Effect of extrusion processing routes on morphology and properties of thermoplastic starch/PBS blends

Abstract The performance of starch-based polymer blends for packaging applications is strongly influenced by processing conditions, particularly when low synthetic polyester content is used to reduce costs and increase renewable content. In this study, thermoplastic starch (TPS), citric acid–crosslinked thermoplastic starch (TPSR), and their blends with poly(butylene succinate) (PBS) containing reduced PBS contents (10–30 wt%) were prepared by extrusion using three distinct processing routes: M1 consisted of a single-step extrusion with simultaneous addition of all blend components, M2 involved reprocessing previously prepared TPS with the remaining components, and M3 employed a two-step process in which TPS containing PBS was first prepared and subsequently blended with the other components. Mechanical testing showed that PBS incorporation significantly enhanced stiffness and tensile strength compared to neat TPS and TPSR; however, the extent of this effect strongly depended on the extrusion route. Route M2 provided a more balanced combination of strength and ductility, whereas route M3 favored higher stiffness at comparable PBS contents. Hydrophobicity increased with both PBS incorporation and starch crosslinking, although no single processing route consistently maximized water resistance across all compositions. Thermogravimetric analysis indicated that all blends exhibited intermediate thermal stability between those of the neat polymers, reflecting the coexistence of starch-rich and PBS-rich phases. Scanning electron microscopy revealed processing-dependent differences in phase dispersion and deformation behavior, in good agreement with the mechanical performance. Because the materials were obtained by reactive melt processing in the presence of citric acid and glycerol, in vitro cytotoxicity assays were conducted as a preliminary safety screening to evaluate potential extractable species generated during processing. The TPSR/PBS blend containing 30 wt% PBS processed by route M3 showed no cytotoxic effects under direct contact conditions and exhibited reduced cell viability only at unrealistically high extract concentrations. Overall, the results demonstrate that extrusion processing routes play a central role in controlling morphology and property balance in thermoplastic starch/PBS blends, enabling meaningful performance gains at reduced polyester contents and providing a viable pathway for the development of cost-effective starch-based materials for packaging applications.