Stress relaxation of tablets is a phenomenon that is known to be related to elastic deformation of particles. Expressions of stress relaxation are tablet expansion and capping. It has been shown that there is a direct relation between the changes in volume of the tablet and the amount of stored energy, calculated from the elastic modulus and the yield strength of the material. The relations are, however, different for the different materials. On the basis of the assumption that the increase in tablet volume is an expression of stress relaxation and stored energy is the driving force for it, there should be a counteracting force that prevents a compact from expansion. This paper shows that the counteracting force is determined by particle bonding, quantified by the Ryshkewitch-Duckworth relation and friction of the tablet with the die wall, quantified by the ejection pressure. The data presented here suggest that the final tablet porosity is unequivocally determined by stored energy, particle attraction and friction of the tablet with the die wall. All tablets that were capped showed both high stored energies and large particle bonding. From this observation it is concluded that porous and capped tablets suffer from the same problem, but the expression stress relaxation is different for the different materials. (C) 1997 Elsevier Science B.V.
