Method for forming multilayer coating film

摘要

This invention provides a method for forming multilayer coating film, which comprises successively carrying out a step of applying an intermediate paint onto a coating object to form an intermediate coating film; a step of forming a base coating film by applying onto the intermediate coating film a water-based base coating paint; a step of forming a clear coating film by applying onto the base coating film a clear coating paint, and a step of heating the uncured intermediate coating film, base coating film and clear coating film to simultaneously cure these three coating films; wherein the intermediate paint has a complex viscosity at 40° C. (η*40) of not higher than 100 Pa·sec under the conditions of shear stress, 1.0 Pa and frequency, 0.1 Hz, and a complex viscosity at 80° C. (η*80) of at least 800 Pa·sec under the conditions of shear stress, 1.0 Pa and frequency, 0.1 Hz.

主权项

1. A method for forming a multilayer coating film, which is characterized by successively carrying out the following steps (1)-(4):
(1) applying an intermediate paint (X) onto a coating object to form an intermediate coating film, (2) forming a base coating film by applying a water-based base coating paint (Y) onto the uncured intermediate coating film as formed in step (1), (3) forming a clear coating film by applying a clear coating paint (Z) onto the uncured base coating film as formed in the step (2), and (4) heating the uncured intermediate coating film, base coating film and clear coating film, which are formed in steps (1)-(3), to simultaneously cure these three coating films, wherein the intermediate paint (X) has a complex viscosity at 40° C. (η*40) of not higher than 100 Pa·sec, under the conditions of shear stress of 1.0 Pa and frequency of 0.1 Hz, and a complex viscosity at 80° C. (η*80) of at least 800 Pa·sec, under the conditions of shear stress of 1.0 Pa and frequency of 0.1 Hz; and wherein the intermediate paint (X) comprises: (A) 40-60 mass % of a carboxyl-containing polyester resin having an acid value in a range of 30-70 mgKOH/g, a hydroxyl value in a range of 50-200 mgKOH/g, and a weight-average molecular weight in a range of 3,000-7,000, (B) 20-30 mass % of a polyisocyanate compound (B-1) and/or a polycarbodiimide compound (B-2),and
(C) 10-35 mass % of a diester compound of a general formula (1): wherein, in general formula 1, R1 and R2 stand for C4-18 hydrocarbon groups independent of each other, R3 stands for a C2-4 alkylene group, and m is an integer of 3-25, said m oxyalkylene units (R3—O) being the same or different,wherein the contents of the components (A)-(C) are percent by weight based on the total solid content of the components (A)-(C), andwherein the carboxyl-containing polyester resin (A) contains 3.0-4.0 mol/kg (solid resin content) of straight chain alkylene group having at least 4 carbon atoms in the molecules.