Development of High‐Toughness Epoxy Composite Insulating Varnish for Cold‐Region Dry‐Type Reactors

ABSTRACT Epoxy insulating varnishes are widely used in motors, transformers, and other electrical equipment because of their excellent electrical properties. However, their inherently rigid molecular structure results in brittleness and insufficient mechanical strength, restricting their applicability under high stress, particularly in cold regions. To prevent insulation cracking in dry‐type air‐core reactors operating in extreme cold regions, this study evaluates the modification of E51 epoxy resin using four agents: Polyaryletherketone (PAEK), epoxy‐terminated butadiene nitrile rubber (ETBN), 4,5‐epoxyhexane‐1,2‐dicarboxylic acid diglycidyl ester (TDE‐85), and dicyclopentadiene (DCPD). Experimental results reveal that TDE‐85 achieves the most significant reinforcement; at an optimal loading of 20 wt%, the flexural strength and modulus reach 151.8 MPa and 4.04 GPa, respectively. PAEK and DCPD exhibit peak flexural strengths of 119 MPa (at 15 wt%) and 122 MPa (at 30 wt%), while ETBN modification leads to a gradual decline in mechanical stiffness. Regarding dielectric properties, all systems maintain a low dissipation factor (tan δ < 0.012). Notably, the TDE‐85 (15 wt%) system exhibits the highest breakdown voltage of 29.15 kV/mm and a high volume resistivity of 7.8 × 10 14 Ω m. Low‐temperature tests at −40°C further demonstrate the superior stability of the TDE‐85 system, which maintains a flexural strength of 133 MPa (a slight increase from 131 MPa at 25°C), whereas the strengths of PAEK, ETBN, and DCPD systems drop significantly to 82 MPa, 66.2 MPa, and 78.2 MPa, respectively. These findings identify TDE‐85 as the optimal candidate for developing high‐toughness, cold‐resistant insulating varnishes for power equipment.