Enhanced flame retardancy of unsaturated polyester resin composites containing ammonium polyphosphate and metal oxides

Three metal oxides (MOs) (Fe₂O₃, Sb₂O₃, Al₂O₃) were incorporated into blends of unsaturated polyester resin (UPR)/ammonium polyphosphate (APP) composites with the aim of studying and comparing their synergistic effect on flame retardancy with APP. The UPR-APP/MOs composites were prepared, then the thermal stability and flame-retardant properties of the UPR composites were evaluated by thermogravimetric analysis (TGA), limiting oxygen index (LOI), vertical burning test (UL-94), and cone calorimetry test (CCT). Besides, residues after CCT were characterized by scanning electron microscopy and laser Raman spectroscopy. The LOI values of the UPR composites with 0.5 wt% MOs increased to around 40%. However, the CCT results indicated that the incorporation of Sb₂O₃ brought an increase in the total heat release. Moreover, these three MOs had different effects on the process of thermal degradation of UPR composites from the TGA results. Based on the above results, Al₂O₃ provided a best promotion on flame retardancy among three MOs.