Strength and morphology of solid bridges in dry granules of pharmaceutical powders

This is an experimental study of the tensile strength of solid bridges between primary particles comprising granules of lactose or mannitol. We report on two systems: granules prepared with ethanol granulating solutions, in which the base powders were at most sparingly soluble, and aqueous granulating solutions, in which the base powder solubility was large. Both systems were studied with and without hydroxypropylcellulose (HPC) or polyvinylpyrrolidone (PVP) or surfactants (Triton-X100, sodium lauryl sulfate or polysorbate 80) added to the granulating solution. The interparticle bridge strength was determined from the granule crush strength with a simple model that assumes that solid bridges form by evaporation of solvent from liquid bridges that maintain their shape during drying.Lactose granules prepared with pure ethanol are very weak, with crush strength comparable to that predicted by JKR theory, consistent with its negligible solubility. Mannitol, which is sparingly soluble, forms granules with bridge strength similar to the theoretical (Griffith) strength of a pure mannitol. Addition of HPC or PVP to the granulating solution produces bridges with strength comparable to that of pure polymer films. In comparison, the behavior of granules prepared with aqueous granulating solutions was much more complex due to the high saturation concentration of base powder. Granules produced with pure water had bridge strength approximately 20% of the theoretical strength. Addition of HPC or PVP to lactose granules increased the bridge strength modestly, but the strength was much smaller than that of the corresponding pure polymer films. Addition of HPC to mannitol granules had little effect on bridge strength, while PVP reduced bridge strength by approximately 30%. Addition of surfactants to the granulating solution also reduced dry bridge strength. These results reflect the complex microstructure and resulting mechanical properties of dry bridges produced by coprecipitation of the sugars and polymers (or surfactants).