Iakovos Vittorias

Predicting extrusion instabilities of commercial polyethylene from non-linear rheology measurements

Processing at the highest possible throughput rates is essential from an economical point of view. However, various flow instabilities and extrudate distortions like sharkskin, stick slip, and gross melt fracture (GMF) may limit the production rate of high-quality products. Predicting the process conditions leading to the occurrence of rheological instabilities is the key for improving product quality, process control, and optimization. Large-amplitude oscillatory shear (LAOS) and FT-rheology were used to quantify the non-linear rheological behavior and instabilities of a series of well-characterized commercial polyethylene (PE). From the latter, we derive the critical non-linearity parameter, F0,c, which corresponds to the normalized intensity of the third harmonic at the critical strain amplitude, γ0,C (defined by the appearance of the second harmonic), normalized by γ0,C. The F0,c is correlated with the high molecular mass fraction of the polymers and with the Deborah numbers. Linear rheological parameters and molecular structures were related to F0,c. An experimental correlation between F0,c of commercial PE melts and pressure fluctuations associated with flow instabilities (sharkskin) was established both for capillary rheometry and extrusion.

On the Use of Indexes for Quantifying Long‐Chain Branching in Polyethylene: Can We Describe the Rheology of LCB PE and Correlate it to Processing Performance by Using a Single Number?

Numerous analytical techniques and methods have been developed in the last decades, to study macromolecular architecture, especially for the case of polyethylene. The goal was to detect with an enhanced sensitivity long‐chain branching (LCB) in polyethylene and accurately quantify the branching degree and structure. These studies resulted in a large number of different methods and LCB indexes (LCBI), derived mainly from rheological techniques, as well as size‐exclusion chromatographic and spectroscopical measurements (GPC‐MALLS, NMR). Within this work, these were applied in a series of polyethylenes, produced by various processes and catalysts, with varying mol. weight distribution and LCB concentration. The combination of GPC‐MALLS, the δ vs. G* plot and elongational rheology was found to be the only possibility to realistically describe the different polyethylene structures and their distribution.