Method of forming multi-layered coating film

Abstract

This invention provides a method for forming multi-layered coating film excelling in appearance, corrosion resistance and chipping resistance, which comprises applying a first coloring paint (B), second coloring paint (C) and clear paint (D) onto cured coating film of a specific electrodeposition paint (A) of low weight loss under heating, wet-on-wet by the order stated; and heat-curing the three-layered coating film simultaneously.

1 . A method of forming multi-layered coating film which comprises applying a first coloring paint (B), second coloring paint (C) and clear paint (D) successively wet-on-wet, onto a cured coating film of an electrodeposition paint (A) showing a heat loss (X) of not more than 5% by weight, said heat loss being calculated according to the following equation:  
         heat loss ( X )=[( Y−Z )/ Y]× 100  [wherein Y is the weight of a dry coating film remaining after removal of the water content from an uncured coating film obtained by electrocoating the electrodeposition paint (A), by heating at 105° C. for 3 hours; and    Z is the weight of the cured film after heating the dry coating film at 170° C. for 20 minutes];    and heat-curing the so formed three-layered coating film simultaneously.    
     
     
         2 . A method as set forth in  claim 1 , in which the heat loss (X) of the electrodeposition paint (A) is not more than 4% by weight.  
     
     
         3 . A method as set forth in  claim 1 , in which the electrodeposition paint (A) comprises base resin (a) obtained through reaction of epoxy resin (a 1 ), amine compound (a 2 ) and phenolic compound (a 3 ), and epoxy resin (b) as a crosslinking agent.  
     
     
         4 . A method as set forth in  claim 2 , in which the epoxy resin (a 1 ) is an epoxy resin of the following formula (1)  
       
         
           
           
               
               
           
         
       
       having at least two epoxy-containing functional groups per molecule.  
     
     
         5 . A method as set forth in  claim 2 , in which the epoxy resin (a 1 ) has an epoxy equivalent within a range of 140-1,000 and a number-average molecular weight within a range of 200-50,000.  
     
     
         6 . A method as set forth in  claim 2 , in which the amine compound (a 2 ) is a primary or secondary amine compound containing primary hydroxyl group(s).  
     
     
         7 . A method as set forth in  claim 2 , in which the phenolic compound (a 3 ) contains at least one phenolic hydroxyl group per molecule.  
     
     
         8 . A method as set forth in  claim 7 , in which the phenolic compound (a 3 ) is a bisphenolic compound.  
     
     
         9 . A method as set forth in  claim 2 , in which the base resin (a) has an amine value within a range of 20-150 mgKOH/g; hydroxyl value within a range of 300-1,000 mgKOH/g; and a number-average molecular weight within a range of 800-15,000.  
     
     
         10 . A method as set forth in  claim 2 , in which the epoxy resin (b) is polyepoxide compound containing at least two epoxy-containing functional groups per molecule on the average, said functional group being formed of epoxy group(s) binding to alicyclic skeletal structure, or glycidyl etherified product of novolak resin.  
     
     
         11 . A method as set forth in  claim 2 , in which the epoxy resin (b) is selected from the group consisting of polyepoxide compounds having recurring units of the following formula (5)  
       
         
           
           
               
               
           
         
       
       polyepoxide polymers having recurring units of the following formula (6)  
       
         
           
           
               
               
           
         
         [in which R 7  is hydrogen or methyl] 
         and a number-average molecular weight within a range of 3,000; and epoxy resins of the following formula (8)  
         
           
             
             
                 
                 
             
           
         
         [in the formula, 
 R 1  and R 2  are same or different, and each stands for hydrogen, C 1 -C 8  alkyl, aryl, aralkyl or halogen; R 3  stands for hydrogen, C 1 -C 10  alkyl, aryl, aralkyl, allyl or halogen; R 4  and R 5  are same or different and each stands for hydrogen, C 1 -C 4  alkyl or glycidyloxyphenyl; R 5  stands for hydrogen, C 1 -C 10  alkyl, aryl, aralkyl, allyl or halogen; and n is an integer of 1-38].  
 
       
     
     
         12 . A method as set forth in  claim 2 , in which the electrodeposition paint (A) contains 0.1-20 mass % of bismuth octanoate, based on the combined solid weight of the base resin (a) and epoxy resin (b).  
     
     
         13 . A method as set forth in  claim 2 , in which the electrodeposition paint (A) contains a rutile type fine particulate titanium dioxide composition which is formed by coating surface of rutile type fine particulate titanium dioxide with 0.5-8.0% by weight (based on TiO 2 ) of zirconium oxide as converted to ZrO 2 .  
     
     
         14 . A method as set forth in  claim 1 , in which the electrodeposition paint (A) forms a cured electrocoated film having an average power spectral value not greater than 70, said value being obtained by power spectral frequency analysis comprising measuring surface roughness of an electrocoated film which has been cured by heating at 170° C. for 20 minutes, with a surface roughness meter over a measuring length of 50 mm at 10 μm intervals, and then Fourier converting the obtained data.  
     
     
         15 . A method as set forth in  claim 1  in which the electrodeposition paint (A) forms a cured electrocoated film having an integrated spectral power value within a wavelength range 0.02-1 mm of not greater than 1.7×10 5 , said integrated value being obtained by power spectral frequency analysis comprising measuring surface roughness of the electrocoated film which has been cured by heating at 170° C. for 20 minutes, with a surface roughness meter over a measuring length of 50 mm at 10 μm intervals, and then Fourier converting the obtained data.  
     
     
         16 . Articles on which multi-layered coating film is formed by any of the methods as described in claims  1 - 15 .