News
18.04.2025

Phosphorus oxidation states and FR effects

pinfa research mini-review on how valency of phosphorus in PIN FRs impacts flame retardancy effectiveness. With continuingly increasing interest in phosphorus-containing PIN flame retardants as a key solution for fire safety in many applications (electronics, batteries, textiles …), researchers are looking at how P oxidation state (valency) impacts fire retardancy effects, in order to support design of future PIN phosphorus FRs. This may also support development of PIN FRs which can be produced without relying on P4 derivates (today, phosphorus chemicals with oxidation state other than +5 are only produced via P4 derivates).

Overall conclusions are that PIN FRs with higher P oxidation state tend to act in the solid phase (char formation) whereas lower P oxidation state FRs act both in the gas phase (PO radicals which engage in complex reactions continuously consuming H and OH flame radicals) and the solid phase. This confirms the conclusion of presentations at FRPM 2023 (pinfa Newsletter n°151) and Zou et al. 2024 (pinfa Newsletter n°161). However, all studies are based on only a few more-or-less similar phosphorus PIN FR compounds, and it seems clear that the effects of differing P valency are less important than effects of interactions with the polymer, and of the PIN FRs degradation temperature (gas phase action only if degradation before polymer decomposition).

In phosphorus compounds (including PIN flame retardants), the phosphorus atom can have different oxidation states (valency), for example:

  • phosphines -3
  • phosphine oxides -1
  • elemental phosphorus 0
  • phosphinates +1
  • phosphites +3
  • phosphonates +3
  • inorganic phosphorus compounds, including phosphoric acid, phosphates, and biological organo-phosphorus compounds (DNA, phospholipids, phytate …) +5

For detail see W. Schipper in ESPP SCOPE Newsletter n°136.

Qin et al. 2024 make detailed chemical modelling of five structurally similar phosphorus flame retardants (all consisting of three benzene rings joined via a phosphorus atom) with different P oxidation states:

  • R1 = triphenylphosphine = PPh3, P-bonds: 3xP-C
  • R2 = triphenyl phosphate oxide = TPPPO, P-bonds: 3xP-C + P=O
  • R3 = phenyl diphenylphosphinate = PDPP, P-bonds: 2xP-C + P-P + P=O
  • R4 = diphenyl phosphate = DPPO, P-bonds: P-C + 2xP-O + P=O
  • R5 = triphenyl phosphate = TPP, P-bonds: 3xP-O + P=O

It is not specified in what oxidation state is the phosphorus in each of these molecules, but it is indicated that the oxidation state is higher as the R-numbers increase.

The decomposition of these different P bonds in the five chemicals constitute 27 different main reaction pathways. In general, the P-C bond in the FRs was more resistant to disassociation than P-O bonds and phenoxy (benzine-O+) was more selective in the flam retardancy process than phenyl (benzine-carbon containing group). However, this general rule show significant variations between the different tested compounds and their different degradation compounds.

The authors conclude that as P oxidation state increases, the FRs act increasingly in the solid phase -char formation) whereas lower P oxidation state act rather in the gas phase (PO radicals which engage in complex reactions continuously consuming H and OH flame radicals). This confirms the conclusion of presentations at FRPM 2023 (pinfa Newsletter n°151) and Zou et al. 2024 (pinfa Newsletter n°161).

Hu, Chu et al. 2022 and 2025 studied different polymeric and reactive phosphorus PIN FRs in unsaturated polyester resins (see above). In the 2025 study, two reactive phosphorus PIN FRs were synthesised by combining itaconic acid (ITA) with DOPO or diphenylphosphine oxide (DPPO), then polymerised with 1-2-propanediol, maleic anhydride and phthalic anhydride to produce phosphorus FR-containing unsaturated polyester (with final 3%P content). The DOPO-ITA had P oxidation state +1 whereas the DPP-ITA had P oxidation state -1. The authors consider that there was little difference in flame inhibition (gas phase action). Fire performance was very similar between the two PIN FRs (both reduce peak heat release by over 40%). The DOPO-ITA (higher oxidation state) however produces more stable char and achieves UL 94 V-1 (3.2 mm). In the 2022 study, five reactive PIN FRs containing phosphorus and sulphur were reacted to unsaturated polyester resins with 8 – 11 %P and 0 – 10 %S. PIN FRs with P oxidation state +3 and sulphur (as sulfone) achieved UL 94 V-0 (3.2 mm), despite again higher P oxidation state of +5 resulting in more charring. The authors conclude that in this case, in unsaturated polyester resin, the gas phase action is most important in improving fire performance and they underline the synergy between phosphorus and sulphur (as sulfone).

Yin et al. 2025 tested PIN FRs derived from DPP (diphenyl phosphate, P-bonds: 2xP-C, P=O, P-OH) and DHP (diphenyl hydrogen phosphate, P-bonds: 2xO, P=O, P-OH) in vinyl ester resins. Both were combined 1:1 with 1- vinylimidazole salts. The resulting ViDPP (low oxidation state +1) and ViDHP (high oxidation state +5) both showed good curing compatibility, thermal stability and flame retardancy effects at 20% loading in vinyl ester, with >50% increase in LOI (limiting oxygen index) and decrease in smoke production. ViDPP (low P oxidation state) showed the best fire performance reducing peak heat release by half and achieving UL 94 V-0 (3.2 mm) at 20% loading, despite significantly lower char residues than for ViDHP.

This again show low P-oxidation state acting in the gas phase and high P-oxidation state acting in the solid phase (char formation).

Denis, Sonnier et al. 2023 tested four phosphorus – carbonate PIN FRs (based on MBDA 4,40-methylenebis(N,N-diglycidylaniline) reacted to polyhydroxyurethanes. These were based on:

  • DOPO (a phosphonate), P oxidation state +1
  • diethyl phosphite DEP, P oxidation state +3
  • diphenyl phosphite DPP, P oxidation state +3
  • dibenzo[d,f][1,3,2]dioxaphosphepine 6-oxide BPPO, P oxidation state +3

This study also concluded that the higher P oxidation state compounds resulted in more char generation. DEP offered the best fire performance, with over 75% reduction in peak heat release rate (2% P loading in final polymer).

Mariappan et al. 2013 tested, in polyurea and in epoxy resin, three PIN phosphorus FRs, as above each with three benzene rings linked by one phosphorus atom: TPPi = triphenylphosphite (P bonds: 3xP-O, oxidation state +3), TPP = triphenyl phosphate (P bonds: 3xP-O + P=O, oxidation state +5 and TPPO = triphenylphosphine oxide (P-bonds: 3xP-C + P=O, oxidation state -1).

The TPP (oxidation state +5) was much more effective at reducing heat release rate in polyurethane, the TPPi (oxidation state +3) was much more effective in epoxy. The effectiveness of TPPi in epoxy was considered to result from generation of non-conventional intumescent char, generated by a tans-esterification between the phosphite group and the epoxy, whereas the TPPi gave poor results in polyurethane because of a plasticisation effect.

This study confirms that P oxidation state is one factor influencing FR effectiveness, it is principally driven by interactions avec the polymer.

Luo et al. 2023 summarise links between P oxidation state and flame retardancy effects in polyurethane elastomers (PUE), noting that for similar molecular structures, the P-content (%P by weight) of molecules tends to decrease with increasing oxidation state. This is within a detailed review of reactive phosphorus PIN FRs in PUE, covering different chemical structures, phosphorus – nitrogen synergies, P-containing chain extenders, P-containing polyols, impacts on PUE mechanical performance, toxicity and environmental safety, burning behaviour.

Modesti et al. 2011, cited by Luo, tested phosphorus PIN FRs with different P oxidation states in rigid polyurethane (PUR) foam:

  • aluminium phosphinate (AlPi), P oxidation state +1
  • dimethylpropanphosphonate (DMPP), P oxidation state +3
  • triethylphosphate (TEP), P oxidation state +5
  • ammonium polyphosphate (APP), P oxidation state +5

This study concluded that when the PIN FR degraded at temperatures below the onset decomposition temperature of PUR (c. 300°C) – as do TMPP and TEP, they acted only in the gas phase irrespective of P oxidation state (because no longer present to cause charring). For TEP and APP, which decompose at c. 350°C, AlPi (lower P oxidation state +1) had both gas and solid phase effects, whereas APP (P oxidation state +5) acted only in the solid phase.

“Influence of oxidation state of phosphorus on the thermal and flammability of polyurea and epoxy resin”, T. Mariappan et al., European Polymer Journal 49 (2013) 3171–3180 http://dx.doi.org/10.1016/j.eurpolymj.2013.06.009

“Theoretical study on the effect of oxidation states of phosphorus flame retardants on their mode of action”, C. Qin et al., Polymer Degradation and Stability 223 (2024) 110735 https://doi.org/10.1016/j.polymdegradstab.2024.110735

“An insight into the effects of the low oxidation states of phosphorous on the combustion behavior of intrinsically flame-retardant unsaturated polyester resins”, Y-D. Hu, F-K Chu et al., Polymer Degradation and Stability 232 (2025) 111156 https://doi.org/10.1016/j.polymdegradstab.2024.111156

“Exploration on structural rules of highly efficient flame retardant unsaturated polyester resins”, F. Chu, Y. Hu et al., Journal of Colloid and Interface Science 608 (2022) 142–157 https://doi.org/10.1016/j.jcis.2021.09.124

“Flame-retardant vinyl ester resins enabled by phosphorus-containing 1-vinylimidazole salts with different phosphorus oxidation states”, Y-Y. Yin et al., J. Materials Science & Technology 205 (2025) 79–88 https://doi.org/10.1016/j.jmst.2024.04.005

“Strategy for Constructing Phosphorus-Based Flame-Retarded Polyurethane Elastomers for Advanced Performance in Long-Term”, Y. Luo et al., Polymers 2023, 15, 3711. https://doi.org/10.3390/polym15183711

“Influence of phosphorus valency on thermal behaviour of flame retarded polyurethane foams”, A. Lorenzetti et al., Polymer Degradation and Stability 96 (2011) 1455e1461 http://dx.doi.org/10.1016/j.polymdegradstab.2011.05.012

“Influence of Phosphorus Structures and Their Oxidation States on Flame-Retardant Properties of Polyhydroxyurethanes”, M. Denis, R. Sonnier et al., Molecules 2023, 28, 611. https://doi.org/10.3390/molecules28020611