An insight into the effects of the low oxidation states of phosphorous on the combustion behavior of intrinsically flame-retardant unsaturated polyester resins

• The effects of low oxidation states of phosphorous on the combustion behaviors of intrinsically flame-retardant UPR were analyzed. • The possible pyrolysis mechanism of intrinsically flame-retardant UPR is revealed. • FRUP2 with a + 1 phosphorus oxidation state exhibits better flame retardancy, achieving a reduction of 46.8% in the value of THR. To mitigate the migration of flame retardants in unsaturated polyester resins (UPR), flame-retardant structures can be integrated into the molecular chain to create intrinsically flame-retardant UPR. Variations in the oxidation states of phosphorus-containing flame retardants can influence the distribution of phosphorus-containing compounds between the gas and condensed phases during combustion, thereby affecting the flame-retardant performance and mechanisms. Building on previous studies of the impact of phosphorus-containing flame retardants with high oxidation numbers on UPR's combustion behavior and flame-retardant mechanism, this research further investigated the effects of phosphorus-containing structures with -1 and +1 oxidation states on the pyrolysis, heat release, smoke emission, and char formation of intrinsically flame-retardant UPR. Two kinds of itaconic acid-based phosphorus-containing dicarboxylic acids were synthesized and incorporated into the polyester backbones of UPR. Pyrolysis and combustion tests were performed to elucidate the combustion behavior regulation mechanisms of intrinsically flame-retardant UPR with low oxidation numbers. Findings indicate that both of the phosphorus-containing dicarboxylic acids exhibit strong flame inhibition, implying a dominant gas-phase mechanism. Moreover, they effectively shorten the duration of the release of the pyrolysis gases. However, the difference in oxidation states didn't result in a huge difference in their flame inhibition, and it's the condensed phase mechanism that makes the difference in their performance. FRUP2 with stable char layers demonstrates better flame retardancy. This study offers practical and efficient guidance for designing intrinsically flame-retardant UPR with high performance by elucidating the structure-performance relationship of phosphorus-containing compounds with low oxidation numbers.