The influence of particle size and shape on the angle of internal friction and the flow factor of unlubricated and lubricated powders

Abstract

The shear properties of 8 different powders, which varied in particle size and shape, were studied using an annular shear cell. The angle of internal friction and the flow factor were determined. Magnesium stearate was added in concentrations between 0.25 and 1.25% w/w, and the change in the shear properties was recorded. For the unlubricated powders, the angle of internal friction was found to depend both on particle size and shape in a nonlinear manner, whereas the flow factor depended only on particle shape. The optimal magnesium stearate content, i.e. the concentration which gave the lowest angle of internal friction, varied for the powders. Both the angle of internal friction and the corresponding concentration of magnesium stearate depended only on particle shape. A large value for the aspect ratio as obtained for needle shaped particles was accompanied by a particularly high angle of internal friction. The optimal magnesium stearate concentration was least for needle shaped or round particles. The optimal magnesium stearate contents for the flow factor, i.e. the concentrations which gave the highest values of flow factor, were partly different from those obtained for the angle of internal friction. While the flow factor depended only on particle shape, the corresponding optimal magnesium stearate concentration was found to depend only on the particle size. For powders of comparable chemical composition such as maize starch and starch 1500, or microcrystalline and microfine cellulose, the angle of internal friction at an optimal lubricant concentration was found to be proportional to the elastic properties of the powders.