Anomalous Residual Stress Behavior and Corresponding Dielectric Properties of Epoxy Resin Induced by Physical Aging

In this article, we report an anomalous behavior of residual stress in epoxy (EP) resin insulation, which decreased with an elevated cooling rate from an initial temperature below the glass transition temperature ( $T_{g}$ ). It was found that an increased cooling rate from 9 to 120 K/h led to a decreased residual stress by 37%. Further results of dielectric relaxation and volumetric strain suggest physical aging as the origin of this anomalous residual stress behavior, which resulted in enhanced chain relaxation on the micro level and volumetric shrinkage on the macro level. It is suggested that the larger residual stress is caused by both greater interaction between chains and larger volumetric shrinkage in the specimens with lower cooling rates. In addition, shortened cooling time also induced insufficient chain curling and thus increased free volume, which can be further supported by enhanced charge mobility and lowered dc breakdown strength. A 13% decrease in breakdown strength and a 38% increase in the charge mobility were found provided the cooling rate enhanced from 9 to 120 K/h. This work proposed a new comprehension of the residual stress from the curing cooling process and the corresponding dielectric properties of EP resin insulation.