TWI872031B - Aqueous resin emulsion, manufacturing method of the same, and aqueous resin composition - Google Patents

Abstract

The present invention provides an aqueous resin emulsion, which comprises a copolymer (X), a polyepoxide (Y) and an aqueous medium (Z), wherein the content of the polyepoxide (Y) is 1 to 40% by mass relative to the total amount of the copolymer (X) and the polyepoxide (Y); the copolymer (X) comprises structural units derived from (meth)acrylate (A) and structural units derived from ethylenic unsaturated carboxylic acid (B), and the content of the structural units derived from (meth)acrylate (A) is 20 to 98% by mass relative to the total amount of the copolymer (X) and the polyepoxide (Y); and the content of the structural units derived from ethylenic unsaturated carboxylic acid (B) is 20 to 98% by mass relative to the total amount of the copolymer (X) and the polyepoxide (Y). The content of the structural units derived from the (meth)acrylate (A) is 0.1 to 10% by mass; the structural units derived from the (meth)acrylate (A) include structural units derived from a hydrophilic (meth)acrylate (A1) containing a (meth)acryloyloxy group, and the carbon number of the part other than the (meth)acryloyloxy group is 2 or less, and the content of the structural units of the hydrophilic (meth)acrylate (A1) is 15 to 98% by mass relative to the total amount of the copolymer (X) and the polyepoxide (Y), and the aqueous resin emulsion is an emulsion in which monomers serving as structural units of the copolymer (X) are emulsified and polymerized in the presence of the polyepoxide (Y) in the aqueous medium (Z), and the content of epoxy groups in the non-volatile components of the aqueous resin emulsion is 0.50×10 ^-4 mol/g or more, and the content of carboxyl groups in the non-volatile components of the aqueous resin emulsion is 0.10×10 ^-4 mol/g or more.

Description

Aqueous resin emulsion and its production method, and aqueous resin composition

The present invention relates to an aqueous resin emulsion and a method for producing the same, as well as an aqueous resin composition. This case is based on Japanese Patent Application No. 2018-212769 filed in Japan on November 13, 2018, and claims priority, and its contents are cited here.

Generally speaking, metal products, such as working machinery or conveying machinery, are often treated on the surface to protect the whole from impact or to prevent rust. In particular, products that are intended to be used outdoors or exposed to water are often coated with anti-rust paint. Since the paints used in the past often contain organic solvents, there is a need for solutions to the impact of volatile organic compounds (VOC) on operators or the surrounding environment. Therefore, the industry has actively shifted its research direction from solvent-based paints to water-based paints, and excellent water-based paints with the same performance as solvent-based paints have attracted much attention.

Patent document 1 describes a thick coating composition, which contains an emulsion composition in which polymer particles are dispersed in an aqueous medium and a skeleton. It is described that the polymer particles are produced by emulsion polymerization of structural units obtained by polymerizing (meth) alkyl acrylate monomers having an alkyl group with 4 to 14 carbon atoms, structural units obtained by polymerizing ethylenic unsaturated carboxylic acid monomers, and structural units obtained by polymerizing other monomers, and a compound having at least two epoxy groups in one molecule in the presence of an alkaline catalyst.

Patent document 2 describes a composition comprising an aqueous dispersion of thermoplastic polymer particles that absorb a thermosetting compound having an ethylene oxide group. It states that the polymer particles have anti-agglomeration functional groups, which can stabilize the latex against aggregation.

Patent Document 3 describes that an acrylic resin absorbing an epoxy compound (acrylic/epoxy emulsion) is formed by mixing an acrylic resin emulsion with an epoxy emulsion.

Patent document 4 describes a water-based resin composition, which contains: a water-based dispersion containing an epoxy resin, an addition-polymerized polyamine, and a carboxylic acid polymer. More specifically, it describes that a free radical polymerizable monomer is polymerized with an epoxy resin in water to obtain an epoxy-modified acrylic emulsion from the water-based dispersion. [Prior art document] [Patent document]

[Patent Document 1] Japanese Patent Publication No. 2011-89092 [Patent Document 2] Japanese Patent Publication No. 2014-65914 [Patent Document 3] International Publication No. 2017/112018 [Patent Document 4] Japanese Patent Publication No. 2005-2353

[The problem that the invention wants to solve]

In Patent Document 1, carboxyl groups and epoxy groups are reacted by an alkaline catalyst. Therefore, the polymer particles contained in the emulsion composition are already cross-linked. Therefore, in Patent Document 1, in order to maintain good dispersion stability, the degree of cross-linking must be precisely controlled.

In the invention described in Patent Document 2, the monomer having an anti-agglomeration functional group has a high hydrophilicity. Therefore, the coating formed using the composition described in this document has insufficient water resistance and rust resistance.

In the invention of Patent Document 3, the epoxy compound is absorbed after the synthesis of the acrylic resin. As a result, since the epoxy compound cannot fully penetrate into the interior of the acrylic resin particles, even after hardening, a large amount of unreacted carboxyl groups will remain inside the particles, resulting in insufficient water resistance and rust resistance. In addition, since the epoxy compound cannot fully penetrate into the interior of the acrylic resin particles, there are places where the acrylic resin and the epoxy resin are not in contact. Therefore, when hardening it using amines, the phase composed only of the low water resistance acrylic resin cannot be protected, and it is judged that the water resistance and rust resistance will be insufficient.

In addition, the composition described in Patent Document 3 is judged to have insufficient water resistance and strength of the coating film formed by curing the composition based on the above reasons.

Patent document 4 discloses an aqueous resin composition, which contains an aqueous dispersion containing an epoxy resin, an addition polymerization type polyamine, and a carboxylic acid polymer. The raw materials of the aqueous dispersion containing an epoxy resin (epoxy-modified acrylic emulsion) are disclosed in the production examples shown in Table 2, etc. There is also an example (Production Example 8) of using a hydrophilic monomer (methyl methacrylate) other than methacrylic acid, but the amount is smaller. In Patent Document 4, when polyamine is added to the aqueous dispersion, gelation can be suppressed by adding a carboxylic acid polymer. When the carboxylic acid polymer is not present, it can be seen from the comparison between Comparative Example 1 and Example 10 that when a polyamine is mixed as a hardener, gelation will proceed rapidly (Tables 8 and 9). However, the addition of the carboxylic acid polymer will lead to an increase in material costs.

In addition, for example, in an emulsion used as a coating, if the particles contained in the emulsion need to be redispersed during the coating step of the product (the object to be coated) or the previous mixing step of the emulsion with a hardener, etc., this will lead to an increase in product cost. In such applications, in order to reduce the manufacturing cost of the object to be coated, the emulsion needs to have excellent dispersion stability.

Furthermore, paints or emulsions used in paints are often placed outdoors in high temperatures, and the temperature inside the containers is often high. When stored in such an environment, in order to fully ensure its quality, the emulsion also needs excellent high temperature stability.

The present invention aims to provide a water-based resin emulsion, a water-based resin composition and a method for producing the water-based resin emulsion, which have excellent high-temperature stability and dispersion stability and can obtain a coating film with high water resistance, rust resistance and high adhesion to metal materials when contained in a coating. [Means for solving the problem]

The present invention for solving the above-mentioned problems is structured as follows. The first aspect of the present invention provides the following water-based resin emulsion. [1] A water-based resin emulsion comprises a copolymer (X), a polyepoxy compound (Y) having no ethylenically unsaturated bonds and having two or more epoxy groups in one molecule, and an aqueous medium (Z), wherein the content of the polyepoxy compound (Y) is 1 to 40% by mass relative to the total amount of the copolymer (X) and the polyepoxy compound (Y); the copolymer (X) comprises: a structural unit derived from a (meth)acrylate (A), and a structural unit derived from an ethylenically unsaturated carboxylic acid (B), wherein the content of the structural unit derived from the (meth)acrylate (A) is 20 to 98% by mass relative to the total amount of the copolymer (X) and the polyepoxy compound (Y); The content of the structural unit of the unsaturated carboxylic acid (B) is 0.1 to 10% by mass. The structural unit derived from the (meth)acrylate (A) includes a structural unit derived from a hydrophilic (meth)acrylate (A1), wherein the carbon number of the moiety derived from the alcohol of the hydrophilic (meth)acrylate (A1) is 2 or less, and the content of the structural unit derived from the hydrophilic (meth)acrylate (A1) is 15 to 98% by mass relative to the total amount of the copolymer (X) and the polyepoxide (Y). The aqueous resin emulsion is an emulsion obtained by emulsifying and polymerizing a monomer as a structural unit of the copolymer (X) in the presence of the polyepoxide (Y) in the aqueous medium (Z), and the content of the epoxy group in the non-volatile component of the aqueous resin emulsion is 0.50×10 ^-4 mol/g or more, and the content of carboxyl groups in the non-volatile components of the aqueous resin emulsion is 0.10×10 ^-4 mol/g or more. The aqueous resin emulsion of the first aspect of the present invention preferably comprises the following features. The following features are also preferably combined in combination of two or more. [2] In [1], the content of carboxyl groups in the non-volatile components of the aqueous resin emulsion is preferably 10×10 ^-4 mol/g or less. [3] In [1] or [2], the content of epoxy groups in the non-volatile components of the aqueous resin emulsion is preferably 50×10 ^-4 mol/g or less. [4] In any one of [1] to [3], the (meth)acrylate (A) is preferably composed of an alkyl (meth)acrylate. [5] In any one of [1] to [4], the ethylenically unsaturated carboxylic acid (B) is preferably a compound having a (meth)acryloyl group and a carboxyl group. [6] In any one of [1] to [5], the polyepoxide (Y) is preferably at least one selected from bisphenol type epoxy compounds, hydrogenated bisphenol type epoxy compounds, diepoxypropyl ethers, triglycidyl ethers, tetraglycidyl ethers, diepoxypropyl esters, triglycidyl esters and tetraglycidyl esters. [7] In any one of [1] to [6], the glass transition point of the copolymer (X) is preferably from -30°C to 100°C. [8] In any one of [1] to [7], the copolymer (X) is preferably composed of structural units derived from (meth)acrylate (A) and structural units derived from ethylenically unsaturated carboxylic acid (B). [9] In any one of [1] to [8], the copolymer (X) preferably comprises structural units (c) derived from an ethylenically unsaturated aromatic compound (C) having a benzene ring and an ethylenically unsaturated bond. [10] In [9], the ethylenically unsaturated aromatic compound (C) is preferably an aromatic vinyl compound. A second aspect of the present invention provides the following aqueous resin composition. [11] An aqueous resin composition comprising: an aqueous resin emulsion (α) as described above in any one of [1] to [10]; and a hardener (β) having a functional group reactive with an epoxy group; the content of the functional group contained in the hardener (β) is not less than 0.01 equivalent and not more than 1.0 equivalent relative to the amount of epoxy group contained in the polyepoxide (Y). The aqueous resin composition of the second aspect of the present invention preferably comprises the following features. [12] In [11], the hardener (β) preferably has at least one selected from the group consisting of an amino group, a carboxyl group and a hydroxyl group. The third aspect of the present invention provides the following method for producing an aqueous resin emulsion. [13] A method for producing an aqueous resin emulsion, comprising the step of emulsifying and polymerizing monomers comprising a (meth)acrylate (A) and an ethylenically unsaturated carboxylic acid (B) in an aqueous medium (Z) in the presence of a polyepoxy compound (Y) having no ethylenically unsaturated bonds and having two or more epoxy groups in one molecule, to obtain an aqueous resin emulsion, wherein: In the aqueous resin emulsion, the amount of the aforementioned polyepoxide (Y) added is 1 to 40% by mass relative to the total amount of the aforementioned monomer and the aforementioned polyepoxide (Y); the amount of the aforementioned (meth)acrylate (A) added is 20 to 98% by mass relative to the total amount of the aforementioned monomer and the aforementioned polyepoxide (Y); and the amount of the aforementioned ethylenically unsaturated carboxylic acid (B) added is 1 to 40% by mass relative to the total amount of the aforementioned monomer and the aforementioned polyepoxide (Y). The addition amount of the aforementioned (meth)acrylate (A) is 0.1 to 10% by mass, the aforementioned (meth)acrylate (A) comprises a hydrophilic (meth)acrylate (A1) having a carbon number of 2 or less in the alcohol-derived portion, the addition amount of the aforementioned hydrophilic (meth)acrylate (A1) is 15 to 98% by mass relative to the total amount of the aforementioned monomer and the aforementioned polyepoxide (Y), the content of epoxy groups in the non-volatile components of the aforementioned aqueous resin emulsion is 0.50×10 ^-4 mol/g or more, and the content of carboxyl groups in the non-volatile components of the aforementioned aqueous resin emulsion is 0.10×10 ^-4 mol/g or more. The method for producing the aqueous resin emulsion of the third aspect of the present invention preferably comprises the following features. [14] In the aforementioned aqueous resin emulsion, the aforementioned emulsion polymerization is preferably carried out at 30 to 90°C. The fourth aspect of the present invention provides a method for producing an aqueous resin emulsion, which comprises the step of emulsifying and polymerizing a monomer comprising a (meth)acrylate (A) and an ethylenically unsaturated carboxylic acid (B) in an aqueous medium (Z) in the presence of a polyepoxide (Y), wherein the amount of the (meth)acrylate (A) added relative to the total amount of the monomer and the polyepoxide (Y) is 20 to 98% by mass, the amount of the ethylenically unsaturated carboxylic acid (B) added relative to the total amount of the monomer and the polyepoxide (Y) is 0.1 to 10% by mass, and the amount of the polyepoxide (Y) added relative to the total amount of the monomer and the polyepoxide (Y) is 1 to 40% by mass. [Effects of the Invention]

According to the present invention, there can be provided an aqueous resin emulsion, an aqueous resin composition and a method for producing the aqueous resin emulsion, which have excellent high temperature stability and dispersion stability and can obtain a coating film with high water resistance, rust resistance and high adhesion to metal materials when contained in a coating.

[Form of implementing the invention]

The following is a detailed description of preferred embodiments of the aqueous resin emulsion, the method for producing the aqueous resin emulsion, the aqueous resin composition, and the method for producing the aqueous resin composition of the present invention.

In addition, the present invention is not limited to the following embodiments. For example, the present invention is not limited to the following examples, and the number, type, position, amount, ratio, material, structure, etc. can be added, omitted, replaced or changed without departing from the scope of the present invention.

(Explanation of terms) "Hardening" refers to the process in which the molecules contained in the raw material are bonded to each other through chemical reactions to form a polymer network structure.

“(Meth)acrylate” refers to acrylate or methacrylate; and “(meth)acrylic acid” refers to acrylic acid or methacrylic acid.

"Ethylenically unsaturated bonds" refer to double bonds between carbon atoms other than carbon atoms forming an aromatic ring.

The “weight average molecular weight” is a value calculated based on standard polystyrene measured by gel permeation chromatography (GPC).

The term "coating film" refers to a coating film formed by applying an aqueous resin composition including the aqueous resin emulsion of the present embodiment, drying the medium, and hardening the resin component, unless otherwise specified.

<1. Aqueous resin composition> The aqueous resin composition of this embodiment includes an aqueous resin emulsion (α) and a hardener (β). The aqueous resin composition of this embodiment is produced by mixing an aqueous resin emulsion (α) and a hardener (β) as described below.

<1-1. Aqueous resin emulsion (α)> The aqueous resin emulsion (α) comprises a copolymer (X), a polyepoxy compound (Y) having no ethylenic unsaturated bonds and having two or more epoxy groups in one molecule, and an aqueous medium (Z). The aqueous resin emulsion (α) is an emulsion in which monomers serving as structural units of the copolymer (X) are emulsified and polymerized in the presence of the polyepoxy compound (Y) in an aqueous medium (Z). This is because when the copolymer is mixed with a curing agent (β) described later and cured, a coating having high strength and high elongation can be obtained.

<1-1-1. Copolymer (X)> Copolymer (X) has a structural unit (a) derived from a (meth)acrylate (A) and a structural unit (b) derived from an ethylenically unsaturated carboxylic acid (B). In addition, the structural unit (a) derived from the (meth)acrylate (A) includes a structural unit (a1) derived from a hydrophilic (meth)acrylate (A1).

The copolymer (X) may also be composed of the structural unit (a) and the structural unit (b) (referred to as copolymer (X1)). The copolymer (X) may also have the structural unit (a), the structural unit (b), and the structural unit (c) derived from the ethylenically unsaturated aromatic compound (C) having a benzene ring and an ethylenically unsaturated bond (referred to as copolymer (X2)). The copolymer (X) may also have a structural unit (d) other than the structural units (a) to (c) (referred to as a structural unit derived from other monomers (D)).

[(Meth)acrylate (A)] (Meth)acrylate (A) is preferably composed of alkyl (meth)acrylate. Examples of alkyl (meth)acrylate preferably have a linear, branched or cyclic alkyl group having 1 to 18 carbon atoms. Specific examples include methyl (meth)acrylate, ethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, tertiary butyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, octadecyl (meth)acrylate and isobornyl (meth)acrylate. These may be used alone or in combination of two or more. Examples of (meth)acrylate (A) may also include examples of hydrophilic (meth)acrylate (A1) described below. In addition, the (meth)acrylate having a carboxyl group is not included in the (meth)acrylate (A) but is included in the ethylenically unsaturated carboxylic acid (B) described later.

The (meth)acrylate (A) preferably contains a low hydrophilic compound because the rust resistance of the coating film can be improved. For the same reason, the (meth)acrylate (A) may also contain a (meth)acrylate having an epoxy group.

Examples of the (meth)acrylate having an epoxy group include glycidyl (meth)acrylate, β-methylglycidyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate glycidyl ether, 3,4-epoxyhexylmethyl (meth)acrylate, 3,4-epoxyhexylethyl (meth)acrylate, and 3,4-epoxyhexylpropyl (meth)acrylate. The structural unit (a) may include only one structural unit derived from these compounds, or may include structural units derived from two or more. In addition, among these compounds, the structural unit (a) preferably includes a structural unit derived from glycidyl (meth)acrylate.

Furthermore, the (meth)acrylate (A) may be a (meth)acrylate that is not a (meth)acrylate alkyl ester or a compound having an epoxy group. Examples of such (meth)acrylates include (meth)acrylates having a hydroxyl group.

Examples of the (meth)acrylate having a hydroxyl group include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 6-hydroxyhexyl (meth)acrylate, 8-hydroxyoctyl (meth)acrylate, and 12-hydroxylauryl (meth)acrylate. Examples include mono(meth)acrylates of polyalkylene glycols such as mono(meth)acrylate of polyethylene glycol and mono(meth)acrylate of polypropylene glycol. These (meth)acrylates having a hydroxyl group may be used alone or in combination of two or more.

The content of the structural unit (a) derived from the (meth)acrylate (A) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is 20% by mass or more. This is because the dispersion stability of the monomers of the copolymer (X) and the polyepoxide (Y) can be improved in the method for producing the aqueous resin emulsion described later. From this point of view, the content of the structural unit (a) derived from the (meth)acrylate (A) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is preferably 35% by mass or more, more preferably 45% by mass or more, and even more preferably 60% by mass or more.

Furthermore, when the copolymer (X) is composed of the structural unit (a) and the structural unit (b), that is, the copolymer (X1), the following ratio is preferred from the viewpoint of improving the dispersion stability. That is, the content of the structural unit (a) derived from the (meth)acrylate (A) relative to the total amount of the copolymer (X1) and the polyepoxide (Y) is more preferably 50% by mass or more, particularly preferably 60% by mass or more.

The content of the structural unit (a) derived from the (meth)acrylate (A) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is 98% by mass or less. This is because if it exceeds 98% by mass, the dispersion stability of the aqueous resin emulsion tends to deteriorate. From this viewpoint, the content of the structural unit (a) derived from the (meth)acrylate (A) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is preferably 92% by mass or less, and more preferably 87% by mass or less.

Furthermore, when the copolymer (X) has the structural unit (a), the structural unit (b) and the structural unit (c), that is, when the copolymer (X) is the copolymer (X2), based on the same viewpoint, the following ratios are preferred. That is, the content of the structural unit (a) derived from the (meth)acrylate (A) in the copolymer (X2) is more preferably 75% by mass or less, and particularly preferably 65% by mass or less.

[Hydrophilic (meth)acrylate (A1)] The aforementioned structural unit derived from (meth)acrylate (A) includes a structural unit derived from hydrophilic (meth)acrylate (A1). The hydrophilic (meth)acrylate (A1) is a (meth)acrylate having a (meth)acryloyloxy group ( _CH2 =CR-COO-, R represents hydrogen or methyl), and the number of carbon atoms of the part derived from the alcohol, that is, the part other than the (meth)acryloyloxy group, is 2 or less. The aforementioned number of carbon atoms of the part other than the acryloxy group may be, for example, 1 or 2. Examples of the hydrophilic (meth)acrylate (A1) include methyl (meth)acrylate, ethyl (meth)acrylate, and 2-hydroxyethyl (meth)acrylate. The hydrophilic (meth)acrylate (A1) is preferably an alkyl (meth)acrylate having a carbon number of 2 or less of the part derived from the alcohol, and more preferably methyl methacrylate.

The content of the structural unit (a1) derived from the hydrophilic (meth)acrylate (A1) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is 15% by mass or more. This is because, if the content of the hydrophilic (meth)acrylate is small, gelation proceeds rapidly when the aqueous resin emulsion (α) is mixed with a hardener containing a polyamine.

The content of the structural unit (a1) derived from the hydrophilic (meth)acrylate (A1) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is 15% by mass or more, preferably 20% by mass or more, more preferably 30% by mass or more, and even more preferably 40% by mass or more. This is because the water resistance and rust resistance of the cured coating can be further improved. The above content may also be 45% by mass or more or 50% by mass or more.

The upper limit of the content of the structural unit (a1) derived from the hydrophilic (meth)acrylate (A1) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is the same as the upper limit of the content of the structural unit (a) derived from the (meth)acrylate (A). However, when the polyepoxide (Y) described later is a hydrophobic compound such as a bisphenol-type epoxy compound, a hydrogenated bisphenol-type epoxy compound, or a phenol novolac-type epoxy compound, the proportion of the structural unit (a1) to the structural unit (a) is preferably 90% by mass or less, more preferably 80% by mass or less, and even more preferably 70% by mass or less. This is because the affinity between the copolymer (X) and the polyepoxide (Y) can be improved.

[Ethylenically unsaturated carboxylic acid (B)] Ethylenically unsaturated carboxylic acid (B) is a compound having an ethylenically unsaturated bond and a carboxyl group. Ethylenically unsaturated carboxylic acid (B) preferably comprises at least one of the group consisting of α,β-unsaturated monocarboxylic acids, α,β-unsaturated dicarboxylic acids, monoalkyl esters of α,β-unsaturated dicarboxylic acids, and vinyl compounds containing carboxyl groups. Examples of α,β-unsaturated mono- or dicarboxylic acids include acrylic acid, methacrylic acid, crotonic acid, citric acid, itaconic acid, maleic acid, maleic anhydride, and fumaric acid. Examples of vinyl compounds containing carboxyl groups include phthalic acid monohydroxyethyl (meth)acrylate and oxalic acid monohydroxypropyl (meth)acrylate. The structural unit (b) may be derived from only one of these compounds, or may include structural units derived from two or more of these compounds. Among these compounds, the ethylenically unsaturated carboxylic acid (B) is preferably composed of a compound having a (meth)acryloyl group and a carboxyl group, and more preferably includes (meth)acrylic acid. That is, the structural unit (b) is preferably composed of a structural unit derived from a compound having a (meth)acryloyl group and a carboxyl group, and more preferably includes a structural unit derived from (meth)acrylic acid.

The content of the structural unit (b) derived from the ethylenic unsaturated carboxylic acid (B) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is 0.1% by mass or more. This is because the dispersion stability of the aqueous resin emulsion (α) can be improved. From this point of view, the content of the structural unit (b) derived from the ethylenic unsaturated carboxylic acid (B) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is preferably 0.3% by mass or more, and more preferably 0.5% by mass or more. The aforementioned content may also be 0.8% by mass or more or 1.0% by mass or more. The content of the structural unit (b) derived from the ethylenic unsaturated carboxylic acid (B) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is 10% by mass or less. This is because the copolymer (X) can be inhibited from forming a gel in a high temperature environment, thereby improving the high temperature stability of the aqueous resin emulsion (α). From this point of view, the content of the structural unit (b) derived from the ethylenically unsaturated carboxylic acid (B) is preferably 7% by mass or less, more preferably 5% by mass or less, relative to the total amount of the copolymer (X) and the polyepoxide (Y). The aforementioned content may be 4% by mass or less or 3% by mass or less.

[Ethylenically unsaturated aromatic compound (C)] The ethylenically unsaturated aromatic compound (C) is a compound that is neither a (meth)acrylate (A) nor an ethylenically unsaturated carboxylic acid (B) and has a benzene ring and an ethylenically unsaturated bond. The ethylenically unsaturated aromatic compound (C) is preferably an aromatic vinyl compound. Examples of the aromatic vinyl compound of the ethylenically unsaturated aromatic compound (C) include styrene, 2-methylstyrene, 3-methylstyrene, 4-methylstyrene, α-methylstyrene, 2,4-dimethylstyrene, 2,4-diisopropylstyrene, 4-tert-butylstyrene, tert-butyloxystyrene, vinyltoluene, divinyltoluene, vinylnaphthalene, monochlorostyrene, dichlorostyrene, monobromostyrene, dibromostyrene, tribromostyrene, fluorostyrene, styrenesulfonic acid and its salts, α-methylstyrenesulfonic acid and its salts, p-hydroxystyrene, m-hydroxystyrene, o-hydroxystyrene, p-isopropenylphenol, m-isopropenylphenol and o-isopropenylphenol. The structural unit (c) may be derived from only one of these compounds or may contain structural units derived from two or more of these compounds. Among these, the structural unit (c) is more preferably composed of a structural unit derived from a hydrocarbon, and particularly preferably a structural unit derived from styrene.

When the copolymer (X) contains a structural unit (c) derived from an ethylenically unsaturated aromatic compound (C), that is, when the copolymer (X) is a copolymer (X2), the content of the structural unit (c) relative to the total amount of the copolymer (X2) and the polyepoxide (Y) is preferably 5% by mass or more. This is because the water resistance of the coating can be improved. From this point of view, the content of the structural unit (c) relative to the total amount of the copolymer (X2) and the polyepoxide (Y) is more preferably 10% by mass or more, and even more preferably 15% by mass or more. The aforementioned content may also be 18% by mass or more, 20% by mass or more, or 23% by mass or more.

When the copolymer (X) is the copolymer (X2), the content of the structural unit (c) relative to the total amount of the copolymer (X2) and the polyepoxide (Y) is preferably 50% by mass or less. This is because the weather resistance of the coating can be improved. From this point of view, the content of the structural unit (c) relative to the total amount of the copolymer (X2) and the polyepoxide (Y) is more preferably 40% by mass or less, and further preferably 35% by mass or less. The aforementioned content may be 33% by mass or less, 30% by mass or less, or 28% by mass or less.

[Other monomers (D)] Other monomers (D) are compounds that are not (meth)acrylates (A), ethylenically unsaturated carboxylic acids (B) and ethylenically unsaturated aromatic compounds (C), and have ethylenically unsaturated bonds that can be copolymerized with the compound used to synthesize the copolymer (X). Examples of other monomers (D) include conjugated diene compounds, maleimide compounds, vinyl ether compounds, allyl ether compounds, dialkyl esters of unsaturated dicarboxylic acids, and vinyl compounds having a cyano group.

Examples of the conjugated diene compound include 1,3-butadiene, isoprene (2-methyl-1,3-butadiene), 2,3-dimethyl-1,3-butadiene, chloroprene (2-chloro-1,3-butadiene), etc. These conjugated diene compounds may be used alone or in combination of two or more.

Examples of the maleimide compound include maleimide, N-methylmaleimide, N-isopropylmaleimide, N-butylmaleimide, N-dodecylmaleimide, N-phenylmaleimide, N-(2-methylphenyl)maleimide, N-(4-methylphenyl)maleimide, N-(2,6-dimethylphenyl)maleimide, N-(2,6-diethylphenyl)maleimide, N-(2-methoxyphenyl)maleimide, N-benzylmaleimide, N-(4-hydroxyphenyl)maleimide, N-naphthylmaleimide, and N-cyclohexylmaleimide. These maleimide compounds may be used alone or in combination of two or more.

Examples of the vinyl ether compound include alkyl vinyl ethers such as methyl vinyl ether and ethyl vinyl ether, and hydroxyl-containing alkyl vinyl ethers in which a part of hydrogen atoms is substituted with a hydroxyl group.

Examples of the allyl ether compound include allyl alkyl ethers such as allyl methyl ether and allyl ethyl ether, hydroxyl-containing allyl alkyl ethers in which a part of hydrogen atoms is substituted with a hydroxyl group, and allyl epoxypropyl ether.

Examples of the aforementioned dialkyl esters of unsaturated dicarboxylic acids include dialkyl esters of unsaturated dicarboxylic acids such as maleic acid, fumaric acid, itaconic acid, liraconic acid, mesaconic acid, maleic anhydride, itaconic anhydride, liraconic anhydride, and tetrahydrophthalic anhydride. These dialkyl esters may be used alone or in combination of two or more. These unsaturated compounds may be used alone or in combination of two or more.

Examples of the vinyl compound having a cyano group include acrylonitrile, methacrylonitrile, α-ethylacrylonitrile, α-isopropylacrylonitrile, α-chloroacrylonitrile and α-fluoroacrylonitrile. These cyano group-containing vinyl monomers may be used alone or in combination of two or more.

<1-1-2. Polyepoxide (Y)> The polyepoxide (Y) is a compound having no ethylenic unsaturated bond and having two or more epoxy groups in one molecule. The polyepoxide (Y) is preferably at least one selected from bisphenol epoxy compounds, hydrogenated bisphenol epoxy compounds, diglycidyl ether, triglycidyl ether, tetraglycidyl ether, diglycidyl ester, triglycidyl ester and tetraglycidyl ester. Examples of the compound having two or more epoxy groups in one molecule include diglycidyl ether of bisphenol A, diglycidyl ether of hydrogenated bisphenol A, diglycidyl ether of bisphenol F, diglycidyl ether of hydrogenated bisphenol F, polyglycidyl ether of glycerol, 1,4-butanediol diglycidyl ether, 1,6-hexanediol diglycidyl ether, diglycidyl ester of phthalic acid, 1,4-cyclohexanedimethanol diglycidyl ether, 1,3-cyclohexanedimethanol diglycidyl ether, and diglycidyl ester of hexahydrophthalic acid. One or more of these compounds may be included.

The polyepoxide (Y) is more preferably a bisphenol epoxy compound or a hydrogenated bisphenol epoxy compound, and even more preferably a bisphenol A epoxy compound or a hydrogenated bisphenol A epoxy compound, because the water resistance and rust resistance of the cured coating can be further improved.

The weight average molecular weight of the polyepoxide (Y) is not particularly limited, but is preferably 1000 or less, more preferably 800 or less, and even more preferably 500 or less. This can improve the compatibility of the polyepoxide (Y) with the copolymer (X), and can produce an emulsion having excellent dispersion stability and storage stability. The lower limit of the molecular weight can be arbitrarily selected, for example, 200 or 300, but is not limited thereto.

The epoxy equivalent of the polyepoxide (Y) (mass of the polyepoxide (Y) per 1 mol of epoxy group) is preferably 500 g/mol or less, more preferably 350 g/mol or less, and even more preferably 250 g/mol or less. This is because the strength of the coating formed by curing the aqueous resin composition described later can be improved. The lower limit of the epoxy equivalent can be arbitrarily selected, for example, it can be 70 g/mol or more or 120 g/mol or more, but it is not limited to these examples.

The content of the polyepoxide (Y) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is 1% by mass or more. This is because the aqueous resin composition hardens and a coating film having excellent rust resistance is obtained. From this point of view, the content of the polyepoxide (Y) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is preferably 5% by mass or more, more preferably 8% by mass or more, and even more preferably 10% by mass or more. Depending on the needs, it can also be 12% by mass or more or 20% by mass or more. The content of the polyepoxide (Y) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is 40% by mass or less. This is because a highly dispersed and stable aqueous resin emulsion (α) can be obtained. From this viewpoint, the content of the polyepoxide (Y) relative to the total amount of the copolymer (X) and the polyepoxide (Y) is preferably 35 mass % or less, more preferably 30 mass % or less.

<1-1-3. Aqueous medium (Z)> The aqueous medium (Z) can be selected arbitrarily, and water is preferably used. However, as long as the dispersion stability of the copolymer (X) and the polyepoxide (Y) is not impaired, for example, a water-soluble solvent added to water can be used as the aqueous medium (Z). The hydrophilic solvent added to water can be selected arbitrarily, and examples thereof include methanol, ethanol, and N-methylpyrrolidone.

<1-1-4. Method for producing aqueous resin emulsion (α)> The method for producing aqueous resin emulsion (α) of the present embodiment is carried out by emulsifying and polymerizing monomers comprising (meth)acrylate (A) and ethylenically unsaturated carboxylic acid (B) (i.e., monomers for constituting copolymer (X)) in aqueous medium (Z) in the presence of polyepoxide (Y). According to the above-mentioned production method of the present embodiment using this method, it is estimated that an aqueous resin emulsion (α) in which polyepoxide (Y) is uniformly dispersed in particles of the generated copolymer (X) can be obtained. The term "uniformly present" here does not necessarily mean that the copolymer (X) and the polyepoxide (Y) are compatible, as long as the structural domain of the polyepoxide (Y) is present impartially on either the center side or the surface side of the copolymer (X) particle. Specific emulsion polymerization methods include a method of pouring the components including the monomers at once, a method of polymerizing while continuously supplying the components, and the like. It is preferred to stir during the polymerization reaction.

The content rate of each raw material in the whole raw materials used for manufacturing the aqueous resin emulsion (α) is the same as the content rate of the structural unit derived from the raw material or the compound corresponding to the raw material in the aqueous resin emulsion (α).

The polymerization is preferably carried out at an arbitrarily selected temperature, for example, 30 to 90° C., more preferably 40 to 80° C., and even more preferably 40 to 70° C. This is because the reaction between the carboxyl group contained in the monomer and the epoxy group contained in the polyepoxide (Y) can be suppressed.

The emulsifier used in the emulsion polymerization can be arbitrarily selected, and examples thereof include nonionic surfactants such as polyoxyalkyl alkyl ethers, polyoxyalkyl alkylphenol ethers, polyoxyalkyl fatty acid esters, polyoxyalkyl sorbitan fatty acid esters, and anionic surfactants such as alkyl sulfates, alkyl benzene sulfonates, alkyl sulfosuccinates, alkyl diphenyl ether disulfonates, polyoxyalkyl alkyl sulfates, and polyoxyalkyl alkyl phosphates. These may be used alone or in combination of two or more. Alkyl benzene sulfonates are preferred as these emulsifiers, and sodium dodecylbenzene sulfonate is more preferred.

In the emulsion polymerization, a polymerization initiator is preferably used. The polymerization initiator is preferably a peroxide. Examples of the peroxide used as the polymerization initiator include persulfates such as potassium persulfate and ammonium persulfate, hydrogen peroxide, etc. Furthermore, a redox initiator using a peroxide and a reducing agent may also be used. Examples of the reducing agent include sodium sulfoxylate formaldehyde, ascorbic acid, sulfite, tartaric acid or its salt, etc. In addition, alcohols and thiols may be used as chain transfer agents as required.

<1-1-5. Characteristics of aqueous resin emulsion (α)> [pH of aqueous resin emulsion (α)] The pH of the aqueous resin emulsion (α) is preferably 2 to 10, more preferably 5 to 9. If the pH is within this range, the mechanical stability and chemical stability of the aqueous resin emulsion (α) can be improved. The pH is a value measured at a liquid temperature of 25°C using a pH meter that uses a hydrogen ion concentration indicator with a glass electrode as a standard electrode. For example, the pH can be adjusted by adding an alkaline substance to the aqueous resin emulsion (α) during or after the emulsion polymerization. Examples of alkaline substances used for pH adjustment include ammonia, triethylamine, ethanolamine, caustic soda, and the like. These may be used alone or in combination of two or more.

[Non-volatile component concentration of aqueous resin emulsion (α)] The non-volatile component concentration of the aqueous resin emulsion (α) is preferably 10-65% by mass, more preferably 15-60% by mass, and even more preferably 20-55% by mass. The above concentration may also be 30-50% by mass or 35-45% by mass. However, if the workability in the mixing step with the hardener (β) described later or the coating step of the aqueous resin composition is taken into consideration, the non-volatile component concentration in the aqueous resin emulsion (α) can be appropriately adjusted by adjusting the amount of the aqueous medium (Y) added.

Here, the non-volatile component concentration is the ratio (mass %) of the mass of the residual component obtained by weighing 1 g of the aqueous resin emulsion (α) on an aluminum scale pan having a diameter of 5 cm and drying the mixture at 105°C for 1 hour in a dryer while circulating air under atmospheric pressure to the mass of the aqueous resin emulsion (α) before drying.

[Viscosity of the aqueous resin emulsion (α)] In this embodiment, the viscosity of the aqueous resin emulsion (α) is measured at 23°C. The viscosity of the aqueous resin emulsion (α) is measured using a B-type viscometer, with a rotation speed of 60 rpm and a rotor corresponding to the viscosity of the aqueous resin emulsion selected. For example, when the viscosity of the aqueous resin emulsion (α) is several mPa・s to several hundred mPa・s, rotor No. 1 is used for measurement. The viscosity may be, for example, 0.1 to 300 mPa・s, 1 to 100 mPa・s, 3 to 50 mPa・s, or 5 to 25 mPa・s.

[Glass transition point of copolymer (X)] The glass transition point Tg of copolymer (X) is calculated based on the glass transition point of the homopolymer of each monomer used in the synthesis of copolymer (X). The specific method for calculating the glass transition point Tg of copolymer (X) is to calculate it according to the following formula (1) from the glass transition point Tg _i of the homopolymer of monomer _Mi (i=1,2,3...,) used as a raw material and the mass fraction _Xi of monomer i in all monomers ( _ΣXi (total monomers)=1). In formula (1), Tg and Tg _i are both calculated as the value of absolute temperature (K). 1/Tg=Σ( _Xi /Tg _i )…(1)

The glass transition point Tg of the copolymer (X) is preferably above -30°C (243K). This is because the strength of the coating can be improved. From this point of view, the glass transition point Tg of the copolymer (X) is more preferably above -10°C (263K), and more preferably above 0°C (273K). This is because, if it is in this range, the strength of the cured coating can be improved. The glass transition point Tg of the copolymer (X) may also be above 5°C or above 10°C. The glass transition point Tg of the copolymer (X) is preferably below 100°C (373K), and more preferably below 80°C (353K). This is because the adhesion of the coating to the substrate can be improved. From this viewpoint, the glass transition point Tg of the copolymer (X) is more preferably 60°C (333K) or less, and particularly preferably 50°C (323K) or less. This is because within this range, the softness of the cured coating can be improved. The glass transition point Tg of the copolymer (X) may also be 40°C or less or 30°C or less.

[Epoxy group content in the non-volatile components of the aqueous resin emulsion (α)] The epoxy group content in the non-volatile components of the aqueous resin emulsion (α) is 0.50×10 ^-4 mol/g or more, preferably 3.0×10 ^-4 mol/g or more, and more preferably 5.0×10 ^-4 mol/g or more. This is because the water resistance, rust resistance, and adhesion to the substrate of the cured coating can be improved. The epoxy group content in the non-volatile components of the aqueous resin emulsion (α) may also be 1.0×10 ^-4 mol/g or more or 6.0×10 ^-4 mol/g or more.

The content of epoxy groups in the non-volatile components of the aqueous resin emulsion (α) is preferably 50×10 ^-4 mol/g or less, more preferably 30×10 ^-4 mol/g or less, and even more preferably 20×10 ^-4 mol/g or less. The content of epoxy groups in the non-volatile components of the aqueous resin emulsion (α) may also be 15×10 ^-4 mol/g or less or 10×10 ^-4 mol/g or less.

The content rate R _EP [mol/g] of epoxy groups in the non-volatile components of the aqueous resin emulsion (α) is a value obtained as follows. If the concentration of the non-volatile components of the aqueous resin emulsion (α) is denoted by _CS [mass %] and the amount of epoxy groups contained per 1 g of the aqueous resin emulsion (α) is denoted by N ₁ [mol/g], the content rate R _EP of epoxy groups is expressed as shown in Formula (2). The method of obtaining _{N 1} is described later in the Examples.

R _EP =N ₁ /(C _S /100)…(2)

[Carboxyl group content in the non-volatile components of the aqueous resin emulsion (α)] The carboxyl group content in the non-volatile components of the aqueous resin emulsion (α) is 0.10×10 ^-4 mol/g or more, preferably 0.50×10 ^-4 mol/g or more, and more preferably 1.0×10 ^-4 mol/g or more. This is because when the aqueous resin emulsion (α) is stored during polymerization and after polymerization, the aggregation of the copolymer (X) can be suppressed.

The carboxyl content in the non-volatile components of the aqueous resin emulsion (α) is preferably 10×10 ^-4 mol/g or less, more preferably 5.0×10 ^-4 mol/g or less, and may be 3.0×10 ^-4 mol/g or less, 2.5×10 ^-4 mol/g or less, or 2.0×10 ^-4 mol/g or less.

Here, the above-mentioned carboxyl group is not only -COOH, but also includes a structure formed by a cationic ion other than a hydrogen ion bonding with ^-COO- . The content of carboxyl groups in the non-volatile components of the aqueous resin emulsion (α) is obtained by subtracting the amount of functional groups that can react with the carboxyl groups in the raw materials before and after polymerization from the content of carboxyl groups in the raw materials, as shown in the following formula. The aforementioned raw materials refer to components used in the synthesis of the aqueous resin emulsion (α). In addition, the functional group that can react with the carboxyl group refers to an epoxy group in the present invention, and a hydroxyl group is not recognized as a functional group that can react with the carboxyl group.

The method for calculating the carboxyl content _RCX [mol/g] in the non-volatile components of the aqueous resin emulsion (α) is described in detail below. The total amount of carboxyl groups in the raw materials (including initiators, solvents, other additives, etc.) is set to _N3 [mol/g], the total amount of epoxy groups in the raw materials (including initiators, solvents, other additives, etc.) is set to _N2 [mol/g], and the amount of epoxy groups contained in 1g of the aqueous resin emulsion (α) is set to _N1 [mol/g]. The concentration of non-volatile components in the aqueous resin emulsion (α) is set to _CS [mass %]. At this time, the carboxyl content _RCX is expressed as formula (3). Examples of how to calculate _N1 and _N2 are described later in the examples. N ₂ can be obtained by calculation.

R _CX ={N ₃ -(N ₂ -N ₁ )}/(C _S /100)…(3)

<1-2. Hardener (β)> The hardener (β) has a functional group reactive to epoxy groups. The hardener (β) is preferably a compound having at least one selected from the group consisting of an amine group, a carboxyl group, and an alkyl group. Examples of the hardener (β) having an amine group include aliphatic polyamines, alicyclic polyamines, aromatic polyamines, polyamides, and tertiary amines. Examples of aliphatic polyamines include ethylenediamine, diethylenetriamine, triethylenetetramine, tetraethylenepentamine, or modified products thereof. Examples of alicyclic polyamines include isophoronediamine, menthanediamine, N-aminoethylpiperazine, diaminodicyclohexylmethane, or modified products thereof. Examples of aromatic polyamines include meta-xylene diamine, diaminodiphenylmethane, meta-phenylenediamine, diaminodiphenylsulfone, or modified products thereof. Examples of polyamides include condensates of dicarboxylic acids such as dimer acid and the aforementioned polyamines. Examples of tertiary amines include compounds containing tertiary amine groups such as dimethylbenzylamine and 2,4,6-tris-dimethylaminomethylphenol, or modified products thereof, or imidazole compounds such as imidazole, 2-methylimidazole, 2-ethyl-4-methylimidazole, and 2-phenylimidazole, or modified products thereof. Examples of other polyamines include dicyandiamide and dihydrazide adipic acid. The carboxyl-containing hardener (β) is preferably a compound having two or more carboxyl groups in the molecule. Examples include phthalic acid, tetrahydrophthalic acid, hexahydrophthalic acid, methyltetrahydrophthalic acid, methylhexahydrophthalic acid, pyromellitic acid, benzophenonetetracarboxylic acid, and 1,2,3,4-butanetetracarboxylic acid. Examples of the hardener (β) having an alkyl group include condensates or polysulfides of thioglycolic acid and polyols. These hardeners may be used alone or in combination of two or more. Polyamine-based hardeners are preferred.

In addition, examples of commercially available hardeners include Epi-cure 8535, 8536, 8537, 8290 and 8292; Anquamine 401; Casamid 360 and 362; Epilink 381, DP660, HZ350, 92-113 and 92-116; Beckopox EH659W, EH623W, VEH2133W; ADEKA HARDENER EH-8051; Fujicure FXI-919; Tohmide TXH-674-B and TXS-53-C; Epotuf 37-680 and 37-681; RIKACID BTW; and Karenz MT BD-1.

The content of the functional group reactive with epoxy groups in the curing agent (β) is preferably 0.01 equivalent or more, more preferably 0.1 equivalent or more, relative to the amount of epoxy groups in the polyepoxide (Y). This is because the rust resistance and adhesion to metal materials of the cured water-based resin composition can be improved.

The content of the functional group reactive with epoxy groups in the curing agent (β) is preferably 1.5 equivalents or less, more preferably 1.0 equivalents or less, relative to the amount of epoxy groups in the polyepoxide (Y). This is because the strength of the coating can be improved. It may also be 0.8 equivalents or less or 0.5 equivalents or less.

<1-3. Other ingredients> The water-based resin composition of this embodiment may also contain a pigment. Examples of the pigment include titanium oxide, talc, barium sulfate, carbon black, iron oxide, calcium carbonate, silicon oxide, talc, mica, kaolin, clay, granulated iron, silica sand, etc. The pigment preferably contains 0.1 to 50% by mass, and more preferably 1 to 40% by mass in the water-based resin composition. This is because it can improve the concealing property of the coating.

The water-based resin composition may also include fillers, organic or inorganic hollow balloons, dispersants (e.g., amine alcohols, polycarboxylates, etc.), surfactants, coupling agents (e.g., silane coupling agents, etc.), defoaming agents, preservatives (e.g., biocides, mycoticides, fungicides, algaecides, and combinations thereof, etc.), flow agents, levelers, neutralizers (e.g., hydroxides, amines, ammonia, carbonates, etc.), and other additives.

The coupling agent is preferably a silane coupling agent. Examples of the silane coupling agent include epoxysilane compounds. Specific examples include 3-glycidoxypropylmethyldimethoxysilane, 3-glycidoxypropylmethyldiethoxysilane, 3-glycidoxypropyltrimethoxysilane, and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane.

The amount of silane coupling agent added is preferably 0.1 to 5 parts by weight, more preferably 0.3 to 3 parts by weight, relative to 100 parts by weight of the water-based resin emulsion. This is because it can enhance the rust resistance and adhesion to metal materials of the hardened water-based resin composition.

<2. Method for forming a coating film> The following is an example of a method for forming a coating film using the aqueous resin emulsion (α) of the present embodiment. The method for forming a coating film comprises: a mixing step of mixing the aqueous resin emulsion (α), a hardener (β) and other components as required to obtain an aqueous resin composition; a coating step of coating the aqueous resin composition on a coating object; and a hardening step of hardening the coated coating film.

The mixing step is to mix and stir the aqueous resin emulsion (α), the hardener (β) and other components as required to obtain an aqueous resin composition in which the components are fully dispersed. The stirring can be performed by, for example, Robomix (manufactured by PRIMIX Co., Ltd.). In order to fully disperse the components, the stirring is preferably performed for more than 5 minutes. In order to suppress the hardening of the resin component, the stirring is preferably performed within 1 hour.

The coating step is to coat the water-based resin composition on the object to be coated. The object to be coated may be, for example, a metal material. The object to be coated may also be subjected to a surface treatment such as a primer or a base coat in advance. The coating method may include, but is not limited to, a method using a brush or a roller. In addition, in order to inhibit the resin component from hardening before the coating step is completed, the coating step is preferably performed within 1 hour after the mixing step is completed.

In the hardening step, the resin components contained in the aqueous resin composition are hardened by drying and aging the coated object. The aging time varies with the temperature of the environment. For example, if it is 20°C, it is preferably more than 5 hours; if it is 40°C, it is preferably more than 1 hour; if it is 60°C, it is more than 5 minutes. <3. Application fields of the present invention> The aqueous resin emulsion (α) and aqueous resin composition of the present invention can be used in various fields, especially in the field of metal coatings. On the coated object, i.e., the article to be coated, in addition to the coating film obtained by using the coating material comprising the aqueous resin emulsion (α) and/or the aqueous resin composition of the present invention, a base coating layer, an upper coating layer, etc. can also be provided as required. The article to which the coating material comprising the aqueous resin emulsion (α) and/or the aqueous resin composition of the present invention is applied, i.e., the object to be coated can be selected arbitrarily. For example, various household goods, recreational facilities in amusement parks or parks, home appliances such as refrigerators, sports goods, buildings (indoor and outdoor), various industrial products or components including transport machinery or working machinery, automobile bodies and chassis, railway vehicle bodies and train underfloor components, ships, offshore container ships, aircraft, etc. can be cited. [Implementation example]

The present invention is described in detail below using embodiments, but the following embodiments do not limit the entire present invention, and all those implemented within the scope of the contents described herein are included in the technical scope of the present invention.

<1. Synthesis of aqueous resin emulsion (α)> <1-1. Examples 1 to 10 and Comparative Examples 1 to 4> 158 parts by mass of ion exchange water were fed into a separable flask equipped with a cooling tube, a thermometer, a stirrer, and a dropping funnel, and the temperature was raised to 60°C. Nitrogen was blown through the contents of the separable flask to deoxygenate. A monomer emulsion consisting of 216 parts by mass of methyl methacrylate (A-1), 115 parts by mass of 2-ethylhexyl acrylate (A-2), 5 parts by mass of methacrylic acid (B-1), 59 parts by mass of hydrogenated bisphenol A epoxy (Y-1), 5 parts by mass of sodium dodecylbenzenesulfonate and 356 parts by mass of ion-exchanged water was added dropwise to a flask over 3 hours. At the same time, 1.2 parts by mass of potassium persulfate as an oxidizing agent was dissolved in 41 parts by mass of ion-exchanged water, and 0.4 parts by mass of sodium hydrogen sulfite as a reducing agent was dissolved in 21 parts by mass of ion-exchanged water, and then added dropwise to the flask over 3.3 hours at 60°C to perform polymerization. After the addition was completed, aging was performed for 1.5 hours. After cooling, 0.8 parts by weight of aqueous ammonia was added to obtain an aqueous resin emulsion (α-1). This was used as Example 1.

In the synthesis of aqueous resin emulsions (α-2) to (α-10) (Examples 2 to 10) and (cα-1) to (cα-4) (Comparative Examples 1 to 4), the types and amounts (by weight) of monomers and polyepoxides (Y) used in the synthesis of copolymers (X) are as shown in Tables 1-1 to 1-4. The amounts of other components added and the preparation method are the same as those of aqueous resin emulsion (α-1). In addition, in Comparative Examples 2 and 3 (cα-3), the polymers agglomerated without being dispersed.

<1-2> Comparative Example 5 Except that the polyepoxide (Y) is not used, the same formula as in Example 1 is used to synthesize the emulsion of the copolymer (X) according to the following method.

30 g of a 70% by mass aqueous solution of polyethylene glycol-tertiary octylphenyl ether (Triton (registered trademark) X-405 (manufactured by Dow Chemical)) and 67 g of ion-exchanged water were stirred until a uniform solution was obtained to obtain an aqueous surfactant solution. While the obtained aqueous surfactant solution was stirred using an overhead stirrer, 129.5 g of a bisphenol A type liquid epoxy resin (Y-5) (D.E.R.331 (epoxy equivalent 187 g/mol, manufactured by Olin)) was added, and further stirred until the droplets of the epoxy resin disappeared from the inner wall of the container. The obtained mixed solution was stirred at 3000 rpm for 20 minutes using an overhead stirrer to obtain an epoxy dispersion. 45.3 g of the obtained epoxy dispersion and 401.5 g of the emulsion of copolymer (X) were placed in a flask and stirred at 60°C for 2 hours to obtain an aqueous resin emulsion (cα-5). The mass ratio of copolymer (X) to epoxy resin in the obtained aqueous resin emulsion (cα-5) was calculated to be 85.0:15.0.

The weight percentages of the components used in the preparation of the aqueous resin emulsion (cα-5) of Comparative Example 5 are shown in Table 2-1 and Table 2-2. These values are calculated based on the total weight percentage of the monomers used in the copolymer (X) being the same as that in Example 1, which is 336 weight percentages.

<2. Evaluation of aqueous resin emulsion (α)> Aqueous resin emulsions (α-1) to (α-10) and (cα-1) to (cα-5) were evaluated as follows. In the following description, aqueous resin emulsions (α-1) to (α-10) and (cα-1) to (cα-5) are collectively referred to as aqueous resin emulsions (α).

<2-1. pH> Measured using a pH meter (HM-30G glass electrode hydrogen ion concentration indicator manufactured by DKK-TOA Co., Ltd.).

<2-2. Non-volatile component concentration> 1 g of the aqueous resin emulsion (α) was weighed on an aluminum scale pan with a diameter of 5 cm, and then dried at 105°C for 1 hour in a dryer under atmospheric pressure while circulating air. The ratio (mass %) of the mass of the residual component to the mass of the aqueous resin emulsion (α) before drying was calculated.

<2-3. Viscosity> The viscosity of the aqueous resin emulsion (α) is measured under the following conditions and equipment. Temperature: 23°C Measurement instrument: B-type viscometer Rotor: No.1 Number of rotations: 60 rpm

<2-4. Glass transition point> The glass transition point Tg of the copolymer (X) is a value calculated according to the above formula (1).

<2-5. Dispersion stability> The state of the aqueous resin emulsion (α) was visually observed and evaluated according to the following criteria. ○ (good): No coagulation, precipitation, separation or gelation was observed. × (poor): At least one of coagulation, precipitation, separation or gelation was observed.

<2-6. High temperature stability> The high temperature stability of the aqueous resin emulsion (α) was evaluated as follows. First, the aqueous resin emulsion (α) was placed in a 70 ml glass bottle and sealed, and then placed at 60°C for 7 days. Thereafter, the state of the aqueous resin emulsion (α) in the glass bottle was visually observed and evaluated according to the following criteria. ○ (good): No coagulation, thickening, precipitation, separation, or gelation was observed. × (poor): At least one of coagulation, thickening, precipitation, separation, and gelation was observed.

<2-7. Residual rate of epoxy groups> The residual rate of epoxy groups in the aqueous resin emulsion (α) is the ratio of the amount N ₁ [mol/g] of epoxy groups contained in the aqueous resin emulsion (α) after synthesis to the total amount N ₂ [mol/g] of epoxy groups contained in the components (including raw materials, initiators, solvents, other additives, etc.) used for the synthesis of the aqueous resin emulsion (α).

The amount N ₁ [mol/g] of epoxy groups in the aqueous resin emulsion (α) after synthesis is determined by adding excess hydrogen chloride to the total amount of epoxy groups contained in the components (raw materials) used for the synthesis of the aqueous resin emulsion (α) to react with the epoxy groups. Next, the amount of residual hydrogen chloride is confirmed by titrating the unreacted hydrogen chloride with potassium hydroxide. In addition, potassium hydroxide is consumed by reacting with acidic components, mainly carboxylic acids, contained in the aqueous resin emulsion (α). Therefore, the amount of acidic components is titrated in advance by a blank test without hydrogen chloride to calibrate the results of the formal measurement. The specific measurement procedure is shown in (i) to (iii) below.

(i) Blank test (confirmation of the amount of acidic components) The aqueous resin emulsion (α) is measured in an amount of W ₁ [g] (5 g in this example and comparative example) in a 100 mL Erlenmeyer flask, 25 g of THF is added and stirred with a magnetic stirrer to form a uniform solution. To this solution is added 0.15 mL of a 0.1 mass % aqueous solution of cresol red as an indicator. Titrate the above solution with a 0.1 M potassium hydroxide/ethanol solution while stirring. The point at which the purple color persists for 30 seconds after the potassium hydroxide/ethanol solution is added is taken as the equivalent point. The amount of potassium hydroxide/ethanol solution used for titration here is set as V _KOH1 [mL].

(ii) Formal measurement The aqueous resin emulsion (α) was measured in an amount of W ₂ [g] (5 g in this example and comparative example) in a 100 mL Erlenmeyer flask, 25 g of THF was added and the mixture was stirred thoroughly with a magnetic stirrer to dissolve. A 0.2 M hydrogen chloride/dioxane solution was added thereto and stirred for 1 hour to form a uniform solution. The amount of hydrogen chloride/dioxane solution added here was set to V _HCl [mL] (25 mL in this example and comparative example). 0.15 mL of a 0.1 mass % aqueous solution of cresol red was added to this solution as an indicator. Titration was performed with a 0.1 M potassium hydroxide/ethanol solution while stirring the solution. The point where the purple color persists for 30 seconds after the potassium hydroxide/ethanol solution is added is taken as the equivalent point. The amount of potassium hydroxide/ethanol solution used for titration at this point is set as V _KOH2 [mL].

From the values obtained in (i) and (ii), the amount of epoxy groups per 1g of the aqueous resin emulsion (α) N ₁ [mol/g] is calculated according to the following formula (4): N ₁ = (0.2×V _HCl /1000-0.1×V _KOH2 /1000)/W ₂ + (0.1×V _KOH1 / 1000)/ W ₁ …(4)

The total amount of epoxy groups N ₂ [mol/g] contained in the components (raw materials) used for the synthesis of the aqueous resin emulsion (α) is obtained from the mass _mi [mass fraction] (i=1, 2, 3, ...) of each component and the epoxy equivalent EP _i [g/mol] according to the following formula (5). The components used for the synthesis of the aqueous resin emulsion (α) here refer to all the components listed in Tables 1-1 to 1-4. N ₂ =Σ(m _i /EP _i )/Σm _i …(5)

In addition, for compounds not containing an epoxy group such as methyl methacrylate and ion exchange water, 1/EP _i =0.

The residual rate of epoxy groups in the aqueous resin emulsion (α) is expressed as 100×N ₁ /N ₂ [mol%] from the amount of epoxy groups thus obtained. In addition, for the aqueous resin emulsion (cα-1), since the total amount of epoxy groups in the raw materials is N ₂ = 0, the residual rate of epoxy groups is expressed as "-" in Table 1-4.

<2-8. Epoxy group content in non-volatile components> The epoxy group content R EP [mol/g] in the non-volatile components of the aqueous resin emulsion (α) is calculated based on the formula ( ₂ ) described above from the non-volatile component concentration obtained by the above method, the epoxy group content N ₁ in the aqueous resin emulsion (α), and the total amount of epoxy groups in the raw materials N ₂ . R _EP = N ₁ /( _CS /100)…(2)

<2-9. Carboxyl group content in non-volatile components> The total amount N ₃ [mol/g] of carboxyl groups contained in the components (raw materials) used for the synthesis of the aqueous resin emulsion (α) is obtained from the mass _mi [part by mass] (i=1, 2, 3, ...) of each component and the carboxyl group equivalent CX _i [g/mol] according to the following formula (6). The components used for the synthesis of the aqueous resin emulsion (α) here refer to all the components described as raw materials of the aqueous resin emulsion (α) in Tables 1-1 to 1-4. N ₃ =Σ(m _i /CX _i )/Σm _i …(6) From the N ₃ obtained here, the carboxyl group content R _CX [mol/g] in the non-volatile components of the aqueous resin emulsion (α) is obtained based on the formula (3) described above. R _CX ={N ₃ -(N ₂ -N ₁ )}/(C _S /100)…(3)

<3. Examples 101 to 114 and Comparative Examples 101 to 104 (Preparation of aqueous resin composition)> In each example and comparative example, 60 parts by mass of ion-exchanged water and a hardener (β) of the type shown in Tables 2-1 and 2-2 were added to 100 parts by mass of the aqueous resin emulsion (α) shown in Tables 2-1 and 2-2 in the amounts (parts by mass) shown in these tables and stirred for 10 minutes to prepare an aqueous resin composition. In addition, in Tables 2-1 and 2-2, "amine equivalent", "carboxyl equivalent", and "hydroxyl equivalent" refer to the mass (g) per 1 mol of amine, carboxyl, and hydroxyl, respectively. The "equivalent weight relative to epoxy groups" of each curing agent is a numerical value indicating the molar ratio of the functional groups contained in the curing agent (β) relative to the amount of epoxy groups based on the raw material contained in the aqueous resin emulsion (α).

<4. Evaluation of coatings> <4-1. Water swelling resistance> The water-based resin compositions obtained in Examples 101 to 114 and Comparative Examples 101 to 104 were cast all over a horizontally placed rectangular polyethylene film of 90 mm × 190 mm. After drying at 23°C for 72 hours, they were aged at 50°C for 24 hours to form a coating. The coating was peeled off from a flat plate and the peeled coating was cut into 10 mm × 10 mm. After weighing the cut coating, it was immersed in ion exchange water at 23°C for 24 hours. The coating was taken out of the ion exchange water, and the coating just taken out was weighed, which was taken as the mass of the coating before drying. After that, the coating was dried at 105°C for 3 hours and weighed again to be the mass of the coating after drying. The value obtained by the mass of the coating before drying and the mass of the coating after drying according to the following formula (7) was taken as the water swelling resistance. {(mass of coating before drying - mass of coating after drying)/mass of coating after drying}×100　…(7)

<4-2. Rust resistance> The water-based resin compositions prepared in Examples 101 to 114 and Comparative Examples 101 to 104 were applied to a cold-rolled steel plate (hereinafter referred to as "substrate") at a unit weight of 50 g/ ^m2 using a brush and dried in a constant temperature bath at 60°C for 10 minutes to form a coating film on the surface of the substrate.

A test piece is prepared by forming a cut formed by two intersecting straight lines (i.e., an X-shape) on the coating film in such a way that the diagonal line of a 30mm×45mm rectangle (in this evaluation, this rectangle is used as the test area) is formed. The cut is formed using a cutting knife in such a way that it reaches deep into the substrate. The test piece with the cut is subjected to a neutral salt water spray test (item 4.2.1) based on JIS Z-2371 (2000). In the test piece after the neutral salt water spray test, the area occupied by the bulge of the coating film in the test area [area %], the size of the bulge [mm], and the size of the flowing rust [mm] from the cut are measured. The size of the bulge is determined as the longest dimension in the area occupied by one independent bulge. In addition, the size of the flow rust is determined as the maximum value of the rust width centered on the cross-cut portion.

<4-3. Adhesion of coating to metal material> Similar to the evaluation of rust resistance mentioned above, a coating was formed on the substrate surface of the cold-rolled steel plate. In accordance with JIS K-5400 (1990) "8.5.2 Checkerboard tape method", the steel plate with the coating formed was used as a test piece, and a cutter was used to cut through the coating to make a checkerboard cut (100 grids) with a spacing of 1 mm, and Cellotape (registered trademark) was attached. After 1 hour, the Cellotape (registered trademark) was peeled off, and the number of grids that the coating did not peel off from the steel plate and remained was counted to evaluate the adhesion of the coating to the metal material.

<4-4. Elongation-stress measurement of coating film> The water-based resin composition obtained in Example 101 and Comparative Example 104 was cast all over a horizontally placed rectangular polyethylene film of 90 mm × 190 mm. After drying at 23°C for 72 hours, it was aged at 50°C for 24 hours to prepare a coating film. The coating film was peeled off from a flat plate, and the peeled coating film was cut into a rectangle of 10 mm × 30 mm to prepare a test piece. The following test was performed with the length direction of the test piece as the stretching direction. The thickness of the test piece was measured using Quick Micro (registered trademark) MDQ-MX manufactured by Mitutoyo Co., Ltd., and the average value of 3 points was taken as the thickness t [mm] of the test piece. The thickness t of the test piece is shown in Tables 3 and 4.

The test was conducted using Autograph AG-X (manufactured by Shimadzu Corporation). The gap (chuck distance) was 10 mm, and the two sides of the specimen were clamped with chucks in the longitudinal direction. The specimen was stretched at a speed of 100 mm/min in an environment of 23°C and 50% RH.

The relationship between the elongation and stress of the coating film obtained from the water-based resin composition of Example 101 is shown in Table 3. The relationship between the elongation and stress of the coating film obtained from the water-based resin composition of Comparative Example 104 is shown in Table 4.

In addition, if the distance between the chucks is L [mm], and the change in the length of the test piece (the difference between the distance between the chucks during the test and the distance between the chucks before the test) is ΔL [mm], the elongation is calculated as 100 × ΔL / L [%]. If the load applied to the test piece (the measured load) is F [N], the width of the test piece is W [mm], and the thickness of the test piece is t [mm], the stress of the test piece is calculated as F / (W × t) [N / mm ² ]. In addition, as mentioned above, W = 10mm.

In addition, the stress integral value ∑ is obtained by integrating the stress value (N/mm ² ) with the elongation (%) value, and is calculated as follows based on the values listed in Tables 3 and 4. The elongation (ΔL/L [%]) value at the nth point is X _n , and the stress (N/mm ² ) is σ _n . If the origin is X ₀ = 0 and σ ₀ = 0, the stress integral value ∑ ₁ from the origin to the first point is (X ₁ ×σ ₁ )/2. The stress integral value ∑ ₂ from the origin to the second point is ∑ ₁ +{(X ₂ -X ₁ )×(σ ₁ +σ ₂ )/2}. Therefore, the stress integral value ∑ _n from the origin to the nth point is ∑ _n-1 +{(X _n -X _n-1 )×(σ _n +σ _n-1 )/2} (i.e., ∑ _n ={(X ₁ ×σ ₁ )/2)}+{(X ₂ -X ₁ )×(σ ₁ +σ ₂ )/2}+…+{(X _n - X _{n- 1} )×(σ _n +σ _n-1 )/2}.

In this evaluation, stress is the load per unit cross-sectional area, and elongation is the rate of change of length. Therefore, the stress integral value ∑ can be said to be the amount of work required to stretch a sample of unit cross-sectional area and unit length (distance between chucks) until it breaks, that is, the amount of energy absorbed by the coating until it breaks.

Fig. 1 is a graph showing the relationship between the elongation and stress of a coating film obtained from the water-based resin composition of Example 101. Fig. 2 is a graph showing the relationship between the elongation and stress of a coating film obtained from the water-based resin composition of Comparative Example 104. In Figs. 1 and 2, × indicates a fracture point.

<5. Evaluation results> It can be seen from Tables 1-1 to 1-4 that the water-based resin emulsions (α-1) to (α-10) of Examples 1 to 10 have excellent dispersion stability and high temperature stability. In addition, it can be seen from Tables 2-1 and 2-2 that the water-based resin emulsions (α-1) to (α-10), when mixed with a hardener (β) of appropriate type and amount, can obtain a coating film with excellent water resistance, rust resistance and high adhesion to metal materials after hardening when contained in the coating.

The water-based resin emulsion (cα-1) of Comparative Example 1 that does not contain the polyepoxide (Y) does not contain a crosslinking component. Therefore, as shown in Table 2-1 and Table 2-2, the water resistance, rust resistance, and adhesion to metal materials are insufficient.

In contrast, in the aqueous resin emulsion (cα-2) of Comparative Example 2 in which the amount of polyepoxide (Y) added was excessive, the polymer agglomerated without being dispersed. In the aqueous resin emulsion (cα-3) of Comparative Example 3 in which the ethylenically unsaturated carboxylic acid (B) was not used as a monomer of the copolymer (X), i.e., the copolymer (X) did not have a structural unit (b) derived from the ethylenically unsaturated carboxylic acid (B), the polymer agglomerated without being dispersed.

In the polymerization of the copolymer (X), an excessive amount of the ethylenically unsaturated monomer (B) is used, that is, the aqueous resin emulsion (cα-4) of Comparative Example 4 in which the copolymer (X) contains an excessive amount of structural units (b) derived from the ethylenically unsaturated carboxylic acid (B) has insufficient high temperature stability.

In Comparative Example 5 in which emulsion polymerization was not performed, the stress integral value of the produced coating (Comparative Example 104), that is, the energy absorbed by the coating until fracture, was smaller.

When a coating film is prepared using an aqueous resin emulsion (cα-5) obtained by mixing the copolymer (X) with a polyepoxide (Y), it is found that the strength of the coating film is significantly reduced compared to the aqueous resin emulsion (α-1).

From the above, it can be seen that the aqueous resin emulsion (α) of the present invention has excellent high temperature stability and dispersion stability.

In addition, it is known that when the water-based resin composition of the present invention is contained in a coating, a coating film having excellent water resistance, rust resistance and high adhesion to metal materials can be obtained. [Industrial Applicability]

According to the present invention, a method for producing an aqueous resin emulsion having excellent high temperature stability and dispersion stability and capable of obtaining a coating film having high water resistance, rust resistance and high adhesion to metal materials when the aqueous resin emulsion is contained in a coating.

[Figure 1] is a graph showing the relationship between the elongation and stress of a coating film obtained from the water-based resin composition of Example 101. [Figure 2] is a graph showing the relationship between the elongation and stress of a coating film obtained from the water-based resin composition of Comparative Example 104.

Claims (18)

A water-based resin emulsion comprises a copolymer (X), a polyepoxide (Y) having no ethylenically unsaturated bonds and having two or more epoxy groups in one molecule, and an aqueous medium (Z), wherein the content of the polyepoxide (Y) is 20% to 30% by mass relative to the total amount of the copolymer (X) and the polyepoxide (Y), and the copolymer (X) comprises: a structural unit derived from a (meth)acrylate (A) and a structural unit derived from an ethylenically unsaturated carboxylic acid (B); the content of the structural unit derived from the (meth)acrylate (A) is 60 to 98% by mass relative to the total amount of the copolymer (X) and the polyepoxide (Y), and the content of the structural unit derived from the (meth)acrylate (A) is 60 to 98% by mass relative to the total amount of the copolymer (X) and the polyepoxide (Y). The content of the structural unit of the carboxylic acid (B) is 1.0 mass % or more and less than 3 mass %, the structural unit derived from the (meth)acrylate (A) includes a structural unit derived from a hydrophilic (meth)acrylate (A1), the carbon number of the moiety derived from the hydrophilic (meth)acrylate (A1) is 2 or less, the content of the structural unit derived from the hydrophilic (meth)acrylate (A1) is 30 mass % or more and 70 mass % or less relative to the total amount of the copolymer (X) and the polyepoxide (Y), the aqueous resin emulsion is an emulsion obtained by emulsifying and polymerizing a monomer as a structural unit of the copolymer (X) in the presence of the polyepoxide (Y) in the aqueous medium (Z), and the content of the epoxy group in the non-volatile component of the aqueous resin emulsion is 0.50×10 ^-4 mol/g or more, and the content of carboxyl groups in the non-volatile components of the aqueous resin emulsion is 0.10×10 ^-4 mol/g or more. The aqueous resin emulsion of claim 1, wherein the content of carboxyl groups in the non-volatile components of the aqueous resin emulsion is 10×10 ^-4 mol/g or less. The aqueous resin emulsion of claim 1 or 2, wherein the content of epoxy groups in the non-volatile components of the aqueous resin emulsion is 50×10 ^-4 mol/g or less. The aqueous resin emulsion of claim 1 or 2, wherein the aforementioned (meth)acrylate (A) is composed of (meth)acrylate alkyl ester. The aqueous resin emulsion of claim 1 or 2, wherein the aforementioned ethylenically unsaturated carboxylic acid (B) is composed of a compound having a (meth)acryloyl group and a carboxyl group. The aqueous resin emulsion of claim 1 or 2, wherein the polyepoxide (Y) is at least one selected from bisphenol epoxy compounds, hydrogenated bisphenol epoxy compounds, diglycidyl ethers, triglycidyl ethers, tetraglycidyl ethers, diglycidyl esters, triglycidyl esters and tetraglycidyl esters. The aqueous resin emulsion of claim 1 or 2, wherein the glass transition point of the copolymer (X) is above -30°C and below 100°C. The aqueous resin emulsion of claim 1 or 2, wherein the aforementioned copolymer (X) is composed of structural units derived from (meth)acrylate (A) and structural units derived from ethylenically unsaturated carboxylic acid (B). The aqueous resin emulsion of claim 1 or 2, wherein the aforementioned copolymer (X) comprises a structural unit (c) derived from an ethylenically unsaturated aromatic compound (C) having a benzene ring and an ethylenically unsaturated bond. As in claim 9, the aqueous resin emulsion, wherein the aforementioned ethylenically unsaturated aromatic compound (C) is an aromatic vinyl compound. As in claim 1, the aqueous resin emulsion, wherein the aforementioned hydrophilic (meth)acrylate (A1) is methyl (meth)acrylate or ethyl (meth)acrylate. The aqueous resin emulsion of claim 1 further comprises an emulsifier, wherein the emulsifier comprises a non-ionic surfactant selected from polyoxyalkyl alkyl ethers, polyoxyalkyl alkylphenol ethers, polyoxyalkyl fatty acid esters, polyoxyalkyl sorbitan fatty acid esters, and/or an anionic surfactant selected from alkyl sulfates, alkylbenzene sulfonates, alkyl sulfosuccinates, alkyl diphenyl ether disulfonates, polyoxyalkyl alkyl sulfates, and polyoxyalkyl alkyl phosphates. A water-based resin composition, comprising: a water-based resin emulsion (α), which is the water-based resin emulsion as described in any one of claims 1 to 12; and a hardener (β), which has a functional group reactive to an epoxy group; the content of the functional group contained in the hardener (β) is 0.01 equivalent or more and 1.0 equivalent or less relative to the amount of epoxy group contained in the polyepoxide (Y). As in claim 13, the water-based resin composition, wherein the aforementioned hardener (β) is a compound having at least one selected from the group consisting of an amino group, a carboxyl group, and a hydroxyl group. A method for producing an aqueous resin emulsion, comprising the step of emulsifying and polymerizing monomers comprising (meth)acrylate (A) and ethylenically unsaturated carboxylic acid (B) in an aqueous medium (Z) in the presence of a polyepoxy compound (Y) having no ethylenically unsaturated bonds and having two or more epoxy groups in one molecule, to obtain an aqueous resin emulsion, wherein the amount of the polyepoxy compound (Y) added to the aqueous resin emulsion is 20 mass % or more and 30 mass % or less relative to the total amount of the monomer and the polyepoxy compound (Y), and the amount of the (meth)acrylate (A) and the ethylenically unsaturated carboxylic acid (B) are 20 mass % or more and 30 mass % or less relative to the total amount of the monomer and the polyepoxy compound (Y). The amount of the acrylate (A) added is 60-98% by mass relative to the total amount of the monomer and the polyepoxide (Y); the amount of the ethylenically unsaturated carboxylic acid (B) added is 1.0% by mass or more and less than 3% by mass relative to the total amount of the monomer and the polyepoxide (Y); the (meth)acrylate (A) contains a hydrophilic (meth)acrylate (A1) having a carbon number of 2 or less in the moiety derived from an alcohol; the amount of the hydrophilic (meth)acrylate (A1) added is 30% by mass or more and 70% by mass or less relative to the total amount of the monomer and the polyepoxide (Y); the content of epoxy groups in the non-volatile components of the aqueous resin emulsion is 0.50×10 ^-4 mol/g or more; and the content of carboxyl groups in the non-volatile components of the aqueous resin emulsion is 0.10×10 ^-4 mol/g or more. The method for producing an aqueous resin emulsion as claimed in claim 15, wherein the emulsion polymerization is carried out at 30-90°C in the aqueous resin emulsion. The method for producing an aqueous resin emulsion as claimed in claim 15, wherein the aforementioned hydrophilic (meth)acrylate (A1) is methyl (meth)acrylate or ethyl (meth)acrylate. A method for producing an aqueous resin emulsion as claimed in claim 15, wherein an emulsifier is used in the aforementioned emulsion polymerization; the aforementioned emulsifier comprises a non-ionic surfactant selected from polyoxyalkyl alkyl ethers, polyoxyalkyl alkylphenol ethers, polyoxyalkyl fatty acid esters, polyoxyalkyl sorbitan fatty acid esters, and/or an anionic surfactant selected from alkyl sulfates, alkylbenzene sulfonates, alkyl sulfosuccinates, alkyl diphenyl ether disulfonates, polyoxyalkyl alkyl sulfates, and polyoxyalkyl alkyl phosphates.

Images