Preparation And Properties Of Thermally Conductive And Electrically Insulating Polypropylene Composites

In recent years, thermally conductive and electrically insulating polymer composites havebeen used to gradually replace the traditional thermally conductive materials or componentparts in the fields of electrical and electronic engineering due to the inherent excellentelectrical insulation, resistance to chemical corrosion, ease of processing and high specificstrength of polymers. The addition of suitable thermally conductive fillers is a simple andfeasible way to prepare polymer-based thermally conductive and electrically insulatingmaterials, which shows a great prospect for industrial applications.Polypropylene (PP) was used as a matrix in this work. Based on the difference betweenthe conduction mechanisms of heat and electricity, the percolation phenomenon associatedwith electrical conductivity in PP composites filled with natural flake graphite (C) having ahigh thermal conductivity was studied to determine the proper content of graphite in PP/Ccomposites. Using boron nitride (BN) and glass flake (GF) as thermally conductive andelectrically insulating fillers and polyethylene-octene elastomer (POE) as a tougheningadditive, the effects of the compositions of PP/C/BN, PP/C/GF and PP/POE/C/BN compositesystems were then investigated on their morphological structures, thermally conductive andelectrically insulating characteristics, mechanical properties, flowability and thermal stability.The studies will lay the foundation for utilizing the hybrid filler system containing a highlythermally and electrically conductive filler in developing polymer-based thermally conductiveand electrically insulating composites.According to the experimental results, the thermal conductivity of PP/C compositesincreases steadily and not abruptly with increasing graphite content. The volume resistivityremains above2×1015·cm for the PP/C composites containing less than27wt%graphiteand decreases dramatically for the PP/C composites containing more than27wt%graphite,showing a percolation phenomenon. Therefore, the graphite content of27wt%is themaximum amount of graphite in the PP/C composites that makes PP still possess a goodelectrically insulating property. The thermal conductivity of the PP/C composite containing27wt%graphite is0.858W/m·K1and still a good electrically insulating material. Based on the above results, the PP/C/BN and PP/C/GF composites containing27wt%graphite were prepared, respectively; their thermally conductive and electrically insulatingcharacteristics, mechanical properties, flowability and thermal stability were characterized;and the effects of BN and GF content on these properties were comparatively analyzed. Thethermal conductivity of PP/C/BN composites increases steadily and the correspondingvolume resistivity exhibits a slow downward trend with an increase in BN content. Thethermal conductivity of PP/C/GF composites increases with an increase in GF content onlywhen it is higher than15wt%, otherwise changes little; and their volume resistivityincreases to some extent with increasing GF content, but can exceed1.3×1015·cm whenGF content is less than25wt%. By comparison, the thermal conductivity of the PP/C/BNcomposites is greater than the corresponding PP/C/GF composites, but their volumeresistivity is lower than the corresponding PP/C/GF composites. As for the mechanicalproperties of both composite systems, the addition of BN and GF will cause a decrease. ThePP/C/BN composite containing25wt%BN exhibits good comprehensive properties. Itsthermal conductivity and volume resistivity can reach1.541W/m·K1and1.0×1014·cm,respectively; its tensile strength, strength, elongation at break, flexural strength and theimpact strength are25.0MPa,5.8%,37.1MPa, and1.67KJ/m2, respectively.In order to improve the machnical properties of PP/C/BN composites filled with a largeamount of fillers, POE was used as a toughening additive and the toughening effect of POE inthe PP/POE/C/BN composite system was investigated under the condition that graphitecontent and BN content were27wt%and25wt%, respectively. The results shows that theelongation at break and impact strength of PP/POE/C/BN composites increase obviously onlywhen the POE content is higher than10wt%; and that their tensile strength and flexuralstrength decrease with increasing POE content. The addition of POE has little impact on thethermal conductivity of PP/POE/C/BN composites, but resulting in a decrease of their volumeresistivity. For the PP/POE/C/BN composite containing15wt%, its tensile strength,elongation at break, flexural strength and impact strength are18.1MPa,11.7%,15.6MPa and2.86KJ/m2, respectively; its thermal conductivity and volume resistivity are1.452W/m·K1and2×1012·cm, respectively, implying it still possesses acceptable electrical insulation formany applications.