Effects of iron oxide-modified hierarchically porous 4A zeolites on flame retardancy of ammonium polyphosphate-treated wood composites

The combination of inorganic porous nanomaterials with metal oxides has shown great prospects as flame retardants or flame-retardant synergists in wood composites. In this work, iron oxide-modified hierarchically porous 4A (FeH4A) zeolites were prepared and applied as synergists of ammonium polyphosphate (APP) for flame retardant treatment of wood composites. The effects of FeH4A zeolites with various degrees of modification (0.5FeH4A, 1FeH4A, 5FeH4A, and 10FeH4A) on flame retardancy of wood composites were investigated. With proper modification, the 0.5FeH4A zeolite favored the decomposition process of APP at low temperatures, leading to better flame retardancy. The peak heat release rate (HRR) and total heat release (THR) of W-APP-0.5 FeH4A were decreased by 15.8% and 16.7%, respectively, compared with those of W-APP-H4A. However, severe modification results in damage of the zeolite framework. This change along with the excess iron oxide weaken the thermal stability of the char residue, which negatively affects flame retardancy. Further investigation of the pyrolysis products and morphologies of the char residue indicates that the FeH4A zeolites play a role in both gaseous and condensed phase flame-retardant mechanisms. This study provides information for the development of novel flame retardants and effective synergists for wood composites.