Valorization of natural fibers in flame retarded poly(lactic acid)

Extensive research has explored natural fiber reinforced composites, typically focusing on a single fiber within a polymer matrix. Comprehensive comparisons across different natural fibers in the same polymer, which are critical for industrial material selection, remain limited. This work presents a systematic comparison of untreated hemp, flax, and sisal fibers incorporated at varying fiber lengths and loadings into flame retarded poly(lactic acid) (PLA). Fire behavior, thermal, and mechanical responses were investigated through thermogravimetry, UL 94, and cone calorimetry, alongside crystallinity, molecular weight (MW), and microstructural analysis. Fiber incorporation reduced the peak heat release rate (pHRR) by up to 30% in 30 wt.% hemp, attributed to protective layer formation, but increased flammability in UL 94. A phytic acid melamine salt combined with expandable graphite and 20 wt.% hemp produced incomplete combustion at 50 kW/m², raising char residue from 4 to 24 wt.% and halving pHRR. Petrella plots revealed that fiber addition alone lowered fire load and flashover propensity as effectively as phytic acid melamine; with hemp, phytic acid and expendable graphite, the flashover hazard and fire load were halved. MW was preserved while crystallinity and modulus increased with fiber content. Hemp delivered the most consistent reinforcement, while optimized processing enabled flax and sisal to improve stiffness. Performance gains were strongest when individual fibers were dispersed via optimized processing, preventing bundle fracture under load. Plasma modification of the fibers improved the maximum tensile strength in the composites. A practical guide is provided for valorizing natural fibers in PLA composites, demonstrating routes to bio-based, compostable materials with improved fire safety and mechanical performance suitable for industrial processing. • Systematic comparison of untreated/valorized hemp, flax, sisal in flame retarded PLA • Flammability, fire behavior, MW, processability, crystallinity, and mechanics studied • Guidelines to develop compostable natural-fiber PLA flame retardant composites