Coating composition and coating method

Abstract

To provide an aqueous coating composition which enables formation of a coating film having no checking.SOLUTION: An aqueous coating composition contains (A) a pigment, and (B) a water-dispersible resin, wherein the average particle size of (A) the pigment is within the range of 0.01 μm to 500 μm, when particle size distribution of the aqueous coating composition is measured, a ratio of a 10 vol.% particle size (D10) to a 60 vol.% particle size (D60), which are determined from the particle size distribution, satisfies a relation of 0<D10 / D60≤0.4.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a coating composition and a coating method, and more particularly to an aqueous coating composition capable of forming a coating film free of checking. [Background technology]

[0002] In recent years, the coating industry has shifted from solvent-based materials using organic solvents to water-based materials using water, and various investigations into water-based coatings have been conducted. It is known that increasing the molecular weight of water-dispersible resins (especially emulsion resins) can improve the coating film performance while maintaining the viscosity of water-based coatings within a range that allows for excellent coating workability. Water-based coatings using water-dispersible resins have therefore been widely investigated. However, water-based coatings using water-dispersible resins often suffer from the problem of coating film cracking upon drying after application, a phenomenon known as checking (also known as mud cracking). Checking is more likely to occur when the coating film becomes thick or when drying is performed under forced heating. This is thought to be due to volume shrinkage near the coating film surface as the solvent water evaporates. Checking not only impairs the appearance of the coating film, but also contributes to a reduction in various performance characteristics such as water resistance, corrosion resistance, and weather resistance.

[0003] JP 2010-138256 A (Patent Document 1) proposes a method for producing an emulsion in which an ethylenically unsaturated monomer (a) is emulsion-polymerized in one or more stages in the presence of an anionic surfactant (I) and a nonionic surfactant (II) to form a polymer (A), and then an ethylenically unsaturated monomer (b) having a different composition from the ethylenically unsaturated monomer (a) is added and emulsion-polymerized to form a polymer (B), in which the mass ratio and total amount of the anionic surfactant (I) and the nonionic surfactant (II) are adjusted within specific ranges. The document describes that an aqueous coating material containing an emulsion obtained by this production method can form a coating film that has excellent blocking resistance, frost resistance, water resistance, mud crack resistance, and weather resistance.

[0004] Maeda Hiroshi and 1 other author, "Relationship between checking and rheological behavior of water-based emulsion paints," DNT Coating Technical Report, Dai Nippon Toryo Co., Ltd., October 2005 (No. 5), pp. 12-16 (Non-Patent Document 1), discovered a correlation between checking and the rheological behavior of emulsions from the results of measuring the rheology of emulsions with different non-volatile contents, and described that in order to suppress checking, it is important to design an emulsion that can alleviate the stress that occurs during volumetric shrinkage during the drying process, in other words, an emulsion with small inter-particle interactions. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-138256 [Non-patent literature]

[0006] [Non-Patent Document 1] Hiroshi Maeda and 1 other author, "Relationship between Checking and Rheological Behavior of Water-Based Emulsion Paints", DNT Coating Technical Report, Dai Nippon Toryo Co., Ltd., October 2005 (No. 5), pp. 12-16 Summary of the Invention [Problem to be solved by the invention]

[0007] In Patent Document 1 and Non-Patent Document 1, emulsion resins are investigated to obtain a coating film free of checking. On the other hand, in aqueous paints containing a water-dispersible resin and a pigment, the main particles dispersed in water are resin and pigment, so it is also important to consider the presence of pigment particles during the drying process in which the dispersed resin particles fuse and form a film. For this reason, it is believed that it would be effective to obtain a coating film free of checking using a method different from that described in Patent Document 1 and Non-Patent Document 1 for aqueous paint compositions containing a water-dispersible resin and a pigment.

[0008] Therefore, an object of the present invention is to provide an aqueous coating composition containing a water-dispersible resin and a pigment, which can form a coating film without checking. Another object of the present invention is to provide a coating method using such a coating composition.

Means for Solving the Problems

[0009] As a result of intensive studies to achieve the above object, the present inventor has found that by combining a pigment having an average particle diameter within a specific range with a water-dispersible resin and adjusting the ratio of the 60 volume% particle diameter (D60) to the 10 volume% particle diameter (D10) obtained from the particle size distribution of the aqueous coating composition to a specific range, a coating film without checking can be formed, and the present invention has been completed.

[0010] Therefore, the coating composition of the present invention is (A) a pigment and (B) a water-dispersible resin, and is an aqueous coating composition, (A) the average particle diameter of the pigment is within the range of 0.01 μm to 500 μm, when the particle size distribution of the aqueous coating composition is measured, the ratio of the 60 volume% particle diameter (D60) to the 10 volume% particle diameter (D10) obtained from the particle size distribution satisfies the relationship of 0 < D10 / D60 ≤ 0.4, and is an aqueous coating composition.

[0011] In a preferred example of the coating composition of the present invention, (A) the pigment contains two or more pigments having different average particle diameters.

[0012] Further, the coating method of the present invention is a coating method for coating the above-described coating composition of the present invention on a substrate, and includes the following steps 1 to 2: Step 1: A step of coating the coating composition on a pre-heated substrate, Step 2: A step of heating the substrate coated with the coating composition to dry the coating film, and is a coating method characterized by including at least one of the steps.

Effects of the Invention

[0013] The coating composition of the present invention can provide an aqueous coating composition capable of forming a coating film free of checking, and the coating method of the present invention can provide a coating method using such a coating composition. [Brief explanation of the drawings]

[0014] [Figure 1] The particle size distribution of the aqueous coating composition of Example 2 is shown in Figure 1. The measurement was carried out three times, and Figure 1 shows the results of the three measurements. [Figure 2] The particle size distribution of the aqueous coating composition of Comparative Example 2 is shown in Figure 2. The measurement was carried out three times, and Figure 2 shows the results of the three measurements. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present invention will be described in detail below. The present invention relates to a coating composition and a coating method.

[0016] The coating composition of the present invention is an aqueous coating composition. An aqueous coating composition is a coating composition that contains water as the main solvent (the solvent with the highest content in the coating). The water that can be used in the coating composition of the present invention is not particularly limited, but examples include tap water, ion-exchanged water, and pure water such as distilled water. Furthermore, when storing the coating composition 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. When the coating composition of the present invention is an aqueous coating composition, the amount of water contained in the coating composition is preferably 30 to 90 mass %, more preferably 50 to 80 mass %.

[0017] The coating composition of the present invention is an aqueous coating composition containing a pigment having an average particle size in the range of 0.01 to 500 μm and a water-dispersible resin. In this specification, the pigment having an average particle size in the range of 0.01 to 500 μm will be referred to as "(A) pigment" or "(A) component," etc., and the water-dispersible resin will be referred to as "(B) water-dispersible resin" or "(B) component," etc.

[0018] When the particle size distribution of the paint composition of the present invention is measured, the ratio of the 60 volume% particle diameter (D60) to the 10 volume% particle diameter (D10) obtained from the particle size distribution satisfies the relationship of 0 < D10 / D60 ≤ 0.4.

[0019] Checking is considered to be caused by volume shrinkage near the surface of the coating film due to the evaporation of water, which is a solvent, as described above. However, the present inventor considered that this volume shrinkage occurs due to the elimination of voids generated in the drying process. In this regard, in "Porosity of a Random Packed Bed of Other Component Particles with a Particle Size Distribution" by Michitaka Suzuki et al. (3 others), The Chemical Society of Japan, Journal of Chemical Engineering of Japan, 1985, Vol. 11, No. 4, p. 438-443, the effect of particle size distribution on porosity was examined using glass beads (spherical particles) and glass fragments (irregularly shaped particles) having the Andreasen (Gaudin-Schuhmann) distribution. It is taught that the minimum value of the porosity exists in the range of the Fuller index of 0.5 to 0.8 for the Andreasen distribution, and in FIG. 9 showing the relationship between the Fuller index and the porosity, the minimum value of the porosity is shown around the Fuller index of 0.6. In the present invention, since the dispersed particles contain resin and the interaction of the resin works or the resin particles deform, when the tendency was read from the actually measured data of the particle size distribution of the aqueous paint composition, it was found that there is a large difference in the value of the 60 volume% particle diameter (D60), which is a particularly small particle diameter on the large particle diameter side (D60 to D90), among the paint compositions. Therefore, the present inventor collected and examined the actually measured data of the particle size distribution of the aqueous paint composition, and found that when the ratio of the 10 volume% particle diameter (D10), which is a particularly small particle diameter on the small particle diameter side (D10 to D40), to the 60 volume% particle diameter (D60), which is a particularly small particle diameter on the large particle diameter side (D60 to D90), in the particle size distribution of the aqueous paint composition satisfies the relationship of 0 < D10 / D60 ≤ 0.4, the occurrence of checking can be suppressed.

[0020] In the present invention, the particle size distribution of the aqueous paint composition is a volume-based particle size distribution measured with the aqueous paint composition as a sample, and can be determined from the particle size distribution measured using a laser diffraction / scattering type particle size distribution measuring device (for example, SALD-7500nano: manufactured by Shimadzu Corporation). The particle diameter here is represented by the equivalent spherical diameter by the laser diffraction and scattering method. And the values of the 60 volume% particle diameter (D60) and the 10 volume% particle diameter (D10) obtained from the particle size distribution of the aqueous paint composition are the average values of the three measurement results.

[0021] In the paint composition of the present invention, the ratio of the 60 volume% particle diameter (D60) to the 10 volume% particle diameter (D10) obtained from the particle size distribution of the paint composition satisfies the relationship of 0 < D10 / D60 ≤ 0.4, and preferably satisfies the relationship of 0 < D10 / D60 ≤ 0.3.

[0022] Here, the 10 volume% particle diameter (D10) is preferably 10 nm to 10 μm. The 60 volume% particle diameter (D60) is preferably 20 nm to 500 μm.

[0023] The amount of the (B) water-dispersible resin contained in the paint composition of the present invention is preferably 40 to 99%, and more preferably 70 to 90% in the volume of the entire film-forming component.

[0024] (A) Pigment (A) The pigment has an average particle diameter in the range of 0.01 to 500 μm, preferably in the range of 0.5 to 300 μm, and more preferably in the range of 0.5 to 100 μm in consideration of the change rate of volume shrinkage with respect to the coating film thickness.

[0025] The average particle size of a pigment is the average particle size that can be determined from the particle size distribution measured using a laser diffraction / scattering particle size analyzer (e.g., SALD-7500nano, manufactured by Shimadzu Corporation). The particle size here is expressed as the spherical equivalent diameter determined by the laser diffraction / scattering method. Note that the term "average particle size of a pigment" used in this specification refers to the average particle size that can be determined from the particle size distribution measured using a laser diffraction / scattering particle size analyzer, excluding the "average particle size (D) of a pigment" used to determine the aspect ratio.

[0026] The (A) pigment preferably has an aspect ratio of 1 or more and less than 100.

[0027] In the present invention, the aspect ratio of a pigment refers to the ratio (D / T) of the average particle diameter (D) to the average thickness (T) of the pigment. Here, the average particle diameter (D) of the pigment refers to the average particle diameter measured using a scanning electron microscope (SEM) for 100 or more pigment particles. The average thickness (T) of the pigment refers to the average thickness measured using a scanning electron microscope (SEM) for 100 or more pigment particles. Here, the particle diameter of the pigment refers to the maximum length of the pigment in an image obtained by the SEM (scanning electron microscope), and is also referred to as the long side of the pigment. The thickness of the pigment refers to the maximum length of the pigment in the direction perpendicular to the long side of the pigment in an image obtained by the SEM (scanning electron microscope), and is also referred to as the short side of the pigment. However, for circular pigments in an SEM image, the particle diameter (long side) and thickness (short side) of the pigment are the same value.

[0028] (A) Pigment particles come in a variety of shapes, including, for example, spherical, chunky, scaly, rod-like, angular, needle-like, fibrous, and irregularly shaped particles. In the present invention, the inclusion of two or more pigments with different shapes makes it easier to adjust the D10 / D60 ratio to a range of 0.4 or less, thereby making it easier to prevent checking. While the inclusion of an appropriate amount of a non-spherical pigment, such as a scaly pigment, imparts structural viscosity to the paint, effectively suppressing checking, if the structural viscosity is too high, the resulting paint film is difficult to apply and level. Therefore, combining two or more pigments with different shapes (particularly pigments with aspect ratios differing by 2 or more) is effective in achieving D10 / D60 0.4 or less and suppressing checking.

[0029] Flake pigments are pigments with a thin, flat, foil-like shape, and specific examples include metal pigments such as zinc, nickel, chromium, tin, copper, silver, platinum, gold, and aluminum, as well as glass flakes, talc, mica, kaolin clay, and micaceous iron oxide. Metal pigments also include alloy pigments such as stainless steel. Furthermore, flake pigments such as talc and mica may be surface-treated with a metal oxide such as titanium oxide.

[0030] The (A) pigment preferably contains two or more pigments with different average particle sizes. Using two or more pigments with different average particle sizes broadens the particle size distribution of the pigments and improves the packing rate. In addition, it is easier to improve the smoothness of the coating film, and the effect of suppressing the occurrence of checking is more pronounced. For example, blending pigments with average particle sizes differing by 1 μm or more, such as barium sulfate with an average particle size of 1.5 μm and silica with an average particle size of 6 μm, results in a wider particle size distribution and improves packing efficiency.

[0031] The (A) pigment preferably has a Mohs hardness of 1 to 8.

[0032] The Mohs scale is a numerical representation of hardness based on how well an object scratches against a standard material. Standard materials are designated on the Mohs scale, ranging from 1 to 10, in order of softest to softest. Specific standard materials are: 1 for talc, 2 for gypsum, 3 for calcite, 4 for fluorite, 5 for apatite, 6 for orthoclase, 7 for quartz, 8 for topaz, 9 for corundum, and 10 for diamond. Mohs hardness can be measured using a Mohs scale in the usual way.

[0033] The specific gravity of the (A) pigment is preferably 2.0 to 5.0 In the present invention, the substance that serves as the standard for specific gravity is water.

[0034] From the viewpoint of packing efficiency, it is preferable to use two or more types of (A) pigment in combination so that the average particle diameters differ by 1 μm or more.

[0035] The amount of (A) pigment contained in the coating composition of the present invention is preferably 10 to 400 parts by mass, more preferably 20 to 200 parts by mass, and particularly preferably 50 to 150 parts by mass, per 100 parts by mass of resin.

[0036] (A) Pigments include anti-rust pigments, extender pigments, coloring pigments, and the like.

[0037] Examples of anti-rust pigments include zinc powder, zinc oxide, barium metaborate, calcium silicate, aluminum phosphate, condensed aluminum phosphate, aluminum tripolyphosphate, zinc phosphate, zinc phosphite, potassium phosphite, calcium phosphite, aluminum phosphite, calcium zinc phosphate, zinc aluminum phosphate, zinc phosphomolybdate, aluminum phosphomolybdate, magnesium phosphate, and vanadate / phosphate mixed pigments.

[0038] Examples of extender pigments include silica, talc, mica, calcium carbonate, and barium sulfate.

[0039] Examples of color pigments include titanium oxide, iron oxide (e.g., red iron oxide), carbon black, yellow lead, molybdate orange, ultramarine, Prussian blue, phthalocyanine blue, phthalocyanine green, quinacridone red, naphthol red, benzimidazolone yellow, Hansa yellow, benzimidazolone orange, and dioxazine violet.

[0040] In the coating composition of the present invention, since the pigment undergoes almost no volume change during drying, the pigment volume concentration (PVC) is preferably 0.1 to 50%, more preferably 1 to 30%, and particularly preferably 10 to 25%.

[0041] In the present invention, the pigment volume concentration (PVC) is the ratio of the total volume of the pigment to the total volume of the film-forming components in the paint composition, and can be calculated from the composition and specific gravity of each component that makes up the film-forming components.

[0042] (B) Water-dispersible resin In the present invention, a "water-dispersible resin" is a resin that can be distributed in water to form a heterogeneous system (e.g., an emulsion or suspension). When the coating composition of the present invention contains a water-dispersible resin, the water-dispersible resin is dispersed in the coating composition of the present invention.

[0043] The water-dispersible resin can be prepared, for example, by emulsifying the water-dispersible resin in water, or by emulsion polymerization of the monomer components, using a surfactant as needed, while applying forced shear force using a high-speed agitator or the like. Alternatively, an aqueous resin dispersion can be prepared by adding a surfactant as needed to a water-dispersible resin polymerized in an organic solvent medium and then subjecting the resin to phase inversion into water, and the organic solvent contained in the aqueous resin dispersion can be removed by distillation or the like as needed. Alternatively, an aqueous resin dispersion can be prepared by polymerization in water using water as the medium.

[0044] Water-dispersible resins can be classified into emulsion resins and dispersion resins. Emulsion resins refer to water-dispersible resins obtained by emulsion polymerization. Dispersion resins refer to self-water-dispersible resins, but emulsion resins are excluded in the present invention. Resins that dissolve in water are water-soluble resins.

[0045] The coating composition of the present invention preferably contains at least one water-dispersible resin selected from the group consisting of water-dispersible acrylic resins, water-dispersible urethane resins, water-dispersible epoxy resins and alkyd resins, water-dispersible fluororesins, and water-dispersible silicone resins. These water-dispersible resins may be modified, such as urethane-modified epoxy resins, amine-modified epoxy resins, isocyanate-modified epoxy resins, acrylic-modified epoxy resins, polyester-modified epoxy resins, dimer acid-modified epoxy resins, alkyd-modified acrylic resins, acrylic-modified alkyd resins, acrylic-modified silicone resins, silicone-modified acrylic resins, fluorine-modified acrylic resins, and acrylic-modified fluororesins. By including an appropriate amount of these water-dispersible resins, it becomes easier to adjust the D10 / D60 ratio to 0.4 or less.

[0046] The glass transition temperature (Tg) of the resin is preferably −50 to 100° C., more preferably −20 to 80° C., because the higher the temperature, the poorer the film-forming property, and the lower the temperature, the less likely checking occurs. In the present invention, the Tg of the resin is calculated using the following FOX formula: [FOX formula] 1 / Tg=W1 / Tg1+W2 / Tg2+···+Wi / Tgi+···+Wn / Tgn In the FOX formula, Tg written in the denominator on the left side represents the glass transition temperature (unit: K) of the polymer component consisting of N types of monomers, Tg(1, 2, i, N) represents the glass transition temperature (unit: K) of each monomer, W(1, 2, i, N) is the mass fraction of each monomer, and the relationship W1 + W2 +··· + Wi +··· + Wn = 1 holds. Here, the glass transition temperature of a monomer means the glass transition temperature of its homopolymer.

[0047] The water-dispersible resin is preferably a resin with a polydisperse particle size. Using a water-dispersible resin with a polydisperse particle size can improve the filling efficiency of the coating film during drying, suppress the occurrence of checking, and also improve corrosion resistance and blocking resistance. In the present invention, "a resin with a polydisperse particle size" refers to a resin with a wide particle size distribution. Methods for obtaining a "resin with a polydisperse particle size" include a method of making the resin polydisperse during polymerization, as well as a method of mixing two or more resins with different average particle sizes. When mixing two or more resins with different average particle sizes, it is preferable to use two or more resins with average particle sizes that differ by 10 nm or more, more preferably two or more resins with average particle sizes that differ by 20 nm or more, and particularly preferably two or more resins with average particle sizes that differ by 30 nm or more.

[0048] The water-dispersible resin preferably has an average particle size of 10 μm or less, more preferably 300 nm or less, and particularly preferably 200 nm or less, and preferably 10 nm or more.

[0049] The average particle size of the water-dispersible resin is the average particle size that can be determined from the particle size distribution measured using a laser diffraction / scattering particle size distribution analyzer (e.g., SALD-7500nano, manufactured by Shimadzu Corporation). The particle size here is expressed as the spherical equivalent diameter measured by the laser diffraction / scattering method. Specifically, the average particle size here is the average value based on a logarithmic scale (geometric mean diameter).

number

number

[0050] The water-dispersible resin may be used alone or in combination of two or more kinds.

[0051] The amount of water-dispersible resin contained in the coating composition of the present invention is preferably 1 to 50 mass %, more preferably 10 to 40 mass %.

[0052] The coating composition of the present invention may contain a film-forming aid. The film-forming aid is a component blended for the purpose of imparting film-forming properties, and includes volatile film-forming aids classified as organic solvents, and non-volatile film-forming aids that act as plasticizers. In the present invention, either a volatile film-forming aid or a non-volatile film-forming aid can be used.

[0053] The boiling point of the film-forming aid is preferably within a range of 120 to 240° C., more preferably within a range of 120 to 190° C. In the present invention, the boiling point refers to the boiling point at 1 atmospheric pressure.

[0054] Examples of the film-forming aid include propylene glycol, propylene glycol monomethyl ether (PGMME), propylene glycol monoethyl ether, propylene glycol monopropyl ether, ethylene glycol monoethyl ether, ethylene glycol monopropyl ether, ethylene glycol monoisopropyl ether, ethylene glycol mono-n-butyl ether, ethylene glycol monoisobutyl ether, ethylene glycol mono-tert-butyl ether, diethylene glycol monoethyl ether, diethylene glycol monopropyl ether, diethylene glycol monoisopropyl ether, diethylene glycol mono-n-butyl ether, diethylene glycol monoisobutyl ether, diethylene glycol mono-tert-butyl ether, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, diethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol monobutyl ether, propylene glycol monomethyl ether, ethylene glycol diethyl ether, ethylene glycol monoethyl ether, ethylene glycol isopropyl ether, polyethylene glycol monomethyl ether, benzyl diglycol, and triethylene glycol.

[0055] The film-forming aids may be used alone or in combination of two or more.

[0056] When the coating composition of the present invention contains a film-forming aid, the amount of the film-forming aid contained in the coating composition is preferably 0.1 to 50 parts by mass, more preferably 0.2 to 15 parts by mass, and particularly preferably 0.5 to 10 parts by mass, per 100 parts by mass of the resin.

[0057] Other components that can be appropriately blended into the coating composition of the present invention include solvents, surface conditioners, wetting agents, dispersants, emulsifiers, thickeners, anti-settling agents, anti-skinning agents, anti-dripping agents, antifoaming agents, anti-color separation agents, viscosity adjusters, rheology control agents, leveling agents, drying agents, plasticizers, preservatives, anti-mold agents, antibacterial agents, insecticides, light stabilizers, ultraviolet absorbers, antistatic agents, and conductivity imparting agents, depending on the purpose.

[0058] The coating composition of the present invention can be prepared by mixing various components appropriately selected as needed. The coating composition of the present invention may be either a one-component type in which the various components are premixed and used as is at the time of application, or a multi-component type (e.g., two-component type) in which two or more components (e.g., base resin and curing agent) that have been stored separately are mixed at the time of application.

[0059] The amount of the film-forming component contained in the coating composition of the present invention is preferably 10 to 70 mass %, more preferably 40 to 65 mass %, and particularly preferably 40 to 60 mass %.

[0060] In the present invention, the film-forming components refer to the components excluding volatile components such as water and organic solvents, and are the components that ultimately form a coating film. In the present invention, the components that remain when the coating composition is dried at 130°C for 60 minutes are considered to be the film-forming components. The mass fraction of the components (film-forming components) that remain when the coating composition is dried at 130°C for 60 minutes is referred to as the heating residue (or non-volatile content NV).

[0061] The coating composition of the present invention preferably has a viscosity of 0.001 to 1000 (Pa s, 23°C) at a shear rate of 0.1 (1 / s). In the present invention, the viscosity is measured using a rheometer (for example, ARES rheometer manufactured by TA Instruments) after adjusting the liquid temperature to 23°C.

[0062] The means for applying the coating composition of the present invention is not particularly limited, and known coating means such as brush coating, roller coating, trowel coating, spatula coating, flow coater coating, spray coating (e.g., air spray coating, airless spray coating, etc.), electrostatic coating, dipping, electrodeposition coating, roll coating, etc. can be used.

[0063] The drying method for the coating composition of the present invention is not particularly limited, and examples include natural drying, heat drying, etc. In the case of heat drying, the drying temperature is preferably 40 to 200° C. Heat drying is carried out by combining various conditions such as preheating and afterheating.

[0064] The coating method of the present invention is a coating method in which the coating composition of the present invention described above is applied to a substrate. Here, the coating composition of the present invention is applied in the following steps 1 to 2: Step 1: applying a coating composition to a preheated substrate; Step 2: heating the substrate coated with the coating composition to dry the coating film; It is preferable to include at least one of the steps.

[0065] In step 1, the surface temperature of the heated substrate is preferably 40 to 200°C. In step 2, the drying temperature of the coating film formed on the substrate is preferably 40 to 200°C.

[0066] By applying the coating composition of the present invention to a substrate, a coating film is formed on the substrate. The coating film thickness is preferably 1 μm to 10 mm, and more preferably 10 μm to 1 mm. In the present invention, the coating film thickness means the thickness of the coating film after application and drying.

[0067] Examples of substrates include plastic substrates such as epoxy resin, ABS resin, polycarbonate, polyvinyl chloride, polystyrene, acrylic resin, e.g., polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), polyolefin, e.g., polypropylene (PP), metal substrates such as steel, galvanized steel, tinned steel, stainless steel, magnesium alloy, aluminum, aluminum alloy, titanium, titanium alloy, etc., inorganic substrates other than metals such as cement, mortar, concrete, slate, gypsum, calcium silicate, glass, ceramic, calcium carbonate, marble, artificial marble, etc., wood substrates such as wood, paper substrates, and composite substrates combining two or more of these substrates. Examples of composite substrates include composite substrates such as 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).

[0068] The substrate may have various shapes, for example, two-dimensional substrates such as films, sheets, and plates, and three-dimensional substrates that are complex three-dimensional objects, etc. The surface of the substrate may be smooth or may have irregularities.

[0069] The surface of the substrate may be subjected to pretreatment such as degreasing, chemical conversion treatment, or polishing, or may be coated with a sealer or primer. [Example]

[0070] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples. In each example, "parts" and "%" are by mass unless otherwise specified. Furthermore, the film thickness of the coating film is based on the applied coating film.

[0071] Production of aqueous coating compositions The materials were thoroughly mixed in a mixer according to the formulations shown in Table 1 below to produce the aqueous coating compositions of Examples 1 to 6 and Comparative Examples 1 to 5. In the compositions in Table 1, the amount of water indicates the total amount of water used in producing the aqueous coating composition, and includes the amount of water used in the resin dispersion or aqueous resin solution. On the other hand, the amount of resin does not indicate the amount of resin dispersion or aqueous resin solution, but rather the amount of resin itself contained in the resin dispersion or aqueous resin solution.

[0072] The descriptions of the pigments, resins and additives shown in Table 1 are as follows: (pigment) Carbon black: "Mitsubishi Carbon Black MA100" manufactured by Mitsubishi Chemical, average particle size 0.024 μm Bengala: "Todacolor 120ED", manufactured by Toda Kogyo Co., Ltd., average particle size 0.14 μm Barium sulfate: "Barium sulfate W-1", manufactured by Takehara Sangyo Co., Ltd., average particle size 1.5 μm Titanium oxide: "R-32", manufactured by Sakai Chemical Industry Co., Ltd., average particle size 0.23 μm Zinc phosphate: "K-WHITE #140W", manufactured by Teika Co., Ltd., average particle size 2.7 μm Talc: "Microace K-1", manufactured by Nippon Talc Co., Ltd., average particle size 8 μm Silica: "SIPERNAT820A", manufactured by EVONIK, average particle size 6 μm Calcium carbonate: "Sunlight SL100", manufactured by Takehara Chemical Industry Co., Ltd., average particle size 6 μm (resin) Acrylic EM-1: "AD69", manufactured by Henkel, acrylic emulsion, average particle size 80 nm Acrylic EM-2: Dai Nippon Toryo Co., Ltd., acrylic emulsion, average particle size 105 nm, emulsion contains acrylic resin, which is a polymer of styrene (St), methyl methacrylate (MMA), n-butyl acrylate (BA), and methacrylic acid (MAA). Acrylic EM-3: "VIACRYL VSC 6254w / 40WA", manufactured by Daicel Allnex Co., Ltd., acrylic emulsion, average particle size 110 nm · Epoxy DI: "BECKOPOX EP2307w / 45WAMP", manufactured by Daicel Ornex Co., Ltd., epoxy dispersion, average particle size 900 nm (Others) · Film-forming aid: glycol-based solvent · Additives: dispersant, rheology control agent, defoaming agent

[0073] <Measurement of D10 / D60> Using a laser diffraction / scattering type particle size distribution measuring device SALD-7500nano (manufactured by Shimadzu Corporation), the volume-based particle size distributions of the aqueous paint compositions of Examples 1 to 6 and Comparative Examples 1 to 5 were measured, and the values of D10, D60, and D10 / D60 obtained from the particle size distributions are shown in Table 1. In the measurement, for each sample (aqueous paint composition), a refractive index at which 50% by volume of the particle size shows the maximum value was searched so that the variation in the particle size distribution obtained by the measurement would be small. The refractive index at which 50% by volume of the particle size shows the maximum value was input into the measuring device, the particle size distribution of the sample was measured, and the 60% by volume particle size (D60) and the 10% by volume particle size (D10) were obtained. The measurement of the particle size distribution of the sample was performed 3 times, the average values of the 60% by volume particle size (D60) and the 10% by volume particle size (D10) were obtained, and the respective average values were taken as the 60% by volume particle size (D60) and the 10% by volume particle size (D10) obtained from the particle size distribution of the aqueous paint composition. The 60% by volume particle size (D60) and the 10% by volume particle size (D10) obtained from the particle size distribution of the aqueous paint composition are shown in Table 1 together with the value of D10 / D60.

[0074] <Checking Test 1> 10% by mass of water was added to each of the paint compositions of Examples 1 to 6 and Comparative Examples 1 to 5 and mixed well. Then, using a plurality of bar coaters (#20 to #100) for forming paint films with different film thicknesses, the paint composition to which water was added was applied to a tin plate (0.3 × 150 × 150 mm) preheated to 70°C, dried, and test plates were prepared. For each test plate, the presence or absence of checking was visually observed. The film thickness was measured with an electromagnetic film thickness gauge, and Checking Test 1 was evaluated based on the following evaluation criteria. The results are shown in Table 1. (Evaluation Criteria) ⊚: No checking occurred in any coating film up to a film thickness of 100 μm. ◯: Checking occurred on the coating when the film thickness was 60 μm or more and 100 μm or less. ×: Checking occurred on the coating film when the film thickness was less than 60 μm.

[0075] <Checking Test 2> 10% by mass of water was added to each of the coating compositions of Examples 1 to 6 and Comparative Examples 1 to 5, and the mixture was thoroughly mixed. The resulting coating composition was then applied to tinplate plates (0.3 × 150 × 150 mm) at a room temperature of 23°C using multiple bar coaters (#20 to #100) that produce coatings of different thicknesses to prepare test plates. The coated test plates were then left to stand for 10 seconds and then heated and dried for 5 minutes in a dryer heated to 100°C. After drying, each test plate was visually inspected for the presence or absence of checking. Film thickness was measured using an electromagnetic film thickness meter, and Checking Test 2 was evaluated based on the following criteria. The results are shown in Table 1. (Evaluation criteria) ⊚: No checking occurred in any coating film up to a film thickness of 30 μm. ◯: Checking occurred on the coating when the film thickness was 20 μm or more and 30 μm or less. ×: Checking occurred on the coating film when the film thickness was less than 20 μm.

[0076] As a result of the tests, in Examples 1 to 4 where the D10 / D60 of the paint composition was 0.1, no checking occurred in either Checking Test 1 or 2. In Examples 5 and 6 where the D10 / D60 was 0.2, checking occurred at some film thicknesses, but no checking occurred below a certain film thickness. In Comparative Examples 1 to 5 where the D10 / D60 was greater than 0.5, checking occurred in all test panels. As a result of comparing the particle size distributions of the examples and the comparative examples, the comparative examples showed a bias in the particle size distribution, and it is considered that checking occurred due to the volume shrinkage of the paint film during drying. On the other hand, the examples had a wide particle size distribution, and it is considered that no checking occurred because stress concentration at specific locations was prevented during the volume shrinkage of the paint film during drying. As an example of a typical particle size distribution, the three measurement results of the particle size distribution of the aqueous paint composition of Example 2 are shown in FIG. 1 and Table 2, and the three measurement results of the particle size distribution of the aqueous paint composition of Comparative Example 2 are shown in FIG. 2 and Table 2.

[0077]

Table 1

[0078] The property values and test results in Table 1 are as follows. 「NV_wt%( / paint)」 indicates the non-volatile content NV (mass%) of the aqueous paint composition produced in the above-mentioned "Production of Aqueous Paint Composition". 「Water ratio_wt%( / coating paint※)」 indicates the non-volatile content NV (mass%) of the aqueous paint composition to which 10% by mass of water was added in the above <Checking Test 1> and <Checking Test 2>. 「Resin ratio_vol%( / paint film)」 indicates the amount (volume%) of resin with respect to the entire film-forming components. 「Refractive index」 indicates the refractive index input to the measuring device when measuring the particle size distribution of the aqueous paint composition. Note that since the "refractive index" is the absolute refractive index of the particles, it is represented by a complex number. 「D10(nm)」 indicates the value of D10 obtained in the above <Measurement of D10 / D60>. 「D60(nm)」 indicates the value of D60 obtained in the above <Measurement of D10 / D60>. "D10 / D60" indicates the value of D10 / D60 obtained from the above <Measurement of D10 / D60>. "Checking 1" indicates the test result of the above <Checking Test 1>. "Checking 2" indicates the test result of the above <Checking Test 2>.

[0079]

Table 2

Claims

1. An aqueous coating composition comprising (A) a pigment and (B) a water-dispersible resin, (A) the average particle size of the pigment is within the range of 0.01 μm to 500 μm; When the particle size distribution of the aqueous coating composition is measured, the ratio of the 60% by volume particle size (D60) to the 10% by volume particle size (D10) determined from the particle size distribution satisfies the relationship 0<D10 / D60≦0.4, and the 10% by volume particle size (D10) is 50 nm to 10 μm, (A) An aqueous coating composition characterized in that the pigment contains two or more types of pigments having different average particle sizes.

2. A coating method for coating a substrate with the coating composition according to claim 1, comprising the following steps 1 to 2: Step 1: Applying a coating composition to a pre-heated substrate; Step 2: heating the substrate coated with the coating composition to dry the coating film; A coating method comprising at least one of the steps.

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