Unloading and postcompression viscoelastic stress versus strain behavior of pharmaceutical solids.

The viscoelastic properties of several compacts composed of drugs and direct compression excipients have been measured during the stress unloading and postcompression phases of the tablet compression process. Measurements of applied strains and the resultant stresses, generated in the tablet structure under compaction, were made using a rotary press. The press was instrumented to measure punch and die wall stresses at normal operating speeds. The three-dimensional viscoelastic theory, used in data analysis, provides for the separate characterization of tablet behavior into its dilation and distortion components. The tablets investigated were found to behave elastically in dilation, but to have both viscous and elastic contributions to their stress/strain relaxation in distortion. This latter behavior could be modeled well as a Kelvin solid. Data derived from an elastic-in-dilation, Kelvin-in-distortion analysis of tablets, compressed at similar machine speeds but at various peak pressures, were found to vary widely depending on tablet composition. Dependence of the viscous and elastic parameters on compression conditions was found to be predictive of conditions under which capping or lamination of the compact would occur.

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.