A molecule combining bismuth with melamine phosphate shows synergistic fire and smoke reduction in epoxy. Phosphoric acid was reacted with pentaerythritol, then with melamine, then with bismuth oxide (Bi2O3, 10% w/w) and additional melamine. The resulting PPMS-Bi [1], and also PPMS (not reacted with bismuth) was tested as a PIN flame retardant in epoxy – polyamide (2:1) resin at FR loadings of 20 – 40%. The neat resin was not rated under UL94, whereas with 30% PPMS or 30% PPMS-Bi a rating V0 (4 mm) was achieved. Peak heat release was reduced by around 75% with 40% PPMS, and by around 85% with 40% PPMS-Bi. Smoke density was around 30% lower with PPMS-Bi than with PPMS only, and was also lower with PPMS-Bi than with PPMS and bismuth oxide added separately to the resin. EDS [2] suggests that the hydrogen bond linking of the bismuth interacts with phosphorus to generate continuous, compact and intumescent char formation.
[1] pentaerythritol phosphate melamine salt hybrid bismuth oxide
[2] EDS = energy-dispersive X-ray spectrometry
“Synergistic effect of bismuth oxide and mono‐component intumescent flame retardant on the flammability and smoke suppression properties of epoxy resins”, Z. Xu et al., Polym Adv Technol. 2019;1–11, https://doi.org/10.1002/pat.4744