Internal friction of compressed pharmaceutical powders observed in terms of the die wall pressure.

Abstract

The internal friction of powders was examined during tablet preparation for lactose and phenacetin as base materials and for potassium chloride, potassium bromide and sodium chloride as model samples. The coefficient of internal friction decreased with the porosity, even if the voids were filled by fragmentation of particles or by plastic flow. When the porosity had reached zero, the state of the whole bed of powder was found to be plastic, where the coefficient of internal friction was zero, for magnesium stearate, phenacetin, potassium bromide and potassium chloride. The plastic behavior of magnesium stearate and phenacetin was considered to be related to the capping of tablets containing them. For lactose granules, the porosity did not reach zero even at a compacting pressure of 4000 kg/cm2, the maximum used in this work, because of its crystal hardness. The coefficient of internal friction was large and decreased only gradually with the porosity. Mixtures of two components exhibited intermediate internal friction, corresponding to the composition. However, when hydroxypropyl cellulose (HPC) was added to phenacetin powder as a binder, HPC increased the yield stress from 2000 to 3100 kg/cm2 even at a content of only 2% w/w on a dry basis. In contrast, lubricants (magnesium stearate and talc) added to granules had little or no effect on the internal friction, although this small amount was sufficient to reduce the wall friction.