| With the development of aerospace,electronics and other high-tech nuclear energy,some motors work at higher than 200℃ over a long period of time.In addition,the temperature and humidity for the working environment change a lot which requires coil wire enamels with high moist resistance and heat resistance.Thus the insulation of electrical products could be ensured in hot and humid conditions.Polyimide(PI)is of the highest rank of organic heat-resistant wire enamels,but PI shows poor storage stability and poor wet and heat resistance,which limits its application in the field of aviation.In this thesis,fluorine-containing poly(ether ether ketone)(FPEEK)was introduced into PI chains to improve their heat and humidity resistance properties.First,polyamide acid(PAA)was synthesized using pyromellitic dianhydride and 4,4-diaminodiphenyl as starting materials,then PI insulating coating was obtained by imidization of PAA film on the tinplate.The structures and properties of PI were characterized by Fourier transform infrared spectroscopy,nuclear magnetic resonance spectroscopy,X-ray diffractometer,thermogravimetric analyzer and universal testing machine.The influences of the monomer ratio on the molecular weight and mechanical properties of PI and properties of PI coatting were discussed.PI with the highest molecular weight of 38436.3 was obtained when the molar ratio of pyromellitic dianhydride and 4,4-diaminodiphenyl ether was close to 1.02: 1.It showed the best thermal stability,and mechanical properties.Its initial thermal degradation temperature tensile strength,Young’s modulus and elongation at breakwere 550℃,84.21 MPa,0.86 GPa and 15.51%,respectively.The heat and humidity resistance,adhesion,flexibility and impact resistance of PI insulating coatings were grade 3,2,0.5 mm and 50 cm,respectively.Then polyimide was modified by FPEEK.Copolymers of PI-co-FPEEK were synthesized through nucleophilic copolycondensation of FPEEK,pyromellitic dianhydride and 4,4-diaminodiphenyl.The influences of FPEEK content and molecular weight on structures and mechanical properties of PI-co-FPEEK and PI-co-FPEEK coatting were investigated.With the increase of FPEEK content,the thermal stability of PI-co-FPEEKdecreased a little bit,while the adhesion of PI-co-FPEEK coatings increased.With the increase of FPEEK molecular weight,the thermal stability of PI-co-FPEEK decreased and mechanical properties were deteriorated due to the low molecular weight of copolymers.When the molecular weight of FPEEK was 3011 and the FPEEK content was 5mol%,the heat and humidity resistance,adhesion,flexibility and impact resistance of PI-co-FPEEK coatings could reach the grade1,1,4mm and 35 cm,respectively. |
