Construction Of Low Percolation Threshold Flake Graphite Doped Polymer Conductive Composite System And Regulation Of Its PTC Performance

Conductive polymer composites（CPCs）composed of an insulating polymer matrix and conductive fillers are considered to be a promising multifunctional material,with very excellent electrical conductivity,thermal conductivity and mechanical properties,and are widely used in many engineering applications.Among them,the composites exhibit an insulator-conductor mutual transformation,which is related to the temperature dependence of resistivity.According to the temperature coefficient,it is divided into the positive temperature coefficient（PTC）effect,which means that the resistivity of the composite material varies with the temperature rises and increases,and the negative temperature coefficient effect（NTC）effect,it means that the resistivity will decrease as the temperature rises.Among them,polymer-based PTC materials with excellent performance have been widely used in fields such as self-regulating heaters,overcurrent,overheat protection,fire alarms,and micro switch sensors.At present,most PTC materials or NTC materials use carbon nanotubes（CNTs）,carbon black（CB）and carbon fibers（CFs）as conductive fillers,which are relatively expensive.There are few reports on conductive polymer composites that use flake graphite as conductive fillers and have a low percolation threshold.So can the composites with low cost,low percolation threshold and excellent PTC performance be prepared?In order to explore the above questions,this article mainly uses flake graphite as conductive fillers,and uses two crystalline polymers as double matrixes to construct different microstructures and form different conductive networks to study and regulate the PTC effect.It mainly includes the following two aspects:（1）In this work,polyurethane（TPU）and ultra-high molecular weight polyethylene（UHMWPE）were used as polymer matrix,flake graphite was used as conductive filler,and melt blending method was used to construct TPU/UHMWPE-FG conductive polymer composites with isolation structure by controlling temperature and mixing sequence.The percolation threshold of the TPU/UHMWPE-FG composite is as low as 1.89 wt%,and it has high electrical conductivity.When the flake graphite loading is 10 wt%,the electrical conductivity of the composite is 10^-1 S·m^-1.Observed by electron microscope（SEM）and polarized light microscope（PLM）,it is found the flake graphite is distributed in the TPU phase with good dispersibility,and the TPU-FG phase is uniformly coated on the surface of the UHMWPE spherical particles;and temperature-resistance effect of the composites was tested,the remarkable PTC effect of the TPU/UHMWPE-FG composites could be observed,and the PTC strength reaches 5 orders of magnitude.At the same time,after 5cycles,the PTC of the composite has excellent repeatability.In summary,this work basically realized the preliminary idea of preparing PTC materials with low percolation threshold using graphite as conductive filler.（2）In the previous work,although the design of composites with the PTC effect with graphite as a conductive filler was successfully realized by constructing an isolation structure,the percolation threshold of the TPU/UHMWPE-FG composites（1.89 wt%）is still too high,and the onset temperature of the PTC effect is low,resulting in a narrow application range.Therefore,in this chapter,high-density polyethylene（HDPE）and polypropylene（PP）were used as the polymer matrix,and FG was used as conductive fillers to prepare HDPE/PP-FG composites by mechanical blending and melt blending.It was found that the percolation threshold（7.02 wt%）of the R-HDPE/PP-FG composites prepared by melt blending was very high.Observed by SEM and PLM,it was found that FG was dispersed in both phases.Although the composites also has the PTC effect,its intensity（PTCI=2.12）is low,which leads to its poor applicability.Observed by SEM and PLM,it is found that the J-HDPE/PP-FG composites prepared by mechanical blending has a bi-continuous structure,and the graphite flakes are evenly distributed in the HDPE phase and the HDPE and PP phases,achieving good electrical conductivity and ultra-low percolation threshold（1.33 wt%）.At the same time,excellent PTC effect and excellent PTC effect repeatability were also observed in the J-HDPE/PP-FG composites,and the PTC intensity was as high as 6.57 orders of magnitude.In addition,the onset temperature of the PTC effect of the J-HDPE/PP-FG composites has reached about 125°C,which has also been greatly improved and has been more widely used.