A novel phenophosphazine based PIN flame retardant (HD-PPA, a ten carbon ring molecule containing four nitrogen and two phosphorus atoms, 7.4% P) was experimentally synthesised and tested in DGEBA epoxy resins. HD-PPA was produced in a one-step reaction from PPA (see previous paper cited), 5,10-dihydro-phenophosphazine-10-oxide, which was produced in two steps by reacting diphenylamine with a phosphorus-carrier then with caustic soda.
The HD-PPA reacts into the epoxy resin during curing. Zero to 3.5% loadings of HD-PPA were tested in epoxy resin, achieving UL94-V0 from 2.5% HD-PPA. Flaming dripping was prevented with 1% HD-PPA. 3.5% HD-PPA increased LOI of the epoxy resin from 24 (zero HD-PPA) to 32. The authors suggest that the HD-PPA functions by accumulating polyphosphate on the resin surface and promoting dehydration and esterification of the epoxy resin under fire conditions, so generating a char barrier. The HD-PPA addition at these levels improved the mechanical properties of the epoxy resin polymer.
“Intumescent flame retardancy of a DGEBA epoxy resin based on 5,10-dihydro-phenophosphazine-10-oxide”, Q. Luo et al, RSC Advances, 2015, 5, 68476-68484 https://dx.doi.org/10.1039/c5ra11847f
“High performance fire-retarded epoxy imparted by a novel phenophosphazine-containing antiflaming compound at ultra-low loading”, Q. Luo et al., Materials Letters 169, 2016, 103-106 http://dx.doi.org/10.1016/j.matlet.2016.01.083