JP2943502B2 - Aqueous paint composition - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a water-based coating composition, and more particularly, to the use of an acrylic resin or an amino resin having a specific structure as a resin component to improve the water resistance and processability of a coating film. It relates to an improved aqueous coating composition.

[0002]

2. Description of the Related Art The outer surfaces of beverage cans containing soft drinks and the like and food cans for packaging food can prevent corrosion of can materials, enhance aesthetic merchandise value, and withstand heat treatment steps during food sterilization. It is covered by a coating film.

Conventionally, such a coating is applied by a roll coater with a solvent type paint mainly composed of an organic solvent solution such as an epoxy-amino resin, an acryl-amino resin, a polyester-amino resin, and a gas oven. It is done by the method of drying in.

However, this method causes a large amount of solvent to evaporate during drying, causing air pollution, and is not preferable in terms of resource saving. In order to solve these problems, various water-based paints have been proposed.

[0005] Known water-based paints are mainly composed of an aqueous film-forming resin such as an acrylic resin or a polyester resin and an amino resin, but have a fundamental disadvantage that the coating film lacks water resistance.

In order to correct this drawback, Japanese Patent Publication No. 52-1735
And JP-A-63-113086 propose an aqueous paint using a film-forming resin having a specific structure. However, although the water resistance of the coating film is improved by such means, its performance is of a level that can withstand boiling water treatment and reaches a level that can withstand pressurized steam treatment (so-called retort treatment). I didn't.

Attempts have also been made to improve the water resistance by increasing the mixing ratio of amino resins such as methyl etherified melamine resin and methyl etherified benzoguanamine resin. In reality, the deterioration of adhesion and workability cannot be avoided, and even in terms of water resistance, it has not yet reached endurance to retort treatment.

As described above, the water-based paints according to the prior art did not provide practically satisfactory retort resistance and workability of coating films.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has as its object to provide water-resistant and various cans that can withstand the heat treatment step of beverage and food can sterilization. It is an object of the present invention to provide an aqueous coating composition capable of forming a coating film having an adhesive property and a processability that can be processed into a form.

[0010]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on paints containing an aqueous film-forming resin and an amino resin, and have found that an acrylic resin having a specific structure and an amino resin have been added. The present inventors have found that a paint is excellent in both the water resistance, adhesion and processability of a coating film, and have completed the present invention.

That is, in order to solve the above problems, the present invention provides: (a) an aqueous resin obtained by copolymerizing the following monomers (I), (II) and (III); (I) an α, β-monoethylenic resin Unsaturated carboxylic acid (II) N- having an alkoxy group having 4 or less carbon atoms
Alkoxymethyl (meth) acrylamide (III) Aromatic vinyl monomer or alkyl (meth) acrylate copolymerizable with (I) and (II), and (b) an amino compound having active hydrogen, formaldehyde, and a compound having 1 to 4 carbon atoms One structural unit (nucleus) of an amino compound having an active hydrogen , which is a reaction product with an alcohol.
The present invention provides an aqueous coating composition comprising an amino resin having at least one imino group per body .

Hereinafter, the composition of the present invention will be described in more detail. The aqueous acrylic resin used in the present invention can be produced, for example, according to the following method.

That is, (I) α, β-monoethylenically unsaturated carboxylic acid, (II) N-alkoxymethyl (meth) acrylamide and (III) a vinyl monomer copolymerizable with the above (I) and (II) A method in which an excess organic solvent is distilled off under reduced pressure from a copolymer solution obtained by radically polymerizing a vinyl monomer mixture containing an organic solvent in an organic solvent, and then aqueousized with a volatile base such as ammonia or an organic amine, or A method in which a volatile base and water are added to a polymer solution, and then an excess organic solvent is distilled off under reduced pressure together with water.

The α, β-monoethylenically unsaturated carboxylic acids include, for example, acrylic acid, methacrylic acid, crotonic acid,
Maleic acid, fumaric acid and the like can be mentioned, and acrylic acid and methacrylic acid are preferred. The use amount of these is preferably in the range of 5 to 20% by weight. If the amount is less than 5% by weight, it is difficult to make the resulting resin aqueous, and if it exceeds 20% by weight, the water resistance of the coating film decreases.

Examples of N-alkoxymethyl (meth) acrylamide include methoxymethyl (meth) acrylamide, ethoxymethyl (meth) acrylamide, n-butoxymethyl (meth) acrylamide, isobutoxymethyl (meth) acrylamide and the like. The amount of these used is preferably in the range of 5 to 40% by weight. If the amount is less than 5% by weight, the retort resistance of the coating film is insufficient due to insufficient curing.

The aromatic vinyl monomer copolymerizable with the above (I) and (II) includes, for example, styrene, p-methylstyrene, α-methylstyrene, vinyltoluene and the like. As the alkyl (meth) acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, cyclohexyl acrylate, methyl methacrylate,
Examples thereof include ethyl methacrylate, butyl methacrylate, stearyl methacrylate, and cyclohexyl methacrylate. The type and copolymerization amount of these monomers can be selected in the range of 40 to 90% by weight in consideration of the hardness and flexibility of the coating film.

The essential monomers for the aqueous resin of the present invention are as described above.
(I), (II), (III), but other than this, also use monomers copolymerizable with (I), (II) such as hydroxyalkyl (meth) acrylate, vinyl carboxylate, vinyl alkyl ether, etc. You can do it. As the solvent used during the polymerization, isopropyl alcohol, n-butyl alcohol, alcohol solvents such as isobutyl alcohol; ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, cellosolve solvents such as ethylene glycol monobutyl ether; 3-methoxybutanol, 3-methyl-3-methoxybutanol and the like can be mentioned.

The amino resin used in the present invention comprises an amino compound having active hydrogen, formaldehyde and a compound having 1 carbon atom.
(1) to (4), which are characterized by containing a considerable amount of imino groups (NH) together with alkoxy groups and methylol groups as functional groups in the molecular structure. Examples of the amino compound include melamine, benzoguanamine,
Compounds such as urea, ethylene urea, acetoguanamine, tetramethylene diguanamine, phthaloguanamine, and spiroguanamine are exemplified. Examples of the alcohol having 1 to 4 carbon atoms include methyl alcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, and n- Butyl alcohol, isobutyl alcohol, sec-butyl alcohol, t-butyl alcohol and the like.

(A) N-alkoxymethyl used in the present invention
(Meth) acrylamide copolymer A preferred use ratio of the aqueous acrylic resin and (b) the amino resin having an imino group is (a) 40 to 80% by weight and (b) 20 to 60% by weight in terms of resin solid content ratio. is there. If (b) is less than 20% by weight, the hardness and water resistance of the coating film are insufficient, and if it exceeds 60% by weight, the processability decreases.

As the solvent that can be used in the aqueous coating composition of the present invention, a solvent mainly composed of water and used at the time of polymerization of the N-alkoxymethyl (meth) acrylamide copolymerized aqueous acrylic resin may be used in combination.

For the purpose of lowering the viscosity of the paint or improving the adhesion of the paint film to the base, low-molecular polyols such as polyether polyol, polyester polyol, polyurethane polyol or an ethylene oxide or propylene oxide adduct of bisphenol A are used. As the third resin component, it can be used in addition to the above (a) and (b). In this case, the use amount of the low molecular polyol is preferably 30% by weight or less based on the total resin solid content from the viewpoint of maintaining the water resistance of the coating film.

To prepare the paint, the above (a), (b) and a polyol are mixed, and if necessary, p-toluenesulfonic acid, dodecylbenzenesulfonic acid, dinonyldinaphthalenedisulfonic acid, or These amine salts may be added as a curing catalyst in an amount of 0.02 to 2.0 parts based on 100 parts of the resin solids.

The water-based coating composition of the present invention may contain conventionally known antifoaming agents, leveling agents, lubricants, pigments and the like.

The aqueous paint of the present invention can be applied to a metal substrate such as an electrotin-plated steel sheet, a chromium-treated steel sheet, and an aluminum plate by a known means such as roll coating, spraying, and brushing.

The aqueous coating composition of the present invention may be used at a temperature of 150 to 200 ° C.-10
A cured film with good performance can be formed under a wide range of drying conditions from a minute condition to a high temperature short time condition of about 250 ° C. for 10 seconds.

[0026]

The present invention will be specifically described below with reference to examples.
In the examples, “parts” and “%” represent “parts by weight” and “% by weight”, respectively.

Synthesis Example 1 (Synthesis of Aqueous Acrylic Resin Solution A-1) Ethylene glycol was placed in a 1-liter four-necked flask equipped with a stirrer, reflux condenser, dropping tank, thermometer, and nitrogen gas inlet tube. Charge 420 parts of monoisopropyl ether, 120
The temperature was raised to ° C. While maintaining the same temperature, 40 parts of acrylic acid, 80 parts of n-butoxymethylacrylamide, and styrene
A mixture of 120 parts, 160 parts of butyl acrylate, and 16 parts of azobisisobutyronitrile was continuously dropped over 4 hours. One hour after completion of the dropwise addition, 3 parts of di-t-butyl peroxide was added, and the reaction was further performed for 2 hours. The resulting solution was cooled to 80 ° C., and 300 parts of the synthesis solvent was distilled off under reduced pressure.40 parts of dimethylaminoethanol and 252 parts of water were added and mixed to form a transparent viscous liquid having a nonvolatile content of 50% and a residual organic solvent of 14.6%. A small acrylic resin aqueous solution A-1 was obtained.

Synthesis Examples 2-3 (Aqueous acrylic resin solution A-2,
According to the method of Synthesis Example 1, an aqueous acrylic resin solution was obtained with the raw material compositions shown in Table 1.

[0029]

[Table 1]

Synthesis Example 4 (Synthesis of methyl-etherified benzoguanamine resin containing imino group B-1) 640 parts of methanol (20 moles) was placed in a 1-liter four-necked flask equipped with a stirrer, a condenser, and a thermometer. ), 92% paraformaldehyde 130.4 parts (4 mol), benzoguanamine 374 parts (1
Mol), 0.3 part of a 10% aqueous sodium hydroxide solution, 70 ° C
Temperature. After performing the reaction at the same temperature for 1 hour, the temperature was lowered to 30 ° C., the pH was adjusted to 3.0 with 36% hydrochloric acid, and the reaction was performed for 2 hours. Next, after adjusting the pH to 8.0 with 50% sodium hydroxide, unreacted methanol and formaldehyde were distilled off under reduced pressure with heating, and the mixture was diluted with ethylene glycol monobutyl ether. The product had a nonvolatile content of 80.1% and a Gardner viscosity of Z7.

Synthesis Example 5 (Synthesis of imino group-containing methyl etherified benzoguanamine resin B-2) The same operation as in Synthesis Example 4 was carried out except that the charged amount of 92% paraformaldehyde was changed to 195.7 parts (6 mol). Min 80.3
%, A product having a Gardner viscosity of Z4.

Synthesis Example 6 (Synthesis of methyl etherified benzoguanamine B-3) 391.3 parts (12 mol) of 92% paraformaldehyde were charged.
The same operation as in Synthesis Example 4 was performed except that
%, Gardner viscosity Z.

Table 2 shows the results of analyzing the functional groups of the amino resins obtained in Synthesis Examples 4 to 6.

[0034]

[Table 2] The numerical values represent the number of moles per mole of benzoguanamine.

The analysis was carried out by electrolytic desorption mass spectrometry (Field Desorpt
ion Mass Spectrometry, hereinafter abbreviated as FD-MS). F
The D-MS spectrum was measured using a JEOL-SX-102 double focusing mass spectrometer with a cathode voltage of 8.0 kV and an emitter current of 0 mA.
At 2 mA / min. And sweeping the magnetic field.

Examples 1-4 The aqueous acrylic resin solutions obtained in Synthesis Examples 1-3, Synthesis Examples 4-6
Amino resin solution obtained in, and BA-8 glycol (Nippon Emulsifier Co., Ltd. bisphenol A ethylene oxide adduct, the solids ratio of each component was prepared as shown in Table 3, ethylene glycol monoisopropyl ether, Water and p-toluenesulfonic acid were added and mixed to prepare a coating material having a nonvolatile content of 35%, an organic solvent content of 15 %%, and a catalyst amount of 0.2 PHR.

Comparative Examples 1 to 5 Aqueous acrylic resin, amino resin, B
Coating materials were prepared so that the solid content ratio of A-8 glycol was as shown in Table 3.

[0038]

[Table 3]

The coating materials obtained in Examples 1 to 4 and Comparative Examples 1 to 5 were subjected to the following coating film physical property tests, and the results are shown in Table 4.

[0040]

[Table 4]

The test method is as follows. 1.Coating film physical property test: A 0.23mm-thick electroplated tin steel plate is dried using a bar coater, and then a paint is applied to a coating thickness of 7μ and dried in a 180 ° C oven for 10 minutes to test the panel. It was created. This panel was subjected to the following tests.

1-1. Boiling water resistance test: After immersing the test panel in boiling water for 30 minutes, the state of the coating film was evaluated.

1-2. Retort resistance test: After exposing the test panel to steam at 130 ° C. in a pressurized container, the state of the coating film was evaluated.

1-3. Workability test: An Erichsen test was performed.

1-4. Impact resistance test: Dupont type, 1/2 inch,
Tested at 500 loads.

1-5. Hardness in hot water: The test panel was immersed in hot water at 80 ° C., and the pencil hardness in the hot water was measured.

The evaluation criteria are as follows. :: good △: slightly bad ×: bad

[0048]

The aqueous coating composition of the present invention has excellent water resistance and workability under a wide range of baking conditions due to the crosslinking reaction between the N-alkoxymethyl group in the acrylic resin and the imino group in the amino resin. Can be formed. Therefore, the aqueous coating composition of the present invention is particularly useful as an outer coating for beverage cans and food cans requiring retort sterilization.

Continuation of front page (56) References JP-A-3-72577 (JP, A) JP-A-3-199211 (JP, A) JP-A-5-279621 (JP, A) JP-A-2-142868 (JP) , A) JP-A-3-273072 (JP, A) JP-A-4-130168 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C09D 133/02, 125/08 C09D 133 / 26,161 / 32

Claims (1)

(57) [Claims] (A) The following monomers (I), (II),
Aqueous resin obtained by copolymerizing (III) (I) α, β-monoethylenically unsaturated carboxylic acid (II) N- having an alkoxy group having 4 or less carbon atoms
Alkoxymethyl (meth) acrylamide (III) Aromatic vinyl monomer or alkyl (meth) acrylate copolymerizable with (I) and (II), and (b) an amino compound having active hydrogen, formaldehyde, and a compound having 1 to 4 carbon atoms One structural unit (nucleus) of an amino compound having an active hydrogen , which is a reaction product with an alcohol.
An aqueous coating composition comprising an amino resin having at least one imino group per body) .