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 , 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  suggests that the hydrogen bond linking of the bismuth interacts with phosphorus to generate continuous, compact and intumescent char formation.
 pentaerythritol phosphate melamine salt hybrid bismuth oxide
 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