Use of Adjustable-Gap On-Line and Off-Line Slit Rheometers for the Characterization of the Wall Slip and Shear Viscosity Behavior of Energetic Formulations

Abstract

It is difficult to characterize the rheological behavior of energetic suspensions due to their viscoplasticity and wall slip. The use of the rectangular slit geometry, as an on-line or off-line rheometer is advantageous provided that the surface to volume ratio of the slit die can be systematically varied to allow the wall slip corrections to be made. Here two rectangular slit rheometers designed and built to handle the rheological behavior of energetic suspensions are presented. The gap of these rheometers is variable to give the user the ability to vary the gap and hence the wall shear rate, thus enabling wall slip corrections to be made. A series of pressure transducers, flush with the wall, are used to determine the pressure drop over the fully developed flow region and hence the wall shear stress directly from the fully developed pressure gradient. The ability to independently vary the mass flow rate and the gap opening allows one to carry-out the wall slip corrections and hence obtain the wall slip velocity versus the shear stress data, which can then be used as the boundary condition during the simulation of the die and extrusion flows and at the same time allow the determination of accurate shear viscosity data. A set of data systematically collected with an on-line slit rheometer with a continuously adjustable gap to characterize the wall slip velocity as well as the shear viscosity material function of a LOVA formulation as a function of deformation rate, solvent concentration, and temperature is used to illustrate the working principles of the on-line and off-line adjustable-gap rheometers.