Coating method and hardener for polyurethane paint

Abstract

[Problem] To increase adhesive strength between an aqueous base coat layer and a clear coat layer in a coating method by a two-coat one-bake system where an aqueous base coat paint is coated, then a clear coat paint is coated, and these two coated films are simultaneously hardened, in comparison with the conventional method. [Solution] Provided is a coating method using a specific hardener in a clear coat paint. Namely, the hardener is characterized in that (1) it includes a reaction product of (i) a derivative of 1,6-hexamethylene diisocyanate including a trimer and virtually not including a diisocyanate monomer, and (ii) a compound having both an active hydrogen and a hydrophilic group; and (2) viscosity at 23 C. is not more than 1,000 mPa.Math.s. The compound having both an active hydrogen and a hydrophilic group herein is a compound having an active hydrogen-containing group and a sulfo group represented by formula (I), and preferably the sulfo group in a hardener is neutralized with a tertiary amine.
XRSO.sub.3HFormula (I)
(in the formula, X is an amino group, alkylamino group, cycloalkylamino group, or hydroxyl group, R is an aliphatic and/or alicyclic hydrocarbon group.)

Claims

1. A coating method comprising applying an aqueous base coat paint, without hardening the aqueous base coat paint by cross-linking, subsequently coating a clear coat paint comprising a hardener and an active hydrogen-containing resin on the base coat paint, hardening the base coat paint and the clear coat paint simultaneously by cross-linking, wherein the clear coat paint is a two-component polyurethane paint with a mole ratio of 0.9 to 2.0 in NCO/H (active hydrogen), wherein (1) the hardener comprises a reaction product of (i) a derivative of 1,6-hexamethylene diisocyanate comprising a trimer, wherein the amount of trimers of 1,6-hexamethylene diisocyanate in the derivative of 1,6-hexamethylene diisocyanate is 55 weight % or more and wherein the amount of diisocyanate monomers included in the derivative of 1,6-hexamethylene diisocyanate is 1 weight % or less, and (ii) a compound having both an active hydrogen and a hydrophilic group; and (2) viscosity of the hardener when it is not diluted with any organic solvent is not more than 1,000 mPa.Math.s/23 C.

2. The coating method of claim 1, wherein the compound having both an active hydrogen and a hydrophilic group is a compound having an active hydrogen-containing group and a sulfo group represented by formula (I), and the sulfo group in a hardener is neutralized with a tertiary amine
XRSO.sub.3Hformula (I) wherein X is an amino group, alkylamino group, cycloalkylamino group, or hydroxyl group, and R is an aliphatic and/or alicyclic hydrocarbon group.

3. The coating method of claim 2, wherein the compound represented by formula (I) is 2-(cyclohexylamino)-ethane sulfonic acid and/or 3-(cyclohexylamino)-propane sulfonic acid.

4. The coating method of claim 1, wherein the aqueous base coat paint is an aqueous baking type paint comprising an active hydrogen.

5. A multilayer coated film comprising a base coat layer and a clear coat layer obtained by the method of claim 1.

Description

EXAMPLES

(1) Next, the present invention is further specifically explained by Examples, but the present invention is by no means restricted by Examples. Representation of % means weight % unless otherwise specified.

(2) Evaluation was carried out in accordance with the following criteria for each characteristic.

(3) <Measurement of Content of Trimer and Dimer in HDI Derivative>

(4) It was obtained by an efflux area percentage obtained through measurement by gel permeation chromatography (GPC). The peak of the molecular weight corresponding to a trimer or dimer of HDI is defined as trimer. The measurement conditions are as follows. Instrument: HLC-8120GPC manufactured by Tosoh Corporation Column: Shodex KF601, 602, 603 each piece Carrier: Tetrahydrofuran Detection method: Differential refractive index detector Pretreatment: Active NCO group was deactivated by the reaction with methanol before the measurement. Molecular weight: Polypropylene glycol (PPG) reduction
<Measurement of Molecular Weight of Acrylic Resin>
Molecular weight of acrylic resin was measured by the same GPC method as described above except the pretreatment of active NCO group.
<Measurement of Viscosity of Hardener>
Viscosity of hardener was measured according to DIN EN ISO 3219/A.3 at 23 C.
<Evaluation on Penetrability of Polyisocyanate from Clear Coat Layer to Base Coat Layer>

(5) A base coat and a clear coat were sequentially coated on a polypropylene plate to obtain a multilayer coated film by a two-coat one-bake system (see the method described in Example 1). The resulting coated film was kept at normal temperature, then, peeled out of the polypropylene plate. Within one hour after baking, infrared absorption spectrum (IR) of coated film of the base coat on the polypropylene plate surface side was measured in a peak height at the following wavelengths, and IR absorption peak height ratios (%) of 2270 cm-1 to 2930 cm-1 and 700 cm-1 were calculated. The higher these ratios, the higher the penetration efficiency of polyisocyanate.

(6) Wavelength 2930 cm-1 (CH stretching vibration absorption)

(7) Wavelength 2270 cm-1 (isocyanate group NCO stretching vibration absorption)

(8) Wavelength 700 cm-1 (aromatic ring absorption)

(9) In the infrared absorption spectrum (IR) measurement, as an instrument, Nicolet 6700FT-IR manufactured by Thermo Fisher Scientific K. K. was used.

(10) Regarding the determination, when the peak height ratio (%) becomes not less than 3 times the blank test that no clear coat is coated (high penetrability), it denotes , and when less than 3 times the blank (low penetrability), x.

(11) <Appearance of Coated Film (Early Phase)>

(12) The appearance of multilayer coated film obtained was measured using a gloss meter (Microtrigloss, 4520) manufactured by BYK Gardner Corp. by gloss values at 20 and 60 in reflecting angle. Further, it was evaluated by visual observation according to the following criteria.

(13) : Smoothness is good.

(14) : Smoothness is intermediate between good and bad.

(15) x: Smoothness is bad.

(16) Water resistance of multilayer coated film was measured by appearance, gloss retention, adhesiveness between multilayers after immersion in water, and cohesion failure property.

(17) <Appearance (Water Resistance)>

(18) On a polypropylene plate coated with a primer for polypropylene, a base coat and a clear coat were sequentially coated to obtain a multilayer coated film by a two-coat one-bake system (see the method described in Example 1). The resulting polypropylene coated film plate was immersed in warm water at 40 C. for 240 hours, and picked up, dried at room temperature for 12 hours, then appearance of the multilayer coated film was evaluated by visual observation in accordance with the following criteria.

(19) : There is no abnormality,

(20) : Blisters are observed overall.

(21) x: Large blisters are observed overall, and/or the whole surface is not glossy.

(22) <Gloss Retention (Water Resistance)>

(23) The appearance of this multilayer coated film was measured using a gloss meter (Microtrigloss, 4520) manufactured by BYK Gardner Corp. by gloss values at 20 and 60 in reflecting angle. From the ratio of these values and the gloss values obtained by the above-described <appearance of coated film> evaluation, gloss retention at respective reflecting angles was calculated, and evaluated in accordance with the following criteria.

(24) : Gloss retention of 90% or more

(25) : Gloss retention of 70% or more and less than 90%

(26) x: Gloss retention of less than 70%

(27) <Adhesiveness Between Multilayers after Immersion in Water (Water Resistance)>

(28) The coated film evaluated by the above-described method was cut with a utility knife so as to reach the substrate, 100 grids of 2 mm2 mm size were made, and an adhesive cellophane tape was attached on the coated surface. After the tape was rapidly peel off at 20 C., the coated surface was evaluated in accordance with the following criteria.

(29) : Grids of coated film remain in 100 pieces.

(30) : Grids of coated film are peeled off by 1 to 10 pieces.

(31) x: Grids of coated film are peeled off by 11 pieces or more.

(32) <Cohesion Failure Property (Water Resistance)>

(33) Regarding the sample after the above-described evaluation of adhesiveness between multilayers after immersion in water, the failure state of the grids of coated film was evaluated in accordance with the following criteria.

(34) : No failure is observed at all inside the base coated film.

(35) x: Failure is clearly observed inside the base coated film.

(36) For Examples and Comparative examples, the following each material was prepared beforehand.

(37) Acrylic polyol A (for base coat)

(38) Bayhydrol A2427 (manufactured by Bayer Material Science AG)

(39) Water-dispersible hydroxyl group-containing acrylic polyol, nonvolatile content 42%, hydroxyl group content 2% (in terms of nonvolatile content), viscosity 100 mPa.Math.s/23 C., pH 8-9, glass transition temperature 87 C., molecular weight Mw/Mn=240,000/22,000

(40) Aluminum flake (for base coat)

(41) EMR D5660 (manufactured by Toyo Aluminum K.K.)

(42) Thickener (for base coat)

(43) Viscalex HV30 (manufactured by Ciba Specialty Chemicals Inc.)

(44) Surfactant A (for base coat)

(45) A mixture of surfactant BYK347 (manufactured by BYK Corp.) and surfactant Disperlon AQ320 (manufactured by Kusumoto Chemicals, Ltd.) by 1:1

(46) Co-solvent (for base coat)

(47) Butyl glycol

(48) Neutralizing agent (for base coat)

(49) Dimethylethanolamine

(50) Acrylic polyol B (for clear coat)

(51) Desmophen A870 (manufactured by Bayer Material Science AG)

(52) Butyl acetate solution with nonvolatile content of 70%, hydroxyl group content 2.95% as it is, glass transition temperature 27 C., viscosity 3,500 mPa.Math.s/23 C., acid value 7.5 mgKOH/g, and molecular weight Mw/Mn=3,400/1,650

(53) Surfactant B (for clear coat)

(54) BYK331 being surfactant (manufactured by BYK Corp.)

(55) Leveling agent (for clear coat)

(56) Modaflow (manufactured by Monsanto Corporation)

(57) Solvent (for clear coat)

(58) Methoxypropyl acetate/butyl acetate=1/1 solution

Synthesis Example 1

Synthesis of HDI Derivative (P-1)

(59) A three-necked flask equipped with a stirring device was filled with nitrogen, 200 g of HDI was put therein and kept at 60 C. Next, as a catalyst, 50% solution of tetrabutyl phosphonium hydrogen difluoride in isopropanol/methanol (2:1) was added dropwise over 4 hours until the NCO content of coarse reaction mixture became 43%. Thereafter, the reaction was stopped by adding dibutyl phosphate of 103 mg. From this, HDI monomer was removed by thin-film distillation at 130 C./0.2 millibars, thereby obtaining a transparent and colorless polyisocyanate with the following data.

(60) Nonvolatile content: 100% (containing no solvent)

(61) NCO content: 23.4%

(62) Viscosity: 680 mPa.Math.s/23 C.

(63) Trimer content: 68 weight %

(64) HDI monomer content: 0.2 weight %

Synthesis Example 2

Synthesis of HDI Derivative (P-2)

(65) A three-necked flask equipped with a stirring device was filled with nitrogen, 1000 g of HDI was put therein and kept at 60 C. To this, 10 g of 1,3-butanediol as a co-catalyst and 3 g of tri-n-butylphosphine as a catalyst were added. Next, this mixture was kept at 60 C., and the reaction was conducted for 4 hours to obtain a raw reaction mixture with an NCO content of 40%. The reaction was stopped by adding methyl p-toluenesulfonate of 2.8 g thereto. After the mixture was subjected to thin-layer distillation at 130 C. and pressure 0.15 mbar, a transparent and colorless polyisocyanate with the following data was obtained.

(66) Nonvolatile content: 100% (containing no solvent)

(67) NCO content: 21.8%

(68) Viscosity: 200 mPa.Math.s/23 C.

(69) Trimer content: 24 weight %

(70) Dimer content 38 weight %

(71) HDI monomer content: 0.2 weight %

Synthesis Example 3

Synthesis of Hardener (H-1)

(72) A three-necked flask equipped with a stirring device was filled with nitrogen, and to a mixture of 800 g of HDI derivative (P-1) obtained in Synthesis example 1 with 200 g of HDI derivative (P-2) obtained in Synthesis example 2, 10 g of 3-cyclohexylaminopropane sulfonic acid and 6 g of dimethylcyclohexylamine were added, and the reaction was conducted under nitrogen stream at 80 C. for 10 hours. After cooling down to room temperature, a transparent and colorless polyisocyanate with the following data was obtained.

(73) Nonvolatile content: 100% (containing no solvent)

(74) NCO content: 22.6%

(75) Viscosity: 650 mPa.Math.s/23 C.

(76) Trimer content: 59 weight % (in HDI derivative)

(77) HDI monomer content: 0.2 weight %

Synthesis Example 4

Synthesis of Hardener (H-2)

(78) A three-necked flask equipped with a stirring device was filled with nitrogen, and to a mixture of 400 g of isocyanurate group-containing polyisocyanate produced from hexamethylene diisocyanate, having an isocyanate content of 21.7% and a viscosity of 3,000 mPa.Math.s (23 C.) with 600 g of HDI derivative (P-1) obtained in Synthesis example 1, 30 g of 3-cyclohexylaminopropane sulfonic acid and 18 g of dimethylcyclohexylamine were added, and the reaction was conducted under nitrogen stream at 80 C. for 10 hours. After cooling down to room temperature, a transparent and colorless polyisocyanate with the following data was obtained.

(79) Nonvolatile content: 100% (containing no solvent)

(80) NCO content: 21.2%

(81) Viscosity: 3,500 mPa.Math.s/23 C.

(82) Trimer content: 58 weight % (in HDI derivative)

(83) HDI monomer content: 0.2 weight %

Synthesis Example 5

Synthesis of Hardener (H-3)

(84) A three-necked flask equipped with a stirring device was filled with nitrogen, and to 1,000 g of isocyanurate group-containing polyisocyanate produced from hexamethylene diisocyanate, having an isocyanate content of 21.7% and a viscosity of 3,000 mPa.Math.s (23 C.), in place of a compound of formula (I), 145 g of monofunctional methoxypolyethylene glycol having an average molecular weight of 350 was added at room temperature while it was stirred. Next, this mixture was heated at 100 C. for 3 hours. After cooling down to room temperature, a practically transparent and colorless polyisocyanate mixture was obtained. The isocyanate content was 17.3% and viscosity was 3,050 mPa.Math.s (23 C.). To 700 g of the resulting polyisocyanate, 300 g of HDI derivative (P-2) was added, and stirred under nitrogen stream at 40 C. for 1 hour. After cooling down to room temperature, a transparent and colorless polyisocyanate with the following data was obtained.

(85) Nonvolatile content: 100% (containing no solvent)

(86) NCO content: 19.0%

(87) Viscosity: 800 mPa.Math.s/23 C.

(88) Trimer content: 43 weight % (in HDI derivative)

(89) HDI monomer content: 0.2 weight %

(90) Characteristics of each hardener such as NCO group content, viscosity and composition are shown in Table 1.

Example 1

(91) Base coat composition BC-1 (nonvolatile content of about 22%, Ford cup No. 4 efflux time of 50 to 60 seconds) shown in Table 1 was prepared. This was spray-coated on a polypropylene plate for the dry film thickness to be 15 to 20 microns, after being at room temperature for 3 minutes, this was further dried at 80 C. for 3 minutes to obtain coated film of the base coat. Next, clear coat composition CC-1 shown in Table 4 was prepared. To this, hardener A was mixed for the NCO/OH mole ratio to be 1.0, obtaining a paint with a nonvolatile content of about 50% and Ford cup No. 4 efflux time of 18 to 22 seconds. This paint was spray-coated on the above-described base coat coated film for the dry film thickness to be about 40 microns. After being left at room temperature for 3 minutes, this was baked at 100 C. for 20 minutes. After cooling down to room temperature, the coated film was peeled off from the polypropylene plate.

(92) The penetration ratio of isocyanate group of base coat coated film on the polypropylene plate surface side was measured by the above-described infrared absorption spectrum (hereinafter, abbreviated as IR) method, and the degree of penetration of polyisocyanate from the clear coat to the base coat was measured.

(93) As a blank test, the coated film obtained by forming a coated film of a base coat alone and drying it was subjected to the same operation to measure the IR absorption ratio.

(94) A primer for polypropylene was prepared by mixing a commercially available chlorinated polyolefin resin with an aqueous polyurethane resin (Dispercoll U54 manufactured by Bayer Material Science AG). This was coated on a polypropylene plate, after being left at room temperature for 3 minutes, dried at 80 C. for 3 minutes. Thereafter, by the method shown above, a base coat and a clear coat were sequentially coated, and baked, thereby obtaining a target coated film. The coated film was evaluated by appearance, gloss retention, adhesiveness between multilayers after immersion in water, and cohesion failure property.

Examples 1 to 3, and Comparative Examples 1 to 4

(95) Experiments were conducted according to Example 1 by changing the loads of each component as shown in the base coat composition, clear coat composition, the use amount of hardener, and NCO/OH mole ratio of Tables 2 to 3.

(96) The results are shown by IR absorption ratio, penetrability evaluation, coated film appearance, gloss retention, adhesiveness between multilayers after immersion in water, and cohesion failure property in Table 4.

(97) TABLE-US-00001 TABLE 1 Hardener H-1 H-2 H-3 P-2 Compound (I) Cyclohexyl- Cyclohexyl- None None aminopropane aminopropane sulfonic acid sulfonic acid Alkoxypoly- None None Methoxy- None alkyleneglycol polyethyl- eneglycol NCO group content 22.5 21.2 19.0 21.8 (%) Viscosity 650 3,500 800 200 (mPa .Math. s/23 C.) Trimer content in 59 58 43 24 HDI derivative (%) HDI monomer 0.2 0.2 0.2 0.2 content (%)

(98) TABLE-US-00002 TABLE 2 Base coat composition (part by weight) BC-1 Acrylic polyol A 32.0 Aluminum flake 5.7 Thickener 3.5 Surfactant A 0.9 Co-solvent 7.4 Neutralizing agent 0.6 Ion-exchanged water 47.4

(99) TABLE-US-00003 TABLE 3 Clear coat composition (part by weight) CC-1 Acrylic polyol B 55.0 Surfactant B 0.55 Leveling agent 0.55 Solvent 38.6

(100) TABLE-US-00004 TABLE 4 Comparative Comparative Comparative Comparative Example 1 Example 2 Example 3 example 1 example 2 example 3 example 4 Hardener H-1 H-1 H-1 H-1 H-2 H-3 P-2 Base coat BC-1 BC-1 BC-1 BC-1 BC-1 BC-1 BC-1 Clear coat CC-1 CC-1 CC-1 CC-1 CC-1 CC-1 CC-1 NCO/OH mole ratio of clear coat 1.0 1.2 1.5 0.7 1.0 1.0 1.0 IR absorption ratio (%) 2270 cm.sup.1/ 47 10 7 49 2930 cm.sup.1 IR absorption ratio (%) 2270 cm.sup.1/ 32 7 6 61 700 cm.sup.1 Penetrability evaluation x x Early Appearance by phase visual observation 20 Gloss value 89 89 87 91 93 87 92 60 Gloss value 100 95 97 98 99 99 99 Water Appearance by resistance visual observation 20 Gloss 100 98 97 85 99 100 57 retention (%) Evaluation on 20 x gloss retention 60 Gloss 100 99 97 92 99 98 85 retention (%) Evaluation on 60 gloss retention Adhesiveness between multi- layers after immer- sion in water Cohesion failure property

(101) From the above-described results, it is known that Examples 1 to 3 show high IR absorption ratio at an NCO/OH mole ratio: 1.0, and excellent water resistance in an NCO/OH mole ratio of 0.9 to 1.5. In particular, the results show that, in the case of using a hardener including a reaction product of 3-cyclohexylaminopropane sulfonic acid and having a viscosity not more than 1,000 mPa.Math.s/23 C., there is a large effect in improving penetrability of coated film.

(102) In contrast to this, in Comparative example 1, since the NCO/OH mole ratio is less than 0.9, water resistance (appearance by visual observation and gloss retention) is low. In Comparative example 2, since the viscosity exceeds 1,000 mPa.Math.s/23 C., penetrability is low. In Comparative example 3, although the viscosity is not more than 1,000 mPa.Math.s/23 C., because of being modified with alkoxypolyoxyethylene glycol not with 3-cyclohexylaminopropane sulfonic acid, penetrability and water resistance (appearance by visual observation) are low. In Comparative example 4, penetrability is high because the viscosity is not more than 1,000 mPa.Math.s/23 C., but since the hardener does not include a reaction product of a compound having both an active hydrogen and a hydrophilic group, water resistance (appearance by visual observation and gloss retention) is low.

INDUSTRIAL APPLICABILITY

(103) The present invention can be used in a coating method called a wet-on-wet system, or a two-coat one-bake system in a coating field for steel plates and plastics of automobiles, home electric appliances, and the like, aiming at shortening of coating process, energy saving, and volatile organic compound (VOC) reduction. In particular, when applied to an aqueous base coat, since a specific polyisocyanate is used in a clear coat layer, it penetrates efficiently into an aqueous base coat layer of the under layer, and reacts with an isocyanate-reactive component in the aqueous base coat layer, adhesive strength between two layers is therefore greatly improved, and excellent physical properties of coated film can be obtained.