JP2003253190A - Aqueous coating composition for can interior - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an aqueous coating composition for a can interior which can form a coating film having a good curability, is excellent in taste retention, and from which does not bleed out bisphenol A. <P>SOLUTION: This aqueous coating composition is prepared by mixing a carboxyl group-containing acrylic resin (A) having an acid value of 30-100 mgKOH/g with a m-cresol-derived resol phenol resin (B) obtained by reacting m-cresol with formaldehyde or the like in the presence of a basic catalyst in a solid content ratio by weight of resin A/resin B of (98/2)-(70/30) and dispersing the resultant mixture in an aqueous medium. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a water-based paint capable of forming a coating film having excellent flavor retention and in which bisphenol A does not elute from the coating film. Among them, a water-based coating suitable for coating the inner surface of a can. It relates to a coating composition.

[0002]

2. Description of the Related Art Conventionally, as a paint for the inner surface of a can, a solvent-type paint in which a bisphenol A type epoxy resin and a phenol resin curing agent are combined has been generally used.

However, due to the problem of the influence of organic solvents on the environment, there is an increasing demand for reduction of the amount of organic solvents in paints, and in recent years, bisphenol A has been cited as one of exogenous endocrine disrupting chemical substances (environmental hormone). As a result, there has been a demand for a paint that does not elute bisphenol A from the paint film.

From the above requirements, epoxy resin-based water-based paints having a low bisphenol A content have been studied (for example, Japanese Patent Laid-Open Nos. 2000-109750 and 2000).
-226542 publication etc.). However, these systems cannot completely eliminate bisphenol A.

As a system which does not use an epoxy resin, a system using an acrylic modified polyester resin is disclosed in JP-A-2000-2000.
No. 327727. However, polyester resin-based paints have inferior coating hardness, and due to problems such as deterioration of storage stability due to hydrolysis of polyester when made into water-based paints, achieve the strict coating performance required for inner surfaces of cans. Is difficult.

Japanese Unexamined Patent Publication (Kokai) No. 7-316489 discloses an aqueous coating composition in which a specific acrylic resin and an amino resin are combined. However, when the can filling is sterilized, the amino resin is hydrolyzed to cause harmful formaldehyde. However, it is not suitable for the inner surface of a can.

An object of the present invention is to provide a water-based coating composition for the inner surface of a can, which has good curability, excellent flavor retention, and can form a coating film in which bisphenol A does not elute. It is intended to provide a coated metal plate and a coating can obtained by applying the coating composition.

[0008]

Means for Solving the Problems As a result of intensive studies to solve the above problems, the present inventors have found that an aqueous coating obtained by combining a methacrylic acid-containing acrylic resin with a meta-cresol-based resol-type phenol resin has the above-mentioned problems. They have found that they can be solved and have completed the present invention.

Thus, the present invention has an acid value of 30 to 100 mg.
KOH / g carboxyl group-containing acrylic resin (A) and meta-cresol-based resol-type phenol resin (B) obtained by reacting meta-cresol with formaldehyde in the presence of a basic catalyst, resin (A) / resin (B) The solid content weight ratio of 1) is mixed within the range of 98/2 to 70/30 and dispersed in an aqueous medium to provide an aqueous coating composition for the inner surface of a can.

Further, the present invention provides a coated metal plate for a can, which is obtained by molding the above-mentioned aqueous coating composition for an inner surface of a can into a metal plate and applying it to the inner surface of the can.

Further, the present invention provides a coated can in which the inner surface of the can is coated with the above-mentioned aqueous coating composition for an inner surface of the can.

The present invention will be described in more detail below.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The aqueous coating composition for an inner surface of a can of the present invention contains a carboxyl group-containing acrylic resin (A) and a metacresol-based resol-type phenol resin (B) as essential components.

Acrylic Resin Containing Carboxyl Group (A) The acrylic resin containing a carboxyl group, which is the component (A) of the aqueous coating composition for an inner surface of a can of the present invention, is obtained by copolymerizing a monomer containing a carboxyl group with another monomer. Obtainable.

Examples of the carboxyl group-containing monomer include acrylic acid, methacrylic acid, crotonic acid,
Examples thereof include itaconic acid, maleic acid and fumaric acid.

Other monomers include, for example, 2-
Hydroxyethyl (meth) acrylate, 2-hydroxypropyl (meth) acrylate, 4-hydroxybutyl (meth) acrylate, 2-hydroxyethyl vinyl ether, 2-hydroxypropyl vinyl ether, 2
Hydroxyl group-containing monomers such as hydroxyethyl allyl ether; vinyl aromatic compounds such as styrene, α-methylstyrene, vinyltoluene and α-chlorostyrene; methyl (meth) acrylate, ethyl (meth) acrylate
, N-propyl (meth) acrylate, i-propyl (meth) acrylate, (n-, i-, t-) butyl (meth) acrylate, hexyl (meth) acrylate
, Cyclohexyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, n-octyl (meth)
Acrylate, decyl (meth) acrylate, lauryl (meth) acrylate, stearyl (meth) acrylate
, An alkyl ester or cycloalkyl ester of acrylic acid or methacrylic acid such as isobornyl (meth) acrylate having 1 to 24 carbon atoms; vinyl acetate, vinyl chloride, vinyl ether, acrylonitrile, methacrylonitrile and the like, Further, a polymer of one or more of these monomers and a so-called macromonomer having a polymerizable unsaturated group at one end can also be mentioned as the copolymerizable monomer. In the present invention, "(meth) acrylate" means "acrylate or methacrylate".

As the copolymerization method, a general vinyl monomer polymerization method can be used, but the solvent type radical polymerization method in an organic solvent is most suitable in consideration of versatility and cost. That is, in the presence of a polymerization initiator such as azobisisobutyronitrile or benzoyl peroxide in an organic solvent, a target polymer is easily obtained by carrying out a copolymerization reaction in the range of usually about 60 to 170 ° C. Obtainable. The carboxyl group-containing acrylic resin used in the present invention has an acid value of 30 to 100 mgKOH / g, particularly 45.
It is within the range of 80 mgKOH / g, including the dispersion stability of the resin in the aqueous medium, the coating film applied,
Each of them tends to improve adhesion to metal, solvent resistance, hygiene when used for the inner surface of a can, and so on, which is preferable.

Metacresol type resol type phenolic tree
Fat (B) The metacresol-based resol-type phenolic resin, which is the component (B) of the aqueous coating composition for the inner surface of a can of the present invention, is obtained by heating metacresol and formaldehyde in the presence of a reaction catalyst to cause a condensation reaction. It is obtained by partially alkylating a methylol group of a methylolated phenol resin obtained by introducing a methylol group with an alcohol.

As the phenols used in the production of the resol type phenol resin cross-linking agent, there are generally used carboxylic acid, orthocresol, para-cresol, bisphenol A or bisphenol F, but when combined with a carboxyl group-containing acrylic resin, carboxylic acid, orthocresol is used. Alternatively, the resole-type phenol resin obtained from para-cresol does not have sufficient curability, and the flavor retention is a problem. Further, in the case of using a resol type phenol resin using bisphenol A or bisphenol F, bisphenol A suspected to be an exogenous endocrine disrupting chemical (environmental hormone) or bisphenol F having a structure similar to that of bisphenol A Elution is inevitable.

The formaldehydes used in the production of the meta-cresol-based resol-type phenol resin include formaldehyde, paraformaldehyde, trioxane and the like, and they may be used alone or in combination of two or more.

The alcohol used for alkyl-etherifying a part of the methylol group of the methylolated phenolic resin has 1 to 8 carbon atoms, preferably 1 to
Four monohydric alcohols can be preferably used.
Suitable monohydric alcohols include methanol, ethanol, n-propanol, n-butanol, isobutanol, etc., but n-butanol is particularly suitable from the viewpoint of the balance between curability and processability.

The compounding ratio of the carboxyl group-containing acrylic resin (A) and the meta-cresol-based resol-type phenolic resin (B) in the water coating composition for the inner surface of a can is (A) / (B) = solid content weight ratio. The range of 98/2 to 70/30, preferably 95/5 to 80/20 is suitable from the viewpoint of the balance between curability and processability.

The aqueous coating composition for the inner surface of a can of the present invention contains a carboxyl group-containing acrylic resin (A) and a meta-cresol-based resol-type phenol resin (B) as essential components. Resol type phenolic resins other than type phenolic resins may be added, and preferred examples include resol type phenolic resins such as carboxylic acid type, orthocresol type, paracresol type, paratertiary butylphenol type and xylenol type. it can.

In addition, an acid catalyst may be added to the water-based coating composition for the inner surface of a can of the present invention, if necessary, in order to accelerate the curing reaction. Specific examples include acid catalysts such as paratoluene sulfonic acid, dodecylbenzene sulfonic acid, dinonylnaphthalene sulfonic acid, dinonyl naphthalene disulfonic acid, phosphoric acid, and amine neutralized products of these acids. Among them, the sulfonic acid compound or an amine neutralized product of the sulfonic acid compound is preferable.

From the viewpoint of the physical properties of the resulting coating film, the amount of the acid catalyst to be blended is the amount of acid (for example, in the case of an amine neutralized product of a sulfonic acid compound, the remaining sulfone obtained by removing the amine from this neutralized product) 0.1 to 5 parts by weight, preferably 0.5 to 2 parts, based on 100 parts by weight of the total solid content of the carboxyl group-containing acrylic resin (A) and the metacresol-based resol-type phenolic resin (B) as the amount of the acid compound). A range of parts by weight is suitable.

The above aqueous coating composition may further contain polyester resin, epoxy resin, additives (lubricity imparting agent, defoaming agent, leveling agent, anti-agglomeration agent, etc.), if necessary.
It can be combined with commonly known raw materials such as pigments (organic pigments, inorganic pigments, bright pigments, extender pigments, rust preventive pigments, etc.) and organic solvents.

The water-based coating composition for the inner surface of a can of the present invention is neutralized with a basic compound to a part or all of the carboxyl groups in the carboxyl group-containing acrylic resin (A), if necessary, and then dispersed in an aqueous medium. However, it can be obtained by adding the meta-cresol-based resol-type phenolic resin (B) and other raw materials as necessary. First, the carboxyl group-containing acrylic resin (A) and the meta-cresol-based resol-type phenolic resin ( It is also possible to first mix B) and, if necessary, other raw materials and then disperse them in an aqueous medium. From the viewpoint of production stability of the resulting water-based coating composition, the latter method is preferred.

As the basic compound that can be used to neutralize the above-mentioned carboxyl group, any of ordinary inorganic bases and organic bases can be used.

Among them, the inorganic base is, for example, sodium hydroxide, potassium hydroxide, sodium carbonate or potassium carbonate, and the organic base is, for example, ammonia and alkylamines such as trimethylamine, triethylamine or butylamine: Examples thereof include alcohol amines such as dimethylaminoethanol, diethanolamine or aminomethypropanol; polyvalent amines such as ethylenediamine and diethylenetriamine; and morpholine.

As the above-mentioned basic compound, ammonia and volatile amines are preferable because they do not remain in the coating film and tend to improve water resistance. The amount of the basic compound used is preferably such that the pH of the dispersion becomes 5 or more.

In the present invention, the aqueous medium means water alone or a mixture with a hydrophilic organic solvent in which at least 10% by weight is water. Examples of the hydrophilic organic solvent that can be used in the present invention include alkyl alcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol and isobutanol; methyl cellosolve, ethyl cellosolve. , Propyl cellosolve, butyl cellosolve, hexyl cellosolve, methyl carbitol, ethyl carbitol, butyl carbitol, and other glycol ethers; or methyl cellosolve acetate, ethyl cellosolve acetate, and other glycol ether esters, such as dioxane, dimethylformamide, and tetrahydrofuran. , Methyl ethyl ketone, diacetone alcohol and the like. Further, a lipophilic organic solvent may be used as long as the storage stability of the aqueous coating composition is not deteriorated.

Coated metal plate The above aqueous coating composition is applied to a flat metal plate by a known coating method such as a roll coater or a curtain flow coater, and baked to form a coating film suitable for the inner surface of a can. You can The coating amount, 40~180mg / 100cm ² in dry coating amount is preferably preferably in the range of 60~150mg / 100cm ^2. The baking condition of the coating film is usually about 15 under the condition that the maximum attainable temperature of the metal plate is about 190 to 300 ° C.
Second to about 30 minutes.

Examples of the material of the can include untreated steel sheet, tin-plated steel sheet, zinc-plated steel sheet, chrome-plated steel sheet, phosphate-treated steel sheet, chromate-treated steel sheet, non-treated aluminum sheet, chromate-treated aluminum sheet. Metal materials such as

Coated can A coated can can be obtained by forming the coated metal plate obtained above into a can with the side coated with the aqueous coating composition of the present invention as the inner surface.

It can also be obtained by coating a metal material which has been previously formed into a can shape with a spray or the like and then baking it.

[0036]

EXAMPLES The present invention will be described more specifically with reference to Examples and Comparative Examples. In the following, "part" and "%"
All are based on weight.

Synthesis of Acrylic Resin Containing Carboxyl Group Synthesis Example 1 Ethylene glycol monobutyl ether 4 was placed in a reaction vessel.
0 part was blended and the temperature was raised to 100 ° C. and maintained. A mixed solution of 10 parts of methacrylic acid, 45 parts of styrene, 45 parts of ethyl acrylate, 1.5 parts of "Perbutyl O" (manufactured by NOF CORPORATION, peroxide polymerization initiator) and 14 parts of ethylene glycol monobutyl ether. Was added dropwise over 3 hours.
After dripping, mature at 100 ° C for 2 hours, solid content 65%
To obtain a carboxyl group-containing acrylic resin solution (a-1). The obtained resin had a number average molecular weight of about 10,000 and a resin acid value of 65 mgKOH / g.

Synthesis Examples 2 to 13 Reactions were carried out in the same manner as in Synthesis Example 1 except that the monomer ratios used were as shown in Table 1 below, and each acrylic resin solution (a) having a solid content of 65% (a -2)
~ (A-13) were obtained. The acrylic resin solutions obtained in Synthesis Examples 10 to 13 are for comparative examples.

[0039]

[Table 1]

Synthesis of meta-cresol type resole resin Synthesis Example 14 100 parts of meta-cresol, 178 parts of 37% aqueous formaldehyde solution and 1 part of caustic soda were added, and after reacting at 60 ° C. for 3 hours, dehydration under reduced pressure at 50 ° C. for 1 hour. did. Then add 100 parts of n-butanol and 3 parts of phosphoric acid, and add 110
The reaction was carried out at ~ 120 ° C for 2 hours. After completion of the reaction, the resulting solution is filtered to remove sodium phosphate produced,
A meta-cresol type resol resin crosslinking agent solution (b-1) having a solid content of about 50% was obtained. The resulting resin had a number average molecular weight of 880, an average number of methylol groups per benzene nucleus of 0.3, and an average number of butoxymethyl groups of 1.2.

Aqueous Coating Composition Example 1 Carboxyl group-containing acrylic resin (a-) having a solid content of 65% obtained in Synthesis Example 1 and held in a reaction vessel at 85 ° C.
1) In 154 parts of solution (100 parts of solid content), the above-mentioned Synthesis Example 1 was added.
After adding 20 parts (solid content 10 parts) of metacresol-type resole resin crosslinking agent (b-1) solution having a solid content of 50% obtained in 5 and 18 parts of n-butanol and stirring and mixing uniformly,
To this, 2.6 parts of N, N-dimethylethanolamine was added and held for 1 hour. Then, 380 parts of deionized water was added dropwise with stirring over 1 hour to obtain an aqueous coating composition having a solid content of about 20%.

Examples 2 to 17, Comparative Example 1 and Comparative Example 1
In Example 1, instead of 154 parts of the carboxyl group-containing acrylic resin (a-1) solution and 20 parts of the meta-cresol type resole resin crosslinking agent (b-1) solution, the types and amounts shown in Tables 2 and 3 below were used. Except for using the carboxyl group-containing acrylic resin solution and the crosslinking agent, the same procedure as in Example 1 was carried out to obtain each aqueous coating composition having a solid content of about 20%. In addition,
In Comparative Example 5, the acrylic resin was poor in water dispersibility and could not be emulsified. In Comparative Example 6, the acrylic resin component was dissolved in deionized water and the insoluble portion floated on the upper layer and separated into two layers.

Performance Test of Aqueous Coating Compositions Using each of the aqueous coating compositions obtained in the above Examples and Comparative Examples except Comparative Examples 5 and 6 in which the acrylic resin dispersion was not successful, No. Bar coater coating was applied to a 5052 aluminum plate so that the dry coating film weight was 50 mg per 100 cm ² , and baking was performed under the condition of holding 200 ° C. for 30 seconds to dry each coated plate. Various coating film performance tests were performed on the obtained coated plates based on the following test methods. The corrosion resistance and flavor retention were tested using a can body prepared by the method described below. The results of these tests are shown in Tables 2 and 3 below.

Coating state: The coating surface of the coated plate was visually observed and evaluated according to the following criteria. ○: The entire coated surface is smooth. Δ: Slight unevenness is observed on the entire coated surface. X: Large unevenness is observed on the entire coated surface.

Adhesiveness: A knife is used to make 11 cuts in each of the vertical and horizontal widths on the coating film of the coated plate in a width of about 1.5 mm, and a cellophane adhesive tape of 24 mm width is adhered to it for strong peeling. The coating film on the eye area at the time of observing was observed and evaluated according to the following criteria. ⊚: No peeling is observed. ◯: Slight peeling is recognized. Δ: Significant peeling is recognized. X: Remarkable peeling is recognized.

Workability: A 180-degree bent portion is provided on the lower part of the coated plate with the coating surface outside, and a special goby folding type DuPont impact tester is used to make a flat contact surface on the bent portion. The length of cracks in the coating film at the bent portion, which occurred when a 1 kg iron weight was dropped from a height of 50 cm, was measured and evaluated according to the following criteria. ◯: less than 5 mm. Δ: 5 mm or more and less than 10 mm. X: 10 mm or more.

Retort resistance: The coated plate was immersed in deionized water at 125 ° C. for 35 minutes in an autoclave and pulled up, and then the whitening state of the coating film was observed and evaluated according to the following criteria. ◯: A slight whitening is observed in the coating film. Δ: Significant whitening is observed in the coating film. X: Significant whitening is observed in the coating film.

Preparation of two-piece can body : Each coating composition was hot-air spray-coated to a dry film thickness of about 5 μm on the inner surface of an aluminum 2-piece can having an internal capacity of 350 cc, and the condition was maintained at 200 ° C. or higher for 30 seconds. Then, it was baked and hardened to prepare a can body of a two-piece can.

Each test solution was placed in the can body of the two-piece can,
The upper lid was wrapped and the following flavor retention and potassium permanganate consumption were tested.

Flavor retaining property: Using each of the above can bodies,
Tap water was filled with 340 cc of water treated with activated carbon, wound, tightened, sterilized at 125 ° C. for 30 minutes, stored at 37 ° C. for 1 month, and then subjected to a flavor test, and evaluated according to the following criteria. ◯: No change is recognized. Δ: Significant change is recognized ×: Significant change is recognized

Potassium permanganate consumption: Using each of the above-mentioned can bodies, filled with 340 cc of deionized water, wound, and treated at 125 ° C. for 30 minutes, and then the content liquid was tested according to the Food Sanitation Law. The amount of potassium permanganate consumed was measured according to the method (Ministry of Health and Welfare No. 434) and evaluated according to the following criteria. It should be noted that the greater the consumption amount of potassium permanganate, the poorer the corrosion resistance. ○: less than 5.0 ppm △: 5.0 ppm or more, less than 10.0 ppm x: 10.0 ppm or more

[0052]

[Table 2]

[0053]

[Table 3]

The raw materials for each of the injections in Tables 2 and 3 have the following contents. (* 1) Shonor BKS377F: Showa High Polymer Co., Ltd., a resole-type phenol resin consisting of carboxylic acid / formaldehyde, solid content = 50%. (* 2) Shonor CKS3873: Showa High Polymer Co., Ltd. resol type phenol resin consisting of paratertiary butylphenol and para-cresol / formaldehyde, solid content = 60%. (* 3) Valcum 29-101: BTL SPECIA
LTY RESINS, xylenol / formaldehyde resol type phenol resin, solid content 10
0%. (* 4) Shonor CKM-1737: Resol-type phenol resin composed of phenylphenol / formaldehyde manufactured by Showa Highpolymer Co., Ltd., solid content = 100%. (* 5) Cymel 303: a melamine resin composed of melamine / formaldehyde manufactured by Mitsui Cytec. (* 6) Mycoat 1128: A benzoguanamine resin composed of benzoguanamine / formaldehyde, manufactured by Mitsui Cytec.

[0055]

EFFECTS OF THE INVENTION An aqueous coating composition containing a carboxyl group-containing acrylic resin and a meta-cresol-based resol-type phenol resin does not contain bisphenol A, which is suspected to be an endocrine disrupter, and has excellent thermosetting properties. Since it is excellent in adhesion, retort resistance, flavor retention and corrosion resistance, it is particularly suitable as a coating composition for the inner surface of a can.

Continued front page    (72) Inventor Yusuke Yamaguchi             Seki, 4-17-1, Higashi-Hachiman, Hiratsuka City, Kanagawa Prefecture             West Paint Co., Ltd. F-term (reference) 3E062 AA04 AB02 AB14 AC03 JA01                       JA07 JB23 JC02 JD01                 4J038 CG001 DA062 GA06 MA08                       MA10 PB04 PC02

Claims (6)

[Claims] 1. A metacresol-based resol-type phenol resin (B) obtained by reacting a carboxyl group-containing acrylic resin (A) having an acid value of 30 to 100 mgKOH / g and metacresol with formaldehyde in the presence of a basic catalyst.
And the resin (A) / resin (B) solid content weight ratio of 98/2
A water-based coating composition for the inner surface of a can, characterized in that it is mixed in the range of 70/30 and dispersed in an aqueous medium. 2. A carboxyl group-containing acrylic resin (A)
10-60 styrene in the polymerizable monomer used in the synthesis of
The water-borne coating composition for the inner surface of a can according to claim 1, wherein the water-based coating composition is contained in a weight percentage. 3. A metacresol-based resol-type phenol resin (B) is obtained by reacting meta-cresol with formaldehyde in the presence of a basic catalyst, and part or all of the methylol groups of the resol-type phenol resin is converted into butyl ether. The water-based coating composition for the inner surface of a can according to claim 1 or 2. 4. A resin composition containing a resol-type phenol resin other than a meta-cresol-based resin.
The aqueous coating composition for the inner surface of a can according to any one of 1. 5. A coated metal plate for a can, which is obtained by molding the aqueous coating composition for an inner surface of a can according to any one of claims 1 to 4 and applying it to the inner surface of the can. 6. A coated can obtained by coating the inner surface of the can with the aqueous coating composition for an inner surface of a can according to any one of claims 1 to 4.