Lourdes de Vargas

Nuclear magnetic resonance imaging of apparent slip effects in xanthan solutions

Nuclear magnetic resonance imaging has been used to investigate the flow of 0.2% aqueous solutions of xanthan gum. Apparent slip was observed in solutions made from the material supplied by UNAM but not in that supplied by Aldrich or Kelco. The apparent slip velocity was a constant fraction of the maximal velocity for a given contraction ratio. The apparent slip velocity also appeared to be independent of L/D but was strongly influenced by the wall stress, the observed apparent slip velocities being comparable with those determined using the Mooney analysis on capillary flows. After exposure to the action of a syringe pump, which reduced the mean molecular weight of the xanthan, the UNAM xanthan solution no longer exhibited apparent slip. Slip appears to be a function of molecular weight, possibly through sensitivity to the aspect ratio of the molecule.

Particle image velocimetry of the unstable capillary flow of a micellar solution

The unstable capillary flow of the micellar system formed by cetylpyridinium chloride 100 mM/sodium salicylate 60 mM (CPyCl/NaSal) in tridistilled water was studied in this work using a combination of particle image velocimetry (PIV) and rheometrical measurements. The experiments were run in a pressure controlled capillary rheometer at a temperature of 26 °C and covered all the different flow regimes occurring in the nonmonotonic flow curve characteristic of micellar solutions. First, we show the suitability of the PIV technique to study the unstable capillary flow of this micellar system. Then, and more important, we provide evidence of the development of shear banding and the velocity profiles for the different flow regimes, including the transition to the high shear branch because of spurt. The velocity profiles at low shear rates exhibited a Newtonian behavior, followed by a still Newtonian with apparent slip up to the onset of spurt. There was a jump of one order of magnitude in the shear rate with a...

Inlet instabilities in the capillary flow of polyethylene melts

Inlet instabilities in the capillary flow of polyethylene melts were studied in this work. Extrudate distortions in branched polyethylenes, produced by unstable upstream flow, were found to be accompanied by pressure oscillations that do not have their origin in the slip phenomenon, but on polymer compressibility. The absence of slip was clearly evidenced in the experiments, and the differences between pressure oscillations occurring in linear and branched polymers are shown.Pressure oscillations in the capillary flow of branched polyethylenes were found to be made up of at least two components of different frequency and amplitude. These two components were identified with different bulk defects appearing in the extrudates. Information about the dynamics of vortices upstream of the contraction and extrudate distortions is obtained from the analysis of pressure oscillations.The influence of capillary entrance angle on flow curves was also investigated. From the results, it is concluded that the extensional component of the flow in the contraction is the main factor responsible for the slope change usually found in the log-log flow curves of both linear and branched polyethylenes.

Temperature-dependent instabilities in the capillary flow of a metallocene linear low-density polyethylene melt

The capillary flow behavior of a metallocene linear low-density polyethylene was studied in a wide temperature range. The critical shear stress for the onset of the unstable spurt flow was found to be dependent on temperature in a nonlinear fashion and it showed a minimum value at a critical temperature, at which unusually long period pressure oscillations were observed. For temperatures above the critical one, the observed decrease of the critical shear stress with decreasing temperature is explained on the basis of an increase in the distance between entanglements. At temperatures below the critical one, the increase in the critical shear stress and the eventual suppression of pressure oscillations as the temperature is further decreased are suggested to be the result of a flow-induced phase change that ends on complete crystallization and suppression of flow. The flow-induced crystallization phenomenon and the extrudate quality were dependent on the contraction ratio. Elimination of surface extrudate d...

Experimental evidence of slip development in capillaries and a method to correct for end effects in the flow of xanthan solutionsa)

Flow experiments using a capillary rheometer with 0.2% xanthan aqueous solutions were carried out to give experimental evidence of the dependence of the slip velocity on the geometrical parameters of the rheometer. The Mooney method was used to evaluate the slip velocity Vs. It was found that Vs is an increasing function of the wall shear stress and also of the length to diameter ratio L/D not considered in the Mooney theory. That is, Vs=Vs(τw,L/D), although, for each τw value, Vs becomes independent of L/D at large L/D. In addition, a method to determine the excess pressure drop due to end effects is outlined and experimental results presented. The differences between this method and the one by Bagley are highlighted.

About the determination of the steady state flow for polymer melts in capillary rheometers

The different parameters influencing the time required to reach steady state flow in polymer melts in a capillary rheometer were investigated. The parameters of the rheometer considered were the contraction ratio, the capillary diameter and its length to diameter ratio. The influence of polymer compressibility and viscosity were also taken into account. It was found that the time required to reach steady state flow for the studied polymers increased when the amount of material in the reservoir, the viscosity of the melt and the contraction ratio for a given L/D capillary ratio were increased and vice versa. It is shown that the main characteristic of the rheometer that determines the time to reach a steady flow is the contraction ratio and not the capillary diameter. This result is explained on the basis of polymer compressibility, its viscosity and the mechanical work done on the system, which depends on the amount of material in the barrel.

On the rheological characterization of polyethylene melts by using glass capillaries

The rheological characterization of polyethylene melts by using glass capillaries was investigated. Different polyethylene melts were characterized in a capillary rheometer by using glass capillaries and the results were compared with those obtained by using standard dies of tungsten-carbide, in addition to some homemade from aluminum. The data from glass capillaries showed an excellent agreement in all the cases with the data obtained with metal capillaries, and differences in the flow curves due to the influence of capillary materials were not detected. These results evidence the good performance and reliability of glass capillaries to perform the rheological characterization of this sort of polymer melt.

Evaluation of end effects in capillary rheometers for solutions of flexible polymers

Flow experiments with a capillary rheometer using 0.3% polyacrylamide and 0.75% polyox aqueous solutions were performed to study flow development and end effects generated by polymer solutions of flexible macromolecules. Several L/D capillary ratios were employed, and a method proposed recently was used to evaluate the corrections due to end effects. The corrections obtained are of the same order of magnitude than those obtained using the Bagley method, but the Bagley corrections have much larger uncertainties. The recently proposed method is superior to the Bagley method when accurate end effects are required, as for correlations with elasticity or to evaluate numerical predictions for the flow behavior of these fluids.