FORCE-TIME CURVES OF A ROTARY TABLET PRESS - INTERPRETATION OF THE COMPRESSIBILITY OF A MODIFIED STARCH CONTAINING VARIOUS AMOUNTS OF MOISTURE

On a rotary tablet press, the force-time curves are segmented into thr ee phases: the compression phase, the dwell phase during which both st ress and strain are variable for plastically deforming materials and t he decompression phase. The following seven parameters were investigat ed: the compression area (A(l)) and the compression slope (Sl(c)) desc ribing the initial phase, the area ratio (AR) and the peak offset time (t(off)) characterizing the dwell time, the decompression area (A(4)) and the decompression slope (Sl(d)) describing the terminal phase and the total area under the force-time curve (A(tot)). AR, t(off); Sl(c) and A(l) (the last with limitations) are used for phase-specific allo cation of the occurrence of plastic flow, which is found to be a funct ion of compression force and moisture content. Tablet strength, tablet porosity and in-die bulk porosity provide additional information for comprehensive interpretation. The values of A(4) for the four starch b atches are not Significantly different. Sld provides somewhat better i nformation about the elastic compact recovery. In general, however, th e short decompression phase seems to be inappropriate for characteriza tion by force-time curve parameters, because it is difficult to separa te machine recovery from that of the tablet. Porosity above the porosi ty limit of the material was found to be a prerequisite for plastic fl ow within the compact. When the porosity limit is reached further dens ification remains elastic and leads to a reduced compact strength duri ng expansion. The area ratio, as a robust in-process control parameter for plastically flowing formulations, is suggested as a means of prev enting this effect.