Valorization of kapok fiber by phosphorylation with a reactive phosphorus- and nitrogen-containing flame retardant for enhanced fire safety

Kapok fiber (KF) with a naturally hollow structure has found an increasing application in filling and composite materials, most of which have a demand of flame retardancy. To realize this high-value utilization, KF was phosphorylated by a reactive phosphorus- and nitrogen-containing flame retardant for enhanced fire safety. The flame retardant was first synthesized by the reaction of urea and diethylene triamine pentakis (methyl phosphonic acid) (a commercially available scale inhibitor) with low cost, and then, applied in the phosphorylation of KF in the presence of dicyandiamide. In thermogravimetric analysis, phosphorylated KF showed 38.1% residue weight in nitrogen and 10.2% residue weight in air at 700°C, whereas the residue weight of KF was 8.4% in nitrogen and 1.5% in air. In microcalorimetric analysis, the heat release capacity and total heat release of phosphorylated KF displayed a reduction of 81% and 55%, respectively. In vertical combustion test, phosphorylated KF was difficult to ignite and had no afterburning or smoldering. The tests indicated that phosphorylated KF had excellent charring properties, low heat release, and good flame retardancy. This work suggests a promising strategy of enhancing the flame retardancy of KF and increasing its applicability in filling and composite materials.