27591-08 GEMP MD284_297x420.indd

Polymers for hot melt extrusion must exhibit thermoplastic characteristics in order to permit the melt extrusion process and they must be stable at extrusion temperatures. Other relevant characteristics are: suitable glass transition temperature (Tg) of 50–180 °C, low hygroscopicity and no toxicity since larger amounts of polymer are used [1]. Extrudability of a polymer is mainly determined by the Tg and melt viscosity [2]. Polymers having a high molecular weight exhibit high melt viscosity and can hardly be extruded. Moreover, a high Tg requires a high processing temperature which can degrade sensitive actives [3]. As a general rule, an extrusion process should be run at temperatures 20–40 °C above the Tg. Most polymers demonstrate thixotropic behavior which means that the viscosity declines as a function of increasing shear stress. Therefore, this study was designed to investigate the infl uence of several plasticizers on the Tg and the melt viscosity of polymers.