Studies On Preparation And Electrical Properties Of Polyimide/Multiwalled Carbonnanotubes Films

Polyimide films are mainly used for manufacturing electrical insulating material, with the rapid development of science and technology, the demandment of insulating materials used in manufacturing electric installations is becoming multi-functional and with high performances. Hence domestic and foreign scholars carried out many related researches on modification of polyimide films. Compared with other inorganic nano-particles, multi-walled carbon nanotubes have two remarkable characteristics: large length-diameter ratio and conductibility. The former can decrease the degrading of mechanical properties caused by inorganic component blended with the matrix polymer; the latter can disperse charge effectively to reduce corona discharge which is the main cause to make polyimide film insulation failure in variable frequency motor. In order to improve the corona-resistance of materials, we used MWNTs to modify polyimide films. At the same time, we must ensure that polyimide films modified byMWNTs can be used as an electrical insulating material through controlling the formation of conductive network. The present work is devoted to studying the relationship between the microstructure and electrical property of PI/MWNTs films. MWNTs were purified and functionalized at first, and then were dispersed in solvent to prepare polyamide acid via In-Situ polymerization, then the polyamide acid was further imidized into polyimide films by heating process.The dispersion of MWNTs in solvent were characterized by means of Scanning Electronic Microscopy before and after functionalization, morphology of the surfaces and the cross section of PI/MWNTs films were investigated by SEM, the change of group in the surface of MWNTs was characterized by means of infrared spectrum, the electrical property of PI/MWNTs films were investigated by high resistance meter, corona-resistance tester, withstand voltage tester and dielectric spectrometer respectively.Experimental results showed that purification and functionalization of MWNTs make the dispersion of MWNTs in solvent and PI matrix improved. The volume resistivity of PI/MWNTs films decreased with the MWNTs content increased, as the doping content reached 11.598wt%, the volume resistivity of PI/MWNTs films sharply decreased to 1.96×108?·m. In this case, semiconductive network has been formed initially. Both dielectric constant and dielectric loss increased as the MWNTs content increased, and both of them decreased at first then increased in the low frequency region. As a whole, dielectric constant decreased as the frequency increased, dielectric loss increased as the frequency increased, compared with high frequency region, dielectric loss increased slowly in the region of low and middle frequency.Breakdown strength decreased with the MWNTs content increased, when doping content is less than 0.6wt%, breakdown strength decreased slightly, with the doping content continuously elevated, breakdown strength decreased significantly. When doping content is more than 2.9wt%, test ceased rapidly when voltage was applied, the trace of breakdown cannot be found in the surface of film. This suggested that breakdown phenomenon was not generated, upper and under electrode were connected directly. Coronaresisitancec time shows no regularity when doping content is below 1wt%, but the coronaresisitancec times of all PI/MWNTs films were higher than pure PI films. When doping content reached 2.9wt%, the phenomenon of corona discharge cannot be found in the surface of film, upper and under electrode were connected directly, the result of coronaresistance time test equates with the result of breakdown strength test.