Elasticity of polymer tablets considered as a network of contacts

The elasticity of tablets as a function of the solid fraction was investiga ted. Theoretically, the network of contacts within the tablet as assumed to be disordered, and the evolution of mechanical rigidity explained in terms of a central-force percolation model. This mechanical version of the perco lation model proposes a power law for the elastic modulus close to the perc olation threshold. The exponent tau in this function was numerically expect ed to be close to four. An universal testing instrument was used for the ma nufacture and subsequent testing of the compacts. Twelve polymer substances were selected on the basis of their ability to form tablets with a broad r ange of relative densities. The modified Young's modulus was determined usi ng an indentation test. For each substance, the elasticity values obtained were set to a power of 0.26 which is an estimate for the inverse elasticity exponent tau(.1) that is predicted by the central-force percolation model. Linear plots were obtained in relation to the tablet solid fraction. The l inearity was demonstrated with all the substances assayed. thus supporting the validity of the percolation theory approach. The values determined for percolation thresholds were found with most substances to be below the rela tive bulk densities. Even though such low threshold values had previously b een reported, it was concluded that an extrapolation to zero elasticity is questionable if it goes far beyond the density range of the co,compressiona l process. Some theoretical arguments are put forward, which indicate that central-force percolation may not be the final solution to how mechanical p roperties are developed in tablets. Nevertheless, it constitutes a simple t heoretical approach that adequately predicts experimental data and provides important insights in the complex physics of the tableting process.