To enhance the flame retardancy of epoxy resin (EP), microcapsules (SiO2@APP) were prepared by encapsulating ammonium polyphosphate (APP) with silicon dioxide (SiO2). Subsequently, a ternary intumescent flame retardant (IFR) system of SiO2@APP/MCA/CaG was formulated, using SiO2@APP as the primary flame retardant and acid source, melamine cyanurate (MCA) as the gas source, and calcium gluconate (CaG) as the carbon source. It was then employed for the flame retardant modification of EP. The thermal stability, flame retardancy, and mechanical properties of the EP composites were investigated through thermogravimetric analysis (TG), limiting oxygen index (LOI), UL-94 vertical burning test, cone calorimeter (CONE), scanning electron microscopy (SEM), and universal material testing machine. The results indicate that the SiO2@APP/MCA/CaG composite flame retardant can enhance the thermal stability and carbon residue rate of EP materials. When the mass ratio of SiO2@APP/MCA/CaG is 4:1:1, the residual carbon rate of EP composites at 600°C is 34.8%, and it gets the highest LOI value of 40.5%, and pass a UL-94 rating of V-0. Furthermore, compared to pure EP, the total heat release (THR) and total smoke production (TSP) are reduced by 68.8% and 66.7%, respectively. SEM testing reveals that the SiO2@APP/MCA/CaG flame retardant system can enhance the graphitization degree of the char residue in EP materials, resulting in a more compact structure. Furthermore, mechanical testing demonstrates that the addition of SiO2@APP/MCA/CaG composite flame retardant reduces the tensile strength of EP, yet it exhibits better tensile performance compared to EP composites with APP flame retardant alone. Based on the comprehensive test results of flame retardancy and mechanical properties, the SiO2@APP/MCA/CaG/EP composite is suitable for applications that require high flame retardancy but have relatively low demands on mechanical properties.
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