CA1148288A - Aqueous coating resin composition - Google Patents

Abstract

Abstract of the Disclosure Disclosed is an aqueous coating resin composition to be applied to a metal substrate, particularly an inner surface of a metal can or can closure on which an undercoating layer is formed. This coating composition comprises an aqueous dispersion of a carboxyl group-excessive epoxy resin-acrylic resin partial reaction product in the presence of ammonia or an amine in an amount sufficient to maintain the pH value of the composition between 5 and 11, said carboxyl group-excessive epoxy resin-acrylic resin partial reaction product being formed by reacting (a) an alkali-neutralizable acrylic resin having a number average mole-cular weight of from 5,000 to 100,000, which is obtained by copolymerizing 12 to 50 % by weight of acrylic acid or methacrylic acid with 50 to 88 % by weight of at least one member selected from the group consisting of styrene, methylstyrene, vinyltoluene and alkyl esters of acrylic acid and methacrylic acid having 1 to 8 carbon atoms in the alkyl group, with (B) an aromatic epoxy group having 1.1 to 2,0 epoxy groups on the average in one molecule and a number average molecular weight of at least 1,400.
This composition is excellent in the storage stability, and provides a coating excellent in the adhesion, processabi-lity, flavor retaining property and resistance to boiling water.

Description

The present invention relates to an aqueous coating resin composition. More particularly, the invention relates to an aqueous coating resin composition useful for forming a coating directly on a tinplate, on aluminum plate, a treated steel plate or the like, or on an under-coating layer of a phenol-epoxy or epoxy-amino paint, which is formed on such metal substrate, particularly the inner surface of a metal can or can closure.
For metal cans, there have heretofore been used a phenol-epoxy or epoxy-amino paint as a base coat paint to be a~plied on the inner surface and a thermoplastic copolymer composed mainly of vinyl chloride and vinyl acetate as a topcoat-forming material. When the base coat alone is applied, the flavor of the content is degraded, and when only the topcoat of a copolymer composed mainly of vinyl chloride and vinyl acetate is formed, the adhesion to the metal substrate and the resistance are insufficient.
Since this copolymer is readily decomposed under heat, the temperature range for the baking treatment is very narrow. Furthermore, since conventional topcoat-forming paints are of the non-aqueous type and contain considerable quantities of organic solvents, polIution of the working atmosphere and other problems are caused and the use of these paints involves a risk of occurrence of a fire.
As means for solving these problems, there have been proposed various aqueous coating compositions to be used instead of the conventional vinyl chloride-vinyl acetate paints. For example, the specification of United States Patent No. 4,021,396 discloses an aqueous coating composition obtained by neutralizing an epoxy resin and an acrylic

- 2 -copolymer formed by copolymerizing 0.5 to 10% of an unsaturated carboxylic acid with other specific monomers, with ammonium or an amine. From experiments made by us, it was found that this aqueous coating composition is poor in the storage stability, particularly at a high temperature (about 50C) and gelation is readily caused at such high temperature, and that at the baking step, there is caused a problem that the physical properties of the resulting coatings vary widely depending on the difference of the baking temperature.
Japanese Patent Application Laid-Open Specification No. 1228/78 proposes a paint formed by grafting an unsatu-rated carboxylic acid-containing monomer to the aliphatic skeleton of an epoxy resin, neutralizing a mixture of this graft polymer and a carboxylic acid-modified functional addition polymer with ammonium or an amine and dispersing the neutralization product in an aqueous medium. Further-more, Japanese Patent Applica~ion Laid-Open Specification No. 1285/78 dlscloses an improvement of the above-mentioned technique, in which the epoxy group of an epoxy resin is reacted with a stopping agent to improve the resistance to hydrolysis.
The proposals made in these Japanese Patent Applica-tion Laid-Open Specifications still involve problems ~o be solved. For example, since it is necessary to effect graft polymerization, expensive and dangerous benzoyl peroxide or a corresponding free radical initiator should be used in large quantities. Moreover, the molecular weight of the addition polymer is reduced, resulting in degradation of the physical properties of the resulting ,~

~8'~8 coatingS. Therefore, there are caused various disadvan-tages with respect to physical properties, adaptability to operations and manufacturing and running costs. Still further, scattering of properties in products is readily caused according to reaction conditions adopted for the graft polymerization.
The most important role of a composition for coating the inner surface of a metal can is to sanitarily protect the content. If dissolution of components of the coating into the content is advanced at the s~erilizing step or during long-time storage, the flavor of the content is degraded and a sanitarily undesirable phenomenon is caused by the extracted components. Accordingly, the coating formed on the inner face of the metal can should have such a characteristic that dissolution of components o the content into the content should be reduced to a level as low as possible under treatment conditions which the metal can actually undergo. ~Ordinarily, dissolution of components of the coating into the content is evaluated depending on the ratio of extraction of the components in water from the coating). All of the aqueous coating compositions according to the above-mentioned proposals are still insufficient in this point. It is therefore a primary object of the present invention to provide an aqueous coating composition which is excellent in the stability and can give coatings having excellent physical properties and in which dissolution of components of the coating into the content can be controlled to a very low level.
From experiments made by us, it was found that in an aqueous coating composi-tion comprising an acrylic resin and an epoxy resin 7 if the molecular weight of the epoxy resin used is low~ the ratio o:~ extraction of the components in water from -the re~sulting coating tend,, to increase.
In -this case 9 -the epoxy group of ~the low--molecular weight epoxy resin chemically reac-ts in the aqueous medium ~uring storage 9 resulting in increase of the viscosity or ocour-rence of gelation. The coating formed by using such composition having an increased viscosity or such gelled composition is inferior in various physical properties.
On the other hand 7 when a high-molecular-weight epoxy resin having a number average molecular weight exceeding 19400 is employed9 -the wa-ter extraction ratio is considerably reduced 7 but the level of the water ex-traction ratio is not so low as -the level attainable by the conventional vinyl chloride copol~mer paintO Although chemical reaction of the epoxy group of the high-molecular-weight epoxy group during storage is reduced~ -the compatibi~ity of the epoxy resin with the acrylic resin is poor and the epoxy resin tends to separate from the acrylic resin during storage.
~urthermore9 in the resul-ting coating9 whitening is readily caused owing to -this poor compatibility 9 and a coating having satisfactory physical properties cannot be obtained~
We made researches with a view to solving the foregoing problems and as a resul-t9 we succeeded in developing a novel aqueous coating resin composition in which the ratio of extraction of the components in water from the coating9 that is9 consumption of potassium permanganate 7 iS remar-kably reduced9 an excellent flavor retaining property can be attained 7 the s-toage stability in the form of a 32~8 paint is very good and the resulting coating is excellent in various physical properties such as the resistance to boiling water and the processability. Thus, we have now completed the present invention.
More specifically, in accordance with the present invention, there is provided an aqueous coating resin composition which comprises an aqueous dispersion of a carboxyl group-excessive epoxy resin-acrylic resin partial reaction product in the presence of ammonia or an amine in an amount sufficient to maintain the pH value of the composition between 5 and 11, said carboxyl group-excessive epoxy resin-acrylic resin partial reaction product being formed by reacting ~A) an alkali-neutralizable acrylic resin having a number average molecular weight of from 5,000 to 100,000, which is obtained by copolymerl~ing 12 to 50% by weight of acrylic acid or methacrylic acid with 50 to 88% by weight of at least one member selected from the group consisting of styrene, methylstyrene, vinyltoluene and alkyl esters of acrylic acid and methacrylic acid having 1 to 8 carbon atoms in the alkyl group, with (B) an aromatic epoxy group having 1.1 to 2.0 epoxy groups on the average in one molecule and a number average molecular weight of at least 1,400.
The characteristic features of the aqueous coating resin composition of the present invention are as follows (1I Since a high-molecular-weight epoxy resin is used and this high-molecular-weight epoxy resin is chemically bonded to an acrylic resin, the ratio of extraction of the components in water from the resulting coating is very low.
Accordingly, the flavor-retaining property under treatment ~ - 6 -~8Z~38 conditions which the metal can pract:ically undergo is very good and consumption of potassium permanganate is remarkably reduced.
(2) Since a high-molecular-weight epoxy resin is used, increase of the viscosity of the paint or gelation owing to chemical reaction of the epoxy group is not caused during storage.

(3) Since a high-molecular-weight epoxy resin and an acrylic resin, between which the compatibility is very low, are chemically bonded to form a carboxyl group-excessive, self-emulsifiable epoxy resin-acrylic resin partial reaction product, the phase separation is not caused in the paint during storage.
(~) Since there are present epoxy groups left at the terminals, the above-mentioned partial reaction product has a self-crosslinking property, and by virtue of not only this self-crosslinking property and a good film-forming property inherent of the acrylic resin, a coating having excellent physical properties can be obtained.
(5) An aqueous coating resin composition including an aromatic epoxy resin having 1.1 to 2.0 epoxy groups in one molecule and a number average molecular weight of 2,000 to 10,000, which is obtained by heating an aromatic epoxy resin having 2 epoxy groups in one molecule and a number average molecular weight of 1,~00 to 5,000 in the presence or absence of an epoxy group-modifying agent has a very industrially advantageous property that variation of properties in the resulting coating, which is due to the variation of the baking temperature at the coating forming step, is drastically reduced. This aqueous coating ,~

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resin composition is excellent over aqueous coating Tesin compositions including an aromatic epoxy resin which has not been subjected to the heat treatment, with respect to other properties of the coating.
Figure 1 is a chart showing the molecular weight distribution determined according to GPC just after mixing (A) a carboxyl group-containing acrylic resin with (B) an epoxy resin solution at room temperature in Example 1.
Figure 2 is a chart showing the molecular weight distribution determined according to GPC when the above-men-tioned mixture has been cooked at 80C for 1 hour.
The alkali-neutralizable acrylic resin (A) that is used in the present invention may be obtained by copolymeriz-ing 12 to 50% by weight of acrylic acid or methacrylic acid with 50 to 88% by weight of at least one member selected from the group consisting of styrene, methylstyrene, vinyltoluene and alkyl esters of acrylic acid and methacrylic acid having 1 to 8 carbon atoms in the alkyl group in a hydrophilic organic solvent having a boiling point of 70 to 230 C, such as ethyleneglycol monoethyl ether or ethylene-glycol monobutyl ether in the presence of a radical poly-merization initiator such as azobisisobutyronitrile or a peroxide at a temperature of 80 to 150C. If the amount of acrylic acid or methacrylic acid used for this copolymerization is smaller than 12% by weight, the dispersion stability of the resulting aqueous coating resin composition is poor. If the amount of acrylic acid or methacrylic acid is larger than 50% by weight, the water resistance of the resulting coating is degraded.
As the alkyl ester of acrylic acid or methacrylic acid, there can be mentioned, for example, methyl acrylate or methacrylate, ethyl acrylate or methacrylate, isopropyl acryla-te or methacrylate, n-butyl acrylate or methacrylate, isobutyl acrylate or methacrylate, n-amyl acrylate or methacrylate, isoamyl acrylate or methacrylate, n-hexyl acrylate or methacrylate, 2-ethylhexyl acrylate or methacryl-ate and n-octyl acrylate or methacrylate.
From the viewpoint of the sanitary effect of the inner coating of a metal can on the content food, it is preferred that the monomer combination for formation of the above-mentioned copolymer be selected from (1) methyl methacrylate/2-ethylhexyl acrylate/acrylic acid, (2) styrene/methyl methacrylate/ethyl acrylate/methacrylic acid and (3) styrene/ethyl acrylate/methacrylic acid.
The alkali-neutralizable acrylic resin (A) has a number molecular weight of 5,000 to 100,000, preferably 20,000 to 40,000. Furthermore, it is preferred that the acid value of the alkali-neutralizable acrylic resin (A) be in the range of from 80 to 350 as calcula~ed as the solid.
Epichlorohydrin/bisphenol type epoxy resins are used in the present invention as the aromatic epoxy resin having 1.1 to 2.0 epoxy groups on the average in the molecule and a number average molecular weight of at least 1,400. For example, there can be mentioned "Epikote* 1004", "Epikote 1007" and~"Epikote 1009" manufactured and sold by Shell Chemical Co. and "Epiclon* 4050" and "Epiclon 7050" manu-factured by Dainippon Ink Chemicals Co. These commer-cially available products have two epoxy groups in one molecule and a number average molecular weight of 1,400 to 5,000. In the present invention, a high-molecular-*Trademark - 9 -weight aromatic epoxy resin obtained by heating and modifying an unmodified aromatic epoxy resin such as mentioned above in the presence or absence of an epoxy group-modifying agent. The resulting modified aromatic epoxy resin has an elevated molecular weight and the aqueous coating resin composition obtained by using such modified aromatic epoxy resin provides a coating having a much reduced water extrac-tion ratio. When the modified epoxy resin obtained by conducting the heat treatment in the presence of an epoxy group-modifying agent is used, the water extraction ratio is further reduced, as compared with the case where the modi-fied epoxy resin obtained by conducting the heat treatment in the absence of an epoxy group-modifying agent is used.
MoreoverJ by the use of such modified aromatic epoxy resin, there can be attained an effect that variation of the water extraction ratio or physical properties of the coating owing to the variation of the baking temperature at the coating-forming step can be remarkably reduced.
As the epoxy group-modifying agent, there can be used, for example, bisphenols such as bisphenol A and bisphenol B, and vegetable oil fatty acids such as dehydrated castor oil, soybean oil fatty acid, cotton seed oil fatty acid, safflower oil fatty acid, tall oil -fatty acid, linseed oil fatty acid, castor oil fatty acid, coconut oil fatty acid and palm oil fatty acid, and mixtures thereof. If necessary, aromatic carboxylic acids such as benzoic acid and p-tert~
butyl benzoate may be used in combination with the above modifiers. Theoretically, the amount of the epoxy group-modifying agent may be up to 45 equivalent % based on the epoxy group of the unmodified aromatic epoxy resin. However, ~.

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since heating is ordinarily necessary for this modification reaction and self-condensation is caused in the aromatic epoxy resin under heating, a modified aromatic epoxy resin having 1.1 to 2.0 epoxy groups in one molecule and a number average molecular weight of 2,000 to 10,000 is practically obtained by using the epoxy group-modifying agent in an amount of 0.5 to 10 equivalent %.
Conditions to be adopted for the heating reaction between the unmodified aromatic epoxy resin and the epoxy group-modifying agent will now be described.
When a bisphenol is used as the modifying agent, predetermined amounts of the epoxy resin and bisphenol are charged in a stirrer-equipped reaction vessel, the inside atmosphere of which has been replaced by nitrogen, and the mixture is cooked in the absence of a solvent or in a hydrophilic organic solvent such as ethyleneglycol monobutyl ether at 150 to 170C for about 5 hours. When a fatty acid is used as the modifying agent, predetermined amounts of the epoxy resin and fatty acid and, if necessary, a small amount of sodium carbonate as the alkali catalyst, are changed in the same stirrer-equipped, nitrogen-substituted reaction vessel as described above, and the mixture is cooked in the absence of a solvent or in a hydrophllic organic solvent such as ethyleneglycol monobutyl ether at 140 to 170C
for about 5 hours. The heat treatment not using an epoxy group-modifying agent is carried out under similar condi-tions. More specifically, heating is conducted in the absence of a solvent or in a hydrophilic organic solvent such as ethyleneglycol monobutyl ether at 140 to 170C for several hours, if desired, in ~he presence of a catalyst such as sodium carbonate.
The modification reaction of unmodified aromatic epoxy resins can be controlled by measuring the content of oxirane according to the hydrobromic acid/acetic acid method described by, for example, "Determination of Epoxide Groups"
written by B. Dobinsonl W. Hofmann and B. P. Stark.
In the present invention, a carboxyl group-excessive epoxy resin-acrylic resin partial reaction product obtained by reacting the above-mentioned alkali-neutralizable acrylic resin (A) with the above-mentioned aromatic epoxy resin (B) is used. Reaction condltions adopted for formation of this partial reaction product will now be described.
The two components (A) and (B) are stirred in a hydro-philic organic solvent such as ethyleneglycol monobutyl ether in the presence or absence of ammonia or an amine as described hereinafter at 60 to 170C for 10 minutes to 2 hours, if necessary, under pressure. The reaction can be controlled by measuring the content of oxirane, examining increase of the viscosity or checking the molecular weight distribution according to gel permeation chromatography (GPC) as described in detail in Example 1 given hereinafter.
In the present invention, the weight ratio (A)/(B) of the alkali-neutrali~able acrylic resin (A) to the aromatic epoxy resin (B) is preferably adjusted in the range of from

4/1 to 1/5. When the amount of the component (A) exceeds beyond this range, a tendency of degradation of physical properties of the resulting coati-ng is observed. On the other hand, when the amount of the component (B) is too large, the ratio of extraction of the components in water from the coating is increased and the stability of the 2~

aqueous coating resin composi~ion tends to decrease. It is preferred that the weight ratio (A)/(B) be adjusted so that the amount of excessive carboxyl groups in the aqueous coating resin composition is such as will provide an acid value of 30 to 200 (as calculated as the solid).
The aqueous coating resin composition of the present invention can be prepared by dispersing the above-mentioned epoxy resin-acrylic resin partial reaction product in an aqueous medium containing ammonia or an amine in such an amount that the pH value of the final coating composition is in the range of from 5 to 11. As the amine, there can be used, for example, alkylamines such as trimethylamine, triethylamine and butylamine, alcohol amines such as 2-dimethylaminoethanol, diethanolamine, triethanolamine, aminomethylpropanol and dimethylaminomethylpropanol, and morpholine. Furthermore, polyvalent amines such as ethylene-diamine and diethylene-triamine can be used.
By the term "aqueous medium" used herein are meant water and a mixture of water and a hydrophilic organic solvent in which the content of water is at least 10% by weight.
As the hydrophilic solvent, there can be mentioned, for example, alkylalcohols such as methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol, tert-butanol and iso-butanol, ather alcohols such as methyl cellosolve*, ethyl cellosolve, propyl cellosolve, butyl cellosolve, methyl carbitol and ethyl carbitol, ether esters such as methyl cellosolve acetate and ethyl cellosolve acetate, and dioxane, dimethylformamide, diacetone alcohol and tetrahydrofurfuryl alcohol.
The aqueous coating resin composition of the present *Trademark - 13 -invention may be applied to a metal plate such as a tinplate, an aluminum plate or a treated steel plate directly or after application of an undercoat or forming processing according to known coating means such as brush coating, spray coating, dip coating, roll coating or electric deposition coating.
The thickness of the coating is not particularly critical, so far as the entire surface of the metal plate is uniformly coated, but ordinarily, the thickness of the coating is adjusted in the range of from 1 to 20 microns.
~Vhen the aqueous coating resin composition is applied to a me-tal plate such as a tinplate, an aluminum plate or a treated steel plate directly or after formation of under-coating of an epoxy-amino resin or the like, a very good adhesion to the metal substrate can be attained, and especially when the aqueous coating resin composition of the present invention is applied to the inner surface of a metal can, the water extraction ratio can be drastically reduced and a coating excellent in the flavor-retaining property, adhesion, resistance to boiling water and processability can be obtained. Moreover, the aqueous coating resin composition of the present invention may be used for the manufacture of aqueous varnishes for dispersing pigments or anticorrosives therein, metal paints and printing inks, and if the kind of the acrylic resin is appropriately chosen, the composition of the present invention may be used as an adhesive, a fiber-processing agent or the like.
The present invention will now be described in detail with reference to the following Examples that by no means limit the scope of the invention. In these Examples, all of "%" and "parts" are by weight.

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Example 1 (A) Preparation of Carboxyl Group-Containing Acrylic Resin Solution:
Styrene 300.0 parts Ethyl acrylate 210.0 parts Methacrylic acid 90.0 parts Ethyleneglycol monobutyl ether 388.0 parts Benzoyl peroxide 12.0 parts 1/4 of a mixture having the above composition was charged in a 4-neck flask, the inside atmosphere of which had been replaced by nitrogen, and the temperature was elevated to 80 to 90C by heating. While the temperature was being maintained at this level, remaining 3/4 of the mixture was gradually added dropwise over a period of 2 hours.
After completion of the dropwise addition, the reaction mixture was stirred at the above temperature for 2 hours and was then cooled to obtain a carboxyl group-containing acrylic resin solution having an acid value of 93 as calculated as the solid (all the acid values mentioned hereinafter are those as calculated as the solid), a solid content of 59.7% and a viscosity of 4,100 cps as measured at 25C ~all the viscosity values mentioned hereinafter are those as measured at 25C).
(B) Preparation of Epoxy Resin Solution:
Epikote 1,007 500 parts Ethyleneglycol monobutyl ether 333.3 parts All of a mixture having the above composition was charged in a 4-neck flask, the inside atmosphere of which had been replaced by nitrogen, and the inside temperature was elevated to 100C by gradual heating and the mixture was stirred for `~
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1 hour to completely dissolve the epoxy resin. Then, the solution was cooled to 80C to form an epoxy resin solution having a solid content of 60%.
~C) Preparation of Aqueous Coating Resin Composition:
(1) Carboxyl group-containing acrylic 100 parts resin solution (A) Epoxy resin solution (B)50 parts (2) 2-Dimethylaminoethanol9.3 parts (3) Deionized water 290.7 parts All of the mixture (1) was charged in a 4-neck flask, and the component (2) was added thereto under stirring to neutralize the contained carboxyl groups substantially equimolarly. Then, the inside temperature was elevated to 80 C and stirring was conducted at this temperature for 30 minutes. Then, the mixture was cooled to room temper-ature. The ratio of decrease of the oxirane content was 63.5%, and the viscosity was 1.5 times the viscosity before this cooking treatment.
The molecular distribution before cooking, determined according to GPC, is shown in the chart of Figure l. As is seen from Figure 1, there are present two peaks of the high-molecular-weight acrylic resin and the low-molecular-weight epoxy resin. In the molecular weight distribution after cooking, shown in the chart of Figure 2, the peak of the low-molecular-weight epoxy resin is not observed. Accordingly, it was confirmed that the epoxy resin was rendered pendant from the acrylic resin.
After the above-mentioned cooking treatment, the component (3) was gradually added under agitation to obtain a slightly milky white dispersion having a solid content ~ - 16 -/ ~

of 19.8% and a viscosity of 360 cps. When the resulting dispersion was stored at 50C for 1 month, no change was observed.
Example 2 (B) Preparation of Fatty Acid-Modified Epoxy Resin Solution:
~1) Epikote 1,007 500.0 parts Coconut oil fatty acid 2.6 parts Sodium carbonate 0.2 part E~hyleneglycol monobutyl ether125.4 parts (2) Ethyleneglycol monobutyl ether 209.4 parts The mixture (1) was charged in a 4-neck flask, the inside atmosphere of which had been replaced by nitrogen, and the inside temperature was elevated to 160 C and cooking was carried out for 4 to 5 hours. The ratio of decrease of the oxirane content was 14%. Then, the reac-tion mixture was cooled to 80C and the component (2) was added thereto to obtain a modified epoxy resin solution having a solid content of 60%.
(C) Preparation of Aqueous Coating Resin Composition:

(1) Carboxyl group-containing acrylic 100.0 parts resin solution (A) prepared in Example 1 Modified epoxy resin solution (B) 50.0 parts t2) 2-Dimethylaminoethanol 9.3 parts (3) Deionized water 290.7 parts All of the mixture (1) was charged in a 4-neck flask, and the component (2) was added thereto under agitation to neutralize 90 mole % of the contained carboxyl groups. Then, the inside temperature was elevated to 100C and cooking was carried out at this temperature for 30 minutes. The ratio of decrease of the oxirane content was found to be X

83.5%.
The component (3) was gradually added to the reaction mixture under agitation to obtain a milky white dispersion having a solid content of 20.1% and a viscosity of 500 cps.
When this dis~ersion was stored at 50C for 1 month, no change was observed.
Examples 3 to 7 and Compa~ative Examples 1 to 6 (A) Preparation of Carboxyl Group-Containing Acrylic Resin Solution:
A carboxyl group-containing acrylic resin solution was prepared according to a recipe shown in Table 1 in the same manner as described in Example 1.

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, ~
o z ~ [same as in Example 4]

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In Table 1, abbreviations have the following meaning:
St: Styrene h~lA: methyl methacrylate 2EHA: 2-ethylhexyl acrylate EA: ethyl acrylate hlAA: methacrylic acid AA: acrylic acid BP0: benzoyl peroxide (B) Preparation of Epoxy Resin Solution and Modified Epoxy Resin Solution:
An epoxy resin or modified epoxy resin solution was prepared according to a recipe shown in Table 2. All the starting materials other than ethyleneglycol monobutyl ether for dilution were charged in a 4-neck flask, and the temperature was elevated to a predetermined level and cooking was carried out for a predetermined time. Then, the reaction mixture was cooled to ab ut 80 C and ethyleneglycol monobutyl ether for dilution was added thereto.

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(C) Preparation of Aqueous Coating Resin Composition:
In all the Examples and Comparative Examples, except Comparative Example 6, aqueous coating resin compositions were prepared according to recipes shown in Table 3 in the same manner as described in Example 1. In Comparative Example 6, the operation was conducted in the same manner as described in Example 1 except that the carboxyl group-containing acrylic resin solution and epoxy resin solution were not cooked but were merely mixed at 25C.

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[same as in O O
Example 1]

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Each of the aqueous coating resin cor,lpositions prepared in Examples 1 through 7 and Comparative Examples 1 through 6 was subjec~ed to the stability test. Further-more, after each composition had been allowed to stand still at room temperature for 5 hours, the composition was roll-coated on a tinplate having an undercoating of an epoxy-urea resin, so that the thickness of the dry coating was 10 to 12 ~, and baking was carried out at 160 or 200 C
for 5 minutes to form a test panel. The so obtained test panel was subjected to various resistance tests. Results of the stability and resistance tests are shown in Table 4.
Each of the above-mentioned coating compositions was spray-coated on the inner surface of a 3-piece tinplate can having an inner diameter of 52.5 mm, a top and bottom lap seam height of 132.8 mm and an inner capacity of 268 cc, and baking was carried out at 160 or 200C for 5 minutes.
Physical properties of the resulting inner surface-coated metal can were tested to obtain results shown in Table 5.
With respect to the results of the resistance tests, there was found no substantial difference between the case where the untreated tinplate was used and the case where the undercoated tinplate was used. Therefore, only results of the resistance tests made on the untreated tinplate are shown.

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The tests mentioned in Tables 4 and 5 were carried out in the following manners.
(1) Adhesion:
Cut lines having a width of about 1.5 mm were formed on the coating by a knife. Namely, cut squares were formed by forming 11 of such cut lines in the lengthwise direction and 11 of such cut lines in the widthwise direction. Then, an adhesive cellophane* tape was applied to the coating and bonded thereto, and the tape was strongly peeled. The 10 number of the unpeeled squares is indicated in the numerator.
(2) Resistance to Boiling Water:
The coating was treated in boiling water at 100 C for 30 minutes, and the resistance was evaluated by the visual observation and the above--mentioned adhesion test using an adhesive cellophane tape.
(3) Processability:
A sample having the lower portion folded in two was set at a special folding type Du Pont impact tester, and an iron weight of 1 Kg having a flat contact surface was let to 20 fall down on the sample from a height of 50 cm and the length of the crack formed on the folded portion of the coating was measured.
O : 0 to 10 mm : 10 to 20 mm X : longer than 20 n~n (4) Storage Stability of Paint:
A sarnple was stored in an incubator maintained at 50 C, and the appearance and resistance to boiling water were examined at predetermined intervals during a period of 1 month.

~ *Trademark - 27 --2~3~

O : good storage stability X : abnormal changes in the dispersion, such as gela-tion, precipitation and phase separation

5) Potassium Permanganate Consumption:
250 mQ of deionized water was filled in an inner face-coated metal can, and after lap seaming, the can was treated at 60C for 30 minutes or at 100C for 30 minutes.
I`he potassium permanganate consumption was determined according to the method described in the Food Sanitation Act.
~6) Flavor-Retaining Property:
250 mQ of deionized water was filled in an inner face-coated can, and after lap seaming, sterilization was ; carried out at 100C for 30 minutes and the can was stored at 50C for 6 months. The content was subjected to the flavor test.
; O : no change : slight change X : considerable change (7) Water Extraction Ratio:
250 mQ of deionized water was filled in an inner face-coated can, and after lap seaming, the can was treated at 60 C for 30 minutes or at 100C for 30 minutes. The content was evaporated by a rotary evaporator and the can was dried in vacuo. The weight of the residue was measured and expressed in terms of the ratio (ppm) to the volume of the content.

-

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An aqueous coating resin composition which comprises an aqueous dispersion of a carboxyl group-excessive epoxy resin-acrylic resin partial reaction product in the presence of ammonia or an amine in an amount suffi-cient to maintain the pH value of the composition between 5 and 11, said car-boxyl group-excessive epoxy resin-acrylic resin partial reaction product being formed by reacting (A) an alkali-neutralizable acrylic resin having a number average molecular weight of from 5,000 to 100,000, which is obtained by copolymerizing 12 to 50% by weight of acrylic acid or methacrylic acid with 50 to 80% by weight of at least one member selected from the group con-sisting of styrene, methylstyrene, vinyltoluene and alkyl esters of acrylic acid and methacrylic acid having 1 to 8 carbon atoms in the alkyl group, with (B) an aromatic epoxy group having 1.1 to 2.0 epoxy groups on the average in one molecule and a number average molecular weight of at least 1,400.

2. An aqueous coating resin composition as set forth in claim 1 wherein the aromatic epoxy resin (B) is an aromatic epoxy resin having a num-ber average molecular weight of 1,400 to 5,000 and two epoxy groups in one molecule.

3. An aqueous coating resin composition as set forth in claim l wherein the aromatic epoxy resin (B) is an aromatic epoxy resin having 1.1 to 2.0 epoxy groups in one molecule and a number average molecular weight of 2,000 to 10,000, which is formed by heating an aromatic epoxy resin having a number average molecular weight of 1,400 to 5,000 and 2 epoxy groups in one molecule in the presence or absence of an epoxy group-modifying agent.

4. An aqueous coating resin composition as set forth in any of claims 1 to 3 wherein the alkali-neutralizable acrylic resin has a number average molecular weight of 20,000 to 40,000.

5. An aqueous coating resin composition as set forth in claim 1 wherein the weight ratio (A)/(B) of the solid of the alkali-neutralizable acrylic resin (A) to the solid of the aromatic epoxy resin (B) is in the range of from 4/1 to 1/5.

6. An aqueous coating resin composition as set forth in claim 3 wherein the epoxy group-modifying agent is a bisphenol or a monovalent fatty acid.