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Five SO42 /TiO2-MXOY solid super acids (SSA: Cu@Ti:SSA,Zr@Ti:SSA,Fe@Ti:SSA,Mn@Ti:SSA,Mo@Ti:SSA) were successfully prepared by sol-gel method,and its chemical structure and element content were characterized by X-ray diffraction (XRD) analysis,Fourier transform infrared spectroscopy (FrIR) and energy dispersive spectroscopy (EDS).Then,a compound flame retardant system containing SSA and ammonium polyphosphate/montmorillonite (AM) were was introduced into an unsaturated polyester resin (UPR) to enhance the flame retardance.The effect of SSA on the flammability and thermal stability of the UPR was evaluated by the limiting oxygen index (LOI),cone calorimeter test (CCT) and thermogravimetric analysis (TGA).The LOI results showed that the flame retardance of the UPR composites was significantly improved with the addition of SSA.Moreover,their heat release rate (HRR),total heat release (THR),the smoke production rate (SPR) and CO and CO2 yield were much decreased.In addition,the initial decomposition temperature of UPR/AM/SSA was delayed,indicating that their thermal stability was increased,and the residual char of UPR/AM/SSA was also increased due to strong catalytic of SSA ability for esteriflcation and dehydration.Furthermore,the microstructure of the residual char after combustion of the UPR composites was studied by the scanning electron microscopy (SEM),and it was found that the char layer structure was more continuous and dense after the addition of the SSA.In sum up,the synergistic effect between SSA and AM was the main factor for the great improvement of flame retardant of UPR composites.