Rheology, Torque, and Oscillatory Rheometers
The rheological characterization of molten materials for pharmaceutical extrusion processes is an important facet in many stages of product development. This chapter presents a review of the fundamentals of both torque and oscillatory rheology by covering their basic principles, setups, and methods. It highlights several applications by utilizing viscoelastic properties of the polymer and active pharmaceutical ingredient (API) -polymer system gathered from a small sample size to assess, guide, and optimize the formulation development via hot melt extrusion (HME). Torque rheometers have been in existence for nearly a century, originally finding application within the baking industry. An evaluation technique quite common to rheograph data includes the observation of a torque signal at significant points (or times) during the testing. These data can be used to compare against a standard material, to indicate physical or chemical state changes, or to determine the work required for processing the material.