Internal and external structure of pellets made in a rotary processor

The internal and external structure of pellets made in a rotary processor by the wet granulation technique was studied using the following techniques: helium pycnometry to determine the true density, krypton adsorption to determine the specific surface area, mercury porosimetry, and scanning electron microscopy. The results of the true density measurements, specific surface area determinations and mercury porosimetry led to the hypothesis that the pellets were open porous sponge structures. Mercury mainly intruded in the macropore range (pores with a radius between 7 and 0.05 μm). The pores and cavities were partially closed as spheronization went on leading to air pockets in the pellets. Scanning electron microscopic pictures provided visual support for the hypothesis that the pellets had a porous surface and that the pellets contained cavities, which could be closed resulting in the formation of air pockets. To reduce the pore volume and smooth the surface, the rotor speed should be high and the spheronization time should be long. The plastic properties of wetted microcrystalline cellulose promoted the smoothing of the surface.