Covalent grafting of reactive phytate salt for durable flame-retardant functionalization of protein silk fabric

The development of sustainable and durable flame-retardant protein silk fabric without compromising its physical properties is of interest but challenging. In this study, a fully biobased reactive flame-retardant, vanillin phytate, was synthesized from biomass phytic acid and vanillin. Subsequently, vanillin phytate was covalently grafted onto silk fabrics along with diethyl phosphite under mild conditions via the Kabachnik-Fields reaction. The chemical structure of vanillin phytate and its potential cross-linking mechanism with silk fibers, thermal stability, combustion behavior, flame retardancy, washing durability, and mode of action of the modified silk fabrics were investigated. The modified silk exhibited a significant reduction in heat and smoke release by 63.8 % and 90 %, respectively, versus pristine silk. The modified silk fabrics also demonstrated excellent self-extinguishing capacity, with a reduced damaged length of 7.0 cm and an increased limiting oxygen index of over 34 %. Furthermore, the modified silk fabric maintained self-extinguishing performance after 25 washing cycles, showing high flame-retardant efficiency and good washing durability. The char residue analyses revealed that the modification primarily exerted its flame-retardant effect in the condensed phase. Interestingly, the present strategy had less influence on the physical performance of silk fabrics, indicating a wide range of practical applications.