High-strength fire- and heat-resistant imide resins containing cyclotriphosphazene and hexafluoroisopropylidene groups

High-strength fire- and heat-resistant polymers were obtained by the thermally induced melt-polymerization of maleimido-phenoxy cyclotriphosphazenes linked by hexafluoroisopropyliden-ediphthalimide groups. These polymers show good thermal stability and high char yields: 78–80% at 800°C in nitrogen and 60–68% in air at 700°C. Graphite-fabric laminates did not burn in pure oxygen, even at 300°C (LOI = 100%), and were tested for shear, flexural, and tensile strengths. Two monomers were synthesized by reacting tris(4-aminophenoxy)-tris(phenoxy) cyclotriphosphazene with maleic anhydride and hexafluoroisopropylidenediphthalic anhydride. The triamine was synthesized by a stepwise reaction of hexachlorocyclotriphosphazene with phenol and 4-nitrophenol to give tris(4-nitrophenoxy)-tris(phenoxy)cyclotriphosphazene and reducing the nitro groups. The structures of cyclic phosphazene-trimeric precursors and the polymers were characterized by FT-IR, ¹H-NMR, ³¹P-NMR, and mass spectroscopy. The curing behaviors of polymer precursors were evaluated by differential scanning calorimetry and thermogravimetric analyses.