STRUCTURE EVOLUTION OF TABLETS DURING COMPRESSION UNLOADING

The evolution of internal structure within compacts composed of selected drugs and direct compression excipients, occurring during the unloading phase of compaction, was studied in an instrumented rotary tablet press. Utilizing three-dimensional viscoelastic analysis applied to successive, sequential segments of the unloading phase, elastic and viscous parameters were obtained that provide quantitative, incremental measures of the changing behavior of the compact during this period. It is apparent from these studies that the principal feature of this phase of compaction is the generation and propagation of internal fractures as a result of the release of internal stresses through expansion. The progression of elastic parameters from positive initial values to negative terminal values during unloading, coupled with the strong relationships they display to corresponding viscous parameters, supports this conclusion. These results follow well-established Dugdale fracture dynamics in which rate-dependent flow, occurring in the immediate surroundings of crack tips, accompanies crack formation and propagation. The extent of viscous flow depends both on the brittle versus plastic nature of the substance and on the extent of crack growth.