Cationic electrodeposition coating composition and method for producing same

Abstract

The purpose of the present invention is to improve corrosion resistance and throwing power, which are properties inherently required for cationic electrodeposition coating compositions, by adding a specific surfactant.　The present invention provides a cationic electrodeposition coating composition comprising a resin emulsion (i) and a pigment dispersion paste (ii), wherein the resin emulsion (i) contains an amine-modified epoxy resin (A) and a blocked isocyanate curing agent (B), the pigment dispersion paste (ii) contains a bismuth compound (C), an inorganic pigment (D), a pigment dispersion resin (E), and a surfactant (F), the pigment dispersion resin (E) has a cationic group, and the surfactant (F) is a nonionic surfactant having a polyoxyalkylene structure and having an HLB value of 14.8 to 18.7 inclusive, a solubility parameter (SP value) of 11.0 to 12.5 inclusive, and a weight average molecular weight of 1500 to 7000 inclusive. The present invention also provides a method for preparing the cationic electrodeposition coating composition.

Description

Cationic electrodeposition coating composition and method for producing the same

[0001] The present invention relates to a cationic electrodeposition coating composition and a method for producing the same.

[0002] Cationic electrodeposition coating compositions are generally used as primer coatings for automobiles, etc., and form coating films with high corrosion resistance. Surfactants have traditionally been added to cationic electrodeposition coating compositions to impart various properties.

[0003] Japanese Patent Laid-Open Publication No. 2008-231142 (Patent Document 1) describes the addition of a bismuth curing catalyst and a specific amount of a nonionic surfactant to a cationic electrodeposition coating composition. The nonionic surfactant is added to improve the dispersibility and dispersion stability of the bismuth curing catalyst. The nonionic surfactant is added during the formation of the cationic electrodeposition coating composition, when the bismuth curing catalyst is dispersed to form a curing catalyst dispersion and then mixed with a binder resin emulsion and a pigment dispersion paste.

[0004] In JP 2020-193281 A (Patent Document 2), a bismuth compound is blended into a pigment dispersion paste, and a surfactant is also blended therein for the purpose of adjusting viscosity.

[0005] JP 2008-231142 A JP 2020-193281 A

[0006] As mentioned above, surfactants are added to cationic electrodeposition coating compositions, but the purpose of adding surfactants is to improve the properties of the surfactant itself, such as dispersibility and viscosity adjustment. The present inventors have investigated the effect of adding surfactants on improving the performance (corrosion resistance and throwing power) of cationic electrodeposition coating compositions, and have found that adding a specific surfactant to a pigment dispersion paste also improves the inherent properties of the cationic electrodeposition coating composition, leading to the development of the present invention. The object of the present invention is to improve the inherent properties of cationic electrodeposition coating compositions, such as corrosion resistance and throwing power, by adding a specific surfactant at a specific time.

[0007] In order to solve the above problems, the present invention provides the following aspects: [1] A cationic electrodeposition coating composition comprising a resin emulsion (i) and a pigment dispersion paste (ii), wherein the resin emulsion (i) contains an amine-modified epoxy resin (A) and a blocked isocyanate curing agent (B), and the pigment dispersion paste (ii) contains a bismuth compound (C), an inorganic pigment (D), a pigment dispersion resin (E), and a surfactant (F), wherein the pigment dispersion resin (E) has a cationic group, and the surfactant (F) is a nonionic surfactant having a polyoxyalkylene structure, with an HLB value of 14.8 to 18.7, a solubility parameter (SP value) of 11.0 to 12.5, and a weight-average molecular weight of 1,500 to 7,000. [2] The cationic electrodeposition coating composition according to [1], wherein the surfactant (F) is contained in an amount of 0.05 to 5 parts by mass per 100 parts by mass of the resin solids content in the cationic electrodeposition coating composition. [3] The cationic electrodeposition coating composition according to [1] or [2], wherein the bismuth compound (C) is dispersed using an organic acid and a polyvalent acid, the organic acid being one or more selected from the group consisting of lactic acid, dimethylolpropionic acid, and methanesulfonic acid, and the polyvalent acid being one or more selected from the group consisting of tartaric acid, citric acid, and malic acid.[4] A method for preparing a cationic electrodeposition coating composition, comprising a step of mixing a resin emulsion (i) and a pigment dispersion paste (ii), wherein the resin emulsion (i) contains an amine-modified epoxy resin (A) and a blocked isocyanate curing agent (B), and the pigment dispersion paste (ii) contains a bismuth compound (C), an inorganic pigment (D), a pigment dispersion resin (E) and a surfactant (F), wherein the pigment dispersion resin (E) has a cationic group, and the surfactant (F) is a nonionic surfactant having a polyoxyalkylene structure, with an HLB value of 14.8 to 18.7, a solubility parameter (SP value) of 11.0 to 12.5, and a weight average molecular weight of 1500 to 7000, further comprising the following steps: a bismuth mixed solution preparation step of mixing the bismuth compound (C), an organic acid, and a solvent to prepare a bismuth mixed solution; and a pigment dispersion paste (ii) preparation step comprising mixing the resulting bismuth mixture, a pigment dispersion resin (E), and a polyacid to prepare a bismuth dispersion, mixing the resulting bismuth dispersion with an inorganic pigment (D), and then adding a surfactant (F) to prepare a pigment dispersion paste (ii). [5] The method for preparing a cationic electrodeposition coating composition according to [4], wherein the surfactant (F) is contained in an amount of 0.05 to 5 parts by mass per 100 parts by mass of the resin solids in the cationic electrodeposition coating composition. [6] The method for preparing a cationic electrodeposition coating composition according to [4] or [5], wherein the organic acid is one or more selected from the group consisting of lactic acid, dimethylolpropionic acid, and methanesulfonic acid, and the polyacid is one or more selected from the group consisting of tartaric acid, citric acid, and malic acid.

[0008] In the present invention, we have discovered that adding a surfactant with specific characteristics and structure to a pigment dispersion paste of a cationic electrodeposition coating composition can improve throwing power while maintaining the corrosion resistance of the electrodeposition coating. The presence of this surfactant in the pigment dispersion paste can also improve the static stability of the pigment paste.

[0009] Fig. 1 is a perspective view showing an example of a box used when evaluating throwing power. Fig. 2 is a cross-sectional view schematically showing a method for evaluating throwing power.

[0010] Definition of Terms In this specification, when a numerical range is expressed as "a to b" (a and b both represent numerical values), it means a to b.

[0011] Cationic Electrodeposition Coating Composition The cationic electrodeposition coating composition of the present invention comprises a resin emulsion (i) and a pigment dispersion paste (ii), wherein the resin emulsion (i) contains an amine-modified epoxy resin (A) and a blocked isocyanate curing agent (B), and the pigment dispersion paste (ii) contains a bismuth compound (C), an inorganic pigment (D), a pigment dispersion resin (E), and a surfactant (F), wherein the pigment dispersion resin (E) has a cationic group, and the surfactant (F) is a nonionic surfactant having a polyoxyalkylene structure and having an HLB value of 14.8 to 18.7, a solubility parameter (SP) of 11.0 to 12.5, and a weight-average molecular weight of 1,500 to 7,000. The present invention also provides a method for preparing the cationic electrodeposition coating composition. The components and the method for preparing the cationic electrodeposition coating composition are described in detail below.

[0012] Resin emulsion (i) The resin emulsion (i) contains an amine-modified epoxy resin (A) and a blocked isocyanate curing agent (B). The resin emulsion (i) may further contain other components as necessary.

[0013] Amine-Modified Epoxy Resin (A) Amine-modified epoxy resin (A) is a film-forming resin that constitutes the electrodeposition coating. Amine-modified epoxy resins obtained by modifying the oxirane ring in the epoxy resin skeleton with an amine compound are preferred. Amine-modified epoxy resins are generally prepared by ring-opening the oxirane ring in the starting resin molecule with an amine compound such as a primary amine, secondary amine, or tertiary amine and / or its acid salt. A typical example of the starting epoxy resin is a polyphenol polyglycidyl ether epoxy resin, which is the reaction product of epichlorohydrin with a polycyclic phenolic compound such as bisphenol A, bisphenol F, bisphenol S, phenol novolac, or cresol novolac. Another example of an epoxy resin starting material is the oxazolidone ring-containing epoxy resin described in JP-A-5-306327. These epoxy resins can be prepared by reacting a diisocyanate compound or a bis-urethane compound obtained by blocking the isocyanate group of a diisocyanate compound with a lower alcohol such as methanol or ethanol with epichlorohydrin.

[0014] The epoxy resin starting material can be chain-extended with a bifunctional polyester polyol, polyether polyol, bisphenol, dibasic carboxylic acid, or the like before the oxirane ring-opening reaction with an amine compound.

[0015] Furthermore, before the ring-opening reaction of the oxirane ring with an amine compound, a monohydroxy compound such as 2-ethylhexanol, nonylphenol, ethylene glycol mono-2-ethylhexyl ether, ethylene glycol mono-n-butyl ether, or propylene glycol mono-2-ethylhexyl ether, or a monocarboxylic acid compound such as octylic acid may be added to some of the oxirane rings for the purposes of adjusting the molecular weight or amine equivalent, improving thermal flow properties, etc.

[0016] An amine-modified epoxy resin can be obtained by reacting the oxirane ring of the epoxy resin with an amine compound. Examples of amine compounds that can be reacted with the oxirane ring include primary amines and secondary amines. Reacting an epoxy resin with a secondary amine can produce an amine-modified epoxy resin having a tertiary amino group. Reacting an epoxy resin with a primary amine can produce an amine-modified epoxy resin having a secondary amino group. Furthermore, an amine-modified epoxy resin having a primary amino group can be prepared by using a secondary amine having a blocked primary amine. For example, an amine-modified epoxy resin having a primary amino group and a secondary amino group can be prepared by blocking the primary amino group with a ketone to form a ketimine before reacting with the epoxy resin, and then introducing this into the epoxy resin and deblocking it. If necessary, a tertiary amine can be used in combination as the amine to be reacted with the oxirane ring.

[0017] Specific examples of primary amines and secondary amines include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, and N-methylethanolamine. Specific examples of secondary amines having a blocked primary amine include ketimine of aminoethylethanolamine and diketimine of diethylenetriamine. Specific examples of tertiary amines that may be used as needed include triethylamine, N,N-dimethylbenzylamine, and N,N-dimethylethanolamine. These amines may be used alone or in combination of two or more.

[0018] The amine compound to be reacted with the oxirane ring of the epoxy resin preferably contains a secondary amine in an amount ranging from 50 to 95% by mass, a secondary amine having a blocked primary amine in an amount ranging from 0 to 30% by mass, and a primary amine in an amount ranging from 0 to 20% by mass.

[0019] The number-average molecular weight of the amine-modified epoxy resin (A) is preferably in the range of 1,000 to 5,000. A number-average molecular weight of 1,000 or more improves the physical properties of the resulting cured electrodeposition coating film, such as solvent resistance and corrosion resistance. On the other hand, a number-average molecular weight of 5,000 or less facilitates viscosity adjustment of the amine-modified epoxy resin, enabling smooth synthesis and facilitating handling of the resulting emulsion dispersion of the amine-modified epoxy resin (A). More preferably, the number-average molecular weight of the amine-modified epoxy resin (A) is in the range of 2,000 to 3,500.

[0020] In this specification, the number average molecular weight is a polystyrene-equivalent number average molecular weight measured by gel permeation chromatography (GPC).

[0021] The amine value of the amine-modified epoxy resin (A) is preferably within the range of 20 to 100 mgKOH / g. When the amine value of the amine-modified epoxy resin (A) is 20 mgKOH / g or more, the emulsion dispersion stability of the amine-modified epoxy resin (A) in the electrodeposition coating composition is improved. On the other hand, when the amine value is 100 mgKOH / g or less, the amount of amino groups in the cured electrodeposition coating film is appropriate, and there is no risk of a decrease in the water resistance of the coating film. The amine value of the amine-modified epoxy resin (A) is more preferably within the range of 20 to 80 mgKOH / g.

[0022] The hydroxyl value of the amine-modified epoxy resin (A) is preferably in the range of 150 to 650 mgKOH / g. A hydroxyl value of 150 mgKOH / g or more results in good curing of the cured electrodeposition coating film, and improved coating film appearance. On the other hand, a hydroxyl value of 650 mgKOH / g or less ensures an appropriate amount of hydroxyl groups remaining in the cured electrodeposition coating film, eliminating the risk of reducing the water resistance of the coating film. The hydroxyl value of the amine-modified epoxy resin (A) is more preferably in the range of 180 to 300 mgKOH / g.

[0023] The use of an amine-modified epoxy resin having a number average molecular weight in the range of 1,000 to 5,000, an amine value of 20 to 100 mgKOH / g, and a hydroxyl value of 150 to 650 mgKOH / g as the amine-modified epoxy resin (A) has the advantage of imparting excellent corrosion resistance to the substrate.

[0024] If necessary, amine-modified epoxy resins with different amine values ​​and / or hydroxyl values ​​may be used in combination as the amine-modified epoxy resin (A). When two or more amine-modified epoxy resins with different amine values ​​and hydroxyl values ​​are used in combination, it is preferable that the average amine value and average hydroxyl value calculated based on the mass ratio of the amine-modified epoxy resins used be within the above-mentioned ranges. Furthermore, the amine-modified epoxy resin (A) used in combination is preferably an amine-modified epoxy resin with an amine value of 20 to 50 mgKOH / g and a hydroxyl value of 50 to 300 mgKOH / g, combined with an amine-modified epoxy resin with an amine value of 50 to 200 mgKOH / g and a hydroxyl value of 200 to 500 mgKOH / g. The use of such a combination has the advantage that the core of the emulsion becomes more hydrophobic and the shell becomes hydrophilic, thereby imparting excellent corrosion resistance.

[0025] The amine-modified epoxy resin (A) may contain an amino group-containing acrylic resin, an amino group-containing polyester resin, or the like, as required.

[0026] Blocked isocyanate curing agent (B) (hereinafter sometimes simply referred to as "curing agent (B)") is a film-forming resin that constitutes the electrodeposition coating film. The blocked isocyanate curing agent (B) can be prepared by blocking a polyisocyanate with a blocking agent.

[0027] Examples of polyisocyanates include aliphatic diisocyanates such as hexamethylene diisocyanate (including trimer), tetramethylene diisocyanate, and trimethylhexamethylene diisocyanate; alicyclic polyisocyanates such as isophorone diisocyanate and 4,4'-methylenebis(cyclohexyl isocyanate); and aromatic diisocyanates such as 4,4'-diphenylmethane diisocyanate, tolylene diisocyanate, and xylylene diisocyanate.

[0028] Preferred examples of the sealant include monohydric alkyl (or aromatic) alcohols such as n-butanol, n-hexyl alcohol, 2-ethylhexanol, lauryl alcohol, phenol carbinol, and methylphenyl carbinol; cellosolves such as ethylene glycol monohexyl ether and ethylene glycol mono-2-ethylhexyl ether; polyether-type both-terminal diols such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol phenol; polyester-type both-terminal polyols obtained from diols such as ethylene glycol, propylene glycol, and 1,4-butanediol and dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, suberic acid, and sebacic acid; phenols such as para-t-butylphenol and cresol; oximes such as dimethyl ketoxime, methyl ethyl ketoxime, methyl isobutyl ketoxime, methyl amyl ketoxime, and cyclohexanone oxime; and lactams represented by ε-caprolactam and γ-butyrolactam.

[0029] The blocked isocyanate curing agent (B) preferably has a blocking rate of 100%, which has the advantage of improving the storage stability of the electrodeposition coating composition.

[0030] As the blocked isocyanate curing agent (B), it is preferable to use a curing agent prepared by blocking an aliphatic diisocyanate with a blocking agent in combination with a curing agent prepared by blocking an aromatic diisocyanate with a blocking agent.

[0031] The blocked isocyanate curing agent (B) reacts preferentially with the primary amine of the amine-modified epoxy resin (A) and then reacts with the hydroxyl group to cure the resin. As the curing agent, at least one curing agent selected from the group consisting of an organic curing agent such as a melamine resin or a phenolic resin, a silane coupling agent, and a metal curing agent may be used in combination with the blocked isocyanate curing agent (B).

[0032] Preparation of Resin Emulsion (i) The resin emulsion (i) can be prepared by dissolving the amine-modified epoxy resin (A) and the blocked isocyanate curing agent (B) in an organic solvent to prepare a solution, mixing these solutions, and then neutralizing the mixture with a neutralizing acid. Examples of the neutralizing acid include organic acids such as methanesulfonic acid, sulfamic acid, lactic acid, dimethylolpropionic acid, formic acid, and acetic acid. In the present invention, it is more preferable to neutralize the resin emulsion containing the amine-modified epoxy resin (A) and the curing agent (B) with one or more acids selected from the group consisting of formic acid, acetic acid, and lactic acid.

[0033] The content of the curing agent (B) must be sufficient to react with active hydrogen-containing functional groups, such as primary amino groups, secondary amino groups, or hydroxyl groups, in the amine-modified epoxy resin (A) during curing to provide a good cured coating film. The content of the curing agent (B) is preferably in the range of 90 / 10 to 50 / 50, more preferably 80 / 20 to 65 / 35, expressed as the solids mass ratio of the amine-modified epoxy resin (A) to the curing agent (B) (amine-modified epoxy resin (A) / curing agent (B)). Adjusting the solids mass ratio of the amine-modified epoxy resin (A) to the curing agent (B) improves the fluidity and curing speed of the coating film (deposited film) during film formation, thereby improving the coating appearance.

[0034] The solid content of the resin emulsion (i) is usually 25 to 50 mass %, and preferably 35 to 45 mass %, based on the total amount of the resin emulsion (i). Here, the "solid content of the resin emulsion" refers to the mass of all components contained in the resin emulsion that remain solid even after removal of the solvent. Specifically, it refers to the total mass of the amine-modified epoxy resin (A), curing agent (B), and other solid components added as needed, contained in the resin emulsion (i).

[0035] The neutralizing acid is preferably used in an amount such that the equivalent ratio of the neutralizing acid to the equivalent of the amino groups in the amine-modified epoxy resin (A) is 10 to 100%, more preferably 20 to 70%. In this specification, the equivalent ratio of the neutralizing acid to the equivalent of the amino groups in the amine-modified epoxy resin (A) is referred to as the neutralization ratio. A neutralization ratio of 10% or more ensures affinity for water and improves water dispersibility.

[0036] Pigment Dispersion Paste (ii) The pigment dispersion paste (ii) used in the method of the present invention contains a bismuth mixed liquid containing a bismuth compound (C), an organic acid, and a solvent; a polyacid; an inorganic pigment (D); a pigment dispersion resin (E); and a surfactant (F). This pigment dispersion paste (ii) is prepared by the following steps: a bismuth mixed liquid preparation step of mixing the bismuth compound (C), the organic acid, and the solvent to prepare a bismuth mixed liquid, and a pigment dispersion paste (ii) preparation step of mixing the obtained bismuth mixed liquid, the pigment dispersion resin (E), and the polyacid to prepare a bismuth dispersion liquid, then mixing the obtained bismuth dispersion liquid with the inorganic pigment (D), and finally adding the surfactant (F) to prepare the pigment dispersion paste (ii).

[0037] Bismuth Mixture The bismuth mixture is a mixture prepared by previously mixing the bismuth compound (C), an organic acid, and a solvent in the preparation of the pigment dispersion paste (ii). By previously mixing the bismuth compound (C), the organic acid, and the solvent in the preparation of the pigment dispersion paste (ii), the bismuth compound is dispersed in a finely divided state, and good catalytic activity can be obtained.

[0038] Bismuth Compound (C) The bismuth compound (C) is a compound containing bismuth metal, and examples thereof include bismuth oxide, bismuth hydroxide, bismuth nitrate, and mixtures thereof. The preferred bismuth compound (C) is at least one selected from the group consisting of bismuth oxide and bismuth hydroxide.

[0039] The bismuth compound (C) is used in powder form. The average particle size of the bismuth compound (C) is preferably 0.5 to 20 μm, more preferably 1 to 3 μm. In this specification, the average particle size refers to the volume average particle size D50, which is measured using a laser Doppler particle size analyzer (manufactured by Nikkiso Co., Ltd., "Microtrac UPA150") by diluting the dispersion with ion-exchange water to obtain an appropriate signal level.

[0040] The amount of bismuth compound (C) contained in the electrodeposition coating composition of the present invention is preferably 0.05 to 1.0 mass % calculated as metal element based on the resin solid content of resin emulsion (i) contained in the electrodeposition coating composition. By using the amount of bismuth compound (C) in this range, the resin component of resin emulsion (i) can be cured well and the storage stability of the electrodeposition coating composition can be maintained well.

[0041] The term "resin solid content of resin emulsion (i)" refers to the total mass of the solid content of the resin components contained in resin emulsion (i), i.e., the aminated resin (A) and the curing agent (B).

[0042] In this specification, the term "metal element conversion" refers to the calculation of the amount of a target metal element by multiplying the content of a metal compound by a metal element conversion coefficient (a coefficient for converting the amount of a metal compound into the amount of a metal element, specifically, the value obtained by dividing the atomic weight of a metal element in a metal compound by the molecular weight of the metal compound). For example, when the bismuth compound (C) is bismuth oxide (Bi ２ O ３, molecular weight 466), the metal element-equivalent bismuth content in an electrodeposition coating composition containing 0.5 mass% of bismuth oxide relative to the resin solid content of resin emulsion (i) is calculated as 0.5 mass% × (418 ÷ 466) to be 0.448 mass%.

[0043] Organic Acids The organic acids are, for example, one or more compounds selected from the group consisting of hydroxymonocarboxylic acids and organic sulfonic acids.

[0044] Examples of hydroxycarboxylic acids include the following compounds: monohydroxymonocarboxylic acids, particularly aliphatic monohydroxymonocarboxylic acids, having a total of 2 to 5 carbon atoms, preferably 2 to 4, such as lactic acid and glycolic acid; dihydroxymonocarboxylic acids, particularly aliphatic dihydroxymonocarboxylic acids, having a total of 3 to 7 carbon atoms, preferably 3 to 6, such as dimethylolpropionic acid (DMPA) and glyceric acid.

[0045] The sulfonic acid is an organic sulfonic acid, and examples thereof include alkanesulfonic acids having a total of 1 to 5, preferably 1 to 3, carbon atoms, such as methanesulfonic acid and ethanesulfonic acid.

[0046] The organic acid is preferably one or more selected from the group consisting of monohydroxymonocarboxylic acids, dihydroxymonocarboxylic acids, and alkanesulfonic acids, and more preferably one or more selected from the group consisting of lactic acid, dimethylolpropionic acid, and methanesulfonic acid.

[0047] The form of the organic acid used is not particularly limited, and examples thereof include a solid form, a liquid form, a solution form in which the organic acid is dissolved in a solvent, and particularly an aqueous solution form.

[0048] The contents of the bismuth compound (C) and the organic acid in the bismuth dispersion are such that the molar ratio of bismuth metal to the organic acid in the bismuth compound (C) (Bi:organic acid) is preferably 1:0.5 to 1:4, and more preferably 1:1 to 1:2.

[0049] Examples of solvents that may be used as needed to prepare the bismuth dispersion include water such as ion-exchanged water, purified water, and distilled water, as well as aqueous solvents containing water as a main component. The aqueous solvent may contain, in addition to water, an organic solvent (e.g., a water-soluble or water-miscible organic solvent such as an alcohol, ester, or ketone) as needed.

[0050] The bismuth dispersion liquid is prepared by mixing the bismuth compound (C), an organic acid, a polyvalent acid, and a solvent by any means. By mixing the bismuth compound (C), an organic acid, and a solvent to prepare a bismuth mixed liquid in advance, the solubility and dispersion stability of the bismuth compound (C) are improved, and an electrodeposition coating composition with excellent coating stability and curability is obtained. The details of this mechanism are not necessarily clear, and without being bound by theory, it is thought that in the bismuth mixed liquid prepared by mixing the bismuth compound (C), an organic acid, and a solvent, some of the bismuth compound dissolves in the organic acid, and some of the bismuth compound disperses with the organic acid (e.g., chelate-like dispersion), resulting in a finely dispersed state of the bismuth compound (C).

[0051] Pigment Dispersion Resin (E) In the preparation method of the present invention, the thus obtained bismuth mixture is mixed with the pigment dispersion resin (E) and a polyvalent acid to prepare a bismuth dispersion. The resulting bismuth dispersion is then mixed with the inorganic pigment (D), and a surfactant (F) is further added and mixed to prepare a pigment dispersion paste (ii) (pigment dispersion paste (ii) preparation step). Mixing the components in the above order improves the dispersion stability of the pigment dispersion paste (ii) containing the bismuth compound (C). Examples of the pigment dispersion resin (E) include an amine-modified epoxy resin (e1) having a hydroxyl value of 20 to 120 mgKOH / g and an amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g. The pigment dispersion resin (E) may contain either or both of the amine-modified epoxy resins (e1) and (e2).

[0052] Examples of the amine-modified epoxy resin (e1) having a hydroxyl value of 20 to 120 mgKOH / g include modified epoxy resins having at least one or more groups selected from a quaternary ammonium group and a tertiary sulfonium group. For example, a modified epoxy resin having a quaternary ammonium group can be prepared by reacting an epoxy resin with a tertiary amine. The aqueous solvent used may be ion-exchanged water or water containing a small amount of alcohol.

[0053] The amine-modified epoxy resin (e1) having a hydroxyl value of 20 to 120 mgKOH / g can be prepared, for example, by reacting a half-blocked isocyanate with the hydroxyl group of an epoxy resin having a hydroxyl group to introduce a blocked isocyanate group.

[0054] The epoxy resin is generally a polyepoxide. This epoxide has an average of two or more 1,2-epoxy groups per molecule. Useful examples of such polyepoxides include the above-mentioned epoxy resins.

[0055] The half-blocked isocyanate used for the reaction with the epoxy resin is prepared by partially blocking a polyisocyanate. The reaction between the polyisocyanate and the blocking agent is preferably carried out in the presence of a curing catalyst (e.g., a tin-based catalyst) as needed, by cooling to 40 to 50°C while stirring and adding the blocking agent dropwise.

[0056] The polyisocyanate is not particularly limited as long as it has an average of two or more isocyanate groups per molecule. Specific examples include polyisocyanates that can be used in preparing the blocked isocyanate curing agent.

[0057] Suitable blocking agents for preparing the above half-blocked isocyanates include lower aliphatic alkyl monoalcohols having 4 to 20 carbon atoms, such as butyl alcohol, amyl alcohol, hexyl alcohol, 2-ethylhexyl alcohol, and heptyl alcohol.

[0058] The reaction between the epoxy resin and the half-blocked isocyanate is preferably carried out by maintaining the temperature at 140° C. for about 1 hour.

[0059] The tertiary amine preferably has 1 to 6 carbon atoms. Specific examples of the tertiary amine include dimethylethanolamine, trimethylamine, triethylamine, dimethylbenzylamine, diethylbenzylamine, N,N-dimethylcyclohexylamine, tri-n-butylamine, diphenethylmethylamine, dimethylaniline, and N-methylmorpholine.

[0060] Furthermore, the neutralizing acid used in combination with the tertiary amine is not particularly limited, and specific examples include inorganic or organic acids such as hydrochloric acid, nitric acid, phosphoric acid, formic acid, acetic acid, and lactic acid. The neutralizing acid is preferably one or more acids selected from the group consisting of formic acid, acetic acid, and lactic acid. The reaction between the resulting neutralized acid salt of a tertiary amine and the epoxy resin can be carried out by a conventional method. For example, the epoxy resin is dissolved in a solvent such as ethylene glycol monobutyl ether, the resulting solution is heated to 60 to 100°C, the neutralized acid salt of a tertiary amine is added dropwise, and the reaction mixture is maintained at 60 to 100°C until the acid value reaches 1.

[0061] The amine-modified epoxy resin (e1) having a hydroxyl value of 20 to 120 mgKOH / g preferably has an epoxy equivalent of 1000 to 1800. This epoxy equivalent is more preferably 1200 to 1700. Furthermore, the amine-modified epoxy resin (e1) having a hydroxyl value of 20 to 120 mgKOH / g preferably has a number average molecular weight of 1500 to 2700.

[0062] The amine-modified epoxy resin (e1) having a hydroxyl value of 20 to 120 mgKOH / g preferably has 35 to 70 meq (milligram equivalents) of quaternary ammonium groups per 100 g, more preferably 35 to 55 meq of quaternary ammonium groups per 100 g. Having the amount of quaternary ammonium groups in this range has the advantages of improving pigment dispersion performance and improving the coating workability of the electrodeposition coating composition.

[0063] Amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g can be prepared by modifying the oxirane ring in the epoxy resin skeleton with an amine compound through reaction. Amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g can be prepared in the same manner as the amine-modified epoxy resin of the amine-modified epoxy resin (A) described above. The amine-modified epoxy resin in the amine-modified epoxy resin (A) may be used as is as the amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g in the present invention. In the present invention, the amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g and the amine-modified epoxy resin of the amine-modified epoxy resin (A) may be the same resin or different resins.

[0064] In preparing the amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g, the amines to be reacted with the oxirane rings of the epoxy resin preferably contain 50 to 95 mass% of a secondary amine, 0 to 30 mass% of a secondary amine having a blocked primary amine, and 0 to 20 mass% of a primary amine.

[0065] The amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g preferably has a number average molecular weight in the range of 1,000 to 5,000. Having a number average molecular weight in this range is preferable, as it allows for good pigment dispersion stability. The number average molecular weight of the amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g is more preferably in the range of 2,000 to 3,500. By having the amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g have a number average molecular weight of 1,000 or more, the physical properties of the resulting cured electrodeposition coating film, such as solvent resistance and corrosion resistance, are improved. Furthermore, by having the amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g have a number average molecular weight of 5,000 or less, the dispersibility and dispersion stability of the resulting pigment-dispersed paste (ii) are improved.

[0066] The amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g preferably has a milligram equivalent of base (MEQ(B)) per 100 g of resin solids of 50 to 350. If the MEQ(B) of the amine-modified epoxy resin (e2) is outside the above range, particularly if the MEQ(B) is less than 50, the storage stability of the pigment-dispersed paste (ii) may be poor. The milligram equivalent of base (MEQ(B)) per 100 g of solids of the amine-modified epoxy resin (e2) can be adjusted by changing the type and amount of the amine compound reacted in the preparation of the amine-modified epoxy resin (e2).

[0067] Here, MEQ(B) is an abbreviation for mg equivalent(base) and is the mg equivalent of base per 100 g of resin solids. This MEQ(B) can be measured by quantitatively determining the amount of base contained in the amine-modified epoxy resin (e2) by precisely weighing out about 10 g of the solids of the electrodeposition coating composition, dissolving it in about 50 ml of solvent (THF: tetrahydrofuran), adding 7.5 ml of acetic anhydride and 2.5 ml of acetic acid, and performing potentiometric titration with a 0.1 N perchloric acid-acetic acid solution using an automatic potentiometric titrator (for example, APB-410 manufactured by Kyoto Electronics Manufacturing Co., Ltd.).

[0068] In the present invention, the amine-modified epoxy resin (e2) used in the preparation of the pigment-dispersed paste (ii) is preferably prepared in the form of a resin emulsion. One embodiment of the resin emulsion preparation method includes a method similar to that for the resin emulsion (i). Specifically, the amine-modified epoxy resin emulsion can be prepared by dissolving the amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g and the blocked isocyanate curing agent (B) in an organic solvent to prepare a solution, mixing these solutions, and then dispersing the mixture in water using a neutralizing acid. Another embodiment of the resin emulsion preparation method includes dissolving the amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g in an organic solvent to prepare a solution, and then dispersing the mixture in water using a neutralizing acid. Examples of neutralizing acids that can be used in preparing the resin emulsion include organic acids such as methanesulfonic acid, sulfamic acid, lactic acid, dimethylolpropionic acid, formic acid, acetic acid, etc. It is more preferable to use one or more acids selected from the group consisting of formic acid, acetic acid, and lactic acid as the neutralizing acid.

[0069] The pigment dispersing resin (E) preferably contains both an amine-modified epoxy resin (e1) having a hydroxyl value of 20 to 120 mgKOH / g and an amine-modified epoxy resin (e2) having a hydroxyl value of 150 to 650 mgKOH / g. When both the amine-modified epoxy resins (e1) and (e2) are contained, the resin solids ratio (e1) / (e2) is preferably in the range of 100 / 1 to 1 / 10, more preferably 100 / 3 to 1 / 1.

[0070] Furthermore, when the pigment dispersing resin (E) contains the above-mentioned amine-modified epoxy resin (e2), the amount of this amine-modified epoxy resin (e2) is preferably 0.1 to 1,000 parts by mass per 100 parts by mass of the solid content of the polyacid described below.

[0071] Polyvalent Acid In this specification, the term "polyvalent acid" refers to a compound having two or more monovalent acid groups or a compound having divalent or higher acid groups. The polyvalent acid is preferably one or more selected from the group consisting of compounds having two or more carboxylic acid groups. Specific examples of polyvalent acids include: compounds having 2 to 6 carbon atoms and having two or more carboxylic acid groups, such as tartaric acid, grape acid, citric acid, malic acid, hydroxymalonic acid, malonic acid, succinic acid, glutaric acid, and adipic acid; and polymers having two or more carboxylic acid groups, such as polyacrylic acid.

[0072] The polyvalent acid is preferably one or more selected from the group consisting of tartaric acid, citric acid, malic acid and polyacrylic acid, and more preferably one or more selected from the group consisting of tartaric acid, citric acid and malic acid.

[0073] The amount of polyvalent acid contained in the pigment dispersion paste (ii) is preferably 0.01 to 10 parts by mass, more preferably 0.08 to 5 parts by mass, and even more preferably 0.09 to 3.5 parts by mass, per 100 parts by mass of the resin solids of the pigment dispersion resin (E). If the amount of polyvalent acid is less than 0.01 part by mass, the horizontal appearance may be deteriorated. On the other hand, if the amount of polyvalent acid is more than 10 parts by mass, the curability may be deteriorated.

[0074] By mixing a bismuth compound (C), an organic acid, and a solvent to obtain a bismuth mixture, with a pigment dispersion resin (E) and a polyacid, followed by an inorganic pigment (D), and then adding and mixing a surfactant (F), the dispersion stability of the resulting pigment dispersion paste (ii) is improved, resulting in an electrodeposition coating composition with excellent coating stability and curability. The details of this mechanism are not entirely clear, and without being bound by theory, it is believed that the bismuth compound (C) is finely dispersed in the bismuth mixture prepared by mixing the bismuth compound (C), an organic acid, and a solvent, with some of the bismuth compound dissolved in the organic acid and some dispersed with the organic acid (e.g., in a chelate-like dispersion). By mixing the bismuth mixture with the pigment dispersion resin (E), at least a portion of the bismuth compound (C) contained in the bismuth mixture is coated with the pigment dispersion resin (E), slightly improving the dispersion stability of the bismuth compound (C). However, the coating state of the bismuth compound at this stage is thought to be insufficient, and adding the inorganic pigment (D) may cause a reaction between the inorganic pigment (D) and the bismuth compound (C), resulting in a breakdown of the coating state. Therefore, by using a polyvalent acid in combination with the pigment dispersion resin (E), the pigment dispersion resin (E) is caused to self-aggregate, and the previously loose coating between the pigment dispersion resin (E) and the bismuth compound (C) is thought to become stronger. Because this self-aggregation force is strong, using a polyvalent acid in combination with the pigment dispersion resin (E) has the advantage of being able to obtain good dispersion stability even when the amount of pigment dispersion resin (E) is reduced.

[0075] Inorganic pigment (D) Inorganic pigment (D) is a pigment that is commonly used in electrodeposition coating compositions. Examples of the pigment include commonly used inorganic pigments and organic pigments, such as coloring pigments such as titanium white (titanium dioxide), carbon black, and red iron oxide; extender pigments such as kaolin, talc, aluminum silicate, calcium carbonate, mica, and clay; and rust-preventive pigments such as iron phosphate, aluminum phosphate, calcium phosphate, aluminum tripolyphosphate, aluminum phosphomolybdate, and aluminum zinc phosphomolybdate.

[0076] The inorganic pigment (D) is preferably used in an amount of 1 to 30% by mass based on the resin solid content of the cationic electrodeposition coating composition.

[0077] Surfactant (F) Specifically, surfactant (F) is a nonionic surfactant having a polyoxyalkylene structure and an HLB value of 14.8 to 18.7, a solubility parameter (SP) of 11.0 to 12.5, and a weight-average molecular weight of 1,500 to 7,000. If the HLB value is less than 14.8, throwing power is not improved and corrosion resistance (salt water immersion test: SDT) deteriorates. If the HLB value is greater than 18.7, throwing power deteriorates and the stability of the pigment paste tends to deteriorate. The HLB value is calculated using the Griffin equation based on weight fraction: HLB = 20 × (MH / M), where MH is the molecular weight of the hydrophilic group moiety and M is the molecular weight of the activator. The HLB value of the surfactant (F) used in the present invention is preferably 15.0 to 17.5, more preferably 15.5 to 16.0.

[0078] The solubility parameter (SP value) of the surfactant (F) of the present invention is 11.0 or more and 12.5 or less. If the solubility parameter (SP value) is less than 11.0, throwing power and pigment paste stability are not improved. If it is more than 12.5, throwing power is not improved and corrosion resistance (salt water immersion test: SDT) is also reduced. The solubility parameter (SP value) of the surfactant (F) is preferably 11.5 to 12.5, more preferably 12.0 to 12.5.

[0079] The solubility parameter (SP value) is an abbreviation for the value of the solubility parameter, and is a measure of solubility. The larger the SP value, the higher the polarity, and conversely, the smaller the SP value, the lower the polarity.

[0080] For example, the SP value can be measured by the following method [Reference: SUH, CLARKE, J. P. S. A-1, 5, 1671-1681 (1967)]. Specifically, a sample is prepared by weighing 0.5 g of an organic solvent into a 100 ml beaker, adding 10 ml of acetone using a volumetric pipette, and dissolving the solvent using a magnetic stirrer. A poor solvent is added dropwise to this sample using a 50 ml burette at a measurement temperature of 20°C, and the amount added is determined as the point at which turbidity occurs. Ion-exchanged water is used as a high SP poor solvent, and n-hexane is used as a low SP poor solvent, and turbidity points are measured for each poor solvent. The SP value δ of an organic solvent is given by the following formula: δ=(V ｍｌ 1 / 2 δ ｍｌ +V ｍｈ １／２ δ ｍｈ ) / (V ｍｌ １／２ +V ｍｈ １／２ ) V ｍ =V １ V ２ / (φ １ V ２ +φ ２ V １ ) δ ｍ =φ1δ1+φ2δ2 V i : Molar volume of solvent (ml / mol) φ i : Volume fraction of each solvent at the clouding point δ i : SP value of solvent ml: low SP poor solvent mixed system mh: high SP poor solvent mixed system

[0081] If the weight-average molecular weight of the surfactant (F) used in the present invention is less than 1,500, throwing power will not improve and corrosion resistance (salt water immersion test: SDT) will also deteriorate. If it is more than 7,000, corrosion resistance (SDT) will also deteriorate. The weight-average molecular weight can be obtained by measurement using gel permeation chromatography. The weight-average molecular weight of the surfactant (F) used in the present invention is preferably 1,500 to 3,500, more preferably 1,900 to 2,500. The surfactant (F) used in the present invention is a nonionic surfactant having a polyoxyalkylene structure. The polyoxyalkylene structure is a polyether of alkylene glycol, and the nonionic surfactant has a structural moiety such as polyethylene glycol or polypropylene glycol. Specific examples of nonionic surfactants include polyoxyethylene lauryl ether, polyoxyethylene alkyl ether, polyoxyethylene phenyl ether, polyoxyethylene polyoxypropylene ether, and mixtures thereof. By adding such a surfactant (F), particularly a nonionic surfactant, throwing power and corrosion resistance are improved, and stability is also excellent.

[0082] The surfactant (F) needs to be blended into the pigment dispersion paste. It is conceivable to blend the surfactant (F) into the electrodeposition coating composition, i.e., blend the surfactant (F) into the resin emulsion (i) described above, or blend the resin emulsion (i) with the pigment dispersion paste (ii) described below and then blend the surfactant (F), but in this case, the expected effects (i.e., throwing power, corrosion resistance, and composition stability) are not obtained. It is believed that this is due to differences in the degree of adsorption of the surfactant (F) to the pigment, but this is merely a hypothesis.

[0083] The amount of surfactant (F) added is easier to handle when considering the composition if it is based on the resin solids content in the final cationic electrodeposition coating composition. The amount of surfactant (F) is 0.1 to 5 parts by mass, preferably 0.5 to 2.5 parts by mass, per 100 parts by mass of the resin solids content of the cationic electrodeposition coating composition. If the amount of surfactant (F) added is less than 0.1 part by weight, throwing power and pigment paste stability will not improve, and if it is more than 5 parts by mass, corrosion resistance (SDT) will deteriorate.

[0084] Preparation of Pigment Dispersion Paste (ii) The pigment dispersion paste (ii) in the present invention is prepared by the steps of first premixing the bismuth compound (C), an organic acid, and a solvent to prepare a bismuth mixed liquid, then mixing the obtained bismuth mixed liquid, the pigment dispersion resin (E), and a polyacid to prepare a bismuth dispersion, then mixing the obtained bismuth dispersion liquid with the inorganic pigment (D), and further mixing the surfactant (F).

[0085] The bismuth compound (C), the organic acid, and the solvent are mixed in advance of the other components to prepare a bismuth mixed solution. By mixing the bismuth compound (C) and the organic acid in advance to prepare a bismuth mixed solution, the solubility and dispersibility of the bismuth compound are improved, which in turn improves catalytic activity, allowing the formation of a coating film with excellent curability and corrosion resistance.

[0086] The bismuth compound (C), organic acid, and solvent can be mixed, for example, by dispersing bismuth compound (C) particles in an aqueous organic acid solution obtained by mixing the organic acid with a solvent (particularly water) through stirring. The mixing conditions, such as temperature and stirring speed, may be those typically used in the production of coating compositions, and can be carried out at a stirring speed sufficient to generate a stirring flow, for example, at 10 to 30°C, preferably at room temperature. The stirring time can be appropriately selected depending on the size of the reaction system, and can be selected, for example, within the range of 0.1 to 24 hours.

[0087] The bismuth liquid mixture thus obtained is mixed with the pigment dispersion resin (E) and a polyvalent acid to prepare a bismuth dispersion. The order of mixing the bismuth liquid mixture, pigment dispersion resin (E), and polyvalent acid may be arbitrary. For example, the bismuth liquid mixture, pigment dispersion resin (E), and polyvalent acid may be added simultaneously and mixed; the bismuth liquid mixture and pigment dispersion resin (E) may be premixed and then the polyvalent acid may be added; or the bismuth liquid mixture and polyvalent acid may be premixed and then the pigment dispersion resin (E) may be added. The mixing conditions, such as temperature and stirring speed, may be those typically used in the production of paint compositions. For example, the mixture may be mixed at a temperature of 10 to 50°C, preferably 20 to 40°C, at a stirring speed sufficient to generate a stirring flow capable of dispersing the bismuth liquid mixture, pigment dispersion resin (E), and polyvalent acid. The stirring time may be selected as desired depending on the scale of the reaction system, the stirring device, and other factors. The stirring time may be, for example, 5 minutes to 1 hour.

[0088] The amount of pigment dispersion resin (E) is preferably in the range of inorganic pigment (D) / pigment dispersion resin (E) = 1 / 0.1 to 1 / 1.5, more preferably inorganic pigment (D) / pigment dispersion resin (E) = 1 / 0.1 to 1 / 1.1, as the ratio (solid content mass ratio) of inorganic pigment (D) to pigment dispersion resin (E) contained in the pigment dispersion paste (ii). If the amount of pigment dispersion resin (E) exceeds the above range, curing performance may be poor. If the amount of pigment dispersion resin (E) is less than the above range, poor pigment dispersion may occur.

[0089] A pigment dispersion paste (ii) is prepared by mixing a bismuth dispersion prepared by mixing a bismuth mixture, a pigment dispersion resin (E), and a polyacid with an inorganic pigment (D), and then adding a surfactant (F). The mixing conditions, such as temperature and stirring speed, may be those typically used in the production of coating compositions, and may be carried out at a temperature of 10 to 50°C, preferably 20 to 40°C, and at a stirring speed sufficient to generate a stirring flow capable of dispersing the pigment. The stirring time is preferably such that the dispersed particle size of the pigment is 10 μm or less. The dispersed particle size of the pigment can be confirmed by measuring the volume average particle size of the pigment.

[0090] Another advantage of the method of the present invention is that the concentration of the bismuth compound (C) contained in the pigment dispersion paste (ii) can be designed to be higher. Adding a bismuth compound directly to a pigment dispersion paste generally reduces the storage stability of the pigment dispersion paste. Furthermore, the concentration of the bismuth compound significantly affects the storage stability of the pigment dispersion paste. For example, the higher the concentration of the bismuth compound, the lower the storage stability of the pigment dispersion paste. In contrast, the pigment dispersion paste containing a bismuth compound prepared by the method of the present invention is characterized by excellent storage stability, despite the high concentration of the bismuth compound contained in the pigment dispersion paste. The concentration of the bismuth compound (C) contained in the pigment dispersion paste (ii) is preferably 1,000 to 30,000 ppm in terms of metal element.

[0091] The amount of the bismuth compound (C) contained in the pigment dispersion paste (ii) relative to 100 parts by mass of the pigment dispersion resin (E) is preferably 0.3 to 30 parts by mass, and more preferably 3 to 25 parts by mass, calculated as the metal element.

[0092] The electrodeposition coating composition of the present invention can be prepared by mixing the resin emulsion (i) and the pigment dispersion paste (ii). The mixing ratio of the resin emulsion (i) to the pigment dispersion paste (ii), in terms of solids mass ratio, is preferably in the range of 1:0.1 to 1:0.4, more preferably 1:0.15 to 1:0.3.

[0093] The electrodeposition coating composition of the present invention may further contain a salt of a rare earth metal selected from the group consisting of neodymium, yttrium, lanthanum, cerium, praseodymium, and ytterbium. Specific examples of the salt of the rare earth metal include acetates, nitrates, sulfates, sulfamates, lactates, formates, and carbonates of the rare earth metals.

[0094] The electrodeposition coating composition contains a rare earth metal salt, which has the advantage of improving corrosion resistance, particularly edge rust prevention. When the electrodeposition coating composition contains a rare earth metal salt, it is preferably contained in an amount of 0.0001 to 0.5 mass % in terms of the rare earth metal element relative to the resin solid content of the resin emulsion (i).

[0095] The rare earth metal salt can be added to the electrodeposition coating composition by any method, for example, by preparing an aqueous solution of the rare earth metal salt in advance and adding it to the electrodeposition coating composition.

[0096] The electrodeposition coating composition of the present invention may contain an amino acid, if necessary. When an amino acid is contained, the amino acid may be further added when the bismuth compound (C), organic acid, and solvent are pre-mixed in the preparation of the pigment dispersion paste (ii). By further adding the amino acid, the amino acid with strong chelating properties can be coordinated to the bismuth compound, thereby improving the dissolution stability of the bismuth compound.

[0097] The amino acid may be, for example, glycine, aspartic acid, or a mixture thereof. When an amino acid is used, it is preferably used in an amount such that the molar ratio of the bismuth compound (C) to the amino acid (Bi:amino acid) is 1:0.5 to 1:4.0, more preferably 1:1 to 1:2.

[0098] The solid content of the electrodeposition coating composition of the present invention is preferably 1 to 30% by mass based on the total amount of the electrodeposition coating composition. If the solid content of the electrodeposition coating composition is less than 1% by mass, the amount of electrodeposition coating film deposited will be small, and it may be difficult to ensure sufficient corrosion resistance. If the resin solid content of the electrodeposition coating composition exceeds 30% by mass, the throwing power or coating appearance may be deteriorated.

[0099] The electrodeposition coating composition of the present invention preferably has a pH of 4.5 to 7. If the pH of the electrodeposition coating composition is less than 4.5, the corrosion resistance will be poor and sludge will be generated during electrodeposition coating. The pH of the electrodeposition coating composition can be set within the above range by adjusting the amount of neutralizing acid used, the amount of free acid added, etc.

[0100] The pH of the electrodeposition coating composition can be measured using a commercially available pH meter having a temperature compensation function.

[0101] The milligram equivalent of acid (MEQ(A)) per 100 g of solid content of the electrodeposition coating composition is preferably 40 to 120. The milligram equivalent of acid (MEQ(A)) per 100 g of resin solid content of the electrodeposition coating composition can be adjusted by the amount of neutralizing acid and the amount of free acid.

[0102] Here, MEQ(A) is an abbreviation for mg equivalent(acid), and is the sum of mg equivalents of all acids per 100 g of solids in the paint. This MEQ(A) can be measured by precisely weighing out about 10 g of the solids of the electrodeposition coating composition, dissolving it in about 50 ml of solvent (THF: tetrahydrofuran), and then performing potentiometric titration using a 1 / 10 N NaOH solution to quantify the amount of acid contained in the electrodeposition coating composition.

[0103] It is preferable that the electrodeposition coating composition is substantially free of both tin compounds and lead compounds. As used herein, "the electrodeposition coating composition is substantially free of both tin compounds and lead compounds" means that the concentration of lead compounds contained in the electrodeposition coating composition does not exceed 50 ppm in terms of lead metal element, and the concentration of organotin compounds does not exceed 50 ppm in terms of tin metal element. The electrodeposition coating composition of the present invention contains a bismuth compound (C). Therefore, there is no need to use lead compounds or organotin compounds as curing catalysts. This makes it possible to prepare an electrodeposition coating composition that is substantially free of both tin compounds and lead compounds.

[0104] The electrodeposition coating composition of the present invention may optionally contain additives commonly used in the coating field, such as organic solvents such as ethylene glycol monobutyl ether, ethylene glycol monohexyl ether, ethylene glycol monoethylhexyl ether, propylene glycol monobutyl ether, dipropylene glycol monobutyl ether, and propylene glycol monophenyl ether, surfactants such as drying inhibitors and antifoaming agents, viscosity modifiers such as acrylic resin microparticles, anti-repellents, and inorganic rust inhibitors such as vanadium salts, copper, iron, manganese, magnesium, and calcium salts. In addition to these, known auxiliary complexing agents, buffers, smoothing agents, stress relaxation agents, gloss agents, semi-gloss agents, antioxidants, and ultraviolet absorbers may also be blended depending on the purpose. These additives may be mixed during the preparation of the resin emulsion (i), during the preparation of the pigment dispersion paste (ii), or during or after the mixing of the resin emulsion (i) and the pigment dispersion paste (ii).

[0105] The electrodeposition coating composition of the present invention may contain other film-forming resin components in addition to the above-mentioned amine-modified epoxy resin (A). Examples of other film-forming resin components include acrylic resins, polyester resins, urethane resins, butadiene-based resins, phenolic resins, and xylene resins. Amine-modified epoxy resins that do not fall under the category of the above-mentioned amine-modified epoxy resin (A) may also be used. Preferred other film-forming resin components that can be contained in the electrodeposition coating composition are phenolic resins and xylene resins. Examples of phenolic resins and xylene resins include xylene resins having 2 to 10 aromatic rings.

[0106] The preparation method of the present invention makes it possible to prepare a cationic electrodeposition coating composition containing a bismuth compound that is excellent in coating stability, curability, coating appearance, etc. Furthermore, the method of the present invention has the advantage that it is possible to more easily prepare an electrodeposition coating composition, since it does not require the preparation of an aqueous solution of a bismuth compound that has low solubility in aqueous media. The preparation method of the present invention makes it possible to easily prepare a cationic electrodeposition coating composition that is excellent in curability, coating stability, and coating appearance, even though it is substantially free of both organotin compounds and lead compounds.

[0107] Electrodeposition coating and electrodeposition coating film formation Electrodeposition coating and electrodeposition coating film formation can be carried out on a substrate using the electrodeposition coating composition of the present invention. In electrodeposition coating using the electrodeposition coating composition of the present invention, the substrate is used as the cathode and a voltage is applied between the cathode and the anode. This causes an electrodeposition coating film to be deposited on the substrate.

[0108] In the electrodeposition coating process, the substrate is immersed in the electrodeposition coating composition and then a voltage of 50 to 450 V is applied to carry out the electrodeposition coating. If the applied voltage is less than 50 V, the electrodeposition may be insufficient, while if it exceeds 450 V, the coating film may be destroyed, resulting in an abnormal appearance. During electrodeposition coating, the bath temperature of the coating composition is usually adjusted to 10 to 45°C.

[0109] The time for applying the voltage varies depending on the electrodeposition conditions, but can generally be set to 2 to 5 minutes.

[0110] The thickness of the electrodeposition coating film is set so that the thickness of the electrodeposition coating film finally obtained by heat curing is preferably 5 to 40 μm, more preferably 10 to 25 μm. If the thickness of the electrodeposition coating film is less than 5 μm, the corrosion resistance may be insufficient. On the other hand, if it exceeds 40 μm, the paint will be wasted.

[0111] After the electrodeposition process is completed, the electrodeposition coating film obtained as described above is heated at 120 to 260°C, preferably 140 to 220°C, for 10 to 30 minutes, either as is or after washing with water, to form a heat-cured electrodeposition coating film.

[0112] The electrodeposition coating composition of the present invention can be applied to a variety of substrates that can be electrically conductive, including, for example, cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electrogalvanized steel sheets, hot-dip galvanized steel sheets, zinc-aluminum alloy-plated steel sheets, zinc-iron alloy-plated steel sheets, zinc-magnesium alloy-plated steel sheets, zinc-aluminum-magnesium alloy-plated steel sheets, aluminum-plated steel sheets, aluminum-silicon alloy-plated steel sheets, and tin-plated steel sheets.

[0113] The present invention will be described in more detail with reference to the following examples, but is not limited thereto. In the examples, "parts" and "%" are by weight unless otherwise specified.

[0114] Production Example 1 Production of Amine-Modified Epoxy Resin (A) 92 parts of methyl isobutyl ketone, 940 parts of bisphenol A epoxy resin (trade name: DER-331J, manufactured by The Dow Chemical Company), 382 parts of bisphenol A, 63 parts of octylic acid, and 2 parts of dimethylbenzylamine were added, and the temperature inside the reaction vessel was maintained at 140° C. while the reaction was carried out until the epoxy equivalent reached 1,110 g / eq. The reaction vessel was then cooled to a temperature of 120° C. Next, a mixture of 78 parts of diethylenetriamine diketimine (a methyl isobutyl ketone solution with a solids content of 73%) and 92 parts of diethanolamine was added, and the reaction was carried out at 120° C. for 1 hour to obtain aminated resin (A-1) (cation-modified epoxy resin).

[0115] Production Example 2: Production of Blocked Isocyanate Curing Agent (B-1) 1,680 parts of hexamethylene diisocyanate (HDI) and 732 parts of methyl isobutyl ketone (MIBK) were charged into a reaction vessel and heated to 60°C. A solution of 346 parts of trimethylolpropane in 1,067 parts of MEK oxime was added dropwise to the vessel at 60°C over 2 hours. After further heating at 75°C for 4 hours, it was confirmed that absorption due to isocyanate groups had disappeared in IR spectrum measurement. After allowing the mixture to cool, 27 parts of MIBK was added to obtain a blocked isocyanate curing agent with a solids content of 80%.

[0116] Production Example 3: Production of blocked isocyanate curing agent (B-2) 1,340 parts of 4,4'-diphenylmethane diisocyanate and 277 parts of MIBK were charged into a reaction vessel and heated to 80°C, after which a solution of 226 parts of ε-caprolactam in 944 parts of butyl cellosolve was added dropwise at 80°C over 2 hours. After further heating at 100°C for 4 hours, it was confirmed in IR spectrum measurement that absorption due to isocyanate groups had disappeared. After allowing to cool, 349 parts of MIBK was added to obtain blocked isocyanate curing agent (B-2) (solids content 80%).

[0117] Production Example 4: Production of Resin Emulsion (i) Containing Amine-Modified Resin and Blocked Isocyanate Curing Agent 350 parts (solids content) of the amine-modified epoxy resin (A) obtained in Production Example 1, 75 parts (solids content) of the blocked isocyanate curing agent (B-1) obtained in Production Example 2, and 75 parts (solids content) of the blocked isocyanate curing agent (B-2) obtained in Production Example 3 were mixed, and ethylene glycol mono-2-ethylhexyl ether was added so that the amount was 3% (15 parts) based on the solids content. Next, formic acid was added so that the amount added was equivalent to a resin neutralization rate of 40%, and the mixture was slowly diluted with ion-exchanged water. Subsequently, methyl isobutyl ketone was removed under reduced pressure until the solids content reached 40%, thereby obtaining Resin Emulsion (i).

[0118] Production Example 5: Production of Pigment Dispersion Resin (E) Preparation of 2-Ethylhexanol Half-Blocked Isophorone Diisocyanate 222.0 parts of isophorone diisocyanate (hereinafter abbreviated as IPDI) was placed in a reaction vessel equipped with a stirrer, a condenser, a nitrogen inlet tube, and a thermometer, and diluted with 39.1 parts of methyl isobutyl ketone (MIBK). Then, 0.2 parts of dibutyltin dilaurate was added. The mixture was then heated to 50°C, and 131.5 parts of 2-ethylhexanol was added dropwise over 2 hours under stirring in a dry nitrogen atmosphere to obtain 2-ethylhexanol half-blocked IPDI (solids content 90.0% by mass). Preparation of Quaternizing Agent 87.2 parts of dimethylethanolamine, 117.6 parts of 75% aqueous lactic acid solution, and 39.2 parts of ethylene glycol mono-n-butyl ether were added in that order to a reaction vessel, and the mixture was stirred at 65°C for 30 minutes to prepare the quaternizing agent. Preparation of Pigment Dispersion Resin (E): 710.0 parts of bisphenol A epoxy resin (trade name: DER-331J, manufactured by Dow Chemical Company) and 289.6 parts of bisphenol A were charged into a reaction vessel and reacted under a nitrogen atmosphere at 150-160°C for 1 hour. The mixture was then cooled to 120°C, and 498.8 parts of the previously prepared 2-ethylhexanol half-blocked IPDI (MIBK solution) was added. The reaction mixture was stirred at 110-120°C for 1 hour, 463.4 parts of ethylene glycol mono-n-butyl ether was added, the mixture was cooled to 85-95°C, and 196.7 parts of the previously prepared quaternizing agent was added. The reaction mixture was maintained at 85-95°C until the acid value reached 1, and then 964 parts of deionized water was added to obtain the desired pigment dispersion resin (E) (solids content 50% by mass).

[0119] Example 1: Preparation of Pigment Dispersion Paste (ii) 5.3 parts of a 50% aqueous lactic acid solution and 6.9 parts of bismuth oxide were stirred and mixed with ion-exchanged water. 60 parts of the pigment dispersion resin (E) obtained in Production Example 5 was added thereto, and the mixture was stirred at room temperature for 1 hour. 0.028 parts of tartaric acid was then added and mixed, followed by 1 part of carbon pigment, 40 parts of titanium oxide, and 59 parts of Satenton (calcined kaolin), and the mixture was stirred at 40°C for 1 hour using a sand mill. 1.07 parts of a nonionic surfactant (F-1: Noigen EA-177, commercially available from Dai-ichi Kogyo Seiyaku Co., Ltd.) was then added and mixed to obtain a pigment dispersion paste (ii) (solids concentration: 47% by mass).

[0120] Preparation of Cationic Electrodeposition Coating Composition To a stainless steel container were added 516 parts of ion-exchanged water, 393 parts of the resin emulsion (i) prepared in Preparation Example 4, and 91 parts of the pigment dispersion paste (ii), and the mixture was then aged at 40°C for 16 hours to obtain an electrodeposition coating composition.

[0121] The HLB value of Noigen EA-177 was 15.6, and the SP value was 12.2. The HLB value was the value listed in the catalog, and the SP value was measured by weighing 0.5 g of sample into a 100 ml beaker, adding 10 ml of acetone using a volumetric pipette, and dissolving uniformly using a magnetic stirrer. A poor solvent was added dropwise to this sample using a 50 ml burette at a measurement temperature of 20°C, and the point at which turbidity occurred was recorded as the amount added. Ion-exchanged water was used as the high SP poor solvent, and n-hexane was used as the low SP poor solvent, and the turbidity point of each was measured.

[0122] The components of Example 1, their amounts, the HLB value, SP value, and weight average molecular weight of the nonionic surfactant (F-1), Noigen EA-177, its addition method, and its amount added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition are shown in Table 1.

[0123] Example 2 A pigment dispersion paste (ii) (solids concentration 47% by mass) was prepared in the same manner as in Example 1, except that the nonionic surfactant (F-1) was changed to 1.64 parts of a nonionic surfactant (F-2: Noigen EA-197D commercially available from Dai-ichi Kogyo Seiyaku Co., Ltd.).

[0124] Using the resulting pigment dispersion paste (ii), an electrodeposition coating composition was obtained in the same procedure as in Example 1. The HLB value of the nonionic surfactant (F-2), Noigen EA-197D, is a catalog value, and the SP value was measured in the same manner as in Example 1. These values ​​are shown in Table 1, along with the weight-average molecular weight of the nonionic surfactant (F-2), its addition method, and its amount added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition. Table 1 also shows the blended components of Example 2 and their blended amounts.

[0125] Example 3 A pigment dispersion paste (ii) (solids concentration 47% by mass) was prepared in the same manner as in Example 1, except that the nonionic surfactant (F-1) was changed to 1.94 parts of a nonionic surfactant (F-3: Noigen EA-207D commercially available from Dai-ichi Kogyo Seiyaku Co., Ltd.).

[0126] Using the resulting pigment dispersion paste (ii), an electrodeposition coating composition was obtained in the same manner as in Example 1. The HLB value of the nonionic surfactant (F-3), Noigen EA-207D, is a catalog value, and the SP value was measured in the same manner as in Example 1. These values ​​are shown in Table 1, along with the weight-average molecular weight of the nonionic surfactant (F-3), its addition method, and its amount added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition. Table 1 also shows the blended components of Example 3 and their blended amounts.

[0127] Example 4 A pigment dispersion paste (ii) (solid content concentration 47% by mass) was prepared in the same manner as in Example 1, except that the amount of nonionic surfactant (F-1) was changed to 0.53 parts and the amount of nonionic surfactant (F-2) was changed to 0.83 parts.

[0128] Using the resulting pigment dispersion paste (ii), an electrodeposition coating composition was obtained in the same procedure as in Example 1. The HLB values ​​and SP values ​​of the nonionic surfactant (F-1) and the nonionic surfactant (F-2) have already been described in Examples 1 and 2. These values ​​are shown in Table 1, along with the weight-average molecular weights of the nonionic surfactants (F-1) and (F-2), their addition method, and the total amount of the nonionic surfactants (F-1) and (F-2) added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition. Table 1 also lists the components and their amounts used in Example 4.

[0129] Example 5 A pigment dispersion paste (ii) was prepared in the same manner as in Example 1, except that the amount of nonionic surfactant (F-1) was changed to 32.39 parts (solids concentration: 47% by mass). Using the resulting pigment dispersion paste (ii), an electrodeposition coating composition was obtained in the same manner as in Example 1. The HLB value and SP value of the nonionic surfactant (F-1), its weight-average molecular weight, its addition method, and its amount added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition are shown in Table 1. Table 1 also shows the ingredients of Example 5 and their amounts.

[0130] Example 6 A pigment dispersion paste (ii) was prepared in the same manner as in Example 1, except that the amount of nonionic surfactant (F-1) was changed to 0.27 parts (solids concentration: 47% by mass). Using the resulting pigment dispersion paste (ii), an electrodeposition coating composition was obtained in the same manner as in Example 1. The HLB value and SP value of the nonionic surfactant (F-1), its weight-average molecular weight, its addition method, and its amount added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition are shown in Table 1. Table 1 also shows the ingredients of Example 6 and their amounts.

[0131] Comparative Example 1 A pigment dispersion paste (ii) (solid content concentration 47% by mass) was prepared in the same manner as in Example 1, except that the nonionic surfactant (F-1) was not used. Using the obtained pigment dispersion paste (ii), an electrodeposition coating composition was obtained in the same manner as in Example 1. In Table 1, the same information as in Example 1 is provided, except for the information on the nonionic surfactant (F-1).

[0132] Comparative Example 2 A pigment dispersion paste (ii) was prepared (solids concentration 47% by mass) in the same manner as in Example 1, except that the bismuth compound (C) was not used. Using the resulting pigment dispersion paste (ii), an electrodeposition coating composition was obtained in the same manner as in Example 1. The HLB value and SP value of the nonionic surfactant (F-1), its weight-average molecular weight, its addition method, and its amount added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition are shown in Table 1. Table 1 also shows the blended components and their blended amounts in Comparative Example 2. However, since the bismuth compound (C) was not blended, it was set to 0.

[0133] Comparative Example 3 A pigment dispersion paste (ii) was prepared in the same manner as in Example 1, except that the amount of nonionic surfactant (F-1) was changed to 41.8 parts (above the upper limit) (solids concentration: 47% by mass). Using the obtained pigment dispersion paste (ii), an electrodeposition coating composition was obtained in the same manner as in Example 1. The HLB value and SP value of the nonionic surfactant (F-1), its weight-average molecular weight, its addition method, and its amount added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition are shown in Table 1. Table 1 also shows the ingredients of Comparative Example 3 and their amounts.

[0134] Comparative Example 4 A pigment dispersion paste (ii) was prepared (solids concentration 47% by mass) in the same manner as in Example 1, except that the amount of nonionic surfactant (F-1) was changed to 0.044 parts (above the lower limit). Using the resulting pigment dispersion paste (ii), an electrodeposition coating composition was obtained in the same manner as in Example 1. The HLB value and SP value of the nonionic surfactant (F-1), its weight-average molecular weight, its addition method, and its amount added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition are shown in Table 1. Table 1 also shows the ingredients of Comparative Example 4 and their amounts.

[0135] Comparative Example 5 A pigment dispersion paste (ii) (solids concentration 47% by mass) was prepared in the same manner as in Example 1, except that the nonionic surfactant (F-1) was replaced with a nonionic surfactant (F-4: Noigen EA-137 commercially available from Dai-ichi Kogyo Seiyaku Co., Ltd.) and the blending amount was changed to 1.1 parts. Using the resulting pigment dispersion paste (ii), an electrodeposition coating composition was obtained in the same manner as in Example 1. The HLB value and SP value of the nonionic surfactant (F-4), Noigen EA-137, were measured in the same manner as in Example 1. The results are shown in Table 1, along with its weight-average molecular weight, its addition method, and its amount added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition. Table 1 also shows the blending components and their blending amounts for Comparative Example 5.

[0136] Comparative Example 6 An electrodeposition coating composition was prepared in the same manner as in Example 1, except that the nonionic surfactant (F-1) was not added to the pigment dispersion paste (ii), but 0.34 parts of the nonionic surfactant (F-1) was added to the resulting electrodeposition coating composition. The HLB value and SP value of the nonionic surfactant (F-1), its weight-average molecular weight, its addition method, and its amount added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition are shown in Table 1. Table 1 also shows the blending components of Comparative Example 6 and their blending amounts.

[0137] Comparative Example 7 A pigment dispersion paste (ii) (solids concentration 47% by mass) was prepared in the same manner as in Example 1, except that the nonionic surfactant (F-1) was replaced with a nonionic surfactant (F-5: SN-001S commercially available from San Nopco Ltd.). Using the resulting pigment dispersion paste (ii), an electrodeposition coating composition was obtained in the same manner as in Example 1. The SP value of SN-001S, the nonionic surfactant (F-5), was measured in the same manner as in Example 1. The results are shown in Table 1, along with its weight-average molecular weight, its addition method, and its amount added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition. Table 1 also shows the components of Comparative Example 7 and their amounts. The HLB value of the nonionic surfactant (F-5) is unknown.

[0138] The electrodeposition coating compositions obtained by the preparation methods of the above Examples and Comparative Examples were subjected to the following evaluations, and the results are shown in Table 1.

[0139] Preparation of Electrodeposition Coated Plates Having Cured Electrodeposition Coatings Cold-rolled steel sheets (JIS G3141, SPCC-SD) were degreased by immersion in Surf Cleaner EC90 (manufactured by Nippon Paint Co., Ltd.) at 50°C for 2 minutes. They were then immersed in Surf Fine GL1 (manufactured by Nippon Paint Co., Ltd.) at room temperature for 30 seconds, and then in Surfdyne 6350 (manufactured by Nippon Paint Co., Ltd.) at 35°C for 2 minutes. They were then rinsed with deionized water. Meanwhile, the cationic electrodeposition coating compositions obtained in the Examples and Comparative Examples were added with the necessary amount of 2-ethylhexyl glycol to achieve a cured electrodeposition coating film thickness of 15 μm. After the steel sheets were completely immersed in the electrodeposition coating composition, voltage application was immediately initiated. The voltage was increased for 30 seconds, reached 180 V, and maintained for 150 seconds, depositing an uncured electrodeposition coating on the substrate (cold-rolled steel sheet). The resulting uncured electrodeposition coating was cured by heating at 160°C for 15 minutes to obtain an electrodeposition coated plate having a cured electrodeposition coating.

[0140] Throwing Power Evaluation Throwing power was evaluated using the so-called four-sheet box method. Specifically, four cold-rolled steel sheets (JIS G3141 SPCC-SD) were degreased by immersion in Surf Cleaner EC90 (Nippon Paint Co., Ltd.) at 50°C for 2 minutes. They were then immersed in Surf Fine GL1 (Nippon Paint Co., Ltd.) at room temperature for 30 seconds and in Surf Dyne 6350 (Nippon Paint Co., Ltd.) at 35°C for 2 minutes. They were then rinsed with deionized water. The four cold-rolled steel sheets obtained were arranged parallel and upright with a 20 mm gap between them, as shown by 11 to 14 in Figure 1 . The lower portions of both sides and the bottom were sealed with insulating material such as adhesive cloth tape to prepare box 10. Steel sheets 11 to 13, except for steel sheet 14, had an 8 mm diameter through-hole 15 at the bottom. Four liters of the electrodeposition coating composition described in each Example or Comparative Example was transferred to a PVC container to form a first electrodeposition tank.

[0141] As shown in FIG. 2 , the box 10 was immersed as an object to be coated in an electrodeposition coating container 20 containing an electrodeposition coating material 21. In this case, the coating material 21 entered the box 10 only through the through-holes 15. The coating material 21 was stirred using a magnetic stirrer (not shown). Then, the steel plates 11 to 14 were electrically connected, and a counter electrode 22 was positioned so that the distance from the nearest steel plate 11 was 150 mm. Electrodeposition coating was performed by applying a voltage to each of the steel plates 11 to 14 as a cathode and the counter electrode 22 as an anode. Coating was performed by increasing the voltage to a level that would result in a coating film formed on side A of the steel plate 11 reaching a thickness of 15 μm within 30 seconds of starting the voltage application, and then maintaining that voltage for 150 seconds in the case of normal electrodeposition.

[0142] After painting, each steel plate was washed with water, baked at 160°C for 15 minutes, and then air-cooled. After that, the thickness of the coating film formed on the A side of the steel plate 11 closest to the counter electrode 22 and the thickness of the coating film formed on the G side of the steel plate 14 farthest from the counter electrode 22 were measured, and the throwing power was evaluated based on the ratio of the coating film thickness (G side) to the coating film thickness (A side) (G / A value).

[0143] Evaluation criteria: ⊚: G / A value is 0.8 or more; ○: G / A value is 0.65 or more and less than 0.8; △: G / A value is 0.5 or more and less than 0.65; ×: G / A value is less than 0.5

[0144] Storage stability of pigment dispersion paste (ii) The pigment dispersion paste (ii) used in preparing the electrodeposition coating compositions of the Examples and Comparative Examples was stored at 40°C for 3 months, and the state of the pigment dispersion paste (ii) after storage was evaluated visually. The evaluation criteria were as follows:

[0145] Evaluation criteria: ◎: No change compared to before storage at 40°C. ○: No separation or sedimentation was observed, and there was almost no increase in viscosity. △: Soft sedimentation was observed, but it returned to a uniform state by stirring, so this did not pose a major problem in use. ×: Hard sedimentation was observed, and the product did not return to a uniform state even when stirred.

[0146] Corrosion Resistance Corrosion Resistance Test (Salt-Solution Dipping Test (SDT)) A scratch was made with a knife in the coating film of a cured electrodeposition-coated cold-rolled steel plate so that it reached the substrate, and the plate was immersed in 5% saline at 55°C for 240 hours. After that, tape was used to peel the plate, and the peel width on both sides was evaluated.

[0147] Evaluation criteria: ⊚: Peel width less than 10 mm; ◯: Peel width 10 mm or more but less than 15 mm; ×: Peel width 15 mm or more

[0148]

[0149] In Table 1, the values ​​for amine-modified epoxy resin (A), blocked isocyanate curing agent (B), bismuth compound (C), inorganic pigment (D), and pigment dispersion resin (E) are added amounts in parts by mass. For surfactant (F), SP is the SP value, HLB is the HLB value, and Mw is the weight-average molecular weight. "Addition method" indicates where it was added; "pigment paste" means it was added to the pigment dispersion paste (ii); "paint" indicates it was added to the cationic electrodeposition coating composition, but not to the pigment dispersion paste. The blank indicates that surfactant (F) was not added. "Addition amount" indicates the amount (parts by mass) added per 100 parts by mass of resin solids in the cationic electrodeposition coating composition.

[0150] In Examples 1 to 6, the surfactant (F) used is a nonionic surfactant that satisfies the following requirements: HLB value of 14.8 to 18.7, solubility parameter (SP value) of 11.0 to 12.5, and weight average molecular weight of 1500 to 7000. Therefore, throwing power and corrosion resistance (SDT) are excellent, and the stability of the pigment dispersion paste is maintained even under harsh conditions at 40°C for three months. Throwing power also maintains a higher G / A value than usual. In Example 6, the amount of surfactant (F) added is small, so throwing power and pigment dispersion paste stability are somewhat worse, but are still within a usable range.

[0151] In Comparative Example 1, the surfactant (F) was not used, so corrosion resistance was maintained to a certain extent, but throwing power and the stability of the pigment dispersion paste were poor. Throwing power and the stability of the pigment dispersion paste were measured under conditions stricter than normal, resulting in significantly poor results. In Comparative Example 2, the bismuth compound was not added, resulting in significantly poor corrosion resistance evaluation. In Comparative Example 3, the amount of surfactant (F) added was high at 6.1 mass%, so throwing power and the stability of the pigment dispersion paste were high, but corrosion resistance was reduced. In Comparative Example 4, the amount of surfactant (F) added was low, resulting in results similar to Comparative Example 1, but with slightly improved stability of the pigment dispersion paste. In Comparative Example 5, the SP value and HLB value of the surfactant (F) were too high and the weight-average molecular weight was low, resulting in very poor throwing power and corrosion resistance. In Comparative Example 6, the surfactant (F) was added to the cationic electrodeposition coating composition rather than to the pigment dispersion paste, resulting in poor throwing power, pigment dispersion paste stability, and corrosion resistance. This shows that the addition of surfactant (F) to the pigment dispersion paste is necessary. In Comparative Example 7, SN-001S, which is commercially available from San Nopco Ltd. and is used in the prior art, was blended, but the SP value of this surfactant was too low, resulting in very poor throwing power and stability of the pigment dispersion paste.

[0152] The cationic electrodeposition coating composition of the present invention can improve throwing power while maintaining the corrosion resistance of the electrodeposition coating by blending a specific surfactant. The coexistence of this surfactant with the pigment dispersion paste can also improve the static stability of the pigment paste. A cationic electrodeposition coating composition with excellent quality and coating appearance can be easily prepared.

Claims

1. A cationic electrodeposition coating composition comprising a resin emulsion (i) and a pigment dispersion paste (ii), wherein the resin emulsion (i) comprises an amine-modified epoxy resin (A) and a blocked isocyanate curing agent (B), the pigment dispersion paste (ii) comprises a bismuth compound (C), an inorganic pigment (D), a pigment dispersion resin (E) and a surfactant (F), the pigment dispersion resin (E) has a cationic group, and the surfactant (F) is a nonionic surfactant having a polyoxyalkylene structure, with an HLB value of 14.8 to 18.7, a solubility parameter (SP value) of 11.0 to 12.5, and a weight-average molecular weight of 1,500 to 7,000.

2. The cationic electrodeposition coating composition according to claim 1, wherein the surfactant (F) is contained in an amount of 0.05 to 5 parts by mass per 100 parts by mass of the resin solid content in the cationic electrodeposition coating composition.

3. A cationic electrodeposition coating composition according to claim 1 or 2, wherein the bismuth compound (C) is dispersed using an organic acid and a polyvalent acid, the organic acid being one or more selected from the group consisting of lactic acid, dimethylolpropionic acid, and methanesulfonic acid, and the polyvalent acid being one or more selected from the group consisting of tartaric acid, citric acid, and malic acid.

4. A method for preparing a cationic electrodeposition coating composition, comprising a step of mixing a resin emulsion (i) and a pigment dispersion paste (ii), wherein the resin emulsion (i) contains an amine-modified epoxy resin (A) and a blocked isocyanate curing agent (B), and the pigment dispersion paste (ii) contains a bismuth compound (C), an inorganic pigment (D), a pigment dispersion resin (E) and a surfactant (F), wherein the pigment dispersion resin (E) has a cationic group, and the surfactant (F) is a nonionic surfactant having a polyoxyalkylene structure, with an HLB value of 14.8 to 18.7, a solubility parameter (SP value) of 11.0 to 12.5, and a weight average molecular weight of 1,500 to 7,000, and further comprising the following steps: a bismuth mixed solution preparation step of mixing the bismuth compound (C), an organic acid, and a solvent to prepare a bismuth mixed solution; and a pigment dispersion paste (ii) preparation step of mixing the obtained bismuth mixture, a pigment dispersion resin (E), and a polyacid to prepare a bismuth dispersion, subsequently mixing the obtained bismuth dispersion with an inorganic pigment (D), and then adding a surfactant (F) to prepare a pigment dispersion paste (ii).

5. The method for preparing a cationic electrodeposition coating composition according to claim 4, wherein the surfactant (F) is contained in an amount of 0.05 to 5 parts by mass per 100 parts by mass of the resin solid content in the cationic electrodeposition coating composition.

6. A method for preparing a cationic electrodeposition coating composition according to claim 4 or 5, wherein the organic acid is one or more selected from the group consisting of lactic acid, dimethylolpropionic acid and methanesulfonic acid, and the polyvalent acid is one or more selected from the group consisting of tartaric acid, citric acid and malic acid.

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