Flame retardancy, mechanical, and viscoelastic properties of GFRP composites filled with aluminium tri-hydroxide and ammonium polyphosphate

Glass fiber-reinforced polymer (GFRP) composites are widely used in transportation and structural applications because of their high specific strength, corrosion resistance, and design flexibility. However, the flammability of the GFRP matrix limits its application at elevated temperatures. This study aimed to improve the flame retardancy of GFRP by adding aluminium tri-hydroxide (ATH), ammonium polyphosphate (APP), and hybrid ATH/APP combinations into an unsaturated polyester (UP) resin matrix. The effects of filler addition were evaluated through fire reaction testing, thermal characterization, Fourier transform infrared spectroscopy (FTIR), flexural testing, morphological observation, and dynamic mechanical analysis (DMA). Fourteen composite variants were fabricated via hand lay-up and press processing: unfilled GFRP as the control, single-filler GFRP containing 10 wt% to 50 wt% ATH or APP, and three hybrid ATH/APP. The +ATH50, +ATH40+APP5, +ATH40+APP10, and +ATH50+APP5 showed that the flame was extinguished before reaching the reference mark used to initiate the measurement of the burning length and time, resulting in a burning rate of 0 mm/min. The SEM observation of the burned surfaces revealed that the hybrid ATH/APP composites formed a thicker and more continuous protective barrier layer than the single-filler composites. The TGA results showed that the maximum mass loss temperature (T max ) of the hybrid ATH/APP composites was higher than that of the single-filler composites, supported by a lower derivative mass change (DMC) relative to unfilled GFRP. The hybrid ATH/APP combination increased T max by up to 16.3%. DSC measurements indicated that this increase in T max was primarily attributed to endothermic heat absorption during ATH decomposition, while APP likely contributed by enhancing the formation of protective barriers. Flexural test results showed that although filler loadings above 40 wt% generally reduced flexural strength, +ATH40+APP5 showed the smallest reduction relative to unfilled GFRP. The DMA results further showed that +ATH40+APP5 possessed the highest storage modulus, loss modulus, and tan δ among the hybrid composites, indicating good resistance to dynamic loading and elevated temperature. Overall, +ATH40+APP5 provided the best balance of flame retardancy, thermal stability, and mechanical performance for structural GFRP applications.