Compact size and mechanical strength of pharmaceutical diluents

The effects of compact size and powder particle size on the axial compressive (σC) and diametral tensile (σT) strength were evaluated, for three compressed pharmaceutical diluents (microcrystalline cellulose, MCC, calcium hydrogen phosphate dihydrate, CHPD, and pregelatinized starch, PGS) differing in deformational behaviour during compaction and testing. Cylindrical specimens of similar aspect ratio (≈1) but different diameter (13, 10 and 5 mm) and total porosity (0.35-0.05) were employed and the extrapolated and interpolated strength values, at zero and 0.15 porosity (σC0 or σT0 and σC0.15 or σT0.15), were predicted by applying an exponential relation. The ratio of σC0.15/σT0.15 was correlated to the corresponding percent radial elastic recovery (%RR). It was found that the changes of compressive and tensile strength with compact size were not characteristic and general. Specimen's size affected both extrapolated and interpolated values of σC and σT most significantly in the case of MCC, for which σT increased while σC decreased, as the compact size became smaller. Less significant was the effect of compact size on the interpolated value of compressive strength (σC) and the extrapolated value of tensile strength (σT) for PGS. For CHPD, no significant and characteristic effect of compact size was observed. Particle size significantly affected the interpolated values of σT for MCC and both σC and σT for CHPD; they all increased with decreasing particle size. The σC0.15/σT0.15 ratio was affected significantly by the compact size only in the case of MCC and seems to be an indication of material elasto-plasticity, since PGS has by far the highest ratio, followed by MCC and CHPD. A correlation between σC0.15/σT0.15 and %RR was established as was its dependence upon deformational behaviour.