Preparation And Properties Of Functional Polyimide Films Composited With Nano Particles

Polyimide（PI）is a high-performance engineering plastic with an aromatic heterocyclic structure,which endows it with excellent thermal stability,insulation and good mechanical properties.Because the motor winding needs to withstand long-term high-frequency pulse impact,the electric field strength is high,and the reliability of insulation directly affects the operation of the motor.In the past two decades,with the progress of science and technology in China,the technical level of high-speed railway motor cars has rapidly improved.At the same time,there are many new technical problems in the development of electrical equipment.The variable frequency speed regulation motor is one of the core components of the whole high-speed railway EMU.Improving its safety and reliability is a hot topic in the industry.If the insulation performance of the interturn wrapping material of the variable frequency speed regulation motor fails to meet the standard,when it is subjected to high-voltage corona discharge,the film will be broken down by high voltage in a short time,resulting in motor failure.Therefore,it is very important to improve the insulation performance of interturn winding materials for variable frequency motors.Polyimide has been used to turn the insulation of variable frequency motors for more than 40 years,but the insulation performance of pure polyimide films has difficulty meeting the requirements of high voltage resistance of modern variable frequency motors.Many studies have shown that doping inorganic fillers into polyimide matrix can effectively improve the insulation properties of polyimide films.The focus of this thesis is to improve the insulation and thermal properties of polyimide.First,a series of films are prepared through different anhydride amine components,and then the performance is tested.After selecting the components with the best performance,a series of binary and ternary nanoparticle composite films are prepared by in-situ polymerization with silicon nitride（Si₃N₄）and hexagonal boron nitride（h-BN）modified by silane coupling agent KH550 as the doping phase.The main research contents of this thesis are as follows:（1）Six kinds of anhydride amine components:diaminodiphenyl ether（ODA）,3,4-diaminodiphenyl ether（3,4′ODA）,p-phenylenediamine（PPD）,3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride（BTDA）,3,3′,4,4′-biphenyltetracarboxylic acid dianhydride（bpda）and pyromellitic dianhydride（PMDA）were selected to synthesize seven types of polyimide films.The properties of the films were characterized by electronic universal testing machine,thermogravimetric analyzer,breakdown voltage tester,thermomechanical analyzer,network analyzer,field emission scanning electron microscope and Fourier infrared spectrometer（FT-IR）.The results showed that the breakdown strength of BPDA based PI films is greater than 300k V/mm,and has ultra-low dielectric constant and dielectric loss,while ODA based PI films have excellent mechanical properties.Finally,the anhydride amine component BPDA-ODA with the best comprehensive performance was selected（2）Through the anhydride amine components selected in the previous chapter,Si₃N₄/PI binary nanocomposite films with different doping percentages were prepared with PI synthesized by ODA-BPDA as the matrix.The phases of Si₃N₄ and a series of PI/Si₃N₄ binary composite films with different components were analyzed by X-ray diffraction（XRD）.It was found that the nanoparticles were successfully compounded into the PI matrix;the dielectric constant,dielectric loss and thermal decomposition temperature of the composite film increased with the doping amount of nanoparticles.When the doping amount is 5%,the dielectric constant,dielectric loss and thermal decomposition temperature are the largest,but the dielectric loss is 0.0059,which remains at a very low value,while the initial thermal decomposition temperature reaches 577.5℃,indicating that the insulation performance and thermal stability of the film are good;When the doping amount of Si₃N₄ is 3%,the maximum breakdown strength is 330.28 k V/mm,which is 1.42 times that of pure PI film,and the maximum tensile strength is 108.4 MPa.Based on the analysis of all the results,it is concluded that the performance of the composite film is the best when the doping amount of Si₃N₄ is 3%（3）The dispersions of hexagonal boron nitride（h-BN）and silicon nitride（Si₃N₄）were blended in equal proportions.The anhydride amine component ODA-BPDA selected in the first chapter was selected to prepare PI/Si₃N₄ BN ternary composite films by in-situ polymerization.The distribution of the two kinds of nanoparticles in the polyimide matrix was studied by scanning electron microscopy.The thermal stability and dielectric properties of the composite film were characterized by thermogravimetric analysis,breakdown voltage testing and network analyzer respectively.When the doping amount of Si₃N₄ and h-BN was 4%（the total doping amount was 8%）,the electrical strength reached the maximum value of 323.8k V/mm.Compared with the pure PI film,the breakdown field strength of the composite film was increased by 39.8%.The tensile properties of the ternary films were tested by an electronic universal testing machine.When the doping amount of Si₃N₄ and h-BN is 3%（the total doping amount is 6%）,the maximum tensile strength reaches 114 MPa and the elongation at break is8.48%.When the particle content continues to increase,the mechanical properties begin to deteriorate.Comprehensive analysis shows that the doping amount of Si₃N₄-BN is between3%～4%（the total doping amount is between 6%～8%）,and the comprehensive performance is the best...