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.