Smoke and toxicity suppression by zinc salts in flame-retardant polyurethane-polyisocyanurate foams filled with phosphonate and chlorinated phosphate

Three types of zinc salts, ZnAl₂O₄, ZnFe₂O₄, and Zn₂SiO₄, were prepared by coprecipitation. Potential smoke and toxicity suppression by zinc salts in flame-retardant polyurethane-polyisocyanurate foams (FPUR-PIR) with dimethylmethylphosphonate (DMMP) and tris (2-chloropropyl) phosphate (TCPP) were investigated. The crystal structure and dispersity of zinc salts in FPUR-PIR were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Smoke density, flame retardancy, and thermal degradation were studied using smoke density rating (SDR), limiting oxygen index (LOI), the cone calorimeter test, and thermogravimetry coupled with FTIR spectrophotometry (TGA-FTIR). The results indicated that pure zinc salts were obtained and evenly dispersed on the cell wall of FPUR-PIR. SDR and the specific extinction area (SEA) were significantly decreased, the time to second heat release rate peak (pk-HRR) of FRUP-PIR was delayed after incorporation of the zinc salts; zinc salts partially inhibited phosphorus oxide release into the gas phase, enhanced the condensed phase effect of phosphorus, reduced, and prolonged the release of isocyanate compound and hydrogen cyanide from FRUP-PIR; due to an increase in the amount of char residues, which indicated the suppression of smoke and toxicity volatiles. ZnFe₂O₄ resulted in better char formation at the initial degradation stage of FPUR-PIR, and ZnAl₂O₄ retained more phosphorus in the solid phase at higher temperature.