“Hydroquinone-based multifunctional unsaturated polyester–epoxy resin systems and its carbon fiber-reinforced composites: Fabrication, characterization, and structure–property relationships”

Unsaturated polyester resins (UPRs) are widely used thermosetting polymers; however, their inherent brittleness and limited thermal stability restrict their application in high-performance structural composites. In this study, a novel tetra-functional epoxy-based modification strategy was developed to enhance the performance of UPR systems. A tetra-functional epoxy resin was synthesized from hydroquinone, formaldehyde, and epichlorohydrin, followed by its reaction with maleic anhydride and various dibasic acids to prepare modified unsaturated polyester resins. Carbon fiber-reinforced composites were fabricated by the hand lay-up technique and cured using methyl ethyl ketone peroxide (MEKP). The modified composites exhibited tensile strengths of 123.4–130.2 MPa, flexural strengths of 212–230 MPa, Rockwell hardness values of 143.1–150, and Izod impact strengths of 12.5–15.2 J/cm. Among the developed composites, HTEC exhibited the highest tensile strength, flexural strength, hardness, and thermal stability, whereas HSEC showed the highest impact strength. Activation energy values ranged from 5.16 to 9.87 kcal mol −1 . Scanning electron microscopy (SEM) revealed improved fiber–matrix interfacial adhesion, confirming the effectiveness of the tetra-functional epoxy modification in enhancing the overall performance of carbon fiber-reinforced composites for advanced structural applications.