Chitosan–Fe(III) Complexes via Green Preparation toward Flame Retardant and High-Mechanical-Strength Epoxy Composites

Biobased flame retardants with facile preparation, high performance in smoke suppression, and mechanical reinforcement toward epoxy resins are worth exploring for feasible sustainability. Although some chitosan-containing flame retardants have been successfully developed so far, the synthesis processes for most of them require the use of large volumes of organic solvents and give low synthetic yields. Herein, we report a facile, green, and high-yielding strategy for the synthesis of chitosan–metal complex flame retardants (CS–Fe) by coordination of chitosan with Fe(III) in water. The results show that incorporating 9 wt % CS–Fe enables the EP composites to pass the UL-94 V-1 rating with a limiting oxygen index (LOI) of 29.5%. At the same time, the peak heat release rate, peak smoke production, total smoke production, peak CO production, and fire growth rate of EP/9CS–Fe are significantly reduced, indicating good flame retardancy. Remarkably, the prepared CS–Fe enhanced both the strength and toughness of EP due to the homogeneous dispersion of CS–Fe and the presence of a reaction between CS–Fe and the EP matrix. This work provides a simple and economic strategy for the green preparation of chitosan-based flame retardants with high efficiency.