STRESS-RELAXATION OF COMPACTS PRODUCED FROM VISCOELASTIC MATERIALS

Stress relaxation of tablets is a phenomenon that is known to be relat ed to elastic deformation of particles. Expressions of stress relaxati on 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 a mount of stored energy, calculated from the elastic modulus and the yi eld strength of the material. The relations are, however, different fo r the different materials. On the basis of the assumption that the inc rease in tablet volume is an expression of stress relaxation and store d energy is the driving force for it, there should be a counteracting force that prevents a compact from expansion. This paper shows that th e counteracting force is determined by particle bonding, quantified by the Ryshkewitch-Duckworth relation and friction of the tablet with th e die wall, quantified by the ejection pressure. The data presented he re 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 ene rgies and large particle bonding. From this observation it is conclude d 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.