Highly Thermally Conductive and Flame-Retardant Waterborne Polyurethane Composites with 3D BNNS Bridging Structures via Magnetic Field Assistance

• A 3D heat conduction pathway is achieved by combining magnetic filler-modified BNNS (M@BNNS) and non-magnetic U-BNNS under a horizontal magnetic field, enabling superior thermal conductivity. • With only 5 wt% U-BNNS, the through-plane thermal conductivity (λ⊥) reaches 2.88 W m−1 K−1, a 194.2% improvement over single-oriented composites. • The 3D-bridging architecture also provides excellent flame retardancy, reducing peak heat release rate and total heat release by 58.9% and 36.9%, respectively, compared to pure WPU. • The composite demonstrates outstanding thermal management capability as a thermal interface material for LED and chip applications, offering broad potential in electronic packaging.