JPH07108962B2 - Aqueous coating composition for outer surface of can - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Purpose of the Invention] (Field of Industrial Application) The present invention relates to a coating material for an outer surface of a can containing an aqueous acrylic resin, a polyether polyol and an amine-formaldehyde condensate, and particularly water resistance and processability. The present invention relates to a water-based coating composition that is excellent in water resistance and can reduce the content of a solvent.

The outer surface of beverage cans containing soft drinks, etc. and food cans for packaging foods prevents corrosion of the can material and enhances aesthetic product value.
Moreover, it is formed by coating with a coating film that can withstand the heat treatment step during food sterilization. Traditionally these paints are epoxy /
A method of coating an organic solvent solution of an amino resin, an acrylic / amino resin, a polyester / amino resin, or the like with a roll coater and baking and curing in a gas oven is used. However, these paints cause a large amount of solvent volatilization at the time of baking, causing air pollution, which is not preferable from the viewpoint of resource saving. Therefore, the advent of water-based paints that can solve these problems is desired.

There are two types of known water-based paints: water dispersible and water soluble.
Many water-dispersible resins are usually synthesized by emulsion polymerization using a surfactant, and the surfactant used has the drawback that it remains in the coating film even after the coating film is formed, reducing the water resistance. It was On the other hand, there is also a method of synthesizing a resin containing an acid component (carboxyl group) by synthesizing in an organic solvent system without using a surfactant, and neutralizing with a volatile base to make the dispersion or water-soluble. The paint has an acid value in the basic resin structure.
Since it requires an acid content of 20 or more, it has the drawback of inferior performance such as water resistance and alkali resistance. In order to improve these drawbacks, a method of mixing a large amount of an aqueous amino resin such as hexamethoxymethylmelamine and methylated benzoguanamine has been used, but the workability is greatly reduced and water resistance and workability are balanced. Was difficult.

On the other hand, the conventional water-soluble paint contains 10% by weight or more of an organic solvent in order to improve the storage stability of the paint and the leveling in the coating film formation, and it is still in view of air pollution and resource saving due to solvent volatilization during baking. It was not enough.

(Problems to be Solved by the Invention) The present invention has been made in view of the above circumstances, and an object thereof is to have water resistance that can withstand the heat treatment process of beverage cans and food cans. The present invention provides a water-based coating composition for an outer surface of a can, which forms a coating film having excellent processability that can be processed into various can forms and has a low organic solvent content.

[Structure of Invention]

(Means for Solving Problems) That is, the present invention comprises: a) Copolymerization with α / β monoethylenically unsaturated carboxylic acid, hydroxyalkyl acrylate or methacrylate having 1 to 5 carbon atoms, and the above monomer Aqueous acrylic resin in which a copolymer of various vinyl monomers is dissolved or dispersed in water with a volatile base 25 to 58% by weight b) Polyether polyol having a molecular weight of at least 300
5 to 19% by weight c) Amine formaldehyde condensate 10 to 56% by weight as a resin component, which is a water-based coating composition for an outer surface of a can.

Examples of the α / β monoethylenically unsaturated carboxylic acid in the present invention include acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid and itaconic acid, and acrylic acid and methacrylic acid are particularly preferable.

Hydroxyalkyl acrylates or methacrylates having 1 to 5 carbon atoms include hydroxymethyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyamyl acrylate, hydroxyhexyl acrylate and corresponding methacrylates, preferably hydroxymethyl acrylate and hydroxyethyl acrylate. Or methacrylate is good.

Vinyl monomers copolymerizable with the above monomers include
For example, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate,
Alkyl esters of acrylic acid or methacrylic acid such as methyl methacrylate, ethyl methacrylate, butyl methacrylate, stearyl methacrylate and cyclohexyl methacrylate, or aromatic vinyl compounds such as styrene, vinyltoluene and α-methylstyrene, vinyl acetate,
N-alkoxyalkyl-substituted amides such as vinyl ethyl ether, N- (methoxymethyl) acrylic amide, N- (n-butoxy) acrylic amide, and the like.

In the present invention, the α / β monoethylenically unsaturated carboxylic acid is preferably 2 to 20% by weight in the copolymer, and if it is less than 2% by weight, it becomes difficult to make it aqueous, and if it is 20% by weight or more, the water resistance is improved. Will not be enough. In addition, hydroxy acrylate or methacrylate is preferably 5 to 65% by weight of the copolymer, and when it is less than 5% by weight, the number of cross-linking points decreases, resulting in poor water resistance of the coating film, and also in the stability of the coating due to few hydrophilic groups. Chipping, and conversely, when it is more than 65% by weight, the flexibility of the coating film is poor as a result of too many crosslinking points.

Further, the monomers other than the above are mixed in the range of 20 to 80% by weight to control the coating film hardness and flexibility of the composition.

The copolymer of the present invention can be obtained by copolymerizing the above monomers in an organic solvent using a peroxide or an azo compound as a catalyst.

To make the copolymer aqueous, use a volatile base such as ammonia or an organic amine, for example, monoethanolamine, dimethylamine, diethylamine, triethylamine, triethanolamine, diethylethanolamine.

After water solubilization, or after mixing polyether polyol and aqueous amine formaldehyde resin, distilling off the organic solvent in the solution under reduced pressure within the range of 40-80 ℃, the water-soluble or water-dispersible resin solution becomes water-soluble. It can be obtained with a difference in power.

Therefore, the boiling point of the organic solvent at the time of synthesis is preferably 150 ° C or less in consideration of the distillation in the later step, and when the temperature at the time of synthesis and the dissolving power of the synthetic copolymer in the organic solvent are taken into consideration, the boiling point is Isopropyl alcohol in the range of 80-150 ℃, n-
Preference is given to alcohol solvents such as butanol, isobutyl alcohol, n-amyl alcohol, isoamyl alcohol and ester solvents such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether and ethylene glycol monoisopropyl ether.

The polyether polyol in the present invention has a molecular weight of at least 300, is soluble or dispersible in water,
Almost all polyether polyols can be used, but preferably ethylene glycol or 1,2 at both ends
A polyether polyol represented by the following general formula in which 1 to 20 mol of propylene glycol is added is preferable.

(However, R ₁ and R ₃ represent a hydrogen atom or a methyl group, and R ₂
Represents a bond composed of ether bonds, and m and n are 1 to 20.
Represents the integer. ) More specifically explaining the constitutional unit of R in the above general formula,
Consisted of ethylene oxide or 1,2-propylene oxide and a polyhydric alcohol, for example, glycerin, trimethylolpropane, hexanetriol, bentaerythritol, bisphenol A, sorbitol, sucrose, or other repeating units, or the polyhydric alcohol alone. To be done. Particularly preferably, R in the above general formula is bisphenol A or polyoxyalkylene bisphenol A composed of bisphenol A and having a molecular weight of 300 to 1600.

The amine-formaldehyde condensate of the present invention is an alkyl etherified melamine resin, an alkyl etherified urea resin,
An alkyl etherified resin of diguanamine having two triazine rings bonded to a phenylene nucleus, an alkyl etherified glycol resin, or the like is used alone or in combination of two or more kinds. Preference is given to fully etherified hexamethoxymelamine.

The amount of the water-based acrylic resin of the present invention is preferably 25 to 85% by weight in the resin component, and if the amount is less than 25% by weight, the adhesion to the base material (for example, electric tin plated steel plate, tin-free stale, aluminum is deteriorated and the gloss of the coating film is reduced If it exceeds 85% by weight, the water resistance will decrease.

The polyether polyol of the present invention is used in an amount of 5 to 19% by weight. If it is less than 5% by weight, the workability is not improved, and if it exceeds 19% by weight, the water resistance is lowered.

The amine-formaldehyde condensate of the present invention contains 10 to 56% by weight.
Is used. If it is less than 10% by weight, crosslinking will be insufficient and the water resistance and coating hardness will decrease, and if it exceeds 56% by weight, workability will decrease.

In the coating composition of the present invention, if necessary, an acid contact blocked with an amine as a curing aid, such as p-toluenebenzenesulfonic acid, dodecylbenzenesulfonic acid, dinonylnaphthalenedisulfonic acid, etc., is added to 100 parts of the resin solid content. 0.1
Add 1 part to make a paint. In addition, water-soluble resins, water-dispersible resins, which are commonly used as resins for water-based paints,
For example, water-soluble polyester resin, maleated fatty acid,
It is also possible to mix a water-soluble or water-dispersible epoxy resin or the like. Similarly, leveling agents, defoaming agents,
A lubricant can also be added. Also, the pigment is kneaded with the solvent-removed copolymer resin solution to prepare a pigment paste,
It can be made into a paint by the same method as described above.

The water-based paint of the present invention can be applied by a known means such as roll coating, spraying or brush coating.

The water-based paint of the present invention can be cured under a wide range of baking conditions, from baking at 150 to 200 ° C for about 10 minutes to high temperature short time baking at about 250 ° C for 10 seconds.

Hereinafter, the present invention will be described with reference to examples. In the examples, “part” means “part by weight” and “%” means “% by weight”.

(Example) Production Example A1 (Production of Aqueous Acrylic Resin Solution A1) 100 parts of n-butanol was charged into a four-necked flask equipped with a thermometer, a stirrer, a reflux condenser, a dropping tank, and a nitrogen gas blowing tube, and nitrogen was added. The temperature is 105 ℃ while stirring while introducing gas.
The mixture of 10 parts of acrylic acid, 15 parts of 2-hydroxymethacrylate, 50 parts of butyl acrylate, 25 parts of ethyl acrylate and 2 parts of benzoyl peroxide was added dropwise from the dropping tank over 3 hours. After that, the temperature was kept at 105 ° C and the reaction was carried out for 1 hour. Then, 0.3 part of benzoyl peroxide was added and the reaction was carried out for 1 hour to complete the reaction. After cooling to 40 ° C or below, 14 parts of triethylamine and 200 parts of water were added, and the synthetic solvent was distilled off together with water at 80 ° C under reduced pressure to obtain a transparent, viscous aqueous solution having a solid content of 60% and residual n-butanol of 10%. An acrylic resin solution A1 was obtained.

Production Example A2 (Production of Aqueous Acrylic Resin Solution A2) In the same manner as in Production Example A1, 100 parts of n-butanol was charged into a flask, the temperature was kept at 105 ° C. with stirring while introducing nitrogen gas, and methacrylic acid was added from the dropping tank. Acid 10 parts, 2
A mixture of 20 parts of hydroxyethyl acrylate, 40 parts of butyl acrylate, 30 parts of ethyl acrylate and 2 parts of benzoyl peroxide was added dropwise over 3 hours. Then 105
The reaction was carried out for 1 hour at ℃, benzoyl peroxide (0.3 parts) was added, and the reaction was continued for 1 hour. After cooling to 40 ° C or lower, 12 parts of triethylamine and 200 parts of water were added, and the synthetic solvent was distilled off together with water at 80 ° C under reduced pressure to obtain a transparent, viscous aqueous solution containing 60% solids and 10% residual n-butanol. An acrylic resin solution A2 was obtained.

Production Example C1 (Production of Pigment Dispersion Paste C1) Aqueous acrylic resin solution A1 (25 parts), rutile-type titanium white (50 parts) and water (25 parts) were mixed and dispersed with an attritor to obtain pigment dispersion paste C1.

Example 1 45 parts of an aqueous acrylic resin solution A1, 45 parts of an ethylene oxide 20 mol adduct of bisphenol A, hexamethoxymelamine (Cymel 303 manufactured by Mitsui Toatsu Chemicals, Inc .; the following examples are the same) 13.5 parts, amine blocked. Paratoluenesulfonic acid (0.1 part), water (37 parts) and ethylene glycol monoethyl ether (0.5 parts) were mixed to form a paint.

Examples 2 to 6 In the same manner as in Example 1, the aqueous acrylic resin solution, the polyether polyol, and the amine-formaldehyde condensate were changed as shown in Table 1 to form a paint.

Example 7 15 parts of aqueous acrylic resin solution A1, pigment dispersion paste C1
60 parts, 3 parts of an ethylene oxide 20 mol adduct of bisphenol A, 9 parts of hexamethoxymelamine, 0.1 part of amine-blocked paratoluenesulfonic acid, 1 part of butyrocellosolve and 12 parts of water were mixed to form a paint.

Comparative examples 1 to 3 will be described below with respect to a white paint obtained by dispersing a pigment-dispersed paste and a paint composed of a known water-soluble acrylic resin and water-soluble aminoplast resin.

Comparative Example 1 A four-necked flask was charged with 40 parts of butyl cellosolve, the temperature was kept at 115 ° C. while stirring while introducing nitrogen gas, and 20 parts of acrylic acid and 50 parts of ethyl acrylate were added from a dropping tank.
Parts, 25 parts of methyl methacrylate, 5 parts of hydroxyethyl acrylate, 1.5 parts of benzoyl peroxide 3 parts
Dropped over time. After that, the temperature was kept at 115 ° C., the reaction was carried out for 1 hour, 0.1 part of benzoyl peroxide was added, and the reaction was carried out for 1 hour to complete the reaction. It was cooled to 70 ° C., diluted with 30 parts of isopropyl alcohol, and further diluted with 25 parts of triethylamine and 60 parts of water to obtain an aqueous acrylic resin solution.

Hexamethoxymelamine was added to the above aqueous allyl resin solution.
67 parts, amine-blocked paratoluenesulfonic acid 0.8
And 94 parts of water were mixed to form a paint.

Comparative Example 2 255 parts of the aqueous acrylic resin solution of Comparative Example 1, 25 parts of hexamethoxymelamine, 0.6 part of amine-blocked paratoluenesulfonic acid and 32 parts of water were mixed to prepare a paint.

Comparative Example 3 32 parts of the aqueous acrylic resin solution obtained in Comparative Example 1, 50 parts of rutile type titanium white and 18 parts of water were mixed and dispersed with an attritor, and then 44.5 parts of acrylic resin solution and 20 parts of hexamethoxymelamine. Then, 0.2 parts of amine-blocked paratoluenesulfonic acid was mixed to form a paint.

The paint test of the paints obtained in Examples 1 to 7 and Comparative Examples 1 to 3, the paint film physical property test after paint baking and curing, and the flow paint property test were performed. The results are shown in Table-2.

Each test method is as follows.

○ Paint stability test: After each paint was stored at room temperature for 2 months, the state of gelation and separation of the resin was observed.

○ Organic solvent content test in paint: The paint was diluted with methyl ethyl ketone and quantified as it was by gas chromatography.

○ Coating film physical property test A 0.23 mm-thick electroplated tin plate was coated by roll coating so that the coating film thickness was 7μ after drying, and a baking coating panel was prepared in a gas oven at an ambient temperature of 190 ° C for 10 minutes.

○ Water resistance test: The coated panel was immersed in water and heat-treated at 100 ° C for 30 minutes, and then the whitening state of the coating film was evaluated.

Workability test: An Erichsen test was conducted.

○ Impact resistance: DuPont type 1/2 inch, tested with a load of 500 g.

○ Pencil hardness test: It was evaluated by the pencil hardness test method according to JIS standard.

O Adhesion test: A peeling test was performed.

○ Gloss: 60 The specular reflectance was measured. The clear coating material was measured after coating it on the baked coating plate of the white coating material of Example 9.

Paintability test ○ Flow: Immediately after coating with a roll coater, baking was performed in a gas oven to evaluate the flow state.

[Evaluation of Invention]

INDUSTRIAL APPLICABILITY The water-based coating composition for an outer surface of a can of the present invention has water resistance capable of withstanding a heat treatment process during food sterilization treatment of beverage cans, food cans, etc., and has excellent processability capable of being processed into various can forms. It has excellent coating stability and flow coating suitability even when it forms a coating film and the organic solvent content is 5% or less of the coating material.

The above examples are all carried out in terms of solid content.

BA10Glycol: Bisphenol A ethylene oxide 10
Molar adduct BA20 Glycol: Bisphenol A ethylene oxide 20
Molar adduct

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Claims (2)

[Claims] 1. A volatile base of a) an α / β monoethylenically unsaturated carboxylic acid, a hydroxyalkyl acrylate or methacrylate having 1 to 5 carbon atoms, and a vinyl monomer copolymerizable with the above monomers. 25 to 85% by weight of aqueous acrylic resin soluble or dispersed in water at 5) b) Polyether polyol having a molecular weight of at least 300 5 to 19% by weight c) Amine formaldehyde condensate 10 to 56% by weight as a resin component A water-based coating composition for the outer surface of a can, which is characterized. 2. A polyether polyol having a molecular weight of 300 to 160.
The water-based paint composition for an outer surface of a can according to claim 1, which is a polyoxyalkylene bisphenol A adduct of 0.