Integrated analysis of Cosmos sulphureus stem fibers as thermally stable and eco-compatible reinforcements in biocomposites

The search for sustainable reinforcements in composite materials has increased interest in lignocellulosic fibers as eco-friendly alternatives to synthetic fibers. This study presents the first systematic evaluation of Cosmos sulphureus (CS) stem fibers, extracted via water retting, with emphasis on their antimicrobial, mechanical, morphological, and thermal properties. Antimicrobial activity: Agar diffusion assays against Staphylococcus aureus demonstrated concentration-dependent inhibition, with zones of 11 ± 0.5 mm (25 µg) and 17 ± 0.7 mm (50 µg), compared to 24 ± 0.6 mm for streptomycin (10 µg). Confocal laser scanning microscopy further confirmed biofilm disruption through degradation of the extracellular polymeric matrix. Mechanical performance: Tensile testing yielded a strength of 11.47 ± 0.42 MPa, elongation at break of 0.82%, and Young's modulus of 1.4 GPa, indicating moderate strength but adequate stiffness for non-structural composite applications. Morphology: SEM micrographs revealed fibrillated surfaces, open lumens, and surface roughness, which are beneficial for polymer infiltration and fiber-matrix interfacial adhesion. Thermal stability: Thermogravimetric analysis showed an onset degradation at 346.62 °C, peak decomposition at 381.42 °C, and a residual char yield of 19.87% at 600 °C, underscoring appreciable thermal resilience. Although CS fibers exhibit lower tensile strength than bast fibers such as jute and flax, their adequate stiffness, intrinsic antimicrobial activity, and superior thermal stability establish them as eco-compatible reinforcements. These multifunctional attributes highlight their potential for non-structural applications, particularly in antimicrobial packaging, biomedical composites (subject to biocompatibility validation), and other sustainable engineering materials.