Method for manufacturing rust-preventive paint composition and rust-preventive coating film

Abstract

The object of this disclosure is to provide a rust-preventive coating composition that can form a coating film exhibiting excellent moisture resistance and excellent rust prevention over a long period of time, especially under acidic environmental conditions, and further possesses good storage stability. [Solution] The rust-preventive coating composition of this disclosure comprises a film-forming resin (A), a crosslinking agent (B), magnesium hydroxide (C), and a metal phosphate salt (D).

Description

[Technical Field]

[0001] This disclosure relates to a rust-preventive paint composition and a method for manufacturing a rust-preventive coating using the paint composition. [Background technology]

[0002] Painted steel sheets, which are made by applying paint to cold-rolled steel sheets or plated steel sheets as a base material, are also called pre-coated metal and are used in a variety of applications, such as outdoor units for air conditioners, exterior components for home appliances such as water heaters, and exterior building materials for roofs, walls, etc. For example, painted steel sheets, including galvanized steel sheets, are usually coated with rust-preventive paint on their surface to improve rust resistance and prevent rusting.

[0003] Traditionally, chromium-containing paints have been commonly used as rust-preventive coatings, and the formation of a chromium-containing coating film can suppress the occurrence of rust. However, concerns about the adverse effects of chromium on the environment are leading to restrictions on its use. Therefore, paint compositions containing vanadium compounds have been proposed as rust inhibitors other than chromium compounds.

[0004] Patent Document 1 describes a paint composition containing a hydroxyl group-containing film-forming resin, a crosslinking agent, and a rust-preventive pigment mixture, wherein the rust-preventive pigment mixture consists of (1) at least one vanadium compound selected from vanadium pentoxide, calcium vanadate, and ammonium metavanadate, (2) a metal silicate, and (3) a phosphate-based calcium salt.

[0005] Patent Document 2 describes a rust-preventive paint composition comprising at least one hydroxyl group-containing film-forming resin selected from specific acrylic resins and specific polyester resins, a bisphenol-type epoxy resin, a curing agent, an epoxy resin having a secondary or tertiary amino group, at least one adhesion-imparting resin selected from acrylic resins having a secondary or tertiary amino group and resol-type phenolic resins, and a rust-preventive pigment. It also states that the rust-preventive pigment contains (1) at least one vanadium compound selected from vanadium pentoxide, calcium vanadate, and ammonium metavanadate, (2) at least one silicon-containing compound selected from metal silicates and silica fine particles, and (3) a phosphate-based metal salt. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2008-222834 [Patent Document 2] Japanese Patent Publication No. 2009-227748 [Overview of the project] [Problems that the invention aims to solve]

[0007] In recent years, there has been a growing demand for the durability of painted steel sheets under various environmental conditions. For example, "acid rain" is a phenomenon that can cause corrosion of painted steel sheets. Acid rain is caused by sulfur dioxide (SO2) and nitrogen oxides (NO2). x Acid rain is caused by substances such as ) dissolving in rain, snow, fog, etc., and exhibits a stronger acidity than usual. Furthermore, after being released, acid rain-causing substances can be transported hundreds to thousands of kilometers across national borders before falling as acid rain, and it is expected that corrosion damage to painted steel sheets will continue to increase over a wide area. Furthermore, if acid rain-causing substances are present in condensation and humid environments, the sulfides and / or nitrides contained in these substances may be exposed to humid conditions, and these substances may accelerate the corrosion reaction of painted steel sheets.

[0008] However, the coatings formed by the paint compositions described in Patent Documents 1 and 2 have insufficient rust prevention properties. For example, their rust prevention is not sufficient for outdoor use, and there is room for improvement in the coatings described in Patent Documents 1 and 2. Furthermore, there is also room for improvement in their rust prevention properties under acidic environmental conditions. Thus, there is a need for a paint composition that can form a coating film that provides rust prevention in acidic environmental conditions and, moreover, provides rust prevention over a longer period of time.

[0009] Furthermore, when used outdoors, the object being coated and the coating film may be exposed to high-humidity environments, so there is a need for a coating composition that can form a coating film that exhibits excellent moisture resistance.

[0010] The object of this disclosure is to provide a rust-preventive coating composition that can form a coating film exhibiting excellent moisture resistance and excellent rust prevention over a long period, especially under acidic environmental conditions, and further possesses good storage stability. In the following, the rust-preventive coating composition may be referred to as "coating composition". [Means for solving the problem]

[0011] This disclosure provides the following: [1] A rust-preventive coating composition comprising a film-forming resin (A), a crosslinking agent (B), magnesium hydroxide (C), and a metal phosphate salt (D). [2] The rust-preventive paint composition according to [1], wherein the metal phosphate salt (D) comprises a condensed metal phosphate salt. [3] The rust-preventive paint composition according to [1] or [2], wherein the metal phosphate salt (D) is a metal tripolyphosphate salt. [4] The rust-preventive paint composition according to any one of [1] to [3], wherein the conductivity of an aqueous solution obtained by mixing 1 g of magnesium hydroxide (C) and 100 g of pure water is 250 μS / cm or less. [5] The pH of an aqueous solution obtained by mixing 1 g of the magnesium hydroxide (C) and 100 g of pure water is 8 or more and 12 or less, and The rust preventive paint composition according to any one of [1] to [4], wherein the average particle diameter of the magnesium hydroxide (C) is 0.5 μm or more and 20 μm or less. [6] The rust preventive paint composition according to any one of [1] to [5], which contains 1 to 150 parts by mass of the magnesium hydroxide (C) with respect to a total of 100 parts by mass of the solid content of the film-forming resin (A) and the solid content of the crosslinking agent (B). [7] The rust preventive paint composition according to any one of [1] to [6], wherein the conductivity of an aqueous solution obtained by mixing 1 g of the metal phosphate (D) and 100 g of pure water is 200 μS / cm or more and 500 μS / cm or less. [8] The rust preventive paint composition according to any one of [1] to [7], wherein the content of the metal phosphate (D) is 20 to 80 parts by mass in a total of 100 parts by mass of the magnesium hydroxide (C) and the metal phosphate (D). [9] The rust preventive paint composition according to any one of [1] to [8], wherein the total content of the magnesium hydroxide (C) and the metal phosphate (D) is 5 to 80 parts by mass with respect to a total of 100 parts by mass of the solid content of the film-forming resin (A) and the solid content of the crosslinking agent (B).

[10] The rust preventive paint composition according to any one of [1] to [9], which further contains a metal vanadate (E). .

[11] The rust preventive paint composition according to

[10] , wherein the total content of the metal vanadate (E) is 0 to 30 parts by mass in a total of 100 parts by mass of the magnesium hydroxide (C), the metal phosphate (D), and the metal vanadate (E).

[12] The rust-preventive coating composition according to any one of [1] to

[11] , wherein the film-forming resin (A) contains at least one selected from the group consisting of an epoxy resin, a polyester resin, an acrylic resin, a urethane resin, and modified products thereof.

[13] The rust-preventive coating composition according to any one of [1] to

[12] , wherein the film-forming resin (A) contains at least one selected from the group consisting of a urethane resin and modified products thereof. The rust-preventive coating composition according to any one of [1] to

[12] , wherein the glass transition temperature of the urethane resin is -50°C or higher and 70°C or lower.

[14] The rust-preventive coating composition according to any one of [1] to

[13] , wherein the film-forming resin (A) contains at least one selected from the group consisting of a urethane resin and modified products thereof. The rust-preventive coating composition according to any one of [1] to

[13] , wherein the urethane resin contains at least one selected from the group consisting of an ester-based urethane resin, an ether-based urethane resin, and a carbonate-based urethane resin.

[15] The rust-preventive coating composition according to any one of [1] to

[14] , wherein the number average molecular weight of the film-forming resin (A) is 1,000 or more and 40,000 or less.

[16] The rust-preventive coating composition according to any one of [1] to

[15] , wherein the film-forming resin (A) contains at least one selected from the group consisting of an ester-based urethane resin, an epoxy resin, a polyester resin, and modified products thereof.

[17] The rust-preventive coating composition according to any one of [1] to

[16] , wherein the solid content acid value of the film-forming resin (A) is 30 mgKOH / g or less.

[18] A coating process of coating an object to be coated with the rust-preventive coating composition according to any one of [1] to

[17] , and A process of curing the rust-preventive coating composition at a temperature of 150°C or higher and 270°C or lower, A method for manufacturing a rust-preventive coating film, comprising the above.

Effects of the Invention

[0012] This disclosure provides a rust-preventive coating composition that can form a coating film exhibiting excellent moisture resistance and excellent rust prevention over a long period, especially under acidic environmental conditions, and further possesses good storage stability. [Brief explanation of the drawing]

[0013] [Figure 1A] This is a schematic diagram showing the cross-section of a painted steel sheet test piece used in the corrosion resistance test. [Figure 1B] This is a schematic diagram showing the cross-cut sections and 1T and 2T bent sections provided on the painted steel sheet test piece used in the corrosion resistance test. [Modes for carrying out the invention]

[0014] We will explain the circumstances that led to this disclosure. Conventionally, chromium-based compounds are used to impart rust-preventive properties to coatings. For example, using a paint composition containing chromium-based compounds can effectively suppress the occurrence of rust on the coated object. On the other hand, in the field of rust-preventive paint compositions, there is a demand for reduction in the burden on human health and the environment, and it is considered essential to reduce or eliminate the use of chromium-based compounds in rust-preventive paint compositions in the future.

[0015] For example, vanadium compounds such as metal vanadate salts are used to reduce the amount of chromium compounds. Increasing the amount of vanadium compounds in a rust-preventive paint composition is considered an effective way to improve the rust-preventive properties of such compositions.

[0016] However, because vanadium compounds, especially vanadates, are highly water-soluble, if a paint composition contains a large amount of vanadium compounds, the paint film becomes more susceptible to moisture absorption. As a result, the moisture resistance of the paint film decreases, and blistering can occur, especially in acidic environments. Such blistering can lead to a decrease in rust prevention and a decrease in the moisture resistance of the paint film. Therefore, when a paint composition contains a large amount of vanadium compounds, it can be difficult to form a paint film that possesses both moisture resistance and long-term rust prevention.

[0017] On the other hand, conventional evaluations of rust prevention have involved short test times, and furthermore, only the molded part (the flat surface of the coated object) has been considered as the evaluation area. However, in recent years, various shapes of coated objects have been manufactured. In coated objects that can have such various shapes, the protective effect of the coating may not be sufficient in areas such as processed parts, end faces, and cross-cut sections, and corrosion of the coated object may occur in these areas. Therefore, there is a need for a paint composition that exhibits excellent rust prevention over a long period of time, even in processed areas, end faces, and cross-cut sections, and moreover, exhibits rust prevention over an extremely long period of time.

[0018] Therefore, the inventors diligently conducted research and completed the present invention. The rust-preventive coating composition of this disclosure comprises a film-forming resin (A), a crosslinking agent (B), magnesium hydroxide (C), and a metal phosphate salt (D).

[0019] The rust-preventive coating composition of this disclosure can be used to form a coating film that exhibits excellent moisture resistance and excellent rust prevention over a long period of time. In particular, it can form a coating film that exhibits excellent rust prevention over a long period of time even under acidic environmental conditions. Therefore, the rust-preventive coating composition of this disclosure can suppress the occurrence of rust that may be caused by, for example, "acid rain". Furthermore, the rust-preventive coating composition of this disclosure has good storage stability. In the following, the coating film formed from the coating composition of this disclosure may be referred to as the coating film of this disclosure.

[0020] The reason why the coating film of this disclosure has the aforementioned rust-preventive properties is thought to be as follows. To suppress corrosion of a substrate (e.g., steel plate), the coating film is required to continuously supply corrosion-inhibiting factors to the areas of the substrate where corrosion occurs. In this disclosure, factors derived from magnesium hydroxide (C) and metal phosphate (D) function as corrosion-inhibiting factors. Magnesium hydroxide (C) is less likely to react on its surface, while metal phosphate (D) is more likely to react on its surface compared to magnesium hydroxide (C). As a result, when using the coating composition of this disclosure, in the initial stages, metal phosphate (D) dissolves and exhibits rust prevention properties, and magnesium hydroxide (C) can remain in the coating film for a long period of time. Consequently, it is considered that when using the coating composition of this disclosure, corrosion-inhibiting factors can be supplied from the coating film to the substrate from the initial stages and over a long period of time. Furthermore, the coating composition of this disclosure, by containing magnesium hydroxide (C), exhibits excellent rust prevention over a long period of time not only on the flat surface of the workpiece but also on the processed parts, end faces, and cross-cut sections, and can exhibit rust prevention on workpieces that may have various shapes.

[0021] Furthermore, the coating film of this disclosure has good moisture resistance and water resistance, and blistering is unlikely to occur in the coating film. As a result, the coating film of this disclosure has good corrosion resistance even when used in an acidic environment. The coating film of this disclosure can also exhibit excellent moisture resistance even when the coated object is exposed to high temperature and high humidity environments, such as when used outdoors. Although it should not be interpreted as being limited to a specific theory, the reasons for obtaining the above effects are thought to be as follows. When moisture or water is contained in the coating film, acid rain-causing substances are absorbed by this moisture or water, and as a result, corrosion of the coated object is thought to progress more easily. However, magnesium hydroxide (C) has low reactivity with water, and as a result, the coating film of this disclosure is thought to have good moisture resistance and water resistance. As a result, the coating film of this disclosure can suppress the absorption of acid rain-causing substances into the coating film, and thus can suppress corrosion of the coated object. In addition, due to the good moisture resistance and water resistance, blistering is unlikely to occur in the coating film of this disclosure, and the reduction in rust prevention performance caused by blistering can also be suppressed. Furthermore, magnesium hydroxide (C) exhibits good dispersibility in paint compositions, which can contribute to improving the storage stability of the paint composition.

[0022] [Magnesium hydroxide (C)] Magnesium hydroxide (C) is an inorganic compound typically represented as Mg(OH)2.

[0023] The oil absorption amount of magnesium hydroxide (C) is preferably 70 g / 100 g or less, more preferably 20 g / 100 g to 65 g / 100 g, and even more preferably 25 g / 100 g to 65 g / 100 g. Although this should not be interpreted as being limited to a specific theory, it is believed that by having the oil absorption amount of magnesium hydroxide (C) within the above range, corrosion inhibitors can be continuously and stably supplied to parts of the coated object where corrosion may occur, thereby forming a coating film with good corrosion resistance.

[0024] In this disclosure, the amount of oil absorbed can be measured in accordance with the provisions of JIS K 5101.

[0025] The BET specific surface area of the magnesium hydroxide is preferably 4.0 m 2 / g or less, more preferably 0.1 m 2 / g or more and 3.5 m 2 / g or less, more preferably 0.2 m 2 / g or more and 3.0 m 2 / g or less, still more preferably 0.3 m 2 / g or more and 2.0 m 2 / g or less, even more preferably 0.3 m 2 / g or more and 1.5 m 2 / g or less. By having such a BET specific surface area, the coating composition of the present disclosure can form a coating film having excellent rust prevention properties over a longer period. More specifically, the coating composition of the present disclosure can form a coating film that exhibits rust prevention properties for a long time, and can also form a coating film that can suppress or greatly reduce corrosion in an acidic environment for a long time. In the present disclosure, the BET specific surface area can be measured using, for example, an automatic specific surface area measuring device Gemini VII 23900 (manufactured by Shimadzu Corporation) or the like. Note that

[0026] In one embodiment, the magnesium metal ion concentration in an aqueous solution obtained by mixing 1 g of magnesium hydroxide (C) and 100 g of an artificial acidic seawater solution is preferably 70 ppm or less, more preferably 10 ppm or more and 69 ppm or less, still more preferably 20 ppm or more and 66 ppm or less, and even more preferably 20 ppm or more and 65 ppm or less. Although it should not be construed as being limited to a particular theory, by including magnesium hydroxide (C) that satisfies such conditions, the coating composition of the present disclosure can supply a corrosion inhibitor to the object to be coated (for example, a steel sheet), making it possible to suppress corrosion of the object to be coated. In addition, it is possible to suppress or greatly reduce a decrease in the moisture resistance of the coating film, and furthermore, it is possible to suppress or greatly reduce the occurrence of blisters in the coating film. For example, by satisfying the above conditions, it is possible to suppress blisters in the coating film and suppress peeling between the coating film and the object to be coated for a long time in a high temperature and high humidity environment. Furthermore, it is possible to suppress or greatly reduce blisters in the coating film at the processed part, end face, and cross-cut part. In this disclosure, "exhibiting corrosion prevention in a high-temperature environment" means exhibiting good corrosion prevention even under conditions where the ambient temperature is high (for example, 40°C or higher), and even at high temperatures of around 80°C.

[0027] In this disclosure, the magnesium metal ion concentration in an aqueous solution obtained by mixing 1 g of magnesium hydroxide (C) with 100 g of an artificial acidic seawater solution can be measured in accordance with the JIS G 0594:2019 cyclic corrosion test method. For example, an aqueous solution based on the acidic salt aqueous solution used in Method B specified in JIS G 0594:2019 cyclic corrosion test method can be used as the artificial acidic seawater solution.

[0028] In one embodiment, the measurement of magnesium metal ions can be carried out, for example, as follows. Prepare artificially acidic seawater by adjusting the pH to 2.5 using a mixture of nitric acid and sulfuric acid as specified in Section 4.1.1 of JIS G 0594:2019. For example, the Marine Art series (manufactured by Tomita Pharmaceutical Co., Ltd.) can be used as a reagent. Next, add 100g of artificially acidic seawater and 1g of magnesium hydroxide (C) to the container, add a stirrer tip, and stir for 4 hours at room temperature (23°C). Afterward, the supernatant liquid is left at room temperature for 24 hours, and the supernatant is collected using a syringe equipped with a syringe filter. The magnesium metal ion concentration can then be measured by measuring the elemental concentration using, for example, an ICP emission spectrometer (ICPS-7510, manufactured by Shimadzu Corporation). Specifically, the value obtained by subtracting the blank from the elemental concentration (magnesium metal ion concentration) contained in the supernatant liquid can be used as the magnesium metal ion concentration in an aqueous solution in which 1 g of magnesium hydroxide (C) is added to 100 g of artificial acidic seawater. Here, the blank is the value obtained by measuring the elemental concentration (magnesium metal ion concentration) of artificial acidic seawater in the same way as the supernatant liquid.

[0029] In one embodiment, the conductivity of an aqueous solution obtained by mixing 1 g of magnesium hydroxide (C) with 100 g of pure water is preferably 250 μS / cm or less, more preferably 10 μS / cm to 210 μS / cm, even more preferably 20 μS / cm to 205 μS / cm, and even more preferably 20 μS / cm to less than 201 μS / cm. By satisfying these conditions, corrosion inhibitors from magnesium hydroxide (C) can be supplied to the steel plate more effectively and stably for a long period of time without depletion, thereby suppressing corrosion. Furthermore, the decrease in the moisture resistance of the coating film can be suppressed or greatly reduced, and the occurrence of blistering on the coating film can also be suppressed or greatly reduced. Moreover, with the rust-preventive coating composition of this disclosure, blistering on processed parts, end faces, and cross-cut parts can also be suppressed or greatly reduced.

[0030] In this disclosure, the conductivity of various materials can be measured as follows: Mix 100g of deionized water with 1g of the target material, add a stirrer tip, and stir at room temperature (23°C) for 4 hours. After that, the conductivity can be measured using, for example, an electrical conductivity meter (CM-42X, manufactured by Toa DKK Co., Ltd.).

[0031] In this disclosure, by having magnesium hydroxide (C) with the aforementioned magnesium metal ion concentration and conductivity, corrosion in acidic environments can be suppressed for an even longer period of time. In addition to suppressing corrosion in acidic environments, it is also possible to stably suppress blistering and peeling of coatings in high-temperature and high-humidity environments for a long period of time. In this disclosure, such combinations of magnesium metal ion concentration and conductivity can be appropriately selected within the range described herein.

[0032] In one embodiment, the pH of an aqueous solution obtained by mixing 1 g of magnesium hydroxide (C) with 100 g of pure water is preferably 8 to 13, more preferably 9 to 13, and even more preferably 10 to 13. In another embodiment, the pH is 8 to 12. By satisfying these conditions, the increase in the corrosion rate of the coated object can be more effectively suppressed, and a coating film with good corrosion resistance can be formed. In this disclosure, the pH value can be measured by a known method. For example, 100 g of deionized water and 1 g of magnesium hydroxide (C) are mixed, a stirrer tip is added, and the mixture is stirred at room temperature (23°C) for 4 hours. After that, the pH can be measured using, for example, a pH meter (bench pH meter F-74, manufactured by Horiba, Ltd.).

[0033] In one embodiment, the average particle size of magnesium hydroxide (C) is preferably 0.5 μm to 20 μm, more preferably 1 μm to 15 μm, and even more preferably 1 μm to 10 μm. If the average particle size is too small, the number of reaction sites on the surface of magnesium hydroxide (C) increases, and corrosion of the coated object may not be sufficiently suppressed. If the average particle size is too large, defects may occur in the coating film. In this disclosure, the average particle diameter can be measured using, for example, a laser diffraction particle size distribution analyzer SALD-2300 (manufactured by Shimadzu Corporation).

[0034] In one embodiment, the magnesium hydroxide (C) content may be preferably 1 to 150 parts by mass, more preferably 10 to 140 parts by mass, based on 100 parts by mass of the total solid content of the coating resin (A) and the crosslinking agent (B). Having the magnesium hydroxide (C) content within this range allows the coating composition of this disclosure to form a coating film that exhibits excellent rust prevention over a long period and also exhibits excellent moisture resistance.

[0035] In this disclosure, the solid content of a component means the heating residue as defined in JIS K 5601-1-2:2008, and the solid content percentage is calculated by measuring the percentage of the mass of the residue after heating at 105°C for 60 minutes relative to the original mass.

[0036] Magnesium hydroxide (C) can function as a rust-preventive pigment in the paint composition according to this disclosure. In one embodiment, magnesium hydroxide (C) is a rust-preventive pigment.

[0037] In one embodiment, the coating composition of the present disclosure may contain other metal compounds, to the extent that they do not impair the effects of magnesium hydroxide (C).

[0038] [Metal phosphate (D)] The metal phosphate salt (D) is typically a salt of a phosphate ion and a metal ion. The phosphate ion may include ions of phosphoric acid or condensed phosphoric acid. Condensed phosphoric acid is a general term for linear polymer phosphates produced by the dehydration condensation of orthophosphate, and examples include pyrophosphate, tripolyphosphate, trimetaphosphate, tetrametaphosphate, and ultraphosphate. The metal ions include alkali metal ions such as sodium ions and potassium ions; alkaline earth metal ions such as calcium ions and magnesium ions; and amphoteric metal ions such as aluminum ions and zinc ions.

[0039] The metal phosphate salt (D) is preferably a condensed metal phosphate salt, and more preferably a tripolyphosphate salt (D1). The amphoteric metal ion is preferably a salt of aluminum or zinc. It is also preferable to treat the phosphate with a metal. The metal used for treatment is preferably an alkaline earth metal, and preferably magnesium.

[0040] In one embodiment, the conductivity of an aqueous solution obtained by mixing 1 g of metal phosphate salt (D) and 100 g of pure water is preferably 200 μS / cm or more, more preferably 200 μS / cm to 500 μS / cm, and even more preferably 200 μS / cm to 300 μS / cm. By satisfying these conditions, corrosion inhibitors can be supplied from the metal phosphate salt (D) from the initial stage, thereby suppressing corrosion.

[0041] The content of the metal phosphate salt (D) is preferably 20 parts by mass or more and 80 parts by mass or less, more preferably 30 parts by mass or more and 70 parts by mass or less, and even more preferably 30 parts by mass or more and 50 parts by mass or less, in 100 parts by mass of the total of the magnesium hydroxide (C), the metal phosphate salt (D), and the metal vanadate salt (E). Having the content of the metal phosphate salt (D) within this range makes it easy to continuously exhibit excellent rust prevention properties from the initial stage over a long period of time.

[0042] The total content of magnesium hydroxide (C) and metal phosphate salt (D) is preferably 5 to 150 parts by mass, more preferably 20 to 80 parts by mass, and even more preferably 40 to 70 parts by mass, based on 100 parts by mass of the total solid content of the coating resin (A) and the crosslinking agent (B). Having the total content of magnesium hydroxide (C) and metal phosphate salt (D) within this range makes it easy to continuously exhibit excellent rust prevention properties from the initial stage over a long period of time, and also makes it easy to suppress blistering of the coating film.

[0043] [Metal vanadate (E)] The aforementioned paint composition may further contain a metal vanadate salt (E). The aforementioned vanadate metal salt (E) may be a salt of a vanadate ion and a metal ion. The valence of vanadium in the vanadate ion is 3, 4, or 5, and vanadate includes both orthovanadic acid and condensed vanadates such as metavanadic acid and pyrovanadic acid. Examples of the aforementioned metal ions include alkaline earth metal ions such as calcium ions and magnesium ions, with calcium vanadate being preferred.

[0044] In one embodiment, the conductivity of an aqueous solution obtained by mixing 1 g of metal vanadate (E) with 100 g of pure water is preferably 200 μS / cm to 2,000 μS / cm, and more preferably 200 μS / cm to 1,000 μS / cm. By satisfying these conditions, a coating film with improved corrosion resistance and moisture resistance can be obtained.

[0045] The content of the vanadate metal salt (E) is preferably 0 to 30 parts by mass, more preferably 5 to 20 parts by mass, and even more preferably 10 to 20 parts by mass, based on 100 parts by mass of the total of the magnesium hydroxide (C), the phosphate metal salt (D), and the vanadate metal salt (E). Having the content of the phosphate metal salt (D) within this range makes it possible to exhibit excellent rust prevention from the initial stage, especially for processed parts.

[0046] The content of the vanadate metal salt (E) is preferably 0 to 30 parts by mass, more preferably 5 to 20 parts by mass, and even more preferably 10 to 20 parts by mass, based on 100 parts by mass of the total of the magnesium hydroxide (C), the phosphate metal salt (D), and the vanadate metal salt (E). Having the content of the phosphate metal salt (D) within this range makes it possible to exhibit excellent rust prevention from the initial stage, especially for processed parts.

[0047] The total content of the magnesium hydroxide (C), the metal phosphate salt (D), and the metal vanadate salt (E) is preferably 5 parts by mass or more and 100 parts by mass or less, more preferably 20 parts by mass or more and 80 parts by mass or less, and even more preferably 40 parts by mass or more and 70 parts by mass or less, based on 100 parts by mass of the total solid content of the coating resin (A) and the crosslinking agent (B). Having the total content of magnesium hydroxide (C), metal phosphate salt (D), and metal vanadate salt (E) within this range makes it easy to continuously exhibit excellent rust prevention properties from the initial stage over a long period of time, and also makes it easy to suppress blistering of the coating film.

[0048] The magnesium hydroxide (C), the phosphate (D), and the vanadate metal salt (E) can all contribute to corrosion resistance. The paint composition of this disclosure may contain, in addition to magnesium hydroxide (C), one or more selected from the phosphate (D) and the vanadate metal salt (E). In one embodiment, the paint composition of this disclosure preferably contains magnesium hydroxide (C), the phosphate (D), and the vanadate metal salt (E).

[0049] [Coating film forming resin (A)] The film-forming resin (A) in the rust-preventive coating composition of this disclosure is not particularly limited as long as it has functional groups that can react with the crosslinking agent (B) and is a resin that has the ability to form a coating.

[0050] Examples of the film-forming resin (A) include epoxy resins and their modified products (such as acrylic-modified epoxy resins); polyester resins and their modified products (such as urethane-modified polyester resins, epoxy-modified polyester resins, and silicone-modified polyester resins); acrylic resins and their modified products (such as silicone-modified acrylic resins); urethane resins and their modified products (such as ester-based urethane resins, ether-based urethane resins, carbonate-based urethane resins, and epoxy-based urethane resins); phenolic resins and their modified products (such as acrylic-modified phenolic resins and epoxy-modified phenolic resins); phenoxy resins; alkyd resins and their modified products (such as urethane-modified alkyd resins and acrylic-modified alkyd resins); and fluororesins. These resins may be used individually or in combination of two or more types.

[0051] In one embodiment, the coating resin (A) includes at least one selected from the group consisting of epoxy resin, polyester resin, acrylic resin, urethane resin, and modified products thereof.

[0052] In one embodiment, the coating resin (A) includes at least one selected from the group consisting of ester-based urethane resins, epoxy resins, polyester resins, and modified products thereof.

[0053] In one embodiment, the coating-forming resin (A) includes at least one selected from the group consisting of epoxy resin, polyester resin, urethane resin, or modified products thereof, from the viewpoint of balancing the bendability of the resulting coating film and the moisture resistance, corrosion resistance, and weather resistance of the resulting coating film.

[0054] In one embodiment, the coating resin (A) includes at least one selected from the group consisting of urethane resins and modified products thereof.

[0055] (Epoxy resin) The epoxy resin may be a resin containing epoxy groups in its molecule and a modified version thereof. The epoxy resin may preferably have hydroxyl groups. In this disclosure, "epoxy resin" may include both epoxy resins and modified versions of epoxy resins.

[0056] The number-average molecular weight (Mn) of the epoxy resin is preferably 1,400 to 20,000, more preferably 2,000 to 10,000, and even more preferably 2,000 to 4,000. Having the above number-average molecular weight allows the crosslinking reaction between the epoxy resin and the crosslinking agent (B) described later to proceed sufficiently, forming a coating film with high moisture resistance and excellent corrosion resistance. Furthermore, the elution of magnesium hydroxide (C) and metal phosphate salt (D) contained in the coating film can be appropriate, allowing the formation of a coating film with good corrosion resistance even in acidic environments. Moreover, it is possible to suppress the crosslinking density of the coating film from becoming too high, allowing the formation of a coating film with sufficient elongation, for example, a coating film with sufficient bendability. In addition, the viscosity of the rust-preventive paint composition is within an appropriate range, resulting in good handling properties such as paint manufacturability and painting workability. In this disclosure, the number-average molecular weight (Mn) is the value obtained by gel permeation chromatography (GPC) on a polystyrene basis.

[0057] The glass transition temperature (Tg) of the epoxy resin may be between 50°C and 120°C, between 55°C and 115°C, or between 55°C and 110°C. By having the glass transition temperature (Tg) of the epoxy resin within these ranges, the moisture permeability of the coating film does not become excessively high, resulting in sufficient moisture resistance and good corrosion resistance. In this disclosure, the glass transition temperature (Tg) can be measured using, for example, a thermal analyzer such as the TMA7100 (manufactured by Hitachi High-Tech Science Corporation).

[0058] The acid value of epoxy resin (including its modified products) may be between 0 mg KOH / g and 30 mg KOH / g. In this disclosure, the acid value refers to the solids acid value and can be measured in accordance with the provisions of JIS K 0070.

[0059] The epoxy resin may be a hydroxyl group-containing epoxy resin (including a hydroxyl group-containing epoxy resin modified product).

[0060] Examples of epoxy resins include resins obtained by condensing epichlorohydrin and bisphenol to a high molecular weight in the presence of a catalyst such as an alkaline catalyst as needed; bisphenol-type epoxy resins such as bisphenol A type and bisphenol F type; and novolac-type epoxy resins.

[0061] Examples of modified epoxy resins include acrylic-modified epoxy resins, urethane-modified epoxy resins, and amine-modified epoxy resins. For example, an acrylic-modified epoxy resin can be prepared by reacting a polymerizable unsaturated monomer component containing acrylic acid or methacrylic acid with the bisphenol-type epoxy resin or the novolac-type epoxy resin. A urethane-modified epoxy resin can be prepared by reacting a polyisocyanate compound with the bisphenol-type epoxy resin or the novolac-type epoxy resin.

[0062] Examples of epoxy resins include jER1004, jER1007, 1255HX30 (bisphenol A skeleton), and YX8100BH30, all manufactured by Mitsubishi Chemical Corporation.

[0063] (Polyester resin) Polyester resin is a resin having ester bonds in its main chain, and may, for example, be a modified version thereof. The polyester resin may also have hydroxyl groups. In this disclosure, "polyester resin" may include both polyester resin and modified polyester resins.

[0064] The number-average molecular weight (Mn) of the polyester resin is preferably 1,400 to 40,000, more preferably 2,000 to 40,000, and even more preferably 2,000 to 30,000. Having the above number-average molecular weight allows the crosslinking reaction between the polyester resin and the crosslinking agent (B) to proceed sufficiently, forming a coating film with high moisture resistance and excellent corrosion resistance. Furthermore, the elution of magnesium hydroxide (C) and metal phosphate salt (D) contained in the coating film is appropriate, allowing the formation of a coating film with good corrosion resistance even under acidic environmental conditions. Moreover, it is possible to suppress the crosslinking density of the coating film from becoming too high, forming a coating film with sufficient elongation, for example, a coating film with sufficient bendability. In addition, the viscosity of the rust-preventive paint composition is within an appropriate range, resulting in good handling properties such as paint manufacturability and painting workability.

[0065] The glass transition temperature (Tg) of the polyester resin is preferably between -35°C and 110°C, more preferably between -30°C and 80°C, and even more preferably between -30°C and 60°C. By having the glass transition temperature (Tg) of the polyester resin within this range, the moisture permeability of the coating film does not become excessively high, resulting in sufficient moisture resistance and good corrosion resistance.

[0066] The acid value of the polyester resin (including its modified products) may be 30 mgKOH / g or less, 0.1 mgKOH / g to 30 mgKOH / g, or 0.3 mgKOH / g to 30 mgKOH / g. Having the acid value within this range allows for improved hydrolysis resistance, for example, and the formation of a moisture-resistant coating. Furthermore, excellent corrosion resistance can be ensured.

[0067] Polyester resins can be obtained by polycondensation of polyhydric alcohols and polybasic acids.

[0068] Specific examples of polyhydric alcohols include, for example, ethylene glycol, diethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, polypropylene glycol, neopentyl glycol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol or 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, hydrogenated bisphenol A, hydroxyalkylated bisphenol A, 1,4-cyclohexanedimethanol, 2,2-dimethyl-3-hydroxypropyl-2,2-dimethyl-3-hydroxypropionate (BASHPN), N,N-bis-(2-hydroxyethyl)dimethylhydantoin, polycaprolactone polyol, glycerin, sorbitol, annitol, trimethylolethane, trimethylolpropane, trimethylolbutane, hexanetriol, pentaerythritol, dipentaerythritol, tris-(hydroxyethyl)isocyanate, and the like. Polyhydric alcohols may be used individually or in combination of two or more types.

[0069] Specific examples of polybasic acids include, for example, phthalic acid, phthalic anhydride, tetrahydrophthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic acid, hexahydrophthalic anhydride, methyltetraphthalic acid, methyltetrahydrophthalic anhydride, hymic anhydride, trimellitic acid, trimellitic anhydride, pyromellitic acid, pyromellitic anhydride, isophthalic acid, terephthalic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, adipic acid, azelaic acid, sebacic acid, succinic acid, succinic anhydride, lactic acid, dodecenyl succinic acid, dodecenyl succinic anhydride, cyclohexane-1,4-dicarboxylic acid, endo anhydride, etc. Polybasic acids may be used individually or in combination of two or more.

[0070] Examples of modified polyester resins include urethane-modified polyester resins, epoxy-modified polyester resins, acrylic-modified polyester resins, and silicone-modified polyester resins. Urethane-modified polyester resin is a resin that has polyester as its main chain, with its ends modified with isocyanate and then urethane-modified. Silicone-modified polyester resins can be prepared by reacting a polyester resin with an organosilicone (for example, an organosilicone with a number average molecular weight of about 300 to 1,000 having -SiOCH3 and / or -SiOH groups as functional groups). The amount of organosilicone used is usually about 5 to 50 parts by mass per 100 parts by mass of polyester resin. A urethane-modified polyester resin can be prepared by reacting the polyester resin with a polyisocyanate compound.

[0071] For example, examples of polyester resins and hydroxyl group-containing polyester resin modified products include Bekkolite 46-118, Bekkolite M-6205-50, Bekkolite M-6401-52, Bekkolite M-6402-50 from DIC Corporation, and Byron 220, Byron UR3500, Byron UR5537, Byron UR8300, Byron UR4410 from Toyobo Co., Ltd.

[0072] (Acrylic resin) The acrylic resin may be a modified product. In this disclosure, "acrylic resin" may include both acrylic resin and modified products of acrylic resin.

[0073] The number-average molecular weight (Mn) of the acrylic resin is preferably 1,400 to 40,000, more preferably 2,000 to 40,000, and particularly preferably 2,000 to 30,000. Having the aforementioned number-average molecular weight allows the crosslinking reaction between the acrylic resin and the crosslinking agent (B) to proceed sufficiently, forming a coating film with high moisture resistance. Furthermore, excellent corrosion resistance can be ensured. For example, although it should not be interpreted in a way that is limited to a specific theory, having the aforementioned number-average molecular weight allows for appropriate elution of magnesium hydroxide (C) contained in the coating film, enabling the formation of a coating film with good corrosion resistance even under acidic conditions. In addition, it is possible to suppress the crosslinking density of the coating film from becoming too high, enabling the formation of a coating film with sufficient elongation, for example, a coating film with sufficient bendability. Furthermore, the rust-preventive paint composition of this disclosure has an appropriate viscosity, resulting in good handling properties such as paint manufacturability and painting workability.

[0074] The glass transition temperature (Tg) of the acrylic resin is preferably between -35°C and 110°C, more preferably between -30°C and 80°C, and even more preferably between -30°C and 60°C. When the glass transition temperature (Tg) of the acrylic resin is within this range, the moisture permeability of the coating film does not become excessively high, resulting in sufficient moisture resistance and good corrosion resistance.

[0075] The acid value of the acrylic resin (including its modified products) may be between 0.1 mg KOH / g and 30 mg KOH / g, or between 0.1 mg KOH / g and 30 mg KOH / g, or between 0.3 mg KOH / g and 30 mg KOH / g. Having the acid value within this range allows for, for example, improved hydrolysis resistance and the formation of a moisture-resistant coating film. Furthermore, excellent corrosion resistance can be ensured.

[0076] Examples of acrylic resins include acrylic resins composed of one or more monomers selected from the following: (meth)acrylic monomers having hydroxyl groups, such as hydroxymethyl (meth)acrylate, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, and N-methylolacrylamide, and their lactone adducts; (meth)acrylic acid; (meth)acrylic acid esters such as alkyl (meth)acrylate; and (meth)acrylonitrile. In addition to the constituent units derived from the monomers, the acrylic resin may also contain constituent units derived from other monomers (for example, carboxyl group-containing ethylenic monomers such as crotonic acid, itaconic acid, fumaric acid, and maleic acid, and vinyl monomers such as styrene). In this disclosure, (meth)acrylic acid refers to acrylic acid or methacrylic acid. Examples of modified acrylic resins include silicone-modified acrylic resins. Silicone-modified acrylic resins can be prepared by reacting acrylic resin with an organic silicone as described above. The amount of organic silicone used is usually about 5 to 50 parts by mass per 100 parts by mass of acrylic resin.

[0077] (urethane resin) In one embodiment, the urethane resin may be a modified product thereof (modified urethane resin). In this disclosure, "urethane resin" may include both urethane resin and modified products of urethane resin.

[0078] The number-average molecular weight (Mn) of the urethane resin may be 1,000 to 30,000, 2,000 to 28,000, 2,500 to 25,000, or 6,000 to 15,000. Having the above number-average molecular weight allows the crosslinking reaction between the urethane resin and the crosslinking agent (B) to proceed sufficiently, forming a coating film with high moisture resistance. Furthermore, excellent corrosion resistance can be ensured. For example, although it should not be interpreted in a way that is limited to a specific theory, having the above number-average molecular weight allows for appropriate elution of magnesium hydroxide (C) contained in the coating film, enabling the formation of a coating film with good corrosion resistance even under acidic conditions. In addition, it is possible to suppress the crosslinking density of the coating film from becoming too high, forming a coating film with sufficient elongation, for example, a coating film with sufficient bendability. Furthermore, the rust-preventive paint composition of this disclosure has an appropriate viscosity, resulting in good handling properties such as paint manufacturability and painting workability.

[0079] In one embodiment, the glass transition temperature (Tg) of the urethane resin may be -30°C to 80°C, -30°C to 60°C, or -30°C to 50°C. In another embodiment, the glass transition temperature (Tg) of the urethane resin is -50°C to 70°C. By having the glass transition temperature (Tg) of the urethane resin within the above range, the moisture permeability of the coating film does not become excessively high, the moisture resistance of the coating film is sufficient, and the corrosion resistance is also good.

[0080] The acid value of the urethane resin (including its modified products) may be between 0.1 mg KOH / g and 30 mg KOH / g, or between 0.1 mg KOH / g and 30 mg KOH / g, or between 0.3 mg KOH / g and 30 mg KOH / g. Having the acid value within this range allows for, for example, improved hydrolysis resistance and the formation of a moisture-resistant coating film. Furthermore, excellent corrosion resistance can be ensured.

[0081] In one embodiment, the concentration of urethane groups (mass%) in the urethane resin in the urethane resin of this disclosure is 2% by mass or more and 20% by mass or less, for example, 5% by mass or more and 17% by mass or less. By having the concentration of urethane groups in the urethane resin within this range, excellent moisture resistance and corrosion resistance can be ensured in the formed coating film.

[0082] Examples of urethane resins include those obtained by reacting a polyol compound with a polyisocyanate compound, followed by further chain extension with a chain extender. The polyol compound is not particularly limited as long as it contains two or more hydroxyl groups per molecule. Examples include ethylene glycol, propylene glycol, diethylene glycol, 1,6-hexanediol, neopentyl glycol, triethylene glycol, glycerin, trimethylolethane, trimethylolpropane, polycarbonate polyol, polyester polyol, polyether polyol such as bisphenol hydroxypropyl ether, polyesteramide polyol, acrylic polyol, polyurethane polyol, or mixtures thereof. The polyisocyanate compound is not particularly limited as long as it contains two or more isocyanate groups per molecule. Examples include aliphatic isocyanates such as hexamethylene diisocyanate (HDI), alicyclic diisocyanates such as isophorone diisocyanate (IPDI), aromatic diisocyanates such as tolylene diisocyanate (TDI), aromatic aliphatic diisocyanates such as diphenylmethane diisocyanate (MDI), or mixtures thereof. The chain extender is not particularly limited as long as it is a compound containing one or more active hydrogen atoms in its molecule, and water or an amine compound can be used. Examples of amine compounds include aliphatic polyamines such as ethylenediamine, propylenediamine, hexamethylenediamine, diethylenetriamine, dipropylenetriamine, triethylenetetramine, and tetraethylenepentamine; aromatic polyamines such as tolylenediamine, xylylenediamine, and diaminodiphenylmethane; alicyclic polyamines such as diaminocyclohexylmethane, piperazine, 2,5-dimethylpiperazine, and isophoronediamine; hydrazines such as hydrazine, succinate dihydrazide, adipic acid dihydrazide, and phthalate dihydrazide; and alkanolamines such as hydroxyethyldiethylenetriamine, 2-[(2-aminoethyl)amino]ethanol, and 3-aminopropanediol.

[0083] In one embodiment, the urethane resin includes at least one selected from the group consisting of ester-based urethane resins, ether-based urethane resins, and carbonate-based urethane resins.

[0084] In one embodiment, the urethane resin is an ester-based urethane resin. In this disclosure, ester-based urethane resin means a resin having both urethane groups and ester groups in its main chain. Furthermore, ester-based urethane resin is defined as a resin in which the number of urethane groups is 5% or more greater than the number of ester groups in the main chain.

[0085] In one embodiment, the urethane resin can be obtained by polycondensation of a copolymer polyester resin having hydroxyl groups and a polyisocyanate compound. In another embodiment, the hydroxyl group-containing polyester resin can be prepared by polycondensation of an acid component such as a polycarboxylic acid and / or an acid anhydride with a polyhydric alcohol.

[0086] In one embodiment, the urethane resin is an ether-based urethane resin. In this disclosure, an ether-based urethane resin means a resin having both urethane groups and ether groups in its main chain. Furthermore, an ether-based urethane resin is defined as one in which the number of urethane groups is 5% or more greater than the number of ether groups, based on a comparison of the number of urethane groups and ester groups in the main chain.

[0087] In one embodiment, the urethane resin is a carbonate-based urethane resin. In this disclosure, a carbonate-based urethane resin means a resin having both urethane groups and carbonate groups in its main chain. Furthermore, a carbonate-based urethane resin is defined as one in which the number of urethane groups is 5% or more greater than the number of carbonate groups in the main chain.

[0088] In one embodiment, the acid value of the urethane resin may be 0.1 mg KOH / g or more and 50 mg KOH / g or less, 0.1 mg KOH / g or more and 40 mg KOH / g or less, or 0.3 mg KOH / g or more and 40 mg KOH / g or less. By having the acid value within such a range, for example, hydrolysis resistance can be improved and a coating film with moisture resistance can be formed. Furthermore, excellent corrosion resistance can be ensured.

[0089] Examples of polycarboxylic acids constituting copolymer polyester resins having hydroxyl groups include aliphatic polycarboxylic acids such as oxalic acid, succinic acid, adipic acid, and azelaic acid, aromatic polycarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid, 4,4"-diphenyldicarboxylic acid, and trimellitic acid, and examples of polyol components include ethylene glycol, propylene glycol, trimethylene glycol, 1,3-butadiol, 1,4-butadiol, 1,5-bentanediol, 1,6-hexanediol, neopentyl glycol, trimethylolpropane, trimethylolethane, and pentaerythritol. The copolymer polyesters of this disclosure are preferably those in which the equivalent ratio of carbonyl groups in the polycarboxylic acid to hydroxyl groups in the polyol component is in the range of 1.0 / 1.001 to 1.0 / 2.0, with an excess of hydroxyl groups, and can be obtained by conventional transesterification or direct esterification reactions. Particularly preferred equivalent ratios are in the range of 1.0 / 1.01 to 1.0 / 1.5.

[0090] The acid anhydrides that may be included in the acid component are not particularly limited, and examples include phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, hymicic anhydride, trimellitic anhydride, pyromellitic anhydride, maleic anhydride, succinic anhydride, dodecenyl succinic anhydride, and the like.

[0091] In the preparation of hydroxyl group-containing polyester resins, other reaction components may be used in addition to the acid component and the polyol component. Examples of other reaction components include monocarboxylic acids, hydroxycarboxylic acids, lactones, drying oils, semi-drying oils, and their fatty acids. More specifically, examples include monoepoxide compounds such as Cardura E (manufactured by Shell Chemical Corporation) and lactones. The lactones can be ring-opened and added to polycarboxylic acid and polyhydric alcohol polyesters to form graft chains, and examples include β-propiolactone, dimethylpropiolactone, butyllactone, γ-valerolactone, ε-caprolactone, γ-caprolactone, γ-caprylolactone, chlorotractone, δ-valerolactone, and δ-caprolactone. Among these, ε-caprolactone is most preferred.

[0092] Ester-based urethane resins can be prepared by reacting a hydroxyl group-containing polyester resin with an aliphatic diisocyanate compound, for example, by a known method.

[0093] The glass transition temperature of the coating-forming resin (A) may be -35°C or higher and 120°C or lower, or -30°C or higher and 110°C or lower.

[0094] The number-average molecular weight of the film-forming resin (A) is preferably 1,000 or more and 40,000 or less. Having a number-average molecular weight within this range allows the coating composition of this disclosure to form a coating film exhibiting excellent corrosion resistance, and furthermore, to form a coating film with excellent adhesion to the substrate and the topcoat coating composition (topcoat paint).

[0095] The solid content acid value of the coating resin (A) may be 0 mg KOH / g or more and 30 mg KOH / g or less.

[0096] In the coating composition of this disclosure, the solid content of the film-forming resin (A) may be preferably 10% to 80% by mass, more preferably 15% to 70% by mass, and even more preferably 15% to 50% by mass, based on 100% by mass of the total amount of the coating composition.

[0097] The coating composition of this disclosure may further contain a thermoplastic resin in addition to the film-forming resin (A). Examples of thermoplastic resins include chlorinated olefin resins such as chlorinated polyethylene and chlorinated polypropylene; homopolymers or copolymers with vinyl chloride, vinyl acetate, vinylidene chloride, etc. as monomer components; cellulose resins; acetal resins; alkyd resins; chlorinated rubber resins; modified polypropylene resins (such as acid anhydride modified polypropylene resins); and fluororesins (for example, vinylidene fluoride resin, vinyl fluoride resin, copolymers of fluorinated olefins and vinyl ethers, copolymers of fluorinated olefins and vinyl esters). Thermoplastic resins may be used individually or in combination of two or more types. By using thermoplastic resins in combination, better coating film properties, such as coating film strength and elongation, can be obtained.

[0098] [Crosslinking agent (B)] The crosslinking agent (B) reacts with the film-forming resin (A) to form a cured coating film. Examples of crosslinking agents (B) include polyisocyanate compounds; blocked polyisocyanate compounds (sometimes referred to as "BI") obtained by blocking the isocyanate group of a polyisocyanate compound with an active hydrogen-containing compound; amino resins; phenolic resins, etc. Of these, the crosslinking agent (B) preferably contains one or more selected from block polyisocyanate compounds and amino resins. By including such a crosslinking agent (B), it is possible to achieve excellent rust prevention over a long period of time and to form a coating film that exhibits excellent moisture resistance.

[0099] The polyisocyanate compound and the polyisocyanate compound constituting the said block polyisocyanate compound are not particularly limited, and conventionally known compounds can be used. Specific examples include, for example, 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 1,12-dodecamethylene diisocyanate, cyclohexane-1,3- or 1,4-diisocyanate, 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethylcyclohexane (also known as isophorone diisocyanate; IPDI), dicyclohexylmethane-4,4'-diisocyanate (also known as hydrogenated MDI), 2- or 4-isocyanatocyclohexyl-2'-isocyanatocyclohexylmethane, 1,3 Examples include - or 1,4-bis-(isocyanatomethyl)-cyclohexane, bis-(4-isocyanato-3-methylcyclohexyl)methane, 1,3- or 1,4-α,α,α'α'-tetramethylxylylene diisocyanate, 2,4- or 2,6-diisocyanatotoluene, 2,2'-, 2,4'- or 4,4'-diisocyanatodiphenylmethane (MDI), 1,5-naphthalene diisocyanate, p- or m-phenylene diisocyanate, xylene diisocyanate, diphenyl-4,4'-diisocyanate, etc. Furthermore, as polyisocyanate compounds, cyclized polymers of each diisocyanate (isocyanurate type), as well as isocyanate biuret derivatives (biuret type) and adduct types may be used. Polyisocyanate compounds may be used individually or in combination of two or more. Isocyanurate-type polyisocyanate compounds are one of those preferably used in this disclosure.

[0100] As the polyisocyanate compound, it is preferable to use an aromatic polyisocyanate compound that contains one or more aromatic functional groups in its molecule. By using an aromatic polyisocyanate compound, the moisture resistance of the coating film can be improved, as well as the strength of the coating film can be improved. Examples of preferred aromatic polyisocyanate compounds include 2,4- or 2,6-diisocyanatotoluene (TDI), 2,2'-, 2,4'- or 4,4'-diisocyanatodiphenylmethane (MDI), xylene diisocyanate (XDI), naphthalene diisocyanate (NDI), and the like.

[0101] The isocyanate group content of the polyisocyanate compound constituting the block polyisocyanate compound, as measured in accordance with JIS K 7301-1995, is typically 3-20%, preferably 5-15%, of the solid content of the polyisocyanate compound. Having the isocyanate group content within this range further improves the curability of the coating film. Furthermore, it can suppress excessively high crosslinking density in the resulting coating film, potentially leading to improved corrosion resistance.

[0102] The active hydrogen-containing compound (blocking agent) used in the blocked polyisocyanate compound is not particularly limited and can include compounds having an -OH group (alcohols, phenols, etc.), an =N-OH group (oximes, etc.), an =NH group (amines, amides, imides, lactams, etc.), a -CH2- group (active methylene group), and azoles. Specific examples include phenol, cresol, xylenol, ε-caprolactam, σ-valerolactam, γ-butyrolactam, methanol, ethanol, n-, i-, or t-butyl alcohol, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, propylene glycol monomethyl ether, benzyl alcohol, formamide oxime, acetaldehyde oxime, acetoxime, methyl ethyl kedoxime, diacetyl monooxime, benzophenone oxime, cyclohexane oxime, dimethyl malonate, ethyl acetoacetate, acetylacetone, pyrazole, and the like. The active hydrogen-containing compound may be used alone or in combination of two or more types.

[0103] The thermal dissociation temperature of a blocked polyisocyanate compound depends on the type of polyisocyanate compound and active hydrogen-containing compound that constitute it, as well as the presence and amount of catalyst. In this disclosure, a blocked polyisocyanate compound having a thermal dissociation temperature (without catalyst) of 120 to 180°C is preferably used. By using a blocked polyisocyanate compound exhibiting a dissociation temperature within this range, the stability of the paint can be improved, and because of its excellent crosslinking reactivity with the film-forming resin (A), a paint film with good moisture resistance can be obtained. Examples of blocked polyisocyanate compounds with a dissociation temperature of 120 to 180°C include Desmodule BL3175 manufactured by Sumika Covestro Urethane Co., Ltd. and Coronate 2554 manufactured by Tosoh Corporation. In this disclosure, blocked polyisocyanate compounds may be denoted as (BI).

[0104] Examples of amino resins include melamine resin and urea resin, with melamine resin being preferred. "Melamine resin" generally refers to a thermosetting resin synthesized from melamine and aldehyde, with three reactive functional groups (-NX) in one triazine molecule. 1 X 2 It has the following characteristics. Examples of melamine resins include four types: a fully alkyl type containing -N-(CH2OR)2 [R is an alkyl group, the same applies hereinafter] as a reactive functional group; a methylol group type containing -N-(CH2OR)(CH2OH) as a reactive functional group; an imino group type containing -N-(CH2OR)(H) as a reactive functional group; and a methylol / imino group type containing both -N-(CH2OR)(CH2OH) and -N-(CH2OR)(H), or containing -N-(CH2OH)(H) as a reactive functional group. In this disclosure, it is preferable to use a melamine resin that has an average of one or more methylol groups or imino groups per molecule, i.e., a methylol-type, imino-type, or methylol / imino-type melamine resin or a mixture thereof. Examples of melamine resins include Mycoat 715, a product manufactured by Ornex Japan Co., Ltd. Amino resins such as melamine resin exhibit excellent crosslinking reactivity with coating film-forming resin (A) even without a catalyst, allowing for the creation of coating films with good moisture resistance. In this disclosure, melamine resin may be referred to as (MF).

[0105] The amount of the crosslinking agent (B) according to this disclosure is preferably 1 to 150 parts by mass, for example, 2 to 150 parts by mass, per 100 parts by mass of solids of the coating resin (A). By including the crosslinking agent (B) under these conditions, it is possible to form a coating film that exhibits excellent rust prevention over a long period of time and also exhibits excellent moisture resistance.

[0106] In one embodiment, the amount of crosslinking agent (B) is 5 to 95 parts by mass in terms of solid content, relative to 100 parts by mass of solid content of the coating resin (A). By including the crosslinking agent (B) under these conditions, the crosslinking reaction between the coating resin (A) and the crosslinking agent (B) proceeds more smoothly, and a coating with even better moisture resistance and corrosion resistance can be formed. In addition, magnesium hydroxide (C) contained in the coating can be stably eluted over a long period of time, and a coating with even better corrosion resistance can be obtained.

[0107] [Body pigments] The paint compositions of this disclosure may further contain at least one extender pigment selected from the group consisting of calcium carbonate, barium sulfate, clay, talc, mica, and silica. By including extender pigments, the strength of the coating film can be further improved, and irregularities can be created on the surface of the coating film without impairing the physical properties of the coating film formed from the coating composition according to this disclosure. This further improves adhesion with the topcoat film. In addition, moisture resistance can be improved.

[0108] In one embodiment, the amount of extender pigment may be 1 to 40 parts by mass, or 10 to 30 parts by mass, based on 100 parts by mass of the total solid content of the coating resin (A) and the crosslinking agent (B). By having the amount of extender pigment within this range, the moisture resistance of the coating film can be improved. Furthermore, it is possible to suppress the excessive moisture permeability of the coating film, for example, by suppressing excessive water penetration into the coating film, thereby improving the moisture resistance of the coating film.

[0109] [Coupling agent] The coating compositions of this disclosure may further include at least one coupling agent selected from the group consisting of silane-based coupling agents, titanium-based coupling agents, and zirconium-based coupling agents. By adding a coupling agent, the adhesion between the substrate (object to be coated) and the coating film formed from the coating composition according to this disclosure can be further improved, and the moisture resistance of the coating film can be further improved.

[0110] The coupling agent is not particularly limited, and conventionally known ones can be used. Specific examples of preferred coupling agents include silane-based coupling agents such as Z-6011 and Z-6040 from Dow-Toray; titanium-based coupling agents such as Orgatics TC-401 and Orgatics TC-750 from Matsumoto Fine Chemical Co., Ltd.; and zirconium-based coupling agents such as Orgatics ZC-580 and Orgatics ZC-700 from Matsumoto Fine Chemical Co., Ltd. Among these, silane-based coupling agents are preferred.

[0111] The amount of coupling agent may be 0.1 parts by mass or more and 20 parts by mass or 0.5 parts by mass or more and 10 parts by mass per 100 parts by mass of the total solid content of the coating-forming resin (A) and the crosslinking agent (B). By keeping the amount of coupling agent within this range, the moisture resistance of the coating film can be improved, and the storage stability of the paint composition can be well maintained.

[0112] [Curing catalyst] When a blocked polyisocyanate compound and / or a polyisocyanate compound is used as the crosslinking agent (B), the coating composition of this disclosure may further contain a curing catalyst.

[0113] Examples of curing catalysts include tin catalysts, amine catalysts, and lead catalysts, with organotin compounds being particularly preferred. Examples of organotin compounds that can be used include dibutyltin dilaurate (DBTL), dibutyltin oxide, and tetra-n-butyl-1,3-diacetoxystanoxane. In one embodiment, the amount of curing catalyst may be 0.1 parts by mass or more and 10 parts by mass or more and 0.1 parts by mass or more and 1.0 part by mass or less, based on 100 parts by mass of the total solid content of the coating resin (A) and the crosslinking agent (B). By keeping the amount of curing catalyst within this range, for example, the storage stability of the paint composition can be well maintained. In another embodiment, the amount of effect catalyst may be 0 parts by mass based on 100 parts by mass of the total solid content of the coating resin (A) and the crosslinking agent (B).

[0114] Furthermore, even when an amino resin is used as the crosslinking agent (B), the coating composition of this disclosure may also contain a curing catalyst. Examples of curing catalysts in this case include acid catalysts such as carboxylic acids and sulfonic acids, among which dodecylbenzenesulfonic acid and p-toluenesulfonic acid are preferably used. The content of the curing catalyst may be 0.1 parts by mass or more and 10 parts by mass or 0.1 parts by mass or more and 1.0 part by mass or less, based on 100 parts by mass of the total solid content of the film-forming resin (A) and the crosslinking agent (B). By keeping the amount of curing catalyst within this range, for example, the storage stability of the paint composition can be well maintained.

[0115] [Other additives] The paint compositions of this disclosure may, if necessary, contain other additives not mentioned above. Other additives include, for example, rust-preventive pigments other than magnesium hydroxide (C); extender pigments other than those mentioned above; colorants such as coloring pigments and dyes; luminous pigments; solvents; ultraviolet absorbers (such as benzophenone-based ultraviolet absorbers); antioxidants (such as phenol-based, sulfoid-based, and hindered amine-based antioxidants); plasticizers; surface modifiers (silicones, organic polymers, etc.); anti-sagging agents; thickeners; lubricants such as waxes; pigment dispersants; pigment wetting agents; leveling agents; color separation inhibitors; precipitation inhibitors; defoaming agents; preservatives; antifreeze agents; emulsifiers; antifungal agents; antibacterial agents; stabilizers, etc. These additives may be used individually or in combination of two or more.

[0116] Examples of rust-preventive pigments other than magnesium hydroxide (C) include non-chromium-based rust-preventive pigments. Non-chromium-based rust-preventive pigments can be used insofar as they do not impair the effects achieved by this disclosure. Examples of non-chromium-based rust-preventive pigments include molybdate pigments (zinc molybdate, strontium molybdate, etc.), phosphomolybdate pigments (aluminum phosphomolybdate pigments, etc.), calcium silicate pigments, silicate-based rust-preventive pigments, and hydroxides or oxides of second elements such as calcium hydroxide and magnesium oxide. These may be used individually or in combination of two or more. The paint compositions of this disclosure exhibit sufficiently high corrosion resistance because they contain a predetermined magnesium hydroxide (C), but may further contain rust-preventive pigments other than the magnesium hydroxide (C) as described above, if necessary.

[0117] In addition to extender pigments, alumina, bentonite, and other pigments may be added, provided that they do not impair the moisture resistance, rust prevention, and bendability of the resulting coating film. These may be used individually or in combination of two or more types.

[0118] Examples of coloring pigments include inorganic pigments such as titanium dioxide, carbon black, graphite, iron oxide, and cold dust; organic pigments such as phthalocyanine blue, phthalocyanine green, quinacridone, perylene, anthrapyrimidine, carbazole violet, anthrapyridine, azo orange, flavanthrone yellow, isoindoline yellow, azo yellow, induthrone blue, dibromanzathrone red, perylene red, azo red, and anthraquinone red; and aluminum powder, alumina powder, bronze powder, copper powder, tin powder, zinc powder, iron phosphide, and finely atomized titanium. These may be used individually or in combination of two or more.

[0119] Examples of luminous pigments include foil pigments such as aluminum foil, bronze foil, tin foil, gold foil, silver foil, titanium metal foil, stainless steel foil, nickel-copper alloy foil, and foil-like phthalocyanine blue. These may be used individually or in combination of two or more.

[0120] Examples of solvents include water; glycol-based organic solvents such as ethylene glycol monobutyl ether (butyl cellosolve), diethylene glycol monobutyl ether, triethylene glycol monoethyl ether, propylene glycol monomethyl ether, propylene glycol monoethyl ether, propylene glycol monopropyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monoethyl ether, and propylene glycol monomethyl ether acetate; alcohol-based organic solvents such as methanol, ethanol, and isopropyl alcohol; ether-based organic solvents such as dioxane and tetrahydrofuran; ester-based organic solvents such as 3-methoxybutyl acetate, ethyl acetate, isopropyl acetate, and butyl acetate; ketone-based organic solvents such as methyl ethyl ketone, acetone, methyl isobutyl ketone, cyclohexanone, and isophorone; and N-methyl-2-pyrrolidone, toluene, pentane, iso-pentane, hexane, iso-hexane, cyclohexane, solvent naphtha, mineral spirits, Solvesso 100, and Solvesso 150 (all aromatic hydrocarbon solvents, manufactured by Shell Chemical Corporation). These may be used individually or in combination of two or more types.

[0121] The paint composition of this disclosure may be a water-based paint or an organic solvent-based paint.

[0122] The paint composition disclosed herein may be applied as an undercoat paint, also known as a primer. It may also be applied as a topcoat paint applied over an undercoat paint. Furthermore, it may be applied as an intermediate coat paint formed between an undercoat paint and a topcoat paint.

[0123] In another embodiment, the coating composition of this disclosure may be used as a coating composition for forming a single coating film rather than for forming a multi-layer coating film.

[0124] The paint composition disclosed herein can exhibit excellent corrosion resistance and moisture resistance regardless of which part of a multi-layer coating it is used on. The primer, topcoat, and intermediate coat paints other than the paint composition disclosed herein may be conventionally known. For example, the primer may be a conventionally known non-chromium rust-preventive paint, and the topcoat and intermediate coat may be a polyester resin-based paint, a fluororesin-based paint, etc.

[0125] The paint composition of this disclosure is preferably used as a primer. When the paint composition of this disclosure is used as a primer, it exhibits particularly good adhesion at the contact surface with the coating film formed by the topcoat or intermediate coat that is in direct contact with the primer.

[0126] [Method for preparing paint composition] The method for preparing the coating composition according to this disclosure is not particularly limited. The coating composition according to this disclosure can be prepared, for example, by mixing a film-forming resin (A), a crosslinking agent (B), magnesium hydroxide (C), and a metal phosphate salt (D), as well as optionally used thermoplastic resins, extender pigments, coupling agents, curing catalysts, and other additives, using a mixer such as a roller mill, ball mill, bead mill, pebble mill, sand grind mill, pot mill, paint shaker, or disper.

[0127] In another embodiment, the coating composition of the present disclosure may be a two-component coating comprising a main component comprising a film-forming resin (A), magnesium hydroxide (C), a metal phosphate salt (D), and optionally a metal vanadate salt (E), and a crosslinking agent component comprising a crosslinking agent (B).

[0128] [Object to be coated] The substrate on which a coating film of the rust-preventive paint composition of this disclosure is formed is not particularly limited as long as corrosion resistance is required. For example, a steel sheet that serves as a base material, such as a pre-coated metal (painted steel sheet), can be used.

[0129] Examples of steel sheets include galvanized steel sheets, cold-rolled steel sheets, stainless steel sheets, aluminum sheets, and Galvalume® steel sheets. Examples of galvanized steel sheets include zinc-containing plated steel sheets that utilize zinc sacrificial corrosion protection, specifically hot-dip galvanized steel sheets, electro-galvanized steel sheets, alloyed hot-dip galvanized steel sheets, aluminum-zinc plated steel sheets, nickel-zinc plated steel sheets, magnesium-aluminum-zinc plated steel sheets, magnesium-aluminum-silica-zinc plated steel sheets, and so on. It is preferable that the steel sheet be surface-treated with a chemical conversion agent before painting. The surface treatment may be carried out by known methods, including chromate treatment, zinc phosphate treatment, and other non-chromate treatments. The surface treatment can be appropriately selected depending on the steel sheet used, but treatments that do not contain heavy metals are preferred.

[0130] [Method for manufacturing rust-preventive coating] In one embodiment, the present disclosure provides a method for manufacturing a rust-preventive coating film, comprising a painting step of applying the coating composition of the present disclosure to an object to be coated, and a step of curing the coating composition at a temperature of 150°C to 270°C.

[0131] Conventional known methods such as roll coaters, airless sprayers, electrostatic sprayers, and curtain flow coaters can be used as methods for applying the coating composition of this disclosure to an object to be coated.

[0132] The coating film of this disclosure can be formed by applying the paint composition to a workpiece such as a steel plate, and then performing a baking treatment by heating the workpiece. The baking temperature (the maximum temperature reached by the workpiece such as a steel plate) is, for example, 150°C to 270°C. By curing the paint composition of this disclosure at such a temperature, a coating film with sufficient strength can be formed. By forming a coating film with sufficient strength, it is possible to further demonstrate excellent rust prevention over a long period of time and to form a coating film that exhibits excellent moisture resistance.

[0133] The curing time is, for example, 10 to 200 seconds. For example, when forming a multi-layer coating consisting of two layers, a primer coating and a topcoat coating, the primer coating composition may be applied and then cured, followed by the application of the topcoat coating composition and subsequent curing of the topcoat coating. Alternatively, the primer coating composition may be applied, and then the topcoat coating composition may be applied wet-on-wet without curing, and both coats may be cured simultaneously. The film thickness (dry film thickness) of the coating film of this disclosure is typically 1 to 30 μm, and for example, in the case of a topcoat coating film, it is preferably 10 to 30 μm.

[0134] [Regarding corrosion resistance] A coating film formed from the coating composition of this disclosure can show good results in both, for example, the "15-cycle immersion-drying test with acidic salt water" and the "2,000-hour CCHC test." Since it can show good results in these tests, it can achieve both long-term stable rust prevention and moisture resistance. Here, the "immersion-drying cycle test with acidic salt water" can be measured based on the method described in Japanese Patent No. 5857156. Good test results indicate that, for example, a coating with rust prevention properties in acidic environments and long-term stable rust prevention can be obtained. Furthermore, the "CCHC test 2,000 hours" is an evaluation test for paint film blistering, known as CCHC (Cleveland Condensing Humidity Cabinet: ASTM D-2247-87-Type A2), which evaluates the paint film by exposing it to a high-temperature, high-humidity environment for 2,000 hours. Furthermore, by extending the test period to 2,000 hours, it is believed that the coating film according to this disclosure can maintain excellent moisture resistance for a long period of time, even when exposed to more severe environmental conditions. The coating composition according to this disclosure can show excellent results in such multiple tests and can form a coating film that achieves both long-term stable rust prevention and moisture resistance. Furthermore, it can achieve both excellent rust prevention and moisture resistance in processed areas, end faces, cross-cut sections, etc. Furthermore, it can suppress the occurrence of rust that may be caused by acid rain, etc. In this disclosure, rust prevention under acidic environmental conditions can be evaluated based on tests using "acidic salt water." Furthermore, in this disclosure, a coating film with excellent rust prevention under acidic environmental conditions can be obtained, and for example, it can exhibit excellent rust prevention against acid rain. [Examples]

[0135] The present disclosure will be further illustrated by the following examples, but will not be limited thereto. In the examples, "parts" and "%" are based on mass unless otherwise specified.

[0136] [Magnesium hydroxide] Table 1 shows the various conditions and characteristic values ​​for magnesium hydroxides (C1) to (C2) used in the examples and comparative examples.

[0137] [Table 1]

[0138] [Preparation of coating-forming resin (A1) (ester-based urethane resin 1)] In a reaction vessel equipped with a thermometer, condenser, and stirrer, 55.0 parts by mass of adipic acid, 6.1 parts by mass of phthalic anhydride, 27.0 parts by mass of neopentyl glycol, and 26.1 parts by mass of propylene glycol were mixed. The mixture was gradually heated to 230°C under a nitrogen atmosphere, and the esterification reaction was carried out for approximately 10 hours while distilling off the generated water until the acid value became 1 or less (amount of dewatered: 14.2 parts by mass). After that, the temperature of the reaction vessel was lowered to 50°C, and 135.0 parts by mass of cyclohexanone and 35.0 parts by mass of 4,4'-diphenylmethane diisocyanate were mixed and the mixture was kept at 80°C for 8 hours to react and obtain ester-based urethane resin 1 (solid content concentration: 50% by mass).

[0139] The film-forming resins (A2) and (A3) (ester-based urethane resins 2 and 3, respectively) were prepared in the same manner as described above, except that the monomer types and quantities were changed as shown in Tables 2A and 2B. The monomer composition and various characteristic values ​​such as molecular weight for each resin are shown in Table 2.

[0140] [Table 2]

[0141] (Example 1) 260 parts by mass of film-forming resin (A1), 20 parts by mass of cyclohexanone, 12 parts by mass of Solvesso 150, and 100 parts by mass of magnesium hydroxide (C1) were mixed and dispersed using a sand mill (dispersion medium: glass beads) until the maximum particle size of the pigment coarse particles was 10 μm or less to prepare dispersion composition 1. To the obtained dispersion composition 1, 38.1 parts by mass of crosslinking agent (B1) (Desmodule BL-3575) and 0.5 parts by mass of TVS KS-1260 (manufactured by Kyodo Yakuhin Co., Ltd.) as a curing catalyst were added and uniformly mixed with a disperser to prepare paint composition 1. The details of the composition of Example 1 are shown in Table 3.

[0142] (Examples 2-19, Comparative Examples 1-3) The paint compositions were prepared in the same manner as in Example 1, except that the types and amounts of each component were changed as shown in Tables 3 to 5.

[0143] [Table 3]

[0144] [Table 4]

[0145] [Table 5]

[0146] The details of each component shown in the tables used in the examples and comparative examples are as follows. [Crosslinking agents (B1), (B2)] Details of the crosslinking agent (B) are as follows: • Crosslinking agent (B1) (Polyisocyanate compound 1): Desmodulo BL3575 (manufactured by Sumika Covestro Urethane Co., Ltd., blocked polyisocyanate), blocked form of hexamethylene diisocyanate (HDI) (isocyanurate type, blocking agent: dimethylpyrazole), isocyanate group content: 10.5% by mass, solid content concentration: 75% by mass • Crosslinking agent (B2) (amino resin); Mycoat 715 (manufactured by Ornex Japan, imino-based melamine resin), solid content concentration: 80% by mass

[0147] [Metal phosphate (D)] The following table shows the various conditions and characteristic values ​​for the metal phosphate salts (D1) to (D4) used in the examples and comparative examples.

[0148] [Table 6]

[0149] [Metal vanadate (E)] • Metal vanadate salt (E1); LF Bowsei CRF-318 (manufactured by Kikuchi Color Co., Ltd., calcium vanadate)

[0150] Details of other ingredients are as follows: • Solvent 1: Cyclohexanone (manufactured by Shoei Chemical Industry Co., Ltd.) • Solvent 2; Solvesso 150 (manufactured by Shell Chemicals) • Curing catalyst: TVS KS-1260 (manufactured by Kyodo Yakuhin Co., Ltd., dibutyltin dilaurate), non-volatile content: 100% by mass

[0151] [Method for preparing evaluation coating panels] After alkaline degreasing a 0.4mm thick aluminum-zinc plated steel sheet, a non-chromium chemical conversion treatment was applied to the front and back surfaces of the steel sheet using Surfcoat EC2310 (manufactured by Nippon Paint Surf Chemicals Co., Ltd.), and then dried. Next, the paint composition 1 obtained above was applied to the back surface of the treated steel sheet so that the dry coating film was 7 μm thick, and the sheet was baked at a maximum temperature of 180°C for 30 seconds to form a coating film on the back surface. On the other hand, one of the paint compositions from Examples 1 to 31 or Comparative Examples 1 to 10 was applied to the surface of the treated steel sheet so that the dry film thickness was 5 μm, and it was baked at a maximum temperature of 200°C for 30 seconds to form a surface primer coating. Furthermore, Nippon Paint Super Coat 300HQ (manufactured by Nippon Paint Industrial Coatings Co., Ltd.; polyester-based topcoat paint) was applied as a topcoat paint on the surface primer coating so that the dry film thickness was 10 μm, and it was baked at a maximum temperature of 210°C for 40 seconds to form a surface topcoat coating, thereby obtaining a painted steel sheet for evaluation.

[0152] The evaluation results of the paint compositions and painted steel sheets obtained in the examples, comparative examples, and reference examples are shown in the table below.

[0153] [Table 7]

[0154] [Table 8]

[0155] [Table 9]

[0156] [Evaluation criteria] (Procedure for measuring the solubility of magnesium hydroxide in artificially acidic seawater) As an acidic solution containing sulfate, we referred to the acidic salt aqueous solution used in Method B as defined in JIS G 0594:2019 Cycle Corrosion Test Method. Marine Art SF-1 (manufactured by Tomita Pharmaceutical Co., Ltd.) was used as artificial seawater, and the pH was adjusted to 2.5 using a mixture of nitric acid and sulfuric acid as specified in JIS G 0594 section 4.1.1 c) to create artificial acidic seawater. Next, 100g of the artificial acidic seawater and 1g of pigment (magnesium hydroxide) were added to a narrow-mouthed polyethylene bottle, a stirrer tip was added, and the mixture was stirred at room temperature for 4 hours. After standing at room temperature for 24 hours, the supernatant was collected using a syringe with a syringe filter, and the elemental concentrations were measured using an ICP emission spectrometer ICPS-7510 (Shimadzu Corporation). The elemental concentrations of the artificially acidic seawater were measured in the same manner. The magnesium metal ion concentration in the aqueous solution prepared by adding 1 g of magnesium hydroxide to 100 g of artificially acidic seawater was defined as the magnesium metal ion concentration in the aqueous solution prepared by subtracting the magnesium metal ion concentration in the artificially acidic seawater from the magnesium metal ion concentration in the supernatant.

[0157] (Procedure for measuring conductivity in pure water) 100g of deionized water and 1g of pigment (magnesium hydroxide) were added to a narrow-mouthed polyethylene bottle, a stirrer tip was inserted, and the mixture was stirred at room temperature for 4 hours. After that, the conductivity was measured using an electrical conductivity meter CM-42X (manufactured by Toa DKK Co., Ltd.).

[0158] (Storage stability test) The paint composition obtained above was left to stand at 50°C for 4 weeks. The state of the paint composition after standing was visually observed, and its storage stability was evaluated according to the following criteria. 5. The paint composition is uniform even without stirring. 4. Although some sedimentation occurs, the paint composition can be easily made uniform by stirring with a spatula. 3: Some sediment has formed, but stirring with a spatula will make the paint composition uniform. 2: Although sedimentation occurs, the paint composition becomes uniform through agitation with the disperser. 1: Sedimentation occurs, and the paint composition does not become uniform even when stirred with a disperser.

[0159] [Evaluation criteria for coating films] 1) Boiling water resistance test The painted steel sheet obtained above was cut into 5cm x 10cm pieces, and the resulting test pieces were immersed in boiling water at approximately 100°C for 2 hours. After being removed, the appearance of the coating on the surface was evaluated according to ASTM D714-56 (flat surface blister evaluation). Here, ASTM D714-56 evaluates the size (average diameter) and density of each blister by comparing them to standard assessment photographs and indicates a grade symbol. For size, there are four grades in the order of 8 (approximately 1 mm in diameter), 6 (approximately 2 mm in diameter), 4 (approximately 3 mm in diameter), and 2 (approximately 5 mm in diameter). For density, there are five grades from smallest to largest: F, FM, M, MD, and D. If there is no blister, it is graded 10. A score of 8FM or higher is considered good. Furthermore, painted steel plate test pieces that had been immersed in boiling water at approximately 100°C for 2 hours were evaluated using a grid tape adhesion test (grid adhesion test). In the grid tape adhesion test, in accordance with JIS K 5400 8.5.2 (1990) grid tape method, the gap between cuts was set to 1 mm, 100 grids were created, cellophane adhesive tape was adhered to the surface, and the number of grids remaining on the painted surface when the tape was rapidly peeled off was examined.

[0160] 2) Humidity resistance test (CCHC test) Painted steel sheets were cut into 5cm x 10cm pieces. The resulting test specimens were left in pure water at 50°C x 98RH for 500 hours. Then, the blistering of the flat surface was evaluated according to ASTM D714-56, in the same manner as the boiling water resistance test. A score of 8FM or higher was considered good. A large wet testing machine (manufactured by Suga Test Instruments Co., Ltd.) was used for this test.

[0161] 3) Corrosion resistance test Painted steel plates were cut to 5cm x 15cm. Cutting was performed alternately from the front and back surfaces, so that each test piece had both an upper burr (cut from the back) and a lower burr (cut from the front) on its cross-section. Next, a narrow 30-degree cross-cut with a 0.5 mm width was made with a utility knife in the center of the surface side, reaching the base material. The upper edge of the painted steel plate was sealed with rust-preventive paint, and a 2T bend was created at the lower end (the two plates were removed after processing). Similarly, a 1T bending section was also created (after processing, one sheet of metal is removed). The 2T bending process involves bending a painted plate with the surface facing outwards, inserting two plates of the same thickness as the painted plate inside, and then bending the painted plate 180 degrees using a vise. After processing, the two plates were removed and used for testing. Similarly, bending a plate with one plate inserted is called the 1T bending process.

[0162] A schematic diagram of the painted steel sheet test specimen obtained as described above is shown in Figure 1. Figure 1(A) schematically shows the cross-sections 20 of the upper burr and 30 of the lower burr in the obtained painted steel sheet test piece 10. The painted steel sheet test piece 10 also has a painted surface 11 and a painted back surface 12. Figure 1(B) is a schematic diagram showing the cross-cut portion 40 and the 2T bent portion 50 provided on the painted steel sheet test piece 10 used in the corrosion resistance test. The painted steel sheet test piece 10 also has an upper burr 21 and a lower burr 31.

[0163] Each painted steel sheet specimen was subjected to a combined cycle corrosion test (CCT) in accordance with JIS K 5600-7-9A JASO M609. One cycle consisted of (spraying with 5% saline solution at 35°C for 2 hours) - (drying at 60°C for 4 hours) - (standing in a humidity-resistant testing chamber at 50°C with an RH of 95% or higher for 2 hours), and 120 cycles (totaling 960 hours) were performed. After this test, the condition of the edges, cross-cut sections, and 1T and 2T bent sections of the painted steel sheet specimens was evaluated based on the evaluation method and criteria described below. A score of 4 or higher for the 2T bent section and 3 or higher for the 1T bent section was considered good. Equipment used: Combined cycle tester CYP-90 (manufactured by Suga Test Instruments Co., Ltd.)

[0164] (Corrosion resistance test: 1T and 2T bent sections) The total length of rusted areas in the 1T and 2T bent sections was determined and evaluated according to the following criteria. 5: No rust was observed. 4: White rust is present, but less than 10mm in size. 3: White rust is 10mm or more but less than 25mm. 2: White rust is 25mm or more but less than 40mm. 1: White rust is 40 mm or more in thickness, or red rust is observed.

[0165] (Corrosion resistance test: edge area) The average value of the edge creep width (width of bulge) on the left and right long sides (i.e., the long side with an upper burr and the long side with a lower burr) of the painted steel sheet test specimen was calculated and evaluated according to the following criteria. 5: Blister width is less than 5mm. 4. The blister width is 5mm or more but less than 10mm. 3: The blister width is 10mm or more but less than 15mm. 2: The blister width is 15mm or more but less than 20mm. 1: The blister width is 20mm or more.

[0166] (Corrosion resistance test: cross-cut section) The corrosion state of the cross-cut section was evaluated according to the following criteria, based on the ratio of the length of white rust occurrence in the exposed base material with a cut width of 0.5 mm, and the average value of the blister width on both sides of the cross-cut section (sum of both sides). 5: White rust occurs in less than 25% of exposed substrate length and blister width is less than 3 mm. 4: White rust occurs in exposed substrate areas with a length ratio of 25% or more but less than 50%, and blister width is less than 3 mm. 3: White rust occurs in 50% or more of the exposed surface area, and the blister width is less than 3 mm. 2: White rust occurs in 50% or more of the exposed substrate area, with a blister width of 3 mm or more but less than 5 mm. 1: White rust occurs in 50% or more of the exposed surface area, with a blister width of 5 mm or more.

[0167] 4) Alkali resistance test Each painted steel sheet was cut into 5cm x 10cm pieces. Each test piece was immersed in a 5% sodium hydroxide aqueous solution at 23°C for 48 hours, then removed, washed with water, and dried at room temperature. The blistering of the flat surface of these painted steel sheet test pieces was evaluated according to ASTM D714-56, in the same manner as the boiling water resistance test. A score of 8FM or higher was evaluated as good.

[0168] 5) Acid resistance test Each painted steel sheet was cut into 5cm x 10cm pieces. The resulting test pieces were immersed in a 5% sulfuric acid aqueous solution at 23°C for 48 hours, then removed, washed, and dried at room temperature. The blistering of the flat surface of these painted steel sheet test pieces was evaluated according to ASTM D714-56, in the same manner as the boiling water resistance test. A score of 8FM or higher was considered good.

[0169] 6) Immersion-drying cycle test with acidic salt solution (Adjustment of the test board) Each of the painted steel plates obtained above was cut into 5cm x 10cm pieces, and a 2T bend was made at the lower end of the resulting test piece in the same manner as in the corrosion resistance test.

[0170] (Preparation of acidic saline solution) As an acidic solution containing sulfate, we referenced the acidic salt aqueous solution used in Method B of the JIS G 0594 Cycle Corrosion Test Method. Specifically, we used Marine Art SF-1 (manufactured by Tomita Pharmaceutical Co., Ltd.) as it is close in composition to the artificial seawater specified in JIS G 0594, and adjusted the pH to 2.5 with a mixture of nitric acid and sulfuric acid as specified in Section 4.2.2 to obtain an acidic salt solution.

[0171] (Test conditions) Each painted steel sheet specimen was subjected to a 15-cycle test (totaling 360 hours) under 23°C conditions, with one cycle consisting of 6 hours of immersion followed by 18 hours of drying. After the test, the condition of the edges, cross-cut sections, and 2T bent sections of the painted steel sheet specimens was evaluated based on the evaluation method and criteria described below. A score of 4 or higher was considered good in all cases.

[0172] (Test using acidic salt solution: 2T bent section) The total length of rusted areas in the 4T bent section was determined and evaluated according to the following criteria. 5: No rust was observed. 4: White rust is present, but less than 10mm in size. 3: White rust is 10mm or more but less than 25mm. 2: White rust is 25mm or more but less than 40mm. 1: White rust is 40 mm or more in thickness, or red rust is observed.

[0173] (Test using acidic salt solution: edge area) The average value of the edge creep width (width of bulge) on the left and right long sides (i.e., the long side with an upper burr and the long side with a lower burr) of the painted steel sheet test specimen was calculated and evaluated according to the following criteria. 5: Blister width is less than 5mm. 4. The blister width is 5mm or more but less than 10mm. 3: The blister width is 10mm or more but less than 15mm. 2: The blister width is 15mm or more but less than 20mm. 1: The blister width is 20mm or more.

[0174] (Test with acidic salt solution: cross-cut area) The corrosion state of the cross-cut section was evaluated according to the following criteria, based on the ratio of the length of white rust occurrence in the exposed base material with a cut width of 0.5 mm, and the average value of the blister width on both sides of the cross-cut section (sum of both sides). 5: White rust occurs in less than 25% of exposed substrate length and blister width is less than 3 mm. 4: White rust occurs in exposed substrate areas with a length ratio of 25% or more but less than 50%, and blister width is less than 3 mm. 3: White rust occurs in 50% or more of the exposed surface area, and the blister width is less than 3 mm. 2: White rust occurs in 50% or more of the exposed substrate area, with a blister width of 3 mm or more but less than 5 mm. 1: White rust occurs in 50% or more of the exposed surface area, with a blister width of 5 mm or more.

[0175] According to the results of the examples, the rust-preventive coating composition of this disclosure exhibits good storage stability and was able to form a coating film that shows excellent rust prevention and excellent moisture resistance over a long period of time. Furthermore, it was able to form a coating film that shows excellent rust prevention over a long period of time even under acidic environmental conditions. Therefore, it can suppress the occurrence of rust that may be caused by, for example, "acid rain". In addition, it was possible to suppress or significantly reduce the decrease in moisture resistance of the coating film, and moreover, it was possible to suppress the occurrence of blistering on the coating film. Moreover, in the examples, the protective effect of the coating film was sufficiently demonstrated even on edges, cross-cut areas, etc.

[0176] Comparative Example 1 was an example that did not contain metal phosphate salt (D), and the resulting coating did not provide satisfactory corrosion resistance (in the bent portion). Comparative Example 2 was an example that did not contain magnesium hydroxide (C), and the resulting coating did not provide satisfactory corrosion resistance (at the edges). Comparative Examples 3 and 4 are examples that do not contain magnesium hydroxide (C) and metal acid salt (D). Of these, Comparative Example 3 was an example containing only the metal vanadate salt (E), and the resulting coating film did not adequately satisfy the requirements for boiling water resistance, alkali resistance, and acid resistance. [Industrial applicability]

[0177] The rust-preventive coating composition disclosed herein can exhibit excellent rust prevention over a long period of time and can also form a coating film that exhibits excellent moisture resistance. Furthermore, the method for manufacturing the coating film according to this disclosure can produce a coating film that exhibits excellent rust prevention over a long period of time and also exhibits excellent moisture resistance. [Explanation of Symbols]

[0178] 10 Painted steel plate test specimens 11. Coating surface 12. Back surface of the coating 20 Cross-section of the upper beam 21 Upper bar 30 Cross-section of the lower beam 31 Lower hook 40 Cross-cut section 50 1T and 2T bending sections

Claims

1. A rust-preventive coating composition comprising a film-forming resin (A), a crosslinking agent (B), magnesium hydroxide (C), and a metal phosphate salt (D).

2. The rust-preventive coating composition according to claim 1, wherein the metal phosphate salt (D) includes a condensed metal phosphate salt.

3. The rust-preventive coating composition according to claim 1, wherein the metal phosphate salt (D) is a metal tripolyphosphate salt.

4. The rust-preventive paint composition according to claim 1, wherein the conductivity of an aqueous solution obtained by mixing 1 g of magnesium hydroxide (C) with 100 g of pure water is 250 μS / cm or less.

5. The pH of the aqueous solution obtained by mixing 1 g of the magnesium hydroxide (C) with 100 g of pure water is 8 or higher and 12 or lower, and The rust-preventive coating composition according to claim 1, wherein the average particle size of the magnesium hydroxide (C) is 0.5 μm or more and 20 μm or less.

6. The rust-preventive coating composition according to claim 1, wherein the magnesium hydroxide (C) is contained in an amount of 1 to 150 parts by mass with respect to 100 parts by mass of the total solid content of the coating resin (A) and the crosslinking agent (B).

7. The rust-preventive coating composition according to claim 1, wherein the conductivity of an aqueous solution obtained by mixing 1 g of the metal phosphate salt (D) with 100 g of pure water is 200 μS / cm or more and 500 μS / cm or less.

8. The rust-preventive coating composition according to claim 1, wherein the content of the metal phosphate salt (D) is 20 parts by mass or more and 80 parts by mass or less in a total of 100 parts by mass of the magnesium hydroxide (C) and the metal phosphate salt (D).

9. The rust-preventive coating composition according to claim 1, wherein the total content of the magnesium hydroxide (C) and the metal phosphate salt (D) is 5 parts by mass or more and 80 parts by mass or less, based on 100 parts by mass of the total solid content of the coating resin (A) and the crosslinking agent (B).

10. The rust-preventive coating composition according to claim 1, further comprising a metal vanadate salt (E).

11. The rust-preventive coating composition according to claim 10, wherein the total content of the vanadate metal salt (E) is 0 parts by mass or more and 30 parts by mass or less in 100 parts by mass of the total of the magnesium hydroxide (C), the phosphate metal salt (D), and the vanadate metal salt (E).

12. The rust-preventive paint composition according to claim 1, wherein the film-forming resin (A) comprises at least one selected from the group consisting of epoxy resin, polyester resin, acrylic resin, urethane resin, and modified products thereof.

13. The coating-forming resin (A) comprises at least one selected from the group consisting of urethane resins and modified products thereof. The rust-preventive coating composition according to claim 1, wherein the glass transition temperature of the urethane resin is -50°C or higher and 70°C or lower.

14. The coating-forming resin (A) comprises at least one selected from the group consisting of urethane resins and modified products thereof. The rust-preventive coating composition according to claim 1, wherein the urethane resin comprises at least one selected from the group consisting of ester-based urethane resins, ether-based urethane resins, and carbonate-based urethane resins.

15. The rust-preventive paint composition according to claim 1, wherein the number average molecular weight of the film-forming resin (A) is 1,000 or more and 40,000 or less.

16. The rust-preventive paint composition according to claim 1, wherein the film-forming resin (A) comprises at least one selected from the group consisting of ester-based urethane resins, epoxy resins, polyester resins, and modified products thereof.

17. The rust-preventive coating composition according to claim 1, wherein the solid content acid value of the coating film-forming resin (A) is 30 mg KOH / g or less.

18. A painting step of applying the rust-preventive paint composition according to any one of claims 1 to 17 to an object to be painted, and A step of curing the rust-preventive paint composition at a temperature of 150°C to 270°C. A method for manufacturing a rust-preventive coating, including the method described above.

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