Effect Of Moisture Absorption On The Dielectric Properties Of Polyethylene Based Nanocomposites

The insulation performance of polymer organic polymer materials is the basic guarantee for the reliability of cable operation.Increasing the breakdown field strength of polymer insulation and prolonging the service life of cables have obvious economic value for reducing the manufacturing cost and maintenance cost of cables.Therefore,it has been the research focus in electrical insulation and related fields to study the influence of molecular chain structure and aggregated structure of polymers on their insulation performance,and to control and optimize the aggregated structure of polymers through technical means to improve the insulation performance of polymers.Over the past two decades,researchers at home and abroad have done a lot of experiments on nanocomposite dielectrics.The results show that nano-MMT/LDPE composites have excellent partial discharge resistance and electrical dendritization resistance,which can prolong the service life of power equipment and reduce operation and maintenance costs.Nano-SiO₂/LDPE composites can significantly improve the breakdown strength of low density polyethylene?LDPE?,improve the voltage level of power cables and reduce manufacturing costs.Therefore,these two nanocomposites have great potential application value in the future power grid construction.In this thesis,SiO₂/LDPE nanocomposite and MMT/LDPE nanocomposite were prepared by melt blending,in which surface-modified nano-SiO₂ and organic montmorillonite?MMT?as the nanofillers and semi-crystalline polymer LDPE as matrix.The effects of nano-MMT and nano-SiO₂ on the crystalline morphology,crystal structure and crystallization process of LDPE were studied by atomic force microscopy?AFM?,polarizing microscopy?PLM?,X-ray diffraction?XRD?and differential scanning calorimeter?DSC?.The results showed that nano-MMT did not play the role of nucleating agent and limited the development of LDPE spherulites.The heterogeneous nucleation of nano-SiO₂results in a large number of nuclei and a large number of small spherulites form more crystalline/amorphous interface.In order to explore the application potential of these two nanocomposites in humid environment,the samples with the same thickness were placed in five environments with different relative humidity for the same time,namely,drying,laboratory,constant humidity,impregnation and Hydropower.The effects of different treatments on the broadband polarization,AC breakdown strength and dielectric properties of LDPE,MMT/LDPE and SiO₂/LDPE composites were investigated experimentally.The results show that the dielectric properties of LDPE,MMT/LDPE composites and SiO₂/LDPE composites decrease after moisture absorption at the same time.Especially,the breakdown strength of LDPE decreases seriously after combined action of hydropower and water.The breakdown strength of MMT/LDPE composites and SiO₂/LDPE composites are higher than that of LDPE.