This research investigates the mechanical, viscoelastic, fatigue and thermal performance of vinyl ester composites reinforced with 30 vol. % natural fiber and varying contents of agro-seed shell (ASS) particles, subjected to alkali-silane surface treatment. The alkali treatment was employed to remove surface impurities and enhance surface roughness while silane modification was used to improve interfacial compatibility between the hydrophilic reinforcements and hydrophobic vinyl ester matrix. Six composite formulations (MU0-MU2 and MT0-MT2) were fabricated alongside a neat resin control (M). Comprehensive characterization included tensile, flexural, impact, shore D hardness, fatigue life at different stress levels, time-dependent creep deformation and thermal conductivity measurements. The results demonstrate significant enhancement in mechanical performance for the treated composites with the MT1 formulation (3 vol. % treated ASS) exhibiting optimum properties including superior tensile and flexural strength, maximum impact resistance, improved fatigue life and reduced creep strain. Thermal conductivity decreased with increasing reinforcement content indicating enhanced insulation capability without compromising structural integrity. The synergistic effect of long-range load transfer by fibers, stress redistribution by ASS particles and strong interfacial bonding achieved through alkali-silane treatment was found to be responsible for the improved overall performance. The findings confirm that controlled surface modification and optimized filler loading substantially enhance the durability and dimensional stability of hybrid vinyl ester composites.
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