The influence of expandable graphite on double-layered microcapsules in intumescent flame-retardant natural rubber composites

To improve both the mechanical properties and flame-retardant performance of flame-retardant natural rubber composite, double-shell co-microencapsulated ammonium polyphosphate and expandable graphite (EG) (M(A&E)) were prepared by using melamine-formaldehyde resin and organic silicon in situ polymerization. The structure of the microcapsules was characterized by Fourier transform infrared spectroscopy and scanning electron microscopy. Due to the presence of synergists EG and organic silicone shell, the NR/M(A&E) composites had much better flame-retardant and mechanical properties than other flame-retardant NR composites. The limited oxygen index value of the NR/M(A&E) composite reached the maximum, and the UL-94 ratings were increased to V-0; in addition, the cone calorimetry indicates that the NR/M(A&E) composite has best flame-retardant properties and the lowest fire risk. Furthermore, the improvement in mechanical properties and flame-retardant behavior of M(A&E) particles filled NR were attributed to the desirable dispersion by the RPA. The occurrence of a synergistic effect between EG and intumescent flame-retardant in the NR composites was proved. As a result, the thermal stability and flame retardancy of NR were enhanced compared to the NR system.