Sustainable green coating for natural fibers: a flame retardant approach for pla-based composites

This study presents a sustainable approach to enhance the fire resistance of natural fiber-reinforced poly(lactic acid) (PLA) composites. A bioinspired approach to functionalize the surface of flax (FF) and basalt (BF) fibers with an innovative organic-inorganic hybrid flame retardant (FR) is proposed. The fibers underwent a three-step coating method: first, ozone (O3) pretreatment enhanced their surface reactivity by forming oxygen-containing functional groups that, in turn, enable the covalent immobilization of gallic acid (GA) units. Ultimately, inspired by the bacterial iron acquisition mechanism, iron phenylphosphonate was complexed with the phenolic groups of gallic acid units, resulting in FR-coated fibers. The coated flax and basalt fibers were used to produce PLA-based composite materials, named PLA/FF-FeP and PLA/BF-FeP, respectively. Thermogravimetric analysis (TGA) results revealed that, despite the low loading (0.1 wt% as assessed by Microwave Plasma Atomic Emission Spectroscopy - MP-AES) of the coating on the fiber surface, both composites exhibited improved thermal stability. The cone calorimetry test (CCT) highlighted an extended time to ignition (TTI), a reduced time to flame out, an 8% decrease in the peak of heat release rate (pHRR) and a 15% decrease in Fire Propagating Index (FPI) for PLA/BF-FeP. Further, the PLA/FF-FeP turned out to be smoke suppressant, showing a decrease in both total smoke release (TSR) and specific extinction area (SEA) of about 87 and, 68%, respectively.