Viscoelastic stress/strain behavior of pharmaceutical tablets: analysis during unloading and postcompression periods.

The processes of nonequilibrium generation and decay of axial and radial stresses within tablet compacts were analyzed in terms of three-dimensional linear viscoelastic theory. A rotary tablet press was instrumented to measure punch and die wall stresses during the compression and postcompression periods. Following compression, tablets were permitted to remain at the compression site within the die, and the die wall stress was followed. Microcrystalline cellulose, spray-processed lactose, and sulfacetamide are known to have different compression characteristics and were found to differ significantly in their viscoelastic parameters. Compacts assumed their final viscoelastic state prior to the time of punch separation. Theory permits separation of material behavior into dilation and distortion components. Dilation, thought to be dependent on voids, was elastic in all cases. Distortion effects could be described well by a Kelvin solid model. Results indicate that viscoelastic properties are functions of compression conditions and may be useful in adjusting compression conditions to avoid problems such as capping.