Metal-ion-chelated lignin derivatives enhance flame retardancy and smoke suppression of silicone-acrylic coatings

Multi-element bio-flame retardants have become an inevitable trend for designing green, ecological, and low-cost coatings, and the metal ions play a critical role in the catalytic charring. Our previous research has explored preparing flame retardant (DAL) by crosslinking 9,10-dihydro-9-oxa-10-phosphaphenanthrene (DOPO) with sodium lignosulfonate (LS), and there is still potential to improve its flame retardancy by chelating iron. Herein, a new Si-P-N-Fe lignin derivative (FDAL) is synthesized through the solvothermal reaction of Fe(NO₃)₃·9H₂O with DAL. A new silicone-acrylic emulsion-based (SAE-based) coating was prepared by introducing FDAL, carboxymethyl chitosan (CMCS) and melamine polyphosphate (MPP). Cone calorimeter shows that there is an appropriate dosage of FDAL (1.30 g, 2.0 wt%), evidenced by the peak of heat release rate (p-HRR) decreased from 237.4 to 92.6 kW·m^-2 and the total smoke release (TSP) diminished from 6.41 to 4.66 m², respectively, as well as the LOI of 34.3% and the V-0 rating of UL-94. Microstructural analysis demonstrates that the doped FDAL transforms into Fe(PO₃)₃ and Fe₂SiO₄-containing resilient residues and enhance the gas/condensed phase fire barrier, leading to an increase in the activation energy rising from 281.77 to 340.65 kJ·mol^-1 during 353-448 °C. It expands the halogen-free flame-retardant techniques and the theory of bio-based functional lignin derivatives.