Aqueous coating composition

Abstract

The present invention relates to a water-based paint composition characterized by containing, as a base resin, a polyester resin having an acid value of 30 mgKOH / g or more and less than 70 mgKOH / g, and a curing agent having a functional group capable of undergoing a cross-linking reaction with a carboxyl group. The water-based paint composition can form a coating film that is excellent in workability and has good curing properties and corrosion resistance, and can be preferably used for a metal container or a metal lid such as a beverage can.

Description

[0001] This application is a divisional application of Chinese patent application No. 201880089309.9, filed on November 13, 2018, entitled "Waterborne Coating Composition". Technical Field

[0002] This invention relates to waterborne coating compositions. More specifically, this invention relates to waterborne coating compositions capable of forming coating films with excellent processability, as well as excellent curing properties and corrosion resistance. Background Technology

[0003] Coating compositions for metal containers or caps, such as beverage cans, are required to have processability that withstands the harsh processing conditions (e.g., necking, beading, scoring, and riveting) encountered during the forming of the metal containers or caps. Furthermore, in some applications, the composition is required to possess various properties, such as good curing properties (hereinafter referred to as "low-temperature curing properties"), which cure even at low temperatures and / or during short firing times. Other properties include corrosion resistance to prevent corrosion of the metal substrate due to the contents, preservation of the flavor of the contents, non-leaching of coating components to provide excellent hygiene, and excellent resistance to boiling. In addition, environmental pollution and the impact on the working environment caused by the volatilization of organic solvents during coating or firing must also be considered.

[0004] Epoxy coatings, such as epoxy-phenolic coatings, epoxy-amino coatings, and epoxy-acrylic coatings, have been widely used as coating compositions for metal containers and metal caps. However, since epoxy coatings are typically manufactured using bisphenol A as a raw material, bisphenol A-free coatings are desirable.

[0005] From this perspective, as a coating composition for metal containers or metal caps, a bisphenol A-free, bisphenol A-free, and environmentally friendly waterborne coating composition has been proposed that takes into account the impact on the working environment.

[0006] An example of such a polyester-based waterborne coating composition is based on an aromatic polyester resin having carboxyl groups, wherein the polyester resin has an acid value (AV) of 10 to 30 mg KOH / g and a number-average molecular weight (Mn) of 3,000 to 10,000. This polyester resin is combined with a curing agent, a neutralizing agent for the polyester resin, and a co-solvent to obtain a coating for metal packaging with excellent curing properties and resistance to boiling (Patent Document 1).

[0007] Examples of proposed waterborne coating compositions include an acrylic-modified polyester resin (C) obtained by grafting a polymerizable unsaturated monomer component (B) onto a polyester resin (A), and a β-hydroxyalkylamide crosslinking agent (D), wherein the acrylic-modified polyester resin (C) and the β-hydroxyalkylamide crosslinking agent (D) are stably dispersed in an aqueous medium. The polyester resin (A) has olefinic double bonds at the resin ends and a number-average molecular weight of 2,000 to 50,000. The polymerizable unsaturated monomer component (B) comprises a carboxyl-containing polymerizable unsaturated monomer (Patent Document 2).

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent No. 4228585

[0011] Patent Document 2: JP-A-2003-26992 Summary of the Invention

[0012] The problem the invention aims to solve

[0013] However, when a phenolic resin that forms a self-condensation compound is used as a curing agent, as in Patent Document 1, hard and brittle regions derived from the self-condensation compound of the phenolic resin are formed in the coating film, which often results in unsatisfactory processability of the coating film.

[0014] Furthermore, when a high-acid-value acrylic-modified polyester resin is used as in Patent Document 2, the number of reaction points (crosslinking points) with the curing agent increases, which significantly improves curability. However, the resin cannot withstand harsh processing and cannot obtain sufficient processability.

[0015] Therefore, the object of the present invention is to provide a polyester resin-based waterborne coating composition capable of forming a coating film with excellent processability and excellent coating film properties such as curability (low-temperature curing) and corrosion resistance.

[0016] Solution for solving the problem

[0017] In this manner, the present invention provides an aqueous coating composition comprising: a polyester resin with an acid value of 30 mg KOH / g or more and less than 70 mg KOH / g; and a curing agent (crosslinking agent) having functional groups capable of crosslinking with carboxyl groups.

[0018] The preferred aqueous coating composition of the present invention is:

[0019] 1. The polyester resin is an unmodified acrylic polyester resin or an acrylic-modified polyester resin in which the content (modification amount) of the acrylic resin component is less than 10% by mass;

[0020] 2. The curing agent is at least one selected from the group consisting of β-hydroxyalkylamide compounds, compounds containing oxazoline groups, and compounds containing carbodiimide groups;

[0021] 3. The curing agent is a β-hydroxyalkylamide compound;

[0022] 4. The β-hydroxyalkylamide compounds are N,N,N',N'-tetrakis(2-hydroxyethyl)hexamethylenediamide and / or N,N,N',N'-tetrakis(2-hydroxypropyl)hexamethylenediamide;

[0023] 5. The blending amount of the solid component of the curing agent is in the range of 1 to 20 parts by weight relative to 100 parts by weight of the solid component of the polyester resin;

[0024] 6. The polyester resin is a mixed polyester resin comprising polyester resin (A) and polyester resin (B) with an acid value lower than that of polyester resin (A), wherein the difference in acid value between polyester resin (A) and polyester resin (B) is 5 mg KOH / g or more; and

[0025] 7. The acid value of polyester resin (A) is in the range of 30 to 80 mg KOH / g, the acid value of polyester resin (B) is above 5 mg KOH / g and less than 30 mg KOH / g, and the mass ratio of solid components of polyester resin (A) to polyester resin (B) is (A):(B) = 95:5 to 5:95.

[0026] The present invention further provides a coated metal sheet having a coating film comprising the above-described water-based paint composition on at least one surface of the metal sheet.

[0027] The present invention further provides a metal container or a metal cap having a coating film comprising the above-described water-based coating composition.

[0028] The effects of the invention

[0029] As a result of in-depth research into improving the processability of coatings formed from polyester-based waterborne coating compositions, the inventors have discovered that coatings with excellent processability capable of withstanding harsh processing can be formed by using a specific polyester resin as the base resin (main component) and a specific curing agent (crosslinking agent). Because coatings formed from the waterborne coating compositions of the present invention exhibit excellent curability and excellent corrosion resistance, achieving sufficient curing even at lower temperatures and / or with shorter firing times, these waterborne coating compositions are suitable for use as coating compositions for metal containers or metal caps, such as beverage cans. Detailed Implementation

[0030] (Polyester resin)

[0031] In the waterborne coating composition of the present invention, the polyester resin to be the base resin (main component) is required to have an acid value of 30 mg KOH / g or more and less than 70 mg KOH / g, preferably in the range of 30 to 60 mg KOH / g, and more preferably in the range of 30 to 45 mg KOH / g, in order to achieve both excellent processability and curability.

[0032] In this invention, the polyester resin to be the base resin (main component) is required to have an appropriate amount of carboxyl groups in order to obtain both curability and processability. When the acid value is below a certain range, the amount of carboxyl groups that serve as crosslinking points with the curing agent (such as β-hydroxyalkylamide compounds) decreases compared to when the acid value is in the above range. This results in poor curability. To achieve sufficient curing, higher temperatures and / or longer firing times are required in the coating process, which reduces productivity. On the other hand, when the acid value is above the above range, the curability is excellent due to the increased number of crosslinking points with the curing agent. However, the crosslinking density becomes too high and the processability deteriorates. Even when the acid value is above the above range, the crosslinking density can be reduced by adjusting, for example, the amount of curing agent blended. However, in this case, free carboxyl groups not used for crosslinking remain in the coating film, which reduces the water resistance of the coating film, resulting in insufficient corrosion resistance.

[0033] In this invention, two or more polyester resins with different acid values ​​can be blended. In this case, the sum of the values ​​obtained by multiplying the acid value and mass fraction of each polyester resin is set as the average acid value of the blend, and the average acid value of the blend is preferably 30 mg KOH / g or more and less than 70 mg KOH / g. When blending a low-acid-value polyester resin with an acid value of less than 5 mg KOH / g, the proportion of the low-acid-value polyester resin relative to the total amount of polyester resin is less than 50% by mass, preferably 40% by mass or less, and more preferably 30% by mass or less. Since the low-acid-value polyester resin has very few carboxyl groups that serve as reaction sites with the curing agent, even if the average acid value is set within the above range by blending with a polyester resin with a higher acid value, the low-acid-value polyester resin itself is likely not to be easily incorporated into the crosslinking structure, and after firing, the low-acid-value polyester resin is likely to remain in the coating film in an uncrosslinked state. Therefore, when the proportion of low-acid-value polyester resin with an acid value of less than 5 mg KOH / g is more than 50% by mass, sufficient curing of the coating film cannot be obtained, and in some cases, resistance to boiling and whitening cannot be obtained.

[0034] Furthermore, in order to achieve both processability and curability of the coating film at a higher level, it is preferable that the polyester resin used as the base resin is a mixed polyester resin comprising polyester resin (A) and polyester resin (B) whose acid value is 5 mg KOH / g or lower than that of polyester resin (A).

[0035] Polyester resin (A) with a relatively high acid value has a relatively high amount of carboxyl groups that act as reaction sites with curing agents (such as β-hydroxyalkylamide compounds). When polyester resin (A) is used alone as the base resin, sufficient curing can be easily achieved in a short time, resulting in excellent curability. However, the resulting coating tends to have a high crosslinking density and poor processability. Polyester resin (B) with a relatively low acid value has a relatively low amount of carboxyl groups. When polyester resin (B) is used alone as the base resin, the crosslinking density decreases, and a coating with excellent processability is easily formed. However, due to the reduced number of reaction sites, the crosslinking reaction requires a longer time, and firing requires a longer time to achieve curability, i.e., the curability tends to be worse. In contrast, when a mixed polyester resin of polyester resin (A) and polyester resin (B) is used, the overall processability of the coating is significantly improved compared to the case of using polyester resin (A) alone, and the characteristics of polyester resin (A) are strongly reflected in the curability. As a result, it exhibits excellent curability and achieves both curability and processability at a higher level compared to using only one polyester resin.

[0036] When forming a coating with the above-mentioned properties, the difference in acid value between polyester resin (A) and polyester resin (B) is, as described above, 5 mg KOH / g or more, preferably 5 to 60 mg KOH / g, and more preferably 10 to 50 mg KOH / g. When the difference in acid value is less than 5 mg KOH / g, it becomes difficult to obtain the above-mentioned effects by combining polyester resins with different acid values.

[0037] To effectively achieve the above-mentioned effect, the difference in acid value between polyester resin (A) and polyester resin (B) is required to be 5 mg KOH / g or more. To meet this condition, it is preferred that polyester resin (A) has an acid value in the range of 30 to 80 mg KOH / g, particularly 30 to 60 mg KOH / g, and that polyester resin (B) has an acid value of 5 mg KOH / g or more and less than 30 mg KOH / g, particularly in the range of 5 to 25 mg KOH / g. Polyester resin (A) and polyester resin (B) are preferably mixed at a mass ratio of (A):(B) = 95:5 to 5:95, more preferably 90:10 to 10:90, more preferably 90:10 to 30:70, and even more preferably 80:20 to 50:50.

[0038] In this invention, it is important to specify the acid value (Av) of the mixed polyester resin formed by mixing polyester resins (A) and (B), as represented by the following formula (1). mix The concentration of KOH is above 30 mg KOH / g and below 70 mg KOH / g.

[0039] Av mix =[Av A ·W A / (W A +W B )+Av B ·W B / (W A +W B )]…(1)

[0040] Among them Av mix The acid value (mgKOH / g) of the mixed polyester resin is represented by Av. A and Av B This indicates the acid value (mgKOH / g) of the polyester resins (A) and (B) used, and W. A and W B This indicates the mass fraction of polyester resins (A) and (B).

[0041] In this embodiment, the two polyester resins (A) and (B) with an acid value difference of 5 mg KOH / g or more can each be a blend of multiple polyester resins. For example, polyester resin (A) can be a mixed polyester resin (A′) formed by mixing multiple polyester resins, and the average acid value of the mixed polyester resin (A′) can be set to have a difference of 5 or more from the acid value of polyester resin (B).

[0042] When using a mixed polyester resin (A') or a similarly prepared mixed polyester resin (B'), the polyester resin constituting the mixed polyester resin (A') is preferably selected from polyester resin (A) with an acid value in the range of 30 to 80 mg KOH / g as described above. Similarly, the polyester resin constituting the mixed polyester resin (B') is preferably selected from polyester resin (B) with an acid value in the range of 5 mg KOH / g or more and less than 30 mg KOH / g as described above.

[0043] Furthermore, in this invention, it is desirable that the polyester resin used as the base resin be an unmodified acrylic polyester resin that has not been modified by acrylic resin.

[0044] In waterborne coating compositions containing polyester resin as the base resin, it has been widely proposed to use acrylic-modified polyester resins modified with acrylic resins by methods such as graft polymerization of polymerizable unsaturated monomers (olefinic unsaturated monomers) to the polyester resin. However, acrylic-modified polyester resins tend to result in poor processability of the formed coating film, and modification increases the number of manufacturing steps and manufacturing costs. For this reason, the polyester resin used in this invention is preferably an unmodified polyester resin (i.e., an unacrylic-modified polyester resin). When using acrylic-modified polyester resins, it is preferable that the content (mass ratio) of the acrylic resin component (the polymer component of the polymerizable unsaturated monomer) in the total acrylic-modified polyester resin is less than 10% by mass.

[0045] In this invention, in addition to using polyester resins having the specific acid value described above and not modified with acrylics, conventionally known water-dispersible or water-soluble polyester resins may also be used.

[0046] Water-dispersible or water-soluble polyester resins are polyester resins that contain hydrophilic groups as components and are dispersed or dissolved in water. These hydrophilic components can be physically adsorbed onto the surface of the polyester dispersion, but particularly preferably, they are copolymerized in the polyester resin backbone.

[0047] Examples of hydrophilic groups include hydroxyl, amino, carboxyl, sulfonic acid, or derivatives thereof, metal salts, and ethers. Polyester resins containing these groups in their molecules can exist in a state that is dispersible in water.

[0048] Specific examples of components containing hydrophilic groups include carboxylic anhydrides such as phthalic anhydride, succinic anhydride, maleic anhydride, trimellitic anhydride, itaconic anhydride, and citraconic anhydride; hydroxyl-containing polyether monomers such as polyethylene glycol, polypropylene glycol, glycerol, and polyglycerol; metal salts of sulfonic acid-containing monomers such as 5-sulfoisophthalic acid and 4-sulfonnaphthalene-2,7-dicarboxylic acid; and ammonium salts, etc.

[0049] In this invention, a carboxyl-containing polyester resin having a carboxyl group as a hydrophilic group can be suitably used as a water-soluble and / or water-dispersible polyester resin.

[0050] There are no particular restrictions on the monomer components that are combined with monomers containing hydrophilic groups for the purpose of forming a water-dispersible polyester resin, as long as they are monomers commonly used in the polymerization of polyester resins.

[0051] The polycarboxylic acid components constituting polyester resins include: aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, and naphthalenedicarboxylic acid; aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanoic acid, and dimer acids; unsaturated dicarboxylic acids such as maleic acid (anhydride), fumaric acid, and terpene-maleic acid adducts; alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid, tetrahydrophthalic acid, hexahydroisophthalic acid, and 1,2-cyclohexenedicarboxylic acid; and trivalent or higher polycarboxylic acids such as trimellitic acid (anhydride), pyromellitic acid (anhydride), and methylcyclohexenetricarboxylic acid. One or more of these can be selected.

[0052] In this invention, from the viewpoints of corrosion resistance, cooking resistance, and flavor characteristics, the proportion of aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, and naphthalic acid in the polycarboxylic acid components constituting the polyester resin is preferably 60 mol% or more, and particularly preferably 80 mol% or more.

[0053] Examples of polyol components constituting polyester resins include, for example, ethylene glycol, propylene glycol (1,2-propanediol), 1,3-propanediol, 1,4-butanediol, 1,2-butanediol, 1,3-butanediol, 2-methyl-1,3-propanediol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 2-ethyl-2-butyl-1,3-propanediol, 2,4-diethyl-1,5-pentanediol, 1-methyl-1,8-octanediol, 3-methyl-1,6-hexanediol, 4 Aliphatic diols such as methyl-1,7-heptanediol, 4-methyl-1,8-octanediol, 4-propyl-1,8-octanediol, and 1,9-nonanediol; ether diols such as diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol; alicyclic polyols such as 1,4-cyclohexanediol, 1,3-cyclohexanediol, 1,2-cyclohexanediol, tricyclodecanediols, and hydrogenated bisphenols; and polyols with three or more ternary groups such as trimethylolpropane, trimethylolethane, and pentaerythritol. One or more of these can be used in combination.

[0054] In this invention, from the viewpoint of hygiene and the like, ethylene glycol, propylene glycol, 1,4-butanediol, 1,4-cyclohexanediol and neopentyl glycol can be appropriately used as components constituting the polyester resin among the above polyol components.

[0055] Carboxyl-containing polyester resins can be produced by known methods, such as polycondensation of one or more of the aforementioned polycarboxylic acid components and one or more polyol components. One example of an alternative method is depolymerization using polycarboxylic acid components such as terephthalic acid, isophthalic acid, trimellitic anhydride, trimellitic acid, and pyromellitic acid, and this method can be carried out after polycondensation. Another example is ring-opening addition of anhydrides such as phthalic anhydride, maleic anhydride, trimellitic anhydride, and ethylene glycol bis(trimeric) dianhydride, and this method can be carried out after polycondensation.

[0056] The glass transition temperature (Tg) of the polyester resin is preferably in the range of -30°C to 120°C, and more preferably in the range of 25°C to 100°C. When the Tg is higher than the above range, the formed coating becomes hard, which leads to poor processability. On the other hand, when the Tg is lower than the above range, the barrier properties of the coating decrease, and the corrosion resistance and boiling resistance deteriorate.

[0057] In this invention, two or more polyester resins with different Tg values ​​can be blended. By blending polyester resins with different Tg values, in some cases a coating film with excellent resistance to embrittlement and impact can be formed compared to using only one polyester resin.

[0058] Even under such circumstances, the Tg of the polyester resin blend calculated by the following formula (2)mix There are no special restrictions, as long as Tg mix It is acceptable if it falls within the range of Tg mentioned above.

[0059] 1 / Tg mix =(W1 / Tg1)+(W2 / Tg2)+...+(Wm / Tgm)...(2)

[0060] W1 + W2 + ... + Wm = 1

[0061] Among them Tg mix The glass transition temperature (K) of the polyester resin blend is represented by Tg1, Tg2, ..., Tgm; the glass transition temperature (K) of each polyester resin (polyester resin 1, polyester resin 2, ..., polyester resin m) is represented by Tg1, Tg2, ..., Tgm; and the mass fraction of each polyester resin (polyester resin 1, polyester resin 2, ..., polyester resin m) is represented by W1, W2, ..., Wm.

[0062] In the waterborne coating composition of the present invention, from the viewpoint of the impact resistance of the coating film, it is preferable to use a mixture of polyester resin (α) with a Tg of 35°C to 100°C and polyester resin (β) with a Tg of -30°C to 25°C. In this case, the mixing ratio by mass is preferably (α):(β) = 98:2 to 10:90, particularly 95:5 to 30:70. Furthermore, from the viewpoint of corrosion resistance and boiling resistance, it is preferable that the glass transition temperature (Tg) calculated by the above formula (2) is... mix (Above 30℃)

[0063] The number-average molecular weight (Mn) of the polyester resin is preferably in the range of 1,000 to 100,000, particularly 1,000 to 50,000, although the invention is not limited to this example. If it is less than this range, the coating film will become brittle and have poor processability. If it is greater than the above range, the stability of the coating will decrease.

[0064] The hydroxyl value of the polyester resin is preferably below 20 mg KOH / g, and more preferably below 10 mg KOH / g, although the invention is not limited to this example. When a β-hydroxyalkylamide compound is used as a curing agent, it reacts with the carboxyl groups of the polyester resin to form crosslinks. However, since this compound is considered to react little or no with hydroxyl groups, most of the hydroxyl groups of the polyester resin remain unreacted in the coating film. Therefore, when the hydroxyl value is greater than the above range, the number of residual hydroxyl groups increases, thereby reducing corrosion resistance.

[0065] In an aqueous medium containing a water-dispersible or water-soluble polyester resin, the average dispersed particle size of the polyester resin is preferably in the range of 10 to 1,000 nm, and particularly in the range of 20 to 500 nm.

[0066] (Curing agent)

[0067] An important feature of this invention is the use of a specific curing agent having functional groups that can crosslink with the carboxyl groups of the polyester resin, which is the base resin.

[0068] Acetylated phenolic and amino resins, commonly used as curing agents in polyester coating compositions, have hydroxymethyl or alkyl-etherified hydroxymethyl groups as functional groups that crosslink with the hydroxyl or carboxyl groups of the polyester resin. These functional groups react with the hydroxyl or carboxyl groups contained in the polyester resin to form crosslinks, while simultaneously tending to undergo self-condensation reactions with other functional groups. As a result, hard and brittle regions composed of self-condensates of acetylated phenolic or amino resins can be formed in the coating film, leading to reduced processability. Furthermore, since the reaction sites (functional groups) of the curing agent are consumed in the self-condensation reaction, the amount of curing agent required to obtain sufficient curing naturally increases, resulting in low efficiency. In addition, a large amount of curing agent contained in the coating film adversely affects coating properties such as processability and impact resistance. On the other hand, when a curing agent with functional groups that do not readily undergo or do not undergo self-condensation reactions is used, the formation of hard and brittle self-condensates can be prevented or reduced, and at the same time, the minimum amount of curing agent corresponding to the amount of carboxyl groups in the polyester resin can be blended, thereby reducing the amount of curing agent in the coating film. As a result, a coating film with excellent processability and impact resistance can be formed.

[0069] For the reasons stated above, preferred curing agents have functional groups that hardly cause self-condensation reactions between curing agents, and are functional groups that can crosslink with the carboxyl groups of polyester resins. Examples of curing agents that can be suitably used include β-hydroxyalkylamide compounds, compounds (polymers) containing carbodiimide groups, and compounds (polymers) containing oxazoline groups. In particular, β-hydroxyalkylamide compounds can be suitably used.

[0070] The functional group equivalent of the curing agent is preferably in the range of 30 to 600 g / eq, and particularly preferably in the range of 40 to 200 g / eq. The functional group equivalent in this invention is a value obtained by dividing the molecular weight by the number of functional groups per molecule of curing agent (here, a functional group refers to a functional group capable of crosslinking with the carboxyl group of the polyester resin used as the base resin), and signifies the molecular weight of each functional group of the curing agent. If the functional group equivalent is less than this range, the distance between crosslinking points cannot be increased, resulting in reduced film flexibility and poor processability. On the other hand, if the functional group equivalent is much greater than the above range, the curing performance will be poor.

[0071] [β-hydroxyalkylamide compounds]

[0072] In the aqueous coating composition of the present invention, a β-hydroxyalkylamide compound can be used as a curing agent. The β-hydroxyalkylamide compound has a β-hydroxyalkylamide group as a functional group capable of crosslinking with a carboxyl group.

[0073] As described above, curing agents composed of β-hydroxyalkylamide compounds are effective because their processability is unlikely to be reduced through self-condensation reactions between curing agents, and sufficient curability can be obtained with the minimum necessary amount. Furthermore, the amount of curing agent in the coating film can be reduced, resulting in a coating film with excellent processability. Moreover, unlike the case using phenolic resins, there is no possibility of the coating film being colored, and a colorless and transparent coating film can be formed, thus achieving a coating film with excellent appearance.

[0074] Examples of β-hydroxyalkylamide compounds used as curing agents include those represented by the following general formula [I].

[0075] General formula [I]:

[0076] [HO-CH(R1)-CH2-N(R2)-CO-] m -A-[-CO-N(R2')-CH2-CH(R1')-OH] n

[0077] [In the formula, R1 and R1' are hydrogen atoms or alkyl groups having 1 to 5 carbon atoms, R2 and R2' are hydrogen atoms or alkyl groups having 1 to 5 carbon atoms or those represented by general formula [II], A is a polyvalent organic group, m is 1 or 2, and n is 0 to 2 (the sum of m and n is at least 2).]

[0078] General formula [II]:

[0079] HO-CH(R3)-CH2-

[0080] [Where, R3 represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms.]

[0081] In general formula [I], A is preferably an aliphatic, alicyclic or aromatic hydrocarbon, more preferably an aliphatic, alicyclic or aromatic hydrocarbon having 2 to 20 carbon atoms, and even more preferably an aliphatic hydrocarbon having 4 to 10 carbon atoms.

[0082] In general formula [I], the sum of m and n is preferably 2, 3 or 4.

[0083] Preferred examples of β-hydroxyalkylamide compounds used as curing agents and represented by the above general formula [I] include: N,N,N',N'-tetratetra(2-hydroxyethyl)hexamethylenediamide [CAS: 6334-25-4, molecular weight: about 320, number of functional groups per molecule: 4, functional group equivalent (theoretical value): about 80 g / eq, product example: Primid XL552 manufactured by EMS-GRILTECH Corporation], and N,N,N',N'-tetratetra(2-hydroxypropyl)hexamethylenediamide [CAS: 57843-53-5, molecular weight: about 376, number of functional groups per molecule: 4, functional group equivalent (theoretical value): about 95 g / eq, product example: Primid QM1260 manufactured by EMS-GRILTECH]. Of these, N,N,N',N'-tetratetra(2-hydroxypropyl)hexamethylenediamide is further preferred from the viewpoint of curability and retortability. Compared to N,N,N',N'-tetra(2-hydroxyethyl)hexamethylenediamide, N,N,N',N'-tetra(2-hydroxypropyl)hexamethylenediamide exhibits higher reactivity with polyester resins and superior curing properties. Furthermore, N,N,N',N'-tetra(2-hydroxypropyl)hexamethylenediamide forms a denser cross-linked structure, resulting in minimal whitening of the coating even during boiling. Consequently, a coating with excellent boiling resistance can be formed.

[0084] [Compounds containing carbodiimide groups]

[0085] As a curing agent, a carbodiimide-containing compound can be used, for example, a resin having a carbodiimide group in its molecule, and specific examples include "CARBODILITE V-02", "CARBODILITE V-02-L2", "CARBODILITE V-04", "CARBODILITE E-01", and "CARBODILITE E-02". These are manufactured by Nisshinbo Chemical Inc.

[0086] [Compounds containing an oxazoline group]

[0087] Examples of oxazoline-containing compounds used as curing agents include water-soluble polymers obtained by polymerizing a monomer composition containing an oxazoline derivative. Examples of oxazoline derivatives include 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline, and 2-isopropenyl-5-ethyl-2-oxazoline. The monomers included in the monomer composition containing the oxazoline derivative, other than the oxazoline derivative, can be any compound capable of copolymerizing with the oxazoline derivative and inert to the oxazoline group, and there are no particular limitations on them. The percentage of structural units derived from the oxazoline derivative in the oxazoline-containing polymer is preferably 5% by mass or more. Specific examples include EPOCROS WS-300 [number average molecular weight: about 40,000, functional group (oxazoline) equivalent: about 130 g / eq] and EPOCROS WS-700 [number average molecular weight: about 20,000, functional group (oxazoline) equivalent: about 220 g / eq], both of which are manufactured by NIPPONSHOKUBAI CO., LTD.

[0088] The curing agent (solid component) is preferably blended in an amount of 1 to 20 parts by weight, preferably 3 to 15 parts by weight, and more preferably 4 to 10 parts by weight, relative to 100 parts by weight of polyester resin (solid component). When the blending amount of curing agent is less than this range, sufficient curability cannot be obtained. When the blending amount of curing agent is greater than this range, if the amount of functional groups of the curing agent is excessive relative to the carboxyl groups of the polyester resin, it is difficult for one molecule of curing agent to react with more than two molecules of polyester resin. This leads to insufficient crosslinking formation and reduced curability. In addition, long-term storage stability is poor.

[0089] When β-hydroxyalkylamide compounds are used as curing agents, it is desirable that the amount of hydroxyl groups derived from the β-hydroxyalkylamide group is in the range of 0.3 to 3.0 equivalents, preferably 0.5 to 2.5 equivalents, and more preferably 0.7 to 2.0 equivalents, relative to the amount of carboxyl groups in the polyester resin (molar ratio of OH groups to COOH groups).

[0090] (Water-based coating composition)

[0091] The waterborne coating composition of the present invention comprises at least the specific polyester resin and curing agent described above as the base resin (main component), and an aqueous medium. In the waterborne coating composition of the present invention, the base resin (main component) is defined as the component included in the largest quantity (by mass ratio) of the solid components (non-volatile components excluding volatile substances such as water and solvents) that form a coating film in the waterborne coating composition. The solid components that form a coating film refer to components that form a continuous layer as a coating film by firing after coating. Components that do not form a continuous layer, such as inorganic pigments and inorganic particles (fillers), do not correspond to these components.

[0092] (Aqueous medium)

[0093] Examples of aqueous media that may be used here include water, which is used in the same way as known waterborne coating compositions, and mixtures of water and organic solvents such as alcohols, polyols or their derivatives.

[0094] When using an organic solvent, it is preferably contained in an amount of 1 to 45% by mass, and particularly preferably 5 to 30% by mass, relative to the total amount of the aqueous medium in the water-based coating composition. When the solvent content is within this range, the film-forming performance can be improved.

[0095] Such organic solvents are preferably amphiphilic, and examples include methanol, ethanol, isopropanol, n-butanol, ethylene glycol, methyl ethyl ketone, butyl cellosolve, carbitol, butyl carbitol, propylene glycol monopropyl ether, propylene glycol ethylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, and 3-methyl-3-methoxybutanol.

[0096] (Alkaline compounds)

[0097] The waterborne coating composition of the present invention preferably contains an alkaline compound capable of neutralizing the carboxyl groups in the polyester resin, thereby improving the water dispersibility of the polyester resin. As the alkaline compound, compounds that volatilize during firing when forming the coating film, i.e., ammonia and / or organic amine compounds with a boiling point below 250°C, are preferred.

[0098] Specific examples include alkylamines such as trimethylamine, triethylamine, and n-butylamine; and alkanolamines such as 2-dimethylaminoethanol, diethanolamine, triethanolamine, aminomethylpropanol, and dimethylaminomethylpropanol. Additionally, polyamines such as ethylenediamine and diethylenetriamine can be used. Furthermore, amines with branched alkyl groups and heterocyclic amines can be used appropriately. As amines with branched alkyl groups, branched alkylamines having 3 to 6 carbon atoms, particularly 3 to 4 carbon atoms, are used, and examples include isopropylamine, sec-butylamine, tert-butylamine, and isopentylamine. As heterocyclic amines, saturated heterocyclic amines containing one nitrogen atom are used, and examples include pyrrolidine, piperidine, and morpholine.

[0099] In this invention, triethylamine or 2-dimethylaminoethanol may be used appropriately. The amount used is preferably 0.5 to 1.5 equivalents relative to the carboxyl group.

[0100] (Lubricant)

[0101] The waterborne coating composition of the present invention may include a lubricant as needed. Preferably, 0.1 to 10 parts by weight of lubricant are added relative to 100 parts by weight of polyester resin. By adding lubricant, scratches on the coating film during molding processes such as can lids can be prevented or reduced, and the slipperiness of the coating film during molding processes can be improved.

[0102] Examples of lubricants that can be added to the waterborne coating compositions of the present invention include fatty acid ester waxes as polyol compounds and fatty acid esters, silicone waxes such as fluorinated waxes like polytetrafluoroethylene, polyolefin waxes such as polyethylene, lanolin waxes, lignite waxes, microcrystalline waxes, carnauba wax, and silicone compounds. These lubricants can be used alone or in mixtures of two or more lubricants.

[0103] (other)

[0104] In addition to the components mentioned above, components that can be blended in conventional coating compositions, such as leveling agents, pigments, and defoamers, can be added to the waterborne coating composition of the present invention according to conventionally known formulations. Furthermore, other resin components can be included together with the polyester resin in an amount that does not impair the purpose of the invention. Examples include water-dispersible or water-soluble resins, such as polyvinyl acetate, ethylene-vinyl acetate copolymers, polyolefin resins, epoxy resins, polyurethane resins, acrylic resins, phenolic resins, melamine resins, polyvinyl chloride resins, polyvinyl chloride-vinyl acetate copolymer resins, polyvinyl alcohol, ethylene-vinyl alcohol copolymers, polyvinylpyrrolidone, polyvinyl ether, polyacrylamide, acrylamide compounds, polyethyleneimine, starch, gum arabic, and methylcellulose.

[0105] The waterborne coating composition of the present invention preferably contains polyester resin in an amount of 5 to 55% by weight, based on solid content. When the resin solid content is less than this range, an adequate coating amount cannot be ensured, and the coatability deteriorates. On the other hand, when the resin solid content is greater than this range, the stability, workability, and coatability of the coating decrease. Furthermore, the coating amount becomes excessive, which can lead to blistering during firing.

[0106] The water-based coating composition of the present invention is applied to a metal sheet or a pre-formed metal container by known coating methods such as roller coating, spray coating, and dip coating, and is fired by heating means such as a coil oven, so that the composition can be used.

[0107] The firing conditions of the waterborne coating composition of the present invention are appropriately adjusted according to the type of polyester resin, curing agent, metal substrate, and coating amount. The waterborne coating composition of the present invention is preferably heated and cured at a temperature of 150°C to 320°C, preferably 180°C to 300°C, for 5 seconds to 30 minutes, preferably 8 seconds to 180 seconds, to obtain sufficient curability.

[0108] The thickness of the formed coating film, measured by dry film thickness, is preferably less than 30 μm, more preferably from 0.5 to 20 μm, more preferably from 1 μm to 15 μm, and even more preferably from 1 μm or more but less than 10 μm. From an economic point of view, coating films thicker than this range are less desirable. Furthermore, the firing time required for the coating film to fully cure will be longer, which will reduce productivity. On the other hand, when the coating film is thinner than this range, the coatability of the coating film will be poor.

[0109] (Painted metal sheet)

[0110] The water-based coating composition of the present invention can be applied to a metal plate by known coating methods such as roller coating, spray coating, and dip coating, and then fired by heating means such as an oven to obtain a coated metal plate.

[0111] Examples of metal sheets include hot-rolled steel sheets, cold-rolled steel sheets, molten zinc-coated steel sheets, electro-galvanized steel sheets, alloy-coated steel sheets, aluminum-zinc alloy-coated steel sheets, aluminum sheets, tin-plated steel sheets, stainless steel sheets, copper sheets, copper-plated steel sheets, tin-free steel sheets, nickel-plated steel sheets, ultra-thin tin-plated steel sheets, and chromium-treated steel sheets, although the invention is not limited to these examples. Various surface treatments, such as chromate phosphate treatment and zirconium-based chemical conversion treatment, can be applied to these steel sheets as needed.

[0112] In addition, a thermoplastic resin film, such as a polyester resin film, can be laminated onto the coating of the coated metal sheet as an organic resin coating layer, thereby forming an organic resin coated metal sheet.

[0113] (Painted metal containers)

[0114] Coated metal containers can be obtained by forming a coating film of the water-based coating composition of the present invention on the inner and / or outer surfaces of the metal container. Since the water-based coating composition of the present invention has particularly excellent processability and corrosion resistance, it is preferable to form a coating film made of the coating composition of the present invention on at least the inner surface of the metal container.

[0115] Any conventionally known metal container can be used as a metal container having a coating thereon, and examples include three-piece cans with side seams and seamless cans (two-piece cans), although the invention is not limited thereto.

[0116] Coated metal containers can be obtained by forming the aforementioned coated metal sheet. In the case of metal containers such as seamless cans formed through rigorous processing, it is preferable to obtain the coated metal container of the present invention by applying the water-based coating composition of the present invention to a pre-formed metal container using methods such as spray coating. Optionally, an organic resin coating layer is formed on the coated metal sheet, and the resulting organic resin coated metal sheet can be formed into a metal container such as a seamless can.

[0117] (Painted metal cap)

[0118] The coated metal cap can be obtained by forming the aforementioned coated metal sheet with the water-based coating composition of the present invention using any conventionally known cap-making method. This cap can be used, for example, as a stay-on-tab type easy-open can cap or a fully open easy-open can cap.

[0119] Example :

[0120] The invention will be described in detail below with reference to embodiments. In the embodiments, the term "parts" means "parts by mass".

[0121] The following describes the measurement methods for various unmodified acrylic polyester resins.

[0122] (Measurement of number-average molecular weight)

[0123] The solids content of the polyester resin was measured using a calibration curve of standard polyethylene via gel permeation chromatography (GPC).

[0124] (Measurement of glass transition temperature)

[0125] The solids content of the polyester resin was measured using a differential scanning calorimeter (DSC) at a heating rate of 20 °C / min.

[0126] (Measurement of acid value)

[0127] The solids of the polyester resin (0.2 g) were dissolved in 20 ml of chloroform and titrated with a 0.1 N KOH ethanol solution to determine the acid value of the resin (mg KOH / g). Phenolphthalein was used as an indicator. If the polyester resin was insoluble in the above solvents, tetrahydrofuran or similar solvents were used.

[0128] (Measurement of monomer composition)

[0129] The solids of the polyester resin (30 mg) were dissolved in 0.6 mL of dichloromethane. ¹H-NMR measurements were performed, and the monomer composition ratio was determined from the peak intensities. The composition ratios, excluding trace components (less than 1 mol% relative to the total monomer composition), were determined.

[0130] (Preparation of coated metal sheets)

[0131] The water-based coating compositions of the Examples, Comparative Examples, and Reference Examples were applied using a bar coater to aluminum plates (Alloy 3104, plate thickness: 0.28 mm, weight of chromium in the surface-treated coating: 20 mg / m³) having surfaces treated with a chromate phosphate base. 2 The coated metal sheet was prepared by firing the film at a predetermined temperature in an oven for a predetermined time to achieve a predetermined thickness. The coating conditions are shown in Tables 1-3. In Examples 1-5, Comparative Examples 1-2, and Reference Examples 1-2, the firing temperature (oven temperature) was set to 200°C, the firing time to 90 seconds, and the thickness of the dried coating film after firing was set to 1.5 μm. In Examples 6-15, Comparative Examples 3-6, and Reference Examples 3-6, the firing temperature (oven temperature) was set to 250°C, the firing time to 60 seconds, and the thickness of the dried coating film after firing was set to 9.0 μm. Various evaluations were performed on the resulting coated metal sheets according to the following evaluation methods.

[0132] (Curing properties)

[0133] The curability of the coated metal sheet was evaluated based on the MEK extraction rate. A 5cm × 5cm sample was cut from the coated metal sheet. The mass of the sample (W1) was measured, and the sample was extracted for 1 hour using 200ml of MEK (methyl ethyl ketone) under reflux at 80°C. After extraction, the coated sheet was dried at 130°C for 1 hour, and the mass of the sample (W2) was measured. Furthermore, the coating was peeled off using concentrated sulfuric acid, and the mass of the sample (W3) was measured. The MEK extraction rate of the coated sheet can be obtained from the following formula (3).

[0134] MEK extraction rate (%) = 100 × (W1 - W2) / (W1 - W3) … (3)

[0135] The evaluation criteria in Table 1 (Examples 1-5, Comparative Examples 1-2 and Reference Examples 1-2) are as follows.

[0136] ◎: Less than 15%

[0137] 〇: 15% or more but less than 25%

[0138] △: 25% or more but less than 40%

[0139] ×:More than 40%

[0140] The evaluation criteria in Table 2 (Examples 6-10, Comparative Examples 3-6 and Reference Example 3) are as follows.

[0141] ◎: Less than 10%

[0142] 〇: 10% or more but less than 15%

[0143] △: 15% or more but less than 20%

[0144] ×:More than 20%

[0145] The evaluation criteria in Table 3 (Examples 11-15 and Reference Examples 4-6) are as follows.

[0146] A: Less than 10%

[0147] B: 10% or more but less than 20%

[0148] C: Over 20%

[0149] (Processability)

[0150] To ensure the aluminum sheet has a long side along the rolling direction, the coated metal sheet is cut into 3.5×3cm pieces. The resulting test pieces are bent parallel to the short side, with the coated surface facing outwards. As spacers, a predetermined number of aluminum sheets, each 0.28mm thick, are placed inside the bent portion (two aluminum sheets in Examples 1-10, Comparative Examples 1-6, and Reference Examples 1-3; one aluminum sheet in Examples 11-15 and Reference Examples 4-6). Bending is performed by applying impacts to each test piece using a seam-fold type DuPont impact tester in an atmosphere at 25°C. A 3kg hammer with a flat contact surface for impact bending is used. The hammer is dropped from a height of 40cm. A 2cm width of the bent tip is brought into contact with a sponge immersed in a 1% sodium chloride aqueous solution. A voltage of 6.3V is applied to the tip, and the current (mA) is measured after 4 seconds.

[0151] The evaluation criteria in Table 1 (Examples 1-5, Comparative Examples 1-2 and Reference Examples 1-2) are as follows.

[0152] ◎: Less than 5mA

[0153] 〇: 5mA or higher and less than 10mA

[0154] △: 10mA or higher and less than 30mA

[0155] ×: Above 30mA

[0156] The evaluation criteria in Table 2 (Examples 6-10, Comparative Examples 3-6 and Reference Example 3) are as follows.

[0157] ◎: Less than 1mA

[0158] 〇: Above 1mA and less than 3mA

[0159] △: Above 3mA and below 10mA

[0160] ×: 10mA or more

[0161] The evaluation criteria in Table 3 (Examples 11-15 and Reference Examples 4-6) are as follows.

[0162] A: Less than 1mA

[0163] B: Above 1mA and below 3mA

[0164] C: Above 3mA

[0165] (Corrosion resistance)

[0166] Test pieces measuring 5cm × 5cm were cut from coated metal sheets and then immersed in a model liquid, which served as the contents of the tank. The degree of corrosion was then visually evaluated according to the following criteria. The immersion conditions were set at 50°C for 14 days. The model liquid used for the test was prepared by adding citric acid to 0.2% sodium chloride to achieve a pH of 2.5. The results are shown in Table 1. The evaluation criteria are as follows.

[0167] 〇: Non-corrosive

[0168] △: Partial corrosion

[0169] ×: Most corrosion

[0170] [Preparation of Waterborne Coating Compositions]

[0171] (Example 1)

[0172] Polyester resin a was used as the polyester resin, and N,N,N′,N′-tetratetra(2-hydroxypropyl)hexamethylenediamide was used as the β-hydroxyalkylamide compound for use as a curing agent. Polyester resin a was an unmodified acrylic polyester resin (acid value: 36 mg KOH / g, Tg: 80℃, Mn=5,000, monomer composition: terephthalic acid component / ethylene glycol component / propylene glycol component = 50 / 14 / 36 mol%). N,N,N′,N′-tetratetra(2-hydroxypropyl)hexamethylenediamide was Primid QM1260 manufactured by EMS-GRILTECH (described as "β-hydroxyalkylamide A" in the table). 333 parts of an aqueous dispersion of polyester resin a (solid content concentration: 30% by mass) (solid content: 100 parts), 50 parts of an aqueous solution of a β-hydroxyalkylamide compound prepared in advance using ion-exchanged water (solid content concentration: 10% by mass) (solid content: 5 parts), 150 parts of 2-propanol, and 517 parts of ion-exchanged water were introduced into a glass container and stirred for 10 minutes to obtain an aqueous coating composition with a solid content concentration of 10% by mass and a solid content blending ratio of polyester resin / curing agent = 100 / 5 (mass ratio).

[0173] (Examples 2-5 and Comparative Examples 1-2)

[0174] Except for replacing the polyester resin with various polyester resins (all of which are unmodified acrylic resins) or changing the solid content blending ratio as shown in Table 1, the waterborne coating composition was prepared in the same manner as in Example 1. As polyester resins, polyester resin b (unmodified acrylic resin, acid value: 31 mg KOH / g, Tg: 40°C, Mn = 5,000), polyester resin c (unmodified acrylic resin, acid value: 17 mg KOH / g, Tg: 40°C, Mn = 8,500, monomer composition: terephthalic acid component / isophthalic acid component / adipic acid / ethylene glycol component / neopentyl glycol component = 28 / 15 / 7 / 25 / 25 mol%), and polyester resin d (unmodified acrylic resin, acid value: 75 mg KOH / g, Tg: 80°C, Mn = 3,000) were used. In Example 5, N,N,N′,N′-tetra(2-hydroxyethyl)hexamethylenediamide (Primid XL552 manufactured by EMS-GRILTECH; described in the table as "β-hydroxyalkylamide B") was used as the β-hydroxyalkylamide compound.

[0175] (Refer to Example 1)

[0176] The waterborne coating composition was prepared in the same manner as in Example 1, except that polyester resin e (Tg: 50℃, acid value: 30mgKOH / g, acrylic component content: 40% by mass) was used as the polyester resin. This polyester resin e is an acrylic-modified polyester resin modified with an acrylic resin.

[0177] (See Example 2 for reference)

[0178] Polyester resin a was used as the polyester resin, and a methyl phenolic resin was used as the curing agent. As the methyl phenolic resin, a m-cresol-based phenolic resin containing butylated hydroxymethyl groups (hydroxymethyl group ratio: 90 mol%, Mn = 1,600) was used. An aqueous coating composition was prepared using 333 parts of an aqueous dispersion of polyester resin a (solid content: 100 parts), 30 parts of a n-butanol solution of the methyl phenolic resin (solid content: 15 parts), 1 part of dodecylbenzenesulfonic acid (curing catalyst), 0.3 parts of triethylamine, 200 parts of 2-propanol, and 635 parts of deionized water (solid content concentration: approximately 10% by mass, solid content blending ratio: polyester resin / curing agent = 100 / 15). As the dodecylbenzenesulfonic acid, "Dodecylbenzenesulfonic acid (soft type) (mixture)" manufactured by Tokyo Chemical Industry Co., Ltd. was used.

[0179] (Example 6)

[0180] A β-hydroxyalkylamide compound was prepared by using polyester resin a as the polyester resin and N,N,N',N'-tetratetra(2-hydroxypropyl)hexamethylenediamide [Primid QM1260 manufactured by EMS-GRILTECH; described in the table as "β-hydroxyalkylamide A"] as the curing agent. 333 parts (solids: 100 parts) of an aqueous dispersion of polyester resin a (resin solids concentration: 30% by mass, 2-propanol concentration: 18% by mass) and 16.7 parts (solids: 5 parts) of an aqueous solution of the β-hydroxyalkylamide compound pre-treated with deionized water were introduced into a glass container and stirred for 10 minutes to obtain an aqueous coating composition with a solids concentration of 30% by mass and a solids blending ratio of polyester resin / curing agent = 100 / 5 (by mass).

[0181] (Examples 7-10 and Comparative Examples 3-6)

[0182] Except for replacing the polyester resin with various polyester resins (all of which are unmodified acrylic resins) or changing the solid content blending ratio as shown in Table 2, the waterborne coating composition was prepared in the same manner as in Example 6.

[0183] Polyester resin f was used in Example 9, and polyester resin g was used in Example 10. Polyester resin f was an unmodified acrylic resin (acid value: 50 mg KOH / g, Tg: 46 °C, Mn = 3,000, monomer composition: terephthalic acid component / isophthalic acid component / sebacic acid component / trimethicone component / ethylene glycol component / neopentyl glycol component / 1,4-butanediol component = 36 / 9 / 4 / 1 / 9 / 25 / 16 mol) and polyester resin g was an unmodified acrylic resin (acid value: 60 mg KOH / g, Tg: 70 °C, Mn = 3,000).

[0184] (Refer to Example 3)

[0185] Except that polyester resin e, which is an acrylic-modified polyester resin modified with an acrylic resin, is used as the polyester resin, the waterborne coating composition is prepared in the same manner as in Example 6.

[0186] (Example 11)

[0187] The polyester resin (A) used here is polyester resin (A)-a (unmodified acrylic resin, acid value: 50 mg KOH / g, Tg: 46℃, Mn=3,000, monomer composition: terephthalic acid component / isophthalic acid component / sebacic acid component / trimethicone component / ethylene glycol component / neopentyl glycol component / 1,4-butanediol component = 36 / 9 / 4 / 1 / 9 / 25 / 16 mol%). The polyester resin (B) used here is polyester resin (B)-a (unmodified acrylic resin, acid value: 23 mg KOH / g, Tg: 80℃, Mn=8,000, monomer composition: terephthalic acid component / ethylene glycol component / propylene glycol component = 50 / 10 / 40 mol%). The β-hydroxyalkylamide compound used as the curing agent is N,N,N',N'-tetratetra(2-hydroxypropyl)hexamethylenediamide [Primid QM1260 manufactured by EMS-GRILTECH; described in the table as "β-hydroxyalkylamide A"]. A mixed polyester resin (Av) was prepared by mixing an aqueous dispersion of polyester resin (A) and an aqueous dispersion of polyester resin (B) at a solid content mass ratio of 50:50. mix 36.5 mg KOH / g, Tg mixA water dispersion (solids concentration: 30% by mass, 2-propanol concentration: 18% by mass) was prepared at 62.1℃. An aqueous solution of the β-hydroxyalkylamide compound (solids concentration: 30% by mass) was pre-adjusted using deionized water. 333 parts of the water dispersion (solids: 100 parts) and 24 parts of the aqueous solution (solids: 8 parts) were introduced into a glass container and stirred for 10 minutes to obtain a waterborne coating composition with a solids concentration of 30% by mass and a solids blending ratio of polyester resin / curing agent = 100 / 8 (by mass).

[0188] (Examples 12-15 and Reference Examples 4-6)

[0189] Except for changing the type of polyester resin and the blending ratio of solid components as shown in Table 3, the waterborne coating composition was prepared in the same manner as in Example 11. In addition to the polyester resins described above, polyester resin (A)-b was used as polyester resin (A), and polyester resins (B)-b, (B)-c, and (B)-d were used as polyester resin (B). Polyester resin (A)-b was an unmodified acrylic resin (acid value: 36 mg KOH / g, Tg: 80 °C, Mn = 5,000, monomer composition: terephthalic acid component / ethylene glycol component / propylene glycol component = 50 / 14 / 36 mol%). Polyester resin (B)-b is an unmodified acrylic resin (acid value: 17 mg KOH / g, Tg: 40℃, Mn = 8,500, monomer composition: terephthalic acid / isophthalic acid / adipic acid / ethylene glycol / neopentyl glycol = 28 / 15 / 7 / 25 / 25 mol%). Polyester resin (B)-c is an unmodified acrylic resin (acid value: 5 mg KOH / g, Tg: 52℃, Mn = 17,000, monomer composition: terephthalic acid / isophthalic acid / adipic acid / ethylene glycol / neopentyl glycol = 23 / 23 / 4 / 24 / 26 mol%). In addition, polyester resin (B)-d is an unmodified acrylic resin (acid value: 11 mg KOH / g, Tg: -25℃, Mn=17,000, monomer composition: terephthalic acid component / isophthalic acid component / sebacic acid component / 1,4-butanediol component = 14 / 17 / 19 / 50 mol%).

[0190] Tables 1, 2, and 3 show the composition (type of polyester resin, type of curing agent, and blending ratio of solid components) of each waterborne coating composition, coating conditions, and evaluation results.

[0191] [Table 1]

[0192]

[0193] [Table 2]

[0194]

[0195] [Table 3]

[0196]

[0197] Industrial availability :

[0198] The water-based coating composition of the present invention can be suitably used as an inner or outer surface coating for metal cans and can lids, etc., because the composition can form a coating film with excellent processability and good curing and corrosion resistance.

Claims

1. A water-based coating composition comprising: Polyester resin; A curing agent having functional groups capable of crosslinking with carboxyl groups is used as the polyester resin in the case where two or more polyester resins with different acid values ​​are blended. The sum of the values ​​obtained by multiplying the acid value and mass fraction of each polyester resin is set as the average acid value of the blend, wherein the average acid value of the blend is 30 mg KOH / g or more and less than 70 mg KOH / g. The polyester resin is a mixed polyester resin comprising polyester resin (A) and polyester resin (B) with an acid value lower than that of polyester resin (A), and the difference in acid value between polyester resin (A) and polyester resin (B) is 10 to 50 mg KOH / g. The polyester resin (A) has an acid value of 30 to 80 mg KOH / g, the polyester resin (B) has an acid value of 5 mg KOH / g or more but less than 30 mg KOH / g, and the solid content mass ratio of the polyester resin (A) to the polyester resin (B) is (A):(B) = 95:5 to 5:

95. The amount of solid component in the curing agent is in the range of 3 parts by weight or more and less than 15 parts by weight relative to 100 parts by weight of solid component in the polyester resin.

2. The waterborne coating composition according to claim 1, wherein the polyester resin is an unmodified acrylic polyester resin or an acrylic-modified polyester resin containing less than 10% by mass of the acrylic component.

3. The waterborne coating composition according to claim 1 or 2, wherein the curing agent is at least one selected from the group consisting of β-hydroxyalkylamide compounds, oxazoline-containing compounds, and carbodiimide-containing compounds.

4. The waterborne coating composition according to claim 3, wherein the curing agent is a β-hydroxyalkylamide compound.

5. The waterborne coating composition according to claim 4, wherein the β-hydroxyalkylamide compound is N,N,N',N'-tetrakis(2-hydroxyethyl)hexamethylenediamide and / or N,N,N',N'-tetrakis(2-hydroxypropyl)hexamethylenediamide.

6. A coated metal sheet having a coating film comprising the water-based coating composition according to any one of claims 1 to 5.

7. A metal container having a coating comprising the aqueous coating composition according to any one of claims 1 to 5.

8. A metal cap having a coating comprising the water-based paint composition according to any one of claims 1 to 5.

Images