Particle interactions and internal tablet structure : Factors affecting the mechanical strength of pharmaceutical compacts

Abstract: A simple tablet model was utilised to identify parameters responsible for tablet compactibility. In this model tablets were considered as homogeneous and isotropic aggregates of individual spherical particles. If the particles were located close enough to each other, interparticulate bonds were assumed to be developed. By testing the tensile strength of tablets made from mixtures of fine particulate polyethylene glycols and coarse sodium chloride or sodium bicarbonate particles it was confirmed that tablets normally fracture around the tablet particles rather than through and that interparticulate bonds are responsible for the coherency of tablets.A method of compressing tablets in media with differing dielectric constants to obtain information on dominating interparticulate bonding mechanisms was evaluated and found to be useful in distinguishing between different bond types. By applying this method it was verified that distance forces probably are the dominating interparticulate bonding mechanism in most pharmaceutical compacts. For stronger bonds (solid bridges) to occur certain prerequisites must be fulfilled such as a simple chemical structure at particle surfaces and a high interparticulate strain during compression.From the simple tablet model it was derived that tablet porosity and specific surface area have a profound effect on tablet strength. By dividing the tablet strength with a factor combining tablet porosity and specific surface area, the results concerning dominating bond types in tablets were confirmed. For some materials, a change in compression speed resulted in a change in tablettensile strength without the expected corresponding changes in tablet porosity or specific surface area. Therefore it was discussed that surface deformability, which not was directly considered in the simple tablet model, might also have a marked effect on tablet strength.The tablet model was also used to discuss the strength enhancing effect of dry binders and the effect of speed of compression on tablet tensile strength.