Moisture effects on FM300 structural film adhesive: Stress relaxation, fracture toughness, and dynamic mechanical analysis

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. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95:1285–1294, 2005