Band 25, Heft 4

We present a numerical study of the film casting process, with a focus on the effect of the draw ratio on the swelling of the extruded sheet. So far, studies regarding film casting have dealt mainly with the phenomenon of neck-in and have neglected swelling of the material as it emerges from the die lips. Knowledge of the amount of swelling is important for accurate determination of the effect of stretching and orientation phenomena. The problem is tackled by studying the gap-wise swelling of the sheet or film as it emerges from a wide rectangular die and is subsequently drawn down under different draw ratios. The material is treated as viscoelastic by utilizing the Linear Phan Thien-Tanner (LPTT) model. Newtonian simulations are also carried out. A decoupled iterative algorithm is used for the determination of the shape of the extruded sheet, based on the fact that the sheet’s surfaces belong to streamlines. Our results are in qualitative agreement with results in the literature, with the latter being limited in number and available mainly for the (similar) process of fiber melt spinning.

There is often a necessity to measure, or at least estimate, true viscosity values using non-standard measuring systems on a rotational rheometer. This may be to replicate a mixing or manufacturing process on a lab scale, to keep a sample dispersed and uniform during a measurement or to measure some rheological property that would be difficult or impossible with a standard configuration. Such measurements can be made easily enough, but without a process for converting torque to shear stress and angular velocity to shear rate only these raw data variables can be reported. In this paper a simple and novel empirical method for determining strain/strain rate C 1 and stress C 2 constants for non-standard measuring systems on a rotational rheometer is presented. This method uses relative torque measurements made with a Newtonian and non-Newtonian material and their corresponding power law fitting parameters to determine C 1 and C 2 using a non-linear regression analysis. Equilibrium flow curves generated for two non-Newtonian fluids using two non-standard mixing geometries show very good agreement with data generated using a standard cone and plate configuration, therefore, validating the approach.

Suspensions of SiO 2 microspheres in glycerine exhibit drastic shear-thickening behavior under steady shear and dynamic oscillatory shear test. The rheological behavior of suspensions agrees with the modified Cox-Merz rules as the dynamic oscillatory rheological behavior at low frequency could be reasonably interpreted in terms of the steady shear behavior. As new insight, the effect of shear history and the relaxation on the rheological behavior was investigated in detail. The result showed that under continuous shear, the viscosity decreases after a “pulse”: The degree of decrease is directly proportional to the shear rate. Similar phenomenon is also found under the continuous stress and dynamic oscillatory shear rate sweep. The shear history shows a non-negligent effect on the rheological behavior, the suspensions with higher viscosity show a lower viscosity under the same shear rate. Moreover, the relaxation time of suspensions shows the direct dependency on the initial viscosity, while the volume fraction of suspensions also affect the relaxation time. For more enlapsed times, also longer relaxation times are needed for the suspensions with lower volume fraction and higher initial viscosity.

The relative intensity and phase of the third harmonic, / 3/1 and Φ 3 −Φ 1 , deduced from Fourier Transform analysis of Large Amplitude Oscillatory Shear (LAOS ) experiments were used to differentiate the effect of irradiation and the effect of multiwalled carbon nanotubes (MWCNT) concentration in PP/MWCNT nanocomposites. Alternatively, studies of elastic and viscous non linearities that give shear thinning and thickening or strain softening and hardening were carried out for the same purpose. Using both methods to analyse LAOS data, the conclusion was the same: The influence of MWCNTs is noticed at low/intermediate γ o strains (10 - 100%), whereas the effect of irradiation is rather observed at strains above 100%. This marks a difference with respect to small amplitude oscillatory flow measurements, which are not valid to distinguish between the respective rheological effects of irradiation and MWCNT in polymer nanocomposites. SEC-MALLS-IR-VI analysis was used to determine the long chain branching degree λ of irradiated polypropylene, but this technique is very difficult to be applied for nanocomposites. Face to this shortcoming, an empirical correlation between λ and the value of the / 3/1 plateau when γ o tends to infinite, found for irradiated neat PP, was used to evaluate the long chain branching degree of nanocomposites.