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Effect of lubricant on die wall friction during the compaction of pharmaceutical powders.

The wall friction of pharmaceutical powders (lactose, sucrose, sodium chloride, potassium chloride and sodium bicarbonate) was measured. The sample powders were compressed against a stainless steel wall. Then, keeping the normal pressure (PN) constant, the wall was slid. During the sliding, the displacement of the wall and the normal and tangential pressures were recorded. For lactose and sucrose powders, the coefficients of wall friction (μw) were higher than those for the other powders. The effects of lubricants (magnesium stearate and talc) on μw and on the sliding process were examined. The values of μw for lubricated powders were 0.22 or less, being slightly dependent on PN. The residual layer on the wall surface and the replicas of particles on the surface of the compact at various stages during the sliding process were examined. It was found that the junctions between the particles and the die wall grew in size during the course of sliding, and that the lubricants prevented this growth, resulting in decreases of the friction and adhesion forces.