Interfacial Self-Polymerization of Tannic Acid for Functional Flame-Retardant Coatings on Silk Fibers

Developing surface finishes that impart flame retardancy to silk while avoiding phosphorus-, metal-, and halogen-containing additives remains challenging, because biobased polyphenols often provide limited protection when directly deposited on fibers. Here, we report an aqueous oxidative self-polymerization strategy that converts tannic acid (TA) into surface-deposited poly(tannic acid) (pTA) particulates on silk. Under mild alkaline conditions, TA underwent self-polymerization and aggregation, and the resulting pTA particles formed a polyphenolic coating on silk fibers through dip coating. In contrast to unpolymerized TA, which showed poor stand-alone flame-retardant efficiency even at higher concentration, pTA-coated silk exhibited self-extinguishing behavior. The damaged length decreased from 30.0 to 10.2 cm, and the limiting oxygen index increased from 23.2% to 27.6%, meeting the B1 classification. Cone calorimetry showed a 38.5% reduction in peak heat release rate and lower smoke production. Thermal, char-residue, and TG-IR analyses indicated that pTA interfacial coating promoted dehydration and carbonization, yielding a compact protective char that limited heat and mass transfer and suppressed combustible volatile release. The coating also imparted antibacterial activity up to 99.9% against Escherichia coli and Staphylococcus aureus. This work links TA self-polymerization, pTA particle deposition, and surface-mediated condensed-phase protection, offering a water-based, single-component route to multifunctional flame-retardant silk.