A new rational model for tablet strength-pressure relation.

Excipients with a dominating or substantial elastic recovery are characterised by a convex curvature in contrast to the linear association observed for inorganic materials or organic monomers. A model is proposed combining two synchronous processes, a positive linear effect, representing the increase in strength with pressure and a curved component characterising the adverse effect of elastic expansion. Three models are investigated where the negative elastic effect is either exponential (exp(P)), power-function (xP) or a simple quadratic form (P2). The three models are evaluated on their fitness ability, internal correlation and relation between parameters and elastic recovery. The elementary polynomial model, TS = aP - b*P2 + c was preferred due to fitness ability and simplicity in calculation. Furthermore a noteworthy linear relationship was observed between the elastic recovery of tablets from 8 polymer materials and a combination of the linear coefficient a and the quadratic coefficient b as b/a2.For brittle materials, like lactose or dicalcium phosphate without significant elastic deformation, the equation reduces to a straight line.