Influence of Alkali-Treated Hemp Stem Fiber on the Structure, Properties, and Soil Biodegradation of Poly(butylene succinate)/Poly(lactic acid) Biocomposites

The development of biodegradable polymer composites from renewable resources is important for reducing dependence on petroleum-based plastics and improving the sustainability of short-life-cycle products. However, poly(butylene succinate) (PBS)/poly(lactic acid) (PLA) blends are generally immiscible, and the reinforcing effect of alkali-treated hemp stem fiber (HSF) on their mechanical, thermal, rheological, morphological, and biodegradation behavior remains insufficiently understood. This study investigates PBS/PLA biocomposites prepared at fixed blend ratios of 90/10 and 80/20 wt.% and reinforced with 5–20 phr alkali-treated HSF. Hemp stem fibers are treated using 5 wt.% NaOH, dried, sieved, and compounded with PBS/PLA blends in an internal mixer at 190 °C and 50 rpm for 15 min, and fabricated by hot compression molding at 190 °C for 13 min. The incorporation of HSF improves composite stiffness, with the highest Young’s modulus of 306 MPa observed for the 80/20/15 composition, representing a 71% increase over neat PBS. The maximum flexural strength reaches 48 MPa for 90/10/20, while the highest flexural modulus reaches 1377 MPa for 80/20/20, representing improvements of 21% and 77%, respectively. In contrast, tensile stress at break and elongation at break generally decrease with HSF incorporation because localized fiber agglomeration, incomplete matrix wetting, and interfacial gaps limit effective stress transfer, particularly at higher HSF loadings. The MFR exhibits composition-dependent, non-monotonic behavior. The addition of PLA initially increases the MFR relative to neat PBS, whereas higher HSF loadings generally reduce the MFR due to restricted polymer chain mobility and increased resistance to melt flow. FT-IR results indicate no strong chemical interactions among PBS, PLA, and HSF, while DSC shows nearly unchanged melting temperatures but composition-dependent changes in PBS crystallinity and crystallization behavior. FESEM confirms a phase-separated PBS/PLA morphology with embedded HSF. Soil burial tests show increased weight loss with higher HSF content and exposure time, confirming enhanced biodegradation.