ABSTRACT:The behavior of cured FM300 epoxy, a structural film adhesive, subjected to partial and full moisture saturation has been evaluated. Three separate but interrelated test methods were used: stress relaxation, fracture toughness, and dynamic mechanical testing. The mechanical response of the epoxy due to increasing moisture content was dependent on the testing method. In stress relaxation testing, the epoxy was plasticized when partially saturated with moisture, but it became more rigid when fully saturated. The plasticization-to-stiffening transition was not observed in the other two test methods. Fracture testing showed that the material toughness increased with increasing moisture concentration: plasticization effects were dominant. Similar changes in the loss modulus were found in dynamic mechanical analysis. We propose that the differences in behavior have been due to differences in load levels and loading rates used in these probing techniques. Stress relaxation testing, at a relatively lower load and loading rate, appeared to be more sensitive to the localized interactions between the absorbed water molecules and the crosslinked structure. Higher loads and loading rates tended to reveal the bulk effects of plasticization only. Nevertheless, there was also strong evidence from glass-transition temperature measurements that these moisture effects were mostly reversible.

Non-Fickian or anomalous diffusion is frequently observed when the absorption of moisture by a polymer is being studied. Different models have been presented in the literature that can accurately predict the trends of the weight-gain curves. However, it is not always clear which of these models yield good predictions of moisture distribution. This article presents a timeresolved moisture distribution study of an epoxy sample immersed in deuterated water (D 2 O) at 708C over a period of 2.5 months. The moisture distribution was measured during that period with a novel high-resolution magnetic resonance imaging technique that is well adapted to the imaging of thin plates. The experimental results showed that the concentration of D 2 O at the surface of the sample increased with time, even after 2.5 months. These results were used to evaluate the performance of several standard diffusion models. Although this study is phenomenological, it appears that a model featuring time-varying boundary conditions yields the best representation of moisture absorption for these samples.

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