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

Abstract

In this paper, we report an anomalous behavior of residual stress in epoxy resin insulation, which decreased with elevated cooling rate from an initial temperature below the glass transition temperature ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ). It was found that an increased cooling rate from 9 K/h to 120 K/h led to a decreased residual stress by 37%. Further results of dielectric relaxation and volumetric strain suggest the 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 rate. 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 K/h to 120 K/h. This work proposed a new comprehending into the residual stress from curing cooling process and corresponding dielectric properties of epoxy resin insulation.