Isothermal physical aging of a fully cured epoxy?amine thermosetting system

Abstract

The rate and effects of isothermal physical aging of a fully cured epoxy—amine/glass fiber composite specimen were studied for a wide range of isothermal aging temperatures (−180 to 200°C) using a freely oscillating torsion pendulum technique: torsional braid analysis (TBA). As assigned from the maxima in the mechanical loss vs. temperature, the glass transition temperature, Tg, was 182°C (0.9 Hz), and the principal glassy-state secondary transition temperature, Tβ, was ≈ −30°C (1.9 Hz). Plots of the increase in the isothermal modulus and of the decrease in the isothermal mechanical loss were linear vs. log aging time; their slopes provided aging rates. It was found that the isothermal aging rate varies with isothermal aging temperature (Ta) and that there are two maxima in the aging rate vs. Ta. A correlation presumably exists between the two maxima in the aging rate and the two transitions. This is not surprising since mechanical loss maxima (i.e., transitions) and aging rate maxima both correspond to specific, localized, and restricted submolecular motions. Effects after isothermal physical aging were investigated vs. temperature in terms of change of modulus of the specimen. The effect of isothermal aging existed primarily in a narrow temperature region localized about Ta. The majority of the isothermal aging effect can be eliminated by heating to temperatures above Ta, but below Tg. Theoretical and practical implications of this observation are discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 55–67, 1997