Mechanical properties of compacts and particles that control tableting success.

The use and limitations of linear-elasticity equations for describing mechanical properties of compacts is discussed; the limitations occur because compacts are porous, viscoelastic, nonhomogeneous, Mohr bodies. Awareness of these properties permits meaningful comparisons to be made. Ignoring limitations may result in unjustified conclusions. Special care during measuring mechanical properties of compacts is required. The mechanical criteria for a successful formulation are good flowability for powders and adequate strength without fracture for compacts. Interparticle attraction is spontaneous but particle contact numbers and size are limited by mechanical preclusion. Plastic deformation and fracture mechanics are controlling mechanisms with the magnitude of elastic constants having little effect on the successful processing. General compaction characteristics can be appraised by using combinations of properties, e.g., specific tableting indices.

Dispersion forces and plastic deformation in tablet bond.

A model for tablet bond formation is developed. It assumes that attraction forces arise from dispersion forces between surfaces with spherical curvature. The radius of these surfaces depends on the amount of plastic deformation produced during compression. The radius of curvature is calculated by using Hertz's equation for elastic deformation, which is assumed to hold during decompression. The number of contact points per unit cross section is taken as a simple multiple of a number deduced from the particle size, the indentation hardness values, and the solid fraction. A similar approach is used to replace the area per contact point produced when under compression. The resulting equation is used to estimate the bonding index. Values corresponding to the lower range of experimentally determined bonding indices are obtained. Arguments are presented to suggest that large values of the bonding index (tensile strength/indentation hardness) probably result principally from plastic (including viscoelastic) deformation during decompression. Nondispersion forces may contribute but are not believed to be of sufficient magnitude to account for the total bond for materials that exhibit a large value of the bonding index.

Rotary press utilizing a flexible die wall.

A die with a flexible wall was constructed and evaluated on a specially modified instrumented rotary tablet press. The design permits an inward deflection of the die wall by a side punch, which rolls past a side compression roll during compression-decompression. The side compression roll is instrumented to monitor the applied side compression roll forces. On decompression, return of the die wall to its original position permits release of residual die wall pressure. The decreased residual die wall pressure can decrease fracture and capping of tablets for problem formulations. The performance was tested on three experimental formulations. For these formulations, tablets made in a conventional die exhibited severe capping problems. However, most tablets compressed in the special die were superior. With proper adjustment of punch and die wall compression forces, excellent tablets could be manufactured. The merits of the special die and modified tablet machine are substantiated, although this initial design did not provide adequate die wall pressure for all formulations. Further engineering efforts could result in practical production equipment.

Physical processes of tableting.

Shear deformation was shown to occur during the decompression of large compacts made by nonisostatic compression. Some materials undergo fracture when these shear stresses are developed. Other materials withstand these stresses but fracture upon ejection when the stress concentrations at the edge of the die are large. Failure to fracture seems to be related to the ability to relieve stresses by plastic deformation. A test was devised to measure this property, called the brittle fracture propensity.