Flame-retardant vinyl ester resins enabled by phosphorus-containing 1-vinylimidazole salts with different phosphorus oxidation states

Different reactive flame retardants have been extensively developed for vinyl ester resins (VERs), but very few of them can yield a flame-retardant resin that meets defined standards (e.g. UL-94 V-0 rating). In this work, phosphorous-containing 1-vinylimidazole salts (called VIDHP and VIDPP) were synthesized through the facile neutralization of the acid and 1-vinylimidazole. VIDHP and VIDPP were then introduced into the resin chain networks of VERs as flame-retardant crosslinking agents. VIDHP/VER and VIDPP/VER showed a high curing activity and can be well cured in moderate temperatures. With 20 wt.% additions of VIDHP and VIDPP, VIDHP20/VER, and VIDPP20/VER presented a limiting oxygen index value of 29.7% and 28.4%, respectively, with the latter achieving a UL 94 V0 rating. In the cone calorimetric test, compared to the unmodified VERs, VIDPP20/VER exhibited large reductions in the peak heat release rate, total heat release rate, and total smoke release rate while VIDHP20/VER demonstrated comparatively inferior performance in terms of the heat release. VIDHP20/VER and VIDPP20/VER showed good thermal stability and presented a little lower glass transition temperature than the control sample. VIDPP with a low phosphorus oxidation state (+1) demonstrated high flame-retardant activities in the gaseous phase, whereas VIDHP with a high phosphorus oxidation state (+5) primarily exhibited efficacy in the condensed phase.