Mechanism of moisture induced variations in true density and compaction properties of microcrystalline cellulose

A single lot of MCC powder (Avicel PH102) was equilibrated at 0%, 11.35%, 21.6%, 38.2%, 52%, 57.5%, 75% and 84.3% relative humidity. Each equilibrated powder was compressed. Tablet density and tensile strength were measured as a function of pressure. Powder true density, tabletability, compressibility, compactibility and plasticity were obtained as a function of water content. The true density of MCC ranged 1.42-1.46 g/cm³ and exhibited a maximum between ∼3% and ∼5% (wt%) moisture. Moisture up to ∼3.3%, corresponding to monolayer coverage, did not induce profound change in MCC plasticity nor bonding strength despite reduced T_g. Consequently, the compaction properties were largely insensitive to moisture variation below 3.3% water. Above 3.3% water, higher moisture content corresponded to improved plasticity, due to the plasticizing effects of water above the critical water content, and consequently larger interparticulate bonding area when compressed. Effects of plasticization by water on bonding area were significant at low compaction pressures but diminish at higher pressures. At above 3.3% water, increasing moisture content also reduced bonding strength. Due to the interplay among the plasticity, compaction pressure and bonding strength, tablet tensile strength peaked in the range of 3.3-5.6% moisture.