The apparent uniaxial extensional viscosity or elongational viscosity, ηE, of several polymer melts in a wide range of temperatures was determined as a function of strain rate, ε̇, using Cogswell's analysis of converging flow at the die entry. In particular, ηE was derived from knowledge of the dependence of the steady-flow viscosity as well as the entrance pressure drop, δP0, on shear rate. Quantitative agreement was found in ηE−ε̇ dependence derived from the convergent flow analysis (CFA) along with that measured from isothermal melt-spinning experiments for melts of high-density and low-density polyethylene, polypropylene, and polystyrene. Further support of CFA comes from the finding here that the normalized entrance pressure drop, δP0/τ, is uniquely related to the vortex angle in a manner similar to that determined from flow visualization studies by Ballenger and White. Here τ is the shear stress at the die wall. Additionally, the flow activation energy in extension is found to be equal to that in shear for melts of low-density polyethylene, polypropylene, and polystyrene. Such observations are in line with the earlier results of Munsted on polystyrene melts. An anomaly exists for high-density polyethylenes since ηE increased with increases in temperature above a critical temperature. Finally, the shapes of extensional flow curves are found to be similar to the published data, although an exception such as low-melt-index, low-density polyethylene, where ηE decreases with ε̇, is cited.

In order to predict the processing behavior of a high density polyethylene resin one must know the resin flow behavior over a wide range of shear rates. Low shear properties are important in applications where melt strength, sagging, etc. are critical. On the other hand, high shear flow properties are a determining factor in applications where melt instability, melt fracture and heat generation are important. The flow behavior of a resin can be established by measuring the zero shear viscosity, η0, the maximum relaxation time, τ0, and the shape of the flow curve. We have measured these basic rheological parameters on a large number of high density polyethylene resins. A shear sensitivity parameter which is independent of molecular weight was derived from a correlation between η0 and τ0. This parameter, together with η0, provide the vital information needed in order to predict the processing behavior of the resin. This method is applicable to other polymer systems provided that the rheological parameters η0 and τ0 can be experimentally obtained.

The processing properties of' low density polyethylene melts, such as drawdown and neck-in, and the final product properties, such as film haze and gloss, have been successfully correlated with rheological functions and the level of longchain branching. The rheological functions employed are the entrance pressure drop AP,, and the swell ratio So, determined at a specified shear stress using an orifice with a length/ diameter (LID) ratio of zero. The calculation of shear stress requires additional measurements using a die with a finite LID e.g. 20. The rheological functions AP,, and S,, are governed by the level of high molecular weight species and/or the level of long-chain branching(LCB). If determined at a constant shear stress, in order to eliminate the effect of viscosity, they are a relative measure of elasticity. Higher AP, and S,, indicate a higher level of LCB and correlate with poorer optical properties and drawdown in films.

SynopsisThe effect of heterogeneity on the flow behavior of high density polyethylene (HDPE) has been studied by systematically homogenizing heterogeneous blends and observing the effect on the basic rheological parameters and on the extrudate surfaw appearance. Ample evidence is presented to show that heterogeneity causes substantial changes in the flow behavior although molecular parameters obtained through solution studies are unchanged. A simplified mathematical model which clearly illustrates the effects af the size of these flow units on the flow behavior is presented.