Mechanical strength of pharmaceutical compacts : Importance of material characteristics, particle characteristics and compaction pressure on interparticulate bonding structure
Abstract: Factors considered important for the interparticulate bonding structure and mechanical strength of pharmaceutical compacts were studied in this thesis.Fractures appear to propagate mainly around rather than through grains during strength testing. Large deviations from theoretical strength values in addition to an effect of particle size were thus obtained when compaction was performed to zero porosity or obtained by extrapolation to zero porosity. When high compaction loads were used, the excess energy was to a large extent used for elastic recovery and/or alteration of the solid-state structure.Filtering out of weak distance forces (intermolecular forces) by compaction in a liquid with a sufficiently high dielectric constant appears to provide reliable information on interparticulate bonding mechanisms. The best correlation between physiochemical properties of the liquids and the gradual decrease in tensile strength of the compacts was achieved using the dielectric constant. The weak distance forces appeared to be screened out when the liquid compaction medium had a dielectric constant of 18. The remaining tensile strength was then believed to be the result of interparticulate bonding by solid bridges for most materials. However, for most pharmaceutical materials, weak distance forces seem to dominate. Of all the materials tested, solid bridges seemed to be the most important bonding mechanism for sodium and potassium chloride. Increasing the particle size and compaction pressure of materials with the capacity to form solid bridges seemed to facilitate the bond formation process. Addition of a dry binder or milling the particles counteracted the formation of solid bridges, probably by reducing the concentration of stress at certain points in the compact, a prerequisite for the establishment of solid bridges.Both the tablet surface area and the interparticulate distance may affect the proportion of external surface area participating in interparticulate bonding. For materials prone to develop solid bridges, the actual surface area involved in bond formation is more important than the space between the particles, i.e. compensation of the tensile strength of a tablet for the surface area and the mean interparticulate distance will probably not reflect the nature of the dominating bond type. However, for the other materials, ranking of the materials according to tensile strength adjusted for surface area and mean interparticulate distance gave a reflection of dominating interparticulate bonding type.
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