Effect of environmental temperature on the polymorphic transformation of phenylbutazone during grinding.

The transformation behavior of phenybutazone polymorphs during grinding at 4 and 35°C and the solid-state stability and dissoluiton behavior of the ground crystals were investigated kinetically. The α, β and δ forms were transformed to a new polymorphic ζ form, which in turn was transformed to the ε form, which was stable at 4°C. On the other hand, during grinding at 35°C, the δ form was not changed, while the α form was transformed to the δ form by was of the ζ form. The β form was apparently transformed directly to the δ form. The ε form, obtained by grinding, was transformed to the δ form under 0% relative humidity at various storage temperatures after an induction time of a few hours. The mechanism of solid-state transformation of the ε form to the δ form was fond to be random nucleation with two-dimensional growth of nuclei (Avrami-Erofeev model) by means of the Hancock and Sharp method, and the activation energy of this transformation was calculated to be 44.5 kcal/mol. The dissolution rate of the ε form was faster than those of other forms in buffer solution, but the ε form was transformed to the δ form during dissolution.