Preparation of γ-phase pigment Red 170 through hydrothermal treatment and kaolin coating modification

Research on the dynamics of crystal transformation can guide production practices and improve the coloration performance of pigment Red 170. As one of the most important azo dyes, the low hiding power, inferior weather resistance, thermal instability, and low flowability of pigment Red 170 limit its applications. To enhance these properties, it is essential to modify the surface of the pigment. Herein, the crystal transformation and isothermal crystallisation kinetics of colour index (C.I.) pigment Red 170 during a hydrothermal process were studied through X-ray powder diffraction. During isothermal crystallisation, the Avrami indexes (n) were 2.65 and 3.01, and the kinetic rate constants (K) were 6.02 × 10^-6 and 8.34 × 10^-6 at 140 and 150 °C, respectively. The apparent activation energies (E) are 10.42 and 24.31 kcal mol^-1 for the incubation period and total transition, respectively. Pigment Red 170 completely transferred from an α-phase to γ-phase upon hydrothermal treatment at 140 and 150 °C for 180 and 90 min, respectively. The effects of heat treatment temperature and time on the colour hue, tinctorial strength, flowability, particle size and distribution, contact angle, thermal stability, and morphology of pigment Red 170 were investigated. In addition, kaolin was used as an inorganic additive to modify γ-phase pigment Red 170. After hydrothermal treatment and kaolin modification, C.I. pigment Red 170 had a small particle size and exhibited a narrow size distribution and improved hydrophilicity. The γ-phase pigments had a tinctorial strength of 189.5%. The flowability and thermal stability of the kaolin-modified pigment were also enhanced. This study promotes the development of pigments with enhanced colour properties, thermal stability, and processability.