Cocoa bean shells as functional fillers in PBS and PBAT biocomposites: Effects of filler content, particle size, and storage on material properties

This study investigated the potential of cocoa bean shells (CBS), a lignocellulosic agro-industrial sidestream, as a functional filler in biodegradable polymer composites based on poly(butylene succinate) (PBS) and poly(butylene adipate-co-terephthalate) (PBAT). The objective was to assess the influence of CBS content, particle size, and storage duration on the structural, mechanical, and surface performance of the resulting materials. Biocomposites containing 30 – 60 wt% CBS with particle sizes < 206 µm and 400 – 570 µm were produced via compounding and injection molding. Thermal, mechanical, and physical properties were analyzed before and after three months of storage under controlled conditions. CBS addition increased stiffness, with elastic modulus rising from 675 to 1130 MPa in PBS and from 51 to 277 MPa in PBAT at 60 wt%, while tensile strength decreased from 34.5 to 11.3 MPa and 18.0–5.5 MPa, respectively. Finer particles improved filler dispersion and reduced water uptake from 11.9 % to 9.2 % in PBS, while water contact angle in PBAT increased from 88.3° to 92.0°, indicating a shift toward hydrophobic surfaces. Thermal and mechanical properties remained stable during storage, with only minor changes in surface characteristics and appearance. These findings demonstrate that CBS is a viable bio-based filler for tailoring biodegradable composites in sustainable product applications. • CBS was incorporated into PBS and PBAT at up to 60 wt% filler content. • CBS particle size had minor effects on thermal and mechanical properties. • Thermal and mechanical properties stayed stable under moderate storage conditions. • CBS is a promising bio-based filler for enhancing sustainability of biopolymers.