263 / 2022-03-14 22:53:10

Hygrothermal ageing assessment of polyesterimide nanocomposites using dielectric spectroscopy

Hygrothermal ageing,nanocomposites,dielectric spectroscopy

Polyesterimide and its nanocomposites are important insulator materials widely applied in motors and generators, which often work in the hygrothermal condition. Dielectric relaxation property of them can be used to characterize ageing，so the dielectric spectroscopy can be used as a useful tool to assess the degree of hygrothermal ageing.



Polyesterimide and its nanocomposites specimens were deposited in the test oven with 80 °C and 100% RH with deionized water for 30 days. The dielectric relaxation properties of specimens were measured at different frequencies and temperatures. In addition, relaxation activation energies of molecular chains and conductivity activation energies were calculated from dielectric spectra at high temperatures.



For unaged specimens, the real permittivity of polyesterimide nanocomposites is lower than that of polyesterimide. With an increase in ageing time, the real permittivity increases. The increment of polyesterimide is higher than that of polyesterimide nanocomposites at high frequency. However，the result is reversed at low frequency. With an increase in temperature, the real permittivity of unaged specimens increases, reaching the peak at high temperature(140 -200°C), and then decreases. Besides, the curve of real permittivity of ageing specimens with temperature also has the other peak at about 80°C. Moreover, the relaxation activation energies of main molecular chains and conductivity activation energies are shown in table in Appendix.



As the aging time increases, the moisture and small molecule products in specimens increase, leading increasing real permittivity. At low frequency, the ultra-high permittivity of aged polyesterimide nanocomposites is due to lots of water absorption. In addition, the peak at about 80 °C may be related to the disappearance of free water of ageing specimens. The activation energies of the specimens decrease after ageing because of the degradation of material molecules.



 

Relaxation activation energy	Unaged	Aged	Conductivity activation energy	Unaged	Aged
Polyesterimide	1.18 eV	0.71 eV	Polyesterimide	1.00 eV	0.59 eV
Polyesterimide nanocomposites	1.00 eV	0.89 eV	Polyesterimide nanocomposites	1.06 eV	1.02 eV