Synergistic action of expandable graphite on fire safety of a self‐intumescent flame retardant epoxy resin

Previously, it was reported that epoxy resin (EP) filled with ammonium polyphosphate (APP) and copper (I) oxide (Cu 2 O) at a mass ratio of 8/2 (APP8‐Cu 2 O2) as a self‐intumescent system demonstrated promising fire retardancy. To further improve the flame retardant efficiency, the possibility of expandable graphite (EG) as an effective synergist for the self‐intumescent EP system was revealed by limiting oxygen index (LOI) test. The results showed that the incorporation of EG increased the LOI value of EP/APP8‐Cu 2 O2 obviously. The highest LOI value was obtained at the EG/APP8‐Cu 2 O2 mass ratio of 3/7, indicating the optimal synergistic effect being achieved. Furthermore, UL‐94 test results showed that the excellent synergistic effect resulted in the addition of 13 wt% EG/APP8‐Cu 2 O2 of 3/7 not only endowed EP a relatively high LOI value of 34.3%, but also made it pass UL‐94 V‐0 rating. Moreover, the main fire hazard parameters obtained from cone calorimeter tests, such as peak heat release rate, total smoke production, and peak CO production were reduced 40.3%, 30.3%, and 46.2%, respectively by the combination effect of EG/APP8‐Cu 2 O2 with mass ratio of 3/7, suggesting the excellent improvement in the fire safety of EP significantly. Finally, a possible action mode, which would be beneficial for developing other flame retardant polymers with high fire safety, was proposed.