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

Published online : 23 September 2014Processing 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 wellcharacterized 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.This work was supported by the Portuguese Foundation for Science and Technology through the Strategic Project PEst-C/ CTM/LA0025/2011 (Strategic Project-LA 25-2011-2012)

Extrusion-instabilities from non-linear rheology for industrial polyethylene

The instabilities that occur in the pressure-driven extrusion of molten polymers are fascinating from the scientific perspective but troublesome and sometimes catastrophic from the industrial one. Flow anomalies like sharkskin, Stick-Slip and Gross Melt fracture can occur in common extrusion operations such as the manufacture of polymeric rods, tubes, sheets, films, tanks or wire coating. Over the years, processors have learned to work around these processing defects by a variety of means: slowing the manufacturing rate, increasing the melt temperature or by- addition of processing additives. All these solutions can be proven extremely costly, thus a significant interest and effort is put in designing and manufacturing polymers with better processing-properties and lower potential to flow-instabilities. Sharkskin, here defined as periodic surface distortions of low amplitude and high frequency, is most commonly observed in polyethylene’s of sufficiently narrow molecular weight. One reason why sharkskin is of great importance is that as the extrusion flow rate increases, it is in many cases the first instability to occur, it is challenging to suppress and affects an important product quality parameter, namely the surface appearance. FT Rheology can be used to study different PE industrial samples with respect to their behavior under the LAOS flow. The resulting non-linearities showed a dependency on both the molecular weight and topology [1]. Filipe et al. [1] found out that for strain amplitudes above a critical value, the stress time signal exhibited an amplitude decay that indicates slip in qualitative agreement with Chen et al. [2] and Hatzikiriakos and Dealy [3]. The second harmonic was thus found to become significant (above noise level) at the onset of the stress amplitude decay and it is found to be a useful indicator of secondary flows or generally instabilities. [1]This work is a master thesis carried out in the frame of the Erasmus Mundus Master Course EURHEO (www.eurheo.eu). The Authors thank the European Union and EACEA for granting project 2008-0099-EURHEO: The Erasmus Mundus Master in Engineering Rheology, and LyondellBasell for sponsoring EURHEO. This work was supported by the Portuguese Foundation for Science and Technology through the Strategic Project PEst-C/CTM/LA0025/2011 (Strategic Project – LA 25-2011-2012)

Influence of Manufacturing Process on the Microstructure, Stability, and Sensorial Properties of a Topical Ointment Formulation

The manufacturing process for ointments typically involves a series of heating, cooling, and mixing steps. Precise control of the level of mixing through homogenization and the cooling rate, as well as temperature at different stages, is important in delivering ointments with the desired quality attributes, stability, and performance. In this work, we investigated the influence of typical plant processing conditions on the microstructure, stability, and sensorial properties of a model ointment system through a Design of Experiments (DoE) approach. Homogenization speed at the cooling stage after the addition of the solvent (propylene glycol, PG) was found to be the critical processing parameter that affects stability and the rheological and sensorial properties of the ointment. A lower PG addition temperature was also found to be beneficial. The stabilization of the ointment at a lower PG addition temperature was hypothesized to be due to more effective encapsulation by crystallizing mono- and diglycerides at the lower temperature. The in vitro release profiles were found to be not influenced by the processing parameters, suggesting that for the ointment platform studied, processing affects the microstructure, but the effects do not translate into the release profile, a key performance indicator. Our systematic study represents a Quality-by-Design (QbD) approach to the design of a robust manufacturing process for delivering stable ointments with the desired performance attributes and properties