Electrical Study On Conductive Multiphase Polymer Composites Filled With Graphite

Polymeric conductive polymer composites can reduce or switch off the electriccurrent at high temperature environment; it can play important part in thermalinsulation or achieve the purpose of over-current and overvoltage protection. Theconductive polymer composites has been widely used in the field of circuit overloadprotection device、automatic temperature-control heating cable、pressure sensitivesensor and electromagnetic shielding materials. But nowadays many conductivemechanism and practical problem of conductive polymer composites is still notsettled，which main focus on the high volume resistivity and the stability of theconductive polymer composites. These shortcomings have restricted the furtherindustrial application and promotion of the conductive polymer composites, how toreduce the conductive filler content while improving electrical properties of materialshas become popular in recent years.This experiment used high density polyethylene (HDPE) and polypropylene (PP)as the substrate material, the naturally abundant cheap graphite was also used as fillermaterial, it can succeed prepared polymeric conductive polymer composites by meltblending. The objective of present study was to investigate the effects of the addingpolypropylene (PP) component to the conductive property and the morphology of theGP/HDPE/PP conductive composites. The dispersed state of GP、volume resistivity ofcomposites、resistivity-temperature properties and thermal cycling stability werediscussed. The conducting mechanism and the best processing technology were alsostudied. The main works and conclusions were as follows:(1) According to volume resistivity testing, the addition of PP into theGP/HDPE conductive composites can significantly decrease volume resistivity of theconductive polymer composites. The volume resistivity of GP/HDPE/PP conductivepolymer composites decreased by l-4orders of magnitude compared to GP/HDPEconductive composites at the same filler content. When the mass proportion of HDPE/PP was1:1, the conductivity of the composite was better than others at samegraphite content.(2) According to SEM observation, graphite particles were dispersed in HDPEmatrix homogeneously at GP/HDPE conductive composites，but some places did notform continuous structure. In GP/HDPE/PP conductive composites, graphite particleswere selectively dispersed in HDPE matrix mostly, only a small amount of graphiteparticles were dispersed in the PP matrix. The double-percolated network was formeddue to two-continuous phases of the PP and HDPE matrix.(3) According to resistance-temperature testing, PTC intensities of highcrystalline GP/HDPE conductive composites were larger than other composites.Compared to GP/HDPE conductive composites, PTC intensities of GP/HDPE/PPconductive composites changed little; PTC-NTC transition temperature becamehigher. The resistance-temperature curve became flat.(4) According to electrical properties stability testing, Room temperatureresistivity significantly decreased about1.5orders of magnitude after several heatingcycle of GP/HDPE system, the reproducibility was poor and PTC intensity becamesmall. Volume resistivity of GP/HDPE/PP conductive composites decreased0.5orders of magnitude after several heating cycle. Oxidation treatment and nucleatingagents can improve electrical properties stability of GP/HDPE/PP conductivecomposites.(5) According to processing technology testing, it was important to the electricalproperties of conductive polymer composites. The best melting temperature ofGP/HDPE/PP conductive composites was from180℃to190℃; The best meltingtime was about5min;The best melting sequence was（GP+HPDE）+PP. Slow coolingprocess can improve volume resistivity of the conductive composites.(6) According to heat treatment technology testing, when the samples weretreated at80℃, the volume resistivity of all the composite system were showing arising trend, With the increase of heat treatment time, it raised slowly.GP/PPconductive composites changed bigger than other conductive composites; when the samples were treated at120℃, the volume resistivity of GP/HDPE conductivepolymer composites significantly decreased0.7orders of magnitude，other conductivecomposites changed little. The volume resistivity of GP/HDPE/PP conductivecomposites decreased0.3orders of magnitude. The best heat treatment temperaturewas at120℃.