Modifications of powders properties by dry-coating: some examples of processes and products characteristics

International audienceDry-coating appears to be an interesting way to create new composite materials in various application areas, avoiding the use of solvents involved in traditional processes, like granulation. Moreover, dry coating processes can be performed on smaller particles sizes. Typical applications include, but are not limited to, altering flowability, solubility, dispersibility, wettability (hydrophilic/hydrophobic properties), electric, electrostatic, magnetic, optical, color, flavor, shape, etc. Particles with relatively large particle size (host particles, 1-500 µm) can be mechanically coated with fine particles (guest particles, 0,1-50 µm) in order to create new functionality or to improve their initial characteristics. In this study we present results obtained with various products and in particular more accurately in the case of silica host particles coated by magnesium stearate using two processes: the Hybridizer NSH0 of the Nara Machinery company and the Cyclomix of Hosokawa company. In this paper we describe the experimental investigation of an application of dry coating technique to change the surface properties of silica gel particles (d50 = 55 µm) coated with different mass ratio of magnesium stearate (MgSt; d50 = 4,6 µm): 1%, 5% and 15%. The products properties were characterised though different methods: particles size distribution (in order to examine the adhesion quality of the fine particles on host particles), flowability, wettability, Atomic Force Microscopy (AFM), and Electron Microscopy (ESEM). The flowability of the samples was characterized by measurements of the tapped and aerated densities. It has been shown in particular that the flowability of silica gel treated with and without MgSt in Hybridizer was not significantly affected and remained good. The wettability of silica gel was determined by measurements of the contact angle between the water drop and the powder bed prepared for each sample. The results obtained have shown that the coating of silica gel powder by hydrophobic magnesium stearate, can improve its hydrophobic properties