Simulation of reflectance from white-anodised aluminium surfaces using polyurethane–TiO 2 composite coatings Visweswara Chakravarthy Gudla 1 • Villads Egede Johansen 1 • Stela Canulescu 2 • Jørgen Schou 2 • Rajan Ambat 1 Received: 12 December 2014 / Accepted: 1 April 2015 / Published online: 18 April 2015 Ó Springer Science+Business Media New York 2015 Abstract Theoretical calculations and experimental studies were carried out on polyurethane (PU)–TiO 2 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 de- signing the optical appearance of anodised surfaces. PU matrix was selected for its matching refractive-index (n = 1.7) with anodic alumina layer. Three different TiO 2 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 in- tegrating sphere-spectrophotometer. Data showed that the coated surfaces have a high diffuse reflectance due to the multiple scattering from TiO 2 particles and the coating– substrate interface. The diffuse reflectance spectra of the coated surfaces varied weakly with TiO 2 particle concen- tration 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 TiO 2 in PU was predicted. The studies pre- sented in this paper provide insight into generating bright white-anodised aluminium surfaces based on aluminium– TiO 2 composites. Introduction White anodising of aluminium has been of great interest to the aluminium surface finishing industry for the past few decades as it finds applications in architecture and marine industry for structural profiles, automobile and consumer goods industry for its pleasing aesthetic appearance and in the aerospace industry due to its high reflectance along with good corrosion resistance [1–4]. However, achieving a bright white- and glossy-anodised Al surface has not been reported to date as the conventional dyeing and colouring process of anodised aluminium surfaces cannot be applied to achieve white appearance [5–9]. White appearance re- quires diffuse and broadband reflection of light generated from high refractive-index scattering centres in a low re- fractive-index medium. In nature, the white-bright beetle displays strong white appearance which arises from mul- tiple scattering of light from a dense randomly structured anisotropic chitin networks that have a very high refrac- tive-index [10]. But traditionally, these high refractive-in- dex scattering centres (white pigments) are an order of magnitude (approx. 150–200 nm) larger than the anodic alumina pore diameters (approx. 10–50 nm) and hence cannot be incorporated into them after anodising [11]. Consequently, various alternative techniques have been studied for achieving enhanced reflection of light from anodised surfaces like plasma electrolytic oxidation (PEO) or micro-arc oxidation (MAO). These processes result in a hard ceramic aluminium oxide surface that is used for applications requiring wear resistance and are not aes- thetically pleasing as they are highly porous, rough and appear diffuse with no gloss [12, 13]. Caustic etching of the Al substrates prior to anodising improves the scattering of light due to the microscopic surface roughness and also retains the surface gloss after anodising, but the obtained & Visweswara Chakravarthy Gudla chakri_gvc@yahoo.co.in; vichg@mek.dtu.dk 1 Department of Mechanical Engineering, Technical University of Denmark, 2800 Kongens Lyngby, Denmark 2 Department of Photonics Engineering, Risø Campus, Technical University of Denmark, 4000 Roskilde, Denmark 123 J Mater Sci (2015) 50:4565–4575 DOI 10.1007/s10853-015-9005-1