Development of a Stable Epoxy Resin System for Composite Repair.

Novel latent epoxy hardeners which show promise for the field repair of composite structures of naval aircraft have been developed and tested. Two hardeners, phthalamides of 1,2-diaminocyclohexane DACP and of m-xylylenediamine, were latent with multifunctional epoxy resins for 4 to 12 months and yield glass transition temperatures of 145C and 130C respectively, when cured at 150C. Fracture toughness KIC of a DACPMY 720 molding is 2.27 kgmm 32. Slightly higher than that of a standard diaminodiphenyl sulfone DDSMY 720 system. Graphite prepreg was made using DACPMY 720 and DDSMY 720 matrix resins, and unidirectional, well-consolidated laminates were prepared at low pressure 100 psi. Short-beam-shear and tensile values for the phthalamide laminate range between 72 and 105 of the values determined for the DDSMY 720 laminate. Dynamic mechanical analysis shows the Tg of the DACP laminate to be 130C and to decrease by about 90C with 1.1 water absorption on aging at 49C and 95 relative humidity. This hotwet conditioning reduces shear and tensile properties much more than for the DDSMY 720 control laminate.