Reaction Condition-Driven Structural Evolution During Epoxy Resin Chain Extension and Its Influence on Insulation Material Properties

Long-chain epoxy resins are increasingly adopted in high-voltage insulation systems due to their low curing exotherm, improved toughness, and superior dielectric properties. These resins are typically synthesized via chain extension between bisphenol A and short-chain epoxy precursors. However, during pilot-scale production, inadequate heat dissipation often leads to thermal overshoot, inducing undesirable side reactions and structural irregularities. In this study, we investigate how reaction-induced thermal fluctuations affect the molecular architecture of chain-extended epoxy resins and, consequently, the performance of their cured insulation materials. Differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) analyses reveal that thermal overshoots exceeding 20°C promote side reactions between hydroxyl and epoxy groups, resulting in increased molecular branching, higher viscosity, and compromised processability. These structural changes further lead to reduced crosslinking uniformity, manifested by lower crosslink density, diminished mechanical toughness (impact strength decreased from 26.4 to 24.1 kJ/m 2 ), and deteriorated dielectric performance (volume resistivity dropped from 2.58 × 10 17 to 1.94 × 10 17 Ω ·cm). This work provides mechanistic insight into the interplay between reaction conditions and structure–property evolution, offering practical guidance for the controlled and scalable production of high-performance epoxy insulation materials.