ABSTRACT

I. INTRODUCTION Ln this book. for the most part. we have considered those situations that involve steady flow. When we did consider time-dependent flow. the time dependence was deliberately imposed. as during the measurement of dynamic mechanical properties in Chap. 6. Often. however. the time dependence is both unintentional and unwelcome. ln particular. a great many instabilities have been found to arise during polymer processing. and the topic has been reviewed on a number of occasions [1-3. for example]. While some of these instabilities are observed with Newtonian and other purely viscous liquids, others are unique to elastic liquids. Some occur in confined flows, others are free surface instabilities. Collectively, the different instabilities can serve to limit production rates in industrial operations and to render unsuitable viscometric measurements made for the purpose of fluid characterization [4]. Examples of unstabie Newtonian fluid behavior include (I) the phenomenon of draw resonance. which arises during fiber or film manufacture by extrusion and subsequent drawdown in cross-sectional area by the use of high-velocity rollers. (2) the formation of ribs on rotating rollers during coating operations. and (3) the occurrence of secondary flows. such as Taylor vortices in rotational Couette 11ow. ln viscoelastic fluids, the addition of fluid elasticity can modify either the point of onset of the instability or the nature of the instability itself. Concerning purely elastic instabilities, one finds, for example, that upon extruding polymer melts or solutions through a circular die at slow to moderate extrusion speeds, the polymer emerges as a smooth extrudate of uniform diameter. As the extrusion speed or shear rate is increased. a critical value is reached above which the extrudate appears distorted in a periodic or

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random manner. This behavior is generally undesirable, and it is known as melt fracture; it may also be accompanied by surges in the flow rate. The phenomenon, though, is very general and is independent of the shape of the die, for it has been observed with slits and tubes of noncircular cross section as well. In addition, a major unifying feature of all these observations is that fracture is an clastic phenomenon, since it has not been reported for Newtonian liquids. This points to the role of fluid rheology in giving rise to secondary flows and unstable flows.