Polylactic Acid Hybrid Composites Reinforced with Thermally Modified Wood Flour and Cellulose Nanofibrils

Thermally modified wood flour (TMW) and cellulose nanofibrils (CNF) are bio-based reinforcements that have been investigated individually in polylactic acid (PLA) composites; however, their combined use in hybrid PLA composites has received comparatively limited attention. This study investigates the simultaneous incorporation of TMW and CNF into PLA and the influence of their relative contents on composite performance. Composite formulations containing 10–30 wt.% total reinforcement, with TMW and CNF contents ranging from 5 to 15 wt.% each, were prepared by twin-screw extrusion followed by injection molding. Mechanical performance was evaluated through tensile, flexural, and impact testing, while thermal, thermomechanical, and morphological characteristics were analyzed using differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The incorporation of TMW and CNF increased the stiffness of PLA, with maximum improvements of approximately 60% and 48% in tensile and flexural moduli, respectively, compared with neat PLA. DMA further demonstrated the reinforcing effect through increased storage modulus values and reduced loss factor (tan δ) peak magnitudes. DSC revealed lower cold crystallization temperatures, increased crystallinity, and bimodal melting behavior in the reinforced composites. TGA showed that the incorporation of TMW and CNF shifted the principal thermal degradation toward lower temperatures while increasing residual mass at elevated temperatures. SEM observations revealed changes in fracture morphology and matrix–filler interactions, with greater morphological heterogeneity at higher reinforcement loadings. Overall, the combined use of TMW and CNF provided substantial stiffness enhancement while largely maintaining tensile strength, without the use of compatibilizers or chemical surface treatments. These results demonstrate the potential of combining TMW and CNF as a fully bio-based hybrid reinforcement system for tailoring the stiffness and overall performance of PLA composites.