Extensional-hardening

Film. By far the largest appHcation for LLDPE resins (over 60% in the United States) is film. Because LLDPE film has high tensile strength and puncture resistance, it is able to compete with HDPE film for many uses. The toughness and low temperature properties of LLDPE film also exceed those of conventional LDPE. Furthermore, because LLDPE resins exhibit relatively low strain hardening in the molten state and lower extensional viscosity, it can be produced at high rates with Httle risk of bubble breaks. 

B.J. Dobraszezyk, Strain Hardening in Biaxial Extension of Wheat Flour Doughs, presented at the 2nd Int. Meeting on Extensional and Shear Flow of Polymer Fluids, St. Andrews, Scotland, June 19-22,1994. 

Turning to Fig. 3.2, Case 1, we see that the tensile force F needed to sustain the applied constant extensional rate e, either levels off to a constant F (s) or exhibits strain hardening increasing with time, occasionally in an unbounded fashion the force is then represented as F (i . t). For this uniform extensional flow... 

M. H. Wagner, H. Bastian, P. Hachmann, J. Meissner, S. Kurtzbeck, H. Miinstedt and F. Langouche, The Strain-hardening Behaviour of Linear and Long-chain-branched Polyolefin Melts in Extensional Flows, Rheol. Acta, 39, 97-109 (2000). 

In Sect. 15.4 it was shown how the shear thinning behaviour of the viscosity could be described empirically with the aid of many suggestions found in literature. It was not mentioned there that the first normal stress coefficient also shows shear thinning behaviour. In this Sect. 15.5 it became clear that also the extensional viscosity is not a constant, but depending on the strain rate upon increasing the strain rate qe the extensional viscosity depart from the Trouton behaviour and increases (called strain hardening) to a maximum value, followed by a decrease to values below the zero extensional viscosity. It has to be emphasised that results in literature may show different behaviour for the extensional behaviour, but in many cases this is due to the limited extensions used,... 

Strands that terminate with a branch point at both of its ends can neither reptate nor completely retract. Relaxation of such strands presumably occurs by more complex, hierarchical processes discussed by McLeish (1988b). Here we simply note that the presence of branch points at both ends of a strand leads to much more strain hardening in extensional flows (Bishko et al. 1997 McLeish and Larson 1998). Low-density polyethylenes (LDPEs), which are highly branched, are well known for their extreme strain hardening behavior in extensional flows (Meissner 1972 Laun 1984) (see Fig. 3-39). The steady-state shear viscosity, as a function of shear rate, seems to be little affected by long-chain branching, however. 

The strain hardening phenomenon also means that any extensional reactivation of CSBs tends to occur by dilatational opening of the margins to form potential fluid flow conduits and sites for the... 

The new metallocene catalysis leads to isomer purity in excess of 96%. It is also possible to produce branched, high melt strength PP, with extensional stress hardening, similar to that of LDPE [Phillips et al., 1992]. The new PP s show the melting point, T = 120-164°C. To enhance the performance, PP is usually blended (in the reactor or outside the reactor) with much more viscous PP-copolymers. As a consequence, one of the most serious industrial problem is homogenization of these materials [Luciani and Utracki, 1996 Utracki andLuciani, 1996]. 

Wagner, M. H., H. Bastian, P Hachmann, J. Meissner, S. Kurzbeck, H. Miinstedt, and R Langouche. 2000. The strain-hardening behaviour of linear and long-chain-branched polyolefin melts in extensional flows. Rheologica Acta 39 97-109. 

Fig. 8.13 A true-stress-true-strain curve of PS, strained to an extensional true strain of 1.4 at T = 296 K, showing the emergence of a strain-hardening stage past the plateau of the flow state, at e = 0.4, with a number of stress removals showing Bauschinger effects (from Hasan and Boyce (1993) courtesy of Elsevier).

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