Water-based coating composition, method for forming multi-layer coating film, and article having multi-layer coating film

A multilayer coating film-forming method comprising the following steps (1) to (4):

• step (1):
• a step of coating an article to be coated with a water-based first pigmented coating material (A) to form a first pigmented coating film, wherein the coating film thickness, as dry film thickness, is from 10 to 100 μm,
• step (2):
• a step of coating the uncured first pigmented coating film formed in step (1) with a water-based second pigmented coating material (B) to form a second pigmented coating film,
• step (3):
• a step of coating the uncured second pigmented coating film formed in step (2) with a clear coating material (C) to form a clear coating film, and
• step (4):
• a step of bake curing the multilayer coating film formed in steps (1) to (3),
• wherein the water-based coating composition as the water-based first pigmented coating material (A) comprises 7 to 60 mass % of an acrylic resin emulsion (a), 10 to 60 mass % of a coating film-forming resin (b) and 10 to 50 mass % of a curing agent (c), based on the total mass of the solid resin content in the water-based coating composition,
• the acrylic resin emulsion (a) has a core-shell structure, the core section comprising, as a copolymerizing component, 0.1 to 10 mass % of a polymerizable unsaturated monomer having two or more polymerizable unsaturated groups in the molecule, based on the total mass of the monomers of the core section, and the shell section comprising, as a copolymerizing component, a hydroxyl group-containing polymerizable unsaturated monomer,
• the hydroxyl value of the core section is 50 to 200 mgKOH/g and the hydroxyl value of the shell section is 50 to 200 mgKOH/g, and
• the curing agent (c) comprises a blocked polyisocyanate compound (c2) having a hydrophilic group, wherein the blocked polyisocyanate compound (c2) having a hydrophilic group is a polyisocyanate compound blocked by both an active hydrogen-containing compound having a polyoxyalkylene group and an active methylene compound as blocking agents, and wherein the active hydrogen-containing compound has a number average molecular weight of 300 to 2,000.