Chromium-free paint composition and paint films obtained by coating same

Abstract

To provide a chromium-free paint composition which can provide paint films which are excellent in terms of corrosion resistance on the edge surfaces, scratched surfaces and worked parts of a pre-coated steel sheet, and the paint films which can be obtained by coating this chromium-free paint composition. A chromium-free paint composition which includes paint film-forming resin (A), anti-rust pigment (B), comprising at least one type of amorphous MgOV.sub.2O.sub.5-based compound, and crosslinking agent (C) which is characterized in that the mass content proportion of the aforementioned anti-rust pigment (B) is from 10 to 80 mass % with respect to the sum of the resin solid fraction mass of the aforementioned paint film-forming resin (A) and the aforementioned crosslinking agent (C), and the overall eluted ion content in a 10% aqueous solution of the aforementioned anti-rust pigment (B) is from 10 ppm to 100 ppm.

Claims

1. A chromium-free paint composition comprising a paint film forming resin (A), an anti-rust pigment (B) that comprises at least one amorphous MgOV.sub.2O.sub.5 compound, and a crosslinking agent (C), wherein the mass content proportion of the anti-rust pigment (B) is in the range of from 20 to 80 mass % with respect to the sum of the resin solid fraction mass of the paint film-forming resin (A) and the crosslinking agent (C), the mol ratio of magnesium with respect to vanadium is in the range of from 1.7 to 5.0, and wherein the overall eluted ion content in a 10% aqueous solution of the anti-rust pigment (B) is from 10 ppm to 100 ppm and the pH of the 10% aqueous solution is in the range of 9.0 to 11.0, and wherein the amorphous MgOV.sub.2O.sub.5 is produced by mixing and reacting magnesium-containing compounds and vanadium-containing compounds in water, drying, and pulverization.

2. The chromium-free paint composition according to claim 1, wherein the magnesium compound which is a raw material for production of the amorphous MgOV.sub.2O.sub.5 compound is at least one magnesium-containing compound selected from the group consisting of magnesium oxide and magnesium carbonate.

3. The chromium-free paint composition according to claim 1, wherein the vanadium-containing compound which is a raw material for production of the amorphous MgOV.sub.2O.sub.5 compound is vanadium pentoxide.

4. The chromium-free paint composition according to claim 1, wherein the paint film-forming resin (A) is at least one paint film-forming resin selected from the group consisting of hydroxyl group-containing epoxy resins of number average molecular weight from 400 to 10,000 and hydroxyl-group containing polyester resins of number average molecular weight from 500 to 20,000.

5. The chromium-free paint composition according to claim 1, wherein the crosslinking agent (C) is at least one crosslinking agent selected from the group consisting of blocked polyisocyanate compounds and amino resins, and wherein the solid fraction mass content proportion of the crosslinking agent (C) is from 3 to 60 mass % with respect to the solid fraction mass of the paint film-forming resin (A).

6. A paint film that has been obtained by coating the chromium-free paint composition according to 1.

7. A pre-coated steel sheet comprising a steel sheet and the chromium-free paint composition according to claim 1 coated thereon.

8. A chromium-free paint composition comprising a paint film forming resin (A), an anti-rust pigment (B) that comprises at least one amorphous MgOV.sub.2O.sub.5 compound, and a crosslinking agent (C), wherein the mass content proportion of the anti-rust pigment (B) is in the range of from 10 to 80 mass % with respect to the sum of the resin solid fraction mass of the paint film-forming resin (A) and the crosslinking agent (C), and wherein the overall eluted ion content in a 10% aqueous solution of the anti-rust pigment (B) is from 10 ppm to 100 ppm, and wherein anti-rust pigment (B) is used without conjoint use of other anti-rust pigments, and wherein the amorphous MgOV.sub.2O.sub.5 is produced by mixing and reacting magnesium-containing compounds and vanadium-containing compounds in water, drying, and pulverization.

9. A chromium-free paint composition comprising a paint film forming resin (A), an anti-rust pigment (B) that comprises at least one amorphous MgOV.sub.2O.sub.5 compound, and a crosslinking agent (C), wherein the mass content proportion of the anti-rust pigment (B) is in the range of from 10 to 80 mass % with respect to the sum of the resin solid fraction mass of the paint film-forming resin (A) and the crosslinking agent (C), and wherein the overall eluted ion content in a 10% aqueous solution of the anti-rust pigment (B) is from 10 ppm to 100 ppm, and wherein anti rust pigment (B) the amorphous MgOV.sub.2O.sub.5 is produced by mixing and reacting magnesium-containing compounds and vanadium-containing compounds in water, drying, and pulverization.

10. The chromium-free paint composition according to claim 9, wherein the anti-rust pigment (B) has a mol ratio of magnesium to vanadium of from 1.7 to 5.0.

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 examples. Moreover, in the absence of any indication to the contrary the terms parts, % and ratio in the examples signify parts by mass, mass % and mass ratio respectively.

Example of Production 1-1: Preparation of Anti-Rust Pigment B1

(2) Magnesium oxide (470 g) and 530 g of vanadium pentoxide were added to 10 L of deionized water and, after raising the temperature to 60 C., the mixture was stirred for 2 hours at the same temperature. The reaction product obtained was de-watered after being washed with water and then dried at 100 C. and pulverized to provide the amorphous MgOV.sub.2O.sub.5-based compound 1. This was the anti-rust pigment B1.

Example of Production 1-2: Preparation of Anti-rust Pigment B2

(3) The amounts of the raw materials used were changed to 530 g of magnesium oxide and 470 g of vanadium pentoxide and amorphous MgOV.sub.2O.sub.5-based compound 2 was obtained with the same method as in Example of Production 1-1. This was the anti-rust pigment B2.

Example of Production 1-3: Preparation of Anti-rust Pigment B3

(4) The amounts of the raw materials used were changed to 570 g of magnesium oxide and 430 g of vanadium pentoxide and amorphous MgOV.sub.2O.sub.5-based compound 3 was obtained with the same method as in Example of Production 1-1. This was the anti-rust pigment B3.

Example of Production 1-4: Preparation of Anti-Rust Pigment B4

(5) The amounts of the raw materials used were changed to 640 g of magnesium oxide and 360 g of vanadium pentoxide and amorphous MgOV.sub.2O.sub.5-based compound 4 was obtained with the same method as in Example of Production 1-1. This was the anti-rust pigment B4.

Example of Production 1-5: Preparation of Anti-rust Pigment B5

(6) The amounts of the raw materials used were changed to 670 g of magnesium oxide and 330 g of vanadium pentoxide and amorphous MgOV.sub.2O.sub.5-based compound 5 was obtained with the same method as in Example of Production 1-1. This was the anti-rust pigment B5.

Example of Production 1-6: Preparation of Anti-rust Pigment B6

(7) The amounts of the raw materials used were changed to 400 g of magnesium oxide and 600 g of vanadium pentoxide and amorphous MgOV.sub.2O.sub.5-based compound 6 was obtained with the same method as in Example of Production 1-1. This was the anti-rust pigment B6.

Example of Production 1-7: Preparation of Anti-Rust Pigment B7

(8) The amorphous MgOV.sub.2O.sub.5-based compound 5 (500 g) and 500 g of the amorphous MgOV.sub.2O.sub.5-based compound 6 were uniformly mixed using a pestle and mortar and the anti-rust pigment B7 was obtained.

Example of Production 1-8: Preparation of Anti-Rust Pigment B8

(9) The amounts of the raw materials used were changed to 730 g of magnesium oxide and 270 g of vanadium pentoxide and amorphous MgOV.sub.2O.sub.5-based compound 8 was obtained with the same method as in Example of Production 1-1. This was the anti-rust pigment B8.

Example of Production 1-9: Preparation of Anti-rust Pigment 89

(10) The amounts of the raw materials used were changed to 250 g of magnesium oxide and 750 g of vanadium pentoxide and amorphous MgOV.sub.2O.sub.5-based compound 9 was obtained with the same method as in Example of Production 1-1. This was the anti-rust pigment B9.

Example of Production 1-10: Preparation of Anti-rust Pigment B10

(11) The amounts of the raw materials used were changed to 740 g of magnesium oxide and 260 g of vanadium pentoxide and amorphous MgOV.sub.2O.sub.5-based compound 10 was obtained with the same method as in Example of Production 1-1. This was the anti-rust pigment B10.

Example of Production 1-11: Preparation of Anti-rust Pigment B11

(12) The amorphous MgOV.sub.2O.sub.5-based compound 8 (600 g) and 400 g of the amorphous MgOV.sub.2O.sub.5-based compound 10 were uniformly mixed using a pestle and mortar and the anti-rust pigment B11 was obtained.

(13) Moreover, the grain sizes of the anti-rust pigments B1 to B11 obtained in the examples of production outlined above were within the range from 10 to 30 m.

(14) A 10 mass % aqueous solution of the anti-rust pigment was prepared with each of the anti-rust pigments B1 to B11 and the total eluted ion contents and pH values were measured with the methods indicated below.

(15) Preparation of a 10 Mass % Aqueous Solution of an Anti-Rust Pigment

(16) The anti-rust pigment (10 g) and 90 g of ion-exchanged water were added to a wide-necked bottle made of polyethylene. The lid was attached and the liquid suspension obtained on shaking vigorously for 1 minute was left to stand for 24 hours at room temperature and a 10 mass % aqueous solution of the anti-rust pigment was obtained.

(17) Measurement of the Total Eluted Ion Content and pH

(18) The supernatant liquid was collected from each of the mass % aqueous solutions of anti-rust pigment obtained with the method outlined above and, using the filtrate obtained by filtration through a rayon filter paper as the sample, the total eluted ion content was measured using ICP emission spectral analysis apparatus (JY-328 Ultrace, produced by the Horiba Seisakujo Co.). Furthermore, the pH was measured with a pH meter (HM-20S, produced by the Toa Denpa Kogyo Co.). The measured results are summarized in Table 1.

(19) TABLE-US-00001 TABLE 1 Total Eluted Ion Content pH of a of a 10 mass % 10 mass % Anti-rust Mg/V Aqueous Solution Aqueous Pigment (B) (mol ratio) (ppm) Solution B1 2.0 43.2 10.7 B2 2.5 39.6 10.8 B3 3.0 33.7 10.8 B4 4.0 20.1 10.9 B5 4.6 13.4 10.8 B6 1.5 70.6 10.5 B7 3.0 27.8 10.8 B8 6.1 6.2 11.8 B9 0.8 144 10.1 B10 3.1 31.9 8.8 B11 4.9 12.3 11.2

Example 1: Production of Chromium-Free Paint Composition CF1

(20) Epoxy resin (trade name jER 1009, bisphenol A-type epoxy resin, produced by the Mitsubishi Kagaku Co., 80 parts) was heated and dissolved in 120 parts of a mixed solvent (aromatic solvent (trade name Solvesso 100, produced by the Exxon Mobil Chemical Co.)/cyclohexanone/n-butanol=55/27/18 (mass ratio)) using a flask which had been furnished with stirring apparatus, a condenser and a thermometer and an epoxy resin solution which formed a paint film-forming component (A) was obtained. Next 30 parts of cyclohexanone and 30 parts of aromatic solvent (trade name Solvesso 150, produced by the Exxon Mobil Chemical Co.) were added to 200 parts of this epoxy resin solution, 15 parts of the anti-rust pigment B1 were added and dispersed in a sand grind mill until the grain sized reached from 20 to 25 m and a mill base was obtained. A blocked isocyanate compound (trade name Desmodur BL-3175, produced by the Sumika Beyer Urethane Co, 21 parts) and 0.3 part of dibutyltin dilaurate (DBTDL) were added to this mill base and mixed uniformly and the chromium-free paint composition CF1 was obtained.

Examples 2 to 34 and Comparative Examples 1 to 11: Production of Chromium-Free Paint Compositions CF2 to 45

(21) The chromium-free paint compositions CF2 to 45 were obtained with the same method as in Example 1 in accordance with the compounding compositions shown in Tables 2 to 8.

(22) TABLE-US-00002 TABLE 2 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Chrome-free Paint Composition CF1 CF2 CF3 CF4 CF5 CF6 CF7 (A) Paint Epoxy resin solution 200 200 200 200 200 200 200 Film- (Note 1) Forming Modified Epoxy Resin Resin (Note 2) Polyester Resin (Note 3) (C) Blocked polyisocyanate 21 21 21 21 21 21 21 Cross- Compound (Note 4) linking Melamine Resin Agent (Note 5) (B) B1 45 Anti-rust B2 45 Pigment B3 15 45 70 B4 45 B5 45 B6 B7 B8 B9 B10 B11 Pigment Titanium Dioxide(Note 9) Kaolin (Note 10) Precipitated Barium Sulfate (Note 11) Finely Powdered Silica (Note 12) Curing DBTDL (Note 13) 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Catalyst Organic Aromatic Solvent 30 30 30 30 30 30 30 Solvent (Note 14) Cyclohexanone 30 30 30 30 30 30 30 (B) Component/{(A) Component + (C) 16% 47% 73% 47% 47% 47% 47% Component} (mass %) (C) Component/(A) Component (mass %) 20% 20% 20% 20% 20% 20% 20% Paint Film Adhesion on Base Steel Sheet Performance Boiling Water Resistance Evaluation (Paint film anomalies) Boiling Water Resistance (Adhesion on Base Steel Sheet) Bending Workability (2T) Bending Workability (3T) Corrosion Resistance (4T Bend Worked Part) Corrosion Resistance (Cross-cut Part) Corrosion Resistance (Edge Part)

(23) TABLE-US-00003 TABLE 3 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Ex. 13 Ex. 14 Chrome-free Paint Composition CF8 CF9 CF10 CF11 CF12 CF13 CF14 (A) Paint Epoxy resin solution 200 200 200 200 200 200 200 Film- (Note 1) Forming Modified Epoxy Resin Resin (Note 2) Polyester Resin (Note 3) (C) Blocked polyisocyanate 21 21 21 21 21 21 16 Cross- Compound (Note 4) linking Melamine Resin 5 Agent (Note 5) (B) B1 Anti-rust B2 Pigment B3 45 45 45 B4 B5 B6 45 B7 45 B8 B9 B10 45 B11 45 Pigment Titanium Dioxide(Note 9) 15 15 Kaolin (Note 10) 5 5 Precipitated Barium Sulfate 5 (Note 11) Finely Powdered Silica 5 (Note 12) Curing DBTDL 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Catalyst (Note 13) Organic Aromatic Solvent 30 30 30 30 30 30 30 Solvent (Note 14) Cyclohexanone 30 30 30 30 30 30 30 (B) Component/{(A) Component + (C) 47% 47% 47% 47% 47% 47% 47% Component} (mass %) (C) Component/(A) Component (mass %) 20% 20% 20% 20% 20% 20% 19% Paint Film Adhesion on Base Steel Sheet Performance Boiling Water Resistance Evaluation (Paint film anomalies) Boiling Water Resistance (Adhesion on Base Steel Sheet) Bending Workability (2T) Bending Workability (3T) Corrosion Resistance (4T Bend Worked Part) Corrosion Resistance (Cross-cut Part) Corrosion Resistance (Edge Part)

(24) TABLE-US-00004 TABLE 4 Ex. 15 Ex. 16 Ex. 17 Ex. 18 Ex. 19 Ex. 20 Ex. 21 Chrome-free Paint Composition CF15 CF16 CF17 CF18 CF19 CF20 CF21 (A) Paint Epoxy resin solution 200 200 Film- (Note 1) Forming Modified Epoxy Resin 200 Resin (Note 2) Polyester Resin 145 145 145 145 (Note 3) (C) Blocked polyisocyanate 50 60 21 21 21 21 Cross- Compound (Note 4) linking Melamine Resin 10 Agent (Note 5) (B) B1 45 Anti-rust B2 Pigment B3 45 45 45 15 45 70 B4 B5 B6 B7 B8 B9 B10 B11 Pigment Titanium Dioxide(Note 9) Kaolin (Note 10) Precipitated Barium Sulfate (Note 11) Finely Powdered Silica (Note 12) Curing DBTDL (Note 13) 0 0.6 0.3 0.3 0.3 0.3 0.3 Catalyst Organic Aromatic Solvent 30 30 30 30 30 30 30 Solvent (Note 14) Cyclohexanone 30 30 30 30 30 30 30 (B) Component/{(A) Component + (C) 52% 38% 36% 16% 47% 73% 47% Component} (mass %) (C) Component/(A) Component (mass %) 8% 47% 56% 20% 20% 20% 20% Paint Film Adhesion on Base Steel Sheet Performance Boiling Water Resistance Evaluation (Paint film anomalies) Boiling Water Resistance (Adhesion on Base Steel Sheet) Bending Workability (2T) Bending Workability (3T) Corrosion Resistance (4T Bend Worked Part) Corrosion Resistance (Cross-cut Part) Corrosion Resistance (Edge Part)

(25) TABLE-US-00005 TABLE 5 Ex. 22 Ex. 23 Ex. 24 Ex. 25 Ex. 26 Ex. 27 Ex. 28 Chrome-free Paint Composition CF22 CF23 CF24 CF25 CF26 CF27 CF28 (A) Paint Epoxy resin solution Film- (Note 1) Forming Modified Epoxy Resin Resin (Note 2) Polyester Resin 145 145 145 145 145 145 145 (Note 3) (C) Blocked polyisocyanate 21 21 21 21 21 21 21 Cross- Compound (Note 4) linking Melamine Resin Agent (Note 5) (B) B1 Anti-rust B2 45 Pigment B3 B4 45 B5 45 B6 45 B7 45 B8 B9 B10 45 B11 45 Pigment Titanium Dioxide(Note 9) Kaolin (Note 10) Precipitated Barium Sulfate (Note 11) Finely Powdered Silica (Note 12) Curing DBTDL (Note 13) 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Catalyst Organic Aromatic Solvent 30 30 30 30 30 30 30 Solvent (Note 14) Cyclohexanone 30 30 30 30 30 30 30 (B) Component/{(A) Component + (C) 47% 47% 47% 47% 47% 47% 47% Component} (mass %) (C) Component/(A) Component (mass %) 20% 20% 20% 20% 20% 20% 20% Paint Film Adhesion on Base Steel Sheet Performance Boiling Water Resistance Evaluation (Paint film anomalies) Boiling Water Resistance (Adhesion on Base Steel Sheet) Bending Workability (2T) Bending Workability (3T) Corrosion Resistance (4T Bend Worked Part) Corrosion Resistance (Cross-cut Part) Corrosion Resistance (Edge Part)

(26) TABLE-US-00006 TABLE 6 Ex. 29 Ex. 30 Ex. 31 Ex. 32 Ex. 33 Ex. 34 Chrome-free Paint Composition CF29 CF30 CF31 CF32 CF33 CF34 (A) Paint Epoxy resin solution Film- (Note 1) Forming Modified Epoxy Resin Resin (Note 2) Polyester Resin 145 145 145 145 145 145 (Note 3) (C) Blocked polyisocyanate 21 21 16 50 60 Cross- Compound (Note 4) linking Melamine Resin 5 10 Agent (Note 5) (B) B1 Anti-rust B2 Pigment B3 45 45 45 45 45 45 B4 B5 B6 B7 B8 B9 B10 B11 Pigment Titanium Dioxide(Note 9) 15 15 Kaolin (Note 10) 5 5 Precipitated Barium Sulfate 5 (Note 11) Finely Powdered Silica 5 (Note 12) Curing DBTDL (Note 13) 0.3 0.3 0.3 0 0.6 0.6 Catalyst Organic Aromatic Solvent 30 30 30 30 30 30 Solvent (Note 14) Cyclohexanone 30 30 30 30 30 30 (B) Component/{(A) Component + (C) 47% 47% 47% 52% 38% 36% Component} (mass %) (C) Component/(A) Component (mass %) 20% 20% 19% 8% 47% 56% Paint Film Adhesion on Base Steel Sheet Performance Boiling Water Resistance Evaluation (Paint film anomalies) Boiling Water Resistance (Adhesion on Base Steel Sheet) Bending Workability (2T) Bending Workability (3T) Corrosion Resistance (4T Bend Worked Part) Corrosion Resistance (Cross-cut Part) Corrosion Resistance (Edge Part)

(27) TABLE-US-00007 TABLE 7 Comp. Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Chrome-free Paint Composition CF35 CF36 CF37 CF38 CF39 CF40 (A) Paint Epoxy resin solution 200 200 200 200 200 200 Film- (Note 1) Forming Modified Epoxy Resin Resin (Note 2) Polyester Resin (Note 3) (C) Blocked polyisocyanate 21 21 21 21 21 21 Cross- Compound (Note 4) linking Melamine Resin Agent (Note 5) (B) B1 Anti-rust B2 Pigment B3 8 B4 B5 B6 B7 B8 45 B9 45 B10 B11 Other Anti- Vanadium Pentoxide (Note 6) 45 20 rust Magnesium Oxide (Note 7) 25 Pigments Calcium Vanadate (Note 8) 45 Pigment Titanium Dioxide(Note 9) Kaolin (Note 10) Precipitated Barium Sulfate (Note 11) Finely Powdered Silica (Note 12) Curing DBTDL (Note 13) 0.3 0.3 0.3 0.3 0.3 0.3 Catalyst Organic Aromatic Solvent 30 30 30 30 30 30 Solvent (Note 14) Cyclohexanone 30 30 30 30 30 30 (B) Component/{(A) Component + (C) 0% 0% 0% 47% 47% 8% Component} (mass %) (C) Component/(A) Component (mass %) 20% 20% 20% 20% 20% 20% Paint Film Adhesion on Base Steel Sheet Performance Boiling Water Resistance X Evaluation (Paint film anomalies) Boiling Water Resistance X X (Adhesion on Base Steel Sheet) Bending Workability (2T) Bending Workability (3T) Corrosion Resistance X X X X (4T Bend Worked Part) Corrosion Resistance X X (Cross-cut Part) Corrosion Resistance X X X X X X (Edge Part)

(28) TABLE-US-00008 TABLE 8 Comp. Comp. Comp. Comp. Comp. Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Chrome-free Paint Composition CF41 CF42 CF43 CF44 CF45 (A) Paint Epoxy resin solution 200 Film- (Note 1) Forming Modified Epoxy Resin Resin (Note 2) Polyester Resin 145 145 145 145 (Note 3) (C) Blocked polyisocyanate 21 21 21 21 21 Cross- Compound (Note 4) linking Melamine Resin Agent (Note 5) (B) B1 Anti-rust B2 Pigment B3 90 8 90 B4 B5 B6 B7 B8 45 B9 45 B10 B11 Other Anti- Vanadium Pentoxide (Note 6) rust Magnesium Oxide (Note 7) Pigments Calcium Vanadate (Note 8) Pigment Titanium Dioxide(Note 9) Kaolin (Note 10) Precipitated Barium Sulfate (Note 11) Finely Powdered Silica (Note 12) Curing DBTDL (Note 13) 0.3 0.3 0.3 0.6 0.3 Catalyst Organic Aromatic Solvent 30 30 30 30 30 Solvent (Note 14) Cyclohexanone 30 30 30 30 30 (B) Component/{(A) Component + (C) 94% 47% 47% 8% 94% Component} (mass %) (C) Component/(A) Component (mass %) 20% 27% 27% 27% 27% Paint Film Adhesion on Base Steel Sheet X X Performance Boiling Water Resistance Evaluation (Paint film anomalies) Boiling Water Resistance X X X (Adhesion on Base Steel Sheet) Bending Workability (2T) Bending Workability (3T) Corrosion Resistance X X X (4T Bend Worked Part) Corrosion Resistance X (Cross-cut Part) Corrosion Resistance X X X X X (Edge Part)

(29) Details of each of the compounded components shown in Tables 2 to 8 are indicated below.

(30) (Note 1) The epoxy resin solution was obtained by heating and dissolving 80 parts of epoxy resin (trade name jER 1009, produced by the Mitsubishi Kagaku Co., bisphenol A-type epoxy resin, resin solid fraction 100 mass %, epoxy equivalent 2,500 to 3,500, number average molecular weight 3,800) in 120 parts of mixed solvent (aromatic solvent (trade name Solvesso 100, produced by the Exxon Mobil Chemicals Co.)/cyclohexanone/n-butanol=55/27/18).
(Note 2) Modified epoxy resin (trade name Epiclon H-304-40, produced by the DIC Co., diethanolamine modified epoxy resin, resin solid fraction 40 mass %, number average molecular weight 3,500)
(Note 3) Polyester resin (trade name LH-822, produced by the Ebonik Degussa Co., resin solid fraction 55 mass %, number average molecular weight 5,000, hydroxyl group value 50 mgKOH/g)
(Note 4) Blocked polyisocyanate compound (trade name Desmodur BL-3175, produced by the Sumika Beyer Urethane Co., HDI isocyanurate type polyisocyanate compound blocked with methyl ethyl ketone oxime, resin solid fraction 75 mass %, NCO approximately 11.1 mass %)
(Note 5) n-Butylated melamine resin (trade name Yuban 122, produced by the Mitsui Kagaku Co., resin solid fraction 60 mass %)
(Note 6) Vanadium pentoxide: commercial reagent
(Note 7) Magnesium oxide: commercial reagent
(Note 8) Calcium vanadate: Commercial reagent
(Note 9) Titanium dioxide (trade name R-960, produced by the DuPont Co.)
(Note 10) Kaolin (trade name Hydrite MS, produced by Imerys Minerals Co.)
(Note 11) Precipitated barium sulfate (trade name SS-50, produced by the Sakai Kagaku Kogyo Co.)
(Note 12) Finely powdered silica (trade name Nipsil E-200A, produced by the Toso Silica Co.)
(Note 13) DBTDL (produced by the Nitto Kase Co., involatile fraction 100%)
(Note 14) Aromatic solvent (trade name Solvesso 150, produced by the Exxon Mobil Chemical Co.)

(31) Test pieces on which a chromium-free paint composition of this invention had been used as an undercoat paint were prepared with the method outlined below with each of the chromium-free paint compositions CF1 to CF45 and performance evaluations of the paint films were carried out.

(32) Preparation of a Test Piece

(33) A chromium-free paint composition of this invention was coated with a bar coater in such a way as to provide a dry film thickness of 5 m on an aluminum/zinc alloy plated steel sheet of thickness 0.35 mm which had been subjected to a chemical forming treatment and baked in a hot-draft drier for 40 seconds with a maximum temperature reached by the sheet of 210 C. and a undercoat paint film was formed. A polyester resin-based top-coat paint (trade name Precolor HD0030, produced by the BASF Coatings Japan Co. Ltd., brown color) was coated over the undercoat paint film with a bar coater in such a way as to provide a dry film thickness of 15 m and baked in a hot-draft drier for 40 seconds with a maximum temperature reached by the sheet of 220 C., a top-coat paint film was formed and a test piece was obtained.

(34) The test piece obtained was subjected to the paint film performance evaluations indicated below and the results have been shown in Tables 2 to 8.

(35) Adhesion on the Base Steel Sheet

(36) A square pattern of one hundred 1 mm1 mm squares was formed in the paint film of a test piece using a cutter knife. The part of the paint film on which the pattern had been formed was pushed out from the reverse side of the test piece with a punch in such a way that the distance from the pushed-out surface of the test piece to the tip of the punch was 6 mm using an Erichsen testing machine. Subsequently cellophane tape was pressed and stuck firmly over the pattern part of the pushed-out paint film and peeled off in one action with the end of the tape at an angle of 45, the state of the pattern was observed and an evaluation was made on the basis of the following criteria:

(37) : No peeling of the paint film was observed.

(38) X: Peeling of the paint film was observed.

(39) Boiling Water Resistance

(40) A test piece was immersed for 2 hours in boiling water and allowed to cool for 2 hours at room temperature in accordance with JIS K 5600-6-2 and then the test piece was evaluated using the methods (1) and (2) below.

(41) (1) Paint Film Abnormality

(42) The paint film of the test piece was observed for abnormalities and evaluated on the basis of the following criteria.

(43) : No paint film abnormality at all was observed.

(44) : Slight bulging of the paint film was observed.

(45) X: Distinct bulging of the paint film was observed.

(46) (2) Adhesion on the Base Metal Sheet

(47) A square pattern of one hundred 1 mm1 mm squares was formed with a cutter knife in the paint film of a test piece, cellophane tape was firmly stuck over the pattern part and peeled away with the end of the tape at an angle of 45 in accordance with JIS-K 5600-5-6, the state of the pattern was observed and an evaluation was made in accordance with Table 1, Classification of Test Results of JIS K 5600-5-6.

(48) : Class 0

(49) : Class 1

(50) X Classes 2 to 5

(51) Bending Workability

(52) A test piece was bent through 180 in such a way that sheets similar to the test piece were inserted. At this time the number of sheets similar to the test piece within the test piece is indicated by 0T, 2T and the like. For example, 0T indicates that the test piece was bent with no sheet similar to the test piece inserted and 2T indicates that the test piece was bent in such a way that two sheets similar to the test piece were inserted. In the performance evaluation of this invention 2T and 3T tests were carried out and cellophane tape was firmly stuck over the apex part after bending, the tape was peeled away in one action with the end of the tape at an angle of 45 and an assessment was made on the basis of the following criteria depending on the state of peeling of the paint film.

(53) : No peeling of the paint film was observed.

(54) : Slight peeling of the paint film was observed.

(55) X: Peeling of the paint film was observed.

(56) Corrosion Resistance

(57) Corrosion resistance test pieces were prepared in accordance with (1) to (3) below.

(58) (1) A test piece of 70 mm150 mm was cut in such a way that the edge part of the test piece had a burr facing the side on which the paint film had been formed (up-burr) and a burr facing the opposite side to that on which the paint film had been formed (down-burr).
(2) The test piece was subjected to 4T bending.
(3) A cross-cut of a depth which reached the base material surface was made in the middle part of the paint film on the side on which the paint film had been formed in such a way that it did not extend over the 4T bend part of (2).

(59) The prepared corrosion resistance test piece was subjected to a salt water spray test (SST) of duration 500 hours in accordance with JIS K5600-7-1 and the state of the edge part, the 4T bend part and the cross cut part of the corrosion resistance test piece after the test were evaluated on the basis of the following criteria:

(60) Edge Part

(61) The widths of the edge creep of the up-burr and the down-burr were measured and evaluated on the basis of the following criteria:

(62) : The average value of the edge creep width of the up-burr and the down-burr was less than 4 mm.

(63) : The average value of the edge creep width of the up-burr and the down-burr was at least 4 mm but less than 10 mm.

(64) X The average value of the edge creep width of the up-burr and the down-burr was 10 mm or more.

(65) 4T Bend Part

(66) The state of occurrence of white rust at the apex where the bending had been carried out was evaluated on the basis of the following criteria:

(67) : Virtually no white rust was observed.

(68) : A little white rust was observed.

(69) X: White rust was observed.

(70) Cross-Cut Part

(71) The state of occurrence of white rusting of the cross cut part was observed and the width of the swollen paint film was measured and evaluated on the basis of the following criteria:

(72) : Virtually no white rust was observed and the width of the bulging paint film was less than 2 mm.

(73) : Some white rust was observed and the width of the bulging paint film was at least 2 mm but less than 5 mm.

(74) X: White rust was observed and the width of the bulging paint film was 5 mm or more.