The Preparation Of LDPE/EP Blends Filled With CNT, BN Or (CNT+BN) And Their Thermal, Electrical And Dielectric Properties

The polymer materials have characteristics of light weight, good anti-corrosion,outstanding process ability and excellent electrical insulation. By filling various kindsof fillers, the polymer-matrix composites can be achieved with high thermalconductivity, low resistivity, and special dielectric properties, and thus they can be usedin microelectronic industry. In this paper, immiscible blends of low density polyethylene(LDPE) and epoxy resin (EP) were used as matrix and boron nitride (BN), carbonnanotubes (CNT), expandable microsphere (DU40) or their mixture were used as thefiller. The composites were synthesized via mold casting or hot pressing method. Theeffect of LDPE/EP volume ratio, the content of CNT and/or BN, CNT/BN volume ratio,DU40content as well as hot-pressing pressure on the thermal, electrical and dielectricproperties of the composites were systemically investigated. The following conclusionscan be obtained:With the increase in LDPE/EP volume ratio at the same filler content, the thermalconductivity of composites filled with12.5vol.%BN first increase sharply as LDPE/EPratio increases to3/7and then slowly, but the thermal conductivity of composites filledwith35vol.%BN show a decreased trend. At both BN contents, the electrical resistivityof the composites gradually increase but the dielectric constant and loss decrease withincrease in LDPE/EP volume ratio. The thermal conductivity, dielectric constant andloss of the composites filled with CNT first increase and then decrease as the LDPE/EPvolume ratio increases, and they reach maximum at LDPE/EP ratio of8/2,5/5and3/7respectively; the electrical resistivity shows opposite trend with LDPE/EP volume ratioand it reach minimum at LDPE/EP ratio of3/7.At fixed LDPE/EP ratio of3/7and volume fraction of12.5%or35%, the thermalconductivity of the composites first increases and then decreases with the increase inCNT/BN volume ratio, and it reach maximum when the CNT/BN ratio is2/10.5or1/34.With the increasing content of BN and (CNT+BN), the thermal conductivity of the composites first linearly increases and then slightly decreases. Comparing thecomposites with different LDPE/EP ratios, it is found that the thermal conductivity ofthe composites with LDPE/EP ratio of8/2is higher than that of3/7when filler contentis less than20vol.%, but shows the opposite result when filler content is more than20vol.%. Compared with the composites filled with BN, it is found that the thermalconductivity of the composites filled with (CNT+BN) is higher at filler content less than35vol%, but lower at filler content more than35vol.%when the LDPE/EP ratio is3/7.When the LDPE/EP ratio is8/2, the thermal conductivity of composites filled with BNis always higher than that with (CNT+BN). The dielectric constant and loss of thecomposites is increased, but the electrical resistivity is decreased when increase theCNT/BN ratio. With increasing the content of BN、(CNT+BN), the electrical resistivityof the composites is decreased and the dielectric constant is increased.During the preparation of composites, the composites become denser with increasein the hot-pressing pressure, which results in higher the thermal conductivity, dielectricconstant and loss as well as lower electrical resistivity. The electrical resistivity of thecomposites is increased, but the thermal conductivity is decreased when adding DU40.There exists an optimum expansion size and density of DU40, at which DU40can fullyexpand and greatly lower the dielectric properties of the composites.The SEM observations indicate that the thermal, electrical and dielectric propertiesof the composites are closely related to the microstructure of the composites, especiallythe thermal conductivity and electrical conductivity. When adding a small amount ofimmiscible LDPE into the EP matrix, the filler can easily contact each other even atlower fillers, which can improve the thermal and electrical properties of the composites.When the filler content is high, the filler distributes in the matrix aggregately and thethermal and electrical paths have formed, and thus the thermal and electrical propertiesof the composites can not be effectively improved by adding immiscible LDPE into EPmatrix. The thermal conductivity of the composites is decreased at higher content ofCNT due to aggregation of the CNT in the matrix and formation of more interface resistance between CNT and matrix.