Influence of the Molecular Weight of Binding Agents on the Physical Properties of Granules and Tablets

To improve the adhesiveness and cohesiveness of powders for granule formation, binding agents are generally used. In the wet granulation process, the binder solution plays an important role in controlling the mechanical properties of granules. In this study, the effects of the type and molecular weight of the binding agent and the effects of the concentration and viscosity of the binder solution on granule characteristics, i.e., average granule size (D50), granule strength (St) and granule compressibility, were investigated.Polyvinylpyrrolidone (PVP) and hydroxypropylmethylcellulose (HPMC) of various molecular weights were used for the preparation of granules by the agitating-fluidized granulation method. In each polymer system, the average granule size (D50) and strength (St) increased with increasing concentration and viscosity of the binder solution and as the molecular weight of the binding agent was increased.The compressibility of granules was evaluated by means of the compressibility constant (K) of a modified version of Kawakita's equation and K decreased with increasing molecular weight. An approximately linear relationship was observed between the reciprocal of the compressibility constant (1/K) and granule strength (St). At the same granule strength, K values in the HPMC system were greater than those in the PVP system. The radial tensile strength (σ) of the tablet, obtained by the diametral compression test, increased with increasing concentrations and viscosity of binder solutions and was also affected by the molecular weight of the binding agent. These results indicate that σ is related to granule strength (St).