A green and facile strategy to enhance thermal stability and flame retardancy of unidirectional flax fabric based on fully bio-based system

Although layer-by-layer (LBL) self-assembly technology played a wide range of applications in the field of flame-retardant profiting from the advantages of various ingredient combinations, working on a variety of substrates and ambient processing, it required repeated assembly to achieve the desired flame retardancy, which increased the time-cost and complexity of the process, restricting further application. In this work, fully bio-based flame-retardant system based on phytic acid (PA) and ammonium alginate (AA) was designed, which endowed flax fabric with excellent flame retardancy with only the assembled layers of 1 biolayer. Compared with pristine flax fabric, the limiting oxygen index (LOI) of Flax-AA/PA reached 32.2% (∼68.6% increase), and revealed faster self-extinguishing behavior. Moreover, the peak heat release rate (PHRR), total heat release (THR), peak smoke release rate (PSPR) and total smoke release (TSR) were decreased by 65.1%, 52.7%, 64.4% and 32.1%, respectively. Both the number of assembled layers and the flame-retardant effect were superior to the previously reported systems through LBL self-assembly technology. Mechanism analysis illustrated that the AA/PA flame-retardant system reduced the production of flammable volatiles (carbonyl compounds, aromatic compounds, hydrocarbons, etc), and promoted the dehydration of flax fabric to form phosphorus-containing hybrid char layer which inhibited the heat and mass transfer between the pyrolysis zone and the combustion zone, thus delaying the spread of combustion. This bio-based flame-retardant system provides a facile LBL self-assembly at an ultra-low number of assembled layers to endow the unidirectional flax fabric with excellent flame retardancy.