Regulating the properties of C.I. Pigment Red 170 by surface modification via hydrous alumina

• Hydrous alumina was coated on P.R. 170 particle surface by hydrolysis of Al2(SO4)3.
• P.R. 170 was coated with three types of structure; dots, floccules and films.
• The relationship between the coating structures and the properties of P.R. 170 are discussed.
• The film coating most significantly improves the properties of P.R. 170.

C.I. Pigment Red 170 (P.R. 170) is one of the widely used organic pigments, and surface modification is essential to improve its thermal stability, and solvent resistance. In this work, hydrous alumina was coated onto P.R. 170 particles by hydrolysis of Al2(SO4)3, and different coating structures/morphologies were obtained including dots, floccules and films with different thickness. The influence of pH, temperature and Al2(SO4)3 content on the hydrous alumina coating structures were investigated by transmission electron microscopy (TEM), ζ-potential analysis, and several spectroscopic techniques. Thermogravimetric analysis (TGA) and pigment bleeding tests indicated that the thermal stability and solvent resistance of the pigments can be remarkably improved by a film coating, and this reveals that the consecutiveness and density of the coating layers should be the key factors for improvement of the organic pigments.