Influence of accelerated hydrothermal aging on physical and mechanical properties of an amine-cured epoxy resin

Amine-cured epoxy resins (EP) are highly hydrophilic, particularly due to alcohol groups in the polymer network. The water absorption in humid environments leads to a reduction in the glass transition temperature (T<inf>g</inf>) and to a deterioration in the mechanical properties. These effects have been extensively studied in the past by exposure experiments carried out at temperatures of no more than 100 °C, usually below the T<inf>g</inf> of the polymer. In this work, however, an amine-cured EP is subjected to water at 100, 150, 200 and 250 °C, all above the polymer’s T<inf>g</inf>. Thus, it is possible to achieve water absorptions of more than 10% in less than three hours. This is due to the reduced density and viscosity of the liquid phase at these temperatures, and the rubbery state of the EP. Test times can therefore be minimized compared to conventional exposure experiments in this regard. The results of dynamic mechanical analyses and tensile tests confirm the expected depression in T<inf>g</inf> (up to nearly 50 %) and decrease in tensile strength and modulus with increasing aging temperature and water absorption, respectively. This change in properties is accompanied by a light to strong yellowing of the polymer. The formation of carbonyl groups responsible for this discoloration is confirmed by infrared spectroscopy. For further application of this accelerated hydrothermal aging, however, these findings still need to be validated by comparison with conventional exposure experiments.