Water-based paints, can lids, and beverage cans

Abstract

To provide an aqueous coating material favorably used for lid inner and outer face-coating capable of forming a coated film excellent in retort resistance even while using a surface active agent which is heretofore considered as a factor for reducing retort resistance, and capable of curing for a short time, capable of forming a coated film excellent in steam sterilization resistance, processability and adhesion.SOLUTION: An aqueous coating material includes: a polymer emulsion (D) obtained by polymerizing using 0.01-1 pts.mass of a nonionic water-soluble polymerization initiator (C) to 100 pts.mass of a mixture (A) of unsaturated ethylene monomers including a mono-functional (meth)acrylic acid ester-based monomer (A-1) having no carboxyl group, a monomer (A-2) having a carboxyl group, and an amide-based monomer (A-3) in the presence of water and a surface active agent (B); and a basic compound (E).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water-based paint, specifically to a water-based paint suitable for use in coating the inner and outer surfaces of can lids for containing beverages or food. More specifically, the present invention relates to a water-based paint capable of forming a coating film that is excellent in steam sterilization resistance (also called "retort resistance"), processability, and adhesion, and further to a coated can obtained by coating the inner and outer surfaces of the lid with the water-based paint. [Background technology]

[0002] BPA-type epoxy resins, synthesized from bisphenol A (hereinafter abbreviated as BPA) and epichlorohydrin, are suitable for use as coatings for the interior and exterior surfaces of can lids because they have the ability to form coating films with excellent steam sterilization resistance, processability, and adhesion. Conventionally, organic solvent-based paints containing BPA-type epoxy resin and phenolic resin as a hardener have been commonly used for can paints, but due to the problem of global environmental pollution caused by organic solvents emitted from paints, organic solvent-based paints have been replaced by water-based paints. For this reason, the use of water-dispersible acrylic-modified epoxy resins, in which BPA-type epoxy resins are modified with acrylic resins and carboxyl groups etc. are introduced into the molecules, has become more widespread.

[0003] In addition, can lids are generally painted using a method known as coil coating, in which paint is continuously applied to a metal coil, such as a coiled steel sheet or aluminum sheet (including aluminum alloy sheet), and then baked. Traditionally, to reduce production costs, coating lines have been speeded up, and the coating method used is the reverse coat method, in which the applicator roll rotates in the opposite direction to the flow of the metal coil to apply the paint. In recent years, as lines have become even faster, the paint baking process has become carried out at higher temperatures and for shorter times than before, creating an increasingly demanding need for paints that can be cured with shorter baking times.

[0004] Therefore, Patent Document 1 discloses an aqueous coating material containing a resin obtained by emulsion-polymerizing an aqueous dispersion of a monomer containing an amide-based monomer, using an aqueous acrylic polymer having a carboxyl group as a high-molecular-weight emulsifier. Furthermore, Patent Document 2 discloses an aqueous coating material characterized by containing core-shell type acrylic resin particles in which ammonium persulfate is preferably used as a polymerization initiator. Furthermore, Patent Document 3 discloses an aqueous coating material characterized by containing core-shell type acrylic resin particles using an azo-based water-soluble initiator in addition to ammonium persulfate. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-193834 [Patent Document 2] Patent No. 5649763 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-113561 DISCLOSURE OF THE INVENTION [Problem to be solved by the invention]

[0006] However, the paint disclosed in Patent Document 1 cannot achieve sufficient curing properties with the recent high-temperature short-time baking. Furthermore, in the paint disclosed in Patent Document 2, an ionic water-soluble initiator such as ammonium persulfate is incorporated into the emulsion resin during the polymerization reaction and behaves as a type of surfactant, which causes a decrease in the steam sterilization resistance of the cured coating film. In addition, the paint disclosed in Patent Document 3 contains substantially the following as a monomer constituting the acrylic resin: Generally, styrene is an essential component, and it is not possible to meet the demand for not using styrene.

[0007] The present invention aims to provide an aqueous paint that is suitable for use in coating the inner and outer surfaces of lids, which can form a coating film that is excellent in retort resistance even while using a surfactant that has traditionally been thought to be a factor in reducing retort resistance, and which can harden in a short time and form a coating film that is excellent in steam sterilization resistance, processability, and adhesion, and to provide a coated can whose lid has been coated with the aqueous paint. [Means for solving the problem]

[0008] In order to address the above-mentioned problems, the inventors conducted extensive research and discovered that a polymer emulsion obtained by polymerizing a specific combination of ethylenically unsaturated monomers in an aqueous medium in the presence of a surfactant can form a coating film that is excellent in steam sterilization resistance, processability, and adhesion, even when a surfactant is used, and that the polymer emulsion can be suitably used as an aqueous paint for coating the inside and outside surfaces of lids, leading to the present invention.

[0009] That is, the present invention relates to an aqueous paint characterized by containing a polymer emulsion (D) obtained by polymerizing, in the presence of water and a surfactant (B), 100 parts by mass of a mixture (A) of ethylenically unsaturated monomers comprising a monofunctional (meth)acrylic acid ester monomer (A-1) having no carboxyl group, a monomer (A-2) having a carboxyl group, and at least one amide monomer (A-3) selected from the group consisting of N-alkoxyalkyl(meth)acrylamide, N-hydroxyalkyl(meth)acrylamide, and (meth)acrylamide, using 0.01 to 1 part by mass of a nonionic water-soluble radical polymerization initiator (C), and a basic compound (E).

[0010] The present invention also relates to the above water-based coating material, which contains 1 to 20 mass % of a monomer (A-2) having a carboxyl group in 100 mass % of the mixture of ethylenically unsaturated monomers (A).

[0011] The present invention also relates to the above aqueous coating material, which contains 0.01 to 10 mass % of the amide monomer (A-3) in 100 mass % of the mixture of ethylenically unsaturated monomers (A).

[0012] The present invention also relates to the above aqueous coating material, wherein the surfactant (B) is an anionic surfactant and the aqueous coating material contains 0.01 to 5 parts by mass of the anionic surfactant per 100 parts by mass of the mixture of ethylenically unsaturated monomers (A).

[0013] The present invention also relates to the above aqueous coating material, wherein the basic compound (E) is contained in an amount such that the neutralization rate is 1 to 50 mol % relative to the carboxyl groups of the polymer emulsion (D).

[0014] The present invention also relates to the above-mentioned water-based paint, which is characterized by containing either a phenolic resin (F) or an amino resin (G).

[0015] Furthermore, the present invention relates to a can lid member having at least one surface coated with the above-mentioned water-based paint.

[0016] Furthermore, the present invention relates to a beverage can comprising the above-mentioned can lid and can body member. [Effects of the Invention]

[0017] The present invention does not use any BPA or styrene-derived components, and has excellent retort resistance even while using surfactants, which have traditionally been thought to be a factor in reducing retort resistance. It has become possible to provide an aqueous paint that is suitable for use in coating the inner and outer surfaces of lids, which can form a coating film that is easy to apply and hardens in a short time and has excellent steam sterilization resistance, processability, and adhesion, and a coated can whose lid has been coated with the aqueous paint. DETAILED DESCRIPTION OF THE INVENTION

[0018] The polymer emulsion (D) contained in the aqueous coating material of the present invention is obtained by radical polymerization of a specific monomer mixture in an aqueous medium using a surfactant and a nonionic water-soluble radical polymerization initiator (C), and is obtained by a method that can also be called an "emulsion polymerization method."

[0019] Next, the various components used in the present invention will be described in detail. "Monomer" refers to a monomer having an ethylenically unsaturated double bond. "(Meth)acrylic" refers to acrylic and methacrylic. "(Meth)acrylate" refers to acrylate and methacrylate.

[0020] The ethylenically unsaturated monomer mixture (A) in the present invention contains a monofunctional (meth)acrylic acid ester monomer (A-1) having no carboxyl group, a monomer (A-2) having a carboxyl group, and at least one amide monomer (A-3) selected from the group consisting of N-alkoxyalkyl(meth)acrylamides, N-hydroxyalkyl(meth)acrylamides, and (meth)acrylamides.

[0021] <Monofunctional (meth)acrylic acid ester monomer (A-1) not having a carboxyl group> Examples of the monofunctional (meth)acrylic acid ester monomer (A-1) having no carboxyl group include alkyl (meth)acrylates such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, hexyl (meth)acrylate, and 2-ethylhexyl (meth)acrylate; Hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate Examples of suitable ethylenically unsaturated monomers include ethylenically unsaturated monomers having a hydroxyl group, such as acrylate, hydroxypentyl (meth)acrylate, and hydroxyhexyl (meth)acrylate. The ethylenically unsaturated monomer mixture (A) preferably contains 70 to 98 mass %, more preferably 80 to 90 mass %, of the monomer (A-1) in a total of 100 mass %. If the content is 70 mass % or more, the appearance, adhesion, and hygienic properties after retort are excellent, while if the content is 98 mass % or less, the polymerization stability and hardness are excellent.

[0022] <Monomer (A-2) Having a Carboxyl Group> Next, the carboxyl group-containing monomer (A-2) will be described. Examples of the carboxyl group-containing monomer (A-2) include (meth)acrylic acid, crotonic acid, itaconic acid (anhydride), maleic acid (anhydride), fumaric acid, citraconic acid, and cinnamic acid, with (meth)acrylic acid being preferred. The carboxyl group-containing monomer (A-2) is preferably contained in an amount of 1 to 20% by mass, and more preferably 5 to 15% by mass, of 100% by mass of the ethylenically unsaturated monomer mixture (A). A content of 1% by mass or more provides excellent polymerization stability and adhesion, while a content of 20% by mass or less provides excellent appearance after retort. The monomer (A-2) having a carboxyl group, together with the surfactant (B), forms monomer droplets containing monomers other than (A-2) during radical polymerization of the monomer mixture (A), or can incorporate other monomers into micelles consisting of the monomer having a carboxyl group. The micelle then incorporates radicals generated from the radical polymerization initiator, thereby initiating polymerization. In other words, the carboxyl group-containing monomer (A-2) and the surfactant (B) provide a site for polymerization in water for the water-insoluble or poorly soluble ethylenically unsaturated monomer. Therefore, the carboxyl group-containing monomer (A-2) has a large effect on the polymerization rate, molecular weight, emulsion particle size, emulsion stability, coating film properties, etc., and is therefore positioned as one of the most important components for the present invention.

[0023] <Amide Monomer (A-3)> Ethylenically unsaturated monomers include N-alkoxyalkyl(meth)acrylamides. , consisting of N-hydroxyalkyl (meth)acrylamide and (meth)acrylamide It is important that the copolymer contains at least one amide monomer (A-3) selected from the group consisting of: By including the amide monomer (A-3), a crosslinkable functional group can be introduced into the polymer dispersed in the polymer emulsion (D) obtained by radical polymerization of the emulsified component (A). By introducing a crosslinkable functional group, the coating material containing the polymer emulsion (D) can undergo self-crosslinking during the baking step after application, forming a strong coating film.

[0024] Examples of the amide monomer (A-3) include N-alkoxyalkyl(meth)acrylamides, N-hydroxyalkyl(meth)acrylamides, and (meth)acrylamides. Examples of N-alkoxyalkyl(meth)acrylamides include N-methoxymethyl(meth)acrylamide, N-ethoxymethyl(meth)acrylamide, N-(n-iso)butoxymethyl(meth)acrylamide ("Nn-butoxymethyl(meth)acrylamide" and "N-isobutoxymethyl(meth)acrylamide" are collectively referred to as "N-(n-iso)butoxymethyl(meth)acrylamide", the same applies below), N-methoxyethyl(meth)acrylamide, N-ethoxyethyl(meth)acrylamide, and N-(n-iso)butoxyethyl(meth)acrylamide. Examples of N-hydroxyalkyl(meth)acrylamides include N-hydroxymethyl(meth)acrylamide, N-hydroxyethyl(meth)acrylamide, and N-hydroxybutyl(meth)acrylamide. Among the monomers (A-3), N-alkoxyalkyl(meth)acrylamides and N-hydroxyalkyl(meth)acrylamides are particularly preferred because they have higher reactivity than (meth)acrylamides.

[0025] The amide monomer (A-3) is preferably contained in an amount of 0.01 to 10% by mass, more preferably 2 to 5% by mass, of 100% by mass of the mixture of ethylenically unsaturated monomers (A). If the amount is 0.01% by mass or more, the appearance and adhesion after retort are excellent, while if it is 10% by mass or less, hygiene is excellent.

[0026] If necessary, for example, polyester resin, acrylic-modified polyester resin, cellulose-based resin, polyvinyl alcohol or a derivative thereof, etc. may be added to the mixture (A) of ethylenically unsaturated monomers in the present invention and subjected to polymerization together with the monomers.

[0027] <Surfactant (B)> The surfactant (B) is preferably used in an amount of 0.01 to 5 parts by mass, more preferably 0.1 to 3 parts by mass, per 100 parts by mass of the mixture of ethylenically unsaturated monomers (A).

[0028] The surfactant is preferably anionic, but nonionic surfactants can also be used.

[0029] Examples of anionic surfactants include higher fatty acid salts such as sodium oleate, alkylarylsulfonates such as dodecylbenzenesulfonic acid, alkyl sulfates such as sodium lauryl sulfate, polyoxyethylene alkyl ether sulfates such as sodium polyoxyethylene lauryl ether sulfate, alkyl sulfosuccinates such as sodium monooctyl sulfosuccinate, sodium dioctyl sulfosuccinate, and sodium polyoxyethylene lauryl sulfosuccinate, and derivatives thereof, and polyoxyethylene distyrenated phenyl ether sulfates.

[0030] <Nonionic water-soluble radical polymerization initiator (C)> Next, the nonionic water-soluble radical initiator (C) will be described. The nonionic radical polymerization initiator in the present invention has a solubility in water of less than 1.0% by mass. is treated as "water-insoluble." In addition, the term "nonionic" in the present invention refers to a property in which no cationic or anionic moieties exist in the molecular structure in water.

[0031] In the polymerization of the ethylenically unsaturated monomer mixture (A), it is important to use a nonionic water-soluble radical polymerization initiator. By using a nonionic water-soluble radical polymerization initiator, the water resistance and retort resistance of the coating film can be improved.

[0032] The nonionic water-soluble radical polymerization initiator (C) is preferably a peroxide or an azo initiator. Examples of the peroxide include hydrogen peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, diisopropylbenzene hydroperoxide, and p-menthane hydroperoxide. Examples of the azo initiator include 2,2'-azobis[2-(2-imidazolin-2-yl)propane], 2,2'-azobis{2-methyl-N-[1,1-bis(hydroxymethyl)-2-hydroxyethyl]propionamide}, and 2,2'-azobis[2-methyl-N-(2-hydroxyethyl)propionamide]. Among these, peroxides are preferred, and hydrogen peroxide and tert-butyl hydroperoxide are more preferred. The nonionic water-soluble radical polymerization initiator (C) can be used alone or in combination of two or more.

[0033] The nonionic water-soluble radical polymerization initiator (C) is used in an amount of 0.01 to 1 part by mass, preferably 0.02 to 0.5 parts by mass, per 100 parts by mass of the mixture of ethylenically unsaturated monomers (A). If the amount is 0.01 part by mass or more, the mixture of ethylenically unsaturated monomers (A) will react sufficiently to obtain a polymer, and if the amount is 1 part by mass or less, the molecular weight of the obtained polymer will be sufficiently large to obtain a coating film with excellent processability and water resistance.

[0034] When polymerizing the monomer mixture, a reducing agent can be used in combination with the polymerization initiator to accelerate the polymerization reaction. Examples of such reducing agents include reducing organic compounds such as ascorbic acid, erythorbic acid, tartaric acid, citric acid, glucose, and metal salts of formaldehyde sulfoxylate, and reducing inorganic compounds such as sodium sulfite, sodium bisulfite, sodium metabisulfite (SMBS), sodium hyposulfite, and ferrous sulfate.

[0035] <Polymer emulsion (D)> As described above, the polymer emulsion (D) in the present invention is prepared by dissolving a monofunctional (meth)acrylic acid ester monomer (A-1) having no carboxyl group, a monomer (A-2) having a carboxyl group, and an N-alkoxyalcohol in the presence of water and a surfactant (B). and at least one amide-based monomer (A-3) selected from the group consisting of alkyl(meth)acrylamide, N-hydroxyalkyl(meth)acrylamide, and (meth)acrylamide. A) is obtained by polymerizing

[0036] In the present invention, the glass transition temperature (hereinafter also referred to as "Tg") of the polymer (hereinafter also referred to as dispersed resin) dispersed in the polymer emulsion (D) is preferably 0°C to 100°C, and more preferably 20°C to 80°C. Generally, the processability of the coating film can be improved by lowering the Tg of the resin constituting the coating material. That is, when an aqueous coating material containing a polymer emulsion in which the dispersed resin has a Tg of 0°C or higher is used to coat the inner and outer surfaces of a can lid, it becomes easier to obtain a coating film that is not scratched during processing. Furthermore, the use of an aqueous coating material containing a polymer emulsion in which the dispersed resin has a Tg of 100°C or lower makes it easier to obtain a coating film with high extensibility that can follow the molding process of the lid. The Tg is a value calculated by the FOX formula based on the weight fraction of each monomer to be polymerized and the Tg of a homopolymer of each monomer.

[0037] <Basic Compound (E)> In the present invention, the basic compound (E) is used to neutralize some or all of the carboxyl groups in the carboxyl group-containing monomer (A-2) and improve the particle stability of the polyemulsion (D).

[0038] The basic compound (E) used in the present invention includes organic amine compounds, ammonia, and hydroxides of alkali metals. Examples of organic amine compounds include monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, triethylamine, monopropylamine, dipropylamine, monoethanolamine, diethanolamine, triethanolamine, N,N-dimethyl-ethanolamine, N,N-diethyl-ethanolamine, 2-dimethylamine -2-methyl-1-propanol, 2-ethanolamine, monoisopropanolamine, Examples include diisopropanolamine and triisopropanolamine. Examples of alkali metal hydroxides include lithium hydroxide, sodium hydroxide, potassium hydroxide, etc. These basic compounds (E) are neutralized at a rate of 1 to 50 mol % relative to the total amount (100 mol %) of carboxyl groups of the dispersed resin in the polymer emulsion (D). It is preferable to use it in this case. If it is 1 mol % or more, the stability of the polymer emulsion (D) is excellent, and if it is 50 mol % or less, a coating material with a viscosity excellent in coatability can be obtained.

[0039] In addition to the polymer emulsion, one or more curing agents such as phenolic resin (F), amino resin (G), polyvinyl alcohol, and derivatives of polyvinyl alcohol may be added to the aqueous coating material of the present invention in order to improve the curing and adhesion of the coating film. The phenolic resin (F) and the amino resin (G) undergo a self-crosslinking reaction, and if the resin in the polymer emulsion (D) has hydroxyl groups, they can also react with the hydroxyl groups, and further, they can also react with the crosslinkable functional groups derived from the amide monomer (A-3). In the present invention, the phenolic resin (F) used may be a trifunctional phenolic compound such as carbolic acid, m-cresol, or 3,5-xylenol, or an o-cresol or p-cresol. , and a bifunctional phenol compound such as p-tert-butylphenol and formaldehyde. Examples include those reacted in the presence of a potassium catalyst. The phenol resin (F) is preferably added in an amount of 0.5 to 20 parts by mass, more preferably 1 to 10 parts by mass, per 100 parts by mass of the solid content of the polymer emulsion (D) (ie, 100 parts by mass of the dispersed resin).

[0040] Examples of the amino resin (G) include compounds obtained by addition reaction of urea, melamine or benzoguanamine with formaldehyde. The amino resin (G) is preferably added in an amount of 0.5 to 20 parts by mass, more preferably 1 to 10 parts by mass, per 100 parts by mass of the solid content of the polymer emulsion (D).

[0041] A hydrophilic organic solvent may be added to the water-based coating material of the present invention for the purpose of improving coating properties. Examples of hydrophilic organic solvents include ethylene glycol monomethyl ether, ethylene glycol dimethyl ether, ethylene glycol monoethyl ether, ethylene glycol diethyl ether, ethylene glycol mono(iso)propyl ether, ethylene glycol di(iso)propyl ether, ethylene glycol mono(iso)butyl ether, ethylene glycol di(iso)butyl ether, ethylene glycol mono-tert-butyl ether, ethylene glycol monohexyl ether, ethylene glycol monohexyl ether, 1,3 -butylene glycol 3-monomethyl ether, 3-methoxybutanol, 3-methyl-3-methoxybutanol, diethylene glycol monomethyl ether, diethylene glycol dimethyl ether, diethylene glycol monoethyl ether, diethylene glycol diethyl ether, diethylene glycol mono(iso)propyl ether, diethylene glycol di(iso)propyl ether, diethylene glycol mono(iso)butyl ether, diethylene glycol di(iso)butyl ether, diethylene glycol monohexyl ether, diethylene glycol dihexyl ether, triethylene glycol dimethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol various ether alcohols or ethers such as glycol mono(iso)propyl ether, propylene glycol mono(iso)butyl ether, propylene glycol dimethyl ether, propylene glycol diethyl ether, propylene glycol di(iso)propyl ether, propylene glycol di(iso)butyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, dipropylene glycol mono(iso)propyl ether, dipropylene glycol mono(iso)butyl ether, dipropylene glycol dimethyl ether, dipropylene glycol diethyl ether, diethylene glycol di(iso)propyl ether, and dipropylene glycol di(iso)butyl ether; Methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, furfuryl alcohol Alcoholic drinks such as cole; Ketones such as methyl ethyl ketone, dimethyl ketone, and diacetone alcohol; Glycols such as ethylene glycol, diethylene glycol, propylene glycol, and dipropylene glycol; Examples include alkoxy esters such as ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, 1-methoxy-2-propyl acetate, and propylene glycol monomethyl ether acetate. These may be used alone or in combination of two or more.

[0042] In addition, various auxiliary agents such as hydrophobic organic solvents, surfactants, and antifoaming agents may be added to the water-based paint of the present invention for the purpose of improving the paintability.

[0043] The aqueous coating material of the present invention can be used not only for the can lids described below, but also for a wide range of general metal materials and metal products, but is particularly suitable as a coating material for coating the inner and outer surfaces of can lids that contain beverages or food, and is particularly suitable for coating the outer surface of lids. Can lid materials include aluminum, tin-plated steel, chrome-treated steel, and nickel-treated steel. Plates and the like are used, and these materials may be subjected to surface treatments such as zirconium treatment or phosphate treatment.

[0044] The water-based paint of the present invention is preferably applied by roll coating, but can also be applied by spray coating such as air spray, airless spray, or electrostatic spray, dip coating, or electrodeposition coating. The water-based paint of the present invention can form a film after application by simply volatilizing the volatile components, but to obtain excellent steam sterilization resistance, workability, and adhesion, it is better to add a baking process. Baking conditions include preferably baking at a temperature of 210°C to 300°C for 10 seconds to 20 minutes.

[0045] A can is obtained by combining a can lid coated with the water-based paint of the present invention with a can body member, and is preferably used to store beverages and food, and is particularly useful as a beverage can. [Example]

[0046] The present invention will be described in more detail below with reference to examples. Unless otherwise specified, "parts" simply means "parts by mass" and "%" means "% by mass".

[0047] [Synthesis Example 1] Synthesis of polymer emulsion (D-1) A reaction vessel equipped with a stirrer, thermometer, reflux condenser, dropping vessel 1, dropping vessel 2, dropping vessel 3, and a nitrogen gas inlet tube was charged with 110 parts of deionized water, and the contents were heated to 50°C while stirring under a nitrogen stream. The temperature rose. A mixture consisting of 61 parts of methyl methacrylate, 25 parts of butyl methacrylate, 9 parts of methacrylic acid, 5 parts of N-butoxymethylacrylamide, 0.2 parts of sodium dodecylbenzenesulfonate as a surfactant, and 40 parts of ion-exchanged water was mixed using a mixer to prepare a monomer pre-emulsion, which was then charged into dropping tank 1 of the reaction vessel. Furthermore, 5 parts of 1% aqueous hydrogen peroxide solution was charged into the dropping tank 2, and 5 parts of 1% aqueous sodium erythorbate solution was charged into the dropping tank 3. The solution was added dropwise from each dropping tank continuously over a period of 2 hours while maintaining the temperature inside the reaction vessel at 50°C. One hour and 1.5 hours after the addition, 0.01 parts of 1% aqueous hydrogen peroxide and 0.01 parts of 1% aqueous sodium erythorbate solution were added, respectively, and the reaction was continued for 2.5 hours after the end of the addition, yielding a polymer emulsion (D-1) of an acrylic copolymer with a glass transition temperature of 70°C (non-volatile content: 39%). Table 1 does not include the additional polymerization initiator and reducing agent.

[0048] "Synthesis Examples 2-15, Comparative Synthesis Examples 1-4" Polymer emulsions (D-2) to (D-3) were prepared in the same manner as in Synthesis Example 1, except that the composition of the monomer pre-emulsion of polymer emulsion (D-1) was changed as shown in Tables 1 and 2. D-18) was obtained. In Comparative Synthesis Example 4, polymerization was attempted according to the composition shown in Table 1, but the polymerization stability was poor and polymer emulsion (D-19) could not be obtained.

[0049] [Comparative Synthesis Example 5a] Synthesis of acrylic copolymer aqueous solution (D-20a) A reaction vessel equipped with a stirrer, thermometer, reflux condenser, dropping tank, and nitrogen gas inlet tube was charged with 8 parts of ethylene glycol monobutyl ether and 18.2 parts of ion-exchanged water, and heating was started to reflux at about 100° C. While maintaining reflux, a mixture of 10 parts of methacrylic acid, 6 parts of methyl methacrylate, 4 parts of ethyl acrylate, and 0.3 parts of benzoyl peroxide was continuously added dropwise from the dropping tank over a period of 4 hours to polymerize. One hour and two hours after the completion of the dropwise addition, 0.03 parts of benzoyl peroxide were added, and the reaction was continued for 3 hours after the completion of the dropwise addition. An acrylic copolymer solution (non-volatile content 41%) with a glass transition temperature of 70°C was obtained. Next, 5.2 parts of dimethylethanolamine was added and stirred for 10 minutes, and then 46.3 parts of ion-exchanged water was added to dissolve the acrylic copolymer in water, thereby obtaining an acrylic copolymer aqueous solution (D-20a) with a nonvolatile content of 20%.

[0050] [Comparative Synthesis Example 5] Synthesis of polymer emulsion (D-20) A reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, dropping vessels 1, 2, and 3, and a nitrogen gas inlet tube was charged with 32 parts of deionized water, and the contents were heated to 50°C while stirring under a nitrogen stream. A mixture consisting of 47 parts of methyl methacrylate, 47 parts of butyl methacrylate, 6 parts of N-butoxymethylacrylamide, and 225 parts of the acrylic copolymer aqueous solution (D-20a) was mixed using a mixer to prepare a monomer pre-emulsion, which was then charged into dropping tank 1 of the reaction vessel. Further, 10 parts of 1% aqueous hydrogen peroxide solution was charged into the dropping tank 2, and 10 parts of 1% aqueous sodium erythorbate solution was charged into the dropping tank 3. The mixture was continuously added dropwise from each dropping tank over a period of 2 hours while maintaining the temperature inside the reaction vessel at 50°C. 1 hour and 1.5 hours after the addition, 0.01 parts of 1% aqueous hydrogen peroxide and 0.01 parts of 1% aqueous sodium erythorbate solution were added, respectively, and the reaction was continued for 2.5 hours after the end of the addition, yielding a polymer emulsion (D-20) of an acrylic copolymer with a glass transition temperature of 27°C (non-volatile content: 39%).

[0051] <Polymerization stability> The polymerization stability of the polymer emulsion (D) was evaluated as follows: The polymer emulsions obtained in the Synthesis Examples and Comparative Synthesis Examples were filtered through a 180-mesh polyester filter cloth, and the amount of residue was visually evaluated. ⊚: Almost no residue is observed. ○: A small amount of residue was observed. △: A lot of residue was observed. ×: Coagulation occurred during polymerization, and no emulsion was obtained.

[0052] [Example 1] 100 parts of the polymer emulsion (D-1) obtained in Synthesis Example 1, 160 parts of ion-exchanged water The neutralization rate of the carboxyl groups in the polymer emulsion (D-1) was equivalent to 15 mol %. A certain amount of dimethylaminoethanol (hereinafter referred to as DMAE) as a basic compound was mixed, and then 4 parts of n-butanol and 6 parts of ethylene glycol monobutyl ether were added to obtain a water-based paint (E-1) with a non-volatile content of 25%.

[0053] [Examples 2 to 17, Comparative Examples 1 to 4] Based on the formulations in Tables 3 and 4, water-based paints (E-2) to (E-21) were obtained in the same manner as in Example 1.

[0054] <Evaluation of coating film> Each of the water-based paints obtained in Examples 1 to 17 and Comparative Examples 1 to 4 was applied to an aluminum plate having a thickness of 0.26 mm to a dry film thickness of 45 mg / dm 2The coating was applied so that the coating thickness was 100 μm, and the coating was baked in a gas oven at an atmospheric temperature of 280° C. for 3 seconds to obtain a test panel for evaluation, and the performance of the coating film was evaluated as follows. Each evaluation method is explained below.

[0055] <Appearance of initial coating> The test panels are visually evaluated. ◎: The coating is smooth and free of lumps and bubbles. ◯: There are a few tiny particles, but this does not pose a problem in practical use. △: There are many bumps on the coating film, which is problematic for practical use. ×: Significant bumps are present over the entire surface of the coating film. Unsuitable for practical use.

[0056] <Appearance after retort> The test panel was immersed in water and retorted in a retort oven at 125°C for 30 minutes, and the appearance of the coating was visually evaluated. ◎: No change from untreated coating 〇: Very slight whitening △: Slightly whitened ×: Significant whitening

[0057] <Adhesion after retort> The test panel was immersed in water and retorted in a retort oven at 125°C for 30 minutes. After cutting a grid pattern on the coating surface with a cutter, cellophane adhesive tape was applied and then peeled off firmly, after which the coating surface was evaluated. ◎: No peeling at all. ○: Less than 5% peeling. △: 5 to 50% peeling. ×: More than 50% peeling

[0058] <Bending processability evaluation> The test panel was cut to a size of 30 mm x 50 mm, and pre-folded by hand with the coating facing outward so that the test area was 30 mm wide. Five 0.26 mm thick aluminum plates were placed between the two folded test pieces, and a 3 kg load was dropped from a height of 40 cm onto the folded part to completely fold it. Next, the bent tip of the test piece was immersed in 1% saline solution, and a current of 6.0 V was passed between the metal part of the test piece that was not immersed in the saline solution and the saline solution for 4 seconds, and the current value was measured. If the coating has poor workability, the coating at the bent portion will crack, exposing the underlying metal plate and increasing the conductivity, resulting in a high current value. ◎: Less than 10mA ○: 10mA or more and less than 20mA △: 20mA or more and less than 50mA ×: 50mA or more

[0059] <Pencil hardness> Indicates the maximum hardness at which the coating does not peel off when measured with Mitsubishi Pencil Co., Ltd.'s "Uni" (registered trademark) in an environment of 25°C. ◎:5H or more 〇:3H-4H △:H-2H ×: F or less

[0060] <Hygiene> The test panel was immersed in water and retorted in a retort oven at 125°C for 30 minutes. The water after retort treatment was analyzed using a "TOC-LCPH" (Shimadzu Corporation) to measure the total organic carbon (TOC) content. The TOC content is the total amount of organic matter present in the water, expressed as the amount of carbon in the organic matter. ◎: Less than 2 ppm (good) ○: 2 ppm or more and less than 5 ppm (usable) ×: 5ppm or more (defective, unusable)

[0061] [Table 1]

[0062] [Table 2]

[0063] [Table 3]

[0064] Table 4

Claims

1. The emulsion contains a polymer emulsion (D) obtained by polymerizing, in the presence of water and a surfactant (B), a mixture of ethylenically unsaturated monomers (A) containing a monofunctional (meth)acrylic acid ester monomer (A-1) having no carboxyl group, a monomer (A-2) having a carboxyl group, and at least one amide monomer (A-3) selected from the group consisting of N-alkoxyalkyl(meth)acrylamide, N-hydroxyalkyl(meth)acrylamide, and (meth)acrylamide, using 0.01 to 1 part by mass of a nonionic water-soluble radical polymerization initiator (C) per 100 parts by mass of the mixture (A), and a basic compound (E), in an amount such that the neutralization rate is 1 to 50 mol % relative to 100 mol % of the total amount of carboxyl groups in the polymer emulsion (D). The aqueous coating composition is characterized in that, based on 100% by mass of a mixture (A) of ethylenically unsaturated monomers, it contains 1 to 15% by mass of a monomer (A-2) having a carboxyl group and 1 to 10% by mass of an amide-based monomer (A-3).

2. 2. The aqueous paint according to claim 1, wherein the surfactant (B) is an anionic surfactant and contains 0.01 to 5 parts by mass of the anionic surfactant per 100 parts by mass of the mixture of ethylenically unsaturated monomers (A).

3. 3. The aqueous paint according to claim 1, wherein the basic compound (E) is contained in an amount such that the neutralization rate is 10 to 40 mol % relative to 100 mol % of the total amount of carboxyl groups in the polymer emulsion (D).

4. 4. The water-based paint according to claim 1, which contains either a phenolic resin (F) or an amino resin (G).

5. A can lid member, at least one surface of which is coated with the water-based paint according to any one of claims 1 to 4.

6. A beverage can comprising the can lid according to claim 5 and a can body member.

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