Hemp hurd torrefaction as an effective strategy to enhance PLA-based composite mechanical and tribological properties

Torrefied hemp hurd (HH) was investigated as a sustainable filler for poly (lactic acid) (PLA)-based composites. HH, a byproduct of the textile industry, underwent torrefaction at 220, 260, and 300 °C to enhance its compatibility with PLA. The resulting fillers (HH220, HH260, HH300) were characterized in terms of morphology, structure, and thermochemical properties. Composites incorporating 2.5 and 5 wt% of each torrefied filler were prepared via melt mixing and characterized for their structural, thermal, mechanical, and tribological behavior. SEM analysis revealed that higher torrefaction temperatures resulted in improved filler dispersion and adhesion to the PLA matrix. While all composites exhibited reduced ductility (i.e., elongation at break), those containing HH300 demonstrated the highest increases in Young's modulus, tensile strength, hardness, and wear resistance. Notably, the HH300 composite containing 5 wt% showed significant improvements over unfilled PLA, including increased stiffness (+ 12%) and toughness (+ 27%), as well as substantial reductions in coefficient of friction (- 83%), and wear rate, that is reduced by about three orders of magnitude. Finally, when compared to the composite containing untreated hemp hurd, stiffness increased by 24%, toughness by 78%, with wear rate still reduced by about three orders of magnitude and friction coefficient lowered by 80%. Torrefied hemp hurd, especially when treated at high temperatures, is thus a promising bio-based reinforcement that improves the performance of PLA composites in structural and tribological applications.