Investigation On Morphology Of Nano-montmorillonite/Polyolefin Composites And Propagating Mechanism Of Electrical Tree

Electrical tree is a common phenomenon of electrical ageing in polymerinsulation, which limits the service life and reliability of insulation and becomesa noticeable factor restricting the development of high voltage electric powerequipments. The phenomenon has been being researched for more than half acentury, and mechanism of electrical tree and the inhibition are always focusedby engineering and scientific of dielectric. The electrical tree resistance ofpolymer could be ameliorated by optimizing insulation structure and improvingprocessing technology. With the development of nanotechnology, many newresearch ideas and methods have been proposed to improve the dielectricproperties and electrical tree resistance of polymer insulating material.In this study, nano-montmorillonite/polyolefin composites (MMT/PE andMMT/PP) are prepared, in which the commonly polyolefin materials ofpolyethylene (PE) and polypropylene (PP) are taken as the matrix, and layeredsilicate nano-montmorillonite (MMT) is taken as nano-filler. The scanningelectron microscopy (SEM) is used to observe the dispersion of MMT in polymermatrix, and chemical structure of organic MMT and composites are check byFourier transform infrared Spectrometer (FTIR). The results show that modifiedMMT is dispersed homogeneously in matrix with nano-scale and reacted withpolymer by coupling agents. The crystal morphology, crystallinity andcrystallization are analyzed by polarized light microscopy (PLM), X-raydiffraction (XRD) and differential scanning calorimetry (DSC), and the resultsshow that MMT adding improves the spherulite structure and crystallinity of PEand PP. The effect of MMT addition on microstructure of semi-crystallinepolymer is demonstrated by crystal structure and filler dispersion of composites. The effect of crystal and interfacial compatibility on electrical tree growth isresearched by analyzing electrical tree characteristics of PE, PP and theircomposites. The spherulites scale decreases and crystallinity increases obviouslyin MMT/PE composite which is prepared by melt intercalation method and with0.5wt%MMT content, and electrical tree resistance of this composite isimproved by improvement of crystalline structure. To investigate the interfacialcompatibility effects on electrical tree growth, electrical tree characteristics ofMMT/PE prepared by solution and melt intercalation and MMT/PP prepared byone-step and two-step melt intercalation are researched. The action of couplingand compatibility agent is promoted respectively in MMT/PE prepared bysolution intercalation and MMT/PP prepared by two-step melt intercalation, andelectrical trees performance is improved significantly in those composites byexcellent interfacial compatibility.The box-counting fractal dimension of electrical tree is calculated by fractalanalysis method, and represents the morphological development electrical treequantitative. The dynamic model is modified based on discharge-avalanchetheory and fractal feature combined with the relevance between crystallinity andelectrical tree growth, and the dynamic mechanisms of electrical tree propagatingin semi-crystalline materials is represented. The consistency of calculation resultsand experimental results present that modeling method could be applied toanalyzing treeing process in semi-crystalline polymer quantitative. The localelectric field of semi-crystalline polymers is simulated according to physicalcharacteristics of spherules structure. The thermodynamic equation is establishedbased on thermodynamic driven action of phase interfacial free energy andelectric field distribution combine with effect of interfacial compatibility onelectrical tree growth. An electrical tree develops along the crystalline interfacein semi-crystalline polymer is confirmed based on the analysis of thermodynamiccharacteristics and observation of tree channel details. According to the dynamicand thermodynamic characteristics of electrical tree propagating in semi-crystalline polymer combining with structure characteristics of nano-montmorillonite/polyolefin composites, the inhibiting mechanism of electricaltree is revealed by barrier MMT layer and crystallization change.The conductive characteristics before and after electrical tree initiation is conducted and the effect of channel conductance on treeing-polymer is analyzedby dynamic process of electrical trees in semi-crystalline polymer combine withcurrent measured before and after electrical tree initiation.The equivalent circuitmodel of treeing polymers is established and the distribution of carriers in treechannel is analyzed according to the changes of Charges and conductive currentbefore and after electrical tree generated.