Durable Flame-Retardant Silk Fabric Based on a Tannic Acid/Soy Protein Isolate/Ferrous Sulfate System

Developing highly effective and durable functional fabrics using polyphenols derived from natural plants is environmentally friendly and sustainable. Tannic acid is widely available and relatively inexpensive, and its abundant phenolic hydroxyl groups promote carbonization during combustion, thereby effectively inhibiting the flame propagation. However, flame-retardant systems based solely on natural polyphenols often suffer from insufficient durability. In this study, a flame-retardant and antibacterial silk fabric was developed through a two-step impregnation method using tannic acid (TA), soy protein isolate (SPI), and ferrous sulfate (Fe2+). It was found that the modified silk possessed greatly improved flame-resistant properties. With increasing amounts of SPI and TA, the char length in the vertical burning tests decreased, while the limiting oxygen index (LOI) increased to 27.1% under the optimal conditions. Even after 20 laundering cycles, the char length remained below 10 cm, demonstrating excellent washing durability. Cone calorimeter tests further revealed significant reductions in both the total heat release rate (THR) and peak heat release rate (pHRR). The synergistic interactions among TA, SPI, and Fe2+ promoted the formation of a stable protective char layer during the combustion. In addition, the modified silk fabric exhibited notable antibacterial activity while maintaining acceptable physical properties for practical use. This work provides a sustainable and effective strategy for developing multifunctional and durable flame-retardant silk textiles.