**3.2 Bulk conductivity**

Bulk conductivity of PI in the three aging stages is reported in **Figure 9**. Before aging, PI samples have a conductivity below 10−15 S/m. During stage 1

**Figure 8.** *Trend of ε" at different frequencies and different aging stages.*

**Figure 9.** *Change of bulk conductivity with aging stage.*

(thermal aging at 270°C) there might be a decrease of conductivity (~25%). After hydrolysis and moisture absorption (stage 2, T = 80°C, RH = 80%), the conductivity increases by an order of magnitude. In stage 3 (thermal aging at 320°C), the moisture is removed, and the chemical structure is restored [21]. The conductivity returns to the initial value, a finding in agreement with the results reported in [19].

*Electrical Endurance of Corona-Resistant Polyimide for Electrical Traction DOI: http://dx.doi.org/10.5772/intechopen.93253*

In summary, the conductivity of PI is stable with thermal aging up to 320°C. However, high humidity levels in the operating environment might cause an increase of conductivity, particularly if the machine has not been used for some time.

**Figure 10.** *Weibull plots of PDIV results in all aging stages.*
