Pressure susceptibility of polymer tablets as a critical property: A modified Heckel equation

The pressure susceptibility (chi(p)), which is defined as the decrease of p orosity (epsilon) under pressure was investigated. Of special interest are compacts obtained at very low pressures, because of the transition between the state of a powder and the state of a tablet. This range was found to be critical in respect to a diverging pressure susceptibility. Above a critic al porosity (epsilon(c)) or below the corresponding relative density (rho(c )), no pressure susceptibility can be defined, because of no rigid structur e exists. To take this into account. a simple function was proposed for the pressure susceptibility: chi(p) proportional to 1/(epsilon(c) - epsilon). This proposal leads to a new porosity vs pressure relationship. The new mod el was compared to the Heckel equation that involves a constant pressure su sceptibility. Various polymers were tested from "out of die" measurements, and the new relationship was found superior to the Heckel equation. As a co nclusion, the pressure susceptibility exhibits a curvature that can be call ed critical at low relative densities. Consequently, a better understanding evolves as to why the Heckel equation is not valid at low pressures. The n ew model has proven to be adequate for polymer tablets but, so far it is no t clear whether other substances exhibit the same performance. Especially t ableting materials exhibiting brittle fracture will be of interest consider ing their importance in compaction technology.