Prediction of the compression behaviour of powder mixtures by the Heckel equation

Abstract

The compression behaviour of plastically flowing microcrystalline cellulose and pregelatinized corn starch and fragmenting α-lactose monohydrate and dicalcium phosphate dihydrate, and their binary mixtures was evaluated using the Heckel equation. Powders were compressed using an instrumented eccentric tablet press. The Heckel plots for both plastically flowing materials and their binary mixtures were linear in shape. In contrast, on the Heckel plots of the fragmenting materials and their mixtures, an obvious curvature was seen. The mean yield pressures were linearly proportional to the mixture composition of two plastically flowing materials. When mixed with a fragmenting material, a plastically flowing component had a greater effect on the compression behaviour of the mixture than a fragmenting mixture component had. This was explained by the more pronounced effect of plastic flow on volume reduction. When two fragmenting materials were compressed, the densification of their binary mixture was strongly influenced by the less compressible material with higher mean yield pressure. It was concluded that in the mixtures of two fragmenting materials the particles of the less compressible material constituted a firm structure, like a skeleton, which tended to dominate the compression behaviour of the binary mixtures. For binary mixtures of fragmenting materials the mean yield pressures were thus not linearly proportional to the mixture composition. Compression behaviour of powder mixtures seemed to be strongly dependent on the deformation properties of plain materials to be compressed. On the other hand, one mixture component may dominate the compression behaviour of the mixture, and the compressibility may change non-linearly with mixture composition. Therefore, caution is needed when the Heckel equation is applied for predicting compression behaviour of powder mixtures.