Phosphorus-functionalized multi-wall carbon nanotubes as flame-retardant additives for polystyrene and poly (methyl methacrylate)

In the present work, we have studied the flame-retardant properties of phosphorus-functionalized multi-walled carbon nanotubes (MWCNTs) created by treating oxidized MWCNTs (O-MWCNTs) with phosphoric acid. These phosphorus MWCNTs (P-MWCNTs), along with pristine MWCNTs and O-MWCNTs, were incorporated into polystyrene (PS) and poly (methyl methacrylate) (PMMA) by solution blending at lower concentrations (0.5-10 mass%) than conventional organophosphorus flame retardants (5-30 mass%). Thermal properties of the PS and PMMA polymer nanocomposites were subsequently investigated through thermogravimetric (TG) analysis and microcombustion calorimetry. Scanning electron microscopy was used to evaluate P-MWCNT dispersion in each polymer matrix. P-MWCNTs were well-dispersed in PS, improved PS thermal stability during nitrogen pyrolysis in proportion to the phosphorus loading, and led to an improvement in PS flammability properties. In contrast, the P-MWCNTs were more aggregated in PMMA and only moderately improved PMMA thermal stability during pyrolysis. In both PS and PMMA, P-MWCNTs were more effective than either MWCNTs or O-MWCNTs at improving each polymer's thermal oxidative stability during combustion in air. Characterization of the P-MWCNT nanoparticles, the PS and PMMA nanocomposites, and TG residues indicated that P-MWCNT flame retardants primarily act in the condensed phase, encouraging char formation of the dispersed MWCNT network as a route to protecting the polymer. By comparing results of polymer composites containing MWCNTs, O-MWCNTs or P-MWCNTs at comparable loadings, data from the current study indicate that phosphorus incorporation can improve the fire-retardant properties of MWCNTs, although optimizing P-MWCNT dispersion in different polymers remains critical to optimizing the flame-retardant effect.