Method for forming electrodeposition coating film using cationic electrodeposition coating material

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

Method for forming electrodeposition coating film using cationic electrodeposition paint

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

[0002] Conventionally, cationic electrodeposition paints have been used as primer paints to impart rust prevention properties to industrial products such as automobiles. Patent Document 1 discloses that electrodeposition coating is performed by immersing an object to be coated, such as an automobile part, in the cationic electrodeposition paint, and the electrodeposition coating film formed on the metal object to be coated is heated to form a cured electrodeposition coating film with a film thickness of 20 μm.

[0003] WO2023 / 120540

[0004] Rust on metal parts, such as metal substrates, progresses when a corrosion circuit forms at the substrate interface and coating 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 electrons flow, corrosion of the substrate begins. Electrodeposition coating is widely used in various industries as a means of inhibiting this corrosion, but in recent years, changes in the market environment have led to a demand for further improvements in rust prevention.

[0005] An object of the present invention is to provide a method for forming an electrodeposition coating film using a cationic electrodeposition paint, which is capable of forming a coating film that has excellent corrosion resistance and can extend the rust prevention life of a metal substrate.

[0006] After careful investigation of the variables that affect the corrosion circuits formed at the interface of the substrate and the coating, it was found that the initiation of corrosion under the coating depends not on the resistance (impedance) of the coating but on the capacitance of the coating. In other words, lowering the capacitance can improve the rust prevention performance, which leads to delaying the onset of early rust on the coated object. To achieve the above object, the present invention includes, for example, the following aspects.

[0007] Item 1. A method for forming an electrodeposition coating film using a cationic electrodeposition paint, comprising the steps of: immersing a metal substrate in a cationic electrodeposition paint containing a cationic resin (A) to perform electrodeposition coating, thereby forming an electrodeposition coating film on the metal substrate; and heating the electrodeposition coating film to form an electrodeposition coating film on the metal substrate with a dry film thickness of 60 μm or more.

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

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

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

[0011] Item 5. The method for forming an electrodeposition coating film according to any one of Items 1 to 4, wherein the curing agent (B) contains a blocked polyisocyanate compound.

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

[0013] Item 7. The method for forming an electrodeposition coating film according to any one of Items 1 to 6, wherein the cationic epoxy resin (A1) comprises 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 formed electrodeposition coating film has a capacitance of 1.5 nF or less as measured by an electrochemical impedance measurement method under the following measurement conditions: <Capacitance Measurement Conditions> Measurement was performed using an electrochemical impedance measurement device (VersaSTAT4 manufactured by Princeton Applied Research) under the following conditions: Electrolyte: 0.1 mass % sodium sulfate aqueous solution Electrolyte immersion time before measurement: 5.5 hours Measurement area: 5.8 cm ２

[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 of the formed electrodeposition coating film in an electrochemical impedance measurement method is 100 minutes or longer.

[0016] Item 10. The method for forming an electrodeposition coating film according to any one of Items 1 to 9, wherein the metal substrate is not subjected to a chemical conversion treatment.

[0017] Item 11. The method for forming an electrodeposition coating film according to any one of Items 1 to 10, wherein in the step of electrodeposition coating on a metal substrate and subsequently heating and drying the resulting uncured deposited electrodeposition coating film, the melt viscosity of the electrodeposition coating film at 80°C is 800 Pa s or less.

[0018] Item 12. The method for forming an electrodeposition coating film according to any one of Items 1 to 11, wherein in the step of electrodeposition coating on a metal substrate and subsequently heating and drying the resulting uncured deposited electrodeposition coating film, the dynamic Tg of the electrodeposition coating film is 80°C or less.

[0019] Item 13. The method for forming an electrodeposition coating film according to any one of Items 1 to 12, wherein the cationic resin (A) contains 10% or more of polyol segments.

[0020] As used herein, the singular forms "a," "an," and "the" are intended to include both the singular and the plural unless otherwise expressly stated herein or otherwise clearly contradicted by context.

[0021] In this specification, the term "comprise" is a concept that also encompasses "consist solely of."

[0022] In the numerical ranges described in stages in this specification, the upper or lower limit of a certain numerical range can be arbitrarily combined with the upper or lower limit of another numerical range. Furthermore, in the numerical ranges described in this specification, the upper or lower limit of the numerical range may be replaced with a value shown in an example or a value that can be unambiguously derived from an example. Furthermore, in this specification, a numerical value connected with "to" means a numerical range that includes the numbers before and after "to" as the upper and lower limits.

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

[0024] In this specification, the term "epoxy resin" refers to both a resin having an epoxy group and a resin obtained by reacting the epoxy group of the epoxy resin with another functional group-containing compound, and does not necessarily contain an epoxy group. That is, in the present invention, the term "epoxy resin" refers to an epoxy resin having an epoxy group and / or a modified epoxy resin not having an epoxy group.

[0025] The amine value in this specification is measured in accordance with JIS K7237-1995. All values ​​are amine values ​​per resin solid content (mg KOH / g).

[0026] In this specification, the number average molecular weight and weight average molecular weight are values ​​calculated by converting the retention time (retention volume) measured using gel permeation chromatography (GPC) into the molecular weight of polystyrene using the retention time (retention volume) of standard polystyrenes of known molecular weights measured under the same conditions.

[0027] The present disclosure provides a method for forming an electrodeposition coating film using a cationic electrodeposition paint, comprising the steps of: immersing a metal substrate in a cationic electrodeposition paint containing a cationic resin (A) to perform electrodeposition coating, thereby forming an electrodeposition coating film on the metal substrate; 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 substrate.

[0028] The cationic electrodeposition paint used in the method for forming an electrodeposition coating film using the cationic electrodeposition paint of the present disclosure contains a cationic resin (A). By using a cationic electrodeposition paint containing the cationic resin (A) as the coating film-forming resin, the method for forming an electrodeposition coating film can be a method for forming an electrodeposition coating film that has excellent corrosion prevention properties and can extend the rust prevention life of a metal substrate.

[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 that is generally used in cationic electrodeposition paints, and known cationic epoxy resins, cationic epoxy resins produced 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 starting epoxy resin molecule with amines such as primary amines, secondary amines, or tertiary amines and / or their acid salts. Typical examples of starting epoxy resins are polyphenol polyglycidyl ether epoxy resins, which are reaction products of epichlorohydrin with polycyclic phenolic compounds such as bisphenol A, bisphenol F, bisphenol S, phenol novolac, and cresol novolac. Other examples of starting resins include the oxazolidone ring-containing epoxy resins described in JP-A-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 groups of a diisocyanate compound with a lower alcohol such as methanol or ethanol.

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

[0033] Similarly, before the ring-opening reaction of the oxirane ring with an amine, 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.

[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 acid salts, such as butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine, N,N-dimethylbenzylamine, and N,N-dimethylethanolamine. Also useful are ketimine-blocked primary amino group-containing secondary amines, such as aminoethylethanolamine and methylisobutylketimine, and diethylenetriamine diketimine. These amines must be reacted in at least an equivalent amount relative to the oxirane ring in order to open all of the oxirane rings.

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

[0036] The number average molecular weight in this specification can be obtained by measuring by GPC (gel permeation chromatography) and using a value converted using polystyrene as a standard.

[0037] The amine value of the cationic epoxy resin (A1) is, for example, within the range of 20 to 150 mgKOH / g. When the amine value of the cationic epoxy resin (A1) is 20 mgKOH / g or more, the dispersion stability of the amine-modified epoxy resin in the electrodeposition coating is improved. On the other hand, when the amine value is 150 mgKOH / g or less, the amount of amino groups in the cured electrodeposition coating film is appropriate, and there is no risk of reducing the water resistance of the coating film. In a specific embodiment, the amine value of the amine-modified epoxy resin is within 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 more results in good curing of the cured electrodeposition coating film. On the other hand, a hydroxyl value of 400 mgKOH / g or less results in an appropriate amount of hydroxyl groups remaining in the cured electrodeposition coating film. In one embodiment, the hydroxyl value of the cationic epoxy resin (A1) is more preferably in the range of 80 to 300 mgKOH / g.

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

[0040] In some embodiments, the cationic epoxy resin (A1) comprises an amino group-containing epoxy resin (A1-1). Use of the amino group-containing epoxy resin (A1-1) in a cationic electrodeposition coating makes it possible to form an electrodeposition coating film with excellent finish and corrosion resistance. 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) comprises an amino group-containing epoxy resin obtained by reacting an amine compound of the following formula (1), preferably an amine compound of the following formula (2), with an epoxy resin:

[0041] X １ -R １ -NH-R ２ -X ２ ...Formula (1) (wherein, R １ and R２ are linear or branched hydrocarbon groups having 1 to 8 carbon atoms, and may be different or the same. １ and X ２ are hydroxyl groups and / or amino groups, and may be different or the same. １ and X ２ When is an amino group, R １ and R ２ At least one of the groups is a hydrocarbon group having 1 to 2 carbon atoms.)

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

[0043] HO-R １ -NH-R ２ -OH Formula (2) Specific examples of the amine compound of the above formula (2) include dialkanolamines such as dibutanolamine, dipropanolamine, diethanolamine, and dimethanolamine. １ and R ２ Those having a hydrocarbon group with 1 to 2 carbon atoms are preferred, and specifically, dimethanolamine and diethanolamine are preferred.

[0044] HO-R １ -NH-R ２ -NH ２... Formula (3) Specific examples of the amine compound of the above formula (3) include alkanolamines having an alkyl group having 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)ethanolamine, N-(aminobutyl)propanolamine, and N-(aminobutyl)butanolamine. Among these, R １ and R ２ Those in which the hydrocarbon group has 1 or 2 carbon atoms are preferred, and specifically, N-(aminomethyl)methanolamine, N-(aminomethyl)ethanolamine, N-(aminoethyl)methanolamine, and N-(aminoethyl)ethanolamine are preferred.

[0045] NH ２ -R １ -NH-R ２ -NH ２ ... Formula (4) The amine compound of the above formula (4) includes R １ and R ２ At least one (preferably both) of the hydrocarbon groups has 1 to 2 carbon atoms, and specific examples include dimethylenetriamine and diethylenetriamine. Among these, diethylenetriamine is preferred. １ and R ２ The hydrocarbon radicals are intended to include all possible isomers, thus, for example, propyl includes isopropyl, butyl includes n-butyl, isobutyl and t-butyl.

[0046] In order to prevent the amine compounds represented by the above formulas (3) and (4) from becoming too high molecular weight during resin synthesis, a preferred production method involves reacting the primary amine at the terminal of the amine compound with a ketone compound to form a ketimine, then reacting the secondary amine with the epoxy group of the epoxy resin, and then hydrolyzing the ketimine back to the primary amine when forming the resin into a paint (emulsification). 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 resin terminal, thereby enabling it to obtain good paint stability in an aqueous paint.

[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] R １ -NH-R ２ -OH...Formula (5) (wherein, R １ and R ２ 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), R １ is usually 1 to 8, preferably 2 to 6, more preferably 3 to 4, and particularly preferably 3, and R2 is usually 1 to 8, preferably 2 to 4, more preferably 3 to 4, and particularly preferably 3,

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

[0051] In addition, the R １ and R ２The hydrocarbon radicals are intended to include all possible isomers, thus, for example, propyl includes isopropyl, butyl includes n-butyl, isobutyl and t-butyl.

[0052] The amine compound of formula (5) can be obtained, for example, by reacting a monoalkylamine with an alkylene oxide. The monoalkylamine is not particularly limited, and examples of the monoalkylamine include linear or branched monoalkylamines having 1 to 6 carbon atoms, such as monomethylamine, monoethylamine, mono-n-propylamine, monoisopropylamine, mono-n-butylamine, monoisobutylamine, mono-sec-butylamine, mono-t-butylamine, mono-n-pentylamine, isopentylamine, and mono-n-hexylamine. Preferred examples of the monoalkylamine include monomethylamine, monoethylamine, mono-n-propylamine, monoisopropylamine, mono-n-butylamine, monoisobutylamine, and mono-t-butylamine. Particularly preferred examples of the alkylene oxide include monomethylamine, monoethylamine, mono-n-propylamine, monoisopropylamine, and mono-n-butylamine. The alkylene oxide is not particularly limited, and examples of the alkylene oxide include ethylene oxide, propylene oxide, and butylene oxide. Preferred examples of the alkylene oxide include ethylene oxide and propylene oxide.

[0053] The ketone used for ketiminization is not particularly limited as long as it reacts with the above-mentioned amine compound to form a ketimine compound and further hydrolyzes in the aqueous coating composition. Examples include methyl isopropyl ketone (MIPK), diisobutyl ketone (DIBK), methyl isobutyl ketone (MIBK), diethyl ketone (DEK), ethyl butyl ketone (EBK), ethyl propyl ketone (EPK), dipropyl ketone (DPK), and methyl ethyl ketone (MEK). Of these, methyl isobutyl ketone (MIBK) is preferred. These ketones can be used alone or in combination of two or more.

[0054] The epoxy resin (b) used as a raw material for the amino group-containing epoxy resin (A1-1) is a compound having at least one, preferably two or more, epoxy groups per molecule. The epoxy resin (b) preferably has a number-average molecular weight of at least 300, preferably 400 to 4000, and more preferably 800 to 2500. The epoxy resin (g) preferably has an epoxy equivalent of at least 160, preferably 180 to 2500, and more preferably 400 to 1500. Examples of such epoxy resins include those obtained by reacting a polyphenol compound with an epihalohydrin (e.g., epichlorohydrin).

[0055] Examples of polyphenol compounds used to form the 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'-dihydroxydiphenyl sulfone, phenol novolac, and cresol novolac.

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

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

[0058] Furthermore, by using a modifier or bisphenol F epoxy resin as a constituent of the amino group-containing epoxy resin (A1-1), the finish and corrosion resistance of the coating film can be more preferably achieved. 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 polymerizing acrylic monomers, xylene formaldehyde compounds, and epoxy compounds. These modifiers can be used alone or in combination of two or more. From the viewpoint of improving the finish and corrosion resistance, the amount of the modifier used is typically 10% by mass or more, preferably 10 to 30% by mass, and more preferably 10 to 20% by mass, based on the solids mass of the amino group-containing epoxy resin (A1-1).

[0059] The cationic acrylic resin (A2) can be prepared by adding an amino group-containing compound to a copolymer resin obtained by radical copolymerization of a hydroxyl group-containing monomer, a glycidyl group-containing monomer, and other monomers. Examples of hydroxyl group-containing monomers used in preparing the 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 alone or in combination of two or more.

[0060] Examples of the glycidyl group-containing monomer include glycidyl (meth)acrylate, (meth)allyl glycidyl ether, etc. These glycidyl group-containing monomers may be used alone or in combination of two or more.

[0061] Examples of other monomers include 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, vinyl toluene, α-methyl styrene, (meth)acrylonitrile, (meth)acrylamide, and vinyl acetate. These other monomers may be used alone or in combination of two or more.

[0062] The hydroxyl group-containing monomer, glycidyl group-containing monomer, and other monomers are radically copolymerized by methods known to those skilled in the art to obtain an acrylic copolymer resin. The amount of hydroxyl group-containing monomer is preferably such that the cationic acrylic resin (A2) has a hydroxyl value of 50 to 250, more preferably 60 to 220. The amount of glycidyl group-containing monomer is preferably such that the cationic acrylic resin (A2) has an amine value of 10 to 100 mgKOH / g, more preferably 15 to 80 mgKOH / g of amino groups. The hydroxyl group value is a value determined from the amount of hydroxyl group-containing monomer.

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

[0064] In the preparation of the cationic acrylic resin (A2), there is also a method of directly synthesizing the cationic acrylic resin (A2) by copolymerizing an acrylic monomer having an amino group with another monomer. 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 above-mentioned glycidyl group-containing monomer, and the 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) thus obtained can be converted into a self-crosslinking cationic acrylic resin (A2) by introducing a blocked polyisocyanate group into the cationic acrylic resin (A2) by addition reaction with a half-blocked diisocyanate compound, if necessary.

[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 more, the physical properties such as finish and corrosion resistance of the resulting cured electrodeposition coating film are good. When the number average molecular weight is 7000 or less, the viscosity of the cationic acrylic resin (A2) can be easily adjusted and the coating film has good finish properties.

[0067] The cationic acrylic resin (A2) is preferably designed so that its hydroxyl value is in the range of 50 to 250. When the hydroxyl value is 50 or more, the coating film is cured well. When the hydroxyl value is 250 or less, the water resistance of the cured electrodeposition coating film is good.

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

[0069] The cationic electrodeposition paint according to the present disclosure may further contain an amino group-containing polyester resin.

[0070] In some embodiments, the total proportion of the cationic epoxy resin (A1) and / or cationic acrylic resin (A2) in the film-forming resins in the cationic electrodeposition paint is preferably 50% by mass or more, 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] Curing Agent (B) The cationic electrodeposition paint may further contain a curing agent (B). In this specification, the cationic resin (A) and the curing agent (B) constitute a film-forming resin. The curing agent (B) contained in the cationic electrodeposition paint according to the present disclosure can be a film-forming resin that undergoes a curing reaction with the cationic resin (A) under heating conditions. As the curing agent (B), a melamine resin or a blocked polyisocyanate (also referred to as a blocked polyisocyanate compound) is preferably used. 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 trimer), tetramethylene diisocyanate, and trimethylhexamethylene diisocyanate; alicyclic polyisocyanates such as isophorone diisocyanate and 4,4'-methylenebis(cyclohexylisocyanate); aromatic diisocyanates such as 4,4'-diphenylmethane diisocyanate, tolylene diisocyanate, and xylylene diisocyanate; and modified products of these diisocyanates (e.g., urethane-modified products, carbodiimide-, uretdione-, uretonimine-, biuret-, and / or isocyanurate-modified products).

[0073] Preferred examples of the blocking 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 both-terminal diols such as polyethylene glycol, polypropylene glycol, and polytetramethylene ether glycol phenol; polyester-type both-terminal polyols obtained from diols such as ethylene glycol, propylene glycol, and 1,4-butanediol and dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, suberic acid, and sebacic acid; phenols such as para-t-butylphenol and cresol; oximes such as dimethyl ketoxime, methyl ethyl ketoxime, methyl isobutyl ketoxime, methyl amyl ketoxime, and cyclohexanone oxime; and lactams represented by ε-caprolactam and γ-butyrolactam.

[0074] The blocked polyisocyanate reacts preferentially with the primary amine of the amine-modified epoxy resin, and then reacts with the hydroxyl group to cure the resin.

[0075] Examples of melamine resins include partially or fully methylolated melamine resins obtained by reacting melamine with formaldehyde, partially or fully alkyl ether melamine resins obtained by partially or completely etherifying the methylol groups of methylolated melamine resins with an alcohol component, imino group-containing melamine resins, and mixed melamine resins thereof. Examples of alkyl ether melamine resins include methylated melamine resins, butylated melamine resins, and methyl / butyl mixed alkyl 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 the cationic electrodeposition coating contains a curing agent (B), the ratio of the cationic resin (A) (particularly the total of the cationic epoxy resin (A1) and / or the cationic acrylic resin (A2)) to the curing agent (B) is preferably (A):(B)=10-90:90-10 in mass ratio.

[0078] Pigment Dispersion Paste The cationic electrodeposition paint according to the present disclosure may contain a pigment dispersion paste, if necessary. The pigment dispersion paste generally contains a pigment dispersing resin and a pigment.

[0079] The pigment dispersing resin is a resin for dispersing pigments, and is used, for example, by dispersing it in an aqueous medium. As the pigment dispersing resin, a pigment dispersing resin having a cationic group, such as a modified epoxy resin having at least one or more groups selected from a quaternary ammonium group, a tertiary sulfonium group, and a primary amine group, can be used. As the aqueous solvent, ion-exchanged water or water containing a small amount of alcohol can be used.

[0080] The pigment is a pigment generally used in electrodeposition paint. 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.

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

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

[0083] In some embodiments, in terms of finish and corrosion resistance, the dynamic coating Tg (°C) of the cationic electrodeposition paint is preferably 80°C or less. The dynamic coating Tg (°C) refers to the temperature at which Tan δ of the data obtained by measuring the melt viscosity at 80°C is maximum.

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

[0085] The lower limit of the melt viscosity of the cationic electrodeposition paint at 80°C is not particularly limited, but is, for example, 100 Pa·s or more, 200 Pa·s or more, or 300 Pa·s or more. The cationic electrodeposition paint disclosed herein has excellent rust prevention performance and can delay the onset of initial rust on metal substrates. Furthermore, even for coatings with a low glass transition temperature (Tg) where the substrate has a large heat capacity and sufficient crosslinking reaction is not achieved, the rust prevention properties can be compensated for by setting the coating thickness thicker, thereby achieving rust prevention properties exceeding conventional coating thicknesses of approximately 15 to 20 μm. Setting a thicker coating than conventional electrodeposition coating specifications extends the rust prevention life of metal parts, thereby reducing the burden of part replacement. Furthermore, by eliminating constraints on barrier properties per unit film thickness, it is possible to significantly relax the paint composition and baking conditions, thereby enabling energy savings toward carbon neutrality. Energy savings here refer to the metal processing energy required to remanufacture parts and the energy required to paint and dry the parts.

[0086] The cationic electrodeposition paint according to the present disclosure can be provided in a form in which a film-forming resin, which is a cationic resin (A) (e.g., an amine-modified epoxy resin, etc.), an optional curing agent (B), and optional other additives are dissolved or dispersed in an aqueous medium. This coating composition is used in a coating bath, and a current is passed through the metal substrate as the cathode or anode to form a deposit coating film on the metal substrate. The deposit coating film is then heated to form a cured electrodeposition coating film.

[0087] Metallic substrates for coating in the present disclosure include, for example, automobile parts, automobile bodies, motorcycle parts, and industrial products made of metal materials such as iron, aluminum, zinc, tin, and alloys thereof.

[0088] The metal substrate may be formed from a variety of electrically conductive materials, such as 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.

[0089] Method for forming an electrodeposition coating film: In the method for forming an electrodeposition coating film using the cationic electrodeposition paint according to the present disclosure, the metal substrate may be subjected 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 as a pre-step for immersing the substrate in the cationic electrodeposition paint. Known processes can be used for each of these steps. Also, known chemical conversion treatment solutions can be used for the chemical conversion treatment.

[0090] The method for forming an electrodeposition coating film using the cationic electrodeposition paint according to the present disclosure includes a step of immersing a metal substrate in a cationic electrodeposition paint containing a cationic resin (A) to perform electrodeposition coating, thereby forming an electrodeposition coating film on the metal substrate.

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

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

[0093] In the step of heating the cationic electrodeposition paint, the heating temperature is, for example, 120° C. or higher and 260° C. or lower, and in one embodiment, 130° C. or higher and 220° C. or lower. In another embodiment, the heating temperature is 140° C. or higher and 220° C. or lower.

[0094] The time for heating or baking the cationic electrodeposition paint at such a heating temperature is, for example, 10 minutes or more and 30 minutes or less.

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

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

[0097] <Conditions for Capacitance Measurement> Measurement was carried out under the following conditions using an electrochemical impedance measuring device (VersaSTAT4 manufactured by Princeton Applied Research).

[0098] Electrolyte: 0.1% by mass sodium sulfate aqueous solution Electrolyte immersion time before measurement: 5.5 hours Measurement area: 5.8 cm ２ In some embodiments, from the viewpoint of corrosion resistance, the capacitance stabilization time (minutes) of an electrodeposition coating film formed by the method for forming an electrodeposition coating film using the cationic electrodeposition paint according to the present 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.

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

[0100] Preparation of Amine-Modified Resins Preparation Example 1: Amino Group-Containing Epoxy Resin (A1-1) In a flask equipped with a thermometer, a stirrer, and a 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, 244 parts of bisphenol F, and 30 parts of methyl isobutyl ketone were added and dissolved by heating. Furthermore, 0.5 parts of dimethylbenzylamine and 10 parts of methyl isobutyl ketone were added at 100°C, and the mixture was reacted at 160°C until the epoxy equivalent reached 500. Thereafter, 110 parts of methyl isobutyl ketone was added, and the mixture was cooled to 105°C. After that, 157.5 parts of diethanolamine and 126 parts of methyl isobutyl ketone of diethylenetriamine were added, and the mixture was reacted until the epoxy completely disappeared, 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] Production Example 2: Amino Group-Containing Epoxy Resin (A1-2) In a flask equipped with a thermometer, a stirrer, and a reflux condenser, 1,026 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 blended and heated to 120°C to dissolve. Thereafter, 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. Thereafter, 158 parts of diethanolamine and 176 parts of methyl isobutyl ketone of diethylenetriamine were added, and the reaction was maintained at 110°C. After confirming that no epoxy groups remained after the reaction, 200 parts of methyl isobutyl ketone was added to obtain an amine-containing modified epoxy resin (A1-2) with a solids content of 78%. The amine-modified epoxy resin (A1-2) had a number average molecular weight of 2,000, a total amine value of 60 mg KOH / g, and a hydroxyl value of 86 mg KOH / g.

[0102] Production Example 3: Amino Group-Containing Epoxy Resin (A1-3) In a flask equipped with a thermometer, a stirrer, and a 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 blended and heated to 120°C to dissolve. Thereafter, 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. Thereafter, 158 parts of diethanolamine and 286 parts of methyl isobutyl ketone 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 was added to obtain an amine-containing modified epoxy resin (A1-3) with a solids content of 78%. The amine-modified epoxy resin (A1-3) had a number average molecular weight of 1,900, a total amine value of 63 mg KOH / g, and a hydroxyl value of 90 mg KOH / g.

[0103] Production Example 4: Amino Group-Containing Epoxy Resin (A1-4) 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 weighed into a flask equipped with a thermometer, a stirrer, and a reflux condenser, and dissolved under heat. 0.5 parts of dimethylbenzylamine and 10 parts of methyl isobutyl ketone were added at 100°C, and the mixture was further heated to 160°C and allowed to react until the epoxy equivalent reached 500. 170 parts of methyl isobutyl ketone were then added, and the mixture was cooled to 105°C. 157.5 parts of diethanolamine and 126 parts of methyl isobutyl ketone of diethylenetriamine were then added, and the reaction was continued until the epoxy was completely consumed, yielding an 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 1,300, a total amine value of 90 mg KOH / g, and a hydroxyl value of 128 mg KOH / g.

[0104] Production Example 5: Amino Group-Containing Acrylic Resin (A2) 20 parts of butyl cellosolve was placed in a reaction vessel equipped with a stirrer, a condenser, a nitrogen inlet tube, a thermometer, and a dropping funnel, and the temperature was raised to 120°C. A solution containing 30 parts of styrene, 30 parts of hydroxyethyl methacrylate, 30 parts of PLACCEL FM-3X (trade name, manufactured by Daicel Corporation, ε-caprolactone-modified hydroxyethyl methacrylate), 10 parts of glycidyl methacrylate, and 5 parts of AIBN was added dropwise over 3 hours. After the dropwise addition was completed, 1 part of methyl isobutyl ketone was added, followed by the dropwise addition of a solution containing 0.5 parts of AIBN dissolved in 10 parts of methyl isobutyl ketone, and the mixture was allowed to mature. After cooling, 7 parts of diethanolamine was added, and the mixture was allowed to react at 130°C for 3 hours. Finally, 7 parts of methyl isobutyl ketone was added to obtain an acrylic resin (A2) with a solids content of 75%. The amino group-containing acrylic resin (A2) had a number average molecular weight of 5,000, a total amine value of 33 mg KOH / g, and a hydroxyl value of 66 mg KOH / g.

[0105] Production of Blocked Polyisocyanate Compounds Production Example 6: Blocked Polyisocyanate (B-1) Into a reaction vessel, 168 parts of hexamethylene diisocyanate and 5 parts of methyl isobutyl ketone were added, and 150 parts of methyl ethyl ketoxime was gradually added, followed by a reaction at 70°C. During the reaction, 13.4 parts of trimethylolpropane was added, and the temperature was raised to 100°C. Thereafter, 0.01 part of bismuth tris(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 continued until the isocyanate value reached 1 or less, 22 parts of methyl isobutyl ketone was added, yielding a blocked polyisocyanate (B-1) with a solids content of 90%.

[0106] Production Example 7: Blocked polyisocyanate (B-2) Into a reaction vessel were placed 267 parts of Sumidur 44V20L (trade name, crude MDI, manufactured by Sumika Covestro Urethane Co., Ltd.) and 10 parts of methyl isobutyl ketone, and the mixture was heated to 60°C. Thereafter, 162 parts of butyl carbitol and 120 parts of butyl cellosolve were gradually added, and the mixture was heated to 120°C. While maintaining this temperature, 0.02 parts of bismuth tris(2-ethylhexanoate) and 2 parts of methyl isobutyl ketone were added, and the mixture was reacted until the isocyanate value reached 1 or less. After that, 17 parts of methyl isobutyl ketone was added, and a blocked polyisocyanate (B-2) with a solids content of 95% was obtained.

[0107] Preparation of Pigment Dispersing Resin Preparation Example 8: 1,200 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 weighed and dissolved in a flask equipped with a stirrer, thermometer, dropping funnel, and reflux condenser. At 100°C, 0.41 parts of dimethylbenzylamine and 6 parts of methyl isobutyl ketone were added, and the mixture was further heated to 160°C and reacted until the epoxy equivalent reached 300. Subsequently, 702 parts of butyl cellosolve was added, and after cooling to 50°C, 126 parts of diethanolamine and 171 parts of dimethylaminopropylamine were added. The reaction was continued 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 dispersing resin with an amine value of 150 and a resin solids content of 60%.

[0108] Preparation of Pigment Dispersion Paste Preparation Example 9 8.3 parts of the pigment dispersion resin having a solids content of 60% obtained in Preparation Example 8 (solids content: 5 parts), 14.5 parts of titanium oxide, 7.0 parts of refined clay, 0.3 parts of carbon black, 2 parts of dioctyltin oxide, and 20.3 parts of deionized water were added and dispersed in a ball mill for 20 hours to obtain a pigment dispersion paste (p-1) having a solids content of 55%.

[0109] Preparation of Cationic Electrodeposition Coating Composition Preparation Example 10: 84.3 parts (solids content: 70 parts) of the amine-modified epoxy resin (A1-1) obtained in Preparation Example 1 and 33.3 parts (solids content: 30 parts) of the blocked polyisocyanate (B-1) obtained in Preparation Example 6 were mixed, and 13 parts of 10% acetic acid was added and stirred uniformly. Deionized water was then added dropwise over a period of approximately 15 minutes with vigorous stirring to obtain an emulsion with a solids content of 34%. Next, 294 parts (solids content: 100 parts) of the above emulsion, 52.4 parts (solids content: 28.8 parts) of the pigment dispersion paste (p-1) obtained in Preparation Example 9, and deionized water were added to prepare a cationic electrodeposition coating composition (X-1) with a solids content of 20%.

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

[0111] Method for Forming Electrodeposition Coating Films Examples 1 to 13, Comparative Examples 1 to 7 Cold-rolled steel sheets (150 mm (length) × 70 mm (width) × 0.8 mm (thickness)) that had been subjected to a 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, and electrodeposition coating was performed using each of the cationic electrodeposition paints (X-1) to (X-5) obtained in Production Examples 10 to 14, adjusting the coating voltage and bath temperature to achieve the film thickness (μm) shown in Table 1. The metal substrates after coating 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> A 5 ​​mm width around the periphery of the test plate was covered with polyester masking tape and immersed in 55°C saltwater with a NaCl concentration of 5% or 0.5%. The presence or absence of blistering was checked every 120 hours, and the test was terminated when even one blister occurred. The test time until blistering occurred was added up to represent the evaluation result.

[0113] The evaluation is "S, A, B" is a passing grade, and "C, D" is a failing grade.

[0114] S: Test time until blistering occurred was 2000 hours or more A: Test time until blistering occurred was 1500 hours or more but less than 2000 hours B: Test time until blistering occurred was 1000 hours or more but less than 1500 hours C: Test time until blistering occurred was 500 hours or more but less than 1000 hours D: Test time until blistering occurred was less than 500 hours <Finish (Voids)> A cross section (1 mm wide) of the coating film of the obtained test panel was observed, and the number of voids (air bubbles) on the coating surface was counted visually.

[0115] The evaluation is "A" and "B" as passing grades, and "C" as failing grades.

[0116] A: No voids, good. B: No large voids (diameter 1 μm or more), but one to three small voids (diameter less than 1 μm). C: At least one large void or four or more small voids.

[0117] Measurement or calculation method of characteristic values ​​<Film thickness> The film thickness of the cationic electrodeposition coating on the test plate can be measured with an electromagnetic film thickness meter. The measurement point of the test plate is a flat area where the entire measuring part of the electromagnetic film thickness meter can contact, and measurements are taken at three randomly changed locations on the flat area, and the average thickness is calculated.

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

[0119] Before measuring the viscoelasticity, the above-mentioned device was calibrated with a viscosity standard liquid (JS-100, manufactured by Nippon Grease) to ensure the correct viscosity.

[0120] 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 coated so that the coating would have a thickness of 0.6 mm after drying, and the temperature was raised to 100°C. The above-mentioned jig was then attached to the cell and inserted into the sample 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 cooling rate of 5°C / min, and the complex viscosity was measured between 100°C and 30°C. The complex viscosity at 80°C was then calculated from the resulting chart as the melt viscosity at 80°C.

[0121] <Dynamic Coating Tg (° C.)> The temperature at which Tan δ of the data obtained in the melt viscosity measurement at 80° C. was maximized was determined as the dynamic coating Tg (° C.).

[0122] <Capacitance (nF)> The capacitance was calculated from the value obtained by the electrochemical impedance method described below. The electrochemical impedance can be measured using a commercially available electrochemical measurement device or potentiostat. An example of such a measurement device is the "VersaSTAT4" manufactured by Ametech Science Instruments Co., Ltd.

[0123] Specifically, the measurement was performed using a 0.1% by mass aqueous solution of sodium sulfate as the electrolyte, a two-electrode system using a coated plate as the working electrode and SUS304 as the counter electrode, and the measurement site of the coated plate was heated to 60°C.

[0124] Electrolyte immersion time before measurement: 5.5 hours, measurement area: 5.8 cm ２ The experiment was carried out under the following conditions.

[0125] The measurement frequency was 1 to 1,000,000 Hz, and the capacitance of the electrodeposition coating film 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] This operation was repeated at 20 minute intervals to measure the capacitance stabilization time.

[0127] <Capacitance stabilization time (minutes)> When the electrochemical impedance measurement was performed continuously in cycles from 1,000,000 Hz to 1 Hz, the total time required until the following value, which is the rate of change in capacitance = (measured value at (n+1)th time - measured value at nth time) / (n+1)th time measured value, became 0.01 or less, 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 mass of all the raw materials constituting the cationic resin. For example, in the case of an amino group-containing epoxy resin (A1-2), the polyol segment content can be calculated as follows:

[0129] Mass of polyol-derived modifier: Polyoxypropylene glycol diglycidyl ether 283 parts Mass of all raw materials constituting the cationic resin: jER-828EL 950 parts + polyoxyethylene glycol diglycidyl ether 268 parts + bisphenol A 456 parts + diethanolamine 158 parts, diethylenetriamine ketimine 286 parts = 2118 parts Polyol segment content = (283 / 2118) x 100 = 13.3%

Claims

1. A method for forming an electrodeposition coating film using a cationic electrodeposition paint, comprising the steps of: immersing a metal substrate in a cationic electrodeposition paint containing a cationic resin (A) to perform electrodeposition coating, thereby forming an electrodeposition coating film on the metal substrate; and heating the electrodeposition coating film to form an electrodeposition coating film with a dry thickness of 60 μm or more on the metal substrate.

2. The method for forming an electrodeposition coating film according to claim 1, wherein the dry film thickness is 65 μm or more.

3. The method for forming an electrodeposition coating film according to claim 1, wherein the dry film thickness is 70 μm or more.

4. The method for forming an electrodeposition coating film according to any one of claims 1 to 3, wherein the cationic electrodeposition paint contains a curing agent (B).

5. A method for forming an electrodeposition coating film according to any one of claims 1 to 4, wherein the curing agent (B) contains a blocked polyisocyanate compound.

6. A method for forming an electrodeposition 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).

7. A method for forming an electrodeposition coating film according to any one of claims 1 to 6, wherein the cationic epoxy resin (A1) comprises an amino group-containing epoxy resin (A1-1).

8. The method for forming an electrodeposition coating film according to any one of claims 1 to 7, wherein the electrostatic capacitance of the formed electrodeposition coating film measured under the following conditions by electrochemical impedance measurement is 1.5 nF or less. <Conditions for Capacitance Measurement> Measurement was carried out using an electrochemical impedance measurement device (VersaSTAT4 manufactured by Princeton Applied Research) under the following conditions: Electrolyte: 0.1 mass % aqueous sodium sulfate Electrolyte immersion time before measurement: 5.5 hours Measurement area: 5.8 cm ２ 9. A method for forming an electrodeposition coating film according to any one of claims 1 to 8, wherein the capacitance stabilization time of the formed electrodeposition coating film in an electrochemical impedance measurement method is 100 minutes or more.

10. The method for forming an electrodeposition coating film according to any one of claims 1 to 9, wherein the metal substrate is not subjected to a chemical conversion treatment.

11. A method for forming an electrodeposition coating film according to any one of claims 1 to 10, wherein in the step of electrodeposition coating a metal substrate and subsequently heating and drying the resulting uncured deposited electrodeposition coating film, the melt viscosity of the electrodeposition coating film at 80°C is 800 Pa·s or less.

12. A method for forming an electrodeposition coating film according to any one of claims 1 to 11, wherein in the step of electrodeposition coating on a metal substrate and subsequently heating and drying the resulting uncured deposited electrodeposition coating film, the dynamic Tg of the electrodeposition coating film is 80°C or less.

13. A method for forming an electrodeposition coating film according to any one of claims 1 to 12, wherein the cationic resin (A) contains 10% or more of polyol segments.

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