Valorization of Olive Pomace-Derived Hydrochar as Filler for PLA Biocomposites: Effect of Hydrothermal Carbonization Conditions on Mechanical, Thermal and Moisture Resistance Properties

The expanding generation of agro-industrial residues demands innovative valorization strategies to support circular bioeconomy goals. Hydrothermal carbonization (HTC) offers an energy-efficient thermochemical pathway to convert wet biomass like olive pomace into carbonaceous fillers without energy-intensive pre-drying. However, the influence of HTC thermal severity on the functional performance of hydrochars when incorporated into poly(lactic acid) (PLA) biocomposites remains insufficiently explored. To address this gap, this study investigates the impact of HTC temperature on the structural, thermal, mechanical, rheological, and moisture resistance properties of PLA biocomposites containing 5 wt.% hydrochar. Two hydrochars were synthesized from olive pomace slurry via microwave-assisted HTC at 180 °C (HC1) and 250 °C (HC2) for 30 min, compounded into a PLA matrix, and shaped via injection molding and continuous filament extrusion. Higher HTC severity induced extensive deoxygenation and hemicellulose degradation, resulting in a hydrochar (HC2) with higher intrinsic thermal stability and reduced moisture affinity. When incorporated into PLA, both hydrochars acted as nucleating agents, increasing matrix crystallinity from 35.3% (neat PLA) to 39.5% (PLA_HC1) and 41.4% (PLA_HC2). Tensile strength and Young’s modulus decreased for both biocomposites compared to neat PLA (62.8 MPa and 3.57 GPa), though PLA_HC1 retained higher tensile strength (55.3 MPa) than PLA_HC2 (49.9 MPa). In addition, PLA_HC2 exhibited a substantial gain in ductility and toughness over PLA_HC1, displaying a higher elongation at break (24.9% vs. 6.2%), higher Charpy impact strength (16.2 kJ/m2 vs. 11.3 kJ/m2), and higher resistance to thermal degradation. After 384 h of water immersion, PLA_HC2 exhibited 30.3% lower water absorption (0.934%) than PLA_HC1 (1.341%), mitigating hydrolytic degradation risks. Melt flow rate testing yielded 27.5 g/10 min for PLA_HC1 and 21.0 g/10 min for PLA_HC2, compared to 26.9 g/10 min for neat PLA. Continuous, dimensionally stable biocomposite filaments were successfully extruded for all formulations, confirming their potential as bio-based feedstocks for fused deposition modeling (FDM) additive manufacturing. Overall, tuning HTC synthesis temperature provides an effective route to tailor hydrochar surface chemistry, balancing stiffness, toughness, and moisture resistance in PLA biocomposites.