JP2020100732A - Cationic electrodeposition coating composition and method for producing cationic electrodeposition coating film - Google Patents

Abstract

To provide a cationic electrodeposition coating composition, that can form a cationic electrodeposition coating film having excellent weather resistance.SOLUTION: Provided is a cationic electrodeposition coating composition, containing an amine-modified epoxy resin and a block polyisocyanate curing agent, and in which, in a molecular weight distribution curve obtained by gel permeation chromatography of the amine-modified epoxy resin, the ratio of the area of a number average molecular weight of 500 or less to the total area of the molecular weight distribution curve is less than 5.0%.SELECTED DRAWING: None

Description

The present invention relates to a cationic electrodeposition coating composition and a method for producing a cationic electrodeposition coating film.

Patent Document 1 discloses an aqueous medium, a binder resin containing a cationic oxazolidone ring-containing epoxy resin and a blocked polyisocyanate curing agent dispersed or dissolved in the aqueous medium, and neutralization for neutralizing the cationic epoxy resin. A lead-free cationic electrodeposition coating composition containing an acid, an organic solvent and a metal catalyst is described.

JP, 2005-194390, A

A cationic electrodeposition coating film formed by electrodeposition coating using a cationic electrodeposition coating composition is usually used as an undercoat coating film for the purpose of preventing rust of a coated object. Then, an intermediate coating film and a top coating film are sequentially formed.

However, the cationic electrodeposition coating film is exposed to visible light or ultraviolet light for a long period of time to cause cleavage or decomposition reaction of the chemical bond of the coating resin component, which is secured by chemical covalent bond or intermolecular force. Also, there is a problem that the interlayer adhesion between the cationic electrodeposition coating film and the intermediate coating film is reduced and the adhesion is impaired. In order to improve the adhesion between the cationic electrodeposition coating film and the intermediate coating film, a method for improving the intermediate coating film is usually adopted, such as increasing the thickness of the intermediate coating film. However, there are parts where the thickness of the intermediate coating film cannot be increased due to problems such as the shape of the vehicle body and the line process. Therefore, not only a method for improving the intermediate coating film, but also a method for improving the cationic electrodeposition coating film to enhance the weather resistance of the cationic electrodeposition coating film is required.

The present invention has been made based on the above circumstances, and provides a cationic electrodeposition coating composition capable of forming a cationic electrodeposition coating film having excellent weather resistance and a method for producing a cationic electrodeposition coating film. The task is to do.

The invention made to solve the above problems, the amine-modified epoxy resin and a blocked polyisocyanate curing agent, in the molecular weight distribution curve obtained by gel permeation chromatography of the amine-modified epoxy resin, the total area of the molecular weight distribution curve The ratio of the area of the region having a number average molecular weight of 500 or less to less than 5.0% is a cationic electrodeposition coating composition.

In addition, "the ratio of the area of the region having a number average molecular weight of 500 or less to the total area of the molecular weight distribution curve" refers to the total area of this molecular weight distribution curve in the molecular weight distribution curve obtained by gel permeation chromatography performed under the following conditions. It is a value obtained by calculating the ratio of the area of a region having a number average molecular weight of 500 or less. The "total area of the molecular weight distribution curve" means the area of the region defined by the molecular weight distribution curve and the horizontal axis.

[Measurement conditions for gel permeation chromatography]
GPC column: TSKgel α-M (7.8 mm ID x 300 mm x 3)
Column temperature: 40°C
Flow rate: 1.0 mL/min Sample concentration: 0.2% by mass
Sample solution injection volume: 100 μL
Eluent: DMF solution of 36 mM LiBr Standard substance: 0.02 mass% polystyrene Detector: RI detector ("2414" manufactured by Waters)

Another invention made to solve the above problems is a step of immersing an article to be coated in an electrodeposition coating solution bath containing the cationic electrodeposition coating composition, and a step of forming an electrodeposition coating film on the article to be coated. And a method for producing a cationic electrodeposition coating film.

According to the cationic electrodeposition coating composition and the method for producing a cationic electrodeposition coating film of the present invention, a cationic electrodeposition coating film excellent in weather resistance can be formed.

<Cationic electrodeposition coating composition>
The cationic electrodeposition coating composition contains an amine-modified epoxy resin and a blocked polyisocyanate curing agent, and in the molecular weight distribution curve obtained by gel permeation chromatography of the amine-modified epoxy resin, the number average for the entire area of the molecular weight distribution curve is obtained. The area ratio of the region having a molecular weight of 500 or less is less than 5.0%.

The cationic electrodeposition coating composition having the above-mentioned constitution can form a cationic electrodeposition coating film having excellent weather resistance. The reason why the cationic electrodeposition coating composition has the above-mentioned configuration and thus exhibits the above-mentioned effects is not necessarily clear, but it can be inferred as follows, for example. That is, when the cationic electrodeposition coating film receives light energy, the C—N bond having small bond energy in the resin component forming the cationic electrodeposition coating film is decomposed, and radicals are easily generated. In addition, the amine-modified epoxy resin has high hydrophilicity when the molecular weight is small, and water-resistant adhesion is likely to be lowered. Therefore, it is considered that the weather resistance is improved by limiting the content ratio of the low molecular weight amine-modified epoxy resin in the cationic electrodeposition coating composition.

[Amine-modified epoxy resin]
The amine-modified epoxy resin is obtained by modifying the epoxy resin with an amine compound. The epoxy resin is not particularly limited, and examples thereof include bisphenol type epoxy resins such as bisphenol A type epoxy resin and bisphenol F type epoxy resin.

The amine compound is not particularly limited, and examples thereof include primary amines such as butylamine, octylamine, monoethanolamine, secondary amines such as diethylamine, dibutylamine, methylbutylamine, diethanolamine, N-methylethanolamine, triethylamine hydrochloride, N. , A tertiary amine acid salt such as N-dimethylethanolamine acetate, a ketiminate of aminoethylethanolamine, a diketiminate of diethylenetriamine, and the like. The amine compound may be used alone or in combination of two or more.

The amine-modified epoxy resin can be obtained by ring-opening addition of the amine compound having active hydrogen to a part or all of the epoxy groups of the epoxy resin.

In the molecular weight distribution curve obtained by gel permeation chromatography of the amine-modified epoxy resin, the ratio of the area of the number average molecular weight of 500 or less to the total area of the above molecular weight distribution curve is less than 5.0%. The upper limit of the area ratio of the region having a number average molecular weight of 500 or less is preferably 4.5%, more preferably 4.0%. The smaller the area ratio of the region having the number average molecular weight of 500 or less, the more preferable. The lower limit is not particularly limited, but it is difficult to set it to 0% in reality, so 0.01% is preferable, and 0% is preferable. 0.1% is more preferable. When the area ratio of the region having the number average molecular weight of 500 or less exceeds the upper limit, the cationic electrodeposition coating film may be deteriorated by being exposed to visible light for a long period of time, which may cause poor adhesion with the intermediate coating film. As a result, a cationic electrodeposition coating film having poor weather resistance may be obtained.

As an upper limit of the number average molecular weight of the amine-modified epoxy resin, 3000 is preferable and 2800 is more preferable. On the other hand, the lower limit of the number average molecular weight of the amine-modified epoxy resin is preferably 1500, more preferably 1700. If the number average molecular weight of the amine-modified epoxy resin exceeds the above upper limit, the appearance of the finished coating film may deteriorate. On the contrary, if the number average molecular weight of the amine-modified epoxy resin is less than the above lower limit, the corrosion resistance may decrease. The number average molecular weight of the amine-modified epoxy resin is a theoretical value calculated based on the monomer composition ratio.

[Blocked polyisocyanate curing agent]
The blocked polyisocyanate curing agent is obtained by blocking isocyanate groups in polyisocyanate with a blocking agent. The term "polyisocyanate" means a compound having two or more isocyanate groups in one molecule.

Examples of the polyisocyanate include aromatic diisocyanates such as tolylene diisocyanate (TDI), diphenylmethane-4,4′-diisocyanate (MDI), p-phenylene diisocyanate and naphthalene diisocyanate, hexamethylene diisocyanate (HDI), 2,2,4. -C3-C12 aliphatic diisocyanates such as trimethylhexane diisocyanate and lysine diisocyanate, 1,4-cyclohexane diisocyanate (CDI), isophorone diisocyanate (IPDI), 4,4'-dicyclohexylmethane diisocyanate (hydrogenated MDI), methyl Cyclohexane diisocyanate, isopropylidenedicyclohexyl-4,4'-diisocyanate, 1,3-diisocyanatomethylcyclohexane (hydrogenated XDI), hydrogenated TDI, 2,5- or 2,6-bis(isocyanatomethyl)-bicyclo [2.2.1] Alicyclic diisocyanates such as heptane (also referred to as norbornane diisocyanate), aliphatic diisocyanates having an aromatic ring such as xylylene diisocyanate (XDI) and tetramethyl xylylene diisocyanate (TMXDI), and the like. And modified products of diisocyanate (urethane, carbodiimide, uretdione, uretoimine, burette, isocyanurate modified product, etc.) and the like. Among these, aromatic polyisocyanates are preferable because they can form a cationic electrodeposition coating film having excellent corrosion resistance. As the polyisocyanate, one type or two or more types can be used.

The blocking agent is added to an isocyanate group and is stable at room temperature, but it can regenerate a free isocyanate group when heated above the dissociation temperature. Examples of the blocking agent include alcohol-based blocking agents such as propanol and butanol, glycol-based blocking agents such as ethylene glycol and propylene glycol, propylene glycol monomethyl ether, diethylene glycol monobutyl ether, 1-butoxyethoxypropanol and ethylene glycol mono-2-ethylhexyl. Ether, ethylene glycol dibutyl ether, glycol ether type blocking agents such as dipropylene glycol monomethyl ether, ε-caprolactam, δ-valerolactam, γ-butyrolactam, lactam type blocking agents such as β-propiolactam, formaldoxime, aceto Examples thereof include oxime-based blocking agents such as aldoxime, acetoxime, methylethylketoxime, diacetylmonooxime and cyclohexaneoxime.

The lower limit of the total content of the amine-modified epoxy resin and the blocked polyisocyanate curing agent relative to the solid content of the cationic electrodeposition coating composition is preferably 60% by mass, more preferably 70% by mass. On the other hand, the upper limit of the total content of the amine-modified epoxy resin and the blocked polyisocyanate curing agent is preferably 90% by mass, more preferably 80% by mass. If the total content ratio is less than the above lower limit, the corrosion resistance may decrease. On the contrary, if the total content ratio exceeds the upper limit, the appearance of the coating film may be deteriorated.

The cationic electrodeposition coating composition is preferably lead-free. The term "lead-free" means that the cationic electrodeposition coating composition contains substantially no lead compound. Specifically, it means that the content ratio of the lead compound contained in the cationic electrodeposition coating composition is less than 50 ppm. When the content ratio of the lead compound exceeds the above upper limit, there is a possibility that the environment is adversely affected.

[Other ingredients]
The cationic electrodeposition coating composition may contain other components such as a pigment, a neutralizing acid, a metal catalyst and an organic solvent as suitable components in addition to the amine-modified epoxy resin and the blocked polyisocyanate curing agent. ..

(Pigment)
Pigments are roughly classified into coloring pigments, extender pigments and rust preventive pigments according to their purposes. Examples of the color pigment include titanium dioxide (titanium white), carbon black, red iron oxide, and the like. Examples of extender pigments include kaolin, talc, aluminum silicate, calcium carbonate, mica and clay. Examples of the rust preventive pigment include zinc phosphate, iron phosphate, aluminum phosphate, calcium phosphate, zinc phosphite, zinc cyanide, zinc oxide, aluminum tripolyphosphate, zinc molybdate, aluminum molybdate, calcium molybdate, and phosphorus. Examples thereof include aluminum molybdate, aluminum zinc phosphomolybdate, bismuth hydroxide, bismuth oxide, basic bismuth carbonate, bismuth nitrate, bismuth benzoate, bismuth citrate, and bismuth silicate. As the pigment, one kind or two or more kinds can be used.

When the pigment is used in the cationic electrodeposition coating composition, it is difficult to disperse the pigment in the cationic electrodeposition coating composition as it is because the pigment is in powder form. It is preferably used in the form of a pigment dispersion paste which is dispersed in an aqueous medium to form a paste.

The pigment dispersion paste can be prepared by dispersing the pigment together with the pigment dispersion resin in an aqueous medium. Examples of the pigment dispersion resin include a cationic or nonionic low molecular weight surfactant, a cationic polymer such as a modified epoxy resin having a quaternary ammonium group or a tertiary sulfonium group, and the like. Examples of the aqueous medium include ion-exchanged water and water containing a small amount of alcohols. Unlike the amine-modified epoxy resin, the pigment-dispersing resin is not a coating film-forming resin that forms a cationic electrodeposition coating film but a resin for dispersing a pigment. Therefore, in calculating the ratio of the area of the region having a number average molecular weight of 500 or less to the total area of the above-mentioned molecular weight distribution curve in the molecular weight distribution curve obtained by gel permeation chromatography of the amine-modified epoxy resin, the amine-modified epoxy resin contains a pigment dispersion. Resin is not included.

(Neutralizing acid)
The neutralizing acid is used to neutralize the amine-modified epoxy resin and improve the dispersibility of the amine-modified epoxy resin in the aqueous medium. Examples of the neutralizing acid include inorganic acids such as hydrochloric acid, nitric acid and phosphoric acid, and organic acids such as formic acid, acetic acid and lactic acid.

(Metal catalyst)
The metal catalyst is used as a catalyst for promoting a urethanization reaction when forming a cured coating film in the curing step, and the metal catalyst is used as a metal ion or an acid salt. Examples of the metal catalyst include metal ions such as tin, bismuth, copper and zinc, or acid salts. As the acid, the same acids as those exemplified for the neutralizing acid can be used.

(Organic solvent)
The organic solvent is used when synthesizing the amine-modified epoxy resin and the blocked polyisocyanate curing agent, etc., and in order to improve the fluidity and smoothness of the coating film when forming the cationic electrodeposition coating film. Is used. Examples of the organic solvent include methyl isobutyl ketone (MIBK), ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, ethylene glycol mono-2-ethylhexyl ether, propylene glycol monobutyl ether, dipropylene glycol monobutyl ether, and propylene glycol monophenyl ether. Are listed.

In addition, the cationic electrodeposition coating composition is usually used in the cationic electrodeposition coating composition within a range that does not impair the effects of the present invention, in addition to the amine-modified epoxy resin, the blocked polyisocyanate curing agent, and the other components described above. The additive may be included. Examples of the additive include a surfactant, an antioxidant, an ultraviolet absorber and an antifoaming agent.

[Method for preparing cationic electrodeposition coating composition]
In the cationic electrodeposition coating composition, the amine-modified epoxy resin, the blocked polyisocyanate curing agent, and, if necessary, the other components (pigment, neutralizing acid, metal catalyst, organic solvent, etc.) are dispersed in an aqueous medium. It can be prepared by

<Method for producing cationic electrodeposition coating film>
The method for producing the cationic electrodeposition coating film comprises a step of immersing an object to be coated in an electrodeposition coating solution bath containing the cationic electrodeposition coating composition (hereinafter, also referred to as "immersion step"), and an electrodeposition to the object to be coated. The method includes a step of forming a coating film (hereinafter, also referred to as "coating film forming step").

The method for producing a cationic electrodeposition coating film having the above configuration can form a cationic electrodeposition coating film having excellent weather resistance.

[Immersion process]
In this step, the article to be coated is dipped in an electrodeposition coating bath containing the cationic electrodeposition coating composition. The article to be coated is not particularly limited as long as it is a conductor that can be a cathode during electrodeposition coating, and examples thereof include cold rolled steel sheet, hot rolled steel sheet, stainless steel sheet, electrogalvanized steel sheet, hot dip galvanized steel sheet, zinc-aluminum alloy system. Examples of the plated steel sheet, zinc-iron alloy-based plated steel sheet, zinc-magnesium alloy-based plated steel sheet, zinc-aluminum-magnesium alloy-based plated steel sheet, aluminum-based plated steel sheet, aluminum-silicon alloy-based plated steel sheet, tin-based plated steel sheet and the like. .. The article to be coated is preferably one which has been subjected to surface treatment such as zinc phosphate treatment in advance.

[Coating film formation step]
In this step, a cationic electrodeposition coating film is formed on the article to be coated by applying a voltage between the article to be coated and the anode.

The time for applying the voltage can be appropriately set depending on the electrodeposition conditions and is not particularly limited, but may be, for example, 2 minutes or more and 4 minutes or less.

The bath temperature of the electrodeposition coating bath at the time of applying a voltage is not particularly limited, but can be, for example, 10° C. or higher and 45° C. or lower.

The method for producing the cationic electrodeposition coating film may include a washing process and a curing process, if necessary, in addition to the dipping process and the coating film forming process.

[Washing process]
In this step, washing with water is carried out for the purpose of washing away the coating liquid adhering to the surface of the cationic electrodeposition coating film formed on the article in the coating film forming step. Further, if necessary, it is preferable to dry after washing with water.

[Curing process]
In this step, the cationic electrodeposition coating film formed on the object to be coated in the coating film forming step is heated and cured. As a maximum of heating temperature, 220 °C is preferred and 200 °C is more preferred. On the other hand, the lower limit of the heating temperature is preferably 150°C, more preferably 160°C. The heating time can be appropriately set according to the heating temperature and is not particularly limited, but can be, for example, 10 minutes or more and 30 minutes or less.

The upper limit of the thickness of the cationic electrodeposition coating film formed by the method for producing the cationic electrodeposition coating film is preferably 40 μm, more preferably 30 μm. On the other hand, the lower limit of the thickness of the cationic electrodeposition coating film is preferably 20 μm, more preferably 15 μm. If the film thickness of the cationic electrodeposition coating film exceeds the upper limit described above, abnormal appearance such as armpits or pinholes may occur due to the influence of reaction desorbed substances. On the contrary, if the thickness of the cationic electrodeposition coating film is less than the above lower limit, the rust preventive property may be insufficient. The film thickness of the cationic electrodeposition coating film means the film thickness of the electrodeposition coating film finally obtained by heat curing.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples. In the following examples, "parts" and "%" are based on mass unless otherwise specified.

<Preparation of cationic electrodeposition coating composition>
[Production Example 1] Preparation of amine-modified epoxy resin A1 A flask equipped with a stirrer, a cooling pipe, a nitrogen introducing pipe, a thermometer and a dropping funnel was charged with 940 parts of liquid epoxy, 61.4 parts of methyl isobutyl ketone and 24.4 parts of methanol. The department was set up. The reaction mixture was heated from room temperature to 40° C. with stirring, and then 0.01 part of dibutyltin laurate and 21.75 parts of tolylene diisocyanate were added. The reaction was continued for 30 minutes at 40°C to 45°C. The reaction was continued until the absorption based on the isocyanate group disappeared in the measurement of IR spectrum.

To the above reaction product, 82.0 parts of polyoxyethylene bisphenol A ether and 125.0 parts of diphenylmethane-4,4'-diisocyanate were added. The reaction was carried out at 55° C. to 60° C. and was continued until the absorption based on the isocyanate group disappeared in the measurement of IR spectrum. Subsequently, the temperature was raised, 2.0 parts of N,N-dimethylbenzylamine was added at 100° C., the temperature was maintained at 130° C., methanol was fractionally distilled using a fractionating tube, and the reaction was carried out. The epoxy equivalent was 284. became.

Then, it was diluted with methyl isobutyl ketone until the nonvolatile content became 95%, the reaction mixture was cooled, and 313.7 parts of bisphenol A and 36.0 parts of 2-ethylhexanoic acid were added. The reaction was carried out at 120°C to 125°C, and when the epoxy equivalent reached 1320, the reaction mixture was cooled by diluting with methyl isobutyl ketone until the nonvolatile content became 85%. 96.2 parts of methyl isobutyl ketone blocked primary amine of diethylenetriamine and 64.4 parts of N-methylethanolamine are added and reacted at 120° C. for 1 hour to prepare amine-modified epoxy resin A1 (resin solid content 85%, A theoretical number average molecular weight of 2400) was obtained.

[Production Example 2] Preparation of amine-modified epoxy resin A2 Amine-modified epoxy resin A2 was prepared in the same manner as in Production Example 1 except that the amount of bisphenol A was 284.1 parts and the amount of 2-ethylhexanoic acid was 73.4 parts. Epoxy resin A2 (resin solid content 85%, theoretical number average molecular weight 2000) was obtained.

[Production Example 3] Preparation of amine-modified epoxy resin B A flask equipped with a stirrer, a cooler, a nitrogen injection tube, a thermometer and a dropping funnel was prepared. To this flask, 250 parts of bisphenol A type epoxy resin having an epoxy equivalent of 250 and 62.5 parts of methyl isobutyl ketone were charged (resin solid content 80%), heated to 50° C. and dissolved, and 105 parts of diethanolamine was divided into 3 parts. I prepared it. At this time, the second charging was performed by cooling to 75° C. or lower, and the third charging was performed at 90° C. or lower. The reaction product was cooled to 80° C., 10.00 parts of 90% acetic acid (neutralization ratio 15%) was added thereto, and the mixture was stirred for 10 minutes. Then, 164 parts of pure water (resin solid content 60%) was added, and after 30 minutes, 118 parts of pure water was added (resin solid content 50%), and after stirring for another 30 minutes, 1065 parts of pure water was added to modify the amine. Epoxy resin B (resin solid content 20%) was obtained. This amine-modified epoxy resin B is used for adjusting the content ratio of the low molecular weight resin component contained in the cationic electrodeposition coating composition in Examples and Comparative Examples described later.

[Production Example 4] Preparation of block polyisocyanate curing agent 1330 parts of crude diphenylmethane-4,4'-diisocyanate and 585.6 parts of methyl isobutyl ketone were charged into a reaction vessel, which was heated to 85°C to 95°C, and then diethylene glycol. 486 parts of monobutyl ether (molecular weight 162) was added dropwise over 2.5 hours. After the dropping was completed, the temperature was kept for 1 hour. Thereafter, 194.8 parts of methyl isobutyl ketone was added and cooled to 50° C., and 532 parts of propylene glycol (molecular weight 76) was added dropwise. After completion of dropping, the temperature was raised to 60° C. and kept for 1 hour. After 0.4 part of dibutyltin laurate was added and the temperature was raised and kept at 70° C. for 1 hour, it was confirmed by IR spectrum measurement that the absorption based on the isocyanate group had disappeared, and the blocked polyisocyanate curing agent was added. Obtained.

[Production Example 5] Preparation of pigment-dispersed resin In a reaction vessel equipped with a stirrer, a cooling tube, a nitrogen introduction tube and a thermometer, 2220 parts of isophorone diisocyanate and 342.1 parts of methyl isobutyl ketone were charged, and the temperature was raised to 50 °C for dibutyl. 2.2 parts of tin laurate was added, and 878.7 parts of methyl ethyl ketone oxime was charged at 60°C. Then, the temperature was kept at 60° C. for 1 hour, it was confirmed that the NCO equivalent was 348, and 890 parts of dimethylethanolamine was added. After confirming that the NCO peak has disappeared by incubating at 60°C for 1 hour, 1872.6 parts of 50% lactic acid and 495 parts of deionized water are added while cooling so that the temperature does not exceed 60°C. I got an agent. Next, 870 parts of tolylene diisocyanate and 49.5 parts of methyl isobutyl ketone were charged into different reaction vessels, and 667.2 parts of 2-ethylhexanol was added dropwise thereto over 2.5 hours so that the temperature did not exceed 50°C. After the dropping was completed, 35.5 parts of methyl isobutyl ketone was added and the temperature was kept for 30 minutes. After that, it was confirmed that the NCO equivalent was 330 to 370, and half-block polyisocyanate was obtained.

A reaction vessel equipped with a stirrer, a cooling tube, a nitrogen introducing tube and a thermometer was diluted with 940.0 parts of liquid epoxy and 38.5 parts of methanol, and then 0.1 part of dibutyltin dilaurate was added thereto. After heating this to 50° C., 87.1 parts of tolylene diisocyanate was added and the temperature was further raised. 1.4 parts of N,N-dimethylbenzylamine was added at 100° C., and the mixture was kept at 130° C. for 2 hours. At this time, methanol was fractionally distilled using a fractionating tube. This was cooled to 115° C., methyl isobutyl ketone was charged until the solid content concentration became 90%, and then 270.3 parts of bisphenol A and 39.2 parts of 2-ethylhexanoic acid were charged and heated and stirred at 125° C. for 2 hours, 516.4 parts of said half block polyisocyanate was dripped over 30 minutes, and after that, it heat-stirred for 30 minutes. 1506 parts of polyoxyethylene bisphenol A ether was gradually added and dissolved. After cooling to 90° C., the above quaternizing agent was added and the temperature was maintained at 70 to 80° C. and an acid value of 2 or less was confirmed to obtain a pigment dispersion resin (resin solid content 30%).

[Production Example 6] Preparation of pigment dispersion paste 106.9 parts of the pigment dispersion resin obtained in Production Example 5, carbon black 1.6 parts, kaolin 40 parts, titanium dioxide 55.4 parts, aluminum phosphomolybdate in a sand grind mill. 3 parts and 13 parts of deionized water were added and dispersed until the particle size became 10 μm or less to obtain a pigment dispersion paste (solid content 60%).

[Example 1] Preparation of cationic electrodeposition coating composition The amine-modified epoxy resin A1 obtained in Production Example 1 and the amine-modified epoxy resin B obtained in Production Example 3 were mixed at a solid content ratio of 98/2. And the blocked polyisocyanate curing agent obtained in Production Example 4 were mixed at a solid content ratio of 70/30 to be uniform. Ethylene glycol mono-2-ethylhexyl ether was added thereto in an amount of 3% with respect to the resin solid content, neutralized with glacial acetic acid so that milligram equivalent of acid per 100 g of resin solid content was 27, and ion-exchanged water was slowly added. And diluted. By removing methyl isobutyl ketone under reduced pressure, an emulsion having a solid content of 36% was obtained.

1730 parts of this emulsion and 295 parts of the pigment dispersion paste obtained in Production Example 6 were mixed with 1970 parts of ion-exchanged water and 10 parts of dibutyltin oxide to obtain a cationic electrodeposition coating composition having a solid content of 20%. ..

[Examples 2 to 4 and Comparative Examples 1 to 5] Preparation of Cationic Electrodeposition Coating Compositions Examples except that the combination of amine-modified epoxy resins used and the solid content ratio thereof were changed as shown in Table 1. In the same manner as in Example 1, the cationic electrodeposition coating compositions of Examples 2 to 4 and Comparative Examples 1 to 5 were obtained.

<Weather resistance test>
[Preparation of test piece]
Each of the cationic electrodeposition coating compositions of Examples 1 to 4 and Comparative Examples 1 to 5 was used as an electrodeposition coating bath, and a cold-rolled steel sheet was used as an object to be coated so that the dry coating film thickness was about 17 μm. The electrodeposition coating conditions were adjusted, and after heating at 200° C. for 25 minutes and cooling, an electrodeposition coating film was formed. Next, an intermediate coating composition (a coating composition excluding the pigment of "OP-30" manufactured by Nippon Paint Automotive Coatings Co., Ltd.) was air-spray coated on the electrodeposition coating film at room temperature to give a film thickness of 30 μm. After that, it was cured at 140° C. for 30 minutes to form an intermediate coating film, and a test piece was prepared.

The test pieces prepared by using the cationic electrodeposition coating compositions obtained in Examples 1 to 4 and Comparative Examples 1 to 5 were evaluated as follows. The evaluation results are shown in Table 1.
[Weather resistance test]
A glass filter (GG420 manufactured by SCHOTT Co., Ltd.) was attached to the test piece prepared above and attached to a superxenon weather meter SX2-75 (manufactured by Suga Test Instruments Co., Ltd.) according to JIS K5600-7-7 Xenon lamp method, and pseudo sun Accelerated exposure to light was performed under the conditions of an illuminance of 180 W/m ² for each time shown in Table 1 below. After that, the test piece was immersed in warm water of 40° C. for 24 hours, dried at room temperature for 1 hour, and then subjected to a tape peeling test to visually confirm the presence or absence of peeling of the intermediate coating film. A (no peeling) and B ( Peeling was confirmed) was determined. The tape peeling test was performed according to JIS K5600. Specifically, an adhesive cellophane tape (registered trademark) was attached to the test piece, and the presence or absence of peeling of the intermediate coating film after the tape was rapidly peeled off at 20° C. was visually confirmed.

In Table 1, "number-average molecular weight of amine-modified epoxy resin A" means the amine-modified epoxy resin A1 used when preparing the cationic electrodeposition coating compositions of Examples 1 to 4 and Comparative Examples 1 to 5. This is a theoretical value calculated from the monomer composition ratio for the amine-modified epoxy resin A2.

Moreover, in Table 1, "ratio of the area of the area|region whose number average molecular weight is 500 or less" is the amine-modified epoxy resin used when preparing the cationic electrodeposition coating composition of Examples 1-4 and Comparative Examples 1-5. For each of the mixture of A1 or amine-modified epoxy resin A2 and amine-modified epoxy resin B, in the molecular weight distribution curve obtained by gel permeation chromatography performed under the following conditions, a number average molecular weight of 500 or less relative to the total area of this molecular weight distribution curve It is a value obtained by calculating the area ratio of the area. The "total area of the molecular weight distribution curve" means the area of the region defined by the molecular weight distribution curve and the horizontal axis.

[Measurement conditions for gel permeation chromatography]
GPC column: TSKgel α-M (7.8 mm ID x 300 mm x 3)
Column temperature: 40°C
Flow rate: 1.0 mL/min Sample concentration: 0.2% by mass
Sample solution injection volume: 100 μL
Eluent: DMF solution of 36 mM LiBr Standard substance: 0.02 mass% polystyrene Detector: RI detector ("2414" manufactured by Waters)

As is clear from the results in Table 1, when the cationic electrodeposition coating compositions of Examples 1 to 4 are used, the adhesion to the intermediate coating film is excellent and the cationic electrodeposition coating is excellent in weather resistance. A film can be formed. On the other hand, when the cationic electrodeposition coating compositions of Comparative Examples 1 to 5 are used, the adhesion with the intermediate coating film is insufficient and the cationic electrodeposition coating film is inferior in weather resistance. From this result, it was found that a cationic electrodeposition coating film excellent in weather resistance can be formed by setting the content ratio of the resin component having a number average molecular weight of 500 or less contained in the amine-modified epoxy resin within a predetermined range.

According to the cationic electrodeposition coating composition and the method for producing a cationic electrodeposition coating film of the present invention, a cationic electrodeposition coating film excellent in weather resistance can be formed.

Claims (4)

Including an amine-modified epoxy resin and a blocked polyisocyanate curing agent,
In the molecular weight distribution curve obtained by gel permeation chromatography of the amine-modified epoxy resin, the ratio of the area of the number average molecular weight of 500 or less to the total area of the molecular weight distribution curve is less than 5.0%. Stuff. The cationic electrodeposition coating composition according to claim 1, wherein the ratio of the area of the region having a number average molecular weight of 500 or less to the total area of the molecular weight distribution curve is 4.5% or less. The cationic electrodeposition coating composition according to claim 1 or 2, wherein the amine-modified epoxy resin has a number average molecular weight of 1500 or more and 3000 or less. A step of immersing an article to be coated in an electrodeposition coating bath containing the cationic electrodeposition coating composition according to claim 1, 2 or 3, and a step of forming an electrodeposition coating film on the article. A method for producing a cationic electrodeposition coating film, comprising: