Halogen-free flame-retardant epoxy resin modified with metal-organic framework-loaded piperazine pyrophosphate

[Objective] Epoxy resin(EP), a critical matrix resin in advanced electronic packaging and aerospace composite systems, exhibits inherent flammability, which significantly limits its engineering application under extreme high-temperature conditions.The enhancement of fire safety in EP materials through innovative multi-level synergistic flame-retardant mechanisms has become a prominent research topic in materials science.In this study, a halogen-free nanocomposite flame-retardant system(UiO@PAPP), based on the chemical bonding of aminated zirconium-based metal-organic framework(UiO-66-NH2)and pyridine phosphoric acid piperazine(PAPP), was designed.[Methods] UiO-66-NH2 was synthesized via a solvothermal method using zirconium chloride and 2-aminoterephthalic acid.PAPP was covalently anchored onto UiO-66-NH2 through a one-step reaction in acetonitrile, forming the hybrid [e-mail] EP composites were prepared:pure EP(EP1), EP with 2.6%(by mass)PAPP(EP2), EP with 1.6%(by mass)PAPP and 1.0%(by mass)UiO-66-NH2(EP3), and EP with 2.6%(by mass)UiO@PAPP(EP4).Structural charac-terization was conducted using Fourier transform infrared spectroscopy(FTIR), X-ray diffraction(XRD), transmission electron microscope(TEM), and scanning electron microscope-X-ray energy dispersive spectrum(SEM-EDS)were used to confirm successful PAPP loading and chemical bonding.Thermal stability was evaluated via thermogravimetric analysis(TGA), while flame-retardant performance was assessed using limiting oxygen index(LOI), UL-94 vertical burning tests, and cone calorimeter test.Post-combustion residue analysis(Raman, FTIR, XRD)and mechanistic studies were conducted to explore char layer evolution and synergistic effects.[Results] UiO@PAPP retained the crystalline structure of UiO-66-NH2 while exhibiting enhanced thermal stability(40.98% char residue rate at 800 ℃ vs 35.72% for UiO-66-NH2).EP4 demonstrated superior flame retardancy:LOI increased from 22.1%(EP1)to 27.1%, achieving UL-94 V-1 rating.Cone calorimeter test revealed that EP4 reduced peak heat release rate(PHRR)by 49.8%(630 vs 1 254 kW/m2), total smoke production(TSP)by 21.0%(20.7 vs 26.2 m2), and peak CO/CO2 production rates by 45.6% and 51.6%, respectively.Char residue rate increased to 22.2%(vs 7.0% for EP1).Structural analysis confirmed that UiO@PAPP synergistically catalyzed the formation of a dense, graphitized char layer(ID/IG:3.04 vs 3.88 for EP1)through ZrO2 and phosphate compounds, which physically blocked heat/gas transfer and quenched free radicals.The chemical bonding between UiO-66-NH2 and PAPP improved dispersion and interfacial compatibility, outperforming physical blends(PHRR reduction of 45.4% for EP3 vs 49.8% for EP4).[Conclusion] This study successfully developed a halogen-free UiO@PAPP nano-hybrid flame retardant via covalent bonding, effectively addressing the limitations of PAPP in EP composites.The hierarchical synergy between UiO-66-NH2 and PAPP enhanced both condensed-phase char formation and gas-phase radical quenching, significantly improving flame retardancy and smoke suppression at only 2% loading.The chemically bonded UiO@PAPP system demonstrated superior efficiency over physical blends, highlighting the critical role of interfacial compatibility in maximizing synergistic effects.The optimized char layer, reinforced by ZrO2 and phosphates, provided robust thermal insulation and structural stability, reducing fire hazards in high-temperature environments.These findings offer a sustainable and effective strategy for designing high-performance flame-retardant EP composites, with promising applications in electronics and aerospace engineering.Future work should explore scalability, long-term stability, and broader applicability in other polymer matrices.