Barrier properties of polyurethane coil coatings treated by microwave plasma polymerization

Abstract

The work investigates the protective properties of polyurethane coil coating with a plasma polymer top layer. Two different gas precursor mixtures [hexamethyldisiloxane (HMDSO)/oxygen and HMDSO/heptadecafluoro-1-decene (HDFD)/Ar] were used for microwave plasma polymer deposition. The barrier properties of the specimens were evaluated by electrochemical impedance spectroscopy (EIS). The evolution of the structure of the plasma polymer-treated coil coating during immersion tests was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Energy-dispersive spectroscopy (EDS) was used to study the change of the top layer composition with the immersion time. Composition of the coil coating surface after plasma activation procedures was analysed by X-ray photoelectron spectroscopy (XPS). Transmission electron microscopy (TEM) was used to reveal the structure and thickness of the plasma polymer layers. The results show that the plasma treatment procedure causes a deterioration of the barrier properties of the polyurethane coil coating due to the formation of pores in the coil coating. However, the fluorine-containing plasma polymer films (obtained using the HMDSO/HDFD/Ar gas mixture) show enhanced stability and better barrier properties when compared to fluorine-free films (obtained using the HMDSO/oxygen gas mixture). The activation of the coil coatings by oxygen plasma treatment can be responsible for degradation of the barrier properties. The argon plasma activation is less destructive in comparison to the oxygen plasma treatment. The plasma polymer treatment of the coil coating using the fluorine-containing precursor gas mixture can be used for modification of the coil coating surface almost without any degradation of the barrier properties.