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.