An evaluation of the techniques employed to investigate powder compaction behaviour

Employing 4 model powders, Avicel, Dipac, mannitol and paracetamol and a single-punch tablet machine, the most common techniques used to investigate powder compaction behaviour are compared and assessed. The use of tablet tensile strength rather than crushing force is found to bias the interpretation of lower punch work (LPW₁)/hardness profiles when compacts of very different thicknesses are produced. Radial versus axial pressure cycles, maximum die wall pressure and lower punch work on recompression (LPW₂) are demonstrated to be insensitive comparative measurements of compaction behaviour. Further, the latter suffers from considerable interpretive difficulties. Stress relaxation measurements are employed to verify data obtained from residual die wall pressure determinations and Heckel plots. Thus, the use of LPW₁ /crushing force profiles, residual die wall pressure versus axial pressure and elastic recovery versus LPW₁ plots are simple and efficient techniques that assist in defining powder compaction behaviour.