Chemical, mechanical and morphological investigation on the hygrothermal aging mechanism of a toughened epoxy

A systematic investigation was implemented in this work to evaluate the effect of hygrothermal aging on the chemistry, mechanics and morphology of a toughened epoxy resin exposed in a 70°C bath. This resin system reached equilibrium moisture absorption within 2530 h with a saturation water uptake of 3.6%, and the moisture diffusion displayed a non-Fick behavior, but can be illuminated well by the Langmuir model. The influence of hygrothermal aging on the glass transition temperature (Tg) was evaluated by Differential Scanning Calorimetry. Chemical changes were monitored by Fourier Transform Infrared spectroscopy, where the evidence for reversible and irreversible chemical changes was observed. The Dynamic Mechanical Analysis was employed to reveal the effect of hygrothermal aging on viscoelasticity, mainly focusing on the Prony moduli and relaxation time, where an interesting phenomenon was noted that the Prony moduli and relaxation time are not affected by hygrothermal aging. The mechanical properties involving tension, compression, and shear were studied detailedly with respect to the dry, hygrothermal, and redry samples. Finally, the fracture morphology of the epoxy under three different hygrothermal aging conditions was analyzed by using Scanning Electron Microscopy. The mechanical results show that for compression, the pre-yield and post-yield both has been strongly weakened after hygrothermal aging, but the pre-yield performance can be recovered by redry; for tension, the nonlinear behavior before final failure has been weakened; for shear, the shear ductility has been significantly enhanced. And the fracture morphology after hygrothermal aging, the representative regions are reduced to two zones: defect and smooth zone.