Force-time curves of a rotary tablet press. Interpretation of the compressibility of a modified starch containing various amounts of moisture.

Abstract

On a rotary tablet press, the force-time curves are segmented into three phases: the compression phase, the dwell phase during which both stress and strain are variable for plastically deforming materials and the decompression phase. The following seven parameters were investigated: the compression area (A1) and the compression slope (Slc) describing the initial phase, the area ratio (AR) and the peak offset time (t(off)) characterizing the dwell time, the decompression area (A4) and the decompression slope (Sld) describing the terminal phase and the total area under the force-time curve (Atot). AR, t(off), Slc and A1 (the last with limitations) are used for phase-specific allocation of the occurrence of plastic flow, which is found to be a function of compression force and moisture content. Tablet strength, tablet porosity and in-die bulk porosity provide additional information for comprehensive interpretation. The values of A4 for the four starch batches are not significantly different. Sld provides somewhat better information about the elastic compact recovery. In general, however, the short decompression phase seems to be inappropriate for characterization by force-time curve parameters, because it is difficult to separate machine recovery from that of the tablet. Porosity above the porosity limit of the material was found to be a prerequisite for plastic flow within the compact. When the porosity limit is reached, further densification remains elastic and leads to a reduced compact strength during expansion. The area ratio, as a robust in-process control parameter for plastically flowing formulations, is suggested as a means of preventing this effect.