JP2008144064A - Waterborne intermediate coating composition for automobiles - Google Patents

Abstract

【選択図】なし
The present invention provides an aqueous intermediate coating composition having excellent base concealing properties and excellent coating workability.
The water-based intermediate coating composition for automobiles of the present invention is at least one selected from the group consisting of a polyester resin, a methylol group-type melamine resin, an imino group-type melamine resin, and a methylol / imino group-type melamine resin. Melamine resin, a polyether polyol having a number average molecular weight of 400 to 1,500, and a carboxylic acid group containing a weight average molecular weight of 50,000 to 1,000,000 represented by the following general formula (1) Polymer compounds.
[Chemical 1]

[Selection figure] None

Description

  The present invention relates to an aqueous intermediate coating composition for automobiles. More specifically, the present invention relates to a water-based intermediate coating composition for automobiles that has excellent base concealing properties and excellent coating workability.

  In the car body painting process, the surface of the steel sheet, which is the object to be coated, is usually subjected to chemical conversion treatment with zinc phosphate, etc., followed by undercoating by electrodeposition coating, intermediate coating by intermediate coating, and top coating. Top coating is performed, and a multilayer coating film is formed on the steel plate. The intermediate coating is usually used for the purpose of concealing minute waviness of the electrodeposition coating film obtained by steel plate or electrodeposition coating and smoothing the coating film obtained.

  An aqueous intermediate coating composition containing a predetermined polyester resin and melamine resin has been proposed as the intermediate coating (see Patent Document 1), but this aqueous intermediate coating composition does not have sufficient base concealing properties. . As means for improving the base concealing property, (1) means for lowering the viscosity of the aqueous intermediate coating composition, and (2) means for increasing the solid content concentration of the aqueous intermediate coating composition are conceivable. The means (1) can improve the flow property, and the means (2) can improve the base concealing property by reducing the volume shrinkage when the coating film is formed. “At the time of coating film formation” includes not only the baking process but also the setting after coating and the preliminary drying process.

However, the above means (1) has a problem that the coating workability is inferior due to a decrease in viscosity. Moreover, in the said means (2), there exists a limit in high solid content concentration. In addition, even if a high solid content concentration can be achieved, there is a problem in painting workability such as dustiness.
JP 2002-294148 A

  The present invention has been made in order to solve the above-described conventional problems, and an object of the present invention is to provide a water-based intermediate coating composition having excellent base concealing properties and excellent coating workability. is there.

（式中、Ｍはカルボン酸基、又は、アンモニア、１級、２級若しくは３級アミン化合物によるカルボン酸中和基であり、ｍは１以上の整数であり、Ｒは水素原子若しくはメチル基である。） The water-based intermediate coating composition for automobiles of the present invention comprises a polyester resin, at least one melamine resin selected from the group consisting of a methylol group-type melamine resin, an imino group-type melamine resin, and a methylol / imino group-type melamine resin. A polyether polyol having a number average molecular weight of 400 to 1,500 and a carboxylic acid group having a structure represented by the following general formula (1) and having a weight average molecular weight of 50,000 to 1,000,000 Containing polymer compound.

(In the formula, M is a carboxylic acid group or a carboxylic acid neutralizing group by ammonia, primary, secondary or tertiary amine compound, m is an integer of 1 or more, and R is a hydrogen atom or a methyl group. is there.)

  In preferable embodiment, the solid content of the said polyether polyol is 3-30 mass parts with respect to 100 mass parts of total resin solid content of an aqueous intermediate coating composition.

  In a preferred embodiment, the solid content of the carboxylic acid group-containing polymer compound is 0.05 to 5.0 parts by mass with respect to 100 parts by mass of the total resin solid content of the aqueous intermediate coating composition.

  In a preferred embodiment, the melamine resin is at least one selected from the group consisting of a methylated melamine resin, a butylated melamine resin, and a methylbutyl mixed melamine resin.

  In preferable embodiment, the hydroxyl value of the said polyether polyol is 30-700.

  According to the present invention, by combining a specific melamine resin, a polyether polyol, and a carboxylic acid group-containing polymer compound, an aqueous intermediate coating composition having excellent base concealing properties and excellent coating workability is provided. can do.

  The aqueous intermediate coating composition of the present invention includes a polyester resin, a specific melamine resin, a polyether polyol, and a carboxylic acid group-containing polymer compound. Hereinafter, each component will be described.

A. Polyester resin As the polyester resin, any suitable polyester resin that can function as a coating film forming component may be employed. For example, a carboxyl group-containing aqueous polyester resin can be mentioned.

  The weight average molecular weight of the carboxyl group-containing aqueous polyester is preferably 3,000 to 80,000. When the weight average molecular weight is less than 3,000, the storage stability may be lowered. If it exceeds 80,000, the base concealing property may be insufficient.

  The solid content acid value of the carboxyl group-containing aqueous polyester resin is preferably 15 to 80. When the acid value is less than 15, the storage stability may be lowered. When it exceeds 80, there exists a possibility that the water resistance of the coating film obtained may fall. The hydroxyl value of the carboxyl group-containing aqueous polyester resin is preferably 40 to 200. When the hydroxyl value is less than 40, the curability may be lowered. When it exceeds 200, there exists a possibility that the water resistance of the coating film obtained may fall.

  The glass transition temperature of the carboxyl group-containing aqueous polyester resin is preferably -40 to 50 ° C, more preferably -40 to 10 ° C. When the glass transition temperature is less than −40 ° C., the hardness may decrease. When it exceeds 50 ° C., the base concealing property may be lowered. The glass transition temperature can be measured with a differential scanning calorimeter (DSC) or the like.

  The carboxyl group-containing aqueous polyester resin is typically obtained by condensing a polybasic acid component and a polyol component by a conventional method to obtain a resin, and then neutralizing with a base and dispersing or dissolving in water. be able to.

  Examples of the polybasic acid component include aromatic polycarboxylic acids and acids such as phthalic anhydride, isophthalic acid, terephthalic acid, trimellitic anhydride, tetrabromophthalic anhydride, tetrachlorophthalic anhydride, and pyromellitic anhydride. Anhydrides: Hexahydrophthalic anhydride, tetrahydrophthalic anhydride, 1,4- and 1,3-cyclohexanedicarboxylic acid and other alicyclic polycarboxylic acids and anhydrides; maleic anhydride, fumaric acid, succinic anhydride, adipine Examples thereof include polybasic acid components such as aliphatic polycarboxylic acids such as acid, sebacic acid and azelaic acid, and anhydrides, and anhydrides thereof.

  Examples of the polyol component include ethylene glycol, propylene glycol, 1,3-propanediol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 2,2-diethyl-1, 3-propanediol, neopentyl glycol, 1,9-nonanediol, 1,4-cyclohexanediol, hydroxypivalic acid neopentyl glycol ester, 2-butyl-2-ethyl-1,3-propanediol, 3-methyl- Diols such as 1,5-pentanediol, 2,2,4-trimethylpentanediol, 1,4- and 1,3-cyclohexanedimethanol, hydrogenated bisphenol A, and trimethylolpropane, trimethylolethane, glycerin, More than trivalent such as pentaerythritol Polyol component, 2,2-dimethylolpropionic acid, 2,2-dimethylolbutanoic acid, 2,2-dimethylolpentanoic acid, 2,2-dimethylolhexanoic acid, 2,2-dimethyloloctanoic acid, etc. The hydroxycarboxylic acid component of can be mentioned.

  Examples of the base include amines such as triethylamine and dimethylaminoethanol. Moreover, the neutralization rate in the case of the said neutralization can be set to arbitrary appropriate values. Typically, it is 80 to 120%. In addition, the aqueous | water-based intermediate coating composition of this invention can also use the said carboxyl group containing water-based polyester resin in combination of 2 or more types.

  The solid content of the polyester resin in the aqueous intermediate coating composition of the present invention is preferably 25 to 75 parts by mass, more preferably 40 to 70, relative to 100 parts by mass of the total resin solid content of the aqueous intermediate coating composition. Part by mass. When the solid content of the polyester resin is less than 25 parts by mass, the coating film performance (for example, chipping resistance, water resistance) may be deteriorated. When it exceeds 75 parts by mass, the hardness of the coating film is insufficient and the solvent resistance may be lowered.

B. Melamine resins general, the melamine resin, the reactive group is _N- (CH _{2 OR)} 2 in which complete alkyl _type, N- (CH 2 OR) methylol group type is a _{CH 2} OH, N- _(CH 2 OR) imino group type is _{H, N- (CH 2 OR)} CH 2 OH and N- _(CH 2 OR) H can be exemplified four methylol / imino group type mixed. The aqueous intermediate coating composition of the present invention contains at least one melamine resin selected from the group consisting of a methylol-based melamine resin, an imino-based melamine resin, and a methylol / imino-based melamine resin. Adopting such a melamine resin and combining a polyether polyol and a carboxylic acid group-containing polymer compound, which will be described later, to provide an aqueous intermediate coating composition with excellent base concealing properties and excellent coating workability. can do.

  R of the reactive group can be any suitable alkyl group. Examples thereof include a methyl group (methylated melamine resin), a butyl group (butylated melamine resin), and a mixture thereof (methylbutyl mixed melamine resin). For example, the use of a butylated melamine resin can improve the water resistance of the resulting aqueous intermediate coating composition.

  The number average molecular weight of the melamine resin used in the present invention is preferably 400 to 1,500, and more preferably 400 to 1,200. By being in such a range, the base concealing property is further improved.

  A commercial item can be employ | adopted as a melamine resin used by this invention. Specific examples include My Coat 723, Cymel 212, Cymel 238 (all manufactured by Nippon Cytec Co., Ltd.), Uban 226 (manufactured by Mitsui Chemicals, Inc.), and the like.

  The solid content of the melamine resin in the aqueous intermediate coating composition is preferably 20 to 50 parts by mass, more preferably 25 to 45 parts by mass with respect to 100 parts by mass of the total resin solid content of the aqueous intermediate coating composition. is there. When the solid content of the melamine resin is less than 20 parts by mass, the hardness of the coating film is insufficient and the solvent resistance may be lowered. When it exceeds 50 parts by mass, the hardness of the coating film becomes too high, and the chipping resistance and impact resistance may be lowered.

C. Polyether Polyol Any suitable polyether polyol can be adopted as long as the polyether polyol has two or more hydroxyl groups in one molecule. By setting it as such a structure, the water resistance and adhesiveness of the coating film obtained can be improved. The upper limit of the number of hydroxyl groups in one molecule of the polyether polyol used in the present invention is preferably 6, and more preferably 4. This is because the coating film can be excellent in curability and water resistance.

  The polyether polyol has a number average molecular weight of 400 to 1,500, preferably 500 to 1,200. When the number average molecular weight is less than 400, the curability of the coating film may be lowered. When it exceeds 1,500, the viscosity becomes high and the compatibility with the polyester resin may be lowered, and the coating film may be dirty.

  The hydroxyl value of the polyether polyol is preferably 30 to 700, more preferably 50 to 500. When the hydroxyl value is outside the above range, the storage stability may be lowered, and various performances of the resulting coating film may be lowered.

  The polyether polyol used in the present invention can be produced by any suitable method. For example, it can be obtained by adding an alkylene oxide to an active hydrogen atom-containing compound in the presence of an alkali catalyst at a temperature of 60 to 160 ° C. under ordinary pressure or increased pressure by a conventional method.

  Examples of the active hydrogen atom-containing compound include water; polyhydric alcohol components such as 1,6-hexanediol and neopentyl glycol; tetrahydric alcohols such as diglycerin and sorbitan; adonitol, arabitol, xylitol, triglycerin and the like. A pentahydric alcohol such as dipentaerythritol, sorbitol, mannitol, iditol, inositol, dulcitol, talose and allose; an octahydric alcohol such as sucrose; a polyhydric alcohol such as polyglycerin; pyrogallol, hydroquinone, phloroglucin And polyhydric phenols of bisphenols such as bisphenol A and bisphenol sulfone; polyhydric carboxylic acid components such as adipic acid and maleic anhydride, and mixtures of two or more thereof. Examples of trihydric or higher alcohols used to form a polyether polyol having 3 or more total hydroxyl groups in one molecule include glycerin, trimethylolethane, trimethylolpropane, pentaerythritol, sorbitan, sorbitol, etc. Is preferred.

  As said alkylene oxide, alkylene oxides, such as ethylene oxide, a propylene oxide, a butylene oxide, are mentioned, for example, These can use 2 or more types together. When two or more types are used in combination, the addition format may be either block or random.

  A commercial item can be employ | adopted as polyether polyol used by this invention. Specific examples include Prime Pole PX-1000, Sanniks GE-600, Sanniks SP-750, PP-400 (all manufactured by Sanyo Chemical Industries), PTMG-650 (manufactured by Mitsubishi Chemical Corporation), and the like.

  The solid content of the polyether polyol in the aqueous intermediate coating composition is preferably 3 to 30 parts by mass, more preferably 5 to 25 parts by mass with respect to 100 parts by mass of the total resin solid content of the aqueous intermediate coating composition. It is. When the solid content of the polyester resin is less than 3 parts by mass, the flowability is insufficient and the base concealability may be lowered. When it exceeds 30 mass parts, the hardness of a coating film is inadequate and there exists a possibility that solvent resistance may fall.

D. Carboxylic acid group-containing polymer compound The carboxylic acid group-containing polymer compound has a structure represented by the following general formula (1).

  In the above formula (1), M is a carboxylic acid group or a carboxylic acid neutralizing group by ammonia, a primary, secondary or tertiary amine compound, m is an integer of 1 or more, and R is a hydrogen atom or It is a methyl group.

  The carboxylic acid group-containing polymer compound has a weight average molecular weight of 50,000 to 1,000,000, preferably 100,000 to 800,000. If it is less than 50,000, the water resistance of the resulting coating film may be reduced. When it exceeds 1,000,000, the compatibility with other components may be reduced.

  The carboxylic acid group-containing polymer compound can be typically obtained by polymerizing a monomer mixed solution containing a neutralized salt of acrylic acid and / or an acrylic acid base. The monomer mixture may contain other components. Other components include, for example, methyl (meth) acrylate, ethyl (meth) acrylate, butyl (meth) acrylate, stearyl (meth) acrylate, hydroxyethyl (meth) acrylate, methoxypolyethylene glycol (meth) acrylate (Meth) acrylic acid ester monomers such as (meth) acrylamide, N-methyl (meth) acrylamide, N-ethyl (meth) acrylamide, N-butyl (meth) acrylamide, N, N-dimethyl (meth) acrylamide, N , N-diethyl (meth) acrylamide, (meth) acrylic acid amide monomers such as N- (hydroxymethyl) (meth) acrylamide, polyethylene glycol monovinyl ether, polyethylene propylene glycol monovinyl ether, methoxypolyethylene Polyalkylene glycol monovinyl ether monomers such as polyglycol glycol monovinyl ether, polyalkylene glycol monoallyl ether monomers such as methoxypolyethylene glycol allyl ether, ethoxypolyethylene glycol allyl ether, and other monomers include (meth) acrylonitrile, acetic acid Examples thereof include vinyl, vinyl chloride, styrene, α-methyl styrene, dimethyl styrene, trimethyl styrene, hydroxymethyl styrene, styrene sulfonic acid and vinyl sulfonic acid. These can be used alone or in combination of two or more.

  Any appropriate method can be adopted as a method for polymerizing the monomer mixture. Examples thereof include solution polymerization in an aqueous medium containing water and, if necessary, an organic solvent such as alcohol, emulsion polymerization using an emulsifier, a polymerization initiator, a chain transfer agent, and the like.

  When M in the general formula (1) is a carboxylic acid neutralizing group, the polymerization may be performed after neutralizing an acrylic acid monomer to form a neutralized salt before the polymerization, or by the polymerization. After obtaining the carboxylic acid group-containing polymer compound, it may be neutralized to obtain a neutralized salt. For the neutralization, for example, ammonia, a primary, secondary, or tertiary amine compound is used. The amine compound may be an alkanolamine or a lower alkylamine having 1 to 4 carbon atoms.

  The solid content of the carboxylic acid group-containing polymer compound in the aqueous intermediate coating composition is preferably 0.05 to 5.0 parts by mass with respect to 100 parts by mass of the total resin solid content of the aqueous intermediate coating composition. More preferably, it is 0.1-3.0 mass parts. When the solid content of the carboxylic acid group-containing polymer compound is less than 0.05 parts by mass, the viscosity of the resulting coating may be insufficient. When the amount exceeds 5.0 parts by mass, the solid content concentration of the paint is lowered, and the workability during coating may be lowered.

E. Others The aqueous intermediate coating composition of the present invention may contain any appropriate other components. For example, a pigment may be included in order to enhance the base concealability and various performances of the resulting coating film. Examples of the pigment include yellow lead, yellow iron oxide, iron oxide, carbon black, titanium dioxide, azo chelate pigment, insoluble azo pigment, condensed azo pigment, phthalocyanine pigment, indigo pigment, perinone pigment, and perylene. Examples thereof include pigments, dioxane pigments, quinacridone pigments, isoindolinone pigments, metal complex pigments and other colored pigments, and extender pigments such as calcium carbonate, precipitated barium sulfate, clay and talc.

  When the aqueous intermediate coating composition of the present invention contains the pigment, the total pigment concentration represented by [total pigment mass / (total pigment mass + total resin solids mass)] × 100 is preferably 20 to 60 mass. %, And more preferably 30 to 55% by mass. When the total pigment concentration is less than 20% by mass, it is difficult to design the color of the paint and the weather resistance of the coating film may be lowered. When it exceeds 60 mass%, there exists a possibility that coating-material stability may fall.

  In addition to the pigment, the other components include organic solvents, curing catalysts, surface conditioners, antifoaming agents, plasticizers, film-forming aids, ultraviolet absorbers, antioxidants, and the like as necessary. obtain.

  Arbitrary appropriate methods can be employ | adopted for the manufacturing method of the aqueous intermediate coating composition of this invention. For example, methods such as kneading a compound such as the above resin and pigment using a kneader or a roll, and dispersing using a sand grind mill, a disper or the like can be used.

F. Method of Use The aqueous intermediate coating composition of the present invention is usually used to form a coating film on an object to be coated by any appropriate method. Examples of the application method include air spray, airless spray, electrostatic coating, and the like. The film thickness of the obtained coating film can be set to any appropriate film thickness depending on the application and the like. In general, the dry film thickness is preferably 15 to 40 μm.

  Furthermore, you may heat-harden the obtained coating film. By curing by heating, the physical properties and various performances of the coating film can be improved. As heating temperature, it can set suitably according to the kind of water-based coating composition of this invention. In general, the temperature is preferably set to 80 to 180 ° C. The heating time can be arbitrarily set according to the heating temperature.

  When the aqueous intermediate coating composition of the present invention is used as an intermediate coating for automobile bodies, the aqueous intermediate coating composition of the present invention is applied to an object to be coated, if necessary, and is baked and cured. After the intermediate coating film is formed, a top coating material is applied on the intermediate coating film, and then heat-cured to obtain a multilayer coating film.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited to these Examples. Unless otherwise specified, parts and% in the examples are based on mass.

(Production Example 1) Production of polyester resin 1 In a reactor, 25.6 parts of isophthalic acid, 22.8 parts of phthalic anhydride, 5.6 parts of adipic acid, 19.3 parts of trimethylolpropane, 26.7 neopentyl glycol Parts, 17.5 parts of ε-caprolactone, and 0.1 parts of dibutyltin oxide were added, and the temperature was raised to 170 ° C. with mixing and stirring. Thereafter, while the temperature was raised to 220 ° C. over 3 hours, water produced by the condensation reaction was removed until the acid value reached 8.
Next, 7.9 parts of trimellitic anhydride was added and reacted at 150 ° C. for 1 hour to obtain a polyester resin having an acid value of 40. Further, after cooling to 100 ° C., 11.2 parts of butyl cellosolve was added and stirred until uniform, and after cooling to 60 ° C., 98.8 parts of ion-exchanged water and 5.9 parts of dimethylethanolamine were added, and the non-volatile content was 50%. A polyester resin 1 having a solid acid value of 40, a hydroxyl value of 110, and a weight average molecular weight of 8,000 was obtained.

(Production Example 2) Production of Colored Pigment Paste 1 50 parts of the polyester resin 1 obtained in Production Example 1, 34.5 parts of rutile titanium oxide, 34.4 parts of barium sulfate, 6 parts of talc, carbon black 0.1 parts and 17.9 parts of ion-exchanged water are mixed and stirred, glass bead medium is added in a paint conditioner, and mixed and dispersed at room temperature for 1 hour to obtain a colored pigment paste 1 having a particle size of 5 μm or less and a non-volatile content of 70%. It was.

(Example 1)
65 parts of the polyester resin 1 obtained in Production Example 1, 142.9 parts of the color pigment paste 1 obtained in Production Example 2, Mycoat 723 as an imino group-type methylated melamine resin (manufactured by Nippon Cytec Co., Ltd., number average) 45 parts molecular weight 420, solid content 100% by mass) Prime Pol PX-1000 (manufactured by Sanyo Chemical Industries, number average molecular weight 1,000, 2 hydroxyl groups in one molecule, solid content 100% by mass) as polyether polyol 10 0.0 part, surfinol 104 (manufactured by Air Products Japan) 3.8 parts, butyl cellosolve 15 parts, and SN thickener N-1 (a carboxylic acid group-containing polymer compound by San Nopco, weight average molecular weight 500,000, effective 0.9 parts of component (25% by mass) were mixed and dispersed uniformly to obtain an aqueous intermediate coating composition.

(Example 2)
35 parts of the polyester resin 1 obtained in Production Example 1, 142.9 parts of the colored pigment paste 1 obtained in Production Example 2, Mycoat 723 as an imino group-type methylated melamine resin (manufactured by Nippon Cytec Co., Ltd., number average) Molecular weight 420, solid content 100% by mass) 25 parts, imino group-type methylbutyl mixed melamine resin Cymel 212 (manufactured by Nippon Cytec Co., Ltd., number average molecular weight 580, solid content 90% by mass) 20 parts, polyether polyol Sannics GE- 600 (manufactured by Sanyo Chemical Industries, number average molecular weight 600, number of hydroxyl groups in one molecule, solid content 100% by mass) 25.0 parts, Surfinol 104 (manufactured by Air Products Japan) 3.8 parts, 15 parts of butyl cellosolve SN thickener N-1 (San Nopco carboxylic acid group-containing polymer compound, weight average molecular weight 5) 1.8 parts of (00,000, active ingredient 25% by mass) were mixed and dispersed uniformly to obtain an aqueous intermediate coating composition.

(Comparative Example 1)
A water-based intermediate coating composition was obtained in the same manner as in Example 1 except that 85 parts of the polyester resin obtained in Production Example 1 was used and Prime Pol PX-1000 was not blended.

(Comparative Example 2)
An aqueous intermediate coating composition was obtained in the same manner as in Example 2 except that SN thickener N-1 was not blended.

(Comparative Example 3)
85 parts of the polyester resin 1 obtained in Production Example 1, 142.9 parts of the color pigment paste 1 obtained in Production Example 2, Cymel 235 (manufactured by Nippon Cytec Co., Ltd., complete alkyl melamine resin, number average molecular weight 650, (Solid content: 100% by mass) 45 parts, Surfinol 104 (manufactured by Air Products Japan) 3.8 parts, and 15 parts of butyl cellosolve were mixed and dispersed uniformly to obtain an aqueous intermediate coating composition.

(Comparative Example 4)
A water-based intermediate coating composition was obtained in the same manner as in Example 1 except that Cymel 235 was used instead of Mycoat 723.

The following evaluation was performed about each of the aqueous | water-based intermediate coating composition obtained above. The evaluation method is as follows. The evaluation results are summarized in Table 1. The obtained aqueous intermediate coating composition was diluted with ion exchange water for 30 seconds (measured at 20 ° C. using a No. 4 Ford cup) and subjected to the following evaluation.
<Finished appearance>
A cationic electrodeposition paint (Power Top U-50, manufactured by Nippon Paint Co., Ltd.) is electrodeposited onto a 300 × 400 × 0.8 mm dull steel plate treated with zinc phosphate so that the dry film thickness is 20 μm. It was cured by heating at 0 ° C. for 30 minutes to obtain an electrodeposition coated plate. On the electrodeposited surface of this electrodeposited coating plate, the aqueous intermediate coating composition obtained above is a cartridge bell (manufactured by ABB Industry Co., Ltd. One stage coating was carried out with an electric coating machine) and preheated at 80 ° C. for 3 minutes. Then, it cooled after heat-hardening for 30 minutes at 150 degreeC, and obtained the test plate.
A solvent-type intermediate coating (manufactured by Nippon Paint Co., Ltd., Olga P-30 Gray) was applied to the dull steel plate in the same procedure as described above, and was cured by heating to obtain a comparative test plate.
The finished appearance of the test plate was visually observed and compared with the observation result of the comparative test plate. The evaluation criteria are as follows. In addition, A is a level with no practical problem.
(Evaluation criteria)
A: Same or better than when using solvent-based intermediate coating B: Slightly inferior to using solvent-based intermediate coating C: Substantially inferior compared to using solvent-based intermediate coating Sex>
Low-wavelength region (W3 value) effective for concealing swells of 320 to 800 μm using Wavescan (Bick Chemie, surface roughness measuring instrument) as the base concealability (coating film smoothness) of the test plate obtained above. ) Was measured. A measured value of 25 or less was accepted.
<Coating workability>
The electrodeposition coating plate having a plurality of holes having a diameter of 5 mm was placed in a horizontal state, and the aqueous intermediate coating composition obtained above was changed on the surface of the plate with a cartridge bell (20 μm). (˜50 μm) gradient coating.
Immediately after coating, the plate was set up vertically, set in that state for 5 minutes, preheated at 80 ° C. for 3 minutes, and then heat-cured at 150 ° C. for 30 minutes. After heat-curing, the film thickness (sag thickness) of the coating film dripping at a point where the length of the coating film hanging from the lower end of the hole was 5 mm was measured. The evaluation criteria was 40 μm, 40 μm or more was accepted, and less than 40 μm was rejected.

  As is apparent from Table 1, the waterborne intermediate coating compositions of the examples of the present invention are excellent in finished appearance and remarkably excellent in both base concealing property and coating workability. On the other hand, the comparative example 1 which does not use polyether polyol has inadequate base concealing property, and is inferior in a finished appearance. The comparative example 2 which does not use a carboxylic acid group containing high molecular compound is inferior to coating workability | operativity. Comparative Example 3 in which neither the polyether polyol nor the carboxylic acid group-containing polymer compound is used is inferior in the base concealing property and the coating workability. Comparative Example 4 in which a polyether polyol and a carboxylic acid group-containing polymer compound are used but the melamine resin is a completely alkyl type is inferior in coating workability. From this, it is guessed that coexistence with base concealability and coating workability is achieved by combining a specific melamine resin, a polyether polyol, and a carboxylic acid group-containing polymer compound. This is a knowledge that can be obtained for the first time when an aqueous intermediate coating composition is actually prepared by combining a specific melamine resin, a polyether polyol, and a carboxylic acid group-containing polymer compound, and is an unexpectedly excellent effect. .

  The aqueous intermediate coating composition of the present invention can be suitably used for coating an automobile body. Moreover, it can be suitably used as a paint in consideration of environmental problems.

Claims (5)

Polyester resin,
At least one melamine resin selected from the group consisting of a methylol-based melamine resin, an imino-based melamine resin, and a methylol / imino-based melamine resin;
A polyether polyol having a number average molecular weight of 400 to 1,500;
An aqueous intermediate coating composition for automobiles comprising a carboxylic acid group-containing polymer compound having a structure represented by the following general formula (1) and having a weight average molecular weight of 50,000 to 1,000,000.

(In the formula, M is a carboxylic acid group or a carboxylic acid neutralizing group by ammonia, primary, secondary or tertiary amine compound, m is an integer of 1 or more, and R is a hydrogen atom or a methyl group. is there.)   The aqueous intermediate coating composition for automobiles according to claim 1, wherein the solid content of the polyether polyol is 3 to 30 parts by mass with respect to 100 parts by mass of the total resin solid content of the aqueous intermediate coating composition.   The solid content of the carboxylic acid group-containing polymer compound is 0.05 to 5.0 parts by mass with respect to 100 parts by mass of the total resin solid content of the aqueous intermediate coating composition. An aqueous intermediate coating composition for automobiles.   The aqueous intermediate coating composition for automobiles according to any one of claims 1 to 3, wherein the melamine resin is at least one selected from the group consisting of a methylated melamine resin, a butylated melamine resin, and a methylbutyl mixed melamine resin. object.   The aqueous intermediate coating composition for automobiles according to any one of claims 1 to 4, wherein the polyether polyol has a hydroxyl value of 30 to 700.