Simulation of reflectance from white-anodised aluminium surfaces using polyurethane–TiO₂ composite coatings

Theoretical calculations and experimental studies were carried out on polyurethane (PU)–TiO₂ composite coatings on bright and matte aluminium surfaces with an aim to understand and tailor the light scattering from particles incorporated into an anodised layer for designing the optical appearance of anodised surfaces. PU matrix was selected for its matching refractive-index (n = 1.7) with anodic alumina layer. Three different TiO₂ particle size distributions were dispersed in PU and spin coated onto bright high-gloss and matte caustic-etched aluminium substrates. The reflectance spectra of coated surfaces in the visible region were analysed using an integrating sphere-spectrophotometer. Data showed that the coated surfaces have a high diffuse reflectance due to the multiple scattering from TiO₂ particles and the coating–substrate interface. The diffuse reflectance spectra of the coated surfaces varied weakly with TiO₂ particle concentration and reached a steady state value at 1 wt% but were dependent on the substrate type used. Using Kubelka–Munk two-stream model, the scattering and absorption coefficient of TiO₂ in PU was predicted. The studies presented in this paper provide insight into generating bright white-anodised aluminium surfaces based on aluminium–TiO₂ composites.