WO2025205412A1 - Cationic electrodeposition coating material composition - Google Patents

Abstract

The present invention addresses the problem of providing a cationic electrodeposition coating material composition having excellent anti-corrosion properties and finishability on an edge portion and a flat portion of a coated article, and the coated article having such excellent coating film performance. As a solution, a cationic electrodeposition coating material composition is characterized in that, if the cationic electrodeposition coating material composition is electrodeposition-coated on a metal substance to be coated and the obtained uncured electrodeposition coating film is heat-dried, a storage modulus (G'), a loss modulus (G"), and a minimum value (G'min) of G' during the heat-drying step of the coating film satisfy specific numerical ranges. Furthermore, provided is the cationic electrodeposition coating material composition containing an amine-modified epoxy resin (A), a blocked polyisocyanate compound (B), and a viscosity modifier (C).

Description

Cationic electrodeposition coating composition

The present invention relates to a cationic electrodeposition coating composition and a method for forming a coating film using the cationic electrodeposition coating composition, which is characterized in that when the cationic electrodeposition coating composition is electrodeposited onto a metal substrate and the resulting uncured electrodeposition coating film is heat-dried, the storage modulus (G'), loss modulus (G"), and minimum G' value (G'min) during the heat-drying process satisfy specific numerical ranges.

Cationic electrodeposition coating compositions offer excellent paintability and the resulting coating films have good corrosion resistance, making them widely used as primers for conductive metal products that require these properties, such as automobile bodies, automobile parts, electrical equipment parts, and other devices.

If the object to be coated has sharp edges, the coating film at the edges may become thin when the paint is heated and cured, resulting in poor corrosion protection. Therefore, when painting objects with edges, there is a need for a means to improve the corrosion protection of the edges.

Patent Document 1 discloses the inclusion of a polyacrylamide resin in an electrodeposition paint as a method for improving the rust prevention properties of edge portions. The inclusion of this resin is thought to control shrinkage caused by heating or to inhibit a decrease in edge covering due to flow through interaction with the coating film components, but the inclusion of a highly polar, soluble resin can sometimes result in poor corrosion prevention properties on flat surfaces.
Patent Documents 2 and 3 disclose the inclusion of a cationic microgel dispersion (epoxy viscosity modifier) in an electrodeposition coating. By including the resin, it is possible to suppress the flow of the electrodeposition coating film due to heat flow at the edge, but there are cases where sufficient corrosion protection at the edge cannot be obtained under severe corrosion conditions.

Japanese Patent Application Laid-Open No. 2017-214572 Japanese Patent Application Laid-Open No. 2018-159032 Japanese Patent Application Publication No. 7-268063

The problem that this invention aims to solve is to provide a cationic electrodeposition coating composition and a coating film formation method that provide excellent corrosion resistance and finish on edge and flat surfaces.

As a result of intensive investigations to solve the above-mentioned problems, the inventors have found that the above-mentioned problems can be solved by a cationic electrodeposition coating composition, which is electrodeposited onto a metal substrate and the resulting uncured electrodeposition coating film is dried by heating, so that the storage modulus (G'), loss modulus (G"), and minimum value of G' in the heat drying step (G'min) of the coating film satisfy the following formulas (1) to (3), and have thus completed the present invention.
G'>1.05×G" ^0.7 ...Formula (1)
G'>60 (Pa)...Formula (2)
G'min<300 (Pa)...Formula (3)
That is, the present invention provides the following cationic electrodeposition coating composition and coated articles obtained by the method for coating a cationic electrodeposition coating film.
Item 1. A cationic electrodeposition coating composition, characterized in that when the cationic electrodeposition coating composition is electrodeposited onto a metal substrate and the resulting uncured electrodeposition coating film is heat-dried, the storage modulus (G'), loss modulus (G"), and minimum value of G' in the heat-drying step (G'min) of the coating film satisfy the following formulas (1) to (3):
G'>1.05×G" ^0.7 ...Formula (1)
G'>60 (Pa)...Formula (2)
G'min<300 (Pa)...Formula (3)
Item 2. The cationic electrodeposition coating composition according to Item 1, characterized in that the cationic electrodeposition coating composition contains an amine-modified epoxy resin (A), a blocked polyisocyanate compound (B), and a viscosity modifier (C).
Item 3. The cationic electrodeposition coating composition according to Item 2, wherein the amine-modified epoxy resin (A) is a reaction product of an epoxy resin and an amine compound.
Item 4. The cationic electrodeposition coating composition according to Item 2 or 3, wherein the amine-modified epoxy resin (A) has a number average molecular weight of 3,000 or more and a glass transition temperature of 100° C. or less.
Item 5. The cationic electrodeposition coating composition according to any one of Items 2 to 4, wherein the viscosity modifier (C) contains a cationic microgel (C-1).
Item 6. The cationic electrodeposition coating composition according to Item 5, wherein the cationic microgel (C-1) contains a silane coupling agent as a constituent component.
Item 7. The cationic electrodeposition coating composition according to any one of Items 2 to 6, wherein the viscosity modifier (C) comprises a (meth)acrylate-modified epoxy (C-2) and/or a hydroxy(meth)acrylate-blocked polyisocyanate compound (C-3).
Item 8. The cationic electrodeposition coating composition according to any one of Items 2 to 7, further comprising a bismuth compound.
Item 9. The cationic electrodeposition coating composition according to any one of Items 2 to 8, wherein the blocked polyisocyanate compound (B) is a reaction product of a polyisocyanate compound and a blocking agent, and the blocking agent contains a diol compound having two hydroxyl groups with different reactivities.
Item 10. A method for forming a coating film, comprising electrodeposition coating a metal substrate with the cationic electrodeposition coating composition according to any one of Items 1 to 9, and subsequently heating and drying the resulting uncured electrodeposition coating film.
Item 11. A method for forming a coating film from a cationic electrodeposition coating composition, comprising electrodeposition coating a metal substrate with a cationic electrodeposition coating composition containing an amine-modified epoxy resin (A), a blocked polyisocyanate compound (B), and a viscosity modifier (C), and subsequently heating and drying the resulting uncured electrodeposition coating film, wherein the storage modulus (G'), loss modulus (G"), and minimum value of G' in the heating and drying step (G'min) of the electrodeposition coating film satisfy the following formulas (1) to (3):
G'>1.05×G" ^0.7 ...Formula (1)
G'>60 (Pa)...Formula (2)
G'min<300 (Pa)...Formula (3)

The cationic electrodeposition coating composition of the present invention provides excellent corrosion resistance and finish on edges and flat surfaces, and maintains good corrosion resistance on edges even under severe corrosive conditions. Automobile bodies coated with the product of the present invention experience little corrosion deterioration even when driven in an environment where snow-melting salt is sprayed.

1 shows an example of viscoelasticity measurement (measurements from 50° C. to 150° C.). These are cross-sectional photographs of the edge portion (left photo: exposed edge portion, right photo: coated edge portion).

Hereinafter, embodiments of the present invention will be described in detail.
It should be understood that the present invention is not limited to the following embodiments, but also includes various modifications that are implemented within the scope of the present invention.
In the present invention, the storage modulus (G') can be abbreviated as "G'", the loss modulus (G") as "G", and the minimum value of G' in the heat drying step (G'min) as "G'min".

The cationic electrodeposition coating composition of the present invention is characterized in that when the cationic electrodeposition coating composition is electrodeposited onto a metal substrate and the resulting uncured electrodeposition coating film is dried by heating, the storage modulus (G'), loss modulus (G"), and minimum value of G' in the heat drying step (G'min) of the coating film satisfy the following formulas (1) to (3). In this specification, the units of G', G" and G'min are "Pa".
G'>1.05×G" ^0.7 ...Formula (1)
G'>60 (Pa)...Formula (2)
G'min<300 (Pa)...Formula (3)
<Method for measuring viscoelasticity>
The storage modulus (G') and loss modulus (G") can be measured (viscoelasticity measurement) using a rotational viscoelasticity measuring device. An example of such a measuring device is a viscoelasticity measuring device (manufactured by TA Instruments, trade name "ARES-G2").
Furthermore, to evaluate the curing properties of the coating film during the heat drying process, measurements were carried out using the jig described in JP 2010-78444 A. The jig is a viscoelasticity measuring jig having a rotating shaft and a circular body attached concentrically to the tip of the rotating shaft, the circular body having a cutout portion covering 60% of the projected area in the direction of the rotating shaft, i.e., the area of the projected outer circumference of the circular body, the diameter of the outer circumference being 40 mm, and the inner diameter being 38 mm.
Before measuring the viscoelasticity, the above-mentioned device was calibrated with a viscosity standard liquid (JS-100, manufactured by Nippon Grease) to ensure the correct viscosity.

The specific measurement conditions were as follows: first, a sample cationic electrodeposition coating composition was applied to a measurement cell of the viscoelasticity measuring device to a thickness of approximately 0.6 mm, and the above-mentioned jig was then placed on top of it and inserted into the sample to a depth of approximately 0.3 mm. The temperature was then raised from 50°C at a frequency of 1.0 Hz and a temperature rise rate of 15°C/min, and the storage modulus (G') and loss modulus (G") were measured between 50°C and 150°C, and the minimum value of the storage modulus (G') between 50°C and 150°C was taken as G'min.
It should be noted that the formulas (1) and (2) are said to be within the ranges when all measured values of the storage modulus (G') and loss modulus (G") between 50°C and 150°C are within the ranges.

For example, FIG. 1 shows an example of a graph of the viscoelasticity measurement results (measured values).
The graph shows the measured values of storage modulus (G') and loss modulus (G") between 50°C and 150°C, with the top rightmost value being the value at 50°C and the bottom leftmost value being the value at 150°C.
The shaded area in FIG. 1 is the range that satisfies both formulas (1) and (2).

According to this invention, by adjusting the dynamic viscoelasticity of the uncured coating film applied during electrodeposition coating, including the storage modulus (G'), loss modulus (G"), and the minimum value of G' during the heat drying process (G'min), it is possible to achieve both corrosion resistance and finish quality on both edge and flat surfaces.

When the electrodeposition coating composition is applied to the substrate by electrodeposition and then heated, the heat causes the coating to fluidize, and the curing reaction then begins. In this process, the storage modulus (G') and loss modulus (G") of the uncured coating prior to the start of the curing reaction, as well as the minimum G' value (G'min) during the heat drying process, are each controlled within specific ranges, thereby achieving both corrosion resistance and finish quality on both the edge and flat surfaces of the resulting cured coating.

If the coating viscosity during the heat drying process is kept above a certain value, edge coverage will be maintained, but problems will arise such as workability (lack of film buildability and finish) and the occurrence of voids (air bubbles) in the cured coating (reduced rust prevention on general surfaces).On the other hand, if the viscosity of the coating is lowered to improve film buildability and suppress voids, the edges will not be covered.
However, it was discovered that even if the viscosity decreases during the heat drying process, the edge coverage can be maintained by increasing the elasticity (G' is higher than G"). [Formula 1]
Furthermore, it was found that edge coverage is good when the storage modulus (G') is always above a certain value [Equation 2], but that finish quality (voids) and corrosion resistance deteriorate unless the minimum value of the storage modulus (G') is below a certain value [Equation 3].

Methods for adjusting the storage modulus (G') and loss modulus (G") include, for example, adding a viscosity modifier (C), adjusting the solids concentration, adjusting the molecular weight of the resin, adjusting the amount of functional groups in the resin, adjusting the glass transition temperature of the resin, and adjusting the pigment concentration. Adjustments can be made by using one or a combination of two or more of these methods. Of these, adjustments can be made easily and effectively by using the viscosity modifier (C), the molecular weight and/or the glass transition temperature of the resin (A).

Cationic electrodeposition coating composition The cationic electrodeposition coating composition of the present invention preferably contains an amine-modified epoxy resin (A), a blocked polyisocyanate compound (B), and a viscosity modifier (C), and preferably further contains a pigment dispersion paste.

Amine-modified epoxy resin (A)
The amine-modified epoxy resin (A) that can be used in the present invention is preferably a reaction product of an epoxy resin and an amine compound. Examples include (1) adducts of an epoxy resin with primary mono- and polyamines, secondary mono- and polyamines, or mixed primary and secondary polyamines (see, for example, U.S. Pat. No. 3,984,299); (2) adducts of an epoxy resin with secondary mono- and polyamines having a ketiminated primary amino group (see, for example, U.S. Pat. No. 4,017,438); and (3) reaction products obtained by etherification of an epoxy resin with a hydroxy compound having a ketiminated primary amino group (see, for example, JP-A-59-43013). Among these, it is preferable that the amine compound that reacts with the epoxy resin contains the ketiminated primary amine of (2) or (3) above.

The epoxy resin (A-1) used in producing the above-mentioned amine-modified epoxy resin (A) is a compound having at least one, preferably two or more, epoxy groups per molecule. Its molecular weight is preferably a number-average molecular weight of at least 300, preferably 400 to 6,000, and more preferably 800 to 4,000, and its epoxy equivalent is preferably at least 160, preferably 180 to 3,000, and more preferably 400 to 2,000. Such epoxy resins can be, for example, those obtained by reacting a polyphenol compound with an epihalohydrin (e.g., epichlorohydrin, etc.).

Examples of polyphenol compounds used to form the epoxy resin include one or more of bis(4-hydroxyphenyl)-2,2-propane [bisphenol A], bis(4-hydroxyphenyl)methane [bisphenol F], bis(4-hydroxycyclohexyl)methane [hydrogenated bisphenol F], 2,2-bis(4-hydroxycyclohexyl)propane [hydrogenated bisphenol A], 4,4'-dihydroxybenzophenone, bis(4-hydroxyphenyl)-1,1-ethane, bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxy-3-tert-butyl-phenyl)-2,2-propane, bis(2-hydroxynaphthyl)methane, tetra(4-hydroxyphenyl)-1,1,2,2-ethane, 4,4'-dihydroxydiphenyl sulfone, phenol novolac, and cresol novolac.

Among the epoxy resins obtained by the reaction of a polyphenol compound with an epihalohydrin, an epoxy resin derived from bisphenol A and represented by the following formula (1) is particularly suitable.
Furthermore, it is also possible to use an epoxy resin having a high molecular weight and/or a multifunctionality obtained by reacting an epoxy resin of the following formula (1) with a polyphenol compound, and among these, bisphenol A is preferred as the polyphenol compound.

Here, n = 0 to 8 is preferred.

Commercially available examples of such epoxy resins include those sold by Mitsubishi Chemical Corporation under the trade names "jER828EL," "jER1002," "jER1004," and "jER1007."

Furthermore, as the epoxy resin (A-1), an epoxy resin containing a polyalkylene oxide chain in the resin skeleton can be used. Typically, such an epoxy resin can be obtained by (α) a method of reacting an epoxy resin having at least one, preferably two or more, epoxy groups with an alkylene oxide or polyalkylene oxide to introduce a polyalkylene oxide chain, or (β) a method of reacting the above polyphenol compound with a polyalkylene oxide having at least one, preferably two or more, epoxy groups to introduce a polyalkylene oxide chain. Alternatively, an epoxy resin already containing a polyalkylene oxide chain may be used (see, for example, JP-A-8-337750).
The alkylene group in the polyalkylene oxide chain is preferably an alkylene group having 2 to 8 carbon atoms, more preferably an ethylene group, a propylene group or a butylene group, and particularly preferably a propylene group.
From the viewpoint of improving paint stability, finish quality and corrosion resistance, the content of the polyalkylene oxide chain is suitably within the range of usually 1.0 to 15.0 mass%, preferably 2.0 to 9.5 mass%, more preferably 3.0 to 8.0 mass%, as the content of the polyalkylene oxide as a constituent component, based on the solids mass of the amine-modified epoxy resin.

The primary mono- and polyamines, secondary mono- and polyamines, or mixed primary and secondary polyamines used in the production of the amine-modified epoxy resin (A) described above in (1) can include one or more of the following: mono- or di-alkylamines such as monomethylamine, dimethylamine, monoethylamine, diethylamine, monoisopropylamine, diisopropylamine, monobutylamine, and dibutylamine; alkanolamines such as monoethanolamine, diethanolamine, mono(2-hydroxypropyl)amine, and monomethylaminoethanol; and alkylenepolyamines such as ethylenediamine, propylenediamine, butylenediamine, hexamethylenediamine, diethylenetriamine, and triethylenetetramine.

The secondary mono- and polyamines having ketiminated primary amino groups used in the production of the amine-modified epoxy resin (A) described above in (2) include, for example, ketimines produced by reacting a ketone compound with diethylenetriamine, dipropylenetriamine, or the like, among the mixed primary and secondary polyamines used in the production of the amine-added epoxy resin described above in (1).

Examples of the hydroxy compound having a ketiminated primary amino group used in the production of the amine-modified epoxy resin (A) described in (3) above include hydroxyl group-containing ketimines obtained by reacting a ketone compound with a compound having a primary amino group and a hydroxyl group, such as monoethanolamine or mono(2-hydroxypropyl)amine, among the primary mono- and polyamines, secondary mono- and polyamines, or mixed primary and secondary polyamines used in the production of the amine-modified epoxy resin (A) described in (1) above.

The amine value of such amine-modified epoxy resin (A) is preferably in the range of 30 to 120 mg KOH/g resin solids, and more preferably 40 to 100 mg KOH/g resin solids, from the viewpoint of improving water dispersibility and corrosion resistance.

Furthermore, the amine-modified epoxy resin (A) can be modified with a modifier, if necessary. Such modifiers are not particularly limited as long as they are resins or compounds that are reactive with the epoxy resin (A-1). For example, one or more of the following can be used as modifiers: polyols, polyether polyols, polyester polyols, polyamidoamines, polycarboxylic acids, fatty acids, polyisocyanate compounds, compounds obtained by reacting polyisocyanate compounds, lactone compounds such as ε-caprolactone, acrylic monomers, compounds obtained by polymerizing acrylic monomers, xylene formaldehyde compounds, and epoxy compounds. These modifiers can be used alone or in combination of two or more.

The addition reaction of the above-mentioned amine compound and modifier to the epoxy resin (A-1) can usually be carried out in an appropriate solvent at a temperature of about 80 to about 170°C, preferably about 90 to about 150°C, for about 1 to 6 hours, preferably about 1 to 5 hours.

The above-mentioned solvents include, for example, hydrocarbons such as toluene, xylene, cyclohexane, and n-hexane; esters such as methyl acetate, ethyl acetate, and butyl acetate; ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, and methyl amyl ketone; amides such as dimethylformamide and dimethylacetamide; alcohols such as methanol, ethanol, n-propanol, and isopropanol; ether alcohol compounds such as ethylene glycol monobutyl ether and diethylene glycol monoethyl ether; and mixtures of these.

The proportion of the modifier used is not strictly limited and can be varied as appropriate depending on the application of the coating composition, but from the perspective of improving finish and corrosion resistance, it is generally appropriate to use a proportion within the range of 0 to 50 mass%, preferably 3 to 30 mass%, and more preferably 6 to 20 mass%, based on the solids mass of the amine-modified epoxy resin.

From the viewpoint of edge corrosion resistance, when the number average molecular weight of the amine-modified epoxy resin (A) is large (the lower limit is usually 2,000 or more, preferably 3,000 or more, and the upper limit is usually 5,000 or less, preferably 4,000 or less) and the glass transition temperature (Tg) is 100°C or less, the increase in G' is rapid and the edge is not exposed, and further, the viscosity is low due to the influence of the glass transition temperature (Tg), so that void (air bubble) formation is suppressed.
Even if the molecular weight is large, if the glass transition temperature (Tg) is high, G' and viscosity will always be high, so although the edge will be coated, voids will be formed, which may result in a decrease in finish and corrosion resistance due to the voids.

Unless otherwise specified, the number-average molecular weight and weight-average molecular weight of an uncrosslinked resin in this specification are values calculated by converting the retention time (retention volume) measured using gel permeation chromatography (GPC) into the molecular weight of polystyrene using the retention time (retention volume) of a standard polystyrene of known molecular weight measured under the same conditions. Specifically, the gel permeation chromatography uses "HLC8120GPC" (trade name, manufactured by Tosoh Corporation) and four columns: "TSKgel G-4000HXL," "TSKgel G-3000HXL," "TSKgel G-2500HXL," and "TSKgel G-2000HXL" (trade names, all manufactured by Tosoh Corporation). Measurements can be performed using a mobile phase of dimethylformamide (containing 0.5% by mass of triethanolamine), a measurement temperature of 40°C, a flow rate of 1 mL/min, and an RI detector.

Blocked polyisocyanate compound (B)
The blocked polyisocyanate compound (B) is a reaction product of a polyisocyanate compound and a blocking agent in approximately stoichiometric amounts. The polyisocyanate compound used in the blocked polyisocyanate compound (B) may be a known compound, such as aromatic, aliphatic, or alicyclic polyisocyanate compounds such as tolylene diisocyanate, xylylene diisocyanate, phenylene diisocyanate, diphenylmethane-2,2'-diisocyanate, diphenylmethane-2,4'-diisocyanate, diphenylmethane-4,4'-diisocyanate, crude MDI [polymethylene polyphenylisocyanate], bis(isocyanatomethyl)cyclohexane, tetramethylene diisocyanate, hexamethylene diisocyanate, methylene diisocyanate, and isophorone diisocyanate; cyclized polymers or biuret products of these polyisocyanate compounds; or combinations thereof.

In particular, aromatic polyisocyanate compounds such as tolylene diisocyanate, xylylene diisocyanate, phenylene diisocyanate, diphenylmethane-2,4'-diisocyanate, diphenylmethane-4,4'-diisocyanate, and crude MDI (preferably crude MDI) are more preferred for their corrosion resistance.

On the other hand, the above-mentioned blocking agent adds to and blocks the isocyanate groups of the polyisocyanate compound, and the blocked polyisocyanate compound produced by addition is stable at room temperature, but it is desirable that when heated to the coating film baking temperature (usually about 100 to about 200°C), the blocking agent dissociates to regenerate free isocyanate groups.

Blocking agents used in the blocked polyisocyanate compound (B) include, for example, one or more of the following: oxime compounds such as methyl ethyl ketoxime and cyclohexanone oxime; phenolic compounds such as phenol, para-t-butylphenol, and cresol; alcohol compounds such as n-butanol, 2-ethylhexanol, phenyl carbinol, methylphenyl carbinol, ethylene glycol monobutyl ether, diethylene glycol monoethyl ether, ethylene glycol, and propylene glycol; lactam compounds such as ε-caprolactam and γ-butyrolactam; and active methylene compounds such as dimethyl malonate, diethyl malonate, ethyl acetoacetate, methyl acetoacetate, and acetylacetone (preferably, alcohol compounds).

Among these, the diol compound (b-1), which is a type of alcohol-based compound, is preferred from the viewpoints of curability (anticorrosion properties and edge anticorrosion properties) and storage stability.
If the diol compound (b-1) has two hydroxyl groups with the same reactivity, the molecular weight may increase, potentially resulting in a deterioration in finished quality, and therefore it is preferable to contain a diol compound (b-2) having two hydroxyl groups with different reactivities, and examples of the diol compound (b-2) include at least one diol compound (b-2) selected from the group consisting of a primary hydroxyl group and a secondary hydroxyl group, a primary hydroxyl group and a tertiary hydroxyl group, and a secondary hydroxyl group and a tertiary hydroxyl group. Furthermore, in order to reduce loss on heating, the molecular weight of the blocking agent is preferably 300 or less, more preferably 200 or less, and even more preferably 150 or less.
Specific examples include one or more of propylene glycol, 1,3-butanediol, 1,2-butanediol, 3-methyl-1,2-butanediol, 1,2-pentanediol, 1,4-pentanediol, 3-methyl-4,3-pentanediol, 3-methyl-4,5-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, 1,5-hexanediol, and 1,4-hexanediol.
Among these, diol compounds having a primary hydroxyl group and a secondary hydroxyl group are preferred, and propylene glycol is suitable from the viewpoints of the curability of the blocked polyisocyanate, reduction in heat loss, storage stability of the coating material, etc. These diol compounds (b-2) usually react with the isocyanate group from the more reactive hydroxyl group to block the isocyanate group.
The diol compound (b-2) can be used in combination with other blocking agents, and the content (mol %) of the diol compound (b-2) having two hydroxyl groups with different reactivities is preferably 50 mol % or more, more preferably 96 mol % or more, and even more preferably 100 mol %.

Viscosity adjuster (C)
The viscosity modifier (C) that can be contained in the cationic electrodeposition coating composition of the present invention can be any known additive capable of adjusting viscosity, and for example, one or more selected from the group consisting of cationic microgels (C-1), (meth)acrylate-modified epoxies (C-2), hydroxy(meth)acrylate-blocked polyisocyanate compounds (C-3), inorganic particles (C-4), and polar polymers (C-5) can be suitably used.
Among these, it is preferable to contain at least one selected from the group consisting of cationic microgel (C-1), (meth)acrylate-modified epoxy (C-2), and hydroxy(meth)acrylate-blocked polyisocyanate compound (C-3).

The amount of viscosity modifier (C) added is typically 0.1 to 40% by mass, preferably 0.1 to 20% by mass, and more preferably 0.5 to 20% by mass, based on the total mass of the solid contents of resin (A) and compound (B).

Cationic microgel (C-1)
The cationic microgel (C-1) is not particularly limited as long as it is a cationic particle in which a resin is crosslinked. Specific examples include cationic epoxy resin crosslinked particles, acrylic resin crosslinked particles, urethane resin crosslinked particles, polyester resin crosslinked particles, and composite particles thereof. One type can be used alone, or two or more types can be used in combination.

The amount of the cationic microgel (C-1) added is typically 0.1 to 40% by mass, preferably 0.1 to 20% by mass, more preferably 0.5 to 20% by mass, even more preferably 1 to 15% by mass, and particularly preferably 2 to 9% by mass, based on the total mass of the solid contents of the resin (A) and compound (B).

Epoxy resin crosslinked particles (C-1-1)
The number average molecular weight of the above-mentioned epoxy resin crosslinked particles (C-1-1), measured under the conditions described below, is preferably in the range of usually less than 100,000, preferably 9,000 or less, and more preferably 5,000 or less as an upper limit, and usually 100 or more, preferably 150 or more, and more preferably 200 or more as a lower limit, from the viewpoints of finish and corrosion resistance of edges.

<Method for measuring number average molecular weight of crosslinked epoxy resin particles>
The crosslinked epoxy resin particles (C-1-1) were diluted with N,N'-dimethylformamide to a solids concentration of 1% by mass and allowed to stand at room temperature for 24 hours. The insoluble components (crosslinked components) were then filtered off using a Myshori filter for GPC (pore size: 0.2 microns), and the number average molecular weight was measured using gel permeation chromatography (GPC) as described below.
In gel permeation chromatography (GPC) measurements, if an insoluble component (in the present invention, a crosslinked component that is not dissolved in a solvent) is present, clogging occurs in the apparatus, which may cause a malfunction. Therefore, it is common to prepare a sample by filtering using a filter.

<Gel permeation chromatography (GPC)>
Apparatus: "HLC8120GPC" (product name, manufactured by Tosoh Corporation)
Columns: 4 columns: "TSKgel G-4000HXL,""TSKgelG-3000HXL,""TSKgelG-2500HXL," and "TSKgel G-2000HXL" (trade names, all manufactured by Tosoh Corporation) Mobile phase: N,N'-dimethylformamide Conditions: Measurement temperature 40°C, flow rate 1 mL/min
Detector: RI

The number average molecular weight is a value calculated by converting the retention time (retention volume) measured using the above-mentioned gel permeation chromatography (GPC) into the molecular weight of polystyrene using the retention time (retention volume) of a standard polystyrene of known molecular weight measured under the same conditions.

When the crosslinked epoxy resin particles (C-1-1) have a high molecular weight, the finish quality deteriorates. Therefore, in the number average molecular weight measurement data measured by gel permeation chromatography (GPC), the peak area (polymer ratio) at a molecular weight of 100,000 or more is preferably less than 40%, more preferably less than 30%, of the total peak area.
In this specification, the peak area corresponding to a molecular weight of 100,000 or more is sometimes referred to as the "polymer ratio."

The proportion of the insoluble components in the epoxy resin crosslinked particles (C-1-1) is preferably 10% by mass or more, more preferably 10 to 90% by mass, even more preferably 10 to 60% by mass, and particularly preferably 15 to 45% by mass, from the viewpoints of corrosion resistance of the edge portions and flat surfaces and finish quality.
If the insoluble components are too high, the finish quality will deteriorate, and if the insoluble components are too low, the corrosion resistance of the edge portion will deteriorate. Therefore, by keeping the content within this range, it is possible to achieve both corrosion resistance and finish quality at the edge portion.
The proportion of the insoluble component (crosslinked component) can be calculated by the following method.

<Method for measuring the proportion of insoluble components (crosslinked components)>
The crosslinked epoxy resin particles (C-1-1) were diluted with N,N'-dimethylformamide to a solids concentration of 1% by mass and allowed to stand at room temperature for 24 hours. The insoluble components (crosslinked components) were then filtered using a Myshori filter for GPC (pore size: 0.2 μm), and the insoluble residue was dried at 130°C for 3 hours to measure the solid mass of the residue. The proportion (% by mass) of the insoluble components (crosslinked components) can be calculated using the following formula:
Proportion (mass%) of insoluble component (crosslinked component) = A/B x 100
A: solid content mass of the filtration residue B: solid content mass of the epoxy resin crosslinked particle (C-1-1) solution before filtration The epoxy resin crosslinked particles (C-1-1) that can be used in the cationic electrodeposition coating composition of the present invention are not particularly limited as long as they are cationic particles obtained by crosslinking an epoxy resin with a crosslinking agent.
Here, the epoxy resin used as the raw material for the epoxy resin crosslinked particles (C-1-1) includes resins obtained by modifying epoxy resins, and also includes resins that do not have epoxy groups obtained by modifying epoxy groups. Examples of crosslinking agents include compounds having one or more (preferably two or more) reactive functional groups such as epoxy groups, isocyanate groups, alkoxysilyl groups, hydroxyl groups, carboxyl groups, and amino groups, and may have two or more types of reactive functional groups.

Examples of the epoxy resin crosslinked particles (C-1-1) include crosslinked epoxy resin particles obtained by reacting an epoxy resin with an amine compound to produce an amine-modified epoxy resin, neutralizing the amine-modified epoxy resin with an acid compound, dispersing the amine-modified epoxy resin in an aqueous solvent, and mixing and reacting the resulting dispersion with a crosslinking agent [e.g., a polyfunctional epoxy resin, an organosilicon compound (silane coupling agent), a polyisocyanate compound, etc.]. The crosslinking agent preferably contains an epoxy resin and/or an organosilicon compound (silane coupling agent), and more preferably contains an organosilicon compound (silane coupling agent) from the perspective of exhibiting viscoelasticity.

The reason why the above-mentioned organosilicon compound (silane coupling agent) has an effect on viscoelasticity is thought to be that during the heat drying process, reactive functional groups such as alkoxysilyl groups remaining in the silane coupling agent react with the primary amino groups, secondary amino groups, and/or hydroxyl groups of the amine-modified epoxy resin (A), resulting in a high molecular weight resin.

One embodiment of the epoxy resin crosslinked particles (C-1-1) is produced by the following production process: (I) reacting an epoxy resin with an amine compound to produce an amine-modified epoxy resin; (II) neutralizing the amine-modified epoxy resin with an acid compound and dispersing it in an aqueous solvent; and (III) mixing and reacting the resulting dispersion with a crosslinking agent containing a silane coupling agent to obtain the epoxy resin crosslinked particles.

<Step (I)>
As a step for producing the amine-modified epoxy resin obtained by reacting an epoxy resin with an amine compound, the same production method as that for the amine-modified epoxy resin (A) described above can be used.

As the epoxy resin, the same as the above-mentioned epoxy resin (A-1) can be used, and among them, an epoxy resin derived from bisphenol A can be preferably used. Furthermore, an epoxy resin which has been made high molecular weight and/or multifunctional by reacting an epoxy resin with a polyphenol compound can be preferably used, and bisphenol A is preferred as the polyphenol compound.
The number average molecular weight of the epoxy resin is preferably 400 to 5,000, and more preferably 700 to 3,000.

As the amine compound, the amine compounds listed in the above-mentioned amine-modified epoxy resin (A) can be suitably used, but it is particularly preferable to include a ketiminated amine compound, and particularly preferable to include a secondary mono- or polyamine having a ketiminated primary amino group.
Examples of the secondary mono- and polyamines having the ketiminated primary amino group include ketimine compounds of the amine compounds represented by the following formula (2). Specific examples include diketimine compounds such as diethylenetriamine, dipropylenetriamine, dibutylenetriamine, bis(hexamethylene)triamine, triethylenetetramine, tetraethylenepentamine, and pentaethylenehexaamine.

(In the formula, _R1 and _R2 are hydrocarbon groups having 1 to 8 carbon atoms and may be different or the same, and n is an integer of 1 to 5.)
The ketiminated amine compound is contained in the amine compound in an amount of preferably 0.1 mol% or more and less than 80 mol%, more preferably 1 mol% or more and less than 50 mol%, even more preferably 2 mol% or more and less than 40 mol%, and particularly preferably 5 mol% or more and less than 30 mol%.
In this specification, the content of the ketiminated amine compound may be referred to as the "ketimine compound content."

The ketiminized, blocked primary amino group of the amine compound is hydrolyzed in the aqueous dispersion step (II) described below, revealing a primary amino group. Then, in step (III), the primary amino group reacts with the epoxy group of the epoxy compound, resulting in a polymerized and/or crosslinked reaction. Therefore, by keeping the amine compound within the above range, the molecular weight, particle size, and/or degree of crosslinking (proportion of insoluble components) of the epoxy resin crosslinked particles (C-1-1) can be kept within optimal ranges.

<Step (II)>
The amine-modified epoxy resin obtained in the above step (I) is then neutralized with an acid compound and further dispersed in an aqueous solvent to obtain a dispersion.
Here, the aqueous solvent refers to a solvent containing water and other solvents that can be contained as necessary. Examples of other solvents include ester-based solvents, ketone-based solvents, amide-based solvents, alcohol-based solvents, and ether alcohol-based solvents, or mixtures thereof.
As the acid compound, known acid compounds can be used without any particular limitation, and among them, organic acids are preferred, and formic acid, lactic acid, acetic acid, or a mixture thereof is more preferred. The neutralization equivalent of the acid compound is preferably 0.2 to 1.5 equivalents, more preferably 0.5 to 1.0 equivalents, per equivalent of amino group.
In addition to the acid compound, additives such as an emulsifier may also be contained.

The dispersion of the amine-modified epoxy resin in the aqueous solvent may be carried out by adding the aqueous solvent to the neutralized amine-modified epoxy resin while stirring, or by adding the neutralized amine-modified epoxy resin to the aqueous solvent while stirring, or by mixing the aqueous solvent and the neutralized amine-modified epoxy resin and then stirring.
The dispersion temperature is preferably less than 100°C, more preferably 40 to 99°C, and even more preferably 50 to 95°C.
The resin solid content concentration of the dispersion is preferably from 5 to 80% by mass, more preferably from 10 to 50% by mass.

<Step (III)>
The dispersion obtained in the above step (II) is then mixed with a crosslinking agent containing a silane coupling agent and further reacted to obtain cationic epoxy resin crosslinked particles (C-1-1). Alternatively, the silane coupling agent can be used in combination with other crosslinking agents (e.g., epoxy resins or polyisocyanate compounds) as the crosslinking agent.
Examples of the silane coupling agent that can be used include amino group-containing silane coupling agents, epoxy group-containing silane coupling agents, (meth)acryloyl group-containing silane coupling agents, mercapto group-containing silane coupling agents, vinyl group-containing silane coupling agents, ureido group-containing silane coupling agents, sulfide group-containing silane coupling agents, and silane coupling agents having a cyclic anhydride structure, and one type may be used alone or two or more types may be used in combination.

Commercially available silane coupling agents include, for example, "KBM-1003" (vinyltrimethoxysilane), "KBM-1083" (7-octenyltrimethoxysilane), "KBM-303" (2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane), "KBM-403" (3-glycidoxypropyltrimethoxysilane), "KBM-4803" (8-glycidoxyoctyltrimethoxysilane), and "KBM-1403" (manufactured by Shin-Etsu Chemical Co., Ltd.). p-styryltrimethoxysilane), "KBM-503" (3-methacryloxypropyltrimethoxysilane), "KBM-5803" (8-methacryloxyoctyltrimethoxysilane), "KBM-5103" (3-acryloxypropyltrimethoxysilane), "KBM-603" (N-2-(aminoethyl)-3-aminopropyltrimethoxysilane), "KBM-903" (3-aminopropyltrimethoxysilane), "KBM-573" (N-phenyl- 3-aminopropyltrimethoxysilane), "KBM-575" (N-(vinylbenzyl)-2-aminoethyl-3-aminopropyltrimethoxysilane hydrochloride), "KBM-6803" (N-2-(aminoethyl)-8-aminooctyltrimethoxysilane), "KBM-9659" (tris-(trimethoxysilylpropyl)isocyanurate), "X-12-1290" (1,3-diallyl-5-(3-(trimethoxysilyl)propyl)-1,3,5-triisopropyltrimethoxysilane), Examples include "dinane-2,4,6-trione," "KBM-803" (3-mercaptopropyltrimethoxysilane), "X-12-967C" (3-trimethoxysilylpropylsuccinic anhydride); "Sila-Ace S810" (3-mercaptopropyltrimethoxysilane) manufactured by Chisso Corporation; "SIM6473.5C" (mercaptomethyltrimethoxysilane) and "SIU9058.0" (N-(3-trimethoxysilylpropyl)urea) manufactured by Azmax Corporation.

In the reaction step, the reactive functional groups (e.g., primary amino groups) of the amine-modified epoxy resin from which the ketiminized blocks have been removed by hydrolysis react with the reactive functional groups (e.g., alkoxysilyl groups) of the silane coupling agent serving as the crosslinking agent, resulting in a polymerization and/or crosslinking reaction.
The reactive functional group equivalent ratio between the amine-modified epoxy resin and the crosslinking agent is preferably 0.5 to 2.0 equivalents, more preferably 0.7 to 1.5 equivalents, of the crosslinking agent per equivalent of the amine-modified epoxy resin.
The reaction temperature is preferably less than 100°C, more preferably 40 to 99°C, and even more preferably 50 to 95°C.
During or after the polymerizing and/or crosslinking reaction, a solvent removal step can be carried out by reducing the pressure at a temperature of 40 to 99°C.

Furthermore, another embodiment of the epoxy resin crosslinked particles (C-1-1) containing a silane coupling agent may be epoxy resin crosslinked particles produced by a process other than steps (I) to (III) above.

The volume average particle diameter of the crosslinked epoxy resin particles (C-1-1) is generally within the range of 10 nm to 1,000 nm, preferably greater than 15 nm, more preferably greater than 20 nm, even more preferably greater than 25 nm, and particularly preferably greater than 30 nm, from the viewpoints of corrosion resistance at the edge and flat surfaces and finish quality, and is preferably smaller than 800 nm, more preferably smaller than 700 nm, even more preferably smaller than 600 nm, and particularly preferably smaller than 500 nm.
The volume average particle size of the cationic microgel (C-1) can be measured by a laser diffraction/scattering measurement device, and the particle size in this specification was measured using "Microtrac UPA250" (trade name, manufactured by Nikkiso Co., Ltd., particle size distribution measurement device).
The amine value of the cationic epoxy resin crosslinked particles (C-1-1) is preferably within a range of 25 to 200 mgKOH/g, and more preferably within a range of 50 to 180 mgKOH/g.
By adjusting the content within the above range, the dispersibility of the particles in the aqueous solvent and the water resistance of the coating film are excellent.

(Meth)acrylate-modified epoxy (C-2)
The (meth)acrylate-modified epoxy (C-2) can be suitably any epoxy resin having at least one (meth)acryloyl group, and can be obtained, for example, by the following method (1) or (2).
(1) It can be obtained by reacting the above-mentioned epoxy resin (A-1) with a compound having at least one reactive functional group and at least one (meth)acryloyl group. Examples of the reactive functional group include a carboxyl group and a primary or secondary amino group, with a carboxyl group being preferred. Specific examples of the compound having at least one reactive functional group and at least one (meth)acryloyl group include (meth)acrylic acid and aminoethyl (meth)acrylate. These can be used alone or in combination of two or more.
(2) It can be obtained by reacting a compound having at least one reactive functional group and at least one (meth)acryloyl group with the hydroxyl group or primary or secondary amino group of the amine-modified epoxy resin (A). Examples of the reactive functional group include a glycidyl group and an isocyanate group. Specific examples of the compound having at least one reactive functional group and at least one (meth)acryloyl group include glycidyl (meth)acrylate and (meth)acryloyloxyethyl isocyanate. These can be used alone or in combination of two or more.

The reason why the above (meth)acrylate-modified epoxy (C-2) has an effect on viscoelasticity is thought to be that during the heat drying process, a Michael addition reaction occurs between the (meth)acryloyl groups of the (meth)acrylate-modified epoxy (C-2) and the primary amino groups, secondary amino groups, and/or hydroxyl groups of the amine-modified epoxy resin (A), resulting in a high molecular weight resin.

When a (meth)acrylate-modified epoxy (C-2) is used as the viscosity modifier (C), it is preferable that the amine compound, which is a constituent of the amine-modified epoxy resin (A), contains a ketimine derivative of a primary amine, in view of the reactivity between the amine-modified epoxy resin and the viscosity modifier.

Hydroxy(meth)acrylate-blocked polyisocyanate compound (C-3)
The hydroxy(meth)acrylate-blocked polyisocyanate compound (C-3) can be obtained by reacting a polyisocyanate compound with a hydroxy(meth)acrylate, and the polyisocyanate compounds exemplified as the blocked polyisocyanate compound (B) can be suitably used as the polyisocyanate compound.
The hydroxy(meth)acrylate may suitably be any compound having at least one hydroxy group and at least one (meth)acryloyl group, and examples thereof include monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms, such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, and 4-hydroxybutyl (meth)acrylate, and ε-caprolactone-modified products of the monoesters of (meth)acrylic acid with dihydric alcohols having 2 to 8 carbon atoms. These may be used alone or in combination of two or more.

In addition, the hydroxy(meth)acrylate-blocked polyisocyanate compound (C-3) can be used in combination with the compounds listed as the blocking agents for the blocked polyisocyanate compound (B) described above, in addition to the above hydroxy(meth)acrylate, as a compound that reacts with the isocyanate group of the polyisocyanate compound.
The ratio of the isocyanate group of the polyisocyanate compound to the reactive group (such as a hydroxyl group) that reacts with the isocyanate group is usually 0.1 to 5.0, preferably 0.5 to 2.0.

The reason why the hydroxy(meth)acrylate-blocked polyisocyanate compound (C-3) is effective in improving viscoelasticity is thought to be that during the heat drying process, a Michael addition reaction occurs between the (meth)acryloyl groups of the hydroxy(meth)acrylate-blocked polyisocyanate compound (C-3) and the primary amino groups, secondary amino groups, and/or hydroxyl groups of the amine-modified epoxy resin (A), resulting in a high molecular weight resin.

When the above-mentioned hydroxy(meth)acrylate-blocked polyisocyanate compound (C-3) is used as the viscosity modifier (C), it is preferable that the amine compound, which is a constituent of the amine-modified epoxy resin (A), contains a ketimine derivative of a primary amine, in view of the reactivity between the amine-modified epoxy resin and the viscosity modifier.

Inorganic particles (C-4)
The inorganic particles include silica and/or clay minerals, and among these, silica is preferred.

The silica is generally a solid substance mainly composed of silicon dioxide. Among them, silica obtained by mixing sodium silicate and acid using a so-called wet method is preferred, and examples of commercially available products include the Sylysia series available from Fuji Silysia Chemical Ltd.
Metal ion-exchanged silica, in which metal ions are introduced into a silica support by ion exchange, can also be used. Examples of the metal ions include calcium ions, magnesium ions, cobalt ions, nickel ions, and lithium ions.

The above silica has excellent affinity with the amine-modified epoxy resin (A), and it is thought that the interaction between it and the polar groups of the amine-modified epoxy resin (A) improves the corrosion resistance of the edge portion.

The clay minerals mentioned above are the main component minerals that make up clay, and include layered silicate minerals (phyllosilicates), calcite, dolomite, feldspars, quartz, zeolites, and others with chain structures (attapulgite, sepiolite, etc.), fibrous structures (palygorskite, etc.), and those without a clear crystalline structure (allophane). Furthermore, the clay minerals preferably contain at least one clay mineral other than clay, and preferably use a combination of clay and at least one clay mineral other than clay. Note that "clay" refers to plate-like or layered clay extracted from clay layers and composed primarily of silica, magnesium, iron, potassium, and sodium, and also includes refined clay. Refined clay is clay that has had impurities dissolved and removed using acid or other methods. Kaolin clay, which contains kaolinite, is the clay used in paint applications.

The shape of the inorganic particles (C-4) is preferably at least one selected from the group consisting of spherical, plate-like, scale-like, layer-like, rod-like, chain-like, needle-like, and fibrous shapes.

The average particle size of the inorganic particles (C-4) is preferably 0.1 to 30 μm, more preferably 0.2 to 20 μm, and even more preferably 0.4 to 10 μm.
The average particle size can be measured by a laser diffraction method (volume basis).

Polar polymer (C-5)
The cationic electrodeposition coating composition of the present invention may contain a polar polymer (C-5).

The content of the polar polymer (C-5) is typically 0.1 to 40% by mass, preferably 0.1 to 20% by mass, more preferably 0.5 to 20% by mass, even more preferably 0.5 to 15% by mass, particularly preferably 0.5 to 10% by mass, and even more preferably 0.5 to 5% by mass, based on the total mass of the solid contents of the resin (A) and compound (B).

In this specification, the term "polymer" refers to a polymer formed by the polymerization (reaction) of two or more monomers.
The weight average molecular weight of the polar polymer (C-5) is, for example, 100 or more, preferably 500 or more, more preferably 1,000 to 10,000,000, even more preferably 2,000 to 8,000,000, and most preferably within the range of 3,000 to 5,000,000, from the viewpoint of viscosity development and water resistance.

The polar polymer (C-5) is a highly polar, high-molecular-weight compound having a polar functional group, such as at least one selected from the group consisting of amide group-containing resins, urea group-containing resins, urethane group-containing resins, polyvinyl alcohol, polyvinyl acetal, polyalkylene ether, polycarbonate, polyester resin, acrylic resin, and polysaccharides, or a composite resin thereof, which can be used alone or in combination of two or more. Among these, amide group-containing resins and polyvinyl alcohol are preferred.
Examples of the amide group-containing resin include polyamide, fatty acid amide, polyhydroxycarboxylic acid amide, polyvinylpyrrolidone, polyvinylacetamide, polyvinylformamide, poly-N-methylvinylacetamide, and poly(meth)acrylamide.
The polar functional group preferably contains at least one polar functional group selected from the group consisting of an amino group, a sulfonic acid group, a carboxyl group, a phosphoric acid group, a polyalkylene ether group, an amide group, a hydroxyl group, an ester group, and an epoxy group.

The polar functional group concentration in the polar polymer (C-5) is typically 0.1 mmol/g or more, preferably 1 to 30 mmol/g, more preferably 2 to 25 mmol/g, even more preferably 5 to 23 mmol/g, and particularly preferably 6 to 20 mmol/g, from the viewpoints of viscosity development and throwing power.

In this specification, the polar functional group concentration is calculated by counting one polar functional group as one. For example, if one polymerizable unsaturated monomer contains two polar functional groups, it is calculated as two.

Regarding the cationic electrodeposition coating composition of the present invention, the blending ratio of the amine-modified epoxy resin (A) and the blocked polyisocyanate compound (B) is preferably within the range of 5 to 95 mass %, preferably 50 to 80 mass %, of component (A) and 5 to 95 mass %, preferably 20 to 50 mass %, of component (B), based on the total mass of the solid contents of the above components (A) and (B), in order to obtain a coated article with good paint stability and excellent finish and corrosion resistance. If the blending ratio falls outside the above range, either the above-mentioned paint properties or the coating film performance may be impaired, which is undesirable.

The cationic electrodeposition coating composition of the present invention is not particularly limited, but is preferably a composition containing, for example, the resin emulsion (I) obtained by thoroughly mixing the resin (A) and compound (B) described above, and, if necessary, various additives such as a viscosity modifier (C), a surfactant, and a surface modifier, and then adding water to obtain a compounded resin, and the pigment paste (II) described below. The composition can be obtained by thoroughly mixing this with water, an organic solvent, a neutralizing agent, etc. Any known organic acid can be used as the neutralizing agent without any particular restrictions, with formic acid, lactic acid, or a mixture thereof being particularly preferred.

Methods of mixing viscosity modifier (C) into the paint composition include incorporating it into the aqueous dispersion when producing resin emulsion (I), incorporating it into the pigment dispersion paste (II) together with the pigment and dispersing resin when producing pigment dispersion paste (II), and adding it to a paint composition containing resin emulsion (I) and pigment paste (II) while stirring. All of these methods are suitable, but it is preferable to incorporate it into resin emulsion (I).

Examples of the shear-type dispersing machine include, but are not limited to, mixers such as Disper, Homomixer, and Planetary Mixer, and homogenizers (such as M Technique's "Clearmix," PRIMIX's "Filmix," and "High Shear Mixer," and Silverson's "Abramix," and "Mixer").

The pigment dispersion paste (II) is a pigment such as a color pigment, anti-rust pigment, or extender pigment that has been dispersed into fine particles in advance. For example, the pigment dispersion paste can be prepared by blending a pigment dispersion resin, a neutralizer, and a pigment, and dispersing the mixture in a dispersion mixer using media such as a ball mill, sand mill, or pebble mill.

The pigment dispersion resin can be any known resin without any particular restrictions, such as epoxy resins or acrylic resins having hydroxyl groups and cationic groups, surfactants, tertiary amine-type epoxy resins, quaternary ammonium salt-type epoxy resins, tertiary sulfonium salt-type epoxy resins, tertiary amine-type acrylic resins, quaternary ammonium salt-type acrylic resins, and tertiary sulfonium salt-type acrylic resins.

The pigment may be any known pigment without any particular limitation, and may include one or more of the following: color pigments such as titanium oxide, carbon black, and red iron oxide; extender pigments such as clay, mica, baryta, calcium carbonate, and silica; and rust-preventive pigments such as aluminum phosphomolybdate, aluminum tripolyphosphate, and zinc oxide (zinc white).

In order to improve the curability of the coating film, it is preferable to contain a bismuth compound as a curing catalyst. As the bismuth compound, for example, one or more of bismuth oxide, bismuth hydroxide, basic bismuth carbonate, bismuth nitrate, bismuth silicate, and organic acid bismuth can be used.
From an environmental standpoint, it is preferable that organotin compounds such as dibutyltin dibenzoate, dioctyltin oxide, and dibutyltin oxide are not contained.
By using the bismuth compound as a curing catalyst, it is possible to improve the curability of the coating film without containing these organotin compounds.
The amount of the pigment to be blended is preferably within a range of 1 to 100 parts by mass, particularly 10 to 50 parts by mass, per 100 parts by mass of the total resin solid content of the resin (A) and the compound (B).
When a bismuth compound is used as a curing catalyst, the amount of the bismuth compound is preferably within the range of 0.1 to 10 parts by mass, particularly 1 to 5 parts by mass, per 100 parts by mass of the total resin solid content of the resin (A) and the compound (B).

The solids concentration of the cationic electrodeposition coating composition is preferably in the range of 5 to 40% by mass, and more preferably 15 to 25% by mass.

Coating Film Formation Method The present invention provides a method for forming a cationic electrodeposition coating film, which comprises the steps of immersing a metal substrate in an electrodeposition bath comprising the above-mentioned cationic electrodeposition coating composition, and passing a current through the metal substrate as the cathode.

Substrates to be coated with the cationic electrodeposition coating composition of the present invention include automobile bodies, motorcycle parts, household appliances, and other equipment, and there are no particular restrictions as long as they are made of metal.

Metal steel sheets that can be used as substrates include cold-rolled steel sheets, alloyed hot-dip galvanized steel sheets, electrogalvanized steel sheets, electrolytic zinc-iron double-layer plated steel sheets, organic composite plated steel sheets, Al materials, Mg materials, and these metal sheets that have been cleaned, if necessary, by alkaline degreasing or other methods, and then subjected to surface treatments such as phosphate conversion treatment, chromate treatment, and zirconium conversion treatment.

The cationic electrodeposition coating composition can be applied to the surface of the desired substrate by cationic electrodeposition coating. The cationic electrodeposition method generally involves diluting the cationic electrodeposition coating composition with deionized water or the like to a solids concentration of approximately 5 to 40% by mass, preferably 10 to 25% by mass, and adjusting the pH to a range of 4.0 to 9.0, preferably 5.5 to 7.0, to form a bath. The bath temperature is typically adjusted to 15 to 35°C, and a current is applied one or more times (preferably once) to the substrate as the cathode at a load voltage of 100 to 400 V, preferably 150 to 350 V. After electrodeposition coating, the substrate is typically thoroughly rinsed with ultrafiltrate (UF filtrate), reverse osmosis water (RO water), industrial water, pure water, or the like to remove any excess cationic electrodeposition coating.

The thickness of the electrodeposition coating is not particularly limited, but can generally be within the range of 5 to 40 μm, preferably 10 to 35 μm, based on the dried coating. Furthermore, the heat drying of the coating is carried out using drying equipment such as an electric hot air dryer or gas hot air dryer to heat the electrodeposition coating to a surface temperature of 110 to 200°C, preferably 140 to 180°C, for 10 to 180 minutes, preferably 20 to 50 minutes. A cured coating can be obtained by the above-mentioned baking drying.

The present invention will be explained in more detail below using production examples, working examples, and comparative examples, but the present invention is not limited to these. In each example, "parts" means parts by mass and "%" means % by mass.

Production of amine-modified epoxy resin Production Example 1
Into a flask equipped with a stirrer, a thermometer, a nitrogen inlet tube, and a reflux condenser, 388 parts of bisphenol A and 0.2 parts of dimethylbenzylamine were added 945 parts of "jER828EL" (trade name, manufactured by Japan Epoxy Resins Co., Ltd., epoxy resin, epoxy equivalent 190, number average molecular weight 350), and the mixture was reacted at 120°C until the epoxy equivalent reached 670.
Next, 188 parts of PLACCEL 212CP was added and the mixture was reacted at 120°C for 2 hours, and then 350 parts of a ketimine compound of diethylenetriamine and methyl isobutyl ketone and 53 parts of diethanolamine were added and the mixture was reacted at 120°C for 4 hours, followed by addition of ethylene glycol monobutyl ether to obtain an amine-modified epoxy resin (a-1) solution with a solids content of 80%. The amine-modified epoxy resin (a-1) had an amine value of 115 mgKOH/g, a number average molecular weight of 2,050, and a glass transition temperature of 90°C.

Production Example 2
Into a flask equipped with a stirrer, a thermometer, a nitrogen inlet tube, and a reflux condenser, 388 parts of bisphenol A and 0.3 parts of dimethylbenzylamine were added 945 parts of "jER828EL" (trade name, manufactured by Japan Epoxy Resins Co., Ltd., epoxy resin, epoxy equivalent 190, number average molecular weight 350), and the mixture was reacted at 120°C until the epoxy equivalent reached 1,050.
Next, 188 parts of PLACCEL 212CP was added and the mixture was reacted at 120°C for 2 hours, and then 500 parts of a ketimine compound of diethylenetriamine and methyl isobutyl ketone was added and the mixture was reacted at 120°C for 4 hours. Ethylene glycol monobutyl ether was then added to obtain an amine-modified epoxy resin (a-2) solution with a solids content of 80%. The amine-modified epoxy resin (a-2) had an amine value of 96 mgKOH/g, a number average molecular weight of 3,100, and a glass transition temperature of 95°C.

Production Example 3
Into a flask equipped with a stirrer, a thermometer, a nitrogen inlet tube, and a reflux condenser, 1,870 parts of "jER828EL" (trade name, manufactured by Japan Epoxy Resins Co., Ltd., epoxy resin, epoxy equivalent 190, number average molecular weight 350), 990 parts of bisphenol A, and 0.3 parts of dimethylbenzylamine were added, and the mixture was reacted at 120°C until the epoxy equivalent reached 1,430.
Next, 300 parts of a ketimine compound of diethylenetriamine and methyl isobutyl ketone and 100 parts of diethanolamine were added and reacted at 120°C for 4 hours, followed by addition of ethylene glycol monobutyl ether to obtain an amine-modified epoxy resin (a-3) solution with a solids content of 80%. The amine-modified epoxy resin (a-3) had an amine value of 70 mgKOH/g, a number average molecular weight of 3,200, and a glass transition temperature of 105°C.

Production Example 4
Into a flask equipped with a stirrer, a thermometer, a nitrogen inlet tube, and a reflux condenser, 2,450 parts of "jER828EL" (trade name, manufactured by Japan Epoxy Resins Co., Ltd., epoxy resin, epoxy equivalent 190, number average molecular weight 350), 1,370 parts of bisphenol A, and 0.4 parts of dimethylbenzylamine were added, and the mixture was reacted at 120°C until the epoxy equivalent reached 1,900.
Next, 350 parts of a ketimine compound of diethylenetriamine and methyl isobutyl ketone and 80 parts of diethanolamine were added and reacted at 120°C for 4 hours, followed by addition of ethylene glycol monobutyl ether to obtain an amine-modified epoxy resin (a-4) solution with a solids content of 80%. The amine-modified epoxy resin (a-4) had an amine value of 60 mgKOH/g, a number average molecular weight of 4,200, and a glass transition temperature of 110°C.

The glass transition temperature (Tg) of the above epoxy resin was measured under the following conditions:

<Measurement of Glass Transition Temperature (Tg)>
The glass transition temperature (Tg) of the epoxy resin was measured after removing the solvent using a viscoelasticity measuring device (manufactured by TA Instruments, trade name "ARES-G2").
Jig: Parallel plates with a diameter of 8 mm Temperature drop: from 130°C to 50°C (5°C/min)
Frequency: 1 Hz
Distortion: Variable between 1 and 60% (automatic control)
Torque: Variable between 0.1 and 23 g cm (automatic control)
In the obtained viscoelasticity spectrum, the peak temperature of the loss tangent (tan δ) was taken as the glass transition temperature.

Production of blocked polyisocyanate compound Production Example 5
Into a reaction vessel, 270 parts of "Cosmonate M-200" (trade name, manufactured by Mitsui Chemicals, Inc., crude MDI, NCO group content 31.3%) and 127 parts of methyl isobutyl ketone were added and heated to 70° C. 236 parts of ethylene glycol monobutyl ether was added dropwise over 1 hour, and the temperature was then raised to 100° C. Sampling was performed over time while maintaining this temperature, and infrared absorption spectroscopy confirmed that absorption due to unreacted isocyanate groups had disappeared, yielding a blocked polyisocyanate compound (b-1) with a resin solids content of 80%.

Production Example 6
A reaction vessel was charged with 152 parts of propylene glycol and 106 parts of methyl isobutyl ketone, and the temperature was raised to 70°C. 270 parts of "Cosmonate M-200" (trade name, manufactured by Mitsui Chemicals, Inc., crude MDI, NCO group content 31.3%) was added dropwise thereto over 1 hour. While maintaining this temperature, sampling was performed over time, and infrared absorption spectroscopy confirmed that no absorption due to unreacted isocyanate remained, yielding a blocked polyisocyanate compound (b-2) with a solids content of 80%.

Production of pigment dispersing resin Production Example 7
A flask equipped with a stirrer, thermometer, dropping funnel, and reflux condenser was charged with 450 parts of nonylphenol and 960 parts of "CNE195LB" (trade name, manufactured by Chang Chun Japan Co., Ltd., a cresol-type novolac epoxy resin, a glycidyl ether of novolac-type phenolic resin), and the mixture was gradually heated with stirring to 160°C to allow the reaction to proceed. Thereafter, 430 parts of ε-caprolactone was charged, and the mixture was heated to 170°C and allowed to react. Furthermore, 105 parts of diethanolamine and 124 parts of N-methylethanolamine were reacted, and after confirming that the epoxy value had reached 0, ethylene glycol monobutyl ether was added to adjust the solids content, yielding a pigment dispersion resin solution with a solids content of 60%.

Preparation of pigment dispersion paste Preparation Example 8
8.3 parts (solids content: 5 parts) of the pigment dispersion resin containing a quaternary ammonium salt group and having a solids content of 60% obtained in Production Example 7, 21.5 parts of titanium oxide, 0.3 parts of carbon black, 2 parts of bismuth hydroxide, and 20.3 parts of deionized water were added and dispersed in a ball mill for 20 hours, thereby obtaining a pigment dispersion paste (p-1) having a solids content of 55%.

Preparation of crosslinked epoxy resin particles Preparation Example 9
A reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux condenser was charged with 413 parts of "jER828EL" (trade name, manufactured by Japan Epoxy Resins Co., Ltd., epoxy resin, epoxy equivalent 190, number average molecular weight 350), 126 parts of bisphenol A, and 0.1 parts of dimethylbenzylamine, and the temperature inside the reaction vessel was maintained at 160°C, allowing the reaction to proceed until the epoxy equivalent reached 490 g/mol. The temperature inside the reaction vessel was then cooled to 140°C, and 1.3 parts of dimethylbenzylamine was added to allow the reaction to proceed until the epoxy equivalent reached 890 g/mol. After that, 177 parts of methyl isobutyl ketone was added while the temperature inside the reaction vessel was cooled to 100°C. Next, a mixture of 44 parts of diethanolamine, 4 parts of N-methylethanolamine, and 35 parts of a diketimine compound of diethylenetriamine and methyl isobutyl ketone (ketimine compound content: 22 mol%) was added, and the mixture was reacted at 115°C for 1 hour to obtain an amino group-containing epoxy resin solution.
305 parts of the resulting amino group-containing epoxy resin solution was added to a new reaction vessel, and the temperature inside the reaction vessel was maintained at 90°C. Next, 26 parts of 88% lactic acid was added to neutralize the acid, and 1,146 parts of deionized water was added to dilute and disperse the mixture. Next, 24 parts of a jER828EL solution adjusted to an 80% solids content with propylene glycol monomethyl ether was added, and the mixture was reacted at 90°C for 3 hours. After that, methyl isobutyl ketone was removed under reduced pressure, and the mixture was diluted with deionized water to obtain a solution of epoxy resin crosslinked particles No. 1 with an 18% solids content. The resulting epoxy resin crosslinked particles No. 1 had a number average molecular weight (Note 1) of 1,000, a polymer ratio (Note 2) of 10%, an insoluble component ratio (Note 3) of 25% by mass, and a volume average particle diameter (Note 4) of 50 nm.
(Note 1) Number average molecular weight: The crosslinked epoxy resin particles were diluted with N,N'-dimethylformamide to a solids concentration of 1% by mass and allowed to stand at room temperature for 24 hours. The insoluble components (crosslinked components) were then filtered out using a Myshori filter for GPC (pore size: 0.2 microns), and the number average molecular weight was measured using gel permeation chromatography (GPC) ["HLC8120GPC" (trade name, manufactured by Tosoh Corporation)].
(Note 2) Polymer ratio (%): In the above molecular weight measurement data, this indicates the ratio (%) of the peak area of molecular weights of 100,000 or more to the total peak area.
(Note 3) Insoluble component ratio (mass %): The crosslinked epoxy resin particles were diluted with N,N'-dimethylformamide to a solids concentration of 1 mass % and allowed to stand at room temperature for 24 hours. The solution was then filtered through a Myshori filter for GPC (pore size: 0.2 microns), and the insoluble component ratio (crosslinked component ratio) was calculated using the following formula:
Proportion of insoluble components (mass%) = A/B x 100
[A: solid content mass of the filtration residue, B: mass of the epoxy resin crosslinked particle solution diluted to 1 mass % solid content/100].
(Note 4) Volume average particle diameter (nm): Epoxy resin crosslinked particles were measured using a "Microtrac UPA250" (product name, manufactured by Nikkiso Co., Ltd., particle size distribution measuring device).

Production Example 10
A reaction vessel equipped with a stirrer, thermometer, dropping funnel, and reflux condenser was charged with 413 parts of "jER828EL" (trade name, manufactured by Japan Epoxy Resins Co., Ltd., epoxy resin, epoxy equivalent 190, number average molecular weight 350), 126 parts of bisphenol A, and 0.1 parts of dimethylbenzylamine, and the temperature inside the reaction vessel was maintained at 160°C, allowing the reaction to proceed until the epoxy equivalent reached 490 g/mol. The temperature inside the reaction vessel was then cooled to 140°C, and 1.3 parts of dimethylbenzylamine was added to allow the reaction to proceed until the epoxy equivalent reached 890 g/mol. After that, 177 parts of methyl isobutyl ketone was added while the temperature inside the reaction vessel was cooled to 100°C. Next, a mixture of 44 parts of diethanolamine, 4 parts of N-methylethanolamine, and 35 parts of a diketimine compound of diethylenetriamine and methyl isobutyl ketone (ketimine compound content: 22 mol%) was added, and the mixture was reacted at 115°C for 1 hour to obtain an amino group-containing epoxy resin solution.
305 parts of the resulting amino group-containing epoxy resin solution was added to a new reaction vessel, and the temperature inside the reaction vessel was maintained at 90°C. Next, 26 parts of 88% lactic acid was added to neutralize the acid, and 1,146 parts of deionized water was added to dilute and disperse the mixture. Next, 12 parts of jER828EL solution (80% solids content) prepared with propylene glycol monomethyl ether and 12 parts of KBM-403 (3-glycidoxypropyltrimethoxysilane, molecular weight 236) were added, and the mixture was allowed to react at 90°C for 3 hours. After removing the methyl isobutyl ketone under reduced pressure, the mixture was diluted with deionized water to obtain a solution of epoxy resin crosslinked particles No. 2 with a solids content of 18%. The resulting epoxy resin crosslinked particles No. 2 had a number average molecular weight (Note 1) of 1,000, a polymer modulus (Note 2) of 12%, an insoluble component ratio (Note 3) of 28% by mass, and a volume average particle diameter (Note 4) of 50 nm.

Preparation of (meth)acrylate-modified epoxy Preparation Example 11
Into a flask equipped with a stirrer, a thermometer, a nitrogen inlet tube, and a reflux condenser, 500 parts of bisphenol A and 0.2 parts of dimethylbenzylamine were added 1,200 parts of "jER828EL" (trade name, manufactured by Japan Epoxy Resins Co., Ltd., epoxy resin, epoxy equivalent 190, number average molecular weight 350), and the mixture was reacted at 130°C until the epoxy equivalent reached 850.
Next, 53 parts of diethanolamine, 108 parts of acrylic acid, 0.04 parts of p-benzoquinone, and 0.4 parts of tetrabutylammonium bromide were added at 100°C and reacted, followed by addition of ethylene glycol monobutyl ether to obtain a (meth)acrylate-modified epoxy solution with a solids content of 80%. The (meth)acrylate-modified epoxy had an amine value of 15 mgKOH/g and a number-average molecular weight of 1,900.

Production Example 12: Production of Hydroxy(meth)acrylate Blocked Polyisocyanate Compound
Into a reaction vessel, 270 parts of "Cosmonate M-200" (trade name, manufactured by Mitsui Chemicals, Inc., crude MDI, NCO group content 31.3%), 127 parts of methyl isobutyl ketone, 0.05 parts of 4-tert-butylcatechol, and 0.004 parts of bismuth tris(2-ethylhexanoate) were added and the temperature was raised to 70°C. While constantly bubbling oxygen into the mixture, 35 parts of 2-hydroxyethyl acrylate were added dropwise over 1 hour, and the temperature was then raised to 100°C and the mixture was allowed to react at this temperature. Further, while maintaining the temperature at 100°C, 150 parts of ethylene glycol monobutyl ether was gradually added, and infrared absorption spectroscopy was used to confirm that the absorption of unreacted isocyanate groups had disappeared. Ethylene glycol monobutyl ether was then added to obtain a hydroxy(meth)acrylate-blocked polyisocyanate compound solution with a solids content of 80%.

Preparation of Cationic Electrodeposition Coating Composition Example 1
87.5 parts (solid content: 70 parts) of the amine-modified epoxy resin (a-1) obtained in Production Example 1 and 37.5 parts (solid content: 30 parts) of the blocked polyisocyanate compound (b-2) obtained in Production Example 5 were mixed, and 13 parts of 10% acetic acid was added and stirred uniformly. Deionized water was then added dropwise over about 15 minutes with vigorous stirring to obtain an emulsion with a solid content of 34%.
Next, 294 parts (solid content 100 parts) of the above emulsion, 52.4 parts of the pigment dispersion paste (p-1) obtained in Production Example 8, 27.8 parts (solid content 5 parts) of viscosity modifier (c-1-1) (epoxy resin crosslinked particles No. 1), and deionized water were added to produce a cationic electrodeposition coating composition (X-1) with a solid content of 20%.

Examples 2 to 11, Comparative Examples 1 to 3
Cationic electrodeposition coating compositions (X-2) to (X-14) were prepared in the same manner as in Example 1, except that the formulations shown in Table 1 below were used.
The results of the evaluation tests described below are shown in Table 1. The cationic electrodeposition coating composition of the present invention must pass all the evaluations ("C" indicates failure).

The results of measuring the viscoelasticity of the electrodeposition coating film are shown in Table 1 below. The viscoelasticity was measured under the conditions of the "Method for measuring viscoelasticity" described in this specification. The meanings in the table are as follows:
"G'>1.05×G" ^0.7 ": When the measured G' value (Pa) was consistently higher than the value of "1.05×G" ^0.7 ", it was marked as "Good", otherwise it was marked as "Poor".
"G'>60 (Pa)": When the measured G' value (Pa) was always higher than 60 (Pa), it was marked as "Good", otherwise it was marked as "Poor".
"G'min<300 (Pa)": Measured G'min (minimum value of G') value (Pa).

All blend amounts in the table are solid content values.
The ingredients in the table are as follows:
Viscosity adjuster (c-1-1): Epoxy resin crosslinked particles No. 1 (Production Example 9)
Viscosity adjuster (c-1-2): Epoxy resin crosslinked particles No. 2 (Production Example 10)
Viscosity modifier (c-2): (meth)acrylate-modified epoxy (Production Example 11)
Viscosity adjuster (c-3): hydroxy(meth)acrylate-blocked polyisocyanate compound (Production Example 12)
Viscosity adjuster (c-4): "Sylysia 710" (product name, manufactured by Fuji Silysia Chemical Ltd., silica, particle size 2.8 μm, specific surface area: 700 m ² /g)
Viscosity adjuster (c-5): Polyvinyl alcohol: saponification degree 88%, weight average molecular weight 180,000.

Evaluation test <Corrosion resistance of edge (48 hours)>
A test plate was prepared by electrodeposition coating a cutter blade (blade angle 20 degrees, length 10 cm, zinc phosphate treated) at a bath temperature of 28°C, adjusting the energization time, so that the film thickness on the general surface was 20 μm.
Next, this was subjected to a 48-hour salt spray resistance test in accordance with JIS Z-2371, and the edge portion at the tip of the cutter blade was evaluated according to the following criteria.
The evaluation is "A" and "B" as passing grades, and "C" as failing grades.
A: No rust occurred. B: The number of rust spots is less than 20/10cm.
C: The number of rust spots is 20 or more per 10 cm

Preparation of test panels: Cold-rolled steel panels (150 mm (length) x 70 mm (width) x 0.8 mm (thickness)) that had been subjected to a chemical conversion treatment (manufactured by Nippon Parkerizing Co., Ltd., trade name "Palbond #3020", a zinc phosphate treatment agent) were used as coating substrates, and were electrodeposited with each of the cationic electrodeposition paints obtained in the Examples and Comparative Examples to a dry film thickness of 17 μm, and then baked and dried at 170° C. for 20 minutes to obtain test panels.

<Corrosion resistance of flat parts>
A cross-cut was made in the coating film with a cutter knife so as to reach the base material of the test plate, and this was subjected to a salt spray test at 35°C for 840 hours in accordance with JIS Z-2371, and the rust and blister width on one side of the cut were evaluated according to the following criteria.
The evaluation is "A" and "B" as passing grades, and "C" as failing grades.
A: The maximum width of the rust and blister is 2.0 mm or less on one side of the cut portion. B: The maximum width of the rust and blister is more than 2.0 mm and 3.0 mm or less on one side of the cut portion. C: The maximum width of the rust and blister is more than 3.0 mm on one side of the cut portion.

<Finishing quality (voids)>
The cross section (1 mm x 17 µm) of the coating film of the obtained test plate was observed, and the number of voids (air bubbles) on the coating surface was visually counted.
The evaluation is "A" and "B" as passing grades, and "C" as failing grades.
A: No voids, good B: No large voids (1 μm or more), but 1 to 3 small voids (less than 1 μm) C: At least 1 large void or 4 or more small voids

Claims (11)

A cationic electrodeposition coating composition, characterized in that when the cationic electrodeposition coating composition is electrodeposited onto a metal substrate and the resulting uncured electrodeposition coating film is dried by heating, the storage modulus (G'), loss modulus (G"), and minimum value of G' in the heat drying step (G'min) of the coating film satisfy the following formulas (1) to (3):
G'>1.05×G" ^0.7 ...Formula (1)
G'>60 (Pa)...Formula (2)
G'min<300 (Pa)...Formula (3) The cationic electrodeposition coating composition according to claim 1, characterized in that the cationic electrodeposition coating composition contains an amine-modified epoxy resin (A), a blocked polyisocyanate compound (B), and a viscosity modifier (C). The cationic electrodeposition coating composition according to claim 2, characterized in that the amine-modified epoxy resin (A) is a reaction product of an epoxy resin and an amine compound. The cationic electrodeposition coating composition according to claim 2, characterized in that the amine-modified epoxy resin (A) has a number average molecular weight of 3,000 or more and a glass transition temperature of 100°C or less. The cationic electrodeposition coating composition according to claim 2, characterized in that the viscosity modifier (C) contains a cationic microgel (C-1). The cationic electrodeposition coating composition according to claim 5, characterized in that the cationic microgel (C-1) contains a silane coupling agent as a constituent component. The cationic electrodeposition coating composition according to claim 2, characterized in that the viscosity modifier (C) contains a (meth)acrylate-modified epoxy (C-2) and/or a hydroxy(meth)acrylate-blocked polyisocyanate compound (C-3). The cationic electrodeposition coating composition according to claim 2, further comprising a bismuth compound. The cationic electrodeposition coating composition according to claim 2, characterized in that the blocked polyisocyanate compound (B) is a reaction product of a polyisocyanate compound and a blocking agent, and the blocking agent contains a diol compound having two hydroxyl groups with different reactivities. A method for forming a coating film, comprising electrodeposition coating a metal substrate with the cationic electrodeposition coating composition described in any one of claims 1 to 9, and then heating and drying the resulting uncured electrodeposition coating film. A method for forming a coating film using a cationic electrodeposition coating composition, comprising electrodeposition coating a metal substrate with a cationic electrodeposition coating composition containing an amine-modified epoxy resin (A), a blocked polyisocyanate compound (B), and a viscosity modifier (C), and subsequently heating and drying the resulting uncured electrodeposition coating film, characterized in that the storage modulus (G'), loss modulus (G"), and minimum value of G'(G'min) in the heating and drying step of the electrodeposition coating film satisfy the following formulas (1) to (3):
G'>1.05×G" ^0.7 ...Formula (1)
G'>60 (Pa)...Formula (2)
G'min<300 (Pa)...Formula (3)