Influence of screw configuration, residence time, and specific mechanical energy in twin-screw extrusion of polycaprolactone/multi-walled carbon nanotube composites

Melt processing of thermoplastic-based nanocomposites is the favoured route to produce electrically conductive or electrostatic dissipative polymer composites containing carbon nanotubes (CNT). As these properties are desired at low filler fractions, a high degree of dispersion is required in order to benefit from the intrinsic CNT properties. This study discusses the influence of screw configuration, rotation speed, and throughput on the residence time and specific mechanical energy (SME) and the resulting macroscopic CNT dispersion in polycaprolactone (PCL) based masterbatches containing 7.5 wt.% multi-walled carbon nanotubes (MWNT) using an intermeshing co-rotating twin-screw extruder Berstorff ZE25.The processing conditions were found to have a strong influence on the residence time (tR) of the extrudates and on the CNT dispersion within the masterbatches as assessed using light microscopy. Both, an increase of rotation speed and throughput resulted in a decrease of tR whereas the use of back-conveying elements and the extension of the processing length showed the opposite effect. As the increase of rotation speed results in higher SME inputs a significant increase of CNT dispersion was found, whereas an increase of throughput resulted in worse dispersion. Beside these machine parameters, the design of the screw can further promote the CNT dispersion especially when using distributive screw configurations containing mixing elements. The lowest area fraction of undispersed primary MWNT agglomerates within PCL masterbatches was found when using an extended distributive screw having a length to diameter ratio L/D of 48 instead of 36. The few remaining macroscopic agglomerates could be entirely dispersed in a subsequent masterbatch dilution process resulting in a very low electrical percolation threshold of 0.24 vol.% MWNT.