Method of forming multilayer coating film including water-borne basecoat on automobile substrate

1. A method of forming a multilayer coating film which comprises applying a water-borne base coating composition to an article to be coated and applying a clear coating composition thereon, followed by curing by heating, said water-borne base coating composition comprising:

(a) an emulsion resin obtained by emulsion polymerization of an α,β-ethylenically unsaturated monomer mixture having an acid value of 3 to 50 and comprising at least 65% by mass, based on a total weight of said α,β-ethylenically unsaturated monomer mixture, of a (meth)acrylate ester whose ester moiety contains one or two carbon atoms,

(b) a coloring component comprising an optically coherent flaky pigment, and

(c) an acrylic polymer compound having a weight average molecular weight of 400,000 to 1,500,000 and represented by the general formula (1):

wherein in the formula, A and B are the same or different and each represents a molecular terminal group resulting from a polymerization reaction producing a resin of the general formula (1), a terminal group resulting from hydrolysis thereof, a hydrogen atom or a hydroxyl group; M represents a carboxylic acid group or a carboxylic acid group neutralized with ammonia or a primary, secondary or tertiary amine compound, N represents a carboxylic acid group, a carboxylic acid ester group whose ester, moiety contains one or more carbon atoms, or a carboxylic acid group neutralized with ammonia or a primary, secondary or tertiary amine compound; m is an integer of not less than 1 and n is an integer of not less than 0, provided that m/9 is not less than n; and R is a hydrogen atom or a methyl group,

the content of said acrylic polymer compound on the solid basis being 0.3 to 4.0% by mass relative to the resin solids in the coating composition.

2. A method of forming a multilayer coating film which comprises applying a water-borne base coating composition to an article to be coated and applying a clear coating composition thereon, followed by curing by heating, said water-borne base coating composition comprising:

(a) an emulsion resin obtained by emulsion polymerization of an α,β-ethylenically unsaturated monomer mixture having an acid value of 3 to 50 and comprising at least 65% by mass, based on a total weight of said α,β-ethylenically unsaturated monomer mixture of a (meth)acrylate ester whose ester moiety contains one or two carbon atoms,

(b) a coloring component comprising an optically coherent flaky pigment,

(c) an acrylic polymer compound having a weight average molecular weight of 400,000 to 1,500,000 and represented by the general formula (1):

wherein in the formula, A and B are the same or different and each represents a molecular terminal group resulting from a polymerization reaction producing a resin of the general formula (1), a terminal group resulting from hydrolysis thereof, a hydrogen atom or a hydroxyl group; M represents a carboxylic acid group or a carboxylic acid group neutralized with ammonia or a primary, secondary or tertiary amine compound, N represents a carboxylic acid group, a carboxylic acid ester group whose ester moiety contains one or more carbon atoms, or a carboxylic acid group neutralized with ammonia or a primary, secondary or tertiary amine compound; m is an integer of not less than 1 and n is an integer of not less than 0, provided that m/9 is not less than n; and R is a hydrogen atom or a methyl group, the content of said acrylic polymer compound on the solid basis being 0.3 to 4.0% by mass relative to the resin solids in the coating composition, and

(d) a polyether polyol having, on an average, not less than 0.02 primary hydroxyl group per molecule, and having a number average molecular weight of 300 to 3,000 and a water tolerance value of not less than 2.0.

3. A method of forming a multilayer coating film which comprises applying a water-borne base coating composition to an article to be coated and applying a clear coating composition thereon, followed by curing by heating, said water-borne base coating composition comprising:

(a) an emulsion resin obtained by emulsion polymerization of an α,β-ethylenically unsaturated monomer mixture having an acid value of 3 to 50 and comprising at least 65% by mass, based on a total weight of said α,β-ethylenically unsaturated monomer mixture, of a (meth)acrylate ester whose ester moiety contains one or two carbon atoms,

(b) a coloring component comprising an optically coherent flaky pigment,

(c) an acrylic polymer compound having a weight average molecular weight of 400,000 to 1,500,000 and represented by the general formula (1):

wherein in the formula, A and B are the same or different and each represents a molecular terminal group resulting from a polymerization reaction producing a resin of the general formula (1), a terminal group resulting from hydrolysis thereof, a hydrogen atom or a hydroxyl group; M represents a carboxylic acid group or a carboxylic acid group neutralized with ammonia or a primary, secondary or tertiary amine compound, N represents a carboxylic acid group, a carboxylic acid ester group whose ester moiety contains one or more carbon atoms, or a carboxylic acid group neutralized with ammonia or a primary, secondary or tertiary amine compound; m is an integer of not less than 1 and n is an integer of not less than 0, provided that m/9 is not less than n; and R is a hydrogen atom or a methyl group, the content of said acrylic polymer compound on the solid basis being 0.3 to 4.0% by mass relative to the resin solids in the coating composition, and

(d) a polyether polyol having, on an average, not less than 0.02 primary hydroxyl group per molecule, and having a number average molecular weight of 300 to 3,000 and a water tolerance value of not less than 2.0, wherein

the polyether polyol has not less than one primary hydroxyl group per molecule and a hydroxyl value of 30 to 700.

4. A method according to any one of claims 1 - 3 , wherein the coated article is an automotive body.