Enhancing mechanical, wear, and UV-shielding properties of unsaturated polyester resin using graphitic carbon nitride

Unsaturated polyester resin (UPR) is the highest-volume thermoset matrix globally yet reports on graphitic carbon nitride (GCN)-reinforced UPR composites remain scarce and largely restricted to hybrid formulations with additional functional additives. This study investigates pristine GCN, synthesized via thermal polycondensation of urea, as a standalone multifunctional nanofiller in UPR across a broad concentration range (0.3–5 wt%), without any supplementary additives. XRD and FTIR analyses confirmed that GCN nanosheets were physically confined within the polyester network through hydrogen bonding and van der Waals interactions, without forming new covalent bonds. SEM revealed increasing filler agglomeration with rising GCN content, directly governing the composites’ mechanical response. At a trace loading of 0.3 wt% (GCN-03), tensile strength and elongation at break simultaneously increased by 17.4% and 43%, respectively, alongside a peak unnotched Charpy impact strength, reflecting efficient stress transfer and crack-deflection mechanisms at low filler content. Beyond this optimum, progressive agglomeration at 1–5 wt% reduced tensile and impact performance, although all GCN-loaded composites still outperformed neat UPR in strength and ductility. Conversely, higher GCN loadings favored functional properties: GCN-5 achieved the lowest abrasive wear loss (367.53 mm³, ∼21% improvement), and all GCN-containing composites met the UL-94 horizontal burning classification, unlike neat UPR. Optical analysis showed that GCN incorporation sharply narrowed the direct and indirect band gaps (from 3.87 to 2.75 eV and 2.96 to 1.79 eV, respectively) and enhanced UV-visible absorption. These findings establish clear structure–property relationships for GCN/UPR composites and demonstrate that optimal filler loading depends on the targeted application, offering a cost-effective, multifunctional alternative to more complex epoxy-based systems for structural and outdoor uses.