Enhanced flame retardancy of flexible polyurethane foam with low loading of liquid halogen-free phosphonium thiocyanate

To achieve enhanced flame retardancy at a low dosage, a room temperature ionic liquid tetrabutylphosphonium thiocyanate (TPT) containing flame-retardant phosphorus and sulfur elements is synthesized and uniformly incorporated into flexible polyurethane foam (FPUF). FPUF containing only 3.4 wt% TPT can pass the Cal. TB117-2000 standard and exhibits a limiting oxygen index value of 23.7% without compromising thermal stability and mechanical performance. Meanwhile, the peak heat release rate of FPUF/TPT in cone calorimeter test decreases by 40.5% in comparison with that of pristine FPUF. It is revealed that the enhanced flame retardancy of FPUF/TPT originates from both the cation and the anion of TPT. The phosphonium cation contributes to the retardancy via the gas-condensed bi-phase action and the thiocyanate anion assisted the retardancy by the fuel dilution effect in the gas phase. The results indicate that the phosphorus-sulfur room temperature ionic liquids possess a great potential to act as flame retardants for fire-safety polymeric foams.