Composites of Layered Double Hydroxide Nanosheets, Hydroxy-Functionalized Carbon Nanotubes, and Hydroxyapatite Nanoparticles as Flame Retardants for Epoxy Resins

Fire safety in civil aviation requires high-performance flame retardants to control the flammability of epoxy (EP) resins used widely in aircrafts. In this study, a novel and nanoscale hybrid flame retardant was designed with incorporated layered double hydroxide (LDH) nanosheets, −OH-functionalized carbon nanotubes (CNTs)(−OH), and hydroxyapatite (HA) nanoparticles. Experimental characterizations and theoretical simulations demonstrated that the interwoven networks of CNTs afford a supporting framework to ensure homogeneous dispersion of LDH nanosheets and HA nanoparticles and also enhance the mechanical strength of the flame-retardant-treated EP resin substrate. This newly designed flame retardant exhibits pronounced flame retardation performance, as shown by the low peak heat release rate of 580 kW/m2, notably small total heat release rate of 20.8%, relatively low CO production of 0.030 g/s, high limit oxygen index of 30.5%, and high char yield of 23.3% at 700 °C. These remarkable flame retarding properties result from synergism among LDH, CNT(−OH), and HA components, which is demonstrated using both theoretical and experimental analyses. This work will provide new insights into the design of highly efficient nanostructured flame retardants for EP resins.