Correlation of Film Structure with Film Formation in Cationic Electropaint Systems: An Ionically Conductive Model

Abstract

Using two cationic electropaint resins, the deposition mechanism including the film structure was studied. The resins introduced methyl methacrylate (MMA system) or methyl acrylate (MA system) into a backbone component, and these systems showed different values of the glass transition temperature The MMA resin had a of 70°C, and the MA resin 5°C. A film was deposited under conditions of both constant voltage and constant current, and the film resistivity was estimated. The surface of the deposited film was also observed by scanning electron microscopy (SEM). For the MMA, the film resistivity was on the order of The MMA formed a dense film as the first layer at the start of deposition, and the electrochemical behavior of the film depended on the electrolysis conditions: constant voltage or constant current deposition. The voltage-current curves for the MMA film exhibited a nonohmic behavior. SEM images of the MMA film thinner than 30 μm revealed that pores up to 0.5 μm diam were scattered in the film. Moreover, by increasing the thickness, the film morphology was transformed into a spongy layer with holes up to several mm diam. However, the resistivity of the MA film was almost constant during deposition, and it was on the order of or less. The voltage-current curves for the MA showed an ohmic behavior. The MA film was nonporous, and such a structure was maintained up to 25 μm thick. As the thickness increased, the deposition morphology was also transformed into a spongy layer from a nonporous layer. The resistivity of both films depended on the concentration of lactic acid in the film and on the morphology of the deposits. We clarified that the film formation mechanism of both systems was similar. A new model proposed as the mechanism is that of ion-permeable film formation. © 2003 The Electrochemical Society. All rights reserved.