Studies on direct compression of tablets XXI. Investigation of bonding mechanisms of some directly compressed materials by strength characterization in media with different dielectric constants (relative permittivity)

Abstract

Three substances with varying properties regarding compactability and volume reduction mechanisms were compressed both in air under ambient conditions and in liquids with different dielectric constants. The relative importance of bonding with intermolecular forces compared with that with solid bridges and mechanical interlocking was evaluated by comparison of the radial tensile strengths of compacts under ambient conditions vacuum, and in liquids with increasing dielectric constant. All investigated materials increased in radial tensile strength in vacuum, which corresponds to a medium with a dielectric constant of unity. The increase was greater for materials with a high surface area, indicating that the increase in strength in vacuum is primarily caused by increased surface interactions due to removal of adsorbed water vapor and surface contaminations which act as a filter, reducing bonding with intermolecular forces in the compact. An increase in the dielectric constant of the liquid generally decreased the radial tensile strength of the investigated compacts. The compact strength decreased monotonically down to a plateau value or to zero. For all materials, these minimum values were obtained at dielectric constants between 10 and 20. It is suggested that the remaining strength seen in the plateau values is predominantly due to solid bridges or interlocking mechanisms. The difference between the radial tensile strength in vacuum and these minimum strength values obtained in liquids was thus used as an indication of the amount of bonding with intermolecular forces. Bonding with intermolecular forces seems to be the dominating bonding mechanism for all the investigated materials. Bonding with solid bridges and mechanical interlocking contributes to a minor degree to the compact strength of the model substances except for the coarse fraction of sodium chloride. The results obtained support the idea that pharmaceutical compacts can generally be regarded as agglomerates of primary particles that are mainly attracted to each other by bonding with intermolecular forces.