Modeling the mean interaction forces between powder particles. Application to silica gel-magnesium stearate mixtures

Dry coating experiments were performed by using the Hybridizer (Nara). Large host silica gel (SG) particles (d50 = 55 μm) were coated with fine invited particles of magnesium stearate (MS, d50 = 4.6 μm) for different contents of MS in the mixture. The real MS mass fraction wI obtained after mechanical treatment has been determined thanks to calibration from TGA measurements. The surface structure and morphology of MS coatings were observed using environmental scanning electron microscopy (ESEM) and atomic force microscopy (AFM) [Y. Ouabbas, A. Chamayou, L. Galet, M. Baron, J. Dodds, A.M. Danna, G. Thomas, B. Guilhot, P. Grosseau, Modification of powders properties by dry coating: some examples of process and products characteristics, Proceedings of CHISA2008, Prague, August 2008, submitted for publication; L. Galet, Y. Ouabbas, A.M. Danna, G. Thomas, P. Grosseau, M. Baron, A. Chamayou, Surface morphology analysis and AFM study of silica gel particles after mechanical dry coating with magnesium stearate, Proceedings of PSA2008, UK, September 2008, submitted for publication].AFM has been also used to measure the adhesion forces between particles. Interaction forces between the material attached to the cantilever (magnesium stearate MS) and the surface of the composite material (silica gel SG or magnesium stearate MS) have been determined at different surface locations. For different compositions wI of the mixture MS-SG, the numeric distribution and the mean value f of the forces fH obtained for MS-SG interactions or fI for MS-MS interactions have been established and the experimental curve showing the evolution of f versus wI has been derived.Models of ordered structures have been developed, implying morphological hypotheses concerning large spherical or cylindrical host particles H and small invited spherical I. Different types of distribution of I materials onto the surface of H have been considered: for examples a discrete monolayer - or multilayers - of monosized particles I on the H surface. The coordinence of MS particles around SG particles has been estimated to calculate the free SG surface fraction through different modelling and to obtain the mean force f versus composition wI. The theoretical force values have been compared to experimental ones. The deviations have been discussed in terms of guest particle distributions on the surface of the large host particles and morphological hypotheses.