Hydrothermal Aging of Composite Materials Part 2: Matrix Aspects

The effects of hydrothermal (100°C in H2O) aging on the epoxy matrix phase of a graphite-epoxy composite is examined in terms of chemical structure and Theological response. Infrared spectra indicate the resin system is a dicyandiamide (DICY) cured diglycyl ether of Bisphenol-A (DGEBA). Hydrothermal aging is shown to modify the resin network structure and increase the magnitude of the low-temperature β transition in short range molecular motion where T β ≃ 45±15°C at frequencies of 3.5 to 110 Hz. In addition to network structure changes the absorbed moisture temporarily lowers the resin glass temperature so that at 100°C in water the resin exhibits a rubbery response state. Evaluation of fracture properties of aged resin shows a loss in fracture toughness and development of internal microcracks in the bulk resin during hydrothermal aging.