Advanced flame-retardant biocomposites: Polylactic acid reinforced with green gallic acid‑iron‑phosphorus coated flax fibers

This study introduces a sustainable approach for enhancing the fire retardancy and smoke suppression of poly(lactic acid) (PLA) composites, contributing to addressing one of the major challenges in biocomposites that limits their application in various engineering fields, as automotive and construction sectors. Flax fibers (FF) were surface functionalized with a novel organic-inorganic hybrid flame retardant (FR), offering a sustainable bioinspired approach that mitigates potential mechanical properties impairment and FR leaching, which can cause environmental concerns and reduced composite durability. The process involves a three-step coating procedure. First, the flax fibers (FF) are pretreated with ozone to promote carboxylic group formation (FF-O₃); subsequently, gallic acid (GA) units are covalently immobilized on the fiber surface (FF-GA); finally, the hybrid FR iron phenylphosphonate is complexed with the phenolic groups of GA units (FF-GA-FeP). Fourier transform infrared (FT-IR) analysis of FF-GA-FeP confirmed the presence of specific absorptions associated with the deposited FR coating. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) revealed changes in fiber morphology and confirmed the incorporation of iron and phosphorus. Solid-state nuclear magnetic resonance (SSNMR) spectroscopy and X-ray WAXS microscopy revealed that fibers' crystallinity was not significantly affected by derivatization. Microwave plasma atomic emission spectroscopy (MP-AES) detected a precise 0.1 wt% iron loading. Using FF-GA-FeP as reinforcement in PLA-based composites (PLA/FF-FeP) resulted in enhanced thermal stability and flame retardancy of the composites, with minimal coating application, as revealed by thermogravimetric analysis (TGA) and cone calorimetry tests (CCT). A decrease in peak of heat release rate (pHRR), total smoke release (TSR), specific extinction area (SEA), and Fire Propagating Index (FPI) of 5, 87, 68, and 9.5 %, respectively, was achieved for PLA/FF-FeP, compared to untreated flax fiber reinforced PLA (PLA/FF). Furthermore, preliminary tensile tests indicate minor changes in tensile strength and a slight increase in stiffness of the PLA/FF-FeP compared to PLA/FF. Hence, in the biocomposite, the immobilization of a minimal amount of iron phenylphosphonate directly on the flax fiber surface proved to be an effective strategy for smoke suppression while preserving the mechanical integrity of the composite.