Flame retardant epoxy/halloysite nanotubes nanocomposite coatings: Exploring low-concentration threshold for flammability compared to expandable graphite as superior fire retardant

Epoxy nanocomposites containing halloysite nanotubes (HNTs) were developed and their low-concentration thresholds for thermal stability and flame retardancy were compared with that of epoxy system containing expandable graphite (EG), as a reference with superior flame retardancy. The effects of HNTs and EG on the peak of Heat Release Rate (pHRR), Total Heat Release (THR), and Time-To-Ignition (TTI) of the prepared samples were subsequently discussed. At low loading level of 3 wt.%, HNTs appeared more effective, as signaled by an enhanced thermal stability compared to the EG-incorporated composite at an identical loading, because of hindered mobility of epoxy chains in a well-cured epoxy network. At higher loadings (6 and 9 wt.%), however, exfoliation of EG because of heat build-up in the system was dominantly hindered the crosslinking of epoxy it the presence of HNTs, which consequently deteriorated thermal stability of epoxy. This was featured by the formation of intumescent flake on the surface of the epoxy that played the role of a physical barrier, and assisted in reduction of the value of pHRR, while it doubled the TTI value. Different functions of HNTs and EG in regard with thermal stability and flame retardancy of epoxy/amine systems were discussed experimentally and mechanistically.