JP2010214283A - Method for forming multilayer coated film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for improving corrosion-resistance in a method for forming a multilayer coated film unnecessarily requiring pretreatment of zinc phosphate by applying voltage in two stages using an aqueous cationic electrodeposition coating composition. <P>SOLUTION: In the method for forming a multilayer coated film including an immersion process of immersing an object to be coated into an electrolytic-decomposition corrosion-resistant metal salt (A), a substrate resin having a cationic group (B) and an aqueous cationic electrodeposition coating composition containing a curing agent (C), an electrolytic decomposition process of applying voltage of below 50 V in the aqueous cationic electrolytic composition by using the object to be coated as a cathode, and an electrodeposition coating process of applying the voltage of 50-450 V by using the object to be coated as a cathode, three of a rare earth metal salt lower in sedimentation pH of the electrolytic-decomposition corrosion-resistance metal salt (A) contained in the aqueous cationic electric-deposition corrosion-resistance coating composition (a1) a rare-earth metal salt having a deposition pH being lower than that of zinc, (a2) a zinc salt and (a3) a metal salt higher in sedimentation pH than that of zinc. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

By using the same aqueous cationic electrodeposition coating composition, the present invention integrates a pretreatment (base treatment) step before electrodeposition coating applied to a metal material, particularly an untreated cold-rolled steel sheet, and an electrodeposition coating step. The present invention relates to a method for forming a multilayer coating film.

  An automobile body is manufactured by forming a metal material such as a cold-rolled steel plate or a galvanized steel plate, coating the metal formed product as an object to be coated, and then performing assembly or the like. In general, such metal molded products are subjected to rust prevention treatment (also called pretreatment) such as zinc phosphate chemical conversion treatment before electrodeposition coating in order to impart adhesion to the electrodeposition coating film. It has been broken.

  Electrodeposition coating using a cationic electrodeposition coating composition is excellent in corrosion resistance and throwing power, and can form a uniform coating film. Therefore, it is widely used mainly for automobile bodies and primer for parts. However, in the conventional cationic electrodeposition coating composition, although it is possible to develop sufficient corrosion resistance by electrodeposition coating for a material in which a pretreatment such as zinc phosphate is applied to the coating object, When the pretreatment (chemical conversion treatment) of the coating is insufficient, there is a problem that it is difficult to ensure corrosion resistance.

  Japanese Patent No. 3168381 (Patent Document 1) discloses a cathode electrodeposition coating composition obtained by dispersing a hydrophilic film-forming resin having a cationic group and a curing agent in an aqueous medium containing a neutralizing agent. 0.1 to 20% by weight of at least one phosphomolybdate selected from aluminum salt, calcium salt and zinc salt, and 0.01 to 2.0% by weight based on cerium compound A cathodic electrodeposition coating composition characterized by comprising is described. Thereby, it is described that the corrosion resistance with respect to the surface untreated cold-rolled steel sheet can be improved.

  Japanese Patent No. 3368399 (Patent Document 2) discloses a cathode electrodeposition coating composition obtained by dispersing a hydrophilic film-forming resin having a cationic group and a curing agent in an aqueous medium containing a neutralizing agent. A cathode battery comprising a total of 0.01 to 2.0% by weight of a copper compound and a cerium compound as a metal, and a copper / cerium weight ratio of 1/20 to 20/1 as a metal A coating composition is described. Similarly, it is described that the corrosion resistance of the surface-untreated cold-rolled steel sheet can be improved.

  However, in any coating using the above electrodeposition coating composition, one-step electrodeposition coating is performed by one-step electrodeposition coating under the condition of an applied voltage of 100 to 450V. Under such electrolysis and electrodeposition conditions, film formation with cerium or cerium-copper is insufficient. For this reason, none of the improvement levels of the corrosion resistance according to these inventions has exhibited the base adhesion comparable to the conventional chemical conversion treatment with the conventional phosphate, and has not reached the level of the corrosion resistance after electrodeposition coating.

  WO 2006-109862 (Patent Document 3) proposes a new multilayer coating film forming method in which the above-described pretreatment process and electrodeposition coating process are integrated. In this multilayer coating film forming method, the object to be coated is immersed in a cationic electrodeposition coating composition containing a rare earth metal compound, and a voltage of less than 50 V is applied as a first step to deposit a rare earth metal electrolytic reaction product. A layer is formed, and then, in the same cationic electrodeposition coating composition, the applied voltage is increased to 50 to 450 V to perform normal electrodeposition coating. In this method, since the pretreatment (electrolytic deposition) step and the electrodeposition coating step are performed in one cationic electrodeposition coating composition, the pretreatment and the electrodeposition coating that have been performed in separate steps so far are performed in one stage. The effect of shortening the painting process is extremely large.

  However, in the method of Patent Document 3, the process can be shortened, and the corrosion test (salt water spray test and salt water immersion test) also shows good rust prevention properties. Further improvements are required.

Japanese Patent No. 3168381 Japanese Patent No. 3368399 WO2006-109862

  An object of the present invention is to further improve the rust prevention property in the multilayer coating film forming method of Patent Document 3 described above.

The present invention
(A) an electrolytic precipitation rust-preventing metal salt, (B) a base resin having a cationic group, and (C) an aqueous cationic electrodeposition coating composition containing a curing agent;
In the aqueous cationic electrodeposition coating composition, a voltage of less than 50 V is applied using the article to be coated as a cathode, and in the aqueous cationic electrodeposition coating composition, 50 to Electrodeposition coating process that applies 450V voltage,
A method for forming a multilayer coating film, comprising:
The electrodepositable rust preventive metal salt (A) contained in the aqueous cationic electrodeposition coating composition is (a1) a rare earth metal salt having a lower precipitation pH than zinc, (a2) a zinc salt and (a3) a precipitation pH lower than zinc. It consists of three kinds of high metal salts,
The total amount of the electrolytically depositable rust-proof metal salt (A) is 100 to 20,000 ppm in terms of metal in the paint,
When the metal amount of each component (a1), (a2) and (a3) in the electrolytically depositable rust-proof metal salt (A) is 100% by mass of the total metal amount, the component (a1) 5 40% by mass, component (a2) 40-90% by mass and component (a3) 5-40% by mass,
A method for forming a multilayer coating film is provided, which achieves the above object.

  The rare earth metal of the rare earth metal salt (a1) having a lower precipitation pH than zinc is preferably yttrium (Y), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb). ), Lutetium (Lu), and mixtures thereof.

  The metal of the metal salt (a3) having a higher precipitation pH than zinc is preferably nickel (Ni), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm). ) And mixtures thereof.

The amount of the electrolytic reaction products of the rare earth metal salt (a1), (B) zinc salt (a2) having a lower precipitation pH than zinc, and metal salt (a3) having a higher precipitation pH than zinc deposited in the electrolytic deposition step is The total amount is preferably 5 mg / m ² or more.

  The energization time in the electrolytic deposition step is preferably 10 to 300 seconds.

  The energization time in the electrodeposition coating process is preferably 10 to 300 seconds.

  This invention also provides the multilayer coating film obtained by said multilayer coating-film formation method.

  The present inventors have conducted a combined environmental corrosion acceleration test in spite of the fact that the coating film formed by the method for forming a multilayer coating film of WO 2006-109862 exhibits a high rust prevention property in the salt water immersion test. It was recognized that good results are not always obtained in the combined cycle corrosion test (CCT). The reason for this is that the vicinity of the surface of the object to be coated must be a layer in which there are many rare earth metal electrolytic reaction products formed by electrolytic deposition at the first low voltage. It was found that the electrodeposition coating resin was a mixed layer. The present inventors conducted an experiment to make the layer near the surface of the object to be coated contain a large amount of electrolytically deposited rust-preventive metal, and mainly a zinc having a high precipitation pH, zinc, It has been found that when a rare earth metal having a precipitation pH lower than that of zinc is allowed to coexist at a predetermined concentration, a layer containing a large amount of a rust preventive metal is formed in the vicinity of the surface of the article to be coated. It has been found that the coating thus formed exhibits not only high performance in the salt spray test and salt water immersion test, but also high corrosion resistance in the combined environmental corrosion acceleration test. The present invention has been completed from such studies.

  Therefore, the multilayer coating film obtained by the multilayer coating film forming method of the present invention (that is, a layer electrolytically deposited at a low voltage and a resin layer deposited by electrodeposition coating) has a rust prevention property in a salt spray test and It has high rust prevention properties not only in a salt water immersion test but also in a combined cycle corrosion test called a combined environmental corrosion acceleration test. Moreover, in the multilayer coating film forming method of the present invention, a rust-preventing metal layer having a high rust prevention property by simply changing the voltage with one cationic electrodeposition coating composition, and an electrodeposition coating film formed by electrodeposition coating Thus, the rust prevention pretreatment of the object to be coated, which was formed as a pretreatment in the conventional method, is unnecessary. Therefore, according to the present invention, a cation according to the present invention is usually applied to a coated product that has been degreased and washed with water without undergoing a pretreatment step (usually a zinc phosphate treatment step) that has been an essential step in the past. The formation of the electrodeposition coating film is completed simply by immersing it in the electrodeposition coating composition and changing the applied voltage. In a normal process, a water washing process is necessary even after pretreatment (zinc phosphate treatment), but this process is also unnecessary. Therefore, the number of processes can be greatly reduced, the processing time can be shortened, and the number of places used for unnecessary processes can be reduced.

Cationic electrodeposition coating composition The multilayer coating film forming method of the present invention comprises an aqueous cationic electrode comprising (A) an electrodepositable rust preventive metal salt, (B) a base resin having a cationic group, and (C) a curing agent. It is carried out using a coating composition. Hereinafter, the cationic electrodeposition coating composition used in the present invention will be described in detail.

  The electrolytically deposited rust-proof metal salt (A) contained in the cationic electrodeposition coating composition used in the present invention comprises (a1) a rare earth metal salt having a lower precipitation pH than zinc, (a2) a zinc salt, and (a3) zinc. It consists of three kinds of metal salts with higher precipitation pH. To measure the precipitation pH, a metal salt solution is first prepared. When a 0.1 N KOH solution is dropped into the metal salt solution, the pH changes. At this time, a titration curve is prepared, and the pH at the inflection point that occurs until the pH rises and stabilizes is defined as the precipitation pH.

  The rare earth metal of the rare earth metal salt (a1) having a lower precipitation pH than zinc is yttrium (Y), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu). ) And mixtures thereof. As the rare earth metal salt (a1) having a lower precipitation pH than zinc, a salt that is water-soluble or hardly soluble in water can be used. In particular, the use of a water-soluble salt having a solubility in water of 1 g / dm 3 or more is preferable because a high corrosion resistance effect can be obtained with a small amount of use. Examples of rare earth metal salts (a1) having a lower precipitation pH than zinc include ytterbium formate, ytterbium acetate, ytterbium lactate, ytterbium oxalate, yttrium formate, yttrium acetate, yttrium lactate, yttrium oxalate, dysprosium formate, dysprosium acetate, lactic acid Dysprosium, dysprosium oxalate, holmium formate, holmium acetate, holmium lactate, holmium oxalate, erbium formate, erbium acetate, erbium lactate, erbium oxalate, thulium formate, thulium acetate, thulium lactate, thulium oxalate, lutetium acetate, lutetium acetate Organic acid salts such as lutetium lactate and lutetium oxalate; ytterbium nitrate, ytterbium tungstate, ytterbium molybdate, amide sulfate Terbium, ytterbium oxide, ytterbium hydroxide, yttrium nitrate, yttrium tungstate, yttrium molybdate, yttrium amidosulfate, yttrium oxide, yttrium hydroxide, dysprosium nitrate, dysprosium tungstate, dysprosium molybdate, dysprosium amidosulfate, dysprosium oxide, water Dysprosium oxide, holmium nitrate, holmium tungstate, holmium molybdate, holmium amidosulfate, holmium oxide, holmium hydroxide, erbium nitrate, erbium tungstate, erbium molybdate, erbium amidosulfate, erbium oxide, erbium hydroxide, thulium nitrate, Thulium tungstate, thulium molybdate, thulium amidosulfate, thulium oxide Um, thulium hydroxide, nitrate lutetium, tungsten lutetium, molybdate lutetium, amide sulfate lutetium, lutetium oxide, and inorganic acid salts or inorganic compounds such as lutetium hydroxide. Among these, a more preferable rare earth metal salt (a1) is each salt of ytterbium (Yb) having a high electrolytic precipitation property.

  The zinc salt (a2) can be used regardless of whether it is water-soluble or sparingly soluble in water. In particular, a water-soluble salt having a solubility in water of 1 g / dm 3 or more is preferable because a high corrosion resistance effect can be obtained with a small amount of use. Examples of the zinc salt (a2) that can be used include water-soluble salts such as carboxylate, zinc nitrate, and zinc sulfate. Further, a composite compound of zinc oxide and condensed zinc phosphate that generates zinc ions in the cationic electrodeposition coating composition, (poly) zinc phosphate, zinc phosphomolybdate, and the like can also be used. These zinc compounds can generally be used as pigments.

  The metal of the metal salt (a3) having a higher precipitation pH than zinc is composed of nickel (Ni), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), and mixtures thereof. Selected from the group. As the metal salt (a3) having a higher precipitation pH than zinc, both salts that are water-soluble or sparingly soluble in water can be used. In particular, when a water-soluble salt having a solubility in water of 1 g / dm 3 or more is used, a high corrosion resistance effect is obtained with a small amount of use, which is preferable. Examples of the metal salt (a3) having a higher precipitation pH than zinc include, for example, neodymium formate, neodymium acetate, neodymium lactate, neodymium oxalate, nickel formate, nickel acetate, nickel lactate, nickel oxalate, cerium formate, cerium acetate, Organic acid salts such as cerium lactate, cerium oxalate, praseodymium formate, praseodymium acetate, praseodymium lactate, praseodymium oxalate, lanthanum formate, lanthanum acetate, lanthanum lactate, lanthanum oxalate, samarium formate, samarium acetate, samarium lactate, samarium oxalate ; Neodymium nitrate, neodymium tungstate, neodymium molybdate, neodymium amidosulfate, neodymium oxide, neodymium hydroxide, nickel nitrate, nickel tungstate, nickel molybdate, nickel amidosulfate, nickel oxide, dihydroxide Kel, cerium nitrate, cerium tungstate, cerium molybdate, cerium amidosulfate, cerium oxide, cerium hydroxide, praseodymium nitrate, praseodymium tungstate, praseodymium molybdate, praseodymium amidosulfate, praseodymium oxide, praseodymium hydroxide, lanthanum nitrate, tungsten Examples include inorganic acid salts or inorganic compounds such as lanthanum acid, lanthanum molybdate, lanthanum amidosulfate, lanthanum oxide, lanthanum hydroxide, samarium nitrate, samarium tungstate, samarium molybdate, samarium amidosulfate, samarium oxide, and samarium hydroxide. be able to. Among these, more preferred rare earth metal compounds are neodymium (Nd) and praseodymium (Pr) compounds having high electrolytic deposition properties.

  The electrolytically depositable rust preventive metal salt (A) of the present invention is 100 to 20,000 ppm, preferably 100 to 5000 ppm, more preferably 200 to 3000 ppm in terms of metal in the cationic electrodeposition coating composition. Blended in quantity. When it is less than 100 ppm, the entire rust prevention property is not sufficient. If the amount exceeds 20,000 ppm, the improvement in rust prevention property is not recognized, and adverse effects may occur.

  When the metal amount of each component (a1), (a2) and (a3) in the electrolytically depositable rust-proof metal salt (A) of the present invention is 100% by mass, the component (a1) Is 5 to 40% by mass, preferably 10 to 30% by mass, component (a2) is 40 to 90% by mass, preferably 40 to 70% by mass, and component (a3) is 5 to 40% by mass, Preferably it is 10-30 mass%. When the amount of the rare earth metal component (a1) is less than 5% by mass, the antirust property against salt spray and salt water immersion is insufficient among the antirust properties. Even if the amount of the component (a1) is more than 40% by mass, no improvement in rust prevention is observed. When the amount of the component (a2) and the component (a3) is less than the lower limit, the rust prevention property against the combined cycle corrosion is insufficient, but conversely, when the amount of the components (a2) and (a3) is larger than the upper limit, The improvement in rust prevention that was seen in the rise will not be seen.

  Introduction of the electrolytically depositable rust-proof metal salt (A) into the aqueous cationic electrodeposition coating composition is not particularly limited, and can be carried out in the same manner as in a normal pigment dispersion method. For example, the electrolytic depositing rust preventive metal salt (A) can be dispersed in advance in a dispersing resin to prepare a dispersed paste, and this dispersed paste can be blended into the aqueous cationic electrodeposition coating composition. When a water-soluble metal compound is used as the electrolytically depositable rust-proof metal salt (A), it may be added as it is after the preparation of the resin emulsion for paint. In addition, as a resin for pigment dispersion, a general one for cationic electrodeposition coating (epoxy sulfonium salt type resin, epoxy type quaternary ammonium salt type resin, epoxy type tertiary amine type resin, acrylic type quaternary ammonium salt) Mold resin).

  The base resin having a cationic group (B) used in the aqueous cationic electrodeposition coating composition of the present invention is a cation-modified epoxy resin obtained by modifying an oxirane ring in a resin skeleton with an organic amine compound. In general, a cation-modified epoxy resin is produced by ring-opening an oxirane ring in a starting material resin molecule by a reaction with an amine such as a primary amine, a secondary amine, or a tertiary amine acid salt. A typical example of the starting material resin is a polyphenol polyglycidyl ether type epoxy resin which is a reaction product of a polycyclic phenol compound such as bisphenol A, bisphenol F, bisphenol S, phenol novolak, cresol novolak, and epichlorohydrin.

  Examples of other starting material resins include the following formula described in JP-A-5-306327.

[Wherein, R represents a residue obtained by removing a glycidyloxy group of a diglycidyl epoxy compound, R ′ represents a residue obtained by removing an isocyanate group from a diisocyanate compound, and n represents a positive integer. An oxazolidone ring-containing epoxy resin represented by the following formula may be used as a cation-modified epoxy resin. This is because a coating film having excellent heat resistance and corrosion resistance can be obtained.

  The starting material resin can be used by extending the chain with a bifunctional polyester polyol, polyether polyol, bisphenol, dibasic carboxylic acid or the like before the oxirane ring-opening reaction with amines.

  Similarly, prior to the ring-opening reaction of the epoxy ring with amines, 2-ethylhexanol, nonylphenol, ethylene glycol mono-monomer for some epoxy rings for the purpose of adjusting the molecular weight or amine equivalent and improving heat flow. Monohydroxy compounds such as 2-ethylhexyl ether and propylene glycol mono-2-ethylhexyl ether can also be added and used.

  Examples of amines that can be used for opening an oxirane ring and introducing an amino group include butylamine, octylamine, diethylamine, dibutylamine, methylbutylamine, monoethanolamine, diethanolamine, N-methylethanolamine, triethylamine And primary, secondary, or tertiary amine salts such as acid salts and N, N-dimethylethanolamine salts. In addition, ketimine block primary amino group-containing secondary amines such as aminoethylethanolamine / methyl isobutyl ketimine and diethylenetriamine / methyl isobutyl diketimine can also be used. When these are modified into a resin, they are easily hydrolyzed to produce a primary amino group when an aqueous cationic electrodeposition coating composition is prepared using the resin. These amines must be reacted with at least an equivalent amount relative to the oxirane ring in order to open all the oxirane rings.

  The number average molecular weight of the cation-modified epoxy resin is 1,000 to 5,000, preferably 1,500 to 3,000. When the number average molecular weight is less than 1,000, physical properties such as solvent resistance and corrosion resistance of the cured coating film may be inferior. On the other hand, when it exceeds 5,000, it is difficult to control the viscosity of the resin solution and it is difficult to synthesize, and it may be difficult to handle in the operation such as emulsification dispersion of the obtained resin. Furthermore, because of its high viscosity, the flowability during heating and curing is poor, and the appearance of the coating film may be significantly impaired.

  The cation-modified epoxy resin is preferably molecularly designed such that the hydroxyl value (mg / g resin solid content in terms of KOH) is in the range of 50 to 250. When the hydroxyl value is less than 50, the coating film is poorly cured. On the other hand, when it exceeds 250, water resistance may be deteriorated as a result of excess hydroxyl groups remaining in the coating film after curing.

  The cation-modified epoxy resin is preferably molecularly designed so that the amine value (KOH conversion mg / g resin solid content) is in the range of 40 to 150. If the amine value is less than 40, the acid neutralization causes poor emulsification and dispersion in an aqueous medium. On the other hand, if it exceeds 150, water resistance may decrease as a result of excess amino groups remaining in the coating film after curing. is there. A more preferred amine value is 50 to 120.

  Moreover, it is more preferable that the amine value based on the primary amino group in the resin is 15 to 50 in order to improve the selective precipitation of the rare earth metal compound in the cathode electrolysis (pretreatment) step.

  Further, (b) a monovalent active hydrogen compound and (c) a divalent active hydrogen compound are added to the epoxy resin containing a plurality of oxazolidone rings in the molecular chain of the above formula [Chemical Formula 1] of the starting resin (a). After reacting in an equivalent ratio of less than 1 to the epoxy group, (d) gallic acid and (e) the secondary monoamine compound remain in the reaction products of (a), (b) and (c). A resin for water-based paints characterized by reacting the reaction product so as to ring-open the epoxy group may be used.

  This is because, in addition to the inclusion of the oxazolidone ring in the epoxy base resin, the chelating effect by gallic acid modification and the extremely high corrosion resistance and heat resistance due to the reduction action to the metal material of the coated material in an alkaline atmosphere can be combined. .

  As the curing agent (C) in the present invention, any kind of curing agent can be used as long as each resin component can be cured at the time of heating. Among them, a block polyisocyanate suitable as a curing agent for an electrodeposition resin. Is recommended.

  Examples of the polyisocyanate that is a raw material of the block polyisocyanate include aliphatic diisocyanates such as hexamethylene diisocyanate (including trimer), tetramethylene diisocyanate, and trimethylhexamethylene diisocyanate, isophorone diisocyanate, 4,4′- Examples thereof include alicyclic polyisocyanates such as methylene bis (cyclohexyl isocyanate), and aromatic diisocyanates such as 4,4′-diphenylmethane diisocyanate, tolylene diisocyanate, and xylylene diisocyanate. The blocked polyisocyanate can be obtained by blocking these with an appropriate sealant.

  Examples of the sealant include monovalent alkyl (or aromatic) alcohols such as n-butanol, n-hexyl alcohol, 2-ethylhexanol, lauryl alcohol, phenol carbinol, and methylphenyl carbinol; ethylene glycol Cellosolves such as monohexyl ether and ethylene glycol mono-2-ethylhexyl ether; polyether-type double-end diols such as polyethylene glycol, polypropylene glycol and polytetramethylene ether glycol phenol; ethylene glycol, propylene glycol, 1,4-butanediol and the like Polyester-type double-ended polyols obtained from diols of the above and dicarboxylic acids such as oxalic acid, succinic acid, adipic acid, suberic acid, sebacic acid; para-t-butylphenol; Phenols such as resol; dimethyl ketoxime, methyl ethyl ketoxime, methyl isobutyl ketoxime, methyl amyl ketoxime, oximes such as cyclohexanone oxime, and ε- caprolactam, lactams typified by γ- butyrolactam is preferably used. In particular, the sealants of oximes and lactams dissociate at a low temperature, and therefore are suitable from the viewpoint of resin curability when co-baking with an intermediate coating film in a subsequent step.

  It is desirable that the block polyisocyanate be blocked beforehand by using a sealing agent alone or by using a plurality of types. The blocking rate is preferably set to 100% in order to ensure the storage stability of the paint unless there is a purpose of denaturing reaction with the resin components.

  The blending ratio of the block polyisocyanate to the base resin having a cationic group (B) varies depending on the degree of crosslinking required for the purpose of use of the cured coating film, but the coating film properties and intermediate coating compatibility are different. In consideration of the solid content, a range of 15 to 40% by mass is preferable. If the blending ratio is less than 15% by mass, the coating film may be poorly cured. As a result, the physical properties of the coating film such as mechanical strength may be lowered, and the coating film may be affected by paint thinner during intermediate coating. May invite. On the other hand, if it exceeds 40% by mass, it may be excessively cured, resulting in poor coating properties such as impact resistance. In addition, block polyisocyanate may be used in combination of two or more kinds for convenience of coating film physical properties, degree of curing and adjustment of curing temperature.

  (B) The base resin having a cationic group is obtained by converting an amino group in the resin to an appropriate amount of inorganic acid such as hydrochloric acid, nitric acid, hypophosphorous acid, or organic such as formic acid, acetic acid, lactic acid, sulfamic acid, and acetylglycine acid. It is prepared by neutralizing with an acid and emulsifying and dispersing in water as a cationized emulsion. When emulsifying and dispersing, usually, emulsion particles containing (C) a curing agent as a core and (B) a base resin as a shell are formed.

  The average particle diameter of the emulsion particles is 0.01 to 0.5 μm, preferably 0.02 to 0.3 μm, and more preferably 0.05 to 0.2 μm. If the average particle size is less than 0.01 μm, the neutralizing agent necessary for water-dispersing the resin component becomes excessive, and the electrodeposition efficiency per a certain amount of electricity decreases. On the other hand, when the average particle diameter exceeds 0.5 μm, the dispersibility of the particles is lowered, and therefore the storage stability of the electrodeposition paint is lowered, which is not preferable.

  The aqueous cationic electrodeposition coating composition used in the coating method of the present invention is not necessarily a necessary component, but may further contain a pigment depending on the purpose. However, the term “pigment” here does not include electrolytically precipitated rust-proof metal salts. As the pigment, any pigment that is usually used in paints can be used without particular limitation. Examples include color pigments such as carbon black, titanium dioxide, and graphite, body pigments such as kaolin, aluminum silicate (clay), talc, calcium carbonate, and inorganic colloids (such as silica sol, alumina sol, titanium sol, and zirconia sol), phosphorus Examples include heavy metal-free rust preventive pigments such as acid pigments (aluminum phosphomolybdate, (poly) zinc phosphate, calcium phosphate, etc.) and molybdate pigments (aluminum phosphomolybdate, zinc phosphomolybdate, etc.).

  Furthermore, silane coupling agents such as vinyltriethoxysilane and γ-methacryloxypropyltrimethoxysilane can also be used. When these inorganic colloids and silane coupling agents are used in combination, there is an advantage that the effect of improving the adhesion of the base coating film and the effect of improving the corrosion resistance as a result.

  Among these, titanium dioxide, carbon black, aluminum silicate (clay), silica, aluminum phosphomolybdate, and zinc polyphosphate are particularly important as pigments used in the aqueous cationic electrodeposition coating composition of the present invention. In particular, titanium dioxide and carbon black are most suitable for electrodeposition coatings because they have high concealability as color pigments and are inexpensive.

  In addition, although the said pigment can also be used independently, it is common to use multiple types according to the objective.

  Mass ratio {P / (P + V)} of the pigment to the total mass (P + V) of the pigment (P) and resin solid content (V) contained in the aqueous cationic electrodeposition coating composition (hereinafter referred to as PWC) ) Is preferably in the range of 5 to 30% by mass. However, it is defined that the pigment as referred to herein does not contain the electrolytically precipitated rust-proof metal salt (A).

  If the above mass ratio is less than 5% by mass, the barrier property against corrosion factors such as water and oxygen with respect to the coating film is excessively reduced due to insufficient pigment, and weather resistance and corrosion resistance at a practical level may not be exhibited.

  However, if such inconvenience does not occur, the pigment concentration may be set to zero as much as possible, and a clear or nearly clear aqueous cationic electrodeposition coating composition may be prepared and used in the present invention.

  Further, if the above mass ratio exceeds 30% by mass, an excess of pigment causes an increase in viscosity at the time of curing, and the flow property is lowered and the appearance of the coating film may be remarkably deteriorated.

  The resin solid content (V) is an electrodeposition coating film containing a pigment dispersion resin in addition to (B) a base resin having a cationic group, which is a main resin of an aqueous cationic electrodeposition coating, and (C) a curing agent. The total solid content of all the resin binders constituting

  The aqueous cationic electrodeposition coating composition is adjusted such that the total solid content concentration is 5 to 40% by mass, preferably 10 to 25% by mass. An aqueous medium (water alone or a mixture of water and a hydrophilic organic solvent) is used to adjust the total solid concentration.

  Further, a small amount of additives may be introduced into the coating composition. Examples of additives include UV absorbers, antioxidants, surfactants, coating surface smoothers, curing catalysts (dibutyltin oxide, dioctyltin dilaurate, dibutyltin dilaurate, dioctyltin dilaurate, dibutyltin diacetate, dibutyltin Organic tin compounds such as dibenzoate or dioctyltin dibenzoate) and curing accelerators (zinc acetate).

Multi-layer coating film forming method In the multi-layer coating film forming method of the present invention, coating is performed by immersing an object to be coated in the aqueous cationic electrodeposition coating composition. And the multilayer coating film formation method of the present invention comprises:

  In the aqueous cationic electrodeposition coating composition, an electrolytic deposition step in which a voltage of less than 50 V is applied using the article to be coated as a cathode, and

  The water-based cationic electrodeposition coating composition includes an electrodeposition coating step in which a voltage of 50 to 450 V is applied using the article to be coated as a cathode.

  Examples of the object to be coated include untreated metal materials such as cold rolled steel sheet, high strength steel, high tensile steel, cast iron, zinc and galvanized steel, aluminum and aluminum alloy. Among these, a cold-rolled steel sheet is a material that can obtain a particularly excellent corrosion resistance effect by the method of the present invention.

  In the aqueous cationic electrodeposition coating composition prepared by the above method, the article to be coated is immersed as a cathode. In the electrolytic deposition step, by applying a voltage of less than 50 V and cathodic electrolysis is performed on the object to be coated, the electrolytic reaction product of the electrolytically precipitated rust-proof metal salt (A) It has been found by the present invention that it can be preferentially deposited.

  When the applied voltage is 50 V or more, the precipitation of the base resin having a cationic group (B) and the curing agent (C), which is a paint vehicle, rather than the precipitation of the composite metal hydroxide becomes prominent. This is not preferable because it is contrary to the purpose of film formation.

  The applied voltage in the electrolytic deposition step that enables selective deposition of the electrolytic reaction product of the electrolytically depositable rust-proof metal salt (A) is preferably 1 to 40V, and more preferably 1 to 20V.

  In the electrolytic deposition step, the bath temperature of the bath containing the aqueous cationic electrodeposition coating composition is preferably adjusted to 15 to 35 ° C. The relationship with the electrodeposition coating process performed continuously after the electrodeposition process is that the electrodeposition process is performed at a temperature similar to the bath temperature normally used in electrodeposition coating performed after the electrodeposition process. This is because it is preferable.

  The energization time in the electrolytic deposition step is usually 10 to 300 seconds, preferably 30 to 180 seconds. If the treatment time is too short, the film is not formed, or even if it is formed, the thickness is insufficient, and the corrosion resistance may be inferior. If the energization time is too long, an appearance defect called matte burn or burnt sometimes occurs. In addition, an excessive treatment time is not preferable because the productivity may be extremely reduced.

A specific high rust preventive film can be formed by setting the amount of the electrolytic reaction product of the electrolytic depositable rust preventive metal salt (A) in the electrolytic deposition step to 5 mg / m ² or more. . A preferable precipitation amount is 10 to 1000 mg / m ² , preferably 10 to 500 mg / m ² .

If it is less than 5 mg / m ² , the underlying adhesion due to the formed film is lowered, so that the necessary rust preventive properties are not exhibited. On the other hand, if it exceeds 1000 mg / m ² , the surface smoothness of the film is impaired, and the appearance after the electrodeposition coating film formation may be deteriorated.

The mechanism by which the electrolytic product is deposited by the electrolytic deposition process of the present invention is considered as follows. Depending on the electrolysis conditions in the electrolytic deposition process, chemical species in the bath such as dissolved oxygen, hydrogen ions, and water are reduced on the metal surface of the cathode to generate hydroxide ions (OH ⁻ ). The hydroxide ions generated on the surface of the metal to be treated first react with the electrolytically precipitated rust-proof metal ions in the vicinity of the metal surface, so that a precipitate of electrolytically precipitated rust-proof metal hydroxide is generated. , Deposited on the metal surface as a film. A film made of the rare earth metal hydroxide, which is an electrolytic product thus deposited, is particularly excellent in adhesion to the base material and the electrodeposition coating film, and at least in the baking and drying process after electrodeposition coating. It is considered that a part of the film changes from a rare earth hydroxide to a film made of oxide dehydrated and exhibits high corrosion resistance.

  Moreover, in the electrolytic conditions of the electrolytic deposition step of the present invention, the electrodeposited metal film is mainly formed preferentially, and (B) electrodeposition by deposition of a base resin having a cationic group and (C) a curing agent. Since the formation of the coating film tends to be suppressed, it is very convenient.

  In the electrodeposition coating process of the present invention, the applied voltage is increased to 50 to 450 V, preferably 100 to 400 V, so that (B) a base resin having a cationic group and (C) a curing agent, which is a paint vehicle, and The corresponding pigment can be preferentially deposited. If the applied voltage is less than 50 V, the precipitation of the vehicle component of the electrodeposition paint may be insufficient. On the other hand, when the applied voltage exceeds 450 V, the above-mentioned vehicle component is deposited in excess of an appropriate amount, and as a result, a film appearance that cannot be put into practical use may be exhibited.

  The energization time is 30 to 300 seconds, preferably 30 to 180 seconds. When the treatment time is shorter than 30 seconds, the electrodeposition coating film is not generated, or even if it is generated, the thickness is insufficient, so that the corrosion resistance may be inferior. Further, excessive treatment time is not preferable because it may cause extremely low productivity.

  By subjecting the uncured multilayer coating film thus obtained to a curing reaction at 120 to 200 ° C., preferably 140 to 180 ° C., an electrodeposition cured coating film having a high degree of crosslinking can be obtained. However, if it exceeds 200 ° C., the coating film becomes excessively hard and brittle, while if it is less than 120 ° C., curing is not sufficient, and film physical properties such as solvent resistance and film strength may be lowered.

  The following examples further illustrate the present invention, but the present invention is not limited thereto. In the examples, “parts” and “%” are based on mass unless otherwise specified.

Production Example 1 ((B) Production of base resin having amino group)
In a reaction vessel equipped with a stirrer, decanter, nitrogen inlet tube, thermometer and dropping funnel, 2400 parts of bisphenol A type epoxy resin (trade name DER-331J, manufactured by Dow Chemical Company) with an epoxy equivalent of 188, 141 parts of methanol, methyl isobutyl 168 parts of ketone and 0.5 part of dibutyltin dilaurate were charged and stirred at 40 ° C. to uniformly dissolve, then 320 parts of 2,4- / 2,6-tolylene diisocyanate (80/20 mass ratio mixture) was added. When dripped over 30 minutes, heat was generated and the temperature rose to 70 ° C. To this was added 5 parts of N, N-dimethylbenzylamine, the temperature in the system was raised to 120 ° C., and the reaction was continued at 120 ° C. for 3 hours until the epoxy equivalent reached 232 while distilling off methanol. Further, 644 parts of methyl isobutyl ketone, 341 parts of bisphenol A and 413 parts of 2-ethylhexanoic acid were added, the temperature in the system was kept at 120 ° C., and the reaction was continued until the epoxy equivalent reached 840, and then the temperature in the system was Cooled to 110 ° C. Subsequently, a mixture of 288 parts of diethylenetriamine diketimine (methyl isobutyl ketone solution having a solid content of 73%), 300 parts of N-methylethanolamine and 314 parts of di (2-ethylhexyl) amine was added and reacted at 120 ° C. for 1 hour. Then, it diluted with 194 parts of methyl isobutyl ketone until it became 80 mass% of non volatile matters, and obtained the cation modified epoxy resin varnish with a solid content of 80 mass%. The number average molecular weight of this resin was 1,800, the amine value was 100, of which the amine value based on the primary amino group was 20, and the hydroxyl value was 160. Moreover, it was confirmed from measurement of an infrared absorption spectrum etc. that it has an oxazolidone ring (absorption wave number; 1750 cm <-1>) in resin.

Production Example 2 (Production of (C) Curing Agent)
Put 222 parts of isophorone diisocyanate in a reaction vessel equipped with a stirrer, nitrogen introduction pipe, cooling pipe and thermometer, dilute with 56 parts of methyl isobutyl ketone, add 0.2 part of butyltin laurate, and heat up to 50 ° C. 17 parts of methyl ethyl ketoxime was added such that the temperature of the contents did not exceed 70 ° C. And it is kept at 70 ° C. for 1 hour until the absorption of the isocyanate residue is substantially disappeared by infrared absorption spectrum, and then diluted with 43 parts of n-butanol to obtain the desired blocked isocyanate curing agent solution (solid content: 70%). Got.

Production Example 3 (Production of pigment-dispersed resin)
A reaction vessel equipped with a stirrer, a cooling tube, a nitrogen introduction tube, and a thermometer was charged with 710 parts of bisphenol A type epoxy resin (trade name Epon 829, manufactured by Shell Chemical Co., Ltd.) having an epoxy equivalent of 198, and 289.6 parts of bisphenol A. The mixture was reacted at 150 to 160 ° C. for 1 hour in an atmosphere, then cooled to 120 ° C., and 406.4 parts of a methyl isobutyl ketone solution of 2-ethylhexanolized half-blocked tolylene diisocyanate (solid content 95%) was added. After maintaining the reaction mixture at 110-120 ° C. for 1 hour, 1584.1 parts of ethylene glycol mono n-butyl ether was added. And it cooled to 85-95 degreeC and made it uniform.

  In parallel with the production of the above reaction product, 104.6 parts of dimethylethanolamine was added to 384 parts of a methyl isobutyl ketone solution of 2-ethylhexanolated half-blocked tolylene diisocyanate (solid content 95%) in another reaction vessel. The mixture was stirred at 80 ° C. for 1 hour, and then 141.1 parts of 75% aqueous lactic acid was added. Further, 47.0 parts of ethylene glycol mono-n-butyl ether was mixed and stirred for 30 minutes to form a quaternizing agent (solid content: 85% ). Then, 620.46 parts of this quaternizing agent is added to the previous reaction product, and the mixture is kept at 85 to 95 ° C. until an acid value of 1 is reached, and a resin solution of pigment dispersion resin (average molecular weight 2200) (resin solid content 56%) )

Production Example 4 (Production of pigment dispersion paste)
Using a sand mill, a pigment paste (solid content 59%) containing the pigment-dispersed resin obtained in Production Example 3 was dispersed and prepared at 40 ° C. until the particle size became 5 μm or less.
Pigment-dispersed resin varnish 53.6 in Preparation Part Production Example 3
Titanium dioxide 54.0
Carbon black 1.0
Aluminum phosphomolybdate 4.0
Clay 11.0
Ion exchange water 46.4

Production Example 5 (Production of aqueous cationic electrodeposition coating composition)
350 g (solid content) of the base resin obtained in Production Example 1 and 150 g (solid content) of the curing agent obtained in Production Example 2 were mixed, and ethylene glycol mono-2-ethylhexyl ether was 3% based on the solid content ( 15 g).

  Next, neutralize glacial acetic acid to a neutralization rate of 40.5%, add ion-exchanged water to dilute slowly, and then remove methyl isobutyl ketone under reduced pressure to a solid content of 36%. did.

  To the thus obtained 2000 g of the emulsion, 460.0 g of the various pigment pastes obtained in Production Example 4, 2252 g of ion-exchanged water, and 1 wt% dibutyltin oxide based on the resin solid content were added and mixed. A 20.0 wt% aqueous cationic electrodeposition coating was prepared.

  In the case of a water-soluble salt, the electrolytically depositable rust-proof metal salt is added directly to the paint, and in other cases, a part of titanium dioxide in the pigment paste is replaced, and added as a metal shown in Tables 1 to 4 Each aqueous cationic electrodeposition coating composition was prepared by adjusting the amount (% by mass).

Examples 1-10 and Comparative Examples 1-10
In the preparation of each aqueous cationic electrodeposition coating composition described in Production Example 5, as shown in Tables 1 to 4, each electrolytically deposited rust-proof metal salt is converted to a metal amount in an amount of 88 to 21,000 ppm. included. The aqueous cationic electrodeposition coating composition thus obtained was poured into a bathtub, and a surface untreated cold-rolled steel sheet was immersed as a cathode. Next, as shown in Tables 1 to 4, by increasing the applied voltage in at least two stages, the electrolytic deposition process (applied voltage 5 V, energizing time 60 seconds) and the cathodic electrodeposition coating process (applied voltage 180 V, energized) Time 150 seconds) was carried out continuously. After coating so that the dry film thickness of the electrodeposition coating film in the electrodeposition coating process was 20 μm, it was cured at 170 ° C. for 20 minutes, and the coating film was evaluated. In Tables 1-4, the result of the Example and comparative example in a coating-film test item was shown.

The procedure of the evaluation test is shown below.

Combined corrosion test (Compound environmental corrosion acceleration test)
The heat-cured coating film was cross-cut with a knife so as to reach the substrate, and JASO M609-91 “Automobile Material Corrosion Test Method” was performed 100 cycles. Thereafter, rust and blistering from the crosscut part and blisters on the flat part were observed. The observation evaluation results are shown in Tables 1 to 4.

Evaluation A: The maximum width of rust or swelling is less than 3 mm from the cut part (both sides).
○: The maximum width of rust or swelling is 3 mm or more and less than 5 mm from the cut part (both sides).
Δ: The maximum width of rust or swelling is 5 mm or more and less than 7 mm (both sides) from the cut part, and some blisters are observed in the flat part.
X: The maximum width of rust or blisters is 7 mm or more from the cut part, or blisters are observed on the entire surface.

Salt water immersion test / salt water immersion test: The heat-cured coating film was subjected to a salt water immersion test in 5% saline at 50 ° C. for 840 hours. The evaluation plate after the test was washed with water and dried, and then the tape was peeled off. The peeled area relative to the evaluation surface was indicated in%.
○: peeling area 10% or less, Δ: peeling area less than 30%, x: peeling area 30% or more

  In Examples 1 to 10, high rust prevention can be confirmed in both the salt water immersion test and the combined cycle corrosion test (an example of a combined environmental corrosion acceleration test). In Comparative Example 1, the electrolytic precipitation rust preventive metal is only zinc, and the rust preventive property is poor. Comparative Examples 2 to 8 are examples in which two types of (a1) rare earth metal salt having a lower precipitation pH than zinc, (a2) zinc salt, and (a3) a metal salt having a higher precipitation pH than zinc are used. When a rare earth metal is included, a satisfactory result is obtained in the salt water immersion test, but a high rust prevention property is not necessarily obtained in the combined cycle corrosion test (CCT). Comparative Examples 9 and 10 are examples including (a1) a rare earth metal salt having a lower precipitation pH than zinc, (a2) a zinc salt, and (a3) a metal salt having a higher precipitation pH than zinc. The total blending amount is small, and Comparative Example 10 is an example in which the entire blending amount is high. In Comparative Example 9, both of the salt water immersion test and the combined cycle corrosion test were bad, and in Comparative Example 10, the appearance defect was remarkable, so that both the salt water immersion test and the combined cycle corrosion test could not be evaluated.

  The multi-layer coating film forming method of the present invention comprises an electrolytic deposition step in which a voltage of less than 50 V is applied using the same aqueous cationic electrodeposition coating composition proposed in WO 2006-109862, and a cationic electrode in which a voltage exceeding it is applied. Further improvement in rust prevention is achieved in a method that eliminates the need for a pretreatment step with zinc phosphate or the like of an object to be coated by performing two-step electrodeposition in the coating process. In particular, although the technology of WO 2006-109862 shows excellent rust prevention in the salt spray test and salt water immersion test, the composite environmental corrosion acceleration test did not necessarily show excellent performance, but the composite environmental corrosion acceleration test according to the present invention. But it came to show high rust prevention.

  In addition, the multilayer coating film forming method of the present invention uses the same aqueous cationic electrodeposition coating composition, and is energized at an applied voltage of at least two stages, so that cathodic electrolysis (electrolytic deposition process) and electrolysis are performed. The coating process can be carried out practically and continuously. By the method of the present invention, a multilayer coating film having excellent coating adhesion and corrosion resistance can be obtained while efficiently integrating the electrolytic deposition step and the electrodeposition coating step. In the method of the present invention, conventional pretreatment and electrodeposition coating can be performed using the same aqueous cationic electrodeposition coating composition. Therefore, the method of the present invention can completely omit at least the cleaning step after the pretreatment step. The method of the present invention is also useful for environmental problems derived from waste liquid treatment and the like.

Claims (7)

(A) an electrolytic precipitation rust-preventing metal salt, (B) a base resin having a cationic group, and (C) an aqueous cationic electrodeposition coating composition containing a curing agent;
In the aqueous cationic electrodeposition coating composition, a voltage of less than 50 V is applied using the article to be coated as a cathode, and in the aqueous cationic electrodeposition coating composition, 50 to Electrodeposition coating process that applies 450V voltage,
A method for forming a multilayer coating film, comprising:
The electrodepositable rust preventive metal salt (A) contained in the aqueous cationic electrodeposition coating composition is (a1) a rare earth metal salt having a lower precipitation pH than zinc, (a2) a zinc salt and (a3) a precipitation pH lower than zinc. It consists of three kinds of high metal salts,
The total amount of the electrolytically depositable rust-proof metal salt (A) is 100 to 20,000 ppm in terms of metal in the paint,
When the metal amount of each component (a1), (a2) and (a3) in the electrolytically depositable rust-proof metal salt (A) is 100% by mass of the total metal amount, the component (a1) 5 40% by mass, component (a2) 40-90% by mass and component (a3) 5-40% by mass,
A method for forming a multilayer coating film.   The rare earth metal of the rare earth metal salt (a1) having a lower precipitation pH than zinc is yttrium (Y), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), lutetium (Lu). ) And a mixture thereof. The method of forming a multilayer coating film according to claim 1 selected from the group consisting of these.   The metal of the metal salt (a3) having a higher precipitation pH than zinc is composed of nickel (Ni), lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), and mixtures thereof. The method for forming a multilayer coating film according to claim 1 selected from the group. The amount of the rare earth metal salt (a1), (B) zinc salt (a2) having a lower precipitation pH than zinc, and the electrolytic reaction product of the metal salt (a3) having a higher precipitation pH than zinc, precipitated in the electrolytic deposition step. The method for forming a multilayer coating film according to claim 1, wherein the total is 5 mg / m ² or more. The method for forming a multilayer coating film according to claim 1, wherein the energization time in the electrolytic deposition step is 10 to 300 seconds. The method for forming a multilayer coating film according to claim 1, wherein the energization time in the electrodeposition coating step is 10 to 300 seconds.   The multilayer coating film obtained by the multilayer coating-film formation method in any one of Claims 1-6.