Particle size-dependent effects of Nano-SiO2 on interfacial regulation and moisture absorption behavior of silane-treated ramie fabric/epoxy composites

The hygroscopicity of ramie fibers and instability of the fiber–matrix interface limit the mechanical performance of ramie fabric/epoxy composites under moisture exposure. This study investigated how nano-SiO 2 particle size regulates interfacial morphology, moisture absorption, and wet-state tensile performance when incorporated into a 3-aminopropyltriethoxysilane (APS) treatment system. SiO 2 particles of 100, 330, and 550 nm were deposited on ramie fabric, followed by surface characterization, mechanical testing, and 20-day immersion in deionized water and 3.5 wt% NaCl solution. Particle coverage changed from incomplete deposition at 100 nm to a relatively uniform layer at 330 nm and localized agglomeration at 550 nm. APS treatment increased the tensile strength from 23.74 to 62.16 MPa. The 330 nm treatment produced the highest flexural strength and modulus, whereas the 550 nm treatment reduced quasi-equilibrium moisture uptake to approximately 10–11%. After deionized-water immersion, untreated RA/EP retained 43.60% of its initial tensile strength, compared with 76.72–88.09% for the surface-treated composites. Following NaCl exposure, the surface-treated composites retained tensile strengths comparable to their initial dry-state values. These findings establish a particle-size–coverage–property relationship and show that controlling SiO 2 distribution is essential for balancing mechanical performance and moisture-exposure stability in ramie fabric/epoxy composites.