JP4777099B2 - Multi-layer coating formation method - Google Patents

Description

  The present invention relates to a method for forming a multilayer coating film comprising a coating base treatment (pretreatment) step and a cathodic electrodeposition coating step suitable for metal materials, particularly untreated cold-rolled steel sheets.

  An automobile body is made into a product by forming a metal material such as a cold-rolled steel sheet or a galvanized steel sheet into a molded product, and painting and assembling the material. In order to impart such adhesion to the base electrodeposition coating film, such a metal molded article has been conventionally subjected to rust prevention treatment such as zinc phosphate chemical conversion treatment in the coating process.

  Electrodeposition paints are excellent in corrosion resistance and throwing power, and can form a uniform coating film. Therefore, electrodeposition paints are widely used mainly for automobile bodies and parts primers. However, conventional cationic electrodeposition paints can exhibit sufficient corrosion resistance for materials that have been completely pretreated, such as zinc phosphate, but for materials that are insufficiently pretreated. It was difficult to ensure the corrosion resistance.

  In particular, in the conventional zinc phosphate treatment, it is uneconomical because a large amount of precipitation per unit area is required to obtain a sufficient base rust prevention effect, and more sludge is deposited. There were practical problems such as adversely affecting the system.

  In addition, as an electrodeposition paint, we designed a paint that can ensure corrosion resistance even for materials with insufficient pretreatment, and in combination with appropriate pretreatment methods, consider environmental protection, and There is a need to construct an economical optimal ground rust prevention system.

  Therefore, Patent Documents 1 and 2 include at least one rare earth metal ion selected from the group consisting of yttrium (Y) ions, neodymium (Nd) ions, samarium (Sm) ions, and praseodymium (Pr) ions, sulfate ions, In addition, an effective pretreatment method is provided that is applied to a coating base treatment of a metal material obtained by electrolysis using a metal to be treated as a cathode in an aqueous solution containing 0.05 g / L or more of zinc ions.

  Patent Document 3 discloses a cathode electrodeposition coating composition in which a hydrophilic film-forming resin having a cationic group and a curing agent are dispersed in an aqueous medium containing a neutralizing agent, based on the solid content of the coating. And 0.1 to 20% by weight of at least one phosphomolybdate selected from an aluminum salt, a calcium salt and a zinc salt, and 0.01 to 2.0% by weight of a cerium compound as a metal. The cathode electrodeposition coating composition is provided to improve the corrosion resistance of the untreated cold-rolled steel sheet.

However, in the above-mentioned patent documents, even in the combination of the pretreatment method and the coating method using the electrodeposition paint described in each of them, the achievement level is such that the substrate adhesion is equivalent to or better than that of the conventional chemical conversion treatment with phosphate. In addition, practical corrosion resistance after electrodeposition coating, particularly, anti-corrosion performance for automobiles was not sufficiently exhibited.
JP-A-9-249990 JP 2000-64090 A JP-A-8-536637

  In view of the above-mentioned present situation, the present invention is a multilayer in which a very small amount of a pretreatment film and an electrodeposition coating film are sequentially formed as compared with a pretreatment process and a cationic electrodeposition coating process with a conventional chemical conversion liquid and electrodeposition paint. By providing a coating film forming method, the objective is to provide a new base rust prevention method that is highly economical and environmentally safe by expressing excellent coating adhesion and corrosion resistance equivalent to or better than those of conventional processes. And

The present invention includes (A) a pretreatment step of immersing an untreated metal substrate in an aqueous solution containing a rare earth metal nitrate and forming a treatment film having a deposition amount of 1 to 100 mg / m ² made of a rare earth metal compound by cathodic electrolysis. And (B) a method for forming a multilayer coating film comprising a step of cathodic electrodeposition coating of an electrodeposition paint containing an organic acid or inorganic acid salt of a rare earth metal.

In the pretreatment step, usually, after adjusting the bath temperature to 15 to 35 ° C. and carrying out cathodic electrolysis at a load voltage of 1 to 20 V, preferably 1 to 10 V, mainly (A) nitrate of a rare earth metal compound is used. It has been found that a pretreatment film from an aqueous solution containing it can be deposited very preferentially on the metal substrate at a deposition amount of 1 to 100 mg / m ² .

  At that time, if the load voltage is less than 1V, the composite metal hydroxide is insufficiently precipitated, and if the load voltage exceeds 20V, the composite metal hydroxide is not electrolyzed but rather by electrolysis of water. Since the generation of hydrogen gas becomes remarkable, it is not preferable because it is contrary to the purpose of forming the pretreatment film.

  The energization time is 10 to 300 seconds, preferably 30 to 180 seconds. When the treatment time is too short, the film is not formed or the corrosion resistance is inferior because the thickness is insufficient even if the film is formed. When the processing time is longer than 300 seconds, an appearance defect called matte burn or burnt sometimes occurs. Further, excessive processing time is not preferable because productivity is extremely lowered.

  Examples of the untreated metal material to which the coating film forming method of the present invention is applied include cold-rolled steel sheet, high-strength steel, high-tensile steel, cast iron, zinc and galvanized steel, aluminum and aluminum alloy, and the like. In particular, a cold rolled steel sheet is a material in which the rust prevention effect is noticeable.

(A) untreated metal substrate was immersed in an aqueous solution containing nitrates of rare earth metal compound, 1 to 100 mg / ^{m 2} a precipitation amount of a rare earth metal compound by cathodic electrolysis, preferably between 5 to 50 mg / ^{m 2} By this, a specifically high rust preventive film can be formed. When the amount of precipitation is less than 1 mg / m ² , the base adhesion due to the formed film is lowered, so that necessary rust preventive properties are not exhibited. On the other hand, when the amount of precipitation exceeds 100 mg / m ² , the surface smoothness of the film is impaired, and the appearance after the electrodeposition coating film formation may be deteriorated.

  Moreover, as said electrodeposition coating process, when mainly performing cathode electrodeposition coating, the load voltage is 50-450V, Preferably it is 100-, maintaining the bath temperature of the said electrodeposition coating composition at 15-35 degreeC. By setting it to 400 V, mainly (B) precipitates from organic acids or inorganic acid salts of rare earth metals, base resins having cationic groups that are paint vehicles, curing agents and pigments are preferentially deposited. When the load voltage is less than 50V, the deposition property of the vehicle component of the electrodeposition paint is insufficient, and when the load voltage exceeds 450V, the vehicle component is deposited in excess of an appropriate amount, resulting in an unusable film appearance. This is not preferable.

  The energization time is 30 to 300 seconds, preferably 30 to 180 seconds. When processing time is too short for 30 seconds, an electrodeposition coating film does not produce | generate, or even if it produces | generates, since thickness is insufficient, corrosion resistance is inferior. Further, an excessive treatment time exceeding 300 seconds is not preferable because productivity is extremely lowered.

The mechanism by which the pretreatment film is obtained by the pretreatment process of the present invention is considered as follows. Under the above electrolysis conditions, chemical species in the bath such as dissolved oxygen, hydrogen ions, and water are reduced on the metal surface of the cathode, and hydroxide ions (OH ⁻ ) are generated. The hydroxide ions generated on the surface of the metal to be treated first react with the rare earth metal ions in the vicinity of the metal surface, whereby a precipitate of the rare earth metal hydroxide is generated and deposited as a film on the metal surface.

  However, (A) a film formed by immersing an untreated metal substrate in an aqueous solution containing a rare earth metal nitrate and cathodic electrolysis of the rare earth metal nitrate has crystallinity. The target conventional chemical conversion treatment level adhesion and rust prevention after electrodeposition coating are not reached.

  In the present invention, in the following electrodeposition coating process, the rare earth metal ions generated from the organic acid or inorganic acid salt of the rare earth metal (B) are more preferential than the electrodeposition paint because they are more precipitated than the resin vehicle components and pigments. As a result of depositing on the pre-treated base material, the gap between the above crystalline films is filled, and a complex continuous continuous conversion film is formed, and further completed by the electrodeposition coating It is presumed that the multi-layer coating film to be exhibited exhibits excellent adhesion and rust prevention after electrodeposition coating, which are not present in the prior art equivalent to or higher than the target conventional chemical conversion treatment level.

  As described above, in the present invention, the treatment film derived from the rare earth metal compound is characterized in that it is designed to be compositely formed in two stages of the pretreatment process and the electrodeposition process.

In the method for forming a coating film of the present invention, as described above, (A) an untreated metal substrate is immersed in an aqueous solution containing a nitrate of a rare earth metal, and the deposited amount of the rare earth metal compound is 1-100 mg / m ² by cathodic electrolysis. A multi-layer coating film forming method comprising a pre-treatment step of forming a treatment film of (B) and (B) cathodic electrodeposition coating of an electrodeposition coating material containing an organic acid or inorganic acid salt of a rare earth metal. The amount of treatment agent is much smaller than that of automobile base rust prevention process, and innovative pretreatment can be performed without sludge generation. Moreover, since the electrodeposition paint has a part of the chemical conversion treatment function, it is possible to improve the coating adhesion and anticorrosion properties equivalent to or better than those of the conventional pretreatment / electrodeposition process by making both processes continuous. It is possible to obtain a multilayer coating film by a treatment film and an electrodeposition coating film.

(A) In the pretreatment step of the present invention, an untreated metal substrate is immersed in an aqueous solution containing a rare earth metal nitrate, and a treatment film having a deposition amount of 1 to 100 mg / m ² made of a rare earth metal compound is formed by cathodic electrolysis. The aqueous solution used is referred to as “pretreatment aqueous solution”. In addition, the untreated metal substrate here generally refers to a metal substrate immediately after the degreasing step in a pretreatment step including a degreasing step, then a surface conditioning step and a chemical conversion treatment step. Hereinafter, the pretreatment agent aqueous solution used in the present invention will be described in detail. The aqueous pretreatment agent solution used in the present invention contains 0.05 to 5% by weight, preferably 0.1 to 3% by weight of a rare earth metal nitrate in terms of rare earth metal. These nitrates are water-soluble or water-dispersible, and a predetermined amount can be easily dissolved or dispersed in pure water and supplied to the practice of the present invention. If it is less than 0.05% by weight, corrosion resistance based on sufficient base adhesion may not be obtained. If it exceeds 5% by weight, the smoothness of the pretreatment film is lowered, resulting in poor skin after electrodeposition. There is a case.

  The rare earth metal nitrate (A) includes at least one rare earth metal selected from the group consisting of cerium (Ce), yttrium (Y), neodymium (Nd), samarium (Sm), and praseodymium (Pr). Nitrate. Among these, particularly preferred rare earth metal nitrates are cerium nitrate (Ce) and neodymium nitrate (Nd).

  The pH of the aqueous pretreatment agent solution is adjusted within the range of 4 to 7, preferably 4.5 to 6.5. When the pH is less than 4, the electrolytic deposition efficiency and the film appearance are lowered. When the pH exceeds 7, the stability of the rare earth metal ions in the composition tends to decrease. The chemicals used for pH adjustment are inorganic acids such as nitric acid or organic acids such as formic acid and acetic acid when the pH is high, and organic bases such as amine or inorganic bases such as ammonia and sodium hydroxide when the pH is low. It is sufficient to add, and the additive chemicals are not limited.

  The appropriate liquid conductivity of the pretreatment aqueous solution used in the present invention is 1 to 100 mS / cm. If the conductivity is less than 1 mS / cm, the pretreatment may be insufficient. Moreover, when it exceeds 100 mS / cm, since there exists a possibility of causing the external appearance defect of a pretreatment film | membrane, it is unpreferable.

  Next, the electrodeposition paint used in the present invention will be described in detail. The electrodeposition paint used in the present invention contains (B) an organic acid or inorganic acid salt of a rare earth metal, and further includes a base resin having a cationic group, a curing agent, and a pigment as main components to be blended. First, (B) the organic acid or inorganic acid salt of the rare earth metal is selected from the group consisting of cerium (Ce), yttrium (Y), lanthanum (La), neodymium (Nd), samarium (Sm), and praseodymium (Pr). An organic acid or inorganic acid salt compound containing at least one rare earth metal selected from the group consisting of at least one selected from acetic acid, formic acid, lactic acid, sulfamic acid, and hypophosphorous acid. Particularly preferred among these are salt compounds of acetic acid, formic acid or sulfamic acid.

  Preferred organic acids or inorganic acid salts of rare earth metals (B) include cerium acetate, yttrium acetate, neodymium acetate, samarium acetate, praseodymium acetate, cerium formate, yttrium formate, neodymium formate, samarium formate, praseodymium formate, cerium lactate, lactic acid Neodymium, cerium sulfamate, neodymium sulfamate, yttrium sulfamate, samarium sulfamate, praseodymium sulfamate, cerium hypophosphite, neodymium hypophosphite, yttrium hypophosphite, samarium hypophosphite, hypophosphorous acid Examples include praseodymium. Among these, particularly preferable rare earth metals are cerium (Ce) and neodymium (Nd).

  In the present invention, (B) the electrodeposition coating containing a water-soluble rare earth metal salt is 0.005 to 2% by weight, preferably 0.01 to 1% by weight of the rare earth metal in terms of the rare earth metal. Contains compounds. (B) If the content of the organic acid or inorganic acid salt of the rare earth metal in the coating is less than 0.005% by weight, corrosion resistance based on sufficient base adhesion may not be obtained, and if it exceeds 2% by weight, The dispersion stability of the electrodeposition coating composition component, the smoothness of the electrodeposition coating film, and the water resistance may be reduced.

  The method for introducing the organic acid or inorganic acid salt of the (B) rare earth metal into the electrodeposition coating composition is not particularly limited, and can be carried out in the same manner as a normal pigment dispersion method. A dispersion paste can be prepared by dispersing a rare earth metal compound in advance in a dispersion resin and then blended. Or it can disperse | distribute or melt | dissolve and mix | blend as it is after preparation of the resin emulsion for paints, or after preparation of a 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 used in the 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 oxazolidone ring-containing epoxy resins described in JP-A-5-306327. This epoxy resin is obtained by a reaction between a diisocyanate compound or a bisurethane compound obtained by blocking an NCO group of a diisocyanate compound with a lower alcohol such as methanol or ethanol, and epichlorohydrin.

  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-monoxide 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 to open an oxirane ring and introduce 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. A ketimine block primary amino group-containing secondary amine such as aminoethylethanolamine methyl isobutyl ketimine can also be used. 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 in the range of 1,500 to 5,000, preferably 1,600 to 3,000. When the number average molecular weight is less than 1,500, 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 the resin solution. Furthermore, because of the 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 so that the hydroxyl value is in the range of 50 to 250. If the hydroxyl value is less than 50, curing of the stomach is inferior. On the other hand, if it exceeds 250, water resistance may decrease 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 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.

  As the curing agent for electrodeposition coating 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, but among them, it is suitable as a curing agent for electrodeposition coating. Blocked polyisocyanates are recommended. Examples of the polyisocyanate used as the raw material for 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 sealing agent include monovalent alkyl (or aromatic) alcohols such as n-butanol, n-hexyl alcohol, 2-ethylhexanol, lauryl alcohol, phenol carbinol, 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, etc. 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.

  It is desirable that the block polyisocyanate is 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 the cationic group varies depending on the degree of cross-linking required for the purpose of using the cured coating film, but it is solid in consideration of coating film properties and intermediate coating compatibility. The amount is preferably in the range of 15 to 40% by weight. If the blending ratio is less than 15% by weight, 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 damaged by paint thinner during intermediate coating. May invite. On the other hand, if it exceeds 40% by weight, 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.

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

  In the electrodeposition coating composition used in the coating method of the present invention, a pigment may be further blended. Any pigment that can be used in paints can be used without particular limitation. Examples include pigments such as carbon black, titanium dioxide and graphite, extender pigments such as kaolin, aluminum silicate (clay), talc, calcium carbonate, and inorganic colloids (silica sol, alumina sol, titanium sol, zirconia sol, etc.), phosphorus 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.) can be mentioned.

  Among these, titanium dioxide, carbon black, aluminum silicate (clay), silica, aluminum phosphomolybdate, and zinc polyphosphate are particularly important as pigments used in the 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.

  The weight ratio {P / (P + V)} of the pigment to the total weight (P + V) of the pigment (P) and the resin solid content (V) contained in the electrodeposition coating composition (hereinafter referred to as PWC) , Preferably in the range of 5 to 30% by weight. If the weight ratio is less than 5% by weight, the barrier property against corrosion factors such as water and oxygen with respect to the coating film may be excessively reduced due to insufficient pigment, and corrosion resistance at a practical level may not be exhibited. However, when such inconvenience does not occur, the pigment concentration may be set to zero as much as possible, and a clear or nearly clear electrodeposition coating composition may be supplied to the present invention. Further, if the weight ratio exceeds 30% by weight, it is necessary to pay attention because excessive pigment causes an increase in viscosity at the time of curing, resulting in a decrease in flowability and a marked deterioration in the appearance of the coating film. However, the resin solid content (V) means the total solid content of all the resin binders constituting the electrodeposition coating film including the base resin, which is the main resin of the water-based paint, and the curing agent as well as the pigment dispersion resin. Indicates.

  The electrodeposition coating composition is adjusted so that the total solid content concentration is in the range of 5 to 40% by weight, preferably 10 to 25% by weight. 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).

  After the above-described electrodeposition coating, an electrodeposition-cured coating film having a high degree of crosslinking can be obtained by performing a curing reaction at 120 to 200 ° C., preferably 140 to 180 ° C. 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 properties such as solvent resistance and film strength are unfavorable.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to these examples.

Production Example 1 (Production Example of Pretreatment Aqueous Solution)
After a predetermined amount of rare earth metal carbonate or hydroxide is dispersed in ion-exchanged water in a reaction vessel equipped with a stirrer, a cooling pipe and a thermometer, nitric acid or the like, which becomes a counter ion of the metal salt, is heated and stirred. An acid such as acetic acid was added and dissolved to prepare a rare earth metal salt aqueous solution having a metal ion concentration of 5%. The obtained solution was adjusted to a pH of 4 to 7 with an aqueous ammonia solution or an aqueous sodium hydroxide solution, and then diluted to a predetermined concentration with ion-exchanged water to obtain a pretreatment agent aqueous solution. In addition, it showed in Tables 2-6 about the pretreatment agent aqueous solution applied to the test, the acid seed | species of a salt compound, and the conductivity of a process liquid.

Production Example 2 (Production of a base resin having a cationic 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 Co.) with an epoxy equivalent of 188, 141 parts of methanol, methyl 168 parts of isobutyl ketone and 0.5 part of dibutyltin dilaurate are added and stirred at 40 ° C. to dissolve uniformly. Then, 320 parts of 2,4- / 2,6-tolylene diisocyanate (80/20 weight ratio mixture) Was added dropwise over 30 minutes, and 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 500 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 1070. Cooled to 110 ° C. Subsequently, a mixture of 241 parts of diethylenetriamine diketimine (methyl isobutyl ketone solution having a solid content of 73%) and 192 parts of N-methylethanolamine was added and reacted at 110 ° C. for 1 hour to obtain a cation-modified epoxy resin. The number average molecular weight of this resin was 2,100, the amine value = 74, and the hydroxyl value was 160. Moreover, it was confirmed from the measurement of an infrared absorption spectrum etc. that the resin has an oxazolidone ring (absorption wave number: 1750 cm ⁻¹ ).

Production Example 3 (Production of curing agent for electrodeposition paint)
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. Later, 17 parts of methyl ethyl ketoxime was added such that the temperature of the contents did not exceed 70 ° C. Then, by the infrared absorption spectrum, the mixture is kept at 70 ° C. for 1 hour until the absorption of the isocyanate residue substantially disappears, and then diluted with 43 parts of n-butanol to obtain the desired blocked isocyanate curing agent solution (solid content: 70%). Got.

Production Example 4 (Production of pigment-dispersed resin)
A reaction vessel equipped with a stirrer, a cooling pipe, a nitrogen introduction pipe, and a thermometer was charged with 710 parts of a bisphenol A type epoxy resin having an epoxy equivalent of 198 (trade name Epon 829, manufactured by Shell Chemical Co., Ltd.) and 289.6 parts of bisphenol A, The mixture was reacted at 150 to 160 ° C. for 1 hour under a nitrogen atmosphere, then cooled to 120 ° C., and 406.4 parts of methyl isobutyl ketone solution of 2-ethylhexanolated half-blocked tolylene diisocyanate (solid content 95%) was added. After holding the reaction mixture at 110-120 ° C. for 1 hour, ethylene glycol mono n-butyl ether 1584.1 was added. And it cooled to 85-95 degreeC and made it uniform.

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

Production Example 5 (Production of pigment dispersion paste for electrodeposition paint)
Using a sand mill, a pigment paste (solid content 59%) having the composition shown in Table 1 including the pigment-dispersed resin obtained in Production Example 4 was dispersed and prepared at 40 ° C. until the particle size became 5 μm or less.

Production Example 6 (Production of electrodeposition coating composition)
350 g (solid content) of the base resin obtained in Production Example 2 and 150 g (solid content) of the curing agent obtained in Production Example 3 are mixed, and 3% of ethylene glycol mono-2-ethylhexyl ether is added to 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 emulsion 2000 g obtained in this way, 460.0 g of various pigment pastes obtained in Production Example 4, 2252 g of ion-exchanged water, and 1% by weight of dibutyltin oxide based on the resin solid content were added and mixed. An electrodeposition paint having a solid content of 20.0% by weight was prepared.

  (B) The organic acid or inorganic acid salt of the rare earth metal was added directly to the paint. Each electrodeposition coating composition was erected by adjusting the addition amount (% by weight) as a metal shown in Tables 2 to 6 below.

(Examples 1-12 and Comparative Examples 1-5)
Surfactor SC-53 (manufactured by Nippon Paint Co., Ltd.) was used as a cathode to each pretreatment agent aqueous solution shown in Tables 2 to 6 prepared by the method described in Production Example 1 as a negative electrode surface-treated cold-rolled steel sheet (JIS G3141, SPCC-SD). After degreasing and washing with water, electrolytic treatment was performed under the conditions shown in the same table. Thereafter, the electrolytically treated base material was sufficiently washed with pure water, and then each electrodeposition paint shown in Tables 2 to 6 prepared in Production Example 6 was applied in the electrodeposition process under the coating conditions shown in the same table. After coating so that the dry film thickness was 20 μm, the film was cured at 170 ° C. for 20 minutes to obtain a coating film. The amount of the treated film deposited was determined by quantifying the dried treated plate after the electrolytic treatment, washing with water, and measuring by fluorescent X-ray measurement.

(Comparative Example 6)
A surface untreated cold-rolled steel sheet (JIS G3141, SPCC-SD) is degreased with surf cleaner SC-53 (manufactured by Nippon Paint Co., Ltd.) and washed with water. An electrodeposition coating film was obtained by electrodeposition coating so that the dry film thickness was 20 μm in the same manner as in Examples 1 to 12 and Comparative Examples 1 to 5 except that the conditions were used.

(Comparative Example 7)
Using electrodeposited paint and coating conditions shown in Table 6 using a zinc phosphate-treated plate obtained by treating a surface untreated cold-rolled steel sheet (JIS G3141, SPCC-SD) with Surfdyne SD-5000 (manufactured by Nippon Paint Co., Ltd.) The electrodeposition coating film was obtained by electrodeposition coating so that the dry film thickness was 20 μm in the same manner as in Comparative Example 6 except that

  About the obtained coating film, the antirust property by a salt spray test (SST: Salt Spray Test), the adhesiveness by an electrolytic peeling test, and the coating film appearance were evaluated as coating film test items, and the results are shown in Tables 2-6. Indicated. The test method was as follows.

(Test method)
(1) Rust prevention evaluation: salt spray test method After cross-cutting the cured electrodeposition coated plate and performing a salt spray test for 1000 hours, tape is peeled off, and one side maximum peeling from the cut part. The width was evaluated. The evaluation criteria were as follows.
Evaluation criteria
A: Stripping width of 3 mm or less
○: peeling width 3 mm to 4 mm
Δ: peeling width 4 mm to 6 mm
×: Stripping width of 6 mm or more

(2) Adhesion evaluation: Electrolytic peeling test Cut the cured electrodeposition coated plate, electrolyze at a current value of 0.1 mA for 72 hours, then peel off the tape, and make both sides to the peeling width. The adhesion was evaluated. The evaluation criteria were as follows.
Evaluation criteria
A: Stripping width of 3 mm or less
○: peeling width 3 mm to 6 mm
Δ: peeling width 6 mm to 10 mm
X: Peeling width of 10 mm or more (3) Appearance of coating film The presence or absence of abnormality was judged visually. The evaluation criteria were as follows.
Evaluation criteria ○: No problem ×: Bad appearance such as rough skin

(Test results)

  As is clear from the results in Tables 2 to 6, the coating films formed using the multilayer coating film forming method of Examples 1 to 12 according to the present invention are based on the salt spray test (SST) as a coating film test item. It was found that the anticorrosion property, the adhesion by electrolysis test and the coating film appearance were all excellent at the same level as the zinc phosphate treated plate of Comparative Example 7.

  The coating films of Comparative Examples 1 to 3 having a small deposition amount of the treatment film in the pretreatment step were very poor in rust prevention and adhesion because the base adhesion by the formed film was lowered.

  Since the coating film of Comparative Example 4 using an aqueous solution containing cerium (Ce) sulfate in the pretreatment step is not nitrate, the antirust and adhesion are very similar to the untreated plate that has not been pretreated. It was bad.

  The coating film of Comparative Example 5 in which the electrodeposition paint did not contain a rare earth metal compound was poor in rust prevention and adhesion because the base adhesion by the formed film was lowered.

  The method for forming a multilayer coating film of the present invention is useful as a method for forming a multilayer coating film comprising a coating base treatment (pretreatment) step and a cathodic electrodeposition coating step suitable for metal materials, particularly untreated cold-rolled steel sheets. The multilayer coating film obtained by the multilayer coating film forming method of the present invention has excellent base adhesion and corrosion resistance (rust resistance) and can be used for automobile applications.

Claims (4)

(A) Nitrate of at least one rare earth metal selected from the group consisting of cerium (Ce), yttrium (Y), neodymium (Nd), samarium (Sm) and praseodymium (Pr) is converted into a rare earth metal. In an aqueous solution containing 0.05 to 5% by weight, an untreated metal base material that has been degreased and washed with water and not subjected to chemical conversion treatment is immersed, and a voltage of 1 to 20 V is applied by cathodic electrolysis for a current application time of 10 to 300. A pretreatment step for forming a treatment film having a deposition amount of 1 to 100 mg / m ² made of a rare earth metal hydroxide , and (B) cerium (Ce), yttrium (Y), neodymium as rare earth metals (Nd), samarium (Sm), and at least one rare earth metal selected from the group consisting of praseodymium (Pr), and acetic acid, formic acid, lactic acid, sulfur 0.005 weight comprising at least one organic acid or an inorganic acid salt selected from the group consisting of amine acid and hypophosphorous acid, an organic acid or inorganic acid salts of rare earth metals, in terms of rare earth metal A method for forming a multi-layer coating film comprising a step of cathodic electrodeposition coating of an electrodeposition coating containing 1 % . The multilayer coating film forming method according to claim 1 , wherein the rare earth metal nitrate is cerium nitrate (Ce) or neodymium nitrate (Nd) . Before Symbol rare earth metal organic or inorganic acid salt comprises a cerium (Ce) or neodymium (Nd), according to claim 1 or 2 multilayer coating film forming method as described. The method for forming a multilayer coating film according to any one of claims 1 to 3 , wherein the electrodeposition paint further contains a cation-modified epoxy resin and a block polyisocyanate curing agent .