Influence of moisture and ionic stresses on the defect-driven failure of PI and PEN films: bulk charge dynamics and surface partial discharge

Abstract Environmental degradation of polymer insulators under moisture and ionic contaminants poses a critical challenge to the reliability of high-voltage applications. This study systematically investigates and compares the dielectric and surface partial discharge degradation mechanisms of polyimide (PI) and polyethylene naphthalate (PEN) films subjected to thermal (90 ∘ C), wet (deionized water), and ionic (5 wt% NaCl) stresses. Time-dependent current integration ( Q ( t ) ) data reveal that while PI suffers from a massive surge in leakage current with an extraordinary coefficient of variation, PEN maintains exceptional bulk insulation stability due to its rigid molecular structure. AC dielectric breakdown analysis shows that environmental stresses cause a severe reduction in the Weibull shape parameter, indicating a transition from intrinsic electronic avalanche processes to a defect-driven failure mode induced by electric field enhancement. Crucially, objective model selection utilizing information criteria reveals a fundamental mechanistic discrepancy between the static inception (PDIV) and dynamic extinction (PDEV) thresholds. While moisture absorption alone preserves the baseline PDIV, localized ionic residues act as severe electric field enhancement sites, triggering premature stochastic inception. Conversely, PDEV is profoundly governed by dynamic surface charge relaxation. It is hypothesized that trace amounts of residual bound water alter surface charge diffusion and trap injected charges, severely degrading the PDEV into a mixed-Weibull framework for both materials. Furthermore, under NaCl treatment, the ion-moisture synergy in PI promotes rapid charge dissipation and PDEV recovery, whereas the hydrophobic nature of PEN sustains isolated charge traps and hinders extinction. These findings phenomenologically link inferred microscopic surface states to macroscopic statistical lifespans, providing vital guidelines for next-generation high-voltage insulation design.