Electrodeposition coating process

Abstract

Disclosed is an electrodeposition coating process which comprises: a first electrodeposition step in which an electrodeposition coated film is partially formed on a substrate surface to be coated with using a lead-free cationic electrodeposition coating composition which is modified so that a coated film deposited on the substrate surface from the coating composition (deposited film) has relatively low glass transition temperature; a second electrodeposition step in which an electrodeposition coated film is formed on an uncoated portion of the substrate surface with using a lead-free cationic electrodeposition coating composition which is modified so that the deposited film has relatively high glass transition temperature; and a step of baking the electrodeposition coated film to cure. The electrodeposition coating process makes difference of film thickness between the interior surface and the exterior surface to be small, makes smoothness of exterior surface to be excellent even if a small amount of coating composition is employed, and exert a little influence on the environment due to its low VOC, and low metal ion content.

What is claimed is:  
     
         1 . An electrodeposition coating process comprising: 
 a first electrodeposition step in which an electrodeposition coated film is partially formed on a substrate surface to be coated with using a lead-free cationic electrodeposition coating composition which comprises an aqueous medium, a binder resin composed of a cationic epoxy resin and a blocked isocyanate curing agent dispersed or dissolved in the aqueous medium, a neutralizing acid in order to neutralize the cationic epoxy resin, an organic solvent, and a metal catalyst, and which has a volatile organic content of 1% by weight or less, a metal ion content of 500 ppm or less and a neutralizing acid amount of 10 to 30 mg equivalent based on 100 g of binder resin solid content, and an electrodeposition coated film deposited on the substrate surface from the coating composition has a glass transition temperature of not more than 0° C.;    a second electrodeposition step in which an electrodeposition coated film is formed on an uncoated portion of the substrate surface with using a lead-free cationic electrodeposition coating composition which comprises an aqueous medium, a binder resin composed of a cationic epoxy resin and a blocked isocyanate curing agent dispersed or dissolved in the aqueous medium, a neutralizing acid in order to neutralize the cationic epoxy resin, an organic solvent, and a metal catalyst, and which has a volatile organic content of 1% by weight or less, a metal ion content of 500 ppm or less and a neutralizing acid amount of 10 to 30 mg equivalent based on 100 g of binder resin solid content, and an electrodeposition coated film deposited on the substrate surface from the coating composition has a glass transition temperature of 5 to 20° C.; and    a step of baking the electrodeposition coated film to cure.    
     
     
         2 . The electrodeposition coating process according to  claim 1 , wherein the first electrodeposition step and the second electrodeposition step are conducted with using separate electrodeposition baths respectively.  
     
     
         3 . The electrodeposition coating process according to  claim 1 , wherein the electrodeposition coated film formed in the first electrodeposition step has a thickness of 8 to 20 um.  
     
     
         4 . The electrodeposition coating process according to  claim 1 , wherein the metal ion of the lead-free cationic electrodeposition coating composition is one or more selected from the group consisting of cerium ion, bismuth ion, copper ion, zinc ion, molybdenum ion, and aluminum ion.  
     
     
         5 . The electrodeposition coating process according to  claim 1 , wherein the neutralizing acid of the lead-free cationic electrodeposition coating composition is one or more selected from the group consisting of acetic acid, lactic acid, formic acid, and sulfamic acid.  
     
     
         6 . The electrodeposition coating process according to  claim 1 , wherein the lead-free cationic electrodeposition coating composition further comprises a pigment in a ratio of 1/9 or less by weight based on a resin solid contained in the coating composition.