Macromolecular P-N-S PIN flame retardant improves fire performance, thermal stability and strength of epoxy. The well-known phosphorus PIN FR molecule DOPO was combined with nitrogen-containing molecules and a phosphorus-sulphur compound and polymerised to generate macromolecular DOPONH2S (14% P content). This was tested at 0 – 10% in DGEBA epoxy. Neat epoxy was not rated and showed dripping in UL 94 (3 mm) whereas with 5 – 10% DOPONH2S V-0 no dripping was achieved. LOI was increased by around one third with 10% DOPONH2S and char production was doubled. The authors attribute the fire performance to solid phase action (char layer) and to gas phase action of phosphorus and sulphur radicals. DOPONH2S improves the tensile and impact strength of the epoxy, probably because amino groups cross-link to epoxy whilst flexible chains in DOPONH2S improve toughness.
“A novel phosphorus-, nitrogen- and sulfur-containing macromolecule flame retardant for constructing high-performance epoxy resin composites”, G. Jiang et al., Chemical Engineering Journal 451 (2023) 137823 https://doi.org/10.1016/j.cej.2022.137823