Preparation and Angle-Dependent Optical Properties of Brown Al/MnO₂ Composite Pigments in Visible and Infrared Region

Traditional low infrared emissivity coatings based on aluminum flakes cannot own low IR emissivity and low lightness simultaneously. Herein, a new simple efficient method for the synthesis of brown Al/MnO₂ composite pigments with low IR emissivity and low lightness is reported, through forming MnO₂ layer on aluminum flakes by thermal cracking, then altering the shape and forming nanoshell by stirring in hot flowing liquid. The results indicate that the MnO₂ particles, which have tetragonal structure with high crystallinity, are needlelike and forming a complete shell on the aluminum flakes. The optical properties of composite pigments can be tuned by mass of KMnO₄ added in precursor and time of hot flowing. Strong angle-dependent optical effects are observed in five different angles through multi-angle reflectance spectrum, while low lightness and low IR emissivity are preserved. This work is expected to provide a new route for the preparation of colored aluminum effect pigments in low infrared emissivity coatings.