JP2024067756A - Aqueous surface treatment composition - Google Patents

Abstract

[Problem] To provide an aqueous metal surface treatment agent capable of forming a film capable of withstanding high-strength workability on metal substrates. [Solution] An aqueous surface treatment composition comprising a polyphenol compound, an epoxysilane compound, a carboxyl group-containing acrylic resin, and a nitrogen-containing phosphate compound consisting of a nitrogen-containing cation and a phosphate anion. It is preferable that the content of the polyphenol compound is 1.0 to 6.0 g/L, the content of the epoxysilane compound is 5.0 to 15 g/L, the content of the carboxyl group-containing acrylic resin is 5.0 to 20 g/L, and the content of the nitrogen-containing phosphate compound is 0.5 to 3 g/L. [Selected Figure] None

Description

The present invention relates to an aqueous surface treatment composition.

Conventionally, chromium-based metal surface treatment agents such as chromate treatment agents and chromate phosphate treatment agents have been known as surface treatment agents for imparting corrosion resistance to metal substrates, and are still widely used today. However, in light of recent trends in environmental regulations, the use of chromium-based metal surface treatment agents may be restricted in the future due to the toxicity of chromium, particularly its carcinogenicity.

Therefore, various chromium-free metal surface treatment agents that exhibit corrosion resistance equivalent to that of chromium-based metal surface treatment agents have been developed. For example, Patent Document 1 discloses a metal surface treatment composition that contains a condensation reaction product of a titanium compound and/or a zirconium compound, an aminosilane, and a polysilyl-functional silane. Patent Document 2 discloses an aqueous rust-preventive surface treatment composition that contains a silane coupling agent, a water-soluble transition metal compound that contains a water-soluble titanium compound or a water-soluble zirconium compound, a condensed phosphate, and a solvent that contains water, and the condensed phosphate contains a highly condensed phosphate that is a salt of a highly condensed product of four or more phosphoric acids.

JP 2011-068930 A JP 2020-73729 A

When a film is formed on a metal substrate using a chromium-free metal surface treatment agent, for example when the metal substrate is used for precoating metal, the formed film is required to have paint adhesion capable of withstanding high-strength processing in addition to corrosion resistance. However, conventional chromium-free metal surface treatment agents do not have sufficient paint adhesion when high-strength processing is performed on the metal substrate after surface treatment, and there is room for improvement. For example, a treatment agent having a silane coupling agent as a main component as described in Patent Document 1 cannot form a uniform film on the metal surface because paint cissing occurs during painting when the metal substrate to be coated has a smooth surface, and the paint adhesion is also insufficient. In addition, an aqueous rust-preventive surface treatment composition containing a condensed phosphate as described in Patent Document 2 is not designed to be painted on the formed film, and does not have sufficient paint adhesion capable of withstanding high-strength processing after painting.

The present invention has been made in view of the above, and aims to provide an aqueous surface treatment composition that can form a film that can withstand high-strength workability on a metal substrate, even when the substrate has a coating film on top of the coating.

(1) An aqueous surface treatment composition comprising a polyphenol compound, an epoxysilane compound, a carboxyl group-containing acrylic resin, and a nitrogen-containing phosphate compound comprising a nitrogen-containing cation and a phosphate anion.

(2) The aqueous surface treatment composition according to (1), in which the content of the polyphenol compound in the aqueous surface treatment composition is 1.0 to 6.0 g/L, the content of the epoxysilane compound in the aqueous surface treatment composition is 5.0 to 15 g/L, the content of the carboxyl group-containing acrylic resin in the aqueous surface treatment composition is 5.0 to 20 g/L, and the content of the nitrogen-containing phosphate compound in the aqueous surface treatment composition is 0.5 to 3 g/L.

(3) The aqueous surface treatment composition according to (1) or (2), further comprising an alcohol compound having 3 or less carbon atoms, the content of the alcohol compound in the aqueous surface treatment composition being 5.0 to 30 g/L.

(4) The aqueous surface treatment composition according to any one of (1) to (3), further comprising an amino compound having one or more amino groups in the molecule, the content of the amino compound in the aqueous surface treatment composition being 0.1 to 10 g/L.

(5) The aqueous surface treatment composition according to any one of (1) to (4), wherein the molecular weight of the polyphenol compound is 1,000 or more.

(6) The aqueous surface treatment composition according to any one of (1) to (5), wherein the epoxysilane compound has a cycloalkyl group.

(7) The aqueous surface treatment composition according to any one of (1) to (6), wherein the weight average molecular weight of the carboxyl group-containing acrylic resin is 80,000 to 1,500,000 and the acid value of the solid content is 150 to 780 mgKOH/g.

(8) The aqueous surface treatment composition according to any one of (1) to (7), wherein the phosphate anion is an orthophosphate ion.

(9) The aqueous surface treatment composition according to (4), wherein the amino compound is a compound having two or more amino groups in the molecule.

(10) An aqueous surface treatment composition according to any one of (1) to (9), having a pH in the range of 2.0 to 8.0.

(11) A surface treatment film obtained by curing the aqueous surface treatment composition described in any one of (1) to (10).

The present invention provides an aqueous surface treatment composition that can form a coating on a metal substrate that can withstand high-strength workability.

The aqueous surface treatment composition according to an embodiment of the present invention will be described below. The present invention is not limited to the description of the following embodiment.

<Aqueous surface treatment composition>
The aqueous surface treatment composition according to the present embodiment contains a polyphenol compound, an epoxysilane compound, a carboxyl group-containing acrylic resin, and a nitrogen-containing phosphate compound consisting of a nitrogen-containing cation and a phosphate anion, and may also contain an alcohol compound and an amino compound described below.

The aqueous surface treatment composition according to this embodiment is preferably a chromium-free aqueous surface treatment composition that does not contain trivalent or hexavalent chromium. The form of trivalent or hexavalent chromium is not particularly limited, and may be metallic chromium, chromium ions, chromium compounds, etc.

(Polyphenol Compounds)
The polyphenol compound improves the adhesion between the film formed by the aqueous surface treatment composition and the substrate on which the film is formed, thereby suppressing the corrosion of the substrate. The polyphenol compound is a compound having two or more phenolic hydroxyl groups bonded to a benzene ring or a condensate thereof. Examples of the compound having two or more phenolic hydroxyl groups bonded to the benzene ring include catechol, pyrogallol, flavonoids (catechin, epicatechin, epigallocatechin, etc.), gallic acid, ellagic acid, curcumin, tannic acid, etc. In addition, tannic acid is a general term for aromatic compounds with a complex structure having a large number of phenolic hydroxyl groups that are widely distributed in the plant kingdom. The polyphenol compound may be used alone or in combination of two or more types.

The condensation product of a compound having two or more phenolic hydroxyl groups bonded to a benzene ring is not particularly limited, and examples thereof include polyphenol resins. Commercially available products can be used as the polyphenol resin. Examples include polyphenol resin "Resitop PL-4012" manufactured by Gun-ei Chemical Industry Co., Ltd. and polyphenol resin "Sumilite Resin NPK-238" manufactured by Sumitomo Bakelite Co., Ltd.

The molecular weight of the polyphenol compound is preferably 1000 or more. In this specification and claims, the molecular weight of the polyphenol compound means the molar molecular weight (g/mol) when the polyphenol compound is a compound (single component) having two or more phenolic hydroxyl groups bonded to the benzene ring, and means the weight average molecular weight (Mw) when the polyphenol compound is a condensate of a polyphenol compound such as the polyphenol resin. The weight average molecular weight is the weight average molecular weight in terms of polystyrene measured by gel permeation chromatography (GPC).

The content of the polyphenol compound in the aqueous surface treatment composition is preferably 1.0 to 6.0 g/L, and more preferably 1.5 to 4.5 g/L. If the content is less than 1.0 g/L, paint adhesion will decrease. If the content is more than 6.0 g/L, paint adhesion will decrease.

(Epoxy silane compound)
The epoxy silane compound crosslinks with a carboxyl group-containing acrylic resin or an amino compound to improve the barrier properties of the film formed by the aqueous surface treatment composition against moisture and chloride ions. The epoxy silane compound is a compound containing an epoxy group and one or more alkoxysilane groups in one molecule, and examples of the epoxy silane compound include aliphatic epoxy silanes such as glycidoxypropylmethyldimethoxysilane, glycidoxypropylmethyldiethoxysilane, glycidoxypropyltrimethoxysilane, and glycidoxypropyltriethoxysilane; epoxy silanes having a cycloalkyl group such as epoxycyclohexylethyltrimethoxysilane, and the like.

The epoxy silane compound preferably has a cyclic cycloalkyl group, which is more hydrophobic than chain silanes, from the viewpoint of improving the hydrophobicity of the film formed by the aqueous surface treatment composition and improving corrosion resistance by forming a film that is difficult for moisture to permeate. Specific examples of epoxy silanes having a cycloalkyl group include 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate, and the like. The epoxy silane compounds may be used alone or in combination of two or more kinds.

The content of the epoxy silane compound in the aqueous surface treatment composition is preferably 5.0 to 15 g/L, and more preferably 7.0 to 13 g/L. If the content is less than 5.0 g/L, it will cause a decrease in paint adhesion. If the content is more than 15 g/L, it will cause a decrease in paint adhesion.

(Carboxy group-containing acrylic resin)
The carboxyl group-containing acrylic resin improves the barrier properties of the film formed by the aqueous surface treatment composition against moisture and chloride ions that induce corrosion. The carboxyl group-containing acrylic resin can be synthesized by (co)polymerizing a carboxyl group-containing acrylic monomer such as acrylic acid, methacrylic acid, itaconic acid, etc. One or more types of monomers can be used as the carboxyl group-containing monomer. In addition to the carboxyl group-containing acrylic monomer, a hydroxyl group-containing acrylic monomer, an acrylic acid ester monomer, or a non-acrylic monomer may be copolymerized as necessary. The carboxyl group-containing acrylic resin may be used alone or in combination of two or more types.

As the carboxyl group-containing acrylic resin, commercially available products can be used. For example, polyacrylic acids "Jurymer SH-8", "Jurymer AC-10H", "Jurymer AC-10L", "Aron A-7185" and the like manufactured by Toagosei Co., Ltd. can be mentioned.

Examples of hydroxyl group-containing acrylic monomers include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 3-hydroxybutyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate.

Examples of acrylic acid ester monomers include methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, 1-methylethyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, phenyl (meth)acrylate, isobornyl (meth)acrylate, cyclohexyl (meth)acrylate, t-butylcyclohexyl (meth)acrylate, dicyclopentadienyl (meth)acrylate, and the like.

Non-acrylic monomers include styrene, α-methylstyrene, t-butylstyrene, vinylnaphthalene, acrylonitrile, methacrylonitrile, ethylene, propylene, vinyl acetate, vinyl propionate, butadiene, isoprene, etc.

The weight average molecular weight of the carboxyl group-containing acrylic resin is preferably 80,000 to 1,500,000, and more preferably 100,000 to 1,000,000. If the weight average molecular weight is less than 80,000, it will cause a decrease in paint adhesion. If the weight average molecular weight is more than 1,500,000, it will be more likely to cause a decrease in storage stability.

The acid value of the solid content of the carboxyl group-containing acrylic resin is preferably 150 to 780 mgKOH/g, and more preferably 180 to 720 mgKOH/g. If the acid value of the solid content is less than 150 mgKOH/g, the corrosion resistance after coating decreases. If the acid value of the solid content is more than 780 mgKOH/g, the corrosion resistance after coating decreases.

The solid content hydroxyl value of the carboxyl group-containing acrylic resin is preferably 0 to 350 mg KOH/g, and more preferably 0 to 300 mg KOH/g.

The content of the carboxyl group-containing acrylic resin in the aqueous surface treatment composition is preferably 5.0 to 20 g/L, and more preferably 7.0 to 17 g/L. If the content is less than 5.0 g/L, the paint adhesion decreases. If the content is more than 20 g/L, the paint adhesion decreases.

The solid content acid value and solid content hydroxyl value of the acrylic resins described above in this specification and claims can be measured in accordance with JIS K 0070.

(Nitrogen-containing phosphate compounds)
The nitrogen-containing phosphate compound is a compound consisting of a nitrogen-containing cation and a phosphate anion. The nitrogen-containing phosphate compound improves the corrosion inhibitory effect of the film formed by the aqueous surface treatment composition. The nitrogen-containing phosphate compound can easily adjust the pH of the aqueous surface treatment composition to a suitable range by containing a nitrogen element in the cation, and can avoid adverse effects such as corrosion caused by the cation by volatilizing the cation during curing of the film. Alternatively, even if the cation is a guanidium ion or the like that does not volatilize and remains in the film, the guanidium ion or the like functions as an inhibitor. Therefore, adverse effects such as corrosion caused by the cation can be similarly avoided. On the other hand, if the cation is a cation containing sodium ion or potassium ion, these cations remain in the film and tend to induce alkali blistering in a corrosion resistance test after painting.

Examples of the nitrogen-containing cation include ammonium ions (NH4 ⁺ ), which volatilize when the coating of the aqueous surface treatment composition hardens and do not remain in the coating, and guanidinium ions, which remain in the coating but can adsorb to the surface of the metal substrate and function as inhibitors.

Examples of the phosphate anion include orthophosphate ion (PO ₄ ³⁻ ), pyrophosphate ion (P ₂ O ₇ ⁴⁻ ), etc., with orthophosphate ion being preferred.

Examples of the nitrogen-containing phosphate compound include ammonium phosphate ( _ammonium monophosphate: _NH4H2PO4 , ammonium _diphosphate : _{(NH4)2HPO4, ammonium triphosphate: (NH4)3PO4 ) , ammonium pyrophosphate} , guanidinium phosphate, etc. The nitrogen-containing phosphate compound may be used alone or in _combination of two or more kinds.

The content of the nitrogen-containing phosphate compound in the aqueous surface treatment composition is preferably 5.0 to 20 g/L, and more preferably 7.0 to 17 g/L. If the content is less than 5.0 g/L, the corrosion resistance after painting decreases. If the content is more than 20 g/L, the corrosion resistance after painting decreases.

(Alcohol compounds)
The alcohol compound improves the storage stability of the aqueous surface treatment composition. The number of carbon atoms in the alcohol compound is preferably 3 or less. Examples of the alcohol compound include methanol, ethanol, 1-propanol, 2-propanol, methanediol, ethylene glycol, propylene glycol, and glycerin. From the viewpoints of cost and convenience, however, monoalcohol compounds such as methanol, ethanol, 1-propanol, and 2-propanol are preferred. The alcohol compound may be used alone or in combination of two or more kinds.

The content of the alcohol compound in the aqueous surface treatment composition is preferably 5.0 to 30 g/L, and more preferably 7.0 to 25 g/L. If the content is less than 5.0 g/L, it will cause a decrease in storage stability. If the content is more than 30 g/L, the corrosion resistance after painting will decrease.

(Amino Compounds)
The amino compound improves the barrier property of the film formed by the aqueous surface treatment composition against moisture and chloride ions that induce corrosion by bonding with the hydroxyl group of the polyphenol compound or reacting with the epoxysilane compound. From the above viewpoint, the amino compound is not particularly limited as long as it has an amino group in the molecule, but it is preferable that the compound has two or more amino groups in the molecule in order to increase the crosslink density and further improve the barrier property. The amino group is preferably a primary amino group or a secondary amino group. The amino compound may be a polymer obtained by polymerizing a monomer. The amino compound may be used alone or in combination of two or more kinds.

Examples of amino compounds include alkylamines, alcoholamines, and amino resins.

Alkylamines may include aliphatic, alicyclic, or aromatic compounds depending on the type of hydrocarbon bonded to the amino group. Examples of alkylamines having two or more amino groups include aliphatic diamines such as ethylenediamine, trimethylenediamine, tetramethylenediamine, pentamethylenediamine, hexamethylenediamine, 2,2,4 (or 2,4,4)-trimethylhexamethylenediamine, and 3,3-diaminodipropylamine; alicyclic diamines such as cyclohexanediamine, methylcyclohexanediamine, and bis-(4,4'-aminocyclohexyl)methane; and aromatic diamines such as xylylenediamine.

Examples of alcohol amines include ethanolamine, N-methylethanolamine, diethanolamine, triethanolamine, and N-methylpropanolamine.

There are no particular limitations on the amino resin, so long as it has two or more amino groups in the molecule, and commercially available products can be used. For example, polyallylamine "PAA-15C" manufactured by Nittobo Medical Co., Ltd. can be used.

The content of the amino compound in the aqueous surface treatment composition is preferably 0.1 to 10 g/L, and more preferably 0.3 to 8.5 g/L. If the content is less than 0.1 g/L, the effect of improving corrosion resistance after painting is reduced. If the content exceeds 10 g/L, this leads to a decrease in storage stability and a decrease in corrosion resistance after painting.

(pH)
The pH of the aqueous metal surface treatment agent according to this embodiment is preferably in the range of 2.0 to 8.0, and more preferably in the range of 2.5 to 7.0. If the pH of the aqueous metal surface treatment agent is less than 2.0, the corrosion resistance after painting is reduced. If the pH of the aqueous metal surface treatment agent is more than 8.0, the storage stability is reduced and the corrosion resistance after painting is reduced.

(Other compounds)
The aqueous metal surface treatment agent according to the present embodiment may further contain other components in addition to the above-mentioned components. Examples of the other components include a crosslinking agent, a leveling agent, a defoaming agent, a pH adjuster, etc.

The aqueous metal surface treatment agent according to this embodiment preferably does not substantially contain zirconium compounds. Being substantially free of zirconium compounds helps to improve storage stability. Note that "substantially free of zirconium compounds" means that the concentration of zirconium compounds in the aqueous metal surface treatment agent is less than 1 mg/L in terms of zirconium atoms.

<Surface treatment film>
The surface treatment film according to this embodiment is formed on a metal substrate that is surface-treated with the aqueous metal surface treatment agent. The shape of the metal substrate is not particularly limited, but examples thereof include a plate shape.

The shape of the metal substrate is not particularly limited, but may be, for example, a plate shape. Examples of the metal substrate include a zinc-plated steel sheet, a zinc alloy-plated steel sheet, and a cold-rolled steel sheet. In addition to the above, an aluminum alloy sheet or a stainless steel sheet may also be used.

Examples of zinc alloy plated steel sheets include zinc-nickel plated steel sheets, zinc-iron plated steel sheets, zinc-chromium plated steel sheets, zinc-aluminum plated steel sheets, zinc-titanium plated steel sheets, zinc-magnesium plated steel sheets, zinc-aluminum-magnesium steel sheets, and zinc-manganese plated steel sheets. Examples of the above plating include electroplating, hot-dip plating, and vapor deposition plating.

Examples of aluminum sheets include 3000 series aluminum alloys, 4000 series aluminum alloys, 5000 series aluminum alloys, 6000 series aluminum alloys, and aluminum-plated steel sheets such as aluminum-based electroplated, hot-dip plated, and vapor-deposited.

A coating film can also be formed on top of the surface treatment film according to this embodiment.

The paint that can be used to form the coating film is not particularly limited, and examples include epoxy-based paints, acrylic-based paints, polyester-based paints, urethane-based paints, and fluorine-based paints. The coating film may be a single-layer film or a laminated film.

When forming a single layer film, a one-coat paint can be used. Examples of one-coat paints include acrylic and melamine paints.

When forming the laminated film, a primer and a top coat can be used. Examples of primers include epoxy-polyester-based primers and phenol-based primers. Examples of top coats include polyester-based top coats.

[Surface treatment method]
The surface treatment method for treating the surface of a metal substrate with the aqueous metal surface treatment agent includes a film forming step of forming a film by treating the surface of the metal substrate with the aqueous metal surface treatment agent.

The film-forming process includes, for example, a coating process in which the aqueous metal surface treatment agent is applied to the surface of the metal substrate, and a drying process in which the applied film is dried. The method for applying the aqueous metal surface treatment agent in the coating process is not particularly limited, but examples include roll coater coating, brush coating, roller coating, bar coater coating, and flow coating.

A known method can be used as the drying method in the drying step. The drying temperature in the drying step is preferably, for example, 60 to 90°C in terms of the peak metal temperature, which is the temperature reached by the surface of the metal substrate.

In the film formation process, the aqueous metal surface treatment agent may be applied to the surface of the metal substrate while being dried. For example, the aqueous metal surface treatment agent may be applied to the surface of a preheated metal and then dried.

The amount of the coating formed after drying in the coating formation step is preferably 0.1 mg/m ² or more and 500 mg/m ² or less, and more preferably 1 mg/m ² or more and 250 mg/m ² or less.

The surface treatment method of this embodiment may further include a coating film forming step of forming a coating film on the coating. The coating film forming step includes, for example, a step of applying paint and a step of baking and drying the applied paint. As the paint, a one-coat paint may be used, or a primer and a top coat may be used.

When using a primer and a top coat, the primer and top coat may be baked and dried after each application, or after the primer and top coat are applied, they may be baked and dried.

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

<Preparation of Aqueous Surface Treatment Composition>
[Examples 1 to 34, Comparative Examples 1 to 10]
A polyphenol compound, an epoxysilane compound, a resin, a phosphate, an alcohol, an amino compound, and ion-exchanged water were mixed and stirred to obtain an aqueous surface treatment composition so as to obtain the contents shown in Table 1. The contents shown in Table 1 indicate the solid content or the amount of active ingredient. Details of the raw materials shown in Table 1 are shown below.

(Polyphenol Compounds)
(A): Gallic acid Molecular weight: 458.4, manufactured by Tokyo Chemical Industry Co., Ltd. (B): Epigallocatechin Molecular weight: 290, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. (C): Pyrogallol Molecular weight: 126, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. (D): Curcumin Molecular weight: 368, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. (E): Tannic acid Molecular weight: 1701, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. (F): Polyphenol resin Weight average molecular weight: 50,000 Resitop PL-4012, manufactured by Gun-ei Chemical Industry Co., Ltd. (G): Polyphenol resin Weight average molecular weight: 70,000 SUMILITE RESIN NPK-238, manufactured by Sumitomo Bakelite Co., Ltd.

(Epoxy silane compound)
(A): KBE-403 3-glycidoxypropylmethyldiethoxysilane manufactured by Shin-Etsu Chemical Co., Ltd. (B): KBM-403 3-glycidoxypropylmethyldimethoxysilane manufactured by Shin-Etsu Chemical Co., Ltd. (C): KBE-402 3-glycidoxypropylmethyldiethoxysilane manufactured by Shin-Etsu Chemical Co., Ltd. (D): KBE-303 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane manufactured by Shin-Etsu Chemical Co., Ltd. (E): 3,4-epoxycyclohexylmethyl 3,4-epoxycyclohexanecarboxylate

(acrylic resin)
(A): Jurymer SH-8 polyacrylic acid Weight average molecular weight: 900,000 Acid value: 640 mg KOH/g Manufactured by Toa Gosei Co., Ltd. (B): Jurymer AC-10H polyacrylic acid Weight average molecular weight: 800,000 Acid value: 550 mg KOH/g Manufactured by Toa Gosei Co., Ltd. (C): Aron A-7185 polyacrylic acid Weight average molecular weight: 500,000 Acid value: 600 mg KOH/g Manufactured by Toa Gosei Co., Ltd. (D): Carboxy group-containing acrylic resin Weight average molecular weight: 500,000 Acid value: 626 mg KOH/g
(E): JURYMER AC-10L Polyacrylic acid Weight average molecular weight: 50,000 Acid value: 620 mg KOH/g Manufactured by Toagosei Co., Ltd.

Synthesis Example of Acrylic Resin (D) Acrylic resin (D) was prepared by the following method. 93.14 parts by mass of ion-exchanged water was charged into a Kolben, and heated to 80°C while stirring and refluxing with nitrogen. Next, while heating, stirring, and refluxing with nitrogen, a mixed monomer solution of 3.86 parts by mass of acrylic acid and 0.94 parts by mass of 2-hydroxyethyl methacrylate, a mixed solution of 0.04 parts by mass of ACVA (4,4'-azobis(4-cyanovaleric acid)) as a polymerization initiator, 0.02 parts by mass of a 25% aqueous ammonia solution, and 2.00 parts by mass of ion-exchanged water were each dropped over 3 hours. After the dropwise addition was completed, heating, stirring, and nitrogen reflux were continued for 2 hours. Thereafter, heating and nitrogen reflux were stopped, and the solution was cooled to 30°C while stirring, neutralized to pH 9 with a 25% aqueous ammonia solution, and filtered through a 200 mesh to obtain a colorless and transparent acrylic resin (D). The resulting acrylic resin (D) had a weight average molecular weight of 500,000, an acid value of the solid content of 626 mgKOH/g, and a hydroxyl value of the solid content of 84 mgKOH/g.

(Phosphate)
(A): Ammonium phosphate monobasic (B): Ammonium phosphate dibasic (C): Ammonium pyrophosphate (D): Sodium phosphate monobasic (E): Orthophosphate

(alcohol)
(A): Ethanol (B): Isopropyl alcohol (C): Methanol (D): n-butanol

(Amino Compounds)
(A): Triethanolamine (B): Diaminobutane (C): Diaminopropylamine (D): Hexamethylenediamine (E): Polyallylamine PAA-15C manufactured by Nittobo Medical Co., Ltd.

[Storage stability test]
The aqueous surface treatment compositions shown in Table 1 were allowed to stand in an incubator at 40° C. for 2 weeks, and then the storage stability was visually evaluated according to the following criteria, with a score of 2 or higher being considered acceptable. The results are shown in Table 2.
3: The hexavalent chromium-free aqueous surface treatment composition is not cloudy. 2: The hexavalent chromium-free aqueous surface treatment composition is slightly cloudy. 1: The hexavalent chromium-free aqueous surface treatment composition is cloudy.

<Preparation of test plate>
The aqueous surface treatment composition shown in Table 1 was used to perform surface treatment on the metal substrates shown in Table 1. The surface treatment was performed according to the following procedure. The metal substrate was degreased by spraying with an alkaline degreaser, Surf Cleaner 155 (manufactured by Nippon Paint Surf Chemicals Co., Ltd.), at 60° C. for 10 seconds, then spray-washed with water and dried. The aqueous surface treatment composition was applied to the metal substrate with a bar coater #3, and then dried at a PMT (peak metal temperature) of 80° C. The metal substrate surface-treated by the above method was baked and dried with Flexcoat 3900NC RI045 (manufactured by Nippon Paint Industrial Coatings Co., Ltd.) as a primer at PMT 250° C., and further baked and dried with SRF05 3704E (manufactured by Nippon Paint Industrial Coatings Co., Ltd.) as a top coat at PMT 265° C. to obtain a test plate. The following evaluations were performed using the test plate.

Details of the metal substrates listed in Table 1 are shown below.
EG: Electrogalvanized steel sheet GI: Hot-dip galvanized steel sheet GL: 55% aluminum-zinc alloy-plated steel sheet

<Evaluation>
[Primary adhesion after bending]
In a 20°C environment, the test plate was bent 180° without a spacer (0TT) or bent 180° with two 0.35 mm aluminum plates as spacers (2TT), and the bent portion was peeled off with tape three times. The degree of peeling was observed with a 20x magnifying glass and evaluated according to the following criteria. A score of 4 or higher was considered to be acceptable, and the results are shown in Table 2.

5: no peeling, 4.5: 1-10% peeling, 4: 11-20% peeling, 3.5: 21-30% peeling, 3: 31-40% peeling, 2.5: 41-50% peeling, 2: 51-60% peeling, 1.5: 61-70% peeling, 1: 71-80% peeling, 0.5: 81-90% peeling, 0: 91-100% peeling

[Secondary adhesion after bending]
The test plate was immersed in boiling water for 2 hours and then left indoors for 24 hours, and evaluated under the same conditions of 0TT and 2TT as in the case of the primary bending adhesion, using the same criteria. A score of 4 or higher was considered to be acceptable, and the results are shown in Table 2.

[SST (Salt Spray Test)]
A test plate with a cross cut was placed in a salt spray corrosion tester specified in JIS Z2317 for 1000 hours, and the average corrosion blister width on one side from the cut and the average corrosion blister width from the edge (upper burr, lower burr) were measured. A cut of 1.0 mm or less was considered acceptable, and an edge of 5.0 mm or less was considered acceptable. The results are shown in Table 2.

[CCT (Combined Cyclic Corrosion Test)]
The test plate with the cross cut was put into the complex cycle corrosion tester shown in JIS K5621, and after 60 complex cycles, the average corrosion blister width from the cut part on one side, the average corrosion blister width from the end face (upper burr, lower burr), and the average corrosion blister width from the processed part that was folded (2TT) were measured in the same manner as the primary adhesion of the folded part. The cut part was rated as passing if it was 0.7 mm or less, and the end face was rated as passing if it was 3.0 mm or less. The processed part was rated according to the following evaluation criteria, and 3.5 or more was rated as passing. The results are shown in Table 2.

5: Peeling area is 0%
4.5: peeled area is less than 10% 4: peeled area is 10% or more and less than 20% 3.5: peeled area is 20% or more and less than 30% 3: peeled area is 30% or more and less than 40% 2.5: peeled area is 40% or more and less than 50% 2: peeled area is 50% or more and less than 60% 1.5: peeled area is 60% or more and less than 70% 1: peeled area is 70% or more and less than 80% 0.5: peeled area is 80% or more and less than 90% 0: peeled area is 90% or more and less than 100%

[Deep drawing]
The test plates were processed under the following conditions using a deep drawing (angle drawing) processing machine (manufactured by Erichsen), and then evaluated according to the following evaluation criteria.
(Conditions) Square cylinder drawing, blank shape: 100 mm square, pressing pressure: 1.5 t, drawing speed: 400 mm/min
(Evaluation criteria) The degree of peeling of the coating film at the four corners after processing was evaluated on a 5-point scale according to the following evaluation criteria, and the average score was calculated. A score of 3.5 or more was considered to be acceptable. The results are shown in Table 2.

5: peeled area is 0% or more and less than 5% 4: peeled area is 5% or more and less than 25% 3: peeled area is 25% or more and less than 50% 2: peeled area is 50% or more and less than 75% 1: peeled area is 75% or more and less than 100%

The following results are evident from Table 2. The test plates according to the Examples have better corrosion resistance and paint adhesion after processing than the test plates according to Comparative Examples 1 to 4, which do not contain any particular component. The aqueous surface treatment compositions according to Comparative Examples 5 to 8 have poor storage stability and were unable to form a film because the contents of each component are excessive and they do not contain alcohol. The test plates according to the Examples have better corrosion resistance and paint adhesion after processing than the test plates according to Comparative Examples 9 and 10, which contain a phosphate other than a nitrogen-containing phosphate compound as the phosphate.

Claims (11)

An aqueous surface treatment composition comprising a polyphenol compound, an epoxysilane compound, a carboxyl group-containing acrylic resin, and a nitrogen-containing phosphate compound consisting of a nitrogen-containing cation and a phosphate anion. The content of the polyphenol compound in the aqueous surface treatment composition is 1.0 to 6.0 g/L;
The content of the epoxysilane compound in the aqueous surface treatment composition is 5.0 to 15 g/L;
The content of the carboxyl group-containing acrylic resin in the aqueous surface treatment composition is 5.0 to 20 g/L;
2. The aqueous surface treatment composition according to claim 1, wherein the content of the nitrogen-containing phosphate compound in the aqueous surface treatment composition is 0.5 to 3 g/L. The aqueous surface treatment composition according to claim 1, further comprising an alcohol compound having 3 or less carbon atoms, the content of the alcohol compound in the aqueous surface treatment composition being 5.0 to 30 g/L. The aqueous surface treatment composition according to claim 1, further comprising an amino compound having one or more amino groups in the molecule, the content of the amino compound in the aqueous surface treatment composition being 0.1 to 10 g/L. The aqueous surface treatment composition according to claim 1, wherein the molecular weight of the polyphenol compound is 1000 or more. The aqueous surface treatment composition according to claim 1, wherein the epoxysilane compound has a cycloalkyl group. The aqueous surface treatment composition according to claim 1, wherein the weight average molecular weight of the carboxyl group-containing acrylic resin is 80,000 to 1,500,000, and the acid value of the solid content is 150 to 780 mgKOH/g. The aqueous surface treatment composition according to claim 1, wherein the phosphate anion is an orthophosphate ion. The aqueous surface treatment composition according to claim 4, wherein the amino compound is a compound having two or more amino groups in the molecule. The aqueous surface treatment composition according to claim 1, having a pH in the range of 2.0 to 8.0. A surface treatment film obtained by curing the aqueous surface treatment composition according to any one of claims 1 to 10.