Aqueous coating composition and coating film

Abstract

An aqueous coating composition containing (A) a polyol, (B) a blocked polyisocyanate that includes a hydrophilic group, (C) a blocking agent dissociation catalyst, and (D) water, wherein the acid value of component (A) is 2-45 mgKOH / g, and component (C) contains a quaternary ammonium salt.

Description

Water-based paint composition and coating film

[0001] This disclosure relates to aqueous paint compositions and coating films.

[0002] A known paint composition for use on automobile bodies and other surfaces is one that combines polyol and polyisocyanate. Since polyisocyanate readily reacts with water, it was previously impossible to use it in water-based paints such as electrodeposition paints. However, a method has been developed to overcome this problem by inactivating the polyisocyanate by reacting it with a blocking agent. The blocked polyisocyanate obtained by this method does not react with the main component (polyol, etc.) at room temperature, but when heated, the blocking agent dissociates, regenerating the isocyanate groups, which then react with the main component to form crosslinks. Therefore, according to the above method, it becomes possible to pre-mix the main component and the curing agent component to form a paint, making it possible to apply polyisocyanate to water-based paints.

[0003] In water-based paints that use blocked polyisocyanate as a curing agent, catalysts such as quaternary ammonium salts (blocking agent dissociation catalysts) are sometimes used to reduce the thermal energy required for the dissociation of the blocking agent (see, for example, Patent Document 1).

[0004] Japanese Patent Publication No. 2014-084426

[0005] Conventionally, paint curing was performed at high temperatures of 150°C or higher. However, in recent years, there has been a demand to cure paint films at lower temperatures (for example, 140°C or lower) to reduce costs, carbon dioxide emissions, and thermal degradation of the coated object during the curing process. Therefore, it is important that water-based paint compositions have good low-temperature curing properties, that is, that even when the curing process is performed at a lower temperature than before, they can sufficiently crosslink and form a paint film with a good gel fraction. In this regard, as described in Patent Document 1 above, the low-temperature curing properties of water-based paint compositions can be improved by using a specific quaternary ammonium salt, but there is still room for improvement in their performance.

[0006] One aspect of this disclosure aims to provide an aqueous coating composition that exhibits excellent low-temperature curing properties.

[0007] This disclosure provides at least the following [1] to

[17] .

[0008] [1] An aqueous paint composition comprising (A) a polyol, (B) a blocked polyisocyanate having hydrophilic groups, (C) a blocking agent dissociation catalyst, and (D) water, wherein the acid value of component (A) is 2 to 45 mg KOH / g, and component (C) contains a quaternary ammonium salt.

[0009] [2] The aqueous paint composition according to [1], wherein the component (A) comprises an acrylic polyol.

[0010] [3] The aqueous coating composition according to [2], wherein the acrylic polyol comprises structural units derived from one or more (meth)acrylic acids and structural units derived from one or more (meth)acrylic acid esters.

[0011] [4] The aqueous paint composition according to [3], wherein the (meth)acrylic acid ester comprises at least one selected from the group consisting of (meth)acrylate hydroxyalkyl esters and (meth)acrylate alkyl esters.

[0012] [5] The aqueous coating composition according to any one of [1] to [4], wherein the component (B) comprises a block polyisocyanate having a structure derived from an aliphatic polyisocyanate or a derivative thereof having an aliphatic hydrocarbon group having 4 to 6 carbon atoms.

[0013] [6] The aqueous paint composition according to any one of [1] to [5], wherein the component (C) comprises a quaternary ammonium salt having a cationic group represented by the following formula (1). [In formula (1), R １ R represents an alkyl group having 1 to 16 carbon atoms, which may have a hydroxyl group, an amino group, or an alkoxy group as a substituent. ２ ~R ４ Each of these independently represents an alkyl group having 1 to 8 carbon atoms.

[0014] [7] The aqueous paint composition according to any one of [1] to [6], wherein the (C) component has at least one cationic group selected from the group consisting of a group represented by the following formula (2) and a group represented by the following formula (3).

[0015] [8] The aqueous paint composition according to any one of [1] to [7], wherein the component (C) has a monoalkyl carbonate group or a hydroxyl group having an alkyl group having 1 to 8 carbon atoms.

[0016] [9] The aqueous paint composition according to any one of [1] to [8], wherein the content of component (C) is 0.5 to 5.0 moles per mole of acidic group contained in component (A).

[0017]

[10] The aqueous paint composition according to any one of [1] to [9], wherein the component (B) comprises a blocked polyisocyanate having isocyanate groups sequestered with an oxime-based blocking agent.

[0018]

[11] The aqueous paint composition according to any one of [1] to

[10] , wherein the component (B) comprises a blocked polyisocyanate having an isocyanate group encapsulated with methyl ethyl ketoxime.

[0019]

[12] The aqueous paint composition according to any one of [1] to

[11] , wherein the hydrophilic group is an alkoxy polyethylene oxide group.

[0020]

[13] The aqueous paint composition according to any one of [1] to

[12] , wherein the hydrophilic group comprises an average of 14 to 50 ethylene oxide units.

[0021]

[14] The aqueous paint composition according to any one of [1] to

[13] , wherein the number average molecular weight of the hydrophilic group is 600 to 2000.

[0022]

[15] The aqueous paint composition according to any one of [1] to

[14] , wherein the content of the hydrophilic group is 5 to 30% by mass, based on the total amount of isocyanate compounds in the isocyanate composition obtained by dissociating the blocking agent from the blocked polyisocyanate.

[0023]

[16] The ratio of the total amount of active isocyanate groups contained in the component (B) to the total amount of hydroxyl groups contained in the component (A) is 0.7 to 1.5 in terms of equivalent ratio, and the aqueous paint composition according to any one of [1] to

[15] .

[0024]

[17] A coating film formed from the aqueous paint composition according to any one of [1] to

[16] .

[0025] According to one aspect of the present disclosure, an aqueous paint composition excellent in low-temperature curability can be provided.

[0026] Hereinafter, exemplary embodiments of the present disclosure will be described. However, the present disclosure is not limited to the following embodiments. In this specification, a numerical range indicated by "~" means a range including the numerical values described before and after "~" as the minimum value and the maximum value, respectively. Also, unless specifically specified, the units of the numerical values described before and after "~" are the same. Also, the upper limit value and the lower limit value described individually can be arbitrarily combined. Also, in this specification, room temperature means 5 to 35°C. In this specification, "(meth)acrylic acid" means one or both of acrylic acid and methacrylic acid. The same applies to similar expressions such as "(meth)acrylic acid alkyl ester".

[0027] <Aqueous paint composition> One embodiment of the present disclosure contains (A) a polyol (hereinafter, also referred to as "component (A)"), (B) a blocked polyisocyanate having a hydrophilic group (hereinafter, also referred to as "component (B)"), (C) a blocking agent dissociation catalyst (hereinafter, also referred to as "component (C)"), and (D) water (hereinafter, also referred to as "component (D)"), and the acid value of the component (A) is 2 to 45 mgKOH / g, and the component (C) contains a quaternary ammonium salt, and it is an aqueous paint composition.

[0028] The above-mentioned aqueous paint composition contains components (A) to (D) and therefore has excellent low-temperature curing properties. Accordingly, the above-mentioned aqueous paint composition can form a coating film with a good gel fraction when baked at a lower temperature than conventional methods (for example, at a temperature of 140°C or lower). Furthermore, the above-mentioned aqueous paint composition contains components (A) to (D) and therefore tends to have excellent 40°C storage stability. Accordingly, the above-mentioned aqueous paint composition makes it easier to form a coating film with a good gel fraction even after being stored for a longer period than conventional methods.

[0029] (Component (A): Polyol) Examples of polyols include acrylic polyols, polyester polyols, polyether polyols, epoxy polyols, polycarbonate polyols, and polylactone polyols. One of these polyols may be used alone, or two or more may be used in combination.

[0030] (A) Component may contain an acrylic polyol, from the viewpoint of easily obtaining an aqueous paint composition with superior low-temperature curing properties. An acrylic polyol is a polymer sometimes called a hydroxyl group-containing acrylic resin, and is a polyol containing at least one structural unit selected from the group consisting of structural units derived from (meth)acrylic acid and structural units derived from (meth)acrylic acid esters.

[0031] (Meth)acrylic acid esters may be (meth)acrylic acid esters having hydroxyl groups (hereinafter referred to as "hydroxyl group-containing (meth)acrylic acid esters") or (meth)acrylic acid esters not having hydroxyl groups (hereinafter referred to as "hydroxyl group-free (meth)acrylic acid esters").

[0032] Examples of hydroxyl group-containing (meth)acrylic acid esters include hydroxyalkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 3-hydroxy-2,2-dimethylpropyl (meth)acrylate, and pentaerythritol tri(meth)acrylate.

[0033] Examples of hydroxyl group-free (meth)acrylic acid esters include alkyl (meth)acrylic acid esters such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate, nonyl (meth)acrylate, decyl (meth)acrylate, dodecyl (meth)acrylate, and cyclohexyl (meth)acrylate, as well as aryl (meth)acrylic acid esters such as phenyl (meth)acrylate and benzyl (meth)acrylate.

[0034] As the (meth)acrylic acid ester, at least one selected from the group consisting of (meth)acrylate hydroxyalkyl esters and (meth)acrylate alkyl esters may be used, from the viewpoint of easily obtaining an aqueous coating composition with superior low-temperature curing properties and easily improving the yellowing resistance of the acrylic polyol. The (meth)acrylate hydroxyalkyl ester is the hydroxyl group-containing (meth)acrylic acid ester described above, and the (meth)acrylate alkyl ester is the hydroxyl group-free (meth)acrylic acid ester described above.

[0035] From the viewpoint of easily obtaining an aqueous coating composition with superior low-temperature curing properties, the acrylic polyol may be a polyol containing structural units derived from one or more types of (meth)acrylic acid and structural units derived from one or more types of (meth)acrylic acid esters. From the viewpoint of easily obtaining an aqueous coating composition with even superior low-temperature curing properties, the polyol may be a polyol containing structural units derived from one or more types of (meth)acrylic acid, structural units derived from one or more types of hydroxyl group-containing (meth)acrylic acid esters and structural units derived from one or more types of hydroxyl group-free (meth)acrylic acid esters. In particular, when the acrylic polyol contains structural units derived from one or more types of (meth)acrylic acid, structural units derived from one or more types of hydroxyalkyl (meth)acrylic acid esters and structural units derived from one or more types of alkyl (meth)acrylic acid-containing esters, an aqueous coating composition with particularly superior low-temperature curing properties can be easily obtained.

[0036] The content of structural units derived from (meth)acrylic acid may be 0.1% by mass or more, 0.3% by mass or more, 1% by mass or more, 1.7% by mass or more, 2.4% by mass or more, 3.4% by mass or more, or 4.7% by mass or more, based on the total mass of the acrylic polyol, and may be 5.9% by mass or less, 5.5% by mass or less, 5% by mass or less, 4.6% by mass or less, 3.2% by mass or less, or 2.3% by mass or less. The amount may be 1.6% by mass or less, or 0.9% by mass or less, and may be 0.1 to 5.9% by mass, 0.3 to 5.5% by mass, 1 to 5% by mass, 1.7 to 5.9% by mass, 2.4 to 5.9% by mass, 3.4 to 5.9% by mass, 4.7 to 5.9% by mass, 0.1 to 4.6% by mass, 0.1 to 3.2% by mass, 0.1 to 2.3% by mass, 0.1 to 1.6% by mass, or 0.1 to 0.9% by mass. If the above content is above the lower limit, it is easier to obtain an aqueous coating composition with superior low-temperature curing properties. If the above content is below the upper limit, it is easier to improve the dispersibility of the acrylic polyol and to obtain an aqueous coating composition with superior storage stability.

[0037] The content of structural units derived from (meth)acrylic acid ester may be 94.1% by mass or more, 95.4% by mass or more, 96.8% by mass or more, 97.7% by mass or more, 98.4% by mass or more, or 99.1% by mass or more, based on the total mass of the acrylic polyol, and may be 99.9% by mass or less, 99% by mass or less, 98.3% by mass or less, 97.6% by mass or less, 96.6% by mass or less, or 95.3% by mass or less, and may be 94.1 to 99.9% by mass, 95.4 to 99% by mass, 96.8 to 98.3% by mass, 97.7 to 99.9% by mass, 98.4 to 99.9% by mass, 99.1 to 99.9% by mass, 94.1 to 97.6% by mass, 94.1 to 96.6% by mass, or 94.1 to 95.3% by mass. When the above content is above the above lower limit, it is easier to obtain an aqueous coating composition that improves the yellowing resistance of the acrylic polyol and has excellent storage stability. When the above content is below the above upper limit, it is easier to obtain an aqueous coating composition that has even better low-temperature curing properties.

[0038] The content of structural units derived from hydroxyl group-containing (meth)acrylic acid ester may be 10% by mass or more, 15% by mass or more, 20% by mass or more, 30% by mass or more, or 40% by mass or more, based on the total mass of the acrylic polyol, and may be 60% by mass or less, 50% by mass or less, 40% by mass or less, or 30% by mass or less, and may be 10 to 60% by mass, 10 to 50% by mass, 15 to 40% by mass, 20 to 30% by mass, 30 to 60% by mass, or 40 to 60% by mass. In one embodiment, the content of structural units derived from (meth)acrylate hydroxyalkyl ester may be within the above range. If the above content is above the lower limit, it is easier to obtain an aqueous coating composition with superior low-temperature curability, and if the above content is below the upper limit, it is easier to improve the dispersibility of the acrylic polyol.

[0039] The content of structural units derived from hydroxyl group-free (meth)acrylic acid esters may be 30% or more by mass, 40% or more by mass, 56% or more by mass, 70% or more by mass, 70.3% or more by mass, 71.6% or more by mass, 72.9% or more by mass, 73.9% or more by mass, 74.5% or more by mass, or 75.2% or more by mass, based on the total mass of the acrylic polyol, and may be 90% or less by mass, 80% or less by mass, 75.1% or less by mass, 74.4% or less by mass, 73.8% or less by mass, or 72.8% by mass. The amount may be less than or equal to % by mass, 71.5% by mass or less, 65% by mass or less, or 55% by mass or less, and may be 30-90% by mass, 40-90% by mass, 56-90% by mass, 70-90% by mass, 70.3-80% by mass, 71.6-75.1% by mass, 72.9-74.4% by mass, 73.9-90% by mass, 74.5-90% by mass, 75.2-90% by mass, 70-73.8% by mass, 70-72.8% by mass, 70-71.5% by mass, 30-65% by mass, or 30-55% by mass. In one embodiment, the content of structural units derived from alkyl (meth)acrylate may be within the above range. When the above content is above the above lower limit, it is easier to improve the yellowing resistance of the acrylic polyol and to obtain an aqueous coating composition with excellent storage stability. When the above content is below the above upper limit, it is easier to obtain a water-based paint composition with superior low-temperature curing properties.

[0040] The structural units constituting the acrylic polyol may include structural units derived from monomers other than (meth)acrylic acid and (meth)acrylic acid esters. Examples of other monomers include maleic acid, itaconic acid, acrylamide, N-methylolacrylamide, diacetoneacrylamide, styrene, vinyltoluene, vinyl acetate, and acrylonitrile.

[0041] Acrylic polyol may be used in an emulsified, dispersed, or dissolved state in water. In other words, an aqueous coating composition may contain an acrylic polyol-containing liquid (solution, dispersion, emulsion, or suspension) obtained by emulsifying, dispersing, or dissolving acrylic polyol in water. The water content may be 100 to 1000 parts by mass per 100 parts by mass of acrylic polyol.

[0042] The acrylic polyol-containing solution may contain a neutralizing agent. Examples of neutralizing agents include ammonia and water-soluble amino compounds (such as triethylamine and tertiary amines like 2-(dimethylamino)ethanol). The amount of neutralizing agent may be 0.1 parts by mass or more, 0.3 parts by mass or more, 0.7 parts by mass or more, 1.1 parts by mass or more, 1.5 parts by mass or more, 2.1 parts by mass or more, or 2.9 parts by mass or more per 100 parts by mass of acrylic polyol, and may be 4 parts by mass or less, 3.5 parts by mass or less, 2.7 parts by mass or less, 2 parts by mass or less, 1.4 parts by mass or less, 1 part by mass or less, or 0.6 parts by mass or less, and may be 0.1 to 4 parts by mass, 0.3 to 3.5 parts by mass, 0.7 to 2.7 parts by mass, 1.1 to 4 parts by mass, 1.5 to 4 parts by mass, 2.1 to 4 parts by mass, 2.9 to 4 parts by mass, 0.1 to 2 parts by mass, 0.1 to 1.4 parts by mass, 0.1 to 1 part by mass, or 0.1 to 0.6 parts by mass. When the above content is above the above lower limit, the dispersibility of the acrylic polyol is easily improved, and when the above content is below the above upper limit, an aqueous coating composition with excellent storage stability is easily obtained.

[0043] The method for obtaining an acrylic polyol is not particularly limited, but may be, for example, a polymerizable monomer containing at least one selected from the group consisting of (meth)acrylic acid and (meth)acrylic acid esters, mixed with a polymerization initiator, and carried out a polymerization reaction.

[0044] There are no particular restrictions on the polymerization initiator, and commonly used ones can be used. Examples of polymerization initiators include azobisisobutyronitrile, 1,1'-azobis(cyclohexanecarbonitride), di-tert-butyl peroxide, tert-butyl hydroperoxide, hydrogen peroxide, potassium peroxodisulfate, benzoyl peroxide, triethylborane, diethylzinc, tert-butyl=2-ethylperoxyhexanoate, and the like. The content of the polymerization initiator may be 1 to 5 parts by mass per 100 parts by mass of polymerizable monomer.

[0045] The polymerization reaction to obtain acrylic polyols may be carried out, for example, in the presence of a solvent. Suitable solvents include aromatic solvents such as toluene and xylene, ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone, ester solvents such as ethyl acetate and butyl acetate, and glycol ether solvents such as ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, and diethylene glycol diethyl ether. The reaction temperature may be, for example, 20 to 200°C. The reaction time may be, for example, 1 to 10 hours.

[0046] After the polymerization reaction, a neutralizing agent may be added to the resulting mixture containing the acrylic polyol. The type and amount of the neutralizing agent may be the same as those described for the neutralizing agent in the acrylic polyol-containing solution above.

[0047] After the polymerization reaction, water may be added to the resulting mixture containing the acrylic polyol. The amount of water added may be the same as the amount in the acrylic polyol-containing solution described above.

[0048] The acrylic polyol content may be 20% by mass or more, 30% by mass or more, 40% by mass or more, 45% by mass or more, 52% by mass or more, 52.5% by mass or more, 54% by mass or more, 55.5% by mass or more, 56.6% by mass or more, 57.5% by mass or more, 58% by mass or more, or 58.5% by mass or more, based on the total solid content of the aqueous coating composition, from the viewpoint of easily obtaining an aqueous coating composition with superior low-temperature curing properties and an aqueous coating composition with superior storage stability. The acrylic polyol content may be 70% by mass or less, 60% by mass or less, 58% by mass or less, 57.3% by mass or less, 56.5% by mass or less, 55% by mass or less, 53.5% by mass or less, 50% by mass or less, or 43% by mass or less, based on the total solid content of the aqueous coating composition, from the viewpoint of easily improving the dispersibility of the acrylic polyol. From these perspectives, the acrylic polyol content may be 20-70% by mass, 30-70% by mass, 40-70% by mass, 45-70% by mass, 52-70% by mass, 52.5-60% by mass, 54-58% by mass, 55.5-70% by mass, 56.6-70% by mass, 57.5-70% by mass, 58-70% by mass, 58.5-70% by mass, 52-57.3% by mass, 52-56.5% by mass, 52-55% by mass, 52-53.5% by mass, 20-50% by mass, or 20-43% by mass, based on the total solid content of the aqueous coating composition. In this specification, "total solid content of the aqueous coating composition" means the amount obtained by subtracting the amount of solvent from the total amount of the aqueous coating composition.

[0049] (A) The content of component (A) may be 20% by mass or more, 30% by mass or more, 40% by mass or more, 45% by mass or more, 52% by mass or more, 52.5% by mass or more, 54% by mass or more, 55.5% by mass or more, 56.6% by mass or more, 57.5% by mass or more, 58% by mass or more, or 58.5% by mass or more, based on the total solid content of the aqueous coating composition, from the viewpoint of easily obtaining an aqueous coating composition with superior low-temperature curing properties and an aqueous coating composition with superior storage stability. (A) The content of component (A) may be 70% by mass or less, 60% by mass or less, 58% by mass or less, 57.3% by mass or less, 56.5% by mass or less, 55% by mass or less, 53.5% by mass or less, 50% by mass or less, or 43% by mass or less, based on the total solid content of the aqueous coating composition, from the viewpoint of easily improving the dispersibility of the polyol. From these perspectives, the content of component (A) may be 20-70% by mass, 30-70% by mass, 40-70% by mass, 45-70% by mass, 52-70% by mass, 52.5-60% by mass, 54-58% by mass, 55.5-70% by mass, 56.6-70% by mass, 57.5-70% by mass, 58-70% by mass, 58.5-70% by mass, 52-57.3% by mass, 52-56.5% by mass, 52-55% by mass, 52-53.5% by mass, 20-50% by mass, or 20-43% by mass, based on the total solid content of the aqueous paint composition.

[0050] The hydroxyl value of component (A) may be 40 mg KOH / g or more, 50 mg KOH / g or more, 60 mg KOH / g or more, 70 mg KOH / g or more, 80 mg KOH / g or more, 90 mg KOH / g or more, 95 mg KOH / g or more, 130 mg KOH / g or more, or 180 mg KOH / g or more, from the viewpoint of easily obtaining an aqueous coating composition with superior low-temperature curing properties. The hydroxyl value of component (A) may be 300 mg KOH / g or less, 270 mg KOH / g or less, 250 mg KOH / g or less, 230 mg KOH / g or less, 220 mg KOH / g or less, 170 mg KOH / g or less, 150 mg KOH / g or less, 140 mg KOH / g or less, 130 mg KOH / g or less, 120 mg KOH / g or less, or 110 mg KOH / g or less, from the viewpoint of easily improving the dispersibility of the polyol. From these perspectives, component (A) may be 40-230 mg KOH / g, 50-300 mg KOH / g, 50-270 mg KOH / g, 50-250 mg KOH / g, 50-220 mg KOH / g, 50-170 mg KOH / g, 50-150 mg KOH / g, 60-140 mg KOH / g, 70-130 mg KOH / g, 80-120 mg KOH / g, 90-110 mg KOH / g, 95-300 mg KOH / g, 130-300 mg KOH / g, or 180-300 mg KOH / g. The hydroxyl value is measured in accordance with JIS K1557-1. If component (A) contains multiple types of polyols, "hydroxyl value of component (A)" means the hydroxyl value of all polyols contained in component (A) (a mixture of all polyols). From the viewpoint of obtaining the above-mentioned effects more significantly, component (A) may contain an acrylic polyol having a hydroxyl value within the above range.

[0051] Component (A) has an acidic group. The acidic group of component (A) can deactivate quaternary ammonium salts remaining in the aqueous coating composition, thereby suppressing yellowing of the cured coating film caused by the quaternary ammonium salts. The acid value of component (A) is 2 to 45 mg KOH / g, and from the viewpoint of easily improving the resistance of yellowing of the cured coating film, it may be 3 mg KOH / g or more, 8 mg KOH / g or more, 13 mg KOH / g or more, 18 mg KOH / g or more, 25 mg KOH / g or more, or 35 mg KOH / g or more. From the viewpoint of easily obtaining an aqueous coating composition with superior low-temperature curing properties and superior storage stability, it may be 40 mg KOH / g or less, 35 mg KOH / g or less, 25 mg KOH / g or less, 17 mg KOH / g or less, 12 mg KOH / g or less, or 7 mg KOH / g or less. From these viewpoints, the acid value of component (A) may be 3-40 mg KOH / g, 8-35 mg KOH / g, 13-25 mg KOH / g, 18-45 mg KOH / g, 25-45 mg KOH / g, 35-45 mg KOH / g, 2-17 mg KOH / g, 2-12 mg KOH / g, or 2-7 mg KOH / g. The acid value is measured in accordance with JIS K1557-5. If component (A) contains multiple types of polyols, "the acid value of component (A)" means the acid value of all polyols contained in component (A) (a mixture of all polyols). From the viewpoint of obtaining the above-mentioned effects more significantly, component (A) may contain an acrylic polyol having an acid value within the above range.

[0052] The glass transition temperature (Tg) of component (A) may be 35°C or higher, 40°C or higher, or 42°C or higher, from the viewpoint of obtaining an aqueous coating composition with superior low-temperature curability, and may be 55°C or lower, 50°C or lower, or 47°C or lower, from the viewpoint of improving the flexibility of the cured coating film. From these viewpoints, the glass transition temperature (Tg) of component (A) may be 35-55°C, 40-50°C, or 42-47°C. The glass transition temperature is determined by measuring the inflection point of the DSC in accordance with JIS K7121. When component (A) contains multiple types of polyols, "the glass transition temperature (Tg) of component (A)" means the glass transition temperature (Tg) of the entire polyol mixture (all polyols) contained in component (A). From the viewpoint of obtaining the above-mentioned effects more significantly, component (A) may contain an acrylic polyol having a glass transition temperature (Tg) within the above range.

[0053] The number-average molecular weight (Mn) of component (A) may be 10,000 or more, 12,000 or more, or 14,000 or more, from the viewpoint of easily obtaining an aqueous paint composition with superior low-temperature curing properties, and may be 20,000 or less, 18,000 or less, or 16,000 or less, from the viewpoint of improving paint stability. From these viewpoints, the number-average molecular weight (Mn) of component (A) may be 10,000 to 20,000, 12,000 to 18,000, or 14,000 to 16,000. The number-average molecular weight is measured using gel permeation chromatography (GPC) and refers to the value determined using polystyrene as a standard substance. If component (A) contains multiple types of polyols, "the number-average molecular weight (Mn) of component (A)" means the number-average molecular weight (Mn) of all polyols contained in component (A) (a mixture of all polyols). From the viewpoint of obtaining the above-mentioned effects more significantly, component (A) may contain an acrylic polyol having a number-average molecular weight (Mn) within the above range.

[0054] (Component (B): Blocked Polyisocyanate) Blocked polyisocyanate is a compound that can be derived from polyisocyanate that does not have isocyanate groups blocked by a blocking agent and hydrophilic groups (hereinafter also referred to as "non-aqueous unblocked polyisocyanate"). Blocked polyisocyanate has, for example, a structure derived from a non-aqueous unblocked polyisocyanate, isocyanate groups blocked by a blocking agent (hereinafter also referred to as "blocked isocyanate groups"), and hydrophilic groups.

[0055] [Non-aqueous unblocked polyisocyanates] Non-aqueous unblocked polyisocyanates are compounds that have multiple isocyanate groups (free isocyanate groups) and do not have isocyanate groups or hydrophilic groups blocked by a blocking agent. Examples of non-aqueous unblocked polyisocyanates include aromatic polyisocyanates, aliphatic polyisocyanates, alicyclic polyisocyanates, and their polyisocyanate derivatives. Examples of polyisocyanate derivatives include isocyanurates, allophanates, and biuretes.

[0056] The polyisocyanate derivative may be an isocyanate group-containing prepolymer obtained by the reaction of the above-mentioned polyisocyanate with a polyol, or a derivative of the prepolymer (e.g., isocyanurate, allophanate, biuret, etc.). As the polyol, for example, a diol having 2 to 9 carbon atoms can be used. Examples of such diols include ethylene glycol, 1,2-propanediol, 1,3-propanediol, 1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol, 1,9-nonanediol, 3-methyl-1,5-pentanediol, and 2-ethyl-1,3-hexanediol.

[0057] Non-aqueous unblocked polyisocyanates do not need to have aromatic rings, from the viewpoint of improving the yellowing resistance of the cured coating film. That is, non-aqueous unblocked polyisocyanates may be non-aromatic polyisocyanates. Examples of non-aromatic polyisocyanates include aliphatic polyisocyanates such as hexamethylene diisocyanate, tetramethylene diisocyanate, 2-methylpentane-1,5-diisocyanate, 3-methylpentane-1,5-diisocyanate, lysine triisocyanate, and trioxyethylene diisocyanate; alicyclic polyisocyanates such as isophorone diisocyanate, cyclohexyl diisocyanate, hydrogenated diphenylmethane diisocyanate, norbornane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylene diisocyanate, and hydrogenated tetramethylxylene diisocyanate, and derivatives thereof.

[0058] The non-aqueous unblocked polyisocyanate may contain an aliphatic polyisocyanate or derivative thereof having an aliphatic hydrocarbon group having 4 to 6 carbon atoms, from the viewpoint of further improving curability, and may also contain a hexamethylene diisocyanate or derivative thereof, from the viewpoint of further improving curability. In other words, component (B) may contain a blocked polyisocyanate having a structure derived from an aliphatic polyisocyanate or derivative thereof having an aliphatic hydrocarbon group having 4 to 6 carbon atoms, and may also contain a blocked polyisocyanate having a structure derived from a hexamethylene diisocyanate or derivative thereof. As a derivative of hexamethylene diisocyanate, at least one selected from the group consisting of isocyanurate, allophanate, and biuret may be used. In particular, when an isocyanurate is used, the gel fraction of the coating film can be further improved. The isocyanurate may be an isocyanurate of hexamethylene diisocyanate, or an isocyanurate of an isocyanate group-containing prepolymer obtained by the reaction of hexamethylene diisocyanate with a polyol (for example, the diol mentioned above).

[0059] When using a non-aqueous unblocked polyisocyanate containing an isocyanurate form, from the perspective of further improving the gel fraction of the coating film, the content of the isocyanurate trimer (isocyanurate trimer content) based on the total mass of the non-aqueous unblocked polyisocyanate may be 50% by mass or more, and the content ratio of the isocyanurate group (isocyanurate group content ratio) to the total of the isocyanurate group and allophanate group (100 mol%) in the non-aqueous unblocked polyisocyanate may exceed 80 mol%. The upper limit value of the above isocyanurate trimer content may be 80% by mass, and the upper limit value of the above isocyanurate group content ratio may be 99 mol%.

[0060] [Hydrophilic group] A hydrophilic group is a functional group having the property of enabling dissolution or dispersion in water. The hydrophilic group is, for example, a functional group derived from a hydrophilic compound. Here, a hydrophilic compound refers to a compound having an octanol / water partition coefficient of less than 1. The functional group derived from a hydrophilic compound may be, for example, a part of a group formed by a urethanization reaction between a hydrophilic compound having a hydroxy group and an isocyanate group.

[0061] The hydrophilic compound may be a nonionic hydrophilic compound from the perspective of being less likely to receive an electrical interaction. Examples of the nonionic hydrophilic compound include polyalkylene glycol, polyalkylene glycol monoalkyl ether, and the like.

[0062] The hydrophilic group may be an alkoxypolyalkylene oxide group from the perspective of easily enhancing the dispersibility of the blocked polyisocyanate. The alkoxypolyalkylene oxide group is a hydrophilic group derived from polyalkylene glycol monoalkyl ether and is represented by the formula: -(OR １ ) ｎ OR ２ where n indicates the number of alkylene oxide units (OR １ ), R １ indicates an alkylene group, and R ２ indicates an alkyl group. The alkoxypolyalkylene oxide group is polyalkylene glycol monoalkyl ether (H(OR １ ) ｎ OR２ The group formed by the reaction of ) with an isocyanate group (-NHCO(OR １ ) ｎ OR ２ It may be part of ).

[0063] Alkylene group (R １ The alkylene group (R １ The number of carbon atoms in the alkylene group (R) is, for example, 2 to 4, and may be 2 to 3 or 2. １ Specific examples of these include the ethylene group, isopropylene group, n-butylene group, etc. Among these, alkylene group (R １ When the group is an ethylene group, the low-temperature curing properties of the aqueous paint composition tend to improve further. In other words, the alkoxypolyalkylene oxide group may be an alkoxypolyethylene oxide group from the viewpoint of further improving the low-temperature curing properties of the aqueous paint composition.

[0064] Alkyl alkyl group (R ２ The alkyl group (R) may be linear, branched, or cyclic. ２ The number of carbon atoms in the alkyl group (R) is, for example, 1 to 18, and may be 1 to 12, 1 to 6, or 1 to 2. ２ Specific examples of alkyl groups include methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, n-pentyl group, n-hexyl group, n-heptyl group, n-octyl group, 2-ethylhexyl group, 2-propylheptyl group, nonyl group, decyl group, isodecyl group, dodecyl group, tridecyl group, tetradecyl group, pentadecyl group, hexadecyl group, heptadecyl group, octadecyl group, etc. Among these, alkyl groups (R ２ When the group is a methyl group or an ethyl group, the low-temperature curing properties of the aqueous paint composition tend to improve further.

[0065] Hydrophilic groups may contain an average of 14 to 50 ethylene oxide units (hereinafter also referred to as "EO units"). "An average of 14 to 50" means that if the block polyisocyanate contains multiple hydrophilic groups, the average value obtained by dividing the total number of EO units contained in those groups by the total number of hydrophilic groups (total number of EO units / total number of hydrophilic groups) is 14 to 50. When calculating the average number of EO units as described above, decimal values ​​shall be rounded to the nearest whole number.

[0066] The average number of EO units contained in the hydrophilic group may be 14 to 45, 15 to 30, or 15 to 25, from the viewpoint of easily obtaining an aqueous coating composition with superior low-temperature curing properties. The average number of EO units contained in the hydrophilic group may be 10 or more, or 20 or more.

[0067] The number-average molecular weight of the hydrophilic group is, for example, 200 to 10,000, and may be 400 to 10,000. From the viewpoint of improving the water dispersibility of the blocked polyisocyanate, the number-average molecular weight of the hydrophilic group may be 600 or more, and from the viewpoint of improving the storage stability of the paint, it may be 2,000 or less or 1,000 or less. From these viewpoints, the number-average molecular weight of the hydrophilic group may be 600 to 2,000 or 600 to 1,000. Here, the number-average molecular weight of the hydrophilic group means the number-average molecular weight of the hydrophilic compound that forms the hydrophilic group. The above number-average molecular weight is measured using gel permeation chromatography (GPC) and determined using polystyrene as a standard substance.

[0068] From the viewpoint of obtaining an aqueous coating composition with superior low-temperature curing properties, the hydrophilic group content may be 1% by mass or more, 5% by mass or more, 6% by mass or more, or 8% by mass or more, based on the total amount of isocyanate compounds in the isocyanate composition obtained by dissociating the blocking agent from the blocked polyisocyanate. From the viewpoint of obtaining an aqueous coating composition with superior low-temperature curing properties, the hydrophilic group content may be 30% by mass or less, 20% by mass or less, or 15% by mass or less, based on the total amount of isocyanate compounds in the isocyanate composition obtained by dissociating the blocking agent from the blocked polyisocyanate. From these viewpoints, the hydrophilic group content may be 1 to 30% by mass, 5 to 30% by mass, 5 to 20% by mass or 5 to 15% by mass, based on the total amount of isocyanate compounds in the isocyanate composition obtained by dissociating the blocking agent from the blocked polyisocyanate. In this specification, "isocyanate compound" means a compound having an isocyanate group.

[0069] (B) The content of alkoxypolyalkylene oxide groups based on the total mass of hydrophilic groups in component (B) may be 90% by mass or more, 95% by mass or more, or 100% by mass, from the viewpoint of superior low-temperature curability. Similarly, the content of alkoxypolyethylene oxide groups based on the total mass of hydrophilic groups in component (B) may be 90% by mass or more, 95% by mass or more, or 100% by mass.

[0070] [Blocked Isocyanate Group] A blocked isocyanate group is an isocyanate group that has been sealed with a blocking agent and has a structure derived from the blocking agent.

[0071] Examples of blocking agents include alcohol-based blocking agents such as methanol, ethanol, n-butanol, isobutanol, 2-ethylhexanol, butyl cellosolve, propylene glycol monomethyl ether, ethylene glycol, and benzyl alcohol; phenol-based blocking agents such as phenol, cresol, ethylphenol, butylphenol, and 2-hydroxypyridine; lactam-based blocking agents such as ε-caprolactam, δ-valerolactam, and γ-butyrolactam; formaldehyde oxime, acetaldehyde oxime, acetone oxime, and Oxime-based blocking agents such as ethyl ketoxime, methyl isobutyl ketoxime, and cyclohexanone oxime; imidazole, 2-methylimidazole, 4-methylimidazole, 2,4-dimethylimidazole, 2-ethylimidazole, 2-propylimidazole, 2-isopropylimidazole, 4-methyl-2-propylimidazole, 2-phenylimidazole, 4-phenylimidazole, 5-phenylimidazole, 2-methyl-4-phenylimidazole, 2-ethyl-4-methylimidazole, and 2-undecylimidazole Imidazole-based blocking agents such as 2-heptadecylimidazole, amine-based blocking agents such as diphenylamine, diisopropylamine, isopropylethylamine, meldramic acid, dimethyl malonate, diethyl malonate, di-n-butyl malonate, di-t-butyl malonate, di-2-ethylhexyl malonate, methyl n-butyl malonate, ethyl n-butyl malonate, methyl s-butyl malonate, ethyl s-butyl malonate, methyl t-butyl malonate, ethyl t-butyl malonate, dibenzyl malonate, diphenyl malonate, benzyl malonate Active methylene-based blocking agents such as ethylphenyl malonate, t-butylphenyl malonate, isopropylidene malonate, alkyl acetoacetate (methyl acetoacetate, ethyl acetoacetate, n-propyl acetoacetate, isopropyl acetoacetate, n-butyl acetoacetate, t-butyl acetoacetate), benzyl acetoacetate, phenyl acetoacetate, 2-acetoacetoxyethyl methacrylate, acetylacetone, ethyl cyanoacetate, pyrazole, 3,5-dimethylpyrazole, 3,5-diisopropylpyrazole, 3,5-diphenylpyrazole, 3,Examples of pyrazole-based blocking agents include 5-di-t-butylpyrazole, 3-methylpyrazole, 4-benzyl-3,5-dimethylpyrazole, 4-nitro-3,5-dimethylpyrazole, 4-bromo-3,5-dimethylpyrazole, and 3-methyl-5-phenylpyrazole. The blocking agent may be an oxime-based blocking agent from the viewpoint of improving storage stability, and may be a methyl ethyl ketoxime from the viewpoint of further improving storage stability.

[0072] Blocked polyisocyanates may have free isocyanate groups, and may not have free isocyanate groups from the viewpoint of improving storage stability. For example, all of the active isocyanate groups in a blocked polyisocyanate may be blocked isocyanate groups. Here, active isocyanate groups mean both free isocyanate groups and blocked isocyanate groups.

[0073] Component (B) can be obtained, for example, by reacting the above-mentioned non-aqueous unblocked polyisocyanate with the above-mentioned hydrophilic compound and the above-mentioned blocking agent. That is, component (B) may be a reaction product of the non-aqueous unblocked polyisocyanate, the hydrophilic compound and the blocking agent. The non-aqueous unblocked polyisocyanate, the hydrophilic compound and the blocking agent may each be used individually or in combination of two or more. If an aromatic polyisocyanate is not used as the non-aqueous unblocked polyisocyanate, the yellowing resistance of the cured coating can be improved.

[0074] The order in which the non-aqueous unblocked polyisocyanate, hydrophilic compound, and blocking agent are reacted is not particularly limited. For example, the non-aqueous unblocked polyisocyanate and hydrophilic compound may be reacted to obtain an isocyanate group-terminated precursor, and then the obtained isocyanate group-terminated precursor may be reacted with the blocking agent.

[0075] The reaction between a non-aqueous, unblocked polyisocyanate and a hydrophilic compound may be carried out, for example, in the presence of a solvent. Suitable solvents include aromatic solvents such as toluene and xylene, ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone, ester solvents such as ethyl acetate and butyl acetate, and glycol ether solvents such as ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, and diethylene glycol diethyl ether. The reaction temperature may be, for example, 20 to 200°C. The reaction time may be, for example, 1 to 10 hours. The amounts of polyisocyanate and hydrophilic compound used may be adjusted so that the hydrophilic group content falls within the above range. Known urethane reaction catalysts can also be used to accelerate the reaction.

[0076] The reaction between the isocyanate group-terminated precursor and the blocking agent can be carried out according to the reaction conditions of a normal blocking reaction. The reaction between the isocyanate group-terminated precursor and the blocking agent may be carried out at room temperature or with heating. Regardless of whether heating is used, the temperature of the reaction solution may be, for example, 20 to 200°C.

[0077] The method for producing blocked polyisocyanates is not limited to the above. For example, if the reaction product obtained after reacting a non-aqueous unblocked polyisocyanate with a hydrophilic compound and a blocking agent has free isocyanate groups, blocked polyisocyanates can also be obtained by reacting the free isocyanate groups of the reaction product with a modifying agent such as an active hydrogen group-containing compound.

[0078] Alternatively, instead of the non-aqueous unblocked polyisocyanate described above, an unblocked polyisocyanate having hydrophilic groups may be used. In this case, a blocked polyisocyanate can be obtained by reacting the unblocked polyisocyanate having hydrophilic groups with a blocking agent.

[0079] Furthermore, blocked polyisocyanates can also be obtained by using blocked polyisocyanates without hydrophilic groups (non-aqueous blocked polyisocyanates) instead of the non-aqueous unblocked polyisocyanates mentioned above. For example, blocked polyisocyanates can be obtained by reacting a blocked polyisocyanate without hydrophilic groups with the hydrophilic compound mentioned above.

[0080] Blocked polyisocyanates may be used individually or in combination of two or more. For example, two or more blocked polyisocyanates derived from different types of non-aqueous unblocked polyisocyanates may be used in combination.

[0081] (B) The content of component (B) may be 35% by mass or more, 35.5% by mass or more, 36.5% by mass or more, 37.5% by mass or more, 38.2% by mass or more, 39% by mass or more, 39.4% by mass or more, 40% by mass or more, 45% by mass or more, or 52% by mass or more, based on the total solid content of the aqueous coating composition, from the viewpoint of easily obtaining an aqueous coating composition with superior low-temperature curing properties. (B) The content of component (B) may be 70% by mass or less, 65% by mass or less, 60% by mass or less, 50% by mass or less, 45% by mass or less, 39.3% by mass or less, 38.6% by mass or less, 38% by mass or less, 37.2% by mass or less, or 36% by mass or less, based on the total solid content of the aqueous coating composition, from the viewpoint of improving the storage stability of the coating. From these perspectives, the content of component (B) may be 35-70% by mass, 35-65% by mass, 35-60% by mass, 35-50% by mass, 35.5-45% by mass, 36.5-39.3% by mass, 37.5-50% by mass, 38.2-50% by mass, 39-50% by mass, 39.4-50% by mass, 40-70% by mass, 45-70% by mass, 52-70% by mass, 35-38.6% by mass, 35-38% by mass, 35-37.2% by mass, or 35-36% by mass, based on the total solid content of the aqueous paint composition.

[0082] (B) The effective isocyanate group content of component (hereinafter referred to as "effective NCO content") may be 4 to 28% by mass, 5 to 22% by mass, or 6 to 16% by mass, from the viewpoint of further improving the curability of the paint. Here, the effective NCO content is expressed in mass percent as the isocyanate groups present in the blocked polyisocyanate that can participate in the crosslinking reaction. The effective NCO content can be rephrased as the content of free isocyanate groups in the polyisocyanate obtained by dissociating the blocking agent from the blocked isocyanate, relative to the total mass of the blocked polyisocyanate (free NCO content). The free NCO content can be determined by reacting the isocyanate groups in the measurement sample (polyisocyanate obtained by dissociating the blocking agent from the blocked isocyanate) with an excess of secondary amine, and then back titrating the unreacted secondary amine with hydrochloric acid.

[0083] The ratio of the total number of active isocyanate groups in component (B) to the total number of hydroxyl groups in component (A) ([NCO / OH]) may be 0.7 or more, 0.8 or more, or 0.9 or more in terms of equivalent weight, from the viewpoint of obtaining an aqueous coating composition with superior low-temperature curing properties. The ratio of the total number of active isocyanate groups in component (B) to the total number of hydroxyl groups in component (A) may be 1.5 or less, 1.4 or less, or 1.3 or less in terms of equivalent weight, from the viewpoint of improving the storage stability of the coating. From these viewpoints, the ratio of the total number of active isocyanate groups in component (B) to the total number of hydroxyl groups in component (A) may be 0.7 to 1.5, 0.8 to 1.4, or 0.9 to 1.3 in terms of equivalent weight.

[0084] (Component (C): Blocking agent dissociation catalyst) Component (C) contains a quaternary ammonium salt. Any quaternary ammonium salt known as a blocking agent dissociation catalyst can be used without particular restriction. In particular, when using a quaternary ammonium salt having a cationic group represented by the following formula (1), it is easier to obtain an aqueous coating composition with superior low-temperature curing properties.

[0085] In formula (1), R １R represents an alkyl group having 1 to 16 carbon atoms, which may have a hydroxyl group, an amino group, or an alkoxy group as a substituent. ２ ~R ４ Each of these independently represents an alkyl group having 1 to 8 carbon atoms.

[0086] R １ The number of carbon atoms in the alkyl group represented by R may be 1 to 10, 1 to 8, or 6 to 8. １ Specific examples of alkyl groups represented by include methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, n-pentyl group, n-hexyl group, n-heptyl group, n-octyl group, 2-ethylhexyl group, 2-propylheptyl group, nonyl group, decyl group, isodecyl group, dodecyl group, hexadecyl group, etc. These groups may be substituted with one or more hydroxyl groups, one or more amino groups, or one or more alkoxy groups. The number of carbon atoms in the alkoxy group may be, for example, 1 to 8, 1 to 6, 1 to 4, or 1 to 2. Examples of alkoxy groups include methoxy group, ethoxy group, butoxy group, propyloxy group, pentyloxy group, hexyloxy group, etc.

[0087] R １ From the viewpoint of easily obtaining an aqueous coating composition with superior low-temperature curing properties, the alkyl group may be an unsubstituted alkyl group. The number of carbon atoms of the unsubstituted alkyl group may be 1 to 10, 1 to 8, or 6 to 8. In particular, when the alkyl group has 8 carbon atoms, the low-temperature curing properties of the aqueous coating composition are more easily improved, and R １ When the group is an n-octyl group, the low-temperature curing properties of the aqueous paint composition tend to be further improved.

[0088] R ２ ~R ４ The number of carbon atoms in the alkyl group represented by R may be 1 to 4, or 1 to 2. ２ ~R ４Specific examples of alkyl groups represented by include methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, t-butyl group, n-pentyl group, n-hexyl group, n-heptyl group, n-octyl group, 2-ethylhexyl group, etc. From the viewpoint of easily obtaining an aqueous coating composition with superior low-temperature curing properties, R ２ ~R ４ All of them may be methyl groups, R １ ~R ４ All of them may be methyl groups.

[0089] From the viewpoint of obtaining an aqueous coating composition with superior low-temperature curing properties, the quaternary ammonium salt may have at least one cationic group selected from the group consisting of the group represented by the following formula (2) and the group represented by the following formula (3). That is, component (C) may contain at least one quaternary ammonium salt selected from the group consisting of a quaternary ammonium salt having a cationic group represented by the following formula (2) and a quaternary ammonium salt having a cationic group represented by the following formula (3). In particular, when the quaternary ammonium salt has a cationic group represented by the following formula (2), an aqueous coating composition with even superior low-temperature curing properties is more likely to be obtained.

[0090]

[0091]

[0092] The anionic group of a quaternary ammonium salt may be a group consisting of an anion derived from an acid such as an organic acid or an inorganic acid, or a group consisting of an anion derived from an ester such as a carbonate ester. The anionic group of a quaternary ammonium salt may be a group that does not fall into any of these categories (for example, a hydroxyl group (a group consisting of a hydroxide ion)).

[0093] Examples of groups consisting of anions derived from organic acids (organic acid groups) include fatty acid groups. The number of carbon atoms in a fatty acid group (aliphatic monocarboxylic acid group) may be, for example, 1 to 12, 1 to 7, or 1 to 3. Specific examples of fatty acid groups include formic acid, acetate, octic acid, lauric acid, cyclohexanecarboxylic acid, and pivalic acid.

[0094] Examples of groups consisting of anions derived from inorganic acids (inorganic acid groups) include halogen groups (fluoro groups, chloro groups, bromo groups, etc.), bicarbonate groups, and carbonate groups.

[0095] Examples of groups consisting of anions derived from esters (ester groups) include monoalkyl carbonate groups. The number of carbon atoms in the alkyl group in the monoalkyl carbonate group may be, for example, 1 to 8, 1 to 4, or 1 to 2. Specific examples of monoalkyl carbonate groups include methyl carbonate groups, ethyl carbonate groups, propyl carbonate groups, and butyl carbonate groups.

[0096] The anionic group of the quaternary ammonium salt may be at least one selected from the group consisting of a monoalkyl carbonate group having an alkyl group with 1 to 8 carbon atoms and a hydroxyl group, from the viewpoint of easily obtaining an aqueous coating composition with superior low-temperature curing properties. In particular, when the anionic group of the quaternary ammonium salt is a monoalkyl carbonate group having an alkyl group with 1 to 8 carbon atoms, an aqueous coating composition with even superior low-temperature curing properties is easily obtained.

[0097] From the above-mentioned viewpoint, the quaternary ammonium salt may be composed of a combination of a cationic group represented by formula (1) and a monoalkyl carbonate group or hydroxyl group having an alkyl group having 1 to 8 carbon atoms. In particular, when at least one selected from the group consisting of a quaternary ammonium salt having a cationic group represented by formula (2) and a monoalkyl carbonate group having an alkyl group having 1 to 8 carbon atoms, and a quaternary ammonium salt having a cationic group represented by formula (3) and a hydroxyl group is used, an aqueous coating composition with even better low-temperature curability is more likely to be obtained. When a quaternary ammonium salt having a cationic group represented by formula (2) and a monoalkyl carbonate group having an alkyl group having 1 to 8 carbon atoms is used, an aqueous coating composition with particularly excellent low-temperature curability is more likely to be obtained.

[0098] Specific examples of quaternary ammonium salts include trimethyl n-octylammonium bicarbonate, tetramethylammonium hydroxide, trimethyl n-octylammonium monomethyl carbonate, trimethyl n-octylammonium carbonate, tetramethylammonium acetate, hexadecyltrimethylammonium hydroxide, trimethyl(2-hydroxypropyl)ammonium 2-ethylhexanoic acid, tetramethylammonium bicarbonate, tetraethylammonium bicarbonate, tetran-propylammonium bicarbonate, tetran-butylammonium bicarbonate, triethylmonomethylammonium bicarbonate, trin-propylmonomethylammonium bicarbonate, trin-butylmonomethylammonium bicarbonate, trin-butylmonoethylammonium bicarbonate, tetramethylammonium monomethyl carbonate, tetraethylammonium monoethyl carbonate, tetran-butylammonium monobutyl carbonate, triethylmonomethylammonium monomethyl carbonate, trin-propylmonomethylammonium monomethyl carbonate, trin-butylmonomethylammonium monomethyl carbonate, trin-butylmonoethylammonium monoethyl carbonate, tetramethylammonium carbonate, tetran-butylammonium carbonate, and tetran-butylammonium carbonate. Among these, when at least one selected from the group consisting of trimethyl n-octylammonium monomethyl carbonate and tetramethylammonium hydroxide is used, an aqueous coating composition with even better low-temperature curing properties is more likely to be obtained, and when trimethyl n-octylammonium monomethyl carbonate is used, an aqueous coating composition with particularly excellent low-temperature curing properties is more likely to be obtained. A quaternary ammonium salt may be used alone, or two or more may be used in combination.

[0099] The blocking agent dissociation catalyst may contain blocking agent dissociation catalysts other than quaternary ammonium salts. The proportion of quaternary ammonium salts in the total blocking agent dissociation catalyst may be 0.0001 to 100% by mass, 0.001 to 100% by mass, or 0.01 to 100% by mass.

[0100] The content of quaternary ammonium salt may be 0.1 parts by mass or more, 0.5 parts by mass or more, 2 parts by mass or more, 4 parts by mass or more, 6 parts by mass or more, 10 parts by mass or more, 14 parts by mass or more, 21 parts by mass or more, or 28 parts by mass or more per 100 parts by mass of block polyisocyanate, from the viewpoint of easily improving low-temperature curability. The content of quaternary ammonium salt may be 38 parts by mass or less, 33 parts by mass or less, 25 parts by mass or less, 20 parts by mass or less, 13 parts by mass or less, 9 parts by mass or less, or 5 parts by mass or less per 100 parts by mass of block polyisocyanate, from the viewpoint of easily improving the storage stability of the paint. From these perspectives, the content of the quaternary ammonium salt may be, for example, 0.1 to 38 parts by mass, 0.5 to 38 parts by mass, 2 to 38 parts by mass, 4 to 33 parts by mass, 6 to 25 parts by mass, 10 to 38 parts by mass, 14 to 38 parts by mass, 21 to 38 parts by mass, 28 to 38 parts by mass, 2 to 20 parts by mass, 2 to 13 parts by mass, 2 to 9 parts by mass, or 2 to 5 parts by mass per 100 parts by mass of block polyisocyanate.

[0101] From the viewpoint of easily improving low-temperature curability, the content of quaternary ammonium salt may be 0.5 moles or more, 0.8 moles or more, 1.0 moles or more, or 1.1 moles or more per mole of acidic groups contained in component (A). From the viewpoint of easily improving the yellowing resistance of the cured coating film, the content of quaternary ammonium salt may be 5.0 moles or less, 3.0 moles or less, 2.0 moles or less, 1.5 moles or less, or 1.3 moles or less per mole of acidic groups contained in component (A). From these viewpoints, the content of quaternary ammonium salt may be 0.5 to 5.0 moles, 0.8 to 3.0 moles, 1.0 to 2.0 moles, 1.1 to 1.5 moles, or 1.1 to 1.3 moles per mole of acidic groups contained in component (A). The number of moles of acidic groups contained in component (A) is calculated from the content of component (A) and the acid value of component (A).

[0102] From the viewpoint of easily improving low-temperature curability, the content of the blocking agent dissociation catalyst may be 0.1 parts by mass or more, 0.5 parts by mass or more, 2 parts by mass or more, 4 parts by mass or more, 6 parts by mass or more, 10 parts by mass or more, 14 parts by mass or more, 21 parts by mass or more, or 28 parts by mass or more per 100 parts by mass of blocked polyisocyanate. From the viewpoint of improving the storage stability of the paint, the content of the blocking agent dissociation catalyst may be 38 parts by mass or less, 33 parts by mass or less, 25 parts by mass or less, 20 parts by mass or less, 13 parts by mass or less, 9 parts by mass or less, or 5 parts by mass or less per 100 parts by mass of blocked polyisocyanate. From these perspectives, the content of the blocking agent dissociation catalyst may be, for example, 0.1 to 38 parts by mass, 0.5 to 38 parts by mass, 2 to 38 parts by mass, 4 to 33 parts by mass, 6 to 25 parts by mass, 10 to 38 parts by mass, 14 to 38 parts by mass, 21 to 38 parts by mass, 28 to 38 parts by mass, 2 to 20 parts by mass, 2 to 13 parts by mass, 2 to 9 parts by mass, or 2 to 5 parts by mass per 100 parts by mass of blocked polyisocyanate.

[0103] (Component (D): Water) The water may be, for example, deionized water, ultrapure water, etc. From the viewpoint of high paint viscosity and good workability, the water content may be 50% by mass or more, 60% by mass or more, 70% by mass or more, or 73.2% by mass or more, based on the total mass of the aqueous paint composition. From the viewpoint of improving the storage stability of the paint, the water content may be 90% by mass or less, 85% by mass or less, 80% by mass or less, or 77% by mass or less, based on the total mass of the aqueous paint composition. From these viewpoints, it may be 50 to 90% by mass, 60 to 85% by mass, 70 to 80% by mass, or 73.2 to 77% by mass.

[0104] (Other components) The aqueous paint composition may contain polyols, blocked polyisocyanates, blocking agent dissociation catalysts, and other components other than water. For example, the aqueous paint composition may contain solvents other than water. As solvents, ketone solvents such as acetone, methyl ethyl ketone, and methyl isobutyl ketone, ester solvents such as ethyl acetate, and glycol ether solvents such as ethylene glycol monoethyl ether acetate, propylene glycol monomethyl ether acetate, and diethylene glycol diethyl ether can be used.

[0105] The content of solvents other than water may be 0 to 5% by mass, based on the total mass of the aqueous paint composition.

[0106] The aqueous paint composition may contain non-aqueous unblocked polyisocyanates (e.g., polyisocyanates remaining as unreacted material) and may contain unreacted blocking agents. The content of non-aqueous unblocked polyisocyanates in the aqueous paint composition may be 5% by mass or less, or 0% by mass, based on the total solid content of the aqueous paint composition. The content of unreacted blocking agents in the aqueous paint composition may be 5% by mass or less, or 0% by mass, based on the total solid content of the aqueous paint composition.

[0107] The water-based paint composition may further contain additives such as pigments, dispersion stabilizers, viscosity modifiers, leveling agents, gelling inhibitors, light stabilizers, antioxidants, UV absorbers, heat resistance improvers, inorganic and organic fillers, plasticizers, lubricants, antistatic agents, reinforcing materials, and catalysts.

[0108] A water-based paint composition may be a one-component composition in which all constituent components are contained in a single liquid, or it may be a multi-component composition in which the constituent components exist separately in multiple liquids. A multi-component water-based paint composition may comprise a first liquid (main component) containing a polyol and a second liquid (curing agent) containing a blocked polyisocyanate. In this case, other constituent components (blocking agent dissociation catalyst and other components) may be contained in the first liquid, in the second liquid, or in liquids different from the first and second liquids.

[0109] The ratio of available isocyanate groups to the total number of hydroxyl groups in the aqueous paint composition ([available isocyanate groups] / [hydroxyl groups]) may be 0.7 to 1.5, 0.8 to 1.4, or 0.9 to 1.3 in terms of equivalent weight.

[0110] Water-based paint compositions can be used as top and intermediate coatings for automobiles, chipping-resistant paints, electrodeposition paints, paints for automobile parts, paints for automobile repairs, pre-coated metals and rust-resistant steel sheets for metal products such as home appliances and office equipment, paints for building materials, paints for plastics, adhesives, adhesion promoters, sealants, etc.

[0111] <Coating Film> Another embodiment of the present disclosure is a coating film formed from the aqueous coating composition of the above embodiment.

[0112] The above coating film includes a cured product of the aqueous coating composition of the above embodiment. The above coating film can be formed by applying the aqueous coating composition onto a substrate using a known method and curing the coating film (uncured coating film) made of the aqueous coating composition. Examples of known methods include roll coating, curtain flow coating, spray coating, electrostatic coating, bell coating, electrodeposition coating, etc. The amount of aqueous coating composition applied, the thickness of the coating film, etc. may be appropriately determined according to the material of the surface to be coated, etc.

[0113] The curing of a coating film made from an aqueous coating composition may be performed by heating the coating film. The heating temperature (baking temperature) may be, for example, 200°C or less. The heating time (baking time) may be, for example, 10 to 180 minutes. According to the aqueous coating composition of this embodiment, even when baking is performed at a low temperature of 100°C or less (for example, 60 to 100°C), a cured coating film with a good gel fraction can be obtained.

[0114] The contents of this disclosure will be described in more detail below using examples and comparative examples, but this disclosure is not limited to the following examples.

[0115] <Example 1> (Production of Acrylic Polyol) 400 g of methyl ethyl ketone (MEK) was placed in a reaction vessel equipped with a four-necked flask containing a stirrer, thermometer, heating device, nitrogen sealing tube, and condenser, and the temperature was raised to 80°C while stirring. Next, a monomer mixture consisting of 111.9 g of methyl methacrylate (MMA), 255.9 g of butyl methacrylate (BMA), 116.2 g of 2-hydroxyethyl methacrylate (HEMA), and 3.2 g of acrylic acid (AA) was mixed with 12.8 g of the peroxide-based polymerization initiator "Perbutyl O (manufactured by NOF Corporation)" and added dropwise over 2 hours to carry out the polymerization reaction. Next, 2.0 g of the above peroxide-based polymerization initiator "Perbutyl O (manufactured by NOF Corporation)" and 100 g of MEK were added dropwise to the reaction vessel over 1 hour to obtain a solution containing acrylic polyol. Subsequently, 2 g of 2-(dimethylamino)ethanol (DMAE) was added to neutralize the mixture, and 2000 g of deionized water was added over 2 hours to obtain an aqueous dispersion of acrylic polyol. Then, stirring was continued, and desolvation was carried out under reduced pressure while raising the temperature to 60°C. Once all of the added methyl ethyl ketone solvent had been removed by distillation, the pressure was returned to atmospheric pressure, and after cooling, the excess deionized water was corrected to obtain an aqueous dispersion of acrylic polyol (1) with a solid content of 20% by mass. The hydroxyl value of acrylic polyol (1) was 100 mg KOH / g, the acid value was 5 mg KOH / g, the glass transition temperature (Tg) was 45°C, and the number-average molecular weight (Mn) was 15,000. The hydroxyl value was measured according to JIS K1557-1, and the acid value was measured according to JIS K1557-5. The number-average molecular weight (Mn) was determined using gel permeation chromatography (GPC) with polystyrene as the standard substance.

[0116] (Production of Block Polyisocyanate) In a four-necked flask equipped with a stirrer, thermometer, heating device, nitrogen seal tube, and condenser, 387 g of Coronate HXR (manufactured by Tosoh Corporation, hexamethylene diisocyanate trimer, NCO content 21.8% by mass, trade name), 43 g of MPEG-1000 (manufactured by Nippon Emulsifier Co., Ltd., polyethylene glycol monomethyl ether, number average molecular weight = 1000, average number of EO units = 22, trade name), and 400 g of diethyldiglycol (manufactured by Nippon Emulsifier Co., Ltd., diethylene glycol diethyl ether, trade name) were charged. The flask was then purged with nitrogen, and the mixture was heated to 80°C while stirring. The reaction was carried out at the same temperature for 4 hours to obtain an isocyanate group-terminated precursor having an alkoxy polyethylene oxide group as a hydrophilic group. Next, 170g of methyl ethyl ketoxime (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was added, keeping the temperature below 80°C, and the mixture was reacted at 70°C for 2 hours. An infrared absorption spectrum (IR measurement) was then used to determine the peak of the NCO group (2270 cm⁻¹). －１ Once the surrounding area had disappeared, the mixture was cooled to room temperature to obtain a solution containing blocked polyisocyanate (1) (60% by mass of solids).

[0117] (Preparation of Aqueous Paint Composition) In a reaction vessel, 59.2 g of the aqueous dispersion of acrylic polyol (1) prepared above (solid content 20% by mass, "AP dispersion" in Table 1), 13.6 g of a solution containing blocked polyisocyanate (1) (solid content 60% by mass, "BPI solution" in Table 1), 0.6 g of trimethyl-n-octylammonium monomethyl carbonate solution (manufactured by Tosoh Corporation, solid content 55% by mass, "TMOA-MC" in Table 1), which is a blocking agent dissociation catalyst, and 27.2 g of deionized water were sequentially charged and thoroughly mixed, and the mixture was stirred at high speed at 2000 rpm for 2 minutes using a homomixer to obtain an aqueous paint composition. The amount of BPI solution was adjusted so that the ratio of the total number of active isocyanate groups in blocked polyisocyanate (1) to the total number of hydroxyl groups in acrylic polyol (1) ([NCO / OH]) was 1.25 in terms of equivalent ratio. Furthermore, the amount of TMOA-MC was adjusted so that the amount of solid content (trimethyl-n-octylammonium monomethyl carbonate) in TMOA-MC was 1.2 moles for every 1 mole of acidic group contained in acrylic polyol (1).

[0118] <Examples 2-8 and Comparative Example 1> (Production of Acrylic Polyols) Solutions containing acrylic polyols (2) to (9) were obtained in the same manner as in Example 1, except that the amounts of MMA, BMA, HEMA, and AA, and the amount of DMAE added were changed as shown in Table 1 (solid content: 20% by mass). The hydroxyl value, acid value, glass transition temperature (Tg), and number average molecular weight (Mn) of the obtained acrylic polyols (2) to (9) were measured in the same manner as in Example 1. As a result, the acid value and hydroxyl value were as shown in Table 1, the glass transition temperature (Tg) was 45°C, and the number average molecular weight (Mn) was 15,000.

[0119] (Preparation of Aqueous Paint Composition) An aqueous paint composition was obtained in the same manner as in Example 1, except that an aqueous dispersion containing any of the acrylic polyols (2) to (9) prepared above (solid content: 20% by mass, "AP dispersion" in Table 1) was used instead of the aqueous dispersion of acrylic polyol (1), and the blending amounts of AP dispersion, BPI solution, TMOA-MC, and deionized water were changed as shown in Table 1. In all examples, the blending amount of TMOA-MC was adjusted so that the amount of solid content (trimethyl-n-octylammonium monomethyl carbonate) in TMOA-MC was 1.2 moles for every 1 mole of acidic groups contained in acrylic polyols (2) to (9).

[0120] <Example 9> (Preparation of aqueous paint composition) 56.8 g of an aqueous dispersion of acrylic polyol (4), 13.0 g of a solution containing blocked polyisocyanate (1), 4.4 g of tetramethylammonium hydroxide solution (manufactured by Tokyo Chemical Industry Co., Ltd., solid content 10% by mass, "TMA-OH" in Table 1), which is a blocking agent dissociation catalyst, were sequentially added and thoroughly mixed. An aqueous paint composition was obtained by high-speed stirring at 2000 rpm for 2 minutes using a homomixer. The amount of BPI solution was adjusted so that the ratio of the total number of active isocyanate groups contained in blocked polyisocyanate (1) to the total number of hydroxyl groups contained in acrylic polyol (4) ([NCO / OH]) was 1.25 in terms of equivalent ratio. The amount of TMA-OH was adjusted so that the amount of solid content (tetramethylammonium hydroxide) in TMA-OH was 1.2 moles for every 1 mole of acidic groups contained in acrylic polyol (4).

[0121] <Evaluation> (Low-temperature curing performance evaluation) The aqueous paint composition prepared above was applied to release paper to a thickness of 200 μm before drying. The resulting coating film was left to stand at room temperature for 60 minutes, and then baked by heating in a constant temperature bath at 80°C for 20 minutes. The baked coating film (cured coating film) was immersed in methyl ethyl ketone at room temperature for 24 hours, and the gel fraction was determined. The gel fraction was calculated using the following formula: Gel fraction (unit: mass%) = Mass of coating film after immersion (mass of undissolved portion) / Mass of coating film before immersion × 100 If the gel fraction measured above was 60% by mass or more, it was evaluated as having excellent low-temperature curing properties. The results are shown in Table 1.

[0122] (Storage Stability) The storage stability of the water-based paint composition was evaluated by observing the dispersion state before and after standing at 40°C. The longer the number of days that the composition remained liquid and did not precipitate after standing, the better the storage stability.

[0123]

Claims

It contains (A) a polyol, (B) a blocked polyisocyanate having hydrophilic groups, (C) a blocking agent dissociation catalyst, and (D) water. The acid value of component (A) is 2 to 45 mg KOH / g. A water-based paint composition wherein component (C) contains a quaternary ammonium salt. 　 The aqueous paint composition according to claim 1, wherein the component (A) comprises an acrylic polyol. 　 The aqueous coating composition according to claim 2, wherein the acrylic polyol comprises structural units derived from one or more (meth)acrylic acids and structural units derived from one or more (meth)acrylic acid esters. 　 The aqueous paint composition according to claim 3, wherein the (meth)acrylic acid ester comprises at least one selected from the group consisting of (meth)acrylate hydroxyalkyl esters and (meth)acrylate alkyl esters. 　 The aqueous coating composition according to claim 1, wherein component (B) comprises a block polyisocyanate having a structure derived from an aliphatic polyisocyanate or a derivative thereof having an aliphatic hydrocarbon group having 4 to 6 carbon atoms. 　 The aqueous paint composition according to claim 1, wherein the (C) component comprises a quaternary ammonium salt having a cationic group represented by the following formula (1). [In formula (1), R １ R represents an alkyl group having 1 to 16 carbon atoms, which may have a hydroxyl group, an amino group, or an alkoxy group as a substituent. ２ ~R ４ Each of these independently represents an alkyl group having 1 to 8 carbon atoms. 　 The aqueous coating composition according to claim 1, wherein the (C) component has at least one cationic group selected from the group consisting of a group represented by the following formula (2) and a group represented by the following formula (3). 　 The aqueous paint composition according to claim 1, wherein the (C) component has a monoalkyl carbonate group or a hydroxyl group having an alkyl group having 1 to 8 carbon atoms. 　 The aqueous paint composition according to claim 1, wherein the content of component (C) is 0.5 to 5.0 moles per mole of acidic group contained in component (A). 　 The aqueous paint composition according to claim 1, wherein component (B) comprises a blocked polyisocyanate having isocyanate groups encapsulated with an oxime-based blocking agent. 　 The aqueous paint composition according to claim 1, wherein component (B) comprises a blocked polyisocyanate having an isocyanate group encapsulated with methyl ethyl ketoxime. 　 The aqueous paint composition according to claim 1, wherein the hydrophilic group is an alkoxy polyethylene oxide group. 　 The aqueous paint composition according to claim 1, wherein the hydrophilic group comprises an average of 14 to 50 ethylene oxide units. 　 The aqueous coating composition according to claim 1, wherein the number-average molecular weight of the hydrophilic groups is 600 to 2000. 　 The aqueous paint composition according to claim 1, wherein the content of the hydrophilic group is 5 to 30% by mass, based on the total amount of isocyanate compounds in the isocyanate composition obtained by dissociating the blocking agent from the blocked polyisocyanate. 　 The aqueous paint composition according to claim 1, wherein the ratio of the total number of active isocyanate groups contained in component (B) to the total number of hydroxyl groups contained in component (A) is 0.7 to 1.5 in terms of equivalent weight. 　 A coating film formed from the aqueous coating composition according to any one of claims 1 to 16.

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