Preparation And Corona Resistance Of PI/（BNNSs+TiO₂） Nanocomposite Film

Polyimide（PI）film is widely used as an insulating material for power equipment such as inverter-fed motors due to its excellent comprehensive properties.However,with the development of electrical engineering towards high voltage and high frequency,corona discharge occurs frequently in power equipment under high frequency pulse voltage,causing premature failure of polyimide under corona aging.Based on1,2,4,5-benzenetetracarboxylic（PMDA）and 4,4’-oxydianiline（ODA）as monomers,the polyimide nanocomposite films with improved comprehensive properties were obtained by introducing boron nitride nanosheets（BNNSs）and exerting the synergistic effect between nanoparticles.The effects and mechanism of single-component and multi-component synergistic modification on the properties of polyimide were studied.The main research contents and results are as follows:（1）Functionalized BNNSs were prepared by chemical liquid phase exfoliation,hydroxylation modification and silane coupling agent modification treatment,with an average size of 0.5～2μm and a thickness of 2～3 nm.The content of successfully grafted silanes coupling agent reaches 1.3 wt%.At the addition amount of 5 wt%,the mechanical properties,thermal stability,breakdown strength and corona resistance of PI/BNNSs composite films are superior to those of PI/h-BN composite films with pristine h-BN.The tensile strength and breakdown field strength are increased to 103.88 MPa and 185.74 k V/mm,and the corona resistance life is 78.4%and 57.1%higher than that of pure PI film and PI/h-BN composite film,respectively.（2）PI/BNNSs and PI/TiO₂ composite films were prepared by in-situ polymerization and thermal imidization.The nanoparticles in the films were well bonded to the matrix interface,and the BNNSs sheets also presented orientation phenomenon.The research results show that PI/TiO₂ composite film has advantages in mechanical properties,and the tensile strength reaches the maximum value of 108.58 MPa at 10 wt%addition,which is 7.3%higher than that of pure PI film.PI/BNNSs composite film has the advantage in breakdown strength,which keeps133.08 k V/mm at a high addition of 20 wt%,43.2%higher than that of PI/TiO₂ composite film.Both nanoparticles are beneficial to improve the thermal stability and corona resistance of the composite films.The corona resistance lifetimes of the PI/BNNSs composite films and PI/TiO₂composite films reach their maximum value at 15 wt%and 20 wt%,respectively,which are 7.9 times and 9.2 times longer than that of pure PI film.（3）The results show that functionalized BNNSs and TiO₂nanoparticles play a synergistic effect to improve the comprehensive performance of PI/（BNNSs+TiO₂）composite films.The tensile strength of the three-phase composite film reaches the maximum value of 113.52MPa at the addition of 10 wt%,which is 27.5%higher than that of the PI PI/BNNSs composite film at the same addition amount.The breakdown strength at the addition of 20 wt%is 122.03 k V/mm,31.3%higher than that of PI/TiO₂ composite film.The synergistic effect between nanoparticles can promote the formation of corona-resistant microstructure,so that the corona resistance performance of the three-phase composite film is better than that of the two-phase composite film under the appropriate nanoparticle content,and its corona resistance life reached 31.7 min at 15 wt%addition,which was 8.6 times longer than that of pure PI film.