JPH06256708A - Aqueous resin composition for metal coating - Google Patents

Abstract

PURPOSE:To obtain an aqueous resin composition useful for coating metal cans for processed foods, providing a coating film having excellent hardness, steam resistance, water resistance, luster, etc., comprising a specific aqueous polymer and an amino resin. CONSTITUTION:This resins composition comprises an aqueous polymer prepared by neutralizing a carboxyl group of a copolymer, consisting of (A) 3-30wt.% of a macromonomer containing a radically polymerizable group at one end of a polymer composed of a hydroxyalkyl (meth)acrylate and a alpha,beta-ethylenic unsaturated monomer, (B) 3-20wt.% of an alpha,beta-ethylenic unsaturated carboxylic acid, (C) 5-40wt.% of a hydroxyalkyl (meth)acrylate and (D) 10-89wt.% of another vinyl monomer based on 100wt.% of the components A+B+C+D, having 20-150 KOHmg/g hydroxyl number, with a base such as an organic amine or ammonium and (E) an amino resin such as an alkyl etherified melamine resin or an alkyl etherified benzogunamine resin.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aqueous resin composition for metal coating, and the aqueous resin composition for metal coating of the present invention is excellent in water resistance, particularly steam resistance. It forms a coating film having high hardness and excellent workability even after being subjected to pressure steam treatment, and is particularly useful as a water-based paint for steel plates and metal cans used in home appliances, automobiles and the like.

[0002]

2. Description of the Related Art Metal coatings, particularly coatings for metal cans for packaging soft drinks or processed foods, are required to have physical properties such as water resistance under heating and processability. As the paint for use, organic solvent type paints such as epoxy / amino resins, acrylic / amino resins and polyester / amino resins that form excellent coating films by heat curing have been used. However, these solvent-based paints have problems such as environmental pollution and waste of resources because a large amount of organic solvent evaporates into the atmosphere during coating.

For this reason, it has been considered to use a water-based paint in which water is used as a medium instead of the above solvent-based paint. However, the conventional water-based paint is inferior in water resistance and is not suitable for hot water treatment such as boiling water. Even if it withstands, it is the current situation that it cannot be put into practical use because it undergoes a treatment (retort treatment) with a pressure steam of 120 ° C. or more, because the physical properties of the coating film are significantly deteriorated.

There are a number of proposals aimed at water-based paints that withstand the above-mentioned pressure steam treatment, for example, Japanese Patent Laid-Open No. 3-72.
No. 577 discloses an aqueous acrylic resin obtained by neutralizing a copolymer having (a) an aromatic vinyl monomer and an alkyl (meth) acrylate as main components and having a carboxyl group and a hydroxyl group in the molecule with a base, ( B) A water-based coating composition comprising a hydroxyl group-containing polyol and (C) amino resin has been proposed, but the physical properties of the resulting coating film have not yet reached a level that is practically satisfactory.

[0005]

Means for Solving the Problems As a result of intensive studies for solving the above problems, the present inventors have found that a monomer mixture containing a monomer having a carboxyl group and another monomer, and a hydroxyl group. Obtained by copolymerizing the following macromonomer having, a resin composition comprising an amino resin and an aqueous copolymer obtained by neutralizing a carboxyl group in a copolymer having a specific constitution with a base, excellent water resistance, The inventors have found that a coating film having excellent hardness and workability can be formed even after treatment with pressure steam of 120 ° C. or higher, and have completed the present invention.

That is, the present invention relates to (a) a macromonomer having a radically polymerizable group at one terminal of a polymer comprising hydroxyalkyl (meth) acrylate and other α, β-ethylenically unsaturated monomers, (B) a copolymer composed of α, β-ethylenically unsaturated carboxylic acid, (c) hydroxyalkyl (meth) acrylate and (d) another vinyl monomer, wherein the components (a) to (( Based on the total amount of each unit based on d), the unit based on the component (a) is 3 to 30% by weight, the monomer unit on the component (b) is 3 to 20% by weight, and the component (c) is the monomer. The carboxyl group in the copolymer having a unit of 5 to 40% by weight, a monomer unit of component (d) of 10 to 91% by weight, and a hydroxyl value of 20 to 150 KOH mg / g is neutralized with a base. Made of water-based copolymer and amino resin It is a coating aqueous resin composition.

The present invention will be described in more detail below. The above-mentioned copolymer before neutralization with a base in the present invention has a component (a), that is, a polymer unit based on a macromonomer as a branch component, and a polymer including other components (b) to (d) as a trunk component. The graft copolymer before neutralization is referred to as a carboxyl group-containing graft copolymer.

As mentioned above, the component (a) in the present invention is a radically polymerizable group at one end of a polymer composed of hydroxyalkyl (meth) acrylate and other α, β-ethylenically unsaturated monomers. The (meth) acryloyl group is preferable as the radically polymerizable group, and the preferable molecular weight of the macromonomer is 2,000 to 30,000 in terms of number average molecular weight. When the number average molecular weight is less than 2000, either the workability or the hardness of the coating film after the pressure steam treatment is likely to be impaired. On the other hand, when it exceeds 30,000, a large amount of unreacted macromonomer remains in the copolymer, resulting in poor physical properties of the coating film. The number average molecular weight in the present invention is gel permeation
It is the number average molecular weight in terms of polystyrene by chromatography.

The preferred proportion of the hydroxyalkyl (meth) acrylate monomer unit in the polymer forming the skeleton of the macromonomer is 5 to 40% by weight based on the total amount of the constituent monomer units, and more preferred Is 15 to 35% by weight. When the proportion of the hydroxyalkyl (meth) acrylate monomer unit is less than 5% by weight, the resulting aqueous resin composition is inferior in water resistance, while when it exceeds 40% by weight, the processability of the coating film is inferior. Examples of hydroxyalkyl (meth) acrylate include hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 3-hydroxypropyl (meth) acrylate, and (meth)
Examples thereof include hydroxybutyl acrylate.

The other α, which forms the polymer backbone of the macromonomer with the hydroxyalkyl (meth) acrylate,
Examples of β-ethylenically unsaturated monomers include methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, and cyclohexyl (meth) acrylate. Examples thereof include styrene derivatives such as (meth) acrylic acid alkyl ester, styrene, α-methylstyrene and p-methylstyrene, nitrile group-containing vinyl monomers such as (meth) acrylonitrile, and (meth) acrylic acid alkylamino ester. .

From the viewpoint of water resistance and weather resistance of the coating film obtained from the resulting aqueous resin composition, alkyl (meth) acrylate having an alkyl group having 4 to 8 carbon atoms is preferable, and the alkyl (meth) acrylate in the polymer skeleton of the macromonomer is The preferred amount of the alkyl (meth) acrylate monomer unit is 50 to 95 based on the total amount of all the monomer units constituting the polymer skeleton.
% By weight.

The proportion of the units based on the above component (a) in the carboxyl group-containing graft copolymer is, as described above, each unit constituting the copolymer, that is, the components (a) to (a).
It is 3 to 30% by weight, and more preferably 5 to 20% by weight, based on the total amount of the units based on (d).
In the case of a copolymer in which the proportion of the component (a) unit is less than 3% by weight, the coating film made of the aqueous copolymer obtained from the copolymer is inferior in water resistance, while in the case of a copolymer exceeding 30% by weight, It is difficult to achieve both water resistance and water resistance.

Examples of the α, β-ethylenically unsaturated carboxylic acid as the component (b) include acrylic acid, methacrylic acid, crotonic acid, maleic acid, maleic anhydride, fumaric acid and itaconic acid, and preferably, Acrylic acid and methacrylic acid. The ratio of the monomer unit of component (b) is 3
If the content of the copolymer is less than 20% by weight, it is difficult to make it water-soluble, while if it exceeds 20% by weight, the coating film has poor water resistance.

As the hydroxyalkyl (meth) acrylate as the component (c), various hydroxyalkyl (meth) acrylates listed as the components of the macromonomer can be used. The ratio of the monomer unit of the component (c), that is, the ratio of the hydroxyalkyl (meth) acrylate monomer unit in the trunk component in the graft copolymer is, as described above, 5 based on the total amount of all the constituent units. ~ 40% by weight,
More preferably, it is 10 to 30% by weight.

The carboxyl group-containing graft copolymer in the present invention contains a human roxyl group derived from the above component (c) and a hydroxyl group derived from a hydroxyalkyl (meth) acrylate monomer unit in the macromonomer. However, the total amount of hydroxyl groups in the copolymer is 20 to 150 KOH mg / g, preferably 40 to 100 KOH mg / g. If the total amount of hydroxyl groups in the graft copolymer is less than 20 KOH mg / g, the processability will be poor, while if it exceeds 150 KOH mg / g, the water resistance will be poor.

Other vinyl monomers of component (d) include methyl (meth) acrylate, ethyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, butyl (meth) acrylate, and (meth) acrylate. Alkyl (meth) acrylate such as cyclohexyl acrylate, styrene, α-
Aromatic vinyl monomers such as methylstyrene, alkylaminoesters of (meth) acrylic acid, N-methoxymethyl (meth) acrylamide, N- (n-butoxy) methyl (meth) acrylamide and the like are mentioned, and these are used alone. Alternatively, two or more kinds can be used in combination.

Among the above other vinyl monomers, a monomer having a functional group reactive with a hydroxyl group such as N-methoxymethyl (meth) acrylamide or N- (n-butoxy) methyl (meth) acrylamide is used. Then, the crosslink density of the coating film can be increased.

The proportion of the monomer unit of component (d) is such that the total amount of the constituent units of the carboxyl group-containing graft copolymer is 100.
It is the amount obtained by subtracting the ratio of the units based on the components (a) to (c) from the weight%, specifically 10 to 89% by weight.
Is. A preferable ratio of the unit based on the monomer having a functional group reactive with a hydroxyl group such as N-methoxymethyl (meth) acrylamide and N- (n-butoxy) methyl (meth) acrylamide is 0 to 30% by weight. is there.

Any of the above polymerizable components (a) to (d) can be polymerized at a polymerization rate of about 100% by a known polymerization method, for example, a polymerization method as described in Examples below. Therefore, in order to obtain the above-mentioned copolymer,
Polymerization of (a) to (d) may be carried out at a ratio corresponding to the constitution.

The outline of the method for synthesizing the carboxyl group-containing copolymer is as follows. That is, isopropyl alcohol, n-butanol, isobutyl alcohol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether or the like is used as a polymerization solvent in the presence of a radical polymerization initiator such as azobisisobutyronitrile at 60 to 150. The polymerizable component may be radically polymerized at ℃.

In the above polymerization, the molecular weight of the resulting copolymer can be adjusted by using an appropriate amount of a chain transfer agent such as n-dodecyl mercaptan, mercaptoacetic acid, thiomalic acid, mercaptoethanol and mercaptopropionic acid. . The preferred carboxyl group-containing copolymer of the present invention has a number average molecular weight of 20.
00-15000, and more preferably 3000-
It is 15,000.

Next, the carboxyl group in the carboxyl group-containing copolymer is neutralized with a base. In the neutralization, all the carboxyl groups in the copolymer may be neutralized, but if necessary, some of the carboxyl groups may be neutralized. As a specific operation for neutralization, it is convenient to add an aqueous solution of a base to an organic solvent solution of the copolymer obtained by the above-mentioned polymerization for neutralization.

As the base, organic amines such as monoethanolamine, dimethylamine, diethylamine, triethylamine, triethanolamine, diethylethanolamine and dimethylethanolamine, and ammonium can be used.

From the aqueous copolymer solution obtained by the above operation, the organic solvent used as a polymerization solvent is distilled off under reduced pressure, and water is added to the solution to obtain a solid content concentration of 20 to 70 suitable for metal coating. A weight percent copolymer aqueous solution or dispersion is obtained.

In the present invention, as the amino resin used in combination with the above-mentioned aqueous copolymer, alkyl etherified melamine such as methyl etherified melamine and butyl etherified melamine; alkyl etherified urea resin; methyl etherified benzoguanamine and ethyl etherified Examples thereof include alkyl etherified benzoguanamine such as benzoguanamine and the like, and these can be used alone or in combination of two or more kinds. The amino resin is preferably selected according to the application of the aqueous resin composition, and for applications requiring high water resistance such as metal cans for food packaging, alkyl etherified benzoguanamine is suitable and has weather resistance. Alkyl etherified melamine is suitable for automobile-related applications where

With respect to the alkyl etherified melamine, alkyl etherified urea resin, alkyl etherified benzoguanamine and the like, dimers, trimers and other multimers can be used in addition to the monomers, and the degree of alkyl etherification Can be any of those in which all or some of the active hydrogen atoms of the amino group are substituted with alkyl ether groups.

In the aqueous resin composition for metal coating of the present invention, the above-mentioned aqueous copolymer used as the main component has good compatibility with any of the above-mentioned amino resins. Even if an amino resin is selected, a uniform aqueous resin composition can be obtained.

The preferred amount of the amino resin used is 10 to 80 parts by weight (amino resin solid content) per 100 parts by weight of the solid content of the aqueous copolymer. In the metal coating aqueous resin composition of the present invention, various coating additives may be blended in addition to the above aqueous copolymer and amino resin. As an example of the additive, a film of ethylene glycol monobutyl ether or the like may be formed. Examples include auxiliary agents.

A resin composition obtained by mixing an aqueous solution or dispersion of an aqueous copolymer and an amino resin in the above proportions,
A cured coating film can be formed by applying it to a metal base material such as aluminum, stainless steel, a zinc-treated steel sheet and other various treated steel sheets and a tin plate, and then heat-curing it at 150 ° C. to 220 ° C. for about 3 to 20 minutes.

Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples. The macromonomer AA-6 used in the following specific examples is Toagosei Chemical Industry Co., Ltd.
Made of polymethylmethacrylate type macromonomer (polymerizable group; methacryloyl group, number average molecular weight; 6000)
Is.

Reference Example 1 (Synthesis of Macromonomer M-1) Cyclohexyl methacrylate 4.9 g, 2-ethylhexyl methacrylate 5
5.0 g and 2-hydroxyethyl methacrylate 2
5.1 g of a mixed solution was prepared, and in a glass flask equipped with a stirrer, two dropping funnels, a gas introduction tube and a thermometer, 1/3 of the above mixed solution, 10.5 g of styrene, and mercaptopropion as a chain transfer agent. 0.9 g of acid and 30 g of toluene were charged and the temperature was raised to 90 ° C. After that, the remaining 2/3 of the mixed solution, 4.5 g of styrene and 2.1 g of mercaptopropionic acid were added dropwise from one dropping funnel over 3 hours, and at the same time, dissolved in 35.0 g of toluene from the other funnel. Azobisisobutyronitrile (0.3 g) was added dropwise over 3 hours.

Thereafter, 0.3 g of azobisisobutyronitrile dissolved in 35.0 g of toluene was added dropwise over 2 hours, and the reaction was continued for another 2 hours to obtain a polymer having a carboxyl group at the terminal. . Next, 0.04 g of methoxybenzoquinone, 2.7 g of tetrabutylammonium bromide and 2.2 g of glycidyl methacrylate were added to the solution of the polymer having a terminal carboxyl group, and the mixture was blown with air at 90 ° C. for 5 hours. It was made to react. Based on the degree of decrease in the acid value of the polymer, the purity of the end group of the methacryloyl group is 99.7%, and the macromonomer M
1 (polystyrene-equivalent number average molecular weight by gel permeation chromatography: 3300) was obtained.

Reference Example 2 (Synthesis of Macromonomer M-2) Except that the total amount of the chain transfer agent mercaptopropionic acid used is 0.79 g,
The procedure of Reference Example 1 was repeated to obtain a macromonomer M-2 having a methacryloyl group terminal group purity of 99.6% and a number average molecular weight of 6000.

Reference Example 3 (Synthesis of Carboxyl Group-Containing Graft Copolymer) The following monomers and 11.0 g of styrene were used for copolymerization.
The following monomers were mixed and used as one liquid. Macromonomer M-1 ────── 10 g Methyl methacrylate (hereinafter referred to as MMA) ────── 1.8 g Ethyl acrylate (hereinafter referred to as EA) ───── 21.0 g n-Butyl acrylate ( Hereinafter referred to as nBA) ───── 38.0 g Acrylic acid (hereinafter referred to as AA) ───── 5.2 g 2-Hydroxyethyl methacrylate (hereinafter referred to as HEMA) ──13.0 g

1/3 of the mixture of the above components, styrene 7.
7 g, mercaptoethanol 0.38 g, ethylene glycol monobutyl ether 6.3 g and isopropyl alcohol 14.7 g were charged into a glass flask equipped with a stirrer, a reflux condenser, two dropping funnels, a gas introduction pipe and a thermometer, 87 The temperature was raised to ° C.

Then, while the remaining 2/3 of the mixed solution, 3.3 g of styrene and 0.38 g of mercaptoethanol were added dropwise from one dropping funnel over 3 hours, at the same time, from the other dropping funnel. , Ethylene glycol monobutyl ether 19.2 g, isopropyl alcohol 44.8 g and 2,2′-azobis (2-methylbutyronitrile) (hereinafter referred to as ABN-E) 0.2 g over a period of 3 hours. Dropped. Then, further ethylene glycol monobutyl ether 4.
5 g, isopropyl alcohol 10.5 g and ABN-
0.46 g of E was added dropwise over 2 hours. Then, stirring was continued for 2 hours to synthesize a carboxyl group-containing graft copolymer. The number average molecular weight of the copolymer was 5,500.

The obtained graft copolymer solution was heated to 40 ° C., isopropyl alcohol was distilled off under reduced pressure, and then dimethylethanolamine (hereinafter referred to as DMEA).
Neutralization was performed by adding 6.4 g and 70 g of distilled water. By the above operation, an aqueous liquid of the graft copolymer A having a solid content concentration of 50% by weight and containing 15% by weight of ethylene glycol monobutyl ether was obtained.

<Reference Examples 4 to 7> Using components shown in Table 1, polymerization was carried out in the same manner as in Reference Example 1, followed by neutralization with DMEA to prepare an aqueous solution of the graft copolymer B (reference: Example 4) -Aqueous liquid of graft copolymer E (Reference Example 7) was obtained. In Table 1, St represents styrene and nBMA represents n-butyl methacrylate.

[0039]

[Table 1]

[0040]

Examples 1 to 5 The copolymer aqueous solutions obtained in Reference Examples 3 to 5 and Cymel 1123 [benzoguanamine resin, manufactured by Mitsui Cyanamide Co., Ltd.] as amino resin or Cymel 303 [hexamethoxymethylol melamine, Mitsui] Cyanamide Co., Ltd.] was used in the combination shown in Table 2 to add 40 parts by weight of amino resin to 100 parts by weight of the solid content of the copolymer to obtain an aqueous resin composition. In the hardener column in Table 2, H1 is Cymel 112.
3 and H2 represents Cymel 303, and the same applies to Tables 3 and 4.

A silicone-based leveling agent, ethylene glycol monobutyl ether and water were added to the above aqueous resin composition to give an organic solvent amount of 15% by weight and a solid content concentration of 3%.
A 0% by weight aqueous resin composition was prepared. This resin composition was applied onto an aluminum plate with a bar coater so as to have a film thickness of 5 to 6 μm, and cured by heating at 200 ° C. for 10 minutes.

Various physical properties were evaluated for the obtained cured coating film and the coating film after pressure steam treatment by a pressure cooker device (standing in steam at 130 ° C. for 30 minutes). The results are shown in Table 2.

[0043]

[Table 2]

The physical properties of the coating films in Table 2 are all JI
It was measured by the measuring method specified in SK 5400. a. Impact resistance: DuPont type impact test (1/2 inch impact core, load 500g). b. Water resistance: A method based on the boiling water resistance test (sample immersion time is 60 minutes). c. Adhesion ──── A method according to the cross-cut tape method test method (evaluated by the coating film residual rate after tape peeling). d. Hardness ──── Test method for pencil scratch test.

<Comparative Examples 1 to 4> Using the aqueous copolymers and amino resins shown in Table 3, an aqueous resin composition was produced in the same manner as in Example 1 and the coating film was evaluated. It was
The results are shown in Table 3. As compared with the examples in Table 2, the aqueous copolymer D had poor compatibility with the benzoguanamine-based curing agent, and a uniform composition could not be obtained. Further, with the combination of the aqueous copolymer D and the melamine-based curing agent, a coating film having a high level of water resistance could not be obtained. On the other hand, with the aqueous copolymer E, the physical properties of the coating film after the pressure steam were remarkably inferior.

[0046]

[Table 3]

[0047]

According to the aqueous resin composition for metal coating of the present invention, a coating film excellent in hardness, water resistance, processability and gloss is a metal-based film due to the characteristics of the aqueous copolymer used as the main component. It can be formed on a material, and the composition is most suitable as a water-based coating material for highly water-resistant precoated steel sheet. Typical examples of the object to be coated are steel plates used for home appliances, automobiles, etc., and metal cans for packaging drinking water or processed foods.

Claims (1)

[Claims] 1. A macromonomer having a radically polymerizable group at one end of a polymer comprising (a) a hydroxyalkyl (meth) acrylate and other α, β-ethylenically unsaturated monomers, and (b) α. , Β-ethylenically unsaturated carboxylic acid, (c) hydroxyalkyl (meth) acrylate and (d) a copolymer of vinyl monomers, which is based on the above components (a) to (d) Based on the total amount of each unit, the unit based on the component (a) is 3 to 30% by weight, the monomer unit of the component (b) is 3 to 20% by weight, and the monomer unit of the component (c) is 5 to 5% by weight. 40% by weight, the component (d) monomer unit is 10 to 89% by weight, and the hydroxyl value is 2
An aqueous resin composition for metal coating comprising an amino resin and an aqueous copolymer obtained by neutralizing a carboxyl group in the copolymer of 0 to 150 KOH mg / g with a base.