MELT ELASTICITY

Polyolefin melts have a high degree of viscoelastic memory or elasticity. First normal stress differences of polyolefins, a rheological measure of melt elasticity, are shown in Figure 9 (30). At a fixed molecular weight and shear rate, the first normal stress difference increases as MJM increases. The high shear rate obtained in fine capillaries, typically on the order of 10 , coupled with the viscoelastic memory, causes the filament to swell (die swell or... 

Melt elasticity is of considerable importance in understanding much of the behaviour of polyethylene when processing by film extrusion techniques and when blow moulding. The complex relationships observed experimentally here have been summarised by the author elsewhere. ... 

PEs, as other polymers, exhibit nonlinear behavior in their viscous and elastic properties under practical processing conditions, i.e., at high-shear stresses. The MFI value is, therefore, of little importance in polymer processing as it is determined at a fixed low-shear rate and does not provide information on melt elasticity [38,39]. In order to understand the processing behavior of polymers, studies on melt viscosity are done in the high-shear rate range viz. 100-1000 s . Additionally, it is important to measure the elastic property of a polymer under similar conditions to achieve consistent product quality in terms of residual stress and/or dimensional accuracy of the processed product. 

Because of the melt-elasticity effects of the material, it does not draw down in a simple proportional manner thus, the draw-down process is a source of errors in the profile. Errors are significantly reduced in a balanced situation such as circular extrudate. These errors must be corrected by modifying the die and takeoff equipment. 

Many of the new plastics, blends, and material systems require special, enhanced processing features or techniques to be successfully injection molded. The associated materials evolution has resulted in new plastics or grades, many of which are more viscoelastic. That is, they exhibit greater melt elasticity. The advanced molding technology has started to address the coupling of viscoelastic material responses with the process parameters. This requires an understanding of plastics as viscoelastic fluids, rather than as purely viscous liquids, as is commonly held... 

However, melt elasticity is more closely related to the third moment known as the z-average molecular weight M. The values of are most often determined using either light-scattering photometry or ultracentrifugation. It is shown mathematically as ... 

Rheological properties of filled polymers can be characterised by the same parameters as any fluid medium, including shear viscosity and its interdependence with applied shear stress and shear rate elongational viscosity under conditions of uniaxial extension and real and imaginary components of a complex dynamic modulus which depend on applied frequency [1]. The presence of fillers in viscoelastic polymers is generally considered to reduce melt elasticity and hence influence dependent phenomena such as die swell [2]. 

Mendelson (169) studied the effect of LCB on the flow properties of polyethylene melts, using two LDPE samples of closely similar M and Mw plus two blends of these. Both zero-shear viscosity and melt elasticity (elastic storage modulus and recoverable shear strain) decreased with increasing LCB, in this series. Non-Newtonian behaviour was studied and the shear rate at which the viscosity falls to 95% of the zero shear-rate value is given this increases with LCB from 0.3 sec"1 for the least branched to 20 sec"1 for the most branched (the text says that shear sensitivity increases with branching, but the numerical data show that it is this shear rate that increases). This comparison, unlike that made by Guillet, is at constant Mw, not at constant low shear-rate viscosity. 

Mendelson and Finger (172) have reported in a later paper the effects of LCB, at constant MW and MWD, on melt elasticity (from an analysis of data on die swelling), and confirm the conclusion that it decreases the melt elasticity as... 

Ham and co-workers (173) compared a LDPE with three HDPE samples, the MJMn ratios of which bracketted that of the LDPE their non-Newtonian behaviour, as measured by the exponent in a power-law relationship between stress and shear rate, also bracketted that of the LDPE. However, the LDPE had considerably higher melt elasticity than the HDPE, which was ascribed to LCB. 

The specific volume was slightly lower for a sample of very high molecular weight and was reflected by an increase in c to 156° C. This effect was attributed to the very high melt elasticity of this polymer. One consequence of the finite value of c is that the internal pressure is not constant as it is for melted polyethylene but decreases with increasing volume. 

S. I. Abdel-Khalik, O. Hassager, and R. B. Bird, Prediction of Melt Elasticity From Viscosity Data, Polym. Eng. Sci., 14, 859 (1974). 

So far in this chapter we have looked into the viscous phenomena associated with the flow of polymer melts in capillaries. We now turn to the phenomena that are related to melt elasticity, namely (a) swelling of polymer melt extrudates (b) large pressure drops at the capillary entrance, compared to those encountered in the flow of Newtonian fluids and (c) capillary flow instabilities accompanied by extmdate defects, commonly referred to as melt fracture. ... 

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