In-process assessment of clay dispersion in PLA during melt compounding: Effects of screw speed and filler content

Abstract

Shear viscosity, clay particle size and volume fraction are measured in-situ during the extrusion of PLA-clay nanocomposites. These in-process measurements are particularly adequate to assess the dispersion of fillers in thermo-mechanically sensitive systems, as the additional degradation associated with sample preparation for off-line characterization is readily avoided. Overall, results point towards the impact of PLA degradation on the clay dispersion mechanism. A better dispersion is reported for composites compounded with more clay. Clay-enhanced PLA degradation reduces the viscosity of the polymer matrix. Thus, melt intercalation is facilitated which favors clay swelling and subsequent exfoliation. Increasing the screw speed does not produce the larger hydrodynamic stresses necessary for improving clay dispersion. Viscous heating is more important at larger screw speeds. This thermally activates the diffusion of clay platelets in the more degraded PLA matrix, and enhances clay dispersion at early stages of the process. However, progressive clay re-agglomeration occurs along the extruder which results in larger volume fractions of bigger clay particles, eventually leading to PNC with larger shear viscosity as the screw speed is increased.