Polymer rheology extrudate swelling

Fig. 12.14 Effect of the method of measurement on the value of D/Do for HDPE. Curve 1, frozen extrudates Curve 2, extrudates annealed at 160°C in hot silicon oil Curve 3, photographs of extrudates emerging from capillary Curve 4, photographs of extrudates in hot silicon oil. [Reprinted by permission from J. L. White and J. F. Roman, Extrudate Swell During the Melt Spinning of Fibers-Influence of Rheological Properties and Take-up Force, J. Appl. Polym. Sci., 20, 1005 (1976).]...

J. L. White and J. F. Roman, Extrudate Swell During the Melt Spinning of Fibers—Influence of Rheological Properties and Take-up Force, J. Appl. Polym. Sci., 20, 1005 (1976). 

Addition of starch has a nucleating effect, which increases the rate of crystallisation. The rheology of starch/PCL blends depends on the extent of starch granule destruction and the formation of thermoplastic starch during extrusion. Increasing the heat and shear intensities can reduce the melt viscosity, but enhance the extrudate-swell properties of the polymer. 

Thermotropic LCPs have high melt elasticity, but exhibit little extrudate swell. The latter has been attributed to a yield stress and to long relaxation times (60). The relaxation times for LCPs are normally much longer than for conventional polymers. Anomalous behavior such as negative first normal stress differences, shear-thickening behavior and time-dependent effects have also been observed in the. rheology of LCPs (56). Several of these phenomena are discussed for poly(benzylglutamate) solutions in the chapter by Moldenaers et al. 

There are several aspects of rheological behavior exhibited by polymeric liquids that set these materials apart from Newtonian fluids. An excellent summary of the differences in fluid response between Newtonian liquids and non-Newtonian polymeric liquids under various scenarios has been given by Bird and Curtis [3]. Two very well-known atypical phenomena exhibited by polymeric liquids are the Weissenberg effect (a polymer melt or solution tends to climb a rotating rod) and extrudate swelling, which are illustrated in Figure 22.4. 

Muksing, N., Nithitanakul, M., Grady, B. P., Magaraphan, R. (2008). Melt rheology and extrudate swell of organobentonite-filled polypropylene nanocomposites. Polym. Test, 17, 470-479... 

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. 

White J L and Roman J F (1976), Extrudate swell during the melt spinning of fibers—influence of rheological prop>erties and take-up force . Journal of Applied Polymer Science, 20,1005-1023. 

H.J. Song, J.L. White, K. Min, N. Nakajima, F.C. Weissert. Rheological properties, extrudate swell, and die extrusion flow marker experiments for rubber carbon black compounds. Adv. Polym. Technol, 8,421-449,1988. 

Rheological test more often used for product quality control is a measurement of the elastic recovery in polymer melts and their compounds. This is usually accomplished by measuring the swell (or shrinkage) of materials undergoing extrusion. The tendency of polymers, whether thermoplastic or rubber, to enlarge when emerging from an extruder die is called die swell. 

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