Method of forming a multi-layer paint film

Abstract

A multi-layer paint film satisfies paint film performance, such as water resistance, impact resistance and the like, required of an automobile paint film and which has excellent paint film appearance is obtained. The method comprises coating an aqueous first base paint (A) over an electrodeposited cured paint film to form a first base paint film, and then coating an aqueous second base paint (B) over the first base paint film without carrying out preliminary drying by heating after forming the first base paint film to form a second base paint film, preliminary drying by heating is carried out after forming the second base paint film, coating a single-liquid type clear paint (C) over the second base paint film to form a clear coat paint film. These three paint film layers are heated and cured at the same time.

Claims

1. A method of forming a multi-layer paint film comprising: forming a first base paint film in which an aqueous first base paint (A) is coated over an electrodeposited cured paint film; forming a second base paint film in which an aqueous second base paint (B) is coated over the first base paint film without carrying out preliminary drying by heating after forming the first base paint film; forming a clear paint film in which preliminary drying is carried out by heating after forming the second base paint film, a single-liquid type clear paint (C) is coated over the second base paint film; and heating and curing in which the first base paint film, the second base paint film, and the clear paint film are heated and cured at the same time, wherein the first base paint (A) includes water-soluble or water-dispersible polyurethane resin (A1) of acid value of less than 15 mg KOH/g and hydroxyl group value from 10 to 100 mg KOH/g and titanium oxide (A2) of specific surface area not more than 13 m.sup.2/g, and the water-soluble or water-dispersible polyurethane resin (A1) includes from 4 to 15 mass % with respect to the resin solid fraction of the water-soluble or water-dispersible polyurethane resin (A1) of at least one of ethylene oxide units and propylene oxide units, and the mass content ratio of the titanium oxide (A2) is from 40 to 60 mass % with respect to the paint solid fraction of the first base paint (A).

2. The method of forming a multi-layer paint film as claimed in claim 1, wherein the water-soluble or water-dispersible polyurethane resin (A1) is a water-soluble or water-dispersible polyurethane resin which has been obtained by the reaction of a polyester polyol and a polyisocyanate compound.

3. The method of forming a multi-layer paint film as claimed in claim 1, wherein a dry film thickness of the first base paint film is from 20 to 40 μm.

4. A paint film which has been formed by means of the method of forming a multi-layer paint film as claimed in claim 1.

Description

ILLUSTRATIVE EXAMPLES

(1) The invention is described in more detail below by means of illustrative examples, but the invention is not limited by these illustrative examples. Moreover, in the absence of any particular indication to the contrary, parts, and ratio in each case indicate parts by mass, mass % and mass ratio.

Example 1-1 of Production

Production of Polyester Resin Solution P-1

(2) Lauric acid (10 parts), 30 parts of phthalic acid anhydride, 6.7 parts of polyethylene glycol mono-methyl ether (trade name Uniox M1000, produced by the Ni. Yu. Co., number average molecular weight 1,000), 14.9 parts of adipic acid, 30.3 parts of neopentyl glycol and 8.1 parts of trimethylolpropane were introduced into a reaction vessel which had been furnished with a reflux condenser which was fitted with a reaction-water separating tube, nitrogen gas delivery apparatus, a thermometer and stirring apparatus and the temperature was raised to 120° C. and the raw materials were dissolved and then, while stirring, the temperature was raised to 160° C. After being maintained at 160° C. for 1 hour the temperature was gradually raised to 230° C. over a period of 5 hours. The temperature was maintained at 230° C. for 2 hours and the reaction continued and after the acid value had reached 5 mgKOH/g the temperature was lowered to 80° C. and then 23 parts of toluene were added and the polyester resin solution P-1 was obtained. The characteristic values of the polyester resin solution P-1 were solid fraction mass %, number average molecular weight 1,500, hydroxyl group value 72 mgKOH/g and acid value 5 mgKOH/g.

Examples 1-2 to 1-5 of Production

Production of Polyester Resin Solutions P-2 to P-5

(3) The polyester resin solutions P-2 to P-5 were obtained with the same method as in Example 1-1 of Production on the basis of the formulations and conditions shown in Table 1. The characteristic values of the polyester resin solutions obtained are summarized in Table 1.

(4) TABLE-US-00001 TABLE 1 Polyester Resin Solution P-1 P-2 P-3 P-4 P-5 Lauric Acid 10 10 10 10 10 Phthalic Acid Anhydride 30 30 30 30 30 Polyethylene Glycol 6.7 16.0 4.5 25.0 6.7 Mono-methyl Ether (Note 1) Adipic Acid 14.9 10.2 16.1 5.0 15.7 Neopentyl Glycol 30.3 23.4 31.6 19.5 27.3 Trimethylolpropane 8.1 10.4 7.8 10.5 10.3 Subtotal 100.0 100.0 100.0 100 100 (Amount of Water Eliminated 8.1 6.8 8.4 5.6 8.0 During Synthesis) Toluene 23.0 23.3 22.9 23.6 23.0 Total 114.9 116.5 114.5 118.0 115.0 Resin Solid Fraction (mass %) 80.0 80.0 80.0 80.0 80.0 Number Average Molecular 1,500 1,500 1,500 1,500 2,000 Weight Hydroxyl Group Value 72 72 72 72 72 (mgKOH/g) Resin Acid Value (mgKOH/g) 5 10 5 5 15 Ethylene Oxide Structural 7.3 16.6 4.9 26.5 7.3 Unit Content (mass %)

(5) The details of each of the compounded components shown in Table 1 and supplementary facts will be described.

(6) (Note 1): Polyethylene glycol mono-methyl ether (trade name Uniox M1000, produced by the Ni. Yu. Co., number average molecular weight 1,000)

Example 2-1 of Production

Production of Polyurethane Resin Solution PUR-1

(7) Polyester resin solution P-1 (83.9 parts), 1.6 parts of dimethylolpropionic acid, 2.3 parts of neopentyl glycol, 22.7 parts of isophorone diisocyanate and 40 parts of methyl ethyl ketone were introduced into a reaction vessel which had been furnished with nitrogen gas delivery apparatus, a thermometer and stirring apparatus and, while stirring, the temperature was raised to 80° C. and each of the components reacted while being maintained at 80° C. When the isocyanate value reached 0.52 meq/g 6.3 parts of trimethylolpropane were added and the reaction was continued at 80° C. Then, when the isocyanate value reached 0.01 meq/g 25.0 parts of butyl cellosolve were added and the reaction was terminated. Subsequently the temperature was raised to 100° C. and the toluene and methyl ethyl ketone were removed under reduced pressure. Then the temperature was dropped to 50° C., 1.3 parts of dimethylethanolamine were added and the acid groups were neutralized and then 258.4 parts of de-ionized water were added and the polyurethane resin solution PUR-1 was obtained. The characteristic values of the polyurethane resin solution PUR-1 were solid fraction 26 mass %, number average molecular weight 4,000, hydroxyl group value 52 mgKOH/g, acid value 10 mgKOH/g.

Examples 2-2 to 2-5 of Production

Production of Polyurethane Resin Solutions PUR-2 to 5

(8) The polyurethane resin solutions PUR-2 to P-5 were obtained with the same method as in Example 2-1 of Production on the basis of the formulations shown in Table 2. The characteristic values of the polyurethane resin solutions obtained are summarized in Table 2.

(9) TABLE-US-00002 TABLE 2 Polyurethane Resin Solution PUR-1 PUR-2 PUR-3 PUR-4 PUR-5 P-1 (Note 2) 83.9 P-2 (Note 2) 83.9 P-3 (Note 2) 83.9 P-4 (Note 2) 83.9 P-5 (Note 2) 83.9 Dimethylolpropionic Acid 1.6 1.6 1.6 1.6 1.6 Neopentyl Glycol 2.3 2.3 2.3 2.3 2.3 Isophorone Diisocyanate 22.7 22.7 22.7 22.7 22.7 Methyl Ethyl Ketone 40 40 40 40 40 (Note 3) Trimethylolpropane 6.3 6.3 6.3 6.3 6.3 Butyl Cellosolve 25 25 25 25 25 Dimethylethanolamine 1.3 1.7 1.3 1.3 2.1 De-ionized Water 258.4 258.0 258.4 258.4 257.6 Total 317.6 317.6 384.7 317.6 317.6 Resin Solid Fraction 26.0 26.0 26.0 26.0 26.0 (mass %) Number Average 4,000 4,000 4,000 4,000 4,000 Molecular Weight Hydroxyl Group Value 52 52 52 52 52 (mgKOH/g) Acid Value (mgKOH/g) 10 13 10 10 17 Ethylene Oxide Unit 5 11 3 18 5 Content (mass %)

(10) The details of each of the compounded components shown in Table 2 and supplementary facts will be described.

(11) (Note 2): The solvent in the polyester resin solution was removed under reduced pressure and so it is not included in the “Total” in the table.

(12) (Note 3): The methyl ethyl ketone was removed under reduced pressure and so it is not included in the “Total” in the table.

Example 3-1 of Production

Production of Aqueous First Base Paint A-1

(13) Titanium oxide T-1 (specific surface area 12 m.sup.2/g, average particle size 0.28 μm, 30 parts), 100 parts of polyurethane resin PUR-1, 10 parts of aqueous polyester resin (trade name Baihidrol E-270, produced by the Sumika Beyer Urethane Co., solid fraction 70%) and 60 parts of de-ionized water were added to a container and dispersed in a mortar mill until the pigment particle size became 10 μm or below. Then 12 parts of melamine resin (trade name Cymel 327, produced by the Cytec Industries Co., methylated melamine resin, solid fraction 90%), 1.5 parts of surface controlling agent (trade name Biketol WS, produced by the Bichemie Co.) and 0.3 part of rheology controlling agent (trade name Primal ASE-60, produced by the Dow Chemical Co.) were added and mixed in a dissolver and the aqueous first base paint A-1 was obtained.

Examples 3-2 to 3-7 of Production

Production of Aqueous First Base Paints A-2 to A-7

(14) The aqueous first base paints A-2 to A-7 were produced with the same method as in Example 3-1 of Production on the basis of the formulations shown in Table 3.

(15) TABLE-US-00003 TABLE 3 Aqueous First Base Paint A-1 A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 T-1 35 65 45 45 45 45 19 82 T-2 (Note 4) 45 PUR-1 100 100 PUR-2 100 100 100 100 PUR-3 100 PUR-4 100 PUR-5 100 Aqueous Polyester Resin (Note 5) 10 10 10 10 10 10 10 10 10 Melamine Resin (Note 6) 12 12 12 12 12 12 12 12 12 Surface Controlling Agent (Note 7) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 Rheology Controlling Agent (Note 8) 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 De-ionized Water 120 150 130 130 130 130 130 100 165 Total 278.8 338.8 298.8 298.8 298.8 298.8 298.8 242.8 370.8 Titanium Oxide Content in the 43.4 58.8 49.7 49.7 49.7 49.7 49.7 30.3 65.2 Paint Solid Fraction (mass %) Paint Solid Fraction (mass %) 28.9 32.6 30.3 30.3 30.3 30.3 30.3 25.9 33.9 The details of each of the compounded components shown in Table 3 and supplementary facts will be described. (Note 4): Titanium oxide T-2 (specific surface area 18 m.sup.2/g, average particle diameter 0.27 μm) (Note 5): Aqueous polyester resin (trade name Baihidrol E-270, produced by the Sumika Beyer Co., solid fraction 70%) (Note 6): Melamine resin (trade name Cymel 327, produced by the Cytec Industries Co., methylated melamine resin, solid fraction 90%) (Note 7): Surface controlling agent (trade name Biketol WS, produced by the Bichemie Co.) (Note 8): Rheology controlling agent (trade name Primal ASE-60, produced by the Dow Chemical Co.)

(16) Paint film performance evaluations (paint film appearance, water resistance, impact resistance, base-concealing properties) were carried out with the methods outlined below. Moreover, the aqueous first base paint and the aqueous second base paint were provided for coating after being diluted with de-ionized water in such a way that the Ford cup #4 viscosity was 40 seconds (20° C.). Furthermore, the temperature and humidity conditions when coating the aqueous first base paint and the aqueous second base paint were set to 25° C. and 75% (relative humidity).

Example 1

(17) Cationic electrodeposition paint (trade name CathoGuard No. 500, produced by the BASF Coating Japan Co. Ltd.) was electrodeposition coated on a mild steel sheet which had been subjected to a zinc phosphate chemical forming treatment in such as way as to provide a dry film thickness of 20 μm and heated and cured for 30 minutes at 160° C. to provide a sheet with an electrodeposited paint film (electrodeposited sheet) for use in the evaluations.

(18) (1) Evaluation of Paint Film Appearance, Water Resistance and Impact Resistance

(19) The aqueous first base paint A-1 was coated on an electrodeposited sheet in such a way as to provide a dry film thickness of 30 μm. After coating, the sheet was left to stand for 5 minutes and then the aqueous second base paint Aqua BC-3 (produced by the BASF Coatings Japan Co. Ltd., silver metallic color) was coated in such a way as to provide a dry film thickness of 12 μm. After coating, the sheet was left to stand for 5 minutes and then preliminary heating was carried out for 3 minutes at 80° C. After being left to cool to room temperature the single-liquid type clear paint Belcoat No. 6100 Clear (trade name, produced by the BASF Coatings Japan Co. Ltd, acrylic resin/melamine curing type) was coated in such a way as to provide a dry film thickness of 35 μm. After coating, the sheet was left to stand at room temperature for 10 minutes and then heating and curing were carried out at 140° C. for 25 minutes and a sheet for evaluation purposes was obtained.

(20) The sheet obtained for evaluation purposes was subjected to the following paint film performance evaluations and the results were as shown in Table 4.

(21) (1-1) Paint Film Appearance

(22) The paint film appearance of the sheet for evaluation purposes was observed visually and evaluated on the basis of the following criteria: ◯: When a fluorescent lamp was reflected by the paint film the outline of the fluorescent lamp was reflected distinctly. X: When a fluorescent lamp was reflected by the paint film the outline of the fluorescent lamp was blurred.
(1-2) Water Resistance

(23) The sheet for evaluation purposes was immersed in warm water at 40° C. for 240 hours and then the surface state of the paint film was observed visually and evaluated on the basis of the following criteria: ◯: No abnormality of the paint film. X: Abnormality such as loss of gloss or the like of the paint film.
(1-3) Impact Resistance

(24) A weight of mass (500±1) g was dropped from a height of 20 cm onto a frame with the sheet for evaluation purposes held between the frame and a receiving table with the painted surface facing upward in accordance with “6. DuPont System” of “JIS K 5600-5-3 Falling-Weight Resistance”. The damaged state of the paint surface was observed visually and evaluated on the basis of the following criteria: ◯: No cracking or peeling of the paint film was seen. X: Cracking or peeling of the paint film was seen.
(2) Evaluation of Base-concealing Properties

(25) A sheet where a commercial white concealing paper (trade name “Rate of Concealment Test Paper”, produced by the Nippon Test Panel Co.) had been stuck on the paint film surface of an electrodeposited sheet and a multi-layer paint film had been formed by coating the aqueous first base paint A-1, the aqueous second base paint and the clear paint under the same coating conditions as for forming a sheet for evaluation purposes was used as a sheet for evaluating the base concealing properties. The sheet for evaluating the concealing properties was observed visually and the extent of concealment of the multi-layer paint film which had been formed over the white concealing paper and a multi-layer paint film which had been formed on an electrodeposited paint film were compared and an evaluation was made on the basis of the following criteria: ◯: Concealment on the white concealing paper and on the electrodeposited paint film are the same. X: Concealment on the electrodeposited paint film was unsatisfactory when compared with the concealment on the white concealing paper.

Examples 2 and 3 and Comparative Examples 1 to 4

(26) Paint film property evaluations were carried out with the same methods as in Example 1 using the aqueous first base paints indicated in Table 3. The results of the paint film performance evaluations are summarized in Table 4.

(27) TABLE-US-00004 TABLE 4 Example 1 Example 2 Example 3 Comp. Ex. 1 Comp. Ex. 2 Comp. Ex. 3 Comp. Ex. 4 Comp. Ex. 5 Comp. Ex. 6 Aqueous First A-1 A-2 A-3 A-4 A-5 A-6 A-7 A-8 A-9 Base Paint Paint Film ◯ ◯ ◯ X ◯ X ◯ ◯ ◯ Appearance Water Resistance ◯ ◯ ◯ ◯ X ◯ ◯ ◯ ◯ Impact Resistance ◯ ◯ ◯ ◯ ◯ ◯ ◯ ◯ X Base Concealing ◯ ◯ ◯ ◯ ◯ ◯ X X ◯ Properties