JP2024113236A - Water-based paint composition and method for forming multi-layered paint film - Google Patents

Abstract

To provide an aqueous coating composition which is excellent in storage stability and curability at low temperatures of 80°C or lower, and is excellent in hardness and appearance when being formed into a resin film.SOLUTION: An aqueous coating composition contains hydroxyl group-containing polyol, a block polyisocyanate composition, at least two kinds of nonionic dispersants and deionized water, in which the nonionic dispersant has a difference between the dispersant having a highest HLB value and the dispersant having a lowest HLB value of 5 or more, and a value obtained by performing weighted average on the HLB values of all the nonionic dispersants contained in the aqueous coating composition is 14 or more and 17 or less.SELECTED DRAWING: None

Description

The present invention relates to a water-based paint composition and a method for forming a multi-layer coating film.

Polyurethane resin paints are known to have excellent abrasion resistance, chemical resistance, and stain resistance. In particular, polyurethane resin paints using polyisocyanates derived from aliphatic diisocyanates or alicyclic diisocyanates have even better weather resistance, and demand for them is showing an increasing trend.

However, polyurethane resin paints are generally two-component, making their use extremely inconvenient. That is, ordinary polyurethane resin paints are made up of two components, polyol and polyisocyanate, and the polyol and polyisocyanate must be stored separately and then mixed when painting. In addition, once the two are mixed, the paint gels in a short time and becomes unusable, which is an issue.

These problems with polyurethane resin paints make them difficult to use in automated painting in fields where line painting is performed, such as automobile painting or low-voltage painting. Also, because isocyanates react easily with water, they cannot be used in water-based paints such as electrodeposition paints. Furthermore, when using paints containing isocyanates, the painters and coating tanks must be thoroughly cleaned at the end of work, reducing work efficiency.

To improve the above-mentioned problems, it has been proposed to use blocked polyisocyanates in which all active isocyanate groups are blocked with a blocking agent. These blocked polyisocyanates do not react with polyols at room temperature. However, by heating, the blocking agent dissociates, and the active isocyanate groups are regenerated and react with polyols to cause a crosslinking reaction, thereby improving the above-mentioned problems. Therefore, many blocking agents have been investigated, and representative examples include phenol and methyl ethyl ketoxime.

However, when using blocked polyisocyanates that use these blocking agents, a high baking temperature of 140°C or higher is generally required. The need for baking at high temperatures is not only disadvantageous in terms of energy, but also requires the substrate to be heat resistant, which limits its applications.

On the other hand, research has been conducted into low-temperature baked blocked polyisocyanates using active methylene compounds such as acetoacetate esters and malonic acid diesters. For example, Patent Documents 1 and 2 disclose blocked polyisocyanate compositions that cure at 90°C.

JP 2002-322238 A JP 2006-335954 A

In recent years, from the viewpoint of protecting the global environment and in order to meet the demand for application to plastics with low heat resistance, there is a strong demand for coating compositions that cure at temperatures below 90°C. Under these circumstances, there is still no known coating composition that has good dispersibility when mixed with an aqueous polyol (water-dispersible polyol) that has hydroxyl groups, does not gel or thicken excessively during storage, cures well at or below 80°C, and provides good coating hardness.

The present invention has been made in consideration of the above circumstances, and aims to provide a water-based paint composition that has excellent storage stability and curing properties at low temperatures of 80°C or less, and that has excellent hardness and appearance when made into a resin film.

（前記一般式（Ｉ）中、Ｒ^１１、Ｒ^１２及びＲ^１３は、それぞれ独立に、ヒドロキシ基及びアミノ基からなる群より選択される１種以上の置換基を含んでもよいアルキル基であり、Ｒ^１１、Ｒ^１２及びＲ^１３の合計炭素数は３以上２０以下であり、Ｒ^１４、Ｒ^１５及びＲ^１６は、それぞれ独立に、水素原子、又はヒドロキシ基及びアミノ基からなる群より選択される１種以上の置換基を含んでもよいアルキル基であり、波線は結合手を表す。）
［４］前記ブロックポリイソシアネート組成物の一部又は全部が、親水性化合物から誘導される構造単位を有する、［１］～［３］のいずれか１つに記載の水系塗料組成物。
［５］電着塗膜の焼付けが完了した自動車車体用の鋼板上に、中塗り塗料組成物、ベース塗料組成物、クリヤー塗料組成物を順次塗装したのち、加熱硬化させて複層塗膜を形成する方法であり、前記中塗り塗料組成物または前記ベース塗料組成物が［１］～［４］のいずれか１つに記載の水系塗料組成物である、複層塗膜の形成方法。
［６］前記加熱硬化の焼付温度が７５℃以上１００℃以下である、［５］に記載の複層塗膜の形成方法。 That is, the present invention includes the following aspects.
[1] A water-based coating composition comprising a blocked polyisocyanate composition, at least two or more types of nonionic dispersants, a hydroxyl group-containing polyol, and deionized water, wherein the difference in HLB value between the dispersant having the highest HLB value and the dispersant having the lowest HLB value is 5 or more, and the weighted average of the HLB values of all of the nonionic dispersants contained in the water-based coating composition is 14 or more and 17 or less.
[2] The aqueous coating composition according to [1], wherein the total amount of the nonionic dispersant is 1 mass % or more and 30 mass % or less relative to the total amount of the blocked polyisocyanate composition.
[3] The blocked polyisocyanate composition has a structure represented by the following general formula (I) or a heterocycle containing one or more nitrogen atoms, the aqueous coating composition according to [1] or [2]:

(In the general formula (I), R ¹¹ , R ¹² and R ¹³ each independently represent an alkyl group which may contain one or more substituents selected from the group consisting of a hydroxyl group and an amino group, the total number of carbon atoms of R ¹¹ , R ¹² and R ¹³ is 3 to 20, R ¹⁴ , R ¹⁵ and R ¹⁶ each independently represent a hydrogen atom or an alkyl group which may contain one or more substituents selected from the group consisting of a hydroxyl group and an amino group, and the wavy line represents a bond.)
[4] The aqueous coating composition according to any one of [1] to [3], wherein a part or all of the blocked polyisocyanate composition has a structural unit derived from a hydrophilic compound.
[5] A method for forming a multilayer coating film, comprising coating an intermediate coating composition, a base coating composition, and a clear coating composition in that order on a steel sheet for an automobile body on which baking of an electrodeposition coating film has been completed, and then heating and curing the composition to form a multilayer coating film, wherein the intermediate coating composition or the base coating composition is the aqueous coating composition described in any one of [1] to [4].
[6] The method for forming a multilayer coating film according to [5], wherein the baking temperature for the heat curing is 75°C or higher and 100°C or lower.

According to the above aspect, it is possible to provide a water-based paint composition that has good storage stability and curability at low temperatures of 80°C or less, and has excellent hardness and appearance when made into a resin film.

The following describes in detail the form for carrying out the present invention (hereinafter, "embodiment"). Note that the present invention is not limited to the following embodiment.

As used herein, "polyol" means a compound having two or more hydroxy groups (-OH).
As used herein, the term "polyisocyanate" refers to a reaction product in which multiple monomer compounds having one or more isocyanate groups (--NCO) are bonded together.

<Water-based coating composition>
The water-based coating composition of this embodiment contains a blocked polyisocyanate composition, at least two or more types of nonionic dispersants, a hydroxyl group-containing polyol, and deionized water.

When a blocked polyisocyanate composition is applied to a water-based paint, a hydrophilic component is added to the blocked polyisocyanate, but if there is a large amount of hydrophilic component, the hardness of the coating film decreases. On the other hand, if there is a small amount of hydrophilic component, the compatibility of the blocked polyisocyanate composition with other components decreases, and aggregates are more likely to form on the coating film surface.

In contrast, the water-based coating composition of this embodiment contains at least two or more nonionic dispersants, and the difference in HLB value between the dispersant with the highest HLB value and the dispersant with the lowest HLB value is 5 or more, and the weighted average of the HLB values of all nonionic dispersants contained in the system is 14 to 17. This makes it difficult for aggregates to form even when there is a relatively small amount of hydrophilic components, and results in excellent appearance and coating film performance when made into a resin film.

Below, we will explain in detail each of the components contained in the water-based paint composition of this embodiment.

<Blocked polyisocyanate composition>
The blocked polyisocyanate composition is a reaction product of a polyisocyanate and a blocking agent. That is, in the blocked polyisocyanate composition, at least a part of the isocyanate groups in the polyisocyanate is blocked with a blocking agent. Hereinafter, the blocked polyisocyanate composition may be abbreviated as "blocked polyisocyanate".

In particular, when the blocked polyisocyanate contains structural units derived from a hydrophilic compound, it is preferable that all of the isocyanate groups are blocked with a blocking agent. Whether all of the isocyanate groups are blocked with a blocking agent can be confirmed, for example, by the disappearance of absorption due to isocyanate using Fourier transform infrared spectroscopy (FT-IR).

The water-based coating composition of this embodiment contains the above-mentioned blocked polyisocyanate, and therefore has good water dispersibility, and when formed into a resin film, it has excellent hardness and low-temperature curing properties.

The blocked polyisocyanate may have one or more functional groups selected from the group consisting of an allophanate group, a uretdione group, an iminooxadiazinedione group, an isocyanurate group, a urethane group, and a biuret group. Of these, it is preferable to have an isocyanurate group, since this has excellent weather resistance.

The blocked polyisocyanate preferably contains a blocked isocyanurate trimer. The term "isocyanurate trimer" used here means a polyisocyanate derived from three molecules of an isocyanate monomer and having an isocyanurate group. The term "blocked isocyanurate trimer" means that at least a portion (preferably all) of the isocyanate groups in the isocyanurate trimer are blocked with a blocking agent. By containing a blocked isocyanurate trimer, the heat resistance of the resin film tends to be superior.

[Polyisocyanate]
The polyisocyanate used in the production of the blocked polyisocyanate is a reaction product obtained by reacting multiple monomer compounds having one or more isocyanate groups (-NCO) (hereinafter, sometimes referred to as "isocyanate monomers").

As the isocyanate monomer, one having 4 to 30 carbon atoms is preferable. Specific examples of the isocyanate monomer include the following. These isocyanate monomers may be used alone or in combination of two or more.

(1) Aromatic diisocyanates such as diphenylmethane-4,4'-diisocyanate (MDI), 1,5-naphthalene diisocyanate, tolylene diisocyanate (TDI), xylylene diisocyanate, and m-tetramethylxylylene diisocyanate (TMXDI).
(2) Aliphatic diisocyanates such as 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate (hereinafter sometimes referred to as "HDI"), 2,2,4-trimethyl-1,6-diisocyanatohexane, 2,4,4-trimethyl-1,6-hexamethylene diisocyanate, 2-methylpentane-1,5-diisocyanate (MPDI), and lysine diisocyanate (hereinafter sometimes referred to as "LDI").

(3) Alicyclic diisocyanates such as isophorone diisocyanate (hereinafter sometimes referred to as "IPDI"), 1,3-bis(diisocyanatemethyl)cyclohexane, 4,4'-dicyclohexylmethane diisocyanate, diisocyanatenorbornane, and di(isocyanatemethyl)norbornane.
(4) Triisocyanates such as 4-isocyanatomethyl-1,8-octamethylene diisocyanate (hereinafter sometimes referred to as "NTI"), 1,3,6-hexamethylene triisocyanate (hereinafter sometimes referred to as "HTI"), bis(2-isocyanatoethyl) 2-isocyanatoglutarate (hereinafter sometimes referred to as "GTI"), and lysine triisocyanate (hereinafter sometimes referred to as "LTI").

Among them, the isocyanate monomer is preferably one or more diisocyanates selected from the group consisting of aliphatic diisocyanates and alicyclic diisocyanates, because they have excellent weather resistance. Moreover, the isocyanate monomer is more preferably HDI or IPDI, because they are easily available industrially. Moreover, the isocyanate monomer is even more preferably HDI, because it reduces the viscosity of the blocked polyisocyanate component.

The blocked polyisocyanate preferably has a structure represented by the following general formula (I) or a heterocycle containing one or more nitrogen atoms.

（前記一般式（Ｉ）中、Ｒ^１１、Ｒ^１２及びＲ^１３は、それぞれ独立に、ヒドロキシ基及びアミノ基からなる群より選択される１種以上の置換基を含んでもよいアルキル基であり、Ｒ^１１、Ｒ^１２及びＲ^１３の合計炭素数は３以上２０以下であり、Ｒ^１４、Ｒ^１５及びＲ^１６は、それぞれ独立に、水素原子、又はヒドロキシ基及びアミノ基からなる群より選択される１種以上の置換基を含んでもよいアルキル基であり、波線は結合手を表す。）

(In the general formula (I), R ¹¹ , R ¹² and R ¹³ each independently represent an alkyl group which may contain one or more substituents selected from the group consisting of a hydroxyl group and an amino group, the total number of carbon atoms of R ¹¹ , R ¹² and R ¹³ is 3 to 20, R ¹⁴ , R ¹⁵ and R ¹⁶ each independently represent a hydrogen atom or an alkyl group which may contain one or more substituents selected from the group consisting of a hydroxyl group and an amino group, and the wavy line represents a bond.)

The alkyl group in R ¹¹ , R ¹² , R ¹³ , R ¹⁴ , R ¹⁵ and R ¹⁶ preferably has 1 or more and 20 or less carbon atoms, more preferably has 1 or more and 8 or less carbon atoms, further preferably has 1 or more and 6 or less carbon atoms, and particularly preferably has 1 or more and 4 or less carbon atoms.

Specific examples of unsubstituted alkyl groups include methyl, ethyl, propyl, isopropyl, n-butyl, tert-butyl, sec-butyl, isobutyl, n-pentyl, isopentyl, neopentyl, tert-pentyl, 1-methylbutyl, n-hexyl, 2-methylpentyl, 3-methylpentyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, n-heptyl, 2-methylhexyl, 3-methylhexyl, 2,2-dimethylpentyl, 2,3-dimethylpentyl, 2,4-dimethylpentyl, 3,3-dimethylpentyl, 3-ethylpentyl, 2,2,3-trimethylbutyl, n-octyl, isooctyl, 2-ethylhexyl, nonyl, and decyl.

Furthermore, when R ¹¹ , R ¹² , R ¹³ , R ¹⁴ , R ¹⁵ and R ¹⁶ are alkyl groups having a substituent, the substituent is a hydroxy group or an amino group.

Examples of alkyl groups containing a hydroxy group as a substituent include a hydroxymethyl group, a hydroxyethyl group, and a hydroxypropyl group.

Examples of alkyl groups containing an amino group as a substituent include an aminomethyl group, an aminoethyl group, an aminopropyl group, and an aminobutyl group.

Examples of alkyl groups containing a hydroxy group and an amino group as a substituent include a hydroxyaminomethyl group, a hydroxyaminoethyl group, and a hydroxyaminopropyl group.

When the blocked polyisocyanate has a heterocycle containing one or more nitrogen atoms, it is preferable that the heterocycle is derived from a compound having a heterocycle containing one or more nitrogen atoms. The compound having a heterocycle containing one or more nitrogen atoms is a blocking agent, and specific examples of such blocking agents include pyrazole-based compounds, triazole-based compounds, and imidazole-based compounds. More specific examples include pyrazole, 3-methylpyrazole, 3,5-dimethylpyrazole, 3,5-dimethyl-1,2,4-triazole, 1,2,4-triazole, 1,2,3-triazole, imidazole, 2-methylimidazole, 4-methylimidazole, 2-ethylimidazole, 2-isopropylimidazole, 2,4-dimethylimidazole, 2-ethyl-4-methylimidazole, 2-phenylimidazole, and 4-methyl-2-phenylimidazole. Among these, from the viewpoint of low-temperature curing, imidazole-based compounds are preferred, and 2-ethylimidazole is more preferred. These blocking agents may be used alone or in combination of two or more.

(Method for producing polyisocyanate)
The method for producing polyisocyanate will be described in detail below.
The polyisocyanate can be obtained by, for example, carrying out an allophanate reaction to form an allophanate group, a uretdione reaction to form a uretdione group, an iminooxadiazinedione reaction to form an iminooxadiazinedione group, an isocyanurate reaction to form an isocyanurate group, a urethanization reaction to form a urethane group, and a biuret reaction to form a biuret group all at once in the presence of an excess of isocyanate monomer, and removing the unreacted isocyanate monomer after the reaction is completed. That is, the polyisocyanate obtained by the above reaction is a reaction product in which a plurality of the above-mentioned isocyanate monomers are bonded, and which has one or more types selected from the group consisting of an allophanate group, a uretdione group, an iminooxadiazinedione group, an isocyanurate group, a urethane group, and a biuret group.
Alternatively, the above reactions may be carried out separately and the resulting polyisocyanates may be mixed in a specific ratio.
From the viewpoint of ease of production, it is preferable to carry out the above reaction at one time to obtain a polyisocyanate, and from the viewpoint of freely adjusting the molar ratio of each functional group, it is preferable to produce them separately and then mix them.

For example, a commonly used catalyst for deriving a polyisocyanate containing an isocyanurate group from an isocyanate monomer is an isocyanurate reaction catalyst.

The isocyanurate reaction catalyst is not particularly limited, but is preferably a catalyst having basicity in general. Specific examples of the isocyanurate reaction catalyst include the following.
1) Hydroxides of tetraalkylammonium such as tetramethylammonium, tetraethylammonium, and tetrabutylammonium, and organic weak acid salts such as acetates, propionates, octylates, caprates, myristates, and benzoates of the above tetraalkylammonium.
2) Hydroxides of aryltrialkylammonium such as benzyltrimethylammonium and trimethylphenylammonium, and organic weak acid salts such as acetates, propionates, octylates, caprates, myristates, and benzoates of the above aryltrialkylammonium.
3) Hydroxyalkylammonium hydroxides such as trimethylhydroxyethylammonium, trimethylhydroxypropylammonium, triethylhydroxyethylammonium, and triethylhydroxypropylammonium, and organic weak acid salts such as acetates, propionates, octylates, caprates, myristates, and benzoates of the above hydroxyalkylammoniums.
4) Metal salts, such as tin, zinc, lead, etc., of alkyl carboxylic acids, such as acetic acid, propionic acid, caproic acid, octylic acid, capric acid, and myristic acid.
5) Metal alcoholates such as sodium and potassium.
6) Aminosilyl group-containing compounds such as hexamethylenedisilazane.
7) Mannich bases.
8) Mixtures of tertiary amines and epoxy compounds.
9) Phosphorus compounds such as tributylphosphine.

Among these, from the viewpoint of preventing the generation of unnecessary by-products, the isocyanurate reaction catalyst is preferably a quaternary ammonium hydroxide or an organic weak acid salt of a quaternary ammonium, and more preferably a tetraalkylammonium hydroxide, an organic weak acid salt of a tetraalkylammonium, an aryltrialkylammonium hydroxide, or an organic weak acid salt of an aryltrialkylammonium.

The upper limit of the amount of the isocyanurate reaction catalyst used is preferably 1000 ppm by mass, more preferably 500 ppm by mass, and even more preferably 100 ppm by mass, based on the mass of the charged isocyanate monomer.
On the other hand, the lower limit of the amount of the isocyanurate reaction catalyst used is not particularly limited, and may be, for example, 10 ppm by mass.

The isocyanurate reaction temperature is preferably 50°C or higher and 120°C or lower, and more preferably 55°C or higher and 90°C or lower. By keeping the isocyanurate reaction temperature below the upper limit, coloration of the polyisocyanate tends to be more effectively suppressed.

When a desired conversion rate (the ratio of the mass of polyisocyanate produced in the isocyanuration reaction to the mass of the charged isocyanate monomer) is reached, the isocyanuration reaction is stopped by adding an acidic compound (e.g., phosphoric acid, an acidic phosphoric acid ester, etc.).
In order to obtain polyisocyanate, it is necessary to stop the reaction at an early stage. However, since the reaction rate of the isocyanurate reaction is very fast at the early stage, it is difficult to stop the reaction at an early stage, and therefore the reaction conditions, particularly the amount and method of adding the catalyst, must be carefully selected. For example, a method of adding the catalyst in portions at regular intervals is recommended as a suitable method.

Therefore, the conversion rate of the isocyanurate reaction to obtain a polyisocyanate is preferably 10% or more and 60% or less, more preferably 15% or more and 55% or less, and even more preferably 20% or more and 50% or less.
By setting the conversion rate of the isocyanurate reaction to the upper limit or less, the viscosity of the blocked polyisocyanate component can be made lower. By setting the conversion rate of the isocyanurate reaction to the lower limit or more, the reaction terminating operation can be more easily carried out.

When deriving a polyisocyanate containing an isocyanurate group, a monohydric to hexahydric alcohol can be used in addition to the above-mentioned isocyanate monomer.
Examples of alcohols having a valence of 1 to 6 that can be used include non-polymerizable alcohols and polymerizable alcohols. The term "non-polymerizable alcohol" used herein means an alcohol that does not have a polymerizable group. On the other hand, the term "polymerizable alcohol" means an alcohol obtained by polymerizing a monomer having a polymerizable group and a hydroxyl group.

Examples of the non-polymerizable alcohol include polyhydric alcohols such as monoalcohols, diols, triols, and tetraols.
Examples of monoalcohols include methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, n-pentanol, n-hexanol, n-octanol, n-nonanol, 2-ethylbutanol, 2,2-dimethylhexanol, 2-ethylhexanol, cyclohexanol, methylcyclohexanol, and ethylcyclohexanol.

Examples of diols include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 2,3-butanediol, 2-methyl-1,2-propanediol, 1,5-pentanediol, 2-methyl-2,3-butanediol, Examples include hexanediol, 1,6-hexanediol, 1,2-hexanediol, 2,5-hexanediol, 2-methyl-2,4-pentanediol, 2,3-dimethyl-2,3-butanediol, 2-ethyl-hexanediol, 1,2-octanediol, 1,2-decanediol, 2,2,4-trimethylpentanediol, 2-butyl-2-ethyl-1,3-propanediol, and 2,2-diethyl-1,3-propanediol.

Examples of triols include glycerin and trimethylolpropane.
An example of the tetraols is pentaerythritol.

Polymerizable alcohols include, but are not limited to, polyester polyols, polyether polyols, acrylic polyols, polyolefin polyols, etc.

The polyester polyols are not particularly limited, but examples thereof include products obtained by a condensation reaction between a dibasic acid alone or a mixture of dibasic acids and a polyhydric alcohol alone or a mixture of polyhydric alcohols.
The dibasic acid is not particularly limited, but examples thereof include at least one dibasic acid selected from the group consisting of carboxylic acids such as succinic acid, adipic acid, sebacic acid, dimer acid, maleic anhydride, phthalic anhydride, isophthalic acid, and terephthalic acid.

The polyhydric alcohol is not particularly limited, but examples thereof include at least one polyhydric alcohol selected from the group consisting of ethylene glycol, propylene glycol, diethylene glycol, neopentyl glycol, trimethylolpropane, and glycerin.
Examples of polyester polyols include polycaprolactones obtained by ring-opening polymerization of ε-caprolactone using the above polyhydric alcohols.

The polyether polyols are not particularly limited, but examples thereof include polyether polyols obtained by adding an alkylene oxide or a mixture of an alkylene oxide to a polyhydric alcohol or a mixture of an alkylene oxide using an alkali metal hydroxide or a strong basic catalyst, polyether polyols obtained by reacting an alkylene oxide with a polyamine compound, and so-called polymer polyols obtained by polymerizing acrylamide or the like using the above-mentioned polyethers as a medium.
Examples of the alkali metal include lithium, sodium, and potassium.
Examples of the strong basic catalyst include alcoholates and alkylamines.
Examples of the polyhydric alcohol include the same ones as those exemplified for the polyester polyols above.
Examples of the alkylene oxide include ethylene oxide, propylene oxide, butylene oxide, cyclohexene oxide, and styrene oxide.
Examples of the polyamine compound include ethylenediamines.

The acrylic polyols are not particularly limited, but examples include copolymers of a single or mixture of ethylenically unsaturated bond-containing monomers having a hydroxyl group and a single or mixture of other ethylenically unsaturated bond-containing monomers copolymerizable therewith.

The ethylenically unsaturated bond-containing monomer having a hydroxyl group is not particularly limited, but examples thereof include hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxybutyl acrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, and hydroxybutyl methacrylate.
The other ethylenically unsaturated bond-containing monomer copolymerizable with the ethylenically unsaturated bond-containing monomer having a hydroxyl group is not particularly limited, and examples thereof include acrylic acid esters, methacrylic acid esters, unsaturated carboxylic acids, unsaturated amides, vinyl-based monomers, and vinyl-based monomers having a hydrolyzable silyl group.
Examples of acrylic esters include methyl acrylate, ethyl acrylate, propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, n-hexyl acrylate, cyclohexyl acrylate, 2-ethylhexyl acrylate, lauryl acrylate, benzyl acrylate, and phenyl acrylate.

Examples of methacrylic acid esters include methyl methacrylate, ethyl methacrylate, propyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-hexyl methacrylate, cyclohexyl methacrylate, 2-ethylhexyl methacrylate, lauryl methacrylate, benzyl methacrylate, and phenyl methacrylate.
Examples of the unsaturated carboxylic acid include acrylic acid, methacrylic acid, maleic acid, and itaconic acid.

Examples of unsaturated amides include acrylamide, methacrylamide, N,N-methylenebisacrylamide, diacetone acrylamide, diacetone methacrylamide, maleic acid amide, and maleimide.

Examples of the vinyl monomer include glycidyl methacrylate, styrene, vinyl toluene, vinyl acetate, acrylonitrile, and dibutyl fumarate.
Examples of vinyl monomers having a hydrolyzable silyl group include vinyltrimethoxysilane, vinylmethyldimethoxysilane, and γ-(meth)acryloxypropyltrimethoxysilane.

Examples of polyolefin polyols include hydroxyl-terminated polybutadiene and its hydrogenated products.

The allophanate formation reaction, the uretdione formation reaction, the iminooxadiazinedione formation reaction, the isocyanurate formation reaction, the urethanization reaction, and the biuret formation reaction may each be carried out successively, or some of them may be carried out in parallel.
After the reaction is completed, unreacted isocyanate monomer can be removed from the reaction solution by thin-film distillation, extraction, or the like to obtain a polyisocyanate.

Furthermore, an antioxidant or an ultraviolet absorber may be added to the obtained polyisocyanate, for example, for the purpose of suppressing coloration during storage.
Examples of the antioxidant include hindered phenols such as 2,6-di-tert-butyl-p-cresol. Examples of the ultraviolet absorber include benzotriazole and benzophenone. These antioxidants and ultraviolet absorbers may be used alone or in combination of two or more. The amount of these added is preferably 10 ppm by mass or more and 500 ppm by mass or less based on the mass of the polyisocyanate.

(Average number of isocyanate functional groups of polyisocyanate)
The average number of isocyanate functional groups of the polyisocyanate is preferably 2.0 or more from the viewpoint of improving the low-temperature curing property when formed into a resin film, and from the viewpoint of achieving both the low-temperature curing property when formed into a resin film and compatibility with a polyhydric alcohol compound, it is more preferably 3.0 or more, further preferably 4.6 or more and 20 or less, and particularly preferably 5 or more and 10 or less.
The average number of isocyanate functional groups of the polyisocyanate can be measured by the method described in the Examples below.

[Blocking agent]
As the blocking agent, a malonic acid diester compound or a compound having a heterocycle containing one or more nitrogen atoms is preferable. These blocking agents may be used alone or in combination of two or more kinds.

The malonic acid diester is not particularly limited, but preferably includes a malonic acid diester having a secondary alkyl group or a malonic acid diester having a primary alkyl group, and a malonic acid diester having a tertiary alkyl group, and more preferably includes a malonic acid diester having a secondary alkyl group and a malonic acid diester having a tertiary alkyl group. The blocking agent may include one type each of a malonic acid diester having a secondary alkyl group, a malonic acid diester having a primary alkyl group, and a malonic acid diester having a tertiary alkyl group, or may include a combination of two or more types.

Examples of malonic acid diesters having a primary alkyl group include dimethyl malonate, diethyl malonate, dipropyl malonate, dibutyl malonate, dicyclohexyl malonate, and diphenyl malonate. Among these, diethyl malonate is preferred as a malonic acid diester having a primary alkyl group.

Examples of malonic acid diesters having a secondary alkyl group include di-sec-butyl malonate, diisopropyl malonate, and isopropylethyl malonate. Among these, diisopropyl malonate is preferred as a malonic acid diester having a secondary alkyl group.

Examples of malonic acid diesters having a tertiary alkyl group include di-tert-butyl malonate, di(2-methyl-2-butyl) malonate, di(2-methyl-2-pentyl) malonate, (tert-butyl)ethyl malonate, (2-methyl-2-butyl)ethyl malonate, (2-methyl-2-butyl)isopropyl malonate, (2-methyl-2-pentyl)ethyl malonate, (2-methyl-2-pentyl)isopropyl malonate, and (2-methyl-2-pentyl)hexylisopropyl malonate. Among these, di(2-methyl-2-butyl) malonate, di(2-methyl-2-pentyl) malonate, (2-methyl-2-butyl) isopropyl malonate, (2-methyl-2-pentyl) ethyl malonate, and (2-methyl-2-pentyl) isopropyl malonate are preferred, and (2-methyl-2-butyl) ethyl malonate, (2-methyl-2-butyl) isopropyl malonate, (2-methyl-2-pentyl) ethyl malonate, and (2-methyl-2-pentyl) hexyl isopropyl malonate are preferred, or di-tert-butyl 3 malonate, (2-methyl-2-butyl) isopropyl malonate, or (2-methyl-2-pentyl) isopropyl malonate are preferred.
The malonic acid diester having a tertiary alkyl group may be a commercially available product, or may be synthesized by the method described in Reference 1 (JP-A-11-130728).

Among these, the malonic acid diester compounds diisopropyl malonate, di-tert-butyl malonate, (2-methyl-2-butyl) isopropyl malonate, and (2-methyl-2-pentyl) isopropyl malonate are particularly preferred because of their ease of availability, the viscosity of the resulting blocked polyisocyanate composition, the curing temperature and curing time, and the curing properties of the resin film at low temperatures of about 80°C or less.

As a compound having a heterocycle containing one or more nitrogen atoms, the heterocycle containing one or more nitrogen atoms described above in the section on "Blocked polyisocyanate composition" can be used.

(Other blocking agents)
The blocking agent used in the production of the blocked polyisocyanate may further contain other blocking agents in addition to the malonic acid diester and the compound having a heterocycle containing one or more nitrogen atoms, within the range that does not impair the storage stability when made into a resin composition and the low-temperature curing property when made into a resin film.

Other blocking agents include, for example, 1) alcohol-based compounds, 2) alkylphenol-based compounds, 3) phenol-based compounds, 4) active methylene-based compounds other than malonic acid esters having a secondary alkyl group and malonic acid diesters having a tertiary alkyl group, 5) mercaptan-based compounds, 6) acid amide-based compounds, 7) acid imide-based compounds, 8) urea-based compounds, 9) oxime-based compounds, 10) amine-based compounds, 11) imide-based compounds, 12) bisulfites, etc. More specifically, the following blocking agents can be used:

1) Alcohol compounds: alcohols such as methanol, ethanol, 2-propanol, n-butanol, sec-butanol, 2-ethyl-1-hexanol, 2-methoxyethanol, 2-ethoxyethanol, and 2-butoxyethanol.
2) Alkylphenol compounds: mono- and di-alkylphenols having an alkyl group having 4 or more carbon atoms as a substituent. Specific examples of the alkylphenol compounds include monoalkylphenols such as n-propylphenol, iso-propylphenol, n-butylphenol, Se-butylphenol, tert-butylphenol, n-hexylphenol, 2-ethylhexylphenol, n-octylphenol, and n-nonylphenol; and dialkylphenols such as di-n-propylphenol, diisopropylphenol, isopropyl cresol, di-n-butylphenol, di-tert-butylphenol, di-Se-butylphenol, di-n-octylphenol, di-2-ethylhexylphenol, and di-n-nonylphenol.

3) Phenolic compounds: phenol, cresol, ethylphenol, styrenated phenol, hydroxybenzoic acid esters, etc.
4) Active methylene compounds: methyl acetoacetate, ethyl acetoacetate, methyl isobutanoylacetate, ethyl isobutanoylacetate, acetylacetone, etc.
5) Mercaptan compounds: butyl mercaptan, dodecyl mercaptan, etc. 6) Acid amide compounds: acetanilide, acetic acid amide, C-caprolactam, 8-valerolactam, γ-butyrolactam, etc.
7) Acid imide compounds: succinimide, maleimide, etc.
8) Urea compounds: urea, thiourea, ethyleneurea, etc.

9) Oxime compounds: formaldoxime, acetaldoxime, acetoxime, methyl ethyl ketoxime, cyclohexanone oxime, etc.
10) Amine compounds: diphenylamine, aniline, carbazole, di-n-propylamine, diisopropylamine, isopropylethylamine, etc.
11) Imine compounds: ethyleneimine, polyethyleneimine, etc.
12) Bisulfite compounds: sodium bisulfite, etc.

[Hydrophilic Compound]
The blocked polyisocyanate contained in the aqueous coating composition of this embodiment may have structural units, some or all of which are derived from a hydrophilic compound.
That is, a part or all of the blocked polyisocyanate contained in the blocked polyisocyanate composition of the present embodiment may be a hydrophilic blocked polyisocyanate.

The ratio of isocyanate groups modified by the hydrophilic compound to the total molar amount of isocyanate groups in the polyisocyanate can also be referred to as the ratio of isocyanate groups modified by the hydrophilic compound to 100 mol% of isocyanate groups in the raw material polyisocyanate (hereinafter sometimes referred to as the "modification rate"), and can also be referred to as the ratio of isocyanate groups to which a hydrophilic group has been introduced to the total molar amount of isocyanate groups to which a hydrophilic group has been introduced and isocyanate groups to which a hydrophilic group has not been introduced.

The modification rate is 0.1 mol% or more and 50 mol% or less, preferably 0.1 mol% or more and 40 mol% or less, and more preferably 0.1 mol% or more and 35 mol% or less. When the modification rate is within the above range, the water dispersibility is good and the hardness when made into a resin film is excellent.

In order for the hydrophilic compound to react with one isocyanate group, it is preferable that the hydrophilic compound has one or more active hydrogen groups per molecule of the hydrophilic compound to react with the isocyanate group of the polyisocyanate. Specific examples of active hydrogen groups include hydroxyl groups, mercapto groups, carboxylic acid groups, amino groups, and thiol groups.

Examples of hydrophilic groups include nonionic hydrophilic groups, cationic hydrophilic groups, and anionic hydrophilic groups. These hydrophilic groups may be used alone or in combination of two or more. Among them, nonionic hydrophilic groups are preferred as hydrophilic groups from the viewpoints of availability and resistance to electrical interaction with the compound, and anionic hydrophilic groups are preferred from the viewpoint of suppressing a decrease in the hardness of the resulting resin film.

Specific examples of hydrophilic compounds having a nonionic hydrophilic group include monoalcohols and compounds in which ethylene oxide is added to the hydroxyl group of an alcohol. Examples of monoalcohols include methanol, ethanol, and butanol. Examples of compounds in which ethylene oxide is added to the hydroxyl group of an alcohol include ethylene glycol, diethylene glycol, polyethylene glycol, and polyethylene glycol monomethyl ether. These hydrophilic compounds having a nonionic hydrophilic group also have an active hydrogen group that reacts with an isocyanate group.

The number of ethylene oxide additions in the ethylene oxide-added compound is preferably from 4 to 30, and more preferably from 4 to 20. When the number of ethylene oxide additions is equal to or more than the above lower limit, water dispersibility tends to be more effectively imparted to the blocked polyisocyanate composition, and when the number of ethylene oxide additions is equal to or less than the above upper limit, the blocked polyisocyanate composition tends to be less prone to precipitate during low-temperature storage.
Among these, as the hydrophilic compound having a nonionic hydrophilic group, a monoalcohol is preferred since it can improve the water dispersibility of the blocked polyisocyanate composition with a small amount used.

From the viewpoint of the aqueous dispersion stability of the blocked polyisocyanate composition, the lower limit of the amount of nonionic hydrophilic groups added to the blocked polyisocyanate (hereinafter, sometimes referred to as the "content of nonionic hydrophilic groups") is preferably 1 mass %, more preferably 2 mass %, even more preferably 3 mass %, and particularly preferably 4 mass %, based on the mass of the solid content of the blocked polyisocyanate composition.

The upper limit of the content of nonionic hydrophilic groups is preferably 30% by mass, more preferably 20% by mass, even more preferably 18% by mass, and particularly preferably 15% by mass, based on the mass of the solid content of the blocked polyisocyanate composition, from the viewpoint of the water resistance and hardness of the resulting resin film.

That is, the content of the nonionic hydrophilic group is preferably 1 mass % or more and 30 mass % or less, more preferably 2 mass % or more and 20 mass % or less, even more preferably 3 mass % or more and 18 mass % or less, and particularly preferably 4 mass % or more and 15 mass % or less, relative to the mass of the non-volatile content of the blocked polyisocyanate composition.
By ensuring that the content of nonionic hydrophilic groups is within the above range, the water-based coating composition disperses better in water, and the water resistance of the resulting resin film tends to be improved.

(Hydrophilic Compound Having a Cationic Hydrophilic Group)
As the hydrophilic compound having a cationic hydrophilic group, specifically, a compound having both a cationic hydrophilic group and an active hydrogen group can be mentioned.Also, a compound having an active hydrogen group such as a glycidyl group and a compound having a cationic hydrophilic group such as a sulfide or phosphine can be used as the hydrophilic compound.
In this case, a compound having an isocyanate group is reacted with a compound having an active hydrogen group in advance to add a functional group such as a glycidyl group, and then a compound such as a sulfide, a phosphine, etc. is reacted. From the viewpoint of ease of production, a compound having both a cationic hydrophilic group and an active hydrogen group is preferred.

Specific examples of compounds having both a cationic hydrophilic group and an active hydrogen group include dimethylethanolamine, diethylethanolamine, diethanolamine, and methyldiethanolamine. Tertiary amino groups added using these compounds can also be quaternized with, for example, dimethyl sulfate or diethyl sulfate.

The reaction between the hydrophilic compound having a cationic hydrophilic group and the polyisocyanate can be carried out in the presence of a solvent. In this case, the solvent is preferably one that does not contain an active hydrogen group, and specific examples include ethyl acetate, propylene glycol monomethyl ether acetate, and dipropylene glycol dimethyl ether.

The cationic hydrophilic group added to the blocked polyisocyanate is preferably neutralized with a compound having an anionic group, such as a carboxy group, a sulfonic acid group, a phosphoric acid group, a halogen group, or a sulfate group.
Specific examples of compounds having a carboxyl group include formic acid, acetic acid, propionic acid, butyric acid, and lactic acid.
A specific example of a compound having a sulfonic acid group is ethanesulfonic acid.
Specific examples of compounds having a phosphoric acid group include phosphoric acid and acidic phosphate esters.
A specific example of the compound having a halogen group is hydrochloric acid.
Specific examples of compounds having a sulfate group include sulfuric acid.
Among them, as the compound having an anionic group, a compound having a carboxyl group is preferable, and acetic acid, propionic acid or butyric acid is more preferable.

(Hydrophilic Compound Having Anionic Hydrophilic Group)
Specific examples of the anionic hydrophilic group include a carboxy group, a sulfonic acid group, a phosphoric acid group, a halogen group, and a sulfate group.
Specific examples of hydrophilic compounds having an anionic hydrophilic group include compounds having both an anionic group and an active hydrogen group, and more specific examples include compounds having a carboxy group of a monohydroxycarboxylic acid or polyhydroxycarboxylic acid as the anionic group.
Examples of monohydroxycarboxylic acids include 1-hydroxyacetic acid, 3-hydroxypropanoic acid, 12-hydroxy-9-octadecanoic acid, hydroxypivalic acid, and lactic acid.

Examples of compounds having a carboxy group of a polyhydroxycarboxylic acid as an anionic group include dimethylol acetic acid, 2,2-dimethylol butyric acid, 2,2-dimethylol pentanoic acid, dihydroxysuccinic acid, and dimethylol propionic acid.
Further, compounds having both a sulfonic acid group and an active hydrogen group are also included, and more specifically, for example, isethionic acid is included.
Among these, hydroxypivalic acid or dimethylolpropionic acid is preferred as a compound having both an anionic group and an active hydrogen group.

The anionic hydrophilic group added to the blocked polyisocyanate is preferably neutralized with an amine compound, which is a basic substance.
Specific examples of the amine compound include ammonia and water-soluble amino compounds.
Specific examples of water-soluble amino compounds include monoethanolamine, ethylamine, dimethylamine, diethylamine, triethylamine, propylamine, dipropylamine, isopropylamine, diisopropylamine, triethanolamine, butylamine, dibutylamine, 2-ethylhexylamine, ethylenediamine, propylenediamine, methylethanolamine, dimethylethanolamine, diethylethanolamine, and morpholine. Tertiary amines such as triethylamine and dimethylethanolamine can also be used. These amine compounds may be used alone or in combination of two or more.

(Method for producing hydrophilic polyisocyanate)
The method for producing the hydrophilic polyisocyanate is not particularly limited, but examples thereof include (i) a method of prepolymerizing an isocyanate monomer and then reacting the resulting prepolymer with a hydrophilic compound, or (ii) a method of reacting an isocyanate monomer with a hydrophilic compound, etc. Among these, the above method (i) is preferred.

The reaction between the prepolymer and the hydrophilic compound may be carried out using an organic metal salt, a tertiary amine compound, or an alcoholate of an alkali metal as a catalyst. Examples of the metal constituting the organic metal salt include tin, zinc, and lead. Examples of the alkali metal include sodium.

The reaction temperature between the prepolymer and the hydrophilic compound is preferably -20°C or higher and 150°C or lower, and more preferably 30°C or higher and 130°C or lower. When the reaction temperature is equal to or higher than the lower limit, reactivity tends to be increased. Furthermore, when the reaction temperature is equal to or lower than the upper limit, side reactions tend to be suppressed more effectively.

It is preferable to completely react the hydrophilic compound with the prepolymer so that no unreacted hydrophilic compound remains. By not leaving any unreacted hydrophilic compound, the water dispersibility of the hydrophilic polyisocyanate can be improved.

(Characteristics of hydrophilic polyisocyanates)
[Average number of isocyanate functional groups]
In the hydrophilic polyisocyanate, the average number of functional groups of the isocyanate groups in the aliphatic polyisocyanate before hydrophilization (before modification with a hydrophilic compound), i.e., the raw material aliphatic polyisocyanate, is preferably 2.5 to 6.0, more preferably 2.7 to 5.8, and even more preferably 2.9 to 5.5. When the average number of functional groups is within the above range, the hardness of the resin film formed tends to be increased.

(Method for producing blocked polyisocyanate composition)
The blocked polyisocyanate composition is not particularly limited, but can be obtained, for example, by reacting the above-mentioned polyisocyanate with the above-mentioned blocking agent.
The blocking reaction between the polyisocyanate and the blocking agent can be carried out regardless of the presence or absence of a solvent, to give a blocked polyisocyanate.

The blocking agent may be one of (A) a compound having a structure represented by the above general formula (I) or (B) a compound having a heterocycle containing one or more nitrogen atoms, or two or more selected from the compound having a structure represented by the above general formula (I) or (B) a compound having a heterocycle containing one or more nitrogen atoms may be used in combination, or other blocking agents as described above may be used in combination.

The blocked polyisocyanate composition represented by the above general formula (I) can be produced by reacting the above polyisocyanate with at least one blocking agent selected from the group consisting of the above malonic acid diester having a tertiary alkyl group, the above malonic acid diester having a secondary alkyl group, and the above malonic acid diester having a primary alkyl group. It can also be produced by adding an alcohol having a chain alkyl group to the obtained reaction product and introducing an alkyl group derived from the alcohol by transesterification of the terminal ester moiety of the reaction product.

The amount of blocking agent added may usually be 80 mol% or more and 200 mol% or less, preferably 90 mol% or more and 150 mol% or less, and more preferably 93 mol% or more and 130 mol% or less, based on the total molar amount of isocyanate groups.

In addition, when a solvent is used, it is sufficient to use a solvent that is inactive to an isocyanate group.
When a solvent is used, the content of non-volatile matters derived from the polyisocyanate and the blocking agent per 100 parts by mass of the blocked polyisocyanate composition may usually be 10 parts by mass or more and 95 parts by mass or less, preferably 15 parts by mass or more and 80 parts by mass or less, and more preferably 20 parts by mass or more and 75 parts by mass or less.

In the blocking reaction, an organic metal salt of tin, zinc, lead or the like, a tertiary amine compound, an alcoholate of an alkali metal such as sodium, or the like may be used as a catalyst.
The amount of catalyst added varies depending on the temperature of the blocking reaction, etc., but is usually from 0.05 to 1.5 parts by mass, and preferably from 0.1 to 1.0 part by mass, per 100 parts by mass of polyisocyanate.

The blocking reaction can generally be carried out at −20° C. or higher and 150° C. or lower, preferably at 0° C. or higher and 100° C. or lower, and more preferably at 10° C. or higher and 90° C. or lower. When the temperature of the blocking reaction is equal to or higher than the above lower limit, the reaction rate can be increased, and when the temperature is equal to or lower than the above upper limit, side reactions can be suppressed.
After the blocking reaction, a neutralization treatment may be carried out by adding an acidic compound or the like.
As the acidic compound, an inorganic acid or an organic acid may be used. Examples of inorganic acids include hydrochloric acid, phosphorous acid, phosphoric acid, etc. Examples of organic acids include acidic phosphoric acid esters, methanesulfonic acid, p-toluenesulfonic acid, dioctyl phthalate, dibutyl phthalate, etc.

When a blocked polyisocyanate is produced using a hydrophilic compound and a blocking agent, for example, the polyisocyanate, the hydrophilic compound, and the blocking agent are reacted to produce the blocked polyisocyanate.
The reaction of the isocyanate group of the polyisocyanate with the hydrophilic compound and the reaction of the polyisocyanate with the blocking agent can be carried out simultaneously, or one of the reactions can be carried out in advance and then the second reaction can be carried out. Among these, it is preferable to carry out the reaction of the isocyanate group with the hydrophilic compound first to obtain a polyisocyanate modified with the hydrophilic compound (hereinafter, sometimes referred to as "modified polyisocyanate"), and then to react the obtained modified polyisocyanate with the blocking agent.

The reaction between the polyisocyanate and the hydrophilic compound may be carried out using an organic metal salt, a tertiary amine compound, or an alcoholate of an alkali metal as a catalyst. Examples of the metal constituting the organic metal salt include tin, zinc, and lead. Examples of the alkali metal include sodium.

The reaction temperature between the polyisocyanate and the hydrophilic compound is preferably -20°C or higher and 150°C or lower, and more preferably 30°C or higher and 130°C or lower. When the reaction temperature is equal to or higher than the lower limit, the reactivity tends to be higher. Also, when the reaction temperature is equal to or lower than the upper limit, side reactions tend to be more effectively suppressed.

It is preferable to completely react the hydrophilic compound with the polyisocyanate so that no unreacted hydrophilic compound remains. This tends to more effectively prevent the water dispersion stability of the blocked polyisocyanate and the deterioration of low-temperature curing properties when made into a resin film.

The blocking reaction between the modified polyisocyanate and the blocking agent can be carried out using the method described above as the blocking reaction.

<Nonionic dispersant>
The water-based coating composition of this embodiment contains at least two or more kinds of nonionic dispersants. Here, "two or more kinds" means a plurality of nonionic dispersants having different HLB values. By containing a plurality of nonionic dispersants having different HLB values, the emulsification property of the blocked polyisocyanate in water can be improved, and the water dispersion stability can be improved. The present inventors have found for the first time that this effect is strongly exhibited when at least two or more kinds of nonionic dispersants are mixed, the difference between the nonionic dispersant having the maximum HLB value and the nonionic dispersant having the minimum HLB value is 5 or more, and the weighted average HLB value of all the nonionic dispersants contained in the water-based coating composition is 14 to 17.

The difference between the nonionic dispersant with the maximum HLB value and the minimum HLB value is preferably 7 or more, more preferably 8 or more, and even more preferably 9 or more. The upper limit is preferably 20 or less, and more preferably 15 or less.

The HLB (hydrophile-lipophile balance) value of a nonionic dispersant is generally used as a value that represents the degree of affinity of the dispersant for oil and water, and can be calculated by the following formula.
HLB value = 20 x sum of formula weights of hydrophilic parts / molecular weight

The nonionic dispersant used is not particularly limited as long as the HLB value calculated by the above method is within a specific range. Specific examples of nonionic dispersants include polyoxyethylene alkyl ether type compounds, polyoxyalkylene derivative type compounds, polyoxyethylene polycyclic phenyl ether type compounds, sorbitan fatty acid ester type compounds, glycerin fatty acid ester type compounds, polyoxyethylene fatty acid ester type compounds, polyoxyethylene castor oil type compounds, and polyoxyethylene alkylamine type compounds. Among these nonionic dispersants, polyoxyethylene alkyl ether type compounds and polyoxyethylene polycyclic phenyl ether type compounds are particularly preferred.

The content of the nonionic dispersant is preferably 1% by mass or more and 30% by mass or less, more preferably 1.5% by mass or more and 20% by mass or less, more preferably 2% by mass or more and 15% by mass or less, and even more preferably 3% by mass or more and 10% by mass or less, based on the total amount of the blocked polyisocyanate. By having the content of the nonionic dispersant be equal to or more than the lower limit, the storage stability of the aqueous coating composition can be improved, while by having the content be equal to or less than the upper limit, the hardness of the resin film can be improved.

When the system contains a nonionic dispersant that satisfies the above conditions, an anionic dispersant may be further used. Specific examples of anionic dispersants include fatty acid salt type compounds, alkyl sulfate ester compounds, polyoxyethylene alkyl ether sulfate ester type compounds, polyoxyethylene alkyl ether sulfate type compounds, polyoxyethylene polycyclic phenyl ether sulfate type compounds, polyoxyalkylene alkenyl ether sulfate type compounds, alkylbenzene sulfonate type compounds, sulfosuccinate type compounds, and alkyl phosphate type compounds. Examples of polyoxyethylene alkyl ether sulfate type compounds include polyoxyethylene alkyl ether ammonium sulfate salts and polyoxyethylene alkyl ether sodium sulfate salts. Examples of polyoxyethylene polycyclic phenyl ether sulfate type compounds include polyoxyethylene polycyclic phenyl ether ammonium sulfate salts and polyoxyethylene polycyclic phenyl ether sodium sulfate salts. Examples of polyoxyalkylene alkenyl ether sulfate type compounds include polyoxyalkylene alkenyl ether ammonium sulfate salts. These anionic dispersants may be used alone or in combination of two or more.

Among these anionic dispersants, polyoxyethylene polycyclic phenyl ether ammonium sulfate, polyoxyethylene polycyclic phenyl ether sodium sulfate, polyoxyethylene alkyl ether ammonium sulfate, and polyoxyethylene alkyl ether sodium sulfate are particularly preferred.

<Hydroxyl-containing polyol>
In this specification, the term "hydroxyl group-containing polyol" refers to a compound having at least two hydroxyl groups (hydroxyl groups) in one molecule. Specific examples of polyols include aliphatic hydrocarbon polyols, polyether polyols, polyester polyols, epoxy resins, fluorine-containing polyols, and acrylic polyols, with acrylic polyols being preferred.

[Aliphatic Hydrocarbon Polyols]
Examples of the aliphatic hydrocarbon polyols include hydroxyl-terminated polybutadiene and hydrogenated products thereof.

[Polyether polyols]
Examples of the polyether polyols include those obtained by any one of the following methods (1) to (3).
(1) Polyether polyols or polytetramethylene glycols obtained by adding an alkylene oxide, either alone or in a mixture, to a polyhydric alcohol, either alone or in a mixture.
(2) Polyether polyols obtained by reacting alkylene oxides with polyfunctional compounds.
(3) Polymer polyols obtained by polymerizing acrylamide or the like using the polyether polyols obtained in (1) or (2) as a medium.

Examples of the polyhydric alcohol in (1) above include glycerin and propylene glycol.
Examples of the alkylene oxide in (2) above include ethylene oxide and propylene oxide.
Examples of the polyfunctional compound in (2) above include ethylenediamine, ethanolamines, and the like.

[Polyester polyols]
Examples of the polyester polyols include the following polyester polyols (1A) and (2A).
(1A) Polyester polyol resins obtained by a condensation reaction between a dibasic acid, or a mixture of two or more kinds thereof, and a polyhydric alcohol, or a mixture of two or more kinds thereof.
(2A) Polycaprolactones obtained by ring-opening polymerization of ε-caprolactone with polyhydric alcohols.
Examples of the dibasic acid include carboxylic acids such as succinic acid, adipic acid, dimer acid, maleic anhydride, phthalic anhydride, isophthalic acid, terephthalic acid, and 1,4-cyclohexanedicarboxylic acid.

Examples of the polyhydric alcohol in (1A) above include ethylene glycol, propylene glycol, diethylene glycol, 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, trimethylpentanediol, cyclohexanediol, trimethylolpropane, glycerin, pentaerythritol, 2-methylolpropanediol, and ethoxylated trimethylolpropane.

[Epoxy resins]
Examples of epoxy resins include novolac type epoxy resins, β-methylepicrotype epoxy resins, cyclic oxirane type epoxy resins, glycidyl ether type epoxy resins, glycol ether type epoxy resins, epoxy type aliphatic unsaturated compounds, epoxidized fatty acid esters, ester type polycarboxylic acids, aminoglycidyl type epoxy resins, halogenated type epoxy resins, resorcinol type epoxy resins, and other epoxy resins, as well as resins obtained by modifying these epoxy resins with amino compounds, polyamide compounds, or the like.

[Fluorine-containing polyols]
Examples of the fluorine-containing polyols include copolymers of fluoroolefins, cyclohexyl vinyl ethers, hydroxyalkyl vinyl ethers, and monocarboxylic acid vinyl esters, which are disclosed in Reference Document 1 (JP-A-57-34107) and Reference Document 2 (JP-A-61-275311), etc.

[Acrylic polyols]
The acrylic polyols can be obtained, for example, by polymerizing a polymerizable monomer having one or more active hydrogens in one molecule, or by copolymerizing a polymerizable monomer having one or more active hydrogens in one molecule with, as necessary, another monomer copolymerizable with the polymerizable monomer.

Examples of the polymerizable monomer having one or more active hydrogens in one molecule include the following (i) to (iii). These may be used alone or in combination of two or more.
(i) Acrylic esters having active hydrogen, such as 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and 2-hydroxybutyl acrylate.
(ii) Methacrylic acid esters having active hydrogen, such as 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, and 2-hydroxybutyl methacrylate.
(iii) (meth)acrylic acid esters having polyvalent active hydrogen, such as acrylic acid monoesters or methacrylic acid monoesters of glycerin, and acrylic acid monoesters or methacrylic acid monoesters of trimethylolpropane.

Examples of other monomers copolymerizable with the polymerizable monomer include the following (i) to (v). These may be used alone or in combination of two or more.
(i) Acrylic acid esters such as methyl acrylate, ethyl acrylate, isopropyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate.
(ii) Methacrylic acid esters such as methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-butyl methacrylate, isobutyl methacrylate, n-hexyl methacrylate, cyclohexyl methacrylate, lauryl methacrylate, and glycidyl methacrylate.
(iii) Unsaturated carboxylic acids such as acrylic acid, methacrylic acid, maleic acid, and itaconic acid.
(iv) Unsaturated amides such as acrylamide, N-methylol acrylamide, and diacetone acrylamide.
(v) Styrene, vinyl toluene, vinyl acetate, acrylonitrile, etc.

Other examples include acrylic polyols obtained by copolymerizing polymerizable ultraviolet-stable monomers, as disclosed in Reference 3 (JP Patent Publication 1-261409 A) and Reference 4 (JP Patent Publication 3-006273 A).

Specific examples of polymerizable ultraviolet-stable monomers include 4-(meth)acryloyloxy-2,2,6,6-tetramethylpiperidine, 4-(meth)acryloylamino-2,2,6,6-tetramethylpiperidine, 1-crotonoyl-4-crotonoyloxy-2,2,6,6-tetramethylpiperidine, and 2-hydroxy-4-(3-methacryloxy-2-hydroxypropoxy)benzophenone.

For example, the above monomer components can be solution polymerized in the presence of a known radical polymerization initiator such as a peroxide or an azo compound, and then diluted with an organic solvent, etc., as necessary, to obtain an acrylic polyol.

Water-based acrylic polyols can be produced by known methods such as a method of solution polymerization of an olefinic unsaturated compound and converting it into an aqueous phase, or emulsion polymerization. In this case, water solubility or water dispersibility can be imparted by neutralizing the acidic moieties of carboxylic acid-containing monomers such as acrylic acid and methacrylic acid, and sulfonic acid-containing monomers, with amines or ammonia.

[NCO/OH]
The molar equivalent ratio (NCO/OH) of the isocyanate groups of the blocked polyisocyanate to the hydroxyl groups of the polyol contained in the aqueous coating composition of this embodiment is determined depending on the required physical properties of the resin film, but is preferably 0.01 or more and 22.5 or less.

[Hydroxyl value]
The hydroxyl value of the polyol is preferably 30 mgKOH/g or more and 250 mgKOH/g or less, more preferably 40 mgKOH/g or more and 200 mgKOH/g or less, and even more preferably 45 mgKOH/g or more and 180 mgKOH/g or less. The hydroxyl value of the polyhydric hydroxyl compound is within the above range, and thus a resin film having various excellent physical properties such as tensile strength can be obtained. The hydroxyl value of the polyhydric hydroxyl compound can be measured, for example, by using the method described in the examples described later.

The content of the blocked polyisocyanate in the aqueous coating composition of this embodiment is preferably 5 parts by mass or more and 200 parts by mass or less, more preferably 6 parts by mass or more and 180 parts by mass or less, and even more preferably 10 parts by mass or more and 150 parts by mass or less, relative to 100 parts by mass of polyol. When the content of the blocked polyisocyanate is within the above range, a resin film having excellent physical properties such as tensile strength can be obtained. The content of the blocked polyisocyanate can be calculated, for example, from the blending amount, or can be calculated by identifying and quantifying using a nuclear magnetic resonance (NMR) method and a gas chromatography/mass spectrometry (GC/MS method).

Deionized water
The aqueous coating composition of the present embodiment contains deionized water. The content of deionized water is preferably 20% by mass or more and 90% by mass or less, and more preferably 30% by mass or more and 80% by mass or less, based on the total amount of the aqueous coating composition.

Other Additives
The water-based coating composition of the present embodiment may further contain other additives.
Other additives include, for example, a curing agent capable of reacting with a crosslinkable functional group in the polyol, a curing catalyst, a solvent, pigments (extender pigments, colored pigments, metallic pigments, etc.), an ultraviolet absorber, a light stabilizer, a radical stabilizer, an anti-yellowing agent that suppresses coloring during the baking process, a coating surface conditioner, a flow conditioner, a pigment dispersant, an antifoaming agent, a thickener, a film-forming assistant, and the like.

Examples of hardeners include melamine resins, urea resins, epoxy group-containing compounds or resins, carboxyl group-containing compounds or resins, acid anhydrides, alkoxysilane group-containing compounds or resins, and hydrazide compounds.

The curing catalyst may be a basic compound or a Lewis acid compound.
Examples of the basic compound include metal hydroxides, metal alkoxides, metal carboxylates, metal acetylacetinates, hydroxides of onium salts, onium carboxylates, halides of onium salts, metal salts of active methylene compounds, onium salts of active methylene compounds, aminosilanes, amines, phosphines, etc. As the onium salts, ammonium salts, phosphonium salts, or sulfonium salts are preferable.
Examples of the Lewis acid compound include an organotin compound, an organozinc compound, an organotitanium compound, and an organozirconium compound.

Examples of solvents include 1-methylpyrrolidone, ethylene glycol monoethyl ether, diethylene glycol monoethyl ether, ethylene glycol monomethyl ether, diethylene glycol monomethyl ether, dipropylene glycol monomethyl ether, propylene glycol monomethyl ether, 3-methoxy-3-methyl-1-butanol, ethylene glycol diethyl ether, diethylene glycol diethyl ether, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dipropylene glycol dimethyl ether, propylene glycol dimethyl ether, methyl ethyl ketone, and a Examples of the solvent include acetone, methyl isobutyl ketone, propylene glycol monomethyl ether acetate, ethanol, methanol, iso-propanol, 1-propanol, iso-butanol, 1-butanol, 2-ethylhexanol, cyclohexanol, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, 1,4-butanediol, 1,3-butanediol, ethyl acetate, isopropyl acetate, butyl acetate, toluene, xylene, pentane, iso-pentane, hexane, iso-hexane, cyclohexane, solvent naphtha, and mineral spirits. These solvents may be used alone or in combination of two or more. From the viewpoint of dispersibility in water, the solvent is preferably one having a solubility in water of 5% by mass or more, and specifically, dipropylene glycol monomethyl ether is preferred.

In addition, known pigments (extender pigments, color pigments, metallic pigments, etc.), ultraviolet absorbers, light stabilizers, radical stabilizers, yellowing inhibitors that suppress coloring during the baking process, coating surface conditioners, flow conditioners, pigment dispersants, defoamers, thickeners, and film-forming aids can be appropriately selected and used.

<Method of producing water-based coating composition>
When producing a water-based coating composition, first, additives such as a curing agent capable of reacting with a crosslinkable functional group in the hydroxyl-containing polyol, a curing catalyst, a solvent, pigments (extender pigments, colored pigments, metallic pigments, etc.), UV absorbers, light stabilizers, radical stabilizers, anti-yellowing agents which suppress coloring during the baking process, coating surface conditioners, flow conditioners, pigment dispersants, defoamers, thickeners, film-forming assistants, etc. are added to a hydroxyl-containing polyol or an aqueous dispersion or solution thereof, as necessary.

Next, the blocked polyisocyanate composition or the aqueous dispersion thereof is added as a curing agent, and the above-mentioned two or more types of nonionic dispersants and deionized water are further added to adjust the viscosity.
The mixture is then forcedly stirred with a stirring device to obtain a water-based coating composition.

<Resin film>
One aspect of the present invention is a resin film formed by curing a water-based coating composition.

The resin film is obtained by applying the above-mentioned water-based coating composition to a substrate using a known method such as roll coating, curtain flow coating, spray coating, bell coating, or electrostatic coating, and then heating to harden it.

From the viewpoints of energy saving and heat resistance of the substrate, the heating temperature is preferably from about 60° C. to about 120° C., more preferably from about 65° C. to about 110° C., and even more preferably from about 70° C. to about 100° C.
From the viewpoints of energy saving and heat resistance of the base material, the heating time is preferably from about 1 minute to about 60 minutes, and more preferably from about 2 minutes to about 40 minutes.

The substrate is not particularly limited, and examples thereof include the outer panels of automobile bodies such as passenger cars, trucks, motorcycles, and buses; automobile parts such as bumpers; the outer panels of household electrical appliances such as mobile phones and audio equipment; and various films, among which the outer panels of automobile bodies or automobile parts are preferred.

The material of the substrate is not particularly limited, and examples thereof include metal materials such as iron, aluminum, brass, copper, tinplate, stainless steel, zinc-plated steel, zinc alloy (Zn-Al, Zn-Ni, Zn-Fe, etc.) plated steel; resins such as polyethylene resin, polypropylene resin, acrylonitrile-butadiene-styrene (ABS) resin, polyamide resin, acrylic resin, vinylidene chloride resin, polycarbonate resin, polyurethane resin, epoxy resin, and various plastic materials such as FRP; inorganic materials such as glass, cement, and concrete; and fibrous materials such as wood, paper, and cloth, among which metal materials and plastic materials are preferred.

The substrate may be the surface of the above-mentioned metal material, or the metal surface of a car body or the like formed from the above-mentioned metal material, which has been subjected to a surface treatment such as phosphate treatment, chromate treatment, complex oxide treatment, or the like, and further may be one on which a coating film has been formed. The substrate on which a coating film has been formed may be one on which a surface treatment has been performed as necessary and on which a primer coating film has been formed, for example, a car body on which a primer coating film has been formed by electrochemical paint. The substrate may be the surface of the above-mentioned plastic material, or the plastic surface of an automobile part or the like formed from the above-mentioned plastic material, which has been subjected to a surface treatment as desired. The substrate may also be one in which a plastic material and a metal material are combined.

<Laminate>
In one embodiment of the present invention, it is also preferable to laminate two or more layers of the resin film to form a laminate. The thickness of each layer of the laminate is preferably 1 μm or more and 50 μm or less. The laminate is formed by laminating various coating films including the resin film on an adherend.

Examples of the adherend include glass, various metals, porous members, members with various coatings, hardened sealants, rubbers, leathers, fibers, nonwoven fabrics, resin films and plates, ultraviolet-cured acrylic resin layers, and layers made of inks. Examples of the various metals include aluminum, iron, galvanized steel sheets, copper, and stainless steel. Examples of the porous members include wood, paper, mortar, and stone. Examples of the various coatings include fluorine coatings, urethane coatings, and acrylic urethane coatings. Examples of the hardened sealants include silicone-based, modified silicone-based, and urethane-based. Examples of the rubbers include natural rubber and synthetic rubber. Examples of the leathers include natural leather and artificial leather. Examples of the fibers include plant-based fibers, animal-based fibers, carbon fibers, and glass fibers. Examples of the resins that are the raw materials for the resin films and plates include polyvinyl chloride, polyester, acrylic, polycarbonate, triacetyl cellulose, and polyolefin. Examples of the inks include printing inks and UV inks.

The laminate can be obtained by applying the above-mentioned water-based coating compositions of different compositions to the substrate using known methods such as roll coating, curtain flow coating, spray coating, bell coating, and electrostatic coating, and then heating and curing each layer, or by applying all layers and then heating and curing them together.

In addition to the resin film, the laminate may contain layers of other known components, such as a primer layer, an adhesive layer, a decorative layer, etc.

<Method of forming multi-layer coating film>
One aspect of the present invention is a method for forming a multi-layer coating film by successively coating an intermediate coating composition, a base coating composition, and a clear coating composition on a steel sheet for an automobile body on which baking of an electrodeposition coating film has been completed, and then heating and curing the composition. In the method for forming a multi-layer coating film of the present invention, the water-based coating composition of the present embodiment is used as the intermediate coating composition or the base coating composition.

In the method for forming a multilayer coating film of this embodiment, the baking temperature for heat curing is preferably 75 to 100°C.

The present embodiment will be described in more detail below with reference to examples and comparative examples, but the present embodiment is not limited in any way to the following examples.

<Test items>
The water-based coating compositions obtained in the Examples and Comparative Examples were subjected to measurement and evaluation of various physical properties according to the methods described below.

[Physical Properties 1-1]
(Isocyanate group (NCO) content)
In order to measure the NCO content of the polyisocyanate, the polyisocyanate before being blocked with a blocking agent was used as a measurement sample.

First, 2 g to 3 g of the measurement sample was weighed out (Wg) in a flask. Next, 20 mL of toluene was added to dissolve the measurement sample. Next, 20 mL of a 2N toluene solution of di-n-butylamine was added, mixed, and left at room temperature for 15 minutes. Next, 70 mL of isopropyl alcohol was added and mixed. Next, this liquid was titrated with a 1N hydrochloric acid solution (Factor F) as an indicator. The titration value obtained was V2 mL. Next, the same operation was performed without the polyisocyanate sample, and the titration value obtained was V1 ml. Next, the isocyanate group (NCO) content (isocyanate group (NCO) content) (mass%) of the polyisocyanate was calculated from the following formula.

Isocyanate group (NCO) content (mass%) = (V1-V2) x F x 42/(W x 1000) x 100

[Physical Properties 1-2]
(Average number of isocyanate functional groups)
The average number of isocyanate functional groups (average NCO number) of the polyisocyanate was calculated by the following formula. In the formula, "Mn" is the number average molecular weight of the polyisocyanate before blocking with a blocking agent for the blocked polyisocyanate composition, and the value measured in "Physical Properties 1-2" above was used. "NCO content" is the isocyanate group content of the polyisocyanate measured before blocking with a blocking agent for the blocked polyisocyanate composition, and the value calculated in "Physical Properties 1-1" above was used.

Average number of isocyanate functional groups = (Mn x NCO content x 0.01) / 42

[Physical Properties 1-3]
(Solid content of blocked polyisocyanate composition)
The solid content of the blocked polyisocyanate composition was determined as follows.
First, an aluminum dish with a bottom diameter of 38 mm was precisely weighed. Next, about 1 g of the blocked polyisocyanate composition produced in the Examples and Comparative Examples was precisely weighed on the aluminum dish (W1). Next, the blocked polyisocyanate composition was adjusted to a uniform thickness. Next, the blocked polyisocyanate composition placed on the aluminum dish was kept in an oven at 105°C for 1 hour. Next, after the aluminum dish reached room temperature, the blocked polyisocyanate composition remaining on the aluminum dish was precisely weighed (W2). Next, the solid content (mass%) of the blocked polyisocyanate composition was calculated from the following formula.

Solid content of blocked polyisocyanate composition [mass %] = W2/W1 x 100

(Coating film appearance)
The obtained water-based coating composition was applied to a polypropylene (PP) plate so that the dry film thickness was 40 μm, and then the plate was dried by heating at 80° C. for 30 minutes to obtain a resin film. The appearance of the coating film on the obtained evaluation plate was evaluated by visual observation according to the following criteria. The evaluation results are the average of the observation results of 20 evaluators.
◎: When a fluorescent light is reflected on the coating, the light is reflected quite clearly.
◯: When a fluorescent light is reflected on the coating film, the fluorescent light is reflected somewhat clearly.
△: When a fluorescent light is reflected on the coating film, the periphery (outline) of the fluorescent light becomes blurred.
×: When a fluorescent lamp is reflected on the coating film, the periphery (contour) of the fluorescent lamp becomes blurred. Aggregates are found on the coating film surface.

(Coating hardness: Konig)
Each water-based coating composition was applied to a glass plate so that the dry film thickness was 40 μm, and then the plate was dried at 80° C. for 30 minutes to obtain a resin film. The Konig hardness (times) of the sample coated plates obtained in each Example and Comparative Example was measured using a pendulum hardness tester manufactured by Erichsen. The coating film hardness: Konig was evaluated according to the following evaluation criteria.
〇: More than 30 times.
△: 20 times or more and 30 times or less.
×: Less than 20 times.

(Storage Stability)
The obtained water-based coating composition was stored in a 20 mL glass bottle at 40° C. for 10 days, after which the condition was observed and evaluated according to the following criteria.
◯: No agglomerates or significant thickening.
Δ: A small amount of aggregates were observed.
×: Aggregates were observed and there was a significant increase in viscosity.

<Synthesis of Polyisocyanate>
[Synthesis Example 1]
(Synthesis of blocked polyisocyanate BL1)
In a four-neck flask equipped with a thermometer, a stirring blade, and a reflux condenser, 100 parts by mass of HDI and 5.1 parts by mass of a polyester polyol derived from a trihydric alcohol and ε-caprolactone (manufactured by Daicel Chemical Industries, Ltd., "Placcel 303" (trade name)) were charged under a nitrogen gas flow, and the temperature inside the reactor was kept at 90°C for 1 hour under stirring to carry out a urethane reaction. Thereafter, the temperature inside the reactor was kept at 60°C, and an isocyanurate catalyst, tetramethylammonium caprylate, was added, and when the yield reached 51% by mass, phosphoric acid was added to stop the reaction. After filtering the reaction liquid, unreacted HDI was removed using a thin-film evaporator to obtain an isocyanurate-type polyisocyanate (hereinafter, sometimes referred to as "polyisocyanate P-1"). The NCO content of the obtained polyisocyanate P-1 was 18.8% by mass, the number average molecular weight was 1130, and the average number of isocyanate groups was 5.1. Furthermore, the obtained polyisocyanate P-1 was subjected to GPC analysis and 1H-NMR analysis, and it was confirmed that an isocyanurate trimer was present.

100 parts by mass of the obtained polyisocyanate P-1, 24 parts by mass of methoxypolyethylene glycol (MPG-081, ethylene oxide repeating units: 15, manufactured by Nippon Nyukazai Co., Ltd.) (9 mol% relative to 100 mol% isocyanate groups), 0.01 parts by mass of 2-ethylhexyl acid phosphate (JP-508T, manufactured by Johoku Chemical Industry Co., Ltd.), and 30 parts by mass of dipropylene glycol dimethyl ether (DPDM) were mixed and reacted at 115°C for 2 hours. The reaction liquid was cooled to 40°C, and di-tert-butyl malonate was added as a blocking agent in a ratio of 1.1 molar equivalents relative to the molar amount of isocyanate groups in the reactant, and 1.0 part by mass of a methanol solution containing sodium methylate (28% by mass) was added as a solution and stirred. The internal bath was kept at 50°C and stirred for more than 6 hours, and the disappearance of the characteristic absorption of isocyanate groups was confirmed by infrared spectroscopy. DPDM was then added in a proportion to give a solid content of 60% by mass, stirred, and cooled to below 40°C.

[Synthesis Example 2]
(Synthesis of blocked polyisocyanate BL2)
Blocked polyisocyanate BL2 was synthesized in the same manner as in Synthesis Example 1, except that the blocking agent was diisopropyl malonate.
[Synthesis Example 3]
(Synthesis of blocked polyisocyanate BL3)
Blocked polyisocyanate BL3 was synthesized in the same manner as in Synthesis Example 1, except that diethyl malonate was used as the blocking agent.

<Production of Water-Based Coating Composition>
[Example 1]
16 g of the blocked polyisocyanate obtained in Synthesis Example 1 was weighed out into a container. Next, nonionic dispersants D1 and D5 were added to the blocked polyisocyanate in such a weight ratio that the total amount was 6% by weight of the resin content of the blocked polyisocyanate, and further in such a ratio that the average HLB value of D1 and D5 was 15. Next, deionized water was added in such a ratio that the solid content was 40% by mass, and the mixture was stirred at 800 rpm for 10 minutes using a propeller blade.
Next, an acrylic polyol water dispersion (hydroxyl value per resin: 130 mg KOH/g, manufactured by the company) was added in such a ratio that the ratio (NCO/OH) of the molar amount of isocyanate groups to the molar amount of hydroxyl groups in the acrylic polyol water dispersion was 0.4. Finally, deionized water was added in such a ratio that the solid content in the coating composition was 30 mass%, and the mixture was stirred at 700 rpm for 10 minutes using a propeller blade to obtain a coating composition. The coating composition thus prepared was evaluated using the method described above. The results are shown in Table 1.

[Examples 2 to 4, Comparative Examples 1 to 8]
The coating compositions of Examples 2 to 4 and Comparative Examples 1 to 8 were obtained in the same manner as in Example 1, except that the type of blocked polyisocyanate, the type and content ratio of the dispersant, and the average HLB value were as shown in Table 1. The prepared coating compositions were evaluated using the methods described above. The results are shown in Table 1.

As shown by the above results, the coating compositions of Examples 1 to 4 satisfying the composition of the present invention had good coating appearance and also had good coating storage stability.
On the other hand, in Comparative Examples 1 to 8, problems remained with respect to the coating appearance or storage stability.

The types of dispersants in Table 1 and the following Tables 2 and 3 are as follows.
(Dispersant)
D1: Newcol 2399S (manufactured by Nippon Nyukazai Co., Ltd., polyoxyethylene alkyl ether-based dispersant, HLB value calculated by the Griffin formula of 19.2), a nonionic dispersant.
D2: Newcol 723 (manufactured by Nippon Nyukazai Co., Ltd., polyoxyethylene polycyclic phenyl ether-based dispersant, HLB value calculated by the Griffin formula of 16.6), a nonionic dispersant.
D3: Newcol 714 (manufactured by Nippon Nyukazai Co., Ltd., polyoxyethylene polycyclic phenyl ether, HLB value calculated by the Griffin formula: 15.0), a nonionic dispersant.
D4: Newcol 710 (manufactured by Nippon Nyukazai Co., Ltd., polyoxyethylene polycyclic phenyl ether, HLB value calculated by the Griffin formula: 13.6), a nonionic dispersant.
D5: Newcol 704 (manufactured by Nippon Nyukazai Co., Ltd., polyoxyethylene polycyclic phenyl ether, HLB value calculated by the Griffin formula of 9.2), a nonionic dispersant.
D6: Homogenol L-95 (manufactured by Kao Corporation, alkyl imidazoline, no HLB value), non-ionic dispersant.

In general, the lower the amount of hydrophilic group modification, the higher the hardness tends to be. In order to improve the hardness shown in Table 1, blocked isocyanates with various amounts of hydrophilic group modification were synthesized and evaluated.

[Synthesis Example 4]
(Synthesis of blocked polyisocyanates BL4 to BL6)
Blocked polyisocyanate compounds BL4 to BL6 were obtained in the same manner as in Synthesis Example 1, except that the hydrophilic group modification amounts were as shown in Tables 2 and 3.

[Examples 5 to 9, Comparative Examples 9 to 23]
The water-based coating compositions of Examples 5 to 9 and Comparative Examples 9 to 23 were obtained in the same manner as in Example 1, except that the types and content ratios of the dispersant components and the average HLB values were as shown in Tables 2 and 3. The prepared water-based coating compositions were evaluated using the methods described above. The results are shown in Tables 2 and 3.

As shown by the above results, the coating compositions of Examples 5 to 9, which satisfied the composition of the present invention, had good coating film appearance and good hardness even in the range where the amount of hydrophilic groups in the blocked polyisocyanate was low.
On the other hand, the coating appearance of Comparative Examples 9 to 11 was poor, and the appearance of Comparative Examples 13 to 16 tended to improve, but the hardness deteriorated.
Even in the region where the amount of hydrophilic groups is extremely low, the appearance of the coating film is improved by satisfying the composition of the present invention.

Claims (6)

A water-based coating composition comprising a blocked polyisocyanate composition, at least two or more nonionic dispersants, a hydroxyl group-containing polyol, and deionized water,
The nonionic dispersant has an HLB value difference between the dispersant having the highest HLB value and the dispersant having the lowest HLB value of 5 or more,
A water-based coating composition, wherein a weighted average of the HLB values of all the nonionic dispersants contained in the water-based coating composition is 14 or more and 17 or less. The aqueous coating composition according to claim 1, wherein the total amount of the nonionic dispersant is 1% by mass or more and 30% by mass or less based on the total amount of the blocked polyisocyanate composition. 3. The aqueous coating composition according to claim 1, wherein the blocked polyisocyanate composition has a structure represented by the following general formula (I) or a heterocycle containing one or more nitrogen atoms:

(In the general formula (I), R ¹¹ , R ¹² and R ¹³ each independently represent an alkyl group which may contain one or more substituents selected from the group consisting of a hydroxyl group and an amino group, the total number of carbon atoms of R ¹¹ , R ¹² and R ¹³ is 3 to 20, R ¹⁴ , R ¹⁵ and R ¹⁶ each independently represent a hydrogen atom or an alkyl group which may contain one or more substituents selected from the group consisting of a hydroxyl group and an amino group, and the wavy line represents a bond.) The aqueous coating composition according to claim 1 or 2, wherein a part or all of the blocked polyisocyanate composition has structural units derived from a hydrophilic compound. A method for forming a multi-layer coating film, in which an intermediate coating composition, a base coating composition, and a clear coating composition are applied in that order to a steel sheet for an automobile body on which baking of an electrodeposition coating film has been completed, and then the intermediate coating composition or the base coating composition is a water-based coating composition according to claim 1 or 2 to form a multi-layer coating film by heating and curing the composition. The method for forming a multi-layer coating film according to claim 5, wherein the baking temperature for the heat curing is 75°C or higher and 100°C or lower.