JPH04351682A - Aqueous resin composition for metal coating - Google Patents

Abstract

PURPOSE:To provide an aqueous resin composition which serves as a non-repair coating material that necessitates no repairing owing to its superior workability and is capable of forming a coating film excellent in adhesion and lid openablility when used as a coating material for can lid. CONSTITUTION:The title composition is prepared by dispersing in an aqueous medium a self-emulsifiable carboxylated epoxy resin which comprises a specific bisphenol F epoxy resin with an epoxy equivalent of 2,000-8,000 and a number average molecular weight of 3,000-8,000.

Description

[Detailed description of the invention]

0001

FIELD OF THE INVENTION The present invention relates to a new and useful aqueous resin composition for coating metals. More specifically, it uses a specific bisphenol F-type epoxy resin to form a coating that has excellent processability and adhesion, especially as a paint for the inner surface of can lids, and does not require repair after processing. The present invention relates to an improved aqueous resin composition for metal coating, which can be used to coat metals.

0002

2. Description of the Related Art Conventionally, metals such as aluminum, tinplate, and tin-free steel have been used as materials for metal cans. To prevent corrosion of these metals, coatings are usually formed on the inner and outer surfaces of the can.

[0003] As paints for the inside of cans, epoxy/phenol resin, epoxy/acrylic resin, or epoxy/
Paints such as amino resin are used.

[0004] However, the above-mentioned paints have insufficient workability, and the base metal is often exposed during the can manufacturing process. For this reason, the exposed metal parts are repaired after the can manufacturing process, but since such repairs complicate the can manufacturing process, it is recommended to use paints with excellent workability that do not require any repairs. is requested.

[0005] As a paint that satisfies these requirements, a vinyl chloride resin-based organosol paint has been developed. By the way, the above-mentioned organosol paint has excellent adhesion to metal materials and processability, and is useful as a paint that does not require repair. However, since it uses an organic solvent as a dispersion medium, it poses health and safety problems. There were many.

In addition, when used as a paint for metal interior surfaces, the epoxidized oil component, which is a plasticizer or stabilizer, is eluted into the contents, especially during the heat sterilization process.
Not only is this undesirable from a food hygiene standpoint, it also has the problem of changing the flavor of the contents. When opening the opening by pulling the opening, it shows the degree of remaining coating film on the inner surface of the opening.The less coating film remaining, the better the opening property.)

Furthermore, recently it has been pointed out that the vinyl chloride monomer remaining in vinyl chloride resin is carcinogenic, and when disposing of cans coated with vinyl chloride resin, the vinyl chloride resin is toxic and corrosive. The use of vinyl chloride resin has become a serious social problem because highly toxic chlorine gas or hydrogen chloride gas is generated, causing environmental pollution and corrosion of incinerators.

[0008] Therefore, as an alternative to organosol paints using vinyl chloride resin, epoxy resins have been replaced with acrylic resins as paints that avoid these problems, especially air pollution, deterioration of the working environment, and the risk of fire and explosion. Various methods have been proposed for dispersing in water a so-called self-emulsifying epoxy resin in which a segment with emulsifying power is introduced into the molecule by modification.

For example, JP-A-53-1228 discloses that a monomer mixture containing a carboxyl group-containing vinyl monomer is polymerized using a free radical generator such as benzoyl peroxide in the presence of an epoxy resin. A method is disclosed in which the resulting grafted epoxy resin is stably dispersed in an aqueous medium containing a base.

[0010] JP-A-55-75460 and JP-A-56-109243 disclose partial reactants with an excess of carboxyl groups, which are obtained by reacting an acrylic resin with an aromatic epoxy resin having a relatively high molecular weight. Dispersed in an aqueous medium in the presence of ammonia or an amine is disclosed.

[0011] JP-A-55-3481 and JP-A-55-3482 disclose methods in which a carboxyl group-containing vinyl polymer is esterified with an epoxy resin in the presence of an amine-based esterification catalyst, respectively. A method is disclosed in which a carboxyl group-containing vinyl polymer-modified epoxy resin, which is substantially free of oxirane groups in the epoxy resin, is neutralized with a base and dispersed in water.

[0012] JP-A-57-105418 and JP-A-58-198513 each disclose an epoxy group in one molecule obtained by partially reacting an aromatic epoxy resin and (meth)acrylic acid. A method has been disclosed in which a monomer mixture containing acrylic acid or methacrylic acid is copolymerized with a low molecular weight compound having an acryloyl group and an acrylic acid group, and the copolymer is neutralized with a base and dispersed in water.

However, although these water-based resin compositions are excellent in terms of food hygiene, they are inferior in coating film adhesion and opening properties, and are also inferior in processability. It was something.

[0014] In particular, Japanese Patent Application Laid-Open No. 3-33169,
Aqueous resin compositions using bisphenol F type epoxy resins with a certain molecular weight have been proposed, but because the molecular weight is too large, when used as a resin for the inner surface of can lids, the coating film The adhesion with the base is poor, and in addition,
The openability and corrosion resistance after hot water treatment were not satisfactory.

[0015]

[Problems to be Solved by the Invention] As described above, there is currently a water-based resin composition that poses few safety and health problems and has excellent coating film adhesion, openness, processability, etc.
The reality is that it has not been discovered.

Therefore, the problem to be solved by the present invention is precisely to provide a novel water-based resin for metal coating, which has the adhesion and processability of conventional vinyl chloride resin organosol paints, and also has safety and hygiene properties. In addition, it can be used as various containers for foods, cosmetics, and medical products, and in particular, even after heat sterilization, the amount of water-soluble resin components present in the coating film is extremely low in elution. The object of the present invention is to obtain an extremely practical aqueous resin composition for metal coating, which has excellent food hygiene properties, flavor retention properties, opening properties, and the like.

[0017]

[Means for solving the problem] Therefore, the present inventors
Aiming at the problems to be solved by the invention as described above, a carboxyl group-containing self-emulsifying epoxy resin obtained using a bisphenol F type epoxy resin having a specific number average molecular weight and epoxy equivalent is Intensive research has been conducted in an effort to provide an improved aqueous metal coating resin composition comprising dispersion in an aqueous medium.

By the way, acrylic modified epoxy resins mainly consisting of bisphenol A type epoxy resins have been used as water-based resins for the inside of cans, but the glass transition point (Tg) of the cured film of such resins ) is about 120
Although it has a high temperature of ℃ and has excellent physical properties after hot water treatment,
The internal stress was large and it was extremely difficult to improve the openability of the can lid.

[0019] To this end, the present inventors have made extensive studies to improve the drawbacks of acrylic-modified epoxy resins mainly based on the above-mentioned bisphenol A type epoxy resins. When using bisphenol F type epoxy resin with As a result, the present invention has been completed.

That is, the present invention provides a carboxyl group-containing self-emulsifying epoxy resin in an aqueous resin composition in which the carboxyl group-containing self-emulsifying epoxy resin is dispersed in an aqueous medium in the presence of a base. As the main component of the raw material components for
00, and the epoxy equivalent is 2,000 to 8,000
It is an object of the present invention to provide an improved aqueous resin composition for metal coating, which uses a bisphenol F type epoxy resin.

That is, the present invention aims to provide an improved aqueous resin composition for metal coating, which comprises using the above-mentioned specific bisphenol F type epoxy resin as the carboxyl group-containing self-emulsifying epoxy resin. It is something.

[0022] Here, the bisphenol F type epoxy resin has an epoxy equivalent of 2,000 or more and 8,
000, more preferably 3,000 or more and less than 6,000, and a number average molecular weight of 3
,000 or more and less than 8,000, particularly preferably 4,000 or more and less than 7,000.

[0023] This number average molecular weight was measured by gel permeation chromatography (GPC) and calculated in terms of polystyrene. Epoxy equivalent is 8,
If it is larger than 000, the curing property will inevitably be poor and the coating film will whiten after hot water treatment.On the other hand, if it is smaller than 2,000, it will inevitably become too hard and difficult to process. Sexuality becomes worse.

On the other hand, if the number average molecular weight is 8,000 or more, the processability will be good, but the adhesion will be poor, and as a result, the opening property will be poor. Furthermore, if the number average molecular weight is 3,000 or less, the processability will be poor and it will not be appropriate to use it as a resin for non-repairable can lids.

[0025] As described above, it is necessary to use a bisphenol F type epoxy resin having an optimum epoxy equivalent weight and molecular weight range as the resin for can lids. In the present invention, bisphenol F type epoxy resin is used as the main component, and furthermore, for the purpose of adjusting Tg, it is also acceptable to use it in combination with bisphenol A type epoxy resin. Of course.

The above-described carboxyl group-containing self-emulsifying epoxy resin used in the present invention [hereinafter also referred to as resin (A)]. ] refers to a compound that has a carboxyl group and can be emulsified in an aqueous medium without using a separate emulsifier.

As described above, any resin (A) can be used as long as it has a carboxyl group and self-emulsifying properties. For example, the following resins can be used. A particularly representative example is a resin prepared by such a manufacturing method, which is neutralized with a basic compound as necessary.

(1) In the presence of bisphenol F type epoxy resin (a-1), a mixture of copolymerizable monomers containing ethylenically unsaturated carboxylic acid as an essential component is added in the presence of a polymerization initiator. (2) A copolymerizable monomer mixture containing bisphenol F type epoxy resin (a-1) and ethylenically unsaturated carboxylic acid as essential components, The carboxyl group-containing acrylic resin (a-2) obtained by copolymerization in an organic solvent is esterified in an organic solvent at a reactive group concentration such that the carboxyl group is in excess of the epoxy group. (3) A method of polymerizing a bisphenol F type epoxy resin into which a (meth)acryloyl group has been introduced and a copolymerizable monomer mixture containing an ethylenically unsaturated carboxylic acid compound as an essential component. A particularly typical method is copolymerization in an organic solvent in the presence of an initiator.

Of course, carboxyl group-containing self-emulsifying epoxy resins obtained by methods other than these, ie, carboxyl group-containing modified epoxy resins, can also be used.

Typical ethylenically unsaturated carboxylic acids include (meth)acrylic acid, crotonic acid, itaconic acid, maleic acid, and fumaric acid.

Furthermore, monoalkyl esters of various ethylenically unsaturated dicarboxylic acids, such as monomethyl maleate, monoethyl fumarate, and mono-n-butyl itaconate, may be used in place of such ethylenically unsaturated carboxylic acids. Of course, that is true.

Other particularly representative copolymerizable monomers include methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, and (meth)acrylate. Various (meth)acrylic esters such as butyl acrylate, pentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, octyl (meth)acrylate, and octadecyl (meth)acrylate;

Various ethylenically unsaturated aromatic monomers (aromatic vinyl monomers; (meth)acrylonitrile and various ethylenically unsaturated nitriles such as; various vinyl esters such as vinyl acetate and vinyl propionate; various vinylidene halides such as vinylidene chloride and vinylidene bromide;

2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 2-methacrylate
various ethylenically unsaturated carboxylic acid hydroxyalkyl esters such as hydroxyethyl; various ethylenically unsaturated carboxylic acid glycidyl esters such as glycidyl (meth)acrylate; or (meth)acrylamide, N-methylol (meth)acrylamide , N-butoxymethylacrylamide, diacetone acrylamide, and various (meth)acrylamide derivatives, which are so-called radically polymerizable compounds.

[0035] In the various production methods (1), (2), and (3) above, the amount of ethylenically unsaturated carboxylic acid used is based on the total weight of the copolymerizable monomers being 100 parts by weight. , 20 parts by weight to 80 parts by weight.

[0036] The amount of ethylenically unsaturated carboxylic acid used is 2
If the amount is within the range of 0 parts by weight to 80 parts by weight, it is used for the dispersion stability of the resin in an aqueous medium, the adhesion of the painted film to metals, the solvent resistance, and the inside of the can. This is preferable because flavor suitability and the like tend to improve in both cases.

[0037] In each of the production methods (1), (2) and (3) above, the so-called aromatic epoxy resin such as bisphenol F type epoxy resin (a-1) and the various types listed above are used. a mixture of copolymerizable monomers,
Alternatively, carboxyl group-containing acrylic resin (a-2
) is used as a solid content weight ratio when the total weight is 100 parts by weight, (a-1)/(a-2) = 5
A range of 0/50 to 90/10 is preferable, and a preferable reactive group concentration range is such that carboxyl groups are in excess of epoxy groups.

Bisphenol F type epoxy resin (a-1
) is preferably used in an amount of 50 to 90 parts by weight, since the adhesion of the coating film to metals is excellent and the dispersion stability of the resulting resin (A) tends to improve.

In the production method (3) above, the usage ratio of bisphenol F type epoxy resin (a-1) and (meth)acrylic anhydride is based on the solid content weight ratio when their total weight is 100 parts by weight. So, resin/acid anhydride=90/1
It is preferably within the range of 0 to 99.95/0.05.

[0040] The polymerization initiators used in each of the above reactions are not particularly limited, but typical examples include common polymerization initiators such as azobisisobutyronitrile and benzoyl peroxide. It is mentioned as.

[0041] The amount of these polymerization initiators used is not particularly limited;
．． A range of 0.01 to 20% by weight is preferred. Examples of particularly typical organic solvents include methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, and tert-butanol.
Various hydrophilic substances such as dioxane, dimethylformamide or diacetone alcohol, including butanol, isobutanol, methyl cellosolve, ethyl cellosolve, propyl cellosolve, butyl cellosolve, ethyl carbitol, butyl carbitol, methyl cellosolve acetate or ethyl cellosolve acetate, etc. various lipophilic organic solvents such as ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone, cyclohexanone, toluene or xylene.

These may be used alone or in combination of two or more, and if necessary, water and these organic solvents may be used in combination. As the basic compound that can be used to neutralize the carboxyl groups of the resin (A) obtained by the above methods (1) to (3), both ordinary inorganic bases and organic bases can be used.

Among these, only particularly representative inorganic bases are exemplified by sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, etc. On the other hand, only particularly representative organic bases are listed. Examples include ammonia, alkylamines such as trimethylamine, triethylamine or butylamine; alcohol amines such as dimethylaminoethanol, diethanolamine or aminomethylpropanol; or morpholine.

Polyvalent amines such as ethylenediamine and diethylenetriamine can also be used. Among the basic compounds mentioned above, ammonia and volatile amines tend to improve water resistance without remaining in the coating film.
preferable.

The amount of the basic compound used is preferably such that the pH of the dispersion is 5 or more. In particular, the carboxyl group-containing self-emulsifying epoxy resin used in the present invention (
After neutralizing at least a portion of the carboxyl groups in A) with a basic compound and dispersing it in an aqueous medium, by heating this resin dispersion, the epoxy groups remaining in the microparticles are removed. By reacting with carboxyl groups, microgel particles having a three-dimensional network structure can be obtained.

However, in the synthesis method (2) of the carboxyl group-containing self-emulsifying epoxy resin (A),
It is necessary to carry out a partial esterification reaction so as to leave an epoxy group.

The reaction for obtaining microgel particles can be carried out in the state of a resin dispersion containing the solvent used in the synthesis of the carboxyl group-containing self-emulsifying epoxy resin (A), or the solvent can be removed by distillation. This can also be done after leaving the country.

[0048] This reaction is normally carried out at 50 to 95°C.
It lasts between 5 and 100 hours. The epoxy resin (A) in the form of microgel particles is particularly effective when baking is performed at high temperatures for a short time or at low temperatures.
Shows good improvement effect on insufficient curing.

[0049] In other words, in terms of processability and adhesion, as well as hygiene, water extraction is reduced, and as a result,
It has the advantage of improving water flavor properties. The carboxyl group-containing self-emulsifying epoxy resin (A) is compatible with various ethers such as tetrahydrofuran; various aprotic polar solvents such as N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide or dimethylsulfoxide, etc. Although it is a soluble non-gel resin,
After dispersion in water, heat treatment results in a microgel with a three-dimensional network structure that is insoluble in solvents.

The reaction rate of this reaction cannot be measured by the method of monitoring the remaining amount of epoxy groups, since the reaction product becomes a microgel and is insoluble in the solvent, but it is not possible to measure the reaction rate by a method of monitoring the remaining amount of epoxy groups. Specifically, it is sufficient to track the acid value when the aqueous medium is cloudy, and the reaction rate can be estimated from this.

According to this method, the carboxyl groups corresponding to the epoxy groups remaining in the reaction system are almost consumed, and the acid value becomes constant thereafter. Even though the reaction is carried out in water containing an amine, the epoxy groups are virtually completely consumed by reacting with the carboxyl groups.

In the present invention, the aqueous medium means at least 10% by weight of water, either alone or in a mixture with a hydrophilic organic solvent. Typical examples of hydrophilic organic solvents that can be used in the present invention include methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-
Alkyl alcohols such as butanol, tert-butanol, isobutanol; methyl cellosolve, ethyl cellosolve, propyl cellosolve, butyl cellosolve,
Glycol ethers such as hexyl cellosolve, methyl carbitol, ethyl carbitol, butyl carbitol; or glycol ether esters such as methyl cellosolve acetate, ethyl cellosolve acetate, dioxane, dimethylformamide, tetrahydrofuran, methyl ethyl ketone, diacetone alcohol, etc. etc.

There is no problem in using a lipophilic organic solvent if necessary. When it is necessary to reduce the organic solvent contained in the aqueous resin composition obtained in this way, when producing the carboxyl group-containing self-emulsifying epoxy resin (A), it is necessary to reduce the amount of organic solvent contained in the aqueous resin composition. If a suitable organic solvent such as acetone, methyl ethyl ketone, n-butanol or butyl cellosolve is dispersed in an aqueous medium in combination with water and then distilled at normal pressure or reduced pressure, the organic solvent can be stably dissolved. It is possible to easily obtain an aqueous resin composition with a low content of .

The aqueous resin composition for metal coating of the present invention includes:
Furthermore, if necessary, a phenolic resin may be added as a curing agent in order to improve the adhesion and openness of the coating film.

Typical examples of such phenol resins include various tetrafunctional phenol compounds such as bisphenol A or bisphenol F, carbolic acid, m-ethylphenol, 3,5-xylenol, Trifunctional phenolic compounds such as m-methoxyphenol or various difunctional phenolic compounds such as p-cresol, o-cresol, p-tert-butylphenol, p-ethylphenol, 2,3-xylenol, and m-methoxyphenol. It is synthesized from phenol and formaldehyde in the presence of an alkali catalyst.

It is also possible to use a phenolic resin in which part or all of the methylol groups contained in the phenol resin are etherified with an alcohol having 1 to 12 carbon atoms.

[0057] It is also possible to add a solvent, a surfactant, an antifoaming agent, etc. to the aqueous resin composition of the present invention, if necessary, in order to improve the coating properties. Furthermore, it is also possible to add various known and commonly used waxes as lubricants to the aqueous resin composition for metal coating of the present invention for the purpose of preventing damage to the coating film during processing.

The aqueous resin composition of the present invention is useful for coating various metal materials or products such as aluminum, tin-plated steel sheets, pretreated metals, and even steel, but wood Of course, it may also be used as a coating agent for other materials or processed products, such as.

The most preferable application is to use it for coating the inner surfaces of various metal containers used for food and drink, etc. The coating film is water resistant and there is no leakage from the coating film to the contents of the can. There is no elution of molecular compounds, and in addition, the coating film is extremely impermeable, so it does not alter the natural taste or aroma of the food or drink stored inside the can, and has excellent processability and opening properties. A coated product is obtained.

Furthermore, the aqueous resin composition for metal coating of the present invention does not foam even under high-temperature and short-time baking conditions, allows thick film coating, and has good processability and corrosion resistance. Therefore, it is also useful as a weld correction paint for welded cans.

[0061] As a coating method for the aqueous resin composition of the present invention, various spray coatings such as air spray, airless spray or electrostatic spray are preferred, but dipping coating, roll coater coating and electrodeposition coating are also possible. be.

[0062] Also, the baking conditions are 120 to 28
The temperature is preferably 10 seconds to 30 minutes at 0°C. The aqueous resin composition of the present invention can be used as a rust-preventing primer, printing ink, anti-corrosion paint, etc. by adding appropriate rust preventive agents, pigments, fillers, etc., depending on the purpose. You can also do it.

Next, the present invention will be explained in more detail with reference to Examples. In the following, unless otherwise specified, all parts and percentages are based on weight. Example 1 (Preparation example of carboxyl group-containing self-emulsifying epoxy resin) (1) n-butanol
1
20 parts (2) Bisphenol F type epoxy resin 150 parts (3)
methacrylic acid
25 copies (
4) Styrene
10
Part (5) Ethyl acrylate
2
Part (6) Benzoyl peroxide

3 parts (7) n-butanol

(1) and (
2) were added and heated to dissolve. Add (3) to this solution.
(7) While stirring each raw material component while keeping the inside of the flask at 110°C,
It was gradually added dropwise over 2 hours.

After completion of the dropwise addition, stirring was continued for 4 hours at the same temperature to obtain a carboxyl group-containing self-emulsifying epoxy resin solution with a solid content of 58%. The bisphenol F type epoxy resin used here has a number average molecular weight of 6,850, an epoxy equivalent weight of 5,700, and a molecular weight distribution of weight average molecular weight (Mw)/number average molecular weight (Mn). The ratio was 4.5.

(Water dispersion of the carboxyl group-containing self-emulsifying epoxy resin) (8) 100 parts of the carboxyl group-containing self-emulsifying epoxy resin solution (9)
dimethylethanolamine
4th part
(10) Ion exchange water

260 parts into a 4-necked flask filled with nitrogen gas (8
), heat it to 100℃, and prepare (9) and (1).
0) was added dropwise over 10 minutes while stirring to obtain the desired aqueous dispersion of the resin.

Furthermore, 130 parts of n-butanol and water were distilled off by azeotropic distillation under reduced pressure, and the nonvolatile content was reduced to 25
A solvent-free aqueous dispersion of the target resin was obtained.

(Painting) (15) The above aqueous dispersion
100 parts (16) phenolic resin
0.6 parts (17) Hexyl cellosolve 6 parts (15) to (17)
A paint was obtained by mixing and stirring thoroughly.

The phenolic resin used here was “EP-
560'' (a resol type phenolic resin manufactured by Monsanto, USA), which was partially butyl etherified and had a nonvolatile content of 72%.

Example 2 An aqueous dispersion was obtained in the same manner as in Example 1, except that the following bisphenol F type epoxy resin was used instead of the bisphenol F type epoxy resin.

Thereafter, coating was carried out in the same manner as in Example 1. The bisphenol F type epoxy resin used here has a number average molecular weight of 5,600 and an epoxy equivalent of 4.9.
00, and Mw/Mn was 4.8.

Example 3 (Synthesis of carboxyl group-containing acrylic resin) (a)
n-butanol 67
0 parts (b) Styrene
180 parts (c) Ethyl acrylate 45 parts (d) Methacrylic acid 225
Part (e) Benzoyl peroxide
3 parts First, (a) was replaced with nitrogen gas 4
The mixture was placed in a neck flask and maintained at 100° C. while stirring and dissolving, into which a mixed and dissolved mixture of (b) to (e) was gradually added dropwise over 2 hours.

After the dropwise addition was completed, stirring was continued for 3 hours at the same temperature to obtain a carboxyl group-containing acrylic resin solution with a solid content of 40%.

(Synthesis of carboxyl group-containing self-emulsifying epoxy resin) (f) Bisphenol F used in Example 1
Type epoxy resin 150 parts (e) The above carboxyl group-containing acrylic resin solution
238 parts (h) n-butanol

250 parts (i) Dimethylethanolamine
20 parts (f) to (h) were first charged into a four-necked flask purged with nitrogen gas, stirred at 100°C for 2 hours to completely dissolve, and then cooled to 80°C.

By adding (i) to this solution and stirring for 1 hour, a solution of a carboxyl group-containing self-emulsifying epoxy resin having a solid content of 48% was obtained. At this point, 56% of the epoxy groups had reacted, and the acid value of the non-gelled resin reactant was 121 in terms of solid content.

(Water dispersion of carboxyl group-containing self-emulsifying epoxy resin) (j) 100 parts of the above self-emulsifying epoxy resin solution (k) Ion-exchanged water
(j) was charged into a four-necked flask filled with 194 parts of nitrogen gas, heated to 50°C, and while stirring, (k) was added dropwise over 30 minutes until the solid content was 1.
An aqueous dispersion of 6.3% carboxyl group-containing self-emulsifying epoxy resin was obtained.

Furthermore, n-butanol and water were distilled off by azeotropic distillation under reduced pressure to obtain a solvent-free aqueous dispersion of carboxyl group-containing self-emulsifying epoxy resin with a nonvolatile content of 25%. Obtained.

[0077] Furthermore, a paint was prepared in the same manner as in Example 1. Example 4 When the aqueous dispersion of the carboxyl group-containing self-emulsifying epoxy resin obtained in Example 1 was kept at 80°C and continued stirring, a microgel insoluble in tetrahydrofuran was formed after 5 hours. When the acid value of the aqueous medium was measured in a cloudy state, it was 87.5.

[0078] After that, the number of microgel bodies increased, and conversely,
The acid value decreased and became constant at 85.5 after 10 hours. After being held for 15 hours, it was cooled to obtain an aqueous resin dispersion containing microgel bodies. Thereafter, it was made into a paint in the same manner as in Example 1.

Example 5 The aqueous dispersion of the carboxyl group-containing self-emulsifying epoxy resin obtained in Example 3 was kept at 80°C and continued stirring to obtain an aqueous resin dispersion containing microgels. .

[0080] Thereafter, a coating was formed in the same manner as in Example 1. Comparative Example 1 A control aqueous dispersion was obtained in the same manner as in Example 1, except that a bisphenol A epoxy resin as described below was used instead of the bisphenol F epoxy resin.

[0081] Furthermore, a paint was formed in the same manner as in Example 1. The bisphenol A type epoxy resin used here has a number average molecular weight of 6,500 and an epoxy equivalent of 5.2
50, and Mw/Mn was 3.1.

Comparative Example 2 A control aqueous dispersion was obtained in the same manner as in Example 1, except that a bisphenol F type epoxy resin as described below was used instead of the bisphenol F type epoxy resin. .

[0083] Furthermore, a paint was formed in the same manner as in Example 1. The bisphenol F type epoxy resin used here has a number average molecular weight of 10,200 and an epoxy equivalent of 4.
000, and Mw/Mn was 2.8.

Next, the paints obtained in Examples 1 to 5 and Comparative Examples 1 and 2 were evaluated by the following method. Those results are
All are displayed first. [Preparation of coated plates for testing] Each of the coated plates obtained in each example was coated on an aluminum plate with a thickness of 0.32 mm using a bar coater so that the dry coating weight was 120 mg/dm2. The paint was applied, baked for 20 seconds in an oven at 260°C, and then cooled to room temperature to obtain a coated board for testing. [Adhesion] Use a cutter to create 100 1 x 1 mm grid lines on the coating film formed on the test painted board, and apply
After applying the adhesive tape, quickly peel off the adhesive tape,
The peeling state of the coating film was observed and evaluated according to the following three grades.

○...No peeling △...1 to 30% of the whole peeled off ×...31 to 100% of the whole peeled off [Workability] Using a coated plate for testing, press temperature The lid was made at a temperature of 20°C and a coated plate temperature of 20°C. In order to investigate the degree of damage to the paint film after processing, we conducted tests on the painted surface of the lid.
Using an enamellator (current testing machine), place the
A 1% saline solution was filled, the lid was used as an anode, and a cathode was inserted into the saline solution, and the current value was measured and evaluated according to the following three steps.

○...The current value is less than 0.5mA△
…The current value is 0.5 to 5 mA × …The current value is 5 mA or more [Openability] Pull the opening tongue of the lid manufactured in the same manner as the workability test Finally, the residual degree of the coating film on the inner surface of the opening was evaluated according to the following three grades.

0087: ○... No paint film remains at all △...
...A little paint film remains ×...A lot of paint film remains [Elution test] In 100 ml of purified water, 160
The above test coated plate of cm2 was immersed and heated at 100°C for 1
After the boiling treatment for several hours, the amount of potassium permanganate consumed in the purified water was measured and evaluated according to the following three levels.

○...Consumption amount is 5 ppm or less △...
...The consumption amount is more than 5 ppm but not more than 10 ppm ×... The consumption amount is more than 10 ppm [Flavor test (taste test)] A hard glass rod with a diameter of 5 mm was prepared from each of the hard glass rods obtained in each example. After soaking in the paint and pulling it out, after the natural dripping has stopped, 26
Baking was performed for 20 seconds in an oven at 0°C. After taking it out and cooling it, 100cm of the surface of the coating film was
It was immersed in purified water at a rate of 100 ml per m2, kept tightly sealed, boiled for 30 minutes, and then cooled to 20°C.

[0089] As a control product (blank), a similar glass rod was used and baked in an oven at 260°C for 20 seconds without being immersed in the paint, and other than that, water for control was prepared in the same manner. The odor and taste of the above test water (purified water) were compared and evaluated using the following three grades.

○… There is virtually no difference in smell and taste △…
...There is a suspicious difference (significant difference) in odor and taste × ...There is a clear difference (remarkable difference) in smell and taste

[0091]

[Effect of the invention] The aqueous resin composition for metal coating of the present invention is
There is little fear of fire or pollution, it is excellent in safety and hygiene, it has excellent paint film adhesion, and it is also extremely easy to process.

In addition, when used as a paint for the inside of a can, it has excellent food sanitation properties as there is very little water extractable from the paint film, and its opening properties are also extremely good. .

Claims (5)

[Claims] Claim 1: A carboxyl group-containing self-emulsifying epoxy resin obtained using a bisphenol F type epoxy resin having a number average molecular weight of 3,000 to 8,000 and an epoxy equivalent of 2,000 to 8,000. , an aqueous resin composition for metal coating, which is dispersed in an aqueous medium in the presence of a base. 2. The aqueous resin composition for metal coating according to claim 1, wherein the carboxyl group-containing self-emulsifying epoxy resin is an acrylic-modified epoxy resin. 3. The aqueous resin composition for metal coating according to claim 2, wherein the carboxyl group-containing self-emulsifying epoxy resin is microgelled. 4. The aqueous resin composition for metal coating according to claim 1, which further contains a phenol resin in addition to the carboxyl group-containing self-emulsifying epoxy resin. 5. The aqueous resin composition for metal coating according to any one of claims 1 to 4, which is used for the inner surface of a can lid.