Bio-based lignin-type flame retardant and ferric alginate synergistic flame-retardant epoxy resin

[Objective] A single flame retardant is no longer sufficient to simultaneously meet the comprehensive requirements of epoxy resin(EP)for high-efficiency flame retardancy, mechanical compatibility, and environmental friendliness.Therefore, the construction of a new flame-retardant system through synergistic effects has become a key direction for overcoming the bottleneck in EP flame retardancy.Against this background, designing a composite flame-retardant system composed of ferric alginate(FeAlg)and lignin is expected to provide an innovative approach to addressing the above challenges.[Method] In this study, a novel bio-based intumescent flame retardant—a lignin-based trinity flame retardant(TLI)—was first synthesized using ammonium polyphosphate(APP), pentaerythritol(PER), and lignin as raw materials.APP, PER, and lignin were mixed in a mass ratio of 4:2:1, added to 50%(by volume)ethanol solution as the solvent, and reacted at 80 ℃ for 4 h.After cooling, the product was filtered, washed, dried at 80 ℃, and then ground using a ball mill to obtain TLI.Subsequently, FeCl3 and sodium alginate(NaAlg)were used to prepare FeAlg.A 2%(by mass)aqueous solution of NaAlg was mechanically stirred at room temperature at 1 000 r/min for 3 h, and then injected into a 5 mol/L FeCl3 aqueous solution.After complete crosslinking(for over 24 h), the resulting precipitate was filtered, washed, and dried at 60 ℃ for 24 h, and then ground to pass through a 200-mesh sieve to obtain the FeAlg required for the experiments.TLI was then compounded with FeAlg and incorporated into EP to prepare the EP/TLI-FeAlg composites.On this basis, the thermal stabilities, flame-retardant performances, and mechanical properties of the EP/TLI-FeAlg composites were further investigated.[Results] The results showed that TLI and FeAlg were successfully synthesized.When the mass ratio of TLI and FeAlg added to EP was 19:1(EP/TLI9.5-FeAlg0.5), the flame-retardant performance was the best.Thermal stability analysis indicated that, compared with pure EP, the char residue rate at 700 ℃(w700 ℃)of the EP/TLI9.5-FeAlg0.5 composite increased from 3.9% to 7.5%, an increase of 92.3%.The maximum decomposition rate(Rmax)decreased from 22.3%/min to 12.6%/min, a reduction of 43.5%.Combustion behavior analysis showed that, compared with pure EP, the peak heat release rate(PHRR), total heat release(THR), peak smoke production rate(PSPR), and total smoke production(TSP)of EP/TLI9.5-FeAlg0.5 decreased by 77.1%, 65.5%, 78.6%, and 74.1%, respectively.Analysis of char residue morphology and structure showed that pure EP left very little and discontinuous residual char, the aluminum foil underneath was burned through, exposing the underlying degreased cotton.The composites had a higher char yield after combustion, with a continuous and relatively dense char layer.A highly expanded char layer was formed, and for composites with FeAlg mass fractions of 0.3%, 0.5%, and 1.0%, the post-combustion char height was over 4 cm.The char of EP/TLI9.5-FeAlg0.5 was relatively flat with fewer cracks and pores.Flame-retardant performance analysis showed that the limiting oxygen index was 27.7%, and the UL-94 fire rating reached V-0.In terms of mechanical properties, compared with the composite without FeAlg(EP/TLI10.0), the tensile strength and flexural strength of EP/TLI9.5-FeAlg0.5 increased by 74.3% and 127.4%, respectively.[Conclusion] This study successfully converted abundantly available lignin into an efficient flame-retardant component and synergistically combined it with FeAlg for flame retardancy.With only 10.0% additive, UL-94 V-0 rating was achieved.In previous studies, when lignin derivatives were used as flame retardants for modifying EP, the focus was often on improving flame retardancy, while mechanical properties were significantly sacrificed.It is difficult to restore mechanical performance using a single flame retardant, often requiring a compromise between flame retardancy and mechanical loss.Although the composite EP/TLI10.0 experienced a severe decline in mechanical properties due to compatibility issues, the combination with FeAlg successfully restored the mechanical performance to a certain extent(tensile strength 18.3 MPa, flexural strength 42.3 MPa).In this system, only 0.5% of FeAlg was added, achieving a significant restorative improvement in mechanical performance while maintaining the same level of flame retardancy, thereby demonstrating high cost-effectiveness and synergistic effects.This provides a new idea for the green functionalization of EP.