Copper-catalyzed pyrolysis of halloysites@polyphosphazene for efficient carbonization and smoke suppression

Nowadays, the underlying mechanism for the flame-retardant property derived from the metal-catalyzed pyrolysis of polyphosphazene composites is unclear, thus limiting the efficient utilization and further development of this high-performance catalytic-type flame retardants. Herein, polyphosphazenes terminated with amino (denoted as PNF) and hydroxyl (denoted as PZF) groups are coated on the surface halloysite nanotubes (HNT), thus affording HNT@PNF and HNT@PZF core-shell structures, respectively. Cu nanoparticles (2.6 +/- 0.8 nm) are anchored on the surface of these two core-shell structures to generate flame retardants named as HNT@PZF-Cu and HNT@PNF-Cu. Cu nanoparticles show high catalytic thermal rearrangement of polyphosphazene for HNT@PZF-Cu instead of HNT@PNF-Cu, which is attributed to the interaction between Cu and HNT@PZF and a large specific surface area of HNT@PZF (93.5 m2/g). Compared with HNT@PNF-Cu, HNT@PZF-Cu exhibits better smoke suppression and flame retardancy for epoxy resin (EP). With 3 wt% loading of HNT@PZF-Cu, the peak heat release rate, smoke production rate, and carbon monoxide production of the EP nanocomposite are reduced by 52.2%, 40.3% and 64.2%, respectively. This is because polyaromatic compounds generated during the pyrolysis of HNT@PZF-Cu promote the rapid carbonization of EP nanocomposites, resulting in the formation of structural stable char residue. Meanwhile, Cu nanoparticles can effectively reduce the release of toxic fumes and the volatilization of volatile organic compounds (VOCs) during the catalytic combustion process. Density functional theory (DFT) calculation reveals that Cu nanoparticles catalyze the purification of toxic gases such as CO and NO.