JP7657044B2 - One-component metal undercoat paint composition - Google Patents

Description

The present invention relates to a one-component undercoat paint composition for metals. More specifically, the present invention relates to a one-component undercoat paint composition for metals that can be applied to a wide range of substrates, can be applied to a wide range of topcoat paint compositions, has excellent rust prevention properties, and can be used for room temperature drying and baking drying.

Undercoat paints that can be applied to metal materials have been developed (Patent Documents 1 to 3). These paint compositions are applied to metal and dried to form a coating. These paint compositions also contain large amounts of rust inhibitors to provide corrosion resistance.

JP 2017-179261 A JP 2012-12497 A Japanese Patent Application Publication No. 3-66766

However, the undercoat paint compositions described in Patent Document 1 and Patent Document 2 require baking drying in the coating film drying process, which reduces the productivity of the coated product. Furthermore, the undercoat paint described in Patent Document 1 requires a large amount of anti-rust pigment to achieve high rust prevention. Furthermore, the anti-rust paint described in Patent Document 3 is limited to a topcoat paint composition that is a powder paint. Therefore, the anti-rust paint described in Patent Document 3 requires equipment suitable for powder paints when applying.

The present invention was made in consideration of these conventional problems, and aims to provide a one-component undercoat paint composition for metal that can be applied to a wide range of substrates, can be used with a wide range of topcoat paint compositions, has excellent rust prevention properties, and can be used for room temperature drying and baking drying.

The one-component metal undercoat paint composition of the present invention, which solves the above problems, mainly contains the following components:

(1) A one-component metal undercoat paint composition comprising an epoxy resin, an isocyanate resin, an anti-rust pigment, and an organic solvent, in which the equivalent ratio (NCO group/OH group) of the hydroxyl groups of the epoxy resin to the isocyanate groups of the isocyanate resin is 0.02 to 0.12, and the mass ratio (A/B) of the content (A) of the anti-rust pigment to the content (B) of the epoxy resin is 0.05 to 0.3.

With this configuration, the one-component metal undercoat paint composition has excellent adhesion to various metal substrates. In addition, the one-component metal undercoat paint composition has excellent adhesion not only to powder paints, but also to various topcoat paint compositions. Furthermore, the coating can be dried at temperatures including room temperature, such as 0 to 40°C, and baking drying is also possible. The resulting coated product has excellent rust resistance and durability.

(2) The one-component metal undercoat paint composition according to (1), wherein the anti-rust pigment comprises a phosphate or phosphite containing at least one metal element selected from the group consisting of zinc and calcium.

With this configuration, the one-component metal primer paint composition can provide the resulting painted product with better rust prevention and improved durability.

(3) The organic solvent includes an oxygen-containing organic solvent, the oxygen-containing organic solvent has a relative evaporation rate of 0.5 or more when the evaporation rate of butyl acetate as a reference is taken as 1, and the content of the oxygen-containing organic solvent in the organic solvent is 10 mass% or more. The one-component metal undercoat paint according to (1) or (2).

With this configuration, the one-component metal undercoat paint composition easily dissolves the resin component. In addition, when the one-component metal undercoat paint composition is applied to a metal substrate and dried, it is easy to dry at drying temperatures including room temperature such as 0 to 40°C, and baking temperatures such as 120 to 200°C, making it highly versatile.

(4) A one-component metal undercoat paint composition according to any one of (1) to (3), having a pigment volume concentration (PVC) of 25 to 35%.

With this configuration, the one-component metal undercoat paint composition is less likely to impair the gloss of the topcoat paint composition, even when it is applied to a metal substrate, dried, and then a topcoat paint composition is applied.

(5) A method for producing a coated product, comprising: a primer coating step of coating a metal substrate with the one-component metal primer coating composition described in any one of (1) to (4); and a primer drying step of drying the applied one-component metal primer coating composition at 0 to 40°C.

In this manner, the method for producing a coated product uses the one-component metal primer paint composition described above, and therefore can form a coating film with excellent adhesion when applied to a metal substrate and dried, even when a drying temperature including room temperature such as 0 to 40°C is used. Therefore, the method for producing a coated product does not require equipment such as a drying oven in the drying process, and the productivity of the coated product is improved.

(6) The method for producing a coated product according to (5), which includes a topcoat step of applying a topcoat paint composition after the undercoat drying step, and a topcoat drying step of drying the applied topcoat paint composition at 0 to 40°C.

In this manner, the method for producing a coated product employs drying temperatures including room temperature, such as 0 to 40°C, not only when applying the undercoat layer but also when applying the topcoat layer. Therefore, the method for producing a coated product does not require equipment such as a drying oven, not only for the drying process of the undercoat layer but also for the drying process of the topcoat layer, improving the productivity of the coated product.

(7) The method for producing a coated product according to (5), which includes a topcoat step of applying a topcoat paint composition after the undercoat drying step, and a baking and drying step of baking and drying the applied topcoat paint composition at 120 to 200°C for 5 to 60 minutes.

In this manner, the method for producing a coated article dries the topcoat paint composition at a baking temperature of 120 to 200°C to form a topcoat layer. As a result, the baking and drying process further accelerates the reaction of the resin in the coating, further improving the coating performance of the resulting coated article.

The present invention provides a one-component undercoat paint composition for metal that can be applied to a wide range of substrates, can be used with a wide range of topcoat paint compositions, has excellent rust prevention properties, and can be dried at room temperature and by baking.

<One-liquid metal undercoat paint composition>
A one-liquid type undercoat paint composition for metals (hereinafter also simply referred to as undercoat paint composition) according to one embodiment of the present invention contains an epoxy resin, an isocyanate resin, an anti-rust pigment, and an organic solvent. The equivalent ratio (NCO group/OH group) of the hydroxyl group of the epoxy resin to the isocyanate group of the isocyanate resin is 0.02 to 0.12. The mass ratio (A/B) of the content (A) of the anti-rust pigment to the content (B) of the epoxy resin is 0.05 to 0.3. Each of these will be described below.

(Epoxy resin)
Epoxy resins harden when dried under various conditions, becoming a component that forms a coating film.

The epoxy resin is not particularly limited. The epoxy resin may be any epoxy resin that dissolves in an organic solvent. Examples of the epoxy resin include bisphenol-type epoxy resins, novolac-type epoxy resins, aromatic epoxy resins, alicyclic epoxy resins, aliphatic epoxy resins, glycidyl ester-type epoxy resins, and glycidyl amine-type epoxy resins. Note that "dissolvable in an organic solvent" refers to dissolving 1 g or more in 100 g of toluene, a typical organic solvent, at 25°C. Epoxy resins may also be used in combination.

Examples of bisphenol type epoxy resins include bisphenol A type epoxy resins, bisphenol F type epoxy resins, bisphenol AD type epoxy resins, bisphenol S type epoxy resins, and their brominated products and water additives. Examples of novolac type epoxy resins include phenol novolac type epoxy resins and cresol novolac type epoxy resins. Examples of aromatic epoxy resins include trisphenolmethane triglycidyl ether.

Examples of alicyclic epoxy resins include 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate, 3,4-epoxy-2-methylcyclohexylmethyl-3,4-epoxy-2-methylcyclohexanecarboxylate, bis(3,4-epoxycyclohexyl)adipate, bis(3,4-epoxycyclohexylmethyl adipate, bis(3,4-epoxy-6-methylcyclohexylmethyl)adipate, 2-(3,4-epoxycyclohexyl-5,5-spiro-3,4-epoxycyclohexanone-meta-dioxane, bis(2,3-epoxycyclopentyl)ether, and the like.

Examples of aliphatic epoxy resins include diglycidyl ether of 1,4-butanediol, diglycidyl ether of 1,6-hexanediol, triglycidyl ether of glycerin, triglycidyl ether of trimethylolpropane, diglycidyl ether of polyethylene glycol, diglycidyl ether of polypropylene glycol, polyglycidyl ether of long-chain polyols including polyoxyalkylene glycols containing alkylene groups with 2 to 9 carbon atoms and polytetramethylene ether glycol, etc.

Examples of glycidyl ester type epoxy resins include diglycidyl ester of phthalic acid, diglycidyl ester of tetrahydrophthalic acid, diglycidyl ester of hexahydrophthalic acid, diglycidyl-p-oxybenzoic acid, glycidyl ether-glycidyl ester of salicylic acid, and glycidyl ester of dimer acid.

Examples of glycidylamine type epoxy resins include triglycidyl isocyanurate, N,N'-diglycidyl derivatives of cyclic alkylene urea, N,N,O-triglycidyl derivatives of p-aminophenol, and N,N,O-triglycidyl derivatives of m-aminophenol.

Among these, the epoxy resin is preferably a bisphenol type epoxy resin, and more preferably a bisphenol A type epoxy resin, because the coating film has excellent weather resistance and flexibility and has excellent adhesion to the substrate.

The content of the epoxy resin is not particularly limited. For example, the content of the epoxy resin in the undercoat paint composition is preferably 10% by mass or more, and more preferably 15% by mass or more. The content of the epoxy resin in the undercoat paint composition is preferably 30% by mass or less, and more preferably 25% by mass or less. When the content of the epoxy resin is within the above range, the undercoat paint composition has excellent barrier properties of the coating film and excellent workability during painting.

The weight average molecular weight (Mw) of the epoxy resin is not particularly limited. As an example, Mw is preferably 10,000 or more, more preferably 30,000 or more. Also, Mw is preferably 100,000 or less, more preferably 50,000 or less. When the Mw of the epoxy resin is within the above range, the resulting coating film is likely to have excellent coating film strength, processability, and smoothness. In this embodiment, Mw can be measured by gel permeation chromatography (GPC) or the like. More specifically, Mw can be calculated in polystyrene equivalent by measuring the difference in refractive index using a GPC device (HLC-8220; manufactured by Tosoh Corporation) equipped with a differential refractometer (RI) detector.

The acid value of the epoxy resin is not particularly limited. As an example, the acid value of the epoxy resin is preferably 5 mgKOH/g or less, and more preferably 1 mgKOH/g or less. When the acid value of the epoxy resin is within the above range, the chemical resistance of the resulting coating film is likely to be improved. In this embodiment, the acid value can be measured by a potentiometric method in accordance with JIS K 0070-1992.

The hydroxyl value of the epoxy resin is not particularly limited. For example, the epoxy resin is preferably 50 mgKOH/g or more, more preferably 100 mgKOH/g or more. The hydroxyl value of the epoxy resin is preferably 1000 mgKOH/g or less, more preferably 500 mgKOH/g or less. When the hydroxyl value of the epoxy resin is within the above range, the coating composition provides a coating film with excellent scratch resistance and coating film strength. In this embodiment, the hydroxyl value can be calculated by expressing the amount of potassium hydroxide required to acetylate the hydroxyl groups in the resin and neutralize the acetic acid required for the acetylation in mg per 1 g of resin.

(Isocyanate resin)
The isocyanate resin hardens when dried under various conditions, and becomes a component that forms a coating film.

The isocyanate resin (polyisocyanate compound) is not particularly limited. For example, the polyisocyanate compound may be an aliphatic diisocyanate compound such as lysine diisocyanate, hexamethylene diisocyanate, or trimethylhexane diisocyanate; an alicyclic diisocyanate compound such as hydrogenated xylylene diisocyanate, isophorone diisocyanate, methylcyclohexane-2,4-diisocyanate, methylcyclohexane-2,6-diisocyanate, 4,4'-methylenebis(cyclohexylisocyanate), or 1,3-(isocyanatomethyl)cyclohexane. Examples of blocking agents include ketoxime-based blocking agents such as ε-caprolactam, methyl ethyl ketoxime, methyl isoamyl ketoxime, methyl isobutyl ketoxime, etc.; phenol-based blocking agents such as phenol, cresol, catechol, nitrophenol, etc.; alcohol-based blocking agents such as isopropanol, trimethylolpropane, etc.; active methylene-based blocking agents such as malonic acid esters, acetoacetate esters, etc.; heterocyclic pyrazole compounds; and the like. Polyisocyanate compounds may be used in combination.

Among these, the isocyanate resin is preferably an aliphatic blocked isocyanate resin, and more preferably a blocked hexamethylene diisocyanate polyisocyanate, because it prevents yellowing of the coating film and has excellent storage stability.

(Anti-rust pigment)
The rust-preventive pigment is not particularly limited. For example, the rust-preventive pigment is a metal oxide and/or a metal nitrate, a metal carbonate, a metal phosphate, a fluorine-containing metal compound, an ammonium complex salt, or zinc powder of one or more metal elements selected from zinc, calcium, aluminum, magnesium, titanium, manganese, strontium, barium, iron, zirconium, cerium, etc. Rust-preventive pigments may be used in combination.

Among these, the anti-rust pigment preferably contains a metal phosphate, metal phosphite, or metal tripolyphosphate containing one of the metal elements zinc, calcium, or aluminum, because this provides the resulting painted product with better anti-rust properties and improved durability, and more preferably contains a phosphate or metal phosphite containing at least one of the metal elements zinc or calcium.

The content of the anti-rust pigment is not particularly limited, so long as the mass ratio (A/B) of the content of the anti-rust pigment (A) to the content of the epoxy resin (B) is 0.05 to 0.3. As an example, the content of the anti-rust pigment in the undercoat paint composition is preferably 1.0 mass% or more, and more preferably 2.0 mass% or more. The content of the anti-rust pigment in the undercoat paint composition is preferably 10.0 mass% or less, and more preferably 5.5 mass% or less. Thus, the undercoat paint composition of this embodiment exhibits excellent anti-rust properties despite the low content of the anti-rust pigment.

The undercoat paint composition of this embodiment may contain other pigments in addition to the above-mentioned anti-rust pigments. The other pigments are not particularly limited. For example, the other pigments are various color pigments, extender pigments, etc. The other pigments may be used in combination.

The color pigment is not particularly limited. Examples of color pigments include titanium oxide, carbon black, and ferric oxide.

The extender pigment is not particularly limited. Examples of extender pigments include talc, barium sulfate, mica, calcium carbonate, etc.

The content of the other pigments is not particularly limited. For example, the content of the other pigments may be determined taking into account the desired color as well as the pigment volume concentration (PVC) described below.

(Organic Solvent)
The organic solvent is blended to dissolve or disperse the epoxy resin and isocyanate resin. The organic solvent is not particularly limited. Examples of the organic solvent include hydrocarbon-based organic solvents such as hexane, benzene, cyclohexane, heptane, toluene, xylene, and mineral spirits; alcohol-based organic solvents such as methyl alcohol, ethyl alcohol, isopropyl alcohol, butyl alcohol, cyclohexanol, propylene glycol monomethyl ether, ethylene glycol monoethyl ether, and ethylene glycol monobutyl ether; ester-based organic solvents such as methyl acetate, ethyl acetate, and butyl acetate; ether-based organic solvents such as 1,4-dioxane, diethyl ether, and tetrahydrofuran; and ketone-based organic solvents such as acetone, methyl ethyl ketone, and cyclohexanone. The organic solvents may be used in combination.

Among these, the organic solvent preferably contains an oxygen-containing organic solvent having a relative evaporation rate of 0.5 or more when the evaporation rate of the reference butyl acetate is taken as 1, and more preferably contains an oxygen-containing organic solvent having a relative evaporation rate of 1.0 or more. Furthermore, the content of the oxygen-containing organic solvent having a relative evaporation rate of 0.5 or more when the evaporation rate of the reference butyl acetate is taken as 1 is preferably 10% by mass or more, more preferably 15% by mass or more, and even more preferably 20% by mass or more in the organic solvent. By having the content of the oxygen-containing organic solvent within the above range, the undercoat paint composition is easy to dissolve the resin component. In addition, when the undercoat paint composition is applied to a metal substrate and dried, it is easy to dry at drying temperatures including room temperature such as 0 to 40 ° C. and baking temperatures such as 120 to 200 ° C., and is highly versatile. The oxygen-containing organic solvent may be an organic solvent containing an oxygen element such as an alcohol, ester, ether, or ketone. Specifically, the evaporation rates of oxygen-containing organic solvents are ethylene glycol monobutyl ether (evaporation rate: 0.09), cyclohexanone (evaporation rate: 0.32), n-butanol (evaporation rate: 0.37), butyl acetate (evaporation rate: 1.00), propylene glycol monomethyl ether (evaporation rate: 0.70), methyl ethyl ketone (evaporation rate: 6.00), etc. In this embodiment, the evaporation rate is expressed as a ratio of the evaporation rate of each organic solvent to the evaporation rate of butyl acetate at a temperature of 23°C and a relative humidity of 50%, which is 1.00. The evaporation rate is measured as a relative rate to the weight loss per unit time of butyl acetate at a temperature of 25°C and a relative humidity of 55%, as specified in the ASTM D 3539-87 standard test method. The higher the evaporation rate of an organic solvent, the easier it is to evaporate.

The flash point of the undercoat paint composition is not particularly limited. For example, the flash point of the undercoat paint composition is preferably 21°C or higher, and more preferably adjusted to 23°C or higher with an organic solvent. When the flash point of the undercoat paint composition is within the above range, the undercoat paint composition is easy to handle in terms of storage method, etc. In this embodiment, the method of adjusting the flash point of the undercoat paint composition is not particularly limited. For example, the flash point of the undercoat paint composition can be adjusted by using multiple organic solvents with different flash points in combination. In this embodiment, the flash point is a flash point measured in accordance with JIS K 2265-1:2007.

The content of the organic solvent is not particularly limited. For example, the content of the organic solvent in the undercoat paint composition is preferably 40% by mass or more, and more preferably 50% by mass or more. The content of the organic solvent in the undercoat paint composition is preferably 70% by mass or less, and more preferably 60% by mass or less. When the content of the organic solvent is within the above range, the undercoat paint composition has excellent smoothness of the coating film, and excellent workability and coating efficiency during painting.

The undercoat paint composition of this embodiment contains the above-mentioned epoxy resin, isocyanate resin, amount of rust inhibitor, and organic solvent, and is characterized by the equivalent ratio (NCO group/OH group) of the hydroxyl group of the epoxy resin to the isocyanate group of the isocyanate resin, and the mass ratio (A/B) of the content of the rust inhibitor pigment (A) to the content of the epoxy resin (B).

(Regarding the equivalent ratio (NCO group/OH group) of the hydroxyl group of the epoxy resin and the isocyanate group of the isocyanate resin)
In the undercoat coating composition of this embodiment, the equivalent ratio (NCO group/OH group) of the hydroxyl group of the epoxy resin and the isocyanate group of the isocyanate resin is 0.02 or more, preferably 0.03 or more. In addition, the equivalent ratio (NCO group/OH group) is 0.12 or less, preferably 0.10 or less. When the equivalent ratio (NCO group/OH group) is less than 0.02, the undercoat coating composition is likely to cause shrinkage on the coating film surface when the undercoat coating composition is applied to a metal substrate to provide a coating film of an undercoat layer, and when a topcoat layer is further provided. On the other hand, when the equivalent ratio (NCO group/OH group) is more than 0.12, the undercoat coating composition is likely to cause poor adhesion between the metal substrate and the undercoat layer, and peeling between the metal substrate and the undercoat layer occurs easily when the undercoat coating composition is applied to a metal substrate to provide a coating film of an undercoat layer. Furthermore, even when a topcoat layer is provided, the adhesion between the undercoat layer and the topcoat layer is poor, and peeling is likely to occur between the undercoat layer and the topcoat layer.

(Regarding the mass ratio (A/B) of the content of the anti-rust pigment (A) to the content of the epoxy resin (B))
In the undercoat paint composition of this embodiment, the mass ratio (A/B) of the content (A) of the rust-preventive pigment to the content (B) of the epoxy resin is 0.05 or more, and preferably 0.1 or more. The mass ratio (A/B) is 0.3 or less, and preferably 0.2 or less. By having the mass ratio (A/B) within the above range, the undercoat paint composition has good rust prevention properties, can protect the metal substrate from corrosion, and has excellent durability.

The primer coating composition of this embodiment has a suitable pigment volume concentration (PVC) of 25 to 35%.

(Regarding pigment volume concentration (PVC))
The pigment volume concentration (PVC) is the volume percentage (%) of the pigment in the total solid content of the total resin content and the total pigment content in the coating composition. In this embodiment, the pigment volume concentration (PVC) is preferably 25% or more, and more preferably 26% or more. In addition, the pigment volume concentration (PVC) is preferably 35% or less, and more preferably 32% or less. By having the pigment volume concentration (PVC) within the above range, even when the undercoat coating composition is applied to a metal substrate and dried, and then a topcoat coating composition is further applied, the gloss of the topcoat coating composition is unlikely to be impaired.

(Additives)
The undercoat coating composition of the present embodiment may contain a thickener, a dispersant, a matting agent, an antifoaming agent, a leveling agent, an anti-sagging agent, a surface conditioner, a viscosity adjuster, an ultraviolet absorber, a wax, etc. Additives may be used in combination.

As described above, according to this embodiment, the undercoat paint composition has excellent adhesion to various metal substrates. The undercoat paint composition also has excellent adhesion not only to powder paints but also to various topcoat paint compositions. Furthermore, the coating film can be dried at drying temperatures including room temperature such as 0 to 40°C, and baking drying is also possible. The resulting coated product has excellent rust resistance and durability. Therefore, the undercoat paint composition of this embodiment is suitable for use in products using various metal substrates such as automobiles, home appliances, and office equipment that are used indoors and outdoors, in environments exposed to sea breezes and rain, and in environments exposed to factory exhaust gases.

The method for preparing the one-component metal undercoat paint composition of this embodiment is not particularly limited. As an example, the undercoat paint composition can be prepared by mixing and stirring the above-mentioned epoxy resin, isocyanate resin, rust-preventive pigment, organic solvent, and appropriate color pigment, extender pigment, additives, etc.

<Manufacturing method of coated products>
The method for producing a coated article according to one embodiment of the present invention includes an undercoat step of applying the above-mentioned one-component metal undercoat paint composition to a metal substrate, and an undercoat drying step of drying the applied one-component metal undercoat paint composition at 0 to 40° C. The undercoat drying step of the one-component metal undercoat paint composition may be a step of baking and drying for 5 to 60 minutes at 120 to 200° C. as appropriate, depending on suitability with the topcoat paint composition, the presence of equipment capable of baking and drying, etc. In addition, the method for producing a coated article may further include applying a topcoat paint composition to form a coating film of a topcoat layer. Each of these will be described below.

(Undercoat process)
The undercoating step is a step of applying the above-mentioned undercoating paint composition to a metal substrate.

The metal substrate is not particularly limited. For example, the metal substrate may be a carbon steel plate, an alloy steel plate, a plated steel plate, or more specifically, a cold-rolled steel plate, a hot-dip galvanized steel plate, an aluminum-containing galvanized steel plate, an electrolytic galvanized steel plate, an alloyed galvanized steel plate, a zinc-nickel plated steel plate, a zinc-cobalt plated steel plate, a vapor-deposition galvanized steel plate, a nickel plated steel plate, a tin plated steel plate (tin plate), a stainless steel plate, or a metal plate other than a steel plate, such as an aluminum plate, a copper plate, a titanium plate, a magnesium plate, or a brass plate.

There is no particular limitation on the method of applying the undercoat paint composition to the metal substrate. For example, the undercoat paint composition can be applied to the surface of the metal substrate by a roll coater method, a spray method, a dipping method, a brush coating method, a curtain coater method, a flow coater method, an immersion method, or the like. The surface of the metal substrate may be solvent degreased before the undercoat paint composition is applied. Solvent degreasing is a process of removing dirt from the substrate surface using various degreasers. In addition, when applying the undercoat paint composition, the viscosity of the undercoat paint composition may be appropriately adjusted with a thinner so as to make it easier to apply.

(Undercoat drying process)
The undercoat drying step is a step in which the applied undercoat paint composition is dried at 0 to 40°C.

In this way, the undercoat drying process employs drying temperatures including room temperature, such as 0 to 40°C. As mentioned above, the undercoat paint composition can be sufficiently dried even at relatively low temperatures compared to the baking temperatures (e.g., 120 to 200°C) used in baking drying. Therefore, equipment such as a drying oven is not necessary, and the productivity of coated products is improved.

The drying temperature should be 0°C or higher, and preferably 10°C or higher. The drying temperature should be 40°C or lower, and preferably 30°C or lower. If it is within the above range, the evaporation rate of the organic solvent is appropriate, so the undercoat paint composition has excellent workability during application and the appearance of the coating film is also excellent.

The drying time is not particularly limited. For example, the drying time may be 10 minutes or more, preferably 30 minutes or more, and more preferably 60 minutes or more. By keeping the drying time within the above range, the applied undercoat paint composition is sufficiently dried to form a coating film (undercoat layer).

The thickness of the undercoat layer is not particularly limited. As an example, the thickness of the undercoat layer is preferably 10 μm or more, and more preferably 20 μm or more. The thickness of the undercoat layer is preferably 50 μm or less, and more preferably 40 μm or less. When the thickness of the undercoat layer is equal to or greater than the above lower limit, the undercoat layer can protect the metal substrate and has excellent corrosion resistance such as water resistance and acid resistance. On the other hand, when the thickness is equal to or less than the upper limit, the undercoat layer has a good coating appearance and is also excellent in workability during painting.

The method for producing a coated product of this embodiment uses the undercoat paint composition described above, so that when the coating is applied to a metal substrate and dried, a coating film with excellent adhesion can be formed even if a low drying temperature, including room temperature, such as 0 to 40°C, is used. Therefore, the method for producing a coated product does not require equipment such as a drying oven in the drying process, and the productivity of the coated product is improved.

In the method for producing a coated article of this embodiment, after the undercoat layer is applied, a topcoat layer may be applied as appropriate to impart design or further improve corrosion resistance. In this case, the topcoat paint composition constituting the topcoat layer may be dried at 0 to 40°C or may be baked dry.

When the topcoat paint composition is dried at 0 to 40°C The method for producing a coated product of this embodiment may include, after the undercoat drying step, a topcoat step of applying a topcoat paint composition, and a topcoat drying step of drying the applied topcoat paint composition at 0 to 40°C.

(Topcoat process)
The topcoat process is a process of applying a topcoat paint composition after the undercoat drying process. The type of the topcoat paint composition is not particularly limited. For example, the topcoat paint composition is a one-liquid solvent paint, two-liquid solvent paint, or powder paint such as phthalic acid resin paint, urethane resin paint, acrylic urethane resin paint, silicone resin paint, melamine alkyd resin paint, acrylic melamine resin paint, polyester resin paint, epoxy resin paint, and fluororesin paint.

There is no particular limitation on the method for applying the topcoat paint composition to the undercoat layer. For example, the topcoat paint composition can be applied to the surface of the undercoat layer by a roll coater method, a spray method, a dipping method, a brush coating method, a curtain coater method, a flow coater method, an immersion method, or the like.

(Topcoat drying process)
The topcoat drying step is a step in which the applied topcoat paint composition is dried at 0 to 40°C.

The drying temperature should be 0°C or higher, and preferably 10°C or higher. The drying temperature should be 40°C or lower, and preferably 30°C or lower. Within the above range, the organic solvent volatilization rate is appropriate, and the topcoat paint composition has good drying efficiency and excellent appearance of the coating film.

The drying time is not particularly limited, and may be any time suitable for the topcoat paint composition. For example, the drying time is preferably 10 minutes or more, more preferably 30 minutes or more, and even more preferably 60 minutes or more. By keeping the drying time within the above range, the applied topcoat paint composition is sufficiently dried to form a coating film (topcoat layer).

The thickness of the topcoat layer is not particularly limited, and may be any thickness suitable for the topcoat paint composition. For example, the thickness of the topcoat layer is preferably 10 μm or more, and more preferably 20 μm or more. The thickness of the topcoat layer is preferably 50 μm or less, and more preferably 40 μm or less. By having a thickness within the above range, and being equal to or greater than the lower limit, the topcoat layer is more likely to protect the metal substrate and has excellent corrosion resistance, such as water resistance and acid resistance. On the other hand, by having a thickness equal to or less than the upper limit, the topcoat layer has a good coating appearance and is also excellent in workability during painting.

In this way, when a process of drying the applied topcoat paint composition at 0 to 40°C is adopted as the topcoat drying process when forming the topcoat layer, the manufacturing method of the coated product of this embodiment adopts drying temperatures including room temperature such as 0 to 40°C when forming not only the undercoat layer but also the topcoat layer. Therefore, the manufacturing method of the coated product does not require equipment such as a drying oven not only for the undercoat layer but also for the drying process of the topcoat layer, improving the productivity of the coated product.

- In the case of drying the topcoat paint composition by baking The manufacturing method of the coated product of this embodiment may include, after the undercoat drying step, a topcoat step of applying a topcoat paint composition, and a baking and drying step of baking and drying the applied topcoat paint composition at 120 to 200°C for 5 to 60 minutes.

(Topcoat process)
The topcoating process is the same as that described above in relation to "drying the topcoat paint composition at 0 to 40°C."

(Baking and drying process)
The baking and drying step is a step in which the applied topcoat paint composition is baked and dried at 120 to 200° C. for 5 to 60 minutes.

The baking and drying temperature should be 120°C or higher, and preferably 130°C or higher. The baking and drying temperature should be 200°C or lower, and preferably 180°C or lower. By keeping the baking and drying temperature within the above range, the resin curing reaction of the topcoat layer is more likely to proceed, the drying efficiency is good, and the appearance of the coating film is also excellent.

There are no particular limitations on the baking and drying time, and it is sufficient if it is a drying time suitable for the topcoat paint composition. As an example, the baking and drying time may be 5 minutes or more, preferably 10 minutes or more, and more preferably 20 minutes or more. Also, the baking and drying time may be 60 minutes or less, and preferably 40 minutes or less. If it is within the above range, the resin curing reaction will proceed, the drying efficiency will be good, and the appearance of the coating film will be excellent.

The thickness of the topcoat layer is not particularly limited. The thickness of the topcoat layer is the same as that of the topcoat layer described above in relation to "When the topcoat paint composition is dried at 0 to 40°C."

When the topcoat layer is applied by baking and drying in this way, the baking and drying process further accelerates the reaction of the resin in the coating, resulting in improved coating performance.

In this embodiment, the undercoat drying step is exemplified by a step of drying the applied undercoat paint composition at 0 to 40°C. Alternatively, the method for producing a painted product in this embodiment can also involve baking and drying the undercoat paint composition.

The present invention will be described in more detail below with reference to examples. The present invention is not limited to these examples. Unless otherwise specified, "%" means "% by mass" and "parts" means "parts by mass."

The raw materials used are shown below.
<Epoxy resin>
Epoxy resin: bisphenol A type epoxy resin, weight average molecular weight (Mw): 35,000, hydroxyl value: 210 mgKOH/g
<Isocyanate resin>
Isocyanate resin: Asahi Kasei Chemicals Corporation, Duranate K6000, NCO: 7.3%
<Anti-rust pigment>
Anti-rust pigment 1: Zinc phosphate/calcium phosphite-based anti-rust pigment Anti-rust pigment 2: Aluminum tripolyphosphate-based anti-rust pigment (organic solvent)
Xylene: Evaporation rate: 0.76
Cyclohexanone: Evaporation rate: 0.32
Butyl acetate: evaporation rate 1.00
n-Butanol: evaporation rate 0.37
Ethylene glycol monobutyl ether: evaporation rate 0.09
Propylene glycol monomethyl ether: evaporation rate: 0.70
Methyl ethyl ketone: evaporation rate: 6.00
*The evaporation rate is a relative evaporation rate when the evaporation rate of butyl acetate as a standard is set to 1 (measurement conditions: in accordance with the ASTM D 3539-87 standard test method).
＜Other＞
Titanium oxide: Furukawa Chemicals Co., Ltd., titanium oxide FR-48
Carbon black: Mitsubishi Carbon Black, manufactured by Mitsubishi Chemical Corporation Calcium carbonate: MC-SII, manufactured by Maruo Calcium Co., Ltd.
Precipitated barium sulfate: precipitated barium sulfate manufactured by SOLVAY BARIO E DERIVATI S.p.A. Surface conditioner: FZ-2122, 10% dilution manufactured by Dow Corning Toray Co., Ltd.

(Examples 1 to 11, Comparative Examples 1 to 4)
According to the formulation shown in Table 1 below, each component was placed in a tank and stirred with a disperser to prepare a one-liquid type metal undercoat paint composition.

The one-component metal primer paint compositions of Examples 1 to 11 and Comparative Examples 1 to 4 were evaluated for drying property, adhesion (lifting property), rust prevention property (neutral salt spray resistance), adhesion (recoatability), 20 degree specular gloss retention, and 60 degree specular gloss retention using the following evaluation methods. The results are shown in Table 1.

(Drying property)
In an environment of 23° C., the one-liquid type metal primer paint composition was applied to the following test panels, and the drying properties of the coating film were confirmed.
Test plate preparation conditions: Test piece: 0.8 mm x 75 mm x 150 mm, dull steel plate Surface treatment: solvent degreasing Coating thickness: 20 to 25 μm
Evaluation method Touch dryness: According to JIS K 5600-1-1, after applying the undercoat paint composition, the center of the coated surface was lightly touched with a fingertip at a certain time (10, 20, 30 minutes) and the time until the fingertip was no longer dirty was measured.

<Evaluation of single undercoat layer (adhesion (lifting property), rust prevention (neutral salt spray resistance))>
The one-liquid type metal primer coating composition was applied to the following test panels, and the performance of the coating film was confirmed.
Test plate preparation conditions: Test piece: 0.8 mm x 75 mm x 150 mm, dull steel plate; Surface treatment: solvent degreasing; Coating system: The undercoat paint compositions of Examples 1 to 8 and Comparative Examples 1 to 4 were dried at 23°C for 10 minutes, and the resulting coating film was evaluated. The undercoat paint composition of Example 9 was dried at 23°C for 30 minutes, and the resulting coating film was evaluated.
Coating film thickness: 20-25μm
Test conditions and evaluation methods Lifting property: The obtained coating film was visually observed to confirm the presence or absence of shrinkage on the coating film surface.
Neutral salt spray resistance: According to JIS K 5400; 1990 8.5.3 X-cut tape method, a test piece with an X-cut scratch was prepared, and the test piece was subjected to a test at 35°C for 120 hours using a 5% NaCl aqueous solution in a neutral salt spray resistance tester (STP-160, manufactured by Suga Test Instruments Co., Ltd.) according to JIS K 5600 7-1. After that, a test piece that peeled off within 3 mm in either direction from the intersection of the X-cut part of the scratch was judged to pass.

<Evaluation when undercoat layer and topcoat layer are provided (adhesion (recoatability), 20 degree specular gloss retention, 60 degree specular gloss retention)>
The one-liquid type metal undercoat paint composition was applied to the following test plate, and the topcoat paint composition was further applied on the resulting undercoat layer to provide a topcoat layer, and the performance was confirmed.
Test plate preparation conditions Test piece: 0.8×75×150 mm, dull steel plate Surface treatment: solvent degreasing Coating system: In Examples 1 to 10 and Comparative Examples 1 to 4, an undercoat paint composition was applied (undercoat process) and dried at 23° C. for 10 minutes (undercoat drying process). Next, a topcoat paint composition was applied to the undercoat layer (topcoat process), and baked and dried at 120° C. for 20 minutes (baking and drying process). In Example 11, a coated product was prepared in the same manner as in Example 1, except that the drying time in the undercoat drying process was 30 minutes.
The topcoat paint composition used was a baking type melamine alkyd resin paint (Tohpe Corporation, Toamelatito #300).
Coating thickness: undercoat layer 20-25 μm, topcoat layer 25-30 μm.
Test conditions and evaluation methods Recoatability: The resulting coating film was visually observed to check for peeling of the topcoat layer.
20 degree specular gloss retention: In accordance with JIS K 5600 4-7, the gloss value of a single film of the topcoat paint composition was used as the standard, and the 20 degree specular gloss of the coating film to which the undercoat paint composition was applied and then the topcoat paint composition was applied was measured, and the retention was calculated (VG7000 gloss meter, manufactured by Nippon Denshoku Industries Co., Ltd.).
60° specular gloss retention: In accordance with JIS K 5600 4-7, the gloss value of a single film of the topcoat paint composition was used as the standard, and the 60° specular gloss of the coating film to which the undercoat paint composition was applied and then the topcoat paint composition was applied was measured, and the retention was calculated (VG7000 gloss meter, manufactured by Nippon Denshoku Industries Co., Ltd.).

As shown in Table 1, the undercoat paint compositions of Examples 1 to 11 had excellent drying properties at 23°C, and excellent adhesion and rust prevention properties. On the other hand, the undercoat paint composition of Comparative Example 1, in which the equivalent ratio (NCO group/OH group) of the hydroxyl group of the epoxy resin to the isocyanate group of the isocyanate resin was less than 0.05, caused shrinkage in the coating film. The undercoat paint composition of Comparative Example 2, in which the equivalent ratio (NCO group/OH group) of the hydroxyl group of the epoxy resin to the isocyanate group of the isocyanate resin was greater than 0.12, had poor adhesion to the substrate. The undercoat paint composition of Comparative Example 3, in which the mass ratio (A/B) of the content of the rust-preventive pigment (A) to the content of the epoxy resin (B) was less than 0.05, had poor rust prevention properties. The undercoat paint composition of Comparative Example 4, in which the mass ratio (A/B) of the content of the rust-preventive pigment (A) to the content of the epoxy resin (B) was greater than 0.3, had poor rust prevention properties.

<Base suitability evaluation>
The one-liquid metal primer coating compositions of Examples 1 and 2 were applied to the substrates shown in Table 2 according to the following test conditions, and the adhesion of the coating film was confirmed.

Test plate preparation conditions Test piece: 0.8 x 75 x 150 mm
Surface treatment: solvent degreasing Coating system: Under normal drying conditions, the coating was dried at 23° C. for 10 minutes to form an undercoat layer. Under baking drying conditions, the coating was dried at 140° C. for 20 minutes to form an undercoat layer.
Coating film thickness: 20-25μm
Test conditions and evaluation method Adhesion: In accordance with JIS K 5600 5-6, tape was applied to a test plate with 1 mm grid cross cuts, the tape was peeled off, and the result was evaluated on a scale of 0 to 5 according to JIS K 5600 5-6 Table 1. Evaluation results of 0 to 1 were considered to be acceptable.
Moisture resistance: A test plate with 1 mm grid cross cuts was prepared according to JIS K 5600 5-6, and tested at 50°C and RH 95% or higher for 240 hours according to JIS K 5600 7-2. Tape was attached to the test plate, and the tape was peeled off, and the plate was evaluated on a scale of 0 to 5 according to JIS K 5600 5-6 Table 1. Evaluation results of 0 to 1 were considered to pass.
Neutral salt spray resistance: According to JIS K 5400; 1990 8.5.3 X-cut tape method, a test piece with an X-cut scratch was prepared, and the test piece was subjected to a test at 35°C for 120 hours using a 5% NaCl aqueous solution in a neutral salt spray resistance tester (STP-160, manufactured by Suga Test Instruments Co., Ltd.) according to JIS K 5600 7-1. After that, a test piece that peeled off within 3 mm in either direction from the intersection of the X-cut part of the scratch was judged to pass.

As shown in Table 2, the one-component metal primer coating composition of the present invention can form a primer layer on various metal substrates not only by baking and drying, but also under normal drying conditions at room temperature, and exhibits excellent adhesion, moisture resistance, and rust prevention properties.

<Appropriate evaluation of topcoat paint composition>
The one-component metal undercoat paint composition of Example 1 was applied to a test plate (metal substrate) in accordance with the following test conditions to form an undercoat layer, and then each topcoat paint composition shown in Table 3 was applied thereto, and the performance of the resulting coating film was confirmed.

Topcoat paint composition Two-liquid type acrylic urethane resin paint: Neo Urethane, manufactured by Tohpe Corporation Baking type melamine alkyd resin paint: Toamelatito #300, manufactured by Tohpe Corporation
One-component baked urethane resin paint: Tohpe Corporation, Toametal Urethane CW
Polyester resin powder coating: Tohpe Corporation, Toa Powder #4000P
Test plate preparation conditions Test piece: 0.8 mm x 75 mm x 150 mm, dull steel plate Surface treatment: solvent degreasing Coating system: Under normal drying conditions, the undercoat paint composition of Example 1 was dried at 23°C for 10 minutes to form an undercoat layer. When the topcoat paint was a powder paint, the undercoat paint composition was baked and dried at 140°C for 20 minutes to form an undercoat layer. Thereafter, various topcoat paint compositions were applied under the drying conditions in Table 3 to form topcoat layers.
Coating thickness: the undercoat layer was 20-25 μm thick, and the various topcoat layers were 25-30 μm thick (when a powder coating (polyester resin-based powder coating) was used, the topcoat layer was 60-80 μm thick).
Test conditions and evaluation methods Finished appearance: In accordance with JIS K 5600 4-7, the 60 degree specular gloss was measured, and a gloss value of 40 to 80 was considered to be acceptable (VG7000 gloss meter, manufactured by Nippon Denshoku Industries Co., Ltd.).
Adhesion: In accordance with JIS K 5600 5-6, tape was applied to a test plate with 1 mm grid cross cuts, the tape was peeled off, and the result was evaluated on a scale of 0 to 5 according to JIS K 5600 5-6 Table 1. Evaluation results of 0 to 1 were considered to be acceptable.
Neutral salt spray resistance: According to JIS K 5400; 1990 8.5.3 X-cut tape method, a test piece with an X-cut scratch was prepared, and the test piece was subjected to a test at 35°C for 120 hours using a 5% NaCl aqueous solution in a neutral salt spray resistance tester (STP-160, manufactured by Suga Test Instruments Co., Ltd.) according to JIS K 5600 7-1. After that, a test piece that peeled off within 3 mm in either direction from the intersection of the X-cut part of the scratch was judged to pass.

As shown in Table 3, the one-component metal undercoat paint composition of the present invention is less likely to affect gloss and exhibits excellent adhesion and rust prevention properties, even when a topcoat layer is formed using various topcoat paint compositions.

Claims (7)

The composition includes an epoxy resin having a hydroxyl group, an isocyanate resin, an anti-rust pigment, and an organic solvent,
an equivalent ratio (NCO group/OH group) of a hydroxyl group of the epoxy resin to an isocyanate group of the isocyanate resin is 0.02 to 0.12;
a mass ratio (A/B) of the content (A) of the rust-preventive pigment to the content (B) of the epoxy resin is 0.05 to 0.2 . The one-component metal undercoat paint composition according to claim 1, wherein the anti-rust pigment comprises a phosphate or phosphite containing at least one metal element selected from the group consisting of zinc and calcium. The organic solvent includes an oxygen-containing organic solvent,
The oxygen-containing organic solvent has a relative evaporation rate of 0.5 or more when the evaporation rate of butyl acetate serving as a reference is taken as 1;
3. The one-component undercoat paint composition for metal according to claim 1, wherein the content of the oxygen-containing organic solvent in the organic solvent is 10 mass % or more. The one-component metal undercoat paint composition according to any one of claims 1 to 3, having a pigment volume concentration (PVC) of 25 to 35%. A primer coating step of applying the one-liquid metal primer coating composition according to any one of claims 1 to 4 to a metal substrate;
and drying the applied one-component metal undercoat paint composition at 0 to 40°C. A topcoat step of applying a topcoat paint composition after the undercoat drying step;
The method for producing a coated article according to claim 5, further comprising a topcoat drying step of drying the applied topcoat paint composition at 0 to 40°C. A topcoat step of applying a topcoat paint composition after the undercoat drying step;
The method for producing a coated article according to claim 5, further comprising a baking and drying step of baking and drying the applied topcoat coating composition at 120 to 200° C. for 5 to 60 minutes.