Depth profiling of wrinkled coil coatings - new insights into gradient structures and coating degradation

For the desired low-gloss appearance of applications such as outdoor roofing systems, organic coatings with a certain degree of surface corrugation are commonly used. Despite various mechanical and chemical mechanisms contributing to wrinkle formation in curing coatings have been extensively discussed in in the past, a comprehensive in-depth characterization of the internal microstructure of textured finishes is yet to be reported. In this work, the interior chemical structure of a state-of-the art wrinkled coil coating system consisting of a polyester-melamine topcoat and a polyester-polyurethane primer applied on a galvanized steel substrate was thoroughly examined by combining advanced surface analysis techniques with cryo-ultra-low-angle microtomy (cryo-ULAM). Plane-view depth-resolved analysis by means of focal-plane-array Fourier-transform infrared (FPA-FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) mappings readily enabled to determine chemical gradients inside the topcoat layer related to the formation of a melamine-enriched and pigment-depleted skin on top of the coating system. Furthermore, correlative depth-resolved analysis additionally allowed to directly assess and compare the degradation of a high-end wrinkle coating system after accelerated artificial ageing as well as after exposure to atmospheric conditions, which consistently showed that the most significant alterations of the coating chemistry induced by progressive weathering particularly occurred at the melamine-enriched coating surface. • Depth profiling of state-of-the-art polyester-melamine/urethane wrinkle coatings • Plane-view analysis of internal gradient structures by means of FPA-FTIR and XPS • Detection of melamine-enriched and pigment-depleted skin at the coating surface • Degradation of wrinkle coatings assessed after artificial and natural weathering