Degradation of fluorinated polyurethane coating under UVA and salt spray. Part Ⅱ: Molecular structures and depth profile

The molecular structures and degradation depth profile of fluorinated polyurethane (FPU) coating on aluminum alloy were studied by Fourier transform infrared spectroscopy (FTIR) and confocal Raman microscopy (CRM) under ultraviolet A (UVA) irradiation and salt spray test (SST), respectively. The results show that UVA irradiation accelerates the breakdown and aggregation of polymer chain, but SST has little influence on the change of polymer structures. Furthermore, the dynamics of coating degradation was investigated based on the depth profile versus time. It is confirmed that the UVA irradiation mainly results in chemical deterioration through the photo-oxidation with the influenced depth of ∼20 μm beneath FPU coating in 2000 h, but SST has no apparent influence on the coating degradation in depth profile. The different degradation mechanisms of FPU under UVA and SST were carefully analyzed.