Cationic electrodeposition coating composition, cationic electrodeposition coating film, and coating method

Abstract

To provide a cationic electrodeposition coating composition which is excellent in both weld part corrosion resistance and edge part corrosion resistance.SOLUTION: A cationic electrodeposition coating composition contains an aminated resin (A) and a block isocyanate curing agent (B), wherein the cationic electrodeposition coating composition further contains a group III element compound (C), an epoxy viscous agent (D), a lactic acid (E) and a high-boiling point solvent (F) having a boiling point of 235°C or higher, the epoxy viscous agent (D) is an epoxy viscous agent obtained by reacting a liquid epoxy resin, a polycyclic phenol resin and an amine compound, the cationic electrodeposition coating composition has electric conductivity of 1,500-2,300 μS / cm2, and the cationic electrodeposition coating composition has melt viscosity of 6,000-15,000 mPa s.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a cationic electrodeposition coating composition having excellent anticorrosive properties and uses thereof. [Background technology]

[0002] On the surface of a substrate such as a metal substrate, multiple coating films having various functions are formed to protect the substrate while at the same time imparting a beautiful appearance. Generally, electrodeposition coating films formed by electrodeposition coating are widely used as coating films to impart corrosion resistance to substrates. Electrodeposition coating can apply coating to even the smallest details of substrates, even those with complex shapes, and can be applied automatically and continuously, so it has been widely put to practical use as a primer coating method, particularly for large substrates with complex shapes such as automobile bodies. Electrodeposition coating using a cationic electrodeposition coating composition is widely used as such electrodeposition coating.

[0003] Electrodeposition paints are fundamentally required to have rust prevention properties, and efforts have been made to ensure rust prevention at the ends (edges) of coated objects, for example. For example, in JP 2018-159031 A (Patent Document 1), a Group 3 element compound and an epoxy tackifier are blended into a cationic electrodeposition paint composition, and the coating viscosity of the deposited electrodeposition coating is controlled to improve rust prevention at the edges. Patent Document 1 achieves this objective, demonstrating excellent edge rust prevention. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-159031 Summary of the Invention [Problem to be solved by the invention]

[0005] Rust prevention is required not only at the edge portions but also at welded portions. At automobile assembly sites, welding robots and the like are used to weld steel sheets, which are used as base materials for automobiles. However, the welded portions are exposed to high temperatures in a short period of time, resulting in significant changes in their surface condition. Typically, steel sheets, which are used as base materials for automobiles, are surface-modified by zinc phosphate treatment or the like. However, changes in the surface condition caused by welding have been reported to cause the surface treatment film on the steel sheet to disappear or denature, which also affects the application of electrodeposition coatings, changing the deposition properties of the electrodeposition coating at the welded portions and preventing sufficient corrosion protection at the welded portions. The present invention aims to improve the rust prevention properties of such welded portions as well as the rust prevention properties of the edge portions. [Means for solving the problem]

[0006] In order to solve the above problems, the present invention provides the following aspects. [1] A cationic electrodeposition coating composition comprising an aminated resin (A) and a blocked isocyanate curing agent (B), the cationic electrodeposition coating composition further comprises a Group 3 element compound (C), an epoxy viscosity modifier (D), lactic acid (E), and a high-boiling point solvent (F) having a boiling point of 235°C or higher; the epoxy tackifier (D) is an epoxy tackifier obtained by reacting a liquid epoxy resin, a polycyclic phenol resin, and an amine compound; The cationic electrodeposition coating composition has an electrical conductivity of 1,500 to 2,300 μS / cm 2 and The cationic electrodeposition coating composition has a melt viscosity of 6,000 to 15,000 mPa·s. [2] The cationic electrodeposition coating composition according to [1], wherein the Group 3 element compound (C) is a lanthanum compound. [3] the epoxy tackifier (D) is an epoxy tackifier obtained by reacting a liquid epoxy resin, a polycyclic phenol resin, and an amine compound; The cationic electrodeposition coating composition according to [1] or [2], wherein the amine compound is a diketimine of 2-ethylaminoethanol, N-methylethanolamine, diethanolamine, and diethylenetriamine, or a mixture thereof. [4] The cationic electrodeposition coating composition according to any one of [1] to [3], wherein the high-boiling point solvent (F) is 2-ethylhexyl diglycol. [5] The cationic electrodeposition coating composition according to any one of [1] to [4], wherein the substrate to be coated with the cationic electrodeposition coating composition is a part having a welded portion. [6] A cationic electrodeposition coating film applied on a part having a welded portion using a cationic electrodeposition coating composition, the cationic electrodeposition coating composition comprises an aminated resin (A), a blocked isocyanate curing agent (B), a Group 3 element compound (C), an epoxy viscosity modifier (D), lactic acid (E), and a high-boiling solvent (F) having a boiling point of 235°C or higher; the epoxy tackifier (D) is an epoxy tackifier obtained by reacting a liquid epoxy resin, a polycyclic phenol resin, and an amine compound; The cationic electrodeposition coating composition has an electrical conductivity of 1,500 to 2,300 μS / cm 2 and The cationic electrodeposition coating composition has a melt viscosity of 6,000 to 15,000 mPa·s. Cationic electrocoating film. [7] A coating method comprising immersing a part having a welded portion in a cationic electrodeposition coating composition, forming a cationic electrodeposition coating film on the part by electrophoresis using the part as the cathode, and then heating and curing the formed film, wherein the cationic electrodeposition coating composition is the cationic electrodeposition coating composition according to any one of [1] to [5]. [Effects of the Invention]

[0007] The present invention provides improved adhesion of electrodeposition coatings at welds by further improving the technology of Patent Document 1. The adhesion of electrodeposition coatings, which is caused by loss or alteration of the zinc phosphate coating at welds due to welding (for example, reduction or non-existence of the zinc phosphate coating), is improved by controlling the flow (heat flow) of the coating during heat curing of the electrodeposition coating, thereby improving film thickness defects at welds. DETAILED DESCRIPTION OF THE INVENTION

[0008] The cationic electrodeposition coating composition of the present invention is a cationic electrodeposition coating composition containing a resin emulsion containing an aminated resin (A) and a blocked isocyanate curing agent (B). The cationic electrodeposition coating composition of the present invention further contains a Group 3 element compound (C), an epoxy thickener (D), lactic acid (E), and a high-boiling point solvent (F) having a boiling point of 235°C or higher, and the epoxy thickener (D) is obtained by reacting a liquid epoxy resin, a polycyclic phenolic resin, and an amine compound. The cationic electrodeposition coating composition has an electrical conductivity of 1,500 to 2,300 μS / cm. 2 The cationic electrodeposition coating composition has a melt viscosity of 6,000 to 15,000 mPa·s.

[0009] Resin emulsion The resin emulsion basically contains an aminated resin (A) and a blocked isocyanate curing agent (B). The electrodeposition coating composition of the present invention contains, in addition to the resin emulsion, a Group 3 element compound (C), an epoxy viscosity enhancer (D), lactic acid (E), and a high-boiling solvent (F) having a boiling point of 235°C or higher. Each component will be described below.

[0010] Aminated resin (A) The aminated resin (A) is a film-forming resin that constitutes the electrodeposition coating. The aminated resin (A) is preferably an amine-modified epoxy resin obtained by modifying the oxirane ring in the epoxy resin skeleton with an amine compound. Amine-modified epoxy resins are generally prepared by 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 resin is a polyphenol polyglycidyl ether epoxy resin, which is the reaction product of epichlorohydrin with a polycyclic phenol compound such as bisphenol A, bisphenol F, bisphenol S, phenol novolac, or cresol novolac. Another example of a starting resin is the oxazolidone ring-containing epoxy resin described in Japanese Patent Laid-Open Publication No. 5-306327. These epoxy resins can be prepared by reacting epichlorohydrin with 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.

[0011] The starting resin 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.

[0012] 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 can be added to some of the oxirane rings for the purposes of adjusting the molecular weight or amine equivalent, improving thermal flow properties, etc.

[0013] Amine-modified epoxy resins 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 with a tertiary amino group. Reacting an epoxy resin with a primary amine can produce an amine-modified epoxy resin with a secondary amino group. Furthermore, an amine-modified epoxy resin with a primary amino group can be prepared by using a secondary amine with a blocked primary amine. For example, an amine-modified epoxy resin with 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 it with the epoxy resin, and then introducing the ketimine 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.

[0014] The primary amine, secondary amine, and tertiary amine can be those mentioned above. 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.

[0015] The amine compound to be reacted with the oxirane ring of the epoxy resin preferably contains 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.

[0016] The number-average molecular weight of the aminated 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 makes it easy to adjust the viscosity of the aminated resin, enabling smooth synthesis and facilitating handling of the resulting emulsion dispersion of the aminated resin (A). The number-average molecular weight of the aminated resin (A) is more preferably in the range of 2,000 to 3,500.

[0017] The amine value of the aminated resin (A) is preferably within the range of 20 to 100 mgKOH / g. When the amine value of the aminated resin (A) is 20 mgKOH / g or more, the emulsion dispersion stability of the aminated 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 aminated resin (A) is more preferably within the range of 20 to 80 mgKOH / g.

[0018] The hydroxyl value of the aminated resin (A) is preferably within 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 aminated resin (A) is more preferably within the range of 150 to 400 mgKOH / g.

[0019] The electrodeposition coating composition of the present invention has the advantage that excellent corrosion resistance can be imparted to the substrate by using an aminated resin (A) 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, more preferably 150 to 400 mgKOH / g.

[0020] If necessary, aminated resins with different amine values ​​and / or hydroxyl values ​​may be used in combination as the aminated resin (A). When two or more aminated resins with different amine values ​​and hydroxyl values ​​are used in combination, the average amine value and average hydroxyl value, calculated based on the mass ratio of the aminated resins used, preferably fall within the above-mentioned ranges. Furthermore, the aminated resins (A) used in combination preferably include an aminated resin with an amine value of 20 to 50 mgKOH / g and a hydroxyl value of 50 to 300 mgKOH / g and an aminated 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.

[0021] The aminated resin (A) may contain an amino group-containing acrylic resin, an amino group-containing polyester resin, or the like, as needed.

[0022] Blocked isocyanate curing agent (B) The 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.

[0023] 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.

[0024] Preferred examples of the sealing agent 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 diols terminated at both ends such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol phenol; polyester-type polyols terminated at both ends 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.

[0025] 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.

[0026] 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.

[0027] The blocked isocyanate curing agent (B) reacts preferentially with the primary amine of the aminated 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 organic curing agents such as melamine resins or phenolic resins, silane coupling agents, and metal curing agents may be used in combination with the blocked isocyanate curing agent (B).

[0028] Preparation of resin emulsion The resin emulsion can be prepared by dissolving the aminated resin (A) and the blocked isocyanate curing agent (B) in an organic solvent to prepare solutions, mixing these solutions, and then neutralizing the mixture with a neutralizing acid. Examples of neutralizing acids 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 aminated 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. The acids used here are neutralizing acids, and lactic acid is included. However, "lactic acid (E)," described below, is used as a pH adjuster and has a different function. Therefore, because the timing of addition and the amount used differ, they are expressed as separate components.

[0029] The content of 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 aminated resin (A) during curing to produce a satisfactory cured coating film. The content of curing agent (B) is preferably in the range of 90 / 10 to 50 / 50, more preferably 80 / 20 to 60 / 40, expressed as the solids mass ratio of aminated resin (A) to curing agent (B) (aminated resin (A) / curing agent (B)). Adjusting the solids mass ratio of aminated resin (A) to curing agent (B) improves the fluidity and curing speed of the coating film (deposited film) during film formation, thereby improving the coating appearance.

[0030] The solid content of the resin emulsion is usually 25 to 50 mass %, and preferably 35 to 45 mass %, based on the total mass of the resin emulsion. Here, "solid content of the resin emulsion" refers to the mass of all components contained in the resin emulsion that remain in solid form even after removal of the solvent. Specifically, it refers to the total mass of the aminated resin (A), curing agent (B), and other solid components added as needed, contained in the resin emulsion.

[0031] The neutralizing acid is preferably used in an amount such that the ratio of the equivalent weight of the neutralizing acid to the equivalent weight of the amino groups in the aminated resin (A) is 10 to 100%, more preferably 20 to 70%. In this specification, the ratio of the equivalent weight of the neutralizing acid to the equivalent weight of the amino groups in the aminated resin (A) is referred to as the neutralization rate. A neutralization rate of 10% or more ensures affinity for water and improves water dispersibility.

[0032] Group 3 element compound (C) The cationic electrodeposition coating composition of the present invention preferably contains a Group 3 element compound (C). By containing the Group 3 element compound (C) in the cationic electrodeposition coating composition, a cured electrodeposition coating film with excellent rust prevention properties can be obtained.

[0033] The Group 3 element compound is preferably one or more selected from the group consisting of lanthanum oxide, lanthanum hydroxide, neodymium oxide, neodymium hydroxide, a mixture of lanthanum oxide and an organic acid, a mixture of lanthanum hydroxide and an organic acid, a mixture of neodymium oxide and an organic acid, and a mixture of neodymium hydroxide and an organic acid.

[0034] The Group 3 element compound (C) is more preferably a lanthanum compound, and the metal element compound (C) is even more preferably one or more selected from the group consisting of lanthanum oxide, lanthanum hydroxide, a mixture of lanthanum oxide and an organic acid, and a mixture of lanthanum hydroxide and an organic acid.

[0035] When at least one selected from the group consisting of lanthanum oxide, lanthanum hydroxide, neodymium oxide, and neodymium hydroxide is used as the Group 3 element compound (C), the Group 3 element compound (C) may be in powder form. When the Group 3 element compound (C) is in powder form, its average particle size is preferably 0.01 to 10 μm, more preferably 0.05 to 2 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") after diluting the dispersion with ion-exchange water to obtain an appropriate signal level.

[0036] When using a mixture of lanthanum oxide and / or lanthanum hydroxide with an organic acid as the Group 3 element compound (C), for example, the mixture can be prepared by premixing the lanthanum oxide and / or lanthanum hydroxide with the organic acid. In the present invention, the organic acid is lactic acid, which is referred to as "lactic acid (E)." Lactic acid may be premixed with the Group 3 element compound (C) or may be added as a separate component when the entire composition is formed.

[0037] When a mixture of lanthanum oxide and / or lanthanum hydroxide with lactic acid is prepared in advance, the ratio of the moles of lanthanum to the moles of lactic acid in the lanthanum oxide and / or lanthanum hydroxide is preferably within the range of 1:1.5 to 1:2.9 (lanthanum:lactic acid). Lanthanum is a rare earth element that forms a trivalent cation. Lactic acid is a monovalent acid. Therefore, when the ratio of the moles of lanthanum to the moles of lactic acid is within the range of 1:1.5 to 1:2.9 (lanthanum:lactic acid), the total valence of the anions due to lactic acid (the number of moles of lactic acid) is less than the total valence of the cations due to the lanthanum metal element (i.e., the number of moles of the metal element × 3). By using lanthanum oxide and / or lanthanum hydroxide and lactic acid in the above ratio, it is possible to prepare a cationic electrodeposition coating composition that provides a cured coating film with particularly excellent edge corrosion prevention performance and excellent coating appearance. The molar ratio of lanthanum:lactic acid is more preferably within the range of 1:1.7 to 1:2.7, and even more preferably within the range of lanthanum:lactic acid=1:1.8 to 1:2.5.

[0038] When a Group 3 element compound (C) is used, the content of the Group 3 element compound (C) contained in the cationic electrodeposition coating composition is preferably 0.02 to 1 mass %, more preferably 0.02 to 0.5 mass %, and particularly preferably 0.05 to 0.3 mass %, calculated as the Group 3 element relative to the total mass of the cationic electrodeposition coating composition. By keeping the amount of the Group 3 element compound (C) within the above range, there is an advantage in that better rust prevention performance can be obtained.

[0039] In this specification, the term "resin solid content of the cationic electrodeposition coating composition" refers to the solid content mass of the coating film-forming resin. Specifically, it refers to the total mass of the resin solid content of the aminated resin (A) and the curing agent (B).

[0040] The term "Group 3 element equivalent" refers to the determination of the amount of the target Group 3 element by multiplying the content of the Group 3 element compound (C) by a Group 3 element conversion factor (a factor for converting the amount of the Group 3 element compound (C) into the amount of the Group 3 element; specifically, this is the value obtained by dividing the atomic weight of the Group 3 element in the Group 3 element compound (C) by the molecular weight of the Group 3 element compound (C)). For example, if the Group 3 element compound (C) is lanthanum oxide (La2O3, molecular weight 325.8), the content of lanthanum in an electrodeposition coating composition containing 0.1% by mass of lanthanum oxide in terms of the Group 3 element is calculated as 0.1% by mass × (277.8 ÷ 325.8), which is 0.0853% by mass.

[0041] Epoxy viscosity agent (D) The cationic electrodeposition coating composition of the present invention contains an epoxy tackifier (D). By including the epoxy tackifier (D), the coating viscosity at 110°C of the electrodeposition of the cationic electrodeposition coating composition can be suitably adjusted to the range of 5,000 to 1,000,000 mPa s.

[0042] The epoxy tackifier (D) preferably has an average particle size of 1 to 200 nm, which has the advantage of improving the dispersibility of the epoxy tackifier (D) in the cationic electrodeposition coating composition.

[0043] In this specification, the average particle size of the epoxy viscosity improver (D) is a volume average particle size. The volume average particle size of the resin emulsion can be measured by a laser light scattering method. An example of an instrument that can be used to measure the average particle size of the resin emulsion is a laser Doppler particle size analyzer, such as Microtrac UPA150 (manufactured by Nikkiso Co., Ltd.).

[0044] In this specification, the number average molecular weight of the epoxy viscosity improver (D) can be measured by gel permeation chromatography (GPC) using a polystyrene standard sample standard after removing water by drying under reduced pressure, etc. Specifically, the number average molecular weight is measured under the following GPC system measurement conditions: Equipment: Alliance 2695 Separations Module Column: Tosoh TSK gel ALPHA-M Flow rate: 0.05ml / min Detector: Alliance 2414 Refractive Index Detector Mobile phase: N,N'-dimethylformamide Standard samples: TSK STANDARD POLYSTYRENE (manufactured by Tosoh Corporation), A-500, A-2500, F-1, F-4, F-20, F-80, F-700, 1-phenylhexane (manufactured by Aldrich)

[0045] The epoxy tackifier (D) preferably has an amine value in the range of 50 to 200 mgKOH / g. When the amine value of the epoxy tackifier (D) is 50 mgKOH / g or more, the emulsion dispersion stability of the epoxy tackifier (D) is improved. On the other hand, when the amine value is 200 mgKOH / g or less, a decrease in the water resistance of the coating film can be prevented. Furthermore, when the amine value of the epoxy tackifier (D) is within the above range, the electrical conductivity of the electrodeposition coating composition can be maintained within an appropriate range, which has the advantage of preventing adverse effects on coating workability.

[0046] The epoxy tackifier (D) preferably has an intramolecular crosslinked structure, and the number-average molecular weight of the crosslinked portion of the crosslinked epoxy tackifier (D) is preferably 100,000 to 10,000,000. A high number-average molecular weight of the epoxy tackifier (D) means that the epoxy tackifier (D) contains a small amount of low-molecular-weight substances. Therefore, the inclusion of the epoxy tackifier (D) in a cationic electrodeposition coating composition does not result in problems such as reduced curability or reduced hardness of the resulting coating film due to the presence of low-molecular-weight substances. Furthermore, the inclusion of the epoxy tackifier (D) in a cationic electrodeposition coating composition can effectively increase the viscosity of the cationic electrodeposition coating composition.

[0047] The epoxy tackifier (D) has a liquid epoxy resin skeleton, particularly a polyphenol polyglycidyl ether structure and a polycyclic phenol structure. These structures are common to the structures of aminated resins, which are film-forming resins commonly used in cationic electrodeposition coating compositions. Therefore, the epoxy tackifier (D) has high compatibility with the film-forming resins of cationic electrodeposition coating compositions. This has the advantage that, for example, even when a relatively large amount of the epoxy tackifier (D) is contained in a cationic electrodeposition coating composition, poor coating appearance (e.g., cissing, crater formation, etc.) due to differences in compatibility can be reduced. The epoxy tackifier (D) can be suitably used as a viscosity adjuster for cationic electrodeposition coating compositions.

[0048] The epoxy viscosity enhancer (D) can be prepared, for example, by the following steps: an amine modification step in which a liquid epoxy resin (particularly, a polyphenol polyglycidyl ether type epoxy resin) and an epoxy resin containing a polycyclic phenol compound are reacted with an amine compound to obtain an amine-modified epoxy resin; an acid neutralization and dispersion step in which part or all of the amino groups of the obtained amine-modified epoxy resin are neutralized with an acid and the resin is dispersed in an aqueous solvent; and a reaction step in which the obtained dispersion is mixed with a polyphenol polyglycidyl ether type epoxy resin to cause a reaction, thereby obtaining an epoxy thickener (D); The epoxy thickener (D) can be produced by a method including the steps of: (1) preparing a polyphenol polyglycidyl ether epoxy resin; (2) preparing a polyphenol polyglycidyl ether epoxy resin; (3) preparing a polyphenol polyglycidyl ether epoxy resin; (4) preparing a polyphenol polyglycidyl ether epoxy resin; (5) preparing a polyphenol polyglycidyl ether epoxy resin; (6) preparing a polyphenol polyglycidyl ether epoxy resin; (7) preparing a polyphenol polyglycidyl ether epoxy resin; (8) preparing a polyphenol polyglycidyl ether epoxy resin; (9) preparing a polyphenol polyglycidyl ether epoxy resin; (10) preparing a polyphenol polyglycidyl ether epoxy resin; (11) preparing a polyphenol polyglycidyl ether epoxy resin; (12) preparing a polyphenol polyglycidyl ether epoxy resin; (13) preparing a polyphenol polyglycidyl ether epoxy resin; (14) preparing a polyphenol polyglycidyl ether epoxy resin; (15) preparing a polyphenol polyglycidyl ether epoxy resin; (16) preparing a polyphenol polyglycidyl ether epoxy resin; (17) preparing a polyphenol polyglycidyl ether epoxy resin; (18) preparing a polyphenol polyglycidyl ether epoxy resin; (19 ...

[0049] Amine modification process in the production of epoxy tackifier (D) The amine modification step is a step in which an amine-modified epoxy resin is obtained by reacting an epoxy resin containing a liquid epoxy resin and a polycyclic phenol compound with an amine compound. The epoxy resin used in the amine modification step contains at least a liquid epoxy resin and a polycyclic phenol compound. This includes a state in which the liquid epoxy resin and the polycyclic phenol compound have reacted (chain extension reaction). In this amine modification step, it is preferable to use an epoxy resin obtained by reacting (chain extension reaction) a polyphenol polyglycidyl ether type epoxy resin with a polycyclic phenol compound. The reaction conditions for the chain extension reaction of the polyphenol polyglycidyl ether type epoxy resin and the polycyclic phenol compound can be appropriately selected depending on the stirring device used and the reaction scale. Reaction conditions include, for example, reaction conditions at 85 to 180°C for 0.1 to 8 hours, more preferably at 100 to 150°C for 2 to 8 hours. The stirring device used can be a stirring device commonly used in the coatings field.

[0050] Examples of polycyclic phenol compounds include bisphenol A, bisphenol F, bisphenol S, phenol novolac, cresol novolac, etc. As the polycyclic phenol compound, it is preferable to use bisphenol A, bisphenol F, or a mixture thereof. A specific example of the polyphenol polyglycidyl ether type epoxy resin is a polyphenol polyglycidyl ether type epoxy resin, which is a reaction product of the above polycyclic phenol compound and epichlorohydrin.

[0051] If necessary, before the ring-opening reaction of the oxirane ring with an amine compound, a part of the epoxy resin may be subjected to a chain extension reaction with a bifunctional polyester polyol, polyether polyol (e.g., a polyol having a polyethylene oxide group, a polyol having a polypropylene oxide group, etc.), a dibasic carboxylic acid, etc. For example, when the chain extension reaction is carried out using a polyol having a polypropylene oxide group, a polypropylene oxide group-containing epoxy resin is obtained.

[0052] In one embodiment of the present invention, the epoxy resin reacted with the amine compound in the amine modification step is an epoxy resin containing a polyphenol polyglycidyl ether-type epoxy resin, a polycyclic phenol compound, and a polypropylene oxide group-containing epoxy resin. An example of this embodiment is a state in which a polyphenol polyglycidyl ether-type epoxy resin, a polycyclic phenol compound, and a polypropylene oxide group-containing epoxy resin have reacted (chain extension reaction). When the epoxy resin reacted with the amine compound in the amine modification step contains a polypropylene oxide group-containing epoxy resin, the content of the polypropylene oxide group-containing epoxy resin is preferably 1 to 40 parts by mass, more preferably 15 to 25 parts by mass, per 100 parts by mass of the epoxy resin. The inclusion of a polypropylene oxide group-containing epoxy resin has the advantages of suppressing an increase in the viscosity of the composition during coating preparation, even when a relatively large amount of epoxy tackifier (D) is contained in the cationic electrodeposition coating composition, and maintaining good coating workability.

[0053] Furthermore, if necessary, the epoxy resins may be used by adding 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, to some of the oxirane rings before the ring-opening reaction of the oxirane rings with an amine compound, for the purposes of adjusting the molecular weight or amine equivalent, improving thermal flow properties, etc.

[0054] By reacting the oxirane ring (epoxy group) of the epoxy resin with an amine compound, amine modification proceeds through a ring-opening reaction of the oxirane ring, and an amine-modified epoxy resin is obtained. The amine compound to be reacted with the oxirane ring may be any of the amine compounds already described in the aminated resin (A).

[0055] In the amine modification step, the amount of the amine compound to be reacted with the epoxy resin is preferably 0.9 to 1.2 equivalents per equivalent of the epoxy group in the epoxy resin. In this "equivalent of the amine compound," 1 mole of diketimine is calculated as 1 equivalent.

[0056] In the present invention, the amine compound reacted in the amine modification step preferably contains diethylenetriamine diketimine in an amount of 20 mol% or more, more preferably 25 to 50 mol%, based on the total molar amount of the amine compound. By including diketimine in the above range, the number-average molecular weight of the crosslinked portion of the resulting epoxy viscosity enhancer can be suitably adjusted to within the range of 100,000 to 10,000,000. If the amount of diketimine relative to the total amount of the amine compound is less than 20 mol%, the amount of residual low-molecular-weight epoxy components will be large, which may result in poor coating film curability.

[0057] As described above, when the amine compound contains diethylenetriamine diketimine (hereinafter sometimes simply referred to as "diketimine") in an amount of 20 mol% or more relative to the total molar amount of the amine compounds, the amount of other amine compounds is 80 mol% or less, preferably 50 mol% or less, of the total molar amount of the amine compounds. When two or more types of amine compounds other than diketimine are used as the amine compound, it is preferable to use these amine compounds in approximately the same amount or similar amounts.

[0058] In the present invention, the amine compound used is preferably (i) a combination of diketimine, 2-ethylaminoethanol, and N-methylethanolamine, or (ii) a combination of diketimine, N-methylethanolamine, and diethanolamine, and the molar ratio of diketimine / other amine compound is 20 / 80 to 80 / 20. When multiple other amine compounds are used, it is preferable that they are used in approximately equal amounts, and for example, a molar ratio of 20 / 80 to 80 / 20 is sufficient.

[0059] The reaction conditions for reacting an epoxy resin with an amine compound to modify the epoxy resin with an amine compound can be appropriately selected depending on the reaction scale, etc. Examples of reaction conditions include reacting at 80 to 150°C for 0.1 to 5 hours, more preferably at 120 to 150°C for 0.5 to 3 hours.

[0060] After the amine compound is introduced into the epoxy resin by the amine modification reaction, the blocked amino groups such as primary amino groups are regenerated by deblocking the blocking agent such as ketone contained in the diketimine. The blocking agent is deblocked by hydrolysis during acid neutralization and dispersion in the acid neutralization and dispersion steps described below.

[0061] Acid neutralization and dispersion process in the production of epoxy tackifier (D) The acid neutralization and dispersion step is a step in which some or all of the amino groups in the amine-modified epoxy resin are neutralized with acid and the resulting resin is dispersed in an aqueous solvent. In this acid neutralization and dispersion step, the acid neutralization rate of the amino groups in the amine-modified epoxy resin is preferably 30 to 100%. Here, the acid neutralization rate refers to the percentage of the number of amino groups neutralized with acid relative to the total number of amino groups in the amine-modified epoxy resin. If the acid neutralization rate of amino groups is less than 30%, aggregation of the reaction product may occur during the preparation of the epoxy thickener. Furthermore, by ensuring that the neutralization rate is within the above range, it is possible to avoid adverse effects on the electrical conductivity of the cationic electrodeposition coating composition when adding the epoxy thickener (D) to the cationic electrodeposition coating composition. Furthermore, by ensuring that the neutralization rate is within the above range, it is possible to effectively increase the coating viscosity, which is directly related to edge rust prevention performance, without adversely affecting coating workability, etc.

[0062] In the present invention, examples of the acid used in the acid neutralization and dispersion step include organic acids such as formic acid, lactic acid, and acetic acid, and inorganic acids such as phosphoric acid and nitric acid. Acetic acid is preferably used as the acid. Lactic acid may also be used in this neutralization and dispersion step, but as described in the section on aminated resin (A), it is considered to be different from the lactic acid used as component (E) described below.

[0063] The amine-modified epoxy resin in which some or all of the amino groups have been neutralized is dispersed in an aqueous solvent. Dispersion in an aqueous solvent may be carried out by adding the aqueous solvent to the partially or fully neutralized amine-modified epoxy resin under stirring, or by adding the partially or fully neutralized amine-modified epoxy resin to the aqueous solvent under stirring. The aqueous solvent contains water and, if necessary, may contain an organic solvent in an amount that does not interfere with the dispersion process.

[0064] By the above dispersion, an oil-in-water (O / W type) dispersion of the amine-modified epoxy resin is obtained. The amine-modified epoxy resin dispersion obtained by this dispersion step preferably has a resin solids concentration in the range of 5 to 30 mass %.

[0065] The aqueous solvent may optionally contain a resin dispersant containing one or more resins selected from the group consisting of quaternary ammonium group-containing epoxy resins and tertiary sulfonium group-containing epoxy resins. As such a resin dispersant, a pigment dispersing resin, which is commonly used in the preparation of cationic electrodeposition coating compositions, can be suitably used.

[0066] When the resin dispersant is contained in the aqueous solvent, the content of the resin dispersant is preferably within the range of 0.1 to 20 parts by mass in terms of resin solid content relative to 100 parts by mass of the resin solid content of the amine-modified epoxy resin.

[0067] Reaction steps in the production of epoxy tackifier (D) The reaction step involves mixing and reacting the dispersion obtained in the dispersion step with a polyphenol polyglycidyl ether epoxy resin to obtain an epoxy thickener (D). The polyphenol polyglycidyl ether epoxy resin used in this reaction step is a resin used in addition to the polyphenol polyglycidyl ether epoxy resin used in the amine modification step. In this reaction step, the polyphenol polyglycidyl ether epoxy resin is additionally reacted. In this reaction step, amino groups such as primary amino groups regenerated by deblocking the blocking agent contained in the diketimine react with the polyphenol polyglycidyl ether epoxy resin, resulting in polymerization. This reaction has the advantage that the number-average molecular weight of the crosslinked portion of the resulting epoxy thickener exceeds 100,000.

[0068] As the polyphenol polyglycidyl ether type epoxy resin to be reacted in this reaction step, the polyphenol polyglycidyl ether type epoxy resins exemplified in the above amine modification step can be suitably used.

[0069] The amount of polyphenol polyglycidyl ether type epoxy resin reacted in the reaction step is preferably 0.5 to 4.0 moles per mole of the ketimine derivative. This amount is preferably 0.5 to 3.0 moles, more preferably 0.6 to 2.5 moles, and even more preferably 0.7 to 2.0 moles. If the amount of polyphenol polyglycidyl ether type epoxy resin is less than the above amount, the number-average molecular weight of the resulting epoxy viscosity enhancer (D) may not fall within the desired range. Furthermore, if the amount of polyphenol polyglycidyl ether type epoxy resin exceeds the above amount, the coating workability of the resulting coating composition may be poor.

[0070] In the present invention, it is preferable to react a polyphenol polyglycidyl ether type epoxy resin with the oil-in-water (O / W type) dispersion of amine-modified epoxy resin obtained in the dispersion step. In this case, a so-called internal crosslinking reaction occurs within the oil-in-water (O / W type) dispersion of amine-modified epoxy resin in the reaction step. Furthermore, it is preferable that the amount of ketimine derivative contained in the amine compound used in the amine modification step of the epoxy resin is 40 to 100 mol %. This has the advantage that a high molecular weight resin, such as an epoxy viscosity enhancer having a number average molecular weight of 100,000 to 10,000,000 and having a partially crosslinked moiety, can be obtained with a low content of low molecular weight compounds.

[0071] The reaction conditions in this reaction step can be appropriately selected depending on the reaction scale, etc. Examples of reaction conditions include reaction at room temperature to 90°C for 3 to 10 hours.

[0072] The above production method has the advantage that it is possible to produce an epoxy thickener having an average particle size of 1 to 200 nm, even when the number average molecular weight is very large as in the above range.

[0073] The cationic electrodeposition coating composition of the present invention contains both a Group 3 element compound (C) and an epoxy viscosity improver (D).

[0074] Lactic acid (E) As mentioned above, in the present invention, lactic acid (E) is used in combination with the Group 3 element compound (C), and is blended into the cationic electrodeposition coating composition. Lactic acid (E) is blended to adjust the pH when the Group 3 element compound (C) is basic. Since lactic acid (E) is added to the cationic electrodeposition coating composition in association with the Group 3 element compound (C), the ratio of lactic acid (E) to the number of moles of the Group 3 element compound (C) added, i.e., the number of moles of Group 3 element in the Group 3 element compound:lactic acid, is in the range of 1:1.5 to 1:2.9, preferably 1.5 to 2.7.

[0075] High boiling point solvents (F) with a boiling point of 235°C or higher As described above, the cationic electrodeposition coating composition of the present invention contains a high-boiling-point solvent (F) having a boiling point of 235°C or higher. By incorporating a high-boiling-point solvent (F) having a boiling point of 235°C or higher into the cationic electrodeposition coating composition, the viscosity is controlled, improving the appearance of the cured coating film. The high-boiling-point solvent (F) constitutes part of the organic solvents typically contained in cationic electrodeposition coating compositions. Examples of organic solvents typically contained in cationic electrodeposition coating compositions include 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. Specifically, 2-ethylhexyl diglycol (boiling point 272°C) is used as the high-boiling-point solvent (F).

[0076] The high-boiling point solvent (F) accounts for 10 to 70% by weight, preferably 20 to 50% by weight, of the organic solvent used. The organic solvent is introduced into the cationic electrodeposition coating composition as a blending component, or is blended to control the solubility of resin components, etc., and is contained in an amount of 0 to 20% by weight, more preferably 3 to 17% by weight, based on the solid content of the cationic electrodeposition coating composition.

[0077] Other ingredients Pigment dispersion paste The cationic electrodeposition coating composition of the present invention may contain a pigment dispersion paste, if necessary. The pigment dispersion paste is a component optionally contained in the electrodeposition coating composition, and generally contains a pigment dispersing resin and a pigment.

[0078] pigment dispersion resin Pigment dispersing resins are resins used to disperse pigments and are dispersed in an aqueous medium before use. Pigment dispersing resins that have cationic groups, such as modified epoxy resins having at least one or more groups selected from quaternary ammonium groups, tertiary sulfonium groups, and primary amino groups, can be used as pigment dispersing resins. Specific examples of pigment dispersing resins include quaternary ammonium group-containing epoxy resins and tertiary sulfonium group-containing epoxy resins. Ion-exchanged water or water containing a small amount of alcohol is used as the aqueous solvent.

[0079] pigment The pigment is a pigment generally used in electrodeposition coating compositions. Examples of the pigment include commonly used inorganic pigments and organic pigments, such as color pigments such as titanium dioxide (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.

[0080] Pigment dispersion paste manufacturing The pigment dispersion paste is prepared by mixing a pigment dispersion resin and a pigment. The content of the pigment dispersion resin in the pigment dispersion paste is not particularly limited, but for example, an amount that provides a resin solid content ratio of 20 to 100 parts by mass per 100 parts by mass of the pigment can be used.

[0081] The solid content of the pigment dispersion paste is usually 40 to 70% by mass, and preferably 50 to 60% by mass, based on the total amount of the pigment dispersion paste.

[0082] In this specification, the "solid content of the pigment dispersion paste" refers to the mass of all components contained in the pigment dispersion paste that remain in a solid state even after removal of the solvent. Specifically, it refers to the total mass of the pigment dispersion resin, pigment, and other solid components added as needed contained in the pigment dispersion paste.

[0083] Preparation of cationic electrodeposition coating composition The cationic electrodeposition coating composition of the present invention can be prepared by mixing a resin emulsion containing an aminated resin (A) and a blocked isocyanate curing agent (B), a Group 3 element compound (C), an epoxy tackifier (D), lactic acid (E), and a high-boiling solvent (F) having a boiling point of 235°C or higher, using a commonly used method. When an epoxy tackifier (D) is used, the amount of epoxy tackifier (D) contained in the cationic electrodeposition coating composition is preferably 0.1 to 15 parts by mass in terms of resin solids per 100 parts by mass of the total resin solids of the resin emulsion and the epoxy tackifier (D). The Group 3 element compound (C), if used as needed, may be pre-dispersed together with the pigment during the preparation of the pigment dispersion paste.

[0084] In this specification, the "solid content of the electrodeposition coating composition" refers to the mass of all components contained in the electrodeposition coating composition that remain in solid form even after removal of the solvent. Specifically, it refers to the total mass of the solid content of the aminated resin (A), curing agent (B), Group 3 element compound (C), epoxy viscosity enhancer (D), pigment dispersing resin, pigment, and other solid components contained in the electrodeposition coating composition.

[0085] The solid content of the cationic 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 solid content of the electrodeposition coating composition exceeds 30% by mass, the throwing power or coating appearance may be deteriorated.

[0086] The cationic 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 amount of acid present in the cationic electrodeposition coating composition will be excessive, which may result in poor coating film appearance or coating workability. On the other hand, if the pH exceeds 7, the filterability of the electrodeposition coating composition may decrease, and the horizontal appearance of the cured electrodeposition coating may deteriorate. 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. The pH is more preferably 5 to 7.

[0087] The pH of the electrodeposition coating composition can be measured using a commercially available pH meter with a temperature compensation function.

[0088] 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.

[0089] 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 determined by precisely weighing out approximately 10 g of the solids of the electrodeposition coating composition, dissolving it in approximately 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.

[0090] The cationic electrodeposition coating composition of the present invention may optionally contain additives commonly used in the coating field, specifically surfactants such as anti-drying agents and antifoaming agents, viscosity modifiers such as acrylic resin fine particles, anti-repellent agents, 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 added during the production of the resin emulsion, during the production of the pigment dispersion paste, or during or after mixing of the resin emulsion and the pigment dispersion paste.

[0091] The cationic electrodeposition coating composition of the present invention may contain other film-forming resin components in addition to the above-mentioned aminated resin (A). Examples of other film-forming resin components include acrylic resins, polyester resins, urethane resins, butadiene resins, phenolic resins, and xylene resins. Preferred examples of other film-forming resin components that can be contained in the electrodeposition coating composition include phenolic resins and xylene resins. Examples of phenolic resins and xylene resins include xylene resins having 2 to 10 aromatic rings.

[0092] The cationic electrodeposition coating composition of the present invention has an electrical conductivity of 1,500 to 2,300 μS / cm 2 Electrical conductivity is also called electrical conductivity and is an index of how easily a substance passes an electric current. The cationic electrodeposition coating composition of the present invention has an electrical conductivity of 1,500 to 2,300 μS / cm. 2 The conductivity is preferably 1,550 to 2,200 μS / cm. 2 and more preferably 1,600 to 2,100 μS / cm 2 The conductivity is 1,500 μS / cm 2 If it is less than 2,300 μS / cm, the throwing power will decrease. 2 If the amount is larger than this, the appearance will be poor. The electrical conductivity can be adjusted by adjusting the amount of the Group 3 element compound and lactic acid.

[0093] The cationic electrodeposition coating composition of the present invention must have a melt viscosity of 6,000 to 15,000 mPa·s. The melt viscosity is the viscosity of the cationic electrodeposition coating composition when molten, and is determined by measuring the melt viscosity of the electrodeposition coating film after applying the cationic electrodeposition coating composition and baking it using a conventional method. The cationic electrodeposition coating composition of the present invention is required to have a melt viscosity of 6,000 to 15,000 mPa·s, preferably 7,000 to 14,000 mPa·s, and more preferably 8,000 to 13,000 mPa·s. If the melt viscosity is lower than 6,000 mPa·s, defects such as sagging will occur during melting. Conversely, if it is higher than 15,000 mPa·s, the flow of the electrodeposition coating film upon heating will be insufficient, resulting in an uneven thickness of the cured electrodeposition coating film.

[0094] Electrodeposition coating and electrodeposition coating film formation An electrodeposition coating film can be formed by electrodeposition coating of a substrate using the cationic electrodeposition coating composition of the present invention. In electrodeposition coating using the cationic 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.

[0095] The cationic 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.

[0096] 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, and if it exceeds 450 V, the appearance of the coating film may be poor. During the electrodeposition coating, the bath temperature of the coating composition is usually adjusted to 10 to 45°C.

[0097] The time for applying the voltage varies depending on the electrodeposition conditions, but can generally be set to 2 to 5 minutes.

[0098] In electrodeposition coating using the cationic electrodeposition coating composition of the present invention, the thickness of the electrodeposition coating film to be deposited is preferably such that the thickness of the electrodeposition coating film finally obtained by heat curing is preferably 5 to 60 μm, more preferably 10 to 25 μm. If the thickness of the electrodeposition coating film is less than 5 μm, the rust prevention properties may be insufficient.

[0099] The electrodeposition coating film deposited as described above can be cured by, if necessary, rinsing with water and then heating for 10 to 30 minutes at, for example, 120 to 260°C, preferably 140 to 220°C, thereby forming a cured electrodeposition coating film.

[0100] The cationic electrodeposition coating composition of the present invention has the advantage that it can provide a cured electrodeposition coating film with excellent edge rust prevention properties even when the cured electrodeposition coating film is provided on an object having edges, and further has the advantage that it can provide a cured electrodeposition coating film with excellent rust prevention properties even in areas having welds.The cationic electrodeposition coating composition of the present invention has the function of retaining the deposited electrodeposition coating film, and therefore can be suitably used when providing a cured electrodeposition coating film on an object having edges or welds.

[0101] In this specification, the corrosion resistance of a cured electrodeposition coating formed on an edged substrate is evaluated by a salt spray test (35°C x 72 hours) in accordance with JIS Z 2371 (2000). When a high-thickness cured electrodeposition coating, for example, a 25-50 μm thick film, is subjected to a salt spray test, the number of rust spots occurring in the edge-coated portion of the cured electrodeposition coating formed on an edged substrate is determined to be within 1 cm of the edge. 2 For example, 3 pieces / cm 2 If the number of rust spots is less than 1cm, it can be said that the coating has excellent corrosion resistance (rust prevention) at the edge. 2 1 piece per cm 2If it is less than this, it can be said that the coating film has extremely excellent edge corrosion resistance. [Example]

[0102] 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.

[0103] Production Example 1: Production of pigment dispersion resin Preparation of 2-ethylhexanol half-blocked isophorone diisocyanate A reaction vessel equipped with a stirrer, a condenser, a nitrogen inlet tube, and a thermometer was charged with 222.0 parts of isophorone diisocyanate (hereinafter referred to as IPDI) and diluted with 39.1 parts of methyl isobutyl ketone (MIBK), after which 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 (solid content 90.0% by mass).

[0104] Preparation of quaternizing agents To a reaction vessel were added 87.2 parts of dimethylethanolamine, 117.6 parts of a 75% aqueous lactic acid solution, and 39.2 parts of ethylene glycol mono-n-butyl ether in that order, and the mixture was stirred at 65°C for 30 minutes to prepare a quaternizing agent.

[0105] Manufacture of pigment dispersing resin A reaction vessel was charged with 710.0 parts of bisphenol A epoxy resin (trade name: DER-331J, manufactured by The Dow Chemical Company) and 289.6 parts of bisphenol A. The mixture was 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, followed by the addition of 463.4 parts of ethylene glycol mono-n-butyl ether. The mixture was then 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 mgKOH / g. Then, 964 parts of deionized water was added to obtain the desired epoxy resin (pigment dispersion resin) with quaternary ammonium groups (solids content: 50% by mass).

[0106] Preparation Example 2-1 Preparation of aminated resin (A-1) A mixture of 92 parts methyl isobutyl ketone, 940 parts bisphenol A epoxy resin (DER-331J, Dow Chemical Company), 382 parts bisphenol A, 63 parts octylic acid, and 2 parts dimethylbenzylamine was added to the reaction vessel, maintained at 140°C, and reacted until the epoxy equivalent reached 1220 g / eq. The reaction vessel was then cooled to 120°C. A mixture of 85 parts diethylenetriamine diketimine (73% solids solution in methyl isobutyl ketone) and 80 parts diethanolamine was then added and reacted at 120°C for 1 hour to yield an aminated resin (cationically modified epoxy resin). The resin had a number-average molecular weight of 2,560, an amine value of 70 mgKOH / g (42 mgKOH / g of which was derived from primary amines), and a hydroxyl value of 236 mgKOH / g.

[0107] Production Example 3-1 Production of Blocked Isocyanate Curing Agent (B-1) 1,680 parts of hexamethylene diisocyanate (HDI) and 732 parts of 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 at 60°C over 2 hours. After further heating at 75°C for 4 hours, IR spectroscopy confirmed that the absorption due to the isocyanate group had disappeared. After allowing to cool, 27 parts of MIBK were added to obtain a blocked isocyanate curing agent (B-1) with a solids content of 78%. The isocyanate value was 252 mgKOH / g.

[0108] Production Example 3-2 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. 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, IR spectroscopy confirmed that the absorption due to the isocyanate group had disappeared. After allowing to cool, 349 parts of MIBK was added to obtain blocked isocyanate curing agent (B-2) (solids content 80%). The isocyanate group value was 251 mgKOH / g.

[0109] Production Example 4-1 Production of Resin Emulsion (1) 350 parts (solids) of the aminated resin (A-1) obtained in Production Example 2-1, 75 parts (solids) of the blocked isocyanate curing agent (B-1) obtained in Production Example 3-1, and 75 parts (solids) of the blocked isocyanate curing agent (B-2) obtained in Production Example 3-2 were mixed, and ethylene glycol mono-2-ethylhexyl ether was added so that the solids content was 3% (15 parts). Next, formic acid was added so that the amount of formic acid 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%, yielding resin emulsion (1).

[0110] Preparation Example 5-1 Preparation of epoxy viscous agent (D-1) 940 parts of bisphenol A epoxy resin (trade name: DER-331J, manufactured by The Dow Chemical Company), 388 parts of bisphenol A, and 2 parts of dimethylbenzylamine were added, and the temperature inside the reaction vessel was maintained at 140°C. The reaction was carried out until the epoxy equivalent reached 800 g / eq, and then the temperature inside the reaction vessel was cooled to 120°C.

[0111] Next, a mixture (amine compound) of 155 parts of diethylenetriamine diketimine (73% solids solution in methyl isobutyl ketone), 44 parts of N-methylethanolamine (MMA), and 44 parts of 2-ethylaminoethanol (MEM) was added and reacted at 120°C for 1 hour to obtain an amine-modified epoxy resin. After cooling to 90°C, ion-exchanged water and acetic acid were added to achieve a 50% neutralization rate of the amine-modified epoxy resin for acid neutralization. Ion-exchanged water was then added to dilute and disperse the mixture to a solids content of 20%. Subsequently, 188 parts of bisphenol A epoxy resin (trade name: DER-331J, manufactured by The Dow Chemical Company) (1.4 moles per mole of the ketimine derivative) was added and reacted at 90°C for 3 hours. Methyl isobutyl ketone was removed under reduced pressure to obtain an epoxy viscosity enhancer (D-1) with a solids content of 20%.

[0112] The number average molecular weight of the resulting epoxy tackifier (D-1) was measured using gel permeation chromatography (GPC) under the following conditions after removing water by drying under reduced pressure, using a polystyrene standard sample as the reference standard. The number average molecular weight of the crosslinked portion of the epoxy tackifier (D-1) was 800,000.

[0113] Equipment: Alliance 2695 Separations Module Column: Tosoh TSK gel ALPHA-M Flow rate: 0.05ml / min Detector: Alliance 2414 Refractive Index Detector Mobile phase: N,N'-dimethylformamide Standard samples: TSK STANDARD POLYSTYRENE (manufactured by Tosoh Corporation), A-500, A-2500, F-1, F-4, F-20, F-80, F-700, 1-phenylhexane (manufactured by Aldrich)

[0114] The average particle size of the epoxy viscous agent (D-1) was measured using a dynamic light scattering particle size analyzer LB-500 (manufactured by Horiba, Ltd.) The average particle size of the epoxy viscous agent (D-1) was 30 nm.

[0115] Preparation Example 5-2 Preparation of epoxy viscous agent (D-2) An epoxy viscosity enhancer (D-2) was produced in the same manner as in Production Example 5-1, except that in the amine modification step, a mixture of 155 parts of diethylenetriamine diketimine (a methyl isobutyl ketone solution with a solids content of 73%), 44 parts of N-methylethanolamine (MMA), and 44 parts of diethanolamine (DEtA) was added and reacted at 120°C for 1 hour to obtain an amine-modified epoxy resin.

[0116] The number average molecular weight, average particle size and polymer ratio of the obtained epoxy viscosity enhancer (D-2) were measured in the same manner as in Production Example 5-1, and the results were as follows. Number average molecular weight of crosslinked part: 1,000,000 Average particle size: 50nm

[0117] Example 1 Manufacture of pigment dispersion paste for cationic electrodeposition paint Using a sand mill, ingredients A to D based on the formulation shown in Table 1 below, including the pigment dispersion resin obtained in Production Example 1, were charged and stirred for 30 minutes, and then the remaining ingredients E to G were charged and stirred for 30 minutes. The resulting mixture was dispersed at 40°C until the volume average particle diameter D50 was 0.6 μm, and prepared to obtain a pigment dispersion paste (solid content 50%). The volume average particle diameter D50 was measured using a laser Doppler particle size analyzer (Nikkiso Co., Ltd., "Microtrac UPA150"), after diluting the dispersion with ion-exchange water to obtain an appropriate signal level. [Table 1]

[0118] A stainless steel container was charged with 2,040 parts of ion-exchanged water, 1,361 parts of the resin emulsion (1) of Production Example 4-1, 465 parts of the pigment dispersion paste for cationic electrodeposition paint prepared above, 114 parts of the epoxy viscous agent (D-1) obtained in Production Example 5-1 (an amount that would result in 4 mass% of the resin solid content relative to the total resin solid content of the resin emulsion and the resin solid content of the epoxy viscous agent), and 20 parts of 2-ethylhexyl diglycol (EHDG) as a high-boiling point solvent (F), and the mixture was aged at 40°C for 16 hours to obtain a cationic electrodeposition paint composition.

[0119] Example 2 Except for changing the amount of lanthanum oxide from 5.6 parts to 6.5 parts and the amount of lactic acid from 12.5 parts to 14.3 parts, In the same manner as in Example 1, a cationic electrodeposition coating composition was prepared.

[0120] Example 3 The amount of lanthanum oxide was changed from 5.6 parts to 4.8 parts, and the amount of lactic acid was changed from 12.5 parts to 10.7 parts. In the same manner as in Example 1, a cationic electrodeposition coating composition was prepared.

[0121] Example 4 A stainless steel container was charged with 2020 parts of ion-exchanged water, 1361 parts of the resin emulsion (2) of Production Example 4-2, 465 parts of the pigment dispersion paste prepared above, 114 parts of the epoxy thickener (D-1) obtained in Production Example 5-1 (an amount that would result in 4 mass% resin solids relative to the total resin solids of the resin emulsion and the epoxy thickener), and 40 parts of 2-ethylhexyl diglycol as a high-boiling point solvent (F), and the mixture was aged at 40°C for 16 hours to obtain a cationic electrodeposition coating composition.

[0122] Example 5 A cationic electrodeposition coating composition was prepared in the same manner as in Example 1, except that the amount of lactic acid was changed from 12.5 parts to 10.6 parts.

[0123] Example 6 A stainless steel container was charged with 2040 parts of ion-exchanged water, 1361 parts of the resin emulsion (1) of Production Example 4-1, 465 parts of the pigment dispersion paste prepared above, 114 parts of the epoxy thickener (D-2) obtained in Production Example 5-2 (an amount that would result in 4 mass% resin solids relative to the total resin solids of the resin emulsion and the epoxy thickener), and 20 parts of 2-ethylhexyl diglycol as a high-boiling point solvent (F), and the mixture was aged at 40°C for 16 hours to obtain a cationic electrodeposition coating composition.

[0124] Comparative Example 1 2060 parts of ion-exchanged water, 1361 parts of the resin emulsion (1) of Production Example 4-1, 465 parts of the pigment dispersion paste prepared above, and 114 parts of the epoxy viscous agent (D-1) obtained in Production Example 5-1 (an amount that would result in 4 mass% of the resin solid content relative to the total resin solid content of the resin emulsion and the resin solid content of the epoxy viscous agent) were added to a stainless steel container and mixed, and the mixture was aged at 40°C for 16 hours to obtain a cationic electrodeposition coating composition.

[0125] Comparative Example 2 A stainless steel container was charged with 2,087 parts of ion-exchanged water, 1,333 parts of the resin emulsion (1) of Production Example 4-1, 465 parts of the pigment dispersion paste prepared above, 95 parts of acrylic microgel (manufactured by Nippon Paint Automotive Coatings Co., Ltd., molecular weight of approximately 1,000,000 or more, solids content 30%, average particle size 50 nm) (an amount that would result in 6 mass% resin solids based on the total resin solids of the resin emulsion and the acrylic microgel), and 20 parts of 2-ethylhexyl diglycol as a high-boiling point solvent (F), and the mixture was aged at 40°C for 16 hours to obtain a cationic electrodeposition coating composition.

[0126] Comparative Example 3 A stainless steel container was charged with 2,076 parts of ion-exchanged water, 1,234 parts of the resin emulsion (1) of Production Example 4-1, 465 parts of the pigment dispersion paste prepared above, 205 parts of acrylic microgel (manufactured by Nippon Paint Automotive Coatings Co., Ltd., molecular weight of approximately 1,000,000 or more, solids content 30%, average particle size 50 nm) (an amount that resulted in 13 mass% of the resin solids based on the total resin solids of the resin emulsion and the acrylic microgel), and 20 parts of 2-ethylhexyl diglycol as a high-boiling point solvent (F), and the mixture was aged at 40°C for 16 hours to obtain a cationic electrodeposition coating composition.

[0127] Comparative Example 4 A cationic electrodeposition coating composition was prepared in the same manner as in Example 1, except that the amount of lanthanum oxide was changed from 5.6 parts to 0 parts and the amount of lactic acid was changed from 12.5 parts to 0 parts.

[0128] Comparative Example 5 A cationic electrodeposition coating composition was prepared in the same manner as in Example 1, except that the amount of lanthanum oxide was changed from 5.6 parts to 12.9 parts and the amount of lactic acid was changed from 12.5 parts to 28.5 parts.

[0129] Comparative Example 6 A cationic electrodeposition coating composition was prepared in the same manner as in Example 1, except that the amount of lactic acid was changed from 17.2 parts to 30.1 parts.

[0130] Measurement of coating viscosity at 110°C The substrate (tinplate) was immersed in the cationic electrodeposition coating composition and electrodeposited for 180 seconds to form an uncured electrodeposition coating film with a thickness of approximately 20 μm. The substrate was then washed with water to remove excess electrodeposition coating composition. After removing excess water from the electrodeposition coating surface, the coating was immediately peeled off without drying. The coating viscosity at 110°C was measured using a dynamic viscoelasticity measuring device (Rheosol-G3000, manufactured by UBM) at a fundamental frequency of 1 Hz and a strain control of 0.5°.

[0131] Paint conductivity The cationic electrodeposition coating composition obtained above was adjusted to 25°C, and the conductivity was measured using a conductivity meter (glass electrode type hydrogen ion concentration indicator, model: MM-60R).

[0132] Formation of cured electrodeposition coating film Cold-rolled steel sheets (JIS G3141, SPCC-SD) were degreased by immersion in Surf Cleaner EC90 (Nippon Paint Surf Chemicals) at 50°C for 2 minutes. They were then immersed in Surf Fine GL1 (Nippon Paint Surf Chemicals) at room temperature for 30 seconds, and then in Surfdyne EC3200 (Nippon Paint Surf Chemicals, zirconium conversion coating agent) at 35°C for 2 minutes. They were then rinsed with deionized water. To the cationic electrodeposition coating composition obtained above, a required amount of 2-ethylhexyl glycol was added so that the thickness of the electrodeposition coating film after curing would be 20 μm. After that, the steel plate was completely immersed in the electrodeposition coating composition, and immediately the application of voltage was initiated, increased for 30 seconds, and after reaching 180 V, the voltage was applied under conditions of being maintained for 150 seconds, depositing an uncured electrodeposition coating film on the substrate (cold-rolled steel plate). The resulting uncured electrodeposition coating film was heat-cured at 160°C for 15 minutes to obtain an electrodeposition-coated plate having a cured electrodeposition coating film with a thickness of 20 μm.

[0133] The cationic electrodeposition coating compositions obtained in the examples and comparative examples were evaluated as follows. The evaluation results are shown in Table 2. Table 2 also lists the components that affect the evaluation and their amounts used, the melt viscosity of the cationic electrodeposition coating composition at 110°C, and the electrical conductivity of the cationic electrodeposition coating composition.

[0134] Appearance of the coating around the weld (untreated) The electrodeposition coated plates having the cured electrodeposition coatings formed in the above Examples and Comparative Examples were visually inspected for the presence or absence of abnormalities in the appearance of the coating in the untreated areas. The evaluation criteria were as follows:

[0135] Evaluation criteria ◎: The coating has a very uniform appearance ○: The coating film has a uniform appearance ○△: Although there are some areas where unevenness is visible, the overall appearance of the coating is almost uniform △: Unevenness is visible ×: The appearance of the coating film is clearly uneven

[0136] Evaluation of edge rust prevention The degreasing, surface treatment, and chemical conversion treatment were carried out in the same manner as above, except that the substrate was changed from cold-rolled steel plate (JIS G3141, SPCC-SD) to an L-type dedicated replaceable blade (LB10K: manufactured by Olfa Corporation, length 100 mm, width 18 mm, thickness 0.5 mm). Next, a cured electrodeposition coating film was formed using the same procedure as in the formation of the cured electrodeposition coating film (1) above, and a cured electrodeposition coating film with a thickness of 20 μm was formed on a substrate having an edge (a dedicated L-type replacement blade), to obtain a test piece.

[0137] The test piece was subjected to a salt spray test (35°C x 72 hours) in accordance with JIS Z 2371 (2000), and the number of rust spots that had developed on the tip of the L-type dedicated replaceable blade was counted. In this test, the "tip of the L-type dedicated blade" refers to the width from the apex of the blade to 5 mm in the direction of the blade body. This width includes both the front and back sides, and the total width of the front and back sides is 10 mm. This "tip of the L-type dedicated blade" corresponds to the "edge portion" in this specification.

[0138] For example, in the following evaluation, if the number of rust spots on the tip of the L-type dedicated blade is 30, the length of the L-type dedicated blade is 100 mm (10 cm), and the width of the tip of the L-type dedicated blade is 10 mm (1 cm width) in total on both the front and back sides, so 2 The number of rust spots per 10cm is 30. 2 =3 pieces / cm 2 In the following evaluation, a score of ○△ or higher is considered a pass.

[0139] Evaluation criteria ◎: No rust ○: Less than 10 (L-type dedicated replacement blade tip 1cm 2 The number of rust particles per square centimeter is 1 particle / cm. 2 less than) ○△: 10 or more but less than 30 (L-type dedicated replacement blade tip 1cm 2 The number of rust particles per square centimeter is 1 particle / cm. 2 More than 3 pieces / cm 2 less than) △: 30 to less than 50 (L-type dedicated replacement blade tip 1cm 2 The number of rust spots per square centimeter is 3. 2 More than 5 pieces / cm 2 less than) △×: 50 to less than 100 (L-type dedicated replacement blade tip 1cm 2 The number of rust spots per square centimeter is 5. 2 More than 10 pieces / cm 2 less than) × :100 pieces or more (L-type dedicated replacement blade tip 1cm 2 The number of rust particles per square centimeter is 10 particles / cm. 2 (End)

[0140] [Table 2]

[0141] In Table 2, the abbreviations are as follows: MEM: 2-ethylaminoethanol Diketimine: Diketimine of ethylenetriamine MMA: N-methylethanolamine DEtA: diethanolamine EHDH: 2-ethylhexyl diglycol

[0142] Within the range of the present invention, there are no defects in the appearance of the welded joints and excellent rust prevention at the edges. Comparative Example 1 does not contain a high-boiling point solvent, and although edge rust prevention is fair, the coating appearance around the welded joints is poor. Comparative Example 2 contains an acrylic microgel instead of an epoxy viscous agent. The melt viscosity (110°C) of the paint is lower than the range of the present invention, resulting in poor edge corrosion prevention and poor coating appearance around the welded joints. Comparative Example 3 also contains a large amount of acrylic microgel instead of an epoxy viscous agent. The melt viscosity (110°C) of the paint is the same as the range of the present invention, but poor edge corrosion prevention and coating appearance around the welded joints are poor. Comparative Example 4 does not contain a lanthanum compound, and poor edge corrosion prevention is poor. Comparative Examples 5 and 6 have coating conductivity values ​​beyond the range of the present invention. Although edge corrosion prevention is good, the coating appearance around the welded joints is poor and unusable. Comparative Example 6 contains a large amount of lactic acid, and has high electrical conductivity and poor appearance. [Industrial Applicability]

[0143] By electrodeposition coating using the cationic electrodeposition coating composition of the present invention, it is possible to form a cured electrodeposition coating film that has excellent rust prevention properties, particularly corrosion prevention properties at welds and edges, and also has excellent coating appearance.

Claims

1. A cationic electrodeposition coating composition comprising an aminated resin (A) and a blocked isocyanate curing agent (B), the cationic electrodeposition coating composition further comprises a Group 3 element compound (C), an epoxy viscosity modifier (D), lactic acid (E), and a high-boiling point solvent (F) having a boiling point of 235°C or higher; the epoxy tackifier (D) is an epoxy tackifier obtained by reacting a liquid epoxy resin, a polycyclic phenol resin, and an amine compound; The cationic electrodeposition coating composition has an electrical conductivity of 1,500 to 2,300 μS / cm ２ and The cationic electrodeposition coating composition has a melt viscosity of 6,000 to 15,000 mPa·s.

2. 2. The cationic electrodeposition coating composition according to claim 1, wherein said Group 3 element compound (C) is a lanthanum compound.

3. the epoxy tackifier (D) is an epoxy tackifier obtained by reacting a liquid epoxy resin, a polycyclic phenol resin, and an amine compound; 3. The cationic electrodeposition coating composition according to claim 1, wherein said amine compound is a diketimine of 2-ethylaminoethanol, N-methylethanolamine, diethanolamine, diethylenetriamine, or a mixture thereof.

4. 3. The cationic electrodeposition coating composition according to claim 1, wherein the high boiling point solvent (F) is 2-ethylhexyl diglycol.

5. 3. The cationic electrodeposition coating composition according to claim 1, wherein the substrate to be coated with the cationic electrodeposition coating composition is a part having a welded portion.

6. A cationic electrodeposition coating film applied on a part having a welded portion using a cationic electrodeposition coating composition, the cationic electrodeposition coating composition comprises an aminated resin (A), a blocked isocyanate curing agent (B), a Group 3 element compound (C), an epoxy viscosity modifier (D), lactic acid (E), and a high-boiling solvent (F) having a boiling point of 235°C or higher; the epoxy tackifier (D) is an epoxy tackifier obtained by reacting a liquid epoxy resin, a polycyclic phenol resin, and an amine compound; The cationic electrodeposition coating composition has an electrical conductivity of 1,500 to 2,300 μS / cm ２ and The cationic electrodeposition coating composition has a melt viscosity of 6,000 to 15,000 mPa·s. Cationic electrocoating film.

7. A coating method comprising immersing a part having a welded portion in a cationic electrodeposition coating composition, forming a cationic electrodeposition coating film on the part by electrophoresis using the part as a cathode, and then heating and curing the formed film, wherein the cationic electrodeposition coating composition is the cationic electrodeposition coating composition according to claim 1 or 2.

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