Infrared Imaging of Pharmaceutical Materials Undergoing Compaction

Abstract

The goal of this study was to use infrared thermography as a new technique to investigate the heat released during compaction and consolidation of pharmaceutical powders and granules. Real-time temperature measurements without physical contact with tablets were provided by a highly sensitive (±0.1°C at 30°C) infrared camera (Agema Infrared Systems, Model 470 with CM-SOFT software). High-resolution images were captured at the takeoff point, i.e., less than 1 sec after compaction, stored on floppy disks, and then analyzed on a regular PC equipped with a VGA color monitor. Thermal surface profiles of tablets were obtained with high geometric and temperature resolution. Reproducibility of the camera readouts was better than 3%. The model granulation used was a direct compression blend of microcrystalline cellulose, spray-dried lactose, and magnesium stearate. This blend was compressed using an instrumented Korsch PH106 rotary press fitted with 1 station of 19.1 × 7.9-mm (0.750 × 0.312-in.) capsule-shaped tools. The effects of compaction force (6–20 kN), rate (130- to 360-msec contact time), and lubricant level (0.5 and 1.0%) on postcompaction temperature rise, caused by heat released during compaction, were investigated. The presence and location of nonhomogeneous heat distribution were assessed as well. Results have shown that the heat released during compaction increases with compaction force. Tablet surface temperatures of 33.8 ± 0.7°C were observed at 20 kN compaction force in contrast to 29.5 ± 0.3°C at 6.7 kN. The compression rate, as determined by the upper punch contact time did not have any significant effect on the heat released during compaction at 15-kN force. However, magnesium stearate level had a significant effect on the heat released during a compaction run. Tablets lubricated with 1.0% magnesium stearate had surface temperatures of 39–40°C after a 20-min run time, as opposed to 50–51°C for tablets lubricated with 0.5% magnesium stearate. Hot spots were seen at tablet edges where the die-wall friction occurs. Tablet cross-sectional thermal profiles revealed a 3–4°C temperature gradient across the tablet. These experiments show that infrared imaging is a unique tool for semiquantitative evaluation of heat released during compaction because it provides direct visualization with good temperature resolution of the heat evolved during the process.