Surface modification of silica particles by dry coating: Characterization and powder ageing

In this study the dry coating experiments were performed with two different processes using a high energy impact coater “Hybridizer” (Nara) and a high shear mixer “Cyclomix” (Hosokawa). Silica gel particles (d50 = 55  µm) were coated with fine particles of magnesium stearate (d50 = 4.6 µm) at two different weight ratio of magnesium stearate, 5% and 15%. Several methods of characterization were used to study the effect of dry coating of the silica gel powder.The uncoated and coated silica gel particles were observed by environmental scanning electron microscopy (ESEM). The images have shown that a more uniform coating was obtained in the case of Hybridizer. The flowability of the different samples obtained with Hybridizer and Cyclomix have been characterized by measurements of the tapped and aerated densities. It has been shown that the flowability of silica gel treated in Hybridizer was not significantly affected whereas the flowability was reduced after treatment in the Cyclomix. The wettability of silica gel was determined by measurements of the contact angle between a water drop and a powder bed prepared for each sample. The results obtained have shown that the coating of silica gel powder by hydrophobic magnesium stearate in the two different devices, Hybridizer and Cyclomix, can improve its hydrophobic properties. The results of the different characterizations have been compared with those concerning the simple mixtures prepared by using a Turbula mixer.To investigate the stability and the ageing of silica gel particles after coating, three samples of silica gel coated with 15% (w/w) of magnesium stearate in the Hybridizer device have been stored under different relative humidities (RH). The temporal evolution of the contact angle and the volume of the drop water on the surface of the bed powder have been determined after 10 days or 36 days storage. The results have shown the influence of the relative humidity on the evolution of the coated silica gel particles. The surface of the host particles recovers its hydrophilic nature after 10 or 36 days storage under different relative humidity. No significant variations of the contact angle and the volume of the drop were recorded for the sample stored under dry atmosphere (RH < 12%). The ESEM observations of the different samples confirmed the results. A mechanism that can explain these ageing effects is proposed.

In this study dry coating experiments were performed using a high energy impact coater “Hybridizer” (Nara) and a high shear mixer “Cyclomix” (Hosokawa). Silica gel particles were coated with fine particles of magnesium stearate at two weight ratio: 5% and 15%. Several methods of characterization were used to study the effect of dry coating of the silica gel powder.Figure optionsDownload as PowerPoint slide