1 The mechanical properties of powders

Abstract

Publisher Summary The vast majority of drugs, when isolated, exist as crystalline or amorphous solids. Subsequently, they may be either milled (comminuted) or admixed with other inactive solids (excipients) and finally filled into capsules or compacted to form tablets. The processes of comminution and compaction involve subjecting the materials to stresses that cause them to undergo deformation. This chapter derives the methods used to measure the mechanical properties of powdered materials in order to be able to predict the comminution and compaction behavior of a material. These methods include Young's modulus of elasticity (E), the yield stress (σ y ), and the critical stress intensity factor (K IC ). Compaction studies, because they mimic the tabletting process, offer an ideal method for assessing the mechanical properties of powders. The moisture content of a material can affect its mechanical properties generally by acting as an internal “lubricant” facilitating slippage and plastic flow. This leads to a decrease in both yield stress and Young's modulus of elasticity. The critical particle size, d crit , is an important factor in predicting comminution behavior as, for a material known to undergo brittle fracture, it should be the limiting size obtained on hammer milling.