Improvement of flame retardancy and thermal conductivity of SBR by piperazine pyrophosphate-modified boron nitride synergistic expandable graphite

Expandable graphite (EG) is widely used for flame retardancy of rubber. Nevertheless, with the increase in the amount of EG, the mechanical properties of rubber materials drop rapidly and they would not be conducive to the practical application of rubber products. In this study, piperazine pyrophosphate-boron nitride (PPAP-BN) was synthesised by in situ polymerization and combined with EG to prepare SBR composite material. The mixture of PPAP-BN/EG with a mass ratio of 1:4 at 25 phr (parts per hundreds rubber) resulted in a slow thermal degradation process of SBR and achieved the best flame-retardant effect: limiting oxygen index (LOI) increased from 19.5% to 28.5%, peak heat release rate (pHRR) and total heat release rate (THR) decreased by 69.9% and 39.4%, respectively, and the peak smoke release rate (pSPR) and total smoke release rate (TSP) also decreased by 68.0% and 48.1%, respectively. This is mainly due to the synergistic effect of PAPP-BN and EG. The phosphorus-containing groups in PAPP-BN have a promoting effect on carbon formation. The piperazine structure can form a cross-linked carbon layer, whilst the layered structure of boron nitride increases the density of the carbon layer, effectively reducing the heat and smoke release of the material. In addition, the thermal conductivity of SBR composite materials increased by 78.8%. Graphical Abstract