Study On Preparation And Properties Of Polypropylene/Flake Graphite Thermal Conductive Composites

As one of the core components of falling film evaporator with horizontal tubes,horizontal evaporation tube plays an important role in the multi-effect distillation seawater desalination technology.However,most of the commonly used horizontal evaporation tubes are made of metal materials,and there are lot of problems such as corrosion,scaling and high maintenance costs,which limit the development of seawater desalination technology.Polymer-based thermal conductive composites have become ideal materials to replacing metal on seawater desalination application because of their corrosion resistance,scale inhibition and easy processing.In this paper,polypropylene(PP),with strong corrosion resistance,good processability and low price,was used as a matrix resin,and flake graphite(FG),with high thermal conductivity and wide source,was used as the thermal conductive filler.Firstly,the composite material was prepared by hot pressing process with a closed mold.The effects of the coupling agent species and loading amount,the particle size and content of the FG,and the PP species on the properties of composites were studied.Secondly,the pressing injection mold and injection mold with special structure were designed and made for the study.By changing the flow mode of the melt in the mold to change the orientation of the FG,the thermal conductivity of the composites in vertical direction was improved significantly.The effect of the flow of materials in the mold,the FG loading amount and the injection condition on the thermal conductivity of the composites were thoroughly studied.The results were as follows:1)The analysis on the composites made by hot pressing showed that the coupling agent could reinforce the mechanical properties of the composites to a certain extent,but the thermal conductivity of the composites would decrease slightly.There are the best mechanical properties of the composites when 0.7 wt%of titanate coupling agent NDZ-201 was added.Differential scanning calorimetry(DSC)and thermogravimetric analysis(TGA)analysis showed that the thermal stability of the composites with NDZ-201 was better than that of the composites without coupling agent.With the increase in FG particle size,the thermal conductivity of composites increased obviously,but the mechanical properties decreased correspondingly.As the increase in the FG loading amount,the thermal conductivity of the composites increased,and after FG content reached 50 wt%,the thermal conductivity increased rapidly.Tensile modulus and flexural modulus also increased with the increasing in FG loading content,while the tensile strain and flexural strain at break decreased,and the tensile strength and flexural strength first decreased and then increased.When the loading amount of FG with 17?m reached 70wt%,the average thermal conductivity of the composites was 22.1 times that of pure PP;Tensile strength and flexural strength were 36.1 MPa and 59.0 MPa,respectively.Tensile modulus and flexural modulus were 10.7 GPa and 13.7 GPa,respectively.Tensile strain at break and flexural strain were 0.32%and 0.49%,respectively.The different PP mixture could effectively improve the toughness of composite materials.2)The analysis on the composites made by pressing injection process showed that it can effectively improve the vertical thermal conductivity(perpendicular to the material surface direction)of composites,but the parallel thermal conductivity(parallel to the material surface direction)decreased,which was due to the melt with folding structure emerged through the special gate of the mold,and then melt with folding structure were further compacted to make FG have higher vertical orientation.In addition,with the increasing in FG loading,the thermal conductive path in the composites became more perfect.SEM and XRD analysis showed that composites made by pressing injection process had higher vertical orientation than that of hot pressing process.The analysis of thermal expansion showed that the average linear expansion coefficient of pressing injection materials is smaller than that of molding composites,further indicating that FG has different orientations in composites.When FG loading amount was increased to 75wt%,the vertical thermal conductivity and parallel thermal conductivity of the composites were 16.6 W·m^-1·K^-1 and 5.89 W·m^-1·K^-1,respectively,while those of composites made of hot pressing process were only 1.35 W·m^-1·K^-1and 26.9 W·m^-1·K^-1,respectively.3)The analysis on composites made by injection molding process showed that it also improved the vertical thermal conductivity of composites,due to the melt with folding structure emerged through the special gate,which improved the vertical orientation of FG.SEM and XRD analysis showed that the vertical orientation degree of the injection molding composites were greater than the horizontal orientation degree,and the vertical thermal conductivity of injection molding composites were the highest with same FG loading amount.Thermal expansion analysis showed that the average linear expansion coefficient of the injection molding composites were the smallest,further indicating that the injection molding composites had a high vertical orientation.With the increase of injection speed,the vertical thermal conductivity decreased linearly,while the parallel thermal conductivity increased linearly.When the injection speed was 20 g/s,the vertical thermal conductivity and parallel thermal conductivity of the composites were 18.4W·m^-1·K^-1 and 2.85 W·m^-1·K^-1,respectively.The vertical thermal conductivity of the composites decreased to 11.4 W·m^-1·K^-1,the parallel thermal conductivity increased to4.32 W·m^-1·K^-1as the injection speed increased to 90 g/s.