Degradation of Epoxy-Steel Single Lap Joints upon Immersion in Water

As environmental exposure has been since long [1] recognised to be the major reason of degradation of adhesive bonds, there is a general demand for testing techniques and data about the response to humidity, taking into account the need for time saving and consistency with real life applications. Accelerated testing conditions [2] such as open-faced technique, exposure to warm moisture or immersion have become customary. The proposed work has the goal to enlarge the knowledge on the environmental response of a bi-component epoxy adhesive (Hysol 3425), which in a previous work [3] was exposed to warm moisture, whilst in this case is immersed in distilled water at room temperature. The specimens are single lap joints of steel substrates, fabricated with the open-faced technique [2] and tested under “dry” conditions (drying was performed before closure of the joints, i.e. application of secondary adhesive and substrate) after exposure times of 1-5 weeks. Such a procedure evidences only the irreversible damaging effects. The results show that the joint strength decreases during the first two weeks of exposure, then remains practically constant, with a final loss of about 30%. This behaviour is in contrast with that observed upon exposure to warm moisture [3], which exhibited a monotonic decrease (tending to zero residual strength) due to the progressive damage of the interface caused by oxidation. Visual and SEM observation of t e surfaces shows a mix of cohesive and interfacial failure. [1] D.M. Brewis, J. Comyn, J.L. Tagg, Int. J. Adh. Adh. 1, 35 (1980). [2] A.K. Moidu, A.N. Sinclair, J.K. Spelt, J. Adhesion 65, 239 (1998). [3] L. Goglio, M. Rezaei, M. Rossetto, J. Adhes. Sci. Technol., in press, DOI:10.1080/01694243.2012.697391 (2012).