“…Recently, halogen-free flame retardants have been developed and applied in the preparation of polyurethane (Zhu et al, 2014;Ding et al, 2016;Jiao et al, 2019;Sykam et al, 2019;Xia et al, 2020). Among them, phosphorus-containing flame retardants, especially 9,10dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives, have attracted much attention (Huo et al, 2020;Huo et al, 2021). 9,10-Dihydro-9-oxa-10-phenanthrene-10oxide (DOPO) and its derivatives were used to improve their use as flame retardants, as these have higher thermal stability, excellent flame retardancy, and low toxicity (Ma et al, 2020;Zhi et al, 2020).…”