An evaluation of a squeeze flow rheometer for the rheological characterization of a filled polymer with a yield stress

Abstract

 This paper is concerned with the evaluation of a squeeze flow rheometer for determining the viscosity of a highly viscous filled viscoplastic fluid which is representative of the behavior of some propellants. Ordinarily materials of this nature are difficult to characterize using conventional capillary and rotary rheometers because of melt and edge fracture. A model system was chosen because its viscosity could still be measured up to reasonable shear rates of about 1 s⁻¹ in a parallel disk rheometer. The power-law, Bingham, and Herschel-Bulkley empiricisms of the generalized Newtonian fluid were fitted to the steady shear viscosity data. The lubrication approximation was then used to predict the squeezing force as a function of gap height and the results compared to experimentally determined values. The best results were obtained for the Herschel-Bulkley model and in this case the lubrication approximation predictions were on the average some 20% lower than the experimental values between gap to radius ratios of 0.15 and 0.35. A finite element simulation of squeezing flow was carried out to assess the limitations of the lubrication approximation. The numerical solution was found to give a good description of the squeezing force for gap to radius ratios between 0.18 and 0.35. It was estimated that the error involved in the lubrication approximation was about 25% at a gap to radius ratio of 0.3 and decreased to below 2% at a gap to radius ratio of 0.05 for the material studied. Experiments were carried out to assess the effect of slip on the squeezing flow measurements using both roughened and lubricated plates. It was found that no significant slip occurred at squeezing speeds up to about 2.4 mm/s. Lubrication was found to reduce the squeezing force by an order of magnitude.