High density polyethylene normal stress difference

For example, Figs. 2.43 and 2.44 present the measured [55] viscosity and first normal stress difference data, respectively, for three blow molding grade high density polyethylenes along with a fit obtained from the Papanastasiou-Scriven-Macosko [59] form of the K-BKZ equation. A memory function with a relaxation spectrum of 8 relaxation times was used. 

Figure 2.44 Measured and predicted first normal stress difference for various high density polyethylene resins at 170° C.

Figure 3.66 shows the steady-shear viscosity for a polymer system at three molar masses. Note the plateau in viscosity at low shear rates (or the zero-shear viscosity). Also note how the zero-shear viscosity scales with to the power 3.4. (This is predicted by Rouse theory (Rouse, 1953).) Figure 3.67 shows the viscosity and first normal-stress difference for a high-density polyethylene at 200 C. Note the decrease in steady-shear viscosity with increasing shear rate. This is termed shear-thinning behaviour and is typical of polymer-melt flow, in which it is believed to be due to the polymer chain orientation and non-affine motion of polymer chains. Note also that the normal-stress difference increases with shear rate. This is also common for polymer melts, and is related to an increase in elasticity as the polymer chain motion becomes more restricted normal to flow at higher shearing rates. 

Normal stress differences for a high density polyethylene (Marlex 6050) at 190°C. Total thrust between parallel disks (solid line)... 

Viscosity and first normal stress difference versus shear for a high density polyethylene at 200°C cone and plate (o), slit die (o), and capillary (A) data. Adapted from Chan et al. (1990). 

Whilst the Vicat test usually gives the higher values the differences are quite modest with many polymers (e.g. those of types A, B and C). For example, in the case of the polycarbonate of bis-phenol A (Chapter 20) the heat distortion temperatures are 135-140°C and 140-146°C for the high and low stress levels respectively and the Vicat softening point is about 165°C. In the case of an acetal homopolymer the temperatures are 100, 170 and 185°C respectively. With nylon 66 the two ASTM heat distortion tests give values as different as 75 and 200°C. A low-density polyethylene may have a Vicat temperature of 90°C but a heat distortion temperature below normal ambient temperatures. 

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