Aqueous coating composition, coating method, coating film, coated article, and coating film forming method

Abstract

The present invention relates to an aqueous coating composition containing a water dispersion of a crystalline acrylic-modified polyester resin (A), wherein the melting point of the crystalline acrylic-modified polyester resin (A) is 125°C or higher.

Description

Water-based coating composition, coating method, coating film, coated article, and coating film forming method

[0001] The present invention relates to an aqueous coating composition, a coating method, a coating film, a coated article, and a coating film forming method.

[0002] As coating materials for cans, various coating compositions such as epoxy resins, polyvinyl chloride resins, and polyester resins have been used from the viewpoint of coating film performance such as corrosion resistance, etc. Among these, coating compositions containing, as a base resin, an epoxy resin produced using raw materials containing bisphenol A (BPA) and the like have been widely and generally used.

[0003] However, from the standpoint of environmental impact, there is a demand for can coating compositions that do not use raw materials containing bisphenol A (including raw materials that may contain residual levels of BPA).

[0004] Therefore, Patent Document 1 discloses an aqueous resin composition with low BPA elution, which is prepared by neutralizing a mixture of an aromatic epoxy resin (A) and a terminal-modified aromatic epoxy resin (B) with an acrylic-modified epoxy resin obtained by subjecting a carboxyl group-containing acrylic resin (C) to a partial esterification reaction in an organic solvent, with a base, and dispersing the resulting mixture in an aqueous medium. The carboxyl group-containing acrylic resin (C) in this aqueous resin composition is a carboxyl group-containing acrylic resin (D) having a Tg of 100°C or higher, a carboxyl group-containing acrylic resin (E) having a Tg of less than 100°C, or a mixture thereof.

[0005] Furthermore, Patent Document 2 discloses an aqueous coating composition that is safe for the human body, in which an acrylic-modified polyester resin (C) is obtained by graft polymerizing a polyester resin (A) having an ethylenic double bond at the resin end and a number-average molecular weight of 2,000 to 50,000 with a polymerizable unsaturated monomer component (B) containing a carboxyl group-containing polymerizable unsaturated monomer, and a β-hydroxyalkylamide crosslinking agent (D) having a specific structural formula are stably dispersed in an aqueous medium.

[0006] Furthermore, Patent Document 3 discloses an aqueous coating composition for can coating, which contains core-shell acrylic resin particles (A) consisting of a shell portion and a crosslinked core portion, and a resol phenolic resin (C1) and / or an amino resin (C2), and does not contain BPA or a resin produced using a raw material containing BPA.

[0007] Japanese Patent Publication No. 2004-331694 Japanese Patent Publication No. 2003-026992 Japanese Patent Publication No. 2016-113561

[0008] In response to this, in recent years, laws and regulations that further expand and strengthen the scope of regulated substances are being enacted, particularly in Europe and the United States. Specifically, not only BPA but also styrene, formaldehyde, isocyanate, etc. may fall under the category of regulated substances.

[0009] In light of the above, the aqueous coating composition of Patent Document 1 has an aromatic epoxy resin as the basic skeleton of the base resin, and therefore contains BPA, which raises concerns about the environment and human health.Furthermore, the aqueous coating composition of Patent Document 3 contains phenolic resin and styrene, and therefore cannot be used under the expanded legal restrictions on restricted substances.

[0010] As a result, the number of regulated substances is increasing, limiting the freedom of choice in the selection of raw materials that can be used in coating compositions. Under these circumstances, the aqueous coating composition of Patent Document 2 is safe for the human body. However, because the aqueous coating composition contains a considerable amount of a β-hydroxyalkylamide crosslinking agent, it is difficult to achieve both coating film properties and water resistance, and better corrosion resistance is also desired.

[0011] Therefore, the problem to be solved by the present invention is to provide an aqueous coating composition that has excellent coating film performance, such as corrosion resistance, processability, and retort resistance, without using raw materials containing legally restricted substances such as bisphenol A. Another object of the present invention is to provide a coating method, coating film, coated article, and coating film formation method that use the aqueous coating composition.

[0012] In order to solve the above-mentioned problems, the present inventors conducted extensive research to develop an aqueous coating composition that does not contain legally restricted substances such as BPA, phenolic resins, and crosslinking agents such as polyisocyanate compounds. As a result, they focused on crystallization and came up with the idea of ​​a crystalline polyester resin. They found that the use of a crystalline polyester resin can achieve good processability. Further research revealed that the use of a crystalline acrylic-modified polyester resin is promising from the perspective of water dispersibility in the coating state. They found that this would achieve even better corrosion resistance and retort resistance, thereby solving the above-mentioned problems, and thus completed the present invention.

[0013] That is, the gist of the present invention is as follows.

[0014] [1] An aqueous coating composition containing an aqueous dispersion of a crystalline acrylic-modified polyester resin (A), wherein the melting point of the crystalline acrylic-modified polyester resin (A) is 125°C or higher. [2] The aqueous coating composition according to [1], wherein the coating film obtained by heating and crystallizing the composition has a heat of fusion of 1.0 to 17 J / g. [3] The coating film obtained by heating and crystallizing the composition has a storage modulus at 50°C of 3.2 x 10 ７ ~2.4 x 10 ８[4] The aqueous coating composition according to any one of [1] to [3] above, wherein the crystalline acrylic-modified polyester resin (A) has a mass solids ratio of the polyester resin component (a1) to the acrylic resin component (a2), expressed as (a1) / (a2), in the range of 60 / 40 to 90 / 10. [5] The aqueous coating composition according to any one of [1] to [4] above, wherein the acrylic resin component (a2) in the crystalline acrylic-modified polyester resin (A) has an acid value of 180 to 400 mg KOH / g and a glass transition temperature of 20 to 160°C. [6] The aqueous coating composition according to any one of [1] to [5] above, wherein the aqueous dispersion of the crystalline acrylic-modified polyester resin (A) contains an organic solvent (a3), and the content of the organic solvent (a3) ​​is 50 parts by mass or more per 100 parts by mass of the total solids content of the crystalline acrylic-modified polyester resin (A). [7] The aqueous coating composition according to any one of the above [1] to [6], further comprising a β-hydroxyalkylamide compound (B). [8] The aqueous coating composition according to any one of the above [1] to [7], for use on cans.

[0015] [9] A coating method comprising applying the aqueous coating composition described in any one of [1] to [8] above to a substrate.

[10] A coating film formed by applying the aqueous coating composition described in any one of [1] to [8] above to a substrate and crystallizing it.

[11] A coated article comprising a substrate and a coating film formed on the substrate, wherein the coating film is a film formed by applying the aqueous coating composition described in any one of [1] to [8] above to a substrate and crystallizing it.

[12] A coating film forming method comprising, in order, applying the aqueous coating composition described in any one of [1] to [8] above to a substrate, volatilizing the aqueous dispersion in the aqueous coating composition, and annealing at 70 to 140°C for 10 to 60 minutes.

[13] A coating film obtained by, in order, applying the aqueous coating composition according to any one of [1] to [8] above to a substrate, volatilizing the aqueous dispersion in the aqueous coating composition, and annealing at 70 to 140°C for 10 to 60 minutes.

[14] A coated article comprising a substrate and a coating film formed on the substrate, wherein the coating film is obtained by applying the aqueous coating composition according to any one of [1] to [8] above, volatilizing the aqueous dispersion in the aqueous coating composition, and annealing at 70 to 140°C for 10 to 60 minutes.

[0016] The aqueous coating composition according to this embodiment can provide a coating film with excellent coating film performance, such as corrosion resistance, processability, and retort resistance, without using raw materials containing legally restricted substances. Therefore, the aqueous coating composition is particularly suitable for can applications. Also provided are an excellent coating method, coating film, coated article, and coating film formation method using the aqueous coating composition.

[0017] The present invention will be described in detail below, but the present invention is not limited to the following embodiments and can be modified as desired without departing from the gist of the present invention. In this specification, the term "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits. In addition, in this specification, parts by mass and parts by weight, and mass% and weight% have the same meaning.

[0018] <<Aqueous Coating Composition>> The aqueous coating composition according to this embodiment contains an aqueous dispersion of a crystalline acrylic-modified polyester resin (A), and the melting point of the crystalline acrylic-modified polyester resin (A) is 125°C or higher.

[0019] The aqueous coating composition of this embodiment can achieve good processability because the acrylic-modified polyester resin has sufficient crystallinity, and furthermore, because the base resin composition is a crystalline acrylic-modified polyester resin with a melting point of 125°C or higher, it can also achieve excellent coating film performance such as good corrosion resistance and retort resistance.

[0020] Specifically, because the polyester resin is acrylic-modified, it has excellent water dispersibility, and the resulting aqueous coating composition also has excellent stability. Furthermore, as will be described in detail below, in an embodiment in which a β-hydroxyalkylamide compound is further contained as a crosslinking agent, corrosion resistance can be further improved. Therefore, the aqueous coating composition according to this embodiment can obtain a coating film with excellent corrosion resistance, processability, and retort resistance without containing raw materials containing a wide range of legally restricted substances, such as bisphenol A, and also has excellent stability as an aqueous coating composition. Therefore, it is particularly suitable for can applications.

[0021] The aqueous coating composition according to this embodiment will be described in detail below.

[0022] <Crystalline Acrylic-Modified Polyester Resin (A)> In this embodiment, the aqueous dispersion of the crystalline acrylic-modified polyester resin (A) is a dispersion liquid in which the crystalline acrylic-modified polyester resin (A) is present in a dispersed state in a medium containing water as a main component.

[0023] In this specification, the term "main component" refers to the component that has the highest content among the components constituting the medium. In other words, the medium is a medium that has the highest content of water among the components. Furthermore, a medium containing water as the main component may be referred to as an "aqueous medium."

[0024] In this specification, the term "crystalline" for an acrylic-modified polyester resin refers to a resin that has a clear endothermic peak, rather than a stepwise endothermic change, in a differential calorimetric curve measured by a differential scanning calorimeter (DSC). Specifically, a clear endothermic peak in this specification means a peak whose half-width is 35°C or less when measured in DSC at a heating rate of 20°C / min.

[0025] The melting point of the crystalline acrylic-modified polyester resin (A) in this embodiment is 125°C or higher, preferably 125 to 180°C. From the viewpoint of obtaining good retort resistance, the melting point is 125°C or higher, may be 130°C or higher, or may be 140°C or higher. There is no particular upper limit, but from the viewpoint of obtaining an aqueous dispersion having high dispersibility, the melting point is preferably 180°C or lower, more preferably 170°C or lower, and even more preferably 160°C or lower.

[0026] In this specification, the melting point of the crystalline acrylic-modified polyester resin (A) is a value measured using a differential scanning calorimeter (manufactured by Hitachi High-Tech Science, DSC600) according to the following procedure. The measurement sample is prepared according to the following procedure. First, an aqueous dispersion of the crystalline acrylic-modified polyester resin (A) is applied to a substrate, the solvent is evaporated, and then the substrate is annealed at 110°C for 30 minutes. Then, each substrate is cut into a size of 4.5 mm in diameter, placed in an aluminum pan with a diameter of 5.2 mm, and further covered with a lid with a diameter of 5.0 mm and crimped to form a measurement sample. Using a differential scanning calorimeter (manufactured by Hitachi High-Tech Science, DSC600), the DSC curve is measured when the measurement sample is heated from -30°C to 200°C at a rate of 20°C / min, and the peak top of the clear endothermic peak located above the glass transition temperature is taken as the melting point.

[0027] The crystalline acrylic-modified polyester resin (A) in this embodiment can be obtained by acrylic-modifying a crystalline polyester resin, and retains crystallinity even after acrylic modification.

[0028] The crystalline acrylic-modified polyester resin (A) in this embodiment can be obtained, for example, by graft polymerizing a polymerizable unsaturated monomer component containing a carboxyl group-containing polymerizable unsaturated monomer onto a crystalline polyester resin containing an ethylenic double bond.

[0029] The crystalline polyester resin containing an ethylenic double bond is mainly an ester of a polybasic acid component and a polyhydric alcohol component.

[0030] The polybasic acid compound in the polybasic acid component is a compound containing two or more carboxy groups in one molecule, including alkyl esters, acid anhydrides, and acid chlorides of polycarboxylic acid compounds. The polyhydric alcohol compound in the polyhydric alcohol component is a compound having two or more hydroxy groups in one molecule.

[0031] Examples of the polycarboxylic acid compound include divalent aliphatic carboxylic acids such as oxalic acid, succinic acid, malonic acid, adipic acid, β-methyladipic acid, pimelic acid, azelaic acid, sebacic acid, nonanedicarboxylic acid, decanedicarboxylic acid, undecanedicarboxylic acid, dodecanedicarboxylic acid, tetradecanedicarboxylic acid, maleic acid, fumaric acid, citraconic acid, diglycolic acid, cyclohexane-3,5-diene-1,2-dicarboxylic acid, malic acid, citric acid, hexahydroterephthalic acid, tartaric acid, and mucic acid; phthalic acid, isophthalic acid, terephthalic acid, tetrachlorophthalic acid, chlorophthalic acid, and nitrophthalic acid. Examples of aromatic carboxylic acids include divalent aromatic carboxylic acids such as naphthalene-1,4-dicarboxylic acid, p-carboxyphenylacetic acid, p-phenylene diacetic acid, m-phenylenediglycolic acid, p-phenylenediglycolic acid, o-phenylenediglycolic acid, diphenyl-p,p'-dicarboxylic acid, naphthalene-1,4-dicarboxylic acid, naphthalene-1,5-dicarboxylic acid, naphthalene-2,6-dicarboxylic acid, anthracene dicarboxylic acid, and dodecenylsuccinic acid; and trivalent or higher aromatic carboxylic acids such as trimellitic acid, pyromellitic acid, naphthalene tricarboxylic acid, naphthalene tetracarboxylic acid, pyrene tricarboxylic acid, and pyrene tetracarboxylic acid.

[0032] From the viewpoint of enhancing the crystallinity of the crystalline polyester resin, the polycarboxylic acid compound preferably contains a divalent aliphatic carboxylic acid. Note that the polycarboxylic acid compound may be used alone or in combination of two or more.

[0033] Examples of the polyhydric alcohol compounds include dihydric straight-chain aliphatic alcohols such as ethylene glycol, propylene glycol (also known as propanediol), butanediol, diethylene glycol, hexanediol, octanediol, decanediol, and dodecanediol; dihydric alicyclic alcohols such as cyclohexanediol; dihydric aromatic alcohols; trihydric or higher hydric aliphatic alcohols such as glycerin and pentaerythritol; and alcohols having a trihydric or higher guanidine skeleton such as hexamethylolmelamine, tetramethylolbenzoguanamine, and tetraethylolbenzoguanamine.

[0034] From the viewpoint of enhancing the crystallinity of the crystalline polyester resin, the polyhydric alcohol compound preferably contains a dihydric aliphatic alcohol, and more preferably a dihydric linear aliphatic alcohol. Note that the polyhydric alcohol compound may be used alone or in combination of two or more types.

[0035] The esterification reaction between the polybasic acid component and the polyhydric alcohol component can be carried out by a known method. Alternatively, the esterification reaction can be carried out by using a lower alkyl ester of the polybasic acid (e.g., methyl ester, ethyl ester, etc.) instead of the polybasic acid. The esterification reaction between the two components can be carried out by a known method.

[0036] As a method for introducing hydroxyl groups into a crystalline polyester resin, a hydroxyl-containing crystalline polyester resin may be first synthesized, and then the resulting hydroxyl-containing crystalline polyester resin may be reacted with an acid anhydride to half-esterify it into a carboxyl- and hydroxyl-containing crystalline polyester resin. Alternatively, a carboxyl-containing crystalline polyester resin may be first synthesized, and then the alcohol component may be added to produce a hydroxyl-containing crystalline polyester resin.

[0037] A method for introducing carboxy groups into a crystalline polyester resin can also be used by first synthesizing a hydroxyl group-containing crystalline polyester resin, and then reacting a polybasic acid with the hydroxyl groups of the resulting hydroxyl group-containing crystalline polyester resin to introduce carboxy groups into the resin, thereby producing a carboxyl group-containing crystalline polyester resin. Examples of the polybasic acid include maleic acid, maleic anhydride, phthalic anhydride, trimellitic anhydride, hexahydrophthalic anhydride, and tetrahydrophthalic anhydride.

[0038] For example, a method for synthesizing a crystalline polyester resin containing an ethylenic double bond may be used by incorporating an unsaturated polybasic acid having an ethylenic double bond as a raw material. Examples of the unsaturated polybasic acid include fumaric acid, maleic acid, itaconic acid, citraconic acid, lower alkyl esters thereof, and acid anhydrides thereof.

[0039] The polybasic acid having an ethylenic double bond may be introduced from the beginning of the reaction, but if introduced from the beginning, it will branch during graft polymerization of the acrylic monomer, making it more likely to gel, and as a result, production stability tends to be poor. Therefore, from the viewpoint of production stability, it is preferable to adopt a method in which a saturated crystalline polyester resin not containing an ethylenic double bond is synthesized, and then a compound having an ethylenic double bond is reacted with a hydroxyl group or a carboxyl group of the saturated crystalline polyester resin. However, this does not exclude the embodiment in which the polybasic acid is introduced from the beginning of the reaction.

[0040] Examples of the synthesis method for synthesizing a saturated crystalline polyester resin containing no ethylenic double bonds by reacting a compound having an ethylenic double bond with the saturated crystalline polyester resin include the following methods 1 and 2.

[0041] Method 1: After synthesizing a saturated crystalline polyester resin containing no ethylenic double bonds, an acid anhydride of a polybasic acid having an ethylenic double bond is added, and added to the hydroxyl groups in the saturated crystalline polyester resin, preferably at 100 to 160°C, to introduce an ethylenic double bond into the terminal of the saturated crystalline polyester resin.

[0042] Method 2: After synthesizing a saturated crystalline polyester resin containing no ethylenic double bonds, a compound having an ethylenic double bond and an epoxy group is added and added to the carboxyl group in the saturated crystalline polyester resin, preferably at 100 to 160°C, to introduce an ethylenic double bond into the terminal of the saturated crystalline polyester resin.

[0043] In the above method 2, examples of the compound having an ethylenic double bond and an epoxy group include glycidyl (meth)acrylate, allyl glycidyl ether, β-methylglycidyl (meth)acrylate, and 3,4-epoxycyclohexylmethyl (meth)acrylate. In this specification, "(meth)acrylate" means "at least one of acrylate and methacrylate." Furthermore, "(meth)acrylamide" means "at least one of acrylamide and methacrylamide."

[0044] Of the above-mentioned methods 1 and 2, in this embodiment, method 2 can be preferably employed from the viewpoint of shortening the reaction time.

[0045] In the above, the saturated crystalline polyester resin preferably has a melting point temperature in the range of 125 to 170°C, particularly 125 to 160°C, from the viewpoints of manufacturability and retort resistance of the aqueous dispersion. Furthermore, the saturated crystalline polyester resin preferably has a glass transition temperature in the range of 20 to 55°C, particularly 30 to 55°C, from the viewpoints of manufacturability and corrosion resistance of the aqueous dispersion. Furthermore, the saturated crystalline polyester resin preferably has a number average molecular weight in the range of 10,000 to 35,000, particularly 14,000 to 30,000, from the viewpoints of manufacturability and corrosion resistance of the aqueous dispersion. Furthermore, the saturated crystalline polyester resin preferably has an acid value in the range of 0 to 15 mgKOH / g, particularly 5.0 to 10 mgKOH / g, from the viewpoints of manufacturability and corrosion resistance of the aqueous dispersion.

[0046] In the present embodiment, it is more preferable that the saturated crystalline polyester resin has two or more of the melting point temperature, glass transition temperature, number average molecular weight, and acid value each within the above range, even more preferable that three or more of the melting point temperature, glass transition temperature, number average molecular weight, and acid value each within the above range, and even more preferable that all four of the melting point temperature, glass transition temperature, number average molecular weight, and acid value each within the above range.

[0047] The polymerizable unsaturated monomer used in the synthesis of the crystalline acrylic-modified polyester resin is, as an essential component, a carboxyl group-containing polymerizable unsaturated monomer such as acrylic acid, methacrylic acid, itaconic acid, or fumaric acid, and other polymerizable unsaturated monomers can be used as needed.

[0048] Examples of other polymerizable unsaturated monomer components include alkyl esters of acrylic acid or methacrylic acid having 1 to 18 carbon atoms, such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, i-butyl (meth)acrylate, t-butyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, benzyl (meth)acrylate, stearyl (meth)acrylate, and cetyl (meth)acrylate; cyclohexyl (meth)acrylate, isobornyl (meth)acrylate; aromatic vinyl monomers, such as styrene, α-methylstyrene, and vinyltoluene; hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, hydroxybutyl (meth)acrylate, hydroxyamyl (meth)acrylate, hydroxypropyl ... Examples of suitable monomers include hydroxyl group-containing polymerizable unsaturated monomers such as hydroxyalkyl (meth)acrylates such as hexyl (meth)acrylate, and caprolactone-modified alkyl (meth)acrylates having a hydroxyl group, which are obtained by ring-opening addition reaction of 1 to 5 moles of ε-caprolactone with 1 mole of the hydroxyalkyl (meth)acrylate; acrylamide-based monomers such as acrylamide, (meth)acrylamide, N-methoxymethyl (meth)acrylamide, N-ethoxymethyl (meth)acrylamide, N-n-propoxymethyl (meth)acrylamide, N-isopropoxymethyl (meth)acrylamide, N-n-butoxymethyl (meth)acrylamide, N-sec-butoxymethyl (meth)acrylamide, and N-tert-butoxymethyl (meth)acrylamide; and acrylonitrile, methacrylonitrile, vinyl acetate, ethylene, and butadiene.

[0049] In the above, it is preferable not to use styrene or α-methylstyrene from the viewpoint of expected future legal restrictions, the effects on the environment and the human body, etc.

[0050] The polymerizable unsaturated monomers may be used alone or in combination of two or more. However, since the resulting crystalline acrylic-modified polyester resin is rendered aqueous by the introduction of a carboxy group, it is preferable to adjust the amount of the carboxy group-containing polymerizable unsaturated monomer to give an acid value of the crystalline acrylic-modified polyester resin in the range of 15 to 100 mgKOH / g, particularly 30 to 80 mgKOH / g.

[0051] A method for synthesizing a crystalline acrylic-modified polyester resin by polymerizing a crystalline polyester resin containing an ethylenic double bond with a polymerizable unsaturated monomer can utilize free radical polymerization in an organic solvent. For example, a method can be employed in which the crystalline polyester resin containing an ethylenic double bond, a polymerizable unsaturated monomer, a radical polymerization initiator, and optionally a chain transfer agent are added and heated. The heating temperature can be, for example, 90 to 120°C. The heating time can be, for example, 1 to 5 hours.

[0052] As the polymerization initiator, organic peroxide-based, azo-based, and other polymerization initiators can be used. Examples of organic peroxide-based polymerization initiators include benzoyl peroxide, t-butylperoxy-2-ethylhexanoate, di-t-butyl peroxide, t-butylperoxybenzoate, and t-amylperoxy-2-ethylhexanoate. Examples of azo-based polymerization initiators include azobisisobutyronitrile and azobisdimethylvaleronitrile.

[0053] Examples of the chain transfer agent include α-methylstyrene dimer and mercaptans.

[0054] The mass solids ratio of the crystalline polyester resin to the polymerizable unsaturated monomer, i.e., the mass solids ratio of the polyester resin component (a1) to the acrylic resin component (a2) in the crystalline acrylic-modified polyester resin (A) of this embodiment, represented by (a1) / (a2), is preferably in the range of 60 / 40 to 90 / 10, more preferably in the range of 60 / 40 to 85 / 15, or more preferably in the range of 65 / 35 to 90 / 10, and even more preferably in the range of 65 / 35 to 85 / 15. Here, the proportion of the polyester resin component (a1) to the total of the polyester resin component (a1) and the acrylic resin component (a2) is preferably 60% by mass or more, more preferably 65% ​​by mass or more, from the viewpoint of obtaining good corrosion resistance and retort resistance. Furthermore, from the viewpoint of processability, the above proportion is preferably 90% by mass or less, more preferably 85% by mass or less.

[0055] From the viewpoints of dispersibility and corrosion resistance, the acrylic resin component (a2) of the crystalline acrylic-modified polyester resin (A) preferably has at least one of a high acid value and a high glass transition temperature (Tg), and more preferably has both a high acid value and a high Tg. The acrylic resin component (a2) preferably has an acid value of 180 to 400 mg KOH / g and a glass transition temperature of 20 to 160°C, and more preferably has an acid value of 180 to 400 mg KOH / g and a glass transition temperature of 20 to 160°C.

[0056] Here, the acid value is preferably 180 mgKOH / g or more, more preferably 200 mgKOH / g or more. The acid value is preferably 400 mgKOH / g or less, more preferably 350 mgKOH / g or less. The glass transition temperature is preferably 20°C or more, more preferably 40°C or more. The glass transition temperature is preferably 160°C or less, more preferably 150°C or less.

[0057] The acid value of the crystalline acrylic-modified polyester resin (A) in this embodiment is preferably 30 to 120 mgKOH / g, and the glass transition temperature is preferably 20 to 80°C, and more preferably satisfies both the acid value of 30 to 120 mgKOH / g and the glass transition temperature of 20 to 80°C.

[0058] The acid value of the crystalline acrylic-modified polyester resin (A) in this embodiment is preferably 30 mgKOH / g or more, and preferably 120 mgKOH / g or less, and more preferably 80 mgKOH / g or less. The glass transition temperature of the crystalline acrylic-modified polyester resin (A) in this embodiment is preferably 20°C or more, and more preferably 30°C or more, and is preferably 80°C or less, and more preferably 70°C or less.

[0059] <Aqueous Dispersion> The aqueous coating composition according to this embodiment contains an aqueous dispersion of the crystalline acrylic-modified polyester resin (A). The aqueous dispersion is obtained by neutralizing the synthesized crystalline acrylic-modified polyester resin (A) and dispersing it in water.

[0060] As a neutralizing agent used for neutralization, amines and ammonia can be preferably used. Examples of the amines include triethylamine, triethanolamine, dimethylethanolamine, diethylethanolamine, and morpholine. Among them, triethylamine and dimethylethanolamine can be particularly preferably used.

[0061] The degree of neutralization of the crystalline acrylic-modified polyester resin (A) is not particularly limited, but from the viewpoint of the manufacturability of the aqueous dispersion, a neutralization degree in the range of, for example, 0.3 to 1.0 equivalents relative to the carboxyl groups in the crystalline acrylic-modified polyester resin (A) is preferred, and a neutralization degree in the range of 0.5 to 1.0 equivalents is more preferred. The above amount is sometimes referred to as the neutralization equivalent.

[0062] In the aqueous dispersion of this embodiment, the aqueous medium in which the crystalline acrylic-modified polyester resin (A) is dispersed must dissolve the crystalline acrylic-modified polyester resin (A) during the synthesis process. Therefore, the aqueous dispersion of this embodiment preferably contains an organic solvent (a3) ​​in addition to the crystalline acrylic-modified polyester resin (A) and water. That is, the aqueous medium of this embodiment is preferably a mixture of water and the organic solvent (a3).

[0063] As the organic solvent (a3), any of the conventionally known organic solvents can be used as long as it does not impair the stability of the crystalline acrylic-modified polyester resin (A) in the aqueous medium.

[0064] Preferred examples of the organic solvent (a3) ​​include alcohol solvents, cellosolve solvents, aprotic polar solvents, amide solvents, carbitol solvents, etc. Specific examples of the organic solvent (a3) ​​include amphiphilic solvents such as alcohol solvents such as n-butanol, cellosolve solvents such as ethylene glycol monobutyl ether (butyl cellosolve), ethylene glycol monoisopropyl ether, ethylene glycol monomethyl ether, and propylene glycol monomethyl ether, aprotic polar solvents such as N-methyl-2-pyrrolidone, amide solvents such as 3-butoxy-N,N-dimethylpropanamide and 3-methoxy-N,N-dimethylpropanamide, and carbitol solvents such as diethylene glycol monoethyl ether.

[0065] In addition, other inert organic solvents that are immiscible with water can also be used as the organic solvent (a3) ​​as long as they do not impair the stability of the crystalline acrylic-modified polyester resin (A) in the aqueous medium. Specific examples of such organic solvents include aromatic hydrocarbon solvents such as toluene and xylene; ester solvents such as ethyl acetate and butyl acetate; and ketone solvents such as methyl ethyl ketone and cyclohexanone.

[0066] Among the above, from the viewpoint of the solubility and dispersibility of the crystalline polyester, N-methyl-2-pyrrolidone, 3-methoxy-N,N-dimethylpropanamide, cyclohexanone, ethylene glycol monobutyl ether (butyl cellosolve), etc. can be suitably used as the organic solvent.

[0067] In the aqueous coating composition according to this embodiment, the content of the organic solvent (a3) ​​relative to 100 parts by mass of the total solids content of the crystalline acrylic-modified polyester resin (A) is, for example, preferably 50 parts by mass or more, more preferably 50 to 200 parts by mass. Here, from the viewpoint of the solubility and dispersibility of the crystalline polyester, the content of the organic solvent (a3) ​​is preferably 50 parts by mass or more, more preferably 70 parts by mass or more, and even more preferably 100 parts by mass or more. Furthermore, from the viewpoint of the storage stability of the resulting aqueous coating composition, the content is preferably 200 parts by mass or less. Note that, when the aqueous dispersion according to this embodiment contains two or more organic solvents (a3), it is preferable that the total content thereof is within the above range.

[0068] In obtaining the aqueous dispersion of this embodiment, the crystalline acrylic-modified polyester resin (A) may be neutralized and dispersed in an aqueous medium by a conventional method, such as a method of gradually adding the crystalline acrylic-modified polyester resin (A) to an aqueous medium containing a neutralizing agent while stirring, or a method of neutralizing the crystalline acrylic-modified polyester resin (A) with a neutralizing agent and then adding the aqueous medium to the neutralized product while stirring, or adding the neutralized product to an aqueous medium while stirring.

[0069] When the aqueous dispersion contains an organic solvent (a3), a dispersion method using pressure emulsification can also be adopted from the viewpoint of reducing the amount of the organic solvent (a3).

[0070] Pressure emulsification is a method in which an aqueous phase component and an oil phase component are pre-emulsified, if necessary, using a homomixer or the like, and then an emulsion having fine emulsion particles is obtained by applying high shear force under pressure using a high-pressure homogenizer such as a Manton-Gaulin, French press, or microfluidizer.

[0071] When the aqueous dispersion of the crystalline acrylic-modified polyester resin (A) is obtained by pressure emulsification, it is preferable to prepare a mixture of water, an organic solvent, and a neutralizing agent, and then stir the mixture under pressure to prepare the aqueous dispersion. The pressure during the pressure application is preferably 0.40 MPa or more, more preferably 0.45 MPa or more, and even more preferably 0.50 MPa or more. The upper limit of the pressure is not particularly limited, but may be, for example, 0.9 MPa or less.

[0072] The aqueous dispersion under pressure may be carried out by adding the mixed solution dropwise as needed. The temperature conditions in this case are preferably, for example, 110 to 150°C. Here, the temperature conditions are preferably 110°C or higher, more preferably 125°C or higher, even more preferably 135°C or higher, and preferably 150°C or lower. The dispersion time in this case is preferably, for example, 20 to 60 minutes. Here, the dispersion time is preferably 20 minutes or longer, more preferably 30 minutes or longer, and preferably 60 minutes or shorter, more preferably 50 minutes or shorter. The stirring speed in this case is preferably, for example, 3000 to 6000 rpm. Here, the stirring speed is preferably 3000 rpm or higher, more preferably 4000 rpm or higher, and preferably 6000 rpm or shorter, more preferably 5000 rpm or shorter. Furthermore, the aqueous dispersion under pressure is preferably carried out by adding the above-mentioned mixed liquid dropwise as needed, at a temperature of 110 to 150°C for 20 to 60 minutes, and under stirring conditions of 3000 to 6000 rpm.

[0073] In this embodiment, the average particle size of the main peak in the aqueous dispersion of the crystalline acrylic-modified polyester resin (A) is preferably, for example, 100 to 400 nm. From the viewpoint of the storage stability of the coating material, the average particle size is preferably 100 nm or more, more preferably 150 nm or more, and preferably 400 nm or less, more preferably 300 nm or less, and even more preferably 250 nm or less. The average particle size of the main peak is the number-based average particle size measured using a particle size measurement device, for example, a particle size distribution analyzer ELSZ-2000 (manufactured by Otsuka Electronics Co., Ltd.).

[0074] <β-Hydroxyalkylamide Compound (B)> While an increasing number of compounds are prohibited from use due to legal restrictions, the aqueous coating composition according to this embodiment allows the use of harmless compounds. For example, from the viewpoint of food contact applications, the aqueous coating composition according to this embodiment may contain a β-hydroxyalkylamide compound (B) as needed to improve corrosion resistance.

[0075] The β-hydroxyalkylamide compound (B) in this embodiment is a compound having an alkyl chain and an amide group (-CONR-, where R represents a hydrogen atom or a hydrocarbon group) and a hydroxy group (-OH), with the hydroxy group being located at the β-position. The β-hydroxyalkylamide compound (B) is a compound having a functional group capable of reacting with the carboxy group contained in the crystalline acrylic-modified polyester resin (A), and examples thereof include compounds represented by the following general formula (1):

[0076]

[0077] (In formula (1), R １ represents a hydrogen atom, a methyl group, or an ethyl group; R ２ is a hydrogen atom, an alkyl group having 1 to 5 carbon atoms, or HOCH(R １ ) CH ２ -, A represents a divalent hydrocarbon group, and a plurality of R １ , R ２ may be the same or different.)

[0078] Examples of the β-hydroxyalkylamide compound (B) include Primid (registered trademark) XL-522, Primid (registered trademark) SF-4510, Priomid (registered trademark) QM-1260 (all manufactured by EMS-GRILTECH); N,N,N',N'-tetrakis(2-hydroxyethyl)adipamide), and the like.

[0079] When the aqueous coating composition according to this embodiment further contains a β-hydroxyalkylamide compound (B), the blending amount (content) thereof is preferably 0.1 to 6 parts by mass per 100 parts by mass of the total solid content of the crystalline acrylic-modified polyester resin (A). From the viewpoint of corrosion resistance, the blending amount is preferably 0.1 part by mass or more, more preferably 0.5 part by mass or more, and is preferably 6 parts by mass or less, more preferably 5 parts by mass or less.

[0080] <Other Optional Components> The aqueous coating composition according to this embodiment may further contain, as necessary, conventionally known raw materials such as antifoaming agents, surfactants, lubricants, waxes, viscosity modifiers, crystal nucleating agents, pigments, etc. Furthermore, these raw materials may be used in combination as appropriate.

[0081] Examples of the antifoaming agent include acrylic, vinyl ether, and dimethylpolysiloxane compounds, and two or more of these can be used in combination.

[0082] The nucleating agent may be an inorganic nucleating agent or an organic nucleating agent.

[0083] Examples of inorganic crystal nucleating agents that can be used include talc, calcium carbonate, mica, boron nitride, synthetic silicic acid, silicate, silica, kaolin, carbon black, zinc oxide, montmorillonite, clay minerals, basic magnesium carbonate, quartz powder, glass fiber, glass powder, diatomaceous earth, dolomite powder, titanium oxide, zinc oxide, antimony oxide, barium sulfate, calcium sulfate, alumina, calcium silicate, and boron nitride. These may be used alone or in combination of two or more. Of these, talc and silica are preferred from the viewpoint of the transparency of the coating film.

[0084] Examples of the organic crystal nucleating agents include the following (1) to (10). These may be used alone or in combination of two or more. (1) Organic carboxylic acids: octylic acid, caprylic acid (octanoic acid), toluic acid, heptanoic acid, pelargonic acid, lauric acid, myristic acid, palmitic acid, stearic acid, behenic acid, cerotic acid, montanic acid, melissic acid, benzoic acid, p-tert-butylbenzoic acid, terephthalic acid, terephthalic acid monomethyl ester, isophthalic acid, isophthalic acid monomethyl ester, rosin acid, 12-hydroxystearic acid, cholic acid, etc.; (2) Organic carboxylic acid alkali metal salts and organic carboxylic acid alkaline earth metal salts: alkali metal salts and alkaline earth metal salts of the organic carboxylic acids described in (1) above, etc.; (3) Polymeric organic compounds having metal salts of carboxyl groups: metal salts of carboxyl group-containing polyethylene obtained by oxidation of polyethylene, carboxyl group-containing polypropylene obtained by oxidation of polypropylene, copolymers of olefins such as ethylene, propylene, butene-1, and acrylic acid or methacrylic acid, copolymers of styrene and acrylic acid or methacrylic acid, copolymers of olefins and maleic anhydride, copolymers of styrene and maleic anhydride, etc.;(4) Aliphatic carboxylic acid amides: oleic acid amide, stearic acid amide, erucic acid amide, behenic acid amide, N-oleyl palmitamide, N-stearyl erucic acid amide, N,N'-ethylene bis(stearamide), N,N'-methylene bis(stearamide), methylol stearamide, ethylene bisoleic acid amide, ethylene bisbehenic acid amide, ethylene bisstearic acid amide, ethylene bislauric acid amide, hexamethylene bisoleic acid amide, hexamethylene bisstearic acid amide, butylene bisstearic acid amide, N,N'-dioleyl sebacic acid amide, N,N'-dioleyl adipic acid amide, N,N'-distearyl adipic acid amide, N,N'-distearyl sebacin acid amides, m-xylylenebisstearic acid amide, N,N'-distearylisophthalic acid amide, N,N'-distearylterephthalic acid amide, N-oleyl oleic acid amide, N-stearyl oleic acid amide, N-stearylerucic acid amide, N-oleylstearinamide, N-stearylstearic acid amide, N-butyl-N'-stearyl urea, N-propyl-N'-stearyl acid urea, N-allyl-N'-stearyl urea, N-phenyl-N'-stearyl urea, N-stearyl-N'-stearyl urea, dimethylitol oil amide, dimethyl lauric acid amide, dimethyl stearic acid amide, N,N'-cyclohexanebis(stearamide), N-lauroyl-L-glutamic acid-α,γ-n-butylamide, and the like; (5) Polymeric organic compounds: polymers of 3-branched α-olefins having 5 or more carbon atoms, such as 3,3-dimethylbutene-1,3-methylbutene-1,3-methylpentene-1,3-methylhexene-1,3,5,5-trimethylhexene-1, and vinylcycloalkanes, such as vinylcyclopentane, vinylcyclohexane, and vinylnorbornane; polyalkylene glycols, such as polyethylene glycol and polypropylene glycol; polyglycolic acid; cellulose; cellulose esters; cellulose ethers; polyesters; and polycarbonates;(6) Organic compounds of phosphoric acid or phosphorous acid and their metal salts: diphenyl phosphate, diphenyl phosphite, sodium bis(4-tert-butylphenyl)phosphate, sodium methylene(2,4-tert-butylphenyl)phosphate, etc.; (7) Sorbitol derivatives such as bis(p-methylbenzylidene)sorbitol and bis(p-ethylbenzylidene)sorbitol; (8) Cholesterol derivatives such as cholesteryl stearate and cholesteryloxystearamide; (9) Thioglycolic anhydride, paratoluenesulfonic acid, lauryl sulfate, paratoluenesulfonic acid amide and their metal salts, etc.; (10) Phenylphosphonic acid and its metal salts, etc.;

[0085] Among these, a nucleating agent consisting of a neutral substance that does not promote the hydrolysis of polyester is preferred because it can prevent the crystalline acrylic-modified polyester resin (A) from being hydrolyzed and thereby reducing its molecular weight. Furthermore, from the viewpoint of preventing the crystalline acrylic-modified polyester resin (A) from being reduced in molecular weight due to transesterification, ester and amide compounds, which are derivatives of carboxyl groups, are more preferred than nucleating agents having carboxyl groups, and similarly, ester and ether compounds, which are derivatives of carboxyl groups, are more preferred than nucleating agents having hydroxyl groups.

[0086] As the nucleating agent, an inorganic nucleating agent and an organic nucleating agent may be used in combination, or a combination of two or more types of nucleating agents may be used.

[0087] The content of the crystal nucleating agent is preferably 0.1 to 20 parts by mass relative to 100 parts by mass of the total solid content of the crystalline acrylic-modified polyester resin (A). Here, the content is preferably 0.1 parts by mass or more, more preferably 0.2 parts by mass or more, and is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 2 parts by mass or less. When two or more crystal nucleating agents are contained, it is preferable that the total content thereof is within the above range.

[0088] <Physical Properties, Characteristics> As described above, the aqueous coating composition according to this embodiment is a dispersion in which the crystalline acrylic-modified polyester resin (A) is dispersed in a medium mainly composed of water. Therefore, the solid content concentration of the aqueous coating composition is preferably 10 to 50% by mass. Here, from the viewpoint of coating workability and storage stability, the solid content concentration is preferably 10% by mass or more, more preferably 15% by mass or more, and is preferably 50% by mass or less, more preferably 40% by mass or less.

[0089] The aqueous coating composition according to this embodiment is cured by heating to crystallize, thereby forming a coating film. The heat of fusion of the coating film is preferably 1.0 to 17 J / g. From the viewpoints of corrosion resistance, retort resistance, and processability, the heat of fusion is preferably 1.0 J / g or more, more preferably 3.0 J / g or more, and is preferably 17 J / g or less, more preferably 10 J / g or less.

[0090] The heat of fusion of the coating film in this specification is a measured value measured as follows for a coating film formed as follows.

[0091] The measurement sample is prepared as follows: An aqueous coating composition containing the crystalline acrylic-modified polyester resin (A) is applied to a substrate, the solvent is evaporated, and then the coating is annealed at 110°C for 30 minutes to form a coating film. The coating film and the substrate are then cut into a size of 4.5 mm in diameter, placed in an aluminum pan with a diameter of 5.2 mm, and then covered with a lid with a diameter of 5.0 mm and crimped to form the measurement sample.

[0092] The measurement sample obtained by the above procedure is measured using a differential scanning calorimeter (DSC600, manufactured by Hitachi High-Tech Science) and the DSC curve is measured when the temperature is raised from -30°C to 200°C at a rate of 20°C / min. The heat of fusion of the coating film is calculated using the following formula from the peak area (ΔHcJ / g) of a significant exothermic peak located above the glass transition temperature and the peak area (ΔHmJ / g) of a significant endothermic peak located above the glass transition temperature in the obtained DSC curve. Formula: (-ΔHm-ΔHc) / weight fraction of the coating film in the measurement sample

[0093] The aqueous coating composition according to this embodiment is cured by heating and crystallizing, thereby forming a coating film. The storage modulus of the coating film at 50°C is 3.2 × 10 ７ ~2.4 x 10 ８ From the viewpoints of corrosion resistance, processability, and retort resistance, the storage modulus is preferably 3.2 × 10 Pa. ７ Pa or more is preferable, and 1.0 × 10 ８ Pa or more is more preferable, and 2.4 × 10 ８ Pa or less, and 2.0 × 10 ８ Pa or less is more preferable.

[0094] The storage modulus of the coating film in this specification is a measured value measured as follows for a coating film formed as follows.

[0095] Measurement samples are prepared as follows: A water-based coating composition containing the crystalline acrylic-modified polyester resin (A) is applied to a substrate, the solvent is evaporated, and then the coating is annealed at 110°C for 30 minutes to form a coating film. The coating film is then peeled off from the substrate and compression-molded to a diameter of 12 mm and a thickness of 0.5 to 0.8 mm to provide the measurement sample.

[0096] The measurement sample obtained by the above procedure is set on the 10 mm parallel plates of a rheometer (TA Instruments, ARES-G2), and after being fully melted at 200°C, a coating film is formed by crystallizing at 110°C for 30 minutes. After that, the sample is rapidly cooled to -100°C, and the dynamic viscoelasticity is measured at a measurement frequency of 1 Hz while the temperature is increased to 200°C at a rate of 20°C / min.

[0097] The aqueous coating composition according to this embodiment can be applied to various substrates, such as untreated or surface-treated metal plates such as aluminum plates, steel plates, and tin plates, as well as metal plates coated with an epoxy or vinyl primer, and cans or other products made from these metal plates.

[0098] The aqueous coating composition according to this embodiment is particularly suitable for use on cans because of its excellent corrosion resistance, processability, and retort resistance. The shape of the can to which the aqueous coating composition is applied is not particularly limited, but examples include two-piece cans consisting of two parts, a lid and a bottom, and a body part integrated with the lid, a three-piece can consisting of three parts, a lid, a bottom, and a body part, and bottle cans. The aqueous coating composition according to this embodiment can be applied to each of the above parts of the can.

[0099] The coating film obtained by applying the aqueous coating composition according to this embodiment is more preferably used for cans, particularly beverage cans, and is particularly suitable for use in coating the inner surfaces of cans.

[0100] In addition to the above, the aqueous coating composition according to this embodiment can also be suitably used for repair painting of seams (joints) on the inside of cans, and for painting the outer surface of cans such as the outer surface of can lids and tabs.

[0101] <<Coating Method, Coating Film, Coating Film Forming Method, and Coated Article>> The coating method according to this embodiment comprises the step of applying the aqueous coating composition described above in <<Aqueous Coating Composition>> to an object to be coated. Preferred aspects of the aqueous coating composition are the same as those described above in <<Aqueous Coating Composition>>.

[0102] The coating film according to this embodiment is a film formed by applying the aqueous coating composition described above in "Aqueous Coating Composition" to a substrate and then crystallizing it. Preferred aspects of the aqueous coating composition are the same as those described above in "Aqueous Coating Composition."

[0103] In the coating film forming method according to this embodiment, a coating film can be formed by applying the aqueous coating composition described above in "Aqueous Coating Composition" to a substrate, volatilizing the water dispersion in the aqueous coating composition, and then crystallizing it. Preferred aspects of the aqueous coating composition are the same as those described above in "Aqueous Coating Composition."

[0104] The coated article according to this embodiment comprises a substrate and a coating film formed on the substrate, the coating film being a film formed by applying and crystallizing the aqueous coating composition described above in "Aqueous Coating Composition." Preferred aspects of the aqueous coating composition are the same as those described above in "Aqueous Coating Composition."

[0105] The aqueous coating composition of this embodiment can be applied to a substrate by any of a variety of known methods. For example, spray coating, roll coater coating, dip coating, electrodeposition coating, etc. are applicable. Among these, spray coating and roll coater coating are preferred.

[0106] The amount of the aqueous coating composition to be applied to the substrate may be appropriately selected depending on the application, but the dry coating thickness may be, for example, 1 to 30 μm. The dry coating thickness may be 1 μm or more, 2 μm or more, or 30 μm or less, or 20 μm or less.

[0107] From the viewpoint of ease of application, it is preferable that the coating viscosity of the aqueous coating composition when applied to a substrate be adjusted to within the range of 15 to 30 seconds using a Ford cup #4 according to ISO 2431 at a liquid temperature of 23°C.

[0108] The aqueous coating composition of this embodiment is a coating composition that forms a coating film primarily by crystallization, and therefore it is preferable to apply the aqueous coating composition of this embodiment to an object to be coated, volatilize the water dispersion in the aqueous coating composition, particularly the solvent, which is a volatile component in the water dispersion, and then anneal the composition to crystallize it and form a coating film.

[0109] The annealing temperature is preferably 70 to 140° C., and the annealing time is preferably 10 to 60 minutes. From the viewpoints of corrosion resistance, retort resistance, and productivity, the temperature is preferably 70° C. or higher, more preferably 80° C. or higher, even more preferably 100° C. or higher, and preferably 140° C. or lower. From the viewpoints of corrosion resistance, retort resistance, and productivity, the annealing time is preferably 10 minutes or longer, more preferably 15 minutes or longer, even more preferably 30 minutes or longer, and preferably 60 minutes or shorter.

[0110] That is, one aspect of the coating film according to this embodiment is a coating film obtained by applying the above-mentioned aqueous coating composition to an object to be coated, volatilizing the water dispersion (solvent) in the aqueous coating composition, and then annealing the resulting coating film preferably at 70 to 140°C for 10 to 60 minutes.

[0111] One aspect of the coating film forming method according to this embodiment comprises, in order, a step of applying the aqueous coating composition to an object to be coated, a step of volatilizing the water dispersion (solvent) in the aqueous coating composition, and a step of annealing the composition preferably at 70 to 140°C for 10 to 60 minutes.

[0112] One aspect of the coated article according to this embodiment includes a substrate and a coating film formed on the substrate, and the coating film is obtained by applying the aqueous coating composition to the substrate, allowing the water dispersion (solvent) in the aqueous coating composition to volatilize, and then annealing the resulting coating preferably at 70 to 140°C for 10 to 60 minutes.

[0113] When the aqueous coating composition is applied to a substrate and heated to volatilize the solvent, and the heating temperature is higher than the annealing temperature, it is preferable to immediately cool the coating film to the annealing temperature after volatilizing the solvent and then anneal it. However, even if annealing cannot be performed immediately after volatilizing the solvent, a similar coating film can be obtained by annealing the coating film even if it is not immediately annealed, as long as the coating film is rapidly cooled to room temperature or below.

[0114] The coating film obtained as described above can be subjected to differential scanning calorimetry, and the presence of a clear exothermic peak above the glass transition temperature can be confirmed to have crystallized portions. Specifically, if the peak top of the clear exothermic peak is the melting point, and this temperature is 125°C or higher, it can be recognized as a coating film obtained from the aqueous coating composition described in the above ``Aqueous Coating Composition'', i.e., an aqueous coating composition containing an aqueous dispersion of crystalline acrylic-modified polyester resin (A) having a melting point of 125°C or higher.

[0115] The present invention will be described in more detail below with reference to examples. Here, "parts" and "%" mean "parts by mass" and "% by mass," respectively. Note that the "parts by mass" of raw materials in the following synthesis examples, production examples, examples, and comparative examples refer to the parts by mass of the solid content (sometimes referred to as active ingredients) of the raw materials.

[0116] Synthesis of Crystalline Polyester Resin Synthesis Example 1 415 parts by mass of terephthalic acid, 125 parts by mass of isophthalic acid, 203 parts by mass of dimethyl 2,6-naphthalenedicarboxylate, 16 parts by mass of trimellitic anhydride, 713 parts by mass of 1,4-butanediol, 60 parts by mass of 1,4-cyclohexanedimethanol, and 0.4 parts by mass of tetra-n-butyl titanate as a catalyst were charged into a 3 L four-neck flask, and an esterification reaction and a transesterification reaction were carried out while gradually increasing the temperature to 240°C over 3 hours. After completion of the reaction, the pressure in the system was gradually reduced, and the pressure was reduced to 10 mmHg over 1 hour to carry out reduced-pressure polymerization, and the temperature was raised to 240°C, and then post-polymerization was carried out for 120 minutes under a vacuum of 1 mmHg or less.

[0117] After the polycondensation reaction was completed, the mixture was cooled to 220 ° C. under a nitrogen atmosphere, and then a predetermined amount of trimellitic anhydride was added. Stirring was continued for 30 minutes at 220 ° C. under a nitrogen atmosphere. After the reaction was completed, the reactant was transferred to a reactor equipped with a stirrer, a reflux condenser, and a thermometer. 30.0 parts of N-methyl-2-pyrrolidone and 29.0 parts of cyclohexanone were added to 100.0 parts of the reactant, and the mixture was heated to 140 ° C. Stirring was then started to completely dissolve the reactant. Next, 0.9 parts of glycidyl methacrylate, 0.1 parts of tetrabutylammonium bromide, and 2.0 parts of cyclohexanone were added, and the mixture was reacted at 140 ° C. for 1 hour to obtain a solution of crystalline polyester resin 1.

[0118] The resulting crystalline polyester resin 1 had a number average molecular weight of 14,400, a glass transition temperature (Tg) of 36°C, a melting point (Tm) of 147°C, an acid value of 6.6 mgKOH / g, and a hydroxyl value of 3.6 mgKOH / g.

[0119] Synthesis Examples 2, 4, 5, 6, 7, 10, 15, and 16 Solutions of crystalline polyester resins 2, 4, 5, 6, 7, 10, 15, and 16 having the characteristic values ​​shown in Table 1 were obtained by carrying out a synthesis reaction in the same manner as in Synthesis Example 1, except that the types and ratios of the monomers used in Synthesis Example 1 were changed.

[0120] In Table 1, crystalline polyester resin 16 is listed. This is a non-crystalline polyester resin obtained by addition reaction of GK-622 (a non-crystalline polyester resin manufactured by Toyobo Co., Ltd.) with glycidyl methacrylate, and is not crystalline. However, for convenience, it is referred to as a crystalline polyester resin.

[0121]

[0122] Synthesis Example 12 557 parts by mass of terephthalic acid, 141 parts by mass of isophthalic acid, 8 parts by mass of trimellitic anhydride, 130 parts by mass of 1,2-propanediol, 500 parts by mass of 1,4-butanediol, 184 parts by mass of 1,4-cyclohexanedimethanol, and 0.4 parts by mass of tetra-n-butyl titanate as a catalyst were charged into a 3 L four-neck flask, and an esterification reaction was carried out while gradually increasing the temperature to 240° C. over 3 hours. After completion of the esterification reaction, the pressure in the system was gradually reduced, and the pressure was reduced to 10 mmHg over 1 hour to carry out reduced-pressure polymerization, and the temperature was raised to 240° C., and further post-polymerization was carried out for 120 minutes under a vacuum of 1 mmHg or less.

[0123] After the polycondensation reaction was completed, the mixture was cooled to 220 ° C. under a nitrogen atmosphere, and then a predetermined amount of trimellitic anhydride was added. Stirring was continued for 30 minutes at 220 ° C. under a nitrogen atmosphere. After the reaction was completed, the reactant was transferred to a reactor equipped with a stirrer, a reflux condenser, and a thermometer. 30.0 parts of N-methyl-2-pyrrolidone and 29.0 parts of cyclohexanone were added to 100.0 parts of the reactant, and the mixture was heated to 140 ° C. Stirring was then started, and the reactant was completely dissolved. Next, 0.75 parts of glycidyl methacrylate, 0.1 parts of tetrabutylammonium bromide, and 2.0 parts of cyclohexanone were added, and the mixture was reacted at 140 ° C. for 1 hour to obtain a solution of crystalline polyester resin 12.

[0124] The resulting crystalline polyester resin 12 had a number average molecular weight of 25,400, a glass transition temperature (Tg) of 35° C., a melting point (Tm) of 104° C., an acid value of 0.0 mgKOH / g, and a hydroxyl value of 3.0 mgKOH / g.

[0125] Synthesis Examples 3, 8, 9, 11, 13, and 14: Solutions of crystalline polyester resins 3, 8, 9, 11, 13, and 14 having the characteristic values ​​shown in Table 2 were obtained by carrying out the synthesis reaction in the same manner as in Synthesis Example 12, except that the types and ratios of the monomers used in Synthesis Example 12 were changed.

[0126]

[0127] In the above, crystalline polyester resins 12 to 16 do not satisfy the requirement of a melting point of 125° C. or higher, and are polyester resins for producing crystalline acrylic-modified polyester resins for comparative examples.

[0128] <<Production of Aqueous Dispersion of Crystalline Acrylic-Modified Polyester Resin (A)>> <Production Example 1> A reactor equipped with a stirrer, reflux condenser, and thermometer was charged with 128.5 parts (80 parts solids) of the solution of crystalline polyester resin 1 obtained in Synthesis Example 1 and heated to 140°C with stirring. Next, 95.2 parts of N-methyl-2-pyrrolidone was added, and the mixture was cooled to 120°C. Next, 4.7 parts of methyl methacrylate, 7.8 parts of n-butyl acrylate, 7.5 parts of acrylic acid, and 0.1 parts of Perbutyl (registered trademark) O (polymerization initiator, manufactured by NOF Corporation) were added, and polymerization was carried out for 30 minutes. Thereafter, 0.09 parts of Perbutyl (registered trademark) O was further charged to react any unreacted monomers, and polymerization was carried out for 1 hour. Next, to perform aqueous dispersion, the reaction mixture was cooled to 95 ° C., and 7.6 parts of dimethylaminoethanol (neutralization equivalent 0.82) and 15.1 parts of butyl cellosolve were added, followed by thorough stirring until uniform. Finally, while maintaining the temperature of the reaction mixture at 95 ° C., 233.9 parts of deionized water was added dropwise over 90 minutes with stirring to obtain an aqueous dispersion of crystalline acrylic-modified polyester resin 1 having a solids concentration of 20% by mass, an acid value (AV) of 59 mg KOH / g, a melting point (Tm) of 145 ° C., and a glass transition temperature (Tg) of 39 ° C. The ratio (a1) / (a2), which is the mass solids ratio of the polyester resin component (a1) to the acrylic resin component (a2), was 80 / 20, and the average particle size of the main peak was 230 nm.

[0129] <Production Examples 2 to 40 and 42 to 53> The synthesis reaction was carried out in the same manner as in Production Example 1, except that the crystalline polyester resin solution and the types and ratios of monomers were changed, to obtain crystalline acrylic-modified polyester resins 2 to 40 and 42 to 53, each having the specific values ​​shown in Tables 3 to 9. Note that blanks in Tables 3 to 9 indicate that no additive was added.

[0130] Although it is referred to as crystalline acrylic-modified polyester resin 53, as mentioned above, since the polyester resin 16, which is non-crystalline, is used, the acrylic-modified polyester resin is also non-crystalline, not crystalline. However, for convenience, it is referred to as crystalline acrylic-modified polyester resin.

[0131] <Production Example 41> 128.5 parts (80 parts solids) of the crystalline polyester resin 1 solution obtained in Synthesis Example 1 was added to a reactor equipped with a stirrer, reflux condenser, and thermometer, and the mixture was heated to 140°C with stirring. Next, 35.7 parts of N-methyl-2-pyrrolidone was added, and the mixture was cooled to 120°C. Next, 10.1 parts of methyl methacrylate, 2.4 parts of n-butyl acrylate, 7.5 parts of acrylic acid, and 0.10 parts of Perbutyl (registered trademark) O (manufactured by NOF Corporation) were added, and polymerization was carried out for 30 minutes. Thereafter, 0.09 parts of Perbutyl (registered trademark) O was further charged to react any unreacted monomers, and polymerization was carried out for 1 hour. Next, to perform aqueous dispersion, the reaction mixture was cooled to 95°C, and 7.6 parts of dimethylaminoethanol (neutralization equivalent 0.82) and 15.1 parts of butyl cellosolve were added, followed by sufficient stirring until uniform. The mixture was then transferred to a pressurizable batch-type disperser, and 293.4 parts of deionized water was added dropwise over 30 minutes under a pressure of 0.5 MPa while maintaining the temperature in the tank at 140 ° C. At this time, by stirring at 4500 rpm using a homogenizer, a water dispersion of crystalline acrylic-modified polyester resin 41 was obtained, having a solids concentration of 20% by mass, an acid value (AV) of 59 mg KOH / g, a melting point (Tm) of 145 ° C, and a glass transition temperature (Tg) of 44 ° C. The ratio (a1) / (a2), which is the mass solids ratio of the polyester resin component (a1) to the acrylic resin component (a2), was 80 / 20, and the average particle size of the main peak was 205 nm. Furthermore, by emulsifying under pressure, it was possible to reduce the amount of solvent by 30 parts by mass per 100 parts by mass of resin solids compared to crystalline acrylic-modified polyester resin 23.

[0132] In Tables 3 to 9, the amount of crystalline polyester resin is the solid content amount. The amount of solvent is in parts (amount) per 100 parts by mass of the crystalline acrylic-modified polyester resin solid content. The value of dimethylethanolamine (neutralizing agent) is the neutralization equivalent to the crystalline acrylic-modified polyester resin.

[0133] Crystalline acrylic-modified polyester resins 42 to 53 are crystalline acrylic-modified polyester resins for comparative examples that do not satisfy the requirement of a melting point of 125° C. or higher. Also, in the table, crystalline acrylic-modified polyester resin 53 is a non-crystalline acrylic-modified polyester resin and is not crystalline, but for convenience it is referred to as a crystalline acrylic-modified polyester resin.

[0134] The productivity of the aqueous dispersions of the crystalline acrylic-modified polyester resins 1 to 53 was evaluated according to the following criteria, and the results are also shown in Tables 3 to 9.

[0135] [Manufacturing] ◎: Water dispersion was possible without any problems. ○: Water dispersion was possible, but there was some varnish lifting. △: Water dispersion was poor, and particles were included. ×: Water dispersion was not possible. If the product was rated ◎ or ○ among the above, it was considered to have passed the test.

[0136] Crystalline acrylic modified polyester resins 47 and 48 were not water-dispersible and were not evaluated for acid value, melting point, main peak average particle size, etc., and are therefore marked with "-" in Tables 8 and 9.

[0137]

[0138]

[0139]

[0140]

[0141]

[0142]

[0143]

[0144] Preparation of Aqueous Coating Composition Example 1 100 parts by weight of the aqueous dispersion of crystalline acrylic-modified polyester resin 1 obtained in Preparation Example 1 was placed in a stirring and mixing vessel, and 2 parts by weight of an aqueous solution of Primid (registered trademark) QM-1260 (butylenediamine-N,N,N',N'-tetra-2-propanol, β-hydroxyalkylamide compound) was added as solids under stirring. Then, with continued stirring, 1 part by weight of DOWSIL (trademark) 71 Additive (defoaming agent, silicone-based, Dow-Toray Industries, Inc.), 1.5 parts by weight of HD-3028 (wax agent, manufactured by Gifu Ceramics Manufacturing Co., Ltd.), and 6 parts by weight of isopropyl alcohol were added. 31.5 parts by weight of deionized water were then added, and the mixture was thoroughly stirred until uniform, thereby producing an aqueous coating composition 1 with a solids concentration of 22% by weight.

[0145] Examples 2 to 43 and Comparative Examples 1 to 10 In Example 1 above, aqueous coating compositions 2 to 53 were produced in the same manner as in Example 1, except that the aqueous dispersions of crystalline acrylic-modified polyester resins 2 to 53 obtained in Production Examples 2 to 53 were used and the compositions shown in Tables 10 to 15 were adopted. Aqueous coating compositions 44 to 53 are for comparative purposes. The blending amounts in Tables 10 to 15 are solid content amounts, excluding isopropyl alcohol.

[0146] In Tables 10 to 15, SCT-275 is a polyether polyurethane thickener manufactured by Dow Chemical Company, and sodium caprylate is a crystal nucleating agent. Blanks in the tables indicate that no additives were added.

[0147] <Evaluation> <Preparation of Test Coated Panels> Each of the aqueous coating compositions 1 to 53 obtained in the above Examples and Comparative Examples was applied to a 0.26 mm thick #5052 aluminum panel using a bar coater so that the cured coating film had a thickness of 5 μm, and the panel was heated at 200°C for 1 minute using a dryer. The coated panel was then immediately removed from the dryer and cooled to a temperature of 25°C or less within 15 seconds. The coated panel was then placed back into the dryer and heated at 110°C for 30 minutes. The reheating step of heating at 110°C for 30 minutes is the annealing step. After annealing, the coated panel was immediately removed from the dryer and cooled to a temperature of 25°C or less within 15 seconds, thereby obtaining each test coated panel.

[0148] Tests were performed on each aqueous coating composition and each of the resulting coated test plates according to the following test methods. The test results are shown in Tables 10 to 15. Additionally, a tinplate coated plate with a release coating was also prepared separately for use as a sample for measuring the storage modulus of the coating film. This tinplate coated plate was obtained in the same manner as the above coated test plate, except that a tinplate was used instead of a #5052 aluminum plate. For measuring the heat of fusion of the coating film, a coated test plate was obtained in the same manner as the above coated test plate using a #5052 aluminum plate, except that the temperature and time in the annealing step were changed to the conditions listed in "Annealing temperature °C" and "Annealing time min" in Table 16.

[0149] [Workability: T-bend workability] A coated test plate was cut into a length of 5 cm in the rolling direction and 4 cm perpendicular to the rolling direction, and then the lower portion was folded in half parallel to the short edge. In a room at 20°C, three 0.26 mm thick aluminum plates were sandwiched between the folded portions of the coated test plate specimen and placed in a special seam-folding DuPont impact tester. A 1 kg iron weight with a flat contact surface was dropped from a height of 50 cm to impact the folded portion of the test plate, and then a voltage of 6.5 V was applied to the folded tip for 6 seconds. The current value (mA) over a 20 mm width of the folded tip was measured and evaluated according to the following criteria: ◎: Less than 10 mA ○: 10 mA or more but less than 40 mA △: 40 mA or more but less than 80 mA ×: 80 mA or more. A rating of ◎ or ○ was considered acceptable.

[0150] [Retort resistance] The test coated plates were immersed in water and treated at 125°C for 30 minutes, after which the whitening of the coating film was visually observed and rated according to the following criteria: ◎: No whitening observed ○: Slight partial whitening observed △: Considerable whitening observed ×: Significant whitening observed Of the above, a rating of ◎ or ○ indicates a pass. Note that the above rating of "○" indicating slight partial whitening means that the area where whitening was observed was approximately 2% or less of the coating film area.

[0151] [Corrosion Resistance] Test coated panels were immersed in a mixed aqueous solution containing 3% each of citric acid and sodium chloride, and stored at 40°C for 2 weeks. The condition of the coated surface was then visually inspected and rated according to the following criteria: ◎: Neither gloss nor corrosion was observed ○: Some gloss was observed, but no corrosion was observed △: Slight corrosion was observed ×: Considerable corrosion was observed A rating of ◎ or ○ above was considered to be acceptable.

[0152] <Paint stability> Each aqueous paint composition was left to stand at room temperature for one week, and the state after standing was visually judged. ◎: No precipitation observed ○: Slight precipitation observed △: A considerable amount of precipitation observed, but dissolved by stirring ×: A considerable amount of precipitation observed, but not dissolved by stirring Of the above, ◎ or ○ is preferable.

[0153]

[0154]

[0155]

[0156]

[0157]

[0158]

[0159] Examples 44 to 52 A coating film was formed using Coating Composition 4 obtained in Example 4. Specifically, a coating film was formed on an aluminum substrate using Coating Composition 4 in the same manner as in the above-described <Preparation of Coated Test Plates>, except that the annealing temperature and time conditions in the above-described <Preparation of Coated Test Plates> were changed to the temperature and time shown in Table 15. The storage modulus of the resulting coating film and samples for measuring the heat of fusion were measured in the same manner as in Example 1, and the aluminum plates on which the coating film had been formed were also subjected to evaluation tests for processability, retort resistance, corrosion resistance, and paint stability in the same manner as in Example 1. The evaluation criteria for each evaluation test were the same as in Example 1.

[0160]

[0161] Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on a Japanese patent application (Patent Application No. 2024-65076) filed on April 15, 2024, the contents of which are incorporated herein by reference.

[0162] The aqueous coating composition according to this embodiment can provide a coating film that is excellent in corrosion resistance, retort resistance, processability, etc., particularly when used for cans. Furthermore, the aqueous coating composition according to this embodiment does not contain raw materials that contain a wide range of legally restricted substances, such as bisphenol A, and also has excellent stability.

Claims

1. An aqueous coating composition comprising an aqueous dispersion of a crystalline acrylic-modified polyester resin (A), wherein the melting point of the crystalline acrylic-modified polyester resin (A) is 125°C or higher.

2. The aqueous coating composition according to claim 1, wherein the heat of fusion of the coating film obtained by heating and crystallizing the composition is 1.0 to 17 J / g.

3. The storage modulus of the coating film obtained by heating and crystallizing it at 50°C is 3.2 x 10 ７ ~2.4 x 10 ８ 3. The aqueous coating composition according to claim 1, wherein the viscosity of the aqueous coating composition is 100 MPa.

4. The aqueous coating composition according to claim 1 or 2, wherein the crystalline acrylic-modified polyester resin (A) has a mass solids ratio of the polyester resin component (a1) to the acrylic resin component (a2), expressed as (a1) / (a2), in the range of 60 / 40 to 90 / 10.

5. The aqueous coating composition according to claim 1 or 2, wherein the crystalline acrylic-modified polyester resin (A) has an acid value of the acrylic resin component (a2) of 180 to 400 mg KOH / g and a glass transition temperature of 20 to 160°C.

6. The aqueous coating composition according to claim 1 or 2, wherein the aqueous dispersion of the crystalline acrylic-modified polyester resin (A) contains an organic solvent (a3), and the content of the organic solvent (a3) ​​per 100 parts by mass of the total solid content of the crystalline acrylic-modified polyester resin (A) is 50 parts by mass or more.

7. The aqueous coating composition according to claim 1 or 2, further comprising a β-hydroxyalkylamide compound (B).

8. The aqueous coating composition according to claim 1 or 2, which is used on cans.

9. A coating method comprising applying the aqueous coating composition according to claim 1 or 2 to an object to be coated.

10. A coating film obtained by applying the aqueous coating composition according to claim 1 or 2 to a substrate and allowing it to crystallize.

11. A coated article comprising a substrate and a coating film formed on the substrate, wherein the coating film is a film formed by applying and crystallizing the aqueous coating composition according to claim 1 or 2.

12. A method for forming a coating film, comprising, in order, applying the aqueous coating composition according to claim 1 or 2 to an object to be coated, volatilizing the water dispersion in said aqueous coating composition, and annealing at 70 to 140°C for 10 to 60 minutes.

13. A coating film obtained by applying the aqueous coating composition according to claim 1 or 2 to an object to be coated, volatilizing the water dispersion in the aqueous coating composition, and annealing at 70 to 140°C for 10 to 60 minutes, in that order.

14. A coated article comprising a substrate and a coating film formed on the substrate, wherein the coating film is obtained by applying the aqueous coating composition according to claim 1 or 2, volatilizing the water dispersion in the aqueous coating composition, and annealing at 70 to 140°C for 10 to 60 minutes, in that order.

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