Viscoelasticity extrudate swell

Profiles are all extruded articles having a cross-sectional shape that differs from that of a circle, an annulus, or a very wide and thin rectangle (flat film or sheet). The cross-sectional shapes are usually complex, which, in terms of solving the flow problem in profile dies, means complex boundary conditions. Furthermore, profile dies are of nonuniform thickness, raising the possibility of transverse pressure drops and velocity components, and making the prediction of extrudate swelling for viscoelastic fluids very difficult. For these reasons, profile dies are built today on a trial-and-error basis, and final product shape is achieved with sizing devices that act on the extrudate after it leaves the profile die. 

Figure 3.11 Extrudate swell for silicone oil (Baysilone M 1000) (A), and for viscoelastic PEO solution (aqueous 1 % PEO solution Polyox WSR 301) (B) when extruded from a die...

Te Nijenhuis K, "Viscoelastic Polymeric Fluids" Chap. 9.1 and "Entrance Correction and Extrudate Swell" Chap. 9.4 in Tropea C, Yarin AL and Foss JF (Eds) "Handbook of Experimental Fluid Mechanics", Springer, Berlin, 2007, Chap. 9.1. 

A study was made of the ability of viscoelastic models to describe the measured material functions of unplasticised PVC during extrusion and to determine whether it was possible to reproduce the elastic properties of the large entrance pressure drop and small extrudate swell during the extrusion of PVC using a capillary rheometer. Models used were the Phan-Thien and Tanner model and the K-BKZ-Wagner model with a single exponential damping... 

In general terms, both of the aforementioned rheological phenomena can be related to the viscoelastic nature of polymeric materials and more specifically to the development of normal stresses and the deformation history of such materials. Extrudate swelling has direct implications... 

Common defects encountered with extrusion include effects associated with the viscoelastic nature of plastic melts. As the melt is extruded from the die for example, it may exhibit sharkskin melt fracture and extrudate (die) swell. Diagrams of these defects are shown in Fig. 1.16. Sharkskin melt fracture occurs when the stresses being applied to the plastic melt exceed its tensile strength. Extrudate swell occurs due to the elastic component of the polymer melt s response to stress and is the result of the elastic rebound of the polymer as it leaves the constraints of the die channel prior to cooling. 

It should be noted that extrudate swelling is not unique to viscoelastic fluids. It can also occur in an inelastic or purely viscous fluid this has been demonstrated experimentally and theoretically. Obviously, in an inelastic fluid, the mechanism of extrudate swell is not an elastic recovery of prior deformation. The swelling is caused by a significant rearrangement of the velocity profile as the polymer leaves the die this is shown in Fig. 7.114. 

Die swell is a complex rheological phenomenon [1], It can be observed as an extrudate with a cross-section (D which is greater than the die cross-section DJ. This effect, also known as extrudate swell, Barus effect, or % memory, is defined as the ratio D /Dq = B and is a feature of polymer melt flow. Die swell is associated with the viscoelastic nature of polymer melts as it exceeds the swelling of constant viscosity (Newtonian) fluids. Accordingly, for laminar flow situations, the swelling due to velocity profile rearrangements or mass balance considerations accounts for only 10-20% and cannot explain the 50-300% increase in extrudate cross-section of the polymer emerging out of a die. 

When a viscoelastic fluid flows through an orifice or a capillary, the diameter of the fluid at the die exit is considerably higher than the diameter of the orifice. This happens because, at the die exit, the viscoelastic fluid partially recovers the deformation it underwent when it was squeezed through the capillary. This type of phenomenon is known variously as extrudate swell, die swell, jet swell, Barus effect or Merrington effect. Metzner [38] discusses the history of extrudate swell... 

Figure 2.9 Extrudate swell effect showing how the viscoelastic fluid swells in diameter when it exits from a die or orifice. (Reprinted from Ref. 34 with kind permission from Chapman Hall, Andover, UK.)...

Basic description of non-Newtoruan fluids is provided so that concepts of shear rate dependent viscosities with or without elastic behavior, yield stress with or without shear rate dependent viscosities and time dependent viscosities at fixed shear rates get classified. The filled polymer systems fall into the category of pseudoplastic fluids with or without yield stress and also often depict the behavior of thixotropic fluids. Their viscoelasticity may give rise to various anamolous effects that are discussed in Chapter 2, such as the Weissenberg effect, extrudate swell, drawn resonance, melt fracture and so on. 

We should expect even greater extrudate swell for a viscoelastic liquid since the first normal stress difference pushes against the walls of the tube until the exit, after which the resistance vanishes. Tanner has developed an approximate theory for extrudate swell based on the elastic recovery when the tube wall is instantly removed, giving the following result ... 

Most concentrated structured liquids shown strong viscoelastic effects at small deformations, and their measurement is very useful as a physical probe of the microstructure. However at large deformations such as steady-state flow, the manifestation of viscoelastic effects—even from those systems that show a large linear effects—can be quite different. Polymer melts show strong non-linear viscoelastic effects (see chap. 14), as do concentrated polymer solutions of linear coils, but other liquids ranging from a highly branched polymer such as Carbopol, through to flocculated suspensions, show no overt elastic effects such as normal forces, extrudate swell or an increase in extensional viscosity with extension rate [1]. 

Viscoelastic behaviour is then covered chapters 13 and 14), first linear viscoelasticity with its manifestations in the time and frequency domain. Then nonlinear displays of viscoelasticity are introduced, these effects being usually encountered in steady-state flow, where overt normal-force effects such as the Weissenberg rod climbing and the extrudate-swell phenomena are seen. 

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