JP7530152B2 - Coating composition for repairing welded cans - Google Patents

Description

The present invention relates to a coating composition for repairing welded cans.

The welds (on the inside of the can) of three-piece cans and the like have exposed metal, and when painting cans with welds, welded can repair paint is used on the welds.

Initially, epoxy resin-based repair paints were common, but they lacked workability and corrosion resistance, so polyester resin-based repair paints were developed to improve workability.

For example, Patent Document 1 discloses an inner surface correction paint for welded cans, which is characterized by containing (A) a polyester resin having a glass transition point (Tg) in the range of 10 to 50°C and a number average molecular weight (Mn) in the range of 10,000 to 20,000, and (B) a thermosetting resin in a weight ratio of (A):(B) 90:10 to 60:40, and also containing (C) an antifoaming agent, (D) a thixotropic agent, and (E) a solvent.

Patent document 2 discloses a laminated can repair paint for the inner surface of a polyester resin laminated can, which contains a polyester resin with a number average molecular weight of 3,000 to 30,000 and a glass transition temperature of 5 to 110°C.

However, although the paints described in Patent Documents 1 and 2 have good processability, they may have insufficient corrosion resistance and blocking resistance.

JP 2002-129096 A JP 2004-210844 A

The problem that the present invention aims to solve is to provide a coating composition for repairing welded cans that has excellent coating film performance such as workability, corrosion resistance, and water resistance, and also has excellent blocking resistance.

As a result of intensive research into solving the above problems, the inventors discovered that the above problems could be solved by a paint composition for repairing welded cans that contains a polyester resin having a number average molecular weight and a glass transition temperature in a specific range, a melamine resin, a resin having a number average molecular weight in a specific range and an aromatic ring structure, and an acid catalyst, and thus completed the present invention.

That is, the present invention relates to the following:

1. A coating composition for repairing welded cans, comprising a polyester resin (A) having a number average molecular weight in the range of 10,000 to 20,000 and a glass transition temperature in the range of 51 to 100°C, a melamine resin (B), a resin having an aromatic ring structure (C), and an acid catalyst (D).

2. The coating composition for repairing welded cans described in item 1 above, in which the melamine resin (B) is a methyl etherified melamine resin.

3. A coating composition for repairing welded cans according to item 1 or 2 above, in which the acid catalyst (D) is a sulfonic acid compound or an amine neutralized product of a sulfonic acid compound.

4. A coating composition for repairing welded cans according to any one of items 1 to 3 above, which contains 60 to 90% by mass of polyester resin (A), 5 to 40% by mass of melamine resin (B), 3 to 20% by mass of resin (C) having an aromatic ring structure, and 0.1 to 3% by mass of acid catalyst (D), based on the total solid content of polyester resin (A), melamine resin (B), and resin having an aromatic ring structure (C).

5. The coating composition for repairing welded cans according to any one of claims 1 to 4, further comprising a solvent composition (E) consisting of a solvent (E1) having a boiling point of less than 100°C, a solvent (E2) having a boiling point in the range of 100°C to 120°C, and a solvent (E3) having a boiling point of more than 120°C.

The aqueous coating composition of the present invention is able to achieve both processability and water resistance because the polyester resin, which is the base resin, has a relatively high molecular weight and a high Tg. In addition, it is presumed that the corrosion resistance is improved due to the improved adhesion caused by the inclusion of a resin with an aromatic ring. In addition, it is presumed that the curing property is good and the blocking resistance is also improved because it is an acid-catalyzed melamine resin curing type.

As described above, the present invention can provide a coating composition for repairing welded cans that can provide a coating film that has excellent coating film performance such as workability, corrosion resistance, and water resistance, and also has excellent blocking resistance.

The present invention relates to a coating composition for repairing welded cans (hereinafter sometimes referred to as the present coating composition for short) that contains a polyester resin (A) having a number average molecular weight in the range of 10,000 to 20,000 and a glass transition temperature in the range of 51 to 100°C, a melamine resin (B), a resin having an aromatic ring structure (C), and an acid catalyst (D).

The contents of the present invention are explained in detail below.

<Coating composition for repairing welded cans>
Polyester resin (A)
The polyester resin (A) is synthesized by direct esterification or transesterification of an acid component and an alcohol component. In order to obtain sufficient processability and water resistance, it is preferable that the polyester resin has a large molecular weight and a relatively high glass transition temperature (Tg). The number average molecular weight is preferably 10,000 to 20,000, particularly 10,000 to 18,000, and the glass transition temperature (Tg) is preferably within the range of 50 to 100°C, particularly 55 to 95°C.

In this specification, the number average molecular weight is measured by gel permeation chromatography (GPC) using a calibration curve of standard polystyrene according to the molecular weight measurement method described below.

The number average molecular weight is the retention time (retention volume) measured by gel permeation chromatography (HLC8120GPC, manufactured by Tosoh Corporation) using tetrahydrofuran as a solvent, converted based on the number average molecular weight of polystyrene. Four columns were used: TSKgel G-4000HXL, TSKgel G-3000HXL, TSKgel G-2500HXL, and TSKgel G-2000XL (all product names manufactured by Tosoh Corporation). The conditions were: mobile phase: tetrahydrofuran, measurement temperature: 40°C, flow rate: 1 ml/min, detector: RI.

In addition, from the viewpoint of crosslinking properties and water resistance, it is preferable that the hydroxyl value is in the range of 1 to 20, and particularly 3 to 15 mgKOH/g.

Examples of the acid components constituting the polyester resin (A) include one or more dibasic acids selected from phthalic anhydride, isophthalic acid, terephthalic acid, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, hexahydroisophthalic acid, hexahydroterephthalic acid, succinic acid, fumaric acid, adipic acid, sebacic acid, maleic anhydride, naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid, etc., and lower alkyl esters of these acids.

If necessary, monobasic acids such as benzoic acid, crotonic acid, and p-t-butylbenzoic acid, and polybasic acids having a valence of three or more such as trimellitic anhydride, methylcyclohexene tricarboxylic acid, and pyromellitic anhydride can also be used in combination.

These acid components can be used alone or in combination of two or more.

Examples of alcohol components constituting polyester resin (A) include dihydric alcohols such as ethylene glycol, diethylene glycol, propylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, 2-methyl-1,3-propanediol, 3-methylpentanediol, 1,4-hexanediol, 1,6-hexanediol, and 1,4-dimethylolcyclohexane.

If necessary, trihydric or higher polyhydric alcohols such as glycerin, trimethylolethane, trimethylolpropane, and pentaerythritol can also be used in combination.

These alcohol components can be used alone or in combination of two or more.

When synthesizing the above acid component and the above alcohol component by the usual direct esterification method or transesterification method, if the reaction is carried out under conditions where the carboxyl groups are in excess of the hydroxyl groups, a polyester resin mainly containing carboxyl groups is obtained, and if the reaction is carried out under conditions where the hydroxyl groups are in excess of the carboxyl groups, a polyester resin mainly containing hydroxyl groups is obtained. Furthermore, by subjecting this polyester resin mainly containing hydroxyl groups to an addition reaction with an acid anhydride, it is possible to adjust the amount of hydroxyl groups and carboxyl groups in the resin.

Examples of acid anhydrides that can be reacted with this polyester resin that mainly contains hydroxyl groups include phthalic anhydride, succinic anhydride, maleic anhydride, hexahydrophthalic anhydride, and trimellitic anhydride.

In the above-mentioned method, the reaction by the direct esterification method or the transesterification method can be accelerated by pressurizing or depressurizing the reaction, or by introducing an inert gas. Furthermore, an organometallic catalyst such as di-n-butyltin oxide can be used as an esterification catalyst during the reaction.

The polyester resin (A) may be an oil-free polyester resin, an oil-modified alkyd resin, or a modified product of these resins, such as a urethane-modified polyester resin or a urethane-modified alkyd resin.

Melamine resin (B)
The melamine resin is a resin obtained by a condensation reaction between melamine and formaldehyde, and is usually obtained by further etherifying the remaining methylol groups with an alcohol.

Before the above-mentioned etherification reaction, the following functional groups are usually present in the melamine resin: (1) a residual amino group (-NH ₂ ), (2) an iminomethylol group (-NHCH ₂ OH), and (3) a dimethylolamino group [-N(CH ₂ OH) ₂ ].

Examples of melamine resin (B) include partially or fully etherified melamine resins in which some or all of the methylol groups of methylol melamine have been etherified with a monohydric alcohol having 1 to 8 carbon atoms, such as methyl alcohol, ethyl alcohol, n-propyl alcohol, i-propyl alcohol, n-butyl alcohol, i-butyl alcohol, 2-ethylbutanol, or 2-ethylhexanol.

As the melamine resin (B), those in which all the methylol groups are etherified or those in which they are partially etherified with methylol groups and imino groups remaining can be used. Specific examples include alkyl-etherified melamines such as methyl-etherified melamine, ethyl-etherified melamine, and butyl-etherified melamine, which can be used alone or in combination of two or more kinds.

Of the above, methyl etherified melamine resins in which at least a portion of the methylol groups have been methyl etherified are preferably used from the viewpoint of blocking resistance.

Commercially available melamine resins can be used as the melamine resin (B).

Specific examples of commercially available products include Cymel 202, Cymel 232, Cymel 235, Cymel 238, Cymel 254, Cymel 266, Cymel 267, Cymel 272, Cymel 285, Cymel 301, Cymel 303, Cymel 325, Cymel 327, Cymel 350, Cymel 370, Cymel 701, Cymel 703, and Cymel 1141 (all manufactured by Allnex Corporation).

Resin having an aromatic ring structure (C)
The resin (C) having an aromatic ring structure can be used without any particular limitation as long as it has an aromatic ring structure in the resin. Specific examples include bisphenol type epoxy resins, novolac type epoxy resins, xylene resins, benzoguanamine resins, and phenol resins.

From the viewpoint of water resistance and corrosion resistance, the number average molecular weight of the resin (C) having an aromatic ring is preferably in the range of 500 to 5,000, more preferably 600 to 4,500.

By including resin (C) having an aromatic ring structure, water resistance and adhesion to the material can be improved, thereby improving corrosion resistance.

As a bisphenol type epoxy resin, for example, one obtained by reacting a bisphenol compound with an epihalohydrin (e.g., epichlorohydrin, etc.) can be used.

Commercially available bisphenol-type epoxy resins include, for example, "jER827", "jER828", "jER828EL", "jER828XA", "jER834", and "jER1007" (all manufactured by Mitsubishi Chemical Corporation), "EPICLON840", "EPICLON840-S", "EPICLON850", "EPICLON850-S", "EPICLON850-CRP", and "EPICLON850-LC" (all manufactured by DIC Corporation), "EPOTOHTO YD-127", and "EPICLON YD-12 Examples of such epoxy resins include bisphenol A type epoxy resins such as "JER806" and "JER807" (all manufactured by Mitsubishi Chemical Corporation), "EPICLON830", "EPICLON830-S", and "EPICLON835" (all manufactured by DIC Corporation), and "Epotohto YDF-170" (manufactured by Nippon Steel Chemical & Material Co., Ltd.).

Novolac type epoxy resins include phenol novolac type epoxy resins, cresol novolac type epoxy resins, and phenol glyoxal type epoxy resins that have many epoxy groups in the molecule.

Commercially available novolac epoxy resins include phenol novolac types such as "jER152" and "jER154" (both manufactured by Mitsubishi Chemical Corporation), EPPN-201 (manufactured by Nippon Kayaku Co., Ltd.), and Epotohto YDPN-638 (manufactured by Nippon Steel Chemical & Material Co., Ltd.), and cresol novolac types such as "jER180S65" and "jER180H65" (both manufactured by Mitsubishi Chemical Corporation), and EOCN-1 02S, 103S, 104S (all manufactured by Nippon Kayaku Co., Ltd.), Epotohto YDCN-701, 702, 703, 704 (all manufactured by Nippon Steel Chemical & Material Co., Ltd.), and other novolac type epoxy resins such as Epotohto ZX-1071T, ZX-1015, ZX-1247, YDG-414S (all manufactured by Nippon Steel Chemical & Material Co., Ltd.).

Examples of xylene resins include the reaction product of xylene and formaldehyde; the reaction product of mesitylene and formaldehyde; the reaction product of xylene, phenol and formaldehyde; etc. If necessary, resins modified with phenols such as phenol or bifunctional phenols such as para-t-butylphenol can also be used.

The xylene resin may be any commercially available product. Specific examples include "Nicanol LLL", "Nicanol LL", "Nicanol L", "Nicanol H", "Nicanol G", "Nicanol Y-50", and "Nicanol Y-100" (all trade names, manufactured by Fudow Co., Ltd.).

Specific examples of benzoguanamine resins include methyl-etherified benzoguanamine resins such as Mycoat 102, 105, and 106 (all manufactured by Allnex Corporation) and BL-60 (manufactured by Sanwa Chemical Co., Ltd.); mixed etherified benzoguanamine resins of methyl ether and ethyl ether such as Cymel 1123 (manufactured by Allnex Corporation); mixed etherified benzoguanamine resins of methyl ether and butyl ether such as Mycoat 136 (manufactured by Allnex Corporation) and BX-4000 (manufactured by Sanwa Chemical Co., Ltd.); and butyl-etherified benzoguanamine resins such as Mycoat 1128 (manufactured by Allnex Corporation).

Examples of phenolic resins include resol-type phenolic resins that are produced by subjecting phenols such as phenolic compounds or bisphenol A to a condensation reaction with aldehydes such as formaldehyde in the presence of a reaction catalyst to introduce methylol groups.

The above phenols may be used alone or in combination of two or more of the following: bifunctional phenolic compounds such as o-cresol, p-cresol, p-tert-butylphenol, p-ethylphenol, 2,3-xylenol, and 2,5-xylenol; trifunctional phenolic compounds such as carbolic acid, m-cresol, m-ethylphenol, 3,5-xylenol, and m-methoxyphenol; and tetrafunctional phenolic compounds such as bisphenol A and bisphenol F.

Examples of the aldehydes include formaldehyde, paraformaldehyde, and trioxane.

The alcohol used to alkyl-etherify a portion of the methylol groups of the methylolated phenolic resin is preferably a monohydric alcohol having 1 to 6 carbon atoms, and preferably 1 to 4 carbon atoms. Suitable monohydric alcohols include methanol, ethanol, n-propanol, n-butanol, and isobutanol.
The resin (C) having an aromatic ring structure may be used alone or in combination of two or more kinds.

Acid Catalyst (D)
The coating composition contains an acid catalyst (D) to accelerate the curing reaction.

Examples of the acid catalyst (D) include acid catalysts such as carboxyl group-containing compounds, phosphoric acid compounds, paratoluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenesulfonic acid, dinonylnaphthalenedisulfonic acid, and cumenesulfonic acid, or amine neutralization products of these acids.

Of the above, from the viewpoint of crosslinking properties, the above sulfonic acid compounds or amine neutralized products of sulfonic acid compounds can be preferably used.

The solid content composition ratio of the polyester resin (A), melamine resin (B), resin having an aromatic ring structure (C) and acid catalyst (D) of the present coating composition is preferably within the range of 60 to 90 mass %, particularly 65 to 85 mass %, of polyester resin (A), 5 to 40 mass %, particularly 10 to 30 mass %, of resin having an aromatic ring structure (C), 3 to 20 mass %, particularly 5 to 15 mass %, of acid catalyst (D) (for example, in the case of an amine neutralization product of a sulfonic acid compound, the sulfonic acid compound remaining after removing the amine from this neutralization product) of 0.1 to 3 mass %, particularly 0.3 to 2 mass %, based on the total solid content of the polyester resin (A), melamine resin (B) and resin having an aromatic ring structure (C) from the viewpoints of processability, water resistance, corrosion resistance and blocking resistance.

This coating composition contains polyester resin (A), melamine resin (B), resin with an aromatic ring structure (C) and acid catalyst (D), but from the viewpoint of paintability, etc., a solvent is usually blended. Furthermore, if necessary, other resins can be blended for the purpose of modifying the coating film, etc., and further, it can contain pigments such as color pigments and extender pigments; paint additives such as anti-aggregation agents, leveling agents, defoamers, waxes, etc.

The above-mentioned solvent can be one that can dissolve or disperse each of the above-mentioned components (A), (B), (C), and (D) and other components such as other resins used as necessary.

Specific examples of such solvents include hydrocarbon solvents such as toluene, xylene, and high-boiling petroleum hydrocarbons; ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, and isophorone; ester solvents such as ethyl acetate, butyl acetate, ethylene glycol monoethyl ether acetate, and diethylene glycol monoethyl ether acetate; alcohol solvents such as methanol, ethanol, and butanol; and alcohol ether solvents such as ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, and diethylene glycol monobutyl ether. These can be used alone or in combination of two or more.

It is preferable to use two or more of the above solvents in combination, and from the viewpoint of improving blocking resistance, it is preferable to contain a solvent composition (E) that satisfies the following requirements in addition to the essential components (A), (B), (C), and (D).

The solvent composition (E) is composed of a solvent (E1) with a boiling point below 100°C, a solvent (E2) with a boiling point in the range of 100°C to 120°C, and a solvent (E3) with a boiling point above 120°C, and satisfies the requirements of 30 to 60 mass% of solvent (E1), 25 to 70 mass% of solvent (E2), and 15 mass% or less of solvent (E3) based on the total amount of solvents (E1), (E2), and (E3).

When the solvent composition (E) is contained, its content is preferably within the range of 90 to 70% by mass, particularly 85 to 75% by mass, based on the total solid content of the polyester resin (A), the melamine resin (B), and the resin having an aromatic ring structure (C), from the viewpoint of improving blocking resistance and ease of coating work.

The coating composition for repairing welded cans of the present invention can be used for repair coating of any known welded can seams. The seams may be formed by either lap welding or butt welding.

The coating composition for repairing welded cans of the present invention can also be applied to welds obtained by overlap laser welding or butt laser welding. In particular, the thickness of the weld obtained by butt laser welding is almost the same as the thickness of the material, and there is no welding step as in overlap welding, making it easy to cover with a repair coating.

As the metal plate constituting the welded can, various surface-treated steel plates are used. Surface-treated steel plates that can be used are cold-rolled steel plates that have been annealed, then secondary cold-rolled, and then subjected to one or more of the following surface treatments: zinc plating, tin plating, nickel plating, nickel-tin plating, electrolytic chromate treatment, chromate treatment, etc. Aluminum-coated steel plates that have been aluminum-plated, aluminum-pressed, etc. can also be used. The thickness of the metal plate varies depending on the type of metal, the use or size of the container, but metal plates having a thickness of 0.10 to 0.50 mm, and particularly 0.10 to 0.30 mm, can be preferably used.

The metal plate used in can manufacturing by welding is preferably laminated with a resin film or painted with paint, except for the edge portion to be welded. Any conventional paint used for metal coating can be used as the paint coated on the metal plate. The thickness of the resin film laminate or the painted coating is preferably in the range of generally 3 to 50 μm, and particularly 5 to 40 μm.

The coating composition for repairing welded cans of the present invention can be applied to the welded seams of a welded can by any means, but is generally preferably applied by spray coating. In the case of welded cans using a resin film laminate plate or a metal plate coated with paint, it is preferable to apply the repair coating so that it straddles the resin laminate film layer or the coated coating layer on both sides of the seam.

The coating thickness of the coating composition for repairing welded cans of the present invention is preferably within the range of 3 to 15 μm, and more preferably 4 to 13 μm.

The heat curing of the coating composition for repairing welded cans of the present invention is preferably carried out by heating at a temperature of 170 to 220°C, particularly 180 to 210°C, for a holding time of 10 to 60 seconds, particularly 15 to 40 seconds.

The present invention will be explained in more detail below with reference to the following examples. In this specification, "parts" and "%" mean "parts by mass" and "% by mass", respectively. Note that the "parts by mass" of raw materials in the following manufacturing examples, examples, and comparative examples refer to the parts by mass of the solid content (or active ingredients) of the raw materials.

<Production of Coating Composition>
Polyester resin (A), melamine resin (B), resin having an aromatic ring structure (C) and acid catalyst (D) were adjusted to have the solid content mass ratio and solvent composition shown in Table 1 to obtain coating composition Nos. 1 to 15 with a mass solid content concentration of 20%. The solvent for Cymel 327 (B1) and Cymel 370 (B2) was isobutanol (boiling point 108°C), and the amount carried over from this was subtracted from toluene to adjust the mass solid content concentration. Coating composition Nos. 12 to 15 are for comparative examples.

In addition, for comparative purposes, an epoxy resin-based paint with the following composition was also prepared as paint composition No. 16.

Epoxy resin-based paint: jER1009 (bisphenol A type epoxy resin, manufactured by Mitsubishi Chemical Corporation) 85 parts/Amidea P-138 (butylated urea resin, manufactured by DIC Corporation) 15 parts/dodecylbenzenesulfonic acid 1 part. The solvent composition was toluene/butyl acetate/methyl ethyl ketone = 1/1/1 (mass ratio), and the mass solids concentration was adjusted to 20%.

Details of each ingredient in the table are as follows:

Polyester resin (A)
Polyester resin (A1): UE9200 (number average molecular weight 15,000, glass transition temperature 65° C., hydroxyl value 6 mgKOH/g, manufactured by Unitika Ltd.)
Polyester resin (A2): Vylon 200 (number average molecular weight 17,000, glass transition temperature 67° C., hydroxyl value 6 mg KOH/g, manufactured by Toyobo Co., Ltd.)
Polyester resin (A3): UE3360 (number average molecular weight 5000, glass transition temperature 55° C., hydroxyl value 25 mgKOH/g, manufactured by Unitika Ltd.)
Polyester resin (A4): Vylon 600 (number average molecular weight 16,000, glass transition temperature 47° C., hydroxyl value 7 mgKOH/g, manufactured by Toyobo Co., Ltd.)
Melamine resin (B)
Melamine resin (B1): Cymel 327 (imino group-containing methyl etherified melamine resin, manufactured by Allnex Co., Ltd.)
Melamine resin (B2): Cymel 370 (methylol group-containing methyl etherified melamine resin, manufactured by Allnex Co., Ltd.)
Melamine resin (B3): Cymel 303 (full methyl etherified melamine resin, manufactured by Allnex Co., Ltd.)
Resin having an aromatic ring structure (C)
Resin having an aromatic ring structure (C1): jER1007 (bisphenol A type epoxy resin, number average molecular weight 2900, manufactured by Mitsubishi Chemical Corporation)
Resin having an aromatic ring structure (C2): jER154 (phenol novolac type epoxy resin, manufactured by Mitsubishi Chemical Corporation)
Resin having an aromatic ring structure (C3): Nikanol LL (xylene formaldehyde resin, manufactured by Fudow Co., Ltd.)
Resin having an aromatic ring structure (C4): BX-4000 (benzoguanamine resin, manufactured by Sanwa Chemical Co., Ltd.)
Acid Catalyst (D)
Acid catalyst (D1): Dodecylbenzenesulfonic acid Acid catalyst (D2): Paratoluenesulfonic acid Acid catalyst (D3): Phosphoric acid Solvent composition Solvent composition a: 50 parts methyl ethyl ketone / 40 parts toluene / Swazol 1000 (*) 5 parts / 5 parts butyl acetate Solvent composition b: 20 parts methyl ethyl ketone / 70 parts toluene / Swazol 1000 5 parts / 5 parts butyl acetate Solvent composition c: 20 parts methyl ethyl ketone / 60 parts toluene / Swazol 1000 10 parts / 10 parts butyl acetate Swazol 1000 (*): Aromatic hydrocarbon solvent, boiling point 160-170°C, manufactured by Maruzen Petrochemical Co., Ltd. <Preparation of test panels>
Each of the coating compositions prepared above was spray-coated onto a cold-rolled steel plate having a thickness of 0.30 mm to a film thickness of 10 μm, and then dried at a plate temperature of 200° C. for 30 seconds to obtain each test plate.

Each test panel obtained above was tested according to the following test method. The results are also shown in Table 1.

Workability The test plate was folded in half while being sandwiched between two cold-rolled steel plates having a thickness of 0.30 mm as a spacer, and the bent portion was observed under an optical microscope.
◯: No cracks or peeling in the coating film △: Microcracks in the coating film ×: Cracks in the coating film and partial peeling Water resistance The test plate was immersed in water and treated at 125°C for 30 minutes, after which the whitening state of the coating film was visually observed and evaluated according to the following criteria.
◯: No whitening observed △: Partial whitening observed ×: Considerable whitening observed Corrosion resistance The unpainted surface of a coated test plate was covered with tape and then bent in the same manner as in the workability test. The plate was then immersed in a mixed aqueous solution containing 3% citric acid and 3% sodium chloride and left at 40°C for 2 weeks. The condition of the coated surface was then visually observed and evaluated according to the following criteria.
◯: No corrosion observed △: Slight corrosion observed ×: Considerable corrosion observed Blocking resistance Each of the coating compositions prepared above was spray-coated onto two cold-rolled steel plates with a thickness of 0.30 mm to a film thickness of 10 μm, and the plates were dried for 30 seconds in a box oven set at 80° C., after which the coated surfaces of the two plates were placed together and left to stand for 1 minute under a load of 200 g/dm2.
◎: The two test plates peel off easily, and no traces are visible. ◯: The two test plates peel off easily, but slight traces are visible. △: There is resistance to peeling off the two test plates, and traces are clearly visible. ×: There is strong resistance to peeling off the two test plates, and part of the coating peels off.

It is possible to provide a coating composition for repairing welded cans that has excellent coating film performance such as workability, corrosion resistance, and water resistance, and also has excellent blocking resistance.

Claims (3)

The composition comprises a polyester resin (A) having a number average molecular weight in the range of 10,000 to 20,000 and a glass transition temperature in the range of 51 to 100° C., a melamine resin (B), a resin (C) having at least one aromatic ring structure selected from a bisphenol-type epoxy resin, a novolac-type epoxy resin, a xylene resin, a benzoguanamine resin, and a phenol resin, and an acid catalyst (D) ,
Further, the solvent composition (E) includes a solvent (E1) having a boiling point of less than 100° C., a solvent (E2) having a boiling point in the range of 100° C. to 120° C., and a solvent (E3) having a boiling point of more than 120° C.;
based on the total amount of solids of the polyester resin (A), the melamine resin (B), and the resin having an aromatic ring structure (C), the polyester resin (A) is 60 to 90 mass%, the melamine resin (B) is 5 to 40 mass%, the resin having an aromatic ring structure (C) is 3 to 20 mass%, and the acid catalyst (D) is 0.1 to 3 mass%,
The solvent composition (E) is a solvent composition that satisfies the requirements of 30 to 60 mass % of solvent (E1), 25 to 70 mass % of solvent (E2), and 15 mass % or less of solvent (E3) based on the total amount of the solvents (E1), (E2), and (E3). The coating composition for repairing welded cans according to claim 1, wherein the melamine resin (B) is a methyl etherified melamine resin. The coating composition for repairing welded cans according to claim 1 or 2, wherein the acid catalyst (D) is a sulfonic acid compound or an amine neutralized product of a sulfonic acid compound.