Grafting modification bamboo kraft lignin and its performance in rigid polyurethane foams

Abstract In order to improve the efficiency of intumescent flame retardant (IFR), bamboo kraft lignin (BKL) was chemically functionalized by grafting melamine (MEL) and diethyl phosphite (DEP) and used for rigid polyurethane (RPU) foam. The BKL, MEL, and DEP in IFR system were used as char forming agent, gas, and acid source, respectively. The FTIR and XPS results indicated that the nitrogen (N) and phosphorus (P) containing BKL was successfully synthesized. The limiting oxygen index (LOI) value of N‐BKL and N/P‐BKL RPU foams were higher than BKL RPU foam, suggesting that N‐BKL and N/P‐BKL improved flame retardancy of the foams. The total heat release (THR), heat release rate (HRR), effective heat of combustion (EHC), and fire growth rate (FIGRA) values of N‐BKL and N/P‐BKL RPU foams were much lower than that of BKL RPU foam. The flame retardancy index value of N/P‐BKL RPU foams was higher comparing to N‐BKL RPU foam. These results indicated that the synergistic interaction between N containing compound of MEL and P containing compound of DEP led to the improvement flame retardant properties. Comparing to BKL RPU foam, the N/P‐BKL RPU foam increased 74°C of maximum weight loss temperature and decreased 18.1 wt% of mass loss, indicating enhanced thermal stability. The morphology of char after cone calorimeter testing showed the N/P‐BKL RPU foam presented more continuous and compact char residues, which could reduce heat and mass transfer, protecting underlying materials from further combustion in a fire, thus resulting in good flame retardancy and thermal stability properties. This work suggests a promising route to enhancing the flame‐retardant performance of RPU foam using nontoxic and more environmentally friendly grafted bamboo lignin.