Powder compaction interpretation using the power law

The effects of strain hardening and axial strain rate, over a wide range of rates (10^–3 to 10⁵ s^–1), on the compaction properties of a variety of pharmaceutical powders have been investigated. The powders tested are: Di Pac sugar, paracetamol d.c., Avicel and lactose. These materials have been assessed using the constants derived from the power law as a criterion to describe their behaviour. All the materials tested show, with varying degrees, a non-linear increase in the yield pressure (flow stress), the constantG, the strain rate exponentm and the strain hardening exponentn as the strain rate increases. These variations are more clear in the materials known to deform plastically, such as Avicel. This is attributable to a change either from ductile to brittle behaviour or a reduction in the amount of plastic deformation due to the time-dependent nature of the plastic flow. This, however, is explained in terms of dislocation and diffusion processes involved in the plastic deformation mechanisms during the compaction process. As the speed of compaction increases the characteristics of deformation, including the value of the strain rate exponent, the shape of the creep curve and the nature of creep rate, suggest that the creep behaviour is therefore controlled by some form of diffusion process. Meanwhile, the creep characteristics of the low and medium rate tests appear to be consistent with dislocation climb and viscous glide. For the materials tested, Avicel is found to be the most strain-rate sensitive material, while paracetamol d.c. is found to be the least strain-rate sensitive material.