Abstract: In this study, the low-cost and high-chroma Co_xM_1-xAl₂O₄/kaolin (M=Zn²⁺, Ca²⁺ or Mg²⁺) composite pigments were prepared by solid-phase method using kaolin, Co₃O₄, α-Al₂O₃ and metal oxides (ZnO, CaO or MgO) as raw materials. The effect of preparation conditions on the color properties of composite pigments was systematically investigated, including calcining temperature, grinding time, the added amounts and the types of different metal ions of Zn²⁺, Ca²⁺ or Mg²⁺. As the calcining temperature, grinding time and molar ratio of Co²⁺ to M²⁺ are 1 200 ℃, 12 h and 3:2, respectively, the as-prepared composite pigments present the best color performance (L^*=53.68, a^*=7.58, b^*=-62.89). Furthermore, the color of composite pigments can be facilely adjusted by incorporating different metal elements. After incorporation of Ca²⁺, the obtained composite pigments present the blue color with a red hue, but Co_xZn_1-xAl₂O₄ composite pigments are blue with a green hue. Based on the relevant characterizations, the possible coloring mechanism was proposed. Due to the similar ion radius of the tetrahedrally coordinated Co²⁺ and Mg²⁺ or Zn²⁺, Mg²⁺ or Zn²⁺ can enter into the tetrahedrally coordinated sites of CoAl₂O₄ to substitute Co²⁺and form the solid solutions ofMgAl₂O₄-CoAl₂O₄ (or ZnAl₂O₄-CoAl₂O₄). However, the ion radius of Ca²⁺ is higher than that of Co²⁺, the introduction of Ca²⁺ led to forming the homogeneous mixtures of CaAl₂O₄ and CoAl₂O₄. Finally, the as-prepared composite pigments are incorporated into organosilicon heat-resistance coating, and the heat-resistance of coating is obviously improved.

Key words: composite pigment, CoAl₂O₄, kaolin, coloring mechanism, heat-resistance coating