(9c) Surface Modification of Silica Particles by Dry-Coating

Dry particle coating has been used to create new-generation materials by combining different powders exhibiting different physical and/or chemical properties. Materials with relatively large particle size (host particles) can be mechanically coated with fine particles (guest particles) in order to create new functionality or to improve their initial characteristics. The coating of a host powder by guest particles can be performed in many different ways ranging from simple stirring of two components, to high energy impact coating.

The purpose of the work is to study the surface modification of silica particles coated with magnesium stearate by dry coating. The coating experiments were performed with the Hybridizer manufactured by Nara Machinery Co. Large particles of silica (median diameter, 55 µm) were coated with fine particles of magnesium stearate (median diameter, 4.6 µm) at different concentrations.

Several methods of characterization were used to study the physico-chemical properties of the coated particles. The uncoated and coated particles were observed by scanning electron microscopy (ESEM). DSC measurements were used to quantify the magnesium stearate after coating. The evaluation of the strength of the interactions between silica and magnesium stearate particles was based on the particle size distributions obtained by using a standard Malvern Mastersizer with dry feed for different dispersing air pressures. Aerated and tapped densities were measured to evaluate the flowability of the uncoated and coated particles. Some surface properties of the silica particles have been examined. In particular, the wettability was determined by measurements of the contact angle between the coated particles and water. Uniaxial compression tests have also been performed to evaluate the compressibility of the particles.

The results of the different characterizations have been compared with those concerning the simple mixtures prepared by using a Turbula mixer.

The influences of the process parameters are discussed according to the desired properties.