Enabling simultaneous flame-retardant and toughening effects in epoxy resins with a low-loading P/N/B hyperbranched polymer from aminoborate esters

Developing high-performance epoxy resins (EP) that integrate outstanding flame retardancy, mechanical reinforcement and optical transparency is a persistent challenge, often hindered by the detrimental effects of conventional additives. Herein, a novel hyperbranched polymeric flame retardant (HPBAE) containing phosphorus and borate ester moieties was synthesized from boric acid, ethanolamine and phenylphosphonic dichloride. Remarkably, incorporation of merely 2 wt% HPBAE substantially boosted the flammability resistance of resin to the UL-94 V-0 rating with 32.9% limiting oxygen index (LOI) and a 23.4% suppression in the peak heat release rate (PHRR). The superior flame retardancy arises from dual-phase action: condensed-phase formation of a P/B-rich thermal barrier char and gas-phase release of phosphorus radicals that quench H · /HO · combustion intermediates. Relative to pure EP, the EP/HPBAE-2 exhibited a 57.9% enhancement in tensile strength while retaining the optical transparency. The synergistic effects imparted by HPBAE not only provide a balanced property profile but also offer a viable design paradigm for developing high-performance, transparent and flame-retardant epoxy composites.