JP2005163068A - Cationic electrodeposition coating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrodeposition coating method using a cationic electrodeposition coating material composition which prevents the generation of gas pinholes, also has excellent laying property, and exerts corrosion preventability to a coating film and stability to a coating material. <P>SOLUTION: The cationic electrodeposition coating method is characterized in that a cationic electrodeposition coating composition in which minimum film formation temperature (MFT) is controlled to 15 to <20°C by adding an organic solvent in which solubility in water is 0.1 to 10 mass%, the value of solubility parameter (SP) is 9.0 to 12.0, and the boiling point is ≥120°C to a coating material composition is electrodeposition-coated. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to a cationic electrodeposition coating method using a cationic electrodeposition coating composition. More specifically, it suppresses the occurrence of gas pinholes on conductive substrates, especially alloyed hot-dip galvanized steel sheets, and provides high throwing power, coating performance such as coating corrosion resistance, and coating stability. The present invention relates to a cationic electrodeposition coating method using a cationic electrodeposition coating composition which is excellent in the above.

Electrodeposition coating can be applied to the details even if the object has a complicated shape, and can be applied automatically and continuously, so it has a large and complicated shape such as an automobile body. However, it is widely used as an undercoating method for objects to be coated that requires high rust prevention.
Cationic electrodeposition coating is performed by immersing an object to be coated in a cationic electrodeposition paint as a cathode and applying a voltage. The majority of the cationic electrodeposition coatings that have been surface-treated from conventional steel sheets have been electrodeposited on galvanized steel sheets with zinc plated on the surface of steel sheets in recent years. I came. In particular, alloyed hot-dip galvanized steel sheets are widely used because of their excellent rust prevention and workability.

However, alloyed hot-dip galvanized steel sheets may have crater-like abnormalities in the electrodeposition coating. In cationic electrodeposition paints, hydrogen gas generated by electrolysis of water is present at the time of painting, and as the film thickness increases, the resistance of the paint film increases and the voltage applied to the paint film increases. The coating resin in the vicinity of the discharge is cured by the discharge heat, the flowability becomes insufficient, and a crater-like abnormality remains. This crater-like abnormality is called a gas pinhole.

As simple methods for preventing gas pinholes, it has been known to increase the amount of organic solvent in the paint and to increase the acid concentration as a neutralizing agent. However, these methods are one of the characteristics of electrodeposition paints. The throwing power (the part that is far from the electrode part may be left unpainted without sufficiently forming an electrodeposition coating, and the ability to reduce this unpainted part) And an increase in the amount of organic solvent is undesirable because it increases the amount of volatile organic compounds (VOC) released to the environment. Therefore, a cationic electrodeposition coating composition containing an ethylene oxide adduct of a secondary alcohol having an HLB value of 10.0 to 13.5 is known as an inhibitor for the generation of paint film craters due to gas (Patent Literature). 1).

However, by adding a plasticizer or the like having the effect of lowering the molecular weight or Tg of such a resin, a method for forming a flexible deposited coating and facilitating the escape of hydrogen gas changes the components of the coating. There was a drawback that the corrosion resistance of the coating film was greatly reduced.
Further, a cationic electrodeposition coating composition is known in which the electric conductivity of the diluted coating is 1000 to 1300 μS / cm ² , and the coulomb efficiency at the time of electrodeposition coating for 3 minutes is 40 mg / coulomb or more (Patent Document) 2). However, with such an adjustment method, it is not only difficult to achieve both the effect on gas pinholes and throwing power, but it may adversely affect other paint workability and bath paint stability. .

In addition, in electrodeposition coating, an electrodeposition paint in which the minimum film-forming temperature of the cationic electrodeposition paint is within ± 5 ° C. of the electrodeposition coating set temperature and the electric conductivity during painting is adjusted to 1000 to 1500 μS / cm, There is also known an electrodeposition coating film forming method in which electrodeposition coating is performed at the electrodeposition setting temperature (see Patent Document 3). However, even if the electric conductivity at the time of painting is used at the set temperature of 25 to 30 ° C. shown in the examples, such an adjustment method does not provide both the effect on the gas pinhole and the throwing power. May adversely affect other paint workability and bath paint stability.

In the cationic electrodeposition coating, the polarization resistance value (a) per unit film thickness is 120 to 300 kΩ · cm ² / μm, and the coating deposition amount (b) per unit quantity of electricity is in the range of 50 to 150 mg / C. A coating film forming method is known which uses a certain cationic electrodeposition coating and is applied at an effective voltage (V) of 230 V or less (see Patent Document 4). However, in such an adjusting method, the effect on the gas pinhole and the throwing power are not sufficiently compatible, which may adversely affect other coating workability and bath paint stability.

In the lead-free cationic electrodeposition coating composition, the cationic epoxy resin contains an amine-modified epoxy resin and a sulfonium-modified epoxy resin, and amino groups contained in a total amount of 100 g of the amine-modified epoxy resin and the blocked isocyanate curing agent. The number of milliequivalents of the acid required for neutralization is 5-30, and the number of milliequivalents of the sulfonium base contained in 100 g of the total amount of the sulfonium-modified epoxy resin and the blocked isocyanate curing agent is 5-30, A coating material having a minimum film-forming temperature (MFT) of a coating film electrodeposited on an object to be coated is 20 to 35 ° C. (see Patent Document 5). However, in this MFT range, the throwing power was good, but the gas pinhole resistance was not sufficiently satisfactory.

Japanese Patent Laid-Open No. 10-36717 JP 2000-204299 A JP 2001-019878 A JP 2002-275690 A JP 2002-356645 A

The present invention relates to a cationic electrodeposition coating composition that prevents gas pinholes from being generated on a conductive substrate such as an alloyed hot-dip galvanized steel sheet, and has excellent throwing power, anticorrosiveness, and coating stability. It aims to provide an electrodeposition coating method using

As a result of intensive studies to solve the above problems, the present inventors have added a special organic solvent to the cationic electrodeposition coating composition in an amount of 0.1 to 1.0% by mass, thereby reducing the minimum film-forming temperature. By electrodeposition coating a cationic electrodeposition coating composition with (MFT) of 15 ° C. or more and less than 20 ° C., it is possible to prevent the occurrence of gas pinholes and obtain a coating film that satisfies other performances. As a result, the present invention has been completed.
That is, the present invention paints an organic solvent having a solubility in water of 0.1 to 10% by mass, a solubility parameter (SP) value of 9.0 to 12.0, and a boiling point of 120 ° C. or higher. A cationic electrodeposition coating composition comprising 0.1 to 1.0% by mass in a composition and electrodeposition coating a cationic electrodeposition coating composition having a minimum film-forming temperature (MFT) of 15 ° C. or higher and lower than 20 ° C. Providing a coating method.

The present invention also provides a cationic electrodeposition coating method wherein the organic solvent has at least one substituent selected from a phenyl ether group, a glycol ether group and a ketone group.
The present invention also provides a cationic electrodeposition coating method, wherein the organic solvent is anisole and / or propylene glycol phenyl ether having a phenyl ether group.
The present invention also provides a cationic electrodeposition coating method wherein the organic solvent is propylene glycol monobutyl ether having a glycol ether group.
Furthermore, the present invention provides the cationic electrodeposition coating method, wherein the organic solvent is isophorone having a ketone group.

With the electrodeposition coating method of the present invention, the occurrence of gas pinholes during coating can be suppressed on conductive substrates such as galvannealed steel sheets, and the throwing power is good. The coating film is excellent in corrosion resistance.

  The cationic electrodeposition coating composition used in the present invention has an organic solubility of 0.1 to 10% by mass in water, an SP value of 9.0 to 12.0, and a boiling point of 120 ° C. or higher. Contains a solvent. By blending this organic solvent, it is possible to effectively and easily lower the MFT without lowering other advantages. The addition of an organic solvent that is insoluble in water significantly reduces the stability of the paint. On the other hand, an organic solvent having high solubility in water has a low effect on lowering MFT, and therefore requires a large amount. In addition, a solvent having an SP value outside this range has a low compatibility with the resin, resulting in poor appearance or reduced paint stability. Further, the solvent having a boiling point of less than 120 ° C. evaporates from the coating composition and is not stable with time.

The solubility of the organic solvent in water is preferably 0.12 to 8% by mass, particularly preferably 0.13 to 7% by mass. The SP value of the organic solvent is preferably 9.2 to 11.8. The upper limit of the boiling point of the organic solvent is not particularly limited, but is preferably 300 ° C. or lower.
In addition, said SP value is a value calculated from the basic structural formula of the compound from the values of Δei and ΔVi at 25 ° C. proposed by Fedors in the following formula (reference: “for engineers” Practical polymer ", P71-77, edited by Kodansha Scientific).
SP value (δ) = √ (ΣΔei / ΣΔVi)
(In the formula, Δei is the evaporation energy of the atom or atomic group, ΔVi is the molar volume of the atom or atomic group, and the unit of the SP value is the half power of (cal / cm ³ ).)

Such an organic solvent preferably has at least one substituent selected from a phenyl ether group, a glycol ether group and a ketone group, and has at least one substituent selected from a phenyl ether group and a glycol ether group. It is particularly preferable to have it. Examples of the organic solvent having a phenyl ether group include anisole and propylene glycol phenyl ether. Examples of the organic solvent having a glycol ether group include propylene glycol butyl ether. Examples of the organic solvent having a ketone group include cyclohexanone, Examples include isophorone. Most preferred are anisole and propylene glycol phenyl ether.
Content of the said organic solvent is 0.1-1.0 mass% with respect to the whole quantity of a cationic electrodeposition coating composition, Preferably it is 0.12-0.95 mass%.

In the cationic electrodeposition coating method of the present invention, the minimum film-forming temperature (MFT) needs to be 15 ° C. or higher and lower than 20 ° C., preferably 15.8 to 19 ° C., particularly preferably 16 to 18 °. ° C. Here, the minimum film-forming temperature refers to the electrodeposition paint bath temperature at which the film thickness becomes a minimum value in the relationship between the bath temperature and the film thickness during electrodeposition coating. When it is 20 ° C. or higher, gas pinholes start to be generated. If it is less than 15 degreeC, throwing power will fall rapidly. In general, a paint considering excellent anticorrosion and throwing power has a high molecular weight, high Tg resin design, resulting in a high MFT. In this case, the occurrence of gas pinholes becomes a problem. It is very difficult to balance and adjust the paint parameters so as to lower the MFT, and other advantageous performances are often lowered.

  The cationic electrodeposition coating composition used in the present invention is preferably a blocked isocyanate curable cationic electrodeposition coating composition. The block isocyanate curable cationic electrodeposition coating composition contains a resin having a cationic charge and a blocked isocyanate curing agent, and performs electrodeposition using the charge of the resin, and blocks the blocked isocyanate curing agent when the coating film is cured. It is a cationic electrodeposition coating composition of a type that cures when the agent is dissociated and the active hydrogen in the resin component reacts.

The resin having a cationic charge is used as a coating film-forming resin, and is a thermosetting resin in which a cationic group imparting cationicity is bonded to the base resin. Cationic groups include primary to quaternary amino groups, and these groups serve as crosslinking points, but groups having other active hydrogens such as hydroxyl groups may be bonded as crosslinking points. Examples of the base resin include resins such as epoxy resins, acrylic resins, alkyd resins, polybutadiene resins, and polyester resins. The base resin may be used alone or in combination of two or more different resins.

As the base resin, an epoxy resin is optimal from the viewpoint of anticorrosion, and an amine-added epoxy resin is generally used in the cationic electrodeposition coating. For example, an adduct of polyglycidyl ether and primary mono or polyamine, secondary mono or polyamine, or primary and secondary mixed mono or polyamine; polyglycidyl ether and 2 having a ketiminated primary amino group And adducts with grade mono- or polyamines.
As the epoxy resin, all low molecular weight and high molecular weight compounds can be used as long as they have an average of more than one glycidyl group per molecule in the present invention. Particularly preferred are polyglycidyl ethers having two or more glycidyl groups.

Examples of the polyglycidyl ether include polyglycidyl ethers of polyphenols such as 1,1-bis (4-hydroxyphenyl) ethane, 2,2-bis ( 4-hydroxyphenyl) propane, 2,2-bis (4-hydroxy-3-tert-butylphenyl) propane, 1-5-dihydroxy-3-naphthalene and the like.
Polyglycidyl ethers of other cyclic polyols other than polyphenols can also be used. Examples of other cyclic polyols include alicyclic polyols, and specific examples thereof include 1,2-cyclohexanediol, 1,4-cyclohexanediol, 1,2-bis (hydroxymethyl) cyclohexane, and hydrogenated products. Bisphenol A and bisphenol F which have been prepared.

  Further, as other polyglycidyl ethers, for example, polyglycidyl ethers obtained by reacting polyglycidyl ethers of polyalkylene glycols such as polyethylene glycol and polypropylene glycol with monophenol compounds, phenolic novolac resins or similar polyphenols. Examples thereof include polyglycidyl ethers of resins and polyglycidyl ethers obtained by reaction of epoxy compounds with carboxyl-terminated butadiene-acrylonitrile copolymers.

  Examples of the primary mono- and polyamines, secondary mono- and polyamines to be reacted with the polyglycidyl ether include mono- and dialkyls such as methylamine, ethylamine, propylamine, butylamine, dimethylamine, diethylamine, dipropylamine, and methylbutylamine. Amines; mono- and dialkanolamines such as ethanolamine, N-methylethanolamine, diethanolamine; and dialkylaminoalkylamines such as dimethylaminoethylamine, diethylaminopropylamine as polyamines; and NN-dimethyl-2-hydroxyethyl- Examples include propylene diamine.

Examples of the mono- and polyamine having a ketiminated primary amino group include N-methylaminopropylamine and a reaction product of diethylenetriamine and methyl isobutyl ketone.
When the cationic electrodeposition coating composition used in the present invention contains a pigment, it is preferable to use a pigment dispersing resin. There is no restriction as a resin for dispersing the pigment, and the resin having the cationic charge can be used. However, a resin having improved pigment dispersibility is preferable, for example, an epoxy quaternary ammonium type or a tertiary amine type, acrylic 4 A quaternary ammonium type resin is preferred.

The coating film-forming resin used in the cationic electrodeposition coating composition in the present invention is preferably composed of the resin having the cationic charge and the pigment dispersing resin, but may contain other resins as necessary.
The pigment used in the present invention is not particularly limited as long as it is a pigment that is usually used in electrodeposition paints, and can be blended as necessary. Examples of such pigments include, for example, colored pigments such as titanium oxide, bengara, carbon black, and yellow iron oxide; extender pigments such as clay, talc, mica, silica, and calcium carbonate; zinc phosphomolybdate, aluminum phosphate, and polyphosphorus Examples include rust preventive pigments such as aluminum oxide.

  The blocked isocyanate curing agent used in the present invention may be a self-crosslinking type contained in the molecule of the base resin, or an external crosslinking type contained outside the base resin. The isocyanate and blocking agent for preparing the blocked isocyanate curing agent used for both the self-crosslinking type and the external crosslinking type are the following polyisocyanate compounds and blocking agents that dissociate when heated at 100 to 200 ° C. Are suitable.

  Examples of the polyisocyanate compound include tolylene isocyanate, xylylene diisocyanate, tetramethylene diisocyanate, 4,4′-diphenylmethane diisocyanate, polymethylene polyphenyl polyisocyanate, p-phenylene diisocyanate, 3,3′-dimethyldiphenyl 4,4 ′. -Aromatic, aliphatic and alicyclic groups such as diisocyanate, 1,5-naphthalene diisocyanate, 1,6-hexane diisocyanate, lysine diisocyanate, 4,4′-dicyclohexylmethane diisocyanate, isophorone diisocyanate and norbornane diisocyanate. These polyisocyanate compounds include, for example, polyols such as ethylene glycol, propylene glycol, tetramethylene glycol, trimethylolpropane, polyethylene glycol, polypropylene glycol, polycaprolactone, polycarbonate polyol, adipic acid, phthalic acid, azelaic acid, sebacin It may be reacted with a carboxylic acid such as acid or maleic acid to have an isocyanate at the end.

  Examples of the blocking agent include aliphatic alcohols such as methanol, ethanol, n-butanol and 2-ethylhexanol, oximes such as methyl ethyl ketoxime, diacetketoxime, butanone-2-oxime and cyclohexanone oxime, phenol, p- Cresol, phenols such as p-tertiary butylphenol, lactams such as ε-caprolactam, ethylene glycol monomethyl ether, ethylene glycol monobutyl ether, ethylene glycol monobutyl ether, propylene glycol monobutyl ether, diethylene glycol monomethyl ether, ethylene glycol monophenyl ether, Ether alcohols such as propylene glycol monophenyl ether, phenyl carbitol Which aromatic alkyl alcohols. Caprolactams and oximes are particularly suitable for coatings that require low-temperature curability.

When a dissociation catalyst is used for dissociation of the block agent of the block polyisocyanate curing agent, organic tin compounds such as dibutyltin laurate, dibutyltin oxide and dioctyltin, amines such as N-methylmorpholine, zinc, cobalt and bismuth Metal salts such as zirconium can be used. The density | concentration of a dissociation catalyst is 0.1-5 mass% normally with respect to the coating-film formation solid content in a cationic electrodeposition coating-material composition.
In the blocked isocyanate-containing cationic electrodeposition coating composition, in the self-crosslinking type having a blocked isocyanate in the base resin, a conventionally known method can be used, for example, in a partially blocked polyisocyanate compound. The blocked isocyanate is introduced into the base resin by, for example, a method of reacting free isocyanate with active hydrogen in the base resin. In the case of the external cross-linking type, the polyisocyanate that is completely blocked is used.

The cationic electrodeposition coating composition used in the present invention is an electrodeposition coating resin blocked isocyanate such as a resin having a cationic charge, and, if necessary, a pigment, a plasticizer, a surfactant, an antifoaming agent, Additives such as repellency inhibitors, antioxidants, and UV absorbers can be blended. The blending ratio of the above components is not particularly limited.
The aqueous electrodeposition coating resin for forming such an aqueous solution or emulsion can be made aqueous by neutralizing with an acid. Examples of the acid used as the neutralizing acid include water-soluble substances such as formic acid, acetic acid, and lactic acid. Mention may be made of organic acids. The amount of these acids used is an amount necessary for neutralization, but the amount of the acid is appropriately selected from the stability of the paint and the coating workability. In general, the acid amount is such that the neutralization rate is 40% or more with respect to the molar amount of the base of the resin measured by neutralization titration.

The cationic electrodeposition coating composition used in the present invention is usually produced using a ball mill, a sand mill, a super mill, a continuous disperser or the like until the pigment is crushed into a desired size and sufficiently wetted and dispersed to produce a pigment paste. In making the resin aqueous, an amount of water sufficient to form a continuous water phase is blended, and an aqueous solution or emulsion is produced using a stirrer, an emulsifying device, or the like.
The cationic electrodeposition coating composition used in the present invention may be added with an aqueous medium such as water if necessary so that the coating solid content is generally 5 to 35% by mass and the pH is 5.0 to 6.5. Adjust and apply electrodeposition. As the coating conditions, for example, it is preferable to perform the coating so that the coating temperature is 25 to 35 ° C., the voltage is 40 to 400 V, and the thickness of the cured coating film is 10 to 40 μm. Furthermore, the range of 100-200 degreeC is suitable for the baking temperature at the time of hardening of a coating film.

The cationic electrodeposition coating composition used in the present invention is a conductive substrate, for example, a metal such as cold-rolled steel, hot-rolled steel, aluminum, magnesium, magnesium alloy, and the surface of these substrates as necessary. It can be applied to a desired one according to the required quality, such as those subjected to treatment, for example, zinc phosphate treatment, iron phosphate treatment, chromate treatment, organic film treatment and the like.
Particularly preferred is galvanized steel such as alloyed hot dip galvanized steel. Although the hot dip galvanized steel sheet has very excellent corrosion resistance, the coating property is likely to cause blistering under the coating film and cause peeling of the coating. In addition, there are cases where the weldability is also lowered. In order to make up for these drawbacks, an alloyed hot-dip galvanized steel sheet is obtained by heating the plated layer to an alloy of Zn-Fe by heating after hot-dip galvanizing. There are types of plating adhesion such as F06 (single-sided adhesion 45 g / m ² ), F08 (single-sided adhesion 60 g / m ² ), F12 (single-sided adhesion 90 g / m ² ), etc. There are “Silver Alloy” manufactured by Nippon Steel Corporation, “Palm Zinc Alloy” manufactured by NKK Corporation, “River Alloy” manufactured by Kawasaki Steel Co., Ltd., “Galba Ace Alloy” manufactured by Kobe Steel Co., Ltd., and the like.
Even when the electrodeposition coating composition used in the present invention is applied to a galvanized steel sheet, pinholes and craters are hardly generated in the coating film, and an excellent appearance can be provided.
The intermediate coating material and the top coating material applied on the electrodeposition coating film obtained by the electrodeposition coating method of the present invention are not particularly limited, and examples thereof include commonly used intermediate coating materials and top coating materials.

Next, although the Example of this invention is described in detail, this patent is not limited to these description. In each example, all the descriptions regarding “parts” and “%” are “parts by mass” or “% by mass” unless otherwise specified.

(Production Example 1 Production of pigment dispersion resin)
2598 parts of bisphenol A-diglycidyl ether having an epoxy equivalent of 188, 787 parts of bisphenol A, 603 parts of dodecylphenol and 206 parts of butyl glycol are reacted at 130 ° C. in the presence of 4 parts of triphenylphosphine to obtain an epoxy equivalent of 856. To do. While cooling, dilute with 849 parts of butyl glycol and 1534 parts of polypropylene glycol diglycidyl ether (Dow Chemical, trade name “DER732”), and at 90 ° C., 266 parts of 2,2-aminoethoxyethanol and N, N— Further reaction with 212 parts of dimethylaminopropylamine. After 2 hours, after the viscosity became constant, diluted with 1512 parts of butyl glycol, neutralized with 201 parts of glacial acetic acid, and further added with 1228 parts of deionized water, a pigment-dispersing resin aqueous solution having a solid content of 60% was obtained. Created.

(Production Example 2 Production of curing agent)
In a reactor equipped with a stirrer, reflux condenser, internal temperature controller, and inert gas supply pipe, 10552 parts of polymethylene polyphenyl polyisocyanate (manufactured by BASF, trade name “Luprant”) was previously placed in a nitrogen atmosphere. Enter. 18 parts of dibutyltin dilaurate are added, and 9498 parts of butyl diglycol are added dropwise little by little with occasional cooling so that the reaction temperature does not exceed 60 ° C. After the addition is complete, the temperature is maintained at 60 ° C. for an additional 60 minutes, and 933 parts of dissolved trimethylolpropane is added at a rate such that the product does not exceed 100 ° C. after dissolution in 7768 parts of methyl isobutyl ketone. When the reaction was further continued for 60 minutes after the addition was completed, no free NCO group was detected. The mixture was cooled to 65 ° C. and simultaneously diluted with 965 parts of n-butanol and 267 parts of methyl isobutyl ketone to obtain a blocked polyisocyanate having a solid content of 70%.

(Production Example 3 Production of binder emulsion)
In a reactor equipped with a stirrer, reflux condenser, internal thermometer and inert gas supply pipe, 6150 parts of epoxy resin based on bisphenol A having an epoxy equivalent of 188 is added to 1400 parts bisphenol A, 335 parts dodecylphenol, p- The mixture is heated to 125 ° C. in a nitrogen atmosphere together with 470 parts of cresol and 441 parts of xylene, and maintained at 125 ° C. for 10 minutes while adjusting the temperature by cooling the exothermic reaction. Subsequently it is heated to 130 ° C. and 23 parts of N, N′-dimethylbenzylamine are added. Maintain this temperature until the epoxy equivalent weight reaches 880. A mixture comprising 7097 parts of the curing agent obtained in Production Example 2 and 90 parts of additive polyether (manufactured by BYK Chemie, trade name “K-2000”) is added and maintained at 100 ° C. After 30 minutes, 211 parts of butyl alcohol and 1210 parts of isobutanol are added.

Immediately after this, diethylenetriamine and methyl isobutyl ketone were mixed, and then heated and refluxed at 130 to 150 ° C. to remove the generated water, and 467 parts of ketimine obtained by cooling when the distillation of the generated water stopped at 150 ° C. A mixture of 520 parts of methylethanolamine is added to the reactor and the temperature is adjusted to 100 ° C. After a further 30 minutes, the temperature is raised to 105 ° C. and 159 parts of N, N′-dimethylaminopropylamine are added. 75 minutes after addition of the amine, 903 parts of a propylene glycol compound (trade name “Plastitit 3060” manufactured by BASF Corporation) is added, diluted with 522 parts of propylene glycol phenyl ether, and cooled to 95 ° C. After 10 minutes, 14821 parts of the reaction mixture are transferred to a disperser. Here, 424 parts of 88% lactic acid is dissolved in 7061 parts of deionized water and added dropwise with stirring. Subsequently, the mixture was homogenized for 20 minutes and then further diluted with 12600 parts of further deionized water to obtain an emulsion having a solid content of 34%.

(Production Example 4 Production of pigment paste)
1750 parts of the pigment dispersion resin obtained in Production Example 1 and 1897 parts of deionized water are mixed in advance. Then 42 parts of carbon black, 2667 parts of titanium oxide, 420 parts of aluminum silicate, and 189 parts of di-n-butyltin oxide are added. After thoroughly mixing this mixture, it was dispersed by a ball mill to a particle gauge of 5 μm or less and deionized water was added to obtain a pigment paste having a solid content of 60%.

(Production Example 5 Production of diluted paint)
To the emulsion obtained in Production Example 1, the pigment paste obtained in Production Example 4 was added with deionized water to prepare a cationic electrodeposition paint having a solid content of 20%.
<Examples 1-4>
After adding various organic solvents shown in Table 1 to the diluted cationic electrodeposition coating composition in the proportions shown in Table 1, the MFT, gas pinhole property, throwing power, and coating stability of the cationic electrodeposition coating composition The coating performance was tested.
<Comparative Examples 1-3>
After adding various organic solvents shown in Table 2 to the diluted cationic electrodeposition coating composition in the proportions shown in Table 2, the MFT, gas pinhole property, throwing power, and coating stability of the cationic electrodeposition coating composition The coating performance was tested.

(Gas pinhole property)
An alloyed hot-dip galvanized steel sheet (made by Nippon Steel Corporation, trade name “Silver Alloy”, 0.8 mm × 70 mm × 150 mm) subjected to chemical conversion treatment with PB-L3020 (manufactured by Nihon Parkerizing) The electrodeposition coating was carried out for 3 minutes by the energization method that reached the specified voltage at the start instant at the same electrodeposition paint bath temperature and coating voltage as in the property test. After painting, it was washed with water, baked at 170 ° C. for 20 minutes, the state of the coating film was observed, and visually judged according to the following criteria.
○: No pinholes are generated. Δ: Pinholes of 10 points or less are generated. ×: Pinholes of 10 points or more are generated.

(Throwing power)
The test method described in the published patent “Japanese Patent Laid-Open No. 2000-204299” was used. That is, the evaluation is based on the so-called four-sheet box method, and as shown in FIG. 1, four zinc phosphate-treated SPC steel sheets (PB-L3020-treated JIS G3141 SPCC-SD).
Steel box, 0.8 mm × 70 mm × 150 mm) 11 to 14 are used in a state where the boxes 10 are arranged in parallel at an interval of 20 mm, and the lower and bottom surfaces of both sides are sealed with an insulator such as cloth adhesive tape. In addition, the steel plates 11 to 13 other than the steel plate 14 are provided with a through hole 15 of 8 mmΦ at the bottom. As shown in FIG. 2, the box 10 is immersed in a container containing the cationic electrodeposition paint 21 so that the diluted paint enters the box only from each hole.

In this state, the steel plates were electrically connected, and the counter electrode 22 was arranged so that the distance from the nearest steel plate 11 was 150 mm. Each steel plate was used as a cathode, and a counter electrode 22 was used as an anode to apply a voltage, and the steel plate was subjected to cationic electrodeposition coating. The coating was boosted to a voltage at which the film thickness of the coating film formed on the A surface of the steel plate 11 reached 20 μm instantaneously from the start of application, and then the voltage was maintained for 3 minutes. The electrodeposition paint bath temperature at this time was adjusted to 30 ° C. Each coated steel sheet was washed with water, baked at 170 ° C. for 20 minutes, and after air cooling, the film thickness of the coating film formed on the A surface of the steel sheet 11 closest to the counter electrode 22 and the steel sheet 14 farthest from the counter electrode 22 The film thickness formed on the G surface was measured, and throwing power was evaluated by the ratio (G / A value) of film thickness (G surface) / film thickness (A surface). It can be evaluated that the greater the value, the better the throwing power.

(Anti-corrosion)
An electrodeposition paint bath temperature of 30 ° C. so that the film thickness becomes 20 μm on an SCP steel sheet (PB-L3020-treated JIS G3141 SPCC-SD steel sheet, 0.8 mm × 70 mm × 150 mm) treated with zinc phosphate. I electrodeposited. The coating film obtained by baking this at 170 ° C. for 20 minutes was used to put scratches reaching the substrate with a cutter, and this was immersed in 5% saline at 55 ° C. for 240 hours, and then the cut part was tape-peeled. The peel width on both sides of the cut part was evaluated according to the following criteria.
○: 3 mm or less Δ: 3 to 5 mm
×: 5 mm or more

表１中の有機溶剤の略号は、以下のものを示す。
ＡＳ：アニソール
ＰＰ：プロピレングリコールモノフェニルエーテル
ＰＢ：プロピレングリコールモノブチルエーテル
ＩＰ：イソホロン

The abbreviations of organic solvents in Table 1 indicate the following.
AS: Anisole PP: Propylene glycol monophenyl ether PB: Propylene glycol monobutyl ether IP: Isophorone

The abbreviations of organic solvents in Table 2 indicate the following.
AS: anisole PM: propylene glycol monomethyl ether BC: butyl cellosolve BA: benzyl alcohol From the results of the examples, the cationic electrodeposition coating method of the present invention has good suitability for galvannealed steel sheets and excellent throwing power. It was also confirmed that there was no decrease in coating film performance and paint stability.

It is a perspective view which shows an example of the box used when evaluating throwing power. It is explanatory drawing which shows the evaluation method of throwing power.

Claims (5)

An organic solvent having a solubility in water of 0.1 to 10% by mass, a solubility parameter value of 9.0 to 12.0, and a boiling point of 120 ° C. or higher is contained in the coating composition in an amount of 0.1 to 1. A cationic electrodeposition coating method comprising electrodeposition-coating a cationic electrodeposition coating composition containing 0.0% by mass and having a minimum film-forming temperature of 15 ° C. or higher and lower than 20 ° C. The cationic electrodeposition coating method according to claim 1, wherein the organic solvent has at least one substituent selected from a phenyl ether group, a glycol ether group and a ketone group. The cationic electrodeposition coating method according to claim 1, wherein the organic solvent is anisole having a phenyl ether group and / or propylene glycol phenyl ether. 2. The cationic electrodeposition coating method according to claim 1, wherein the organic solvent is propylene glycol monobutyl ether having a glycol ether group. The cationic electrodeposition coating method according to claim 1, wherein the organic solvent is isophorone having a ketone group.

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