Investigation On The Thermal Properties Of Polypropylene/graphene Nanoplatelets Composites

Polymer-based graphene nanoplatelets composites can be used as a heat dissipation material,and in the process of production,the performance of composites is influenced greatly by the exfoliation and the distribution of GNP in polymer matrix,because the GNP such layered nanometer piece has structured two-dimensional layer board structure.The atoms in the layers are bound together by strong covalent bonds,and the layers are combined with van der Waals force and electrostatic force interaction,this unique structure makes the GNP easy to aggregate.This layered aggregation is not beneficial to its own excellent performance such as high electrical and thermal conductivity,but easily leads to defects in polymer-based nanocomposites and reduces the properties of the composites.On the contrary,if GNP can be uniformly distributed in the polymer matrix in the form of thinner sheets,when the content of GNP increases to a certain value,it can form a filler network in the polymer matrix,and then obtain excellent properties.In order to realize the enhancement of the properties of the polyethylene filled GNP,one of the key problems is how to control the exfoliation and distribution of GNP in the polyethylene.Therefore,this paper investigated the relationship between the morphology of GNP and the properties of PP/GNP composites,the effects of fillers structure on the thermal conductivity of PP/GNP/BNF composites,and the exfoliation of GNP under the elongational flow field.Scanning electron microscope,optical microscope,transmission electron microscope,differential scanning calorimeter,X-ray diffraction,rheological behaviors,electric properties and thermal properties are carried out to analyze the relationship among processing,morphology and properties.The specific contents are as follows:(1)Polypropylene(PP)nanocomposites reinforced with different types of graphene nanoplatelets(GNP)and their hybrid systems are prepared via melt extrusion.On the basis of experimental analysis and simulation,the factors of thermal property of the PP/GNP nanocomposites,including GNP size,weight filling ratio and proportion of various sizes,are systematically investigated.At high GNP content(9wt%,12wt%),GNP are widely distributed in PP matrix and the thermal paths are basically formed.The thermal conductivity of composites is determined by the size and thermal properties of GNP.At low GNP content(6wt%),for single system,the larger diameter with moderate distribution would be more conducive to achieve the higher thermal conductivity,indicating the formation of thermal paths dramatically affects the thermal conductivity.For hybrid system,the PP filled with medium and small diameter GNP obtains the highest thermal conductivity at the ratio of medium diameter GNP to small diameter GNP is 8:2,and is 23.8%higher than the single system of PP filled with small diameter GNP.More precisely,the small diameter GNP plays a role in connecting the scattered medium diameter GNP,as mass thermal paths are formed.This shows that the distribution state by combining the synergistic effect of various GNP significantly affects the thermal conductivity of PP/GNP nanocomposites.Moreover,a numerical simulation dealing with the synergistic effect of different GNP,is developed on the thermal conductivity of GNP-reinforced PP matrix.The heat flux images demonstrate the existence of synergistic effect between different type of GNP.(2)Highly thermally conductive but electrically insulating polymer composites play an increasingly important role in thermal management applications due to their easy processing and lightweight.Herein,a unique segregated network by using graphene nanoplatelets(GNP)and boron nitride fibers(BNF)as fillers,was constructed in polypropylene(PP)-based composites via two-screw extrusion.And the high thermal conductivity with electrical insulation of the composites could be well controlled by adjusting the content and size of GNP and BNF.When PP was filled with 9wt%GNP and 30wt%short BNF,the highest thermal conductivity of 1.32 W/m K was achieved in this work,about 6 times compared to neat PP,while remained a good electrical insulating(as low as 2.98×10^-9 S/m).It proposed that the unique segregated network,where the BNF play as a bridge to connect the scattered GNP,could benefit for phonons transmission but effectively interrupt electron conduction.Our work provides a facile method to design and fabricate the highly thermally conductive but electrically insulating composites applied in thermal management materials.(3)Three extrusion dies were designed to produce elongational force fields.The microscopic morphology,electrical and thermal conductivity of polypropylene(PP)/graphene nanoplatelets(GNP)nanocomposites were studied.The effect of the distribution of GNP in PP matrix on the performance of composite materials was investigated.The experimental results show that the elongational force field generated by the convergent flow channel has a peeling and dispersing effect on GNP and reduces the agglomeration of GNP,which is beneficial to construct an electrically conductive and thermally conductive network,and improving the electrical and thermal conductivity of the composite.When the GNP content was 13wt%,the in-plane thermal conductivity of PP/GNP composites were increased by103.3%in the extrusion dies of J3 compared to the extrusion die J0.