Method for forming an electrodeposited coating using cationic electrodeposition paint

Abstract

Disclosed is a method for forming an electrodeposition coating film using a cationic electrodeposition coating material, the method comprising: immersing a metal article to be coated in a cationic electrodeposition coating material containing a cationic resin (A) to perform electrodeposition coating, thereby forming an electrodeposition coating film on the metal article; and heating the electrodeposition coating film to form an electrodeposition coating film having a dry film thickness of 60 μm or more on the metal article.

Description

Technical Field

[0001] The present disclosure relates to a method for forming an electrodeposited coating film using a cationic electrodeposition paint.

Background Art

[0002] Conventionally, cationic electrodeposition paints have been used as undercoat paints for imparting rust prevention properties to industrial products such as automobiles. In Patent Document 1, it is disclosed that an object to be coated such as an automobile part is immersed in a cationic electrodeposition paint for electrodeposition coating, and the electrodeposited coating film formed on the metal object to be coated is heated to form a cured electrodeposited coating film with a film thickness of 20 μm.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Rust on metal parts such as metal substrates progresses due to the formation of a corrosion circuit at the interface of the substrate and the coating film in the presence of oxygen, water, and corrosion-promoting ions. In this circuit, a concentration gradient of oxygen and corrosion-promoting ions occurs at the substrate interface, and when an electron flow can occur, corrosion of the substrate begins. As a means of suppressing this corrosion, electrodeposition coating is widely used in various industries, but in recent years, due to changes in the market environment, further improvement in rust prevention properties is required.

[0005] An object of the present invention is to provide a method for forming an electrodeposited coating film using a cationic electrodeposition paint, which can form a coating film having excellent corrosion resistance and capable of extending the rust prevention life of a metal object to be coated.

Means for Solving the Problems

[0006] Through diligent investigation of variables affecting the corrosion circuits formed at the substrate interface and the coating film, it was found that the initiation of corrosion beneath the coating film depends not on the coating film's resistance (impedance) but on its capacitance. In other words, lowering the capacitance can improve rust prevention performance and delay the initial rust formation on the coated object. To achieve the above objectives, the present invention includes, for example, the following embodiments.

[0007] Section 1. A method for forming an electrodeposited coating film using cationic electrodeposition paint, A process of immersing a metal workpiece in a cationic electrodeposition paint containing a cationic resin (A) to perform electrodeposition coating and form an electrodeposited coating film on the metal workpiece, and The process involves heating the electrodeposited coating to form an electrodeposited coating with a dry thickness of 60 μm or more on the metal workpiece. A method for forming an electrodeposited coating film, including the method described above.

[0008] Section 2. The method for forming an electrodeposited coating film according to item 1, wherein the dry film thickness is 65 μm or more.

[0009] Section 3. The method for forming an electrodeposited coating film according to item 1, wherein the dry film thickness is 70 μm or more.

[0010] Section 4. The method for forming an electrodeposited coating film according to any one of items 1 to 3, wherein the cationic electrodeposition coating contains a curing agent (B).

[0011] Section 5. A method for forming an electrodeposited coating film according to any one of claims 1 to 4, wherein the curing agent (B) comprises a blocked polyisocyanate compound.

[0012] Section 6. A method for forming an electrodeposited coating film according to any one of claims 1 to 5, wherein the cationic resin (A) comprises a cationic epoxy resin (A1) and / or a cationic acrylic resin (A2).

[0013] Section 7. The method for forming an electrodeposition coating film according to any one of items 1 to 6, wherein the cationic epoxy resin (A1) contains an amino group-containing epoxy resin (A1-1).

[0014] Item 8. The method for forming an electrodeposition coating film according to any one of items 1 to 7, wherein the capacitance in the electrochemical impedance measurement method measured under the following measurement conditions of the formed electrodeposition coating film is 1.5 nF or less. <Capacitance measurement conditions> Measured under the following conditions using an electrochemical impedance measuring instrument (VersaSTAT4 manufactured by Princeton Applied Research). Electrolyte: 0.1 mass% aqueous sodium sulfate solution Electrolyte immersion time before measurement: 5.5 hours Measurement area: 5.8 cm , , , , , ,

[0018] , ,

[0017] , ,

[0019]

[0015] Item 9. The method for forming an electrodeposition coating film according to any one of items 1 to 8, wherein the capacitance stabilization time in the electrochemical impedance measurement method of the formed electrodeposition coating film is 100 minutes or more.

[0016] [[ID=ID=]] Item 10. The method for forming an electrodeposition coating film according to any one of items 1 to 9, wherein the metal object to be coated is not subjected to chemical conversion treatment.

[0017] Item 11. In the step of electrocoating a metal object to be coated and then heat-drying the obtained uncured deposited electrocoating film, the melt viscosity of the electrocoating film at 80°C is 800 Pa·s or less. The method for forming an electrodeposition coating film according to any one of items 1 to 10.

[0018] Item 12. In the step of electrocoating a metal object to be coated and then heat-drying the obtained uncured deposited electrocoating film, the dynamic Tg of the electrocoating film is 80°C or less. The method for forming an electrodeposition coating film according to any one of items 1 to 11.

[0019] Item 13. The electrodeposition coating method according to any one of items 1 to 12, wherein the cationic resin (A) contains 10% or more polyol segments. [Modes for carrying out the invention]

[0020] In this specification, the singular form includes both singular and plural forms unless otherwise explicitly stated herein or the context clearly contradicts it.

[0021] In this specification, the term "contains" also encompasses the concept of "consisting of only."

[0022] In the numerical ranges described stepwise in this specification, the upper or lower limit of a numerical range in one step can be arbitrarily combined with the upper or lower limit of a numerical range in another step. Furthermore, in the numerical ranges described in this specification, the upper or lower limit of a numerical range may be replaced with values ​​shown in the examples or values ​​that can be uniquely derived from the examples. Moreover, in this specification, numbers connected by "~" mean a numerical range that includes the numbers before and after "~" as the lower and upper limits.

[0023] In this specification, "cationic electrodeposition coating" can be used interchangeably with "cationic electrodeposition coating composition."

[0024] In this specification, "epoxy resin" refers to both a resin having epoxy groups and a resin obtained by the reaction of the epoxy groups of the epoxy resin with other functional group-containing compounds, and does not necessarily have to contain epoxy groups. That is, in the present invention, "epoxy resin" means an epoxy resin having epoxy groups and / or a modified epoxy resin that does not have epoxy groups.

[0025] The amine values ​​used herein are measured in accordance with JIS K7237-1995. All values ​​are amine values ​​per resin solids (mgKOH / g).

[0026] In this specification, the number-average molecular weight and weight-average molecular weight are values ​​obtained by converting the retention time (retention capacity) measured using gel permeation chromatography (GPC) to the molecular weight of polystyrene using the retention time (retention capacity) of standard polystyrene with a known molecular weight measured under the same conditions.

[0027] This disclosure relates to a method for forming an electrodeposited coating film using a cationic electrodeposition paint, A process of immersing a metal workpiece in a cationic electrodeposition paint containing a cationic resin (A) to perform electrodeposition coating and form an electrodeposited coating film on the metal workpiece, and The process involves heating the electrodeposited coating to form an electrodeposited coating with a dry thickness of 60 μm or more on the metal workpiece. The present invention provides a method for forming an electrodeposited coating film, including the following:

[0028] Cationic electrodeposition coating The cationic electrodeposition paint used in the method for forming an electrodeposited coating film using the cationic electrodeposition paint of this disclosure contains a cationic resin (A). By using a cationic electrodeposition paint containing a cationic resin (A) as the coating film forming resin, the method for forming an electrodeposited coating film can be made capable of forming an electrodeposited coating film that has excellent corrosion resistance and can extend the rust prevention life of the metal workpiece.

[0029] Cationic resin (A) In some embodiments, the cationic resin (A) comprises a cationic epoxy resin (A1) and / or a cationic acrylic resin (A2).

[0030] The cationic epoxy resin (A1) is not particularly limited as long as it is an amine-modified epoxy resin commonly used in cationic electrodeposition coatings. Known cationic epoxy resins, cationic epoxy resins manufactured by known methods, and commercially available epoxy resins modified with amines can be used.

[0031] For example, cationic epoxy resin (A1) is a resin obtained by modifying the oxirane ring in the resin skeleton with an amino group-containing compound. Cationic epoxy resin (A1) can be prepared by ring-opening the oxirane ring in the epoxy resin molecule, which is the starting material, through a reaction with amines such as primary amines, secondary amines, or tertiary amines and / or their salts. Typical examples of epoxy resins used as starting materials are polyphenol polyglycidyl ether type epoxy resins, which are reaction products of polycyclic phenol compounds such as bisphenol A, bisphenol F, bisphenol S, phenol novolac, and cresol novolac with epichlorohydrin. Another example of a starting material resin is the oxazolidone ring-containing epoxy resin described in Japanese Patent Publication No. 5-306327. These epoxy resins can be prepared by reaction of a diisocyanate compound, or a bisurethane compound obtained by blocking the isocyanate group of a diisocyanate compound with a lower alcohol such as methanol or ethanol, with epichlorohydrin.

[0032] The epoxy resin used as the starting material mentioned above can be used after chain extension with a bifunctional polyester polyol, polyether polyol, bisphenol, dibasic carboxylic acid, etc., before the ring-opening reaction of the oxirane ring by amines. In particular, bisphenols may be used during the ring-opening reaction of the oxirane ring by amines to extend the chain.

[0033] Similarly, prior to the ring-opening reaction of the oxirane ring by amines, monohydroxy compounds such as 2-ethylhexanol, nonylphenol, ethylene glycol mono-2-ethylhexyl ether, ethylene glycol mono-n-butyl ether, and propylene glycol mono-2-ethylhexyl ether, or monocarboxylic acid compounds such as octic acid, can be added to some of the oxirane rings for purposes such as adjusting the molecular weight or amine equivalent, or improving the thermal flowability.

[0034] Examples of amines that can be used to open the oxirane ring and introduce an amino group include primary, secondary, or tertiary amines and / or their salts, such as butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine, N,N-dimethylbenzylamine, and N,N-dimethylethanolamine. Additionally, secondary amines containing ketimine-blocked primary amino groups, such as aminoethylethanolaminemethylisobutylketimine, and diethylenetriaminediketimine can also be used. These amines must be reacted with the oxirane ring in an amount at least equivalent to the oxirane ring in order to open all of them.

[0035] The number-average molecular weight of the cationic epoxy resin (A1) described above is preferably in the range of 800 to 5000. When the number-average molecular weight is 800 or higher, the finish and corrosion resistance of the cured electrodeposited coating obtained by electrodeposition coating are good. When the number-average molecular weight is 5000 or lower, viscosity adjustment is easy and the finish of the coating is good.

[0036] The number-average molecular weights used herein can be obtained by measuring them by GPC (gel permuration chromatography) and using the converted values ​​based on polystyrene standards.

[0037] The amine value of the cationic epoxy resin (A1) is, for example, in the range of 20 to 150 mgKOH / g. An amine value of 20 mgKOH / g or higher for the cationic epoxy resin (A1) ensures good dispersion stability of the amine-modified epoxy resin in the electrodeposition coating. On the other hand, an amine value of 150 mgKOH / g or lower ensures an appropriate amount of amino groups in the cured electrodeposition coating, preventing a decrease in the water resistance of the coating. In a specific embodiment, the amine value of the amine-modified epoxy resin is in the range of 20 to 100 mgKOH / g.

[0038] The hydroxyl value of the cationic epoxy resin (A1) is, for example, in the range of 50 to 400 mgKOH / g. A hydroxyl value of 50 mgKOH / g or higher ensures good curing in the cured electrodeposited coating. On the other hand, a hydroxyl value of 400 mgKOH / g or lower ensures an appropriate amount of hydroxyl groups remaining in the cured electrodeposited coating. In one embodiment, it is more preferable that the hydroxyl value of the cationic epoxy resin (A1) is in the range of 80 to 300 mgKOH / g.

[0039] The cationic epoxy resin (A1) may be of one type, or two or more cationic epoxy resins (A1) with different amine values ​​and / or hydroxyl values ​​may be used in combination.

[0040] In some embodiments, the cationic epoxy resin (A1) includes an amino group-containing epoxy resin (A1-1). By using the amino group-containing epoxy resin (A1-1) in the cationic electrodeposition coating, an electrodeposition coating film with excellent finish and corrosion resistance can be formed. The amino group-containing epoxy resin (A1-1) is a resin obtained by reacting an amine compound (a) with an epoxy resin (b). For example, the amino group-containing epoxy resin (A1-1) includes an amino group-containing epoxy resin obtained by reacting an amine compound of formula (1), preferably the amine compound of formula (2), with an epoxy resin.

[0041] X1-R1-NH-R2-X2...Formula (1) (In the formula, R1 and R2 are linear or branched hydrocarbon groups having 1 to 8 carbon atoms, and may be different or the same. X1 and X2 are hydroxyl groups and / or amino groups, and may be different or the same. However, if X1 and X2 are amino groups, then at least one of R1 and R2 is a hydrocarbon group having 1 to 2 carbon atoms.)

[0042] Examples of amine compounds of formula (1) include the amine compounds represented by the following formulas (2) to (4).

[0043] HO-R1-NH-R2-OH...Formula (2) Examples of the amine compound of formula (2) above include, for example, dialkanolamines such as dibutanolamine, dipropanolamine, diethanolamine, and dimethanolamine. Among these, those in which the hydrocarbon groups R1 and R2 have 1 to 2 carbon atoms are preferred, and specifically, dimethanolamine and diethanolamine are preferred.

[0044] HO-R1-NH-R2-NH2...Formula (3) Examples of the amine compound of formula (3) above include alkanolamines having an alkyl group with 1 to 4 carbon atoms, such as N-(aminomethyl)methanolamine, N-(aminomethyl)ethanolamine, N-(aminomethyl)propanolamine, N-(aminomethyl)butanolamine, N-(aminoethyl)methanolamine, N-(aminoethyl)ethanolamine, N-(aminoethyl)propanolamine, N-(aminoethyl)butanolamine, N-(aminopropyl)methanolamine, N-(aminopropyl)ethanolamine, N-(aminopropyl)propanolamine, N-(aminopropyl)butanolamine, N-(aminobutyl)methanolamine, N-(aminobutyl)propanolamine, and N-(aminobutyl)butanolamine. In particular, those in which the hydrocarbon groups R1 and R2 have 1 to 2 carbon atoms are preferred, and specifically, N-(aminomethyl)methanolamine, N-(aminomethyl)ethanolamine, N-(aminoethyl)methanolamine, and N-(aminoethyl)ethanolamine are preferred.

[0045] NH2-R1-NH-R2-NH2...Formula (4) The amine compound of formula (4) above has at least one (preferably both) hydrocarbon groups of R1 and R2 with 1 to 2 carbon atoms. Specifically, examples include dimethyltriamine and diethylenetriamine. Among these, diethylenetriamine is preferred. Furthermore, the hydrocarbon groups R1 and R2 indicated above shall include all possible isomers. Therefore, for example, propyl shall include isopropyl, and butyl shall include n-butyl, isobutyl, and t-butyl.

[0046] The amine compounds represented by formulas (3) and (4) above are preferably manufactured in a way that prevents high molecular weight formation during resin synthesis. This is achieved by reacting the primary amine at the end of the amine compound with a ketone compound to ketimine it, then reacting the secondary amine with the epoxy group of the epoxy resin, and finally converting it back to the primary amine by hydrolysis during paint formation (emulsion). The amino group-containing epoxy resin (A) used in the present invention has a primary amino group, which is a water-dispersible group, at the end of the resin, thereby achieving good paint stability in aqueous paints.

[0047] The amino group-containing epoxy resin (A1-1) may further contain an amino group-containing resin obtained by reacting an amine compound of formula (5) with an epoxy resin.

[0048] R1-NH-R2-OH...Formula (5) (In the formula, R1 and R2 are linear or branched hydrocarbon groups having 1 to 8 carbon atoms, and may be different or the same.)

[0049] In the above formula (5), it is preferable that the number of carbon atoms in R1 is usually 1 to 8, preferably 2 to 6, more preferably 3 to 4, and particularly preferably 3, and that the number of carbon atoms in R2 is usually 1 to 8, preferably 2 to 4, more preferably 3 to 4, and particularly preferably 3.

[0050] Specifically, examples include monomethylethanolamine, monomethylpropanolamine, monomethylbutanolamine, monoethylethanolamine, monoethylpropanolamine, monoethylbutanolamine, monopropylethanolamine, monopropylpropanolamine, monopropylbutanolamine, monobutylethanolamine, monobutylpropanolamine, and monobutylbutanolamine, which can be used individually or in combination of two or more.

[0051] Furthermore, the hydrocarbon groups R1 and R2 indicated above shall include all possible isomers. Therefore, for example, propyl shall include isopropyl, and butyl shall include n-butyl, isobutyl, and t-butyl.

[0052] The amine compound of formula (5) can be obtained, for example, by the reaction of a monoalkylamine with an alkylene oxide. There are no particular restrictions on the monoalkylamine, but linear or branched monoalkylamines having 1 to 6 carbon atoms can be used, such as monomethylamine, monoethylamine, mono-n-propylamine, monoisopropylamine, mono-n-butylamine, monoisobutylamine, monosec-butylamine, mono-t-butylamine, mono-n-pentylamine, isopentylamine, and mono-n-hexylamine. Preferably, monomethylamine, monoethylamine, mono-n-propylamine, monoisopropylamine, mono-n-butylamine, monoisobutylamine, and mono-t-butylamine can be used, and particularly preferably monomethylamine, monoethylamine, mono-n-propylamine, monoisopropylamine, and mono-n-butylamine can be used. There are no particular restrictions on the alkylene oxide, but for example, alkylene oxides such as ethylene oxide, propylene oxide, and butylene oxide can be used, and preferably ethylene oxide and propylene oxide can be used.

[0053] The ketone used for ketimination is not particularly limited as long as it reacts with the amine compound to form a ketimine compound and is further hydrolyzed in the aqueous paint composition. Examples include methyl isopropyl ketone (MIPK), diisobutyl ketone (DIBK), methyl isobutyl ketone (MIBK), diethyl ketone (DE K), ethyl butyl ketone (EBK), ethyl propyl ketone (EPK), dipropyl ketone (DPK), and methyl ethyl ketone (MEK). Among these, methyl isobutyl ketone (MIBK) is preferred. These ketones can be used individually or in combination of two or more.

[0054] The epoxy resin (b) used as a raw material for the above-mentioned amino group-containing epoxy resin (A1-1) is a compound having at least one epoxy group, preferably two or more, in one molecule. The epoxy resin (b) preferably has a number-average molecular weight in the range of at least 300, preferably 400 to 4000, and more preferably 800 to 2500. The epoxy resin (g) preferably has an epoxy equivalent in the range of at least 160, preferably 180 to 2500, and more preferably 400 to 1500. Such an epoxy resin can, for example, be obtained by the reaction of a polyphenol compound with an epihalohydrin (e.g., epichlorohydrin).

[0055] Examples of polyphenol compounds used for forming epoxy resin (b) include bis(4-hydroxyphenyl)-2,2-propane [bisphenol A], bis(4-hydroxyphenyl)methane [bisphenol F], bis(4-hydroxycyclohexyl)methane [hydrogenated bisphenol F], 2,2-bis(4-hydroxycyclohexyl)propane [hydrogenated bisphenol A], 4,4'-dihydroxybenzophenone, bis(4-hydroxyphenyl)-1,1-ethane, bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxy-3-tert-butyl-phenyl)-2,2-propane, bis(2-hydroxynaphthyl)methane, tetra(4-hydroxyphenyl)-1,1,2,2-ethane, 4,4'-dihydroxydiphenylsulfone, phenol novolac, and cresol novolac.

[0056] Among the epoxy resins (b) obtained by the reaction of a polyphenol compound with an epihalohydrin, bisphenol A or F type epoxy resins are particularly preferred.

[0057] Examples of commercially available epoxy resins (b) include those sold by Mitsubishi Chemical Corporation under the product names jER-806, jER828EL, jER1002, jER1004, and jER1007.

[0058] Furthermore, by using a modifier or a bisphenol F type epoxy resin as a component of the amino group-containing epoxy resin (A1-1), the finish and corrosion resistance of the coating film can be more favorably satisfied. Examples of modifiers that can be used include polyols, polyether polyols, polyester polyols, polyamidoamines, polycarboxylic acids, fatty acids, polyisocyanate compounds, compounds obtained by reacting polyisocyanate compounds, lactone compounds such as ε-caprolactone, acrylic monomers, compounds obtained by polymerization of acrylic monomers, xyleneformaldehyde compounds, and epoxy compounds. These modifiers can be used individually or in combination of two or more. From the viewpoint of improving finish and corrosion resistance, the amount of the above-mentioned modifier used is usually within the range of 10% by mass or more, preferably 10-30% by mass, and more preferably 10-20% by mass, based on the solid content mass of the amino group-containing epoxy resin (A1-1).

[0059] Cationic acrylic resin (A2) can be prepared by adding an amino group-containing compound to a copolymer resin obtained by radical copolymerization of hydroxyl group-containing monomers, glycidyl group-containing monomers, and other monomers. Examples of hydroxyl group-containing monomers used in the preparation of cationic acrylic resin (A2) include 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, and addition products of these monomers with ε-caprolactone. These hydroxyl group-containing monomers may be used individually or in combination of two or more.

[0060] Examples of glycidyl group-containing monomers include glycidyl (meth)acrylate and (meth)allyl glycidyl ether. These glycidyl group-containing monomers may be used individually or in combination of two or more.

[0061] Other monomers include, for example, acrylic monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate, cyclohexyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate, and isobornyl (meth)acrylate; and non-acrylic monomers such as styrene, vinyltoluene, α-methylstyrene, (meth)acrylonitrile, (meth)acrylamide, and vinyl acetate. These other monomers may be used individually or in combination of two or more.

[0062] Acrylic copolymer resins can be obtained by radical copolymerizing the above-mentioned hydroxyl group-containing monomers, glycidyl group-containing monomers, and other monomers using methods known to those skilled in the art. The amount of hydroxyl group-containing monomer is preferably such that the hydroxyl value of the cationic acrylic resin (A2) is 50 to 250, and more preferably 60 to 220. The amount of glycidyl group-containing monomer is preferably such that the amine value of the cationic acrylic resin (A2) is 10 to 100 mgKOH / g, and more preferably such that it has 15 to 80 mgKOH / g of amino groups. The hydroxyl value is a value that can be determined from the amount of hydroxyl group-containing monomer used.

[0063] The oxirane ring of the acrylic copolymer resin obtained in this way can be reacted with an amino group-containing compound such as a primary amine, secondary amine, or tertiary aminate salt to open the ring, thereby obtaining a cationic acrylic resin (A2). As the amino group-containing compound used for ring opening, the amino group-containing compounds listed in the above-mentioned cationic epoxy resin (A1) can be used.

[0064] Furthermore, in the preparation of cationic acrylic resin (A2), there is also a method of directly synthesizing cationic acrylic resin (A2) by copolymerizing an acrylic monomer containing an amino group with other monomers. In this method, an amino group-containing acrylic monomer such as N,N-dimethylaminoethyl (meth)acrylate or N,N-di-t-butylaminoethyl (meth)acrylate is used instead of the glycidyl group-containing monomer mentioned above, and cationic acrylic resin (A2) can be obtained by copolymerizing this with a hydroxyl group-containing acrylic monomer and other acrylic monomers and / or non-acrylic monomers.

[0065] The cationic acrylic resin (A2) obtained in this way can, if necessary, be modified to introduce blocked polyisocyanate groups by addition reaction with a half-block diisocyanate compound to form a self-crosslinked cationic acrylic resin (A2).

[0066] The number-average molecular weight of the cationic acrylic resin (A2) is preferably in the range of 1500 to 7000. When the number-average molecular weight is 1500 or higher, the resulting cured electrodeposited coating has good physical properties such as finish and corrosion resistance. When the number-average molecular weight is 7000 or lower, the viscosity of the cationic acrylic resin (A2) is easy to adjust, and the finish of the coating is good.

[0067] It is preferable that the cationic acrylic resin (A2) be designed to have a hydroxyl value in the range of 50 to 250. When the hydroxyl value is 50 or higher, the coating film hardens well. When the hydroxyl value is 250 or lower, the water resistance of the cured electrodeposited coating film is good.

[0068] The cationic acrylic resin (A2) may be of one type, or two or more cationic acrylic resins (A2) may be used in combination.

[0069] The cationic electrodeposition coating relating to this disclosure may further contain an amino group-containing polyester resin or the like.

[0070] In some embodiments, the total proportion of cationic epoxy resin (A1) and / or cationic acrylic resin (A2) among the film-forming resins in the cationic electrodeposition coating is preferably 50% by mass or more, and is 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or 100% by mass.

[0071] Hardener (B) The cationic electrodeposition coating may further contain a curing agent (B). In this specification, the cationic resin (A) and the curing agent (B) constitute the film-forming resin. The curing agent (B) contained in the cationic electrodeposition coating according to this disclosure can be a film-forming resin that undergoes a curing reaction with the cationic resin (A) under heated conditions. Melamine resin or blocked polyisocyanate (also referred to as a blocked polyisocyanate compound) is preferably used as the curing agent (B). A blocked polyisocyanate that can be preferably used as the curing agent (B) can be prepared by blocking a polyisocyanate with a blocking agent.

[0072] Examples of polyisocyanates include aliphatic diisocyanates such as hexamethylene diisocyanate (including trimers), tetramethylene diisocyanate, and trimethylhexamethylene diisocyanate; alicyclic polyisocyanates such as isophorone diisocyanate and 4,4'-methylenebis(cyclohexyl isocyanate); aromatic diisocyanates such as 4,4'-diphenylmethane diisocyanate, tolylene diisocyanate, and xylylene diisocyanate; and modified products of these diisocyanates (such as urethanes, carbodiimides, uretdiones, uretonimines, biuretes, and / or isocyanurate modified products).

[0073] Examples of blocking agents that are preferably used include monohydric alkyl (or aromatic) alcohols such as n-butanol, n-hexyl alcohol, 2-ethylhexanol, lauryl alcohol, phenolcarbinol, and methylphenylcarbinol; cellosolves such as ethylene glycol monohexyl ether and ethylene glycol mono-2-ethylhexyl ether; polyether-type terminal diols such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycolphenol; polyester-type 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.

[0074] Blocked polyisocyanates preferentially react with the primary amines in amine-modified epoxy resins, and then react with hydroxyl groups to cure.

[0075] Examples of melamine resins include partially or completely methylolated melamine resins obtained by reacting melamine with formaldehyde, partially or completely alkyl ether type melamine resins obtained by partially or completely etherifying the methylol groups of methylolated melamine resin with an alcohol component, imino group-containing melamine resins, and mixed melamine resins thereof. Examples of alkyl ether type melamine resins include methylated melamine resins, butylated melamine resins, and methyl / butyl mixed alkyl type melamine resins.

[0076] As the curing agent (B), at least one curing agent selected from the group consisting of organic curing agents such as phenolic resins, silane coupling agents, and metal curing agents may be used in combination with the melamine resin and / or blocked polyisocyanate.

[0077] When a cationic electrodeposition coating contains a curing agent (B), the ratio of cationic resin (A) (particularly the sum of cationic epoxy resin (A1) and / or cationic acrylic resin (A2)) to curing agent (B) is preferably (A):(B) = 10 to 90:90 to 10 by mass ratio.

[0078] Pigment dispersion paste The cationic electrodeposition coatings relating to this disclosure may optionally include a pigment dispersion paste. The pigment dispersion paste generally includes a pigment dispersion resin and a pigment.

[0079] Pigment dispersion resins are resins used to disperse pigments, and are used, for example, by being dispersed in an aqueous medium. As pigment dispersion resins, those having cationic groups, such as modified epoxy resins having at least one or more groups selected from quaternary ammonium groups, tertiary sulfonium groups, and primary amine groups, can be used. As aqueous solvents, ion-exchanged water or water containing small amounts of alcohols can be used.

[0080] Pigments are commonly used in electrodeposition coatings. Examples of pigments include commonly used inorganic and organic pigments, such as coloring pigments like titanium white (titanium dioxide), carbon black, and red iron oxide; extender pigments like kaolin, talc, aluminum silicate, calcium carbonate, mica, and clay; and rust-preventive pigments like iron phosphate, aluminum phosphate, calcium phosphate, aluminum tripolyphosphate, and aluminum phosphomolybdate and aluminum zinc phosphomolybdate.

[0081] The method for producing cationic electrodeposition coatings is not particularly limited, but for example, it can be obtained by thoroughly mixing the above-mentioned cationic resin (A), an optional curing agent (B), and various optional additives, then dispersing them in water, and then thoroughly mixing them with a pigment dispersion paste, a solvent such as water or an organic solvent, and an optional neutralizing agent. As the neutralizing agent, any known organic acid can be used without particular limitation, and formic acid, lactic acid, acetic acid, or mixtures thereof are particularly preferred.

[0082] By using a cationic electrodeposition paint containing the above-mentioned cationic resin (A) to perform the method for forming an electrodeposited coating film using the cationic electrodeposition paint according to this disclosure, and by keeping the dry film thickness of the electrodeposited coating film formed on a metal workpiece within the range of this disclosure, it is possible to form a coating film with good finish and excellent corrosion resistance.

[0083] In some embodiments, the dynamic film Tg (°C) of the cationic electrodeposition coating is preferably 80°C or lower in terms of finish and corrosion resistance. The dynamic film Tg (°C) refers to the temperature at which the Tanδ of the data obtained from the melt viscosity measurement at 80°C is maximum.

[0084] In some embodiments, the melt viscosity (Pa·s) of the cationic electrodeposition coating at 80°C is preferably 800 Pa·s or less. Keeping the melt viscosity of the cationic electrodeposition coating within this range improves the finish. The melt viscosity of the cationic electrodeposition coating at 80°C can be measured using a rotary viscoelasticity measuring device, for example, by the measurement method described in the examples.

[0085] The lower limit of the melt viscosity of cationic electrodeposition coatings at 80°C is not particularly limited, but for example, it may be 100 Pa·s or higher, 200 Pa·s or higher, or 300 Pa·s or higher. The cationic electrodeposition coating disclosed herein has excellent rust prevention performance and can delay the initial rust formation on metal substrates. Furthermore, even with coatings that have a large heat capacity and a low glass transition temperature (Tg) where sufficient crosslinking reaction cannot be obtained, rust prevention can be compensated for by setting a thicker coating film thickness, making it possible to achieve rust prevention performance exceeding that of conventional film thicknesses of approximately 15-20 μm. By setting a thicker film compared to conventional electrodeposition coating specifications, the rust prevention life of metal parts can be extended, thereby reducing the burden of part replacement. Furthermore, since there are no constraints on the barrier performance per unit film thickness, it is possible to significantly relax the paint composition and baking conditions, and for these reasons, energy conservation toward carbon neutrality can be achieved. Energy conservation here refers to the metal processing energy required when remanufacturing parts and the energy required to paint and dry those parts.

[0086] Method for forming an electrodeposited coating using cationic electrodeposition paint The cationic electrodeposition coating according to this disclosure may be provided in a form in which a film-forming resin, which is a cationic resin (A) (for example, an amine-modified epoxy resin), an optional curing agent (B), and an optional other additive are dissolved or dispersed in an aqueous medium. This coating composition is used in a coating bath, and an electric current is passed through the metal workpiece with the metal workpiece as the cathode or anode to form a deposited coating film on the metal workpiece. After that, the deposited coating film is heated to form a cured electrodeposited coating film.

[0087] Metal coated object Examples of metal-coated objects in this disclosure include automobile parts, automobile bodies, motorcycle parts, and industrial products made from metal materials such as iron, aluminum, zinc, tin, and alloys thereof.

[0088] Metal-coated objects are formed from various electrically conductive materials. Examples include cold-rolled steel sheets, hot-rolled steel sheets, stainless steel, electro-galvanized 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.

[0089] Method for forming electrodeposited coatings The method for forming an electrodeposited coating film using cationic electrodeposition paint according to this disclosure may involve, as a pre-step to immersing the metal workpiece in the cationic electrodeposition paint, subjecting the workpiece to at least one step selected from the group consisting of an alkaline degreasing step, a water washing step, and a chemical conversion treatment step. Each of these steps can be a known step. Furthermore, a known chemical conversion treatment solution can be used for the chemical conversion treatment.

[0090] The method for forming an electrodeposited coating film using cationic electrodeposition paint according to this disclosure includes the step of immersing a metal workpiece in cationic electrodeposition paint containing a cationic resin (A) and performing electrodeposition coating to form an electrodeposited coating film on the metal workpiece.

[0091] For example, electrodeposition coating may involve immersing a metal workpiece in a cationic electrodeposition coating and applying a voltage of 50 to 450 V to it for a predetermined time, using the immersion as the cathode. The voltage application time varies depending on the electrodeposition conditions, but is, for example, 1 to 8 minutes. An electrodeposited coating film is formed on the metal workpiece so that the dry coating film thickness is 60 μm or more.

[0092] The method for forming an electrodeposited coating using cationic electrodeposition paint in this disclosure includes the step of heating the electrodeposited coating deposited by electrodeposition coating to form an electrodeposited coating with a dry thickness of 60 μm or more on a metal substrate.

[0093] In the process of heating the cationic electrodeposition coating, the heating temperature is, for example, between 120°C and 260°C, and in one embodiment, between 130°C and 220°C. In another embodiment, the heating temperature is between 140°C and 220°C.

[0094] At such heating temperatures, the time for heating or curing the cationic electrodeposition coating by baking is, for example, between 10 and 30 minutes.

[0095] The dry film thickness of the electrodeposited coating can be determined, for example, by cross-sectional observation using an electromagnetic film thickness gauge or a video microscope. In particular, the dry film thickness of the electrodeposited coating refers to the average film thickness measured by the method described in the examples. When the dry film thickness of the electrodeposited coating is 60 μm or more, the dried and cured electrodeposited coating exhibits excellent corrosion resistance. When the dry film thickness of the electrodeposited coating is less than 60 μm, the corrosion resistance of the dried and cured electrodeposited coating is poor. In terms of corrosion resistance, the dry film thickness of the electrodeposited coating is preferably 65 μm or more, and more preferably 70 μm or more. The upper limit of the dry film thickness of the electrodeposited coating is not particularly limited, but for example, it is 120 μm or less, or 100 μm or less.

[0096] In some embodiments, in terms of corrosion resistance, the capacitance measured by the electrochemical impedance measurement method using the cationic electrodeposition paint according to the present disclosure is preferably 1.5 nF or less, more preferably 0.7 nF or less, more preferably 0.6 nF or less, and more preferably 0.5 nF or less, of the electrodeposited coating film formed by the electrodeposited coating film formation method according to the present disclosure under the following measurement conditions. The lower limit of the capacitance is not particularly limited, but for example, it is 0.3 nF or more. By lowering the capacitance, the corrosion resistance (especially rust prevention) of the coating film formed from cationic electrodeposition paint can be improved, and the initial rust formation of the coated object can be delayed.

[0097] <Capacitance Measurement Conditions> The measurements were taken using an electrochemical impedance analyzer (VersaSTAT4, Princeton Applied Research) under the following conditions.

[0098] Electrolyte: 0.1% by mass sodium sulfate aqueous solution Immersion time in electrolyte solution before measurement: 5.5 hours Measurement area: 5.8 cm² 2 In some embodiments, in terms of corrosion resistance, the capacitance stabilization time (in minutes) of an electrodeposited coating formed by the method for forming an electrodeposited coating using the cationic electrodeposition paint according to this disclosure is preferably 100 minutes or more, more preferably 120 minutes or more, and even more preferably 140 minutes or more. The upper limit of the capacitance stabilization time is not particularly limited, but is, for example, 250 minutes or less, or 200 minutes or less. The capacitance stabilization time can be measured by the method described in the examples. [Examples]

[0099] The present invention will be further described below with reference to examples. Here, 'parts' and '%' mean 'parts by mass' and '% by mass', respectively.

[0100] Manufacturing of amine-modified resins Manufacturing Example 1: Amino group-containing epoxy resin (A1-1) In a flask equipped with a thermometer, stirrer, and reflux condenser, 755 parts of jER-806 (trade name, manufactured by Mitsubishi Chemical Corporation, bisphenol F type epoxy resin, epoxy equivalent 170 g / eq) were added, along with 244 parts of bisphenol F and 30 parts of methyl isobutyl ketone, and heated until dissolved. Further, at 100°C, 0.5 parts of dimethylbenzylamine and 10 parts of methyl isobutyl ketone were added, and the mixture was reacted at 160°C until the epoxy equivalent reached 500. Subsequently, 110 parts of methyl isobutyl ketone were added, and the mixture was cooled to 105°C. Then, 157.5 parts of diethanolamine and 126 parts of methyl isobutyl ketonate of diethylenetriamine were added, and the mixture was reacted until the epoxy was completely gone, yielding an amine-modified epoxy resin (A1-1) with a resin solids content of 83%. The amine-modified epoxy resin (A1-1) had a number-average molecular weight of (1200), a total amine value of (99) mgKOH / g, and a hydroxyl value of (142) mgKOH / g.

[0101] Manufacturing Example 2: Amino group-containing epoxy resin (A1-2) In a flask equipped with a thermometer, stirrer, and reflux condenser, 1026 parts of jER-828EL (trade name, manufactured by Mitsubishi Chemical Corporation, bisphenol A type epoxy resin, epoxy equivalent 190 g / eq), 283 parts of polyoxypropylene glycol diglycidyl ether (epoxy equivalent 471), 456 parts of bisphenol A, and 200 parts of methyl isobutyl ketone were mixed and heated to 120°C to dissolve. Then, 1 part of dimethylbenzylamine was added and the reaction was carried out at 120°C for 4.5 hours to obtain an epoxy compound with an epoxy equivalent of 880. Subsequently, 158 parts of diethanolamine and 176 parts of methyl isobutyl ketonate of diethylenetriamine were added and the reaction was maintained at 110°C. After confirming that no epoxy groups had reacted and remained, 200 parts of methyl isobutyl ketone were added to obtain an amine-containing modified epoxy resin (A1-2) with a solid content of 78%. The amine-modified epoxy resin (A1-2) had a number-average molecular weight of (2000), a total amine value of (60) mgKOH / g, and a hydroxyl value of (86) mgKOH / g.

[0102] Manufacturing Example 3: Amino group-containing epoxy resin (A1-3) In a flask equipped with a thermometer, stirrer, and reflux condenser, 950 parts of jER-828EL (trade name, manufactured by Mitsubishi Chemical Corporation, bisphenol A type epoxy resin, epoxy equivalent 190 g / eq), 268 parts of polyoxyethylene glycol diglycidyl ether (epoxy equivalent 268), 456 parts of bisphenol A, and 200 parts of methyl isobutyl ketone were mixed and heated to 120°C to dissolve. Then, 1 part of dimethylbenzylamine was added and the reaction was carried out at 120°C for 4.5 hours to obtain an epoxy compound with an epoxy equivalent of 830. Subsequently, 158 parts of diethanolamine and 286 parts of methyl isobutyl ketonate of diethylenetriamine were added and the reaction was maintained at 110°C. After confirming that no epoxy groups remained after the reaction, 70 parts of methyl isobutyl ketone were added to obtain an amine-containing modified epoxy resin (A1-3) with a solid content of 78%. The amine-modified epoxy resin (A1-3) had a number-average molecular weight of (1900), a total amine value of (63) mgKOH / g, and a hydroxyl value of (90) mgKOH / g.

[0103] Manufacturing Example 4: Amino group-containing epoxy resin (A1-4) In a flask equipped with a thermometer, stirrer, and reflux condenser, 768 parts of jER-828EL (trade name, manufactured by Mitsubishi Chemical Corporation, bisphenol A type epoxy resin, epoxy equivalent 190 g / eq), 233 parts of bisphenol A, and 70 parts of methyl isobutyl ketone were measured out and heated to dissolve. At 100°C, 0.5 parts of dimethylbenzylamine and 10 parts of methyl isobutyl ketone were added and the mixture was heated further to 160°C until the epoxy equivalent reached 500. Then, 170 parts of methyl isobutyl ketone were added, and after cooling to 105°C, 157.5 parts of diethanolamine and 126 parts of methyl isobutyl ketonate of diethylenetriamine were added and the mixture was reacted until the epoxy was completely consumed, yielding epoxy resin (A1-4) with a resin solids content of 78%. The amine-modified epoxy resin (A1-4) had a number-average molecular weight of (1300), a total amine value of (90) mgKOH / g, and a hydroxyl value of (128) mgKOH / g.

[0104] Manufacturing Example 5: Amino group-containing acrylic resin (A2) In a reaction vessel equipped with a stirrer, condenser, nitrogen inlet tube, thermometer, and dropping funnel, 20 parts of butyl cellosolve were added and the temperature was raised to 120°C. A solution of 30 parts styrene, 30 parts hydroxyethyl methacrylate, 30 parts Plaxel FM-3X (trade name, manufactured by Daicel Corporation, ε-caprolactone-modified hydroxyethyl methacrylate), 10 parts glycidyl methacrylate, and 5 parts AIBN was added dropwise over 3 hours. After the dropwise addition was complete, 1 part methyl isobutyl ketone was added, and then a solution of 0.5 parts AIBN dissolved in 10 parts methyl isobutyl ketone was added dropwise and allowed to mature. After further cooling, 7 parts diethanolamine was added and the reaction was carried out at 130°C for 3 hours. Finally, 7 parts methyl isobutyl ketone was added to obtain acrylic resin (A2) with a solid content of 75%. The number-average molecular weight of the amino group-containing acrylic resin (A2) was (5000), the total amine value was (33) mgKOH / g, and the hydroxyl value was (66) mgKOH / g.

[0105] Production of blocked polyisocyanate compounds Manufacturing Example 6: Blocked Polyisocyanate (B-1) 168 parts of hexamethylene diisocyanate and 5 parts of methyl isobutyl ketone were added to a reaction vessel, and 150 parts of methyl ethyl ketoxime were gradually added, and the reaction was carried out at 70°C. Midway through the reaction, 13.4 parts of trimethylolpropane were added and the temperature was raised to 100°C. Then, 0.01 parts of bismastris(2-ethylhexanoate) and 10 parts of methyl isobutyl ketone were added, and the dropwise addition of methyl ethyl ketoxime was completed. After the reaction was carried out until the isocyanate value was 1 or less, 22 parts of methyl isobutyl ketone were added to obtain a blocked polyisocyanate (B-1) with a solid content of 90%.

[0106] Manufacturing Example 7: Blocked Polyisocyanate (B-2) 267 parts of Sumijoule 44V 20L (trade name, manufactured by Sumika Covestro Urethane Co., Ltd., Crude MDI) and 10 parts of methyl isobutyl ketone were added to the reaction vessel and the temperature was raised to 60°C. Then, 162 parts of butyl carbitol and 120 parts of butyl cellosolve were gradually added and the temperature was raised to 120°C. While maintaining this temperature, 0.02 parts of bismastris (2-ethylhexanoate) and 2 parts of methyl isobutyl ketone were added and the mixture was reacted until the isocyanate value was 1 or less. After that, 17 parts of methyl isobutyl ketone were added to obtain blocked polyisocyanate (B-2) with a solid content of 95%.

[0107] Manufacturing of pigment dispersion resins Manufacturing Example 8 In a flask equipped with a stirrer, thermometer, dropping funnel, and reflux condenser, 1200 parts of jER-828EL (Mitsubishi Chemical product, epoxy resin, epoxy equivalent 190, number average molecular weight 350), 180 parts of bisphenol A, and 20 parts of butyl cellosolve were measured out and heated to dissolve. At 100°C, 0.41 parts of dimethylbenzylamine and 6 parts of methyl isobutyl ketone were added and the mixture was heated further to 160°C until the epoxy equivalent reached 300. Subsequently, 702 parts of butyl cellosolve were added, and after cooling to 50°C, 126 parts of diethanolamine and 171 parts of dimethylaminopropylamine were added and the mixture was reacted until the epoxy was completely consumed, yielding an epoxy resin with a resin solids content of 70%. Finally, 125 parts of acetic acid and 270 parts of deionized water were added to obtain a pigment dispersion resin with an amine value of 150 and a resin solids content of 60%.

[0108] Manufacturing of pigment dispersion paste Manufacturing Example 9 8.3 parts (5 parts solids) of pigment dispersion resin with a solid content of 60% obtained in Production Example 8, 14.5 parts titanium dioxide, 7.0 parts purified clay, 0.3 parts carbon black, 2 parts dioctyl tin oxide, and 20.3 parts deionized water were added and dispersed in a ball mill for 20 hours to obtain a pigment dispersion paste (p-1) with a solid content of 55%.

[0109] Manufacturing of cationic electrodeposition coating compositions Manufacturing Example 10 84.3 parts (70 parts solids) of the amine-modified epoxy resin (A1-1) obtained in Production Example 1 and 33.3 parts (30 parts solids) of the blocked polyisocyanate (B-1) obtained in Production Example 6 were mixed, 13 parts of 10% acetic acid were added and the mixture was uniformly stirred. Then, deionized water was added dropwise over approximately 15 minutes while vigorously stirring to obtain an emulsion with a solid content of 34%. Next, 294 parts (100 parts solids) of the emulsion, 52.4 parts (28.8 parts solids) of the pigment dispersion paste (p-1) obtained in Production Example 9, and deionized water were added to produce a cationic electrodeposition coating composition (X-1) with a solid content of 20%.

[0110] Manufacturing Examples 11-14 Cationic electrodeposition coating compositions (X-2) to (X-5) were prepared in the same manner as in Production Example 10, except that the types and amounts of amine-modified resin and curing agent were as shown in Table 1 below. The numerical values ​​of the formulations in Table 1 represent the solid content.

[0111] [Table 1] Method for forming electrodeposited coatings Examples 1-13, Comparative Examples 1-7 Cold-rolled steel sheets (150mm (length) x 70mm (width) x 0.8mm (thickness)) treated with chemical conversion treatment (product name, Palbond #3020, manufactured by Nippon Parkerizing Co., Ltd., zinc phosphate treatment agent) or untreated cold-rolled steel sheets were used as metal substrates. Electrodeposition coating was performed using the respective cationic electrodeposition coatings (X-1) to (X-5) obtained in Production Examples 10 to 14, adjusting the coating voltage and bath coating temperature to achieve the film thickness (μm) shown in Table 1. The coated metal substrates were baked and dried at 170°C or 110°C for 20 minutes to obtain test plates No. 1 to No. 20.

[0112] Evaluation Test <Corrosion Resistance> The test plates were covered with polyester masking tape with a 5mm width around the perimeter and immersed in 55°C salt water with a NaCl concentration of 5% or 0.5%. The presence or absence of blisters was checked every 120 hours, and the test was terminated when even one blister appeared. The total test time until blister formation was used as the evaluation result.

[0113] The evaluation system uses "S, A, B" for passing grades and "C, D" for failing grades.

[0114] S: Test time until blister formation is over 2000 hours A: Test time until blister formation is between 1500 hours and 2000 hours. B: Test time until blister formation is between 1000 hours and 1500 hours. C: Test time until blister formation is 500 hours or more but less than 1000 hours. D: Test time until blister formation is less than 500 hours <Finish quality (voids)> The cross-section (1 mm wide) of the coating film on the obtained test plate was observed, and the number of voids (air bubbles) on the coating surface was visually counted.

[0115] The evaluation system is such that "A" and "B" are passing grades, and "C" is a failing grade.

[0116] A: There are no voids, and the condition is good. B: There are no large voids (diameter 1 μm or larger), but there are one to three small voids (diameter less than 1 μm). C: There is at least one large void, or four or more small voids.

[0117] Methods for measuring or calculating characteristic values <film thickness> The film thickness of the cationic electrodeposition coating on the test coating plate can be measured using an electromagnetic film thickness meter. The measurement area on the test coating plate should be a flat area on which the entire measuring part of the electromagnetic film thickness meter can make contact, and the average value of three measurements taken at randomly selected locations within this flat area should be taken.

[0118] <Melting viscosity at 80°C (Pa·s)> The melt viscosity (Pa·s) of a cationic electrodeposition coating composition at 80°C can be measured using a rotary viscoelasticity measuring device. An example of such a device is a viscoelasticity measuring device (manufactured by TA Instruments, product name "ARES-G2"). A parallel plate with a diameter of 8 mm was used as the measuring jig.

[0119] In the above apparatus, calibration was performed using a viscosity standard solution (JS-100, manufactured by Nippon Grease Co., Ltd.) to ensure the correct viscosity before measuring viscoelasticity.

[0120] The specific measurement conditions were as follows: First, the measurement cell of the viscoelasticity measuring device was immersed in an electrodeposition bath containing each cationic electrodeposition coating composition, and the coating was applied to a thickness of 0.6 mm after drying, and the temperature was raised to 100°C. The jig was then placed in close contact with the sample and inserted to a depth of approximately 0.3 mm. The temperature was then lowered from 100°C under conditions of a frequency of 1.0 Hz and a temperature cooling rate of 5°C / min, and the complex viscosity between 100°C and 30°C was measured. From the resulting chart, the complex viscosity at 80°C was determined as the melt viscosity at 80°C.

[0121] <Dynamic coating Tg (°C)> The temperature at which Tanδ is maximized in the melt viscosity measurement at 80°C was determined as the dynamic coating film Tg (°C).

[0122] <Capacitance (nF)> Capacitance was calculated using a formula derived from the values ​​obtained by the electrochemical impedance method described below. Electrochemical impedance can be measured using commercially available electrochemical measuring devices or potentiostats. An example of such a measuring device is the "VersaSTAT4" manufactured by AMETEK Science Instruments.

[0123] The specific measurements were performed using a two-electrode system with a 0.1% by mass sodium sulfate aqueous solution as the electrolyte, a painted plate as the working electrode, and SUS304 as the counter electrode, with the measurement area of ​​the painted plate heated to 60°C.

[0124] Immersion time in electrolyte solution before measurement: 5.5 hours, measurement area: 5.8 cm² 2 It was carried out under the following conditions.

[0125] Measurements were taken at frequencies from 1 to 1,000,000 Hz, and the capacitance of the electrodeposited coating was calculated from the capacitive reactance at 10,000 Hz using the following formula.

[0126] C = 1 / (2π·F·Zc) C: Capacitance [F] F: Frequency [Hz] Zc: Capacitive reactance [Ω] Furthermore, this operation was performed at 20-minute intervals, and the capacitance stabilization time was measured.

[0127] <Capacitance stabilization time (minutes)> When the above electrochemical impedance measurement is performed continuously in a cycle from 1,000,000 Hz to 1 Hz, Change rate of capacitance = ((n+1)th measurement - nth measurement) ÷ (n+1)th measurement The total time taken until the value fell below 0.01 was measured.

[0128] <Polyol segment content> In this specification, the polyol segment content in a cationic resin is approximated by the mass ratio of the polyol-derived modifier to the total mass of the manufacturing raw materials constituting the cationic resin. For example, in the case of an amino group-containing epoxy resin (A1-2), it can be calculated as follows.

[0129] Mass of polyol-derived modifiers 283 parts of polyoxypropylene glycol diglycidyl ether Mass of all manufacturing raw materials that make up cationic resin jER-828EL 950 parts + polyoxyethylene glycol diglycidyl ether 268 parts + bisphenol A 456 parts + diethanolamine 158 parts, ketimine of diethylenetriamine 286 parts = 2118 parts Polyol segment content = (283 / 2118) × 100 = 13.3%

Claims

1. A method for forming an electrodeposited coating film using cationic electrodeposition paint, A process of immersing a metal workpiece in a cationic electrodeposition coating containing an amino group-containing epoxy resin (A1-1) containing a modifier or bisphenol F type epoxy resin as a constituent component, and / or a cationic resin (A2) having a number average molecular weight of 7000 or less, and performing electrodeposition coating to form an electrodeposited coating film on the metal workpiece, and The process of heating the electrodeposited coating to form an electrodeposited coating on the metal workpiece having a dry film thickness of 80 μm or more. A method for forming an electrodeposited coating film, including the method described above.

2. The method for forming an electrodeposited coating film according to claim 1, wherein the cationic electrodeposition coating contains a curing agent (B).

3. The method for forming an electrodeposited coating film according to claim 2, wherein the curing agent (B) comprises a blocked polyisocyanate compound.

4. The capacitance of the electrodeposited coating film formed above, measured by electrochemical impedance measurement under the following measurement conditions, is 1.5 nF or less. The capacitance of the electrodeposited coating is, A method for forming an electrodeposited coating film according to claim 1, wherein the capacitive reactance at 10,000 Hz is calculated by the following formula. C=1 / (2π・F・Zc) C: Capacitance [F] F: Frequency [Hz] Zc: Capacitive reactance [Ω] <Capacitance Measurement Conditions> The measurements were taken using an electrochemical impedance analyzer (VersaSTAT4, Princeton Applied Research) under the following conditions. Electrolyte: 0.1% by mass sodium sulfate aqueous solution Immersion time in electrolyte solution before measurement: 5.5 hours Measurement area: 5.8 cm ２ Temperature at the measurement site: 60°C Measurement frequency: 1 to 1,000,000 Hz

5. The capacitance stabilization time in the electrochemical impedance measurement method of the electrodeposited coating film formed is 100 minutes or more. The capacitance stabilization time is determined when the electrochemical impedance of the formed electrodeposited coating is measured continuously at 20-minute intervals in a cycle from 1,000,000 Hz to 1 Hz. The rate of change in capacitance is calculated as ((n+1)th measurement - nth measurement) ÷ the total time required until the (n+1)th measurement is 0.01 or less. The method for forming an electrodeposited coating film according to claim 4.

6. The method for forming an electrodeposited coating film according to claim 1, wherein the metal object to be coated is not subjected to chemical conversion treatment.

7. The method for forming an electrodeposited coating according to claim 1, wherein the step of electrodepositing a metal workpiece and then heating and drying the obtained uncured deposited electrodeposited coating film is such that the melt viscosity of the electrodeposited coating film at 80°C is 800 Pa·s or less.

8. The method for forming an electrodeposited coating according to claim 1, wherein the step of electrodepositing a metal workpiece and then heating and drying the obtained uncured deposited electrodeposited coating film is such that the dynamic Tg of the electrodeposited coating film is 80°C or less.

9. The electrodeposition coating method according to claim 1, wherein the cationic resin (A) contains 10% or more polyol segments.

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