Viscosity Controlled Strategy via <scp>BN</scp> @ <scp>TPU</scp> Structural Engineering on Epoxy Composites with High Thermal Conductivity

ABSTRACT Dry‐type transformer encapsulation materials commonly suffer from a trade‐off between processability and multifunctional performance, as high filler loadings improve thermal conductivity and mechanical reliability but sharply increase viscosity and impair casting behavior. In this work, a structure‐regulated strategy based on boron nitride/thermoplastic polyurethane (BN@TPU) granulation is proposed to address this issue. Spherical BN@TPU granules were prepared by assembling BN platelets with a small amount of TPU binder, creating a hierarchical structure with BN as the main framework and TPU localized at interfaces. Rheological analysis demonstrates that the BN@TPU system reduces the initial viscosity by 40% compared with the pristine BN‐filled epoxy system, significantly extending the processing window. Owing to the optimized structure, the composites exhibit enhanced tensile and flexural strength compared with pristine BN‐filled systems while maintaining a continuous thermally conductive network, leading to markedly improved thermal conductivity at high filler loadings. Dynamic mechanical analysis confirms increased stiffness and restricted chain mobility without sacrificing thermal stability. In addition, the composite achieves a thermal conductivity of 0.74 W·m −1 ·K −1 at a filler loading of 35 wt%, while maintaining a high volume resistivity on the order of 10 14 –10 15 Ω·m, demonstrating excellent electrical insulation performance. This work provides a promising strategy for balancing processability and multifunctional performance in highly filled polymer composites.