JP2008297379A - Aqueous coating material and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain an aqueous coating material that forms a polymer emulsion and a coating film having excellent steam sterilization resistance, processability, adhesivity and corrosion resistance without using a BPA-derived constituent component at all and not substantially utilizing a surfactant being a causative agent to deteriorate steam sterilization resistance of cured coating film after baking and is suitably used for coating the inside of a can. <P>SOLUTION: The aqueous coating material comprises a polymer emulsion (F) produced by radically polymerizing a component (A) to be emulsified containing an ethylenic unsaturated monomer (A1) with a cationic water-soluble radical initiator (E) in the presence of an aqueous solution or emulsion (1) of a polymer comprising a carboxy group-containing component (B) containing a carboxy group-containing acrylic copolymer (B1), a basic compound (C) and water (D). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a water-based paint, and more particularly to a water-based paint suitably used for coating an inner surface of a can containing a beverage or food. More specifically, the present invention relates to a water-based paint capable of forming a coating film having excellent hygiene and steam sterilization resistance, processability, and adhesion, and further relates to a coated can formed by coating the inner surface of the can.

  BPA type epoxy resin synthesized from bisphenol A (hereinafter abbreviated as “BPA”) and epichlorohydrin has a function of forming a coating film excellent in steam sterilization resistance, workability and adhesion. It is suitably used for coating materials for inner and outer surfaces. Conventionally, water-dispersed acrylic modified epoxy resins in which a BPA type epoxy resin is modified with an acrylic resin and a carboxyl group or the like is introduced into the molecule are mainly used for water-based coatings for can inner surface coating.

  Other than BPA type epoxy resins, examples of resins having processability and adhesion similar to those of BPA type epoxy resins include emulsion type acrylic resins synthesized by emulsion polymerization. Emulsion type acrylic resin synthesized by emulsion polymerization method is generally known to have a very high molecular weight compared to acrylic resin synthesized by solution polymerization method. It is thought that processability and adhesion can be obtained.

  The emulsion polymerization method is a polymerization reaction described by a special mechanism in which a solubilized monomer reacts with a polymerization initiator in a micelle, and then the monomer is sequentially supplied from dispersed monomer droplets. In general, since a surfactant is used as an emulsifier in the emulsion polymerization method, naturally, the surfactant is also contained in the emulsion resin obtained by this polymerization reaction. Therefore, when the emulsion resin produced by the emulsion polymerization method is used as an aqueous paint, the cured coating film made of this paint deteriorates the steam sterilization resistance due to the influence of the surfactant in the emulsion resin, Causes blistering (dotted peeling).

  In addition, anionic water-soluble initiators such as ammonium persulfate and potassium persulfate are preferably used as the polymerization initiator in the emulsion polymerization method. These ionic water-soluble initiators are used in the polymerization reaction as an emulsion. Since it is incorporated in the resin and exhibits a behavior as a kind of surfactant, it becomes a factor of lowering the steam sterilization resistance of the cured coating film as in the case of the aforementioned surfactant.

  As described above, the emulsion type acrylic resin synthesized by the usual emulsion polymerization method has a surfactant and an anionic water-soluble initiator, which deteriorate the steam sterilization resistance of the cured coating film. However, at this stage, it has not been commercialized.

  Therefore, instead of using a surfactant that causes deterioration of steam sterilization resistance, instead of using an aqueous acrylic polymer having a carboxyl group and a crosslinkable functional group other than a carboxyl group and a base compound, A method in which a mixture is dispersed in an aqueous medium in advance to obtain an aqueous dispersion (pre-emulsion) of the monomer, and the aqueous dispersion of the monomer is radically polymerized in the presence of a separately prepared aqueous resin having a carboxyl group. (Hereinafter also referred to as “pre-emulsification method”) has been proposed (Patent Document 1). That is, Patent Document 1 states that a soap-free acrylic resin emulsion obtained by dropwise polymerization of an aqueous dispersion of an acrylic monomer in the presence of an aqueous resin having a carboxyl group can be used in an aqueous coating composition for cans. It is described that the water-based coating composition can form a coating film excellent in processability, adhesion, and boiling resistance. However, depending on the type of contents, the can covered with the paint for cans may be subjected to a steam sterilization process more severe than boiling. When a can coated with an aqueous coating composition obtained by the method disclosed in Patent Document 1 is steam sterilized, the coating film may be whitened or blistered (dotted peeling).

  Furthermore, in can paints, especially beverage can applications, various processes may be performed on the can body in order to accommodate filling of various contents. In particular, the lid portion is subjected to complicated and advanced processing to prevent deformation of the can body after filling the contents and corrosion caused thereby. It should be noted that this advanced processing is performed after the cured coating is provided on both sides of the lid. Naturally, the coating does not cause coating defects even under this processing. Sex is required. However, when the aqueous coating composition obtained by the method described in Patent Document 1 is used, the required workability level cannot be ensured.

In addition, the dispersion stability deteriorated due to alcohol added at the time of preparing the water-based paint, and an aqueous paint having a stable viscosity and viscosity could not be obtained.
JP 2002-155234 A

  An object of the present invention is to use a polymer emulsion without using any component derived from BPA and substantially not using a surfactant that is a causative substance that deteriorates the steam sterilization resistance of a cured coating film after baking. A water-based paint that can be formed and can form a coating film excellent in steam sterilization resistance, processability, adhesion, and corrosion resistance, and is suitably used for coating the inner surface of a can, and formed by coating the inner surface of the can It is to provide a coated can.

  As a result of repeated studies by the present inventors on the above problems, a polymer emulsion obtained by radical polymerization of an ethylenically unsaturated monomer in an aqueous medium in the presence of a carboxyl group-containing component contains a component derived from BPA. In addition, it is possible to form a coating film excellent in steam sterilization resistance, processability, adhesion, and corrosion resistance without substantially using a surfactant, and is suitable for water-based paints for can inner surface coating. As a result, the present invention has been completed.

  That is, the first invention is a polymer containing a carboxyl group-containing component (B) containing a carboxyl group-containing acrylic copolymer (B1), a basic compound (C), and water (D). Polymer emulsion obtained by radical polymerization of an emulsified component (A) containing an ethylenically unsaturated monomer (A1) with a cationic water-soluble radical initiator (E) in the presence of an aqueous solution or emulsion (1) It is a water-based paint characterized by containing F).

  In the second invention, the cationic water-soluble radical initiator (E) is 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [2- (2-imidazoline-2- Yl) propane] dihydrochloride, 2,2′-azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane} dihydrochloride, and 2,2′-azobis [1-imino −1-pyrrolidino-2-ethylpropane] dihydrochloride is at least one selected from the group consisting of dihydrochloride.

  In the third invention, the ethylenically unsaturated monomer (A1) is at least an amide monomer selected from the group consisting of N-alkoxyalkyl (meth) acrylamide, N-hydroxyalkyl (meth) acrylamide, and (meth) acrylamide. One type of water-based paint according to the first or second aspect of the invention.

  A fourth invention is the water-based paint according to any one of the first to third inventions, wherein the acrylic copolymer (B1) having a carboxyl group has a number average molecular weight of 5,000 to 100,000.

  A fifth invention is the water-based paint according to any one of the first to fourth inventions, wherein the acid value of the acrylic copolymer (B1) having a carboxyl group is 50 to 500 mgKOH / g.

  A sixth invention is the water-based paint according to any one of the first to fifth inventions, wherein the polymer emulsion (F) has a glass transition temperature of 0 to 100 ° C.

  A seventh invention is the water-based paint according to any one of the first to sixth inventions, characterized in that it contains at least one component selected from the group consisting of a phenol resin and an amino resin.

  An eighth invention is the water-based paint according to any one of the first to seventh inventions, characterized in that it is for covering a can material for containing a beverage or food.

  According to a ninth aspect of the present invention, there is provided a coated can characterized in that a can material for containing a beverage or food is coated with the aqueous paint of the eighth aspect of the invention.

  The tenth invention is an aqueous solution of a polymer comprising a carboxyl group-containing component (B) containing a carboxyl group-containing acrylic copolymer (B1), a basic compound (C), and water (D). Polymer emulsion characterized by radical polymerization of an emulsified component (A) containing an ethylenically unsaturated monomer (A1) with a cationic water-soluble radical initiator (E) in the presence of the emulsion (1) It is a manufacturing method.

  According to the present invention, an aqueous paint that is excellent in hygiene, can form a coating film excellent in steam sterilization resistance, processability, adhesion, and corrosion resistance, can be suitably used for can inner surface coating, and has excellent stability is provided. can do.

  The polymer emulsion (F) contained in the water-based paint of the present invention is a polymer containing a carboxyl group-containing component (B) essentially comprising an acrylic copolymer having a carboxyl group, instead of using a so-called general surfactant. The emulsified component (A) containing the ethylenically unsaturated monomer (A1) is radically polymerized with a cationic water-soluble radical initiator (E) in an aqueous medium without pre-emulsification in advance. It is obtained by a method that should be called a “monomer dropping method”.

Next, various components used in the present invention will be described in detail.
<Emulsified component (A)>
The emulsified component (A) is mainly composed of an ethylenically unsaturated monomer (A1), many of which are insoluble or sparingly soluble in water, and are usually solution polymerization using an organic solvent or a surfactant. The emulsion is subjected to emulsion polymerization. The present invention provides a polymer emulsifier containing a carboxyl group-containing component containing an acrylic copolymer having a carboxyl group, without using a so-called general low molecular weight surfactant, a monomer insoluble or hardly soluble in water. And is radically polymerized with a cationic water-soluble radical initiator (E). For the emulsified component (A), in addition to the ethylenically unsaturated monomer (A1), for example, a polyester resin, an acrylic-modified polyester resin, a cellulose resin, polyvinyl alcohol or a derivative thereof is mixed with the ethylenically unsaturated monomer (A1). In the state, it can also use for radical polymerization as an emulsified component (A).

Of the emulsified component (A), the ethylenically unsaturated monomer (A1) will be described. The ethylenically unsaturated monomer (A1) is an ethylenically unsaturated monomer that is emulsified when acrylic emulsion polymerization is performed using an ethylenically unsaturated monomer or a surfactant used in normal acrylic solution polymerization. The thing similar to a monomer can be used. As an example of the ethylenically unsaturated monomer (A1), (meth) acrylic acid ["acrylic acid" and "methacrylic acid" are collectively referred to as "(meth) acrylic acid". The same applies below. ], Ethylenically unsaturated monomers having a carboxyl group such as (anhydrous) itaconic acid, (anhydrous) maleic acid;
Methyl (meth) acrylate, ethyl (meth) acrylate, isopropyl (meth) acrylate, n-propyl (meth) acrylate, n-butyl (meth) acrylate, isobutyl (meth) acrylate, t-butyl (meth) acrylate, hexyl ( Alkyl (meth) acrylates such as (meth) acrylate and ethylhexyl (meth) acrylate,
Hydroxyl groups such as hydroxymethyl (meth) acrylate, 2-hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, hydroxypentyl (meth) acrylate, and hydroxyhexyl (meth) acrylate Ethylenically unsaturated monomers having:
Aromatic monomers such as styrene and methylstyrene;
N-hydroxyalkyl (meth) acrylamides such as N-hydroxymethyl (meth) acrylamide, N-hydroxyethyl (meth) acrylamide, N-hydroxybutyl (meth) acrylamide;
N-methoxymethyl (meth) acrylamide, N-ethoxymethyl (meth) acrylamide, N- (n-, iso) butoxymethyl (meth) acrylamide, N-methoxyethyl (meth) acrylamide, N-ethoxyethyl (meth) acrylamide N-alkoxyalkyl (meth) acrylamides such as N- (n-, iso) butoxyethyl (meth) acrylamide;
And amide monomers such as (meth) acrylamide.

  The ethylenically unsaturated monomer (A1) contains at least one amide monomer selected from the group consisting of N-alkoxyalkyl (meth) acrylamide, N-hydroxyalkyl (meth) acrylamide, and (meth) acrylamide. It is preferable. The amide monomers are all self-crosslinking monomers and can easily self-condense under heating or acidic conditions. Further, it is known to react with these functional groups in the presence of a carboxyl group or a hydroxyl group. That is, the polymer emulsion obtained by radical polymerization of the emulsified component (A) containing the amide monomer can have self-crosslinking property in the particles.

  In the present invention, the crosslinking reaction of the polymer emulsion obtained by radical polymerization of the emulsified component (A) containing an amide monomer proceeds in the baking step during radical polymerization and during coating. The crosslinking reaction at the time of radical polymerization contributes to high molecular weight and three-dimensional structure in the emulsion particles, and improves the workability and corrosion resistance of the resulting coating film. Moreover, in a baking process, it contributes to bridge | crosslinking between polymer emulsion particles, can improve film forming property, and can form a firm coating film.

  The ethylenically unsaturated monomer (A1) does not contain an amide monomer selected from the group consisting of N-alkoxyalkyl (meth) acrylamide, N-hydroxyalkyl (meth) acrylamide, and (meth) acrylamide. If a saturated monomer (A1) is used and an acrylic copolymer (B1) having a carboxyl group, which will be described later, does not contain a crosslinkable functional group derived from the amide monomer, the amide-free polymer is used. An emulsion can also be obtained.

<Carboxyl group-containing component (B)>
Next, the carboxyl group-containing component (B) containing the acrylic copolymer (B1) having a carboxyl group will be described. The carboxyl group-containing component (B) functions as an emulsifier component with respect to the emulsified component (A). When the carboxyl group-containing component (B) radically polymerizes the emulsified component (A), it forms monomer droplets using the emulsified component (A) containing the ethylenically unsaturated monomer (A1) as droplets. In addition, the emulsified component (A) is solubilized in the micelle composed of the carboxyl group-containing component (B), or the polymerization is started in a state in which the radical generated from the radical polymerization initiator is taken into the micelle. That is, the carboxyl group-containing component (B) provides a place for polymerization in water to the ethylenically unsaturated monomer (A1) that is insoluble or hardly soluble in water. Accordingly, the carboxyl group-containing component (B) has a great influence on the polymerization rate, molecular weight, particle size, emulsion stability, coating film physical properties and the like, and is therefore positioned as one of the most important components for the present invention.

  The number average molecular weight of the acrylic copolymer (B1) having a carboxyl group contained in the carboxyl group-containing component (B) is preferably 5,000 to 100,000, and the number average molecular weight is 10,000 to 70,000. Is more preferable. When the number average molecular weight is less than 5,000, the stability of the polymer emulsion obtained by polymerizing the component to be emulsified (A) is inferior, and the occurrence of bumps and sedimentation of the emulsion are likely to occur. When the number average molecular weight exceeds 100,000, the viscosity of the aqueous solution or emulsion (1) containing the acrylic copolymer (B1) is increased, and a gel product is easily generated.

  As the carboxyl group-containing component (B) containing the acrylic copolymer (B1) having a carboxyl group, in addition to the acrylic copolymer (B1), a polyester resin containing a carboxyl group, an acrylic modified polyester resin, Components that can be dissolved or dispersed in water such as cellulose resin, polyvinyl alcohol, or derivatives thereof can also be used.

  In general, an emulsifier in emulsion polymerization plays a role of emulsifying a hydrophobic component in an aqueous medium. Therefore, it is essential to have a hydrophilic part and a hydrophobic part. The acrylic copolymer (B1) having a carboxyl group has a carboxyl group as a hydrophilic portion. The acrylic copolymer (B1) having a carboxyl group is obtained by copolymerizing an ethylenically unsaturated monomer having a carboxyl group. The ethylenically unsaturated monomer having a carboxyl group is preferably contained at least 10% by weight or more preferably 20 to 80% by weight in 100% by weight of the monomer used for copolymerization. It is also important that the acrylic copolymer (B1) having a carboxyl group has a hydrophobic portion. Therefore, it is preferable to have an ethylenically unsaturated monomer having an aromatic ring or an ethylenically unsaturated monomer having an alkyl chain having 6 or more carbon atoms in the copolymer component.

  The acrylic copolymer (B1) having a carboxyl group can be obtained by a usual method, that is, solution polymerization in a solvent, and can be synthesized by polymerization in an aqueous medium or bulk polymerization if desired. is there.

  Examples of the ethylenically unsaturated monomer having a carboxyl group that is a constituent component of the acrylic copolymer (B1) having a carboxyl group include (meth) acrylic acid, (anhydrous) itaconic acid, (anhydrous) maleic acid and the like. . In addition, when the acrylic copolymer (B1) having a carboxyl group has no crosslinkable functional group other than the carboxyl group, the acrylic copolymer (B1) is obtained by bulk polymerization without using an organic solvent. It can also be synthesized.

  In general, emulsion polymerization is often performed in the absence of an organic solvent, and even when an organic solvent is present, it is often preferable that the proportion of the organic solvent in the entire system is not so large. When the proportion of the organic solvent is large, the conversion rate of the polymerization may be deteriorated, or the physical properties of the coating film may be deteriorated. Therefore, when radically polymerizing the emulsified component (A), in order to use the acrylic copolymer obtained by solution polymerization as an emulsifier, it is necessary to distill off the organic solvent during the acrylic copolymer polymerization. There are many cases. As a method of distilling off the organic solvent, a method of removing the solvent using a normal decompression method is used.

  Moreover, as components other than the ethylenically unsaturated monomer having a carboxyl group that can constitute the acrylic copolymer (B1) having a carboxyl group, the same components as the ethylenically unsaturated monomer (A1) can be exemplified. For example, the acrylic copolymer (B1) having a carboxyl group contains at least an amide monomer selected from the group consisting of N-alkoxyalkyl (meth) acrylamide, N-hydroxyalkyl (meth) acrylamide, and (meth) acrylamide. It can also be set as the copolymer formed by copolymerizing the monomer containing 1 type. The functional group possessed by the amide monomer selected from the group consisting of N-alkoxyalkyl (meth) acrylamide and N-hydroxyalkyl (meth) acrylamide undergoes a self-crosslinking reaction or a carboxyl group-containing component when forming a cured coating film. It can also undergo a crosslinking reaction with the carboxyl group of (B), the phenol resin described later, and the like. These also undergo a crosslinking reaction with (meth) acrylamide. Thereby, the sclerosis | hardenability of a coating film and other physical properties improve more.

  The acid value of the acrylic copolymer (B1) having a carboxyl group is preferably 100 to 400 mgKOH / g. More preferably, it is 100-300 mgKOH / g. When the acid value is less than 100 mgKOH / g, there is a disadvantage that the emulsifying power is lowered during radical polymerization and the radical polymerization becomes unstable. When the acid value is larger than 400 mgKOH / g, the water resistance of the resulting cured coating film tends to decrease. In the present invention, the carboxyl group-containing component (B) is dissolved or dispersed in water using a basic compound (C) and water (D) as will be described later. The emulsified component (A) is added, but it is preferable to keep it in a state dissolved in water.

<Basic compound (C)>
In the present invention, the basic compound (C) neutralizes part or all of the carboxyl groups in the carboxyl group-containing component (B) to obtain an aqueous solution or emulsion (1) of the carboxyl group-containing component (B). It is used for.

  Examples of the basic compound (C) used in the present invention include organic amine compounds, ammonia, alkali metal hydroxides, etc. Examples of organic amine compounds include monomethylamine, dimethylamine, trimethylamine, and monoethylamine. , Diethylamine, triethylamine, monopropylamine, dipropylamine, monoethanolamine, diethanolamine, triethanolamine, N, N-dimethyl-ethanolamine, N, N-diethyl-ethanolamine, 2-dimethylamino-2-methyl- 1-propanol, 2-amino-2-methyl-1-propanol, N-methyldiethanolamine, N-ethyldiethanolamine, monoisopropanolamine, diisopropanolamine, triisopropanolamine, etc. That. Examples of the alkali metal hydroxide include lithium hydroxide, sodium hydroxide, and potassium hydroxide. These basic compounds (C) are preferably used in a proportion of 20 to 70 mol% with respect to 100 mol% of the total amount of carboxyl groups in the carboxyl group-containing component (B).

<Water (D)>
When water (D) obtains the polymer emulsion (F), it forms an aqueous solution or emulsion (1) of the polymer together with the carboxyl group-containing component (B) and the basic compound (C). Water (D) is preferably 100 to 1000 parts by weight and more preferably 200 to 600 parts by weight with respect to 100 parts by weight of the emulsified component (A). A water-soluble solvent can be added for the purpose of assisting emulsification. In general, emulsion polymerization is often performed in the absence of a solvent, but a water-soluble solvent may serve to supplement the emulsification.

<Cationic water-soluble radical initiator (E)>
Subsequently, the cationic water-soluble radical initiator (E) will be described. The cationic radical initiator in the present invention is defined as “water-soluble” as long as the solubility in water is 1.0% or more under 20 ° C., and the solubility in water is less than 1.0%. Is treated as “water-insoluble”. In addition, “cationic” in the present invention indicates a property that an anionic moiety cannot be generated in the molecular structure in water.

  Water-soluble radical initiators are roughly classified into three types, cationic, anionic and nonionic, in terms of their molecular structure. These two types of water-soluble radical initiators are common in that they react with an ethylenically unsaturated monomer in emulsion polymerization to form a polymer emulsion, but there is a great difference when focusing on the properties of the polymer emulsion itself. . For example, when emulsion polymerization is performed with an anionic water-soluble radical initiator having a persulfate structure as an anionic functional group in the molecule, such as ammonium persulfate, the anionic functional group derived from the radical initiator is a polymerization reaction. Through the polymer emulsion. Since this anionic functional group is rich in hydrophilicity, the anionic functional group is present at the surface of the polymer emulsion, that is, at the interface with water and exhibits a kind of surface active property, which contributes to the improvement of the hydrophilicity of the polymer emulsion. Naturally, since the anionic functional group derived from the radical initiator is present at the particle interface, the hydrophilicity of the cured coating film made of the polymer emulsion is also improved. As a result, the water resistance and resistance of the cured coating film are improved. Steam sterilization deteriorates, and water intrusion into the particle interface causes whitening of the coating and blistering (dotted peeling).

  In that respect, when emulsion polymerization is carried out using a cationic water-soluble radical initiator, there is no anionic functional group derived from the initiator that improves hydrophilicity in the polymer emulsion comprising this, Since there is little improvement in the hydrophilicity of the cured coating film, it is possible to form a good coating film having excellent water resistance and steam sterilization resistance.

  Examples of the cationic water-soluble radical initiator (E) include 2,2′-azobis [2- (phenylamidino) propane] dihydrochloride (VA-545, manufactured by Wako Pure Chemical Industries), 2,2′-azobis. {2- [N- (4-chlorophenyl) amidino] propane} dihydrochloride (VA-546, manufactured by Wako Pure Chemical Industries), 2,2′-azobis {2- [N- (4-hydroxyphenyl) amidino] propane} Dihydrochloride (VA-548, manufactured by Wako Pure Chemical Industries), 2,2′-azobis [2- (N-benzylamidino) propane] dihydrochloride (VA-552, manufactured by Wako Pure Chemical Industries), 2,2′-azobis [ 2- (N-allylamidino) propane] dihydrochloride (VA-553, manufactured by Wako Pure Chemical Industries), 2,2′-azobis (2-amidinopropane) dihydrochloride (V-5) 0, manufactured by Wako Pure Chemical Industries, Ltd.), 2,2′-azobis {2- [N- (4-hydroxyethyl) amidino] propane} dihydrochloride (VA-558, manufactured by Wako Pure Chemical Industries), 2,2′-azobis [ 2- (5-Methyl-2-imidazolin-2-yl) propane] dihydrochloride (VA-041, manufactured by Wako Pure Chemical Industries), 2,2′-azobis [2- (2-imidazolin-2-yl) propane] Dihydrochloride (VA-044, manufactured by Wako Pure Chemical Industries), 2,2′-azobis [2- (4,5,6,7-tetrahydro-1H-1,3-diazepin-2-yl) propane] dihydrochloride ( VA-054, manufactured by Wako Pure Chemical Industries, Ltd.), 2,2′-azobis [2- (3,4,5,6-tetrahydropyrimidin-2-yl) propane] dihydrochloride (VA-058, manufactured by Wako Pure Chemical Industries), 2,2'-azobi [2- (5-Hydroxy-3,4,5,6-tetrahydropyrimidin-2-yl) propane] dihydrochloride (VA-059, manufactured by Wako Pure Chemical Industries), 2,2′-azobis {2- [1- (2-Hydroxyethyl) -2-imidazolin-2-yl] propane} dihydrochloride (VA-060, manufactured by Wako Pure Chemical Industries, Ltd.), 2,2′-azobis [1-imino-1-pyrrolidino-2-ethylpropane] And dihydrochloride (VA-067, manufactured by Wako Pure Chemical Industries).

  In the present invention, 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [2- (2-imidazolin-2-yl) is used as the cationic water-soluble radical initiator (E). ) Propane] dihydrochloride, 2,2′-azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane} dihydrochloride, and 2,2′-azobis [1-imino- At least one selected from the group consisting of 1-pyrrolidino-2-ethylpropane] dihydrochloride can be particularly preferably used.

  In the present invention, the cationic water-soluble radical polymerization initiator (E) can be included in the emulsified component (A) during the polymerization reaction, or the reaction vessel together with the carboxyl group-containing component (B) and the like. It can also be put in, and when adding or to add an emulsified component (A), it can also add separately in a reaction tank. The polymerization initiator may be added intermittently or continuously, or may be added all at once.

  In the present invention, redox polymerization can be performed by using the cationic water-soluble radical initiator (E) as an oxidizing agent and a reducing agent in combination with the cationic water-soluble radical initiator (E). This facilitates acceleration of the polymerization rate and radical polymerization at a low temperature. Examples of reducing agents used for redox polymerization include reducing organic compounds such as metal salts such as ascorbic acid, erythorbic acid, tartaric acid, citric acid, glucose, formaldehyde sulfoxylate, sodium thiosulfate, sodium sulfite, and the like. And reducing inorganic compounds such as sodium bisulfite and sodium metabisulfite, ferrous sulfate, ferrous chloride, Rongalite, thiourea dioxide, and the like. The oxidizing agent and the reducing agent are preferably used in amounts of about 0.01 to 1 part by weight and about 0.01 to 2 parts by weight, respectively, with respect to 100 parts by weight of the ethylenically unsaturated monomer (A1).

<Polymer emulsion (F)>
As described above, the polymer emulsion (F) in the present invention comprises the polymer of the emulsified component (A) and the carboxyl group-containing component (B) as the polymer constituent components. And it is thought that the polymer of the to-be-emulsified component (A) mentioned above and the carboxyl group-containing component (B) can be mainly formed in the core part and the shell part of the polymer emulsion (F) particles, respectively.

  In the present invention, the glass transition temperature (hereinafter also referred to as “Tg”) of the polymer emulsion (F) is preferably 0 ° C. to 100 ° C., and Tg is more preferably 0 ° C. to 70 ° C. Generally, the processability of the coating film can be improved by lowering the Tg of the resin constituting the paint. In an acrylic resin containing a polymer emulsion, the Tg can usually be lowered by using a large amount of a monomer having a long alkyl group. However, a coating film made of a coating composition containing a resin containing a large amount of a monomer having a long alkyl group tends to have a low coating film hardness, and can withstand various complicated processes applied to the can after coating. It is difficult to obtain a coating film hardness that can be obtained. That is, when an aqueous paint containing a polymer emulsion having a dispersion resin Tg of less than 0 ° C. is used for coating the inner surface of the can, it becomes difficult to obtain a coating film that is not damaged during processing. On the other hand, in the case of an aqueous paint containing a polymer emulsion having a dispersion resin Tg of more than 100 ° C., the processability of the coating film tends to be inferior.

  In the case of the present invention, the Tg of the polymer emulsion can be determined according to a conventional method using each Tg of the constituent components and the composition ratio. For example, only the ethylenically unsaturated monomer (A1) is used as the emulsified component (A) without containing a polyester resin, and the acrylic copolymer having a carboxyl group without containing a polyester resin as the carboxyl group-containing component (B) When using only the polymer (B1), Tg of each homopolymer that can be formed from each monomer constituting the acrylic copolymer (B1) and each monomer of the ethylenically unsaturated monomer (A1), It can be determined by calculation according to a conventional method from the composition ratio of each monomer constituting the composite polymer in the polymer emulsion. In the case where a polyester resin or the like is contained as the emulsified component (A) or the carboxyl group-containing component (B), the same applies based on the Tg of the polyester resin or the like and the ratio of the polyester resin or the like contained in the polymer emulsion. Can be requested.

  In other words, in the present invention, the Tg of the whole polymer emulsion can be controlled by the Tg and the composition ratio of the resin obtained by polymerizing the emulsified component (A) and the carboxyl group-containing component (B), whereby the polymer It is preferable to control the Tg of the emulsion to be 0 to 100 ° C.

  In general, emulsion polymerization in which a monomer is polymerized using an emulsifier micelle in an aqueous medium can obtain a high molecular weight polymer that cannot be obtained by solution polymerization because of its unique polymerization mechanism. Also in the present invention, the polymer emulsion has a higher molecular weight than in the case of mere solution polymerization. As a result, the water-based paint containing the polymer emulsion forms a coating film having good processability and corrosion resistance. It is considered that it was possible to do.

  In the present invention, the carboxyl group-containing component (B) is preferably 5 to 300 parts by weight, more preferably 10 to 200 parts by weight, with respect to 100 parts by weight of the emulsified component (A). More preferably, it is -100 weight part. When the carboxyl group-containing component (B), which is an emulsifier component, containing the carboxyl group-containing acrylic copolymer (B1) with respect to 100 parts by weight of the component to be emulsified (A) is less than 5 parts by weight, A) and the composite polymer after polymerization tend to be less emulsified. When the carboxyl group-containing component (B) as an emulsifier component is more than 300 parts by weight with respect to 100 parts by weight of the emulsified component (A), improvement in workability of the coating film after baking and curing cannot be expected so much. The component to be emulsified (A) is a component having a very high molecular weight by radical polymerization, and this contributes to the improvement of the workability of the cured coating film. Therefore, when the carboxyl group-containing component (B) as an emulsifier component is more than 300 parts by weight, the emulsified component (A) is relatively reduced, and as a result, improvement in the workability of the cured coating film can hardly be expected.

  In addition to the polymer emulsion described above, the water-based paint of the present invention further improves the curability and adhesion of the coating film as required, such as phenol resin, amino resin, polyvinyl alcohol, polyvinyl alcohol derivatives, etc. One or more curing agents can be added.

  The phenol resin and amino resin can react with the carboxyl group in the carboxyl group-containing component (B) in addition to the self-crosslinking reaction. When the polymer formed from the emulsified component (A) or the carboxyl group-containing component (B) has a hydroxyl group, the phenol resin or amino resin can also react with those hydroxyl groups. Furthermore, when the ethylenically unsaturated monomer (A1) has a crosslinkable functional group derived from an amide monomer, it can also react with these crosslinkable functional groups.

  Examples of the phenol resin used in the present invention include trifunctional phenol compounds such as carboxylic acid, m-cresol, and 3,5-xylenol, and bifunctional phenol compounds such as o-cresol, p-cresol, and p-tert-butylphenol, and formaldehyde. And the like in the presence of an alkali catalyst.

  Examples of the amino resin used in the present invention include urea, melamine, and benzoguanamine added with formalin.

  As the phenol resin and amino resin, those obtained by etherifying some or all of the methylol groups generated by addition of formalin with alcohols having 1 to 12 carbon atoms are also preferably used. When using a phenol resin or an amino resin, it is preferable to add 0.5 to 20 parts by weight, and more preferable to add 2 to 10 parts by weight with respect to 100 parts by weight of the resin solid content of the polymer emulsion.

  If necessary, a lubricant such as wax may be added to the water-based paint of the present invention for the purpose of preventing the coating film from being damaged in the can-making process. Examples of waxes include animal and plant waxes such as carnauba wax, lanolin wax, palm oil, candelilla wax, and rice wax, petroleum waxes such as paraffin wax, microcrystalline wax, and petrolatum, polyolefin wax, and Teflon (registered trademark) wax. A synthetic wax or the like is preferably used.

A hydrophilic organic solvent can be added to the water-based paint of the present invention for the purpose of improving paintability. 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, 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 mono (iso) propyl ether, propylene glycol mono (iso) butyl ether, propylene glycol dimethyl ether, propylene glycol diethyl ether, propylene glycol di (iso) propyl ether, Pyrene 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 Various ether alcohols or ethers such as 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, isobutyl alcohol, sec-butyl alcohol, tert-butyl alcohol, furfuryl alcohol;
Ketones such as methyl ethyl ketone, dimethyl ketone, diacetone alcohol;
Glycols such as ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol;
Examples include alkoxy esters such as ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, 1-methoxy-2-propyl acetate, propylene glycol monomethyl ether acetate, etc. These may be used alone or 2 Can be used in combination with more than one species.

  In addition, for the purpose of improving the paintability, the water-based paint of the present invention may contain various auxiliary agents such as a hydrophobic organic solvent, a surfactant, and an antifoaming agent. When a cationic water-soluble radical initiator (E) is used, the stability when these organic solvents are added is particularly excellent.

  The water-based paint of the present invention can be widely used not only for the following cans but also for general metal materials or metal products, etc., but can be suitably used as a paint for coating inner and outer surfaces of cans for containing beverages and foods. Suitable for inner coating. As the material of the can, aluminum, tin-plated steel plate, chrome-treated steel plate, nickel-treated steel plate or the like is used, and these materials may be subjected to surface treatment such as zirconium treatment or phosphoric acid treatment.

  As a coating method of the water-based paint of the present invention, spray coating such as air spray, airless spray, electrostatic spray or the like is desirable, but roll coater coating, dip coating, electrodeposition coating, or the like can also be applied. Although the water-based paint of the present invention can form a film only by volatilization of volatile components after coating, it is better to add a baking step in order to obtain excellent steam sterilization resistance, processability and adhesion. As conditions for baking, baking at a temperature of 150 ° C. to 280 ° C. for 10 seconds to 30 minutes is desirable.

  The present invention will be specifically described below with reference to examples. In the examples, “parts” represents “parts by weight” and “%” represents “% by weight”.

[Synthesis Example 1] Synthesis of Acrylic Copolymer (B1-1) Having Carboxyl Group Ethylene glycol monobutyl ether in a reaction vessel equipped with a stirrer, thermometer, reflux condenser, dropping tank, and nitrogen gas introduction tube 100 parts and 100 parts of ion-exchanged water were charged, and the temperature was raised to 90 ° C. While maintaining the temperature in the reaction vessel at 90 ° C., a mixture consisting of 96 parts of methacrylic acid, 52 parts of styrene, 42 parts of ethyl acrylate, 10 parts of N-butoxymethylacrylamide, and 2 parts of benzoyl peroxide was added from the dropping tank for 4 hours. It was dripped continuously over. After 1 hour and 2 hours from the end of dropping, 0.2 part of benzoyl peroxide was added, and the reaction was continued for 3 hours from the end of dropping. The number average molecular weight was 45000, the glass transition temperature was 73 ° C, and the acid value was 312 mgKOH / g. An acrylic copolymer solution (solid content 50%) was obtained. Next, after adding 29.8 parts of dimethylethanolamine and stirring for 10 minutes, 368.2 parts of ion-exchanged water was added to make it water-soluble, and the aqueous acrylic copolymer having a carboxyl group with a nonvolatile content of 25% was added. An aqueous solution was obtained. This is designated as an aqueous acrylic copolymer (B1-1) aqueous solution.

[Synthesis Example 2] Synthesis of Acrylic Copolymer (B1-2) Having Carboxyl Group Ethylene glycol monobutyl ether in a reaction vessel equipped with a stirrer, thermometer, reflux condenser, dropping tank, and nitrogen gas inlet tube 100 parts and 100 parts of ion-exchanged water were charged and the temperature was raised to 90 ° C. While maintaining the temperature in the reaction vessel at 90 ° C., a mixture of 96 parts of methacrylic acid, 52 parts of styrene, 42 parts of ethyl acrylate, 10 parts of N-butoxymethylacrylamide, and 8 parts of benzoyl peroxide was added from the dropping tank for 4 hours. It was dripped continuously over. After 1 hour and 2 hours from the end of dropping, 0.2 part of benzoyl peroxide was added, and the reaction was continued for 3 hours from the end of dropping. The number average molecular weight was 11,000, the glass transition temperature was 73 ° C., and the acid value was 312 mgKOH / g. An acrylic copolymer solution (solid content 50%) was obtained. Next, after 29.8 parts of dimethylethanolamine was added and stirred for 10 minutes, 361.8 parts of ion-exchanged water was added to make it water-soluble, and the aqueous acrylic copolymer having a carboxyl group with a nonvolatile content of 25% was added. An aqueous solution was obtained. This is designated as an aqueous acrylic copolymer (B1-2) aqueous solution.

Synthesis Example 3 Synthesis of Acrylic Copolymer (B1-3) Having Carboxyl Group Ethylene glycol monobutyl ether in a reaction vessel equipped with a stirrer, thermometer, reflux condenser, dropping tank, and nitrogen gas inlet tube 100 parts and 100 parts of ion-exchanged water were charged, and the temperature was raised to 90 ° C. While maintaining the temperature in the reaction vessel at 90 ° C., a mixture of 130 parts of methacrylic acid, 36 parts of styrene, 24 parts of ethyl acrylate, 10 parts of N-butoxymethylacrylamide, and 2 parts of benzoyl peroxide was added from the dropping tank for 4 hours. It was dripped continuously over. After 1 hour and 2 hours from the end of dropping, 0.2 part of benzoyl peroxide was added, and the reaction was continued for 3 hours from the end of dropping. The number average molecular weight was 50000, the glass transition temperature was 94 ° C, and the acid value was 425 mgKOH / g. An acrylic copolymer solution (solid content 50%) was obtained. Next, after 29.8 parts of dimethylethanolamine was added and stirred for 10 minutes, 361.8 parts of ion-exchanged water was added to make it water-soluble, and the aqueous acrylic copolymer having a carboxyl group with a nonvolatile content of 25% was added. An aqueous solution was obtained. This is designated as an aqueous acrylic copolymer (B1-3) aqueous solution.

[Example 1]
In a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, a dropping tank, and a nitrogen gas introduction tube, 120 parts of an aqueous solution of carboxyl group-containing acrylic copolymer (B1-1) obtained in Synthesis Example 1, ion exchange 88.9 parts of water was charged, and the temperature was raised to 60 ° C. with stirring while introducing nitrogen gas. Next, 39.0 parts of styrene, 23.5 parts of ethyl acrylate, and 7.5 parts of N-butoxymethylacrylamide were charged into the dropping tank 1, and 1% 2,2′-azobis (2-amidino) was added to the dropping tank 2. Propane) Dihydrochloride (V-50, manufactured by Wako Pure Chemical Industries, Ltd.), 3.0 parts, was charged dropwise over 3 hours while maintaining the temperature in the reaction vessel at 60 ° C., and the theoretical Tg was 54 ° C. A polymer emulsion was obtained. Thereafter, 114.3 parts of ion-exchanged water, 50 parts of n-butanol and 50 parts of ethylene glycol monobutyl ether were added, filtered through a 5 μm filter to take out the contents, and an aqueous paint (1) having a nonvolatile content of 20% was obtained. Obtained.

[Example 2]
In a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, a dropping tank, and a nitrogen gas introduction tube, 120 parts of an aqueous solution of carboxyl group-containing acrylic copolymer (B1-1) obtained in Synthesis Example 1, ion exchange 88.9 parts of water was charged, and the temperature was raised to 60 ° C. with stirring while introducing nitrogen gas. Next, 43.0 parts of styrene and 27.0 parts of ethyl acrylate were charged into the dropping tank 1, and 1% 2,2′-azobis (2-amidinopropane) dihydrochloride (V-50, sum) was added to the dropping tank 2. (Manufactured by Koyo Pharmaceutical Co., Ltd.) 3.0 parts of an aqueous solution was charged over 3 hours while the temperature in the reaction vessel was kept at 60 ° C., and the mixture was added dropwise with stirring to obtain a polymer emulsion having a theoretical Tg of 52 ° C. Thereafter, 114.3 parts of ion-exchanged water, 50 parts of n-butanol and 50 parts of ethylene glycol monobutyl ether were added, filtered through a 5 μm filter, and the content was taken out to obtain an aqueous paint (2) having a non-volatile content of 20%. Obtained.

[Example 3]
A method similar to Example 1 except that the aqueous solution of carboxyl group-containing acrylic copolymer (B1-2) obtained in Synthesis Example 2 was used instead of the aqueous solution of carboxyl group-containing acrylic copolymer (B1-1). Then, a water-based paint (3) having a theoretical Tg of 54 ° C. and a non-volatile content of 20% was obtained.

[Example 4]
The same method as in Example 1 except that the aqueous solution of carboxyl group-containing acrylic copolymer (B1-3) obtained in Synthesis Example 3 was used instead of the aqueous solution of carboxyl group-containing acrylic copolymer (B1-1). Thus, a water-based paint (4) having a theoretical Tg of 58 ° C. and a non-volatile content of 20% was obtained.

[Example 5]
In a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, a dropping tank, and a nitrogen gas introduction tube, 120 parts of an aqueous solution of carboxyl group-containing acrylic copolymer (B1-1) obtained in Synthesis Example 1, ion exchange 88.9 parts of water was charged, and the temperature was raised to 70 ° C. with stirring while introducing nitrogen gas. Next, 15.0 parts of styrene, 47.5 parts of ethyl acrylate, and 7.5 parts of N-butoxymethylacrylamide were charged into the dropping tank 1, and 1% 2,2′-azobis (2-amidino) was added to the dropping tank 2. Propane) dihydrochloride (V-50, manufactured by Wako Pure Chemical Industries, Ltd.), 3.0 parts, was charged dropwise with stirring while maintaining the temperature in the reaction vessel at 60 ° C. over 3 hours, and the theoretical Tg was 23 ° C. A polymer emulsion was obtained. Thereafter, 114.3 parts of ion-exchanged water, 50 parts of n-butanol and 50 parts of ethylene glycol monobutyl ether were added, filtered through a 5 μm filter to take out the contents, and an aqueous paint (5) having a nonvolatile content of 20% was obtained. Obtained.

[Example 6]
In a reaction vessel equipped with a stirrer, a thermometer, a reflux condenser, a dropping tank, and a nitrogen gas introduction tube, 120 parts of an aqueous solution of carboxyl group-containing acrylic copolymer (B1-1) obtained in Synthesis Example 1, ion exchange 88.9 parts of water was charged, and the temperature was raised to 60 ° C. with stirring while introducing nitrogen gas. Next, 52.0 parts of styrene, 10.0 parts of ethyl acrylate, and 7.5 parts of N-butoxymethylacrylamide were charged into the dropping tank 1, and 1% 2,2′-azobis (2-amidino) was added to the dropping tank 2. Propane) dihydrochloride (V-50, manufactured by Wako Pure Chemical Industries, Ltd.), 3.0 parts, was charged dropwise with stirring while maintaining the temperature in the reaction vessel at 60 ° C. over 3 hours, and the theoretical Tg was 75 ° C. A polymer emulsion was obtained. Thereafter, 114.3 parts of ion-exchanged water, 50 parts of n-butanol and 50 parts of ethylene glycol monobutyl ether were added, and the content was taken out by filtration through a 5 μm filter, and an aqueous paint (6) having a nonvolatile content of 20% was obtained. Obtained.

[Example 7]
Instead of 2,2′-azobis (2-amidinopropane) dihydrochloride (V-50, manufactured by Wako Pure Chemical Industries, Ltd.) as the cationic water-soluble radical initiator (E), 2,2′-azobis {2- [ 1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane} dihydrochloride (VA-060, manufactured by Wako Pure Chemical Industries, Ltd.) was used in the same manner as in Example 1 except that A water-based paint (7) having a theoretical Tg of 58 ° C. and a non-volatile content of 20% was obtained.

[Example 8] (Phenolic resin added)
8 parts of a 20% butanol solution of “PR-C-101” (manufactured by Sumitomo Bakelite Co., Ltd., phenol resin) was added to 200 parts of the water-based paint (1) obtained in Example 1 with stirring. The content was removed by filtration to obtain an aqueous paint (7) having a nonvolatile content of 20%.

[Example 9] (Amino resin added)
Aqueous paint obtained in Example 1 (1) 2.6 parts of a 62% solution of “Melan 11E” (manufactured by Hitachi Chemical Co., Ltd., amino resin) was added under stirring of 200 parts and filtered through a 5 μm filter. Then, the contents were taken out to obtain an aqueous paint (9) having a nonvolatile content of 20%.

[Comparative Example 1]
The same as Example 1 except that 1% ammonium persulfate aqueous solution was used instead of 1% 2,2′-azobis (2-amidinopropane) dihydrochloride (V-50, manufactured by Wako Pure Chemical Industries) aqueous solution. By the method, an aqueous paint (8) having a theoretical Tg of the dispersion resin of 54 ° C. and a nonvolatile content of 20% was obtained.

[Filterability of paint]
The filterability when filtered through a 5 μm filter was evaluated according to the following evaluation criteria.
A: Filtration of 500 g of paint can be completed within 10 minutes.
○: Filtration of 500 g of paint can be completed within 10 minutes to 20 minutes.
Δ: Filtration of 500 g of paint can be completed within 20 minutes to 30 minutes.
X: Filtration of 500 g of paint cannot be completed even after 30 minutes.

[Evaluation of coating film]
Each water-based paint obtained in Examples 1 to 9 and Comparative Example 1 was applied to an aluminum plate having a thickness of 0.26 mm so that the dry film thickness was 5 to 6 μm, and the ambient temperature was 200 ° C. using a gas oven. Was baked for 3 minutes to obtain a test panel for evaluation, and the performance of the coating film was evaluated as follows. In addition, about the transparency of the coating film in anti-retort whitening evaluation, each water-based paint obtained in Examples 1-9 and Comparative Example 1 was coated and baked on a tin plate having a thickness of 0.23 mm in the same manner as an aluminum plate. And an evaluation tin plate test panel was obtained and evaluated. The results are shown in Table 1.

  Each evaluation method will be described below.

<Corrosion resistance>
After cutting the test panel to 40 x 80 mm, applying the DuPont impact (1/2 inch, 500 g, 30 cm) with the coating film on the outside (convex), the test panel was immersed in a commercially available sports drink and retort Retort treatment was performed at 125 ° C. for 30 minutes in the kettle. Then, it was immersed at 50 ° C., taken out after 4 days, and the blisters of the flat surface portion and the DuPont impact portion were evaluated.

◎: No blister occurred ○: Blister occurred in the DuPont impact area, less than 50% of the processed area △: Blister occurred in the DuPont impact area, 50% or more of the processed area ×: Blister occurred in the flat area

<Retort resistance>
While the test panel was immersed in water, retort treatment was performed in a retort kettle at 130 ° C. for 1 hour. After cross-cutting the coated film surface with a cutter, a cellophane pressure-sensitive adhesive tape was stuck and peeled strongly, and the coated film surface was evaluated.

◎: No peeling ○: Less than 5% peeling △: 5-50% peeling ×: More than 50% peeling

<Retort whitening-visual test>
While the test panel was immersed in water, a retort treatment was performed at 130 ° C. for 1 hour in a retort kettle, and the appearance of the coating film was visually evaluated.

◎: Untreated coating and no change ○: Very thin whitening △: Slightly whitening ×: Remarkably whitening

<Retort Whitening-Transparency of Coating Film (Haze Value)>
While the tin plate test panel was immersed in water, a retort treatment was performed in a retort kettle at 130 ° C. for 1 hour. Next, the coating film was peeled off from the retort-treated tin plate test panel by the amalgam method using mercury to form a film, and the transparency (haze value) of the obtained coating film was measured with a haze meter (Nippon Denshoku Industries ( It was measured by “NDH-500W” manufactured by Co., Ltd.). The lower the haze value, the better the transparency of the coating film, and the higher the haze value, the poorer the transparency. Since the coating film whitened by the retort treatment reduces transparency, a high haze value is obtained.

◎: Haze value is less than 2.0% ○: Haze value is 2.0% or more and less than 5.0% △: Haze value is 5.0% or more and less than 10.0% ×: Haze value is 10.0% or more

<Processability after retort>
While the test panel was immersed in water, retort treatment was performed in a retort kettle at 130 ° C. for 1 hour. Using the treated test panel, the test panel was cut into a size of 30 mm × 50 mm, the coating film was on the outside, and the test part was previously folded by hand so that the test site had a width of 30 mm. Two aluminum plates having a thickness of 0.26 mm were sandwiched between them, and a load of 1 kg was dropped from a height of 40 cm onto the bent portion and completely bent. Next, the bent tip of the test piece was immersed in a 1% concentration saline solution, and 6.0 V × 6 seconds was passed between the metal portion of the test piece not immersed in the saline solution and the saline solution. The current value was measured. When the processability of the coating film is poor, the coating film in the bent portion is cracked, the underlying metal plate is exposed, and the conductivity is increased, so that a high current value can be obtained.

◎: Less than 10 mA ○: 10 mA or more and less than 20 mA Δ: 20 mA or more and less than 50 mA ×: 50 mA or more

  As shown in Table 1, the aqueous paints of Examples 1 to 9 using a polymer emulsion obtained by radical polymerization with a cationic water-soluble radical initiator (E) have good filterability derived from the stability of the paint. While the corrosion resistance, retort adhesion, retort whitening resistance and workability after retort were good, the water-based paint of Comparative Example 1 was poor in either filterability or retort whitening resistance. Nothing was obtained that was all good.

Claims (10)

  Presence of an aqueous solution or emulsion (1) of a polymer comprising a carboxyl group-containing component (B) containing a carboxyl group-containing acrylic copolymer (B1), a basic compound (C), and water (D) Below, it contains a polymer emulsion (F) obtained by radical polymerization of an emulsified component (A) containing an ethylenically unsaturated monomer (A1) with a cationic water-soluble radical initiator (E). Water-based paint.   Cationic water-soluble radical initiator (E) is 2,2′-azobis (2-amidinopropane) dihydrochloride, 2,2′-azobis [2- (2-imidazolin-2-yl) propane] dihydrochloride 2,2′-azobis {2- [1- (2-hydroxyethyl) -2-imidazolin-2-yl] propane} dihydrochloride, and 2,2′-azobis [1-imino-1-pyrrolidino-2 The water-based paint according to claim 1, which is at least one selected from the group consisting of -ethylpropane] dihydrochloride.   The ethylenically unsaturated monomer (A1) contains at least one amide monomer selected from the group consisting of N-alkoxyalkyl (meth) acrylamide, N-hydroxyalkyl (meth) acrylamide, and (meth) acrylamide. The water-based paint according to claim 1 or 2, characterized by the above.   The water-based paint according to any one of claims 1 to 3, wherein the acrylic copolymer (B1) having a carboxyl group has a number average molecular weight of 5,000 to 100,000.   The water-based paint according to any one of claims 1 to 4, wherein the acrylic copolymer (B1) having a carboxyl group has an acid value of 100 to 400 mgKOH / g.   The water-based paint according to any one of claims 1 to 5, wherein the polymer emulsion (F) has a glass transition temperature of 0 to 100 ° C.   The aqueous paint according to any one of claims 1 to 6, comprising at least one component selected from the group consisting of a phenol resin and an amino resin.   The water-based paint according to any one of claims 1 to 7, wherein the water-based paint is used for covering a can for containing a beverage or food.   A coated can comprising a can material for containing a beverage or food with the aqueous paint according to claim 8.   Presence of an aqueous solution or emulsion (1) of a polymer comprising a carboxyl group-containing component (B) containing a carboxyl group-containing acrylic copolymer (B1), a basic compound (C), and water (D) A method for producing a polymer emulsion, wherein the emulsified component (A) containing an ethylenically unsaturated monomer (A1) is radically polymerized with a cationic water-soluble radical initiator (E).