The effect of moisture content on the compression and bond-formation properties of amorphous lactose particles

A lactose solution was spray-dried and the particles obtained were stored at relative humidities of 0, 11, 22 and 33%. The moisture content (gravimetric analysis), crystallinity (microcalorimetry), particle density (air pycnometry) and glass transition temperature (differential scanning calorimetry) of the particles were determined and compacts were prepared (50-200 MPa). From porosity-applied pressure profiles, the yield pressure for the materials was calculated and the tensile strength and porosity of the compacts were determined. Compacts were also examined in an electron microscope. The moisture content of the amorphous material varied between 0 and 6.2 wt.% and an increased moisture content related rectilinearly to a reduced particle density, a reduced glass transition temperature and a reduced yield pressure. An increased moisture content gave an increased tablet tensile strength and a reduced tablet porosity. Different relationships between tablet strength and porosity were obtained dependent on the moisture content of the particles, while a single relationship seemed to exist between tablet strength and a contact area coefficient. To conclude, amorphous lactose powders reduced in volume due to particle deformation, and powder compression was facilitated by an increased moisture content due to an increased deformability of the particles in their glassy state. The particle deformability controlled the area of contact formed between the particles during compression, which controlled the tablet tensile strength. Particles seemed to bind by adsorption bonding and, at high compaction pressures, by solid bridges. (C) 1997 Elsevier Science B.V.