A research paper shows synergies between the organic and inorganic nitrogen-phosphorus PIN flame retardants in the performance polymer ABS ( acrylonitrile butadiene styrene), using 15 % wt. HNTP (hexakis(4-nitrophenoxy) cyclotriphosphazene and 15% APP (ammonium polyphosphate). Neat ABS did not pass UL94-V1, showed flaming dripping and LOI (limiting oxygen index) of 18. The PIN flame retardants enabled UL94-V0 (3.2 mm), no flaming dripping and LOI increased to over 25. Tensile strength tests indicated that this PIN FR loading was compatible with high temperature ABS applications. The authors conclude that the PIN FR fire performance was due to generation of cohesive and dense intumescent char on the polymer surface during fire.
“High efficiency intumescent flame retardancy between Hexakis (4-nitrophenoxy) cyclotriphosphazene and ammonium polyphosphate on ABS”, X. Cao et al., Polymer Degradation and Stability 143 (2017) 259-265 http://dx.doi.org/10.1016/j.polymdegradstab.2017.07.022