Innovative Design and Preparation of Hierarchical BP–OH@HAP Structure: Study on Flame Retardancy and Mechanical Characteristics of UPR Nanocomposites

The flammability and brittleness of unsaturated polyester resin (UPR) were two serious problems that limited its application in high-precision fields. Here, the rod-shaped hydroxyapatite (HAP) was anchored on the surface of hydroxylated black phosphorus nanosheets (BP–OH) through a hydrothermal reaction to obtain a highly stable black phosphorus-based nano flame retardant (BP–OH@HAP). Owing to the exposure of many hydroxyl groups, BP–OH@HAP was well dispersed in the UPR matrix, and UPR nanocomposites with 0.5 wt % BP–OH@HAP realized a 71% increase in impact strength. The presence of BP–OH@HAP also greatly inhibited the combustion of UPR nanocomposites. In detail, the UPR composites with 2 wt % BP–OH@HAP achieved a 47.0% decrease in peak heat release rate (PHRR) along with 23.1% reductions in total heat release (THR), revealing the excellent ability of BP–OH@HAP to inhibit polymer combustion. In addition, UPR/BP–OH@HAP 2.0 achieved a 46 s increase in the time to PHRR (tPHRR) and a 62% reduction in the fire growth index (FGI), indicating that the fire spread of UPR/BP–OH@HAP 2.0 was significantly suppressed. Therefore, this work obtained the UPR/BP–OH@HAP nanocomposite with high fire safety through the innovation of inorganic nanotechnology, which provided new research ideas for improving the toughness and flame-retardant properties of UPR-based nanocomposites.