JPH0826271B2 - Aqueous coating composition - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Purpose of the Invention] (Field of Industrial Application) The present invention relates to an aqueous coating composition containing an aqueous acrylic resin, a bisphenol A alkylene oxide adduct and an amine formaldehyde condensate, and particularly to water resistance. The present invention relates to an aqueous coating composition which has excellent processability and can reduce the content of a solvent.

The outer surface of beverage cans containing soft drinks and food cans is coated with a coating that prevents corrosion of the cans, enhances aesthetic commercial value, and can withstand the heat treatment process during food sterilization. Has been done. Conventionally, these paints are coated with an organic solvent solution of epoxy / amino resin, acrylic / amino resin, polyester / amino resin, etc. by a roll coater and baked and cured in a gas oven. 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, and water-dispersible resins are usually synthesized by emulsion polymerization using a surfactant, and the surfactant used forms a coating film. After that, it remained in the coating film, and there was a drawback that the water resistance was reduced. 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 carried out, but the workability is largely decreased, while the workability is improved. When the amount of water-based amino resin is reduced, the coating film hardness tends to decrease, making it difficult to balance water resistance, coating film hardness, and processability.

In addition, conventional water-soluble paints contain 10% by weight or more of organic solvent in order to improve the storage stability of the paint and the leveling in the coating film formation. It was still insufficient.

(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 capable of withstanding a heat treatment process of beverage cans and food cans. A water-based coating composition for the outer surface of a can, which forms a coating film with high hardness that is not easily damaged during the process of transporting the can and has excellent processability that can be processed into various can forms, and has a low organic solvent content. is there.

[Structure of Invention]

(Means for Solving the Problems) That is, the present invention comprises: a) Aromatic vinyl monomer 15-45 wt% ethyl acrylate or butyl acrylate 15-60 based on the copolymer.
wt% α / β monoethylenically unsaturated carboxylic acid 2 to 15 wt% and a vinyl monomer copolymer containing 5 to 35 wt% hydroxyalkyl acrylate or methacrylate having 1 to 5 carbon atoms can be dissolved in water with a volatile base. Aqueous acrylic resin dissolved or dispersed 32 to 70 wt% b) Bisphenol A polyalkylene oxide adduct 5
-18% by weight and 10-50% by weight of c) amine-formaldehyde condensate as a resin component.

The aromatic vinyl monomer in the present invention is styrene,
Examples include vinyltoluene and α-methylstyrene, and styrene is particularly preferable. Aromatic vinyl monomer is used in the copolymer in an amount of 15 to 45 wt%. If it is less than 15 wt%, the water resistance and coating hardness will be reduced, and if it exceeds 45 wt%, the flexibility of the coating will be poor.

Ethyl acrylate or butyl acrylate is a necessary component to improve the flexibility of the coating film, and 15 to 60 wt% of the copolymer is used.

Examples of the α / β monoethylenically unsaturated carboxylic acid include acrylic acid, methacrylic acid, crotonic acid, maleic acid, fumaric acid and itaconic acid, with acrylic acid and methacrylic acid being particularly preferred. The α / β monoethylenically unsaturated carboxylic acid is used in an amount of 2 to 15 wt% in the copolymer. If it is less than 2 wt%, it becomes difficult to make it aqueous, and if it is 15 wt% or more, the water resistance of the coating film becomes poor.

Hydroxyalkyl acrylates or methacrylates having 1 to 5 carbon atoms include hydroxymethyl acrylate, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyamyl acrylate, hydroxybutyl acrylate and corresponding methacrylates, preferably hydroxymethyl acrylate,
Hydroxyethyl acrylate or methacrylate is preferred. As for these monomers, 5 to 35 wt% of the copolymer is used, and if the amount is less than 5 wt%, the number of crosslinking points decreases,
The water resistance of the coating film is poor, and the coating stability is poor due to the small number of hydrophilic groups. On the other hand, if it exceeds 35 wt%, the number of cross-linking points becomes too large, resulting in poor flexibility of the coating film.

The above-mentioned monomer is blended in the acrylic copolymer of the present invention as an essential component, but a copolymerizable vinyl monomer may also be used. Examples of these vinyl monomers include acrylic acid or methacrylic alkyl esters such as 2-ethylhexyl acrylate, cyclohexyl acrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, stearyl methacrylate, cyclohexyl methacrylate, vinyl acetate, vinyl ethyl ether, N. Examples include N-alkoxyalkyl-substituted amides such as-(methoxymethyl) acrylic amide and N- (n-butoxy) acrylic amide.

The acrylic copolymer of the present invention can be obtained by an ordinary solution polymerization method. That is, the above monomer mixture is radically polymerized in an organic solvent using a peroxide such as benzoyl peroxide or an azo compound such as 2,2′azobisisobutyronitrile as a catalyst.

To make an aqueous acrylic resin, a volatile base such as ammonia or an organic amine is added together with water to the solution of the copolymer obtained by solution polymerization. The volatile base may be in an amount that at least partially neutralizes the carboxylic acid in the acrylic copolymer. Examples of organic amines include monoethanolamine, dimethylamine, diethylamine, triethylamine, triethanolamine, diethylethanolamine and dimethylethanolamine. Then, the organic solvent is distilled off from the solution under reduced pressure within the range of 40 to 80 ° C to obtain an aqueous acrylic resin soluble or dispersed in water. Therefore, the boiling point of the organic solvent at the time of synthesis is preferably 150 ° C or less, considering the distillation in the later step.
Taking into consideration the temperature during synthesis and the solubility of the synthetic copolymer in organic solvents, isopropyl alcohol, n-butanol, isobutyl alcohol, n-amyl alcohol, isoamyl alcohol, etc., with boiling points within the range of 80 to 150 ℃ Alcohol solvents and ester solvents such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoisoprol ether, and ethylene glycol monobutyl ether are preferred.

The water-based acrylic resin of the present invention is used in an amount of 32 to 70 wt% in the resin solid matter of the entire coating composition. If it is less than 32 wt%, the adhesion to the metal base material is lowered and the gloss of the coating film is lowered.

The bisphenol A polyalkylene oxide adduct of the present invention is obtained by adding about 2 to 30 mol of ethylene oxide or propylene oxide to bisphenol A. Mixtures of ethylene oxide and propylene oxide may be used. Bisphenol A polyalkylene oxide adduct is 5 to 18 wt% in the resin solid content of the entire coating composition.
If it is less than 5 wt%, no improvement in the workability of the coating film is observed, and if it exceeds 18 wt%, the water resistance of the coating film decreases.

The amine-formaldehyde condensate of the present invention includes 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, etc., or two kinds thereof. The above is mixed and used. Preference is given to fully etherified hexamethoxymelamine. The amine-formaldehyde condensate is used in an amount of 10 to 50 wt% in the resin solid content of the entire coating composition. If it is less than 10 wt%, the crosslinking is insufficient and the water resistance and the hardness of the coating film are reduced. Processability is reduced.

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

The water-based paint of the present invention can be applied to a substrate by a known means such as roll coating, spraying or brush coating.
As the base material, there are metal base materials such as electric tin-plated steel plate, tin-free steer, and aluminum.

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, “parts” means “parts by weight” and “%” means “% by weight”.

(Example) Production Example 1 (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. While introducing gas, stir the temperature to 105
The mixture was kept at 0 ° C and a mixture of 17 parts of styrene, 45 parts of ethyl acrylate, 8 parts of acrylic acid, 10 parts of 2-hydroxyethyl acrylate, 19 parts of methyl methacrylate and 3 parts of benzoyl peroxide was added dropwise from a dropping tank over 3 hours. afterwards
The reaction was carried out at 105 ° C for 1 hour, 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, 9.9 parts of dimethylethanolamine and 200 parts of water were added, and n-butanol was distilled off together with water at 80 ° C under reduced pressure to obtain a solid content.
A transparent and viscous aqueous acrylic resin solution A1 containing 60% and residual n-butanol 10% was obtained.

Production Examples 2 to 7 According to the same method as in Production Example 1, aqueous acrylic resin solutions A2 to A5 were obtained using the monomer compositions shown in Table 1 and dimethylethanolamine.

Production Example 8 (Production of Pigment Dispersion Paste B1) Aqueous acrylic resin solution A1 25 parts, rutile titanium white 50
And 25 parts of water were mixed and dispersed by an attritor to obtain a pigment dispersion paste B1.

Production Example 9 (Production of Pigment Dispersion Paste B2) Aqueous acrylic resin solution A2 25 parts, rutile titanium white 50
And 25 parts of water were mixed and dispersed by an attritor to obtain a pigment dispersion paste B2.

Example 1 40 parts of aqueous acrylic resin solution A1, 4 parts of bisphenol A ethylene oxide 10 mol adduct, 12 parts of hexamethoxymelamine (Cymel 303 manufactured by Mitsui Toatsu Chemicals, Inc .; the following examples are the same), amine blocked. Paratoluenesulfonic acid 0.08 part, ethylene glycol monobutyl ether 0.
5 parts and water 43.42 parts, and silicone leveling agent
0.1 part was mixed to form a paint.

Examples 2 to 10, Comparative Examples 1 to 3 According to the same method as in Example 1, a coating composition using an aqueous acrylic resin solution, a polyalkylene oxide adduct of bisphenol A, and hexamethoxymelamine in the formulation shown in Table 2 was used. I got a thing.

Example 11 13.75 parts of aqueous acrylic resin solution A1, 55.0 parts of pigment dispersion paste B1, ethylene oxide of bisphenol A
2.75 parts of 10-mol addition product, 8.25 parts of hexamethoxymelamine,
Amine-blocked paratoluenesulfonic acid 0.05 parts, ethylene glycol monobutyl ether 1.75 parts, and water
A white paint was obtained by mixing 18.45 parts.

Example 12 A white coating material was obtained according to the formulation shown in Table 2 according to the same method as in Example 11.

The paint test, the paint film physical property test and the paintability test of the paints obtained in the respective examples and comparative examples were performed.

 The results are shown in Table-3.

 Each test method is as follows.

Paint test ○ Paint stability test: After the paint was stored at room temperature for 2 months, the state of gelation 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. The scratch resistance was evaluated by preparing a 3-piece can in which a coated plate was molded so that the coated surface was the outer surface of the can.

○ 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 exen test was performed.

○ 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 based on JIS plan.

O Adhesion test: A peeling test was performed.

○ Scratchability The contents were packed in a can, and the outer surface of the can was contacted in hot water at 80 ° C to evaluate the degree of damage to the paint on the outer surface of the can.

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

[Effects of the Invention] The water-based coating composition for an outer surface of a can of the present invention has water resistance that can withstand a heat treatment process during food sterilization treatment of beverage cans, food cans, etc. , It forms a coating film with excellent processability that can be processed into various can forms, and has excellent coating stability and flow coating suitability even if the organic solvent content is 5% or less of the coating. .

Claims (1)

[Claims] 1. A) Aromatic vinyl monomer 15-45 wt% ethyl acrylate or butyl acrylate 15-60 based on the copolymer.
wt% α / β monoethylenically unsaturated carboxylic acid 2 to 15 wt% and a vinyl monomer copolymer containing 5 to 35 wt% hydroxyalkyl acrylate or methacrylate having 1 to 5 carbon atoms can be dissolved in water with a volatile base. Aqueous acrylic resin dissolved or dispersed 32 to 70 wt% b) Bisphenol A polyalkylene oxide adduct 5
-18 wt% and c) amine formaldehyde condensate 10-50 wt% as a resin component.