Dynamic mechanical analysis of epoxy composites plasticized by water : artifact and reality

The plasticization of a glass/epoxy unidirectional composite by water has been investigated by Dynamic Mechanical Thermal Analysis (DMTA). The analysis of the thermomechanical spectra of the water aged material revealed the appearance of a splitting of the tan 6 peak associated with the α transition. This splitting was found to be dependent on the experimental conditions, i.e., the temperature scan rate, the straining frequency, and the initial water content on the composite. A thermogravimetric analysis (T.G.A.) of the wet specimens has shown that this effect could be attributed to an enhanced drying of the specimens above the glass transition, during the DMTA temperature scan. A differential plasticization of the epoxy network at the aging temperature can therefore not be invoked from the appearance of a splitting of the DMTA spectra. In conditions where the sample drying was minimized, plasticization effects on the thermomechanical properties of the composite were found to be temperature independent and reversible after re-drying of the aged material. Owing to the filling of hygrothermally induced cracks with water, the composite sensitivity to plasticization can be underestimated significantly at elevated aging temperature.

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