Flame Retardant Epoxy Resins containing Aromatic Poly(phosphonamides)

Abstract

As part of a program to develop more survivable aircraft, flame-retardant epoxy resins were investigated for their potential as fire-resistant exterior composite structures for future subsonic commercial and general aviation aircraft. Four different poly(phosphonamide)s were prepared by low temperature NMR condensation and characterized by Fourier transform infrared spectroscopy, 1H-NMR spectroscopy, 31P-NMR, differential scanning calorimetry, viscometry and gel-permeation chromatography. The poly(phosphonamides) were used as toughening agents (with 4,4'-diaminodiphenyl sulfone) to partially cure a commercially available unmodified liquid epoxy resin. The resulting cured epoxy formulations were evaluated for water uptake, flame retardance and fracture toughness. The formulations show good flame retardation with phosphorus content as low as 1.6% by weight, but exhibited high moisture uptake compared to the baseline epoxy. The fracture toughness of the cured formulations showed no detrimental effect due to phosphorus content (∼ 1.5% P). The heat of combustion of the pyrolysis gases, hc = 23.5 ± 1.3 kJ g—1 for the poly(phosphonamide) formulations were essentially the same as the N, N, N', N'-tetraglycidylether of -4,4'-methylene dianiline/4,4'-diaminodiphenyl sulfone epoxy formulation, hc = 24 kJ g—1.