Efficient flame-retardant hybrid coatings on wood plastic composites by layer-by-layer assembly

Fire retardant treatments that reduce the heat release rate of wood plastic composites (WPCs) have been intensively studied. However, few studies have reported on increasing the ignition time using precise control of nanostructured hybrids coating on WPC surfaces. Here, a nanocoating layer, composed of polyethyleneimine, Namontmorillonite (MMT) and graphitized multiwalled carbon nanotubes, is deposited on the WPC surface via a layer-by-layer assembly (LbL). The nanocoating layer has favorable pull-off adhesion performance in dry and wet conditions. The protective layer has a "brick-mortar-rebar" nanostructure generating a thermal and gas barrier. This nanostructure prevents oxygen permeation and volatile gases release during material combustion. The thermal kinetics and decomposition reactions are restricted, increasing the ignition time from 49 s to 237-286 s, and 30% char formation due to the thermal shielding nature of MMT and the high charring capability of the coating nanostructure. The coating layer does not affect the WPC mechanical properties negatively. We foresee that this efficient multilayer hybrid coating method for improving fire-retardancy and smoke suppression could be applied to other materials, such as wood, foams, aerogels and polymer substrates.