Synergistic enhancement of flame retardancy in epoxy resin via triazine-based covalent organic framework and ammonium polyphosphate

[Objective] Epoxy resin(EP)is widely used in electronics, aerospace, and construction due to its excellent mechanical strength, electrical insulation, and chemical resistance.However, its high flammability severely restricts its applications in fire-sensitive environments.Conventional halogen-based flame retardants pose environmental and health hazards, whereas halogen-free systems such as ammonium polyphosphate(APP)often require high loadings that deteriorate the mechanical performance of EP.Covalent organic framework(COF)offers high thermal stability and structural ordering, presenting potential as novel flame-retardant synergists.This study investigates the synergistic flame-retardant effect between a triazine-based COF and APP in an EP matrix.The objective was to elucidate the synergistic effect between COF and APP in enhancing the fire safety of EP and to provide a new strategy for high-performance, halogen-free flame-retardant modification.[Methods] A COF was synthesized at room temperature through a Schiff-base condensation between 4-aminobenzonitrile-derived 2, 4, 6-tri(4-aminophenyl)-1, 3, 5-triazine(TAPT)and terephthalaldehyde(TPAL), yielding an imine-linked triazine-based COF.The structure and morphology of the COF were characterized by Fourier-transform infrared spectroscopy(FTIR), powder X-ray diffraction(PXRD), scanning electron microscopy(SEM), and elemental analysis.The COF was incorporated with APP at a total loading of 10%(by mass), with varying m(APP):m(COF)=9:1, 6:1, and 3:1, to prepare EP/AC9, EP/AC6, and EP/AC3 composites.Flame retardancy was evaluated using cone calorimetry under a heat flux of 35 kW/m2 and UL-94 vertical burning tests, while Raman spectroscopy was used to analyze the char residues.The mechanical properties of the composites were also evaluated using a tensile testing machine.[Results] The triazine-based COF synthesized in this experiment exhibited low crystallinity but high nitrogen content and a porous morphology.The COF/APP flame-retardant system exhibited significantly better flame retardancy than APP alone.The EP/AC3 composite(7.5% APP+2.5% COF)exhibited a peak heat release rate(PHRR)of 684 kW/m2, a 40% reduction compared to the pure EP(1 140 kW/m2)and a 26% reduction compared to the EP/10APP(10% APP)composite(927 kW/m2).The peak CO2 production rate(0.23 g/s)decreased by 50% and 32% compared to the pure EP(0.46 g/s)and the EP/10APP composite(0.34 g/s), respectively.The peak CO production rate(0.018 g/s)dropped by 50% and 25% compared to the pure EP(0.036 g/s)and the EP/10APP composite(0.024 g/s), respectively.The COF addition also delayed the time to ignition(TTI)(71 s for EP/AC3)compared to the EP/10APP composite(60 s).The fire growth rate index(FIGRA)decreased by 39% relative to the EP/10APP composite, while the fire performance index(FPI)improved by 60%.UL-94 tests confirmed the synergistic effect:EP/AC3 achieved a V-0 rating with first and second ignition durations(t1 =3 s, t2 =6 s)and no dripping, overcoming the typical limitation of APP systems that fail at low loadings.As confirmed by Raman spectroscopy, the ID/IG ratio of char residues decreased from 2.62(pure EP)to 2.11(EP/AC3), indicating enhanced graphitization and a more thermally stable char barrier.This dense carbonaceous barrier effectively isolated heat and oxygen, retarded decomposition, and inhibited the release of combustible volatiles.Although the introduction of COF improved fire resistance, mechanical performance was compromised.The tensile strength decreased from 61.3 MPa(pure EP)to 38.7 MPa for EP/AC3, and the elongation at break declined from 5.0% to 2.4%.This reduction was attributed to COF particle aggregation within the EP matrix.Additionally, total smoke production(TSP)was not improved.The EP/AC composites(15.5-16.5 m2)produced slightly more smoke than the EP/10APP composite(14.2 m2).Despite these drawbacks, the synergistic COF/APP system achieved superior flame retardancy at significantly lower phosphorus loadings compared with conventional APP-only systems.[Conclusion] This study demonstrates that a triazine-based COF acts as an effective synergist with APP, enhancing EP's flame retardancy.The synergistic mechanism is attributed to improved condensed-phase char quality.This work presents a novel approach to achieving high-performance flame-retardant EP.However, the increase in TSP and the notable deterioration of tensile strength highlight the need for further optimization.Future research should focus on surface modification or nanoscale dispersion of COF to improve compatibility with the EP matrix and minimize mechanical loss, as well as on strategies to suppress smoke evolution.