Dubrocq Baritaud, Claire (2008) Mechanisms of action of Polymer Processing Aids during the extrusion of linear low-density polyethylene: experiments and interpretations. PhD thesis Sciences et Génie des Matériaux, CEMEF - Centre de Mise en Forme des Matériaux, ENSMP p.263.

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Abstract

One of the main limitations during extrusion process of linear polymers is the occurrence at very low flow rate of surface defects, also called “sharkskin” defect. In industry, fluoropolymer based Polymer Processing Aids (PPAs) are commonly used to suppress this defect but their actions are not well understood. Three PPAs are studied in an extrusion process of a linear-low density polyethylene (LLDPE) with a prononced surface defect. A transparent slit die enables to follow die pressure, flow velocity by Laser Doppler Velocimetry (LDV) and to collect die surface at the end of 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 heterogeneously coats 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, particle size) 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. A microscopic model is then developed and compared to experiments to explain the heterogeneous coating at die walls.

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