Influence of processing parameters during ultrasound assisted extrusion on the properties of polycarbonate/carbon nanotubes composites

Polycarbonate (PC)/carbon nanotube (CNT) composites containing 0.4 to 3 wt% CNT were prepared using an ultrasound assisted twin screw extruder at two different flow rates. The CNTs used in this study were multi-walled carbon nanotubes with trade name Baytubes C150P, a material which had relatively high bulk density and therefore was easy to handle in industrial production. The effects of flow rate and ultrasonic amplitude on the thermal stability of PC extrudates and PC/CNT composites were studied. The electrical, morphological, rheological and mechanical properties of the untreated and ultrasonically treated composites were characterized. Results showed that the difference of thermal stability between different PC extrudates was very small, while the incorporation of CNT significantly decreased the thermal stability of PC. During the extrusion of PC/CNT composites, both the shearing forces caused by rotating screws and the bubble cavitation caused by ultrasound helped dispersing CNT into the polymer matrix. At low flow rate, CNT can be well dispersed, but this induced more degradation of polymer matrix. At high flow rate, the dispersion of CNT was not as good as that at low flow rate. With respect to this observation, ultrasonic treatment showed superior capability and efficiency in improving the dispersion and distribution of CNT in a short treatment time and increasing the throughput at the same time which is especially important and beneficial for industrial production of polymer/nanofiller composites. At high flow rate with increasing ultrasonic amplitude, the electrical percolation threshold of the composites decreased, while the storage modulus and complex viscosity generally increased. At low flow rate, ultrasonic treatment showed an improvement of the Young's modulus, yield stress and elongation at break.