Syntheses and Characterization of Four Phosphaphenanthrene and Phosphazene-based Flame Retardants

Four novel intumescent flame retardants containing phosphaphenanthrene and phosphazene groups were synthesized by using the classic Atherton-Todd reaction and Kabachnik-Fields reactions. 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was firstly reacted with hexa-aminophenoxy-cyclotriphosphazene (compound 2) in the presence of carbon tetrachloride and triethylamine (Atherton-Todd reaction) to obtain hexa-(phosphaphenanthrene-anmino-phenoxyl)-cyclotriphosphazene (compound 3). Moreover, the condensation of hexa-aminophenoxy-cyclotriphosphazene with formaldehyde, salicylaldehyde, 4-hydroxybenzaldehyde respectively, followed by the addition of DOPO (Kabachnik-Fields reaction) to the imine linkage to obtain hexa-(phosphaphenanthrene-methyl-amino-phenoxyl)-cyclotriphosphazene (compound 4), hexa-(phosphaphenanthrene-methyl-(o-hydroxybenzal)-amino-phenoxyl)-cyclotriphosphazene (compound 5), and hexa-(phosphaphenanthrene-methyl-(p-hydrobenzal)-amino-phenoxyl)-cyclotriphosphazene (compound 6). The structures of compounds 3-6 were determined by FTIR, NMR, and elemental analysis. A preliminary investigation on compound 5 as a flame retardant for PLA has been carried out, and the result shows that the limiting oxygen index (LOI) of the PLA sample can reach up to 30% at the presence of only 5% compound 5.