Preparation of environmental friendly coatings based on natural shellac modified by diamine and its applications for copper protection

An environmental friendly coating based on natural product—shellac was prepared by using aliphatic diamine as a crosslinking agent and ethanol as a solvent. The amide group appeared in FTIR spectra in the modified shellac structure suggested the 1,3-propanediamine could successfully react with the acid groups in shellac. The enhancement in anticorrosive performance of the modified shellac could be observed directly from the color changes of the surface images of copper, copper coated with shellac and modified shellac after 60 h salt spray test. The EIS results indicated that after 24 h immersion in 0.5 M NaCl solution, the ionic Cl− and water had reached the copper electrode and the shellac lost its protection to copper. However, the corrosive behavior of the copper electrode coated with modified shellac immersed in 0.5 M NaCl solution for 24 h still showed a characteristic of middle period of immersion and the coating layer hindered the ionic and water penetrating onto the surface of the copper; while after 72 h immersion, the modified shellac began to lose its protection to copper. The improvement in anticorrosive performance of modified shellac was attributed to the formation of network structure and the introduction of hydrophobic aliphatic group in shellac structure, which slowed down the speed of the ionic and water moving onto the copper surface.