JPH06256707A - 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 resin 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 (meth)acrylonitrile/styrene copolymer, (B) 3-20wt.% of an alpha,beta-ethylenic unsaturated carboxylic acid, (C) 5-40wt.% of a hydroxyalkyl (meth)acrylic acid hydroxyalkyl and (D) 10-89wt.% of a vinyl monomer based on 100wt.% of the components A+B+C+D, 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 an aqueous paint for metal cans.

[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-mentioned problems, the present inventors have found that a monomer mixture consisting of a monomer having a carboxyl group and other monomers and the following macro A resin composition composed of an amino resin and an aqueous copolymer obtained by copolymerizing a monomer with a carboxyl group in a copolymer having a specific constitution and a base is excellent in water resistance, and has a temperature of 120 ° C. or higher. The inventors have found that a coating film having excellent hardness and workability can be formed even after treatment with pressure steam, and completed the present invention.

That is, the present invention provides (a) a macromonomer having a radically polymerizable group at one end of a (meth) acrylonitrile / styrene copolymer, (b) an α, β-ethylenically unsaturated carboxylic acid, c) a copolymer of hydroxyalkyl (meth) acrylate and (d) another vinyl monomer, based on the total amount of each unit based on the above components (a) to (d), The unit based on the component (a) is 3 to 30% by weight, and the monomer unit on the component (b) is 3 to 2%.
0% by weight, 5 to 40% by weight of the component (c) monomer unit, and 10 to 89% by weight of the component (d) monomer unit, the carboxyl group in the copolymer is neutralized with a base. Is an aqueous resin composition for metal coating comprising an aqueous copolymer and an amino resin.

The present invention will be described in more detail below. In addition, in the following description, the copolymer before neutralization with a base is referred to as a carboxyl group-containing copolymer.

As described above, the component (a) in the present invention is a macromonomer having a radical-polymerizable group at one terminal of the (meth) acrylonitrile / styrene copolymer, and the radical-polymerizable group is ( The (meth) acryloyl group is preferable, and the number average molecular weight of the macromonomer is preferably 2000 to 30,000. 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 a polystyrene equivalent number average molecular weight measured by gel permeation chromatography.

As the (meth) acrylonitrile / styrene type copolymer which is the skeleton of the macromonomer, an acrylonitrile / styrene binary copolymer or a methacrylonitrile / styrene binary copolymer is preferable, but those monomers are preferable. In addition to the units, other radically polymerizable monomer units,
A (meth) acrylonitrile / styrene copolymer containing up to 20% by weight based on the total amount of constituent monomer units can also be used. (Meth) acrylonitrile / styrene 2
When the terpolymer is a skeleton of a macromonomer, the preferable ratio of acrylonitrile or methacrylonitrile and styrene in the binary copolymer is (meth) acrylonitrile / styrene = 5 to 50% by weight / 50 to 95% by weight. Is.

Radical polymerizable monomers that can be used other than (meth) acrylonitrile and styrene include α-
Styrene derivatives such as methylstyrene and p-methylstyrene, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, cyclohexyl (meth) acrylate, etc. Examples thereof include (meth) acrylic acid ester.

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

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.

Examples of the hydroxyalkyl (meth) acrylate as the component (c) include hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate,
Examples thereof include 3-hydroxypropyl (meth) acrylate and hydroxybutyl (meth) acrylate. In the case of a copolymer in which the proportion of the monomer unit of component (c) is less than 5% by weight, the coating film has a low crosslinking density and poor water resistance.
If the amount of the copolymer exceeds 5% by weight, the processability of the coating film is poor.

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

Among the above-mentioned 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 the component (d) is an amount obtained by subtracting the proportion of the units based on the components (a) to (c) from the total amount of the constituent units of the carboxyl group-containing copolymer of 100% by weight. And, specifically, 10 to 89% by weight.
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 synthesis method of 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, the amino resin used in combination with the above aqueous copolymer includes alkyl etherified melamine such as methyl etherified melamine and butyl etherified melamine; alkyl etherified urea resin; methyl etherified benzoguanamine and ethyl etherified resin. 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. Alkyl etherified benzoguanamine is preferable in terms of excellent water resistance.

With respect to the above-mentioned alkyl etherified melamine, alkyl etherified urea resin, alkyl etherified benzoguanamine and the like, dimers, trimers and other multimers can be used in addition to their 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, since the aqueous copolymer used as the main component has good compatibility with any of the above-mentioned amino resins, any of the amino resins is used. Even if a 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-mentioned 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 macromonomers AN-6 and AA-6 used in each example are both manufactured by Toagosei Chemical Industry Co., Ltd., and AN-6 is a poly (acrylonitrile / styrene) type macromonomer (polymerizable group; Methacryloyl group, average molecular weight: 6000, ratio of acrylonitrile monomer unit to styrene monomer unit = 25 wt% to 75 wt%), and AA-6 is a polymethylmethacrylate type macromonomer (polymerizable group; methacryloyl group). ,
The number average molecular weight is 6000).

Reference Example 1 (Synthesis of Aqueous Copolymer) The following monomers and styrene 1
Copolymerization was carried out using 3.5 g. The following monomers were mixed and used as one liquid. Poly (acrylonitrile / styrene) type macromonomer: AN-6-10 g n-butyl methacrylate (hereinafter referred to as nBMA) ──── 11.6 g n-butyl acrylate (hereinafter referred to as nBA) ─────46 2.8 g acrylic acid (hereinafter referred to as AA) 5.2 g 2-hydroxyethyl methacrylate (hereinafter referred to as HEMA) -12.9 g

1/3 of the mixture of the above components, styrene 9.
5 g, mercaptoethanol 0.18 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, 4.1 g of styrene and 0.27 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 copolymer. The number average molecular weight of the copolymer was 8,000 in terms of polystyrene equivalent by gel permeation chromatography.

The obtained carboxyl group-containing copolymer solution was heated to 40 ° C., isopropyl alcohol was distilled off under reduced pressure, and then dimethylethanolamine (hereinafter referred to as DME).
6.4 g of A) and 70 g of distilled water were added for neutralization. By the above operation, a translucent viscous aqueous liquid of 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 2 to 5> Using the components shown in Table 1, polymerization was carried out in the same manner as in Reference Example 1 and then neutralized with DMEA to obtain an aqueous solution of a copolymer.

[0034]

[Table 1]

[0035]

Examples 1 to 5 The copolymer aqueous solutions obtained in Reference Examples 1 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 content of 15% by weight and a solid content 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.

[0038]

[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.

[0041]

[Table 3]

[0042]

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 the material. Typical examples of the object to be coated are pre-coated steel plates, home appliances such as air conditioners and electric washing machines, and metal cans for drinking water or processed foods.

Claims (1)

[Claims] 1. A macromonomer having a radical-polymerizable group at one terminal of (a) a (meth) acrylonitrile / styrene copolymer, (b) an α, β-ethylenically unsaturated carboxylic acid, and (c) (meth). ) A copolymer comprising hydroxyalkyl acrylate and (d) another vinyl monomer, wherein the component (a) is based on the total amount of each unit based on the above components (a) to (d). 3 to 30 based on
% By weight, 3 to 20% by weight of the component (b) monomer unit, 5 to 40% by weight of the component (c) monomer unit, and 10 to 89% by weight of the component (d) monomer unit. An aqueous resin composition for metal coating, comprising an amino resin and an aqueous copolymer obtained by neutralizing a carboxyl group in the copolymer with a base.