Effect
of filling factor on the DETERMINATION OF SHEAR RATE AND VISCOSITY FROM BATCH
MIXER
Ticiane Sanches Valera, Nicole Raymonde Demarquette* and Samuel Marcio Toffoli
Materials Engineering Department – Escola Politécnica
da USP
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 160ºC and the other polymers were processed at
200ºC. The torque
data obtained were used to calculate the shear rate in the batch mixer and
corresponding viscosity of the polymer using Bousmina’s model [Journal of
Rheology 43, 415 (1999)], which provides a quantitative correlation
between torque data and viscosimetric properties of a polymer. Bousmina’s 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 Bousmina’s model and an emulsion model should be used
sequentially to infer viscosimetric data from torque data obtained using a
partially filled mixing chamber.
Keywords: torque rheometer,
viscosimetric data.