Study of Hybrid Epoxy Resin Composites Reinforced with E-CR Glass Fiber/Polypropylene: Moisture Behavior, Mechanical Strength, and Acoustic Damage Mitigation Under Marine Aging Conditions

The durability of fiber-reinforced epoxy composites in marine environments depends on the interaction among interfacial characteristics, moisture transport, and damage evolution. However, it remains poorly understood whether E-CR/PP hybridization merely provides a secondary reinforcing phase or also modifies moisture-transport and load-transfer mechanisms. This study investigated the effects of incorporating E-CR glass fibers and polypropylene (PP) fibers on moisture absorption, mechanical response, and damage development in an epoxy matrix. The reinforced composites were subjected to short-term seawater immersion for up to 30 days. FTIR-ATR, AFM, micromechanical testing, moisture absorption and diffusion analysis, flexural testing, and acoustic emission were combined to relate surface characteristics and interfacial behavior to moisture transport and mechanical performance. The neat epoxy matrix showed progressive moisture uptake and a 22.03% reduction in acoustic-wave propagation velocity, consistent with moisture-induced plasticization and reduced effective stiffness. The incorporation of 5 wt.% PP was associated with lower moisture uptake in composites containing equivalent E-CR contents and with a reduction in the estimated effective diffusion coefficient. The CPPV formulation, containing 50 wt.% E-CR and 5 wt.% PP, exhibited the lowest measured moisture uptake (1.20%) and effective diffusion coefficient (2.96 × 10−12 m2 s−1) among the formulations investigated during the early immersion period. After 30 days of seawater immersion, CPPV exhibited a residual flexural strength of approximately 600 MPa, compared with approximately 420 MPa for the corresponding 50 wt.% E-CR composite without PP (CFV). These findings indicate that PP does not act solely as a secondary reinforcing phase; rather, E-CR/PP hybridization modifies the effective moisture-transport environment and load-transfer response of the epoxy composite. Within the 30-day exposure period investigated, the hybrid architecture therefore provided improved resistance to moisture ingress and higher residual flexural strength. Longer-term exposure studies are required to establish whether these advantages persist under prolonged marine aging.