Use of the hole pressure data to obtain N1 at high shear rates for polymer melts

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Virginia Polytechnic Institute and State University


A slit die with a rectangular slot placed transverse to the flow direction has been used for both flow visualization and direct pressure measurements of the hole pressure (Ph). The results from these measurements have been used to evaluate the Higashitani-Pritchard-Baird (HPB) equation which has been proposed for determining the magnitude of the primary normal stress difference (N1) from the values of the hole pressure. The slit die was run at higher shear rates than those used by Pike. Both the tracer method and flow birefringence technique were applied to visualize the streamlines and stress field, respectively, in the slot region . Effects of the slot on the flow pattern and on the stress field were examined by changing the slot width and by rounding the corners of the slot. The validity of the HPB equation, which is derived from the Higashitani-Pritchard theory (H-P theory), was tested by comparing values of N1 predicted by the HPB equation and slit die data with that obtained from the cone-and-plate rheometer. The validity of the HPB equation was also tested by changing the slot dimensions.

Flow visualization experiments were performed for polystyrene (Styron-678) and polycarbonate (Lexan) melts. It was found that both the flow and stress fields are asymmetrically distributed about the slot centerline and that secondary flows exist in the upper part of the slot. However, no visible vortices was found for LOPE even though σw was increased to 70 Kpa. The shear rate at which the vortices became visible is lower for a wider slot. Rounding both corners of the slot seems to have an effect on smoothing the streamlines across the slot. Polycarbonate, which exhibits lower fluid elasticity than polystyrene does, the streamlines and stress field are more symmetrically distributed about the slot centerline. Five polymer melts were used in measuring Ph . It was found that the values of N1 predicted from the HPB equation correlate well with those obtained by the C&P apparatus at low shear rates. The predicted values of N1 also agree well with 2G' even though the shear rate was increased to 70 sec⁻¹ for most of the polymers investigated. Changing the width of the slot did not have a significant effect on the magnitude of Ph , whereas the magnitude of Ph depended largely on the polymers investigated. The measured Ph was nearly zero for polycarbonate at σw = 40KPa and was about 70 KPa for polystyrene at σw = 8O KPa. This was attributed to the significantly lower fluid elasticity of polycarbonate relative to polystyrene.