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.