High density polyethylene extensional viscosity

For polymer melts where the low shear rate limiting viscosity value is r ), r 3t]0 (14). Examples of extensional viscosity growth, either to a steady t](i ) value or to a strainhardening-like mode, are shown in Fig. 3.6 for the linear nonbranched polystyrene (PS), a high density polyethylene (HDPE) that is only slightly branched with short branches, and a long chain-branched low density polyethylene (LDPE) (15). 

Another effect of the variation of the extensional viscosity is the maximum extend-ibility. For polymers like high-density polyethylene, the rapid increase of the extensional viscosity during the spinning process limits the obtainable spin-draw ratio that is the ratio between the winding velocity and the velocity in the orifice. Examples can be found in an article of Han and Lamonte (1972). 

Figure 3.39 Uniaxial extensional viscosity rj as a function of time following start-up of steady uniaxial extension at the extension rates e indicated. Data are shown for an unbranched polystyrene (PS I), a high-density polyethylene with short, unentangled side branches (HOPE I), and two low-density polyethylenes (LDPE III and lUPAC A), with long side branches. (From Laun 1984, with permission from the Universidad Nacional Autonoma de Mexico.)------------------------------...

In order to improve the tensile properties of low-density polyethylene. Mead and Porter [100] added high density polyethylene fibers and film strips. This resulted in an increase in the extensional viscosity and consequently, the tensile modulus of the composite was increased by a factor of 10. The effect of different mineral fillers (e.g. talc, mica, clay, dolomite) on the rheological properties of low density polyethylene films was studied by Arina et al. [17]. It was found that the fillers increased the extensional viscosity of a polymer matrix in concurrence with the earlier observations of Han and Kim [86] as well as Mead and Porter [100]. 

Figure 9.10 Variation of relative extensional viscosity with extensional rate for glass fiber filled high density polyethylene at 180°C with different levels of filler loading as indicated. (Reprinted from Ref. 14 with kind permission from John Wiley Sons, Inc.,...

FIG. 24 Extensional viscosity growth function vs time for Phillips Marlex 5502, high-density polyethylene. Strain rate, s 1, 0.0022 2, 0.005 3, 0.055 4, 1.10. (Data from Ref 61.)... 

Figure 11.23 Extensional viscosity (e) for three polyethylene melts at 150 "C.The squares are for a high density polyethylene (HDPE M = 152,000 and MJM = 13.8) that is believed to lack long-chain branching, while the other two samples (LDPE III = 256,000 and...

The emphasis to this point has been on viscous behavior in shearing modes of deformation. However, any operation which reduces the thickness of a polymeric liquid must do so through deformations that are partly extensional and partly shear. In many cases polymers respond very differently to shear and to extension. A prime industrial example involves low density and linear low density polyethylenes, i.e., LDPE and LLDPE, respectively (Section 9.5.3). LDPE grades intended for extrusion into packaging film have relatively low shear viscosities and high elonga-tional viscosities. As a result, extrusion of tubular film involves reasonable power... 

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. 

Phan-Thien and Tanner used data of Meissner for the low-density polyethylene in Figure 9.9 to test the model, with = 0.2 and s = 0.01. The fit to steady shear viscosity and normal stresses is good, and the transients are fit reasonably well the shear data are insensitive to the value of s as expected. The fit to the extensional data at three stretch rates is shown in Figure 9.13. The agreement is quite good, especially when it is recalled that there is probably an experimental artifact at short times that causes the data to be high. The model predicts an approach to a steady extensional viscosity that scales as 1/e, but the data do not extend sufficiently far to test the prediction. 

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