JP2020070422A - Aqueous coating composition and method for producing the same - Google Patents

Abstract

To provide an isocyanate curable aqueous coating composition having both excellent low-temperature curability and good long-term storage stability.SOLUTION: The aqueous coating composition contains: a polyol; a polyisocyanate; a hydrophobic curing catalyst which comprises an organometallic compound containing at least one kind of metal atom selected from the group consisting of Sn, Bi, Zr, Ti and Al and has a solubility in water of 1 g/100 mL or less under an atmospheric pressure condition at 20°C; and a nonionic surfactant having an HLB value of 10-15 as determined by Griffin's method.SELECTED DRAWING: None

Description

  The present invention relates to an aqueous coating composition and a method for producing the same, and more particularly to an isocyanate-curable aqueous coating composition and a method for producing the same.

In the coating of automobiles and the like, designing the baking temperature of the thermosetting paint to be low is an important issue from the viewpoint of reducing energy in the baking furnace and reducing CO ₂ emission. Therefore, it has been attempted to lower the baking temperature by incorporating a curing catalyst into the paint.

  For example, JP-A-2017-193705 (Patent Document 1) describes a coating composition containing a curing catalyst composed of a polyol, a polyisocyanate, a bismuth compound, and a carboxylic acid having 8 or more carbon atoms. It is also described that the coating composition has excellent storage stability and excellent curability under high humidity conditions. However, when the coating composition described in Patent Document 1 is stored for a long time, gelation occurs even at a temperature extremely lower than the baking temperature (for example, 40 ° C.), and thus it is difficult to store for a long time.

  Further, Japanese Patent Laid-Open No. 2017-43759 (Patent Document 2) discloses that at least one curing catalyst selected from the group consisting of polyepoxide, diazabicycloundecene salt, and diazabicyclononene salt, and nonionic surfactant. A one-component curing type epoxy emulsion containing an agent is described, and it is also described that the one-component curing type epoxy emulsion has good long-term storage stability and low temperature curability.

JP, 2017-193705, A JP, 2017-43759, A

  However, an isocyanate-curable aqueous coating composition containing a polyol and a polyisocyanate is blended with a curing catalyst and a nonionic surfactant described in Patent Document 2, and the curing catalyst is treated with the nonionic surfactant. Even if encapsulated in the formed micelle and protected, since the curing catalyst is hydrophilic, the curing catalyst diffuses to the outside of the micelle over time and accelerates the reaction between the polyol and the polyisocyanate. The present inventors have found that there is a problem that excellent storage stability cannot be obtained.

  The present invention has been made in view of the above problems of the prior art, and provides an isocyanate-curable aqueous coating composition having both excellent low-temperature curability and good storage stability for a long time, and a method for producing the same. The purpose is to do.

  As a result of intensive studies to achieve the above-mentioned object, the present inventors have mixed a hydrophobic curing catalyst with a nonionic surfactant having an HLB value within a specific range in advance to obtain the hydrophobic curing. By protecting the catalyst with the nonionic surfactant and incorporating the hydrophobic curing catalyst protected with the nonionic surfactant into an aqueous coating composition containing a polyol and a polyisocyanate, excellent It was found that an isocyanate-curable aqueous coating composition having both low-temperature curability and good long-term storage stability can be obtained, and the present invention has been completed.

  That is, the aqueous coating composition of the present invention comprises a polyol, a polyisocyanate, and an organometallic compound containing at least one metal atom selected from the group consisting of Sn, Bi, Zr, Ti and Al. It is characterized by containing a hydrophobic curing catalyst having a solubility in water of 1 g / 100 ml or less under conditions of ° C and atmospheric pressure, and a nonionic surfactant having an HLB value of 10 to 15 determined by the Griffin method. It is what

  In the aqueous coating composition of the present invention, it is preferable that the hydrophobic curing catalyst is encapsulated in the micelle formed by the nonionic surfactant. The aqueous coating composition of the present invention preferably contains no organic solvent, or contains an organic solvent having a solubility in water at 20 ° C of 10 g / 100 ml or more and a boiling point of 50 to 200 ° C.

  Further, the method for producing the aqueous coating composition of the present invention comprises an organometallic compound containing at least one metal atom selected from the group consisting of Sn, Bi, Zr, Ti and Al, at 20 ° C. and atmospheric pressure. A hydrophobic curing catalyst having a solubility in water of 1 g / 100 ml or less under the conditions and a nonionic surfactant having an HLB value of 10 to 15 determined by the Griffin method are mixed to obtain a curing catalyst-containing surface active agent. After the preparation of the aqueous solution of the agent, the polyol, the polyisocyanate, and the aqueous solution of the surfactant containing the curing catalyst are mixed.

  In the method for producing an aqueous coating composition of the present invention, when mixing the polyol, the polyisocyanate, and the curing catalyst-containing surfactant aqueous solution, no organic solvent is mixed, or water at 20 ° C. is added. It is preferable to further mix an organic solvent having a solubility of 10 g / 100 ml or more and a boiling point of 50 to 200 ° C.

The HLB value is a value representing the degree of hydrophilicity or lipophilicity (hydrophobicity) of the surfactant, and the following formula:
HLB value = 20 × sum of formula weights of hydrophilic group portion / molecular weight of nonionic surfactant It can be determined by the Griffin method.

  Further, the reason why the water-based coating composition of the present invention has both excellent low temperature curability and good storage stability for a long time is not always clear, but the present inventors speculate as follows. That is, in the water-based coating composition of the present invention, the hydrophobic curing catalyst is treated with a nonionic surfactant having a predetermined HLB value (preferably, it is encapsulated in the micelle formed by the nonionic surfactant). Protected). As described above, in the hydrophobic curing catalyst protected by the nonionic surfactant, the molecular movement of the curing catalyst is suppressed at a temperature from room temperature to around 40 ° C. Therefore, in the hydrophobic curing catalyst, contact with the polyol and the polyisocyanate is suppressed, and the development of the catalytic function is suppressed. Therefore, the aqueous coating composition of the present invention is used for a long time at a temperature of 40 ° C. or lower. It is presumed that it can be stored, that is, it exhibits good storage stability for a long time. On the other hand, the hydrophobic curing catalyst protected by the nonionic surfactant is released from the protection by the nonionic surfactant at a temperature of 50 ° C. or higher (when micelles are formed, , The stability of the micelles is reduced and the micelles are disintegrated and released from the protection by the nonionic surfactant, so that the molecular movement becomes active. Therefore, the hydrophobic curing catalyst is activated by contacting with the polyol and the polyisocyanate, so that the aqueous coating composition of the present invention undergoes a curing reaction even at a low temperature (for example, 100 ° C.), resulting in excellent curing. It is presumed that it shows sex. In the present invention, such a property, that is, the property of suppressing the expression of the catalytic function at a certain temperature or lower and the catalytic function at a certain temperature or higher is referred to as “switching property”.

  According to the present invention, an isocyanate-curable type having excellent low-temperature curability (for example, curability at 100 ° C.) and good storage stability for a long time (for example, storage stability at 40 ° C. for 8 hours) is used. It becomes possible to obtain an aqueous coating composition.

1 is a graph showing ¹ H-NMR spectrum of a curing catalyst-containing surfactant aqueous solution prepared in Example 1. 4 is a graph showing ¹ H-NMR spectrum of a curing catalyst-containing surfactant aqueous solution prepared in Example 1 and Comparative Example 3.

  Hereinafter, the present invention will be described in detail with reference to its preferred embodiments.

  First, the water-based coating composition of the present invention will be described. The aqueous coating composition of the present invention comprises a polyol, a polyisocyanate, and an organometallic compound containing at least one metal atom selected from the group consisting of Sn, Bi, Zr, Ti and Al, and 20 ° C. It contains a hydrophobic curing catalyst having a solubility in water of 1 g / 100 ml or less under atmospheric pressure conditions, and a nonionic surfactant having an HLB value of 10 to 15 determined by the Griffin method. In such an aqueous coating composition of the present invention, at a temperature from room temperature to around 40 ° C., the hydrophobic curing catalyst is treated with the nonionic surfactant (preferably, the hydrophobic curing catalyst is treated with the nonionic surfactant). It has good storage stability for a long time because it is protected (by being encapsulated in micelles formed by a surfactant), and at a temperature of 50 ° C or higher (for example, 100 ° C), it has the above-mentioned properties. The hydrophobic curing catalyst is released from protection by the nonionic surfactant (when micelles are formed, the stability of the micelles is reduced and the micelles are disintegrated by the nonionic surfactant). Since it is released from protection), it exhibits excellent low temperature curability.

  The polyol is not particularly limited as long as it is a polymer polyol used as a base resin in an isocyanate-curable aqueous coating composition, and examples thereof include a hydroxyl group-containing acrylic resin (acrylic polyol), polyester resin (polyester polyol), Examples thereof include alkyd resin (alkyd polyol), epoxy resin (epoxy polyol), urethane resin (polyurethane polyol) and the like. These polyols may be used alone or in combination of two or more. Among these polyols, an acrylic resin (acrylic polyol), a polyester resin (polyester polyol), and a urethane resin (polyurethane polyol) containing a hydroxyl group are preferable from the viewpoint of ensuring sufficient coating film performance. Further, the content of such a polyol is preferably 20 to 90% by mass, and 30 to 85% by mass based on 100% by mass of the total amount of the polyol and the polyisocyanate, from the viewpoint of ensuring sufficient coating film performance. Mass% is more preferable, and 40-80 mass% is especially preferable.

  The polyisocyanate is not particularly limited as long as it is a polyisocyanate used as a curing agent in an isocyanate-curable aqueous coating composition, and examples thereof include an isocyanate compound, a blocked isocyanate compound, an isocyanate resin, and a blocked isocyanate resin. These polyisocyanates may be used alone or in combination of two or more. Among these polyisocyanates, isocyanate compounds are preferable from the viewpoint of ensuring sufficient coating film performance. Further, the content of such a polyisocyanate is preferably 10 to 80 mass% with respect to 100 mass% of the total amount of the polyol and the polyisocyanate, from the viewpoint of ensuring sufficient coating film performance. 70 mass% is more preferable, and 20 to 60 mass% is especially preferable.

  The hydrophobic curing catalyst used in the present invention has a solubility in water at 20 ° C. and atmospheric pressure of 1 g / 100 ml or less. By using such a hydrophobic curing catalyst, the curing catalyst is formed by the nonionic surfactant (preferably, the curing catalyst is formed by the nonionic surfactant described below) at a temperature from room temperature to around 40 ° C. It becomes easier to be protected (by being encapsulated in micelles), and the aqueous coating composition can be stored for a long time. On the other hand, when the solubility of the curing catalyst in water (at 20 ° C. under atmospheric pressure) exceeds the above upper limit, the hydrophilicity of the curing catalyst becomes high, and therefore the nonionic surfactant described later (preferably, curing catalyst (Because they are encapsulated in micelles formed by a nonionic surfactant described later), the curing catalyst is released from the protection and easily diffuses in the water solvent, and even at a temperature from room temperature to around 40 ° C. Due to the action, the reaction between the polyol and the polyisocyanate proceeds, and the storage stability for a long time decreases. Further, the hydrophobicity of the curing catalyst becomes high, and at temperatures ranging from room temperature to around 40 ° C., the curing catalyst is formed by a nonionic surfactant (preferably, the micelle formed by the nonionic surfactant described below is used). From the viewpoint that the water-based coating composition is more easily protected and the storage stability of the water-based coating composition is improved, the hydrophobic curing catalyst has a solubility in water (20 ° C., under atmospheric pressure) of 0. It is preferably 0.5 g / 100 ml or less.

  The hydrophobic curing catalyst used in the present invention is composed of an organometallic compound containing at least one metal atom selected from the group consisting of Sn, Bi, Zr, Ti and Al. By using the organometallic compound containing the metal atom as a curing catalyst, the reaction between the polyol and the polyisocyanate can proceed at a relatively low temperature (for example, 100 ° C.), and an isocyanate-curable aqueous coating composition can be obtained. It becomes possible to bake the resulting coating film at a relatively low temperature (for example, 100 ° C.).

  The carbon number of the organic chain constituting the organometallic compound is preferably 3 or more, more preferably 5 or more, and particularly preferably 8 or more. This improves the hydrophobicity of the curing catalyst (reduces the solubility in water) and improves the storage stability of the aqueous coating composition. On the other hand, when the carbon number of the organic chain is less than the lower limit, the hydrophobicity of the curing catalyst tends to decrease (solubility in water increases), and the storage stability of the aqueous coating composition tends to decrease. The upper limit of the number of carbon atoms in the organic chain is not particularly limited, but is preferably 20 or less from the viewpoint of ensuring the stability of the curing catalyst in the aqueous coating composition.

  Further, it is preferable that the organometallic compound contains an oxygen atom. As a result, the switchability is easily exhibited regardless of the carbon number of the organometallic compound.

  As such a hydrophobic curing catalyst, specifically, dibutyltin dilaurate (0.1 g / 100 ml or less), bismuth 2-ethylhexanoate (0.1 g / 100 ml or less), zirconium tetraacetylacetonate (0. 1 g / 100 ml or less), tetraoctyl titanate (0.1 g / 100 ml or less), titanium ethyl acetoacetate (0.1 g / 100 ml or less), aluminum trisacetylacetonate (0.1 g / 100 ml or less), etc. (parentheses) Solubility in water at 20 ° C and atmospheric pressure.

  The content of such a hydrophobic curing catalyst is 0.001 to 10% by mass based on the total amount of the aqueous coating composition from the viewpoint of achieving both storage stability of the aqueous coating composition and coating film performance. Is preferable, 0.01-5 mass% is more preferable, and 0.01-1.0 mass% is especially preferable.

  The nonionic surfactant used in the present invention has an HLB value of 10 to 15 determined by the Griffin method. The nonionic surfactant having such an HLB value protects the hydrophobic curing catalyst in an aqueous solution at a temperature from room temperature to around 40 ° C. (preferably, forming a micelle to form the hydrophobic curing catalyst. Can be protected by encapsulating), and the aqueous coating composition can be stored for a long time. Further, the nonionic surfactant releases the hydrophobic curing catalyst at a temperature of 50 ° C. or higher (for example, 100 ° C.) (in the case where micelles are formed, the stability of the micelle decreases. Since the hydrophobic curing catalyst is released by the disintegration of micelles), the water-based coating composition of the present invention exhibits excellent low temperature curing property. On the other hand, when the HLB value of the nonionic surfactant is less than the lower limit, the hydrophobic curing catalyst is formed by the nonionic surfactant (particularly, the hydrophobic curing catalyst is formed by the nonionic surfactant). They are not protected) because they are not incorporated into the micelles, which reduces the long-term storage stability of the aqueous coating composition. On the other hand, when the HLB value of the nonionic surfactant exceeds the above upper limit, the hydrophobic curing catalyst is not released even at a temperature of 50 ° C. or higher (for example, 100 ° C.) (in the case where micelles are formed, micelles Does not disintegrate stably, so the hydrophobic curing catalyst is not released), and the low temperature curability of the aqueous coating composition is reduced. Further, at a temperature from room temperature to around 40 ° C., it becomes easier to protect the hydrophobic curing catalyst in an aqueous solution (preferably, easier protection by forming micelles and encapsulating the hydrophobic curing catalyst in micelles). The storage stability of the water-based coating composition is improved, and the hydrophobic curing catalyst is released at a temperature of 50 ° C. or higher (for example, 100 ° C.) (if micelles are formed, the From the viewpoint that the low temperature curability of the water-based coating composition is improved by releasing the hydrophobic curing catalyst by decreasing stability and disintegrating micelles, the HLB value of the nonionic surfactant is Is preferably 12 to 14.5, more preferably 13 to 14.

  The molecular weight of the nonionic surfactant is preferably 100 to 5000, more preferably 300 to 1500. When the molecular weight of the nonionic surfactant is less than the lower limit, the micelles tend to disintegrate at temperatures from room temperature to around 40 ° C, while when it exceeds the upper limit, it is difficult to uniformly dissolve in water, Further, it tends to be hard to disintegrate at a temperature of 50 ° C. or higher (for example, 100 ° C.).

  As such a nonionic surfactant, specifically, polyoxyethylene-oleyl ether (for example, Nonion E-212 (HLB value: 13.3) manufactured by NOF CORPORATION, Nonion E-215 (HLB)). Value: 14.2)), polyoxyethylene-isodecyl ether (for example, NOF Corporation NONION ID-206 (HLB value: 12.5), NONION ID-209 (HLB value: 14.3)). , Polyoxyethylene-2-ethylhexyl-ether (for example, Nonion EH-208 (HLB value: 14.6) manufactured by NOF CORPORATION), polyoxyethylene-stearyl ether (for example, Nonion S manufactured by NOF CORPORATION). -215 (HLB value: 14.2)) and the like.

  The content of such a nonionic surfactant is not particularly limited as long as the content of the nonionic surfactant in the resulting aqueous coating composition is a critical micelle concentration or more, but the coating film From the viewpoint of ensuring the water resistance of 0.1 to 20% by mass, preferably 0.1 to 10% by mass, more preferably 0.1 to 5% by mass, based on the entire aqueous coating composition. ..

  In addition, the aqueous coating composition of the present invention may contain an organic solvent. As such an organic solvent, an organic solvent having a solubility in water at 20 ° C. of 10 g / 100 ml or more and a boiling point of 50 to 200 ° C. is preferable. By using an organic solvent having a solubility in water and a boiling point in the above range, it is possible to suppress the generation of bubbles when applying the aqueous coating composition, and even when the aqueous coating composition contains an organic solvent. At the same time, both excellent low temperature curability (for example, curability at 100 ° C.) and good long-term storage stability (for example, storage stability at 40 ° C. for 8 hours) are maintained. This is because the organic solvent having a solubility and boiling point in water in the above range is highly hydrophilic and has a low boiling point, and the organic solvent is difficult to be incorporated into the nonionic surfactant protecting the hydrophobic curing catalyst. , The curing catalyst is stably protected without being released from protection by the nonionic surfactant (in the case where the nonionic surfactant forms micelles, the hydrophobic curing catalyst is included). This is because the organic solvent is hard to be taken into the existing micelles, so that the curing catalyst is not released from the micelles and is stably protected).

  Examples of the organic solvent having the solubility in water and the boiling point in the above range include 1-propanol, 2-propanol, 1-butanol, 1-methoxy-2-propanol, and ethylene glycol monobutyl ether. Moreover, as content of such an organic solvent, 30 mass parts or less are preferable with respect to 100 mass parts of resin components in a water-based coating composition, 10 mass parts or less are more preferable, and 5 mass parts or less are still more preferable. ..

  In addition, the water-based coating composition of the present invention may contain conventionally known coloring pigments, bright pigments and the like in a conventionally known range, if necessary. Further, in order to adjust various physical properties, various additives such as a viscosity control agent, a surface modifier, a thickener, an antioxidant, an ultraviolet absorber and an antifoaming agent may be blended within a conventionally known range.

  In such an aqueous coating composition of the present invention, the reaction rate constant k (100 ° C.) at 100 ° C., which is determined by measuring the relative storage elastic modulus (Er ′) of the coating film, is preferably 0.09 or more. , 0.10 or more is more preferable. An aqueous coating composition having a reaction rate constant k (100 ° C.) in such a range is excellent in low temperature curability (for example, curability at 100 ° C.).

  Further, in the water-based coating composition of the present invention, the reaction rate constant k at 70 ° C. (70 ° C.) and the reaction rate constant k at 100 ° C. (100 ° C.), which are determined by measuring the relative storage elastic modulus (Er ′) of the coating film. The ratio [k (70 ° C.) / K (100 ° C.)] is preferably 0.3 or less, more preferably 0.2 or less. The aqueous coating composition having k (70 ° C.) / k (100 ° C.) in the above range has excellent storage stability.

  Furthermore, in the water-based coating composition of the present invention, the rate of increase in viscosity after standing for 8 hours at a temperature of 40 ° C. with respect to the initial viscosity is preferably 20% or less, and more preferably 10% or less. It is preferably 5% or less, and particularly preferably 5% or less. An aqueous coating composition having an increase rate of viscosity within the above range is excellent in long-term storage stability.

  The baking temperature (curing temperature) of the aqueous coating composition of the present invention is not particularly limited and is usually 40 to 200 ° C, preferably 80 to 160 ° C.

  Next, a method for producing the aqueous coating composition of the present invention will be described. The method for producing a water-based coating composition of the present invention comprises preparing a curing catalyst-containing surfactant aqueous solution by mixing the hydrophobic curing catalyst and the nonionic surfactant, and then preparing a polyol, a polyisocyanate and the curing catalyst. It is a method of obtaining the above-mentioned aqueous coating composition of the present invention by mixing with an aqueous surfactant-containing solution.

  In the method for producing a water-based coating composition of the present invention, first, the hydrophobic curing catalyst and the nonionic surfactant are mixed to prepare a curing catalyst-containing surfactant aqueous solution. As a result, the hydrophobic curing catalyst is, at a temperature from room temperature to about 40 ° C., surrounded by the nonionic surfactant (preferably by being encapsulated in the micelle formed by the nonionic surfactant). ) Being protected, the resulting aqueous coating composition has excellent long-term storage stability. Further, at a temperature of 50 ° C. or higher (for example, 100 ° C.), the hydrophobic curing catalyst protected by the nonionic surfactant is released, so that the obtained aqueous coating composition has excellent low temperature curability. (For example, curability at 100 ° C.) is shown.

  When preparing the aqueous solution of the surfactant containing the curing catalyst, it is preferable to mix the hydrophobic curing catalyst and the nonionic surfactant, and then heat the resulting mixture at 30 to 60 ° C. for 8 hours or more. .. This ensures that the hydrophobic curing catalyst is included in the micelle formed by the nonionic surfactant.

  Next, the curing catalyst-containing surfactant aqueous solution thus prepared is mixed with the polyol and the polyisocyanate. Thereby, the hydrophobic curing catalyst protected by the polyol, polyisocyanate, and the nonionic surfactant (preferably, the hydrophobic curing catalyst included in the micelle formed by the nonionic surfactant). ) And an aqueous coating composition of the present invention containing

  Hereinafter, the present invention will be described more specifically based on Examples and Comparative Examples, but the present invention is not limited to the following Examples. The polyols used in Examples and Comparative Examples were synthesized by the following method.

(Synthesis example 1)
First, butyl acrylate 99.0 parts by mass, butyl methacrylate 85.5 parts by mass, styrene 45.0 parts by mass, 2-hydroxyethyl acrylate 162.0 parts by mass, acrylic acid 13.5 parts by mass, methyl methacrylate. 45.0 parts by mass, n-dodecyl mercaptan as a chain transfer agent 4.5 parts by mass, ion-exchanged water 170 parts by mass, and polyoxyalkylene alkenyl ether ammonium sulfate as an emulsion polymerization agent ("Latemul PD-104" manufactured by Kao Corporation). 25.0 parts by mass were mixed and stirred with a mixer to emulsify to prepare a monomer pre-emulsion.

  Then, 320 parts by mass of ion-exchanged water and polyoxyalkylene alkenyl ether as an emulsion polymerization agent were added to a usual reaction vessel for producing an acrylic resin emulsion, which was equipped with a stirrer, a thermometer, a dropping funnel, a reflux condenser, a nitrogen introducing tube, and the like. 5.0 parts by mass of ammonium sulfate (“Latemuru PD-104” manufactured by Kao Co., Ltd.), and 1.0 part by mass of ammonium persulfate aqueous solution (ammonium persulfate (APS, manufactured by Aldrich) and 10.0 parts by mass of deionized water) as a polymerization initiator. (Mixture of parts) and the temperature was raised to 80 ° C. with stirring. Next, 5% by mass of the monomer pre-emulsion was added to this reaction container and kept at 80 ° C. for 10 minutes. Then, the remaining monomer pre-emulsion was added dropwise to the above reaction vessel with stirring over 3 hours. After completion of dropping, stirring was continued at 80 ° C. for 1 hour for further reaction. After that, 500 parts by mass of ion-exchanged water was added and cooled to room temperature. After cooling, 33.4 parts by mass of a 50% by mass dimethylethanolamine aqueous solution was added and stirred for 10 minutes to prepare a hydroxyl group-containing acrylic emulsion (polyacrylic polyol emulsion).

<HLB value>
The HLB values of the nonionic surfactants used in Examples and Comparative Examples have the following formula:
HLB value = 20 × sum of formula weight of hydrophilic group portion / molecular weight of nonionic surfactant.

(Example 1)
First, a container was charged with 80 parts by mass of ion-exchanged water and 20 parts by mass of a nonionic surfactant having an HLB value of 13.3 (“Nonion E-212” manufactured by NOF CORPORATION), and the nonionic surfactant was charged. Was dissolved, 2 parts by mass of dibutyltin dilaurate (DBTL, manufactured by Tokyo Chemical Industry Co., Ltd.) was added as a curing catalyst, and the mixture was stirred at 60 ° C. for 8 hours to prepare a curing catalyst-containing surfactant aqueous solution S-1. did. The solubility of dibutyltin dilaurate in water at 20 ° C. and atmospheric pressure was 0.1 g / 100 ml or less.

  Next, 33.0 parts by mass of the polyacrylic polyol emulsion obtained in Synthesis Example 1, 10.5 parts by mass of polyisocyanate (“Bernock DNW5500” manufactured by DIC Corporation), and 10.5 parts by mass of ion-exchanged water were mixed. To the obtained aqueous solution, 3.0 parts by mass of the curing catalyst-containing surfactant aqueous solution S-1 was added to prepare an aqueous coating composition (curing catalyst content: 0.10% by mass).

(Example 2)
Curing catalyst in the same manner as in Example 1 except that 20 parts by mass of a nonionic surfactant having an HLB value of 14.2 (“Nonion E-215” manufactured by NOF CORPORATION) was used as the nonionic surfactant. Example 1 except that a containing surfactant aqueous solution S-2 was prepared and 3.0 parts by mass of this curing catalyst containing surfactant aqueous solution S-2 was added instead of the curing catalyst containing surfactant aqueous solution S-1. An aqueous coating composition (curing catalyst content: 0.10% by mass) was prepared in the same manner as in.

(Example 3)
Curing catalyst in the same manner as in Example 1 except that 20 parts by mass of a nonionic surfactant having an HLB value of 12.5 (“Nonion ID-206” manufactured by NOF CORPORATION) was used as the nonionic surfactant. Example 1 except that the containing surfactant aqueous solution S-3 was prepared and 3.0 parts by mass of this curing catalyst containing surfactant aqueous solution S-3 was added instead of the curing catalyst containing surfactant aqueous solution S-1. An aqueous coating composition (curing catalyst content: 0.10% by mass) was prepared in the same manner as in.

(Example 4)
Curing catalyst in the same manner as in Example 1 except that 20 parts by mass of a nonionic surfactant having an HLB value of 14.3 (“Nonion ID-209” manufactured by NOF CORPORATION) was used as the nonionic surfactant. Example 1 except that a containing surfactant aqueous solution S-4 was prepared and 3.0 parts by mass of this curing catalyst containing surfactant aqueous solution S-4 was added in place of the curing catalyst containing surfactant aqueous solution S-1. An aqueous coating composition (curing catalyst content: 0.10% by mass) was prepared in the same manner as in.

(Example 5)
Curing catalyst in the same manner as in Example 1 except that 20 parts by mass of a nonionic surfactant having an HLB value of 14.6 (“Nonion EH-208” manufactured by NOF CORPORATION) was used as the nonionic surfactant. Example 1 except that a containing surfactant aqueous solution S-5 was prepared and 3.0 parts by mass of this curing catalyst containing surfactant aqueous solution S-5 was added instead of the curing catalyst containing surfactant aqueous solution S-1. An aqueous coating composition (curing catalyst content: 0.10% by mass) was prepared in the same manner as in.

(Example 6)
A curing catalyst-containing surfactant aqueous solution S-6 was prepared in the same manner as in Example 1 except that 2 parts by mass of bismuth 2-ethylhexanoate mineral spirit solution (2EHBi, manufactured by Wako Pure Chemical Industries, Ltd.) was used as a curing catalyst. In the same manner as in Example 1 except that 12.0 parts by mass of the curing catalyst-containing surfactant aqueous solution S-6 was added instead of the curing catalyst-containing surfactant aqueous solution S-1, the aqueous coating composition (curing catalyst) was added. Content: 0.36% by mass) was prepared. The bismuth 2-ethylhexanoate mineral spirit solution had a solubility in water at 20 ° C. and atmospheric pressure of 0.1 g / 100 ml or less.

(Example 7)
A curing catalyst-containing surfactant aqueous solution was prepared in the same manner as in Example 1 except that 4 parts by mass of zirconium tetraacetylacetonate (Zr (acac) ₄ , "Organix ZC-700" manufactured by Matsumoto Fine Chemical Co., Ltd.) was used as a curing catalyst. Aqueous solution was prepared in the same manner as in Example 1 except that S-7 was prepared and 12.0 parts by mass of this curing catalyst-containing surfactant aqueous solution S-7 was added instead of the curing catalyst-containing surfactant aqueous solution S-1. A coating composition (curing catalyst content: 0.70% by mass) was prepared. The solubility of zirconium tetraacetylacetonate in water at 20 ° C. and atmospheric pressure was 0.1 g / 100 ml or less.

(Comparative Example 1)
Curing catalyst in the same manner as in Example 1 except that 20 parts by mass of a nonionic surfactant having an HLB value of 4.9 (“Nonion E-202” manufactured by NOF CORPORATION) was used as the nonionic surfactant. Example 1 except that the containing surfactant aqueous solution CS-1 was prepared and 3.0 parts by mass of this curing catalyst containing surfactant aqueous solution CS-1 was added instead of the curing catalyst containing surfactant aqueous solution S-1. An aqueous coating composition (curing catalyst content: 0.10% by mass) was prepared in the same manner as in.

(Comparative example 2)
Curing catalyst in the same manner as in Example 1 except that 20 parts by mass of a nonionic surfactant having an HLB value of 9.0 (“Nonion E-205” manufactured by NOF CORPORATION) was used as the nonionic surfactant. Example 1 except that a surfactant-containing aqueous surfactant solution CS-2 was prepared and 3.0 parts by mass of this curing-catalyst-containing surfactant aqueous solution CS-2 was added instead of the curing catalyst-containing surfactant aqueous solution S-1. An aqueous coating composition (curing catalyst content: 0.10% by mass) was prepared in the same manner as in.

(Comparative example 3)
Curing catalyst in the same manner as in Example 1 except that 20 parts by mass of a nonionic surfactant having an HLB value of 16.6 (“NONION E-230” manufactured by NOF CORPORATION) was used as the nonionic surfactant. Example 1 except that the containing surfactant aqueous solution CS-3 was prepared and 3.0 parts by mass of this curing catalyst containing surfactant aqueous solution CS-3 was added in place of the curing catalyst containing surfactant aqueous solution S-1. An aqueous coating composition (curing catalyst content: 0.10% by mass) was prepared in the same manner as in.

(Comparative example 4)
Curing catalyst as in Example 6 except that 20 parts by mass of a nonionic surfactant having an HLB value of 9.0 (“Nonion E-205” manufactured by NOF CORPORATION) was used as the nonionic surfactant. Example 1 except that the containing surfactant aqueous solution CS-4 was prepared and 12.0 parts by mass of this curing catalyst containing surfactant aqueous solution CS-4 was added instead of the curing catalyst containing surfactant aqueous solution S-1. An aqueous coating composition (curing catalyst content: 0.36% by mass) was prepared in the same manner as in.

(Comparative example 5)
Curing catalyst in the same manner as in Example 6 except that 20 parts by mass of a nonionic surfactant having an HLB value of 16.6 (“NONION E-230” manufactured by NOF CORPORATION) was used as the nonionic surfactant. Example 1 except that a containing surfactant aqueous solution CS-5 was prepared and 12.0 parts by mass of this curing catalyst containing surfactant aqueous solution CS-5 was added instead of the curing catalyst containing surfactant aqueous solution S-1. An aqueous coating composition (curing catalyst content: 0.36% by mass) was prepared in the same manner as in.

(Comparative example 6)
Curing catalyst in the same manner as in Example 7 except that 20 parts by mass of a nonionic surfactant having an HLB value of 9.0 (“Nonion E-205” manufactured by NOF CORPORATION) was used as the nonionic surfactant. Example 1 except that the containing surfactant aqueous solution CS-6 was prepared and 12.0 parts by mass of this curing catalyst containing surfactant aqueous solution CS-6 was added in place of the curing catalyst containing surfactant aqueous solution S-1. An aqueous coating composition (curing catalyst content: 0.70% by mass) was prepared in the same manner as in (1).

(Comparative Example 7)
Curing catalyst in the same manner as in Example 7 except that 20 parts by mass of a nonionic surfactant having an HLB value of 16.6 (“Nonion E-230” manufactured by NOF CORPORATION) was used as the nonionic surfactant. Example 1 except that a containing surfactant aqueous solution CS-7 was prepared and 12.0 parts by mass of this curing catalyst containing surfactant aqueous solution CS-7 was added instead of the curing catalyst containing surfactant aqueous solution S-1. An aqueous coating composition (curing catalyst content: 0.70% by mass) was prepared in the same manner as in (1).

(Comparative Example 8)
A curing catalyst-containing surfactant aqueous solution CS-8 was prepared in the same manner as in Example 1 except that 2 parts by mass of diazabicycloundecene p-toluenesulfonate (DBU salt, manufactured by Aldrich) was used as a curing catalyst. In the same manner as in Example 1 except that 3.0 parts by mass of the curing catalyst-containing surfactant aqueous solution CS-8 was added in place of the curing catalyst-containing surfactant aqueous solution S-1, the aqueous coating composition (curing catalyst) was added. Content of 0.10% by mass) was prepared. The solubility of diazabicycloundecene p-toluenesulfonate in water at 20 ° C. and atmospheric pressure was 0.5 g / 100 ml or more.

(Comparative Example 9)
A curing catalyst-containing surfactant aqueous solution CS-9 was prepared in the same manner as in Example 1 except that 2 parts by mass of diazabicycloundecene (DBU, manufactured by Tokyo Kasei Kogyo Co., Ltd.) was used as a curing catalyst. An aqueous coating composition (content of the curing catalyst: in the same manner as in Example 1) except that 3.0 parts by mass of this curing catalyst-containing surfactant aqueous solution CS-9 was added instead of the surfactant aqueous solution S-1. 0.10 mass%) was prepared. The solubility of diazabicycloundecene in water at 20 ° C. and atmospheric pressure was 0.5 g / 100 ml or more.

(Comparative Example 10)
A curing catalyst-containing surfactant aqueous solution CS-10 was prepared in the same manner as in Example 1 except that 2 parts by mass of diazabicyclononene (DBN, manufactured by Tokyo Chemical Industry Co., Ltd.) was used as a curing catalyst, and the curing catalyst-containing interface was prepared. An aqueous coating composition (curing catalyst content: 0 was added in the same manner as in Example 1 except that 3.0 parts by mass of this curing catalyst-containing surfactant aqueous solution CS-10 was added instead of the activator aqueous solution S-1. 10% by mass) was prepared. The solubility of diazabicyclononene in water at 20 ° C. and atmospheric pressure was 0.5 g / 100 ml or more.

(Comparative Example 11)
33.0 parts by mass of the polyacrylic polyol emulsion obtained in Synthesis Example 1, 10.5 parts by mass of polyisocyanate (“Bernock DNW5500” manufactured by DIC Corporation), and 10.5 parts by mass of acetone were mixed. To the resulting solution was added 0.06 parts by mass of dibutyltin dilaurate (DBTL, manufactured by Tokyo Chemical Industry Co., Ltd.) as a curing catalyst to give an aqueous coating composition (content of the curing catalyst: 0.10% by mass). Prepared.

(Comparative Example 12)
33.0 parts by mass of the polyacrylic polyol emulsion obtained in Synthesis Example 1, 10.5 parts by mass of polyisocyanate (“Bernock DNW5500” manufactured by DIC Corporation), and 10.5 parts by mass of ion-exchanged water are mixed to prepare an aqueous coating composition. A product (content of curing catalyst: 0% by mass) was prepared.

(Example 8)
Instead of 10.5 parts by mass of ion-exchanged water, 0.1 parts by mass of 1-methoxy-2-propanol (PGME, solubility in water (20 ° C.): 10 g / 100 ml or more, boiling point: 120 ° C.) and ion-exchanged water 10 Aqueous coating composition (content of curing catalyst: 0.10% by mass, content of organic solvent: 5 parts by mass relative to 100 parts by mass of resin content) in the same manner as in Example 1 except that 4 parts by mass was used. ) Was prepared.

(Example 9)
2-propanol (IPA, solubility in water (20 ° C.): 10 g / 100 ml or more, boiling point: 83 ° C.) 0.2 parts by mass and ion exchanged water 10.3 parts by mass instead of ion-exchanged water 10.5 parts by mass An aqueous coating composition (content of curing catalyst: 0.10% by mass, content of organic solvent: 10 parts by mass with respect to 100 parts by mass of resin content) was prepared in the same manner as in Example 1 except that was used. ..

(Example 10)
Instead of 10.5 parts by mass of ion-exchanged water, 0.2 parts by mass of 1-propanol (PrOH, solubility in water (20 ° C.): 10 g / 100 ml or more, boiling point: 97 ° C.) and 10.3 parts by mass of ion-exchanged water. An aqueous coating composition (content of curing catalyst: 0.10% by mass, content of organic solvent: 10 parts by mass with respect to 100 parts by mass of resin content) was prepared in the same manner as in Example 1 except that was used. ..

(Example 11)
Instead of 10.5 parts by mass of ion-exchanged water, 0.2 parts by mass of 1-butanol (BuOH, solubility in water (20 ° C.): 10 g / 100 ml, boiling point: 118 ° C.) and 10.3 parts by mass of ion-exchanged water were added. An aqueous coating composition (content of curing catalyst: 0.10% by mass, content of organic solvent: 10 parts by mass relative to 100 parts by mass of resin content) was prepared in the same manner as in Example 1 except that the components were used.

(Example 12)
Instead of 10.5 parts by mass of ion-exchanged water, 0.2 parts by mass of 1-methoxy-2-propanol (PGME, solubility in water (20 ° C.): 10 g / 100 ml or more, boiling point: 120 ° C.) and ion-exchanged water 10 Aqueous coating composition (content of curing catalyst: 0.10% by mass, content of organic solvent: 10 parts by mass relative to 100 parts by mass of resin content) in the same manner as in Example 1 except that 3 parts by mass was used. ) Was prepared.

(Example 13)
Instead of 10.5 parts by mass of ion-exchanged water, 0.2 parts by mass of ethylene glycol monobutyl ether (EGBE, solubility in water (20 ° C.): 10 g / 100 ml or more, boiling point: 171 ° C.) and 10.3 parts by mass of ion-exchanged water An aqueous coating composition (content of curing catalyst: 0.10% by mass, content of organic solvent: 10 parts by mass relative to 100 parts by mass of resin content) was prepared in the same manner as in Example 1 except that parts were used. did.

(Example 14)
Instead of 10.5 parts by mass of ion-exchanged water, 0.4 parts by mass of 1-methoxy-2-propanol (PGME, solubility in water (20 ° C.): 10 g / 100 ml or more, boiling point: 120 ° C.) and 10 parts of ion-exchanged water Aqueous coating composition (content of curing catalyst: 0.10% by mass, content of organic solvent: 20 parts by mass per 100 parts by mass of resin content) in the same manner as in Example 1 except that 1 part by mass was used. ) Was prepared.

(Example 15)
Instead of 10.5 parts by mass of ion-exchanged water, 0.6 parts by mass of 1-methoxy-2-propanol (PGME, solubility in water (20 ° C.): 10 g / 100 ml or more, boiling point: 120 ° C.) and ion-exchanged water 9 Aqueous coating composition (content of curing catalyst: 0.10% by mass, content of organic solvent: 30 parts by mass per 100 parts by mass of resin content) in the same manner as in Example 1 except that 9 parts by mass was used. ) Was prepared.

<Curing behavior of water-based coating composition>
The obtained aqueous coating composition was applied onto a stainless steel plate (40 mm × 50 mm, thickness 0.5 mm) so that the film thickness after baking would be 35 ± 5 μm. Specifically, the stainless steel plate was placed on a horizontal table, and an adhesive tape having a thickness of 70 μm was attached to each region of about 5 mm from the edges of the two opposite sides of the stainless steel plate. In the area surrounded by the adhesive tape, after dropping an appropriate amount of the water-based coating composition, slide a knife with a straight blade edge on the adhesive tape to form the water-based coating composition in the gap between the stainless steel plate and the blade edge of the knife. I smeared it in.

  Rigid body pendulum type physical property tester equipped with an annular pendulum having a diameter of 74 mm with a knife edge having a blade edge angle of 40 ° after 7 ± 1 minutes from forming a coating film made of the aqueous coating composition on the stainless steel plate. (Using "RPT-5000 type" manufactured by A & D Co., Ltd., the temperature rises from room temperature (25 ° C) to the baking temperature (70 ° C, 100 ° C, or 140 ° C) at a rate of 20 ± 4 ° C / min. The relative storage elastic modulus (Er ′) of the coating film was measured by a temperature program of heating and then maintaining at a baking temperature (70 ° C., 100 ° C., or 140 ° C.) from the following inflection point. It continued until 15 minutes or more passed.

The measured value of the obtained relative storage elastic modulus (Er ′) was plotted against time, and the point at which the curve convex downward was changed to the curve convex upward (hereinafter, this changed point is referred to as “ 15 minutes from the "inflection point"), the following formula:
Er '= A _{[1-exp {k (t-} t d)} ]
(In the formula, A is a constant, k is a reaction rate constant, t is time, and t _d is reaction start time.)
Was applied and the reaction rate constant k was determined by the non-linear least squares method. The results are shown in Tables 1 to 4. The larger the value of k is, the more the curing is progressing.

<Storage stability of aqueous coating composition>
The obtained water-based coating composition was allowed to stand in an oven set at 40 ° C. for 8 hours, and the viscosity of the water-based coating composition before and after the standing was measured by a viscoelasticity measuring device (“ARES Instruments” “ARES”). -G2 rheometer), temperature: 25.0 ± 0.1 ° C., shear rate: 1000 s ⁻¹ , geometry: cone plate having a diameter of 25 mm, angle 0.04 rad, gap: 50 μm. The results are shown in Tables 1 to 4.

  As shown in Tables 1 to 3, a hydrophobic curing catalyst having a solubility in water (at 20 ° C. under atmospheric pressure) of 1 g / 100 ml or less and a nonionic surfactant having an HLB value of 10 to 15. It was confirmed that the isocyanate-curable water-based coating composition containing and has both high curability at 100 ° C. and high storage stability at 40 ° C. (Examples 1 to 7). This is because at 40 ° C., the hydrophobic curing catalyst was taken in and protected by the micelle formed by the nonionic surfactant, and the catalytic function was not exhibited, whereas at 100 ° C. It is considered that the hydrophobic curing catalyst was released due to the disintegration and the catalytic function was exhibited (the switchability was exhibited).

  On the other hand, it was found that when the HLB value of the nonionic surfactant was 9 or less, the storage stability at 40 ° C was lowered (Comparative Examples 1-2, 4, 6). It is considered that this is because the hydrophobic curing catalyst was not incorporated into the micelle and the switchability was not exhibited. Further, it was found that when the HLB value of the nonionic surfactant was 16.6, the curability at 100 ° C. decreased (Comparative Examples 3, 5, and 7). It is considered that this is because the hydrophobic curing catalyst was not released from the micelles even when the temperature reached 100 ° C. and the catalytic function was not exhibited.

  It was also found that the storage stability at 40 ° C. was lowered even when the amine-based curing catalyst was used (Comparative Examples 8 to 10). It is considered that this is because the amine-based curing catalyst, which has a high solubility in water, was distributed not only in the micelles but also in the water solvent, and the switchability was not exhibited.

  Further, it was found that the storage stability at 40 ° C. was lowered even when no surfactant was used (Comparative Example 11). It is considered that this is because micelles were not formed and the switchability was not expressed due to the absence of the surfactant.

  Further, when neither the curing catalyst nor the surfactant is contained, the storage stability at 40 ° C. is secured, but the curability at 100 ° C. is low, and 100% of the aqueous coating composition obtained in Example 1 is obtained. It was found that it was necessary to bake at 140 ° C. in order to obtain curability equivalent to that at 0 ° C. (Comparative Example 12).

  Furthermore, as shown in Table 4, even in an isocyanate-curable aqueous coating composition containing an organic solvent having a solubility in water (20 ° C.) of 10 g / 100 ml or more and a boiling point of 50 to 200 ° C., high at 100 ° C. It was confirmed that both curability and high storage stability at 40 ° C. were compatible (Examples 8 to 15). This is because the highly hydrophilic organic solvent is hard to be taken into the micelle formed by the nonionic surfactant, so that the hydrophobic curing catalyst is not released from the micelle and is stably protected in the micelle. It is thought to be a tame.

<NMR Measurement of Curing Catalyst-Containing Surfactant Aqueous Solution>
¹ H-NMR spectra of the curing catalyst-containing surfactant aqueous solution S-1 prepared in Example 1 and the curing catalyst-containing surfactant aqueous solution CS-3 prepared in Comparative Example 3 were measured. Specifically, an aqueous solution of a surfactant containing a curing catalyst is put into an NMR sample tube (diameter: 5 mm) for ordinary measurement, and a Fourier transform nuclear magnetic resonance (NMR) device (“JNM-ECA500” manufactured by JEOL Ltd.) is used. The ¹ H-NMR spectrum was measured using the curing catalyst-containing surfactant aqueous solution S-1 in the temperature range from room temperature to 90 ° C and the curing catalyst-containing surfactant aqueous solution CS-3 at 50 ° C. The results are shown in FIGS. 1 and 2.

As shown in FIG. 1, in the ¹ H-NMR spectrum of the curing catalyst-containing surfactant aqueous solution S-1, the peak derived from DBTL was unclear at room temperature to 40 ° C. It was found that the temperature became clear at 50 ° C or higher. This result indicates that at room temperature to 40 ° C., DBTL was encapsulated in and protected by micelles formed by a nonionic surfactant, and the molecular motion was suppressed, while the temperature was 50 ° C. or higher. Then, the micelles collapsed, the protection of DBTL was released, and the molecular motion of DBTL became active. Therefore, in the aqueous coating composition of the present invention containing a nonionic surfactant having an HLB value within a predetermined range, at a temperature of 40 ° C. or lower, the nonionic surfactant suppresses the molecular motion of the curing catalyst. Since the contact between the polyol and polyisocyanate and the curing catalyst is suppressed, the curing reaction does not proceed, whereas at a temperature of 50 ° C. or higher, the curing catalyst is released from protection by the nonionic surfactant. It is considered that the curing reaction proceeds even at 100 ° C. because the molecular motion becomes active and the polyol and polyisocyanate come into contact with the molecules to be activated.

On the other hand, as shown in FIG. 2, in the ¹ H-NMR spectrum of the curing catalyst-containing surfactant aqueous solution CS-3, the peak derived from DBTL was unclear even at a temperature of 50 ° C. This result shows that even when the temperature reaches 50 ° C., the protection of DBTL encapsulated in the micelle formed by the nonionic surfactant is not released and the molecular motion is suppressed. Therefore, in an aqueous coating composition containing a nonionic surfactant having an HLB value higher than a predetermined value, the curing catalyst is not released from protection by the nonionic surfactant even when the temperature reaches 50 ° C. Since the molecular motion is suppressed and the contact between the polyol and polyisocyanate and the curing catalyst is suppressed, it is considered that the curing reaction does not proceed.

  As described above, according to the present invention, a hydrophobic curing catalyst containing a hydrophobic curing catalyst protected by a nonionic surfactant (preferably by being included in a micelle formed by the nonionic surfactant) is contained. It is possible to obtain an isocyanate-curable water-based coating composition.

  Therefore, the water-based coating composition of the present invention has both excellent low-temperature curability and good storage stability for a long time, so that it can be used for vehicle bodies such as passenger cars, trucks, buses and motorcycles, and parts thereof. It is useful as a coating composition used for coating.

Claims (7)

  Polyol, polyisocyanate, organometallic compound containing at least one metal atom selected from the group consisting of Sn, Bi, Zr, Ti and Al, and solubility in water at 20 ° C. and atmospheric pressure Of 1 g / 100 ml or less, and a nonionic surfactant having an HLB value of 10 to 15 as determined by Griffin's method.   The aqueous coating composition according to claim 1, wherein the hydrophobic curing catalyst is included in micelles formed by the nonionic surfactant.   The aqueous coating composition according to claim 1 or 2, which does not contain an organic solvent.   The aqueous coating composition according to claim 1 or 2, which contains an organic solvent having a solubility in water at 20 ° C of 10 g / 100 ml or more and a boiling point of 50 to 200 ° C.   It is composed of an organometallic compound containing at least one metal atom selected from the group consisting of Sn, Bi, Zr, Ti and Al, and has a solubility in water at 20 ° C. and atmospheric pressure of 1 g / 100 ml or less. A hydrophobic curing catalyst and a nonionic surfactant having an HLB value of 10 to 15 determined by the Griffin method are mixed to prepare a curing catalyst-containing surfactant aqueous solution, and then the polyol, polyisocyanate, and the curing are performed. A method for producing an aqueous coating composition, which comprises mixing with a catalyst-containing surfactant aqueous solution.   The method for producing an aqueous coating composition according to claim 5, wherein an organic solvent is not mixed when the polyol, the polyisocyanate, and the curing catalyst-containing surfactant aqueous solution are mixed.   When mixing the polyol, the polyisocyanate, and the curing catalyst-containing surfactant aqueous solution, further mixing an organic solvent having a solubility in water at 20 ° C. of 10 g / 100 ml or more and a boiling point of 50 to 200 ° C. The method for producing the aqueous coating composition according to claim 5, which is characterized in that.