| Pharmaceutical tablets were studied using the theories of mechanics and thermodynamics to interpret stress/strain compaction data collected from an instrumented tablet machine. The effect of lubrication, strain rate, compression pressure, and formulation composition, on tablet structure during unloading was investigated. Thermodynamic and viscoelastic constitutive parameters obtained from stress/strain data were used to characterize material behavior of pharmaceutical tablets. The basis of this research is the analysis of stress/strain data collected from an instrumented rotary tablet machine. The punch and die were instrumented with bonded electrical resistance strain gages for the measurement of stress. The response from the strain gages was processed by a Wheatstone bridge and collected by a PC equipped with analog to digital converters. The data were fitted to a Kelvin-in-dilation, Kelvin-in-distortion viscoelastic model employing a singular value decomposition algorithm. Using viscoelastic models, the energy input can be broken into stored and dissipative energy. Then by dividing the stress and strain tensors into deviatoric and dilational components, the energy input can be divided into four parts: deviatoric stored, deviatoric dissipated, dilational stored, and dilational dissipated. The distribution of energy into these four parts was used to characterize material behavior and the process occurring during compaction. To account for the evolution of tablet structure these viscoelastic and energy parameters were calculated for overlapping three percent strain intervals. Using these parameters and the first law of thermodynamics, the total energy input can be divided among the various energy transforming process occurring during that interval. |
