(197a) Compaction Properties of Pregelatinized Starch (Starch 1500™): Self-Lubrication

It has been shown repeatedly that incorporation of partially pre-gelatinized maize starch (Starch 1500TM) markedly reduces sticking, binding and ejection forces of direct compression formulations in tablet punch and dies. The origins of the self-lubrication properties of Starch 1500TM seem to lie in its viscoelastic behavior i.e., its time-dependent deformation. The scope of this work was a preliminary characterization of the viscoelastic behavior of this excipient. The effect of punch velocity over the range 0.08-10 mm.s-1 on the compaction and ejection of Starch 1500TM tablets was studied using the yield stress derived from the Heckel equation and the compact maximum ejection force as parameters. There was an increase in the yield stress and maximum ejection force with punch velocity attributable to the reduction in the amount of plastic deformation due to the time-dependent nature of plastic flow. An attempt to quantify the viscoelastic behavior of Starch 1500TM was made by the study of stress relaxation profiles. These showed that deformation mechanism and rate are quasi independent of imposed strain and that stress relaxation rate decays quickly with time to a constant value and eventually zero. Initial and final stress relaxation rates were found to be within the orders of magnitude of 2.4 MPa.s-1 and 3.74?10-3 MPa.s-1 respectively.