Smoke and toxicity suppression properties of ferrites on flame-retardant polyurethane–polyisocyanurate foams filled with phosphonate

Three types of ferrites zinc ferrite, copper ferrite and nickel ferrite were successfully synthesized via coprecipitation. The morphology of ferrites and foams was studied by X-ray diffraction (XRD), transmission electron microscopy and scanning electron microscopy. The flame retardancy and potential smoke and toxicity suppression effects of ferrites on flame-retardant polyurethane–polyisocyanurate (FPUR–PIR) foams with dimethylmethylphosphonate and tris (2-chloropropyl) phosphate were investigated by thermogravimetric analysis (TG), limiting oxygen index, smoke density rating (SDR), cone calorimeter and X-ray photoelectron spectroscopy. Meanwhile, the detailed reaction between ferrite and phosphonate, ferrite and polyurethane–polyisocyanurate was examined by thermogravimetry coupled with FTIR spectrophotometry, XRD, TG and FTIR. The results showed that ferrites significantly decreased the smoke production of FPUR–PIR, and reduced SDR per unit mass by 25.1–41.6 %. Notably, ZnFe₂O₄ (ZF) showed an excellent performance in smoke and toxicity suppression in this study. Moreover, ZF not only can restrained the release of groups containing phosphorus and kept more P in condensed phase, but also can accelerated the dehydration of PUR–PIR and facilitated the crosslinking of pyrolysis fragments to form thermal oxidation stable char layer. This study has provided insight into possible modes of smoke and toxicity suppression action in FPUR–PIR incorporated ferrites.