Valorization of Eggshell Waste in Woven Sisal Fiber/Polyester Composites: Synergistic Enhancement of Physical, Mechanical, and Thermal Performance

This study investigates the influence of waste eggshell powder (ESP) on the physical, mechanical, and thermal properties of woven sisal fiber-reinforced polyester composites. Five formulations containing 0–8 wt.% ESP were evaluated to identify the optimum filler concentration. The results indicated that 6 wt.% ESP provided the best overall mechanical performance, achieving tensile strength, flexural strength, impact energy, and interlaminar shear strength of 93.80 MPa, 110.87 MPa, 5.98 J, and 18.31 MPa, respectively. Compared with the unfilled composite, tensile and flexural strengths increased by 18.8% and 19.4%, while water absorption decreased by approximately 16.1%. Scanning electron microscopy (SEM) observations indicated a comparatively compact fracture morphology and improved apparent fiber–matrix contact in the 6 wt.% ESP composite. Thermal conductivity and heat deflection temperature increased progressively with ESP content, reaching 0.372 W/m·K and 84.98 °C, respectively, at 8 wt.% ESP. However, several mechanical properties declined slightly at this concentration, suggesting that excessive filler incorporation may compromise composite integrity. One-way analysis of variance (ANOVA) revealed statistically significant differences among the five composite formulations for all eight evaluated properties (p < 0.001), confirming the significant influence of ESP content on composite performance. The findings identify 6 wt.% ESP as a promising formulation for balancing mechanical performance and moisture resistance, while higher ESP loading favors thermal performance. This approach offers potential for converting eggshell waste into a value-added filler for sustainable natural-fiber polymer composites.