Aqueous coating composition and multilayer coating film forming method

Abstract

To provide an aqueous coating composition capable of exhibiting superior coating film appearance, coating film drying performance, and coating film strength, and a multilayer coating film forming method.SOLUTION: An aqueous coating composition comprising water, a resin, at least one of a lustrous pigment and a coloring pigment, and an extender pigment, wherein the extender pigment has a particle diameter D50 of 0.5 μm or more and a density of 4.2 g / cm3 or more, the resin is at least one selected from an acrylic resin and a urethane resin, and the content of the extender pigment in the aqueous coating composition is 0.1 mass% or more and less than 25 mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a water-based coating composition and a method for forming a multi-layer coating film. [Background technology]

[0002] Traditionally, organic solvent-based paints have generally been used to paint plastic materials (see, for example, Patent Document 1). Organic solvent-based paints contain organic solvents that etch the surface of plastic materials, and the coating film they form adheres well to the etched surface due to its anchoring effect, making them suitable as paints for plastics. However, the organic solvents (hereinafter also referred to as VOCs) in paints are released into the atmosphere during painting or drying, causing air pollution and becoming a major environmental problem. Along with the trend toward stricter VOC regulations, various industries are actively moving toward solvent-free (VOC-free) paints.

[0003] Therefore, in recent years, the paint industry has been shifting from organic solvent-based paints to water-based paints that contain little or no VOCs, against the backdrop of these global environmental issues. Also, in the field of automotive interiors, the introduction of water-based paints is being promoted, taking into consideration the effects on the human body of organic solvents released from the coatings and adhesives applied to interior materials in rooms that tend to be enclosed spaces, as well as the impact on people who suffer from chemical sensitivity.

[0004] Plastic materials are primarily used in automotive interior parts, home appliances, and other products, but water-based paints do not contain sufficient organic solvents that etch the surfaces of plastic materials as described above, and therefore tend to have lower adhesion to plastic surfaces than organic solvent-based paints.

[0005] For example, Patent Document 2 discloses an aqueous coating composition, which is a glitter pigment dispersion containing water, a scaly aluminum pigment, and a cellulose-based viscosity modifier, and a method for forming a multilayer coating film using the glitter pigment dispersion.

[0006] Patent Document 3 discloses an aqueous multi-component coating composition comprising a base component (I) and a curing agent (II), wherein the base component (I) contains a hydroxyl-containing resin (A), which contains a colloidal dispersion-type acrylic resin (A1) and a water-soluble acrylic resin (A2), and wherein the colloidal dispersion-type acrylic resin (A1) accounts for 50% by mass or more of the total resin solids contained in the base component (I). It is disclosed that this aqueous multi-component coating composition provides a coating film that is suitable for large substrates, with excellent drying properties, paint handling, storage properties, weather resistance, and gloss finish. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 2018-069185 [Patent Document 2] International Publication No. 2017 / 175468 [Patent Document 3] Patent Publication No. 2021-130812 Summary of the Invention [Problem to be solved by the invention]

[0008] However, the invention described in Patent Document 2 is limited to metallic water-based paints containing a cellulose-based viscosity modifier. Also, the invention described in Patent Document 3 improves the coating appearance and coating workability of a two-component water-based paint composition, but does not propose solutions to the problems associated with one-component water-based paint compositions. In aqueous coating compositions, water is the main solvent and has low volatility, which means that the liquid tends to drip and the workability during application is poor. Furthermore, when water is the main solvent, the solid content during application tends to be low, which can result in poor drying of the coating film. As described above, conventional aqueous coating compositions and coating film forming methods can result in coating films that dry poorly. In such cases, the appearance and strength of the coating film after application are not fully satisfactory, and there is still room for improvement. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a water-based coating composition and a method for forming a multi-layer coating film that are excellent in coating film appearance, coating film drying properties, and coating film strength. [Means for solving the problem]

[0009] The present invention relates to a water-based paint composition containing water, a resin, at least one of a bright pigment and a color pigment, and an extender pigment, wherein the resin is at least one of an acrylic resin and a urethane resin, and the extender pigment has a particle size D50 of 0.5 μm or more and a density of 4.2 g / cm 3 The water-based paint composition has the above-mentioned properties, and the content of the extender pigment in the water-based paint composition is 0.1% by mass or more and less than 25% by mass.

[0010] The oil absorption of the extender pigment is preferably 21 ml / g or less.

[0011] The extender pigment is preferably barium sulfate.

[0012] The aqueous coating composition further contains a thixotropic agent, and when the viscosity of the aqueous coating composition is adjusted to 430 mPa s at a measurement temperature of 25°C, the solid content of the thixotropic agent in the aqueous coating composition is preferably 0.1% by mass or more and 1.5% by mass or less.

[0013] When the viscosity of the aqueous coating composition is adjusted to 430 mPa·s at a measurement temperature of 25°C, the solids concentration of the aqueous coating composition is preferably 5% by mass or more and 50% by mass or less.

[0014] The method for forming a multilayer coating film of the present invention comprises a base coating film forming step of applying the aqueous coating composition of the present invention onto an object to be coated to form a wet base coating film, a topcoat coating film forming step of applying a topcoat coating composition onto the obtained wet base coating film to form a wet topcoat coating film, and a drying step of simultaneously drying the obtained wet base coating film and wet topcoat coating film to form a multilayer coating film. [Effects of the Invention]

[0015] According to the present invention, it is possible to obtain a water-based coating composition and a method for forming a multi-layer coating film that are excellent in coating film appearance, coating film drying properties, and coating film strength. DETAILED DESCRIPTION OF THE INVENTION

[0016] The water-based coating composition of the present invention will be described in detail below.

[0017] [Water-based paint composition] The water-based paint composition of the present invention is a water-based paint composition containing water, a resin, at least one of a bright pigment and a color pigment, and an extender pigment, wherein the extender pigment has a particle size D50 of 0.5 μm or more and a density of 4.2 g / cm 3 The water-based paint composition is as described above, wherein the resin is at least one of an acrylic resin and a urethane resin, and the content of the extender pigment in the water-based paint composition is 0.1% by mass or more and less than 25% by mass. First, the components of the water-based coating composition will be described below.

[0018] <Water> In this specification, the term "water-based coating composition" refers to a coating composition containing water as the main solvent. By "main solvent" it is meant that the proportion of the solvent used, i.e. water, is at least 50% by mass. The water used in the aqueous coating composition of the present invention is not particularly limited, and examples thereof include pure water such as ion-exchanged water and distilled water, groundwater, tap water, industrial water, etc. Furthermore, when the aqueous coating composition is to be stored for a long period of time, water that has been sterilized by ultraviolet irradiation or the like may be used to prevent the growth of mold and bacteria.

[0019] (organic solvent) The aqueous coating composition of the present invention may contain an organic solvent. The content of the organic solvent in the aqueous coating composition is preferably 15% by mass or less, and most preferably 10% by mass. From the viewpoint of reducing VOCs, a low organic solvent content is preferable. However, the use of a small amount of organic solvent may improve adhesion to plastic substrates, and in the case of an aqueous metallic coating composition, may favorably maintain dispersibility of the scaly metal pigment in the composition and film-forming properties of the aqueous metallic coating composition. The organic solvent that can be used in the water-based coating composition of the present invention is not particularly limited, and various hydrophilic or hydrophobic organic solvents can be used.

[0020] As the hydrophilic organic solvent, it is preferable to use, for example, an alcohol solvent, a glycol ether solvent, etc. Examples of the alcohol solvent include isobutyl alcohol, n-butyl alcohol, isoamyl alcohol, n-amyl alcohol, hexyl alcohol, 2-ethylbutyl alcohol, methylamyl alcohol, cyclohexanol, 2-ethylhexyl alcohol, octyl alcohol, benzyl alcohol, dimethylaminoethanol, butanol, etc. Examples of the glycol ether solvent include ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, tetraethylene glycol dimethyl ether, etc.

[0021] Examples of hydrophobic organic solvents include 2-ethylhexanol, dipropylene glycol monobutyl ether, and ethylene glycol monobutyl ether acetate. Of the organic solvents in the water-based coating composition, the ratio of hydrophilic organic solvents to hydrophobic organic solvents is preferably 100:0 or more and 33:67 or less.

[0022] When the aqueous coating composition of the present invention is applied directly to a plastic substrate, it is preferable that the coating composition contain 0.5% by mass to 5.0% by mass of an organic solvent that has a high boiling point and is highly soluble in the substrate. Water-based coatings have a lower organic solvent content than organic solvent-based coatings, and are therefore considered to have a smaller etching effect on the surface of plastic substrates. However, by using an organic solvent that has a high boiling point and is highly soluble in the substrate, the highly soluble solvent remains on the substrate for a longer period after coating, and it is therefore believed that even a small amount of the organic solvent added will provide a high etching effect.

[0023] The high-boiling organic solvent preferably has a boiling point of 160° C. or higher, particularly preferably in the range of 180° C. or higher and 300° C. or lower. Examples of such organic solvents include N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, 1,3-dimethyl-2-imidazolidinone, butyl glycol acetate, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, diethylene glycol monopropyl ether, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol ethyl methyl ether, diethylene glycol dibutyl ether, diethylene glycol butyl methyl ether, dipropylene glycol monomethyl ether, triethylene glycol monomethyl ether, triethylene glycol butyl methyl ether, triethylene glycol dimethyl ether, tripropylene glycol monomethyl ether, tetraethylene glycol dimethyl ether, and polyethylene glycol dimethyl ether.

[0024] An organic solvent with high solubility in a substrate is an organic solvent that can dissolve the surface of a plastic substrate when dropped onto it, and has an SP value (solubility parameter) of 20.0 (J / cm 3 )1 / 2 It is preferable that the saturation is 18.0 (J / cm 3 ) 1 / 2 It is more preferable that the organic solvent is one of the following: Preferably, the type of organic solvent is a glycol-based organic solvent, and more preferably a water-soluble glycol-based organic solvent. Examples of organic solvents having a high boiling point and high solubility in the substrate include triethylene glycol butyl methyl ether (boiling point: 261°C, SP value: 16.6), diethylene glycol diethyl ether (boiling point: 189°C, SP value: 17.8), diethylene glycol dimethyl ether (boiling point: 162°C, SP value: 18.2), diethylene glycol butyl methyl ether (boiling point: 212°C, SP value: 16.1), and tetraethylene glycol dimethyl ether (boiling point: 275°C, SP value: 17.5).

[0025] The SP value is a guideline for determining compatibility, and there are various calculation and measurement methods. In this specification, the SP value is the Hansen solubility parameter value, and is calculated using the Y-MB method.

[0026] -Calculation method of average SP value- When a plastic substrate is made up of n types of polymerizable compounds, the SP values ​​of each polymerizable compound are SP1, SP2, ..., SPn, and the mass fractions of each polymerizable compound are W1, W2, ..., Wn (W1 + W2 + ... + Wn = 1), and the average SP value can be calculated using the following formula: Average SP value = SP1 x W1 + SP2 x W2 + ... + SPn x Wn

[0027] <Resin> The aqueous coating composition of the present invention contains at least one of an acrylic resin and a urethane resin as a resin. By including at least one of an acrylic resin and a urethane resin, the orientation of the pigment and the workability of the coating are improved. In addition to these resins, the composition may contain, for example, an epoxy resin, an alkyd resin, a polyester resin, a silicone resin, etc. Furthermore, the composition may contain, as a crosslinking component, a known resin such as an amino resin, a melamine resin, an isocyanate resin, or a urea resin. In the present invention, the resin may be used alone or in combination of two or more kinds.

[0028] (acrylic resin) The acrylic resin contained in the aqueous coating composition of the present invention may be any resin having one or more (meth)acrylic monomers as constituent units, and may contain monomers other than (meth)acrylic monomers as constituent units as long as the effects of the present invention are not impaired. The (meth)acrylic monomer refers to a monomer having a (meth)acryloyl group, such as (meth)acrylic acid, (meth)acrylic acid ester, etc. For example, the constituent monomer of the acrylic resin used in the present invention is preferably an alkyl ester of (meth)acrylic acid, and the alkyl group in this ester preferably has 1 to 10 carbon atoms, particularly preferably 1 to 8 carbon atoms, and a functional group may be introduced into this alkyl group, or this alkyl group may be a cyclic group. Examples of such constituent monomers include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, i-propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, hexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, tridecyl (meth)acrylate, stearyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, glycidyl (meth)acrylate, cyclopentyl (meth)acrylate, and cyclohexyl (meth)acrylate.

[0029] Examples of the resin form of the acrylic resin include an acrylic emulsion resin that can be produced by emulsion polymerization, and an aqueous acrylic dispersion resin that can be produced by solution polymerization in a common organic solvent.

[0030] - Weight average molecular weight Mw of acrylic resin - The weight average molecular weight Mw of the acrylic resin is not particularly limited, but is preferably 50,000 or more, and more preferably 100,000 or more. The weight-average molecular weight Mw of the acrylic resin can be measured by SEC (size exclusion chromatography, trade name "HLC-8420GPC", manufactured by Tosoh Corporation) using a TSKgel column SuperMultiporeHZ-M (manufactured by Tosoh Corporation) equipped with a differential refractive index (RI) detector. SEC conditions include using tetrahydrofuran as the developing solvent, a flow rate of 0.35 ml / min, and a temperature of 40°C, and using TSK standard polystyrene (manufactured by Tosoh Corporation) as the standard. Generally, emulsion resins obtained by emulsion polymerization tend to have large molecular weights, and emulsion resins with crosslinked structures in particular cannot be accurately measured by gel permeation chromatography. Therefore, in this specification, the molecular weight is defined as 100,000 or more.

[0031] -Hydroxyl value of acrylic resin- When the aqueous coating composition of the present invention is used as a one-component coating, the hydroxyl value of the acrylic resin solid content is preferably 2% or less. If the hydroxyl value of the acrylic resin solid content exceeds 2%, many hydroxyl groups will remain in the coating film, resulting in a decrease in water resistance.

[0032] (urethane resin) The urethane resin contained in the aqueous coating composition of the present invention can be any conventionally known urethane resin without any particular limitation. Examples include urethane resins obtained by reacting at least one diisocyanate compound selected from the group consisting of aliphatic diisocyanate compounds, alicyclic diisocyanate compounds, and aromatic diisocyanate compounds with at least one polyol compound selected from the group consisting of polyether polyols, polyester polyols, and polycarbonate polyols; and urethane resins obtained by reacting the above-mentioned acrylic resins, polyester resins, or epoxy resins with dipolyisocyanate compounds to obtain high molecular weight urethane resins.

[0033] <Luminous pigments or colored pigments> The water-based coating composition of the present invention contains at least one of a bright pigment and a color pigment. The luster pigment and coloring pigment are appropriately selected depending on the color and luster of the coating film, the ease of application, the strength of the coating film, the physical properties, and the like.

[0034] (bright pigment) Luster pigments refer to pigments that impart metallic luster to coating films through the reflection and interference of light. Examples of luster pigments include aluminum powder pigments, nickel powder pigments, gold powder, silver powder, bronze powder, copper powder, stainless steel powder pigments, mica pigments, graphite pigments, glass flake pigments, metal-coated glass powder, metal-coated mica powder, metal-coated plastic powder, and scaly iron oxide pigments. Aluminum powder pigments are particularly preferred. Luster pigments may be used alone or in combination of two or more, and are preferably present in an amount of 5% by mass to 30% by mass, more preferably 7% by mass to 20% by mass, of the solid content of the aqueous coating composition.

[0035] (color pigments) The color pigment is not particularly limited, and pigments commonly used in the paint industry can be used. Examples include inorganic pigments such as titanium dioxide, iron oxide, and carbon black, and organic pigments such as phthalocyanine pigments and azo pigments. The color pigment may be used alone or in combination of two or more, and preferably accounts for 30% to 70% by mass, and more preferably 40% to 60% by mass of the solid content of the aqueous paint composition.

[0036] <Extender pigment> The water-based paint composition of the present invention contains an extender pigment. The extender pigment has a particle size D50 of 0.5 μm or more and a density of 4.2 g / cm 3 By using an extender pigment having a particle size and density within the above ranges, it is possible to suppress an increase in the viscosity of the paint and increase the NV value (nonvolatile content) during application. The particle size D50 of the extender pigment is 5.0 μm or less, and the density is 4.2 g / cm 3 By having a particle size D50 of 5.0 μm or less, it is possible to prevent unevenness caused by the pigment from appearing on the coating film surface and to prevent the arrangement of other pigments from being disturbed, thereby achieving a good appearance. The particle size D50 is measured using an X-ray transmission sedimentation particle size distribution analyzer in accordance with JIS Z 8825 "Particle size analysis - laser diffraction and scattering method."

[0037] The content of the extender pigment in the aqueous paint composition of the present invention is 0.1% by mass or more and less than 25% by mass. When the content of the extender pigment is 0.1% by mass or more, the drying properties of the coating film are good. Furthermore, when the content of the extender pigment is less than 25% by mass, the appearance of the coating film is good. The content of the extender pigment in the aqueous paint composition is preferably 5% by mass or more and 15% by mass or less, and more preferably 5% by mass or more and 10% by mass or less. The extender pigments may be used alone or in combination of two or more.

[0038] The oil absorption of the extender pigment is preferably 5 ml / g or more and 30 ml / g or less, and more preferably 10 ml / g or more and 20 ml / g or less. When the oil absorption of the extender pigment is in the above range, the NV value can be increased while suppressing an increase in the viscosity of the paint, which improves workability during application and also improves drying properties. The oil absorption of extender pigments shall be the value determined in accordance with ISO-787-5 "General test methods for pigments and extender pigments - Part 5: Determination of oil absorption."

[0039] Examples of extender pigments include silica, talc, mica, calcium carbonate, barium sulfate, clay, etc. Among these, barium sulfate is preferred.

[0040] <Other ingredients> The aqueous coating composition of the present invention may contain various additives commonly used in the coating field, such as crosslinking components, thixotropic agents, dispersants, thickeners, neutralizing agents, antifoaming agents, surface conditioners, emulsifiers, surfactants, silane coupling agents, pH adjusters, and preservatives, within the range that does not impair the effects of the present invention.

[0041] (crosslinking component) When the substrate to be coated is metal, the aqueous coating composition of the present invention can be baked at high temperatures, and therefore may contain acrylic resins, amino resins, melamine resins, urea resins, etc., which have a high hydroxyl value. Furthermore, a blocked polyisocyanate can be used in the aqueous coating composition of the present invention when improving the storage stability of the coating at room temperature or when coating and drying on a plastic material such as a polyimide having excellent heat resistance. From the viewpoints of storage stability and workability, such as ease of uniform mixing with the aqueous acrylic resin, it is more preferable to use a water-dispersible blocked polyisocyanate.

[0042] (thixotropic agent) The aqueous coating composition of the present invention can use any conventionally known thixotropic agent as the thixotropic agent without any particular limitation. Examples include polymer association thixotropic agents, alkali-thickening thixotropic agents, polyamide-type thixotropic agents, polyethylene-type thixotropic agents, and bentonite-type thixotropic agents. Among these, it is preferable to include at least one of polymer association thixotropic agents, alkali-thickening thixotropic agents, and polyamide-type thixotropic agents. Thixotropic agents impart thixotropic properties to the coating, thereby reducing sagging during application and enabling a thicker coating thickness per coat. Thixotropic agents are added for the purposes of improving workability by reducing the viscosity of the coating during application. Thixotropic agents may be used alone or in combination of two or more.

[0043] -Solid content of thixotropic agent- When the viscosity of the aqueous coating composition is adjusted to 430 mPa s at a measurement temperature of 25°C, the solids content of the thixotropic agent in the aqueous coating composition is preferably 0.1 to 1.5 mass%, more preferably 0.3 to 0.9 mass%. When the content of the thixotropic agent is within this range, the viscosity behavior described below is more easily achieved, making it possible to form a coating film with excellent coating workability and excellent appearance.

[0044] Commercially available thixotropic agents include, for example, trade names "AS1130", "AS1137", and "AS1180" (all manufactured by BASF), trade names "RHEOBYK H7625VF", "RHEOBYK L1400VF", "RHEOBYK-420", and "RHEOBYK-425" (all manufactured by BYK-Chemie), trade names "ACRYSOL RM-8W" and "ACRYSOL RM-12W" (all manufactured by DOW Chemical), trade names "Thixol 53L", "Viscoatex 46", "Viscoatex 146", "Viscoatex 730", "Viscoatex 146", "Lheotech 4800", "Coapur XS83", "Coapur 6050", and "Coapur 830W" and "BR100P" (all manufactured by COATEX Co., Ltd.), trade name "Nopal 3303" (manufactured by San Nopco Ltd.), trade name "Disparlon 4200-20" (manufactured by Kusumoto Chemicals Co., Ltd.), and the like.

[0045] (dispersant) The dispersant is not particularly limited, and commercially available dispersants for water-based paints such as the BYK series (manufactured by BYK-Chemie), the EFKA series (manufactured by EFKA Chemical Co., Ltd.), and the FLOWEN series (manufactured by Kyoeisha Chemical Co., Ltd.) can be used.

[0046] (Neutralizer) The aqueous coating composition of the present invention is preferably neutralized with ammonia or an amine-based neutralizing agent in terms of water resistance, dispersion stability, reactivity, weather resistance, contamination resistance, etc. The amine-based neutralizing agent is not particularly limited as long as it can neutralize acidic groups, and examples thereof include diethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, dimethylethanolamine, diethylethanolamine, isopropylethanolamine, diisopropanolamine, 2-amino-2-methylpropanol, 2-(dimethylamino)-2-methylpropanol, morpholine, N-methylmorpholine, and N-ethylmorpholine. The neutralizing agent is preferably added in an amount such that the pH of the aqueous coating composition after neutralization is 6.5 to 9.0.

[0047] (Antifoaming agent) The aqueous coating composition of the present invention may contain an antifoaming agent. Commercially available antifoaming agents can be used as appropriate, but it is preferable to use one with a low odor. Antifoaming agents are generally classified into mineral oil-based, silicone-based, polyether-based, etc. depending on the carrier oil, and usually contain, in addition to the carrier, a nucleating agent such as hydrophobic silica, wax, amide, or metal soap as a constituent component. The amount of the antifoaming agent in the aqueous coating composition of the present invention is preferably 0.01% by mass or more and 5% by mass or less. The antifoaming agents may be used alone or in combination of two or more.

[0048] (surface conditioner) Examples of the surface conditioner include silicone-based, acrylic-based, vinyl-based, and fluorine-based surface conditioners. From the viewpoint of the metallic luster and water resistance of the resulting coating film, silicone-based surface conditioners are preferred. Examples of silicone-based surface conditioners include polydimethylsiloxane and modified silicones obtained by modifying polydimethylsiloxane. Examples of modified silicones include polyether-modified silicone, acrylic-modified silicone, and polyester-modified silicone. The surface conditioners may be used alone or in combination of two or more. Examples of commercially available surface conditioners include "SURFYNOL SF104BC" (acetylene dialcohol-based surface conditioner, manufactured by EVONIK).

[0049] <Solid content concentration of water-based paint composition> The solids concentration of the aqueous paint composition of the present invention is preferably 5% by mass or more, more preferably 10% by mass or more, and even more preferably 15% by mass or more, when the viscosity at 25°C is adjusted to 430 mPa·s. Furthermore, the solids concentration of the aqueous paint composition of the present invention is preferably 50% by mass or less, more preferably 40% by mass or less, and even more preferably 35% by mass or less, when the viscosity at 25°C is adjusted to 430 mPa·s. By keeping the solids concentration in the aqueous paint composition within the above range, it is possible to improve the orientation of the pigment and the sagging resistance during application.

[0050] <Viscosity> The viscosity of the aqueous coating composition of the present invention varies depending on the coating method and conditions, but for example, in the case of rotary disk coating using a bell-type coating gun, it can be suitably used within the range of about 300 mPa·s to about 580 mPa·s at a measurement temperature of 25° C., and in the case of air spray coating, it can be suitably used within the range of about 100 mPa·s to about 180 mPa·s at a measurement temperature of 25° C. A diluent such as water can be used to adjust the viscosity. If the viscosity of the water-based coating composition at the time of application is 430 mPa·s or more, it is suitable for application to uneven or flowing substrates such as automobile parts. The viscosity of the water-based coating composition is a value measured using a Stormer viscometer at a measurement temperature of 25°C in accordance with JIS K 5600-2-2:1999.

[0051] The aqueous coating composition of the present invention can be prepared by mixing various components appropriately selected as needed. The aqueous coating composition of the present invention may be a one-component or two-component type, but is preferably used as a one-component type from the viewpoint of eliminating the need to consider the labor of mixing during application and the usable time.

[0052] [Multi-layer coating film formation method] The method for forming a multilayer coating film of the present invention is a method for forming a multilayer coating film, comprising: a base coating film forming step of applying the aqueous coating composition of the present invention onto an object to be coated to form a wet base coating film; a top coating film forming step of applying a top coating composition onto the obtained wet base coating film to form a wet top coating film; and a drying step of simultaneously drying the obtained base coating film and top coating film to form a multilayer coating film.

[0053] <Object to be coated> Substrates that can be coated with the aqueous coating composition of the present invention include plastic substrates such as epoxy resins, ABS resins, polycarbonate, polyvinyl chloride, polystyrene, acrylic resins, such as polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), and polyolefins, such as polypropylene (PP); metal substrates such as steel, galvanized steel, tinned steel, stainless steel, magnesium alloys, aluminum, aluminum alloys, titanium, and titanium alloys; inorganic substrates other than metals, such as cement, mortar, concrete, slate, gypsum, calcium silicate, glass, ceramics, calcium carbonate, marble, and artificial marble; wood substrates, and composite substrates that combine two or more of these substrates. Examples of composite substrates include wood fiber-reinforced cement boards, fiber-reinforced cement boards, and fiber-reinforced cement-calcium silicate boards; metal substrates that have been subjected to various surface treatments, such as oxidation treatments; and plastic substrates whose surfaces are coated with inorganic substances (e.g., glass-coated plastic substrates). Furthermore, the substrate may be subjected to various surface treatments, and at least a portion of the substrate surface may have a primer coating film such as a conductive primer, or an old coating film (a coating film that has already been formed before the application of the aqueous coating composition of the present invention).

[0054] <Topcoat paint composition> The topcoat paint composition is not particularly limited, and any commonly used topcoat paint can be used. Examples of resins used in the topcoat paint composition include acrylic resins, fluorine-based resins, urethane resins, epoxy resins, organically modified silicone resins (e.g., acrylic silicone resins), polyester resins, and melamine resins, with acrylic resins and urethane resins being particularly preferred. In addition, in order to take advantage of the appearance of the coating film formed from the aqueous paint composition of the present invention, the topcoat paint composition is preferably a clear paint.

[0055] The method for applying the water-based paint composition and topcoat paint composition is not particularly limited, and known methods such as air spray coating, airless spray coating, rotary disk coating, etc. Alternatively, a method in which electrostatic charge is applied during coating to increase the coating efficiency may be used.

[0056] The dry film thickness of the aqueous coating composition of the present invention is preferably, for example, 5 μm to 40 μm, more preferably 5 μm to 30 μm. A dry coating film of 5 μm or more makes it easier to obtain a continuous, uniform film that is not too thin and can completely conceal the color of the base. On the other hand, a dry coating film of 40 μm or less makes it less likely to develop defects such as popping or sagging during drying, resulting in a normal dry coating film.

[0057] The base coating film forming step of the present invention may be preceded by a step of forming a primer on the substrate. For example, a conductive primer is preferably used as the primer from the viewpoint of the coating efficiency of electrostatic coating. When the coating composition of the present invention is applied as a base coat paint, it is preferable to apply the conductive primer so that the dry film thickness is 10 μm or more and 100 μm or less. [Example]

[0058] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited thereto.

[0059] [Examples 1 to 18, Comparative Examples 1 to 4] The preparation of the coating compositions of the examples and comparative examples will now be described. The raw materials and blending amounts used in the coating compositions are shown in Tables 1 to 4. The values ​​in the tables are in mass %. Here, the solid content refers to the components remaining in the coating film when the coating film is formed. Each of the raw materials shown in Tables 1 to 4 was mixed in a disperser so as to be uniformly dispersed, to obtain each of the coating compositions.

[0060] Details of the raw materials listed in Table 1 are as follows: (1) Resin Acrylic resin: NeoCryl A639 (manufactured by DSM, solid content: 45%, hydroxyl value: less than 1.0%) Urethane resin 1: DAOTAN TW6450 / 30WA (manufactured by Daicel Allnex Co., Ltd., solid content: 35%) Urethane resin 2: Bayhydrol UH2606 (manufactured by Covestro, solid content: 35%) Urethane resin 3: Watersol WTW-315 (DIC Corporation, solid content: 41.5%)

[0061] (2) Hardener Isocyanate: Duranate WR80-70P (manufactured by Asahi Kasei Corporation, solid content: 70%) Melamine resin: Maprenal MF 927 / 70IB (Prefere Resins, solids content: 70%)

[0062] (3) Pigments (i) Luster pigment: (Silica-coated aluminum pigment (product name "EMR-DZ460", solid content 56%, manufactured by Toyo Aluminum Co., Ltd.) (ii) Color pigments Color pigment 1: White pigment (titanium dioxide, product name "R960", manufactured by DuPont) Color pigment 2: Black pigment, carbon black, product name "COLOUR BLACK FW200 BEADS", manufactured by Orion Engineered Color pigment 3: Red pigment, DPP Red (trade name "Irgazin Rubine L 4030", manufactured by BASF) (iii) Extender pigment ·Extender 1: Barium sulfate (product name: "Ultra-fine precipitated barium sulfate", particle size D50: 0.50~0.70μm, oil absorption: 10~15g / cm 3 , Density: 4.5g / cm 3 , manufactured by Shaanxi Fuhua Chemical Co., Ltd.) Extender pigment 2: Barium sulfate (product name "BARIFINE BF-21"), particle size D50: 0.05 μm, oil absorption: 15-19 g / cm 3 , Density: 3.9g / cm 3 , manufactured by Sakai Chemical Industry Co., Ltd.) Extender pigment 3: Calcium carbonate (product name "Sunlight 1000"), particle size D50: 2.5-5.5 μm, oil absorption: 25-29 g / cm 3 , density 2.71g / cm 3 , manufactured by Takehara Chemical Industry Co., Ltd.)

[0063] (4) Thixotropic agents Alkaline thickening thixotropic agent 1: Product name "AS1137", manufactured by BASF, solid content 30% Alkaline thickening thixotropic agent 2: Product name "Viscoatex 146", manufactured by COATEX, NV: 28% Urethane associative type: BYK-425, manufactured by BYK, solid content 50%

[0064] (5) Other additives Dispersant 1: Product name "XL250", manufactured by Allnex, anionic wetting agent, solid content 47% Dispersant 2: Product name "DISPERBYK-190", manufactured by BYK, dispersant, solid content 40% Defoamer: BYK024, manufactured by BYK-Chemie, solids content 96% Surface conditioner: SURFYNOL SF104BC, acetylene dialcohol-based surface conditioner, manufactured by EVONIK, solids content 49%

[0065] (6) Solvent Water-soluble organic solvent 1: Ethylene glycol monobutyl ether (boiling point: 171°C, SP value: 20.8) Water-soluble organic solvent 2: Dimethylaminoethanol Water-soluble organic solvent 3: Butanol Water-soluble organic solvent 4: Triethylene glycol butyl methyl ether (boiling point: 261°C, SP value: 16.6) Water-soluble organic solvent 5: Diethylene glycol diethyl ether (boiling point: 189°C, SP value: 17.8) Non-water-soluble organic solvent 1: Diethylene glycol butyl methyl ether (boiling point: 212°C, SP value: 16.1) Water-soluble organic solvent 6: Ethylene glycol monomethyl ether (boiling point: 124°C, SP value: 24.8) Water-soluble organic solvent 7: Ethylene glycol dimethyl ether (boiling point: 85°C, SP value: 17.6)

[0066] [Evaluation of paint properties and coating appearance] The coating compositions obtained in the above Examples and Comparative Examples were evaluated as follows. The evaluation results are shown in Tables 1 to 4. The "paint properties after viscosity adjustment" refer to the paint properties when water is added to 100 parts by mass of each paint composition until the Stormer viscosity reaches 430 mPa s, and the amount of water added is the dilution amount. The viscosity after viscosity adjustment was measured at a measurement temperature of 25°C using a digital Stormer viscometer (KU2, manufactured by Brookfield). The measurement temperature refers to the liquid temperature at the time of viscosity measurement. The shear viscosity at each time point was measured using a rheometer (such as the MCR rheometer manufactured by Anton Parr) after adjusting the liquid temperature to 25°C.

[0067] (Appearance of the coating film when painted with bell paint) The viscosity of each of the coating compositions obtained in the above Examples and Comparative Examples was adjusted to 430 mPa·s, and then the coating was applied to polypropylene plates using a "Mighty Robobell 21 rotary atomizer electrostatic coater" (manufactured by CFT Ransberg) under the following coating conditions so that the dry film thickness would be 10 μm or 20 μm. <Painting conditions> Temperature / Humidity: 25℃ / 70RH% Bell cup diameter: Φ70mm Bell rotation speed: 40,000 rpm Shaving air (in): 200kPa Shaving air (out): 200kPa Applied voltage: -60KV

[0068] (Appearance of the coating film after bell painting) The finished appearance of the 20 μm thick coating film was visually observed and evaluated according to the following criteria. <Evaluation criteria> A: The paint film has no uneven color when visually inspected, and no irregularities can be seen on the surface of the paint film, giving it an excellent paint appearance. B: Some unevenness in the color of the coating film is visible to the naked eye, but no irregularities are visible on the coating surface, and there are no practical problems with the appearance of the paint. C: Unevenness in the color of the coating film is visually observed, and irregularities can be seen on the surface of the coating film.

[0069] (Paint film drying property 1) Each of the coating compositions obtained in the above Examples and Comparative Examples was applied under the above coating conditions to a film thickness of 10 μm. Immediately after application, the plate was placed upright and left to stand for 180 seconds, after which sagging was evaluated according to the following criteria. <Evaluation criteria> A: There is no sagging and the surface is smooth and uniform. B: Slight sagging of paint is observed but does not amount to dripping. C: Dripping occurs and the base coat is partially visible.

[0070] (Paint film drying property 2) Each of the coating compositions obtained in the above Examples and Comparative Examples was applied under the above coating conditions to a film thickness of 20 μm. Immediately after application, the plate was placed upright and left to stand for 180 seconds, after which sagging was evaluated according to the following criteria. <Evaluation criteria> A: There is no sagging and the surface is smooth and uniform. B: Dripping has occurred and sagging of the paint is observed. C: Dripping has occurred and the base coat is visible.

[0071] (Appearance of spray-painted film) Each of the coating compositions obtained in the above Examples and Comparative Examples was diluted with water to a viscosity that allowed application by air spray, and then applied to a polypropylene plate using an air spray at 25°C / 70% RH to a dry film thickness of 20 μm. The finished appearance was observed and evaluated according to the following criteria. <Evaluation criteria> A: The paint film has no uneven color when visually inspected, and no irregularities can be seen on the surface of the paint film, giving it an excellent paint appearance. B: Some unevenness in the color of the coating film is visible to the naked eye, but no irregularities are visible on the coating surface, and there are no practical problems with the appearance of the paint. C: Unevenness in the color of the coating film is visually observed, and irregularities can be seen on the surface of the coating film.

[0072] (adhesion to substrate) Each coating composition obtained in the above Examples and Comparative Examples was diluted with water to a viscosity suitable for application by air spray. It was applied to an ABS (acrylonitrile butadiene styrene resin) plate (150 mm x 70 mm x 2 mm) using an air spray at 25°C / 70% RH to a dry film thickness of 20 μm, and then dried at 100°C for 30 minutes to prepare a test specimen. In accordance with JIS K5600-5-6:1999, the coating surface of each test specimen was cut with a cutter to create 100 2mm squares, and adhesive tape was then applied to the squares, followed by a forced peeling test. The number of remaining squares out of the 100 was counted and evaluated according to the following criteria. Here, 100 / 100 indicates that the peeled area of ​​the coating film is 0%, and for example, 80 / 100 indicates that the peeled area of ​​the coating film is 20%. <Evaluation criteria> AA:100 / 100 A: 90 / 100~99 / 100 B:70 / 100~89 / 100 C:69 / 100 or less

[0073] (Coating strength) The evaluation was carried out according to item 6 (DuPont method) of JIS K 5600-5-3. AA: 500g 50cm no cracks A: 500g 40cm no cracks B: 500g 30cm no cracks C: 500g 30cm cracked

[0074] [Table 1]

[0075] [Table 2]

[0076] [Table 3]

[0077] [Table 4]

[0078] As shown in Tables 1 to 4, the aqueous coating compositions of the present invention in Examples 1 to 18 gave good results in all evaluation items, including coating appearance by bell coating, coating drying properties, coating appearance by spray coating, adhesion to substrate, and coating strength.

Claims

1. A water-based coating composition containing water, a resin, at least one of a bright pigment and a color pigment, and an extender pigment, the resin is at least one of an acrylic resin and a urethane resin, The body pigment has a particle diameter D50 of 0.5 μm or more and a density of 4.2 g / cm ３ That's all, The water-based paint composition has a content of the extender pigment in the water-based paint composition of 0.1% by mass or more and less than 25% by mass.

2. 2. The water-based paint composition according to claim 1, wherein the oil absorption of said extender pigment is 21 ml / g or less.

3. 2. The water-based paint composition according to claim 1, wherein said extender pigment is barium sulfate.

4. The aqueous coating composition further contains a thixotropic agent, 2. The aqueous coating composition according to claim 1, wherein when the viscosity of the aqueous coating composition is adjusted to 430 mPa s at a measurement temperature of 25°C, the solids content of the thixotropic agent in the aqueous coating composition is 0.1 mass% or more and 1.5 mass% or less.

5. 2. The aqueous coating composition according to claim 1, wherein the aqueous coating composition has a solids concentration of 5% by mass or more and 50% by mass or less when the viscosity of the aqueous coating composition is adjusted to 430 mPa s at a measurement temperature of 25°C.

6. a base coating film forming step of applying the aqueous coating composition according to any one of claims 1 to 5 onto an object to be coated to form a wet base coating film; a topcoat film forming step of applying a topcoat paint composition onto the obtained undried base coating film to form an undried topcoat film; a drying step of simultaneously drying the obtained undried base coating film and the undried top coating film to form a multi-layer coating film; A method for forming a multilayer coating film comprising the steps of:

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