at shear rates above 1000 s−1 . This is typical for many polymer
melts and occurs as a result of lower values of shear thinning index (i.e. higher degree of shear thinning) at lower temperatures. It can also be observed from Fig. 3 that viscosity above shear strain rates
greater than 1000 s−1 deviated from the power law model. This
can be seen more clearly from plots of wall shear stress against wall shear rate as shown in Fig. 4. Wall shear stress increased with rate in all cases with a gradually diminishing gradient due to the shear thinning nature of the melts. However, at rates significantly
above 1000 s−1 step changes or discontinuities in the curves were
observed in several cases. This indicates that the melt flow in this region may no longer follow the power law model so care should be taken when using it to make pressure drop or flow rate predic- tions in this region of shear strain rates. The observed step change in the shear stress versus shear strain rate appeared to become more pronounced with increasing molecular weight and decreasing tem- perature, i.e. with increasing viscosity. Deviation from the power law is likely to result from a breakdown of the assumptions on which the model is based, namely (i) zero fluid velocity at the wall, (ii) fluid streamlines are parallel to the wall, (iii) uniform hydrostatic pressure across any radial section of the capillary and (iv) flow is vis- cometric. The most likely causes of deviation from power law at high rates are melt flow instabilities and non-zero velocity at the die wall (i.e. wall slip). Surface roughness and in some cases foaming was observed for extrudate exiting the capillary dies at high shear rates
as shown in images obtained from scanning electron microscopic (SEM) images shown in Fig. 5, which suggests that flow instabilities and/or slip occurred. The onset of surface instabilities correlated with the discontinuities observed in Figs. 3 and 4, and therefore occurred at lower rates for higher molecular weight grades. Gen- eration of instabilities during melt extrusion of polymers has been reported for many polymers used in the plastic industry [22,23] and have been observed to occur above a critical value of wall shear stress. Deviations from the power law model have been reported to correlate with flow instabilities [24] and from the zero wall velocity assumption [25]. The onset of foaming was found to be independent of rate by reversing the order of test stages shown in Table 1. Insta- bilities were formed at high strain rates as with tests carried out in the original order, but foaming occurred in the low rate stages at the end of the test, suggesting a dependence upon time and/or pressure rather than rate.
The relationship between K and average molecular weight is shown in Fig. 6. At each of the measured melt temperatures, K increased with increasing molecular weight. The rise appeared to be exponential in nature. Sensitivity of shear viscosity to set melt temperature was also found to increase with molecular weight, as shown in Fig. 7. This finding is in agreement with previous work [18] which concluded that the temperature and pressure sensitivity of a polymer melt was directly related to the size and complexity of the polymer molecule.