The effect of waxes, hydrolysed gelatin and moisture on the compression characteristics of paracetamol and phenacetin.

Stearic acid or hard paraffin added to crystals of paracetamol and phenacetin reduced capping of tablets prepared by direct compression but did not produce acceptable tablets because the inter-particular bonds were very weak. The pressure cycle that can be constructed form the measurement of the axial pressure and the corresponding die wall pressures offers information that is useful in the formulation of the tablets. The behaviour of paracetamol or phenacetin and their mixtures with gelatin hydrolysate or water or both shows a similarity to a Mohr body and it appears that the maximum die wall pressure is affected by the particle size of the material compressed and also by the additives present. Good transmission of radial force implies that the material can be initially consolidated, but alone it does not indicate that the tablet formed is physically stable. When the tablet formed remains coherent after the axial pressure is removed the residual die wall pressure remains high. Measurement of the residual die wall pressure might therefore be a useful indicator for identifying satisfactory formulations of substances that cap readily. Hydrolysed gelatin or water or both together produced paracetamol and phenacetin mixtures with satisfacotry compression characteristics.