From straw waste to circular composites: A systematic comparison of untreated fillers in PBS, PBAT and their blends through performance, morphology and sustainability assessment

In this study, wheat straw (WS) and rice straw (RS) were added up to 30 wt% in maleic anhydride grafted polybutylene succinate (PBS) and poly(butylene adipate-co-terephthalate) (PBAT) using a co-rotating twin-screw extruder. Injection molded samples were analyzed using tensile, flexural, impact, Shore D hardness, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), heat deflection temperature (HDT), and melt mass flow rate index (MFR). Increasing lignocellulosic filler content reduced the thermal stability of PBAT up to 26.6% for 30 wt% RS, while the associated decline in crystallinity at higher filler contents indicates that neither RS nor WS act as nucleating agents. All formulations demonstrated enhanced tensile and flexural stiffness with increasing contents of RS and WS. Tensile strength peaked at 41.3 MPa (+22%) for PBS and 15.7 MPa (+43%) for PBAT filled with 30 wt% WS. WS provided higher performance compared to RS, owing to its superior fraction of fine particles and higher specific surface (increase of 260%). The rough surface and the presence of maleic anhydride improved mechanical interlocking and adhesion at the matrix/filler interface. The fillers also enhanced mechanical reinforcement, increasing the storage modulus. The heat deflection temperature (HDT-A) increased to 82.0 °C (+38%) for PBS with 30 wt% RS. PBAT reached a maximum of 55.1 °C (+12%) for 30 wt% WS. Higher filler content reduced the MFR to 2–16 g/10 min, while remaining suitable for additive manufacturing processing.