Method of forming multilayer coating film

Patent metadata

Field	Value
Publication number	US-11213854-B2
Application number	US-201916409145-A
Country	US
Kind code	B2
Filing date	May 10, 2019
Priority date	May 11, 2018
Publication date	Jan 4, 2022
Grant date	Jan 4, 2022

Abstract

A method of forming a multilayer coating film with excellent flip-flop properties and smoothness and allowing formation of a high-design multilayer coating film. In a method of forming a multilayer coating film comprising: a low-brightness intercoating film; a first base coating film; a second base coating film formed on an object to be coated; and a clear coating film formed on the second base coating film; the first base coating film is obtained by applying a first aqueous base coating material, the second base coating film is obtained by applying a second aqueous base coating material containing a brightness pigment, the clear coating film, with specific compositions and properties, is formed on a low-brightness intercoating film by a wet-on-wet method and the multilayer coating film is heat cured to form a cured multilayer coating film.

First claim

The invention claimed is:

1. A method of forming a multilayer coating film comprising a low-brightness intercoating film formed on an object to be coated, a first base coating film formed on the low-brightness intercoating film, a second base coating film formed on the first base coating film and a clear coating film formed on the second base coating film, the method including:
(1) a first base coating film-forming step in which a thermosetting first aqueous base coating material is coated onto the low-brightness intercoating film to form an uncured first base coating film;
(2) a second base coating film-forming step in which a thermosetting second aqueous base coating material containing a brightness pigment is coated onto the uncured first base coating film to form an uncured second base coating film;
(3) a preheating step in which the uncured first base coating film and second base coating film are preheated;
(4) a clear coating film-forming step in which a thermosetting clear coating material is coated onto the preheated second base coating film to form an uncured clear coating film; and
(5) a baking step in which the multilayer coating film comprising the preheated first base coating film and second base coating film and the uncured clear coating film is subjected to heat curing to form a cured multilayer coating film, wherein:
(a) the L* value of the low-brightness intercoating film is 30 or smaller,
(b) the first aqueous base coating material has a coating material solid concentration in the range of 16 to 45 mass %, with respect to the total mass of the first aqueous base coating material, and contains a water-dispersible hydroxyl group-containing acrylic resin (a) and a melamine resin (b),
the water-dispersible hydroxyl group-containing acrylic resin (a) having a core/shell-type multilayer structure composed of a core of a copolymer (I) obtained by copolymerizing 0.1 to 30 mass % of a polymerizable unsaturated monomer with two or more polymerizable unsaturated groups in a monomer (a1) and 70 to 99.9 mass % of another polymerizable unsaturated monomer (a2), the mass % of the polymerizable unsaturated monomer (a1) and the polymerizable unsaturated monomer (a2) each being based on the total mass of the monomer (a1) and the monomer (a2) combined, and a shell of a copolymer (II) obtained by copolymerizing 5 to 50 mass % of a vinyl aromatic compound (a3) and 50 to 95 mass % of another polymerizable unsaturated monomer (a4), the mass % of the compound (a3) and the monomer (a4) each being based on the total mass of the compound (a3) and the monomer (a4) combined, and the copolymer (I)/copolymer (II) solid mass ratio being in the range of 10/90 to 90/10, and
the weight-average molecular weight of the melamine resin (b) being in the range of 1,000 to 3,000,

(c) the viscosity of the first aqueous base coating material at 90 seconds after coating is in the range of 15,000 to 300,000 mPa·s as measured under conditions with a temperature of 25° C. and a shear rate of 0.1 sec −1 ,
(d) the black-white concealing film thickness of the cured coating film of the first aqueous base coating material is 50 μm or greater, and the haze of the cured coating film with a film thickness of 15 μm is in the range of 0 to 15%,
(e) the second aqueous base coating material has a coating material solid concentration in the range of 5 to 15 mass % with respect to the total mass of the second aqueous base coating material, and contains a brightness pigment in the range of 10 to 40 parts by mass with respect to 100 parts by mass as the solid content of the second aqueous base coating material,
(f) the film thickness of the cured second base coating film is in the range of 2 to 8 μm, and
(g) the ratio between the film thicknesses of the cured first base coating film and second base coating film is in the range of 1.5:1 to 6:1.

2. The method of forming a multilayer coating film according to claim 1 , wherein the first aqueous base coating material does not contain a brightness pigment.

Assignees

• Kansai Paint Co Ltd
• Toyota Motor Co Ltd

Inventors

• Ono Keita
• Omura Masahiro
• Asai Masayoshi
• Nishiguchi Tetsuya

Classifications

• B05D2502/005

  modified · CPC title

• B05D2508/00

  Polyesters · CPC title

• C09D133/066

  containing -OH groups · CPC title

• B05D3/0209 Primary

  Multistage baking · CPC title

• C09D161/28

  with melamine · CPC title