Application of tannic acid and ferrous ion complex as eco-friendly flame retardant and antibacterial agents for silk

Traditional flame retardants (FRs) for textiles are limited by toxicity, environmental persistence and bioaccumulation. There is an urgent need to develop environmentally benign FRs. For this purpose, the combination of tannic acid (TA, a specific form of hydrolysable tannin) and ferrous ion was applied to simultaneously impart durable FR and antibacterial functions to silk fabric. A ternary TA-ferrous ion-silk complex was fabricated by adsorption of TA on silk followed by mordanting with ferrous salt. After treated with 20% TA (on the weight of fabric) and 2 g/L ferrous sulfate, silk fabric exhibited an obvious increase in limiting oxygen index from 23.6% to 27.5%, a significant decrease in damaged length from 30.0 to 11.2 cm after vertical burning, and a significant increase in antibacterial rate from 22% to 95%. Even after 20 washes, the treated fabric still had the same FR ability as the unwashed one, and an antibacterial rate of above 90%, exhibiting excellent washing durability. The high thermal stability of the treated fabric in nitrogen indicated the formation of a ternary TA-ferrous ion-silk complex which was responsible for excellent washing durability. The observation on the morphological structure of charred products from vertical burning revealed the good charring capability of the treated fabric which contributed to enhanced FR behavior. This research proposes an eco-friendly FR treatment for silk and a novel application of hydrolysable tannins in the FR modification of textiles.