JP7806840B2 - Water-based paints, can lids, and beverage cans - Google Patents

Description

The present invention relates to a water-based paint, and more specifically to a water-based paint suitable for use in coating the interior and exterior 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 exhibits excellent steam sterilization resistance (also known as "retort resistance"), processability, and adhesion, and further relates to a coated can in which the interior and exterior surfaces of the lid are coated with the water-based paint.

BPA-type epoxy resins, synthesized from bisphenol A (hereinafter abbreviated as BPA) and epichlorohydrin as raw materials, have the ability to form coating films that are excellent in steam sterilization resistance, processability, and adhesion, and are therefore suitable for use as coating materials for the inner and outer surfaces of can lids.
Conventionally, organic solvent-based paints containing BPA-type epoxy resins and phenolic resins as hardeners have been commonly used as 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of global environmental pollution caused by organic solvents emitted from paints, organic solvent-based paints have been replaced by water-based paints.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 and the like are introduced into the molecules, has become widespread.

Generally, can lids are often 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, coating lines have been speeded up to reduce production costs, 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. In recent years, as lines have become even faster, the coating baking process has become carried out at higher temperatures and for shorter times than before, creating an ever-increasing demand for paints that harden with shorter baking times.

Therefore, Patent Document 1 discloses an aqueous paint 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 a water-based paint 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 that contains core-shell type acrylic resin particles using an azo-based water-soluble initiator in addition to ammonium persulfate.

JP 2015-193834 A Patent No. 5649763 JP 2016-113561 A

However, the paint disclosed in Patent Document 1 cannot achieve sufficient hardening 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.
Furthermore, the paint disclosed in Patent Document 3 essentially contains styrene as an essential component of the acrylic resin as a monomer, and is therefore unable to meet the demand for not using styrene.

The present invention aims to provide a water-based paint that is suitable for use as a coating for the inner and outer surfaces of lids, and which can form a coating film with excellent retort resistance despite using surfactants that have traditionally been thought to reduce retort resistance, and which can be cured in a short time to form a coating film with excellent steam sterilization resistance, processability, and adhesion, as well as a coated can whose lids are coated with the water-based paint.

In order to address the above issues, 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 exhibits excellent steam sterilization resistance, processability, and adhesion, even when using a surfactant, and that the polymer emulsion can be suitably used as a water-based coating for the interior and exterior surfaces of lids, leading to the present invention.

Specifically, 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), a mixture of ethylenically unsaturated monomers (A) including a monofunctional (meth)acrylic acid ester monomer (A-1) not having a 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. and a basic compound (E).

The present invention also relates to the above-mentioned aqueous coating material, which contains 1 to 20 mass% of a carboxyl group-containing monomer (A-2) relative to 100 mass% of the ethylenically unsaturated monomer mixture (A).

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

The present invention also relates to the above aqueous paint, in which the surfactant (B) is an anionic surfactant and the content of the anionic surfactant is 0.01 to 5 parts by mass per 100 parts by mass of the mixture of ethylenically unsaturated monomers (A).

The present invention also relates to the above-mentioned aqueous paint, in which 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).

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).

Furthermore, the present invention relates to a can lid in which at least one surface of a can lid component is coated with the above-mentioned water-based paint.

Furthermore, the present invention relates to a beverage can comprising the above-mentioned can lid and can body member.

The present invention has made it possible to provide an aqueous paint that is completely free of BPA- and styrene-derived components, and that can form a coating film that is excellent in retort resistance even while using surfactants that have traditionally been thought to be a factor in reducing retort resistance, and that can harden in a short time and form a coating film that is excellent in steam sterilization resistance, processability, and adhesion, and that is suitable for use as a coating for the inner and outer surfaces of lids, as well as a coated can whose inner and outer surfaces are coated with the same.

The polymer emulsion (D) contained in the aqueous paint 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), a method that can also be called an "emulsion polymerization method."

Next, the various components used in this 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.

The ethylenically unsaturated monomer mixture (A) in the present invention contains a monofunctional (meth)acrylic acid ester monomer (A-1) that does not have a carboxyl group, a monomer (A-2) that has 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.

<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;
Examples of such ethylenically unsaturated monomers include hydroxyl group-containing ethylenic unsaturated monomers such as hydroxymethyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, hydroxypentyl (meth)acrylate, and hydroxyhexyl (meth)acrylate.
The ethylenically unsaturated monomer mixture (A) preferably contains 70 to 98% by mass, more preferably 80 to 90% by mass, of the monomer (A-1) in a total of 100% by mass. If the content is 70% by mass or more, the appearance, adhesion, and hygiene after retort are excellent, while if the content is 98% by mass or less, the polymerization stability and hardness are excellent.

<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.
When the monomer mixture (A) is radically polymerized, the carboxyl group-containing monomer (A-2) together with the surfactant (B) forms monomer droplets containing monomers other than (A-2) or solubilizes other monomers in micelles composed of the carboxyl group-containing monomer, thereby initiating polymerization with radicals generated from the radical polymerization initiator incorporated into the micelles. In other words, the carboxyl group-containing monomer (A-2) together with the surfactant (B) provides a site for polymerization in water for ethylenically unsaturated monomers that are insoluble or poorly soluble in water.
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.

<Amide Monomer (A-3)>
It is important that the ethylenically unsaturated monomer contains at least one amide monomer (A-3) selected from the group consisting of N-alkoxyalkyl(meth)acrylamides, N-hydroxyalkyl(meth)acrylamides, and (meth)acrylamides.
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 coating, forming a strong coating film.

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 ("N-n-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.

Of 100% by mass of the ethylenically unsaturated monomer mixture (A), the amide monomer (A-3) preferably accounts for 0.01 to 10% by mass, and more preferably 2 to 5% by mass. A content of 0.01% by mass or more provides excellent appearance and adhesion after retort, while a content of 10% by mass or less provides excellent hygiene.

If necessary, for example, polyester resin, acrylic-modified polyester resin, cellulose-based resin, polyvinyl alcohol or its derivatives, etc. may be added to the ethylenically unsaturated monomer mixture (A) of the present invention and subjected to polymerization together with the monomers.

<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).

Anionic surfactants are preferred. Nonionic surfactants can also be used.

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 and derivatives thereof such as sodium monooctyl sulfosuccinate, sodium dioctyl sulfosuccinate, and sodium polyoxyethylene lauryl sulfosuccinate, and polyoxyethylene distyrenated phenyl ether sulfates.

<Nonionic Water-Soluble Radical Polymerization Initiator (C)>
Next, the nonionic water-soluble radical initiator (C) will be described. In the present invention, the nonionic radical polymerization initiator having a solubility in water of less than 1.0 mass % is regarded 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.

When polymerizing the ethylenically unsaturated monomer mixture (A), it is important to use a nonionic, water-soluble radical polymerization initiator. Using a nonionic, water-soluble radical polymerization initiator can improve the water resistance and retort resistance of the coating film.

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.

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.

When polymerizing the monomer mixture, a reducing agent can be used in combination with the polymerization initiator. This accelerates 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.

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

In the present invention, the glass transition temperature (hereinafter also referred to as "Tg") of the polymer dispersed in the polymer emulsion (D) (hereinafter also referred to as the dispersed resin) 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 from the weight fraction of each monomer to be polymerized and the Tg of a homopolymer of each monomer, according to the FOX formula.

<Basic Compound (E)>
The basic compound (E) is used in the present invention 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).

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-dimethylethanolamine, N,N-diethylethanolamine, 2-dimethylamino-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 preferably used in a neutralization ratio of 1 to 50 mol % relative to 100 mol % of the total amount of carboxyl groups of the dispersed resin in the polymer emulsion (D).
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 having a viscosity excellent in coatability can be obtained.

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 properties and adhesion of the coating film.
The phenol resin (F) and the amino resin (G) undergo a self-crosslinking reaction, and also react with the hydroxyl groups of the resin in the polymer emulsion (D) if such resin has such groups, and further react with the crosslinkable functional groups derived from the amide monomer (A-3).
Examples of the phenolic resin (F) used in the present invention include those obtained by reacting a trifunctional phenolic compound such as carbolic acid, m-cresol, or 3,5-xylenol, or a bifunctional phenolic compound such as o-cresol, p-cresol, or p-tert-butylphenol with formaldehyde in the presence of an alkali catalyst.
The phenolic 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) (i.e., 100 parts by mass of the dispersed resin).

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).

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;
Alcohols such as methyl alcohol, ethyl alcohol, propyl alcohol, isopropyl alcohol, butyl alcohol, sec-butyl alcohol, tert-butyl alcohol, and furfuryl alcohol;
Ketones such as methyl ethyl ketone, dimethyl ketone, and diacetone alcohol;
Glycols such as ethylene glycol, diethylene glycol, propylene glycol, and dipropylene glycol;
Examples of such alkoxyesters include 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.

In addition, various auxiliary agents such as hydrophobic organic solvents, surfactants, and defoamers can be added to the water-based paint of the present invention to improve its coatability.

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.
Materials used for can lids include aluminum, tin-plated steel sheets, chromium-treated steel sheets, and nickel-treated steel sheets, and these materials may be subjected to surface treatments such as zirconium treatment and phosphate treatment.

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 simply by volatilizing the volatile components after application, but to obtain excellent steam sterilization resistance, workability, and adhesion, it is better to add a baking process. Baking conditions include baking at a temperature of 210°C to 300°C for 10 seconds to 20 minutes.

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 or food, but is particularly useful as a beverage can.

The present invention will be explained in more detail below using examples. Unless otherwise specified, "parts" simply means "parts by mass" and "%" means "% by mass."

Synthesis Example 1 Synthesis of Polymer Emulsion (D-1) 110 parts of deionized water was charged into a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, dropping tanks 1, 2, and 3, and a nitrogen gas inlet tube, and the temperature of the contents was raised to 50°C while stirring under a nitrogen stream.
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.
Further, 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 contents were added dropwise from each dropping tank continuously over 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 a 1% aqueous hydrogen peroxide solution and 0.01 parts of a 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 having a glass transition temperature of 70° C. (non-volatile content: 39%).
Table 1 does not include the additional polymerization initiator and reducing agent.

"Synthesis Examples 2 to 15, Comparative Synthesis Examples 1 to 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.

Comparative Synthesis Example 5a Synthesis of Acrylic Copolymer Aqueous Solution (D-20a) A reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, a dropping tank, and a 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 carry out polymerization.
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. The mixture was then cooled to obtain a solution of an acrylic copolymer (non-volatile content 41%) having a number average molecular weight of 25,000 and a glass transition temperature of 70°C.
Next, 5.2 parts of dimethylethanolamine was added, and the mixture was 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 aqueous acrylic copolymer solution (D-20a) with a nonvolatile content of 20%.

Comparative Synthesis Example 5 Synthesis of Polymer Emulsion (D-20) 32 parts of deionized water was charged into a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, dropping tanks 1, 2, and 3, and a nitrogen gas inlet tube, and the temperature of the contents was raised to 50°C while stirring under a nitrogen stream.
A mixture 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 contents were added dropwise from each dropping tank continuously over 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 a 1% aqueous hydrogen peroxide solution and 0.01 parts of a 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 (non-volatile content: 39%) having a glass transition temperature of 27° C.

<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.
x: Coagulation occurred during polymerization, and no emulsion was obtained.

[Example 1]
100 parts of the polymer emulsion (D-1) obtained in Synthesis Example 1, 160 parts of ion-exchanged water, and dimethylaminoethanol (hereinafter, DMAE) as a basic compound in an amount corresponding to a neutralization rate of 15 mol% relative to the carboxyl groups of the polymer emulsion (D-1) were mixed, and 4 parts of n-butanol and 6 parts of ethylene glycol monobutyl ether were further added to obtain an aqueous coating material (E-1) with a non-volatile content of 25%.

[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.

<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 with a thickness of 0.26 mm so that the dry film thickness would be 45 mg/ ^dm² , and the plate was baked in a gas oven at an atmospheric temperature of 280°C for 3 seconds to obtain a test panel for evaluation. The performance of the coating film was evaluated as follows.
Each evaluation method is explained below.

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

<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 film was visually evaluated.
◎: No change from untreated coating ○: Very slight whitening △: Slight whitening ×: Significant whitening

<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 firmly peeled off, after which the coating surface was evaluated.
⊚: No peeling at all.
◯: Less than 5% peeling occurred.
△: 5 to 50% peeling.
×: More than 50% peeled off

<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 aluminum plates with a thickness of 0.26 mm were sandwiched between the two halves of the test piece, and a 3 kg load was dropped from a height of 40 cm onto the bent 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 but less than 20mA △: 20mA or more but less than 50mA ×: 50mA or more

<Pencil hardness>
This 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 ○: 3H-4H
△: H-2H
×: F or less

<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)

Claims (6)

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. 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. 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. The water-based paint according to claim 1, which contains either a phenolic resin (F) or an amino resin (G). 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 . A beverage can comprising the can lid according to claim 5 and a can body member.