<![CDATA[Natural fiber-reinforced composites have emerged as promising alternative materials due to their low density, environmental friendliness, and favorable mechanical properties. Sisal fiber (Agave sisalana) is one of the most widely used natural fibers as a composite reinforcement because of its excellent mechanical performance. However, its hydrophilic nature results in poor interfacial bonding with polymer matrices, which adversely affects the mechanical performance of the composite. Alkali treatment using a 5% sodium hydroxide (NaOH) solution can improve the fiber surface characteristics, while the fiber volume fraction plays a significant role in determining load transfer efficiency and the composite's ability to absorb impact energy. This study aims to investigate the effects of sisal fiber soaking time in a 5% NaOH solution (120, 180, and 240 minutes) and fiber volume fractions of 35%, 50%, and 65% on the impact strength of polyester-based composites. The composites were fabricated using the hand lay-up method with a vertical fiber orientation. The results showed that the highest impact strength of 0.22409 J/mm² was obtained for the composite treated with a 240-minute soaking time and a 35% sisal fiber volume fraction with 65% polyester matrix. This improvement in impact strength indicates that the selected combination of immersion time and fiber volume fraction enhanced the interfacial bonding between the fiber and the matrix, thereby improving load transfer efficiency and impact energy absorption.]]>
