JP7172125B2 - Method for producing coated metal substrate, coated metal plate and coated metal can - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a coated metal substrate, and more particularly, to a method for producing a metal substrate on which a coating film having excellent workability, curability and corrosion resistance is formed, and excellent workability. The present invention relates to a coated metal plate and a coated metal container having a coating film with toughness, curability and corrosion resistance.

The coating composition used for metal containers such as beverage cans or metal lids should be able to withstand severe processing (e.g., neck-in processing, bead processing, score processing, riveting) during molding of metal containers or metal lids. Workability is required. In addition, depending on the application, it must have good curability, have corrosion resistance that can prevent corrosion of the metal substrate due to the contents, etc., do not impair the flavor of the contents, and paint ingredients. Various properties are required, such as excellent sanitation without the elution of ions, and excellent retort resistance. Furthermore, it is also necessary to consider the environmental pollution and the working environment caused by volatilization of the organic solvent during painting and baking.

Epoxy-based paints such as epoxy-phenol-based paints, epoxy-amino-based paints, and epoxy-acrylic-based paints have been widely used as coating compositions for metal containers or metal lids. Since many of them are manufactured as raw materials, paints that do not contain bisphenol A are desired.

From this point of view, as a coating composition for metal containers or metal lids, there has been proposed a polyester-based water-based coating composition that does not use bisphenol A and also takes into consideration the effects on environmental pollution and working environment. .
Such a polyester-based water-based coating composition, for example, is mainly composed of an aromatic polyester resin having a carboxyl group, and has an acid value (AV) of 10 to 30 mgKOH / g and a number average of 3000 to 10000 molecular weight (Mn) and is combined with a curing agent, a neutralizing agent for the polyester resin, and a co-solvent, and has excellent curability and retort resistance. It has been proposed (Patent Document 1).

Further, as a water-based coating composition, for example, a polyester resin (A) having a number average molecular weight of 2,000 to 50,000 having an ethylenic double bond at the resin end, a polymerizable polymerizable monomer containing a carboxyl group-containing polymerizable unsaturated monomer An aqueous coating composition is proposed in which an acrylic-modified polyester resin (C) obtained by graft polymerization of an unsaturated monomer component (B) and a β-hydroxyalkylamide cross-linking agent (D) are stably dispersed in an aqueous medium. (Patent Document 2).

Patent No. 4228585 JP-A-2003-26992

However, when a resol-type phenolic resin that forms a self-condensate is used as a curing agent as in Patent Document 1, a hard and brittle domain derived from the self-condensate of the phenolic resin is formed in the coating film. Therefore, the processability of the coating film was not sufficiently satisfactory. As in Patent Document 2, when a polyester resin with a high acid value is used, the number of reaction points (crosslinking points) with the curing agent increases, resulting in excellent curability, but the crosslink density becomes too high. At the time of molding, the coating film was easily cracked and could not withstand severe processing, and sufficient workability could not be obtained.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a method for producing a coated metal substrate having a coating with excellent workability, curability and corrosion resistance using a specific polyester-based water-based coating composition.
Another object of the present invention is to provide a coated metal sheet and coated metal container having a coating with excellent workability and corrosion resistance.

According to the present invention, a polyester resin having an acid value of 17 mgKOH/g or more and less than 30 mgKOH/g and a curing agent having a functional group capable of cross- linking with a carboxyl group of the polyester resin are contained, and the functional group is is a β-hydroxyalkylamide group, and a water-based coating composition containing 2 to 8 parts by mass of the curing agent (solid content) with respect to 100 mass of the polyester resin (solid content) . A coated metal characterized by comprising a coating step of applying onto a substrate, and a baking step of heating the water-based coating composition applied onto the metal substrate in the coating step at a temperature higher than 200°C and not higher than 320°C. A method of manufacturing a substrate is provided.
In the method for producing a coated metal substrate of the present invention,
1. the functional group equivalent weight of the curing agent is 30 to 500 g/eq;
2. that the molecular weight of the curing agent is 1000 or less;
3 . the curing agent is a β-hydroxyalkylamide compound;
4 . wherein the metal substrate is a metal plate or a metal can;
is preferred.

According to the present invention, a polyester resin having an acid value of 17 mgKOH/g or more and less than 30 mgKOH/g and a β-hydroxyalkylamide compound as a curing agent for the polyester resin are added to 100 mass of the polyester resin (solid content). and a coating film containing 2 to 8 parts by mass of the curing agent (solid content) and having a thickness of less than 20 μm on at least one surface.
According to the present invention, a polyester resin having an acid value of 17 mgKOH/g or more and less than 30 mgKOH/g and a β-hydroxyalkylamide compound as a curing agent for the polyester resin are added to 100 mass of the polyester resin (solid content). On the other hand, there is provided a coated metal can characterized by having, on at least one side thereof, a coating film containing 2 to 8 parts by mass of the curing agent (solid content) and having a film thickness of less than 20 μm.

The present inventors have made intensive studies on improving the workability of a coating film formed from a polyester-based water-based coating composition. By using a water-based coating composition consisting of a combination, coating it on a metal substrate, and heating it at a temperature higher than 200°C and not higher than 320°C, it is possible to sufficiently cure it in a short time and achieve an appropriate crosslink density. It has become possible to form a coating film capable of realizing excellent workability and corrosion resistance.

(Aqueous paint composition)
An important feature of the water-based coating composition used in the method for producing a coated metal substrate of the present invention is that it uses a polyester resin having a specific acid value and a specific curing agent, which are contained in the aqueous medium. It is a coating composition formed by

(polyester resin)
In the coating composition of the present invention, it is important that the polyester resin, which is the main ingredient, has an acid value of 5 mgKOH/g or more and less than 30 mgKOH/g, particularly in the range of 10 to 29 mgKOH/g.
That is, in the present invention, it is important for the polyester resin that is the main ingredient to have an appropriate amount of carboxyl groups (acid value) in order to achieve curability, workability, and adhesion of the coating film. If the acid value is smaller than the above range, the number of carboxyl groups serving as cross-linking points with the curing agent is small, and sufficient curability cannot be obtained. Since the number of groups is small, the adhesion of the coating film becomes poor. On the other hand, if the acid value is larger than the above range, the number of cross-linking points with the curing agent increases, resulting in excellent curability, but the cross-linking density tends to be excessively high, the coating film becomes hard, and workability is poor. become. Even if the acid value is higher than the above range, it is possible to keep the crosslink density low by adjusting the amount of the curing agent to be reduced. Since the carboxyl group remains in the coating film, the coating film becomes inferior in water resistance, and as a result, sufficient corrosion resistance cannot be obtained.

In addition, in the present invention, two or more polyester resins having different acid values may be blended and used, and in that case, the value obtained by multiplying the acid value of each polyester resin and the blending ratio (mass ratio) is defined as the average acid value of the blend, and the average acid value may be in the range of 5 mgKOH/g or more and less than 30 mgKOH/g. (Even if a polyester resin having a single acid value outside the above range is used, the average acid value of a blend with another polyester resin having a different acid value may be within the above range.)

The polyester resin used in the present invention may be a known water-dispersible polyester resin and/or water-soluble polyester resin used in coating compositions, except that it has the acid value described above.
The water-dispersible polyester resin and the water-soluble polyester resin are polyester resins containing hydrophilic groups as components, and these components may be physically adsorbed on the surface of the polyester dispersion, but are copolymerized in the polyester resin skeleton. It is particularly preferred that
Examples of hydrophilic groups include hydroxyl groups, amino groups, carboxyl groups, sulfonic acid groups, derivatives thereof, metal salts, ethers, and the like. By including these in the molecule, they can exist in a water-dispersible state. .
Specific examples of components containing hydrophilic groups include carboxylic anhydrides such as phthalic anhydride, succinic anhydride, maleic anhydride, trimellitic anhydride, itaconic anhydride, and citraconic anhydride, polyethylene glycol, polypropylene glycol, hydroxyl group-containing polyether monomers such as glycerin and polyglycerin, metal salts of sulfonic acid-containing monomers such as 5-sulfoisophthalic acid, 4-sulfonaphthalene-2,7-dicarboxylic acid and 5(4-sulfophenoxy)isophthalic acid, or An ammonium salt etc. can be mentioned.
It may also be an acrylic resin-modified polyester resin obtained by graft-polymerizing a vinyl-based monomer having a hydrophilic group to a polyester resin, and the vinyl-based monomer having a hydrophilic group includes a carboxyl group, a hydroxyl group, a sulfonic acid group, an amide group, and the like. Examples of groups that can be converted to hydrophilic groups include those containing an acid anhydride group, a glycidyl group, a chloro group, and the like. However, a polyester resin modified with an acrylic resin may increase the number of manufacturing steps due to its modification, and the manufacturing cost may be high. Resin is preferred.
In the present invention, a carboxyl group-containing water-dispersible polyester resin having a carboxyl group as a hydrophilic group and/or a carboxyl group-containing water-soluble polyester resin can be suitably used as the polyester resin.

Moreover, the monomer component for forming the polyester resin in combination with the monomer containing the hydrophilic group is not particularly limited as long as it is a monomer commonly used for polymerization of the polyester resin.
Examples of the polyvalent carboxylic acid component constituting the polyester resin include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid and naphthalenedicarboxylic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, and dodecanedione. acids, aliphatic dicarboxylic acids such as dimer acid, (anhydrous) maleic acid, fumaric acid, unsaturated dicarboxylic acids such as terpene-maleic acid adducts, 1,4-cyclohexanedicarboxylic acid, tetrahydrophthalic acid, hexahydroisophthalic acid, Alicyclic dicarboxylic acids such as 1,2-cyclohexenedicarboxylic acid, (anhydrous) trimellitic acid, (anhydrous) pyromellitic acid, trivalent or higher polyvalent carboxylic acids such as methylcyclohexene tricarboxylic acid, and the like. One or more of these can be selected and used.
In the present invention, from the viewpoint of corrosion resistance, retort resistance, flavor properties, etc., the ratio of aromatic dicarboxylic acid (terephthalic acid, isophthalic acid, naphthalenedicarboxylic acid, etc.) to the polyvalent carboxylic acid component constituting the polyester resin is 60 mol. % or more, and particularly preferably 80% or more.

The polyhydric alcohol component constituting the polyester resin is not particularly limited, and may be ethylene glycol, propylene glycol (1,2-propanediol), 1,3-propanediol, 1,4-butanediol, 1,2-butane. diol, 1,3-butanediol, 2-methyl-1,3-propanediol, neopentyl glycol, 1,5-pentanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, 2 -ethyl-2-butyl-1,3-propanediol, 2,4-diethyl-1,5-pentanediol, 1-methyl-1,8-octanediol, 3-methyl-1,6-hexanediol, 4 -aliphatic glycols such as methyl-1,7-heptanediol, 4-methyl-1,8-octanediol, 4-propyl-1,8-octanediol, 1,9-nonanediol, diethylene glycol, triethylene glycol , polyethylene glycol, polypropylene glycol, ether glycols such as polytetramethylene glycol, 1,4-cyclohexanedimethanol, 1,3-cyclohexanedimethanol, 1,2-cyclohexanedimethanol, tricyclodecane glycols, hydrogenated bisphenol , trihydric or higher polyalcohols such as trimethylolpropane, trimethylolethane, pentaerythritol, and the like, or a combination of two or more thereof.
In the present invention, among the above polyhydric alcohol components, ethylene glycol, propylene glycol, 1,4-butanediol, and 1,4-cyclohexanedimethanol are suitable as components constituting the polyester resin from the viewpoint of hygiene and the like. can be used for

The polyester resin can be obtained by polycondensing one or more of the above polycarboxylic acid components and one or more of the polyhydric alcohol components, or polycondensing polyhydric carboxylic acid components such as terephthalic acid, isophthalic acid, and trianhydride after polycondensation. A method of depolymerizing with mellitic acid, trimellitic acid, pyromellitic acid, etc., and after polycondensation, acid anhydrides such as phthalic anhydride, maleic anhydride, trimellitic anhydride, ethylene glycol bis trimellitate dianhydride, etc. It can be produced by a known method such as ring-opening addition.

The glass transition temperature (Tg) of these polyester resins is preferably in the range of -30°C to 120°C, particularly 15°C to 100°C, although not limited thereto. If the Tg is higher than the above range, the formed coating film will be hard and may be poor in workability. On the other hand, if the Tg is lower than the above range, the barrier properties of the coating film may deteriorate, resulting in poor corrosion resistance and retort resistance.

Moreover, in the present invention, it is preferable to use a blend of two or more polyester resins having different Tg. By blending polyester resins with different Tg, it may be possible to form a coating film that has excellent impact resistance and is less likely to cause coating film defects even when subjected to external impact, compared to the case where only one type of polyester resin is used. .
Even in that case, Tg of the polyester resin blend calculated by the following formula (1)
_mix should be within the above Tg range.
1/Tg _mix =(W1/Tg1)+(W2/Tg2)+...+(Wm/Tgm)
... (1)
W1+W2+...+Wm=1
In the formula, Tg _mix represents the glass transition temperature (K) of the polyester resin blend, and Tg1, Tg2, . Represents temperature (K). W1, W2, . . . , Wm represent the weight fraction of each polyester resin (polyester resin 1, polyester resin 2, .

In the aqueous coating composition of the present invention, in particular, a mixture of a polyester resin (A) having a Tg of 35° C. to 100° C. and a polyester resin (B) having a Tg of −30° C. to 25° C. is used as the polyester resin. is particularly preferred from the viewpoint of the impact resistance of the coating film. In that case, it is preferable that the mass ratio of (A):(B) is 98:2 to 10:90, particularly 95:5 to 30:70. Moreover, it is preferable that the glass transition temperature (Tg _mix ) calculated by the above formula (1) is 30° C. or higher from the viewpoint of corrosion resistance and retort resistance.

Although the number average molecular weight (Mn) of the polyester resin is not limited to this, it is preferably in the range of 1,000 to 100,000, particularly 3,000 to 50,000. If it is smaller than the above range, the coating film may become brittle and may be inferior in workability, and if it is larger than the above range, the coating stability may decrease.

The average dispersed particle size of the polyester resin in the water-dispersible polyester resin is preferably in the range of 10 to 1,000 nm, particularly 20 to 500 nm.

Although the hydroxyl value of the polyester resin is not limited to this, it is preferably 20 mgKOH/g or less, more preferably 10 mgKOH/g or less. As the curing agent, when using a compound that reacts with the carboxyl group of the polyester resin but is unlikely to react with, or does not react with, the hydroxyl group, such as a β-hydroxyalkylamide compound, the hydroxyl group of the polyester resin is large. A portion will remain in the coating film unreacted. Therefore, when the hydroxyl value is larger than the above range, the number of remaining hydroxyl groups increases, and there is a risk that the corrosion resistance will decrease.

(curing agent)
An important feature of the present invention is the use of a specific curing agent having a functional group capable of cross-linking with the carboxyl group of the main polyester resin.
The functional group equivalent weight of the curing agent used in the present invention is preferably 30 to 500 g/eq, more preferably 40 to 200 g/eq. The functional group equivalent in the present invention is a value obtained by dividing the molecular weight by the number of functional groups per molecule of the curing agent (the functional group referred to here refers to a functional group capable of cross-linking with the carboxyl group of the main polyester resin). , means the molecular weight per functional group of the curing agent, and is expressed, for example, as an epoxy equivalent. If the functional group equivalent weight is less than the above range, the distance between cross-linking points cannot be increased, resulting in decreased flexibility of the coating film and poor workability. On the other hand, if it is larger than the above range, the curability will be insufficient, resulting in poor workability and retort resistance.
Also, the average molecular weight of the curing agent is preferably 1,000 or less. If it is larger than the above range, the compatibility with the polyester resin of the main agent may decrease, and the reactivity may decrease.
Furthermore, the average number of functional groups per molecule of the curing agent is preferably 3 or more for obtaining good curability.

Moreover, as the curing agent, a curing agent that is difficult to undergo a self-condensation reaction, particularly one that does not undergo a self-condensation reaction is preferable. Generally, resol-type phenol resins, amino resins, and the like, which are used as curing agents for polyester-based coating compositions, tend to cause self-condensation reactions between curing agents, and self-condensation products, which are hard and brittle domains, are formed during coating film formation. As a result, the coating film may become hard and workability may be reduced. In addition, since the reaction points (functional groups) of the curing agent are consumed in the self-condensation reaction, the amount of the curing agent required to obtain sufficient curability naturally increases, resulting in inefficiency. The presence of large amounts of hardener in the coating can adversely affect coating properties such as processability and corrosion resistance. On the other hand, when a curing agent that is difficult to undergo self-condensation reaction or does not undergo self-condensation reaction is used, the formation of a hard and brittle self-condensate can be suppressed, and the minimum amount corresponding to the amount of carboxyl groups in the polyester resin is used. It is efficient because it is only required to be blended, and the amount of curing agent in the coating film can be reduced. As a result, a coating film having excellent workability and corrosion resistance can be formed.
Therefore, as the curing agent, a curing agent having a functional group that hardly induces a self-condensation reaction between curing agents is preferable for the reason described above as a functional group having reactivity with the carboxyl group of the polyester resin. Examples of the group include epoxy group, carbodiimide group, oxazoline group, amino group, and hydroxyl group (however, hydroxyl group derived from self-condensing methylol group contained in resol-type phenolic resin, amino resin, etc., self contained in silane coupling agent, etc.). excluding hydroxyl groups derived from condensable silanol groups), β-hydroxyalkylamide groups, and the like. Among them, curing agents having an epoxy group, an oxazoline group, or a β-hydroxyalkylamide group are preferred, and curing agents having a β-hydroxyalkylamide group are particularly preferred.

(β-hydroxyalkylamide group-containing curing agent)
The curing agent having a β-hydroxyalkylamide group includes β-hydroxyalkylamide compounds such as those represented by the following general formula [I].
general formula [I];
[HO-CH(R ₁ )-CH ₂ -N(R ₂ )-CO-] _m -A-[-CO-N(R ₂ ')
—CH ₂ —CH(R ₁ ′)—OH] _n
[In the formula, R ₁ and R ₁ ' are hydrogen atoms or alkyl groups having 1 to 5 carbon atoms; R ₂ and R ₂ ' are hydrogen atoms or alkyl groups having 1 to 5 carbon atoms; As shown, A represents a polyvalent organic group, m is 1 or 2, and n is 0 to 2 (the sum of m and n being at least 2). ]

general formula [II]; HO—CH(R ₃ )—CH ₂ —
[In the formula, R ₃ represents a hydrogen atom or an alkyl group having 1 to 5 carbon atoms. ]

A in the general formula [I] is preferably an aliphatic, alicyclic or aromatic hydrocarbon, more preferably an aliphatic, alicyclic or aromatic hydrocarbon having 2 to 20 carbon atoms, Aliphatic hydrocarbons of numbers 4 to 10 are more preferred.
Also, the sum of m and n in the general formula [I] is preferably 2, 3, or 4.
Among those represented by the above general formula [I], β-hydroxyalkylamide group-containing curing agents (β-hydroxyalkylamide compounds) used as curing agents include N,N,N′,N′-tetrakis (2 -hydroxyethyl) adipamide [CAS: 6334-25-4, molecular weight: about 320, functional group equivalent: about 80 g/eq, number of functional groups per molecule: 4, product example: Primid XL552 manufactured by EMS] and N, N ,N',N'-tetrakis(2-hydroxypropyl)adipamide [CAS: 57843-53-5, molecular weight: about 376, functional group equivalent: about 95 g/eq, number of functional groups per molecule: 4, product example: Primid QM1260 manufactured by EMS Co., Ltd.] is preferred. Among these, it is more preferable to use N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide from the viewpoint of curability and retort resistance. Compared to N,N,N',N'-tetrakis(2-hydroxyethyl)adipamide, N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide is more reactive with polyester resins. By forming a more dense crosslinked structure, the coating film is less likely to whiten even during retort treatment, and a coating film with excellent retort resistance can be formed.

[Epoxy group-containing curing agent]
Examples of the curing agent having an epoxy group include water-soluble polyepoxy compounds such as polyethylene glycol diglycidyl ether, sorbitol tetraglycidyl ether, trimethylolpropane diglycidyl ether, trimethylolpropane triglycidyl ether, and polyglycerol polyglycidyl ether. Preferably, specifically, Nagase ChemteX Co., Ltd. Denacol EX-314 [molecular weight: about 320, functional group (epoxy) equivalent: 144 g / eq], EX-421 [molecular weight: about 440, functional group (epoxy) equivalent : about 159 g/eq], EX-611 [molecular weight: about 630, functional group (epoxy) equivalent: 167 g/eq].

[Oxazoline group-containing curing agent]
The curing agent having an oxazoline group includes, for example, a water-soluble polymer obtained by polymerizing a monomer composition containing an oxazoline derivative. Such oxazoline derivatives include, for example, 2-vinyl-2-oxazoline, 2-vinyl -4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5- methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline and the like. Further, the monomer other than the oxazoline derivative contained in the monomer composition containing the oxazoline derivative is not particularly limited as long as it is a compound that copolymerizes with the oxazoline derivative and is inert to the oxazoline group. . The ratio of structural units derived from oxazoline derivatives in the oxazoline group-containing polymer is preferably 5% by mass or more. Specific examples include Epocross WS-300 [functional group (oxazoline) equivalent: about 130 g/eq] and Epocross WS-700 [functional group (oxazoline) equivalent: about 220 g/eq] manufactured by Nippon Shokubai Co., Ltd.

The curing agent is preferably blended in an amount of 1 to 10 parts by mass, more preferably 2 to 8 parts by mass, and even more preferably 3 to 7 parts by mass with respect to 100 parts by mass of the polyester resin (solid content). If the amount of the curing agent is less than the above range, sufficient curability cannot be obtained. If the functional groups of the curing agent are excessively large relative to the amount of carboxyl groups in the polyester resin, it becomes difficult for one molecule of the curing agent to react with two or more molecules of the polyester resin, resulting in insufficient cross-linking. On the contrary, curability may decrease. In addition, there is a possibility that the stability of long-term storage is inferior.
Further, the amount of functional groups (β-hydroxyalkylamide group, etc.) of the curing agent relative to the amount of carboxyl groups of the polyester resin is preferably in the range of 0.2 to 3.0 equivalents, more preferably in the range of 0.5 to 2.5 equivalents. is more preferred.

(aqueous medium)
The aqueous coating composition of the present invention contains the polyester resin and curing agent described above, and an aqueous medium. As the aqueous medium, water or a mixture of water and an organic solvent such as an alcohol, a polyhydric alcohol, or a derivative thereof can be used as the aqueous medium in the same manner as a known aqueous coating composition. When an organic solvent is used, it is preferably contained in an amount of 1 to 45% by mass, particularly preferably 5 to 30% by mass, based on the total aqueous medium in the aqueous coating composition. By containing the solvent in the above range, the film-forming performance is improved.
As such an organic solvent, those having amphipathic properties are preferable, and examples thereof include methyl alcohol, ethyl alcohol, isopropyl alcohol, n-butanol, ethylene glycol, methyl ethyl ketone, butyl cellosolve, carbitol, butyl carbitol, and propylene glycol monopropyl. ether, propylene glycol ethylene glycol monoblu ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, 3-methyl-3-methoxybutanol and the like.

(basic compound)
In order to impart water-dispersibility or water-solubility to the polyester resin, the water-based coating composition of the present invention preferably contains a basic compound capable of neutralizing the carboxyl groups of the polyester resin. The basic compound is preferably a compound that volatilizes during baking during coating film formation, ie, ammonia and/or an organic amine compound having a boiling point of 250° C. or less.
Specifically, alkylamines such as trimethylamine, triethylamine and n-butylamine, and alcoholamines such as 2-dimethylaminoethanol, diethanolamine, triethanolamine, aminomethylpropanol and dimethylaminomethylpropanol are used. Polyvalent amines such as ethylenediamine and diethylenetriamine can also be used. Furthermore, amines having branched chain alkyl groups and heterocyclic amines are also preferably used. As the amine having a branched chain alkyl group, branched chain alkylamines having 3 to 6 carbon atoms, particularly 3 to 4 carbon atoms such as isopropylamine, sec-butylamine, tert-butylamine and isoamylamine are used. As heterocyclic amines, saturated heterocyclic amines containing one nitrogen atom such as pyrrolidine, piperidine and morpholine are used.
In the present invention, among the above, triethylamine or 2-dimethylaminoethanol can be preferably used, and the amount used is preferably 0.5 to 1.5 equivalents relative to the carboxyl group.

(Curing catalyst)
The aqueous coating composition of the present invention may optionally contain a curing catalyst for the purpose of promoting the cross-linking reaction between the polyester resin and the curing agent. As the curing catalyst, conventionally known curing catalysts can be used, and acid catalysts such as p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenedisulfonic acid, or amine-blocked products thereof, organic tin compounds, and organic titanium compounds. compounds, organometallic compounds such as organozinc compounds, organocobalt compounds, and organoaluminum compounds; metal oxides such as zinc oxide, titanium oxide, and aluminum oxide; alkali metal hypophosphites; alkali metal phosphites; Phosphorus compounds such as phosphoric acid and alkylphosphinic acid can be used. The curing catalyst is preferably blended in the range of 0.1 to 5 parts by mass with respect to 100 parts by mass of the solid content of the polyester resin. If the amount of the curing catalyst to be blended is less than the above range, the curing reaction acceleration effect obtained by blending the curing catalyst cannot be sufficiently obtained. On the other hand, if the amount of the curing catalyst is more than the above range, no further effect can be expected, resulting in poor economy.

(lubricant)
The aqueous coating composition of the present invention may contain a lubricant if necessary. It is preferable to add 0.1 to 10 parts by mass of a lubricant to 100 parts by mass of the polyester resin.
By adding a lubricant, it is possible to suppress damage to the coating film during molding of can lids and the like, and to improve the lubricity of the coating film during molding.

Lubricants that can be added to the aqueous coating composition of the present invention include, for example, fatty acid ester waxes, which are esters of polyol compounds and fatty acids, silicone waxes, fluorine waxes, polyolefin waxes such as polyethylene, and lanolin waxes. , montan wax, microcrystalline wax, carnauba wax, and silicon compounds. These lubricants can be used singly or in combination of two or more.

(others)
In addition to the components described above, the water-based paint composition of the present invention may also contain leveling agents, pigments, antifoaming agents, and the like, which have conventionally been blended in paint compositions, according to conventionally known formulations.
In addition, other resin components may be contained in addition to the polyester resin as long as the object of the present invention is not impaired. For example, polyvinyl acetate, ethylene/vinyl acetate copolymer, polyolefin resin, epoxy resin, polyurethane Resin, acrylic resin, phenol resin, melamine resin, polyvinyl chloride resin, polyvinyl chloride-vinyl acetate copolymer resin, polyvinyl alcohol, ethylene-vinyl alcohol copolymer, polyvinylpyrrolidone, polyvinyl ethyl ether, polyacrylamide, acrylamide Compounds, polyethylenimine, starch, gum arabic, and water-dispersible or water-soluble resins such as methylcellulose may also be included.
In the water-based coating composition of the present invention, it is preferable that the polyester resin is contained in an amount of 5 to 55% by mass as a solid content. If the resin solid content is less than the above range, an appropriate coating amount cannot be secured, resulting in poor coverage. On the other hand, when the resin solid content is larger than the above range, workability and coatability may be inferior.

(Method for producing coated metal substrate)
The method for producing a coated metal substrate of the present invention comprises a coating step of applying the water-based coating composition described above onto a metal substrate, and heating the water-based coating composition applied on the metal substrate in the coating step to a temperature higher than 200°C to 320°C. It consists of a baking process that heats at the following temperature.

(Painting process)
In the coating step, the above-mentioned aqueous coating composition is coated on the metal substrate by a known coating method such as roll coater coating or spray coating.
It should be noted that it is desirable to coat so that the dry film thickness is less than 20 μm, particularly 0.5 to 15 μm.

(Baking process)
The baking method can be carried out by conventionally known heating means according to the shape of the metal substrate and the like.
In the baking step, it is preferable to heat the water-based coating composition applied to the metal substrate at a temperature higher than 200°C and lower than or equal to 320°C. The temperature referred to here refers to the ambient temperature (inside temperature of the oven) during baking.
By heating at a temperature in the above range, even when using a water-based coating composition composed of a polyester resin with a relatively low acid value having an acid value of 5 mgKOH/g or more and less than 30 mgKOH/g and a specific curing agent, it can be cured for a short time. It becomes possible to obtain sufficient curability. If the baking temperature is lower than the above range, a sufficient degree of hardening may not be obtained if the baking time is short. On the other hand, if the baking temperature is higher than the above range, the polyester resin may be thermally decomposed due to excessive heating. The baking time is 5 seconds or longer, preferably 5 to 180 seconds, particularly preferably 10 to 120 seconds. If the baking time is shorter than the above range, a sufficient degree of hardening may not be obtained, and if the baking time is longer than the above range, economy and productivity are poor.
The baking method can be carried out by conventionally known heating means according to the shape of the metal substrate and the like.

(Painted metal substrate)
In the present invention, a coated metal substrate can be formed on which a coating film made of the water-based coating composition described above is formed. Examples of metal substrates on which such coating films are formed include metal plates, metal cans, metal lids, and the like.

(painted metal plate)
The coated metal sheet of the present invention is obtained by forming a coating film of the water-based coating composition described above on at least one surface of a metal sheet.
Examples of metal sheets include, but are not limited to, hot-drawn steel sheets, cold-rolled steel sheets, hot-dip galvanized steel sheets, electro-galvanized steel sheets, alloy-plated steel sheets, aluminum-zinc alloy-plated steel sheets, aluminum sheets, tin-plated steel sheets, stainless steel sheets, Copper plate, copper-plated steel plate, tin-free steel, nickel-plated steel plate, ultra-thin tin-plated steel plate, chromium-treated steel plate, etc., and various surface treatments such as phosphate chromate treatment and zirconium chemical conversion treatment are applied to these as necessary. You can use what you have.
The thickness of the coating film on the coated metal sheet is preferably less than 20 μm, particularly preferably in the range of 0.5 to 15 μm.
Further, a thermoplastic resin film such as a polyester resin film may be laminated as an organic resin coating layer on the coating film of the coated metal sheet to form an organic resin-coated coated metal sheet.

(painted metal can)
The coated metal can of the present invention may be obtained by directly coating at least one side of a preformed metal can with the water-based coating composition described above, or the coating film-formed surface of the coated metal sheet. It may be a three-piece can (welded can) molded as an inner surface, or a metal can such as a seamless can is formed from the organic resin-coated coated metal plate in which an organic resin coating layer is formed on the above-mentioned coated metal plate. can also Such a seamless can can be formed by drawing and ironing, drawing, drawing and stretching, and drawing and stretching and ironing from an organic resin-coated painted metal sheet.
Also in the coated metal can, the thickness of the coating film is preferably less than 20 μm, particularly in the range of 0.5 to 15 μm.

EXAMPLES The present invention will be specifically described below with reference to examples. In the examples, "parts" means "parts by mass".

Various measurement items of the polyester resin were measured according to the following methods. For the measurement, a solid obtained by vacuum drying after removing the aqueous medium from the aqueous dispersion of the carboxyl group-containing polyester resin using an evaporator was used.
(Measurement of number average molecular weight)
Measured by gel permeation chromatography (GPC) using a standard polystyrene calibration curve.
(Measurement of glass transition temperature)
Measurement was performed using a differential scanning calorimeter (DSC) at a heating rate of 20°C/min.
(Measurement of acid value)
0.2 g of a polyester resin was dissolved in 20 ml of chloroform and titrated with a 0.1N KOH ethanol solution to determine the resin acid value (mgKOH/g). Phenolphthalein was used as an indicator. When the polyester resin was not dissolved, a solvent such as tetrahydrofuran was used.
(Measurement of monomer composition)
The monomer composition of the polyester resin was obtained by dissolving 30 mg of the vacuum-dried resin in 0.6 ml of deuterated chloroform, performing ¹ H-NMR measurement, and determining the composition ratio from the peak intensity. The composition ratio was determined except for a very small amount of components (less than 1 mol % with respect to all monomer components).

(Preparation of coated metal plate)
Each water-based coating composition of Examples, Comparative Examples, and Reference Examples was applied to a chromate phosphate-based surface-treated aluminum plate (3104 alloy, plate thickness: 0.28 mm, weight of chromium in the surface treatment film: 20 mg/m ² ). Coating was performed with a coater so that the coating thickness after baking was a predetermined film thickness, and oven baking was performed at a predetermined baking temperature and time to prepare a coated metal plate. The coating conditions are as shown in Table 1. In Example 1, the baking temperature (inside oven temperature) was 260° C., the baking time was 60 seconds, and the thickness of the baked coating was 1.5 μm. Various evaluations were performed on the obtained coated metal sheet based on the following evaluation methods.

(Curability)
Curability of the coated metal plate was evaluated by MEK extraction rate. A test piece having a size of 5 cm×5 cm was cut out from the coated metal plate, and after measuring the mass of the sample (W1), extraction was performed at room temperature for 1 hour using 200 ml of MEK (methyl ethyl ketone). The coated plate after extraction was dried under the conditions of 130° C. for 1 hour, and the mass (W2) of the sample after extraction was measured. Further, the coating film was peeled off by a decomposition method using concentrated sulfuric acid, and the mass (W3) of the sample was measured. The MEK extraction rate of the coated plate is obtained by the following formula (3). Table 1 shows the results.
MEK extraction rate %=100×(W1−W2)/(W1−W3) (3)
Evaluation criteria are as follows.
◎: Less than 10% ○: 10% or more and less than 30% △: 30% or more and less than 50% ×: 50% or more

(workability)
The coated metal plate was cut into a size of 3.5 x 4 cm so that the rolling direction of the aluminum plate became the long side, and the test piece was bent parallel to the short side so that the coated surface of the test piece faced outward. Two 0.28 mm aluminum plates were sandwiched inside the bent portion in an atmosphere of 25° C. and subjected to impact bending using a seam-fold type DuPont impact tester. An iron weight with a flat contact surface to be impact-bent has a weight of 3 kg and is dropped from a height of 40 cm. It was brought into contact with the soaked sponge, a voltage of 6.3 V was applied, and measurement was performed 4 seconds later. Table 1 shows the results.
◎: Less than 5 mA ○: 5 mA or more and less than 20 mA △: 20 mA or more and less than 30 mA ×: 30 mA or more

(corrosion resistance)
After cutting out a test piece of 5 cm x 5 cm size from the coated metal plate, using a DuPont impact tester, the coated surface of the test coated plate under the conditions of 1/4 inch diameter of the tip of the striking core, weight load of 120 g, and drop weight height of 30 cm. A test piece subjected to impact bulging was immersed in a model liquid that simulates the contents of a can, and the degree of corrosion was visually evaluated according to the following criteria. The immersion conditions were 37° C. for 14 days.
The model solution used in the test was prepared by adding 0.2% sodium chloride and adding citric acid to adjust the pH to 2.5. Table 1 shows the results.
◎: No corrosion ○: Slightly corroded △: Partially corroded X: Mostly corroded

(Retort resistance)
The coated metal plate was placed in an autoclave, subjected to retort treatment at 125° C. for 30 minutes, and the whitening state (whitening property) of the coating film was visually evaluated. Table 2 shows the results.
◎: No whitening ○: Slightly whitening △: Slightly whitening ×: Significantly whitening

[Preparation of aqueous coating composition]
(Example 1)
Polyester resin A (acid value: 23 mg KOH / g, Tg: 80 ° C., Mn = 7,500, monomer composition: terephthalic acid component / ethylene glycol component / propylene glycol component = 50/10/40 mol%) as a polyester resin, β- N,N,N',N'-tetrakis(2-hydroxyethyl)adipamide [manufactured by Tokyo Chemical Industry Co., Ltd.; indicated as "β-hydroxyalkylamide A" in the table] was used as a hydroxyalkylamide group-containing curing agent. 333 parts (solid content: 100 parts) of an aqueous dispersion of polyester resin A (solid content concentration: 30 wt%), and an aqueous solution of a β-hydroxyalkylamide group-containing curing agent (solid content: 100 parts) prepared in advance using deionized water concentration: 10 wt%), 150 parts of 2-propanol, and 517 parts of ion-exchanged water were placed in a glass container and stirred for 10 minutes, and the solid content concentration was 10% by mass. obtained a water-based coating composition of polyester resin/curing agent=100/5 (mass ratio).

(Examples 2 to 23 )
A water-based coating composition was prepared in the same manner as in Example 1 except that the various polyester resins, curing agents, and solid content blending ratios (mass ratios) shown in Tables 1 and 2 were used. Polyester resin B (Tg: 67°C, Mn = 9,000, acid value: 18 mgKOH/g, monomer composition: terephthalic acid component/isophthalic acid component/ethylene glycol component/neopentyl glycol component = 36/14/ 24/26 mol%), polyester resin C (Tg: 40 ° C., Mn = 8,500, acid value: 17 mg KOH / g, monomer composition: terephthalic acid component / isophthalic acid component / adipic acid / ethylene glycol component / neopentyl glycol component = 28/15/7/25/25 mol%), polyester resin D (Tg: 20°C, Mn = 11,000, acid value: 11 mgKOH/g, monomer composition: terephthalic acid component/isophthalic acid component/sebacic acid component/ Ethylene glycol component/neopentyl glycol component = 31/7/12/30/20 mol%), polyester resin E (Tg: 8°C, Mn = 19,000, acid value: 12 mgKOH/g, monomer composition: terephthalic acid component/ Isophthalic acid component/sebacic acid component/ethylene glycol component/neopentyl glycol component = 30/5/15/22/28 mol%), polyester resin F (Tg: -25°C, Mn = 17,000, acid value: 12 mg KOH/ g, monomer composition: terephthalic acid component/isophthalic acid component/sebacic acid component/1,4-butanediol component = 14/17/19/50 mol%), polyester resin G (Tg: 40°C, Mn = 5,000, Acid value: 29 mgKOH/g), polyester resin H (Tg: 52°C, Mn = 17,000, acid value: 5 mgKOH/g, monomer composition: terephthalic acid component/isophthalic acid component/adipic acid component/ethylene glycol component/neo Pentyl glycol component = 23/23/4/24/26 mol%) was used.
In Examples 20, 21, and 23 , N,N,N',N'-tetrakis(2-hydroxypropyl)adipamide [Primid QM1260 manufactured by EMS Co., Ltd.; Alkylamide B”] was used.

(Comparative Examples 1 to 6 )
A water-based coating composition was prepared in the same manner as in Example 1, except that the various polyester resins shown in Table 1 and the solid content blending ratio (mass ratio) were prepared, and a coated metal plate was produced.

(Reference example 1)
Polyester resin A (acid value: 23 mgKOH/g, Tg: 80°C, Mn = 7,500) was used as the polyester resin, and a resol-type phenolic resin was used as the curing agent. As the resol-type phenolic resin, a meta-cresol-based phenolic resin in which methylol groups were butyl-etherified (ratio of etherified methylol groups: 90 mol %, Mn=1,600) was used. 333 parts of aqueous dispersion of polyester resin A (solid content: 100 parts), 40 parts of n-butanol solution of the resol-type phenolic resin (solid content: 20 parts), 1 part of dodecylbenzenesulfonic acid (curing catalyst), 0.3 triethylamine Parts, 200 parts of 2-propanol, and 635 parts of ion-exchanged water were used to prepare an aqueous coating composition (solid content concentration: 10% by mass, solid content mixing ratio: polyester resin/curing agent=100/15). As dodecylbenzenesulfonic acid, "dodecylbenzenesulfonic acid (soft type) (mixture)" manufactured by Tokyo Chemical Industry Co., Ltd. was used. A coated metal plate was produced in the same manner as in Example 1 using the obtained aqueous coating composition.

(Reference example 2)
Reference Example except that the blending amounts of the n-butanol solution of the curing agent and the deionized water were adjusted so that the solid content blending ratio in the water-based coating composition was polyester resin / curing agent = 100/5 (mass ratio) A water-based coating composition was prepared in the same manner as in 1 to prepare a coated metal plate.

Tables 1 and 2 show the composition of each water-based coating composition (type of polyester resin, type of curing agent, solid content mixing ratio) and coating conditions (baking temperature, baking time, coating thickness) and evaluation results.

The method for producing a painted metal substrate of the present invention can produce a painted metal substrate having excellent workability, curability, and corrosion resistance. Moreover, the coated metal sheet and coated metal can of the present invention have excellent workability, curability, and corrosion resistance, and can be suitably used for beverage can applications that require high workability and corrosion resistance.

Claims (7)

Contains a polyester resin having an acid value of 17 mgKOH/g or more and less than 30 mgKOH/g, and a curing agent having a functional group capable of cross- linking with the carboxyl group of the polyester resin ,
the functional group is a β-hydroxyalkylamide group,
A coating step of applying an aqueous coating composition containing 2 to 8 parts by mass of the curing agent (solid content) to a metal substrate with respect to 100 mass of the polyester resin (solid content) , and the coating and a baking step of heating the aqueous coating composition applied on the metal substrate in the step to a temperature higher than 200°C and lower than or equal to 320°C. 2. The method for producing a coated metal substrate according to claim 1, wherein the curing agent has a functional group equivalent weight of 30 to 500 g/eq. 3. The method for producing a coated metal substrate according to claim 1, wherein the curing agent has a molecular weight of 1,000 or less. The method for producing a painted metal substrate according to any one of claims 1 to 3 , wherein the curing agent is a β-hydroxyalkylamide compound. The method for producing a coated metal substrate according to any one of claims 1 to 4 , wherein the metal substrate is a metal plate or a metal can. A polyester resin having an acid value of 17 mgKOH/g or more and less than 30 mgKOH/g and a β-hydroxyalkylamide compound as a curing agent for the polyester resin are added to 100 mass of the polyester resin (solid content), and the curing agent (solid content ) in an amount of 2 to 8 parts by mass and having a film thickness of less than 20 μm on at least one surface. A polyester resin having an acid value of 17 mgKOH/g or more and less than 30 mgKOH/g and a β-hydroxyalkylamide compound as a curing agent for the polyester resin are added to 100 mass of the polyester resin (solid content), and the curing agent (solid content ) in an amount of 2 to 8 parts by mass and having a coating film thickness of less than 20 μm on at least one side thereof.