阅读说明：本技术 水性涂料组合物和涂装金属基体的生产方法 (Aqueous coating composition and process for producing coated metal substrate ) 是由 柏仓拓也 樱木新 张楠 山本宏美 于 2018-05-10 设计创作，主要内容包括：本发明涉及适合用于如饮料罐等金属容器或金属盖的聚酯系水性涂料组合物。该聚酯系水性涂料组合物被构造成包含酸值为5mgKOH/g以上且小于30mgKOH/g的聚酯树脂,和具有能够与聚酯树脂的羧基反应的官能团的固化剂。结果,该聚酯系水性涂料组合物能够形成具有优异的加工性、固化性和耐腐蚀性的涂膜。(The present invention relates to a polyester-based water-based coating composition suitable for use in a metal container or a metal lid such as a beverage can. The polyester-based water-based coating composition is configured to contain a polyester resin having an acid value of 5mgKOH/g or more and less than 30mgKOH/g, and a curing agent having a functional group capable of reacting with a carboxyl group of the polyester resin. As a result, the polyester-based aqueous coating composition can form a coating film having excellent processability, curability and corrosion resistance.)

1. An aqueous coating composition comprising a polyester resin having an acid value of 5mgKOH/g or more and less than 30mgKOH/g, and a curing agent having a functional group capable of undergoing a crosslinking reaction with a carboxyl group of the polyester resin.

2. The aqueous coating composition according to claim 1, wherein the functional group is at least one selected from a β -hydroxyalkylamide group, an epoxy group, and an oxazoline group.

3. An aqueous coating composition according to claim 1 or 2, wherein the curing agent is a β -hydroxyalkylamide compound.

4. An aqueous coating composition according to claim 3, wherein the β -hydroxyalkylamide compound is N, N, N ', N' -tetrakis (2-hydroxyethyl) adipamide and/or N, N, N ', N' -tetrakis (2-hydroxypropyl) adipamide.

5. The aqueous coating composition of claim 1, wherein the curing agent has a functional group equivalent weight of 30 to 500 g/eq.

6. The aqueous coating composition according to claim 1, wherein the curing agent has a molecular weight of 1,000 or less.

7. The aqueous coating composition according to claim 1, wherein a blending amount of a solid content of the curing agent is 1 to 10 parts by mass with respect to 100 parts by mass of a solid content of the polyester resin.

8. The aqueous coating composition according to claim 1, wherein the polyester resin is a mixed polyester resin comprising a polyester resin (a) and a polyester resin (B) having an acid value larger than that of the polyester resin (a), and the difference in acid value between the polyester resin (a) and the polyester resin (B) is 5mgKOH/g or more.

9. The aqueous coating composition according to claim 8, wherein the acid value of the polyester resin (a) is 3 to 20mgKOH/g, the acid value of the polyester resin (B) is 10 to 60mgKOH/g, and the solid content mass ratio of the polyester resin (a) to the polyester resin (B) is (a): 95:5 to 5: 95.

10. The aqueous coating composition according to claim 1, wherein the polyester resin is a mixed polyester resin comprising a polyester resin (α) having a glass transition temperature (Tg) of 35 ℃ to 100 ℃ and a polyester resin (β) having a Tg of-30 ℃ to 25 ℃, and the solid content mass ratio of the polyester resin (α) to the polyester resin (β) is (α): 98:2 to 10: 90.

11. A coated metal sheet having a coating film of the aqueous coating composition according to claim 1 formed on at least one surface thereof.

12. A coated metal container having formed thereon a coating film of the aqueous coating composition according to claim 1.

13. A method of producing a coated metal substrate comprising the steps of:

a coating step of coating a metal substrate with an aqueous coating composition comprising a polyester resin having an acid value of 5mgKOH/g or more and less than 30mgKOH/g, and a curing agent having a functional group capable of undergoing a crosslinking reaction with a carboxyl group of the polyester resin; and

a step of firing the aqueous coating composition applied on the metal substrate in the coating step at a temperature higher than 200 ℃ and 320 ℃ or lower.

14. A process for producing a coated metallic substrate according to claim 13, wherein the curing agent is a β -hydroxyalkylamide compound.

15. The production method of a coated metal substrate according to claim 13 or 14, wherein the metal substrate is a metal plate or a metal can.

Technical Field

The present invention relates to an aqueous coating composition for metal cans. More specifically, the present invention relates to an aqueous coating composition capable of forming a coating film excellent in processability and having excellent curability and corrosion resistance, and to a method for producing a coated metal substrate by using the aqueous coating composition.

Background

Coating compositions for metal containers and metal lids such as beverage cans must have workability capable of withstanding severe working (e.g., necking working, hemming working, scoring working, and riveting working) in forming the metal containers and metal lids. Further, depending on the application, the coating composition must have various characteristics such as good curability, corrosion resistance to prevent corrosion of the metal substrate by the contents, characteristics to prevent deterioration of the flavor of the contents, characteristics to maintain excellent hygienic properties without elution of coating components, and excellent retort resistance. Further, in recent years, attention must be paid to the influence on the environment and the working environment due to volatilization of an organic solvent during the work of coating and firing.

Heretofore, as coating compositions for metal containers and metal lids, epoxy-based coatings such as epoxy phenol-based coatings, epoxy amino-based coatings, and epoxy acrylic coatings have been widely used. However, in many cases, bisphenol a is used as a raw material to produce epoxy-based coatings. Accordingly, it would be desirable to provide coatings that do not contain bisphenol a.

From the above-mentioned viewpoints, a polyester-based water-based coating composition has been proposed as a coating composition for metal containers and metal lids, which is free from bisphenol a and is also noticeable in terms of environmental pollution and the working environment.

As a polyester-based aqueous coating composition, a coating for a metal package mainly comprising a carboxyl group-containing aromatic polyester resin having an Acid Value (AV) of 10 to 30mgKOH/g and a number average molecular weight (Mn) of 3,000 to 10,000, further in combination with a curing agent, a neutralizing agent for the polyester resin, and a co-solvent has been proposed, which has characteristics of excellent curability and retort resistance (patent document 1).

Further, as an aqueous coating composition, there has been proposed an aqueous coating composition obtained by stably dispersing a β -hydroxyalkylamide crosslinking agent (D) and an acrylic modified polyester resin (C) obtained by graft polymerizing a polyester resin (a) having an ethylenic double bond at the terminal of the resin 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 in an aqueous medium (patent document 2).

Disclosure of Invention

Problems to be solved by the invention

However, as taught in patent document 1, when a phenolic resin forming a self-condensate is used as a curing agent, hard and brittle regions derived from the self-condensate of the phenolic resin are formed in the coating film. Therefore, the coating composition is still not completely satisfactory from the viewpoint of processability of the coating film. On the other hand, as taught in patent document 2, when a polyester resin having a large acid value is used, the reaction point (crosslinking point) with the curing agent increases, contributing to improvement in curability. However, the crosslinking density becomes so high that the coating film tends to crack during forming and becomes no longer able to withstand severe processing, thus making it difficult to obtain sufficient processability.

Accordingly, an object of the present invention is to provide a polyester resin-based aqueous coating composition capable of forming a coating film excellent in processability and having characteristics such as excellent coating film properties such as curability and corrosion resistance.

Means for solving the problems

According to the present invention, there is provided an aqueous coating composition comprising a polyester resin having an acid value of 5mgKOH/g or more and less than 30mgKOH/g, and a curing agent having a functional group capable of reacting with a carboxyl group of the polyester resin.

In the aqueous coating composition of the present invention, it is desirable that:

1. the functional group is at least one selected from beta-hydroxyalkylamide, epoxy and oxazoline;

2. the curing agent is a beta-hydroxyalkylamide compound;

3. the beta-hydroxyalkylamide compound is N, N, N ', N' -tetrakis (2-hydroxyethyl) adipamide and/or N, N, N ', N' -tetrakis (2-hydroxypropyl) adipamide;

4. the curing agent has a functional group equivalent of 30 to 500 g/eq;

5. the molecular weight of the curing agent is 1,000 or less;

6. the blending amount of the solid content of the curing agent is 1 to 10 parts by mass relative to 100 parts by mass of the solid content of the polyester resin;

7. the polyester resin is a mixed polyester resin comprising a polyester resin (a) and a polyester resin (B) having an acid value larger than that of the polyester resin (a), and the difference in acid value between the polyester resin (a) and the polyester resin (B) is 5mgKOH/g or more;

8. the acid value of the polyester resin (A) is 3 to 20mgKOH/g, the acid value of the polyester resin (B) is 10 to 60mgKOH/g, and the solid mass ratio of the polyester resin (A) to the polyester resin (B) is (A): 95:5 to 5: 95; and

9. the polyester resin is a mixed polyester resin comprising a polyester resin (α) having a glass transition temperature (Tg) of 35 to 100 ℃ and a polyester resin (β) having a Tg of-30 to 25 ℃, and the mass ratio of the polyester resin (α) to the polyester resin (β) is (α): 98:2 to 10: 90.

Further, according to the present invention, there is provided a coated metal sheet having a coating film of the above aqueous coating composition formed on at least one surface thereof.

Further, according to the present invention, there is provided a coated metal container or a coated metal lid having formed thereon a coating film of the above-mentioned aqueous coating composition.

Further, according to the present invention, there is provided a production method of a coated metal substrate, comprising the steps of:

a coating step of coating a metal substrate with an aqueous coating composition containing a main agent of a polyester resin having an acid value of 5mgKOH/g or more and less than 30mgKOH/g, and a curing agent having a functional group capable of causing a crosslinking reaction with a carboxyl group of the polyester resin; and

a step of firing the aqueous coating composition applied to the metal substrate in the coating step at a temperature higher than 200 ℃ and not higher than 320 ℃.

In the production method of a coated metal substrate of the present invention, it is desirable that:

1. the curing agent is a beta-hydroxyalkylamide compound; and

2. the metal substrate is a metal plate or can.

ADVANTAGEOUS EFFECTS OF INVENTION

The present inventors have intensively studied to improve the processability of a coating film formed by using a polyester-based aqueous coating composition, and found that, when a specific polyester resin and a curing agent are used, a coating film having excellent processability capable of withstanding even severe processing can be formed. Further, a coating film formed by using the aqueous coating composition of the present invention has excellent curability and corrosion resistance, and can be advantageously used for metal containers such as beverage cans and metal lids.

Detailed Description

(polyester resin)

In the aqueous coating composition of the present invention, it is important that the acid value of the polyester resin as the main agent is 5mgKOH/g or more and less than 30mgKOH/g, particularly, in the range of 10 to 29 mgKOH/g.

That is, in the present invention, it is important that the polyester resin as the main agent has an appropriate amount of carboxyl groups (acid value) from the viewpoint of achieving curability, processability, and adhesion of the coating film. When the acid value is less than the above range, only a small amount of carboxyl groups which become crosslinking points with the curing agent is present. Therefore, curability is insufficient and crosslinking is also insufficient. In this case, it becomes difficult to improve the properties (heat resistance, processability, etc.) of the coating film by crosslinking with the curing agent. Further, since the number of carboxyl groups participating in the adhesion between the coating film and the metal substrate is reduced, the adhesion of the coating film is deteriorated. On the other hand, when the acid value is more than the above range, the crosslinking point with the curing agent increases, and excellent curability is exhibited. However, in this case, the crosslinking density becomes too high, and the processability becomes poor.

Even when the acid value is larger than the above range, the crosslinking density can be suppressed by adjusting the blending amount of the curing agent to be small. However, in this case, free carboxyl groups that do not participate in crosslinking remain in the coating film, and thus the water resistance of the coating film deteriorates. Therefore, as a result, sufficient corrosion resistance cannot be obtained.

According to the present invention, a mixed polyester resin obtained by mixing two or more polyester resins having different acid values together may also be used. In this case, the sum of values obtained by multiplying the acid value of each resin by its blending ratio (mass ratio) is regarded as the acid value (Av) of the mixed polyester resin_mix) And the acid value should be in the range of 5mgKOH/g or more and less than 30mgKOH/g (even if a polyester resin which itself shows an acid value outside the above range is used, it is permissible if the average acid value is in the above range after blending with other polyester resins having different acid values).

As the polyester resin used as the main agent, it is desirable to use a mixed polyester resin of a mixture of the polyester resin (a) and the polyester resin (B) which has an acid value larger than that of the polyester resin (a) and in which the difference in acid value between the polyester resin (B) and the polyester resin (a) is 5mgKOH/g or more, from the viewpoint of achieving both the processability and curability of the coating film at a higher level.

When only the polyester resin (a) having a relatively small acid value, i.e., having a relatively small amount of carboxyl groups which become reaction points with the curing agent (β -hydroxyalkylamide compound) is used as a main agent, the density of crosslinking points is suppressed and a coating film having characteristics excellent in processability can be easily formed, however, since the number of reaction points is small, a prolonged crosslinking reaction time is required. Therefore, an extended firing time is required before a high level of curing is achieved, or in other words, curability tends to deteriorate. On the other hand, when only the polyester resin (B) having a relatively large acid value, i.e., having a relatively large number of carboxyl groups, is used as the main agent, a sufficient degree of curing can be achieved in a short time, or in other words, excellent curability is achieved, however, this results in an increase in the density of crosslinking points in the formed coating film, and the processability tends to deteriorate.

Therefore, when the mixed polyester resin is used by mixing the polyester resin (a) and the polyester resin (B) together, the coating film as a whole shows significantly improved curability as compared with when only the polyester resin (a) is used. In addition, regarding processability, the characteristics of the polyester resin (a) are strongly reflected, and excellent processability is achieved. Therefore, both curability and processability can be achieved at a higher level than when only a polyester resin alone is used.

Here, from the viewpoint of forming a coating film having the above-described characteristics, it is important that, as described above, the difference in acid value between the polyester resin (a) and the polyester resin (B) is 5mgKOH/g or more, preferably in the range of 5 to 55mgKOH/g, more preferably in the range of 7 to 40mgKOH/g, and further preferably in the range of 10 to 35 mgKOH/g. When the difference in acid value is less than 5mgKOH/g, it is difficult to obtain the above-mentioned effects based on the combined use of polyester resins having different acid values.

In order to effectively exhibit the above-mentioned effects, under the condition that the difference in acid value between the polyester resin (A) and the polyester resin (B) is 5mgKOH/g or more, it is desirable to use, as the polyester resin (A), a polyester resin having an acid value in the range of 3 to 20mgKOH/g, particularly in the range of 5 to 15mgKOH/g, and to use, as the polyester resin (B), a polyester resin having an acid value in the range of 10 to 60mgKOH/g, particularly in the range of 15 to 45 mgKOH/g. That is, it is desirable to use the polyester resin (a) and the polyester resin (B) mixed together in a ratio (mass ratio) of (a) to (B) in the range of 95:5 to 5:95, particularly 90:10 to 10: 90.

Here, in the present invention, it is important that the mixed polyester resin obtained by mixing the above-mentioned polyester resins (a) and (B) together has an acid value (Av) represented by the following formula (1)_mix) In the range of 5mgKOH/g or more and less than 30mgKOH/g,

Av_mix＝[(Av_A·W_A/(W_A+W_B)+(Av_B·W_B/(W_A+W_B)]---(1)

wherein, Av_mixThe acid value (mgKOH/g), Av, of the mixed polyester resin was_AAnd Av_BThe acid values (mgKOH/g) of the polyester resins (A) and (B) used, and W_AAnd W_BIs the mass fraction of the polyester resins (A) and (B). In the present invention, the two polyester resins (A) and (B) having a difference in acid value of 5mgKOH/g or more may each be a blend of a plurality of polyester resins. For example, the polyester resin (A) may be used asThe mixed polyester resin (a ') of a mixture of a plurality of polyester resins may have an acid value of 5 or more different from that of the polyester resin (B) in the mixed polyester resin (a') obtained according to the above formula (1).

When the mixed polyester resin (a ') or similarly the mixed polyester resin (B') is used, the polyester resin constituting the mixed polyester resin (a ') should desirably be selected from the polyester resins (a) having an acid value in the range of 3 to 20mgKOH/g as described above, and similarly, the polyester resin constituting the mixed polyester resin (B') should desirably be selected from the polyester resins (B) having an acid value in the range of 10 to 60mgKOH/g as described above.

As the polyester resin used in the present invention, any known water-dispersible polyester resin and/or water-soluble polyester resin used in a coating composition may be used if the above acid value is not considered.

The water-dispersible polyester resin and the water-soluble polyester resin are polyester resins containing a hydrophilic group as a component. These components may be physically adsorbed by the surface of the polyester dispersion, but it is particularly preferable that they are copolymerized in the backbone of the polyester resin.

As the hydrophilic group, a hydroxyl group, an amino group, a carboxyl group, a sulfonic acid group, their derivatives, metal salts and ethers may be exemplified. When the hydrophilic group is contained in the molecule, the polyester resin can be dispersed in water.

As the hydrophilic group-containing component, there can be specifically exemplified carboxylic acid anhydrides such as phthalic anhydride, succinic anhydride, maleic anhydride, trimellitic anhydride, itaconic anhydride, and citraconic anhydride; hydroxyl group-containing polyether monomers such as polyethylene glycol, polypropylene glycol, glycerin and polyglycerin; and metal salts of sulfonic acid-containing monomers such as 5-sulfoisophthalic acid, 4-sulfonaphthalene-2, 7-dicarboxylic acid, and 5 (4-sulfophenoxy) isophthalic acid, and ammonium salts thereof.

It is also possible to use a polyester resin modified with an acrylic resin obtained by graft polymerizing a polyester resin with a vinyl monomer having a hydrophilic group. As the vinyl-based monomer having a hydrophilic group, those having a carboxyl group, a hydroxyl group, a sulfonic acid group, or an amide group may be exemplified. As the group that can be converted into a hydrophilic group, those having an acid anhydride group, a glycidyl group, and a chlorine group can be exemplified. However, the polyester resin modified with the acrylic resin is obtained through an increased number of steps due to the modification, thus often resulting in an increase in production costs. Therefore, the present invention uses a polyester resin that is not modified with an acrylic resin.

The present invention preferably uses a water-dispersible carboxyl group-containing polyester resin having a carboxyl group as a hydrophilic group, and/or a water-soluble carboxyl group-containing polyester resin as a polyester resin.

There is no particular limitation on the monomer component that forms the polyester resin together with the monomer containing the hydrophilic group, provided that the monomer component is a monomer component generally used for polymerization of the polyester resin. As the polycarboxylic acid component constituting the polyester resin, there may be exemplified, for example, aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid and naphthalenedicarboxylic acid; aliphatic dicarboxylic acids such as succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedioic acid, and dimer acid; unsaturated dicarboxylic acids such as (anhydrous) maleic acid, fumaric acid and terpene-maleic acid adduct; alicyclic dicarboxylic acids such as 1, 4-cyclohexanedicarboxylic acid, tetrahydrophthalic acid, hexahydroisophthalic acid and 1, 2-cyclohexenedicarboxylic acid; for example, one or two or more kinds of three or more kinds of polycarboxylic acids such as (anhydrous) trimellitic acid, (anhydrous) pyromellitic acid, and methylcyclohexene tricarboxylic acid are selected and used. In the present invention, it is desirable that aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid and naphthalenedicarboxylic acid be present in the polycarboxylic acid component constituting the polyester resin in a proportion of 60 mol% or more, particularly 80 mol% or more, from the viewpoints of corrosion resistance, retort resistance and flavor retention.

There is no particular limitation on the polyol component constituting the polyester resin. That is, examples of the polyol component include ethylene glycol, propylene glycol (1, 2-propanediol), 1, 3-propanediol, 1, 4-butanediol, 1, 2-butanediol, 1, 3-butanediol, 2-methyl-1, 3-propanediol, neopentyl glycol, 1, 5-pentanediol, 1, 6-hexanediol, 3-methyl-1, 5-pentanediol, 2-ethyl-2-butyl-1, 3-propanediol, 2, 4-diethyl-1, 5-pentanediol, 1-methyl-1, 8-octanediol, 3-methyl-1, 6-hexanediol, 4-methyl-1, 7-heptanediol, 4-methyl-1, 8-octanediol, and mixtures thereof, Aliphatic diols such as 4-propyl-1, 8-octanediol and 1, 9-nonanediol; ether glycols such as diethylene glycol, triethylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol; alicyclic polyols such as 1, 4-cyclohexanedimethanol, 1, 3-cyclohexanedimethanol, 1, 2-cyclohexanedimethanol, tricyclodecanediol and hydrogenated bisphenols; and ternary or higher polyhydric alcohols such as trimethylolpropane, trimethylolethane, and pentaerythritol, and one of them or two or more of them may be used in combination. In the present invention, among the above-mentioned polyol components, ethylene glycol, propylene glycol, 1, 4-butanediol and 1, 4-cyclohexanedimethanol can be preferably used as components constituting the polyester resin from the viewpoint of hygienic properties.

The water-dispersible carboxyl group-containing polyester resin and/or the water-soluble carboxyl group-containing polyester resin can be produced by a known method, for example, by polycondensing one or more of the above-mentioned polycarboxylic acid components with one or more polyol components, and after the polycondensation of the polyester resin, depolymerizing the polyester resin with a polycarboxylic acid component such as terephthalic acid, isophthalic acid, anhydrous trimellitic acid, or pyromellitic acid; or after polycondensation of the polyester resin, the polyester resin is subjected to a ring-opening addition reaction with an acid anhydride such as phthalic anhydride, maleic anhydride, trimellitic anhydride, or ethylene glycol bistrimellitic dianhydride.

Further, it is desirable that the glass transition temperature (Tg) of the polyester resin is in the range of-30 ℃ to 120 ℃, particularly in the range of 15 ℃ to 100 ℃, but not limited thereto. When Tg is higher than the above range, the formed coating film becomes so hard that processability is deteriorated. On the other hand, when the Tg is less than the above range, the barrier property of the coating film becomes so low that the corrosion resistance and retort processing resistance become poor.

In the present invention, it is desirable to use two or more polyester resins having different Tg, blended together. When polyester resins having different Tg, blended together, are used, it is often possible to form a coating film having more excellent impact resistance and generating less coating film defects despite external impact than when only one polyester resin is used.

In this case, too, the polyester resin blend should have Tg calculated according to the following formula (2)_mix，Tg_mixWithin the above-mentioned range of Tg, the,

1/Tg_mix＝(W1/Tg1)+(W2/Tg2)+···+(Wm/Tgm)---(2)

W1+W2+···+Wm＝1

wherein

Tg_mixIs the glass transition temperature (K) of the polyester resin blend,

tg1, Tg2,. cndot. Tgm is the glass transition temperature (K) of each polyester resin (polyester resin 1, polyester resin 2,. cndot. polyester resin m), and W1, W2,. cndot. Wm is the weight fraction of each polyester resin (polyester resin 1, polyester resin 2,. cndot. polyester resin m).

In the aqueous coating composition of the present invention, it is particularly desirable to use, as the polyester resin, a polyester resin (. alpha.) having a Tg of 35 ℃ to 100 ℃ and a polyester resin (. beta.) having a Tg of-30 ℃ to 25 ℃ mixed together from the viewpoint of the impact resistance of the coating film. The blending ratio in this case is a mass ratio of (α): (β) 98:2 to 10:90, particularly 95:5 to 30: 70. From the viewpoint of corrosion resistance and retort treatment resistance, it is further desirable that the glass transition temperature (Tg) calculated according to the above formula (2)_mix) Is above 30 ℃.

The number average molecular weight (Mn) of the polyester resin is desirably in the range of 1,000 to 100,000, particularly in the range of 3,000 to 50,000, but is not limited thereto. When it is less than the above range, the coating film becomes brittle and is often poor in processability. When it is more than the above range, the coating material may have reduced stability.

The average dispersed particle diameter of the water-dispersible polyester resin and/or the water-soluble polyester resin in the aqueous medium should be in the range of 10 to 1,000nm, particularly 20 to 500 nm.

The hydroxyl value of the polyester resin should be 20mgKOH/g or less, and more preferably 10mgKOH/g or less, but is not limited thereto. When a curing agent such as a β -hydroxyalkylamide compound which reacts with the carboxyl group of the polyester resin but is considered to react little or not with the hydroxyl group is used, most of the hydroxyl groups of the polyester resin remain in an unreacted state in the coating film. Therefore, when the hydroxyl value is more than the above range, much hydroxyl groups remain, and a decrease in corrosion resistance is caused.

(curing agent)

In the present invention, it is an important feature that a specific curing agent having a functional group capable of causing a crosslinking reaction with a carboxyl group of a polyester resin as a main agent is used.

The functional group equivalent of the functional group of the curing agent used in the present invention is preferably in the range of 30 to 500g/eq, particularly 40 to 200 g/eq. Here, the functional group equivalent referred to 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 herein is a functional group capable of undergoing a crosslinking reaction with the carboxyl group of the polyester resin as a main agent). That is, the functional group equivalent means a molecular weight of each functional group of the curing agent, and is expressed, for example, as an epoxy equivalent. When the functional group equivalent is less than the above range, the distance between crosslinking points cannot be extended. Therefore, the coating film shows reduced flexibility and may have poor processability. On the other hand, when the functional group equivalent is more than the above range, curability may be deteriorated.

Further, it is desirable that the average molecular weight of the curing agent is 1,000 or less. When it is more than the above range, compatibility with the polyester resin as a main agent may be reduced, and reactivity may often be reduced. Further, from the viewpoint of obtaining good curability, it is desirable that the average number of functional groups per molecule of the curing agent is 3 or more.

As the curing agent, it is desirable to use a curing agent which does not undergo a self-condensation reaction with each other, particularly a curing agent which does not undergo a self-condensation reaction. In general, a resol-type phenol resin or an amino resin used as a curing agent of a polyester-based coating composition tends to easily undergo a self-condensation reaction between the curing agents, and to form a self-condensate as a hard and brittle region when a coating film is formed. Therefore, the coating film often hardens and workability is reduced. Further, since the reaction site (functional group) of the curing agent is consumed in the self-condensation reaction, it is needless to say that the curing agent must be used in an increased amount in order to obtain sufficient curability, and the efficiency is lowered. In addition, the presence of a large amount of a curing agent in the coating film may adversely affect the properties of the coating film, such as processability and corrosion resistance. On the other hand, when a curing agent which rarely or not undergoes a self-condensation reaction is used, the formation of a hard and brittle self-condensate can be suppressed. Further, the curing agent needs to be added in an amount that satisfies the minimum limit of the amount of carboxyl groups of the polyester resin, which makes the method effective. Further, the amount of the curing agent in the coating film can be reduced, and as a result, a coating film having excellent processability and corrosion resistance can be formed.

Therefore, as the curing agent having a functional group capable of reacting with the carboxyl group of the polyester resin, it is desirable to use a curing agent having a functional group which hardly causes a self-condensation reaction between the curing agents. As such a functional group, an epoxy group, a carbodiimide group, an oxazoline group, an amino group, a hydroxyl group (but excluding a hydroxyl group derived from a self-condensing methylol group contained in a resol-type phenol resin or an amino resin, and a hydroxyl group derived from a self-condensing silanol group contained in a silane coupling agent), and a β -hydroxyalkylamide group can be exemplified. Among them, it is desirable to use a curing agent having an epoxy group, an oxazoline group, and a β -hydroxyalkylamide group, particularly a curing agent having a β -hydroxyalkylamide group.

(curing agent comprising beta-hydroxyalkylamide group)

Examples of the curing agent having a β -hydroxyalkylamide group include, for example, β -hydroxyalkylamide compounds represented by the following general formula [ I ],

general formula [ I ];

[HO-CH(R₁)-CH₂-N(R₂)-CO-]_m–A-[-CO-N(R₂’)-CH₂-CH(R₁’)-OH]_n

wherein R is₁And R₁' is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms, R₂And R₂' is a hydrogen atom, an alkyl group having 1 to 5 carbon atoms or a compound represented by the formula [ II]Those represented by A is a polyvalent organic group, m is 1 or 2, and n is 0 to 2 (the sum of m and n is at least 2).

General formula [ II]；HO-CH(R₃)-CH₂-

Wherein R is₃Is a hydrogen atom or an alkyl group having 1 to 5 carbon atoms.

In the above general formula [ I ], a is desirably an aliphatic, alicyclic or aromatic hydrocarbon, more preferably an aliphatic, alicyclic or aromatic hydrocarbon having 2 to 20 carbon atoms, and further preferably an aliphatic hydrocarbon having 4 to 10 carbon atoms.

Further, in the above general formula [ I ], the sum of m and n is desirably 2, 3 or 4.

Among those represented by the above general formula [ I ], the β -hydroxyalkylamide group-containing curing agent (β -hydroxyalkylamide compound) used as a curing agent is specifically N, N' -tetrakis (2-hydroxyethyl) adipamide [ CAS: 6334-25-4, molecular weight: about 320, functional group equivalent: about 80g/eq, number of functional groups per molecule: 4, product: primid XL 552 produced by EMS-GRILTECH co. or N, N' -tetrakis (2-hydroxypropyl) adipamide [ CAS: 57843-53-5, molecular weight: about 376, functional group equivalent: about 95g/eq, number of functional groups per molecule: 4, product: primid QM1260 manufactured by EMS-GRILTECH co. Among them, N' -tetrakis (2-hydroxypropyl) adipamide is more desirably used from the viewpoint of curability and retort resistance. N, N '-tetrakis (2-hydroxypropyl) adipamide has higher reactivity with a polyester resin than N, N' -tetrakis (2-hydroxyethyl) adipamide, can be cured excellently, forms a dense crosslinked structure, and hardly causes whitening of a coating film during retort treatment, thus contributing to formation of a coating film having excellent retort treatment resistance.

(epoxy group-containing curing agent)

Examples of the curing agent having an epoxy group include polyepoxy compounds dissolved in water, such as polyethylene glycol diglycidyl ether, sorbitol tetraglycidyl ether, trimethylolpropane diglycidyl ether, trimethylolpropane triglycidyl ether, and polyglycerol polyglycidyl ether. Specific examples include denacols ex-314 produced by Nagase Chem-Tecks co. [ molecular weight: about 320, functional group (epoxy) equivalent: about 144g/eq ], EX-421[ molecular weight: about 440, functional group (epoxy) equivalent: about 159g/eq ] and EX-611[ molecular weight: about 630, functional group (epoxy) equivalent weight: about 167g/eq ].

(oxazoline group-containing curing agent)

As the oxazoline group-containing curing agent, a water-soluble polymer obtained by polymerizing a monomer composition containing an oxazoline derivative can be exemplified. As oxazoline derivatives, 2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline, 2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-5-methyl-2-oxazoline, and 2-isopropenyl-5-ethyl-2-oxazoline can be exemplified. As the monomer other than the oxazoline derivative contained in the oxazoline-derivative-containing monomer composition, any compound may be used without any limitation, provided that it is capable of copolymerizing with the oxazoline derivative and is inert to the oxazoline group. It is desirable that the structural unit derived from the oxazoline derivative is present in the oxazoline group-containing polymer in a proportion of 5 mass% or more. Specifically, EPOCROS WS-300[ number average molecular weight: about 40,000, functional group (oxazoline) equivalent: about 130g/eq ] and EPOCROS WS-700[ number average molecular weight: about 20,000, functional group (oxazoline) equivalent: about 220g/eq ].

The curing agent is used in an amount of preferably 1 to 10 parts by mass, more preferably 2 to 8 parts by mass, and further preferably 3 to 7 parts by mass, relative to 100 parts by mass of the polyester resin (solid content). When the amount of the curing agent is less than the above range, curability may not be sufficiently achieved. On the other hand, when the amount of the curing agent is more than the above range, it becomes economically disadvantageous. In addition, in the case where the amount of the functional group of the curing agent becomes much larger than the amount of the carboxyl group of the polyester resin, it becomes difficult for one molecule of the curing agent to react with two or more molecules of the polyester resin. As a result, in this case, crosslinking is formed rather poorly, and curability tends to decrease. In addition, the stability during long-term storage may be reduced.

The amount of the functional group (β -hydroxyalkylamide group) of the curing agent is in the range of 0.2 to 3.0 equivalents, more preferably in the range of 0.5 to 2.5 equivalents, relative to the amount of the carboxyl group of the polyester resin.

(aqueous medium)

The aqueous coating composition of the present invention comprises the above polyester resin, a curing agent 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 in the case of a conventional aqueous coating composition. When an organic solvent is used, it is desirable that the amount thereof is 1 to 45 mass%, particularly 5 to 30 mass% with respect to the entire aqueous medium in the aqueous coating composition. The solvent contained in an amount within the above range contributes to the improvement of the formation of the coating film.

As the organic solvent, those having amphiphilicity are desirably used, such as methanol, ethanol, isopropanol, n-butanol, ethylene glycol, methyl ethyl ketone, butyl cellosolve, carbitol, butyl carbitol, propylene glycol monopropyl ether, propylene glycol monobutyl ether, propylene glycol monomethyl ether, propylene glycol monobutyl ether, dipropylene glycol monomethyl ether, dipropylene glycol monobutyl ether, tripropylene glycol monomethyl ether, and 3-methyl-3-methoxybutanol.

(basic Compound)

It is desirable that the aqueous coating composition of the present invention contains a basic compound capable of neutralizing the carboxyl group of the polyester resin to render the polyester resin dispersible in water or soluble in water. As the basic compound, it is desirable to use a compound that volatilizes by firing when forming a coating film, that is, ammonia and/or an organic amine compound having a boiling point of 250 ℃ or lower.

Specifically, for example, alkylamines such as trimethylamine, triethylamine and n-butylamine; and alcamines such as 2-dimethylaminoethanol, diethanolamine, triethanolamine, aminomethylpropanol, and dimethylaminomethylpropanol. Polyamines such as ethylenediamine and diethylenetriamine may also be used. Further, amines having a branched alkyl group and heterocyclic amines can be preferably used. As the amine having a branched alkyl group, branched alkylamines having 3 to 6 carbon atoms, particularly 3 to 4 carbon atoms, such as isopropylamine, sec-butylamine, tert-butylamine, and isopentylamine, can be used. As the heterocyclic amine, saturated heterocyclic amines having a nitrogen atom such as pyrrolidine, piperidine, and morpholine can be used.

Among them, the present invention preferably uses trimethylamine or 2-dimethylaminoethanol in an amount of 0.5 to 1.5 equivalents with respect to the carboxyl group.

(curing catalyst)

The aqueous coating composition of the present invention may be blended with a known curing catalyst as necessary to promote a crosslinking reaction between the polyester resin and the curing agent.

As the curing catalyst, a known curing catalyst, that is, an acid catalyst such as p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenedisulfonic acid, or an amine-terminated compound thereof; organic metal compounds such as organic tin compounds, organic titanium compounds, organic zinc compounds, organic cobalt compounds, or organic aluminum compounds; metal oxides such as zinc oxide, titanium oxide, or aluminum oxide; or phosphorus-based compounds such as alkali metal hypophosphites, alkali metal phosphites, hypophosphorous acid, and alkylphosphinic acids.

The curing catalyst is used in an amount in the range of 0.1 to 5 parts by mass relative to 100 parts by mass of the solid components of the polyester resin. When the amount of the curing catalyst is less than the above range, the effect of promoting the curing reaction may not be sufficiently exerted in spite of the use of the curing catalyst. On the other hand, even when the amount of the curing catalyst is larger than the above range, further effects cannot be expected, but only economical disadvantages are brought about.

(additives)

According to the present invention, the aqueous coating composition contains tannic acid as an additive as necessary. When tannic acid is added, a chelate is formed by tannic acid and the metal substrate to be coated, which tends to improve the adhesion of the interface between the metal substrate and the coating film. Therefore, improved adhesion at the interface between the metal substrate and the coating film can be expected particularly under a high-temperature and high-humidity environment such as sterilization treatment. Tannic acid is added in an amount of preferably 0.01 to 10 parts by mass, particularly 0.1 to 5 parts by mass, relative to 100 parts by mass of the polyester resin.

(Lubricant)

According to the present invention, the aqueous coating composition contains a lubricant as necessary. The lubricant is added in an amount of preferably 0.1 to 10 parts by mass with respect to 100 parts by mass of the polyester resin.

The use of the lubricant helps prevent the coating film from being scratched when the can and lid are formed, and also helps improve the slidability of the coating film when formed.

As the lubricant which can be added to the aqueous coating composition of the present invention, fatty acid ester wax, silicon-based wax, fluorine-based wax, polyolefin wax such as polyethylene, lanolin-based wax, montan wax, microcrystalline wax, carnauba wax and silicon-based compound, which are esterification products of polyol compounds and fatty acids, can be exemplified. These lubricants may be used singly or as a mixture of two or more kinds.

(others)

In addition to the above components, the aqueous coating composition of the present invention may further comprise components heretofore used in coating compositions, such as leveling agents, pigments and defoaming agents, according to well-known formulations.

Further, the aqueous coating composition of the present invention may contain, in addition to the polyester resin, other water-dispersible or water-soluble resin components such as polyvinyl acetate, ethylene-vinyl acetate copolymer, polyolefin-based resin, epoxy resin, polyurethane resin, acrylic resin, phenol resin, melamine resin, polyvinyl chloride-based resin, polyvinyl chloride-vinyl acetate copolymer resin, polyvinyl alcohol, ethylene-vinyl alcohol copolymer, polyvinyl pyrrolidone, polyvinyl ethyl ether, polyacrylamide, acrylamide-based compound, polyethyleneimine, starch, gum arabic, and methyl cellulose, within a range not impairing the object of the present invention.

In the aqueous coating composition of the present invention, it is desirable that the polyester resin is contained in an amount of 5 to 55 mass% in terms of solid content. When the solid content of the resin is less than the above range, a suitable coating film amount cannot be secured, and coatability is deteriorated. On the other hand, when the solid content of the resin is more than the above range, workability and coatability may be deteriorated.

The aqueous coating composition of the present invention may be applied to a metal plate depending on a known coating method such as coating by using a roll coater, spray coating, or dip coating, and fired by a heating means such as a coil oven. The firing conditions of the aqueous coating composition of the present invention may vary depending on the kind of the polyester resin, the curing agent, the metal substrate and the amount of the coating. However, the aqueous coating composition may be heated and cured under conditions of, for example, a firing temperature of 150 ℃ to 320 ℃ and a firing time of 5 seconds to 30 minutes, which may be appropriately adjusted.

(method for producing coated Metal substrate)

The production method of the coated metal substrate comprises the following steps: a coating step of coating the aqueous coating composition of the present invention on a metal substrate, and a step of firing the aqueous coating composition coated on the metal substrate in the coating step at a temperature of more than 200 ℃ and 320 ℃ or less.

[ coating step ]

In the coating step, a metal substrate is coated with the above aqueous coating composition by a known coating method such as coating using a roll coater or spray coating.

Here, it is desirable that the aqueous coating composition is applied in such a manner that the thickness of the coating film after drying is less than 20 μm, particularly 0.5 to 15 μm.

[ firing step ]

In the firing step, it is desirable that the aqueous coating composition applied on the metal substrate is heated at a temperature higher than 200 ℃ and 320 ℃ or lower. The temperature referred to herein means the temperature of the atmosphere during firing (the furnace temperature of the oven).

When heating is performed at a temperature within the above range, it becomes possible to obtain sufficient curability in a short time even by using an aqueous coating composition containing a polyester resin having a relatively low acid value of 5mgKOH/g or more and less than 30mgKOH/g and a specific curing agent. When the firing temperature is lower than the above range, if the firing time is short, sufficient curability may not be obtained. On the other hand, when the firing temperature is higher than the above range, the polyester resin tends to be thermally decomposed due to excessive heating. The firing time should be 5 seconds or more, preferably 5 to 180 seconds, particularly 10 to 120 seconds. When the firing time is shorter than the above range, curing cannot be sufficiently performed. On the other hand, when the firing time is longer than the above range, disadvantages are generated in economy and productivity.

The firing method may be performed depending on a conventional heating means depending on the shape of the metal substrate and the like.

(coating of Metal substrate)

According to the present invention, a coated metal substrate such as a coated metal plate, a coated metal container, and a coated metal lid can be produced by forming a coating film on a metal substrate depending on the production method described above. As the metal substrate on which the coating film is to be formed, a metal plate, a metal container (metal can), and a metal lid can be exemplified.

[ coated Metal sheet ]

The aqueous coating composition of the present invention is applied to a metal sheet by a known coating method such as coating using a roll coater and spray coating. The aqueous coating composition is then fired by heating means such as an oven, to obtain a coated metal sheet.

As the metal plate, although not limited thereto, a hot-rolled steel plate, a cold-rolled steel plate, a hot-dipped zinc-plated steel plate, a zinc-plated steel plate, an alloy-plated steel plate, an aluminum-zinc alloy-plated steel plate, an aluminum plate, a tin-plated steel plate, a stainless steel plate, a copper-plated steel plate, a tin-free steel plate, a nickel-plated steel plate, an ultra-thin tin-plated steel plate, and a chromium-treated steel plate may be exemplified. These steel sheets may be surface-treated with, for example, phosphoric chromate (phosphoric chromate), or may be conversion-treated with zirconium, as required.

On the coating film of the above-described coated metal sheet, a thermoplastic resin film such as a polyester resin film may be further laminated as an organic resin coating layer to obtain a coated metal sheet further coated with an organic resin.

[ coated Metal Container ]

A coated metal container can be obtained by forming a coating film of the aqueous coating composition of the present invention on the inner surface and/or the outer surface of the metal container. Specifically, the aqueous coating composition of the present invention is excellent in processability and corrosion resistance. Therefore, it is desirable to form a coating film of the coating composition of the present invention on at least the inner surface of the metal container.

As the metal container on which the coating film is to be formed, all conventional metal containers such as three-piece cans and seamless cans (two-piece cans) having a seam on the side thereof can be exemplified.

The coated metal container can also be obtained by using the above-described coated metal sheet. In the case of a metal container to be formed by severe working such as a seamless can, a previously formed metal container may be spray coated with the aqueous coating composition of the present invention. A metal container such as a seamless can may be formed from a coated metal plate further coated with an organic resin, that is, the coated metal plate further coated with an organic resin.

[ coated Metal Cap ]

From the above-mentioned coated metal sheet coated with the aqueous coating composition of the present invention, a coated metal cap can be obtained depending on a conventional arbitrary cap production method. The painted metal lid can be used as, for example, a tab-type (stay-on-tab type) easy-open can lid and a full-open type easy-open can lid.