Study On Modification And Properties Of Polypropylene Dielectric Material

Polymer film capacitors play a critical role in fields such as advanced power electronic systems,pulse power technology and new energy vehicles due to advantages involving ultrahigh power density、high voltage resistance and low loss.However,the most commercialized polymer film capacitors consist of biaxially-oriented polypropylene(BOPP)film as the dielectric material,of which low energy storage density suffers from low dielectric constant(only 2.2).In recent years,with the rapid development of advanced electronic devices and power systems,the demand for polymer-based dielectric film materials with high energy storage capacity get increasingly urgent.Therefore,this thesis combinates theoretical simulation and experimental characterization to study how to modify polypropylene dielectric materials for improving comprehensive properties.The main research contents and results are as follows:(1)Through theoretical simulation,the optimal content of barium titanate(BT)nanoparticles filled in the experiment is revealed as below 37.79 wt%volume fraction.Experiments exhibit uniform dispersity of BT filled in the polypropylene(PP)matrix below this content.A certain content of BT nanoparticles added to PP by melt blending method improves the dielectric constant of the composites.In the BT/PP two-phase dielectric composite before biaxially stretched,at frequency of 1 kHz and BT content of 30 wt%,the dielectric constant of the composite film is about 3.44,which presents an increase of 52.89%from 2.25,that of pure PP.(2)The problems caused by the addition of excess BT,including inorganic-organic twophase interface defects and poor film formation,can be suppressed by introducing the third phase thermoplastic elastomers ethylene-propylene-diene rubber(EPDM)and styreneethylene-butylene-styrene block copolymer(SEBS).It is found that the dielectric properties,breakdown properties,mechanical properties and energy storage properties of the composites can be improved by introducing thermoplastic elastomers.Eventually this work obtains a PPbased dielectric composite material with excellent energy storage properties and mechanical processing properties.For example,in the SEBS/BT-30 wt%/PP three-phase dielectric composite,when the SEBS content is 16 wt%and the electric field strength is 200 MV/m,the discharge energy density is 0.76 J/cm3,higher than 0.49 J/cm3 of the pure PP,that is improved by 55.10%.(3)By comparing the properties of the composites before and after biaxial stretching,it is found that biaxial stretching has an important influence on the microstructure,dielectric,breakdown,energy storage,thermodynamics and other properties of the material.Taking pure PP as an example,the melting point of unstretched pure PP is 162.80℃,while the film biaxially-stretched by 2.2 times reaches 169.19℃.In the BT/PP two-phase composite,when the BT content is 10 wt%,the breakdown strength of the unstretched composite is 357.28 MV/m,and that after biaxial stretching is 452.41 MV/m.When the BT content is 30 wt%,the unstretched composite has a crystallinity of 34.64%,and approaches 52.61%after biaxially stretching.(4)BT/PP,EPDM/BT-30wt%/PP and SEBS/BT-30wt%/PP composite dielectric materials are successfully prepared by melt blending and pressing combined with biaxiallystretching process.This solvent-free method advances simpleness and easiness to implement,resembles the current industrial production method of polymer dielectrics,and provides a basis for subsequent large-scale industrialization.