Biodegradable PBS/TPS Bioplastic Composites With Spent Coffee Grounds: Thermal, Melt Flow, and Mechanical Properties

Biopolymer composites have gained increasing attention for sustainable applications using residual materials. This study investigates the incorporation of treated spent coffee grounds (SCG) into poly(butylene succinate) (PBS) and thermoplastic starch (TPS) blends at a fixed ratio of 80/20 wt.%, aiming to improve their mechanical, thermal, and biodegradation performance. The novelty of this work lies in utilizing silane-treated SCG as a natural filler within PBS/TPS blends, systematically examining its effect on modulus, strength, flowability, and soil biodegradability. Adding 10 phr SCG enhanced the stiffness of the composites, increasing the flexural modulus by 40% and Young's modulus by 24% compared to the PBS/TPS (80/20/0) formulation. Furthermore, SCG incorporation significantly accelerated biodegradation, with up to ~38% weight loss after 60 days. However, the results also revealed drawbacks: tensile strength and flexural strength decreased with increasing SCG fractions, primarily due to particle agglomeration and limited interfacial adhesion, as confirmed by FE-SEM analysis. Thermal analysis indicated slight reductions in melting enthalpy and crystallinity without significant changes in glass transition temperature. Overall, PBS/TPS/SCG composites demonstrate potential as biodegradable packaging materials, offering both stiffness enhancement and improved degradability. Nonetheless, future research should address the observed surface modification strategies to better balance stiffness, strength, and durability.