Water-Based Coating Composition, Method for Forming Multi-layer Coating Film, and Article Having Multi-layer Coating Film

Abstract

A water-based coating composition and method for forming a multi-layer coating film by which it is possible to form a coating film having excellent smoothness, image clarity, and chipping resistance in a 3C1B format. The composition is characterized by containing 7 to 60 mass % of an acrylic resin emulsion (a), 10 to 60 mass % of a film-forming resin (b), and 10 to 50 mass % of a curing agent (c), on the basis of the total mass of the solid resin content in the composition; the acrylic resin emulsion (a) having a core/shell structure where the core section contains, as a copolymer component, 0.1 to 10 mass % of a polymerizable unsaturated monomer having two or more polymerizable unsaturated groups in one molecule, on the basis of the total mass of monomer of the core section, and 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.

1 . A water-based coating composition to be used as a water-based first pigmented coating material (A) in a multilayer coating film-forming method in which the following steps (1) to (4) are carried out in order: 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, 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 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 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.   
     
     
         2 . A water-based coating composition according to  claim 1 , wherein the core section comprises, as a copolymerizing component, 30 to 90 mass % of a copolymerizable unsaturated monomer with a C4 or greater hydrocarbon group, based on the total mass of the monomers of the core section, the acid value of the acrylic resin emulsion (a) is 5 to 25 mgKOH/g, and its glass transition temperature (Tg) is 20° C. or higher. 
     
     
         3 . A water-based coating composition according to  claim 1 , wherein the coating film-forming resin (b) comprises at least one type selected from among polyester resins (b1), water-soluble acrylic resins (b2) and urethane resins (b3). 
     
     
         4 . A water-based coating composition according to  claim 1 , wherein the curing agent (c) comprises an amino resin (c1) and/or a blocked polyisocyanate compound (c2), and the water-based coating composition includes 5 to 30 mass % of an amino resin (c1) and/or 5 to 30 mass % of a blocked polyisocyanate compound (c2), based on the total mass of the solid resin content in the water-based coating composition. 
     
     
         5 . A water-based coating composition according to  claim 1 , wherein the blocked polyisocyanate compound (c2) comprises an active methylene-type blocked polyisocyanate compound (c3) with at least one type of blocked isocyanate group selected from the group consisting of blocked isocyanate groups represented by the following formula (I): 
       
         
           
           
               
               
           
         
         (wherein R 1 , R 2 , R 4  and R 5  each independently represent a C1-12 hydrocarbon group and R 3  represents a C1-12 straight-chain or branched alkylene group), 
         blocked isocyanate groups represented by the following formula (II): 
       
       
         
           
           
               
               
           
         
         (wherein R 2 , R 3 , R 4  and R 5  have the same definitions as above), 
         and blocked isocyanate groups represented by the following formula (III): 
       
       
         
           
           
               
               
           
         
         (wherein R 2 , R 3 , R 4  and R 5  have the same definitions as above, and R 6  represents a C1-12 hydrocarbon group). 
       
     
     
         6 . A water-based coating composition according to  claim 1 , wherein the water-based coating composition includes, as a pigment, titanium dioxide and/or barium sulfate in a total range of 50 to 150 mass % based on the total mass of the solid resin content. 
     
     
         7 . 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,   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 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.   
     
     
         8 . The method according to  claim 7 , wherein the core section comprises, as a copolymerizing component, 30 to 90 mass % of a copolymerizable unsaturated monomer with a C4 or greater hydrocarbon group, based on the total mass of the monomers of the core section, the acid value of the acrylic resin emulsion (a) is no greater than 15 mgKOH/g, and its glass transition temperature (Tg) is 20° C. or higher. 
     
     
         9 . An article with a multilayer coating film, formed by the method according to  claim 7 .