An experimental P-based PIN FR for rigid polyurethane foam shows both gas phase and solid phase fire inhibition effects. Researchers synthesised (in two steps) a bi-functional, reactive phosphorus flame retardant for rigid polyurethane foam: PDEP, containing a DOPO and a phosphate (DEP, derived from diethyl phosphite). With12% PDEP and 8% expanded graphite, LOI was increased to 27% (from 18.5% neat foam) and UL94-V0 vertical burn test (10 mm thickness sample) was achieved. Analysis showed that the FR effect resulted from successive decomposition of DEP and then DOPO with increasing temperature. High levels of PO and PO2 radicals released to the gas phase exert flame inhibition effect. In the solid phase, viscous P-based char bonded and integrated other chars, in particular from graphite, to generate an effective fire barrier.
“A phosphorous-based bi-functional flame retardant for rigid polyurethane foam”, J. Wang et al., Polymer Degradation and Stability 186 (2021) 109516 https://doi.org/10.1016/j.polymdegradstab.2021.109516