In this work, the effect of thermo-oxidative aging on organic montmorillonite/intumescent flame retardant/long glass fiber–reinforced polypropylene composites was investigated for different exposure times at 140°C. Limiting oxygen index, Underwriters Laboratories-94 tests, cone calorimeter test, and thermogravimetric analysis were used to evaluate the flammability and thermal stability. The results of limiting oxygen index values, Underwriters Laboratories 94 test, and cone calorimeter test show that aging performs negative effect on the flame retardancy of organic montmorillonite/intumescent flame retardant/long glass fiber–reinforced polypropylene composites. Thermal oxidation aging markedly changes the decomposition process of organic montmorillonite/intumescent flame retardant/long glass fiber–reinforced polypropylene composites. The scanning electronic microscopy images of the external surface of composites indicate that many ground particles and micro-scale cracks are scattered in the surfaces of the composites after aging. The sharp micro-scale cracks and crazing formed on the surface promote the heat and oxygen to penetrate into the bulk of polypropylene matrix. According to the mechanical test results, the thermal oxidation aging reduces the tensile, flexural, and notched impact strengths of organic montmorillonite/intumescent flame retardant/long glass fiber–reinforced polypropylene composites.
