Promoting Dispersion Of Expanded Graphite In Polypropylene By Water-assisted Mixing Extrusion And Its Mechanism

In this dissertation,the mixing index of melt in assembled screw element in twin screw extruder was numerically simulated.The flow state of water and melt in the barrel was observed by the visualization system to analyze the process of droplet deformation.The thermal conductivity,microstructure,crystallization behavior,rheological properties and electrical properties of polypropylene(PP)/expanded graphite(EG)composites prepared by WAME were investigated and the relationship of “rheological properties–microstructure–properties” was analyzed.The mechanism of WAME promoting exfoliation and dispersion of EG and then enhancing thermal conductivity of prepared PP/EG composites was analyzed.The numerical simulation result shows that the injected water during WAME increases the mixing index of melt in assembled screw element.The visual observation shows that under the shear mixing of the screw,the injected water is stretched,deformed and refined into water droplets,finally dissolves into the PP melt.Compared to PP/EG composites prepared by common mixing extrusion,PP/EG composites prepared by WAME exhibit higher thermal degradation temperature,crystallinity,elastic modulus,complex viscosity,storage modulus,Young's modulus and dielectric permittivity.This may be attributed to the enhanced interaction between the PP molecular chains and EG layers,and improved exfoliation and dispersion of EG in PP matrix during WAME.Better EG exfoliation and dispersion,stronger interaction between PP molecular chains and EG layers,and higher crystallinity facilitate to enhance the thermal conductivity of PP/EG composites.PP/EG composite with 15 wt% EG prepared by WAME has a thermal conductivity of about 1.42 W/m K,which is about 56% and 576% higher than those of PP/EG composite(0.91 W/mK)with 15 wt% EG and PP(0.21 W/mK)prepared by common mixing extrusion,respectively.