DEVELOPMENT OF AN ULTRACENTRIFUGE TECHNIQUE TO DETERMINE THE ADHESION AND FRICTION PROPERTIES BETWEEN PARTICLES AND SURFACES

The extension of a centrifuge technique to measure adhesion and friction forces to an ultracentrifuge has been described. The equipment and procedure provide many experimental possibilities of which the adhesion of single particles to flat compacted powder surfaces has been used to measure the adhesion and friction force of starch microspheres to microcrystalline cellulose. The equipment used allows the positioning of the adhesion samples in the rotor in such a way that any angle between the centrifugal force vector and the flat sample surface can be obtained, and hence both adhesion and friction forces can be measured. The adhesion strength between starch microspheres and microcrystalline cellulose could initially be increased by applying a higher press-on force. However, a maximum plastic deformation and hence maximum contact area between the spheres and the surfaces was eventually reached, and any further application of press-on force appeared to lead only to more elastic deformation and hence not to an increase in adhesion strength. The friction between the starch microspheres and the compacted microcrystalline cellulose surfaces at a maximum deformation of the spheres is still very low, so that starch microspheres could be used as excipient in mixtures including microcrystalline cellulose for example in tabletting.