The effect of compression on the capillary microstructure of tablets

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Abstract
The effect of compaction pressure on the capillary characteristics of compacts of a crystalline drug and of a pure polymer was studied by means of penetration and porosimetry measurements. Not only the constant but also the exponent m of the general penetration volume-time relationship V = Kmtm proved to be strongly dependent on the pressure: as the compaction pressure is increased, the value of m tends to decrease. The capillary penetration data also showed the development of a marked heterogeneity in the structure of the polymer compacts at the lower compaction pressures: at these pressures the surface layers showed a much lower penetrability than the inner core of the compacts, whereas at the higher pressures the structure of the compacts was homogeneous. Furthermore, on the basis of a generally used equation describing the powder consolidation process, it was possible to relate the most relevant variation of the kinetics of capillary penetration into the drug compacts to the deformation of the particles. It is concluded that the capillary penetration measurements can give useful information in the understanding of the behaviour of pharmaceutical powders under compression.