JPH07305022A - Aqueous resin composition for coating metal - Google Patents

Abstract

PURPOSE:To obtain a composition, useful for a water-based coating of steel plates for household appliances, automobiles, etc., and metal cans, comprising a specific aqueous copolymer and an amino resin and capable of forming a coating, excellent in water resistance, having high hardness after pressurized steam treatment and excellent in processability. CONSTITUTION:This aqueous resin composition for coating metals comprises (A) an aqueous copolymer containing a copolymer, comprising (i) 3-30wt.% micromonomer having a radically polymerizable group at one end of a copolymer comprising a hydroxyalkyl (meth)acrylate and other alpha, beta-ethylenically unsaturated monomers, (ii) 3-25wt.% N-alkoxymethyl(meth)acrylamide, (iii) 3-20wt.% alpha, beta-ethylenically unsaturated carboxylic acid, (iv) 5-20wt.% hydroxyalkyl (meth)acrylate and (v) 5-86wt.% other vinyl monomers and prepared by neutralizing carboxylic groups therein with a base and (B) an amino resin such as an alkyl etherified benzoguanamine or an alkyl etherified melamine.

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 Paints for metals, particularly paints 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, 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. A resin composition comprising an amino resin and an aqueous copolymer obtained by copolymerizing with a specific macromonomer having a carboxyl group in a copolymer having a specific structure, which is excellent in 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) N-alkoxymethyl (meth) acrylamide, (c) α, β-ethylenically unsaturated carboxylic acid, (d) hydroxyalkyl (meth) acrylate, and (e) a copolymer of vinyl monomers. The unit is 3 to 30% by weight based on the total amount of each unit based on the components (a) to (e), and the monomer unit based on the component (b) is 3 to 30% by weight. -25 wt%, component (c) monomer unit 3-20 wt%, component (d) monomer unit 5-20 wt%, and component (e) monomer unit 5-86 wt% And the carboxyl group in the copolymer is Aqueous copolymer formed by the sum, as well as metal-coated aqueous resin compositions comprising an amino resin.

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 (e) 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 units in the polymer that serves as the skeleton of the macromonomer is 5 to 40% by weight based on the total amount of the monomer units constituting (a). And more preferably 15 to 35% by weight. When the proportion of the hydroxyalkyl (meth) acrylate monomer unit is less than 5% by weight, the resulting water-based resin composition is inferior in water resistance.
If it exceeds 5% by weight, the processability of the coating film is poor. Examples of hydroxyalkyl (meth) acrylate include hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 3-hydroxypropyl (meth) acrylate, and hydroxybutyl (meth) 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).
3 to 30 based on the total amount of each unit based on (e)
%, And more preferably 5 to 20% by weight. 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 component (b), N-alkoxymethyl (meth) acrylamide, include N-methoxymethyl (meth) acrylamide and N- (n-butoxy) methyl (meth) acrylamide. It is effective in increasing the crosslinking density and the crosslinking rate of, especially in increasing the hardness of the coating film in hot water. The ratio of the monomer unit of the component (b) is 3 based on the total amount of each unit constituting the copolymer.
-25% by weight, more preferably 5-20% by weight
Is. When the amount of the copolymer is less than 3% by weight, the crosslink density of the coating film is not sufficient, while when it exceeds 25% by weight, the coating film becomes brittle.

Examples of the α, β-ethylenically unsaturated carboxylic acid as the component (c) 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 component (c) monomer unit 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 (d), various hydroxyalkyl (meth) acrylates mentioned as the components of the macromonomer can be used. The ratio of the monomer unit of the component (d), 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. ~ 20% by weight,
More preferably, it is 5 to 15% by weight. If the amount is less than 5% by weight, the water resistance of the obtained aqueous resin composition is poor, while on the other hand, 20
If it exceeds 5% by weight, the processability of the coating film is poor.

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

Other vinyl monomers of component (e) 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-
Examples thereof include aromatic vinyl monomers such as methylstyrene, and alkylamino esters of (meth) acrylic acid. These can be used alone or in combination of two or more.

The proportion of the monomer unit of the component (e) is such that the total amount of the constituent units of the carboxyl group-containing graft copolymer is 100.
The amount is the amount obtained by subtracting the proportion of the units based on the components (a) to (d) from the weight%, specifically 5 to 86% by weight, and preferably ~ 65% by weight.

Any of the above polymerizable components (a) to (e) can be polymerized at a polymerization rate of almost 100% by a known polymerization method, for example, a polymerization method as described in Examples described later. Therefore, in order to obtain the above-mentioned copolymer,
Polymerization of (a) to (e) may be performed in 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 molecular weight of the carboxyl group-containing copolymer in the present invention is 2 in terms of number average molecular weight.
000 to 15,000, more preferably 3,000
˜15,000.

Next, the carboxyl group in the carboxyl group-containing copolymer is neutralized with a base to convert the copolymer into a water-soluble or water-dispersible copolymer. 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

As the alkyl etherified melamine, alkyl etherified urea resin, alkyl etherified benzoguanamine and the like, multimers such as dimers and trimers can be used in addition to those monomers. Regarding the degree of alkyl etherification, those in which all of the active hydrogen atoms of the amino group are substituted with alkyl ether groups, such as tetramethylbenzoguanamine, or those in which a portion is substituted with alkyl ether groups, such as dimethyliminobenzoguanamine, are also available. Can be used.

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.

[0030]

EXAMPLES The present invention will be described more specifically below with reference to Examples and Comparative Examples.

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, a macromonomer M-1 (polystyrene-equivalent number average molecular weight by gel permeation chromatography: 3300) having a purity of the methacryloyl group end group of 99.7% was obtained. It was

(Synthesis of Carboxyl Group-Containing Graft Copolymer) The following monomers and 24.3 g of styrene were used for copolymerization. The following monomers were mixed and used as one liquid. Macromonomer M-1 25.0 g N-methoxymethyl acrylamide (hereinafter referred to as NMMA) 7.5 g Acrylic acid (hereinafter referred to as AA) 6.7 g 2-hydroxyethyl methacrylate (hereinafter referred to as HEMA) 8.7 g n-butyl acrylate (Hereinafter referred to as BA) 27.8g

1/3 of the mixture of the above components, 1 styrene
7.0 g, mercaptoethanol 0.16 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. The temperature was raised to 87 ° C.

Then, while adding the remaining 2/3 of the mixed solution, 7.3 g of styrene and 0.38 g of mercaptoethanol over one hour from the one dropping funnel, the other dropping funnel was simultaneously added. From ethylene glycol monobutyl ether 19.2 g, isopropyl alcohol 44.8 g and 2,2′-azobis (2
-Methylbutyronitrile) (hereinafter referred to as ABN-E)
A polymerization initiator solution consisting of 0.2 g was added dropwise over 3 hours. Thereafter, 4.5 g of ethylene glycol monobutyl ether, 10.5 g of isopropyl alcohol and 0.46 g of ABN-E were 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 is 9800.
Met.

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 8.3 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 Example 2, Comparative Reference Examples 1 and 2> Using the components shown in Table 1, the aqueous solution of the graft copolymer B (Reference Example 2) and the graft copolymer were prepared in the same manner as in Reference Example 1. Polymer C
Aqueous liquid (Comparative Reference Example 1) and an aqueous liquid of Graft Copolymer D (Comparative Reference Example 2) were obtained. In addition, in Table 1, NBM
A, HMA, BMA and St are each N- (n-
Butoxy) methyl acrylamide, 2-ethylhexyl methacrylate, n-butyl methacrylate and styrene.

[0039]

[Table 1]

<Examples 1 and 2, Comparative Examples 1 and 2> The aqueous copolymer solutions obtained in Reference Examples 1 and 2 and Comparative Reference Examples 1 and 2 and an amino resin were added to 100 parts by weight of the solid content of the copolymer. 40 parts by weight of the amino resin was blended to prepare an aqueous resin composition for coating. As the amino resin, Mycote 106 [dimethylimino-type benzoguanamine, manufactured by Mitsui Cyanamide Co., Ltd.] was used in each example.

A silicone-based leveling agent, ethylene glycol monobutyl ether and water were added to the above aqueous resin composition to obtain an aqueous resin composition having an organic solvent amount of 15% by weight and a solid content concentration of 30% by weight. 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.

The physical properties of the coating films shown in Table 2 were measured according to JIS K 5400. a. Impact resistance: DuPont impact test (1/2 inch impact core, load 500g). b. Adhesion: A method according to the cross-cut tape method test method (evaluated by the coating film residual rate after tape peeling). c. Hardness: Testing machine method for pencil scratch test. d. Hardness in hot water: A tester method for a pencil scratch test in hot water after dipping the test plate in hot water at 80 ° C. for 3 minutes.

[0044]

[Table 2]

[0045]

According to the aqueous resin composition for metal coating of the present invention, a coating film excellent in hardness, water resistance and processability is obtained by the synergistic action of the aqueous copolymer used as the main component and the amino resin. It can be formed on a metal substrate. In particular, due to the synergistic action of the crosslinkable functional group and the macromonomer introduced into the aqueous copolymer, a tough coating film having a high crosslink density is formed, so that it is excellent in hot water hardness after steam treatment. A high performance coating film is obtained, and it is most suitable as a water-based paint for metals with high water resistance and high steam resistance. As a typical example of the coating object,
There are metal cans for packaging drinking water or processed foods, steel sheets used for home appliances, automobiles, and the like.

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) N. A copolymer of alkoxymethyl (meth) acrylamide, (c) α, β-ethylenically unsaturated carboxylic acid, (d) hydroxyalkyl (meth) acrylate and (e) other vinyl monomers, , Based on the total amount of each unit based on the above components (a) to (e), the unit based on the component (a) is 3 to 30% by weight, and the monomer unit on the component (b) is 3 to 25% by weight. 3 to 20% by weight of the component (c) monomer unit, 5 to 20% by weight of the component (d) monomer unit, and 5% of the component (e) monomer unit.
A water-based resin composition for metal coating, which comprises an amino resin and an aqueous copolymer in which the carboxyl group in the copolymer is neutralized with a base.