EFFECT OF FILLING FACTOR ON THE DETERMINATION OF SHEAR RATE AND VISCOSITY FROM BATCH MIXER

In this work, the effect of filling factor on torque data obtained with a torque rheometer was studied. Torque data were obtained for polypropylene (PP), high density polyethylene (HDPE), and polystyrene (PS), for filling degrees ranging from 60 to 90%. The processing temperature used for HDPE was 160?C and the other polymers were processed at 200?C. The torque data obtained were used to calculate the shear rate in the batch mixer and corresponding viscosity of the polymer using Bousminaʹs model [Journal of Rheology 43, 415 (1999)], which provides a quantitative correlation between torque data and viscosimetric properties of a polymer. Bousminaʹs model considers the mixer chamber and rotor blades as concentric cylinders of internal and external radii Ri and Re, respectively. The model was used making different assumptions for the rheological nature of the polymers. The polymers were considered either Newtonian or power law fluids. The calculated values of viscosity versus shear rate were compared to results obtained by rotational and capillary rheometry. The experimental results showed that the torque increases exponentially with increasing filling factor. The results of the comparison indicated that in order to evaluate viscosity versus shear rate from torque rheometer data, the experimental chamber should be calibrated to obtain RiPC (internal radius for calibration polymer) using a first polymer. This value of RiPC should fulfill RiPC / Re > 0.9, otherwise the calibration should be made with another polymer. This value of RiPC can then be used together with torque data obtained with a filling degree of 90% or more for a second polymer assuming the polymer as a Newtonian fluid. If the torque data are obtained with a filling degree less than 90%, the Newtonian hypothesis is not valid any more and Bousminaʹs model and an emulsion model should be used sequentially to infer viscosimetric data from torque data obtained using a partially filled mixing chamber.