A High‐Efficiency Flame‐Retardant Coating for Flexible Polyurethane Foam via Layer‐by‐Layer Self‐Assembly of Tannic Acid and Metal Ions

ABSTRACT Flexible polyurethane foam (FPUF) is widely used in daily applications and thermal insulation materials owing to its excellent stability and comfort. However, its high flammability and the substantial production of molten droplets during combustion limit its use. Layer‐by‐layer self‐assembly (LBL) has emerged as an efficient strategy for imparting flame retardancy to FPUF, particularly through coatings based on metal ions‐polyphenol coordination networks; it can efficiently suppress heat and smoke release during combustion. Here, we have developed an alternating layer composed of polyethyleneimine (PEI)/tannic acid (TA) + Fe(NO 3 ) 3 /polyacrylic acid (PAA) + montmorillonite (MMT). This coating not only markedly reduces the peak heat release rate (PHRR) at low add‐on levels but also efficiently suppresses smoke emission from FPUF. Cone calorimetry results show that five layers foam a modest weight gain of only 6.6 wt% achieves reductions of 43% in PHRR, 59% in peak smoke production rate (PSPR), and 48% in total smoke production (TSP) compared to pristine FPUF. Moreover, the coating exhibits excellent water resistance: after 15 days of water immersion, the PHRR and TSP of the coated foam remain reduced by 35% and 27%, respectively, relative to uncoated FPUF. Further analysis of the flame‐retardant mechanism, offering valuable insights for the rational design of high‐performance flame‐retardant coatings for FPUF.