Green Fiber-Reinforced Laminates: Styrene-Free UPe with VTES-Functionalized ZrO₂ and Flax Fabrics

Natural fiber-based composites are gaining attention as sustainable alternatives to synthetic fiber-reinforced materials. Herein, styrene-free unsaturated polyester (UPe) nanocomposites and flax-fabric laminates reinforced with vinyl-triethoxy-silane (VTES) functionalized zirconia nanoparticles (ZrO₂-VS) were studied. Nanoparticles were dispersed by high-shear mixing, and ZrO₂-VS was benchmarked against unmodified ZrO₂ and neat UPe. Fourier-transform infrared spectroscopy (FTIR) tracked cure conversion; scanning electron microscopy (SEM/EDS), tensile testing, and dynamic mechanical analysis (DMA) evaluated structure-property relationships. ZrO₂-VS improved dispersion and interfacial adhesion, yielding higher tensile strength and storage modulus compared with unmodified ZrO₂. In flax-fabric laminates, ZrO₂-VS/UPe achieved a tensile strength of 72.2 ± 3.6 MPa, exceeding both unmodified ZrO₂/UPe and neat UPe controls. DMA showed pronounced increases in storage modulus across temperature with small, non-significant changes in Tg. These results highlight a low-styrene-hazard UPe matrix and natural fiber reinforcement pathway to improved mechanical performance via silane-mediated nanoparticle-matrix-fiber bridging.