Film formation from cationic electropaint systems containing resins with different glass transition temperature

Some phenomena in the deposition process of electropainting have not been well elucidated till now. In this paper, to investigate an influence of glass transition temperature (T_g) on film formation, the deposited film was observed with an atomic force microscope and the electrochemistry was investigated, using two kinds of cationic acrylate resin with different T_g (methyl methacrylate system (MMAs): T_g=70°C, and methyl acrylate system (MAs): T_g=5°C). Electrodeposition was performed under constant voltage or current condition.

At constant voltage, the deposition behavior in the two resin systems differed extremely. The MMAs, the resin with high T_g, produced a high resistance film. The MAs, the resin with low T_g, was deposited forming a film at a voltage lower than 20 V. At constant current, the film formation did not result in a rise in voltage. It behaved like a conductive film. When the resin with high T_g was used, particulate deposits were observed by AFM even in the induction period. The resin with low T_g formed flat deposits. These results suggest that paint deposition is initiated once electrolysis of water starts. In addition, there are two types of film formation on the cationic electropainting: high resistance film formation for the resin with high T_g, and ion-permeable film formation for the resin with low T_g. In both cases, film growth occurs at the film/bulk solution interface.