Reactive diluent-induced trade-offs: unraveling the interplay between resin flow and volatility for multilayer GFRP

• Effect of reactive diluent on glass fiber-reinforced polymer (GFRP) is examined. • Optimal 20 wt% diluent boosts Charpy impact by 19.4% and shear strength by 48.1% • Diluent content imbalance causes voids, degrading GFRP’s structural integrity. • Balancing viscosity and volatility is key to maximizing GFRP performance. • Simulations reveal void-induced stress concentration under impact and shear loading. The incorporation of reactive diluents is essential for improving the processability and performance of glass fiber-reinforced polymers (GFRPs), particularly in advanced applications. This study systematically explores the effect of reactive diluent (styrene) content (0, 20, 40, and 60 wt%) on the structural and mechanical properties of GFRPs fabricated via vacuum-assisted resin transfer molding. Rheological, thermal, and mechanical analyses reveal that 20 wt% diluent optimally enhances Charpy impact strength and interlaminar shear strength while minimizing porosity and improving fiber–matrix bonding. However, excessive diluent content (40–60 wt%) increases volatility, resulting in higher porosity and mechanical deterioration. Representative volume element-based simulation accurately predicts macroscopic mechanical properties, while unit-cell-based modeling demonstrates localized stress concentration at voids under impact and shear loading, elucidating failure mechanisms. Microstructural assessments using field-emission scanning electron microscopy, micro-computed tomography, burn-off, and Archimedes techniques confirm the correlation between void formation and mechanical degradation. This study reveals a critical balance: while increasing the reactive diluent content lowers resin viscosity for better fiber impregnation, it also heightens volatility, causing void formation. These findings highlight the importance of balancing viscosity reduction and volatility control, offering key insights into optimizing reactive diluent content for high-performance GFRPs.