Preparation And Properties Of Polyethylene Based Organic/Inorganic Composites

Cross-linked polyethylene?XLPE?is widely used in the manufacture of high-voltage direct current?HVDC?transmission cables because of its excellent insulation property,stable mechanical property,high heat resistance and light weight etc.Recently,it has been reported that the various properties of XLPE can be improved effectively by adding inorganic nanoparticles?NPs?into the PE matrix.In this thesis,composite films with different NPs were prepared by solution casting method,and their thermal and dielectric properties as well as breakdown strength were investigated under the influence of the different proportion of NPs.The specific research results are listed as follows:?1?Organic montmorillonite?OMMT?was modified with octadecyltrimethylammo-nium bromide.The results show that the diffraction angle of the characteristic crystal plane move to smaller angle,the interplanar spacing is enlarged,the interlayer space increases obviously and the MMT layer become loose after modification.Then,the OMMT/XLPE composite films have been prepared by solution casting method,and OMMT keeps intact and disperses well in the PE matrix.Differential Scanning Calorimetry?DSC?and Thermo-gravimetric Analysis?TGA?exhibit that the addition of OMMT has little effect on the crystallinity and melting temperature of composite films.Especially,the thermal stability of the composite films increase with the increase of OMMT at a weight loss of 5%.Dielectric constant and dielectric loss characterization shows that the dielectric constant of OMMT/XLPE composite films is lower than the pure XLPE film when the content of OMMT is 0.5 or 1.0 phr,and the dielectric loss of composite films is slightly increased.By characterizing the DC breakdown field strength and dielectric properties of different OMMT/XLPE composite films,the breakdown strength of the composite films exhibits a trend of rising first and then falling,and which is higher than the pure XLPE film when the content of OMMT is 0.5 or 1.0 phr,the breakdown field strength reaches 440.9 kV/mm when OMMT is 0.5 phr,which is 19.2%higher than the pure XLPE film.?2?The Al₂O₃/XLPE composite has been prepared by solution casting method.It is found that the peaks of Al₂O₃ NPs and Al₂O₃ nanofibers are sharp and there are no impurity peaks,which are highly consistent with the peaks of the?-Al₂O₃ standard map in XRD spectra.Moreover,two kinds of Al₂O₃ both have excellent dispersibility in the PE matrix,and the addition of alumina has little effect on the crystallinity and melting temperature of the composite film.With increase of Al₂O₃,the thermal decomposition temperature and the dielectric constant of composite films both have been increased.Compared with OMMT/XLPE composite films,the dielectric loss of the Al₂O₃-doped composite films is much smaller.When the content of Al₂O₃ NPs reaches 1.0 phr,the breakdown strength of Al₂O₃/XLPE film reaches 448.2 kV/mm,which is 21.2%higher than the pure XLPE film.Nevertheless,the breakdown strength of the composite film can reach 468.3 kV/mm when the content of Al₂O₃ nanofiber is 2.0 phr,which is higher than the NPs.?3?The bulk boron nitride has been peeled off to form boron nitride nanosheets?BNNS?by liquid nitrogen without changing its crystal structure.The BNNS/XLPE composite films were prepared by solution casting method.DSC and TGA analysis shows that the addition of BNNS to the PE matrix has less effect on the crystallinity and melting temperature of the composite films,and the thermal stability of the composite films is improved.The dielectric constant and dielectric loss characterization shows that the dielectric constant of composite films decreases slightly when the content of BNNS is low,the dielectric loss of composite films increases slightly with the increase of inorganic fillers.Moreover,the breakdown strength of BNNS/XLPE composite film reaches 488.6 kV/mm when the content of BNNS is 3.0 phr,which is 32.1%higher than the pure XLPE film.?4?On the basis of the above results,ternary composite film Al₂O₃/BNNS/XLPE has been synthesized by compounding Al₂O₃ nanofiber and BNNS at a mass ratio of 2:3 with XLPE.This ternary composite films exhibit an excellent thermal stability and its crystallinity and melting temperature only has a little change.Meanwhile,the dielectric constant and dielectric loss of Al₂O₃/BNNS/XLPE both increase slightly with the increase of the content of inorganic NPs.In addition,the breakdown strength of composite film can reach 502.5kV/mm when the content of compounded NPs is 2.0 phr,which is 35.9%higher than the pure XLPE film.In short,the breakdown strength of ternary composite film Al₂O₃/BNNS/XLPE is indeed superior compared with the binary composite and the pure XLPE.