Percolation theory and compactibility of binary powder systems.

Defined size fractions of polyethyleneglycol powder (MW = 10,000) were mixed with defined size fractions of alpha-lactose monohydrate in order to study the effect of compaction as a function of the weight ratios of the two excipients. For a precise control of the compression cycle, tablets were compressed on a Universal Testing Machine (Zwick 1478). Tablet tensile strength sigma T was quantified as a function of compressional stress sigma c and relative density rhor r using a two-parameter model with sigma Tmax = maximal tensile strength at zero porosity and gamma = compressibility. The results have been analyzed on the basis of the percolation theory. As soon as the component with the lower mechanical stability is percolating the powder system, tablet hardness is controlled entirely by this component. The percolation threshold is a function of the geometrical arrangement of the particles in the compressed powder system. The expected two percolation thresholds can be distinguished as a function of the composition weight ratios if the particle size distributions of the two components differ enough.