Preparation of Acid-Resisting Ultramarine Blue by Novel Two-Step Silica Coating Process

Acid-resisting ultramarine blue pigment was prepared by a novel two-step silica coating process which is dense liquid coating followed by a sol–gel process. X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), and BET surface area analysis were used to characterize the composition of elements, surface structure, and morphology on the coating films. Uniform, smooth, and dense silica films on the irregular particles of ultramarine pigment were obtained via novel two-step coatings. Acid resistance of the silica-coated ultramarine blue was evaluated by immersing the pigment in 10 wt % hydrochloric acid. The silica-coated ultramarine blue prepared by the novel two-step process shows the 10th grade of acid resistance, which exhibits much higher acid resistance than that by dense liquid, sol–gel, and conventional two-step coatings. Furthermore, leaching experiments indicated that no sulfur was leached out for the sample prepared by novel two-step coatings, while hydrogen sulfide concentrations from 0.018 to 0.34 mg/L were detected in the leaching solutions for the samples prepared by the conventional two-step coating, dense liquid coating, and sol–gel process.