The utilization of surface free-energy parameters for the selection of a suitable binder in fluidized bed granulation

Surface free energy was determined for model substances pentoxyfilline, acyclovir, lactose and binding agents (that were used in the granulation process) hydroxypropilmethyl cellulose (HPMC) and polyvinylpyrrolidone (PVP) were determined by contact angle measurements. The methods of Wu, Good-van Oss and Della Volpe were used for solid-surface free-energy calculation. Spreading coefficients (S) were calculated and correlated with granulate properties. Granulates consisted of model drug and binding agent, and were produced in fluid bed granulator Glatt powder coater granulator GPCG1 by means of spraying the colloidal solution of binder on the model substance. Granules contained either 5% or 10% binder. Inverse granules, however, were also produced by spraying the model drug (i.e. pentoxyfilline and lactose) on the binding agent (HPMC, PVP). Particle size distribution, friability, true density, bulk density and tapped density of the granulates were determined. Although many different parameters influence the granule properties, it has been found that the interactions between the drug and the binder play a very important role. Spreading coefficients were found to be in good correlation with the friability of granulates. Positive spreading coefficient values of the binder over the model substance correlate well with the low friability of the granules containing lower amount of binder, i.e. 5%. In the group of the same binder, the spreading coefficient values decrease from pentoxyfilline over lactose to acyclovir. Friability results show that, for the system under consideration, PVP offers certain advantages over the grade of HPMC employed. The increase of the binder amount from 5 to 10% resulted in more friable granulates. Lower work of cohesion of the binder (PVP and HPMC) than the work of adhesion between binder and the model substances is considered responsible for the higher friability of the granules. The inverse granulation process, where the suspension of the model substance was sprayed over the solid binder particles, proved more efficient with HPMC than with PVP. According to the spreading coefficient results, the binder should spread over the drug. However, the kinetics of wetting appears to play an important role in the granulation process. According to these results, the conclusion was made that water wets HPMC much faster than PVP.