DETERMINATION OF PERCOLATION THRESHOLDS IN MATRIX-TYPE CONTROLLED-RELEASE SYSTEMS - APPLICATION OF A RESISTANCE ANALYSIS TECHNIQUE

In this work, matrix tablets have been prepared with binary mixtures o f the inert polymer Eudragit(R) RS 100 and a soluble, power-conductor model substance, sodium chloride. The sodium chloride content comprise d between 20 and 80% w/w. A technique based on the measurement of the resistance of the matrix tablets has been used to achieve easy estimat ion of the values of the percolation thresholds, as a function of the sodium chloride loadings. The results obtained have been expressed in terms of resistivity and evaluated on the basis of percolation theory. The tablet resistivity has been considered as a critical property of the system. A change in this property and the presence of the percolat ion threshold were observed to appear simultaneously. From the data ob tained, we have determined a first percolation threshold comprising be tween 30 and 40% (w/w) undergo a disintegration process. The results o btained have been corroborated by scanning electron microscopy.