THERMOPHYSICAL PROPERTIES OF SOME PHARMACEUTICAL EXCIPIENTS COMPRESSED IN TABLETS

Purpose. Thermophysical properties of three tableting excipients; micr ocrystalline cellulose, lactose and dicalcium phosphate dihydrate were observed to evaluate their ability to resist temperature induced chan ges in tablet form. Methods. Two thermophysical parameters, thermal di ffusivity and specific heat, were measured by a pulse heating method. The materials were also evaluated by differential scanning calorimetry (DSC). Results. Microcrystalline cellulose in tablet form was found t o be rather insensitive to heating and cooling treatments, even though the tablets seemed to remain in a stressed state four weeks after tab leting. This stress, indicated by low temperature anomalies, was obser ved by the pulse method, but not by DSC. When magnesium stearate was i ncorporated as a lubricant within the microcrystalline cellulose powde r, the thermophysical parameters indicated that the internal structure of the tablets changed with heating and cooling. Magnesium stearate e liminated the low temperature anomalies as well. The heat treatment ch anged the thermophysical properties of tablets made of the crystalline excipients lactose and dicalcium phosphate dihydrate, permanently cau sing irreversible structural changes. Conclusions. The melting of the lubricant together with enhanced stress relaxation in the structure of microcrystalline cellulose most probably caused the improved thermal diffusivity. The observed thermophysical changes with the crystalline excipients were due to changes in tablet's structure and material. The combination of methods used was found to be an accurate and reliable way to obtain useful information on the structural changes and materia l relaxations of intact tablets during temperature treatment and age-r elated changes in material properties.