Methodology for a better evaluation of the relation between mechanical strength of solids and polymorphic form

In order to evaluate the role played by polymorphism in the mechanical strength of solid dosage forms (e.g. compressed tablets) and minimize the influence of other factors (such as compaction force, porosity, particle size, and possibly crystal habit), a melted disc technology was developed. With this technique, tablet-shaped discs of zero porosity were prepared by melting powder and subsequent crystallization in the desired modifications. Taking phenobarbitone as a model drug, different methods were used to get discs of forms I, II and III and the amorphous form. Mechanical properties of the discs were assessed, primarily through their bending strength. The Vickers hardness number was also determined for some specimen discs and monocrystals. Results showed that the amorphous form and form III of phenobarbitone gave the toughest discs and would therefore the most suitable materials to manufacture coherent tablets. Moreover, the various preparation methods used resulted in discs of different internal structures. Both crystal size and crystal habit significantly affected the physical properties of the tested materials.