adjusted by controlling the ratio (excess rate: equivalent ratio of hydroxyl groups to acidic groups) of acidic groups derived from the polycarboxylic acid and hydroxyl groups derived from the polyol. During the polycondensation reaction (esterification reaction) of polycarboxylic acids and polyols, antioxidants may be used to ensure polymerization stability. Examples include phenolic antioxidants (including hindered phenolic antioxidants). Preferably, the antioxidant is used in amounts of 0.1 to 10 parts by mass per 100 parts by mass of the total of the polycarboxylic acid and polyol. Furthermore, in order to adjust the acid value, a new polycarboxylic acid with a valency of trivalent or higher may be added during the synthesis reaction, and the reaction may proceed until the desired acid value is achieved.

[0034] It is preferable to use polyester resin in a mass ratio of acrylic resin to acrylic resin such that acrylic resin / polyester resin = 10 / 90 to 90 / 10, and more preferably acrylic resin / polyester resin = 30 / 70 to 70 / 30. If the mass ratio of polyester resin is 10 or higher, the processability and adhesion of the resulting coating film tend to improve. Also, if the mass ratio of polyester resin is 90 or lower, the hardness of the resulting coating film tends to improve.

[0035] <Amino resin> The amino resin in this invention is a resin obtained by adding an aldehyde compound to some or all of the amino groups of an amino component to generate N-methylol groups, and then etherifying some or all of the resulting N-methylol groups by dehydrating them with an alcohol to generate N-alkoxymethyl groups. Because it can react with the aforementioned acrylic resin and polyester resin, it acts as a curing agent. Examples of amino resins include urea resin, melamine resin, benzoguanamine (also known as 2,4-diamino-6-phenyl-1,3,5-triazine) resin, acetoguanamine resin, steroguanamine resin, spiloganamine resin, and dicyandiamide resin. Melamine-benzoguanamine cocondensation resins, obtained using the reaction product of melamine and benzoguanamine as a raw material, are also an example. Examples of aldehyde compounds include formaldehyde, paraformaldehyde, acetaldehyde, and benzaldehyde. The alcohol used for etherification of the N-methylol group is preferably a monoalcohol having 1 to 6 carbon atoms, and more preferably methyl alcohol, ethyl alcohol, isobutyl alcohol, or n-butyl alcohol. The alcohol can be used alone or in combination of two or more types. From the viewpoint of the hardness and processability of the resulting coating film, it is preferable to use melamine resin, melamine-benzoguanamine cocondensation resin, or benzoguanamine resin as the amino resin, and in particular, from the viewpoint of abrasion resistance, it is more preferable to use melamine resin.

[0036] The amino resin preferably has a mass ratio of melamine resin to benzoguanamine resin of 30 / 70 to 100 / 0, and more preferably 50 / 50 to 100 / 0. If the mass ratio of melamine resin is 30 or higher, sufficient crosslinking density can be obtained, improving curability and the hardness of the resulting coating film. In addition, the proportion of benzoguanamine resin, which has low solubility in water, is reduced, making it easier to ensure paint stability when converted to a water-based solution. Amino resins can be used individually or in combination of two or more types.

[0037] Commercially available amino resins may be used. Examples include Cymel 232, 303LF, 325N, 370N, 659E, 1123, Mycoat 106, 137, 159, 212, and 779 from Allnex; Luwipal 014, 015, 018, 066, 070, 052, and B017 from BASF; and Amidia L-105-60, Amidia L-109-65, Amidia L-110-60, Amidia TD-126, and Amidia 15-594 from DIC.

[0038] In this invention, it is preferable to use 25 to 185 parts by mass of amino resin per 100 parts by mass of acrylic resin and polyester resin combined, and more preferably 40 to 100 parts by mass. If the amount is 25 parts by mass or more, the crosslinking density is improved, and the curability and abrasion resistance of the resulting coating film are improved. If the amount is 185 parts by mass or less, an appropriate crosslinking density is easily obtained, and the workability and adhesion of the coating film are improved.

[0039] <Other resins> The can coating of the present invention may include, as other resins, for example, polyether polyol resins, polyester polyol resins, and modified epoxy resins obtained by adding and modifying the glycidyl groups of epoxy resins with amines, phosphoric acid, etc.

[0040] <Basic compounds> The paint for cans of the present invention is preferably a water-based paint, that is, a water-soluble paint or a water-dispersible paint. In the present invention, the carboxyl groups in acrylic resins and polyester resins can be neutralized to make the resins water-soluble or water-dispersible. When neutralizing, it is preferable to use basic compounds such as amine compounds, ammonia, or alkali metal hydroxides. Examples of the amine compounds include monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, monopropylamine, dipropylamine, monoethanolamine, diethanolamine, triethanolamine, N,N-dimethylethanolamine, N,N-diethylethanolamine, 2-dimethylamino-2-methyl-1-propanol, 2-amino-2-methyl-1-propanol, N-methyldiethanolamine, N-ethyldiethanolamine, monoisopropanolamine, diisopropanolamine, and triisopropanolamine. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, and potassium hydroxide. Basic compounds can be used individually or in combination of two or more.

[0041] <Water> The paint for cans of the present invention preferably contains 10 to 45% by mass of water per 100% by mass of the paint for cans, and more preferably contains 20 to 35% by mass, for the purpose of protecting the environment and ensuring a safe working environment. If the water content is between 10 and 45% by mass, paint stability can be ensured when the paint is water-based, and the amount of organic solvent discharged during the paint curing process can be reduced.

[0042] <Organic solvents> Water-based paints for cans may contain 5 to 40% by mass of an organic solvent to improve paintability and storage stability. It is preferable to use a hydrophilic organic solvent. Hydrophilic organic solvents include, for example, alcohols such as methanol, ethanol, n-propanol, iso-propanol, n-butanol, sec-butanol, iso-butanol, t-butanol, and diacetone alcohol; Examples of glycol ethers include ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol monopropyl ether, and dipropylene glycol monobutyl ether. Organic solvents can be used alone or in combination of two or more types.

[0043] <Curing catalyst> The paint for cans of the present invention may optionally contain a curing catalyst. An acid catalyst is preferred as the curing catalyst, which allows for appropriate adjustment of the curing rate. Examples of curing catalysts include p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenedisulfonic acid, phosphoric acid, and block forms thereof. The curing catalyst can be added in an amount of 0.001 to 1 part by mass per 100 parts by mass of the total resin component of the paint for cans (including amino resin used as a curing agent).

[0044] <Lubricant> The can coating of the present invention may optionally contain a lubricant. The lubricant improves the slipperiness of the coating film and can suppress, for example, damage to the coating film when coated cans come into contact with each other. The lubricant is preferably, for example, a wax or a silicone-based lubricant. Waxes include natural waxes and synthetic waxes. Natural waxes include, for example, animal-derived waxes such as lanolin, beeswax, and whale wax; Plant-based waxes such as carnauba wax, candelilla wax, and rice wax; Examples include paraffin wax, microcrystalline wax, and mineral waxes such as montan wax. Examples of synthetic waxes include polyolefin waxes, fluorine waxes, and fatty acid ester waxes. Examples of silicone-based lubricants include dimethylpolysiloxane and its modified forms. Lubricants can be used alone or in combination of two or more types. The lubricant may be included in amounts of 0.001 to 10 parts by mass per 100 parts by mass of the total resin component of the paint for cans (including amino resins used as curing agents, etc.).

[0045] Conventional can coatings sometimes used fluorine wax to provide excellent scratch resistance, abrasion resistance, and slipperiness. However, the organofluorine compounds contained in fluorine wax are a concern due to their poor decomposition and bioaccumulation potential, and are subject to some regulations in Europe. Therefore, for the purpose of protecting the environment, the can coating of the present invention preferably does not contain fluorine atom-containing compounds such as fluorine wax. Even without containing fluorine atom-containing compounds such as fluorine wax, the can coating of the present invention can form a coating film with good scratch resistance and slipperiness. The can coating of the present invention may contain fluorine atom-containing compounds such as fluorine wax, and if it does, it is preferably 2 parts by mass or less, and more preferably 1.5 parts by mass or less, per 100 parts by mass of the total resin content of the can coating (including amino resin used as a curing agent). By limiting it to 2 parts by mass or less, the precipitation of fluorine atom-containing compounds with a high specific gravity, such as polytetrafluoroethylene wax, is suppressed, the redispersibility of the wax when using the can coating after static storage is improved, and a good surface coating film with fewer aggregates can be obtained.

[0046] <Hardening agent> The can coating of the present invention may contain a curing aid. Examples of curing aids include polyisocyanates. Examples of polyisocyanates include blocked isocyanates using activated methylene, MEK oxime, ε-caprolactam, etc., as blocking agents.

[0047] <Preparation of paint for cans> The paint for cans of the present invention can be prepared by stirring and mixing the materials that constitute it. The can coatings of the present invention are preferably used as, for example, colored coatings containing colorants such as pigments and dyes, clear coatings without colorants, and topcoats to protect colored coatings formed by applying colored coatings. Pigments include chromatic pigments (e.g., quinacridone) and achromatic pigments (e.g., titanium dioxide, aluminum pigments). Colored paints are preferably prepared by mixing a colorant with a dispersant as needed, and using a disperser such as a sand mill or disperser.

[0048] <Painted board> The painted plate of the present invention comprises a metal plate and a coating film which is a cured product of the paint for cans of the present invention. The painted plate can be obtained by coating the paint for cans onto a metal plate and allowing it to dry and harden. Suitable materials for the metal sheet include electroplated tinned steel sheets, aluminum steel sheets, and stainless steel sheets, as well as laminated steel sheets obtained by laminating these with a polyester film (e.g., polyethylene terephthalate film, polybutylene terephthalate film, etc.). The metal plate is not limited to a flat plate, but may also be a three-dimensional object such as a can body. By applying the can coating of the present invention to a can body, a coated can can be obtained. Methods for applying paints in cans include, for example, roll coating, spraying, and brush application. The drying and curing conditions are typically 140-240°C for 5 seconds to 10 minutes, and the mass thickness of the cured coating film is 10 mg / dm². 2 ~150 mg / dm 2 It is to that extent.

[0049] <Coated can> The coated can of the present invention comprises a can body and a coating film which is a cured product of the can coating of the present invention. The can body is preferably made of the metal plate described above. Examples of the can body of a coated can include a two-piece can consisting of one lid and one can body member, and a three-piece can consisting of two lid members (upper and lower) and one can body member. The can body member of the two-piece can is a bottomed cylindrical shape. Furthermore, examples of coated cans include so-called bottle cans, which consist of a cap that can be opened and closed, and a bottle section. The bottle can has a screw-on drinking spout onto which the cap can be attached. The coating on the can body can be formed at any stage of the process, such as when the metal plate is a flat plate, or when the flat plate is formed into, for example, a cup shape. The coated can of the present invention can be used, for example, as a food and beverage can, an industrial can such as an 18L can, or an art can, but it is preferable that it be used as a food and beverage can. The can coating of the present invention is preferably used as an inner or outer coating for coated cans, and more preferably as an outer coating. It goes without saying that this can coating can also be used for coating film formation applications other than coated cans. [Examples]

[0050] The present invention will be described more specifically below with reference to examples, but the present invention is not limited to these examples. Unless otherwise specified in the examples, "parts" refers to "parts by mass" and "%" refers to "percentage by mass".

[0051] [Manufacturing Example 1] <Synthesis of acrylic resin (A-1)> In a reaction vessel equipped with a thermometer, stirrer, reflux condenser, dropping tank, and nitrogen gas inlet, 100 parts of ethylene glycol monobutyl ether were charged. Heating was started with stirring while introducing nitrogen gas, and the internal temperature was 120°C. A mixture consisting of 15 parts N-butoxymethylacrylamide, 35 parts methyl methacrylate, 25 parts lauryl methacrylate, 15 parts styrene, 5 parts 2-hydroxyethyl methacrylate, 5 parts acrylic acid, and 5 parts benzoyl peroxide was uniformly added dropwise from the dropping tank over 2 hours. The internal temperature was then maintained at 120°C and the reaction was allowed to proceed for 30 minutes. Next, 0.2 parts of benzoyl peroxide were added and the reaction was allowed to proceed for 30 minutes. A further 0.2 parts of benzoyl peroxide were added and the reaction was allowed to proceed for another 30 minutes to complete the reaction. Then, under reduced pressure at 110°C, the ethylene glycol monobutyl ether was removed by distillation until the non-volatile content of the reaction system was 65%. Subsequently, 6.2 parts of N,N-dimethylethanolamine and an appropriate amount of deionized water were added to obtain a solution of acrylic resin (A-1) with a non-volatile content of 50%. The Tg of acrylic resin (A-1) was 35°C, and the weight-average molecular weight was 15,000.

[0052] The weight-average molecular weight was measured using a Tosoh Corporation GPC instrument 8020 series (THF (tetrahydrofuran) solvent, column temperature 40°C, polystyrene standard). Four columns from Tosoh Corporation (G1000HXL, G2000HXL, G3000HXL, and G4000HXL) were connected in series, and measurements were taken at a flow rate of 1.0 ml / min.

[0053] [Manufacturing Examples 2-4] <Synthesis of acrylic resins (A-2) to (A-4)> Solutions of acrylic resins (A-2) to (A-4) were obtained using the same method as in Production Example 1, except that the types and amounts (parts) of each component were changed as shown in Table 1.

[0054] [Manufacturing Example 5] <Synthesis of polyester resin (B-1)> In a reaction vessel equipped with a thermometer, stirrer, nitrogen gas inlet, fractional distillation apparatus, and condenser, 22 parts of 1,4-butanediol, 10 parts of diethylene glycol, 11.3 parts of trimethylolpropane, 29.8 parts of isophthalic acid, 16.4 parts of hexahydrophthalic anhydride, and 10.5 parts of adipic acid were added. The esterification reaction was carried out at 220°C under nitrogen gas injection with stirring until the acid value reached 30 mg KOH / g, at which point the reaction was stopped and the mixture was cooled. When the internal temperature reached 100°C, 25 parts of ethylene glycol monobutyl ether were added to obtain a polyester resin solution (B-1) with a weight-average molecular weight of 3,000 and a non-volatile content of 65%.

[0055] The acid value was measured using the following method. 0.2 g of resin (based on non-volatile content) was dissolved in 20 ml of THF (tetrahydrofuran), titrated with a 0.1 N KOH ethanol solution, and the acid value was calculated using the following formula (Equation 2). Acid value (mgKOH / g)=5.611×A×F / 0.2 (Formula 2) A: Titration volume of KOH ethanol solution (ml) F: Potency of 0.1N KOH ethanol solution

[0056] [Manufacturing Example 6] <Synthesis of amino resin (C-1)> In a reaction vessel equipped with a thermometer, stirrer, reflux condenser, dropping tank, and nitrogen gas inlet, 187 parts benzoguanamine, 187.5 parts 80% paraformaldehyde, and 518 parts n-butanol were charged. The pH was adjusted to 9.0 with 25% sodium hydroxide aqueous solution, and the mixture was heated to 100°C and reacted for 3 hours. Subsequently, 60% nitric acid aqueous solution was added until the reaction solution reached pH 4, and the reaction was continued for another 3 hours under reflux dehydration. After the reaction was complete, the mixture was neutralized with 25% sodium hydroxide aqueous solution, and n-butanol and water were removed under reduced pressure. Ethylene glycol monobutyl ether was added to the product to obtain a solution of amino resin (C-1) with a non-volatile content of 80%.

[0057] [Example 1] 22 parts of the acrylic resin (A-1) obtained in Production Example 1 (on a non-volatile content basis), 42 parts of the polyester resin (B-1) obtained in Production Example 5 (on a non-volatile content basis), 36 parts of Cymel303LF (Allnex, methylated melamine-based amino resin) as an amino resin, 0.2 parts of p-toluenesulfonic acid as an acid catalyst, 0.5 parts of "BYK302" (Vic Chemie) as a silicone-based leveling agent, and 0.6 parts each of "CERACOL604" (Vic Chemie, carnauba wax) and "CERAFLOUR1050" (Vic Chemie, polyethylene wax) as waxes were mixed. Furthermore, appropriate amounts of deionized water and ethylene glycol monobutyl ether were added and mixed to obtain a water-based paint for cans, with a non-volatile content of 45% and a water concentration of 30%. [Example 2]~[Example 16] Can paint was obtained in the same manner as in Example 1, except that the types and amounts (parts) of each component were changed as shown in Tables 2 and 3.

[0058] Note that in Tables 2 and 3, the blending amounts of acrylic resin (A-1) to (A-4), polyester resin (B-1), amino resins "Cymel303LF", "Cymel232", "Mycoat106", and amino resin (C-1) are calculated based on non-volatile content. The materials used in Tables 1, 2, and 3 are listed below. Cymel232: Methylbutylated melamine amino resin manufactured by Allnex (non-volatile content: 97%) Cymel303LF: Methylated melamine amino resin manufactured by Allnex (non-volatile content: 98%) Mycoat106: Methylated benzoguanamine amino resin manufactured by Allnex (non-volatile content: 77%) BYK302: Polyether-modified polydimethylsiloxane leveling agent manufactured by Bic Chemie. CERACOL604: Carnauba wax manufactured by Big Chemie (non-volatile content concentration 20%) CERAFLOUR1050: Polyethylene wax manufactured by Big Chemie Co., Ltd. CERAFLOUR981: Polytetrafluoroethylene wax manufactured by Bic Chemie. N-BMA: Nn-butoxymethylacrylamide (homopolymer Tg: 50°C) MMA: Methyl methacrylate (homopolymer Tg: 105°C) nBA: n-butyl acrylate (homopolymer Tg: -48℃) LMA: Lauryl methacrylate (Tg of homopolymer: -65℃) St: Styrene (Tg of homopolymer: 100℃) 2-HEMA:2-hydroxyethyl methacrylate (homopolymer Tg: 55℃) AA: Acrylic acid (Tg of homopolymer: 57℃) p-TSA: p-toluenesulfonic acid

[0059] [Comparative Example 1]~[Comparative Example 5] A paint for cans was obtained in the same manner as in Example 1, except that the type and amount (parts) of each component were changed as shown in Table 3.

[0060] <Evaluation of coating film properties> Using the paints obtained for cans in each example, painted boards were prepared and evaluated using the following method. The aluminum can body component for a two-piece can (a bottomed cylindrical component in which the body and bottom are integrated) has a coated film thickness of 50 mg / dm² after drying by a roll coater. 2 The can paint was applied in this manner, baked in a gas oven at 180°C for 52 seconds, and then further cured by baking at 200°C for 2 minutes. After that, the can body was cut open, and the painted portion of the can body was flattened to create a painted panel for evaluation. The following evaluations were performed on the obtained painted panels. The results are shown in Tables 2 and 3.

[0061] <Average friction coefficient of the coating, maximum scratch mark depth, hardness> The average coefficient of friction, maximum scratch depth, and hardness of the coating film were measured on the coating surface using a Hyditron TI Premier (manufactured by Bruker) according to the conditions described in [Modes for Carrying Out the Invention].

[0062] <Abrasion resistance test> The coating on the obtained painted boards was tested using a NORMAN TOOL MODEL7 RCA Abrasion Wear TESTER, and the number of abrasion cycles at which the underlying material became visible was recorded and evaluated. The evaluation criteria are as follows. ◎: 25 times or more. Good. ○: 10 to 24 times. No practical problems. ×: 9 times or less. Not practical.

[0063] <Processing Adhesion> The obtained painted boards were subjected to impact testing using a DuPont impact testing machine, with the coating facing upwards. After the test, commercially available cellophane tape was applied to the processed area of ​​the coating and strongly peeled off. The peeling state of the coating surface was then visually evaluated. The impact testing was performed by dropping a 300g weight from a height of 50cm using a 3 / 8-inch diameter impact pin. The evaluation criteria are as follows. ◎: No peeling in the processed area. Good. ○: Peeling is present in less than 5% of the processed area. No practical problems. ×: More than 5% of the processed area is peeled off. Not suitable for practical use. <Curability> The coating on the obtained painted boards was rubbed back and forth with a 2-pound hammer wrapped in gauze impregnated with methyl ethyl ketone, and the number of rubbings required until the coating dissolved or peeled off was measured. The evaluation criteria are as follows. ◎: More than 200 times. Good. ○: Between 100 and 199 times. No practical problems. ×: 99 times or less. Not practical. <Wax redispersibility> The paint for cans obtained above was stored at 25°C for 30 days, then stirred to prepare painted boards. The condition of the paint film was then visually evaluated. The evaluation criteria are as follows. ◎: No aggregates are observed in the coating. Good. ○: Slight aggregates are visible in the coating. No practical problems. ×: Numerous aggregates are visible in the coating. Unsuitable for practical use.

[0064] [Table 1]

[0065] [Table 2]

[0066] [Table 3]

[0067] As shown in Tables 2 and 3, the can coatings of Examples 1 to 16 all exhibited good physical properties, whereas the can coatings of Comparative Examples 1 to 5 all exhibited poor physical properties in at least one area, and none exhibited all properties as being good.

Claims

1. A paint for cans that satisfies (i) and (ii) in the following nanoscratch test. <Nano-scratch test> A sample obtained by applying can paint to an aluminum can body and baking it at 180°C for 52 seconds and 200°C for 2 minutes was measured at 23°C using a nanoindenter. (i) The average coefficient of friction is 2 or less, and (ii) the maximum scratch mark depth is 120 nm or less.

2. The paint for cans according to claim 1, satisfying (iii) in the following indentation test. <Indentation Test> A sample obtained by applying can paint to an aluminum can body and baking it at 180°C for 52 seconds and 200°C for 2 minutes was measured at 23°C using a nanoindenter. (iii) The film hardness is 0.35 GPa or less.

3. The paint for cans according to claim 1, wherein the paint for cans comprises an acrylic resin, a polyester resin, and an amino resin, and the amino resin is present in an amount of 25 to 185 parts by mass per 100 parts by mass of the total of the acrylic resin and the polyester resin.

4. The amino resin comprises a melamine resin and a benzoguanamine resin, or comprises only a melamine resin. The paint for cans according to claim 3, wherein the mass ratio of the melamine resin to the benzoguanamine resin is melamine resin / benzoguanamine resin = 30 / 70 to 100 / 0.

5. The paint for cans according to claim 1, wherein the content of the fluorine atom-containing compound in the paint for cans is 2 parts by mass or less with respect to 100 parts by mass of the total resin content of the paint for cans.

6. A painted plate comprising a metal plate and a coating film which is a cured product of a paint for cans according to any one of claims 1 to 5.

7. A coated can comprising a metal plate and a coating film which is a cured product of a can coating according to any one of claims 1 to 5.

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