Paradox effects of flake carbonyl iron on the photodegradation behaviors of epoxy-based wave-absorbing coatings: Photo-catalytic and UV blocking

Polymer-based electromagnetic wave-absorbing (EWA) coatings that can effectively dissipate electromagnetic radiation are in high demand. While researchers on the development of lighter and more efficient EWA coatings have been carried out extensively, there is limited information on their durability and the underlying degradation mechanisms. In this work, epoxy (EP)/flake carbonyl iron powder (FCI) coatings were selected as the model EWA coating. UV aging experiments of specimens were conducted up to about 240 h to study the effects of additional FCI on the photo-oxidative behaviors of coatings. The evolutions in the surface topography, chemical structure, hydrophilicity of aged coatings were systematically studied by Scanning electron microscopy (SEM), Laser scanning confocal microscopy (LSCM), Fourier transform infrared spectroscopy (FTIR), and static contact angle measurement, and further combined with micro-FTIR to investigate the degradation diffusion behavior from the exposed surface to the bulk of coatings. The results showed that the addition of FCI significantly accelerated the photo-oxidation of EP coatings, which was attributed to the corrosion of FCI during aging and the concomitant photo-catalytic degradation effects of its corrosion products. Moreover, the cross-sectional analysis revealed a non-uniform distribution of EP degradation species across the thickness direction, while it is interesting to find that the degradation depth of EP/FCI coating was lowered compared with that of pure EP coating, which could be due to the UV blocking effect of FCI. This work clearly revealed the underlying mechanisms for the distinct dependence of surface and cross-sectional degradation of coatings on the incorporation of FCI, which has laid crucial groundwork for durability evaluation and performance optimization of EWA coatings.