Enhancing Natural Fiber‐Reinforced Biocomposites via Biodegradable Polycaprolactone ( PCL ) Coatings

This study introduces a novel interfacial modification strategy for flax fiber‐reinforced bio‐epoxy composites via polycaprolactone (PCL) coating. Bidirectional flax fabrics were first alkali‐treated with NaOH and subsequently immersed in PCL/dichloromethane solutions with systematically varied polymer concentrations (2.5, 5, and 10 wt%) and immersion times (10, 20, and 40 s). SEM characterization revealed that alkaline pretreatment enhanced fiber surface energy and exposed reactive hydroxyl groups. Achieving a continuous coating over the rough fiber surface required a PCL concentration of at least 5 wt%. While solution concentration governed total polymer deposition and fiber thickness, longer immersion times yielded a denser, lower‐porosity PCL coating. Notably, although surface porosity enhanced mechanical interlocking and fiber–matrix compatibility, it slightly compromised fiber thermal stability. The mechanical performance of composites was governed primarily by PCL concentration, whereas immersion time exerted minimal influence on interlaminar stress transfer. Coatings obtained at or above 5 wt% PCL substantially enhanced toughness and damping behavior. Overall, among all parameters evaluated, the 10 wt% PCL coating applied for 10 s emerged as the optimal condition, maximizing interfacial adhesion and composite performance without requiring prolonged processing.