| With the development of the electronic and electrical industry,polymer-based conductive composites have been increasingly used because of their advantages of low cost,easy processing,good formability and adjustable conductivity.The conductivity of the composite material is determined by the conductive fillers’dispersion in the polymer matrix.Three-screw extruder is very suitable for preparing polymer-based conductive composites because of its excellent mixing performance.In this thesis,the mixing characteristics of tri-screw extruder were studied by numerical simulation.Then High-density polyethylene（HDPE）and Polypropylene（PP）were selected as the polymer matrix,and Graphite nano platelet（GNP）was used as the conductive filler,HDPE/GNP and HDPE/PP/GNP conductive composites were prepared by tri-screw extruder.The effects of the raw material ratio,extruder process parameters on the microstructure,electrical conductivity,mechanical properties,and rheological properties of the composites were investigated.Numerical simulation results show that flow rate and shear rate of the flow field in the homogenization section of tri-screw extruder increase approximately linearly with the increase of the screw speed.Shear stress also increases with the increase of the screw speed,but the increament gradually slow down.Dispersion index have no dependence on the screw speed.High shear stress region in the flow field mainly occurs at the screw-to-barrel gap and the meshing area between the three screws.High dispersion index zone appears at the interface between the high shear rate region and the low shear rate region.Increase of simulation processing time and increase of screw rotation speed are favorable for more material particles to experience high shear stress,and the effect of screw rotation speed is more significant.The probability distribution of the particles undergoing the maximum dispersion index distributes towards higher dispersion index with the increase of simulation processing time,but does not depend on the screw rotation speed.HDPE/GNP composites reached the conductivity percolation threshold when the GNP content was about 9wt%,and the electrical conductivity reached the magnitude of 10-5S/cm.When the GNP content was 15wt%,the electrical conductivity reached the magnitude of 10-3S/cm.With the increase of the GNP content,the tensile and impact properties of the composite material are greatly reduced,and the bending performance is continuously improved.Melt shear viscosity increases with increasing GNP content,but the effect is weak at high shear rates.The processing parameters of the tri-screw extruder have a certain influence on the properties of the composites.When the processing temperature is 205°C-225°C,and the screw speed is 80rpm,the electrical and mechanical properties of the composites are relatively good.In the microstructure of HDPE/PP/GNP composites,GNP is preferentially distributed in the HDPE phase which forms a double percolation structure,and the percolation permeation threshold is reduced to about 6 wt%compared to HDPE/GNP composites.With the increase of GNP content,the tensile and impact properties of composites are greatly reduced,and the bending properties are increased first and then decreased.The polymer matrix ratio has a significant effect on the electrical conductivity of the composites.When the mass ratio of HDPE/PP is 50:50,the electrical conductivity reaches the highest.The processing parameters of the tri-screw have a significant influence on the structure and properties of the composites.When the processing temperature is 185°C and the screw speed is 80 rpm,the composite has relatively good conductivity and mechanical properties. |
