Physical, Mechanical, and Flammability Properties of Glass Fiber/Unsaturated Polyester Resin Composites: The Effect of Post-Curing Time

Abstract

Post-curing is commonly used to increase the degree of cure in thermoset composites. However, prolonged heat exposure can degrade composite performance. This study examines how post-curing time affects the chemical, thermal, morphological, physical, mechanical, and combustion properties of Glass Fiber-Reinforced Unsaturated Polyester Resin (GF/UPR) composites. The GF/UPR laminates were fabricated by hand lay-up using a CSM–WR–CSM stack-up and then post-cured at 140 °C for 60, 90, 120, and 150 min. The composites were characterized using Fourier Transform Infrared Spectroscopy (FTIR), Thermogravimetric Analysis (TGA), Scanning Electron Microscopy (SEM), density and porosity measurements, flexural testing, impact testing, and horizontal burning tests. FTIR results showed that post-curing did not alter the main functional groups or form new dominant chemical structures in the GF/UPR composites. The best performance was achieved at 60 min, with the highest density of 1.22 g/cm³, the lowest porosity of 1.555%, the highest flexural strength of 103.13 MPa, the highest impact strength of 18.15 kJ/m², the longest ignition time of 88.6 s, and the lowest burn rate of 0.301 mm/s. In contrast, post-curing for 150 min reduced the flexural strength to 71.39 MPa, the impact strength to 6.83 kJ/m², and the ignition time to 71.4 s, while increasing the porosity and burning rate. SEM results confirmed that this decline in performance was due to delamination and fiber pull-out, resulting from a weakened fiber-matrix bond. These findings indicate that post-curing for 60 min at 140 °C is the most effective condition for achieving a balance among thermal stability, structural integrity, mechanical properties, and fire resistance in GF/UPR laminates.