Synthesis of Tubular Iron Oxide Pigments with Bright Red Color Deposited on Phosphorylated Cellulose Nanofibers

Hematite α-Fe2O3, called “bengara”, is widely used as a red pigment because it is abundant, economical and extremely low in toxicity. However, its redness is not so high at around a*=+30. In this study, we attempted to improve the redness by controlling the microstructure of iron oxide particles and adding the Al element. To achieve this goal, we focused on fibrous hematite particles. The synthesis of fibrous particles was carried out by the co-precipitation method. That is, ammonium was dropped into a mixed aqueous solution of iron chloride and aluminum chloride, and the mixture was adsorbed and precipitated on phosphorylated cellulose nanofibers (P-CNF). By heat-treating the precipitate, the P-CNF was burned and at the same time the hydroxide precipitate crystallized into hematite while maintaining the fibrous shape. The sample with 23% Al added showed an excellent reddish yellow color with a*=42, b*=52, and L*=50 after heat-treatment at 800°C.