Char formation and smoke suppression mechanism of montmorillonite modified by ammonium polyphosphate/silane towards fire safety enhancement for wood composites

The low efficiency of montmorillonite (MMT) as a nano-flame retardant has limited its widespread application. In this work, a clay-based flame retardant was developed by modifying MMT with ammonium polyphosphate (APP) and 3-Aminopropyltriethoxysilane (SCA). Subsequently, wood composites treated with the clay-based flame retardant were prepared, and their char formation and smoke suppression behavior were investigated. MMT sheet effectively absorbed a significant amount of APP, and the broken edges of the sheet were successfully grafted with SCA. This promoted the formation of polyphosphoric acid and improved the interface compatibility among the components of wood composites. The clay-treated wood composites exhibited a reduction in total heat release (by over 27.0%) and a significant increase in char residues (up to 111.9%) compared to the control. Moreover, the second peak of the smoke production rate and mean CO yield were decreased by up to 43.2% and 63.2%, respectively. The formation of continuous, compact, and cross-linking (e.g. C-Si and Si-O-P) char layers endowed wood composites with thermal insulation, delayed the spread of flammable or poisonous gases (e.g. CH4 and CO), and suppressed the release of toxic smoke. Therefore, a simple and effective method for fabricating a clay-based flame retardant was proposed, which holds potential application in wooden construction materials.