Method of removing acid formed during cathodic electrodip coatings

Abstract

A method of removing, by oxidation at the anode, the acid liberated in cathodic dip-coating in the course of the deposition of the coating film, using anodes coated with a layer of ruthenium oxide, iridium oxide or tin oxide or with a mixture of these oxides.

1 . A method of removing the acid liberated in cathodic electrodeposition coating in the course of the deposition of the coating film, which comprises breaking down the acid by oxidation at an anode which is coated with a layer of ruthenium oxide, iridium oxide or tin oxide or with a mixture of these oxides.  
     
     
         2 . The method as claimed in  claim 1 , wherein said oxidation takes place in an anolyte circuit.  
     
     
         3 . The process as claimed in  claim 1 , wherein anode and cathode are not separated from one another by a membrane in the electrodeposition bath and wherein in addition to the anodic oxidation a further separation of acid takes place with the aid of a membrane method.  
     
     
         4 . The method as claimed in  claim 3 , wherein an ultra-filtration circuit is present in the electrodeposition coating bath and the membrane method is carried out with the ultrafiltrate.  
     
     
         5 . The method as claimed in either of claims  3  and  4 , wherein the membrane method is a dialysis, electro-dialysis, osmosis, reverse osmosis or a second ultrafiltration.  
     
     
         6 . The method as claimed in one of  claims 1  to  5 , wherein the anodes are employed in electrodeposition coating baths whose binders comprise synthetic resins that have cationic groups.  
     
     
         7 . The method as claimed in one of  claims 1  to  6 , wherein the synthetic resins are protonated reaction products of epoxide-functional synthetic resins and amines.  
     
     
         8 . The method as claimed in one of  claims 1  to  7 , wherein formic acid, acetic acid, lactic acid and dimethylol-propionic acid are oxidized at the anode.  
     
     
         9 . The method as claimed in one of  claims 1  to  8 , wherein anodes are employed whose substrate comprises metal or conductive plastic, the metal used preferably being titanium, tantalum, niobium or an alloy of these metals such as titanium with 1-15% by weight molyb-denum, for example.  
     
     
         10 . The use of an anode coated with a layer of ruthenium oxide, iridium oxide or tin oxide or with a mixture of these oxides to remove, by means of oxidation, the acid liberated in cathodic electrodeposition coating in the course of the deposition of the coating film.  
     
     
         11 . The use of the anode as claimed in  claim 10  to oxidize formic acid, acetic acid, lactic acid and dimethylolpropionic acid.