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Mécanismes d'action de "Polymer Processing Aids" fluorés durant l'extrusion d'un polyéthylène basse densité linéaire : études expérimentales et interprétations

Abstract : One of the main extrusion process limitations of linear polymers is the occurrence at very low flow rate of surface defect, also called sharkskin defect. In industry, Polymer Processing Aids (PPAs) are commonly used to suppress the sharkskin defect but theirs actions are not well understood. Three PPAs are studied in an extrusion process of a linear-low density polyethylene (LLDPE) with an important surface defect. A transparent slit die enables to follow die pressure, flow velocity by Laser Doppler Velocimetry and to collect die surface after PPA extrusion. LLDPE extrusions with PPA show that after a delay time, pressure tends to decrease and sharkskin disappears. Analyses of die surface by Scanning Electronic Microscopy (SEM) reveal that fluoropolymer coats heterogeneously the die surface. Velocity profiles indicate that wall slip appears during the pressure decrease, and depends on the location in the die, in agreement with coating observations. The influence of extrusion parameters (flow rate, die roughness and die nature) and relatives to PPA (concentration, nature, dispersion) are studied in order to understand the mechanism of PPA action. We demonstrate that local wall slip is directly correlated to deposit area of PPA, which leads to the die pressure decrease. Two models are then developed and compared to experiments and literature: the microscopic one explains the heterogeneous coating at die walls, and the macroscopic one proposes a kinetic of deposits.
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https://pastel.archives-ouvertes.fr/pastel-00005020
Contributor : Ecole Mines Paristech <>
Submitted on : Thursday, April 23, 2009 - 8:00:00 AM
Last modification on : Thursday, September 24, 2020 - 5:22:54 PM
Long-term archiving on: : Friday, September 10, 2010 - 1:34:36 PM

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  • HAL Id : pastel-00005020, version 1

Citation

Claire Dubrocq-Baritaud. Mécanismes d'action de "Polymer Processing Aids" fluorés durant l'extrusion d'un polyéthylène basse densité linéaire : études expérimentales et interprétations. Sciences de l'ingénieur [physics]. École Nationale Supérieure des Mines de Paris, 2008. Français. ⟨NNT : 2008ENMP1596⟩. ⟨pastel-00005020⟩

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