THE VISCOELASTIC DEFORMATION OF SOME TABLETING MATERIALS AS ASSESSED BY INDENTATION RHEOLOGY

The time-dependent deformation of compacts prepared from four tablet c ompression bases has been assessed by microindentation rheology. Compa cts were made from Avicel PH101, Emcompress, Emdex, and Starch 1500 at compaction pressures of 30, 60, and 90 MPa. A spherical indenter, 1.5 5 mm diameter, was allowed to indent into the compacts under a Feed lo ad of 5.89 N (600 g) and the changing depth of indentation was recorde d. The corresponding creep compliance versus time curve was calculated and analyzed by discrete mechanical analysis to yield Maxwell and Voi gt ''spring and dashpot'' mechanical models. This yielded quantitative delta on instantaneous elastic compliance and modulus, the time-depen dent viscoelastic compliances, viscosities, and retardation times of e ach Voigt unit, and the Newtonian viscosity of the compacts. In genera l, the deformation of the compacts was in the rank order Starch 1500 > Avicel PH101 > Emdex > Emcompress. Emcompress showed negligible visco elasticity and plasticity. For the other compacts, the number of Voigt units (a reflection of the magnitude of viscoelasticity) generally de creased with increasing compaction pressure between 30 and 90 MPa. For example, Avicel PH101 compacts required five Voigt units to define th eir deformation when compressed at 30 MPa, three at 60 MPa and two at 90 MPa. The corresponding numbers for Emdex compacts were 4, 3, and 2, while Starch 1500 compacts showed a high degree of viscoelasticity at high compaction pressure, exhibiting 5 (at 30 MPa), 4 (at 60 MPa), an d still 4 Voigt units even at 90 MPa compaction pressure. The rank ord er for the viscosity values was Starch 1500 < Avicel PH101 < Emdex << Emcompress compacts. Long-term indentation rheology proved to be a val uable tool in assessing the viscoelastic characteristics of pharmaceut ical compacts and will be useful in predicting time-dependent scale-up problems during the change from single-punch development presses to h igh-speed rotary production machines.