Properties and Potential of Green Natural Rubber Composites Filled with Biofillers from <i>Cassia bakeriana</i> Craib and <i>Cassia fistula</i> L. Pods

This study investigates Cassia bakeriana (PS) and Cassia fistula (GS) pod powders as sustainable biofillers for natural rubber (NR), aiming to offer eco-friendly alternatives to conventional fillers. The powders were characterized and incorporated into NR to evaluate their effects on bound rubber content, cure behavior, cross-link density, and mechanical and dynamic properties. Both fillers were rich in organic components; cellulose, hemicellulose, lignin, proteins, and fatty acids with minor calcium oxalate and surface hydroxyl groups that promoted rubber-filler interaction. Nitrogen-containing compounds in the powders contributed to cure acceleration. GS powder outperformed PS in terms of scorch and cure times, hardness, modulus, and reinforcement, indicating more effective filler-rubber interaction. However, both fillers exhibited lower reinforcing efficiency than carbon black (CB), mainly due to their larger particle sizes. Tensile strength peaked at 20 phr for PS (20 MPa), about 11 and 22% lower than GS and CB, respectively. GS-filled composites also showed higher storage modulus and lower tan δ, suggesting greater stiffness and reduced energy loss. These results confirm that PS and GS powders can function as renewable, partially reinforcing fillers in NR, with GS offering superior mechanical and dynamic performance. Their use also shortens cure time and supports sustainable waste utilization. Given their moderate reinforcement and environmental benefits, PS and GS powders are suitable for noncritical rubber products such as shoe soles, mats, gaskets, and general-purpose molded goods, where cost efficiency and sustainability are prioritized over high-performance requirements.