Preparation of phosphorylated kapok fiber using ammonium dihydrogen phosphate and its thermal degradation and flame retardancy

Reducing the flammability of kapok fiber (KF) is an important strategy to increase its applicability. In this work, the phosphorylation of KF with ammonium dihydrogen phosphate (ADP) was used to reduce its flammability, the conditions of phosphorylation reaction were studied in detail, and the chemical groups, crystalline structure, thermal stability, heat release capability, flammability and charring ability of phosphorylated KF were analyzed. KF showed significantly higher phosphorylation level than cotton, and its phosphorylation greatly depended on ADP and urea dosage, reaction temperature and time, and pH. Urea addition, pH 6, 150 ℃ and 1 h rendered high phosphorylation with acceptable whiteness. Phosphorylated KF exhibited good char-forming, low heat release and excellent flame retardant performances during thermal degradation and combustion as evidenced by thermogravimetric, microcalorimetric, vertical combustion and combustion residue morphology analyses. The residue rate of phosphorylated KF subjected to thermal degradation exceeded 50% in nitrogen, and 25% in air. This investigation demonstrates that phosphorylated KF is an intrinsically flame retardant material. Graphic abstract