Synergistic effect of modified unsaturated polyester and epoxy hybrid matrix on the thermomechanical and chemical performance of glass fiber-reinforced composites

In this study, β-naphthol-based tri-functional epoxy resins were synthesized and subsequently reacted with maleic anhydride and various dibasic acids to prepare the modified unsaturated polyester resins (UPRs). Glass fiber-reinforced composites were fabricated via the hand lay-up technique using methyl ethyl ketone peroxide as a curing initiator. The mechanical, thermal and chemical properties of the cured composites were systematically evaluated. UPRs derived from aromatic dibasic acids exhibited higher molecular weight, enhanced thermal stability and superior mechanical performance compared to those derived from aliphatic acids. The developed composites showed tensile strength in the range of 104.2–111.9 MPa and flexural strength of 179.5–185.2 MPa, indicating good load-bearing capacity and bending resistance. Rockwell hardness values (67.4–73.2) suggested adequate surface rigidity, while Izod impact strength (7.10–9.21 J/cm) demonstrated satisfactory energy absorption capability. The activation energy in the range of 4.65–9.05 kcal/mol indicates moderate thermal stability and curing reactivity of the composite system. Scanning electron microscopy analysis revealed improved fiber–matrix interfacial adhesion in the modified systems. Overall, the results clearly establish that β-naphthol modification significantly enhances the mechanical properties of glass fiber-reinforced composites compared to conventional epoxy systems.