Rheology extrudate swell

Rheological parameters, such as relaxation time, shear modulus, and stored elastic energy, are determined from the extrudate swell and stress-strain data as previously described. Representative examples of the variation of these parameters with blend ratios for both blends are shown in Figs. 16-18. Figure 16 shows that relaxation time for both preblends without heating and... 

J.A. Menjivar, E.N. Chang and L. Maciel, Extrudate Swell Behaviour of Wheat Flour Doughs, in "Theoretical and Applied Rheology", P.Moldenaers and R. Kennings (eds.), Elsevier Science B.V., 1992, pp. 714-716. 

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). 

R. B. Bird, R. K. Prud homme, and M. Gottlieb, Extrudate Swell as Analyzed by Macroscopic Balances, The University of Wisconsin, Rheology Research Center Report RRC-35, 1975. 

FIG. 15.33 Extrudate swell ratio as a function of L/Da for high-density polyethylene at 18 °C for shear rates as indicated. From Han et al. (1970). Courtesy Society of Rheology. 

FIG. 15.37 Extrudate swell ratio vs. shear stress at the wall, for melts of polystyrenes of broad and narrow molecular-weight distributions. Filled symbols the broad polystyrene mentioned in Fig. 15.34-15.36 Mw = 2.2 x 105 and Mw/Mn = 3.1. Open symbols narrow polystyrene Mw = 1.6 x 105 and M /Mn < 1.1. From Greassley et al. (1970). Courtesy Society of Rheology. 

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. 

The fundamental assumption of the classical rheological theories is that the liquid stmcture is either stable (Newtonian behavior) or its changes are well dehned (non-Newtonian behavior). This is rarely the case for flow of multiphase systems. For example, orientation of sheared layers may be responsible for either dilatant or pseudoplastic behavior, while strong interparticle interactions may lead to yield stress or transient behaviors. Liquids with yield stress show a plug flow. As a result, these liquids have drastically reduced extrudate swell, B = d/d (d is diameter of the extrudate, d that of the die) [Utracki et al, 1984]. Since there is no deformation within the plug volume, the molecular theories of elasticity and the relations they provide to correlate, for example either the entrance pressure drop or the extmdate swell, are not applicable. 

Two types of rheological phenomena can be used for the detection of blend s miscibility (1) influence of polydispersity on the rheological functions, and (2) the inherent nature of the two-phase flow. The first type draws conclusions about miscibility from, e.g., coordinates of the relaxation spectmm maximum cross-point coordinates (G, CO ) [Zeichner and Patel, 1981] free volume gradient of viscosity a = d(lnT]) / df the initial slope of the stress growth function S = d(lnr +g)/dlnt the power-law exponent n = d(lnOj2)/dlny = S, etc. The second type involves evaluation of the extrudate swell parameter, B = D/D, strain (or form) recovery, apparent yield stress, etc. 

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. 

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... 

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... 

ASTM D3835/2000 test method measures rheological properties of thermoplastic (and thermosetting) melts by using a capillary rheometer [4], The test method includes measurements of viscosity, shear rate, shear stress, swell ratio, and percent of extrudate swell. Assuming a newtonian fluid, to calculate melt viscosity j, use... 

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. 

Keywords die swell, extrudate swell, blow molding, modelling of die swell, rheology, finite element modelling (FEM), creep fiber spinning. 

The rheological responses measured at low values of strain better reflect the effects of the blend structure. For multiphase systems, there are serious disagreements between the predictions of continuum-based theories and experiments, that is, between the small and large deformation behavior. For example, the identity of zero-deformation rate dynamic and steady state viscosity is seldom found, and so is the Trouton rule. Similarly, the derived by Cogswell, relationship between the extensional viscosity and the capillary entrance pressure drop, and derived by Tanner equation for calculating the first normal stress difference from the extrudate swell, are rarely valid. 

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