Characterisation of stearate/stearic acid coated fillers

Selected magnesium hydroxide and calcium carbonate fillers were dry coated with stearic acid and calcium, magnesium, and zinc stearates. The course of the coating reaction was monitored using a quantitative diffuse reflectance Fourier transform infrared spectroscopy method and by thermal analysis. The coated fillers were also examined by wide angle X-ray diffraction and by X-ray photoelectron spectroscopy. These techniques were shown to produce complementary information. When the dry coating process was useded, complex reactions occurred. Stearic acid reacted to produce stearate, while the stearates melted and formed coatings, but in some cases they also appeared to form polymorphic metal stearates. Stearic acid produced the best coverage on calcium carbonate, but the poorest on magnesium hydroxide. Of the metal stearates, the best filler coverage was produced when zinc stearate was used, while calcium stearate was the least effective coating.