Masters Theses

Date of Award

3-1988

Degree Type

Thesis

Degree Name

Master of Science

Major

Polymer Engineering

Major Professor

Donald C. Bogue

Committee Members

Joseph E. Spruiell, Larry C. Wadsworth

Abstract

Rheological properties of a series of low molecular weight polypropylenes (being used in the manufacture of melt blown fabrics and also in the melt spinning studies) were studied with particular emphasis on elongational properties. Shear viscosities and normal stresses in the low shear rate region were measured using a cone-andplate rheometer and shear viscosities in the high shear rate region were measured using a capillary rheometer. One of the samples studied was remelted web of fabrics prepared from PP-300 MFR at 500 °F. Comparison of shear viscosity data of PP-300 MFR with the remelted web sample obtained from it suggest that significant thermal degradation occurred in the screw extruder of the melt blown unit. Because of the very thin nature of the materials studied, it is difficult to measure small stretching forces; therefore, isothermal spinning under the force of gravity (gravity spinning) was carried out.

The shear viscosity and the normal stress data were correlated by analytical expressions provided by the convected Maxwell model, with deformation rate dependent relaxation times as proposed by Bogue and White. The zero shear viscosity data were used for theoretical predictions for gravity spinning assxaming Newtonian behavior. Such predictions depicted many features of gravity spinning and provided the base case for later comparison. For the deformation rates observed in the gravity spinning experiments, the apparent elongational viscosities for PP-700 MFR and PP-IAOO MFR in the upper part of the spinline were close to three times the zero-shear viscosities, in agreement with Trouton's viscosity result for Newtonian fluids. But the corresponding results for PP-300 MFR were lower than three times the zero-shear viscosity. However, to understand the dynamics of the entire spinline, viscoelastic modelling is necessary. The convected Maxwell model provides a convenient framework for analyzing viscoelastic fluids. But the use of a deformation rate dependent relaxation times (as used in the Bogue-White model and the Phan-Thien and Tanner model) is necessary to account for the shear thinning behavior and for quantitative predictions of the gravity spinning data. The viscoelastic modelling shows that the time constants obtained from fitting the shear data cannot be used to predict the gravity spinning data quantitatively. A higher value of the average relaxation time is therefore necessary. These higher values of the average relaxation times give quantitative fits of the elongational flow data.

Under certain conditions (those giving higher elongation rates than observed for stable spinning), draw resonance, a kind of spinline instability, was observed in all the materials studied. Draw resonance data were correlated semi-quantitatively.

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