Method for making multilayer coating film
This invention provides a multilayer coating film-forming method which comprises applying a first clear paint onto a coating object, applying onto so formed first clear coating film which is either uncured or cured by heating, a second clear paint such that its heat-cured film thickness falls within a range of 2-15 μm, and forming the second clear coating film, (a) the first clear paint and second clear paint comprising hydroxyl-containing resin as the base resin and polyisocyanate compound as the crosslinking agent, and (b) when the coating film formed by applying a paint to a cured film thickness of 40 μm and curing it by heating at 140° C. for 30 minutes is measured of its dynamic viscoelasticity at a frequency of 11 Hz, the first clear paint forming the first clear coating film having a glass transition temperature of lower than 70° C. and tanδ (=loss elastic modulus/storage elastic modulus) at 80° C. of less than 0.4, and the second clear paint forming the second clear coating film having a glass transition temperature of at least 70° C.
1. A method for forming a multilayer coating film, comprising applying a first clear paint onto a coating object to form a first clear coating film, applying onto the first clear coating film which is either uncured or cured by heating, a second clear paint to form a second clear coating film such that the second clear coating film has a heat-cured film thickness within a range of 2-15 μm,
wherein:
(a) the first clear paint and the second clear paint comprise hydroxyl-containing resin as a base resin and polyisocyanate compound as a crosslinking agent, and
(b) when the first clear paint is applied and cured by heating at 140° C. for 30 minutes to form a first clear coating film having a cured thickness of 40 μm, and a dynamic viscoelasticity of the so formed first clear coating film is measured at a frequency of 11 Hz, the so formed first clear coating film has a glass transition temperature of lower than 70° C. and a tan δ (=loss elastic modulus/storage elastic modulus) at 80° C. of less than 0.4, and
when the second clear paint is applied and cured by heating at 140° C. for 30 minutes to form a second clear coating film having a cured thickness of 40 um, and a dynamic viscoelasticity of the so formed second clear coating film is measured at a frequency of 11 Hz, the so formed second clear coating film has a glass transition temperature of at least 70° C.
2. The multilayer coating film-forming method according to claim 1 , in which the surface of the coating object is a surface coated with an uncured or cured coating film formed by application of coloring paint.
3. The multilayer coating film-forming method according to claim 1 , in which the first clear paint contains, as the hydroxyl-containing resin, hydroxyl-containing acrylic resin or hydroxyl-containing polyester resin.
4. The multilayer coating film-forming method according to claim 1 , in which the first clear paint contains, as the hydroxyl-containing resin, a hydroxyl-containing acrylic resin having a glass transition temperature within a range of from −60° C. to 30° C.
5. The multilayer coating film-forming method according to claim 1 , in which the first clear paint contains, as the polyisocyanate compound, at least one of aliphatic polyisocyanate, alicyclic polyisocyanate, and derivatives of these polyisocyanates.
6. The multilayer coating film-forming method according to claim 1 , in which the second clear paint contains, as the hydroxyl-containing resin, a hydroxyl-containing acrylic resin having a glass transition temperature within a range of from −30 to 70° C.
7. The multilayer coating film-forming method according to claim 1 , in which the second clear paint contains polyfunctional polyisocyanate compound having at least three isocyanate groups per molecule.
8. The multilayer coating film-forming method according to claim 1 , in which the first clear paint forms a first clear coating film having a glass transition temperature within a range of at least 20° C. but less than 70° C.
9. The multilayer coating film-forming method according to claim 1 , in which the first clear paint forms a first clear coating film having the tanδat 80° C. within a range of 0.05-0.35.
10. The multilayer coating film-forming method according to claim 1 , in which the second clear paint forms a second clear coating film having a glass transition temperature within a range of 70-130° C.
11. The multilayer coating film-forming method according to claim 1 , in which the first clear coating film has a Young's modulus within a range of 800-10,000 Kgf/cm 2 .
12. An article coated by the method as described in claim 1 .
13. An article coated by the method as described in claim 2 .
14. An article coated by the method as described in claim 3 .
15. An article coated by the method as described in claim 4 .
16. An article coated by the method as described in claim 5 .
17. An article coated by the method as described in claim 6 .
18. An article coated by the method as described in claim 7 .
19. An article coated by the method as described in claim 8 .
20. An article coated by the method as described in claim 9 .
21. An article coated by the method as described in claim 10 .
22. An article coated by the method as described in claim 11 .