Molecular engineering of a Rigid-Flexible phosphorus Macromolecule for Transparent, Mechanically Reinforced, and Flame-Retardant epoxy resins

Achieving flame retardancy together with high transparency and mechanical integrity remains difficult for epoxy resins (EP), particularly when polymeric flame retardants trigger phase separation and light scattering. Here, a DOPO-derived macromolecule (HEPOMA-DPMEA) was synthesized via solution copolymerization and used to modify EP. At 10 wt% loading, EP/HEPOMA-DPMEA achieves UL-94 V-0 and an LOI of 36.8% while retaining high optical clarity (92.5% transmittance at 800 nm for 3.0 mm specimens) and markedly improving tensile strength by 62.5% to 155.4 MPa. Cone calorimetry further shows a 42.3% reduction in peak heat release rate, accompanied by an increased water contact angle (97.0°). TG-FTIR and char analyses support a dual-phase flame-retardant action involving phosphorus-containing volatiles in the gas phase and the formation of a more protective char layer in the condensed phase. This work provides a macromolecular flame-retardant strategy to mitigate the transparency–mechanical–fire-safety trade-off, enabling durable transparent epoxy materials for protective coatings and electronic encapsulation.