Coating method and coating film

Abstract

A coating method in which a first coating composition is applied to the surface of an object to be coated to obtain a first uncured coating film, a second coating composition is applied to the first uncured coating film to obtain a second uncured coating film, and the first uncured coating film and the second uncured coating film are then cured simultaneously by heating, wherein the first coating composition contains a hydroxyl group-containing resin component and the second coating composition contains an isocyanate component containing a triisocyanate represented by general formula (I) (in the formula, the multiple Y.sup.1 each independently are a single bond or a C1-20 divalent hydrocarbon group which optionally contains one or more selected from the group consisting of an ester group and an ether group, and R.sup.1 is a hydrogen atom or a C1-12 monovalent hydrocarbon group) and the above-mentioned hydroxyl group-containing resin component, is provided. ##STR00001##

Claims

1. A coating method comprising: obtaining a first uncured coating film by applying a first coating composition to a surface of an object to be coated; obtaining a second uncured coating film by applying a second coating composition to the first uncured coating film; and curing simultaneously the first uncured coating film and the second uncured coating film by conducting heating, wherein the first coating composition comprises a hydroxyl group-containing resin component and the second coating composition comprises: an isocyanate component (a) comprising a triisocyanate of general formula (I); the hydroxyl group-containing resin component; and a polyisocyanate component (b) having an isocyanurate structure formed by at least one isocyanate selected from the group consisting of aliphatic isocyanates and alicyclic isocyanates, wherein a mass ratio (a):(b) of the isocyanate component (a) and the polyisocyanate component (b) is 4:6 to 9:1, ##STR00009## in the general formula (I), multiple Y.sup.1 each independently represents a single bond or a C1-20 divalent hydrocarbon group which may comprise at least one selected from the group consisting of an ester group and an ether group, the multiple Y.sup.1 are identical to or different from each other, and R.sup.1 is a hydrogen atom or a C1-12 monovalent hydrocarbon group.

2. The coating method according to claim 1, wherein the first uncured coating film and the second uncured coating film are cured simultaneously by the isocyanate component (a).

3. The coating method according to claim 1, wherein, in the second coating composition, a molar ratio (NCO/OH) of an isocyanate group (NCO group) in the isocyanate component, relative to a hydroxyl group (OH group) in the hydroxyl group-containing resin component, is 1.0 to 5.0.

4. The coating method according to claim 1, wherein the triisocyanate comprises an ester group.

5. The coating method according to claim 1, wherein the triisocyanate is a lysine triisocyanate.

6. The coating method according to claim 1, wherein, the first coating composition further comprises an isocyanate component, and, in the first coating composition, a molar ratio (NCO/OH) of an isocyanate group (NCO group) in the isocyanate component, relative to a hydroxyl group (OH group) in the hydroxyl group-containing resin component, is 1.0 or less.

7. The coating method according to claim 6, wherein the isocyanate component comprised in the first coating composition comprises a blocked isocyanate.

8. The coating method according to claim 1, wherein the object to be coated is an uncured coating film.

9. The coating method according to claim 8, wherein the first uncured coating film, the second uncured coating film and the object to be coated are cured simultaneously by the isocyanate component (a).

10. The coating method according to claim 8, wherein, in the second coating composition, a molar ratio (NCO/OH) of an isocyanate group (NCO groups) in the isocyanate component, relative to a hydroxyl group (OH group) in the hydroxyl group-containing resin component, is 1.0 to 5.0.

11. The coating method according to claim 8, wherein the triisocyanate comprises an ester group.

12. The coating method according to claim 8, wherein the triisocyanate is a lysine triisocyanate.

13. The coating method according to claim 8, wherein, the first coating composition further comprises an isocyanate component, and, in the first coating composition, a molar ratio (NCO/OH) of an isocyanate group (NCO group) in the isocyanate component, relative to a hydroxyl group (OH group) in the hydroxyl group-containing resin component, is 1.0 or less.

14. The coating method according to claim 13, wherein the isocyanate component comprised in the first coating composition comprises a blocked isocyanate.

15. The coating method according to claim 1, wherein the triisocyanate is 4-isocyanatemethyl-1,8-octamethylene diisocyanate.

Description

EXAMPLES

(1) Hereinafter, the present embodiment will be described further specifically with reference to specific examples and comparative examples. However, the present embodiment is not limited to the following examples and comparative examples unless the present embodiment exceeds the gist thereof. The physical properties of isocyanate components and evaluation of coating films were conducted as shown below. The terms “part” and “%” indicate “part by mass” and “% by mass” respectively, unless particularly mentioned.

(2) <Method for Measuring Physical Properties>

(3) [Physical Property 1] Viscosity of Isocyanate Component

(4) The viscosity of the isocyanate component was measured at 25° C. using an E-type viscometer (manufactured by Tokimec, Inc.). In the measurement, a standard rotor (1°34′×R24) was used. The number of rotation was described below.

(5) (Number of Rotation)

(6) 100 rpm (at less than 128 mPa.Math.s)

(7) 50 rpm (at 128 mPa.Math.s or more but less than 256 mPa.Math.s)

(8) 20 rpm (at 256 mPa.Math.s or more but less than 640 mPa.Math.s)

(9) 10 rpm (at 640 mPa.Math.s or more but less than 1280 mPa.Math.s)

(10) 5 rpm (at 1280 mPa.Math.s or more but less than 2560 mPa.Math.s)

(11) [Physical Property 2] NCO Content of Isocyanate Component

(12) Isocyanate groups in each isocyanate component were neutralized with an excess amount of 2N amine, followed by conducting back titration using 1N hydrochloric acid to determine the NCO content (% by mass) of the isocyanate component.

(13) [Physical Property 3] Calculated NCO Content of Isocyanate Component

(14) The NCO content of an isocyanate compound used to synthesize a blocked isocyanate was determined by the method described in the [physical property 2], and the mass of NCO was determined from the mass of the isocyanate compound charged. Then, the calculated NCO content was determined using the following formula.
Calculated NCO content (% by mass)=100×the mass of NCO/the total mass of the isocyanate compound charged
[Physical Property 4] Number-Average Molecular Weight (Mn) of Isocyanate Component

(15) The number-average molecular weight (Mn) of an isocyanate component was determined based on polystyrene by conducting gel permeation chromatography (hereinafter, may be referred to as “GPC”) measurement using the following device and conditions.

(16) (Measurement Conditions)

(17) Device: “HLC-8120 GPC” (trade name) manufactured by Tosoh Corporation

(18) Column used: Manufactured by Tosoh Corporation “TSK gel Super H1000” (trade name)×1 column. “TSK gel Super H2000” (trade name)×1 column, “TSK GEL Super H3000” (trade name)×1 column.

(19) Carrier: Tetrahydrofuran

(20) Detection method: Differential refractometer

(21) Sample concentration: 5 mass/% by volume

(22) Outflow: 0.6 mL/min,

(23) Column temperature: 40° C.

(24) [Physical Property 5] Average Number of Isocyanate Groups (Fn) in Isocyanate Component

(25) The average number of isocyanate groups (Fn) in an isocyanate component was determined by the following formula.
Average number of isocyanate groups (Fn)=[Number-average molecular weight (Mn)×NCO content (% by mass)×0.01]/42
[Physical Property 6] Concentration of Difunctional Groups (Difunctional Group Content) in Isocyanate Component

(26) The concentration of difunctional groups in an isocyanate component was determined by conducting GPC measurement using the same device and conditions as those used to measure the number-average molecular weight, followed by calculating the ratio (%) of the area of diisocyanate monomers and diisocyanate dimers, relative to the total area of the isocyanate component.

(27) <Evaluation Method>

(28) [Evaluation 1] Coating Film Hardness

(29) The Koenig hardness of a sample coating plate obtained in each Examples and Comparative Examples was measured at 23° C. using a Koenig hardness tester (manufactured by BYK). As the evaluation criteria, 28 times or more were evaluated as ⊚+, 24 times to 27 times were evaluated as ⊚, 20 times to 23 times were evaluated as ∘, 15 times to 19 times were evaluated as Δ, and 14 times or less were evaluated as x.

(30) [Evaluation 2] Penetration Property into Lower Layer

(31) The cross section of multilayered coating films of a sample coating plate obtained in each Examples and Comparative Examples was processed by broad ion beam. The sample coating plate was cooled until immediately before conducting processing so as to suppress thermal damage when processing was conducted. Specifically, the sample coating plate was left still for 12 hours in a cooling device at −20° C. Thus, a smooth coating film cross section was obtained.

(32) The resultant was subjected to endoscopic IR measurement at the vicinity of the adhered surface between the resultant coating film and the substrate to obtain values A and B of the peak absorbance at 1730±50 cm.sup.−1 and 2960±50 cm.sup.−1, respectively, and then the ratio A/B was evaluated in accordance with the following criteria: the penetration property into lower layer was evaluated as ⊚ when the ratio A/B was 5.5 or more; the penetration property into lower layer was evaluated as ∘ when the ratio A/B was 5.2 or more but less than 5.5; the penetration property into lower layer was evaluated as Δ when the ratio A/B was 4.9 or more but less than 5.2; and the penetration property into lower layer was evaluated as x when the ratio A/B was less than 4.9.

(33) [Evaluation 3] Adhesiveness

(34) A sample coating plate obtained in each Examples and Comparative Examples was soaked in water at 23° C., and, after 1 day passed, the adhesiveness test was conducted in accordance with JTS K5600-5-6. The results were evaluated in accordance with the following evaluation criteria: the peeling number of 0 was evaluated as ⊚ the peeling number of 1 or more but less than 10 was evaluated as ∘; the peeling number of 10 or more but less than 50 was evaluated as Δ; and the peeling number of 50 or more but 100 or less was evaluated as x.

(35) [Evaluation 4] Yellowing Property when Baked

(36) A white tile plate was used as a substrate instead of JIS G 3141 (SPCC, SD) cationic electrodeposition coated plate used in each Examples and Comparative examples to obtain b value of a sample coating plate obtained in each Examples and Comparative Examples using a color-difference meter (manufactured by KONICA MINOLTA OPTICS, INC., color-difference meter “CR-231”). The yellowing property when baked was evaluated using the resultant b value in accordance with the following evaluation criteria: the yellowing property was evaluated as ∘ when the b value was 0.0; and the yellowing property was evaluated as x when the b value was 0.1 or more.

[Synthesis Example 1] Synthesis of GTI

(37) 275 g of glutamic acid hydrochloride, 800 g of ethanolamine hydrochloride, and 150 ml of toluene were put into a four-necked flask equipped with a stirrer, a thermometer, and a gas inlet tube, and then the mixture was heated to reflux at 110° C. for 24 hours while blowing a hydrogen chloride gas thereinto until azeotropy of water was not confirmed. The resultant reaction mixture was recrystallized in a mixture liquid composed of methanol and ethanol to obtain 270 g of bis(2-aminoethyl)glutamate trihydrochloride, 85 g of the bis(2-aminoethyl)glutamate trihydrochloride was suspended in 680 g of o-dichlorobenzene, and then the temperature of the reaction liquid was raised while conducting stirring. When the temperature thereof reached 135° C., a phosgene was blown thereinto at a rate of 0.8 mol/hour for 13 hours. The resultant was subjected to filtration, condensation under reduced pressure, and then purification with a thin-film evaporator to obtain 54 g of GTI. The viscosity of the GTI was 200 mPa.Math.s/25° C., the NCO content thereof was 40% by mass, the average number of isocyanate groups was 3.0, and the concentration of difunctional groups was 0%.

[Synthesis Example 2] Synthesis of LTI

(38) 122.2 g of ethanolamine, 100 nil of o-dichlorobenzene, and 420 ml of toluene were put into a four-necked flask equipped with a stirrer, a thermometer, and a gas inlet tube, and then hydrogen chloride gas was introduced therein under ice-cooling to convert the ethanolamine into a hydrochloride. Then, 182.5 g of lysine hydrochloride was added to the resultant, and then the reaction liquid was heated at 80° C. to dissolve the ethanolamine hydrochloride therein, followed by introducing a hydrogen chloride gas thereinto to obtain lysine dihydrochloride. In addition, a hydrogen chloride gas was passed therethrough at a rate of 20 ml/minute to 30 ml/minute, and the reaction liquid was heated at 116° C. until water was not distilled off. The resultant reaction mixture was recrystallized in a mixture liquid composed of methanol and ethanol to obtain 165 g of a lysine β-aminoethyl ester trihydrochloride. 100 g of the lysine β-aminoethyl ester trihydrochloride was pulverized to fine powders, and then suspended in 1200 ml of o-dichlorobenzene, followed by heating the reaction liquid while conducting stirring. When the temperature thereof reached 120° C., phosgene was blown into the reaction liquid at a rate of 0.4 mol/hour for 10 hours, followed by heating the reaction liquid to 150° C. The suspension was almost dissolved. After the resultant was cooled and then subjected to filtration, the dissolved phosgene and solvent were distilled off under the reduced pressure, and then the resultant was subjected to vacuum distillation to obtain 80.4 g of a colorless and transparent LTI having a boiling point of 155° C./0.022 mmHg to 157° C./0.022 mmHg. The viscosity of the LTI was 26 mPa.Math.s/25° C., the NCO content was 47% by mass, the average number of isocyanate groups was 3.0, and the concentration of difunctional groups was 0%.

[Synthesis Example 3] Synthesis of NTI

(39) 1060 g of 4-aminomethyl-1,8-octamethylene diamine (hereinafter, may be referred to as “triamine”) was dissolved in 1500 g of methanol in a four-necked flask equipped with a stirrer, a thermometer, and a gas inlet tube, and then 1800 ml of 35% concentrated hydrochloric acid was added dropwise into the resultant solution gradually while conducting cooling. The resultant was concentrated by removing methanol and water therefrom under reduced pressure, and then dried at 60° C./5 mmHg for 24 hours to obtain a white solid triamine hydrochloride. 650 g of the resultant triamine hydrochloride was pulverized to fine powders, and then suspended in 5000 g of o-dichlorobenzene, followed by raising the temperature of the reaction liquid while conducting stirring. When the temperature reached 100° C., a phosgene was blown thereinto at a rate of 200 g/Hr, and risig temperature was kept until the temperature reached 180° C., followed by maintaining the temperature at 180° C. while blowing the phosgene thereinto for 12 hours. The dissolved phosgene and solvent were distilled off under reduced pressure, followed by conducting vacuum distillation to obtain 420 g of a colorless and transparent NTI having a boiling point of 161° C./1.2 mmHg to 163° C./1.2 mmHg. The viscosity of the NTI was 10 mPa.Math.s/25° C., the NCO content was 50% by mass, the average number of isocyanate groups was 3.0, and the concentration of difunctional groups was 0%.

[Synthesis Example 4] Synthesis of Polyisocyanate P-1

(40) 50 g of LTI and 0.05 g of isobutabol were charged in a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube under nitrogen atmosphere, and then the temperature thereof was maintained at 80° C. for 2 hours. Then, tetramethyl ammonium caprate was added thereto to allow the reaction to proceed, and then, when the NCO content became 37% by mass, dibutyl phosphate was added thereto to terminate the reaction. Then, the reaction liquid was maintained at 120° C. for 15 minutes to obtain a polyisocyanate P-1. The viscosity of the polyisocyanate P-1 was 400 mPa.Math.s/25° C., the NCO content was 37% by mass, the average number of isocyanate groups was 6.2, and the concentration of difunctional groups was 0%.

[Synthesis Example 5] Synthesis of Polyisocyanate P-2

(41) 50 g of HDI was put in a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube under nitrogen atmosphere, and the temperature in the reactor was maintained at 80° C. while conducting stirring. Tetramethyl ammonium caprate was added thereto to allow the reaction to proceed, and, when the yield became 40%, dibutyl phosphate was added thereto to terminate the reaction. Then, the resultant was maintained at 120° C. for 15 minutes to obtain a polyisocyanate P-2. The viscosity of the polyisocyanate P-2 was 5 mPa.Math.s/25° C., the NCO content was 43% by mass, the average number of isocyanate groups was 2.5, and the concentration of difunctional groups was 60%.

[Synthesis Example 6] Synthesis of Polyisocyanate P-3

(42) Unreacted HDI was removed from the polyisocyanate P-2 obtained in the synthesis example 5 using a thin-film evaporator to obtain a polyisocyanate P-3. The viscosity of the polyisocyanate P-3 was 2300 mPa.Math.s/25° C., the NCO content was 21.5% by mass, the average number of isocyanate groups was 3.3, and the concentration of difunctional groups was 0%.

[Synthesis Example 7] Synthesis of Polyisocyanate P-4

(43) 50 g of HDI was put in a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube under nitrogen atmosphere, and the temperature in the reactor was maintained at 60° C. while conducting stirring. 0.5 g of trisdiethylaminophosphine was added thereto to allow the reaction to proceed. After 4 hours passed, 0.4 g of phosphoric acid was added to the resultant and then the resultant was maintained at 60° C. for 1 hour to confirm that the reaction was terminated and the reaction was obtained. After the reaction liquid was subjected to filtration, unreacted HDI was removed using a thin-film evaporator to obtain a polyisocyanate P-4. The viscosity of the polyisocyanate P-4 was 52 mPa.Math.s/25° C., the NCO content was 24% by mass, the average number of isocyanate groups was 2.1, and the concentration of difunctional groups was 76%.

[Synthesis Example 8] Synthesis of Polyisocyanate P-5

(44) 50 g of HDI, 10.7 g of trimethyl phosphate, 10.7 g of methyl cellosolve, and 0.25 g of water were put in a four-necked flask equipped with a stirrer, a thermometer, a reflux condenser, and a nitrogen inlet tube under nitrogen atmosphere. Then, the temperature in the reactor was maintained at 120° C. for 2 hours, and then further maintained at 160° C. for 4 hours to allow the reaction to proceed, while conducting stirring. Then, the resultant was cooled to room temperature to obtain a reaction liquid. After the reaction liquid was subjected to filtration, and then unreacted HDI was removed using a thin-film evaporator to obtain a polyisocyanate P-5. The viscosity of the polyisocyanate P-5 was 1100 mPa.Math.s/25° C., the NCO content was 24% by mass, the average number of isocyanate groups was 3.0, and the concentration of difunctional groups was 13%.

Example 1

(45) Acrylic polyol (manufactured by DIC Corporation under the trade name of “ACRYDIC A-801”) diluted with butyl acetate/xylene (mass ratio 1/1) such that the resin solid content became 18% by mass was applied as a first coating composition on a JIS G 3141 (SPCC, SD) cationic electrodeposition coated plate used as a substrate such that the dried coating film thickness became 20 μm, and then preheated at 80° C. for 3 minutes. After conducting preheating, a second coating composition obtained by blending acrylic polyol (manufactured by Allnex Ltd., under the trade name of “SETALUX 1767”) and LTI at a molar ratio, isocyanate group/hydroxyl group (NCO/OH), of 1.5, and then diluting the mixture with butyl acetate such that the resin solid content become 50% by mass was applied thereon such that the dried coating film thickness became 35 μm. Thereafter, the resultant was heated at 80° C. for 30 minutes to obtain a sample coating plate on which multilayered coating films are coated. The resultant sample coating plate was evaluated by the above-mentioned method in terms of the coating film hardness, the penetration property into lower layer and the adhesiveness. As the result, the coating film hardness was evaluated as ∘, the penetration property into lower layer was evaluated as ⊚, and the adhesiveness was evaluated as ⊚. The results are also shown in Table 1.

Examples 2 to 13 and Comparative Examples 1 to 4

(46) The sample coating plate was obtained by the same method as that of Example 1 except that the kind of the isocyanate component, and the blend ratio of the isocyanate component and the hydrogen group-containing resin component are shown in Table 1. Each of the resultant sample coating plate was evaluated by the same method as that mentioned above in terms of the coating film hardness, the penetration property into lower layer and the adhesiveness. The results are shown in Table 1.

(47) Two kinds of isocyanate components were used in Examples 10 to 13. In Example 10, isocyanate components in which NTI and P-3 were blended at a mass ratio. NTI/P-3, of 5/5 was used. In Example 11, isocyanate components in which NTI and P-3 were blended at a mass ratio, NTI/P-3, of 3/7 was used. In Example 12, isocyanate components in which NTI and P-3 were blended at a mass ratio, NTI/P-3, of 4/6 was used. In Example 13, isocyanate components in which NTI and P-3 were blended at a mass ratio, NTI/P-3, of 9/1 was used.

Example 14

(48) An acrylic polyol dispersion (manufactured by Allnex Ltd., under the trade name of “SETAQUA 6520”) was applied on a JIS G 3141 (SPCC, SD) cationic electrodeposition coated plate used as a substrate such that the dried coating film thickness became 35 μm. Thereafter, an acrylic polyol emulsion (manufactured by Allnex Ltd., under the trade name of “SETAQUA 6515”) diluted with deionized water such that the resin solid content became 20% by mass was applied thereon as a first coating composition such that the dried coating film thickness became 20 μm. Thereafter, the resultant was preheated at 80° C. for 3 minutes. After conducting preheating, a second coating composition obtained by blending acrylic polyol (manufactured by Allnex Ltd., under the trade name of “SETALUX 1767”) and LTI at a molar ratio, isocyanate group/hydroxyl group (NCO/OH), of 1.5, and then diluting the mixture with butyl acetate such that the resin solid content become 50% by mass was applied thereon such that the dried coating film thickness became 35 μm. Thereafter, the resultant was heated at 80° C. for 30 minutes to obtain a sample coating plate on which multilayered coating films are coated. The resultant sample coating plate was evaluated by the above-mentioned method in terms of the coating film hardness, the penetration property into lower layer and the adhesiveness. As the result, the coating film hardness was evaluated as ∘, the penetration property into lower layer was evaluated as ⊚, and the adhesiveness was evaluated as ⊚. The results are also shown in Table 2.

Examples 15 to 26 and Comparative Examples 5 to 8

(49) The sample coating plate was obtained by the same method as that of Example 14 except that the kind of the isocyanate component, and the blend ratio of the isocyanate component and the hydrogen group-containing resin component are shown in Table 2. Each of the resultant sample coating plate was evaluated by the same method as that mentioned above in terms of the coating film hardness, the penetration property into lower layer and the adhesiveness. The results are shown in Table 2.

(50) Two kinds of isocyanate components were used in Examples 23 to 26. In Example 23, isocyanate components in which NTI and P-3 were blended at a mass ratio, NTI/P-3, of 5/5 was used. In Example 24, isocyanate components in which NTI and P-3 were blended at a mass ratio, NTI/P-3, of 3/7 was used. In Example 25, isocyanate components in which NTI and P-3 were blended at a mass ratio, NTI/P-3, of 4/6 was used. In Example 26, isocyanate components in which NTI and P-3 were blended at a mass ratio, NTI/P-3, of 9/1 was used.

Examples 27 to 32

(51) The sample coating plate was obtained by the same method as that of Example 14 except that the kind of the isocyanate component, and the heating temperature are shown in Table 3. Each of the resultant sample coating plate was evaluated by the same method as that mentioned above in terms of the coating film hardness, the penetration property into lower layer, the adhesiveness, and the yellowing property when baked. The results are shown in Table 3.

(52) TABLE-US-00001 TABLE 1 Ex. Ex. Ex. Ex. Ex. Ex. 1 2 3 4 5 6 Isocyanate LTI ◯ — — ◯ ◯ ◯ GTI — ◯ — — — — NTI — — ◯ — — — P-1 — — — — — — P-2 — — — — — — P-3 — — — — — — P-4 — — — — — — P-5 — — — — — — Viscosity [mPa .Math. s] 26 200 10 26 26 26 NCO content [%] 47 40 50 47 47 47 NCO group number 3.0 3.0 3.0 3.0 3.0 3.0 [number] Difunctional group 0 0 0 0 0 0 content [%] Ratio NCO/OH 1.5 1.5 1.5 1.1 2.2 4.2 Coating film hardness ◯ ◯ ◯ ◯ Δ Δ Penetration property ⊚ ◯ ⊚ ◯ ⊚ ⊚ into lower layer Adhesiveness ⊚ ⊚ ⊚ ◯ ⊚ ⊚ Ex. Ex. Ex. Ex. Ex. Ex. 7 8 9 10 11 12 Isocyanate LTI — — — — — — GTI — — — — — — NTI ◯ ◯ — ◯5 ◯3 ◯4 P-1 — — ◯ — — — P-2 — — — — — — P-3 — — — ◯5 ◯7 ◯6 P-4 — — — — — — P-5 — — — — — — Viscosity [mPa .Math. s] 10 10 400 150 200 170 NCO content [%] 50 50 37 36 30 33 NCO group number 3.0 3.0 6.2 3.2 3.2 3.2 [number] Difunctional group 0 0 0 0 0 0 content [%] Ratio NCO/OH 1.1 2.2 1.5 1.5 1.5 1.5 Coating film hardness ◯ ◯ ◯ ⊚+ Δ ⊚ Penetration property ◯ ⊚ ◯ ⊚ ⊚ ⊚ into lower layer Adhesiveness ◯ ⊚ ⊚ ⊚ ⊚ ⊚ Ex. C. Ex. C. Ex. C. Ex. C. Ex. 13 1 2 3 4 Isocyanate LTI — — — — — GTI — — — — — NTI ◯9 — — — — P-1 — — — — — P-2 — ◯ — — — P-3 ◯1 — — — ◯ P-4 — — — ◯ — P-5 — — ◯ — — Viscosity [mPa .Math. s] 20 52 1100 5 2300 NCO content [%] 47 24 24 43 21.5 NCO group number 3.0 2.1 3.0 2.5 3.3 [number] Difunctional group 0 76 13 60 0 content [%] Ratio NCO/OH 1.5 1.5 1.5 1.5 1.5 Coating film hardness ⊚ X Δ X Δ Penetration property ⊚ ⊚ X ◯ X into lower layer Adhesiveness ⊚ Δ X X X (Ex.: Example; C. Ex.: Comparative Example)

(53) TABLE-US-00002 TABLE 2 Ex. Ex. Ex. Ex. Ex. Ex. 14 15 16 17 18 19 Isocyanate LTI ◯ — — ◯ ◯ ◯ GTI — ◯ — — — — NTI — — ◯ — — — P-1 — — — — — — P-2 — — — — — — P-3 — — — — — — P-4 — — — — — — P-5 — — — — — — Ratio NCO/OH 1.5 1.5 1.5 1.1 2.2 4.2 Coating film hardness ◯ ◯ ◯ ◯ Δ Δ Penetration property ⊚ Δ ⊚ Δ ⊚ ⊚ into lower layer Adhesiveness ⊚ ◯ ⊚ Δ ⊚ ⊚ Ex. Ex. Ex. Ex. Ex. Ex. 20 21 22 23 24 25 Isocyanate LTI — — — — — — GTI — — — — — — NTI ◯ ◯ — ◯5 ◯3 ◯4 P-1 — — ◯ — — — P-2 — — — — — — P-3 — — — ◯5 ◯7 ◯6 P-4 — — — — — — P-5 — — — — — — Ratio NCO/OH 1.1 2.2 1.5 1.5 1.5 1.5 Coating film hardness ◯ ◯ ◯ ⊚+ Δ ⊚ Penetration property Δ ⊚ ◯ ⊚ ◯ ⊚ into lower layer Adhesiveness Δ ⊚ ◯ ⊚ ⊚ ⊚ Ex. C. Ex. C. Ex. C. Ex. C. Ex. 26 5 6 7 8 Isocyanate LTI — — — GTI — — — — — NTI ◯9 — — — — P-1 — — — — — P-2 — ◯ — — — P-3 ◯1 — — — ◯ P-4 — — — ◯ — P-5 — — ◯ — — Ratio NCO/OH 1.5 1.5 1.5 1.5 1.5 Coating film hardness ⊚ X Δ X Δ Penetration property ⊚ ⊚ X Δ X into lower layer Adhesiveness ⊚ X X X X (Ex.: Example)

(54) TABLE-US-00003 TABLE 3 Ex. Ex. Ex. Ex. Ex. Ex. 27 28 29 30 31 32 Isocyanate NTI ◯ ◯ ◯5 ◯5 ◯ ◯5 P-3 — — ◯5 ◯5 — ◯5 Ratio NCO/OH 1.5 1.5 1.5 1.5 1.5 1.5 Heating temperature 60 150 60 150 170 170 [° C.] Coating film hardness ◯ ◯ ⊚+ ⊚+ ⊚ ⊚+ Penetration property ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ into lower layer Adhesiveness ⊚ ⊚ ⊚ ⊚ ⊚ ⊚ Yellowing property ◯ ◯ ◯ ◯ X X when baked (Ex.: Example)

(55) The sample coating plates prepared using the second coating compositions containing the isocyanate component containing the triisocyanate as shown in Tables 1 and 2 were excellent in terms of the penetration property of the isocyanate component into the lower layer and the adhesiveness.

(56) The coating film hardness of the sample coating plates prepared using the second coating compositions containing the polyisocyanate components each having an isocyanurate structure formed from LTI or HDI was further improved.

(57) In addition, the adhesiveness and the coating film hardness were improved, and the yellowing property when baked was reduced by controlling the heating temperature.

INDUSTRIAL APPLICABILITY

(58) The coating method according to the present embodiment makes it possible to obtain a coating film having excellent coating film hardness, because the penetration property into a lower layer is favorable.