An insight into pyrolysis and flame retardant mechanism of unsaturated polyester resin with different valance states of phosphorus structures

• The effect of phosphorus valence states on combustion behaviors of UPR was analyzed. • Flame retardant UPR achieved UL-94 V0 with only 15 wt% of PBPG. • +3 phosphorus valance of PBEG was more helpful to improve flame retardancy of UPR. • The preparation of flame retardant UPR was mainly based on gaseous phase mechanism. This work analyzes the effect of different valence states in phosphorus structures on thermal stability, pyrolysis kinetics and flame retardancy of unsaturated polyester resin (UPR). Then, the corresponding mechanisms are systematically analyzed. The poly(ethylene phenylphosphonate) (PBEG) and poly(ethylene phenoxyphosphate) (POEG) were synthesized, and the combustion behaviors of UPR, such as pyrolysis, ignition, heat suppression and charring capacity were studied. The phosphorus structures with +5 valence states in POEG was more conducive to formation of carbon layers than that of +3 valence states in PBEG. The better charring capacity made pyrolysis activation energy ( E k ) of UPR/POEG different from UPR/PBEG in F-W-O and DAEM fitting methods. The E k of UPR/POEG was larger than pure UPR at high mass conversions, but E k of UPR/PBEG was less than pure UPR during entire pyrolysis process, due to different phosphorus-containing pyrolysis products. For PBEG, more phosphorus-containing species existed in gaseous phase to capture H· radicals; By contrast, POEG left more products in condensed phase, promoting the formation of thermally stable carbon layers. Different phosphorus-containing products also affected flame retardancy of UPR. UPR reached UL-94 V0 with only 15 wt% of PBEG, where peak heat release rate (PHRR), total heat release (THR), fire growth index (FGI) were obviously reduced. However, flame retardant efficiency of POEG were lower than PBEG. This indicates that +3 phosphorus valance of PBEG is more helpful to improve flame retardancy of UPR, and the essential reasons have been systematically analyzed. Meanwhile, the selection of high-efficiency flame retardant systems for UPR materials can be based on gaseous-phase mechanism and supplemented by condensed-phase mechanism.